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NIST TECHNICAL NOTE 1292

U.S. DEPARTMENT OF COMMERCEATechnology Administration/National Institute of Standards and Technology

NIST Reactor:

Summary of Activities

July 1990 Through June 1991

QC

100

.U5753

NO. 1292

1992
NATIONAL LN'STITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20899

DATE DUE

Demco, Inc. 3«-2 93
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NIST Technical Note 1292 /^,r

NIST Reactor: T^

Summary of Activities

July 1990 Through June 1991

C. O'Connor, Editor

Reactor Radiation Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

January 1992

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U.S. DEPARTMENT OF COMMERCE

Robert A. Mosbacher, Secretary

Technology Administration

Robert M. White, Under Secretary for Technology

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards and Technology Technical Note 1292

Natl. Inst. Stand. Technol. Tech. Note 1292, 231 pages (Jan. 1992)

CODEN: NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1992

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
FOREWORD

This has been a year of steady progress in the Reactor Radiation Division

(RRD), with a reactor availability of better than 90% of scheduled time.

Although operating power had to be reduced to 15 MW in April because of

repeated problems with one main heat exchanger, the productivity of the facility

was high, as can be seen from the number of reports in this summary. New

replacement heat exchangers have been ordered, and are now being fabricated,

with delivery scheduled for 1992, This year also marks the first use of a fifth generation of fuel element for the NIST reactor, which will improve fuel

utilization beyond its already high level.

The dominant feature of the program of research supported by the reactor

is the great breadth and diversity of the effort, from measurements of fundamental

properties of the neutron to condensed matter physics to polymers to art history

and restoration to neutron standards. This year, we have attempted to group

contributions into broad general classes, but as is common with such schemes,

there remain many ambiguities. We hope that the change is productive, in spite

of the somewhat arbitrary nature of the subdivisions. In fact, the very diversity

that is the highlight and strength of the program renders it somewhat hard to assign to categories. Out of a multitude of possibilities, however, the following

examples will perhaps serve to highlight various areas. Work on the fundamental

properties of oxide superconductors continues to provide new insights into these

important materials, which offer a rich variety of magnetic and structural

phenomena. In particular, the studies of magnetic and atomic dynamics is essential to clarification of the mechanisms that lead to high T^. Studies of

magnetic multilayers, including new work in magnetic semiconductors which

provide a broad range of new phenomena, continues at a high level. Neutron

studies of C^o (Buckminsterfullerene) have shown definitively that the disorder in

the high temperature phase is due to rotational diffusion, and have also shown that

there is residual disorder in the low temperature orientationally ordered phase.

The studies of hydrogen in Nb-V alloys have revealed the occupation of

octahedral sites, with the lowest excitation energy ever measured in any metal

hydride. The availability of the 30-m SANS has allowed new studies of

"superanomalous" flow in liquids under shear, and systematic studies of the

effects of other process variables. The use of neutron reflectivity as a probe of

surfaces and interfaces continues to grow, with increasing sophistication of

analysis and technique. The effort to develop maximum entropy methods as an

essential tool in biological crystallography has been successfully completed, with

computer programs for application of the method developed and running. Finally, a new research collaboration in neutron scattering with the Johns Hopkins

University was successfully initiated.

Ill
The Cold Neutron Project continues to progress well. The 30-m NIST/

Exxon/U. Minn SANS was dedicated in May 1991, and the performance

continues to be improved as the inevitable problems are corrected. The neutron

lifetime measurements are under way with calibration of the flux measurement

systems (both foil and calorimeter) now being established. The analytical

chemistry instruments are on line (depth profiling and prompt gamma), and a

station to develop neutron focusing methods being used to characterize

components and try out concepts. The Center for High Resolution Neutron

Scattering (CHRNS) SANS (funded by NSF) is being assembled, and NG-3,

which will serve it, is being installed, with completion scheduled for February

1992. Other instruments are in various stages of construction and design, and the

supermirror technology is sufficiently developed to allow an order to be placed for

guides 1, 2, and 4. As a result of Monte Carlo modeling, a new hydrogen source geometry has been selected which promises a 50% improvement in flux; as a

result, detailed design is delayed, while analytical and experimental studies of the

new design are being performed at NIST, Boulder. We have initiated the user program at the Cold Neutron Research Facility (CNRF) with a call for proposals,

and the response has been quite satisfactory. The proposals are now out for

review.

The non-RRD programs at the reactor, both CNRF and thermal neutron,

continue to be healthy and productive, and a new program centered in the

Polymer Division is now in operation at the 8-m SANS in the guide hall.

In closing, I want again to recognize the dedication and professionalism of

the entire staff associated with the reactor—both from within RRD and outside.

It gives me great pleasure to congratulate Ted Prince and Tawfik Raby on their

receipt of the Department of Commerce Gold Medal for outstanding service to the

Nation, and I am sure that they would both want me to add that the work for

which they are recognized would not have been possible without the efforts of

everyone in the Division. A special word of thanks is due once again to Carol

O'Connor for her work on this report. She was ably assisted by Lisa Poole and

Linda Clutter.

Michael Rowe

Chief, Reactor Radiation Division

January 1992

IV
ABSTRACT

This report summarizes all the programs which use the NIST reactor. It covers the period

for July 1990 through June 1991. The programs range from the use of neutron beams to study

the structure and dynamics of materials through nuclear physics and neutron standards to sample

irradiations for activation analysis, isotope production, neutron radiography, and nondestructive

evaluation.

KEY WORDS: activation analysis; cold neutrons; crystal structure; diffraction;

isotopes; molecular dynamics; neutron; neutron radiography;

nondestructive evaluation; nuclear reactor; radiation.

DISCLAIMER

Certain trade names and company products are identified in order to adequately specify the

experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the

products are necessarily the best available for the purpose.

CONTENTS

FOREWORD iii ABSTRACT v

A. CONDENSED MATTER PHYSICS AND THEORY 1

Crystallographic and Magnetic Structure of ^^2^Qi'j^2.& ^ Temperature Dependence of Magnetic Order in UPdSn and UPtGe 2

Antiferromagnetic Ordering of [(CD3)3ND]FeCl3-2D20,

A One-Dimensional Ising Ferromagnet 4

Polarization Analysis of the Magnetic Excitations in Invar Feg6Bj4 6

Magnetoelastic Effects in Er/Lu Thin Films 8

Polarized Neutron Grazing Angle Diffraction 10

Anisotropic Spin Coupling in Gd/Y Superlattices 11 Magnetic Properties of Dy/Lu Superlattices 13

Normal Modes and Structure Factor for A Canted Spin System:

The Generalized Villain Model 15

Strain and Other Symmetry Breaking Effects in Magnetic Semiconductors 17 Small-Angle Neutron Scattering of Nanodispersed Magnetic Particles 19

Scaling Behavior of the Generalized Susceptibility in La2.xSxCu04+y 21

Magnetic Correlations and Energy Gap in Superconducting

YBa2Cu306 6 With T^ = 53 K 23

Magnetic Order of Cu in Nd2.xCexCu04 25

Two-Dimensional Magnetic Correlations and Magnetic Ordering

of Dy in DyBa2Cu307 27

Two-Dimensional Bilayer Magnetic Order of Dy Ions in Dy2Ba4Cu70i5 29

Magnetic Ordering of Sm in Sm2Cu04 30

Observation of the Magnetic Flux Lattice in DyBa2Cu307 32

Neutron Powder Diffraction Study of the Crystal Structures of YSr2(CoCu2)07, (Yi.xCax)Sr2(CoCu2)07, and YSr2(GaCu2)07 33

Neutron Powder Diffraction Study of the Structure of YSr2(AlCu2)07 35

Cu Chain and Plane Ordering of Single Crystal PrBa2Cu306+x ^^ Phonon Density-of-States in the Superconductors

(Nd-Ce)2Cu04, and (Pr-Ce)2Cu04 38

Antiferromagnetic Ordering of BaPr03 40

Quantitative Determination of Impurity Phases in YBa2Cu307.5 42

Neutron Powder Diffraction Study of the Nuclear and Magnetic Structures of YBa2Fe30g at Room Temperature 42

Have the Intermediate Bosons Responsible for Pairing in the HighTemperature Superconducting Copper-Oxide Materials Been Found? 43

Structural Study of a-FeCr Near the o -Phase Transition 45

Collective Excitations of Liquid Helium in Restricted Geometries 46

Toward Non-Gaussian Random Models of Surface Roughness for

Neutron and X-Ray Reflectivity 48

Theory of Fractal Behavior in the Leveled Wave Model of Random Morphologies 50

Theory of Coherent Neutron Scattering From n Itinerant Scatterers Embedded in Crystal Lattices 52

vu
B. CHEMICAL PHYSICS AND CRYSTALLOGRAPHY 53

Observation of C-Axis Mode-Splitting in a-LuHg ^9 53

Quantum Hopping of Protons in a-ScHj^ and a-YH^ 54

Vibrational Modes and Lineshapes in a-ScHg 34 and oc-ScDq 34 56

Octahedral Site Occupation in BCC NbV Alloys 57

Neutron Diffraction Studies of Solid C^q 58

Vibrational Spectroscopy of C50 and Graphitic Carbons 60

Quasielastic Neutron Scattering Study of the Rotations of C^q in the Solid Phase 62

Neutron Scattering Studies of Cs-Ammonia Intercalated Graphite 63

Lattice Dynamics of Hexagonal Boron Nitride 65

Low-Energy Network Dynamics of Silica Aerogels 66

Structure and Dynamics of Potassium Silicate Glass 68

Structure of Nitrogen in Porous Vycor Glass 69

Self-Diffusion in a Microporous Medium 71

Suspension Structure Under Shear Flow: SANS Study of the Origin

of Superanomalous Flow 72

The Vibrational Spectroscopy of Ammonium Ions in Titanyl Phosphates 73

Neutron Spectroscopic Studies of the Dynamics in Sodalites 75

Structure of the Sodalites Nag_,_j^[AlSi04]6(OH)2*nH20 and Deuterated Analogs 77

Structures and Properties of New Zeolite X-Type Beryllophosphate

and Zincophosphate Molecular Sieves 79

Sr2Bi205: A Structure Containing only 3-Coordinated Bismuth 81

The Crystal Structure of a Defect Pyrochlore, NaW206 5

• D2O 81

The Crystal Structure of Na^WOg^^/z ' y^z^ ...•• 82

Structure of Mg-Exchanged BePO-X 83

Structure of Lithium Chabazite 84

The Low Temperature Crystal Structure of Acetonitrile 85

Phase Determination for Macromolecular X-Ray Diffraction Data

by Maximum Entropy 86

Exploiting Symmetry in Crystallographic Fast Fourier Transforms 87

NIST*Lattice—Determination and Critical Evaluation of Symmetry 88

NIST Crystal and Electron Diffraction Data Center 89

Interlayer Diffusion in Langmuir-Blodgett Multilayers Containing

Layers of Two Types 90

Microstructure and Interlayer Diffusion in Langmuir-Blodgett Multilayers

of a Rod-Like Copolymer 92

Neutron Reflectivity Studies of Mitochondrial Outer Membrane 93

C. MATERIALS SCIENCE 95

SANS From PSD/PVME/PSH Ternary Polymer Blends 95

Small Angle Neutron Scattering From Deuterated Polystyrene in Dioctyl Phthalate Solution Under Shear 97

Structure Factor for "Dendrimer" Polymer Gels 98

Quasielastic Neutron Scattering Study of the Dynamics of Water in

Curing Cement 100

Small Angle Neutron Scattering Study of the Miscibility of SBR/PB Blends 101

Thermodynamics of Hydrocarbon Polymer Blends by SANS 102

Phase Separation in Deuterated Poly(Carbonate)/Protonated Poly(Methylmethacrylate)

Blends Near Glass Transition Temperature 104

vni
Neutron Reflectivity Study of Finite Thickness Effects in the Thermodynamics

of Weakly Segregating Diblock Copolymer Thin Films 106

Order/Disorder in Thin Diblock Copolymer Films 107

Neutron Reflectivity Studies of Copolymer/Homopolymer Mixtures 109

Neutron Reflectivity Study of a Polymer-Solid Interface 110

Segment Density Profile of Grafted Polymer Brushes Near the

Solid-Solution Interface 112

Phase Behavior of Aqueous Poly(vinyl-Alcohol) Solution

—A Small Angle Neutron Scattering Study 113

Diffuse Scattering in Icosahedral Phase Alloys 115

Textures of Tantalum Metal Sheets by Neutron Diffiraction 117

Residual Stress Determination in a Steel Fatigue-Test Specimen 118

Neutron Activation Autoradiography Study of Paintings ' 120

D. NUCLEAR ANALYTICAL CHEMISTRY 122

Nuclear Methods Group: Overview 122

Determination of Trace Element Impurities in High Temperature Superconductor

Starting Materials 125

Determination of Fluorine in Vegetation Reference Materials by INAA 127

Determination of Lithium in Geochemical Reference Materials by Thermal Neutron

Capture Mass Spectrometry 128

Bulk Lithium Concentration by Thermal Neutron Transmission 130

Inclusion of Marine Mammal Tissues in the National Biomonitoring Specimen Bank 132

Boron and Lithium in Total Mixed Diets of Countries Participating in the IAEA

Project on Dietary Intake of Minor and Trace Elements 133

Dietary Intake of Minor and Trace Elements in the United States 134

Neutron Activation Analysis at the Food and Drug Administration 137

Atmospheric and Environmental Studies by Instrumental and Neutron-Capture

Prompt y-Ray Activation Analysis 139

Effects of Target Shape and Neutron Scattering by Hydrogen on

Neutron-Capture Prompt y-Ray Activation Analysis (PGAA) 139

Submicrometer Size Distributions of Airborne Particles Bearing

Various Trace Elements 141

Sources of Airborne Particles In and Near Maryland and Their Wet Deposition 142

Elemental Concentrations in Airborne Particles Depositing in the Chesapeake Bay 144

E. NEUTRON INTERACTIONS AND DOSIMETRY 145

Dosimetry for Material Performance Assessment 145

Radiation Protection Dosimetry 147

Research and Technology Assistance 147

Irradiation and Calibration Facilities 149

Cold Neutron Physics and Metrology 151

Neutron Beta Decay 151

Absolute Determination of Neutron Flux 152

Neutron Interferometry 153

The Elastic Deformation of a Monolithic Perfect Crystal Interferometer:

Implications for Gravitational Phase Shift Experiments 155

Theoretical Analysis of Fundamental Tests of Quantum Mechanics

Via Neutron Interferometry 157

IX
F. FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT 159

Cold Neutron Project 159

The CHRNS-SANS Instrument at NIST 160

The 30 Meter NIST/EXXON/University of Minnesota Small Angle

Scattering Instrument 161

Multiple Disk Chopper Time-of-Flight Spectrometer 163

Cold Neutron Depth Profiling 164

Prompt-Gamma Activation Analysis With Cold Neutrons 165

Preliminary Design of the Liquid Hydrogen Cold Neutron Source 167

Development of a Focusing Mirror for SANS 169

Transmission Properties of Curved-Straight Neutron Guide Systems 171

Transmission of Neutron Benders 173

Studies on Silicon Wafer Microguides for Neutron Focusing 175

Deflection of Neutron Beams Using Capillary Optics 177

Correcting Time-of-Flight Neutron Diffraction Data for Chopped Delayed

Neutron Backgrounds 179

Neutron Reflectivity Fit Programs 179

Profile Refinement in Grazing Angle Diffraction 181

Resolution Considerations for Polarized Triple-Axis Spectrometry 183

Scattering Effects in Spheres . 185

Reactor Operations and Engineering 186

Practical Applications of Nuclear Research Reactors 187

Activation Determination for NBSR Irradiated Fuel Element Hardware

(Nonfiiel Sections) and Spent Shim Arms Assemblies 189

Neutron Activation Database 191

Response Tests for a NBSR Secondary System on-Line H^^ Monitor 192

Reactor Building Area Monitoring Using Thermoluminescent Dosimeters 193

G. PERSONNEL ROSTER 195

H. PUBLICATIONS 210
A. CONDENSED MATTER PHYSICS AND THEORY

A great deal of the work in this area is related to studies of oxide superconductors, with

a threefold emphasis—structure studies, magnetic

properties, and, to a lesser extent, dynamics and

mechanisms of superconductivity. This work

has benefited from a strong interaction with the

superconductivity center at the University of

Maryland and from collaborative studies with

Brookhaven, MIT, AT&T Bell Laboratories, and

DuPont. Our efforts to improve methods to quantitatively determine impurity phases in oxide

superconductors provides a necessary tool for

characterization of these materials.

Other programs in magnetic semiconductors

(with the University of Notre Dame), layered

materials (with the University of Illinois), and

structural phase transitions continue to contribute

to the effort in condensed matter physics. The

experiment on the dynamics of liquid helium in restricted geometries (with the University of

Pennsylvania) represents a new departure with

interesting results. The development of new theoretical models

for surface roughness and the extensions of an

earlier model for bicontinuous structures to

fractal structures represents an important step in our understanding of a class of phenomena that are of increasing interest (see reports on

reflectivity elsewhere in this report).

CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF Pr2Fei7N2 g

J. K. Stalick and J. A. Gotaas

and

S. F. Cheng, J. Cullen, and A. E. Clark (Naval Surface Warfare Center, Silver Spring, MD)

There has been recent interest in the use of

nitrided rare earth-iron compounds of the type

R2Fej7N3.j as permanent magnets. Coey and

coworkers [1-3] and Buschow et al. [4] have

reported that treatment of R2^^n compounds

with nitrogen results in a volume expansion of

about 7% along with a significant increase in Curie temperature and room temperature

magnetization. All compounds exhibit easyplane anisotropy at room temperature, while

retaining the original Th2Nij7 or Th2Znj7

structure type, except for samarium, which is easy-axis [1,2]. Pr2Fej7 is highly magnetostrictive at low temperatures; it is plausible to think that raising the Curie temperature by

nitrogenation will result in a compound with a

large room-temperature magnetostriction. This,

taken together with the easy-plane magnetic

anisotropy, could make Pr2Fe^7N^ compounds

very attractive transducer materials. We have

thus undertaken the structural analysis of

Pr2FeiYN3_g to determine the position of the

interstitial nitrogen for this structure type as well

as the magnetic structure of the Pr-containing

material.

Alloys of composition Pr2Fej7 were

nitrided at 793 K using flowing nitrogen gas. Thermal magnetic measurements showed a change of Curie temperature from 285 K for

Pr2Fe^Y to 715 K for the nitrided material.

Neutron diffraction data were obtained on the

NIST high-resolution powder diffractometer at room temperature and at 4 K, and the structure was refined using the Rietveld method. The N

atom was placed in the 9e site as suggested by

the structure determination of Nd9Fe2-7Co 5 by

Helmholdt and Buschow [5]. The occupancy of

the N atom was refined, along with the

occupancies of potential N atoms in the 3a

position as suggested by Luo et al. [6,7] and in the 18h position (x = 0.115, z = 0.05) as suggested by Stadelmaier et al. [8] for similar

carbides, as well as in the 18g site reported by

Jaswal et al. for Nd2Fe^7Nj^ [9]. The regions of

scattering of the impurity phase a-Fe were

excluded from the refinements.

The structure was found to be the expected

Th2Zn^7 structure type, space group R3 m, with a = 8.776(1) A, c = 12.649(2) A at 4 K and a = 8.794(1) A, c = 12.668(2) A at 295 K. The
CONDENSED MATTER PHYSICS AND THEORY

N atom was found to occupy the interstitial 9e

site, while the occupancies of the alternative 3a,

18h, and 18g sites refined to zero. The

occupancy of the 9e site at 4 K is 8.4(1) yielding

a composition of P^2^^n^2.s ^^^ ^'^ sample,

close to the ideal composition Pr2Fe^7N3 if the

9e site is fully occupied. This confirms the

results of Coey et al. [3], who deduced the

nitrogen position in Sm2Fei7N2 from EXAFS

data. The nitrogen atom is octahedrally

coordinated by two Pr and four Fe atoms; the

Pr-N distance of 2.542(1) A and the Pr-Fe

distances of 1.912(1) A and 1.934(2) A at 295 K

are consistent with the sums of the covalent

radii. The alternative sites for the N atom that have been suggested would result in Fe-N

distances of 1.21 - 1.49 A, considerably shorter

than the sum of the radii of N (0.70 A) and Fe

(1.24 A).

The magnetic moments of the Pr and Fe

atoms are ferromagnetically aligned perpendicular to the c-axis. The magnetic moment of

3. 4(1) Jig for Pr at 4 K is close to the free ion

moment of 3.2jig, although the actual moment

should be determined by the crystal field anisotropy in the system. At room temperature,

the reduced moment of 1.9(1)hb is the result of

thermal disorder. The refined magnetic

moments of the Fe atoms range from 2.1(2) -

2.9(2)jib at 4 K and from 2.4(2) - 3.1(1)iib at 295 K. They agree within the standard

deviations reported with those found for

Nd2Fej7Co5 by Helmholdt and Buschow [5],

and are close to the typical value of 2.2^3 for Fe.

References

[1] J. M. D. Coey and H. Sun, J. Magn. Magn. Mater.

87, L251 (1990).

[2] H. Sun, J. M. D. Coey, Y. Otani, and D. P. F. Hurley, J. Phys. Condens. Matter 2, 6465 (1990).

[3] J. M. D. Coey, J. F. Lawler, H. Sun, and J. E. M.

Allan, J. Appl. Phys. 69, 3007 (1991).

[4] K. H. J. Buschow, R. Coehoom, D. B. de Mooij,

K. de Waard, and T. H. Jacobs, J. Magn. Magn.

Mater. 92, L35 (1990).

[5] R. B. Helmholdt and K. H. J. Buschow, J. Less- Common Met. 155, 15 (1989).

[6] S. Luo, G. Zhang, Z. Liu, X. Pel, W. Jiang, and W. Ho, J. Magn. Magn. Mater. 70, 311 (1987).

[7] S. Luo, Z. Liu, G. Zhang, X. Pei, W. Jiang, and W. Ho, IEEE Trans. Magn. 23, 3095 (1987).

[8] H. H. Stadelmaier, E.-Th. Henig, G. Schneider, and

B. Grieb, Mater. Lett. 7, 155 (1988).

[9] S. S. Jaswal, W. B. Yelon, G. C. Hadjipanayis, Y.

Z. V^'ang, and D. J. Sellmyer, Phys. Rev. Lett. 67,

644 (1991).

TEMPERATURE DEPENDENCE OF MAGNETIC ORDER IN UPdSn and UPtGe

R. A. Robinson and A. C. Lawson (Los Alamos National Laboratory, Los Alamos, NM)

J. W. Lynn (University of Maryland, College Park, MD and Reactor Radiation Division)

and

K.H.J. Buschow (Philips Research Laboratories, Eindhoven, The Netherlands)

The intermetallic compounds UPdSn and

UPtGe have been studied by means of neutron

diffraction. Both materials had been studied

previously at LANSCE [1,2] and the intent of

the NIST work was to study the temperature

dependence of magnetic order in more detail. UPdSn is one of the set of ternary (1:1:1)

uranium-based compounds and crystallizes in the

space group P63mc with the hexagonal GaGeLi

structure, which is an ordered form of the Caln2

structure type. At low temperature it exhibits

the noncollinear antiferromagnetic structure shown in figure 1, with a uranium moment of

2.05 +/- 0.13 Hb' and where the canting angle

4) = 54° and 6 = 45°. Susceptibility measure- ments indicated that there might be two

transitions, the N6el point at = 40 K and a second transition at ~ 25 K. The previous

neutron measurements showed very different

diffraction patterns above and below 25 K, and

we thought the intermediate phase might be a simpler structure in which = 0. This would be

indicated by the absence of the (010) reflection,

which is the strongest at low temperature.

However, the (010) reflection did seem to be

present, but only just, above 25 K. To resolve
CONDENSED MATTER PHYSICS AND THEORY

QFigure 1. The magnetic structure of UPdSn. The right- hand panel shows the projection onto the orthorhombic a,b

plane, which is the same as the hexagonal basal plane. The

hexagonal cell is shown by the dashed lines. The left-hand panel shows a projection onto the orthorhombic b,c plane.

The angle <|> is = 54° and at the lowest temperature, the angle 6 is = 45°. The atoms are plotted with half their respective atomic radii; in order of increasing radius, they

are Pd, Sn and U.

whether this remnant was due to temperature

hysteresis, and to clarify the nature of the

magnetic phase transitions, we undertook a

careful temperature-dependent study.

The results, in terms of the three model

parameters, ^, 4> and 0, are shown in figure 2. We show results extracted from integrated

intensities, taken both in cooling and heating, in

panel (a), while in panel (b) we show similar

results extracted from measurements of peak

intensities. The statistical precision of the latter results is much greater, but the measurement is more susceptible to systematic errors associated

with background subtraction and possible

variation in linewidth, so the results in panel (a) are more reliable for absolute values of the

model parameters. We can make the following

conclusions: first, there is no temperature

hysteresis; second, there is excellent agreement

with the LANSCE results; third, there are no

sharp transitions below the N6el point and the

model parameters vary continuously. Indeed the

moment behaves quite normally and <|) is almost

temperature-independent. 6 does indeed rotate towards the b-c plane progressively, but at 35 K

it appears to move back again.

It is now quite a puzzle to correlate our

results with the two peaks in the susceptibility. In addition, the anomalous rotation near the N6el

point is also very strange: it may be that we are including critical scattering in our measurement,

and as the canting angles become ill-determined

there the results are unreliable. We now plan a single crystal experiment to resolve these

questions.

As for UPtGe, it is orthorhombic with the

NiSiTi structure, and orders magnetically at 52

K. We had previously shown that it exhibited

incommensurate magnetic order with q =

(0,0.555,0). In our present work at NIST, we

have shown that this q-vector does not change

with temperature and that there is a single order

parameter, the uranium moment. Figure 3

shows the variation with temperature of the

000+/- magnetic reflection.

o 50 ••••-

}

(") M

: 3o . °«= • ^ JO '..•

10

|60 • •

»

o

70

.: • ° 1

o •

° o 4 .•is

„ 4 3 10 — «

a • KOti'mt loi

o Udiog

J

Di

20 M

««M«)««»V^ o'W ^V o oo A ftx" V, "f ^•""^^-^

«»»,,<ba,«-»*«*v«~^°'^'^

Wrf",,X.

1.

•%>

•\.*,\

o

o*

rf*

*ipov

Figure 2. The variation of model parameters

for UPdSn, uranium moment, 6 and <(), with temperature as extracted (a) from integrated

intensities and (b) from peak intensities. The

solid circles represent data taken on cooling while the open circles represent data taken on

heating. The crosses represent the data reported in ref. 1.
3000 OOOt

^o

2500 """^N^o

OQo

2000 ... ^

CONDENSED MATTER PHYSICS AND THEORY

References

[1] R. A. Robinson, A. C. Lawson, K.H.J. Buschow, F. R. de Boer, V. Sechovsky and R. B. Von Dreele, J. Magn. Mater, (accepted for publication), Los Alamos

preprint LA-UR-90-3875.

[2] R. A. Robinson, A. C. Lawson and K.H.J. Buschow, LANSCE experiment report (1990).

20 40

T(K)

60

Figures. The variation of intensity in the 000 + /- magnetic

reflection in UPtGe, with temperature. The line is merely

a guide to the eye.

ANTIFERROMAGNETIC ORDERING OF [(CD3)3ND]FeCl3-2D20,

A ONE-DIMENSIONAL ISING FERROMAGNET

N. Rosov and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

J. J. M. Williams and C. P. Landee (Clark University, Worcester, MA)

Recently much attention has been focused on synthesizing organic-based magnets with

various desirable properties [1]. Initially, this work focussed on tailoring magnets to have

reduced magnetic exchange along one or two

dimensions [2]. The series of compounds

[(CH3)3NH]MX3 • 2H2O, where X = CI or Br

and M = Mn, Fe, Co, Ni, or Cu (referred to as MTAX) is characterized by 1-d anisotropic

Heisenberg-magnetic behavior. All of the

MTAX compounds, with the exception of

MnTAB and CuTAC, are orthorhombic, of

space group Pnma. The transition metal ions are

at the center of distorted octahedra of four

chlorines and two oxygens, with the CP ions

bibridging the M^"^ ions into chains along the b

axis, and the waters of hydration providing c

axis hydrogen bonding of the chains into planes.

The large trimethylammonium groups isolate the

planes along the a axis. The title compound,

[(CH3)3NH]FeCl3-2H20, FeTAC, is a good ex- ample of a 1-d Ising ferromagnet. Susceptibility

measurements [3] indicate the ferromagnetic in trachain exchange is J^/kg = 17.7 K and that the

small total interchain exchange is antifer- romagnetic, J7J^ == -0.0013. Just above the

N6el temperature, Tj^, the easy (b) axis susceptibility is a factor of 10"^ greater than the a or c

axis susceptibilities.

Approximately 3.5 g of powdered, fuUydeuterated FeTAC were placed in a flat-plate

holder in a pumped ^He cryostat at BT-9.

Figure 1 shows the diffraction pattern at 10 K

and the difference between scans at T = 0.4 K

and 10 K, over the range 2° ^ 26 ^ 62°. The

difference pattern indicates the development of

long-range magnetic order, which is confirmed

by the temperature dependence of the 26 = 8.1°

peak (fig. 2). The ordering is extremely sharp

and suggests that the transition is lowdimensional in nature. For T < 2.95 K, the

temperature dependence of the order parameter

is well represented by the exact Onsager solution

for an anisotropic 2-d Ising lattice, where [4]
CONDENSED MATTER PHYSICS AND THEORY

MZ2 = {l-sinh"V/^^7)sinh"V/^B7)}^^^

and J and J' are the two in-plane magnetic

exchange constants. In the present case, we

have chosen a ratio jJVJI = 1.3 x 10'^. This

fit gives a value of Tj^ = 3.02 K, in close

agreement with Tj^ = 3.06 K determined by dc

magnetization measurements of the deuterated

compound. A fit to the square 2-d Ising

magnetization gives Tj^ = 3.13 K and an

intensity curve which is not as sharp as the data. Given the large difference in the intra- and inter- chain exchange, we expect that the anisotropic 2-

d Ising result will be more consistent with the

data far from Tj^j. The rounding of the observed

order parameter in the vicinity of Tj^ is probably

due to the inclusion of critical fluctuations in the

scattering from a powder; this should be greatly

improved by planned single crystal measure- ments of the Bragg peak intensity.

The observed pattern of magnetic

diffraction peaks suggest that the spin

configuration is ferromagnetic along the b axis

and staggered along the [101] direction. A

collinear structure accounts for most of the

observed magnetic peaks, with the exception of

the peak at 20 = 20.42(3)°. This extra peak

suggests that there is canting in the magnetic

structure, in agreement with conclusions based on susceptibility and Mossbauer measurements

[3]. Further analysis and single crystal

measurements are planned to determine the nature of the canting.

E « « 3

^'V'v'v' W '\^. A :^ V

-^—I—I—I—(—I—I—1—I—1- I I

I I—I I I

I I I I

WW^

;' 'vyt^yv*^ W'J^hA^'J^i^W*''^^^

16000

2e(deg) Figure 1. (top) 28 scan of FeTAC powder at T = 10 K

with A = 2.3775 A neutrons, (bottom) Difference between

28 scans at T = 0.4 K and 10 K.

4000

Figure 2. Temperature dependence of the 28 = 8.1(1)° peak intensity, indicating Tj,^ = 3.16(2) K. The solid line

is the exact Onsager solution for the magnetization of a

anisotropic 2-d Ising lattice with J/kg = 10.2 K and

J7J = 1.3 X 10 , as described in the text. The dotted line

is the exact Onsager solution for a square 2-d Ising lattice.

Referaices

[1] see, e.g., J. M. Manriquez, G. T. Yee, R. S. McLean, A. J. Epstein, J. S. Miller. Science 252. 1415 (1991).

[2] R. D. Willett, R. M. Gaura, and C. P. Landee, in Extended Linear Chain Compounds , v. 3, J. Miller,

ed., Plenum (1983).

[3] R. E. Greeney, C. P. Landee, J. H. Zhang, and W.

M. Reiff. Phys. Rev. B 39, 12200 (1989).

[4] L. Onsager. Nuovo Cimento Suppl. 6, 261 (1949).
CONDENSED MATTER PHYSICS AND THEORY

POLARIZATION ANALYSIS OF THE MAGNETIC EXCITATIONS

IN INVAR Feg^Bj4

J. W. Lynn and N. Rosov

(University of Maryland, College Park, MD and Reactor Radiation Division)

Q. Lin (Beijing Institute of Physics, Beijing, China)

C.-H. Lee (Korea Advanced Energy Institute, Taejon, Korea)

and

G. Fish (Allied Signal, Inc., Morristown, NJ)

The spin dynamics of isotropic ferromagnets

at modest temperatures is known to be well

described by linear spin wave theory. In the

long wavelength hydrodynamic limit there is a Goldstone mode with dispersion relation given

by the familiar expression E^^ = Z)(T)q^. The

quantitative value of the "stiffness" constant D

depends on the details of the interactions and the nature of the magnetism, such as whether the

magnetic electrons are localized or itinerant, or

the structure is amorphous or crystalline, but the

general form of the spin wave dispersion

relation, and hence the spin wave density of

states, is invariant. The leading order

temperature dependence to the magnetization is then given by M(T) = M(0) [1 - B T^^^], where

the coefficient B is related to the spin wave

dispersion relation by

B =

C(3/2)gtXg

M(0) 4nD

3/2

(2)

A measurement of the spin wave dispersion

relation can then be directly related to the bulk

magnetization, and vice versa. These relationships have been found to be in excellent accord with experimental observations

for the vast majority of isotropic ferromagnetic

materials, with the singular exception of Invar systems [1,2]. In all the Invar materials,

whether they be amorphous or crystalline, eq.

(1) is found to fail in a major way, with the

observed stiffness constant as much as a factor

of two larger than inferred from magnetization

measurements. In an attempt to understand the

origin of this discrepancy, we previously carried out extensive unpolarized neutron measurements on the amorphous Invar Fe-B system in order to make a detailed comparison between spin wave

theory and experiment [2]. We found that spin

wave theory worked remarkably well in describing the long wavelength spin dynamics of

this Invar alloy system.

The conventional explanation for the

observed behavior is that there are additional

"hidden" excitations which participate in reducing the magnetization. The magnetization

and neutron measurements already put stringent

conditions on the form that such excitations

might take, since there is no freedom to change

the form of the theory, viz the T^''^ behavior for

the magnetization, the T^''" behavior for Z)(T),

etc. Hence we must have a density of "hidden"

excitations which has precisely the same form as the conventional spin wave excitations themselves. One possibility which has been

suggested [3] is that the (transverse) spin wave

excitations couple to the longitudinal fluctuations, yielding propagating longitudinal

excitations which peak at the transverse spin wave energies. Thus in an unpolarized beam

experiment they appear together, and cannot be

separated. We have been carrying out inelastic polarized neutron measurements on the Fe85Bi4

Invar system to explicitly separate the

longitudinal spin fluctuation spectrum (S^S^)

from the usual spin wave excitations represented

by S- = S^ + /S^. The measurements reveal the

presence of longitudinal excitations, not only in the vicinity of Tq, but substantially below the

ordering temperature as well.

Due to the amorphous nature of the sample,

all the present data have been taken in the small wave vector regime, where the spin waves are well defined at low temperatures. The Curie

temperature is 556 K, and the low T spin

stiffness coefficient is -120 meV-A^ [2]. The

polarization analysis technique as applied to this problem is in principle straightforward. All the transverse spin wave scattering causes a reversal
CONDENSED MATTER PHYSICS AND THEORY

of the neutron spin. These spin-flip cross

sections are denoted by^(-l--) and (-+). If the

neutron polarization V is parallel to the

momentum transfer (5, P || (^, then we may create a spin wave in the (-+) configuration, or destroy

a spin wave in the (+-) configuration.

Longitudinal fluctuations, on the other hand, are

invisible in this situation. Figure 1 shows a measurement with the (+-) configuration. The

strong peak on the energy gain side (E < 0)

corresponds to the destruction of spin wave

excitations. The weak peak on the energy loss side (E > 0) is caused by the imperfect

polarization of the instrument, and the fact that P and Q were not precisely parallel.

300-

200-

o

100

O OFF/ON

Figure 1. The (+-) spin- flip scattering cross section observed at 295 K and q = 0.06 A' . The energy gain

side shows a strong spin wave excitation.

In the configuration where P±Q, the spin wave scattering still causes a neutron spin-flip,

but it shows up with equal intensity in the energy

gain and energy loss cross section. The nonspin-flip {++) or (~) scattering, on the other

hand, is directly related to the longitudinal (S^S^)

scattering. Figure 2 shows a measurement in

this vertical field configuration. The spin-flip

scattering clearly shows spin waves in energy

gain and energy loss, as expected, while the

non-spin-flip data also display peaks at the spin wave energies. The scattering at the spin wave

positions is - 1/3 the strength of the spin-flip

scattering, while the flipping ratio is ~ 10. We

make the following remarks about this nonspinO OFF/OFF O ON/OFF

Figure 2. Observed scattering in the vertical field configuration (PxQ). The spin-flip scattering exhibits the usual spin wave excitations, while tlie non-spin-flip

scattering also reveals excitations near the spin wave

energies.

flip scattering: 1) The peak in energy obeys a q^ dependence. 2) The ratio of the intensity of

the spin-flip to nonspin-flip scattering did not change when experimental improvements

doubled the flipping ratio. 3) The ratio did not change significandy as a function of q, while the

resolution effects change substantially.

These data strongly suggest that there are longitudinal propagating excitations in this Invar system. These are just the type of excitations

which would be needed to explain the "Invar anomaly". However, a word of caution is in order, as the longitudinal and transverse

excitation energies are very close to each other,

and this is the most likely situation under which

"spurious" longitudinal scattering might be

found. Therefore, ftirther experimentation is warranted before unambiguous conclusions can be drawn.

References

[1] See J. W. Lynn and J. J. Rhyne, in: Spin Waves

and Magnetic Excitations, A. S. Borovik-Romanov

and S. K. Sinha, eds. (North-Holland, Amsterdam,

1988), Chap. 4, p. 177.

[2] J. A. Femandez-Baca, J. W. Lynn, J.J. Rhyne and

G. Fish, Phys. Rev. B36, (1987) 8497, and

references therein.

[3] See, for example, R. Raghavan and D. L. Huber,

Phys. Rev. B14, 1185 (1976); J. K. Bhattacharjee,

Phys. Rev. B27, 3058 (1983).
CONDENSED MATTER PHYSICS AND THEORY

MAGNETOELASTIC EFFECTS IN Er/Lu THIN FILMS

R. S. Beach, A. Matheny, M. B. Salamon and C. P. Flynn

(University of Illinois at Urbana-Champaign)

J. Rhyne (University of Missouri Research Reactor and Reactor Radiation Division)

and

J. A. Borchers and R. W. Erwin

In an earlier study of erbium thin films

grown epitaxially on nonmagnetic yttrium

baselayers [1], it was found that the ferromagnetic phase, which develops in bulk Er

below 20 K, is completely suppressed. Its absence was explained by a model for the

magnetoelastic energy in which the Er basal

plane is stretched toward the Y lattice through

elastic coupling of the two elements. This effect

inhibits both the basal plane contraction and

c-axis expansion that accompany the first-order

ferromagnetic transition in bulk Er and shifts the

energy balance away from ferromagnetism. As

a test of the model, we have studied a series of

Er films (400 A - 9500 A) [2] grown epitaxially on Lu baselayers which should induce

compression of the Er basal plane.

Magnetic neutron scattering experiments

performed on a triple axis spectrometer show

that all of the Lu based films exhibit magnetic

phases similar to bulk erbium. Specifically, the

paramagnetic phase of elemental Er gives way at 85 K to one in which the moments order

sinusoidally along the c axis with a period near Tq. At 54 K a helical spin component develops

in the basal plane and the c-axis modulation

(CAM) begins to square up. At 20 K the

competition between the exchange and magnetoelastic energies drives a first-order transition to a cone state, ferromagnetic along the c axis

while remaining helical in the basal plane.

For the Er/Lu films the CAM wavevector was extracted from the positions of the magnetic

satellites of the (1010) reflection. The resulting

phase angle falls up to 3° below the bulk values.

This behavior contrasts with turn angles for

Er/Y films, [3] which are as much as 2.5° per

layer larger than bulk. In figure 1 the 30 K

phase angle data plotted as a function of Er

thickness demonstrate that the CAM periodicity

approaches bulk behavior as the film thickness is increased.

^^0) 2-47

Q> l«

V

*

4 , , . ,

356 •S

2000 4000 Thickness (A)

2000 Thickne

4000

(A)

Figure 1. Er thickness dependence of the phase angle of

the c-axis modulated spin structure at 30 K along with the Curie temperature upon heating (open circles) and cooling

(shaded circles). Also plotted are the a-axis lattice parameter and out-of-plane mosaic of the Er/Lu films. The

solid lines mark the corresponding bulk values.

Contrasting with the Er/Y films, the CAM

collapses to a conical ferromagnet near 20 K in

all of the Lu based films except for one 800 A

thick. The plot of the transition temperature

versus film thickness in figure 1 shows that T^

is enhanced from the 20 K Er value for the

400 A and 600 A films and reduced from bulk

for thicker films. In addition the hysteresis

through the transition ranges from 5 K to 10 K

relative to the 1.5 K hysteresis observed in

elemental Er [4]. Structural characteristics of the films

obtained by x-ray diffraction techniques correlate

strongly with the magnetic properties. The

a-axis spacing of the Er layer at room

temperature is plotted as a function of Er layer

thickness in figure 1. For films thinner than

800A, the lattice parameter approaches, but does

not equal, the smaller Lu value; the Er and Lu

lattices are not coherent in the growth plane.

The erbium layer relaxes sharply with increasing

film thickness, as expected. The slope of

approach changes near 800 A coinciding with the cross-over in the Curie temperature and turn angle data. Similar to the yttrium based films

[1], bulk Er spacings are not fully recovered for even the thickest 9500 A film.
CONDENSED MATTER PHYSICS AND THEORY

The out-of-plane mosaic determined from

the width of rocking curves through the (0002)

reflection provides an indirect measure of the

density of misfit dislocations. As demonstrated

for ErAs films on GaAs substrates [5], an

increase of the mosaic generally accompanies

strain relief. Instead the room temperature

values for the Er/Lu films plotted in figure 1 fall off exponentially with increasing Er thickness to a value near 0.1° for the 9500 A film. The

relaxation of the Er lattice is apparently

associated with the dilution or "healing", rather

than the formation, of defects. The properties of the Er/Lu films

summarized in figure 1 can clearly be divided

into three regions: t < 800 A, t « 800 A and t > 800 A. In the first, the lattice is significantly

strained by the Lu base layer, the phase angle of

the spin order is reduced from bulk and T^-, is enhanced from 20 K. In the second, the lattice

is partly relaxed and the ferromagnetic phase is suppressed. In the third region, the strain is relieved and Tq is slightly reduced from 20 K.

The modified transition temperatures

observed in the Er/Lu films are consistent with

a magnetoelastic energy model developed to explain the suppression of ferromagnetism in the

Er/Y films [1]. Assuming epitaxial growth of

the Er on the Lu layer, an elastic energy

contribution from the Lu lattice is added to the

magnetoelastic energy tern for bulk Er. The Er

and Lu basal plane lattice parameters are constrained to be equal at the interface, and the

total magnetoelastic energy is minimized with

respect to strain. The difference between the

magnetoelastic energy of the ferromagnetic and

antiferromagnetic configurations can then be

calculated as a function of the lattice strains and

compared to the opposing exchange barrier [6] of 1.01 J/cm^. In figure 2 the driving energy

plotted as a function of e^^ is compared to the

energy equivalent of the low temperature critical field or to the difference between the measured

and bulk Tq for the Er/Lu and Er/Y films.

Clearly the strain induced by epitaxial growth on

the smaller Lu lattice or bigger Y baselayer is responsible for the thickness dependence of the

magnetic properties observed in both systems.

Figure 2. Magnetoelastic driving energy for the ferromagnetic transition plotted as a function of c-axis

strain. The energy was calculated from the model

described in the text. Energy equivalents of the low

temperature critical fields or the differences between the measured and bulk T^-i upon cooling for the Er/Y films and

superlattices [1] (open circles) and Er/Lu films (closed

circles) are plotted for comparison.

The effects of epitaxial strain on the

magnetic behavior of Er on Y carry smoothly

over to the Er/Lu systems where the Er is compressed rather than stretched in the basal

plane. The Er/Lu results suggest the

introduction of a "cross-over" thickness near 800 A at which the Er lattice is significantly

relaxed. Additional structural characterization is in progress to identify the mechanism

responsible for strain relief.

References

[1] J. A. Borchers, M. B. Salamon, R. W. Erwin, J. J. Rhyne, R. R. Du and C. P. Flynn, Phys. Rev. B 43,

3123 (1991).

[2] R. S. Beach, J. A. Borchers, R. W. Erwin, J. J. Rhyne, A. Matheny, C. P. Flynn and M. B. Salamon, J. Appl. Phys. 69, 4531 (1991).

[3] J. A. Borchers, M. B. Salamon, R. W. Erwin, J. J. Rhyne, G. L. Nieuwenhuys, R. R. Du, C. P. Flynn

and R. S. Beach, Phys. Rev. B (to be published).

[4] M. Habenschuss, C. Stassis, S. K. Sinha, H. W.

Deckman and F. H. Spedding, Phys. Rev. B 10,

1020 (1974).

[5] P. F. Miceli, C. J. Palmstrom and K. W. Moyers,

Appl. Phys. Lett, (submitted).

[6] M. Rosen, D. Kalir and H. Klimker, Phys. Rev. B

8, 4399 (1973).
CONDENSED MATTER PHYSICS AND THEORY

POLARIZED NEUTRON GRAZING ANGLE DIFFRACTION

J. F. Ankner, C. F. Majkrzak, and D. A. Neumann

and

A. Matheny and C. P. Flynn (University of Illinois at Urbana-Champaign)

We have carried out the first measurements

employing the grazing-angle diffraction geometry

using polarized neutrons and observed the

magnetization of an Y/Gd film. Magnetic

materials (both films and multilayers) have

received a great deal of attention both because of

their inherent technological utility and for the

richness of behavior they exhibit. A number of

experimental techniques, of varying quantitative

value, have evolved to observe this behavior

(e.g., magneto-optic Kerr effect [MOKE], spinpolarized low-energy electron diffraction

[SPLEED], and spin-polarized photoemission).

We have for the past several years been

exploring the possible uses of grazing-angle

neutron diffraction for the study of interfacial problems [1,2]. Neutron diffraction methods

possess the advantage that the extraction of

quantitative information is straightforward

(described by the first Born approximation for

most materials). In grazing-angle diffraction, a finely

coUimated beam strikes the surface of a flat sample at an angle of incidence near the critical angle for total external reflection. Glancing

angles of incidence and exit ensure that the

probe intensity is confined to a thin layer at the

interface. Weakly interacting waves such as x

rays and neutrons can in this way be used to study physics and chemistry at surfaces and

interfaces.

Our experiment was performed on the

BT-7 reflectometer at NIST [3]. In this in- strument, a focussing graphite monochromator

produces a monochromatic neutron beam of

wavelength 2.37 + 0.02 A, which is collimated

by slits to an angular divergence of about 1 arcmin. A unique polarization of the beam is selected by reflection from a ferromagnetic FeSi

supermirror. For this experiment we suspended

a shielded ^He tube above the sample table, used

the arc motions of the goniometer head to align

the sample in-plane, and manually positioned the

detector to specify the diffraction angle. The

data were collected as a function of either

incident angle or rocking angle of the

goniometer head and integrated over diffracted

exit angle. For this experiment, we used a 3-in

diam film grown by molecular beam epitaxy,

consisting of a 150 A Y(OOOl) film atop 5000 A

of Gd and studied the (1120) in-plane Bragg

peak. Figure 1 shows a plot of intensity versus

incident wavevector k^ = (27r/X)sin (j), where ^

is the angle of incidence of the neutron beam

onto the sample with the detector set at 20g = 82°. The two spin states exhibit the same intensity at small k^, because the neutrons

only illuminate the topmost atoms (those in the

non-magnetic yttrium layer). As the wavevector

increases, the ferromagnetically aligned

gadolinium atoms are probed and cause a

splitting of the polarization states that falls away

at the largest incident angles as the crystal moves away from the Bragg condition.

60

40

20

"1 I I 1 \

1 r

i

I

{

•

0.00 J I I I III* 0.01 0.02 0.03 0.04

ICo (A-')

Figure 1. Rod scan of Y/Gd(1120) at 7 = 150 K. We have measured diffraction using

polarized neutrons in the grazing-angle

geometry. The next step in the development of

this technique is the extraction of quantitative

information from the data. We have outlined a formalism for this data analysis [3,4] and are obtaining a position-sensitive detector capable of

resolving the exit diffracted intensity into its wavevector components (an essential requirement

for the determination of structural parameters).

This improvement in instrumentation coupled

10
CONDENSED MATTER PHYSICS AND THEORY

with the theoretical formalism should produce

quantitative results in the near future.

References

[1] J. F. Ankner, H. Zabel, D. A. Neumann, and C. F. [3] Majkrzak, Phys. Rev. B 40, 792 (1989).

[2] J. F. Ankner, H. Zabel, D. A. Neumann, C. F. [4] Majkrzak, J. A. Dura, and C. P. Flynn, J. Phys.

(Paris) 50, C7-189 (1989).

J. F. Ankner, C. F. Majkrzak, D. A. Neumann, A. Matheny, and C. P. Flynn, Physica B, in press.

J. F. Ankner, in Proceedings of the Second

International Conference on Surface X-ray and Neutron Scattering, Springer-Verlag, in press.

ANISOTROPIC SPIN COUPLING IN Gd/Y SUPERLATTICES

R. W. Erwin and J. A. Borchers

T. F. Tsui, C. P. Flynn and M. B. Salamon

(University of Illinois at Urbana-Champaign)

and

J. J. Rhyne (Missouri University Research Reactor, Columbia, MO and Reactor Radiation Division)

Studies of c-axis rare-earth superlattices

such as Gd/Y [1] and Dy/Y [2] have demonstrated that the magnetic order propagates

through nonmagnetic interlayers with an

interaction strength ranging from 1 to 10

K/magnetic ion. Specifically, for the Gd/Y

system the alignment of the ferromagnetic Gd

layers fluctuates from ferromagnetic to antiferromagnetic with a period of 25 A as the

yttrium layer thickness is increased. The

strength, range and oscillatory nature of this spin

coupling is consistent with a simple model

describing the indirect RKKY interaction

resulting from the polarization of the Y

conduction band by the adjacent gadolinium

moments [3]. This interaction, however, should

decay sharply along the a- and b-axis directions

due to anisotropic nesting features in the yttrium

Fermi surface [4]. To probe the symmetry of

the RKKY coupling mechanism, we have

examined three epitaxial b-axis Gd/Y

superlattices, t)-[Gd43^ | Y52A.185

b-[Gd47^|Y42A]5o and b-[Gd60A I Y26a]80, W

neutron diffraction techniques.

Typical scans along the b* direction for

b-[Gd43^ I Y52a]85 tbe sample with the thickest

Y interlayer, are shown jn figure la. The large

central peak is the (1010) reflection from the

yttrium substrate. The 315 K data show distinct

superlattice satellites separated from each other

by AQ = 2n7r/A where A == 95 A is the period of

the superlattice. Below T^-, the presence of

magnetic peaks with a periodicity of 2A suggests

that the Gd layer moments anti-align across the

Y interlayers. For contrast, diffraction data for b-[Gd60A I '^26aJsO' ^be sample with the thinnest

Y interlayer, are shown in figure lb. The extra

intensity evident at the superlattice satellite positions at 90 K indicates that the Gd layer

moments are aligned. Antiferromagnetic order

develops, however, at temperatures greater than

120 K. At 90 K the third sample, b-[Gd47 ^|

Y42a]5o exhibits a spin state with mixed

ferromagnetic and antiferromagnetic order.

For all three samples the strength of the

antiferromagnetic coupling at 150 K is proportional to the c-axis saturation fields which

equal 80 G, 35 G and 20 G for b-[Gd43^|

Ys2a]85, b-[Gd47A I Y42a]50

Y26a]80 respectively.

and, b-[Gd60Al

The fields decrease

slightly with decreasing Y thickness and are

significantly smaller than those measured for

c-axis Gd/Y systems (ie. ~ 1 kG) [1]. The reduction of the saturation fields may

follow from the anisotropic nature of the

RKKY-like exchange interaction. In previous

studies of b-axis Dy/Y superlattices [5] it was

suggested that the range of the coupling along

the b-axis is a factor of 10 less than that along

the c-axis. For comparison, we have derived a

11
CONDENSED MATTER PHYSICS AND THEORY

600

200

I 1 r

a) G<i(43i)/Y(52A)

1200

400-

2.2

Figure 1. a) Neutron diffraction scans along the b*

direction forb-[Gd42A| Y^oA-IsS ^^ ^^ ^ (circles) and 315

K (triangles). At 80 K peaks develop with twice the bilayer

periodicity indicating that the Gd layer moments are

anti-aligned. b) Neutron diffraction scans along the b*

direction for b-[GdgQ^| Y2g^]go at 90 K (circles) and

315K (triangles). The extra intensity evident at the

superlattice satellite positions at 90 K indicates that the Gd

layer moments are aligned.

relationship for the strength of the dipolar

coupling between two finite ferromagnetic

sheets. Because the magnitude of the interaction

tends to zero as the width of the slabs

approaches infinity, it is only necessary to include contributions from the moments at the

ends of each sheet. The resulting expression for

the critical field is

H = 4M^ln( ^ ) (3)

where M is the magnitude of the Gd moment, W

is the width of the Gd slabs and t^^ and ty are

the thicknesses of the Gd and Y layers

respectively. The strength of the field required

to align sheets (~ 4000 A x 4000 A) is the same

order as the measured saturation fields and falls off gradually as the spacing between the sheets

is increased as shown in figure 2. For the b-axis

Gd/Y systems we suggest that the exchange

coupling, which favors ferromagnet alignment of

the Gd sheets, competes with the dipolar

coupling which favors antiferromagnetic

alignment. In 3-d both the dipolar and the ideal

RKKY interaction fall off as 1/r^. Since the

RKKY interaction depends on conduction

electron transport, mean free path effects give

rise to damping terms. The range of the dipolar

interaction is thus greater than that of the RKKY

interaction. A similar effect in lower dimensions

may explain why the Gd moments have a tendency to anti-align for thicker Y layers.

.ecu

1 1 1 1

230 o\

O N

-

190 - -

_ ^\^^^ _

150 1

1

1 1

1 30 110 190

ty (A) 270

Figure 2. Applied field required to align two fmite ferromagnetic sheets plotted as a function of the separation

distance ty The calculation assumes dipolar coupling

between sheets 4000 A x 4000 A with t^^ = 29 A. The

solid line corresponds to the approximation described in the

text, and the points were determined by a direct summation

technique.

The results for the b-axis Gd/Y superlattices clearly confirm the asymmetric nature of

the RKKY spin interaction. In addition, the spin

alignment is apparently determined by a competition between the exchange and dipolar

coupling which are of comparable magnitude.

Additional samples are being prepared to characterize further the nature of the spin

coupling.

References

[1] C. F. Majkrzak, J. W. Cable, J. Kwo, M. Hong, D.

B. McWhan, Y. Yafet, J. V. Waszczak and C.

Vettier, Phys. Rev. Lett. 56, 2700 (1986); J. Kwo,

M. Hong, F. J. DiSalvo, J. V. Waszczak and C. F. Majkrzak, Phys. Rev. B 35, 7295 (1987).

[2] R. W. Erwin, J. J. Rhyne, M. B. Salamon, J. A.

Borchers, S. Sinha, R. Du, J. E. Cunningham and

C. P. Flynn, Phys. Rev. B 35, 6808 (1987).

[3] Y. Yafet, J. Kwo, M. Hong, C. F. Majkrzak and T. O'Brien, J. Appl. Phys. 63, 3453 (1988).

[4] R. R. Du, F. Tsui and C. P. Flynn, Phys. Rev. B

38, 2941 (1988).

[5] F. Tsui, C. P. Flynn, M. B. Salamon, R. W. Erwin,

J. A. Borchers and J. J. Rhyne, Phys. Rev. B 43 . 13320 (1991).

12
CONDENSED MATTER PHYSICS AND THEORY

MAGNETIC PROPERTIES OF Dy/Lu SUPERLATTICES

J. A. Borchers and R. W. Erwin

R. S. Beach, A. Matheny, M. B. Salamon and C. P. Flynn

(University of Illinois, Urbana-Champaign, IL)

and

J. Rhyne (Missouri University Research Reactor, Columbia, MO and Reactor Radiation Division)

The evolution of molecular beam epitaxy

(M.B.E.) has led to the ability to tailor materials on the atomic scale. The one dimensional

periodic confinement of alternating materials in a superlattice is the simplest step in that direction. Since the first metallic superlattices were constructed in the early 1980's, a variety of

magnetic superlattices including Dy/Y [1], Gd/Y

[2] and Fe/Cr [3] have been studied to characterize the origin and nature of spin coupling

across nonmagnetic interlayers. As a continuation of this effort, we have probed the structural and magnetic properties of a series of epitaxial

dysprosium/lutetium superlattices using neutron

and x-ray scattering techniques.

In elemental Dy, at Tjq = 178 K ferromagnetic planes perpendicular to the c-axis form

a spiral structure; the spins rotate from one sheet

to the next by a turn angle of 43°. The phase

advance decreases to 26.5° at 85 K, where a spontaneous first order transition to a-axis

ferromagnetism is driven by the competition

between the exchange and magnetoelastic

energies. This is accompanied by a 0.5%

orthorhombic distortion of the basal plane.

Previous studies of c-axis Dy/Y

superlattices show that the Dy spiral order

propagates across the intervening nonmagnetic

yttrium [1] via a spin-density wave stabilized in the Y conduction band. In addition, the

ferromagnetic phase of Dy is completely

suppressed by the epitaxial clamping of the Dy

to the nonmagnetostrictive Y layers and

substrate. Though the Lu band structure is similar to that of Y, epitaxial strains produced in a superlattice with Dy layers should be quite

different. While Dy expands to accommodate

the Y atomic spacing, the 1.2% lattice mismatch

between Dy and Lu compresses the Dy in the

basal plane.

Room temperature x-ray scans along the b*

direction verify that the Dy and Lu layers are lattice-matched in the basal plane. The coherent

growth gives rise to a periodic strain gradient

along the c-axis that can be characterized from

fits of the (000/) scans to a damped rectangle wave model [1]. Using the Hendricks-Teller

approximation [4], the model has been modified

to include various types of structural disorder

[5]. A typical scan and fit are shown in figure

1 for the superlattice [Dy2i | Lu^qIvo- According

to the fit this sample consists of an average of 21

planes of Dy and 10 planes of Lu in each bilayer

with interfacial diffusion over 4-5 lattice planes.

The number of Dy and Lu layers per bilayer

vary locally with a full-width of 2 planes and 1 plane respectively. The resulting profile of the

c-axis spacings for a single bilayer is shown in the inset. 900

CO

IO

4.2 4.4 4.6 4.8 Q (A-^) Figure 1. Room temperature x-ray diffraction scan through the (0004) reflection for IDy^i | Lui Q]-yQ. The

splitting results from al and a2 radiation produced by the Cu source. The inset shows the profile of the c-axis lattice parameter through two bilayers obtained from fits of the data to the model described in the text.

Figure 2 shows complementary neutron

scans through the (0002) reflection for

[Dyi5|Lui5]40 at 160 K and 15 K. In addition

to the central structural peaks, the 160 K data

exhibit the signature of the Dy spiral phase. In

fact all five superlattices examined display long

range spiral order below Tj^ » 175 K. The helix

13
CONDENSED MATTER PHYSICS AND THEORY

in the superlattices advances in the Dy at no more than 31°/layer, with the smallest turn angle of 26° observed at the ferromagnetic

transition. The phase angle is smaller at any

given temperature than in Dy/Y superlattices or

in elemental Dy. The spiral order propagates

coherently through the Lu with a phase advance

of roughly 45° per Lu layer. This value is close

to the maximum in the generalized susceptibility

of the nonmagnetic element [6], as is the case in

the Dy/Y system.

2.2 2.4 2.6

Figure 2. Neutron diffraction scans through the (0002)

reflection for [Dy, jILuijI^q at 160 K and 15 K.

In figure 2 the presence of magnetic peaks

between the structural superlattice peaks at 15 K

corresponds to a doubling of the chemical unit

cell; each Dy layer is ferromagnetic, but

alternating layers are oppositely aligned across

the Lu interlayers. We observe that the three

superlattices with the thickest Lu interlayers

order with anti-aligned Dy layers at 160 K, 150

K and 145 K for [Dy^^ | Lu2o]6o. i^Yis I Lui5]4o

and [Dy2i ILujoIvq, respectively. In contrast the

Dy layer moments in the two samples with the

thinnest Lu, [Dyi4|Lu8]7o and [Dyi4|Lu5 jlgo, are ferromagnetically aligned below ~ 140 K.

We note that both ferromagnetic and

antiferromagnetic interlayer coupling were also

evident in c-axis Gd/Y superlattices [2]. In comparison to the bulk Dy value of 85

K, Tq is enhanced by as much as 80 K. A

model for the magnetoelastic energy [1], which

successfully predicted the critical fields for Dy/Y

superlattices, was also able to account for the

transition temperature as it related to strain in a

series of Er films grown on Lu buffer layers [7]

The same model, applied to the Dy/Lu series,

does predict a small jump in Tq.

The model assumes that the Dy layers are highly strained and forced to follow the normal

thermal expansion of the Lu interlayers. X-ray

data obtained at the NSLS, however, reveal a

distortion of the entire superlattice basal plane,

approximately 85% of the anomalous strain

observed in bulk Dy, accompanying the

spiral-ferromagnetic transition. Additional

synchrotron and neutron measurements are in progress to correlate changes in the structural and magnetic order through the ferromagnetic

transition.

References

[1] R. W. Erwin, J. J. Rhyne, M. B. Salamon, J. A.

Borchers, S. Sinha, R. Du, J. E. Cunningham and

C. P. Flynn, Phys. Rev. B 35, 6808 (1987); R. W.

Erwin, J.J. Rhyne, J. A. Borchers, M. B. Salamon,

R. Du and C. P. Flynn, Mat. Soc. Symp. Proc. 166. 133 (1990).

[2] C. F. Majkrzak, J. W. Cable, J. Kwo, M. Hong, D.

B. McWhan, Y. Yafet, J. V. Waszczak and C.

Vettier, Phys. Rev. Lett. 56, 2700 (1986); J. Kwo,

M. Hong, F. J. DiSalvo, J. V. Waszczak and C. F. Majkrzak, Phys. Rev. B 35, 7295 (1987).

[3] S.S.P. Parkin, N. More and K. P. Roche, Phys. Rev. Lett. 64, 2304 (1990).

[4] Sterling Hendricks and Edward Teller, J. Chem.

Phys. 10, 147 (1942).

[5] E. E. Fullerton, 1. K. SchuUer, H. Vanderstraeten and Y. Bruynseraede, Phys. Rev. B (submitted).

[6] S. H. Liu, R. P. Gupta and S. K. Sinha, Phys. Rev. B 4, 1100(1971).

[7] R. S. Beach, J. A. Borchers, R. W. Erwin, J. J. Rhyne, A. Matheny, C. P. Flynn and M. B. Salamon, J. Appl. Phys. 69, 4531 (1991).

14
CONDENSED MATTER PHYSICS AND THEORY

NORMAL MODES AND STRUCTURE FACTOR FOR A CANTED SPIN SYSTEM:

THE GENERALIZED VILLAIN MODEL

W. M. Saslow (Texas A&M University, College Station, TX)

and

R. W. Erwin

We have studied the generalized Villain

model (GVM), which is one of the simplest

periodic frustrated systems. Because it is a

frustrated spin system, it contains a key

ingredient of any model claiming to describe

reentrant-spin-glass behavior (RSG), and because

it is periodic we can obtain exact solutions of the

spin-wave dynamics. This may eventually be

useful in separating the effect of randomness

from other possible mechanisms for spin-wave

damping, such as the coupling of transverse and

longitudinal degrees of freedom due to canting.

Furthermore, for a certain parameter range, the

GVM has a phase transition from a collinear to a canted state. This is of interest because there

has been some controversy regarding the extent of the breaicup of ferromagnet order in the RSG

phase [1-2].

The Villain model (VM) consists of spins on a square lattice, with nearest neighbor

interactions. Vertical bonds are all J, and the

horizontal bonds are alternately J and - 7,

according as they lie on even or odd numbered

rows. Every square (or plaquette) of spins has

a single antiferromagnetic bond, and thus a system of Ising spins is frustrated. Berge et al. have also defined a generalization of this model, the GVM, in which

the - / bonds take on the values J' = - r\J, and

for 7 > the quantity ri was varied [3] . We

consider, at zero temperature, the normal modes

and their structure factor for the GVM, treating

three-component spins in the linear spin-wave

approximation.

This problem is in the class of models

described by the Hamiltonian

2 ij u I S. -X'S.

J I I H (1)

which consists of an exchange part and a Zeeman part. We assume that the spins have

been equilibrated, and now turn to the question

of the spin dynamics.

Linearizing the equations of motion for transverse spin deviation components, a^ and B,-, defined with respect to the local equilibrium spin

direction, one finds eqs. (3.9) of Walstedt and

Walker [4]

«/ -\^i-Jij[(h • ^j)<^j -(P/ • Py)P,]. (2)

^i=-X.a. + y..[(5^. • ^j)aj i^i

' Py)Py]- (3)

Note that the relative canting angles appear in the coefficients of these equations. Applying

orthonormalization conditions, the structure

factors for a given eigenmode can be calculated

as.

S(^) '£e-^^''^iSi\^. (4)

As an example of our calculations, the

energies and structure factors are shown for

T) = 0.5 in figure 1. Here the frustration is sufficiently strong to produce a canted ground

state, with acoustic modes having zero energy at the reciprocal lattice points produced by the

canting. Furthermore, there are two acoustic

branches, one with ferromagnetic character and one with antiferromagnetic character, since the

system has a net magnetization with antiferromagnetic order of the ;c-components of the spins.

For T| > 1/3, (1,0) is a magnetic reciprocal

lattice point, and as expected the antiferromagnetic branch indeed has a diverging structure

factor there. Note that there are several mode

crossings which should produce strong mode

couplings. Furthermore, there are now two

optic branches. We have also shown that the

effect of a uniform magnetic field, which

produces an excitation gap for the ferromagneticlike modes, results in strong shifts of the mode

crossing points compared to the zero-field case.

15
CONDENSED MATTER PHYSICS AND THEORY

While these calculations are interesting as a demonstration of the feasibility of such

numerical studies, they also contain insights

about the behavior of canted systems. There are two important differences between the spin

dynamics of the ferromagnetic and the

reentrant-canted phases, which could have been

predicted without the above calculations. First,

mode softening occurs in the zone boundary

region of reciprocal space, since the magnetic

unit cell doubles in going from the ferromagnetic

(r| < 1/3) to the canted (t| > 1/3) phase.

Second, additional nonferromagnetic (AFM)

modes appear. Our calculations of the structure

factor show that the AFM S(q) increases linearly

from zero at the zone center, and at fixed finite wavevector it increases as the canting angles

increase. As expected, the AFM amplitude

diverges at the new AFM-like Bragg peaks in the

canted state. Such antiferromagnetic modes have

long been predicted for spin-glasses and RSG

systems on the basis of hydrodynamic arguments

[5], although their experimental detection has

proved elusive. This is perhaps because for

spin-glasses the wave-vector is well-defined as a good "quantum number" only for small q, and in

that regime the structure factor 5(q) is small.

We also conclude that certain qualitative

features of the GVM should persist in the

randomized canted systems. These features

include mode softening near quasi-Bragg peaks

at values of momentum transfer corresponding to the next-nearest-neighbor spacings (AFM-like

periodicity). These soft modes should have both

ferromagnetic and AFM character. In addition

optic modes should appear having their largest structure factor near the AFM-like quasi-Bragg

peaks. The amplitude of the AFM acoustic

mode vanishes as the origin of wave-vector

space is approached, whereas its amplitude at

finite wave-vectors is correlated with the canting

angles. Predictions concerning the behavior of

the ferromagnetic acoustic modes at the zone

center, must await further theoretical work on

the finite temperature dynamics.

GVM 77=0.50 H/J=0

(q,.q,) (Va A-')

Figure 1. Spin-wave dispersion and structure factor for the generalized Villain model with Ti = 0.5, which has a canted ferromagnetic ground state. Once the system cants there is an antiferromagnetic mode with infmite structure factor along with a ferromagnetic mode at (1,0). There are also optic modes with finite structure factor near (1,0).

References

[1] G. Aeppli, S. M. Shapiro, R. J. Birgeneau, and H.

S. Chen, Phys. Rev. B 28, 5160 (1983).

[21 M. Hennion, B. Hennion, I. Mirebeau, S. Lequien,

and F. Hippert, J. Appl. Phys. bf 63, 4071 (1988).

[3] B. Berge, H. T.Diep, A. Ghazali, and P. Lallemand,

Phys. Rev. B 34. 3177(1986).

[4] L. R. Walker and R. E. Walstedt, Phys. Rev. B 22,

3816 (1980).

[5] B. I. Halperin and W. M. Saslow, Phys. Rev. B 16, 2154 (1977).

16
CONDENSED MATTER PHYSICS AND THEORY

STRAIN AND OTHER SYMMETRY BREAKING EFFECTS IN MAGNETIC

SEMICONDUCTORS

P. Klosowski, T. M. Giebultowicz, N. Samarth, H. Luo, and J. K. Furdyna

(University of Notre Dame, Notre Dame, IN)

and

J.J. Rhyne (University of Missouri, Columbia, MO)

Zinc blende (ZB) magnetic semiconductors

(MS) provide prototypical systems for study of

Heisenberg frustrated antiferromagnets [1]. In the ideal case such systems are Type III antiferromagnets with first order paramagnetantiferromagnet phase transition [2] . However,

if the cubic symmetry is broken (e.g., by a

strain field or by dilution of the magnetic

sublattice with nonmagnetic ions), the system is expected to form a rich variety of magnetic

ground state configurations [3]; at the same time

its critical behavior is expected to change from

first order to second order transition [4]. The molecular beam epitaxy (MBE) grown

magnetic semiconductor systems provide good

opportunity for such studies. First, a very large

strain can be introduced into the sample by

growing heterostructures composed of inter- leaving layers of materials with different lattice constants. The choice of a larger or smaller

lattice constant of the non-magnetic spacers

results in achieving either compressive or tensile

strain. Secondly, the composition of the samples

can be controlled to achieve the magnetic

dilution. It has been well established that MBE-grown samples are of good crystalline

quality with clean interfaces, and that they are as good models for a magnetism study as the

hypothetical bulk-grown samples.

Previous to the work described in this report we investigated ZnSe/MnSe and

ZnTe/MnTesuperlattices with compressive strain

[5]. Since then, we extended our work to ZnTe/MnSe superlattices exhibiting tensile strain

and to Zn^.^MUj^Te weakly diluted epitaxial

layers (0.7 ^ x ^ 1). We have confirmed the crossover to the second order magnetic phase

transition in all ZnTe/MnSe samples and in Zn^.^Mn^^Te epilayers for x < 0.94 (fig. 1).

The second order transition is evident in the

71% and 84% Zn^.^Mn^Te samples, with N6el

temperatures of 46 K and 57 K, respectively.

However, in 93% and 100% samples there was

a much sharper falloff of the magnetization

below the N^el temperature (respectively 62 K

and 66 K), indicating the crossover to a first order transition. There is a supporting evidence

of that in the magnetization hysteresis observed

near the transition point.

The ZnTe/MnSe superlattices differ from

other MS systems by exhibiting an unusual

helical AF structure that can be described as a generalization of the Type III fee AF order seen

in unstrained ZB MS systems such as MnSe/ZnSe and MnTe/ZnTe superlattices with

compressive strain. This structure has a magnetic cell whose one dimension is incommensurate with the chemical cell dimensions, although it is nearly triple its size

(fig. 2).

Such helicoidal structure is favored by the

Dzyaloshinskii-Moriya interaction present in the

MS systems. However, since there is a considerable strain and strain-induced anisotropy

in the nearest-neighbor exchange interaction, we

offer a simpler explanation of the origin of this structure, based on the energy analysis of

distorted AF bonds [6]. These results show the variety of physical

model situations that can be realized in MBE-grown MS systems. We plan to pursue

the studies of magnetic phenomena in these

samples, and in particular to study the effects of

pressure-induced strain. The interface and

dimensionality effects will be also investigated.

17
CONDENSED MATTER PHYSICS AND THEORY

20 40 60 Temperature (K)

Figure 1. Temperature dependence of the squared

sublattice magnetization, as determined from the magnetic

scattering intensity. For the lower concentration samples,

the transition is of the second order, and it agrees well with

the square Brillouin function mean field formula for spin

(solid lines). The 93% and 100% concentration samples

exhibit different behavior, analogous to the first order

transition in fl-MnS (ref. [7]). The insert shows the slow

(1 K/hr) scan increasing (open circles) and decreasing (dark

circles) near the transition temperature in the MnTe sample,

showing the magnetization hysteresis (figure adapted from

[8]).

UnSe/ZnTe multilayer — (f.1.0) scan

0.62 0.68 0.70 0.74 Q—vector component f

0.78

Figure 2. Examples of (5,1,0) diffraction scan data from

a MnSe/ZnTe superlattice at various temperatures showing

a shift of the magnetic peak from the commensurate (Va, 1 ,0) position, and the dependence of this shift on T.

Referraices

[1] P. Klosowski et al., Antiferromagnetism in epUayers and superlattices containing zinc-blende MnSe and

MnTe, submitted to the proceedings of the MMM'91

in Pittsburgh.

[2] P. Bak, S. Krinsky, and D. Mukamel, Phys. Rev.

Lett. 36, 52 (1976).

[3] S. E. Larson and C. L. Henley, Ground state selection in type III fee vector antiferromagnets, submitted to Phys. Rev. B, 1990.

[4] M. Kerszberg and D. Mukamel, Phys. Rev. B 23,

3953 (1981).

[5] P. Klosowski et al., J. Appl. Phys. 69, 6109 (1991), Proceedings of the 35th Annual Conference on Magnetism and Magnetic Materials, San Diego,

November 1990.

[6] T. M. Giebultowicz et al. Incommensurate

antiferromagnetic order in strained layer MnSe/ZnTe

superlattices, Submitted to the proceedings of the ICNS '91.

[7] J. Hastings, L. Corliss, W. Kunnmann, and D. Mukamel, Phys. Rev. B 24, 1388 (1981).

[8] P. Klosowski et al.. Investigation of Zuj^Mn^Te

(0.7 ^ X i 1) weakly diluted fee magnetic

semiconductor, submitted to the proceedings of the ICNS '91.

18
CONDENSED MATTER PHYSICS AND THEORY

SMALL-ANGLE NEUTRON SCATTERING OF

NANODISPERSED MAGNETIC PARTICLES

J. R. Childress and C. L. Chien (Johns Hopkins University, Baltimore, MD)

R. W. Erwin

and

J. J. Rhyne (Missouri University Research Reactor, Columbia, MO and Reactor Radiation Division)

Granular magnetic materials, which consist

of fine magnetic particles dispersed in an amorphous insulating matrix, display a number

of unique properties as a result of their

nanometer-scale microstructure [1]. However,

many properties of granular magnetic composites

are still poorly understood at the fundamental

level, such as the exact nature of the magnetic

ordering and magnetic excitations within and

between the grains. A well known property of

single-domain magnetic particles is the occurrence of superparamagnetic relaxation,

resulting from the thermallyactivated fluctuation

of the particle moment. Superparamagnetism

can be observed on different time scales t by

using different magnetic probes, such as SQUID

magnetometry (t = 10 sec) and ^'^Fe Mossbauer

spectroscopy (MS) (t = 10'^ sec). In contrast, it may be possible using small-angle neutron

scattering (SANS) to measure the instantaneous

spin-pair correlation function. Thus SANS

provides a unique window to understanding the

basic magnetic structure and dynamics in these

nanoscale materials.

The sample examined in this study was

prepared using RF magnetron sputtering, by

co-deposition of Fe and AI2O3 from a single

mixed target onto thin AI2O3 substrates kept at 200 °C, resulting in a film approximately 7^

thick. The granular microstructure was

confirmed by transmission electron microscopy,

which reveals a narrow distribution of Fe

particle sizes around 3 nm in diameter. The film

composition of 35 vol% of Fe is well below the

granular percolation concentration (= 60 vol%),

and, therefore, contains well-isolated particles

[2]. X-ray diffraction analysis shows that the

particles retain the body-centered cubic structure of bulk a-Fe. Magnetic hysteresis loops obtained

at T = 5 K show a clear ferromagnetic

behavior, with 60% remanent magnetization and

a coercivity of about 600 Oe. For our sample.

the blocking temperatures Tg above which the

particles become superparamagnetic are 75 Kfor

SQUID measurements (peak in the

zero-field-cooled susceptibility and onset of

irreversibilities) and 210 K for MS

measurements (onset of magnetic hyperfine

splitting). Above Tg, little information on the

magnetic ordering can be obtained by these

methods.

The SANS samples consisted of many film

layers superposed in an aluminum sample

holder, resulting in a total sample thickness of more than I00\i . The temperature of the sample was varied between 15 K and 350 K, and the

scattering intensity was obtained over the range

of wavevector transfers (Q) from 0.17 to 1.5 nm'^ using an incident neutron wavelength of

0.5 nm. A background spectrum was obtained

at 425 K and subtracted from the lowtemperature data, so that only the temperaturedependent magnetic scattering was analyzed. In the high-Q region, the data is well described by

a single Lorentzian-squared scattering function.

Preliminary results on data obtained at longer

neutron wavelengths suggest that this scattering

is essenfially wavelength-independent, i.e., that no inelasticity corrections are required in the

analysis. We also expect the inter-particle

fluctuations to be sufficiently slow (based on

Mossbauer and bulk magnetitation) that the

SANS can measure the instantaneous correlation

function between particles. If the

Lorentzian-squared cross-section is a true representation of the instantaneous inter-particle

correlation function, it is different than the

Lorentzian form seen in bulk magnetic systems

[3]. This may be related to the finite extent of

the strongly-correlated magnetic regions in our

granular sample.

At the smallest wavevector transfers, we

find additional intensity, which we model by

19
CONDENSED MATTER PHYSICS AND THEORY

adding a simple 1/Q^ term to the scattering.

Thus excellent fits to the data are obtained using:

KQ) = (Q^ ^kY

^4 (^)

where k = 1/$ is the inverse correlation length.

The 1/Q^ term corresponds to a Lorentzian

cross-section with a near-zero inverse correlation

length, thus reflecting correlations on a scale

larger than what can be measured in our

experiment (roughly a few tens of nm's). It is important to note, however, that the form of this cross-section is not unique, and that further

refining of the data analysis is necessary to pinpoint the functional form and correlation

length associated with the low-Q scattering.

The Lorentzian-squared amplitude A is shown in figure 1 as a function of temperature.

This amplitude is seen to decrease with

decreasing temperature, suggesting that this component of the scattering is dynamic in origin.

The decrease is sharp below 200 K, flattening out near zero below about 120 K. In figure 1 we also plot the correlation length $ = 1/k

which shows a related temperature dependence.

Starting at room temperature in the 1 nm range,

$ increases only slightly with decreasing

temperature until about 200 K, below which a rapid increase is observed. However, instead of

a divergence of $, we observe a saturation at about 4 nm, very close to the average particle

size. By contrast, the amplitude B of the

Lorentzian component to the scattering is constant and nearly zero at high temperatures.

As T is decreased below 200 K, B begins to increase and rises sharply below 75 K, a behavior typical of order parameter scattering.

Because of the long correlations lengths

associated with this scattering, it is suggestive of

inter-particle ordering.

The SANS results (fig. 1) suggest that the

magnetic scattering at high temperatures

(T > 200 K) is due to spin fluctuations within

the particles. Below 200 K, these fluctuations are slowly frozen out, with increasing correlation

of the spins until the correlation length saturates near the particle size. As the intraparticle order

Figure 1. Amplitude A (filled circles) and correlation length 5 (open circles) associated with the Lorentzian squared term in the small-angle neutron magnetic scattering

from 3 nm Fe particles (see eq. 1 in text), as a function of

temperature. At low temperatures, the decrease of A

suggests that this component is dynamic in origin, and a

saturation of I is observed near the particle size. The lines are drawn to guide the eye.

increases, correlations develop between particles

and increase as the temperature is lowered.

Further SANS experiments, including a study of

the dependence of the scattering on an externally

applied magnetic field, are already under way to better characterize the low-Q component of the

scattering, extract an Fe particle size distribution

and to further understand the role of spin

fluctuations in the scattering.

References:

[1]

[21

[3]

[41

C. L. Chien, Physical Phenonmena in Granular

Materials G. D. Cody, T. H. Geballe, and P. Sheng,

eds.. Mat. Res. Soc. Symp. Proc. vol.195, p. 411

(1990)

B. Abeles, Appl. Solid State Sci. 6, 1 (1976). See, e.g., H.E. Stanley, Introduction to Phase

Transitions and Critical Phenomena , p. 94 (Oxford

University Press, 1971) and references therein. See, e.g., I. S. Jacobs and C. P. Bean, in Magnetism . G. T. Rado and H. Suhl, eds. vol.3

p. 271 (1963).

20
CONDENSED MATTER PHYSICS AND THEORY

SCALING BEHAVIOR OF THE GENERALIZED SUSCEPTIBILITY

INLa2.,Sr^Cu04^y

B. Keimer, R. J. Birgeneau, A. Cassanho, and M. A. Kastner

(Massachusetts Institute of Technology, Cambridge, MA)

Y. Endoh (Tohoku University, Sendai, Japan)

R. W. Erwin

and

G. Shirane (Brookhaven National Laboratory, Upton, NY)

Motivated by the observation in both the

La2.x»Srj(^Cu04+y and YBa2Cu306^y series that the spin fluctuations at low energies have

comparable strengths in antiferromagnetic, spin

glass and superconducting samples, we have

made a detailed study of the instantaneous and

dynamic spin fluctuations for a crystal of

Laj ggSro 04CUO4 which is in the intermediate

spin glass regime. We find that the instanta- neous correlations are well described by theory

for the 2-d Heisenberg model in the quantum

disordered regime [5,6]. The spin-spin

correlation length is ~ 40 A and is temperature

dependent below 300 K. Studies of the

dynamics in the CUO2 plane reveal that the

fluctuations are centered about the (n ,71) position

(square lattice, unit lattice constant notation)

with negligible weight at other high symmetry

positions such as (7r,0) and (2ir,0). Our essential new result is that we find that the integrated

response around (Ti,ir) as a function of

temperature T exhibits a simple scaling behavior

in w/T for temperatures 10 K ^ T ^ 500 K and

energies 4.5 meV ^ w ^ 12 meV.

The static measurements were carried out on

a two axis spectrometer (BT-4) at NIST with

incident neutron energy Ej = 30.5 meV and

collimator configuration 40'-20'-S-20'. The

sample was oriented in the (h,k,0) plane and

scans were carried out across (h,0.38,0). As

discussed previously (ref. 1) in this geometry one automatically integrates over the 2-d spin

fluctuations which have energies -T < o < 30.5

meV. The inelastic measurements were carried out on a triple axis spectrometer (BT-9) with

fixed incoming neutron energy Ej = 30.5 meV

and collimator configuration 40'-40'-S-40'-80'.

The sample was oriented with [0, 1 ,0] vertical so

that the entire 2-d in-plane reciprocal lattice

could be studied. Pyrolytic graphite (002) was

used as both monochromator and analyzer.

For a Heisenberg system, the neutron

magnetic cross section is proportional to

S(q,i^

1

l-e

-dT

ImxiQM- (1)

A full-integration of equation 1 over energy

yields the instantaneous spin-spin correlation

function

Z7t -

r ^Sg-Sp. (2)

Our experimental technique and method of

analysis for the energy integrating measurements

are identical to those discussed in ref. 1 . We

find that the static structure factor is peaked at (1,0,0) indicating that the local magnetic order is commensurate with the lattice. Fits of the

measured structure factor to a 2-d Lorentzian

show that the inverse correlation length k is nearly independent of temperature from 10 K to 300 K and then increases gradually with

increasing temperature above room temperature.

The spin correlation length below room

temperature is about 40 A and there are no

evident effects on the spin correlations from the

weak localization effects which typically set in below approximately 80 K.

We performed a series of inelastic scans at 6 meV at temperatures between 10 K and 300 K.

We observed that the peaks are sharp and

symmetric and that the background is relatively

flat and featureless. Thus we are able to extract integrated intensities quite reliably. We carried out scans in both the longitudinal and transverse

21
CONDENSED MATTER PHYSICS AND THEORY

directions through (100) for temperatures

between 10 K and 500 K at energies of 4.5, 6.0,

9.0 and 12.0 meV. We also surveyed reciprocal

space extensively at 10 K and 300 K. We found no significant scattering above background at any

other place in reciprocal space including

especially the (1/2, 0, 1/2) and (1,0,1) positions.

The resolution corrected 2-d integrated

intensities as a function of temperature, In each

case, peak at a temperature of T = 2o). This

suggests tiiat one should replot the data in a scaled form in terms of w/T. Since most

theories and many derivative quantities involve

/mx""((^,o)) we have used eq. (1) to convert the

measure^^/^^d Os""(0,o)) tp„J„)d Imx(d ,")•

We have normalized the data and have chosen

the overall scale such that the integrated

susceptibility for w/T > 1 is ~ 1. The results are plotted in figure 1

1.25

1^1.00 -

0.75 -

•2 0.50

g

^0.25 -

-

. I

*

• ^ //

" . /-- o i

.0

- fiy

/ / Id

o

6

4.5meV 6meV 9meV

l2meV

- /a - Varma el al.

• (Lorenlzian)"

/a.t . 1 0.5 1.0 iE/kgT

1.5 2.0

Figure 1 . Normalized integrated spin susceptibility around

(71, It) as a function of the scaling variable AE/kBT. The

lines are the predictions of marginal Fermi liquid theory

(solid line) and a fit to a heuristic power of Lorentzian form

as discussed in the text (broken line). We see that all of the data for temperatures

between 10 K and 500 K and energies between

4.5 meV and 12 meV fall on a single universal curve when plotted in this fashion. This scaling occurs in spite of the fact that the correlation

length is nearly independent of temperature over

the entire temperature interval. We also note

that there is no difference for data at temperatures above and below 80 K. Thus

/mx(Q,o)), along with the static correlations, is insensitive to effects of weak localization, as one

might have expected on physical grounds since

weak localization largely involves the behavior

of the carrier wave functions at large distances.

There also exists a more limited set of data

for YBa2Cu30g^. at energies of 3, 6, and 8.3

meV for y = 0.45, 0.4 and 0.5 respectively (ref

2). All three samples exhibit disordered

magnetic states and are, at least in part,

superconducting at low T. These data, which

cover the range 0. 1 ^ w/T ^ 5 fall on the same

universal curve within the errors as that shown

in figure 1 . Thus our results appear to be quite

general.

The only model we are aware of which

predicts behavior anything like that shown in

figure 1 is the "marginal" Fermi liquid

hypothesis of Varma et al. [3]. Their model

assumes,

Imx(QM~{ ifor\J\>T ^^

and that x is weakly Q dependent. Clearly,

since /mx is sharply peaked about the (n,n)

position the latter assumption is incorrect for low

carrier concentrations. The hypothesized general

behavior in w/T is, nevertheless, reasonably

close to our experimental results. It is, however, clear from figure 1 that for co/T < 1 the Varma et al. ad hoc assumption that /mx ~

o)/T is not quantitatively correct. Instead our

data require /mx ~ (a/T)" with a = 3 although

Imx ~ C^ljf' this exponent is poorly determined

so that a could range between 2 and 5. Thus,

the effect of the 4% holes on /mx(Q,w) is threefold. First, the spin-spin correlation length

is reduced to 4 A for T < 300 K. Second, the

l/o) spin wave divergence as w - is suppressed. Third, the spin fluctuations with

energies w < T are severely diminished in

amplitude like (co/T)" with a - 3.

References

[1]

m

[3]

R. J. Birgeneau et al.; Phys. Rev. B 38. 6614 (1988); T. R. Thurston et al., Phys. Rev. B 40, 4-585 (1989);

J. M. Tranquada et al., Phys. Rev. Lett. 64, 800

(1990).

P. Gehring, private communication.

C. M. Varma et al., Phys. Rev. Lett. 63, 1996

(1989).

22
CONDENSED MATTER PHYSICS AND THEORY

MAGNETIC CORRELATIONS AND ENERGY GAP IN SUPERCONDUCTING

YBa2Cu30^ g WITH T^ = 53 K

P. M. Gehring, J. M. Tranquada, and G. Shirane (Brookhaven National Laboratory, Upton, NY)

J. R. D. Copley and R. W. Erwin

and

M. Sato and S. Shamoto (Nagoya University, Nagoya, Japan)

Over the past two years, inelastic-neutronscattering studies have demonstrated the

existence of dynamic magnetic correlations in superconducting YBa2Cu306^.x single crystals

[1-4]. The data show that the imaginary part of

the spin susceptibility, x" (Q,ti)), is characterized

in reciprocal space by a broad peak centered

about the 2-d antiferromagnetic (AF) zone

center, which in three dimensions forms the

tetragonal rod Q^aj;(V2,y2,£) measured in reciprocal lattice units (r.l.u.). For x ^ 0.5 this peak

width corresponds to a spin-spin correlation

length on the order of twice the lattice spacing.

As for the frequency dependence, the usual l/o)

behavior observed for spin waves in the anti- ferromagnetic phase is strongly suppressed at low frequencies. It has been shown that the

observed magnetic cross section is consistent

with a model in which antiferromagnetic spin

fluctuations are heavily overdamped due to the

short correlation length. In addition to the overdamping of the

low-frequency excitations, Rossat-Mignod et al.

[5] have reported a temperature-dependent

energy gap in superconducting YBa2Cu306^j^

crystals with x = 0.51 and 0.69 (T^, = 47 and

59 K, respectively). They observed the gap in measurements of the frequency dependence of x"

at Q^p. Recently, Bulut and Scalapino [6] have

pointed out that they obtain a pseudogap in x"

precisely at Q^p from a random-phase approximation (RPA) analysis of the normal-state

properties of a 2-d Hubbard model; the gap

occurs for kinematic reasons and does not appear

at the incommensurate positions where their

theoretical x" peaks. We have performed

measurements on a single crystal with x = 0.6

and T^, = 53 K which convincingly demonstrate

that the energy gap exists not only at Q^p, but

all along the zone diagonal [7]. At 10 K ^e Qdependent magnetic peak vanishes (within

experimental error) for energies AE ^ 5 meV.

Figure 1 shows several representative

constant-E scans as a function of the in-plane

momentum transfer taken along the zone

diagonal (h,h,1.8) at energy transfers of 5, 9,

and 12 meV at 10 K. Each scan was fit to a Gaussian line shape plus a constant sloping

background, the sum of which is indicated by a

solid line. Clearly the most striking feature is the absence of a measurable Q-dependent

magnetic cross section at 5 meV. An identical scan at 3 meV also revealed no sign of a peak.

In spite of the low counting rates associated with

the magnetic signal, a clear and well-defined

cross section is seen to emerge at 9 meV,

becoming even more prominent at 12 meV. The

difficulty inherent in these measurements

becomes apparent when one realizes that the

magnetic signal at 9 meV, counting 15 mins per

point, is only ~ 30 counts above a background

of ~ 60 counts. The low counting rate stems in

100 -

50

I

1

c3c

100

50

100

50

r/i.Ai.8) Al > 5.0 miY

i it ^ ^^I^^Iiil^il ^^^ '' '

('^.^,l.e) A^ = 9.0 meV r-ioK

I I I I '

AE - 12.0 meV r-lOK

:hh«

i

I I I I 1 I i t I I 1 I I I I I I I I r 02 0.3 0.4 0.5 0.6 0.7 0.8 h (r.l.u.)

Figure 1. Constant-E scans measured along (h,h,1.8) at 5, 9, and 12 meV at 10 K, using a fixed incident neutron energy of 30.5 meV. Solid lines represent best fits to a Gaussian line shape, h is measured in reciprocal lattice units (r.l.u.), where 1 r.l.u. = 23nm" .

23
CONDENSED MATTER PHYSICS AND THEORY

part from the very short spin-spin correlation

lengths $ present in superconducting samples.

For X = 0.6, the constant-E scans at 10 K have

full widths at half maximum (FWHM) of order

-0.12 + 0.02 r.l.u. which implies a correlation

length of only $ ~ 0.7 - 0.9 nm. The complete set of measurements at 10 K

is summarized in figure 2 where the 1-d

q-integrated intensity of constant-E (h,h,1.8) scans is plotted as a function of energy transfer. The magnetic intensity is zero within

experimental error up to 5 meV at 10 K. Above

this energy, the spectral weight of the spin

fluctuations begins to increase until it appears to saturate between 12 and 15 meV. The size of

the gap is therefore taken to be - 5 meV (- 60

K), which is of order T^. These data are

consistent with those presented by Rossat- Mignod [5] for two samples corresponding to x = 0.51 and 0.69. The gap energy of 5 meV for

X = 0.6 is nicely bracketed by these two

samples having both higher and lower oxygen

contents. The size of the gap observed at low

temperature in these samples appears to change

rapidly with oxygen content, but shows little correlation with T^.

Rossat-Mignod et al. [5] point out the

similarity of the temperature dependence of

X"(QaF'^) for small « and the unusual decrease

of l/r^ below 100 K for ^^Cu measured the

NMR nuclear relaxation rate in 60-K material

[8], but associating the spin-fluctuation gap with

the normal-state behavior of l/T^ raises more

questions than it answers. If the lowtemperature gap develops above T^,, is it related

to the superconductivity or is it a normal-state

property? Bulut et al. [8] found a gaplike

depression for integrated inelastic-scattering

intensities at low temperature in their RPA

analysis of a 2-d Hubbard model. In a more

recent study, Bulut and Scalapino [6] found a pseudogap in x"(Q,<^) at Q = Q^f- By contrast,

the data in figure 2 appear to indicate a true gap

at 10 K in the X = 0.6 sample along the entire zone diagonal, not just at the point Q =

(V2,V2,1.8). If the gap observed by neutron

scattering is connected to superconductivity, why

is it so small and why does it change so rapidly

with small changes in T^.? Much more experimental work will be needed before these

questions can be resolved.

I. «

v>a 4

I

1

e

c

T r

ih.h,0)

vmtimsmztmmtio (0.5,0.5,1)

12 16 A^ (meV)

Figure 2. Frequency dependence of the magnetic cross

section. Data points are the integrated intensities from

Gaussian fits to (h,h,1.8) constant-E scans as shown

schematically in the inset. Each scan was corrected for a constant sloping background and corrected for the spectro- meter resolution. The solid line is a guide to the eye.

References

[1] G. Shirane, J. Als-Nielsen, M. Nielsen, J. M.

Tranquada, H. Chou, S. Shamoto, and M. Sato, Phys. Rev. B 41, 6547 (1990).

J. M. Tranquada, W. J. L. Buyers, H. Chou, T. E. Mason, M. Sato, S. Shamoto, and G. Shirane, Phys. Rev. Lett. 64, 800 (1990). Henry Chou, J. M. Tranquada, G. Shirane, T. E. Mason, W. J. L. Buyers, S. Shamoto, and M. Sato, Phys. Rev. B 43, 5554 (1991).

J. Rossat-Mignod, L. P. Regnault, M. J. Jurgens, C.

Vettier, P. Burlet, J. Y. Henry, and G. Lapertot, Physica B 163, 4 (1990).

J. Rossat-Mignod, L. P. Regnault, C. Vettier, P. Burlet, J. Y. Henry, G. Lapertot, Physica B 169, 58

(1991). N. Bulut and D. J. Scalapino (unpublished).

P. M. Gehring, J. M. Tranquada, G. Shirane, J. R. D. Copley, R. W. Erwin, M. Sato and S. Shamoto,

Phys. Rev. B. 44, 2811 (1991).

[8] W. W. Warren, Jr., R. E. Walstedt, G. F. Brennert, R. J. Cava, R. Tycko, R. F. Bell, and G. Dabbagh,

Phys. Rev. Lett. 62, 1193 (1989). N. Bulut, D. Hone, D. J. Scalapino, and N. E.

Bickers, Phys. Rev. Lett. 64, 2723 (1990).

[2]

13]

14]

15]

16] m

19]

24
CONDENSED MATTER PHYSICS AND THEORY

MAGNETIC ORDER OF Cu IN Nd2.xCexCu04

S. Skanthakumar and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

J. L. Peng and Z. Y. Li (University of Maryland, College Park, MD)

The magnetic properties of the electron

superconducting systems R2.^Ce^CuO^ (R = Nd,

Pr or Sm) have been recently studied by neutron

diffraction techniques [1-4]. The Cu spins in the

parent materials i?2Cu04 order antiferromagnetically at relatively high temperatures

(-280 K). For the Nd2Cu04 system two spin

reorientation transitions have been observed at 75 and 30 K, and there is a substantial inter- action between the Cu and Nd sublattices [2]. However, these spin reorientation transitions are not observed in Pr2Cu04 and Sm2Cu04 [4]. Here, we report measurements of the Cu

magnetic ordering in Nd2.xCe^Cu04. It is found

that increasing the Ce concentration reduces the

N6el temperature and site-averaged ordered Cu

moment, and superconductivity is suppressed for

X i 0.14. This behavior is qualitatively similar

to the behavior observed in other oxide superconductors, but the rate of suppression is substantially less than in the related hole-type

systems such as La2.xSrxCu04.

Unpolarized neutron diffraction data were

taken on single crystals of Nd2.xCe^Cu04 with

X = 0.0, 0.025, 0.11, 0.17 and 0.22. Longrange antiferromagnetic order for Cu is observed

in crystals with x = 0.0, 0.025 and 0.11. All

the observed magnetic Bragg peaks in these

crystals can be indexed as {h/2 k/2 /) based on

the tetragonal chemical unit cell. The nearest neighbor spins in the Cu-0 layers are anti- parallel; however, the spin direction between

layers can be either collinear or noncollinear [5]. The N^el temperatures (Tj^j^) were obtained from

the temperature dependence of the (Va V2 1) Bragg

peak. The N6el temperature decreases with

increasing Ce concentration x, which is shown in the phase diagram in figure 1. This phase

diagram is slightly different from the one for

La2.xSr^Cu04 [6] (hole doped), where an

increase in Sr doping destroys the anti- ferromagnetic ordering more rapidly. The

site-averaged ordered moment of the Cu spins

also decreases with increasing x. The

temperature dependence of the {^hViX) magnetic

Bragg peak was measured for both cooling and

warming in the pure sample, and there was no

indication of thermal hysteresis effects, in contrast to a previous report [3].

0)

c_D

-1-) rac_

03D.E

CU

4-1C

o

enc

(TJ C

V0.1 0.2 0.3 Ce concentration,

X

Figure 1 . Magnetic phase diagram for Nd2.j^Cej^Cu04 as a function of Ce concentration x. Shown are the regions of

antiferromagnetic order (AF) and paramagnetic (P) behavior. The spin reorientation transition temperatures are taken as the mid point in the transition region, while the bars indicate the width of the transitions.

In the pure sample two spin reorientation

transitions are observed at Tj^2 = 75 K and at Tf^3 = 30 K. At T = Tj^2 ^^ th^ ^u spins

rotate by Till about the c axis, and they rotate back to their original direction at T = Tj^g.

Both spin reorientation transition temperatures

increase with magnetic field at the rate of about

1.2 and 0.8 K/T, respectively [7]. These

transitions can be most easily observed by

looking at the temperature dependence of the

(V2V2O) peak intensity because this Bragg peak

has non-zero intensity only between Tj^2 ^^

T,N3-

25
CONDENSED MATTER PHYSICS AND THEORY

The temperature dependence of the ('/2 V2O)

magnetic Bragg peak is shown in figure 2 for x = 0, 0.025, and 0.11. In addition to the

decrease in T^ and the ordered moment via Ce

doping, we see that the ('72 '^O) intensity is strongly reduced by adding Ce. In fact in order

to compare these data on the same graph, we

have scaled the intensities by dividing by the

square of the maximum site-averaged ordered Cu

moment observed at each Ce concentration.

Even with this normalization we see that the

intensity drops off rapidly with increasing Ce

concentration. We interpret this as a reduction

in the fraction of the Cu spins which participate

in the spin reorientation. The data also show

that the transitions for the doped samples are not

nearly as sharp as for the pure sample (x =

0.0), which is likely caused by the site randomness of the Ce in the sample. The spin

reorientation transition temperatures Tj^2 ^^^

Tjs43 ^^^ ^'^^ shown in figure 1 , where Tj^2 ^<^

Tj^3 are taken as the center point in the transition

region, and the bars indicate the start and end

points in the transition region.

Nds-xCe^CuO^ (1 ^ 0]

1000

nj 800

A

N 600

400

-• X = 0.0

: A X = 0.025 II J ^ i

200

-200

150

Hi

V

-P

-H

if) c

0) 100^

,^, 50

r

• •S|**« ** *A***

-50

X = 0.11

20 40 .60 80 100 Temperature (K)

Figure 2. Temperature dependence of (WAO) magnetic

Bragg peak for Nd2_j,Ce CuO^ for various Ce concentration X. These plots show the spin reorientation transitions.

Referaices

[1] S. Skanthakumar, H. Zhang, T. W. Clinton, W.-H. [4] Li, J. W. Lynn, Z. Fisk, and S.-W. Cheong, Physica

C 160, 124 (1989); S. Skanthakumar, H. Zhang, T. W. Clinton, I. W. Sumarlin, W-H. Li, and J. W.

Lynn, Z. Fisk, S.-W. Cheong, J. Appl. Phys. 67, [5] 4530 (1990).

[2] J. W. Lynn, L W. Sumarlin, S. Skanthakumar, [6] W.-H. Li, R. N. Shelton, J. L. Peng, Z. Fisk, S.-W. Cheong, Phys. Rev. B 41, 2569 (1990).

[3] Y. Endoh, M. Matsuda, K. Yamada, K. Kakurai, Y. [7] Hidaka, G. Shirane and R. J.Birgeneau, Phys. Rev. B 40, 7023 (1989).

D. E. Cox, A. 1. Goldman, M. A. Subramanian, J. Gopalakrishnan, and A. W. Sleight, Phys. Rev. B 40

6998 (1989); S. Skanthakumar, J. W. Lynn, J. L. Peng, and Z. Y. Li, J. Appl. Phys 69, 4866 (1991).

S. Skanthakumar and J. W. Lynn, Physica C, 170 . 175 (1990). See, for example, High Temperature Superconductivity, J. W. Lynn, ed. (Springer, New York,

1990), Chap. 8. S. Skanthakumar, J. W. Lynn, J. L. Peng, and Z. Y. Li (preprint).

26
CONDENSED MATTER PHYSICS AND THEORY

TWO-DIMENSIONAL MAGNETIC CORRELATIONS

AND MAGNETIC ORDERING OF Dy IN DyBa2Cu307

T. W. Clinton and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

J. Z. Liu, Y. X. Jia, and R. N. Shelton (University of California, Davis, CA)

The magnetic ordering of the rare earth ions

in the layered superconducting oxides has been

of particular interest because of the 2-d magnetic

behavior these materials display [1-3]. In the

RB?L2Cu20j (R = rare earth) materials of direct

interest here, the 2-d behavior originates

naturally from the crystallography; there is only one rare earth ion per chemical unit cell, and the

c-axis is three times as long as the a and b axes. Powder neutron diffraction experiments on ErBa2Cu307 [2] were the first to confirm 2-d

behavior of the rare earth ions in the 7?Ba2Cu30Y

system. Though the Er magnetic system is 2-d

in character, when strong 2-d correlations

develop in the a-b plane a weak interaction along

the c-axis induces 3-d long range order at T^ =

0.618 K [1]. The Dy [4], Nd [5] and Gd [6] systems have also been investigated with powder

neutron diffraction, but these studies have mainly

concentrated on the 3-d magnetic structures. In this article we report measurements on a single crystal of DyBa2Cu307. Above the 3-d

ordering temperature we observe a rod of

scattering along {V2 Vi q^.), which directly

demonstrates the 2-d character of the system. At

low T the system is characterized by an anti- ferromagnetic structure with wave vector of the

form ('/2 '/2 V2), corresponding to antiferromagnetically coupled spins along all three

crystallographic directions.

Above the ordering temperature the 2-d rod

of scattering originates from critical fluctuations,

while below Tj>^ it originates from low energy

magnetic excitations. In the quasielastic approximation the critical scattering is proportional to the wave-vector-dependent magnetic susceptibility x(q)- For a 2-d system in the OrnsteinZernike approximation

i(q)di ffuse c(q) (k2 + q^2 + Qv")

2vl

where k is the inverse correlation length within

the a-b plane. Thus, in a 2-d system we expect

to see critical scattering about the reciprocal

lattice positions as in the 3-d case, with the

important difference that in the 2-d case the

critical scattering should be independent of q^, so one should see a rod of scattering extending

along q^.

Figure la shows a scan across the rod, just

above the 3-d ordering temperature. This strong

rod of scattering develops for temperatures <. 2

K. The linewidth of this peak is a measure of

the inverse 2-d correlation length k. Thus, by

scanning across the rod at various temperatures

we have observed the evolution of the magnetic

correlations in the a-b plane. Also shown in this figure is a scan along the rod, which demonstrates that the scattering intensity is independent

of q^. Hence there are no significant correlations between spins in adjacent a-b planes, even though we are just above Tjv^. Figure 2 shows the intensity at a point on the

rod, ('/2 V2 1.2), versus temperature. The rod

starts to develop below about 2 K, and the

intensity steadily increases with decreasing

temperature until it peaks at Tjs^, when long

range order sets in. Below Tj^j the intensity

quickly decreases, presumably due to the sharp

increase in the energy of the magnetic

excitations, and the consequent decrease in their

thermal population.

The temperature dependence of the sublattice

magnetization obtained from the 3-d Bragg peak

(V2 ^/i V2) is shown in figure 3, where the

ordering temperature can be seen to be about

0.91 K. The 3-d ordering is simply being

induced as a necessary consequence of the onset of long range order within the a-b planes [1]. This behavior is analogous to the 2-d and 3-d

magnetic ordering of Er observed in ErBa2Cu30Y [1]. We have also observed

27
CONDENSED MATTER PHYSICS AND THEORY

cH

E

O

QJa

if)

o

u

350 DyBcJaCu.BO7

(1/2.1/2.0.85) 1=0.93 K

250 /

^

150 /

( '\

50. /

n1 KV T

-no . . . L . . .. _ _.J ^

i •

0.436 0.468 0.500 0.532 0.5E4

C

•rH E

O»^C

GJa

CO

o

u

350

250

150

50

-50

(1/2. 1/2. QJ

T

T T

. J- ..

T=0.93 K

I il

0.3 0.7 1.1 Q,

1.5

Figure 1. (a) A scan across the rod of scattering in reciprocal space, showing the 2-d character just above Tj^ = 0.91 K and (b) a scan along the rod, showing that the

scattering intensity does not vary significantly along Q . Hence there are no significant spin correlations between

planes, even just above Txj. The slight decrease in intensity with increasing Q^ is due to the magnetic form

factor.

magnetic Bragg peaks characterized by the wave

vector (V2 Vi 0), corresponding to ferromagnetically coupled spins along the c-axis and

antiferromagnetic along the a and b directions,

but this structure was only observed in oxygen

deficient samples of Dyl23. Therefore, we

believe that the correct ground state spin

configuration for fully oxygenated DyBa2Cu307

is ('/2 '/2 Vl).

References

[1] J. W. Lynn, T. W. Clinton, W.-H. Li, R. W.

Erwin, J. Z. Liu, K. Vandervoort and R. N.

Shelton, Phys. Rev. Lett. 63, 2606 (1989).

100

DyBaaCUgOy

(1/2.1/2.1.2)

0.91 K

0.6 0.8 1.0 1.2 1.4 Temperature (K)1.6 l.B

Figure 2. Rod intensity as a function of

temperature.

1.2 (1/2.1/2,1/2)

o Tn - 0.91 K

s.\ l.U Oo OOo

h- °°^

z

oM

0.8 %

\ H 0.6 Q <N 8 M

1- 0.4 8

lll y o

CO< 0.2 s ——1—1—1—i—1—1—1—.—«—.—1

0.2 0.4 0.6 0.8 1.0 1.2 Temperature 1/1^

Figure 3. Sublattice magnetization vs temperature obtained from the {}h. '/i '/i) 3-d magnetic Bragg peak, with 7^=0.91 K.

[2] J. W. Lynn, W.-H. Li, Q. Li, H. C. Ku, H. D. Yang and R. N. Shelton, Phys. Rev. B36, 2374

(1987).

[3] H. Zhang, J. W. Lynn, W.-H. Li, T. W. Clinton,

and D. E. Morris Phys. Rev. B41, 11229 (1990).

[4] A. I. Goldman, B. X. Yang, J. Tranquada, J. E. Crow, and C.-S. Jee, Phys. Rev. B36, 7234 (1987);

P. Fischer, K. Kakurai, M. Steiner, K. N. Clausen,

B. Lebechi, F. Hulliger, H. R. Ott, P. Bruesch and

P. Untemahrer, Physica C152, 145 (1988).

[5] K. N. Yang, J. M. Ferreira, B. W. Lee, M. B. Maple, W.-H. Li, J. W. Lynn and R. W. Erwin,

Phys. Rev. B40, 10963 (1989).

[6] D. McK. Paul, H. A. Mook, A. W. Hewit, B. C.

Sales, L. A. Boatner, J. R. Thompson, and M.

Mostoller, Phys. Rev. B37, 2341 (1988).

28
CONDENSED MATTER PHYSICS AND THEORY

TWO-DIMENSIONAL BELAYER MAGNETIC ORDER

OF Dy IONS IN Dy2Ba4Cu70j5

H, Zhang and J. W. Lynn (University of Maryland, College Park, MD and Reactor Radiation Division)

and

D. E. Morris (Morris Research, Berkeley, CA)

A considerable amount of research has been

done on the magnetic properties of the oxide

superconductors [1] because on one hand the Cu

spin fluctuations still stand as a candidate for the

electron pairing mechanism, and on the other

hand the rich phenomena of 2-d and 3-d

magnetic order in these materials have stimulated

considerable interest. We have been systematically investigating the nature of the rare-earth

ordering in the i?Ba2Cu307 and 7?Ba2Cu40g

systems, and found that they are prototypical 2-d

systems [2-4].

The present article reports our recent measurements on the Dy2Ba4Cu70i5 (2-4-7)

material, which is directly related to the

DyBa2Cu307 (1-2-3) and Dy2Ba4Cu80i6 (2-4-8)

systems. The Dy sublattice is shown in figure 1, where a = 3.864 k, b = 3.879 A, and c =

50.26 A [5]. The crystallographic anisotropy

leads naturally to highly anisotropic magnetic

Figure 1. The Dy sublattice in the Dy2Ba4Cu7025

compound. Every other layer is shifted along the b-axis by

b/2, which is responsible for the cancellation of interplane

magnetic interactions and ultimately the 2-d coupled-bilayer

magnetic structure.

interactions and hence 2-d-like behavior. In

addition, every other a-b plane is shifted along

the b direction by b/2. This, together with the

crystal field anisotropy which forces the Dy

spins along the c-axis, results in a cancellation of

magnetic interactions between the shifted and

unshifted layers.

The magnetic neutron powder diffraction

data were taken with the standard subtraction

technique, in which the scattering well above the

ordering temperature is subtracted from the

scattering at low T, to isolate the magnetic

response of the system. A dilution refrigerator was employed to cool the powder samples down

as low as 0.09 K. The samples were powder

pellets with (onset) superconducting transition

temperatures of ~ 60 K. The 32 pellets have a

total weight of 10 gm.

Figure 2 shows the magnetic scattering

profile at T = 0.09 K. The magnetic spin

configuration is shown in the inset, where the

spins are along the c-axis, and coupled

antiferromagnetically in both a and b directions,

but ferromagnetically in the c direction. The

solid curve in figure 2 is the theoretical

calculation based on such a 2-d coupled-bilayer

magnetic structure shown in the inset. The N^el

temperature is -1.25 K.

The spin structure in the a-b planes

observed in the Dy2Ba4Cu7025 compound is the same as that found in the DyBa2Cu307 [6,7] and

DyBa2Cu408 compounds [3]; nearest neighbors

are antiparallel. When the Dy magnetic

moments order at Tj^, the behavior is 2-d in nature because the magnetic interaction in the c

direction is much smaller than along a or b. This is due to the same crystallographic

anisotropy (the Dy-Dy distance in c is ~ 3 times

that in a-b plane).

Once the Dy spins are ordered two

dimensionally, a coupling is induced between the

two unshifted layers, which results in the

29
CONDENSED MATTER PHYSICS AND THEORY

Dy2Ba4Cu,0j.

^-^ ' ' /\i c o 1200

I*- (1/2.1/2) Jd\?in e " c ^ 900 1

in rt) 600 dL -i tr 300 ^4r:

p ^ o a t^^tTr- iJlrlllj ^ o mwlwSm^£mfIk I f 2

'^ -300 ^^^^^^^

-500 t WW^ |Pr - ^s

20 30 40 50 Scattering Angle 29 (degree)

Figure 2. Magnetic scattering intensity for Dy2Ba4Cu'7025

at T = 0.09 K. The solid curve is the calculated scattering

intensity for a 2-d coupled bilayer system in which spins are

pointing along the c-axis and antiferromagneticaUy coupled

in both a and b directions, while along the c direction they

are ferromagnetically coupled. The inset shows the magnetic structure, which is in principle repeated ad

infinitum in the a and b directions, but is restricted to two

layers along the c axis.

modulation of the usual "saw-tooth" shape of a 2-d powder diffraction profile. Between the

shifted and unshifted layers, the magnetic (either

dipolar or exchange) interactions are canceled by

symmetry. Therefore no long range 3-d order

occurs around the 2-d Tj^. Indeed in our

experiment no 3-d order was observed to the

lowest temperatures measured ( — 90 mK). An

absence of 3-d order has also been found in the

Dy 2-4-8 case [3], where a similar cancellation occurs between individual a-b layers.

References

[1] A review of both theory and experiment which pertain

to the oxide supjerconductors is given High

Temperature Superconductivity . J. W. Lynn, ed., (Springer-Verlang, New York 1990).

[2] J. W. Lynn, T. W. Clinton, W.-H. Li, R. W. Erwin,

J. Z. Liu, K. Vandervoort, and R. N. Shelton, Phys. Rev. Lett. 63, 2606 (1989).

[3] H. Zhang, J. W. Lynn, W.-H. Li, T. W. Clinton,

and D. E. Morris, Phys. Rev. B 41, 11229 (1990).

[4] T. W. Clinton, J. W. Lynn, J. Z. Liu, R. N. Shelton,

J. Appl. Phys. (to be published).

[5] D. E. Morris, N. G. Asmar, J. Y. T. Wei, J. H.

Nickle, R. L. Sid, J. S. Scott, J. E. Post, Phys. Rev. B40, 11406(1989).

[6] A. I. Goldman, B. X. Yang, J. Tranquada, J. E. Crow, and C.-S Jee, Phys. Rev. B 36, 7234 (1987).

[7] P. Fisher, K. Kakurai, M. Steiner, K. N. Clausen, B. Lebech, P. Hulliger, H. R. Ott, P. Bruesch, and P. Unternahrer, Physica C 152, 145 (1988).

MAGNETIC ORDERING OF Sm IN Sm2Cu04

I. W. Sumarlin, S. Skanthakumar, and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

J. L. Peng and Z. Y, Li (University of Maryland, College Park, MD)

The discovery of superconductivity in a new

class of copper-oxide systems, R2.^CeyJCuO^ {R = Nd, Pr, Sm) [1], has attracted considerable

interest since these are the only copper-oxide

systems known to date that carry electrons as the

charge carriers, in contrast to the hole

conduction observed in other high T^, copper-oxide systems. In addition to their

transport properties, these materials together

with their parent insulating compounds R2CUO4

exhibit a variety of interesting magnetic behavior

involving both the rare earth and the copper

spins [2-6]. Two members of these

electron-doped systems (Nd,Sm) [2,3] have

displayed a coexistence of rare-earth magnetism

and superconductivity, thus providing an

interesting situation to study the interplay

between the two cooperative phenomena,

particularly since there is no clear separation

between the magnetic and the superconducting

subsystems. It is, therefore, of central concern

to elucidate the magnetic properties in these

systems.

Neutron diffraction techniques have been

carried out to investigate the magnetic properties

of the R2CUO4 compounds [3-6]. The Cu spins

in these materials order at relatively high

temperatures (~ 280 K) in a simple antiferromagnetic arrangement: the rare-earth ordering

in the Nd2Cu04 compound is particularly

30
CONDENSED MATTER PHYSICS AND THEORY

interesting since it is found that the ordered Cu

sublattice exhibits a substantial coupling to the

Nd sublattice, thereby inducing the ordering of

Nd ions [3]. As for the related Pr2Cu04

compound, it has been suggested that a small Pr

moment is induced, similar to that found in

Nd2Cu04 [5]. In this report we present the

results of our investigations of the nature of the

Sm ordering in the Sm2Cu04 compound from

neutron diffraction measurements.

A thin plate-like high-quality single crystal

of Sm2Cu04 weighing 66 mg was used, which is the same crystal used in our earlier studies of the

Cu ordering [6]. Its growth and preparation

techniques can be found in the literature [6]. The crystal needed to be very thin because the

experiments must be done in transmission

geometry, and Sm is highly absorbing.

Unpolarized neutron diffraction data were taken

with a wavelength of 2.358 A on the BT-2

triple-axis spectrometer with a pyrolytic graphite

PG(002) monochromator and a PG filter. The

crystal was mounted in the (h,OJ) scattering

plane and enclosed in a ^He cryostat.

Figure 1 shows the temperature dependence

of the (101) Bragg peak intensity, and reveals a phase transition at Tj^j = 5.95 K associated with

long-range antiferromagnetic order of the Sm

spins. This value of Tj^^ is in good agreement

widi specific heat data [7] . The magnetic Bragg

peaks below Tjs^ share the same reciprocal lattice positions with the nuclear Bragg peaks, in fact,

the scattering intensity of 275,000 counts/10 min

shown in figure 1 is due to the (101) nuclear

Bragg peak. Since the Miller indices of the

magnetic Bragg peaks are integers, the magnetic

unit cell is identical to the chemical unit cell. The Sm spin configuration consists of

ferromagnetic sheets within the a-b planes, with

the spins in alternate layers aligned antiparallel.

This spin configuration differs from the

rare-earth spin structure observed in the related

Nd2Cu04 and Pr2Cu04, and in fact the

ferromagnetic sheets of rare-earth ions in the a-b

plane are not found in any other copper-oxide

systems. This prompts an interesting question as

to how these ferromagnetic sheets affect the

electrons in the Cu-0 layers, which are involved

in the superconductivity in the doped

superconductor Sm2.xCexCu04.

We have also determined that the Sm spin

direction is along the c-axis, which is perpendicular to the Cu spin directions; the Cu

sublattice has already ordered at 280 K [6]. We

observe no coupling between the Sm and the Cu

spins, which is not surprising since the spin

directions are orthogonal and the magnetic

structures are different. This contrasts with the

behavior observed in the related Nd2Cu04

system, where the magnetic structures and spin

directions are the same, and strong coupling is observed. We have also observed an increase in the Sm magnetic Bragg intensities below -0.8

K, which is currently under further investigation.

SmjCuO^

310000

270000L12 3 4 5 6 7 Temperature (K)

Figure 1. Temperature dependence of (1,0,1) Bragg peak

intensity. The scattering intensity of 275,000 counts/ 10 min is due to the nuclear Bragg peak. The magnetic

contribution can be seen as a rise in the intensity as the temperature goes below Tkj — 5.95 K.

Rcfexeaces

[1] Y. Tokura, H. Takagi, and S. Uchida, Nature 337,

345 (1989).

[2] Y. Dalichaouch, B. W. Lee, C. L. Seaman, J. T. Markert, and M. B. Maple, Phys. Rev. Lett. 64, 599

(1990); M. B. Maple, N. Y. Ayoub, T. Bjomholm,

E. A. Early, S. Ghamaty, B. W. Lee, J. T. Markert,

J. J. Neumeier, and C. L. Seaman, Physica C 164 . 296 (1989).

[3] J. W. Lynn, L W. Sumarlin, S. Skanthakumar, W.-H

Li, R. N. Shelton, J. L. Peng, Z. Fisk, S.-W. Cheong, Phys. Rev. B 41, 2569 (1990).

[4] S. Skanthakumar, H. Zhang, T. W. Clinton, I. W.

SumarUn, W.-H. Li, J. W. Lynn, Z. Fisk and S.-W. Cheong, J. Appl. Phys. 67, 4530 (1990); D. E. Cox,

A. L Goldmann, M. A. Subramaniam, J. Gopalakrishnan, and A. W. Sleight, Phys. Rev. B 40,

6998 (1989).

[5] M. Matsuda, K. Yamada, K. Kakurai, H. Kadowaki,

T. R. Thurston, Y. Endoh, Y. Hidaka, R. J. Birgeneau, M. A. Kastner, P. M. Gehring, A. H.

31
CONDENSED MATTER PHYSICS AND THEORY

Moudden and G. Shirane, Phys. Rev. B 42, 10098

(1990).

[6] S. Skanthakumar, J. W. Lynn, J. L. Peng and Z. Y.

Li, J. Appl. Phys. 69, 4866 (1991).

[7] M. F. Hundley, J. D. Thompson, S.-W. Cheong, Z. Fisk and S. B. Oseroff, Physica C 158, 102 (1989); C. L. Seaman, N. Y. Ayoub, T. Bjemholm, E. A.

Early, S. Ghamaty, B. W. Lee, J. T. Markert, J. J. Neumeier, P. K. Tsai and M. B. Maple, Physica C

159 . 391 (1989).

OBSERVATION OF THE MAGNETIC FLUX LATTICE IN DyBa2Cu307

Huai Zhang, J. W. Lynn, T. W. Clinton, and N. Rosov

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

J. Z. Liu, Y. X. Jia, and R. N. Shelton (University of California, Davis, CA)

It is well known that in type-II superconductors when the applied magnetic field exceeds Rq^ the magnetic flux becomes

quantized inside the superconductor with flux

quantum 4)^ = h/2e, and the flux quanta form a 2-d triangular magnetic flux lattice (MFL). This

MFL can be investigated by using ^iSR techniques, the Bitter decoration method, STM and

small angle neutron scattering (SANS), for

example. Each technique has its own merits, but

neutron scattering data are the most representative of the bulk properties of the MFL.

Although the MFL has been well studied for the

low Tg superconductors [1], it is considerably more difficult to make measurements for the

high Tg oxide superconductors because the

London penetration depths in these materials are much longer and therefore the flux quanta

overlap, making the lattice much more difficult to observe.

The observation of MFL in the oxide

superconductors by neutron scattering was first reported by Forgan et al. in a single crystal of

YBa2Cu307.^ [2]. In this report we present the

results of our preliminary measurements in a single crystal DyBa2Cu307 sample.

DyBajCujO^ was chosen because the Dy ion

carries a magnetic moment and the crystal

electrical field forces the moment direction to be

along the c-axis. Thus when an external

magnetic field is applied along the c-axis the Dy

magnetic moments can be aligned, increasing the

magnedc scattering. More importantly, it is the

total magnetic flux that should be quantized, and

this leads to a number of possible new phases

[3]. It will also be interesting to investigate how

the MFL lattice interacts with the Dy ions. As

a first step toward answering these questions, we

carried out an exploratory experiment at the 8-m

SANS. A wavelength of A. = 6 A was used

because Dy has a large absorption cross section

and the neutron absorption rate is exponentially

proportional to the wavelength. The coarse

resolution configuration was used to achieve high

incident beam intensity. The sample-detector

distance was 360 cm. Under these conditions

the minimum q observable is typically ~ 0.0083

A'\ and in order to bring the (10) MFL

diffraction peak signal out of the shadow of the

beam stop q^Q needs to exceed this, where

^10 = ^^ B

2^3" 4>,

1/2

We therefore chose H = 5 kOe, which gives q^g = 0.0105 A'^ In addition, we moved the beam

stop away from the usual beam stop center in order to reach smaller q's on the side of the

detector where the diffraction peak was

expected.

The sample was a fully oxygenated single

crystal weighing 250 mg, and had the approximate dimensions of 5 x 5 x 2 mm'^ with a superconducting (onset) transition temperature — 90 K. This is the same crystal used in our

diffraction measurements of the Dy magnetic

ordering with Tj^ = 1 K (see p. 27 in this report). Two sets of data were collected at each

temperature, one with the sample field-cooled

from above T^, and another with the sample

zero-field-cooled. The zero-field-cooled data

32
CONDENSED MATTER PHYSICS AND THEORY

were then subtracted from the field-cooled data

to isolate the magnetic response. The observed

signal was highly anisotropic, and an appropriate

sector average was taken. The subtracted data

taken at T = 10 K are shown in figure 1, and

reveal vortex scattering in the vicinity of q -

0.01 A'^ as expected. This scattering intensity

also decreased with increasing temperature as expected for a vortex lattice. The sample used in the present measure- ments was much thicker than desired for these

vortex (SANS) experiments, so that substantial

improvements in the intensity can be expected in

the next phase of the experiment. It will also be

important to compare these data with data for

YBa2Cu307 where there is no rare earth

moment. Further work is in progress.

References

[1] See, for example, D. K. Christen, H. R. Kerchner,

S. T. Sekula, and P. Thorel, Phys. Rev. B 39, 102

(1980).

[2] E. M. Forgan, D. McK. Paul, H. A. Mook, P. A. Timmins, H. Keller, S. Sutton, and J. S. Abell, Nature 343, 735-737 (1990).

DyBaaCUgOy

-500

0.01 0.02 0.03 0.04 0.05 0.06 Momentum transfer Q (A"M

Rguie 1. Field-cooled data subtracted from the zero-field-cooled data, for a temperature of T = 10 K. The applied field is H = 5 kOe, which yields a qiQ =

0.0105 A"^. The data were collected for 3 1/2 hrs at each

temperature.

[3] See, for example, A. I. Buzdin and A. Ya. Simonov,

Physica C 167, 388 (1990).

NEUTRON POWDER DIFFRACTION STUDY OF THE CRYSTAL STRUCTURES OF

YSr2(CoCu2)07, (Yj.j^Cax)Sr2(CoCu2)07, AND YSr2(GaCu2)07

Q. Huang and A. Santoro

and

R. J. Cava (AT&T Bell Laboratories)

The structures of YSr2(Co Cu2)07 and

(Yj_j^Cax)Sr2 (Co Cu2)07 have been analyzed by

neutron powder diffraction techniques. Both

materials crystal ize with the symmetry of space

group Ima2 and lattice parameters a = 22.7900

(5), b = 5.4515 (1), c = 5.4097 (1) A for

YSr2(Co Cu2)07 and a = 22.8266(7),

b = 5.4312(2), c = 5.4048(2) A for

(Yi_J22i^Sr2(CoCu2)0'j. In the non-doped

compound the Co ions substitute exclusively the

copper ions located on the chain sites of the

1,2,3, parent structure, YBa2Cu306^x- ^®

coordination of Co is tetrahedral and the C0O4

tetrahedra form chains propagating in a zigzag

fashion along the c-axis of the structure. The

oxygen atoms of the CoO layers were found to be disordered over two sets of positions, 0(l)a

at 1/4, y^, Zq and 0(l)b at 1/4, y^, l-z^, with

occupancy factors of 0.62(4) and 0.38 (4). In the case of die compound YSr2(Ga Cu2)07 [1], our measurements show that this type of

disordering is much less pronounced, with

occupancy factors of 0.88(2) and 0.12(2). The

presence of oxygen on the sites 0(l)a and 0(l)b

creates a configuration of the chains of C0O4

tetrahedra such that two types of chains coexist, one being the mirror image of the other. The

unusually high temperature factor of Co has been

attributed to a splitting of the atom between two

positions, symmetrical with respect to z = 0,

33
CONDENSED MATTER PHYSICS AND THEORY

along the c-axis. This configuration can be

easily explained in terms of the disordering of

the oxygen atoms over the positions 0(1 )a and

0(1 )b and the coordination requirements of

cobalt (figs. 1 and 2). Our experimental results are consistent with the distribution of oxygen

atoms on the layers CoO shown in figure 3. It can be shown that such configuration is closely

related to that of the oxygen atoms located on

the basal plane of the 1,2,3 superconductor for

the 0-j composition. The structure of the doped

compound (Yj.j^Cax)Sr2(CoCu2)07 is considerably more complicated tiian that of the nondoped

material. Calcium substitutes yttrium, but cobalt seems to replace copper both on the chain sites and at the CUO2 planar sites. The oxygen atoms on the CoO layers seem to be disordered not only over the 0(l)a and 0(l)b sites, but also over the two sites generated from these by the

operation of the mirror x, 1/2, z, perpendicular

to the b-axis. Models for the local structure of

this compound are still under investigation.

Q04 04

N N

X-0.25

Figure 1. Atomic configuration of the

layers CoO at x = 1/4. In this figure

N = Co. The oxygen atoms 0(1 )a and

0(l)b are co-planar with N, but the atoms 0(4) belong to the SrO layers

and are located above and below the plane of figure. The two diagrams

show the two possible tetrahedra which

form chains running parallel to the caxis.

1.772(5)

0(1.)

99.8(8) 2.874(7)

3.404(5)

.2.93(1)

1.772(5) 2.93(1)

(a)

1.772(5)

CXlb)

147.7(8)

2.77(1) 1.772(5)

3.096(6) 2.164(8)

2.93(1)

'91(1) 89.0(2)

88.7(5)^ 0(2)_

2.77(2) 0(3)

1.92(1)' 2.706(1

95.9(3)

92.6(3)

(b)

1.93(2) ,2.705(1)

,1.94(2) •0(2)

-90.0(5)

3.17(1)

3.10(1) 99.4(3) 2.341(3)"^'^^'

• i

2.45(1

2.45(1)2.705(1)

0(2)

2.41(1)

0(2)

(d)

0(2) 0(2)

0(4)

Z385(4:

rr70(2) 2-9M(6)

^•2.401(5)

«>»> 0(lb)

(e)

Figiure 2. Coordination polyhedra of the metal ions in the structure of YSr2(CoCu2)0-7. Figure 2a represents the CoO^

tetrahedra forming the regular chains on the CoO layers and figure 2b the CoO^ tetrahedra interconnecting chains perpendicular

to one another.

34
CONDENSED MATTER PHYSICS AND THEORY

® Co o 0. x=0.25

Figure 3. Model of the atomic structure of the CoO layer

at X = 1/4. The atoms 0(4) are not shown for clarity. The comer-sharing tetrahedra of oxygen atoms form two

types of chains which are specular to one another. A

possible way of interconnecting the chains is also shown in the figure.

References

[1] R. Roth et al., J. de Physique, preprint (1990).

NEUTRON POWDER DIFFRACTION STUDY OF THE STRUCTURE OF

YSr2(AlCu2)07

Q. Huang and A. Santoro

and

S. Sunshine and R. J. Cava (AT&T Bell Laboratories, Murray Hill, NJ)

The structure of YSr2(AlCu2)07 has been

analyzed by neutron powder diffraction techniques. The compound crystallizes with the

symmetry of space group P4/mmm and lattice parameters a = 3.8645(1) and c = 11.1139(3)A.

A schematic representation of the unit cell is shown in figure 1. This structure is closely

related to that of the 1,2,3 superconductor and to those of YSr2(GaCu2)07 and YSr2(CoCu2)07.

Like the Ga and Co ions in these last two

compounds, also Al replaces exclusively the

chain copper ions, assuming tetrahedral

coordination and forming chains similar to those

made by the Ga04 and C0O4 tetrahedra.

Refinements of the structure show that practically all the oxygen atoms are disordered over

the sets of positions indicated in figure 1 . The

models refined, and the order in which the

refinements were made, are specified in table 1

.

The first case is the same as the structure of the

parent 1,2,3 compound. In model 2, the oxygen

atoms 0(1) and 0(4) are split along the a-axis,

U

Figure 1. Unit cell of the compound YSr2(AlCu2)07,

showing the splitting of the oxygen atoms around their idealized positions corresponding to the structure of the 1,2,3 superconductor.

35
CONDENSED MATTER PHYSICS AND THEORY

and in model 3 the extra atom 0(42) is introduced with further improvement of the R-factors. In 4 the oxygen atoms 0(1) are shifted along the

mesh's diagonal, and finally, in model 5, based on space group P4/mmm, the atoms 0(2) are

split vertically, i.e. an extra atomic position is introduced in the structure for these atoms. The

atomic disordering present in this structure

results in many possible local configurations.

For example, the possible coordination polyhedra

of the Al ions are illustrated in figure 2. Based on reasonable bond distances and angle, of these

four possibilities only the tetrahedra II and III have to be taken into consideration (the other

two have a too short 0-0 distance of 2.310 A).

The analysis of models for the coordination polyhedra of the other metal ions is still under way.

Table 1. Shifts of the oxygen atoms in Sr2YAlCu20-7.

01 0, 0, z X, 0, z X, 0, z X, X, z X, X, z 02 1/2, 0, z 1/2, 0, z 1/2, 0, z 1/2, 0, z 1/2, 0, z** 03 0, 1/2, z 0, 1/2, z 0, 1/2, z 0, 1/2, z 041 0,1/2,0 X, 1/2, X, 1/2, X, 1/2, X, 1/2, 042 1/2, y, 1/2, y, a 3.8644(1) 3.8645(1) 3.8645(1) 3.8645(1) 3.8645(1)

c 11.1142(3) 11.1139(3) 11.1141(3) 11.1139(3) 11.1139(3)

V 165.97(1) 165.98(1) 165.98(1) 165.98(1) 165.98(1)

Rn 12.56 9.35 8.26 7.87 5.97 Rp 9.29 8.34 7.84 7.65 7.09 Rw 12.29 11.17 10.26 10.03 9.49 Re 4.49 4.49 4.49 4.49 4.49 X 2.74 2.49 2.29 2.23 2.11 *Pmmm symmetry is used in 1,2,3, and 4, but set a = b, z(02) = z(03), and x(041) = y(042).

P4/mmm symmetry is used in 5. **This atom is split along c.

o

^ 80.1(1). iD

u

'U

o

o^

©rrp 0~ KD 103.4(2)' 2.310(4) //|V.l.593(5)

\-2.795(4)

"a

^

Al-O square (S) Al-0 Tetrahedron I (Tl)

2.795(4)

©01 O

Al-0 tetrahedron II (Til) Al-0 tetrahedron III (Till)

Figure 2. Possible coordination polyhedra around the Al ions. The first two cases have to be excluded because they result

in unreasonably short O-O distances.

36
CONDENSED MATTER PHYSICS AND THEORY

Cu CHAIN AND PLANE ORDERING OF SINGLE CRYSTAL PrBa2Cu306+

N. Rosov and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

G. Cao, J. W. O'Reilly, P. Pernambuco-Wise, and J. E. Crow

(MARTECH, Florida State University, Tallahassee, PL)

Alone of all the single-phase layered

perovskites, PrBa2Cu307 (Prl237) is not superconducting [1]. Two possible explanations have

been advanced to explain this behavior. One

suggests that the Pr is mixed-valent with the

electrons resulting from the oxidation of Pr"'"^ to

Pr"^"*" filling holes in the conduction band [2]. The other explanation proposes that the /

electrons of Pr^"*" hybridize with the conduction

electrons and thereby suppress the superconductivity [3]. In an effort to understand the effect of the Pr ion on the R1236-I-X system, we are studying the Cu and Pr magnetism in Prl236-l-x.

Previous neutron diffraction measurements on powder Prl237 [4] showed that the Pr ordered at Tj^ = 17 K, with a moment of 0.74 /xg directed

along the c axis. We have measured a 1 1 .5 mg single crystal

of as-grown Prl236-l-x on BT-2 and BT-6 over

the temperature range 4 K < T < 340 K. The

crystal was aligned with hhl reflections in the

scattering plane. Figure 1 shows room temperature transverse scans of the (006) and (113)

structural Bragg peaks which indicate that the

single crystal is of high quality.

(113)

/

;

HiA- \ •/ . Xm., . -0.5

Figure 1. Transverse scans of the (006) and (113) reflec- tions of Prl236+x.

The Cu ordering in Y12364-X and

Ndl236-l-x has been shown, by neutron diffraction [5], to take two forms (fig. 2). For plane

ordering, in which only the Cu spins in the plane

sites are ordered, only (—1) reflections ("wholeintegral" reflections) are observed. For chain

ordering, in which the Cu spins in both the plane

and chain sites are ordered, ( ^^^ ) reflections

("half-integral" reflections) are observed while

the whole-integral reflections are greatly reduced

in magnitude. For small x, the planes order for T » 500 K and the chains order below 100 K.

Figure 2. The basic spin structure for the Cu moments:

(a) only plane Cu ordered and (b) both plane and chain Cu

ordered.

For the present Prl236-l-x crystal, we

observed half-integral peaks at all the temperatures studied, which indicates that both the Cu

chains and planes are ordered up to T = 340 K.

This is quite different from the behavior of the

other R1236+X compounds, but a similar

increase in the chain ordering temperature has

been observed in the Co-substituted

YBa2(Co,Cu)305+x system, where the Co ions

preferentially occupy the chain sites [6]. Hence

these data suggest that the presence of Pr not only affects the superconductivity but also

strongly affects the Cu magnetism. The temperature dependence of the (^ ^ -^ ) reflection is shown in figure 3, indicating a Tj^^ near 390 K.

37
CONDENSED MATTER PHYSICS AND THEORY

600

500

400

300

S" 200

100

100 200

T{IO 300 400

113

Figure 3. Temperature dependence of the ( _ _ _ ) integrated intensity, indicating Tjsj « 390 K. We observed a small significant decrease in the intensity of the (44^) reflection as T was

decreased from 20 K to 5 K. This may be due

to the onset of magnetic order for the Pr; however, further analysis is required to confirm

this.

References

[1] L. Soderholm, K. Zhang, D. G. Hinks, M. A. Beno,

J. D. Jorgensen, C. U. Segre, and I. K. Schuller, Nature 328, 604 (1987).

[2] A. Kebede, C. S. Jee, J. Schwegler, J. E. Crow, T.

Mihalisin, G. H. Myer, R. E. Salomon, P. Schlottmann, M. V. Kuric, S. H. Bloom, and R. P. Guertin, Phys. Rev. B 40, 4453 (1989).

[3] L. Soderholm, C.-K. Loong, G. L. Goodman, and

B. D. Dabrowski. Phys. Rev. B 43, 7923 (1991).

[4] W-H. Li, J. W. Lynn, S. Skanthakumar, T. W.

Clinton, A. Kebede, C.-S. Jee, J. E. Crow, and T.

Mihalisin. Phys. Rev. B 40, 530 (1989).

[5] W-H. Li, J. W. Lynn, and Z. Fisk. Phys. Rev. B

4i, 4098 (1990).

[6] P. F. Miceli, J. M. Tarascon, L. H. Greene, P. Barboux, M. Giroud, D. A. Neumann, J. J. Rhyne,

L. F. Schneemeyer, and J. V. Waszczak. Phys. Rev. B 38, 9209 (1988).

PHONON DENSITY-OF-STATES EV THE SUPERCONDUCTORS

(Nd-Ce)2Cu04 AND (Pr-Ce)2Cu04

I. W. Sumarlin and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

D. A. Neumann and J.J. Rush

and

J. L. Peng and Z. Y. Li (University of Maryland, College Park, MD)

Since the discovery of high-T^, superconductors the mechanism of the electron pairing

in these systems has remained an open question.

However, there are observations which suggest

that phonons may play a major role in the

formation of the superconducting state, particularly in the "moderate-T^" (-30 K) systems.

For example, a substantial isotope effect has

been reported in the La2.xM^Cu04 (M = Sr,

Ba)[l] system, and measurements of a^F(<o) in (Ba-K)Bi03 from point-contact-tunneling have

been found to be in reasonable agreement with

the measured phonon density-of-states from

neutron scattering [2]. We have been investigating the phonon density-of-states in the 2-1-4

electron-superconductor systems using inelastic neutron scattering techniques. For the case of

(Nd-Ce)2Cu04 we have found reasonable agreement at low energies between the point-contact

tunneling density-of-states data and our neutron

measurements, suggesting that phonons are involved in the pairing mechanism in these

systems [3]. We present here our measure- ments of the generalized phonon density-ofstates (GPDOS) in the Nd2.xCe^Cu04 and

Pr2.xCe^Cu04 superconducting materials, as well

as the related insulating undoped systems.

Powder samples of Nd2.xCej^Cu04,

Pr2.xCe^Cu04, Nd2Cu04, and Pr2Cu04, each

weighing ~ 42 g, were prepared by the usual

methods found in the literature [4] . The neutron

scattering experiments were carried out on the

BT-4 triple-axis spectrometer and all measure- ments were done at 4.2 K. The measurements were taken in the energy loss mode with a fixed

final energy using a Be-graphite-Be filter

38
CONDENSED MATTER PHYSICS AND THEORY

analyzer assembly and employing either a pyrolytic graphite PG(002) monochromator for

low energy transfers (^40 meV) or a Cu(220)

monochromator for higher energy transfers (s:36

meV). In the case of (Nd-Ce)2Cu04, the data were supplemented by the usual triple-axis

technique for energies :s23 meV, employing

these two monochromators. The triple-axis

technique has the advantage at low energy

transfers of being able to collect data over a range of momentum transfers, Q, and at larger

values of | Q |

Figure la shows the observed scattering for

energy transfers up to 105 meV in (Nd-Ce)2Cu04. The region between 17 meV

and 35 meV is dominated by two magnetic peaks

whose intensities go beyond the scale of the

figure thus prohibiting the determination of the

phonon density-of-states in this region of energy.

These magnetic peaks, as well as the 93 meV

peak shown in the figure, originate from crystalfield excitations of the Nd ions. The phonon

density-of-states spectrum extends only up to ~84 meV. The observed phonon peak at 13

meV and another peak at —40 meV are in reasonable agreement with the peaks measured in the tunneling data [3]. The correspondence at higher energies between the tunneling and our

data is at present not good, but we believe this reflects the extreme difficulty in obtaining

quantitative tunneling data in the oxidesuperconductors.

Figure lb shows the density-of-states for

(Pr-Ce)2Cu04. The crystal-field transitions out of the ground state for the Pr ions occur at 88

meV and at ~ 18 meV; the intensity of the low

energy crystal field peaks for both systems goes

beyond the scale of the figures. As illustrated there are substantial changes in the peak

intensities in a region of energy between 30 and

60 meV as the material is changed from

(Nd-Ce)2Cu04 to (Pr-Ce)2Cu04. The broad

feature around 53 meV narrows into a peak at 53

meV and the 40 meV peak intensity increases

relative to the 53 meV peak. There is also a peak at -36 meV in the (Pr-Ce)2Cu04

compound which is not observed in the

(Nd-Ce)2Cu04 data. Infrared reflectivity

measurements on polycrystalline (Pr-Ce)2Cu04

show active IR modes at 33.6, 53.1, 62.6, and

68.4 meV corresponding to the motions parallel

to the c-axis [5]. However, direct identification

with the GPDOS is difficult since IR techniques

generally measure zone-center phonon modes,

while substantial dispersion in all the phonon

branches is expected, analogous to the related

La2Ni04 [6] where the dispersion relations have

been measured in detail. We also have observed substantial decreases

in some of the phonon peak intensities as the

material is changed from the parent insulating

Pr2Cu04 to superconducting (Pr-Ce)oCu04. It will be interesting to see the correspondence

between our data for (Pr-Ce)2Cu04 and the

tunneling data when they become available.

-pc

D

O

u

600 r

400

200

10 20 30 40 50 60 70 80 90 100

*

nil 111 llll 1 ll. Kil Oillllll 1

-r ^ (b):

} -

%,,

10 20 30 40 50 60 70 80 90 100 Energy (meV)

Figure 1(a). Generalized phonon density-of-states

(GPDOS) in (Nd-Ce)2Cu04, whose spectrum extends up to — 84 meV. The region between 17 - 35 meV is dominated

by two crystal- field peaks of the Nd ions, whose intensities go beyond the scale of the figure. The peak at 93 meV is also a crystal-field peak. Figure 1(b). GPDOS in (Pr-Ce)2Cu04. Two crystal-field transitions are observed

at 88 meV and 18 meV; the intensity of the 18 meV

transition is 4400 counts. There are substantial changes in the peak intensities in the 30 - 58 meV region as the material changes from (Nd-Ce)2Cu04 to (Pr-Ce)2Cu04.

References

[1] M. K. Crawford, M. N. Kunchur, W. E. Farneth,

E. M. McCarron, III, and S. J. Poon, Phys. Rev. B

41, 282 (1990).

[2] Q. Huang, J. F. Zasadzinski, N. Tralshawala, K. E. Gray, D. G. Hinks, J. L. Peng, and R. L. Greene, Nature (London), 342, 369 (1990).

[3] J. W. Lynn, L W. Sumarlin, D. A. Neumann, J. J. Rush, J. L. Peng, and Z. Y. Li, Phys. Rev. Lett. 66, 919 (1991).

39
CONDENSED MATTER PHYSICS AND THEORY

[4] J. L. Peng and R. L. Greene, Physica C 172 143

(1990); J. W. Lynn, I. W. Sumarlin, S. Skanthakumar, W.-H. Li, R. N. Shelton, J. L. Peng,

Z. Fisk, and S. W. Cheong, Phys. Rev. B 41, 2569

(1990).

[5] M. K. Crawford, G. Bums, G. V. Chandrashekar,

F. H. Dacol, W. E. Fameth, E. M. McCarron, III,

and R. J. Smalley, Solid State Comm. 73, 507

(1990).

[6] L. Pintschovius, J. M. Bassat, P. Odier, F. Gervais, G. Chevrier, W. Reichardt, and F. Gompf, Phys. Rev. B 40, 2229 (1989).

ANTIFERROMAGNETIC ORDERING OF BaPrOj

N. Rosov, J. W. Lynn, Q. Lin, and S. Skanthakumar

(University of Maryland, College Park, MD and Reactor Radiation Division)

Q. Huang

and

G. Cao, J. W. Reilly, P. Pernambuco-Wise, and J. E. Crow

(Florida State University, Tallahassee, FL)

The superconducting properties of the

layered perovskites RBa2Cu306^x (R123)

change very little on substitution of the trivalent rare earth ion, R [1]. An important exception to

this behavior is Pr, where the superconducting

transition temperature decreases with increasing

Pr concentration; superconductivity is completely

suppressed for more than 60% Pr in the rare

earth site [2]. Two proposed mechanisms exist

to account for the suppression of superconductivity in Prl23: one suggests that the Pr is mixedvalent Pr-^"'"/Pr'^"^ where the electrons oxidized

from Pr-^"^ fill conduction band hole states [2]. The other suggests tiiat there is hybridization

between the Pr^"*" / shell and the conduction

electrons [3]. In order to understand the

behavior of Pr in Prl23, we are studying

tetravalent Pr in BaPrOg, an orthorhombicallydistorted perovskite which is a precursor to the more complicated layered-perovskite high-T^.

materials [4]. Reitveld refinements of T = 300, 17, and

5 K powder diffraction patterns of BaPr03 taken on BT-1 confirm that the structure is space

group Pbnm as reported previously [5], with

lattice parameters a= 6.2137 A, b= 6.1787 A,

and c= 8.7261 A. The Pr ions are surrounded

by nearly symmetric oxygen octahedra, with

Pr-0 distances of 2.2252(6) A along the c axis

and 2.223(3) A in the ab plane. The orthorhombic asymmetry is reversed from the previous

incorrectly reported structural determination [5],

with the present lattice parameter a approximately equal to the previously reported lattice parameter b and vice versa. Diffraction scans at T = 5 K and 17 K

were taken on BT-2 with X = 2.3556 A to study

the magnetic order. Magnetic (difference) peaks

appear at 26 = 27(1)° and 53.0(1.4)° (fig. 1);

1000

500

Figure I. Difference between 26 scans at T = 5 K and 17 K. Note the magnetic peaks at 26 = 27(1)° and 53(1)°

and the incomplete cancellation of the structural peak at 26 = 55°.

difference peaks were also observed at 72(2)°

and 89(2)°.

The temperature dependence of the 27°

peak intensity (fig. 2) confirms that this peak is due to magnetic order with a N^el temperature

Tj^ = 11.7(2) K, in agreement with previous

magnetic susceptibility, magnetization and

specific heat measurements [4]. The magnetic

reflections are coincident with nuclear Bragg

peaks implying that the magnetic unit cell and

structural unit cell are the same size. The spins

are therefore staggered along the < 110> and

[001] directions. With the exception of the

40
CONDENSED MATTER PHYSICS AND THEORY

26 = 27° peak, these difference peaks are ap- proximately 1 % of the intensity of the structural

intensities at the same scattering angle.

Since the a and b axes of BaPr03 differ by

less than 1%, the spin direction can be

determined with respect to the c axis, barring

any accidental degeneracies. However, a '^b ^

citl and so reflections for which 2Q?+l?)+fis a constant cannot be resolved in the present

experiments. Thus all magnetic diffraction peaks

are composed of reflections from many different

directions in reciprocal space and the peak

intensity ratios depend not on the spin direction

but on the multiplicity of the contributing re- flections.

The intensity ratios for the 26 = 27°,53°,

and 88° reflections of 1:4.4(4):5. 3(1.1) agree

relatively well with the calculated multiplicity

ratios of 1:3:4. Since the intensities do not depend on the spin direction, we can determine

that the low temperature value of the moment is

(ji^) = 0.35(5) /ig. The expected saturated moment of Pr"^"*" is 0.43 ^g, assuming a + Vi doublet

ground state of the crystal-field split '^'^^12 multiplet [3]. Since the magnetic and nuclear Bragg peaks

occur at the same diffraction angles in this system, the technique of subtracting the high T

data from the data at low T could in principle

include both magnetic and nuclear components.

Hence, the discrepancy between the observed

difference intensity ratios and the intensity ratios

expected on the basis of the multiplicity may be

due in part to the inclusion of the effect of small

structural changes in the nuclear peaks. In fact the uncertainty in the refined peak intensities,

with the exception of the 26 = 27° peak, can be

as large as one-half the observed difference

intensity. As a result, it is impossible to distinguish between the small magnetic

components and intensity differences due to small structural changes. In fact, the difference

peak observed at 26 = 72° clearly has been

significantly distorted by this effect. Further

work will have to wait for suitable single

crystals.

CM

II

4800

4600

4400 -

O 4200

4000

- Q*^-

K*

c \

c •-

: ^

\

^ a

- \* t *

i V

-

i . 1 1 1

1 ,

1 , . , 1 , , , 10 T(K)

12 14 16

Figure 2. Temperature dependence of the 28 = 27° peak

intensity, indicating T^ = 11.7(2) K. Inset: Antiferro- magnetic ordering inferred from magnetic diffraction pattern, where the open circles indicate a spin pointing in some definite, undetermined, direction and the closed

circles indicate a spin pointing antiparallel to that direction. The Pr moments are antiferromagnetically ordered along tlie <110> and [001] directions.

References

[1] P. H. Hor, R. L. Meng, Y. Q. Wang, L. Gao, Z.

J. Huang, J. Bechtold, K. Forster, and C. W. Chu.

Phys. Rev. Lett. 58, 1891 (1987).

[2] A. Kebede, C. S. Jee, J. Schwegler, J. E. Crow, T.

Mihalisin, G. H. Myer, R. E. Salomon, P. Schlottmann, M. V. Kuric, S. H. Bloom, and R. P. Guertin. Phys. Rev. B 40, 4453 (1989).

[3] L. Soderholm, C.-K. Loong, G. L. Goodman, and

B. D. Dabrowski. Phys. Rev. B 43, 7923 (1991).

[4] G. Cao, T. Yuen, P. Pernambuco-Wise, J. E. Crow,

J. W. O'Reilly, M. V. Kuric, R. P. Geurtin, N. Rosov, and J. W. Lynn. J. Appl. Phys. (to be

published)

[5] A. J. Jacobson, B. C. Tofield, and B. E. F. Fender. Acta Cryst. B28, 956 (1972).

41
CONDENSED MATTER PHYSICS AND THEORY

QUANTITATIVE DETERMINATION OF IMPURITY PHASES CV YBa2Cu307.5

J. K. Stalick

Neutron Rietveld refinement is a powerful

technique in the study of high-T^, superconductor powders, owing to the relative

sensitivity of neutrons to oxygen content and the

amenable powder line shape [1]. This method

has been used as a tool for the detection and

quantitative measurement of minor impurity

phases in superconducting YBa2Cu307.3

samples, even when no resolved impurity peaks

are evident.

Experimental neutron diffraction patterns were obtained for the common impurity phases

CuO, Y2BaCu05, BaCOj, and BaCu02. The

structures were refmed to provide the best values

for obtaining a calculated neutron pattern;

however, the structure of BaCu02 could not be

completely refmed owing to as yet unresolved

difficulties in structure solution. High-resolution

powder diffraction data were then collected on a

series of YBa2Cu307,5 samples prepared under

different processing conditions; the structures were refmed, along with scale factors for the

intensities of potential impurity phases which were included as a contribution to background.

The impurity phase commonly found was CuO, ranging in concentration from 17 wt. %

(obvious) to 0.5(1) wt. %, which is not visually

discernable in a plot of the data. In all cases the

agreement factors x = ^wp/Re ^^'^ ^B improved

statistically when the contributions of the

impurity phase were included as part of the

background calculation.

Reference

[1] R. J. HiU, Powder Diffraction 6, 74 (1991).

NEUTRON POWDER DIFFRACTION STUDY OF THE NUCLEAR AND MAGNETIC

STRUCTURES OF YBa2Fe308 AT ROOM TEMPERATURE

Q. Huang, A. D. Mighell, V. L. Karen, and A. Santoro

P. Karen and A. Kjekshus (University of Oslo, Blindern, Oslo, Norway)

and

J. W. Lynn and N. Rosov

(University of Maryland, College Park, MD and Reactor Radiation Division)

The nuclear and magnetic structures of

YBa2Fe308 have been investigated by powder

neutron diffraction at room temperature. The

nuclear structure of the compound has the

symmetry of space group P4/mmm and lattice parametersa= 3.9170(1) and c = 11.8252(4)A.

The configuration of the atoms in the unit cell is very similar to that of the superconductor

YBa2Cu307, with the exception that the iron

ions corresponding to the Cu chain ions have

octahedral coordination, rather than square

planar; the octahedra thus are arranged in layers

rather than in chains. This significant difference

is a consequence of the fact that all possible

oxygen sites in these layers are fully occupied,

resulting in an oxygen content of eight rather

than seven atoms per formula unit. A second

consequence of the full occupancy of the oxygen

sites is that the Ba ions have a twelvefold

cuboctahedral coordination in the iron

compound, rather than tenfold. The magnetic

structure is based on a unit cell related to that of

the nuclear structure by an axis transformation of

matrix (110/110/002). The magnedc origin of

the extra intensities and the basic spin

configuration were determined by polarized

neutron diffraction measurements. The iron

moments are coupled antiferromagnetically

within each Fe02 layer, as well as along the caxis. The magnetic moments of all the iron ions are the same with a value of 3.49(2)jig. This

configuration results in the magnetic symmetry

Ij,mm'm.

42
CONDENSED MATTER PHYSICS AND THEORY

Figure 1. (a) Nuclear and magnetic structures of YBa2Fe20o. For simplicity, only the nuclear cell is shown

in the figure. The magnetic cell is related to the nuclear one by the transformation of axes matrix (11 0/110/002).

The magnetic moments of the iron atoms lie in the planes

perpendicular to c. Their direction in the plane, however,

cannot be determined in our experiment and has been fixed

arbitrarily along the a-axis of the magnetic cell, (b) Unit

cell of the compound YBa^CuoOT shown for comparison

with the iron compound.

(a) YBaafejOs (b) VBazCujO?

HAVE THE INTERMEDIATE BOSONS RESPONSIBLE FOR PAIRING

IN THE HIGH-TEMPERATURE SUPERCONDUCTING COPPER-OXIDE

MATERIALS BEEN FOUND?

R. C. Casella

For some time I have been considering

models of high-temperature superconductivity in the layered copper-oxide compounds which

involve the exchange of some heretofore

unspecified high-energy (o > > o) (phonon))

intermediate bosons (IB) [1,2]. As discussed in the previous progress report [3], double IB

exchange between pair constituents in one band

via a third fermion in another band leads (when

the IB and third fermion coordinates are

integrated out) to effective pair correlations in each band. Moreover, the assumption that one

band is associated with orbital overlap mainly in the ab planes while the other (very narrow) band

(of width < 0(100 meV)) is associated with the

overlap of hybrid orbitals along c (i.e., between

planes), led to a satisfactory explanation of the

superconducting gap anistropy observed in the

compound Bi2SrCaCu203. A basic assumption of

the model is that interband scattering of the pair

constituents via exchange of the IBs can be

neglected. This assumption was originally

introduced in an ad hoc way in order to maintain

the identity of a given pair as belonging to one

or the other of the two bands rather than

considering pairs with constituents, one from

each of the two bands. It seemed rather natural

within the context of the formalism but appeared

somewhat mysterious, having no other physical

basis, theoretical or experimental. Moreover,

the nature of the IB remained a mystery,

although in order to fit the model to the

observed (anisotropic) tunneling gaps it is necessary to assume that the IB energy w >

0(0.3 eV). It now appears possible to link both

mysteries and to understand them

simultaneously. Moreover, it appears that an IB

of comparable energy and similar origin has

been found in the Y-I23 system, suggesting

further experimental tests of the model.

Subsequent systematic examination of the

infrared data for the Bi compound (keeping in mind that to be consistent with the tunneling data

the model implies o) > 0(0.3 eV)) suggested

identifying the broad infrared-active excitation

centered at » 3000 cm'^ [4] as a candidate IB. Since the IB couples to photons, it must possess

nontrivial spectral density near q = 5. But a large momentum transfer is required to scatter an approximately ab-moving fermion from near

43
CONDENSED MATTER PHYSICS AND THEORY

the Fermi surface of the energetically wide

quasi-two-dimensional ab band to an approximate c-mover in the energetically narrow c band.

Hence, the identification of the high energy IB

with a low-q excitation "justifies" a posteriori

the neglect of interband scattering of the

fermions at each stage of the double-IB

exchange. The pair constituents (k!,- kl) in a given band are indirectly coupled by the

exchange of these high-E low-q IB's with the

third fermion in the other band. At each stage

the active pair constituent and the third fermion

scatter very little but (virtually) exchange

considerable energy. Two other remarks: (i) Large-q low-E phonons can lead to interband

scattering between approximate ab- and c- movers, but this process presumably does not produce the high-T^ condensate. (Such phononmediated interband transitions likely lead to pair

breaking as T -* T^ from below.) (ii) Although

not essential, the IB, if composite, might be an

i.r. -active broad unbound electron-hole

resonance, possibly accounting for the large

width of the i.r. absorption. Thus the IB itself might be associated with a low-q interband

transition of a fermion between two other energy

bands in these complex high-T^. systems.

Are there other experimental tests and can

the model be applied to other high-T^, crystals?

The existence of a broad i.r. -coupled excitation

in Y-123 centered at - 3300 cm'^ [5] (or - 2100

cm'^ [6]) suggests the latter may be true, if the

reported tunneling-gap anisotropy can be

confirmed. As to other tests, the high density of

states associated with the narrow c band might

be seen by angle-resolved photoemission on

crystals oriented such that the component of £

along c is conserved, assuming suftlcient energy

resolution can be achieved. (I am aware only of

angle-resolved work in which k^^j, is conserved.)

If such a peak shows up, it may still be difficult to disentangle it from one which would result from the high-mass component m^^ of a single

anisotropic band. Nonetheless such studies

would be useful. Returning to the i.r. experiments, the gap values for Y-123 as reported,

e.g., in [7], have been the subject of controversy

[4,5], quite apart from the disagreement with

tunnel values. The i.r. detectability and even the

existence of gaps in both the Y- and Bi- CUO2

systems have been questioned. (It is conceivable

that the gaps reported via interpretation of

tunneling data are real, but not i.r. detectable,

possibly due to the existence of states within the

gaps.) I nevertheless consider the implications

of the value lA^^/kT^ - 8, inferred from the i.r. data for Y-123 in [7] as well as elsewhere.

Setting this ratio equal to 8.0 and setting co =

0.4 eV (based upon our identification of the IB

with the i.r. absorption centered at 3300 cm"^),

the model then implies 2A^,/kT^ < 2.7. The

prediction lies somewhat below the result 2A^/kTg - 3, inferred in [7]. The experimental

situation requires clarification since the model

predictions are unambiguous. If 2A^jj/kT^ =

8.0, then to obtain a value as large as 2.9 for the

ratio 2A^/kT^ within the model requires that 00 - 5 eV, clearly at odds with our identification of

the IB. (Table 1.)

Table 1. Computed values for the effective coupling g, intermediate boson cut-off energy 0), narrow band width Bq

and the parameter x = Bc/4kT, for the Y-123

compound as a function of the ratio a)/Bp = R/2, given

that the critical temperature T^. = 92 K and that the in- plane pair-excitation gap 2A^jj = 2A£) = 63.4 meV.

Results are presented for four values of the parameter r =

Aq/Aq = A t^/A where 2A is the pair-excitation gap along

the c-axis. (Data from [7].)

io/Bq g

ca[eV] B(.[meV] Xc-^

r = 3.5 (2A^/kt^ = 2.3)

4 0.439 0.24 60.6 1.91 5 0.380 0.40 78.5 2.48 10 0.268 1.75 175 5.52

15 0.229 4.20 280 8.83

r = 3.2 (2A^/kt^ = 2.5)

5 0.515 0.21 42.4 1.34 10 0.366 0.71 71.1 2.24 20 0.283 2.38 119 3.75 30 0.250 4.83 161 5.08

r = 3.0 (2A,/kt, = 2.7)

25 0.330 1.83 73 2.30 50 0.274 5.28 106 3.33

r = 2.8 (2A^/kt^ = 2.9)

75 0.321 4.69 63 1.98 85 0.312 5.57 66 2.07

44
CONDENSED MATTER PHYSICS AND THEORY

To conclude, I have examined in greater

depth the physical ramifications of the high-T^.

modifications to the canonical two-band theory

of superconductivity, which I introduced earlier

[1]. The model has been applied to reported

tunneling-gap anisotropies. This has led to identifying the observed broad i.r. absorptions

at 0.3 eV to 0.4 eV in Bi2Sr2CaCu208 and in Y-123 as candidates for the exchanged IBs. This

identification is consonant with our earlier

assumptions, which seem to imply a two-stage

pairing mechanism. Experimental tests have

been proposed and the results submitted for

publication [8].

References

[1] R. C. Casella, Nuovo Cimento DIO, 1439 (1988).

[2] R. C. Casella, Solid State Comm. 70, 75 (1989); Appl. Phys. Lett. 55. 908(1989).

[3] R. C. Casella, NIST Tech. Note 1285, C. O'Connor, ed. p. 44 (1990).

[4] L. Forro et al., Phys. Rev. Lett. 65, 1941 (1990).

[5[ K. Kamaras et al., Phys. Rev. Lett. 64, 84 (1990).

[6] T. Timusk et al., Phys. Rev. B 38, 6683 (1988).

[7] Z. Schlesinger et al., Phys. Rev. B A\, 11237

(1990).

[8] R. C. Casella, Bui. Am. Phys. Soc. 36, 770 (1991)

and Solid State Comm. 78 No. 5, 377-379 (1991).

STRUCTURAL STUDY OF a-FeCr NEAR THE a-PHASE TRANSITION

J. L. Robertson and D. A. Neumann

Reinhardt (Lawrence Livermore National Laboratory, Livermore, CA)

and

S. C. Moss (University of Houston, Houston, TX)

A single crystal of the binary alloy

a-Fe 53Cr 4Y was quenched to room temperature

after annealing at 1 108 K (5 K above the o-phase

transition for four days. The a -phase, a body

centered cubic solid solution, which exists over

a wide temperature and concentration range may

exhibit percursory structural fluctuations in the

local atomic arrangements (i.e. short range

order, local atomic arrangements) near the

o-phase equilibrium boundary.

X-ray synchrotron measurements of the

diffuse scattering were made at beamline X-14

Oak Ridge National Laboratory (ORNL) at the

National Synchrotron Light Source at Brookhaven National Laboratory. These data

indicate a clustering tendency [1] (Fe-Fe and

Cr-Cr nearest neighbors are more probable than

Fe-Cr neighbors). Inverse Monte Carlo and

Krivoglaz-Clapp-Moss determinations of pair

interaction energies are self consistent and

compare favorably with recent theoretical

calculations [2]. The average individual Fe-Fe

and Cr-Cr distances were also determined and

reveal a contraction of both the Fe-Fe and the

Cr-Cr nearest neighbor distances. This result disagrees with a conventional treatment of

atomic size effects in solid solutions and must be

understood using species dependent solute-lattice

couplings.

Elastic and inelastic neutron scattering

measurements are currently under way to determine the more subtle structural features

such as the static displacements which cannot be

readily obtained from the X-ray measurements.

A detailed analysis requires accurate values for the atomic force constants and the elastic constants. These were obtained by measuring

the phonon dispersion curves (fig. 1) and fitting

them to a 5 nearest neighbor Born-Von Karman

model [3] and are listed in table 1. These

measurements also provided an opportunity to search for soft modes indicating precursory

o-phase fluctuations (thus far none have been

observed that cannot be attributed to other know

effects in bcc crystals). Elastic diffuse neutron

and Huang scattering measurements are planned

in the near future.

45
CONDENSED MATTER PHYSICS AND THEORY

Table 1. Atomic force constants in dynes/cm.

0X0 oa 0£0 0.75 100 r Reduced Wave Vector HP(K»/27r)

Figure 1. Phonon dispersion cures fit to a 5 nearest neighbor Bom-Von Karman model.

Ill xx: 13403 xy: 13879

200 xx:20105 yy: -376 220 xx: 1442 zz: -744 xy: 634

311 xx: -488 yy: -24 yz: -86 222 xx: 971 xy: 1169

xy:-183

References

[1] L. Reinhardt, J. L. Robertson, S. C. Moss, G. E.

Ice, and C. J. Sparks (to be publisiied).

[2] P. E. A. Turchi, M. Sluiter and G. M. Stocks, Proc.

Fall MRS (1990); and private communication.

[3] Computer algorithm for Born-Von Karman fit provided by M. Mostoller (ORNL).

COLLECTIVE EXCITATIONS OF LIQUID HELIUM

IN RESTRICTED GEOMETRIES

P. E. Sokol and J. Byrne (The Pennsylvania State University, University Park, PA)

and

S. K. Satija

The effects of restricted geometries on the

properties of liquid behave generated considerable experimental and theoretical interest

recently. Of particular interest [1] has been the

effect of confinement and random surface

potentials on the properties of the superfluid

phase. Measurements [2] have been carried out

in porous glasses such as vycor, aerogel, and

xerogel and a variety of effects have been

observed. In vycor, which has a small and

relatively uniform pore size distribution, the

superfluid transition is substantially suppressed,

but the critical exponent for the superfluid

density is not measurably changed. Alternately,

in aerogel, which has a much more open

structure and a wide range of characteristic pore

sizes, little suppression of the superfluid

transition is observed. However, the critical exponent is substantially different from the bulk

liquid case, which indicates that this systems

may belong to a different universality class. Previous studies have concentrated primarily on the macroscopic properties of the confined

liquid such as the heat capacity and superfluid

fraction [2]. However, there has been little work on the effects of the confining geometry on

the microscopic properties of the liquid.

Therefore, we have undertaken a study of the

collective excitations of superfluid helium

confined in aerogel glass using inelastic neutron

scattering.

The confining medium used for these studies was a highly porous aerogel glass. The glass

had a porosity of ~ 95% and consists of highly

interconnected pores with sizes ranging from 10s

to 1000s of angstroms. Aerogel glass was used

so that a large amount of helium could be placed

in the pores allowing a good signal to noise ratio to be obtained for the confined liquid. Previous

measurements have shown that the superfluid

transition temperature is only slighfly depressed

by confinement in the aerogel but that the critical exponents for the superfluid transition are

significanfly modified.

The measurements were carried out using

the BT-2 and BT-4 spectrometers with an

incident energy of 5 meV. Data were collected on both bulk liquid helium and helium in aerogel

for momentum transfers between 0.4 and

2.3 A'\ covering the phonon, maxon, and roton regimes. The collective excitations of the bulk

liquid at low temperatures are very sharp and

46
CONDENSED MATTER PHYSICS AND THEORY

provide a measure of the instrumental resolution.

The temperature dependence of the helium in aerogel was also measured for temperatures

between 1.4 and 2.1 K.

The collective excitation spectrum in bulk

superfluid helium exhibits a sharp response over

a broad range of momentum transfer, Q, that is unlike that observed in conventional liquids [3]. The collective excitation spectrum can be divided

into three different regions. At low Q, the

collective excitations are phonons and the energy

is very nearly linearly, proportion to Q. At

intermediate Q the collective excitation spectrum

shows a peak known as the maxon. At larger

(2's the dispersion relation shows the characteristic dip associated with the roton. The energy of the collective excitations was

measured in both the bulk and confined superfluids for several (2's at 1.5 K. Within the

statistical accuracy of the measurements there was no difference in the energies of the

collective excitations. However, the widths of

the excitations are substantially different. Figure

1 shows the widths of the collective excitation

for bulk He and He in aerogel. In bulk He, the

excitation is very sharp and provides a measure

of the instrumental resolution. The widths for

He in aerogel, as seen in die figure, are

systematically larger than for the bulk.

The finite width of the excitations of the

confined liquid can be qualitatively understood.

In the bulk liquid the width is due to excitation-excitation scattering and, at low

temperatures, is very small. However, for the

confined liquid an additional method for

broadening the excitations exists-scattering from

the walls of the aerogel. This simple picture

also provides, again qualitatively, the correct Q

dependence for the broadening. The phonons,

which are long wavelength excitations, are more

sensitive to the walls and are broadened much

more than the rotons, which are thought to be

highly localized excitations.

The temperature dependence of the

broadening is also of interest since this provides

information on the scattering of excitations.

Figure 2 shows the temperature dependence for

the roton (Q= 1.9 A'^ ) for the helium in the

aerogel and for the bulk liquid. In the bulk, the

increase in width with temperature is due to the

thermal excitation of the collective excitations.

This mechanism will also be present in the

confined liquid. At ~ 2 K the widths for the

confined and bulk superfluids become

comparable indicating that thermal broadening is the dominant feature at this temperature.

0.20 -

Q(A-^)

Figure 1. The width of the collective excitation peak as a function of momentum transfer at a temperature of 1.5 K. The stars are the results for the bulk liquid. At this temperature the true width of the excitations is vanishingly small and these results give the instrumental broadening for the spectrometer. The open circles are the results for liquid

helium in aerogel glass. The peaks are significantly broader

than the instrumental resolution due to the effects of the aerogel on the liquid.

U.6U .

'

' '/

2 0.16

II /:

^ 0.12 A '

% 0.08

e 0.04

/

0.00 —

1

1 1 1—1_ 1.4 1.6 1.6 T (K)

2.0

Figure 2. The open circles show the temperature variation of the width of the roton peak (Q =1.9 A" instrumental broadening. The solid line is the temperature variation in the bulk hquid.

References

[1] A. B. Harris, J. Phys. C 7, 1671 (1974); P. B. Weichman and M. E. Fisher, Phys. Rev. B 34, 7652

(1986).

[2] M.H.W. Chan, K. I. Blum, S. Q. Murphy, G.K.S. Wong, J. D. Reppy, Phys. Rev. Lett. 61, 1950

(1988) and references therein.

[3] W. G. Stirling and H. R. Clyde, Phys. Rev. B 4i,

4224 (1990) and references therein.

47
CONDENSED MATTER PHYSICS AND THEORY

TOWARD NON-GAUSSIAN RANDOM MODELS OF SURFACE ROUGHNESS FOR

NEUTRON AND X-RAY REFLECTIVITY

N. F. Berk

Most theoretical treatments of surface

roughness in neutron and x-ray reflectivity

studies assume that the surface height function

z(r2), where T2 is the transverse position vector,

is a Gaussian random function [1]. This is mathematically convenient but possibly

restrictive in the scope of physical models of

roughness it actually encompasses. We are

interested in extending the analysis of surface

sensitive scattering to non-Gaussian roughness

models and have begun by devising a particular

class of models that is amenable to some degree

of exact treatment. We note to begin with that in the Born

approximation, valid not too close to the critical angle, the scattered (reflected) intensity depends on the surface height function according to

/ (Q) = 4 J

^^P ^^'^2 • ^2)

q:

(1)

<exp {z(2, [z (r2) -z (O2)]} > d\

where A is a constant. We are considering a random terrace model for z{t2) in the form

zir^) - ^ E Cp(r,)], (2)

where the C; in (2) represent independent

samples of a random step function, C(r2) = +V2.

Thus the sum in (2) takes on the M = 2J 4- 1 values J, J - 1, ..., -J, for J = Vz, 1,—. Also,

<z(r2)> =0and<z(r2)> = h^. In the limit

J -* 00 the model for z(r2) tends to a Gaussian

random function, for then at each r2 it is an

infinite sum of random variables to which the

central limit theorem applies. In this case, as

for any Gaussian random variable with zero mean,

< exp {iQ^ [z (r^) - z (O^)]] > =

2. . - (3)

where, in the given model,

<[z(^2) - zmf> = 2h\\ - YCr,)], (4)

with

Y(r2) =4<C(r2)C(02)>, (5)

and yCOi) ~ ^- ^^^ finite J, however, the

model z(r2) is not a Gaussian random function,

and the cumulant expansion implicit in (3) does

not truncate. Nevertheless, we have derived an

exact expression for the right hand side of (3). Indeed, if we define

F/r2) = <exp{/^^5: [C/A-2)-C/02)]} >

V-^ i'=i (6)

then, under the conditions of the model,

F/r2)={l-sin2((2/2/v/27")[l-C(r2)]}^. (7)

Using a standard limit, we have almost at once from (7) that

lim F//-2) = exp{-(2/[l - Y(/-2)]}, (8)

which retrieves the correct result for a Gaussian

random height function, as seen from (3) and

(4).

From (1) the scattered intensity is A

Q

I{Q) = -^ J

exp(/(22 • r2)Fj{r2)d\.

(9)

exp{-i G^ <[z(r2) -z(02)]^>}

The specular contribution to the intensity is defined by the limit of Fj(r2) as 12 -* 0°. Thus

from (7), assuming Y(r2) "* in the limit, we

have

48
CONDENSED MATTER PHYSICS AND THEORY

Qz (10)

Thus, using the formalism already in place for the 3-d leveled wave method [2], we get

y(r^) = farcsin[T(r2)], (14)

Compare this to the Gaussian model, for which where

^spec(Q)

q:

(11)

Thus for finite, not too large, values of J, the

specular scattering from the random terrace surface oscillates strongly with Q^. Moreover,

the non-specular scattering will also vary

strongly with Q^ and have generally complicated

line shapes as a function of the transverse wavevector Q2.

Up to this point we have not had to specify

a particular model for the random step function,

C(r2). One possibly useful class of such

functions is a 2-d analogue of the leveled wave

construction developed originally for random 3-d

structures [2,3]. Thus let

Cir^) - e[S(r)] - ^,

(12)

where 6(x) is the Heaviside function and

Sir) = -1 Y: co,ik^n' ^2-*«)- (^^^

^fN n=l

The indexed quantities in (13) are independent random variables. The phase

constants 4) are uniformly distributed in (0,2k)

for homogeneity, while the directions of k-, are

distributed according to the degree of isotropy

required of the terraces. For example, totally

isotropic terraces correspond to a uniform

distribution of k2-directions. The magnitudes of

k2 are distributed with probability density P(k2).

(^2) = Jj ^(^)^(^)^^- (^5)

this scheme, then, analysis of non-specular

scattering provides information about the model

distribution function, P(k2). Figure 1 shows an example of a leveled wave terrace for J = 1, corresponding to M = 3 distinct surface levels. We are currently developing the leveled wave

random terrace model for application to reflectivity data from real surfaces. Extensions to scattering from multilayers are also planned.

IfSl -T 54 #"^3^ ^> * 'V *4 " '

Figure 1. Computer simulation of leveled wave, isotropic

random terrace for J = 1 . The three levels are indicated by white (level 1), grey (level 0) and black (level -1).

References

[1] S. K. Sinha, E. B. Sirota, S. Garoff and H. B. Stanley, Phys. Rev. B 38, 2297 (1988).

[2] N. F. Berk, Phys. Rev. Lett. 58, 2718 (1987).

[3] N. F. Berk, elsewhere in this report (1991).

49
CONDENSED MATTER PHYSICS AND THEORY

THEORY OF FRACTAL BEHAVIOR IN THE LEVELED WAVE MODEL

OF RANDOM MORPHOLOGIES

N. F. Berk

The leveled wave model of random bicontinuous morphologies has been extended to

fractal structures. Beyond the mathematical

interest of the technique, it may be relevant to the observed deviations from Porod's law in

small angle scattering experiments on various

porous glass systems which usually are attributed

to fractal roughness. A model that adds fractal roughness to the surfaces of a bicontinuous

structure in a geometrically consistent way has

been lacking up to now.

In the leveled wave scheme of random

morphologies [1], continuous interfaces are mathematically modeled by level sets—2-d

contours—of a stochastic standing wave, S(r),

which is defined by

1 ^

S(r) = E cos(/:„ • r+4)„) (1)

where the subscripted quantities on the right are taken to be independent random variables. A

corresponding density function, p(r), can be

defined pointwise by p(r) = 1 or p(r) = 0, say, according to whether S(r) > 6 or S(r) < B,

respectively. One way of writing this is

p{r) = (Sir) - P), (2)

where 0(x) is the Heaviside function: 0(x) = 1 for X ^ 0, and 0, otherwise. The phase

constants, 4)^, are chosen from a uniform

distribution over (0, 2k). This makes S(r)

spatially homogeneous; no special value of r, including the origin, is singled out. For an

isotropic morphology, the directions of the

random wavevectors, k^^, are uniformly

distributed over solid angle 4:t. All other

properties of the construction are then

determined by the distribution, P(k), of the

magnitudes of kj^ which is normalized by

^P{k)dk = 1 (3)

Scattering from the morphology is obtained from

00 I(Q) = 4nVc(l - c)jr^y(ry^(Qr)dr, (4)

where c = (1 - erf(P))/2 and yW is the

Porod-Debye correlation function.

c(l -c)Y(r) =c^ - <p(r)p(0)> (5)

The mathematical properties of this model

have been developed in detail [1, 2]. In

particular, the surface-to-volume ratio of the

interface is proportional to the second moment of

P(k), indicating that sufficiendy long-tailed

k-distributions generate very rough surfaces,

including those which are fractal [2]. To

develop this idea analytically we considered that P(k) was contained in the convex family of

functions

P{k) - [1 - ii(e)]P^,(k) + \i(e)Pi,(k), (6)

where n(e) e [0,1]. We treat Pst(k) as a

short-tailed function possessing all moments, and

Pjf(k) as a long-tailed function which behaves

asymptotically (k - «>) the same as

Pu(k) =

IIqG

(1 - kl^)

(2e+l)-

where l^ is a fixed but arbitrary length and

e = 3 - D, 2 < D < 3,

(7)

(8)

is the codimension of the fractal dimension, D.

The long tail of Pit(k), weighted by the mixing

parameter \i{e), provides the mechanism for

fractal behavior. We require that both P^t^k) and

Pjf(k) are normalized so that P(k) is normalized

for all functions in this set. The asymptotic

behavior of P(A:) ultimately fixes the asymptotic

law of the scattering. Specifically, we have

found that for Qij, > > 1,

50
CONDENSED MATTER PHYSICS AND THEORY

/(0 - A(e)Q

-^-3

<2- (9)

corresponding the Bale and Schmidt "6 - D" law

for scattering from fractal surfaces, where, for p = 0, the proportionality is given by

A(e) 2v^(e + 1)

7C

^

2evi{e)i:^^ tan(^)

(10)

The behavior of the square root in (10) at the

Euclidean limits, D - 2 and D - 3, requires

discussion. First, the tangent diverges as e -* 1, which can be interpreted as expressing the

implicit discontinuity in the moment expansion

of long-tailed functions. The second moment,

< k > , does not exist for functions acting as

Pl[(k) when e < 1 , but it is finite if e > 1 is allowed. Thus, for the scattering to be finite at D = 2, we require ji(€) -* at least as fast as 1

- e as G -» 1 and that \i(e) = for e >. 1 . Then

^(1) = -^\l\\i\l)\ , (11)

where \i ' is the derivative of \i

.

The nature of fractal scattering for D - 3

(e - 0) is also subtle and in other contexts has

been subject to interesting debate. The basic

issues are: according to (10), the (leading)

asymptotic scattering from a "surface" fractal vanishes indentically as D - 3, rather than

approaching the finite Q"^ dependence expected

for scattering from a "volume" fractal; and if the

leading Q"-^ scattering does indeed vanish, than

what is the mathematical nature of the

asymptotic scattering? The leveled wave method

follows a scenario in which the structure, while

space-filling in the limit, nonetheless retains its topological character as an interface. The

asymptotic scattering vanishes because the

contrast between the two sides of the interface

effectively disappears in the limit. For the

second point it has been argued that the leading

order nonvanishing scattering in the D = 3 limit

should be Q"^ scattering associated with the

"ordinary" length scale of the porous structure.

To deal with this question in the leveled wave

model, it is necessary to extend the expansion in

(10). We have found that

KQ) <

s/2l

3e

(Qlo)'

N

(12)

(QioY

for QIq ^ 00 and e - 0. Thus at D = 3 the right

hand side of (12) vanishes as 0(6), and the

scattering from the leveled wave morphology

falls faster than algebraically (i.e. , exponentially)

as Q - 00. Near D = 3 the large-Q scattering

falls algebraically but not as a well-isolated

power law.

In the leveled wave model, therefore, there

is no evidence in the asymptotic scattering of

nonfractal length scales, even though such scales are implicit in the structure from Pst(k) and will

contribute structure to the scattering at small Q.

Leveled wave "fractal" structure, seen as a planar slice through the 3-d image, is shown in figure 1. For this computer simulation, the

short-tailed component of the k-distribution has

the form, Pst(k) = 6(k - k^), where k^

determines the overall scale, and the long-tailed

component is given by (7) with D = 2.5.

Figure 1. Computer simulation of leveled wave fractal pores for D = 2.5 and 50% porosity.

References

[1] N. F. Berk, Phys. Rev. Lett. 58, 2718 (1987).

[2] N. F. Berk, Phys. Rev. A 58, in press (1991).

51
CONDENSED MATTER PHYSICS AND THEORY

THEORY OF COHERENT NEUTRON SCATTERING FROM n ITINERANT

SCATTERERS EMBEDDED IN CRYSTAL LATTICES

R. C. Casella

Some years ago, in an attempt to understand the rather large widths reported by Rush et

al. [1] and Magerl et al. [2] for the peaks

associated with the inelastic scattering of

neutrons from dilute hydrogen atoms in the bcc

metals Nb, Ta, and V, I was led to consider the

possibility that the neutrons scatter from

hydrogen atoms, which (in the excited-oscillator

final state) move in protonic energy bands [3]. These bands are analogous to, but generally

much narrower than, the electronic energy bands

which typically occur in solids. Their essential

feature is that the phase of the proton wave

function remains coherent as the (shielded)

proton (or modified hydrogen atom) traverses

several interstial sites, whence its wave function

(in the excited-oscillator state) is Bloch-like

(although possibly modified by incoherent

lifetime effects [4]). Such (limited) phase

coherence plays a role in this theory, even when

the neutron scatters from a single itinerant

proton. In contrast, when a neutron scatters from more than one hydrogen (be they quasilocalized or itinerant) the average over initial spin states of the hydrogens leads to well-known

spin incoherence. That is, the cross section for

a neutron to scatter from n hydrogens is essentially n time the average cross section for it to scatter from only one, even for the case when

the neutron scatters elastically from hydrogens,

each of which, for a given spin, is in a spacially

(quasi) coherent Bloch-like state. What happens

when the spin incoherence is not a factor as would be the case in the elastic scattering of a neutron from n itinerant deuterium atoms, were

the overlap between the deuterium oscillator

wave functions sufficient to produce Bloch-like

phase coherence in the individual deuterium wave functions even in the ground state? It has

recently been claimed in the literature that a

factor rp- enters in the cross section for processes

such as this. I believe this result is incorrect,

except for the very special case where the n

scatterers are confined within a volume of linear

dimensions each < 0(X) where k is the

wavelength of the incoming neutron. My belief

is based, in part, on my earlier work on the

distinct but somewhat analogous situation

involving the possibility of coherent scattering of

reactor-generated neutrinos from N quasilocalized atomic nuclei in crystal materials. I showed then that, as in the well-known case of

Bragg scattering of neutrons from crystals, the

cross section for coherent neutrino scattering is proportional to N rather than N^ [5,6], contrary

to the N" behavior claimed in the literature at the time. I plan to demonstrate analyfically that, in the present case, the neutron cross section is proportional to n, not n^.

References

[1] J. J. Rush, A. Magerl, and J. M. Rowe, Bull Am.

Phys. Soc. 26, 337 (1981).

[2] A. Magerl, J. J. Rush, J. M. Rowe, D. Richter, and H. Wipf, Phys. Rev. B 27, 927 (1983).

[3] R. C. Casella, Phys. Rev. B 24, 2913 (1981).

[4] R. C. Casella, Phys. Rev. B 27, 5943 (1983).

[5] R. C. Casella, in Intersections between Particle and

Nuclear Physics, Lake Louise, Canada 1986, ed. D.

F. Geesman (Am. Inst. Physics. Conf. Proc. 150. New York, 1986) p. 1040.

[6] R. C. Casella, Nuovo Cimento A 94, 42 (1986).

52
B. CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

The work described in these areas generally

relates to materials whose primary interest or

functionality is chemical. The diversity of

systems investigated by neutron scattering and

crystallographic techniques continues to grow.

Neutron scattering group scientists, in cooperation with U. Penn, Perm State and

Exxon, have carried out the first elastic and

inelastic scattering studies of buckminster

fullerene, which have revealed critical information on the dynamics, orientational

disorder and rotator phase transition in this fascinating new molecular architecture. Our

longstanding research efforts in layered

materials, catalysts and hydrogen in metals have

during the past year yielded interesting new

results, for example, on hydrogen potentials and

dynamics in rare earth and refractory metalhydrogen systems and on the dynamics and

structure of boron nitride and of new molecular

complexes in intercalated graphite. Important

progress has been made in the extension of

maximum entropy methods and related fast Fourier transform codes for macromolecular

phase determination. Studies have also been

extended to systematic investigations of the

dynamics of glasses and aerogels, to the

structure of Langmuir Blodgett multilayers and

to the structure and flow of colloidal suspensions

under shear.

OBSERVATION OF C-AXIS MODE-SPLITTING IN c-LuHq ^9

T. J. Udovic and J. J. Rush

I. S. Anderson (Paul Scherrer Institut, Villigen, Switzerland)

J. N. Daou and P. Vajda (University Paris-Sud, Orsay, France)

and

O. Blaschko (Institut fiir Experimentalphysik der Universitat Wien, Vienna, Austria)

In recent years, we have investigated, by

incoherent inelastic neutron scattering (IINS), the

structure and dynamics of hydrogen in two rare- earth hep metals, Y and Sc, that form a-phase

solid solutions. As mentioned in this report on

p. 56, although hydrogen occupies tetrahedral

interstices (i.e., t sites) in these metal host

lattices, IINS investigations have indicated that an anharmonic, anisotropic soft mode exists for

vibrations polarized along the c-axis, 25-30%

lower in energy than the doubly-degenerate

basal-plane modes. Moreover, this c-axis mode

possesses a spectral lineshape that is much

broader than the instrumental resolution, and for

high H concentrations and low temperatures, is distinctly split into two components. This

splitting is caused by hydrogen pairs dynamically

coupled across metal atoms along the c-axis.

To make a more thorough comparison of

hydrogen dynamics in rare earth metals, an

investigation was undertaken of hydrogen in Lu,

namely, a-LuHg 19, which lies very close to the

a-LuH^/li-LuH2 phase boundary. Previous

work on a-LuDg ^9 [1] has suggested a very

unusual, large (~8 meV) splitting for the

c-axis-polarized D vibration. In the present

work, spectra were measured with the BT-4

spectrometer using both the Be-filter (lowresolution) and Be-graphite-Be filter (highresolution) analyzers. Low-resolution H

vibrational spectra establish the c-axis and

basal-plane mode energies at 102.5 and 144

meV, respectively, and a ratio of 1.78 for the

second- to first-excited-state energies of the

c-axis vibration, again verifying the anisotropy

and anharmonicity of the rare-earth t-site potential along the c-axis. Figure 1 shows

high-resolution IINS spectra of the c-axis

vibration as a function of temperature. In agreement with the other rare-earth studies, the

c-axis peak possesses a broad multicomponent

lineshape indicating hydrogen-pairing-induced

splitting at low temperature. As the temperature

is increased, this splitting disappears, suggestive

of increased H disorder within the metal lattice. Figure 2 compares the low-temperature c-axis

53
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

spectra for the three investigated H/rare-earth aphases in the order of decreasing lattice constant

c from Y (0.573 nm) to Lu (0.555 nm) to Sc

(0.527 nm). Proceeding from Y to Sc, the overall lineshape becomes broader and die splitting

becomes less distinct, consistent with decreasing

H-pair order as the lattice constant c decreases.

Further isotope and concentration-dependent

studies are under way for this system.

100 110 Energy Loss (meV)

120

Figure 1. High-resolution IINS spectra of the c-axis mode

for o-LuHq ^g as a function of temperature. Solid lines are drawn to guide the eye. The instrumental resolution (fwhm) is illustrated by the horizontal bar beneath the

spectra.

100 no

Energy Loss (meV)

120

Figure 2. A comparison of high-resolution IINS spectra below 10 K of the H c-axis mode for Y, Sc, and Lu

high-H-concentration a-phases. Solid lines are drawn to guide the eye. The instrumental resolution (fwhm) is illustrated by the horizontal bar beneath the

spectra.

Reference

[1] O. Blaschko, G. Krexner, L. Pintschovius, P. Vajda,

J. N. Daou, Phys. Rev. B 38, 9612 (1988).

QUANTUM HOPPEVG OF PROTONS IN a-ScH^ AND a-YH,

N. F. Berk, J. J. Rush, T. J. Udovic

and

I. S. Anderson (Institut Laue-Langevin, Grenoble, France)

We are continuing our studies of local hydrogen dynamics in rare earth metals using

quasielastic neutron scattering. Previously [1] we had measured a striking minimum in the

proton hopping rate near T = 100 K in ScHq ^5, the first such observation in a pure metalhydrogen system. In addition, the hopping rate

at lower temperatures, which approaches 10^^ s"^ near T = 10 K, is faster than any previously seen in such systems. We have extended these

measurements to several concentrations of

hydrogen in scandium and have also measured

quasielastic scattering from low concentrations of

hydrogen in yttrium. The quasielastic line

widths for these measurements are collected in figure 1 . The previous data for ScHq ^5 are also

shown. A minimum quasielastic linewidth and

"hopping" rate near T = 100 K is clearly

evident at both the lower and higher concentrations of hydrogen in scandium. Figure 1 also

54
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

presents linewidth data for YHqqi, which also

shows a definite minimum near T = 150 K and

at somewhat slower local hopping rate than

observed for the ScH^^^ alloys. The combined

evidence in figure 1 indicates the phenomenon

of the hopping rate minimum in generic in these

systems.

•

•

>

ISO -

*

•

1

• ScH.... ScH.j,

10 ^

10 IOC 110 T (»)

lO" »

Figure 1. Quasielastic neutron scattering linewidth T vs T. Top: aSU^ forx = 0.05, 0.15, and 0.25, Bottom: a-YH^^

forx = 0.01.

As described elsewhere [1,2], we have

attributed the anomolous temperature dependence

of the hopping rate to coupling of the interstitial protons to the metal conduction electrons, as originally developed theoretically by Kondo and

others. These non-classical effects on the

motion are amplified in the rare-earth metals for

those "labile" protons able to hop rapidly back

and forth between nearest neighbor tetrahedral

sites along the c-axis of the hep metal lattice. At

low temperatures, in our picture, most of the

protons, in fact are rendered "non-labile" on the

time scale of the measurement because of

coherent strain interactions that produce strong

short range order. The population of the labile

species that exhibits the anomolous hopping thus

decreases rapidly as temperature decreases. In our model the labile and non-labile populations

are energetically operated by an effective binding

energy. This energy can be inferred from ±'j

temperature dependence of the elastic component

of the quasielastic measurement, known as the

EISF or elastic incoherent structure factor. In figure 2 we show model fits to the measured

EISF from YH^ for x = 0.01 and x = 0.1. These fits determine short ranged ordering

energies of approximately 50 meV, which is consistent with independent estimation of

"pairing" energies in the systems from analysis

of thermodynamic measurements.

1.0

COw 0.5-

0.0

1 1 1 1 1

^=0.10

1

- a:=0.01 X

- YH,

^>-^

""

1 1 1 1 1

1 100 200 300

T(°K)

Figure 2. Elastic incoherent structure factor EISF vs T for a-YH with x = 0.01 and x = 0.1. The solid line is a fit to a model in which labile and non-labile proton populations

are separated in energy by — 50 meV.

References

[1] I. S. Anderson, N. F. Berk, J. J. Rush, T. J. Udovic,

R. G. Bames, A. Magerl, and D. Richter, Phys. Rev.

Lett. 65, 1439 (1990).

[2] N. F. Berk, J. J. Rush, T. J. Udovic, and I. S. Anderson, J. of Less Common Metals, in press.

55
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

VIBRATIONAL MODES AND LINESHAPES IN o-ScHq 34 and a-ScD^ 34

T. J. Udovic, J. J. Rush, and N. F. Berk

and

I. S. Anderson (Paul Scherrer Institut, Villigen, Switzerland)

In a-ScHj^ (^max ~0-35) solid solutions,

hydrogen occupies the tetrahedral interstices

(i.e., t sites) of the Sc metal lattice. Previous

studies of a-ScH^^ at H concentrations x ^ 0.25

[1] have indicated that the H vibration along the

c-axis direction (at ca. 103 meV) is over 30%

lower in energy than the doubly-degenerate

basal-plane modes (at ca. 148 meV) and

possesses considerable anharmonicity, as reflected by the ratio (1.76) of the second- to first-excited-state energies. High-resolution

neutron spectra for the c-axis vibrations revealed

a peak at all hydrogen concentrations that was much broader (ca. 14 meV full width half

maximum intrinsic linewidth, x = 0.25) than the

instrumental resolution, yet devoid of any

resolved fine structure indicative of a distinct

local-mode splitting due to the dynamic coupling

of hydrogens paired across Sc atoms along the

c-axis as was observed previously for a-YH^^ [2]. Although the broad overall widths of the a-ScH,^

c-axis peaks at these H concentrations were

consistent with the presence of H pairing across Sc atoms along the c-axis at low temperatures,

the lack of a distinct splitting suggested that the

ordering of the hydrogen pairs themselves along

the c-axis was less extensive than in a-YH,^.

Recently, the H(D) vibrational spectra of

a-ScHg 34 and a-ScDg 34 have been measured

using incoherent inelastic neutron scattering

(IINS) methods in order to further characterize

the bonding potential of H(D) in the t sites very

close to the a-ScH(D)^/15-ScH(D)2 phase

boundary. Spectra were measured with the BT-4

spectrometer using both the Be-filter (lowresolution) and Be-graphite-Be-filter (highresolution) analyzers. Low-resolution H(D)

vibrational spectra locate the c-axis mode

energies at 103.5(78.5) meV and the basal -plane

mode energies at 147.5(106.8) meV. Figure 1 illustrates the high-resolution c-axis vibrational

spectrum for a-ScHg 34 compared with that for

a-ScHg 25 and a-YHgig- In effect, increasing

the H concentration has led to a more fully

developed ordering of hydrogen pairs along the

c-axis as indicated by the growth of scattering

intensity on the higher-energy side of the c-axis

peak, leading to the emergence of a faint local-mode splitting of ca. 4-5 meV, similar to that found for a-YHg ^g. Figure 2 illustrates the

high- resolution c-axis vibrational spectrum for

oc-ScDq 34. The shape of this spectrum suggests

the presence of a broad multicomponent feature

and is consistent with the results for a-ScHg 34.

"1 I I I

1 [ r

** c-axis mode

; V;.

*^ • • •

(8K)

a-YH,

••.., (8K)

90 100 110 Energy Loss (meV)

120

Figure 1. High-resolution IINS spectra of the c-axis mode

below 10 K for c-ScHq 34 (closed circles), o-ScHq 25

(open circles) [1] and a-YHn 10 (closed squares) [2]. The

instrumental resolution (fwhm) is illustrated by the horizontal bar beneath the spectra.

56
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

—

1

1

1

c—axis mode

70 75 80 65 Energy Loss (meV)

Figure 2. High-resolution IINS spectrum of the c-axis mode at 8 K for c-ScDq 34. The instrumental resolution (fwhm) is illustrated by the horizontal bar beneath the spectrum.

Referraices

[1] T. J. Udovic, J. J. Rush, I. S. Anderson, R. G.

Barnes, Phys. Rev. B 41, 3460 (1990).

[2] I. S. Anderson, N. F. Berk, J. J. Rush, T. J. Udovic, Phys. Rev. B 37, 4358 (1988).

OCTAHEDRAL SITE OCCUPATION IN BCC NbV ALLOYS

T. J. Udovic and J. J. Rush

and

R. Hempelmann and D. Richter

(Institut fiir Festkorperforschung, Forschungszentrum Jiilich, Germany)

Nbjoo-x^x^y '^ ^ random bcc alloy system

with a hydrogen sublattice disordered with

respect to site energies. Our previous incoherent

inelastic neutron scattering (IINS) investigation

[1] ascertained both octahedral and tetrahedral

site occupation by hydrogen for x = 5, 10, and

50. On the basis of spectroscopic and statistical arguments, the low temperature vibrational peaks

in the Nb-rich Nb-V-H alloys at low H

concentration (y = 1) were assigned to Nb4V2-

octahedral and NbVj-tetrahedral sites. To more thoroughly investigate the regime

of low H concentration and the effects of

deuterium substitution, further IINS spectra were measured for NbgsVjHy (y = 0.5,0.8),

^^95^5027, and Nb95V5 with the BT-4

spectrometer using both the Be-filter (lowresolution) and Be-graphite-Be-filter (highresolution) analyzers. Figure 1 shows the

hydrogen vibrational spectra for Nb95V5H at 4 K, indicating that the vibrational feature at 38 meV becomes more predominant as the H

concentration is decreased. This feature is assigned to the doubly-degenerate hydrogen

vibrations in the Nb4 plane of the Nb4V2

octahedral interstitial site depicted in figure 2. To our knowledge, this is the only known case of octahedral site occupation by hydrogen in a bcc metal (alloy) lattice. Moreover, it is the

lowest observed vibrational energy for hydrogen

in any metal hydride. Additional IINS measure- ments of Nb95V5D2 7 locate the analogous

deuterium vibrational energy at 29 meV, yielding an H to D vibrational energy ratio of 1.31. The higher-energy peaks in the IINS spectra of

figure 1 are due to tetrahedral-site occupation as well as an overtone of the 38 meV fundamental

vibration. Measurements are under way to determine the energy of the nondegenerate

vibration along the V-V axis for H and D in this Nb4V2 octahedral site. For monoclinic B-V2H,

the analogous octahedral H vibration occurs at 223 meV, much higher than the perpendicular

vibrations in the 50-60 meV range [1].

57
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

1 1 1 1 1 -T"

NbggVgHy (4K)

^\

1 1 1 1 1 1 50 100 150 Energy Loss (meV)

Figure 1. IINS difference spectra of Nbg^V^H - Nb^^V^

(y = 0.5, 0.8) illustrating the dominant vibrational peak at 38 meV assigned to the doubly-degenerate hydrogen

vibrations in the Nb^ plane of the Nb4V2 site in figure 2.

Figure 2. Hydrogen-occupied Nb4V2 octahedral interstitial

site in a random Nb-V bcc alloy (V atoms are shaded.)

Reference

[1] R. Hempelmann, D. Richter, J. J. Rush, J. M.

Rowe, J. Less-Common Metals 172-174 . 281

(1991).

NEUTRON DIFFRACTION STUDIES OF SOLID Cgg

J. R. D. Copley, D. A. Neumann, and E. Prince

R. L. Cappelletti (Ohio University, Athens, OH)

W.A. Kamitakahara (University of Maryland and Reactor Radiation Division)

N. Coustel, J. P. McCauley Jr., N. C. Maliszewskyj, J. E. Fischer, and A. B. Smith III (University of Pennsylvania, Philadelphia, PA)

and

K. M. Creegan and D. M. Cox (Exxon Research and Engineering Company, Annandale, NJ)

We have made neutron diffraction measure- ments of the structure of C^q below and above

the order-disorder transition temperature T^, =

249 K [1]. The two samples used in these

experiments were prepared by standard

techniques, taking precautions to minimize the

amount of hydrogenous material in the final product. Their combined weight was approximately 2.4 g. The measurements were

performed at the BT-1 spectrometer using

1.545 A neutrons, with the sample in a vanadium container. The measured diffraction

patterns, parts of which are shown (on the same

intensity scale) in figures 1 and 2, are similar to previously published x-ray measurements [1],

but because the neutron form factor is independent of scattering angle the high angle

scattering pattern is more clearly apparent.

The most remarkable feature in the room

temperature diffraction pattern is the diffuse

scattering, which peaks at wave vector transfers

Q of -3.4 and -5.8 A'^ with a broad shoulder at -4.8 A'^ This scattering, which is also visible

in the x-ray measurements [1], is disorder

scattering associated with randomly oriented

molecules located on a crystal lattice. The

complete scattering may be written (per

molecule) [2] as

Sdis(Q)= <F^(Q)> + <F(Q)>-[Sc(Q)-l],

58
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

where F(Q) is the molecular form factor, S^CQ)

represents scattering associated with molecular

centres, and <...> denotes an average over

molecular orientations. Note that as Q - <», < f2(Q) > - 60, Sc(Q) - 1 , and < F(Q) > ^ - 0. The calculated profile in figure 1 is simply

SjjgCQ) for a face-centred cubic lattice with aQ =

14.17 A [1] plus a flat background, convoluted

into the instrumental resolution and scaled; intra- pentagon and inter-pentagon nearest C-C bond

distances were fixed at 1.45 and 1.40 A,

respectively. The agreement with experiment,

including the shoulder, is remarkably good.

This model satisfactorily describes the large Q

pulsed neutron diffraction results of Li et al. [3]. The 14 K pattern (fig. 2) indexes on a simple cubic lattice [1]. Following

Sachidanandam and Harris [4] we assume that the space group of the ordered structure is Pa3 . In Rietveld refinements we have found that the

"background" under the 14 K diffraction pattern

has the same general shape as the diffuse

scattering observed at room temperature. We

have therefore modelled the scattering as a sum

of three terms: (i) Sqj.j(Q), which is the

diffraction pattern for ordered C^g molecules

located as described in ref. [4], with F (cf. ref

[1]) equal to 24°, slq = 14.04 A [1], and the

temperature factor B = 0.3 A^, (ii) S^-^^iQ)

(using the same C-C distances as at 295 K), and

(iii) a constant background. These three terms,

appropriately scaled and broadened, together

give a very good account of the observed

scattering, as seen in figure 2. Our calculations

suggest that about 30% of the structure is orientationally disordered at 14 K (cf. refs. [1]

and [4]). In order to ascertain the nature of the

orientational disorder we have also made

quasielastic scattering measurements, described on page 62 in this report.

Figure 1. Measured (points) and calculated (line) scattering patterns for C^q at 295 K. Several peaks are labelled with

the quantity (h^+k^ + f^). Peaks at -2.7 and -3.1 A"^

are due to aluminum in the beam. The lower line shows

[<f2> - <F>2].

Figure 2. Measured (points) and calculated (line) scattering patterns for C^q at 14 K. Several peaks are labelled with

the quantity _(h^+k^+f^). The lower line shows

[<f2> <F>\

References

[1] P. A. Heiney, J. E. Fischer, A. R. McGhie, W. J. Romanow, A. M. Denenstein, J. P. McCauley, Jr., A. B. Smith, III, and D. E. Cox, Phys. Rev. Lett. 66 2911 (1991).

[2] A. Guinier, X-Ray Diffraction (W. H. Freeman, San

Francisco, 1963), p. 74.

[3] F. Li, D. Ramage, J. S. Lannin, and J. Conceicao,

Phys. Rev. B (in press).

[4] R. Sachidanandam and A. B. Harris, Phys. Rev.

Lett. 67, 1467 (1991).

59
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

VIBRATIONAL SPECTROSCOPY OF C^^ AND GRAPHITIC CARBONS

W. A. Kamitakahara (University of Maryland, College Park, MD and Reactor Radiation Division)

R. L. Cappelletti (Ohio University, Athens, OH)

J. R. D. Copley

and

J. S. Lannin and Fang Li (Penn State University, University Park, PA)

Methods [1] for making bulk quantities of

a remarkable new form of elemental carbon, C^q

or buckminsterfuUerene, have recently been

developed, stimulating intensive research on the

properties of the C50 solid. The structure of the

Cgo molecule is a truncated icosahedron, in

which the C-C bonds form a pattern exactly like

the seams on a soccer ball made up of twenty

hexagonal and twelve pentagonal panels.

Considering only molecular symmetry, all C

atoms are equivalent and are sited on the surface

of a perfect sphere. There are many other kinds

of interesting carbon solids. Among the latter are so-called glassy carbon, actually a nanocrystalline material, carbon aerogels, which can have densities as low as 0.04 g/cn?, and various

crystalline and amorphous carbons made by thin

film deposition. We are in the process of

making a systematic study of the vibrational

densities of states of a wide variety of carbon

solids. So far, we have completed neutron

measurements of vibrational spectra of pure

CgQ-fullerite [2], glassy carbon, and graphite.

"Glassy" carbon is made by high-temperature

decomposition of organic polymers to form a hard, brittle, disordered phase. Our sample has

a diffraction pattern resembling graphite but

consisting of much broader peaks consistent with

crystalline order over a few nanometers. The

present results indicate a number of important

similarities as well as differences in the

vibrational spectra.

Our sample of Cgg was prepared at Penn

State University, while our glassy carbon sample was manufactured by Sloan, Inc. Most of the

data at 14 K were obtained on a triple-axis

spectrometer at NIST, using Be or Be/graphite

filter analyzers. Low energy data for glassy

carbon and for graphite powder were obtained at room temperature on a time-of-flight spectrometer at the Institut Laue-Langevin, Grenoble.

The time-of-flight data for glassy carbon were

corrected for multiphonon scattering and

converted to a vibrational density of states (VDOS). Although no such conversions or

corrections, apart from subtraction of background and container scattering, were applied to the Be-filter data, the latter are expected to give

directly the general shape of the VDOS. The

two data sets on glassy carbon were scaled to match the peaks between 40 and 80 meV.

Our results are shown in figure 1. Open

and closed circles represent data taken with the

time-of-flight and Be-filter methods respectively.

The spectrum for glassy carbon, shown in figure

1(a), has four major features above 30 meV, at 57, 80, 103, and 177 meV. We also made Befilter and time-of-flight measurements on pure

graphite powder. The result was nearly identical

to figure 1(a) for glassy carbon, except that the

shoulder in the latter result, which starts at about

18 meV, becomes a sharp cusp at 16 meV,

corresponding to the zone boundary interlayer

longitudinal mode of graphite. Thus the glassy

carbon spectrum is essentially that of graphite,

with only a rounding of a low energy Van Hove

singularity to mark the presence of disorder.

Very broad diffraction peaks, with widths of

about 0.3 A'^ (3 nm"^), were obtained for the

glassy carbon sample, indicating that crystalline

order only extends over a few nm. Nonetheless,

it is more reasonable to regard glassy carbon as a highly disordered crystalline material than as amorphous, since the hexagonal layered structure of graphite is preserved out to several

coordination shells. The CgQ results are shown in figure 1(b).

Of the 46 distinct frequencies [3] for the 174

internal modes of each molecule, only 4 are

infrared-active and 10 are Raman-active; 8 of the

10 Raman modes have been unambiguously

60
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

oo

(«l

1.

! 1 /

1 1 1 1/w\/\

1 ^^ 1 'l '

1 (c) ' ' ' -chII |ti|tll|l||. - illllODH'

APS...ll|l|!f i)ll .

1 1 1

1 1

illtlnili Ui

II ntli tHl

\ w m \\\

1 1 40 80 120 160 200

Energy (meV)

Figure 1. (a) Vibrational density of states (VDOS) for glassy carbon, and (b) intramolecular vibrational spectrum

for CgQ. Lines through points are guides to the eye. Upward (downward) pointing arrows represent observed [4] infrared- (Raman-) active modes, (c) Calculated vibrational frequencies for C^q, according to refs. 5, 6, and 7. In both (b) and (c), the length of a line or arrow is proportional to the degeneracy of the mode.

identified to date [4]. Neutron spectroscopy

provides sensitivity to all modes, and the

spectrum in ref. 2, most of which is shown here.

is thought to be a complete representation of the

intramolecular vibrational modes. There have

been several quantum mechanical model calculations of the vibrational modes of the C^g

molecule. Results of three calculations [5-7] are shown in figure 1(c). When folded with experimental resolution, the calculations yield spectra

qualitatively similar to experiment, but with

significant discrepancies. For example, the

theories predict a deep minimum near 180-195

meV, whereas none is observed.

The major features of the spectrum in figure 1(b) occur at 53, 69, 95, and 170 meV.

The spectrum of C^q thus looks very much like that of a glassy carbon (or graphite) but is redshifted an average of 8%. The resemblance no doubt arises from the similar three-coordination

of atoms in these materials.

References

[1] W. Kratschmer, L. D. Lamb, K. Fostiropoulos, and D. R. Huffman, Nature (London) 342. 354 (1990).

[2] R. L. Cappelletti, J. R. D. Copley, W. A. Kamitakahara, Fang Li, J. S. Lannin, and D. Ramage, Phys. Rev. Lett. 66, 3261 (1991).

[3] D. E. Weeks and W. G. Harter, J. Chem. Phys. 90

(1989) 4744.

[4] D. S. Bethune, G. Meijer, W. C. Tang, H. J. Rosen, W. G. Golden, H. Seki, C. A. Brown, and M. C. deVries, Chem. Phys. Lett. 179, 181 (1991).

[5] C. Z. Wang, C. H. Xu, C. T. Chan, and K. M. Ho,

private communication.

[6] R. E. Stanton and M. D. Newton, J. Phys. Chem.

92, 2141 (1988).

[7] G. B. Adams, J. B. Page, O. F. Sankey, K. Sinha,

J. Menendez, and D. R. Huffman, private

communication.

61
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

QUASIELASTIC NEUTRON SCATTERING STUDY OF THE ROTATIONS

OF C^Q IN THE SOLID PHASE

D. A. Neumann and J. R. D. Copley

R. L. Cappelletti (Ohio University, Athens, OH)

W. A. Kamitakahara (University of Maryland and Reactor Radiation Division)

K. M. Creegan and D. M. Cox (Exxon Research and Engineering Co., Annandale, NJ)

and

N. Coustel, J. P. McCauley, Jr., N. C. Maliszewskyj, J. E. Fischer, and A. B. Smith III (University of Pennsylvania, Philadelphia, PA)

The existence of an orientational orderdisorder transition in the solid phase of C^ has

been clearly demonstrated by the neutron

diffraction results of Copley et al. [1] (see pg.

58 in this report) and by the x-ray results of

Heiney et al. [2]. In order to ascertain whether

the orientational disorder above T^. is static or dynamic, quasielastic scattering measurements

have been made using the BT-2 and BT-9 triple

axis spectrometers at the NBSR with an energy

of 14.8 meV and an elastic energy resolution of

0.48 meV. The rather high incident energy was

necessary to reach the first peak in the diffuse

scattering. The background was measured by

offsetting the analyzer by 5' and the average

value was subtracted. The data obtained with

the fixed final energy configuration were

corrected for second order contamination in the

monitor, while those collected with the fixed

initial energy configuration were corrected for

the resolution volume of the secondary

spectrometer.

Spectra obtained below and above T^. =

249 K [2] are shown in figure 1 . Closed circles are the experimental data whereas the solid line

is a fit to a 6 -function plus a Lorentzian,

convoluted with a Gaussian resolution function.

In the ordered state, the scattering is purely

elastic within the resolution of the spectrometer,

but above T^ there is clearly a Lorentzian

component shown as the dashed line in figure 1

The unmistakable conclusion is that the C^q

molecules are rapidly rotating in the high

temperature phase. The width of the Lorentzian

component indicates that the rotational time scale

is 1-10 ps; the exact value depends on details of

the rotational motion. This result is consistent

with values of 12 ps inferred from NMR

measurements at 300 K [3] and 5 ps from

molecular dynamics simulations [4]. Note that the intensity scales are not directly comparable.

Furthermore, no blank has yet been measured so

that it is not clear whether the 6 -function

component is due to scattering from the can,

intrinsic to the C^g molecules, or a combination.

These uncertainties will be resolved in future

experiments.

Q=3.6A" T=240K

2 4 Energy (meV)

Figure 1. Quasielastic neutron scattering spectra above and

below the orientational order-disorder transition.

The points in figure 2 show the integrated

intensity of the Lorentzian component as a function of Q at a temperature well above T^.. The apparent agreement with the calculated

diffuse scattering function, <F^> - <F>^,

further confirms that the high temperature

diffuse scattering is dynamic in origin. Here F

is the molecular form factor and the angle

brackets indicate an average over all orientations. On the other hand, the nature of

the diffuse scattering at low temperatures is not

62
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

as obvious, and experiments to examine this question are in progress.

Figure 2. Integrated intensity of the Lorentzian component

as a function of Q (solid points), and the quantity

[<F^> - <F>2] (solid line).

Referraices

[1] J. R. D. Copley, D. A. Neumann, R. L. Cappelletti,

W. A. Kamitakahara, E. Prince, N. Coustel, J. P. McCauley, Jr., N. C. Maliszewskyj , J. E. Fischer, K. M. Creegan, and D. M. Cox, Physica B (in press).

[2] P. A. Heiney, J. E. Fischer, A. R. McGhie, W. J. Romanow, A. M. Denenstein, J. P. McCauley, Jr., A. B. Smith, III, and D. E. Cox, Phys. Rev. Lett. 66, 2911 (1991).

[3] R. Tycko, G. Dabbagh, R. M. Fleming, R. C. Haddon, A. V. Makhija, and S. M. Zahurak, Phys. Rev. Lett. 67, 1886 (1991).

[4] A. Cheng and M. L. Klein, J. Phys. Chem. (in press).

NEUTRON SCATTERING STUDIES OF Cs-AMMONIA INTERCALATED

GRAPHITE

D. A. Neumann

S. F. Trevino (ARDEC, Picatinney Arsenal, NJ and Reactor Radiation Division)

and

Ph. Depondt (Universite Pierre et Marie Curie, Paris, France)

Neutron scattering is an ideal tool for the

study of the dynamics of alkali-ammonia graphite

intercalation compounds. Previous work [1] on

the K-ND3 system has revealed a hybridization

of the [OOL] longitudinal acoustic phonons with

a mode of the "liquid" intercalate layer at an

energy of about 7 meV. To date three different

models for this intercalate mode have been

proposed. The first model is that the excitation

is a libration of the ammonia molecule about its C3 symmetry axis which lies in the basal plane

[2]. This model seems doubtful in light of

results which show that the activation energy in the low temperature solid intercalate phase in both K-ammonia [3] and Cs-ammonia [4] intercalated graphite is about 6 meV. Thus at room temperature (about 25 meV), one would

expect the ammonia molecule to be "freely"

rotating and would therefore not hybridize with

the phonons in the manner observed. In the

second model, the mode is a libration of the

ammonia in the potential created by the screened

charge-dipole interaction responsible for the

formation of the K-ammonia complexes observed

by Solin and co-workers [2] . In the third model

the K ion vibrates along the c-axis due to the

relatively small size of the K ion compared to the hydrogen plane of the ammonia molecule [5]

Thus the second and third possibilities involve

the alkali ion either directly or indirectly so that changing the alkali ion should alter the mode

which couples to the phonons. We have

therefore undertaken a neutron scattering study

of CsC24(ND3)3 7 devised as a test of these

models.

Figure 1 shows in-plane elastic scans of the

liquid intercalate for KC24(ND3)4 3 (solid line)

and CsC24(ND3)3 7 (closed circles). Note that the peak at about 20 nm'^ is narrower for the Cs

compound than for the K one. This indicates

that the ammonia-ammonia correlations, which

dominate the scattering, are somewhat longer

range in Cs-ammonia intercalated graphite than

for its K counterpart. In addition, the peak at about 8.5 nm'^ in the K-compound is replaced

for the Cs case by substantial small angle

scattering. This shows that while some sort of

clusters have formed in the Cs compound, they

63
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

are different than the fourfold complexes found

in the K analog. Thus the potential due to the

screened charge-dipole interaction responsible

for the complex formation has, to at least some

extent, been modified.

4500

3000-

branch is much larger in CsC24(ND3)3 7 than in

KC24(ND3)4.3.

a

oo

1500

Figure 1. In-plane elastic scan of the liquid intercalate

structure. The closed circles are the data obtained for CsC24(ND3)3 -7 while the solid line is the same scan for KC24(ND3)4 3 taken from reference [2].

Figure 2 shows the [OOL] longitudinal

acoustic phonon branch for both the K-ammonia

(open circles) and Cs-ammonia intercalated

graphite (closed circles). Here the solid (Cs)

and dashed lines (K) represent fits of the

dispersion to a 1-d Born-von Karmen model

coupled to an Einstein oscillator of energy a. The fit parameters reflect three features readily

observed in the data. First, the observation that the CsC24(ND3)3 7 is somewhat stiffer than

KC24(ND3)4 3 is exhibited in the values of the

force constant (f) which were found to be 2.31

N/m for the Cs compound compared to 2.02

N/m for the K one. The reason for this change

in the force constant may be the reduced charge

back-transfer from the graphite pi orbitals to the

intercalate layer of the Cs-ammonia system. The

second fit parameter shows that a is only slightly

larger (7.1 meV compared to 6.8 mev) for the

Cs-ammonia system. This indicates that the

mode is not simply a motion of the alkali ion

and suggests that the alkali-ammonia interaction

has not been appreciably altered even though the

intercalate structure seems to be quite different or alternatively that this interaction is not the

origin of the librational potential of the

molecule. Finally, the splitting of the acoustic

10.0

=S

CI

Figure 2. [OOL] longitudinal acoustic phonon dispersion obtained for CsC24(ND3)3 j

(solid circles) and for KC24(ND3)4 3 (open circles) taken from reference [1]. The solid and dashed hues represent fits to the data.

In summary, the [OOL] longitudinal

acoustic phonons in CsC24(ND3)3 -7 and a hybridization of these phonons with a librational

mode of the ammonia have been observed.

Through comparison of these results with those

in the analogous K compound, we have been

able to show that this mode does not involve an

excitation of the alkali ion. In addition, we take

the large change in the liquid diffraction pattern

of the intercalate layer as an indication that the

alkali-ammonia interaction is not the principal source of the librational potential. Thus this mode is almost certainly a libration in a potential

created by either graphite-ammonia or ammonia-ammonia interactions or both.

References

[1] D. A. Neumann, H. Zabel, Y. Fan, S. A. Solin, and J. J. Rush, Phys. Rev. B37. 8424 (1988).

[2] Y. Fan, S. A. Solin, D. A. Neumann, H. Zabel,

and J. J. Rush, Phys. Rev. B36, 3386 (1987).

[3] D. A. Neumann, H. Zabel, J. J. Rush, Y. Fan, and S. A. Solin, J. Phys. C20, L761 (1987).

[4] J. White, I. Jamie, and C. Carlile, in ISIS 1990. ed by C. Wilson (Swinden Press Ltd, 1990) 70.

[5] H. A. Resing, R. M. Fronko, T. Tsang, and S. A. SoUn, Synthetic Metals 23, 297 (1988).

64
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

LATTICE DYNAMICS OF HEXAGONAL BORON NITRIDE

W. A. Kamitakahara (University of Maryland, College Park, MD and Reactor Radiation Division)

D. A. Neumann

and

A. W. Moore (Union Carbide Corporation, Parma, OH)

Graphite is the prototypical layered solid,

consisting of planar hexagonal sheets of threecoordinated carbon atoms linked with very

strong covalent bonds, and much weaker van der

Waals interactions between layers. A close

analogue to graphite is hexagonal boron nitride

(h-BN), formed from the neighboring elements

to carbon in the periodic table. In contrast to graphite, which is a semimetal, h-BN is an

insulator with a 5 eV bandgap. It is an

extensively used ceramic in high-temperature

applications. The structure of h-BN comprises

planar layers in which each atom type, B or N,

is three-coordinated with atoms of the other

type. The interlayer structure forms strings of

alternating atoms BNBNBN.. spaced 3.35 A

along the c-axis. This differs to some extent from graphite, in which the stacking places half

the atoms in such strings, with the others facing

voids at hexagon centers instead of atoms in adjacent layers. The chief microscopic

difference between graphite and h-BN is the

partially ionic character of the latter. Single crystals of h-BN of the size required

for neutron inelastic scattering are unavailable.

However, a c-axis-aligned form, referred to as highly-oriented pyrolytic boron nitride, or HOPBN, in analogy to the HOPG form of

graphite, has recently been successfully prepared

at the Parma, Ohio laboratories of Union

Carbide. The best quality samples of HOPBN

have c-axis mosaic spreads of ~ 2°. Using a composite sample comprising three such pieces

of HOPBN, we have been able to measure phonon dispersion curves for (i) [001]L rigidlayer modes with wave vectors parallel to the caxis, and (ii) layer-bending modes propagating

parallel to the layers.

We find (fig. 1) that the [001]L modes have

significantly lower frequencies than the analogous modes of graphite. This is somewhat surprising, since the ionic character and interlayer

stacking would both have led one to expect

0.4 q (27r/c)

Figure 1. [OOIJL phonon dispersion of hexagonal boron

nitride (points and solid line), and of graphite (dashed line, from ref. 1).

increased interlayer interactions. The dispersion

for layer-bending modes [2] (fig. 2) is characteristic of a layered compound, with a pronounced

upward curvature to the acoustic branch.

Higher-resolution measurements on this branch

are needed to extract elastic constant C44, which

characterizes the resistance of the layers to a

rigid-layer transverse shearing motion. Also

planned are measurements of out-of-plane vibrational modes in which nearest-neighbor B and N

atoms within the same layer are displaced in opposite directions along the c-axis. Although it

is important to include such modes to obtain a

realistic picture of the lattice dynamics of

HOPG, for that material they are difficult to measure, since their structure factor goes to zero

for Q parallel to c . The different scattering

lengths of B and N should give them an

appreciable structure factor for the case of

HOPBN. Thus we will be able to characterize

both intralayer and interlayer interactions in a straightforward manner.

65
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

20-

15- >01

fe 10 c

1 1 1 1

i

-

\

1

-

1 -

1 1

1 1

References

[1] R. M. Nicklow, N. Wakabayashi, and H.G. Smith,

Phys. Rev. 5, 4951 (1972).

[2] H. Zabel and W. A. Kamitakahara, Phys. Rev. B

26, 5919 (1982).

5-

0.0 0.2 0.4

Figure 2. Phonon dispersion for layer bending modes of HOPBN.

LOW-ENERGY NETWORK DYNAMICS OF SILICA AEROGELS

B. J. Olivier, D. W. Schaefer, and C. S. Ashley (Sandia National Laboratories, Albuquerque, NM)

D. Richter (KFA Jiilich, Institut fur Festkorperforschung, Jiilich, Germany)

B. Frick and B. Farago (Institut Laue-Langevin, Grenoble, France)

and

W. A. Kamitakahara (University of Maryland, College Park, MD and Reactor Radiation Division)

Silica aerogels are random-network, lowdensity microstructures whose dynamics are currently not well understood. These materials

are chemically similar to amorphous silica (a-Si02) but their novel microstructure gives rise

to very different low energy dynamics.

Specifically, the density of network excitations

below 1 meV can be as much as two orders of

magnitude larger than in a-Si02.

In the present study we investigate a series

of porous silica aerogels having qualitatively

different microstructures and connectivities in order to characterize their relative low-energy

dynamic responses. Neutron spin-echo (NSE)

and time-of-flight (TOF) spectrometers were

used to measure an effective density of states, g(E), covering four decades in energy transfer. E. TOF diffraction measurements were also

carried out to characterize the atomic arrangements of this same aerogel series for wave

vectors up to 40 A'

Two primary types of microstructures exist

in our aerogel series. One is a polymeric

(polymer-like) network, while the other is a

colloidal (particle-like) network [1]. The

difference, as determined using small-angle

neutron scattering (SANS) techniques, is shown

in figure 1 . The polymeric aerogel has a Porod

slope greater than - 3 thus indicating that the

corresponding net-work is self-similar (fractal) on a length scale between ~ 10 to 100 A. In

contrast, the colloidal microstructure shows a Porod slope of - 4, indicative of a nonfractal,

smooth particle network.

The low energy dynamics, as measured by

NSE, of these contrasting microstructures show

markedly different behavior. For example, the

colloidal aerogel has a peak in g(E) for network

66
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

excitations in the 10 - 20 fieV range (see fig. 2).

Polymeric aerogels show no such peak, but only

a continuum of network excitations which can be

interpreted in terms of the fracton model [2]. The polymeric aerogel is power-law from 10 - 100 neV with a slope of ~ 0.5.

>-t

COz

ILI

r

: POLYMERIC

-2.5

r BBBB slope

^

^COLLOIDAL ^O^C^

0.01

1 ^0.1 Q(A-^)

Rgure 1. SANS curves for the polymeric vs. colloidal aerogel microstructure. The slope greater than - 3 indicates the polymeric aerogel has a self-similar network.

S

kma

lu

POLYMERIC

L^L_L. I

COLLOIDAL

J I I I 1 1

10

E (ueV)

100

Figure 2. Effective density of states, g(E), for the polymeric vs. colloidal microstructure as calculated in the NSE (IN 11) energy window. Note the g(E) are not actual data but rather fits to the measured intermediate scattering function S(Q,t) (see ref. [1]).

The intermediate energy dynamics, as measured by TOF, of the colloidal vs. polymeric

microstructure also show power-law behaviors

that are markedly different from one another. In figure 3 are the TOF energy transfer spectra of

the two contrasting microstructures. The

polymeric aerogel shows slightly less than a decade of power-law behavior in g(E), which

may also be interpreted within the fracton

model. However, the slope here,

0.1 < E < .75meV, is ~ 0.75. The origin of

the change in slope from 0.5 to 0.75 is not understood. The polymeric aerogel shows

evidence of an additional crossover at E ~ 0.75 meV. The origin of this crossover lies in changes in the solution precursor chemistry.

The colloidal aerogel shows a more extensive

power-law regime (slightly more than a decade).

The linear behavior in g(E) for this material,

however, cannot be interpreted within the fracton

model since the microstructure, from SANS,

shows no evidence of a fractal network.

The SANS data were obtained at the Oak

Ridge and Los Alamos National Laboratories.

All inelastic measurements were made at the

Institut Laue-Langevin in Grenoble, France,

using a combination of the IN6 (TOF) and IN 1

(NSE) spectrometers.

c

TO, g"3

POLYMERIC

10.0 E (meV)

Figure 3. Effective density of states, g(E), for the polymeric vs. colloidal microstructure as seen in the TOF

energy window. Data were taken on IN6 at 300 K.

References

[1] D. W. Schaefer, C. J. Brinker, D. Richter, B. Farago, and B. Frick, Phys. Rev. Lett. 64, 2316

(1990).

[2] S. Alexander and R. Orbach, J. Phys. (Paris) Lett. 43, L625 (1982).

67
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

STRUCTURE AND DYNAMICS OF POTASSIUM SILICATE GLASS

B. J. Olivier, D. W. Schaefer (Sandia National Laboratories, Albuquerque, NM)

J.R.D. Copley

W. A. Kamitakahara (University of Maryland, College Park, MD and Reactor Radiation Division)

K. D. Keefer (University of Cincinnati, Cincinnati, OH)

S. Martin (Iowa State University, Ames, lA)

Alkali silicate glasses, produced by the

addition of Na20, Li20, and K2O to Si02, have

structures and dynamical properties substantially

different from the parent Si02 glass. For

example, the addition of the above oxides

changes the viscosity, softening temperature, and

thermal expansion coefficient of the a-Si02

network [1]. The effect of these modifiers is to introduce excess nonbridging oxygen atoms, i.e., oxygens which do not link adjacent Si02

tetrahedra. The K"*" cations, moreover, are not

covalently bonded to the network and can, under

certain conditions, migrate around these nonbridging oxygen atoms. The relationship

between the structure of the disrupted networks

and the vibrational dynamics is not fully

understood.

We have begun a systematic study on a

series of silica based glasses (potassium silicates) in order to expand current understanding of the

dynamics of these important amorphous materials. We have chosen the series (Si02)i.x(K20)

for several reasons. First, the concentration of

K2O cations can be varied over a wide range,

0.1 < X < 0.55, so that trends can be followed

into a regime where large changes from the

a-Si02 structure are present. Some of our early

results from the BT-4 spectrometer (fig. 1), indicate significant changes in the dynamic

response for the energy transfer range,

60 < E < 160 meV. For example, the entire

spectrum of the 50% K2O appears down shifted

by ~ 20 meV from undoped a-Si02.

We believe that the substitution of the

potassium atoms is random, in contrast to earlier

findings for lithium and sodium silicates [2]. We have taken diffraction data, some of which

is shown in figure 2, in order to explore this question of random vs. nonrandom substitution.

The diffraction data were also taken on the BT-4

spectrometer and they reveal a radical modification of the unadulterated a-Si02 structure. Unfortunately, the short range in Q prohibits us

c

3

(0

uT

a-SIO,

(SIO2) (K2O)

^ 0.7 ^ 0.3

(SIO2), JK2O)

OS^ ^ 'OS

».*Ia..i...i...

60 80 100 120 140 160 E (meV)

Figure 1. The effective density of states, G(E), for amorphous silica and the given potassium silicates. The

data were taken at 50 K on the BT-4, triple axis spectrometer, in the beryllium fdter mode.

from accurately Fourier transforming the data

into the corresponding radial distribution

function. We have recently obtained data on a pulsed source (HIPD at Los Alamos), hence a much broader range in Q, so that the question of

correlations after substitution can be more

accurately explored.

If the substitution of the potassium atoms

into the silica network is random, we expect the

disruption of the network by the potassium to lead to dynamics somewhat different than the

dynamics of correlated substitutions. Existing

models are currently being applied to our

inelastic data. More data, however, is desirable

at higher energy transfers, E > 150 meV.

68
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

c€(9

3

a

' 1 "." 1 " ' 1 " " 1

1 " ' 1 •

i- . •j

i- • • - •

-:

• •. . •--..•• . "

f

• • ".

•• a-SiO,

2

*•• ' r«% ": •

j L

J" •• j

: • •

: .' • ; T :

i- « • J

L •^ :

: •* (SfO,>„<K,0)„ ~

'^ 1

r

•

:

'- r- • • -

: : r • '»* ''^' -j

: % :

- y

'

^•* J 12 3 4 5 6 7 Q(A-^)

Figure 2. The elastic structure factor, S(Q, E = 0), for a-SiO^ and the potassium silicates shown in figure 1. The data were taken at 300 K on the BT-4 triple axis spectrometer.

Referraices

[1] N. E. Cusack, The Physics of Structurally

Disordered Matter (Adam Hilger, Philadelphia, 1987).

[2] M. Misawa, D. L. Price, K. Suzuki, J. Non-Cryst.

Solids 37, 85 (1980).

STRUCTURE OF NITROGEN IN POROUS VYCOR GLASS

P. E. Sokol and J. Byrne (The Pennsylvania State University, University Park, PA)

and

S. K. Satija

The properties of liquids and solids

confined in small pores has generated significant

interest recently [1]. A number of interesting

effects have been observed which are common to many systems. Among the most interesting [2]

is the observation of supercooling, a depression

of the temperature of phase transitions below

their bulk value, and hysteresis of the phase

transitions between cooling and warming. These

effects have been observed for both liquid-solid

and solid-solid phase transitions. The degree of

supercooling is inversely related to the pore size

and has been interpreted using a geometrical

supercooling model . The collective properties of

confined liquids and solids are also significantly

different from those of the bulk system.

Despite the great amount of experimental

work on these systems there is little direct

information on the microscopic structure of the

liquid or solid in the pores, such as obtained

from X-ray or neutron scattering measurements.

In the liquid, the local environment and bonding

of a particle is directly reflected in the short

range structure. In the solid, the spatial

arrangement of the atoms or molecules

composing the solid can be obtained. Thus

structural studies address such basic questions as whether the solid phase in the porous media is crystalline or amorphous and whether the

structure is the same as the bulk structure. We have studied the structure and phase

transitions of nitrogen adsorbed in porous vycor

glass. Vycor glass is formed by spinodal

decomposition of a boro-silicate glass at high

temperature followed by leaching out of the

boron rich phase. The porosity of the resulting

glass is ~ 0.31 and the network of pores is highly interconnected. In addition, the pores

have a fairly narrow size distribution with a nominal pore diameter of 70 A. Nitrogen

69
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

provides an interesting system for study since it has both a liquid-solid and solid-solid phase

transitions. In the bulk, nitrogen solidifies into a hep structure at 63 K and undergoes a first order phase transition to a fee structure with

rotational ordering at 35 K.

The measurements were carried out on the

BT-6 spectrometer with a fixed incident energy

of 14.8 meV. Data was collected for momentum

transfers between 0.5 and 4.3 A'^ at increments

of 0.01 A"^ The vycor sample, produced by

Corning Glass Co., was cleaned by slowly

heating the glass to 600 C under vacuum and

occasionally flushing with oxygen. The sample was transferred to an aluminum sample cell in a glove box to prevent contamination. The pores were filled with nitrogen at 70 K, in the liquid

phase, to - 90% of their capacity. Slightly

underfilling the pores prevented the formation of

bulk regions in the sample.

Figure 1 shows the scattering from nitrogen

in vycor, with the background from the Al cell and the vycor glass subtracted, at three different

temperatures. The scattering at 70 K, which

corresponds to the bulk liquid phase, exhibits the

characteristic scattering for a liquid.

Comparisons to the bulk liquid structure are currently under way.

N2 IN VYCOR

W09i

' ' '

I 1 ' ' ' ' 1I ' ' i — at 10 K

b — at 50 K •

120IB \ ' — at 70 K -

IBBBl i

-

8e«i \\. 1

, lal'

6900 A V

A tb)-

4BII

: Jf\^--~^

^^

2«SI / V^ {c\

1 _„___-/

^~-—

Q( INVERSE ANGSTROM)

Figure 1. The scattering from nitrogen in vycor at temperatures of 10, 50, and 70 K. These correspond to the bulk fee, hep, and liquid phases. The background from the aluminum cell and the vycor glass has been subtracted. The zero of the scattering has been shifted to show the

scattering more clearly. A sharp transition, marked by the

appearance of diffraction peaks, occurs at 59 K,

well below the bulk liquid-solid transition

temperature. This supercooling of the liquid

solid transition has been observed for many other

systems [2]. The diffraction peaks are a clear

and unambiguous indication that long range

positional correlations are present and that

solidification has occurred. The location of the

diffraction peaks agree well with those for the

bulk beta phase of nitrogen, indicating that this phase has the same symmetry as the bulk solid. The intensities of the peaks are different,

possibly due to different orientational

correlations of the nitrogen in the vycor.

The hep to fee transition in the bulk, which

occurs at 35 K, is not observed for the nitrogen

in vycor down to temperatures of 4.5 K. The

location of the peaks is always consistent with a hep, and not fee, structure. Thus, the solid-solid

transition is suppressed in the presence of Vycor

glass. We do, however, observe a fairly rapid

change in the relative intensities of the

diffraction peaks below 35 K. We are currently

exploring the nature of these changes. One

possibility is that orientational ordering occurs, but without the lattice change that is observed in

the bulk systems. Whether these changes

represent a phase transition or simply a continuous change in the orientational order is not yet clear. The observed diffraction peaks are

significantly broader than the instrumental resolution. This broadening can be due to several

factors such as the finite crystallite size, strains,

defects, etc. The contributions of these various

effects depends on the detailed nature of the

crystallites, the strains, etc. However, a rough

estimate gives an average size [3] on the order

of 400 A, much larger than the average pore

diameter. This implies that crystalline regions

must extend a substantial length down the pores

or even between pores.

References

[1] Molecular Dynamics in Restricted Geometries . J. Klafter and J. M. Drake, eds., John Wiley and Sons,

New York, 1989.

[2] D. D. Awschalom and J. Wamock, Phys. Rev. B

35, 6779 (1987). and references therein.

[3] The Powder Method in X-ray Crystallography . L. V.

Azaroff and J. Berger, McGraw Hill, NY 1958 pp

254-256.

70
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

SELF-DIFFUSION IN A MICROPOROUS MEDIUM

M. Y. Lin, B. Abeles, J. S. Huang, H. Stasiewski, and Q. Zhang

(Exxon Research and Engineering Company, Annandale, NJ)

There has been a great deal of progress in recent years in the understanding of flow of

liquids in porous media and its relation to the

microstructures of the materials, due to the

importance of the subject in both scientific understanding and technological applications.

The permeability for viscous flow is given by

K = b^cP'lx where Z? is a dimensionless factor

whose value depends on the shape of the crosssection, 4) is the porosity, a is an average pore

radius and t is the tortuosity. While cj) and a

can be determined from measurements of the

isotherms, the tortuosity t must be determined

from transport measurements. The most direct

way of determining x is from the relationship t = D/D where D and D are the diffusion

coefficients of the liquid in the bulk state and in the porous medium respectively. In this work, we use small angle neutron

scattering (SANS) to determine the self-diffusion

coefficient D of hexane and decane in Vycor.

Vycor is a model microporous system with a porosity (j) = 0.3 and an average pore radius of a = 30 A. In the range of the momentum

transfer Q that is accessible with SANS, the

microstructure of Vycor is shown in figure 1 to have the characteristic peak-like structure factor

whose amplitude is proportional to the contrast between the solid material (Si02) and the material in the pores (either air, vacuum, or fluid)

squared, I~(p^-pJ^ where p^, and p are the

neutron scattering length densities of the solid

and of the material in the pores respectively.

The Vycor sample was first imbibed with a contrast matching (i.e., p = p^ mixture of

C^y^ and 050^4. The normal hexane {C^^^^

and the deuterated hexane {C^-^^ have the same diffusion coefficient but very different

neutron scattering length densities, which makes

the contrast matching possible with the liquids of

the same diffusion coefficient. The self-diffusion

coefficient D was then determined by observing

the rise in the SANS intensity / as a function of

time, as the matching fluid in the sample was

exchanged through diffusion of C5HJ4 whose

Pp*Ps-> from a reservoir in contact with the

sample. In figure 1, we plot the SANS intensity

I(Q) as a function of the momentum transfer Q

at three different times. The amplitude of the

1{Q) as a function of time t is then fitted with a

theoretical function calculated for the particular

geometry used in the experiment, with the diffusion coefficient D as the fitting parameter. The

fitting is shown in figure 2 for the case of

another paraffin ^i^^l (decane). The values of

Dp determined at 23°C were 9.9 x 10'^ and 3.2

X 10"^ cm"/sec for C^Hj^ and C^gHo-., respectively. The tortuosity thus determined is x =

4.0 + 0.1 for both hexane and decane, independent of the carbon number. We believe that indicates that the size of the alkane molecules is

still small enough that the surface-molecule

interaction is not significant in these cases.

1(0)

H E+5

I E*5

S E't

E E*t

r-'"'

1

1 -^~.,^ ; 1 1 , , , 1 _

1

1 Ett

S Et3 L ^

>-. """--. "

1 Et3

5 E»a

2 E.2

1 1 1 1 , , , , 1 '~

1 E a e E-E 5 E-E 1 E-1 Q(A') Figure 1. The characteristic structure factor of Vycor is shown as the neutron scattering intensity l(Q) as a function of the momentum transfer Q. The three curves were taken

at three different times during the diffusion of the non- matching liquid into the Vycor sample. The lowest curve was taken at t = 0, and as more nonmatching liquid diffuses in, the intensity increases with the shape

unchanged.

71
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

1 1 1 > 1 . > 1 1 1 1 1 1 1 - ^^'^^\

H. — /

""^

\

3. — X

-

1.

- y

r

-

e.e

^ ^r^< , \ , . , , 1 , , 10. 20.

Figure 2. A fit (solid line) to the SANS intensity data

(squares) as a function of time of the diffusion process. The experiment lasts for more than a day. The self- diffusion coefficient of decane obtained from the fitting is 3.2 X 10"^cm^/sec.

SUSPENSION STRUCTURE UNDER SHEAR FLOW: SANS STUDY

OF THE ORIGIN OF SUPERANOMALOUS FLOW

L. B. Chen and C. F. Zukoski (University of Illinois, Urbana, IL)

B. J. Ackerson (Oklahoma State University, Stillwater, OK)

J. M. Hanley and G. C. Straty (Thermophysics Division)

and

J. G. Barker and C. J. Glinka

A variety of colloidal suspensions

consisting of strongly interacting particles exhibit an abrupt drop in steady state viscosity under

certain conditions of shear flow. This behavior,

in which an increase in shear rate results in a decrease in shear stress, is referred to as superanomalous flow, or discontinuous shear

thinning, and has been the focus of recent SANS

measurements using the new 30 m NIST/Exxon/

U. of MN SANS instrument. The measurements were made on a suspension of charged 229 nm

diam polystyrene spheres (at a volume fraction

of 53%) that filled the 0.5 mm gap between the

stationary and rotating coaxial cylinders of a

quartz shear cell. When properly prepared such

suspensions can form highly ordered structures over macroscopic dimensions and serve as model

systems for relating mechanical response and

suspension microstructure.

Figure 1 shows four representative SANS

patterns that were recorded with the incident

neutron beam perpendicular to the cylindrical

walls of the shear cell at shear rates spanning the

region of superanomalous flow. The patterns are gray scale representations of the scattered

neutron intensity as recorded on the SANS

instrument's 2-d detector. The pattern in figure

la) was measured with the suspension at rest

after shearing at a low rate for several minutes

to induce ordering. The characteristic sixfold

spot pattern seen in figure la) has been observed

[1] in a variety of ordered latex suspensions and

results from close packed hexagonal layers of

particles oriented parallel to the walls of the cell with their close-packed direction parallel to the

flow direction. At finite shear rates, below the

region of anomalous flow, the SANS pattern

evolves to one of nearly uniform concentric

rings, figure lb), which are too sharply defined

for a liquid-like structure and indicate, therefore,

that the suspension has broken up into nearly

randomly oriented polycrystals. As the shear

rate is increased into the superanomalous region,

discrete diffraction spots start to reappear in the

SANS data, figure Ic), indicating a return of

longer range order which is restored gradually as the shear rate is further increased. At a shear

rate of 35 sec'\ a well-defined sixfold spot

pattern is observed once again, figure Id), that remains sharp to shear rates in the vicinity of

2000 sec"^ where the sample shear melts, loses

long range order and becomes dilatent. From

these data, together with data taken with the

beam shifted off the axis of the shear cell (to

72
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

probe the structure along the direction of the

velocity gradient), it can be inferred that at low

shear rates the suspension has long range

ordered close-packed layers that have a random

registered stacking in the velocity gradient

direction. When long range order is reestablished above the region of anomalous

flow, extended oriented layers of close-packed

particles form with no correlation in position

perpendicular to the layers.

These measurements demonstrate that anomalous flow in this system is associated with

a nonequilibrium structural phase transition

between two distinct states with long range

order. Further experiments are planned on

related systems in order to explore the generality

of this result.

Reference

[1] B. J. Ackerson, J. B. Hayter, N. A. Clark, and L.

J. Cotter, J. Chem. Phys., 84, 2344 (1986).

Figure 1. Small angle neutron scattering patterns from an aqueous suspension of 239 nm polystyrene latex spheres

measured on the 30 m NIST/Exxon/U. of MN SANS

instrument. The patterns correspond to shear

a) sec"^ b) 0.08 sec"^ c) 0.3 sec" , and d) 35 sec

rates:

1

THE VIBRATIONAL SPECTROSCOPY OF AMMONIUM IONS

IN TITANYL PHOSPHATES

J. M. Nicol (University of Maryland, College Park, MD and Reactor Radiation Division)

J. J. Rush

and

W.T.A. Harrison, T. E. Gier and G. D. Stucky (University of California, Santa Barbara, CA)

Thepotassium titanyl phosphate (KTiOP04:

KTP) family of phases are currently the subject

of great interest due to their technologically

important nonlinear and electro-optical properties

[1]. In KTP, it is possible to effectively modify

the SHG (second harmonic generation) characteristics by replacement of the potassium ions by

ion-exchange with other cations, such as NH4'*',

Na"^, Rb+. While the ammonium (NH4TiOP04:

NTP) derivatives, for example, have a similar

SHG response to the prototype material, other

cations such as Na"*", can drastically reduce the

nonlinear properties. The object of the present

study was to examine the vibrational

spectroscopy and dynamics of the ammonium

derivatives of KTP by inelastic neutron

scattering (INS) and relate the results to the

structure and SHG-response of the materials [2]. The KTP structure is characterized by TiO^

chains crosslinked by phosphate bridges forming

a framework with 1-d channels, in which the K"^

cations are located [1]. The Ti-0 bonds of the

0-Ti-O chains may be regarded as alternate

'single' and 'double', and the resulting polarization difference is believed to be a crucial

feature in determining SHG-response. In NTP,

neutron diffraction [2], has found two crystallographically distinct NH4'^ ion sites. Both

types of NH4"'" ions form hydrogen bonds to framework oxygens via two of their four

protons. When NTP is partially deammoniated

to (NH4)o 5H0 5TP (NHTP), loss of NH4+ is

73
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

site selective. However, a substantial structural rearrangement takes place. In particular, the

long-short Ti-0 bond alternation in the chains is lost and the remaining NH^"*" shows a different

coordination to the oxygens. The SHG response

of NHTP is much reduced compared to NTP.

INS spectra were measured for a series of

titanyl phosphates including NTP,

K.6i(NH4).39TP (KNTP), K 8(NH4) 2TP,

NHTP, and KTP using the BT4 spectrometer

with either a beryllium (645-1800 cm"^) or Begraphite-Be (100-1130 cm"^) low-energy-bandpass filter placed before the detector. All

spectra were obtained with the samples at 80 K,

unless otherwise specified and were corrected for

fast neutron background contributions. A single

vibrational energy spectrum was obtained by

scaling the different energy ranges according to

their overlapping regions.

In the high energy INS spectrum of

K ^^(NH^) 39TP the characteristic symmetric and

antisymmetric bending modes of the NH^"*" ion were observed at 1440 and 1670 cm"^ respectively, in agreement with those found for other

ammonium-containing materials [3]. An intense

band peaked at ca. 339 cm"^ in the low energy

spectra of KNTP and NTP (figures 1) is due to the torsional mode of the NH4''" ions. This peak

energy compares well with those found for other

ammonium salts, e.g. NH4CI and NH^Br [4] and

suggests a comparable reorientational barrier in

the range 10-20 kJ mol'^ Spectra recorded at 298 K for NTP (and NHTP) show the expected

damping of this mode with temperature,

suggestive of rapid NH4''" ion reorientations.

The shape of the torsional-mode peak is observed to be broad with evidence for a shoulder on the low energy side. This shoulder,

together with weak features on the high energy

side are possibly due to enhanced scattering

intensity from lattice modes which derive their

increased intensity from coupling to the NH4'^

ions. Indeed, the INS spectrum of KTP displays

features due to lattice vibrations in this energy

range. The most significant contributions to the

observed broadness of the torsional peak may

arise from either an asymmetric torsional

potential or from the presence of different

structural environments for the NH4"'" ions.

The sharp peak observed in NTP and

KNTP at ca. 155 cm'^, which broadens on

ffavenumbera (era" ) 500 750 1000

30 60 90 Energy Loss (meV)

Wavenumbors (ctn~*) 500 750

30 60 90 120 Energy Loss (meV)

Figure 1. INS spectra of NTP and NHTP at 295 K (open

circles) and 80 K (closed circles) collected using the Begraphite-Be fdter. The data have been corrected for fast neutron background contributions. The solid line represents

a guide to the eye.

increasing the temperature, is assigned to an

optical translatory mode of the NH4"'" ions in the

lattice. Similar modes have been observed in the

ammonium halide salts [4]. This mode is noticeably absent in the INS spectra of NHTP.

The INS spectra of NHTP (figure 1) show

different characteristics compared to those

observed for either NTP or NHTP. Although

the torsional-mode peak of the NH4^. ions is observed at ca. 323 cm"\ an energy very similar

to that found for NTP, the peak is considerably

broadened and a separate translatory-mode peak

is not observed. These observations all suggest

that the NH4"'" ions in NHTP experience a

different environment from those in either NTP

or KNTP. The broadening of the torsional mode

and the disappearance of the optical translatory

mode on deammoniation indicate an NH4'^ ion

that is more 'floppy', i.e. less tightly bound.

The vibrational energy of the torsional mode,

however, suggests that the average rotational

barrier is not significantly altered by

74
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

deammoniation. These observations are supported by crystallographic data [2]. For,

although only NH4''' ions at one crystallographic

site are depleted during deammoniation, the

NH4'^ ions remaining are crystallographically

distinct from those of NTP, both in their siting

and coordination to the oxygens of the titanyl

phosphate framework.

From the torsional energy of 339 cm'\ an

estimate of the rotational barrier can be made

using the harmonic approximation and a simple

cosine potential:

2/

Where E is the torsional energy, I the moment

of inertia and Vq the height of the n-fold barrier.

Taking an average value for the N-H bond

length of 0.1029 nm, and assuming a fourfold

barrier and 90° jump distances, so that n = 4,

then an average barrier of 14.5 kJ mol"^ is obtained. A similar fourfold barrier has been

observed previously for the halide salts. The

calculated harries for NTP and KNTP falls between those observed for the halide salts

where NH4'''

those where NH4"^

experience a strong barrier [4] and

experience a weak potential

as in the cages of zeolite rho [3]. It should be

noted, however, that the different N-H...0

bonds experienced by the NH4"'" ions in the

titanyl phosphate actually precludes a symmetric

fourfold potential, as experienced by the NH4''"

ions in the bcc ammonium halides, so that intercomparisons are only quantitatively useful. The rotational barrier for NHTP may also

be estimated from the torsional energy of

323 cm'\ yielding a value of 13.1 kJ mol"^,

assuming n = 4. This supports the conclusion

that the NH4"'" ions in NHTP are experiencing a weaker barrier to rotation than those in NTP and

KNTP.

References

[1] G. S. Stucky et al. Chem. Mater. 1, 492 (1989).

[2] W.T.A. Harrison, T. E. Gier, A. J. Schultz, J. M.

Nicol, J. J. Rush and G. S. Stucky, in preparation.

[3] T. J. Udovic et al. J. Phys. Chem, £1, 5968 (1987).

[4] G. Venkataraman et al. J. Phys. Chem. Solids, 27,

1102 (1966).

NEUTRON SPECTROSCOPIC STUDIES OF THE DYNAMICS IN SODALITES

O. Elsenhans (Swiss National Science Foundation Fellow at NIST)

J. M. Nicol (University of Maryland, College Park, MD and Reactor Radiation Division)

T. J. Udovic

and

W. Buhrer (ETH Zurich, Villigen, Switzerland)

J. Felsche and P. Sieger (University of Konstanz, Konstanz, Germany)

Inelastic incoherent neutron scattering

experiments were performed for the first time on

three members of the hydroxy-sodalite series

Na6 + x[AlSi04]6(OH)x.nH20 with the

compositions x = 2,n = ("820 sodalite"),

X = 2, n = 2 ("822 sodalite), and x = 0, n = 8

("608 sodalite") as well as on the corresponding

deuterated equivalents. The framework AlSi04

tetrahedra of these sodalites are alternatively

connected via oxygen bridges forming a closepacked arrangement of p cages. The nonframework constituents Na, OH and H2O are located

inside this cage-like structure where they

compensate the negative framework charge.

The sodalite structures have been studied by

neutron and X-ray diffraction ([1] and references

therein). For all three sodalites the sodium

atoms are located on the edges of a tetrahedron

in a structure averaged in space and time. In the

820 sodalite, the center of gravity of the OH lies in the center of the sodium cage. The oxygen of

the water "molecules" in the 608 sodalite are located outside of the sodium tetrahedra on 8(e)

positions and the corresponding hydrogen or deuterium atoms on 24(i) positions of the space

group P43n. In this case, the 6 Na atoms are

statistically distributed on the 8(e) positions.

The 822 sodalite shows a much more

75
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

complicated structure involving a H2O-HO

complex rather than distinguished OH and HjO

molecules. This sodalite is the only one in the

series studied which shows a phase transition (at 150 K). Only the high temperature structure could be well determined, and is much more

complex than either of the other two sodalites.

The structure is cubic but disordered and

associates one hydrogen atom with a 8(e)

position of the space group P43n near to the

center of the sodium tetrahedra. The other

hydrogen atoms and the oxygen atoms are

statistically located on 8(e) and on 24(i)

positions.

Inelastic neutron scattering data in the

energy range 15-230 meV were collected at low

temperatures in the range 4.2 - 12 K on the BT4 spectrometer in the Be- and Be-graphite filter analyzer modes. One characteristic spectrum for

each sodalite is shown in figure 1

The well defined line at 72 meV in the

spectrum of the 820 sodalite can be attributed to a local transversal vibration of the hydrogen [2]. Since the line at 90 meV is observed in each

spectrum, it can be associated with an Al-0 or Si-0 stretching mode or with a combination of

the mentioned transversal vibration with a low

energy mode of the Na4-0H complex. The

contribution at 140 meV is likely an overtone of

the 72 meV line. The spectrum of the 608 sodalite exhibits a

line at 210 meV, which can be attributed to the

bending mode of the water molecules. Broad

intensity contributions in the range 30-80 meV

are assigned to stretching modes of the hydrogen

bridges between the water oxygen and the

framework oxygen. These modes are probably

influenced by the absence of one sodium atom in

each p cage. The importance of this absence is also demonstrated by recent results from

quasielastic investigations, which showed

rotational diffusion for only one water molecule

[3]. Since the OH and the H2O molecules

cannot be distinguished, but are assumed to be a complex in the 822 sodalite, one cannot expect

to observe a superposition of the 608 and the

820 spectra. Indeed, the measured 822 spectrum

is far more complex. Intramolecular vibrations

probably cause the observed spectral lines in the

a) Sodalite 8-2-0 T=4.2K 8/10/90

b)

70 110 150 190 Energy Transfer [meV]

Sodalite 6-0-8 T=4^K 8/10/90

>, 800

C 400

C)

200

70 no 150 190 Energy Transfer [nieV]

Sodalite 8-2-2 T=4J2K 8/10/90

1 1 1 1 T rTrl Cu-it col 40 '/<0' _

•s-*

70 110 150 190 EMergy Transfer [meV]

Figure 1. Inelastic incoherent neutron intensities as function of energy transfer for a) 820 sodalite, b) 608

sodalite and c) 822 sodalite.

energy range above 110 meV. The two wellresolved lines between 50 and 110 meV

areassigned to the collective vibrations of the

H3O2 complex. The line at 54 meV is an

instrumental artifact. A qualitative analysis of the dynamics in these sodalites is underway using the program

CLIMAX [4]. This program uses the Wilson

GF matrix method to calculate the vibrational

frequencies and amplitudes and thus the

corresponding neutron spectrum, which can be

fitted directly to the observed data. Actually, it

is unclear, whether or not this method of

76
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

resolving the dynamics of these sodalites is adequate. There are some indications that an approach which minimizes the energy in real potentials rather than one which adjusts force

constants would be more successful. In the

former case, one can in theory resolve the

structure, the dynamics and the diffusion

simultaneously.

References

[1] O. Elsenhans et al. , "Structure of the Sodalites," (See

following report).

[2] W. Buhrer, J. Felsche, S. Luger, J. Chem. Phys. 87,

2316 (1987).

[3] W. Buhrer, private communication.

[4] G. J. Kearley, private communication.

STRUCTURE OF THE SODALITES Na6+JAlSi04]6(OH)2.nH20

AND DEUTERATED ANALOGS

O. Elsenhans (Swiss National Science Foundation Fellow at NIST)

J. M. Nicol (University of Maryland, College Park, MD and Reactor Radiation Division) T. J. Udovic

W. Buhrer (ETH Zurich, Villigen, Switzerland)

and

J. Felsche and P. Sieger (University of Konstanz, Konstanz, Germany)

Three members of the hydroxy-sodalite

series Na6+x[AlSi04]6(OH)x.nH20 with the

composi-tions x = 2, n = ("820 sodalite"),

X = 2, n = 2 ("822 sodalite), and x = 0, n = 8

("608 sodalite"), as well as the corresponding

deuterated equivalents were the subject of a powder neutron diffraction study at various

temperatures ranging from 4.2 K to 180 K.

From previous studies [1,2] it is known that the

aluminosilicate framework is completely ordered

in the aluminosilicate sodalite phase. Recent

neutron diffraction investigations [3,4,5] have

found the structural positions of the nonframework constituents Na, OH, H2O and H3O2. For

the 822 sodalite, a phase transition from the

cubic disordered room temperature structure to the low temperature orthorombic structure occurs

at 150 K. No phase transitions were observed

for the two remaining sodalites. These studies

[3,4,5] showed rather large temperature factors

for the nonframework constituents, which

indicates, at least for the heavier atoms, possible

static disorder. Furthermore, recent quasielastic

neutron scattering measurements on the 820

sodalite have investigated the thermally activated

jump diffusion [6] of the hydrogen atom, which

occupies statistically one of the four possible

positions in the (Na4) tetrahedron. From these

experiments a jump distance is found which is

significantly shorter than the corresponding

values obtained from the neutron diffraction

experiment [3] . The aim of the present neutron

diffraction experiments was to reconsider the

positions of the nonframework constituents in order to obtain better agreement with the

observed diffusional and vibrational behavior of

the studied sodalites.

Neutron diffraction measurements were

carried out using the BT-1 diffractometer.

Samples of the 820, 822, and 608 sodalites were

synthesized by J. Felsche et al. The relatively

low hydrogen concentration in the 820 and 822

sodalites, made these samples suitable for study

without deuteration. For the 608 sodalite a deuterated sample was required, due to the high

hydrogen concentration and the resulting background from the large hydrogen incoherent cross

section. Due to only a partial deuteration of the

608 sodalite, no quantitative analysis of the

structure could be performed. However, a measurement using a fully deuterated sample is currently in progress.

The neutron diffraction data of the

deuterated 822 sodalite at 4.2 K and 180 K

clearly showed the existence of two different

phases, as found for the protonated sodalite [5]. While the high temperature diffractogram of the

deuterated sample can be explained with the

77
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

structural model in [5] using the space group

P43n, no agreement has been found for the low

temperature phase, which is based on the space

group P222.

Figure 1 shows the calculated, and

measured neutron diffraction patterns of the

protonated and the deuterated 820 sodalites. The new refinement of the structure (using program

MINREF [7]) resulted in an modified view of

the structure. The oxygen position of the OH

group was found not to occupy the central

position of the sodalite cage as suggested in earlier work [3]. On the contrary, both, the

oxygen as well as the hydrogen atom of the OH

group seems to be located statistically on the 8(e)

positions of the space group P43n. In this case

the center of gravity of the OH lies at the center of the sodalite cage. Furthermore, a smaller

H-O distance of 0.096 nm was found for this new position, leading to a shorter length for the

jump distance of the hydrogen atom. This seems

to be compatible with the new quasielastic

measurements. As indicated by the remaining

discrepancies between the calculated and the

observed neutron diffractograms (in particular

for the deuterated 820 sample), further

modifications of the structural model are needed.

The introduction of a possible trigonal

deformation of the sodium cage, which is correlated with the position of the hydrogen

atom on one of the sides of the sodium cage

faces, leads to a refinement with the space group

R3c. In fact, it is not possible to distinguish

between a undeformed and a deformed sodium

cage. So, the trigonal deformation can not be

Table 1. Structure parameter of NaQ[AlSi04]g(OH)2

at 4.2 K

Lattice constant: a = 0.8737(1) nm

Space group: P43n (undeformed Na cage)

Refinement Factors: R^eighted profile = 0.096,

^profile = 0075

^expected = 0.045

Atom Pos. X n B;ISO

Al

SiOl

02

H

Na

6(c) 6(d)

24(i) 8(e)

8(e)

0.250

0.250

0.137

0.013

-0.051 8(e)0.159

0.500

0.000

0.428

0.013

-0.051 0.159

0.000

0.500

0.147

0.013

-0.051 0.159

1.00

1.00

1.00 0.25 0.25

1.00

1.1

1.1

1.1 2.6

2.8

1.9

excluded. Such a deformation would give the

preference for a two-site model for the rotational

diffusion of the hydrogen, which is also reflected

in the quasielastic data. Further refinements and

measurements are in progress.

a) 3700

. TALC o oes

t 2775

Sj

I I I I I I III III III Ml II I Mill III III 111 III I n II ill II. IIIIMI

SODALITE 8-2-0 •4.2K

MrMWai JJJijiiUlw^l

'''*»>--.,^,,,,^Ji,.T--,Jlr^\..4^

b) 3700

10 20 30 40 SO 60 70 60 00 100 110

. CALC o OBS

V....,._sJ^>c~Jr'fv4.-.fy^-^

I —,—I—.—I—.—I—.—I—' I —I—'—r —I

' I ' 1 10 20 30 40 50 60 70 80 90 100 110

Figure 1. Observed and calculated neutron diffraction pattern of a) Nag[AlSi04]^(OH)2and of b) Na8[AlSi04]g(OD)2 at 4.2 K.

Space groupe: R3c (deformed Na cage)

Refinement Factors: Rv^,eighted profile^O.104,

Vofile - 0.082,

^expected = 0-047

Atom Pos. X Y Z n B,

Al 6(b) 0.250 0.500 0.000 1.00 0.44

Si 6(b) 0.250 0.000 0.500 1.00 0.44 01 6(b) 0.137 0.428 0.144 1.00 0.44 02 6(b) -0.137 -0.428 0.144 1.00 0.44 03 6(b) -0.137 0.428 -0.144 1.00 0.44 04 6(b) 0.137 -0.428 -0.144 1.00 0.44 Nal 2(a) 0.178 0.178 0.178 1.00 1.23 Na2 6(b) -0.157 -0.157 0.157 1.00 1.23 05 2(a) 0.010 0.010 0.010 1.00 2.27 H 2(a) -0.054 -0.054 -0.054 1.00 2.47

78
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

References

[1] L. PauUng, Z. KristaUogr. 74, 213 (1930).

[2] J. Loens and H. Schulz, Acta Crystallogr. 23, 434

(1967).

[3] S. Luger, J. Felsche and P. Fischer, Acta Cryst. C43 . 1 (1987).

[4] J. Felsche, S. Luger and P. Fischer, Acta Cryst. C43, 809 (1987).

[5] J. Schefer, J. Felsche, P. Sieger, M. Wiebcke,

progress report Laboratory of Neutron Scattering,

ETH Zurich, Switzerland, 102 (1990).

[6] W. Biihrer, private communication.

[7] O. Elsenhans, J. of Appl. Cryst. 23, 73 (1990).

STRUCTURES AND PROPERTIES OF NEW ZEOLITE X-TYPE

BERYLLOPHOSPHATE AND ZINCOPHOSPHATE MOLECULAR SIEVES

W.T.A. Harrison, T. E. Gier, K. L. Moran, H. Eckert, and G. D. Stucky

(University of California, Santa Barbara, CA)

and

J. M. Nicol (University of Maryland, College Park, MD and Reactor Radiation Division )

The solid state chemistry of nonaluminosilicate molecular sieves has recently undergone

a renaissance, following the discovery of a number of phases in which framework aluminum

and silicon are replaced by phosphorus, gallium

and beryllium. Some of these AlPOs, GaSiOs,

and BePOs are isostructures of known aluminosilicates, while others have new framework

topologies. We have recently characterized the

structures of the new beryllophosphate (BePO-X)

and zincophosphate (ZnPO-X) phases that are analogues of the industrially important zeolite-X

framework using neutron and x-ray diffraction

[1]. Zeolite-X is a synthetic isostructure of the

mineral faujasite. Faujasite-type phases contain one of the largest pore openings of any known

zeolite (ca. 8 A) and are of industrial importance

for petroleum cracking and other reactions [2]. BePO-X (Na96(BeP04)96. I92H2O) and

ZnPO-X (Na67TMAi2Zn8(ZnP04)96. I92H2O

(TMA = Tetramethylammonium cation)) were

prepared at ambient conditions [3]. For

BePO-X, a sample was dehydrated under vacuum at 150 K overnight and sealed in a vanadium can. The data were collected, with

the sample at 10 K, using the high resolution

five-detector powder diffractometer at NIST.

For ZnPO-X, high resolution powder x-ray data were collected on a Scintag PAD X

diffractometer.

Rietveld profile refinement was carried out with the program GSAS [4], using the starting

framework model of aluminosilicate zeolite-X

[5], space group Fd3 (No. 203). Our BePO-X

and ZnPO-X models assumed a fully ordered 1 :

distribution of Be/Zn and P over the framework

tetrahedra, and strict alternation of Be/Zn and P

between adjacent tetrahedra, as would be the case for an X-type zeolite with a 1:1 Si:Al ratio,

showing perfect cation alternation. For each

phase the usual profile parameters were refined.

For BePO-X, Na cations were positioned at the

sites found in previous studies of dehydrated

faujasite-type materials. For ZnPO-X, extra framework species were progressively located by

difference Fourier syntheses and added to the

refinement. The largest difference peaks were

assigned to sodium cations and those remaining

to extraframework water molecules. No reasonable geometry corresponding to the tetramethylammonium guest cation could be established.

For both BePO-X and ZnPO-X, the occupancies

and thermal factors of the guest were refined to ensure convergence to the best possible

minimum. No bond-distance restraints were employed for the BePO-X refinement. The final cycle of least-squares calculations converged to give residuals of i?„ = 6.20% and/?^„ = 8.17%

,2 _

wp

(x = 1 .96) for 67 variable parameters and 3495

observations. The final atomic positional

parameters for BePO-X are listed in table 1,

79
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

with selected bond distances/angle data in table

2. The final observed, calculated and difference

profiles for one detector are illustrated in figure

1. For ZnPO-X, residuals of R^ = 7.23% and

R^ = 9.25% (x^ = 6.29) for 45 variable parameters and 3002 observations were obtained.

Final atomic positional parameters and selected

bond distances/angle data for ZnPO-X are given

in ref. 1, together with details of MAS NMR,

thermogravimetric and analytical results for both

samples.

These new BePO-X and ZnPO-X phases

crystallize in the faujasite-type zeolite X

structure, which consists of cuboctahedral

sodalite (P) cages linked via 6-rings or hexagonal

prisms in a diamond-like array that surrounds

large supercages. The framework consists of 1 Zn/Be atom and IP and 40 atom sites, all on

general crystallographic positions. The

framework atom's unit cell content is (Zn/Be)96P9603g4^^', with charge compensation

provided by the extra framework species: for

BePO-X sodium cations and for ZnPO-X,

sodium, TMA and possibly zinc cations.

For BePO-X, the framework-atom geometry

is in good accordance with the expected behavior

of these species (d^^(Be-0) = 1.61 (5) A, d^^

(P-0) = 1.55 (6) A). The three distinct

locations for the sodium atoms, all on the body

diagonal of the cubic cell, are similar to those

found in studies of dehydrated sodium

aluminosilicate-X phases [5]. The total unit-cell occupancy of the Sj, Sy and Sjj is ca. 63,

leaving 33 sodium atoms unaccounted for. No

other significant site populations were established

from Fourier maps, suggesting that the other

sodiums may be dispersed over a number of sites in the supercage.

Table 1. Final Atomic Coordinates for BePO-X (Cubic,

Space Group Fd3 , a = 23.368 (2) A)

Table 2. Selected Bond Distance (angstroms) and Angle

(degrees) Data for BePO-X

Atom lOOU:.

Be(l) -0.0525(6)

P(l) -0.0547(8)

0.1071(6)

0.066(6)

0.0279(4)

0.0730(5)

0(1)

0(2)

0(3)

0(4)

Na(l)

Na(2) 0.067(2)

Na(3) 0.2442(6)

0.1247(7)

0.0370(8)

0.0071(6)

-0.0013(6)

0.0687(7)

0.0768(8)

0.067(2)

0.2442(6)

^Fractional site occupany

0.0357(6)

0.1226(10)

0.1043(6)

0.1477(4)

0.0694(7)

0.1740(7)

0.067(2)

0.2442(6)

1.1(2)

1.1(2)

1.1(2)

1.1(2)

1.1(2)

1.1(2) 1.08(7) 2.8(6) 0.47(4) 2.8(6) 1.04(7) 2.8(6)

Be(l)-0(1) 1.62 (3) Be(l)-0(2) 1.64 (3) Be(l)-0(3) 1.63 (3) Be(l)-0(4) 1.55 (3) P(l)-0(1) 1.47 (3) P(l)-0(2) 1.55 (3) P(l)-0(3) 1.58 (4) P(l)-0(4) 1.58 (3) Na(l)-0(3) 2.372 (8) X 6 Na(2)-0(3) 2.22 (3) X 3 Na(3)-0(2) 2.28 (2) X 3

0-Be(l)-0^ 109 (2) Be(l)-0-P(l)'' 136(2)0-P(l)-0^ 110 (4)

^Tetrahedral average. Bridging oxygen average.

These new BePO-X and ZnPO-X phases are representatives of an extensive new class of

zeolite materials not based on group Ilia or group IVa metal atom framework compositions.

Further work on the preparation, modification

and characterization of these and related systems

is in progress.

SODIUM BEPO-X lOK NBS OATi

LilueD* 1.5530 A. L-S CYCLE

NICOL/HARBISON

09S0 ano oiff. profiles

-I 1— 1—

10

2-THETA. DEC

Figure 1. Final observed (crosses), calculated (line), and

difference profile plots, with reflection positions indicated by tickmarks for BePO-X (first detector, low-angle neutron

data).

References

[1] W.T.A. Harrison, T. E. Gier, K. L. Moran, J. M.

Nicol, H. Eckert and G. D. Stucky, Chem. Mater. 3 27 (1991).

[2] J. S. Magee and J. J. Blazek, Zeolite Chemistry and

Catalysis, American Chemical Society, Washington

D.C., 1976; p 615.

[3] T. E. Gier and G. D. Stucky, Nature (London) 349 . p 508 (1991).

[4] A. C. Larson and R. B. Von Dreele, GSAS User

Guide; Los Alamos National Laboratory, Los Alamos,

NM (1985-1988).

[5] J. V. Smith, Zeolite Chemistry and Catalysis,

American Chemical Society, Washington D.C., 1976; pi.

80
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

Sr2Bi205: A STRUCTURE CONTAINING ONLY 3-COORDINATED BISMUTH

C. C. Torardi (DuPont Experimental Station, Wilmington, DE)

J. B. Parise (State University of New York at Stoney Brook, Stoney Brook, NY)

C. J. Rawn, R. S. Roth, and B. P. Burton (Ceramics Division)

and

A. Santoro

The bismuth (III) strontium oxide Sr2Bi205

has been synthesized in the low-temperature

form, and studied using single-crystal x-ray and

powder neutron diffraction techniques. The

compound crystallizes with the symmetry of

space group Pnma with Z = 4 and x-ray lattice parameters a = 14.261(3), b = 6.160(2), c =

7.642(3) A. All bismuth ions are in threefold

coordination with oxygen and form . . .Bi(l)-0(1)-

Bi(2)...Q..Bi(l)-0(l)-Bi(2)... chains propagating in the direction of the c-axis, as shown in figure 1 (the symbol D indicates an oxygen

vacancy). These chains are stacked along the a- axial direction and are connected by Bi-0 bonds

between the Bi ions in the chains and oxygen

ions in the Sr-0 slabs, and by Sr-0 bonds

between the Sr(l) ions in the slabs and 0(1)

atoms in the chains. Strontium is seven- coordinated to six oxygen ions forming a

trigonal prism and to one oxygen ion belonging

to the chains. The Bi ions along the chain are not evenly spaced, thereby contributing to a "superstructure", along the c-axial direction,

observed in long exposure precession photographs. The geometry around the bridging

oxygen ion 0(1) is trigonal planar because the

four atoms Bi(l), Bi(2), Sr(l), and 0(1) lie on

the same mirror plane. There are no bonds to 0(1) out of the mirror and 0(1) can therefore

vibrate more or less freely in the direction

perpendicular to the plane.

Rgure 1. Representation of the structure of Sr9Bi205.

The filled circles indicate the Bi and Sr ions. The Sr-0

slabs and the Bi-0 chains are connected by Sr(l)-0(1) bonds.

THE CRYSTAL STRUCTURE OF A DEFECT PYROCHLORE, NaW205 5

• D2O

K. P. Reis and M. S. Whittingham (State University of New York at Binghamton, Binghamton, NY)

and

E. Prince

Neutron powder diffraction and Rietveld

refinement [1] were used to refine the crystal structure of a deuterated sample of a defect

pyrochlore with the composition NaW20g 5

• D2O. This compound crystallizes in the space

group Fd3m, with the lattice parameter a =

10.3125(2) A. The structure is built from

slightly distorted WO^ octahedra that share

corners to form a three dimensional framework.

This rather rigid framework determines the

overall crystal structure. It contains a set of

hexagonal channels that intersect, forming

substantial sized cavities, but their sizes and

shapes are not particularly well suited to accommodating the additional oxygen, the

sodium atoms, and the water molecules. As a result the remaining structure is highly

disordered, with no one crystallographic site
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

being more than half fiill, and some of the

deuterium occupying only six out of 192

equivalent sites of a general position. The

coordinates and occupancies of the structure that gives the best fit to the data (R^ = 6.58%, Rg = 5.30%, Rg = 9.88%) are given in table 1.

Table 1. Parameters for NaW20g ^

• D2O. Space group Fd3m

Atom site X V z B N

W 16c 0.8(2) 1.0 01 48f 0.3125(5) 1/8 1/8 0.6(1) 1.0 02 32e 0.4073(8) X X 1.7(5) 0.385(1)

Na 16d 1/2 1/2 1/2 1.0 0.5(2) Dl 192i 0.404(6) 0.195(4) 0.137(6) 1.0 0.028(2)

D2 96g 0.902(2) X 0.476(3) 1.0 0.13(1)

Reference

[1] H. M. Rietveld, J. Appl. Cryst. 2, 65 (1969).

THE CRYSTAL STRUCTURE OF NaxW03^^/2 • yH20

K. P. Reis and M. S. Whittingham (State University of New York at Binghamton, Binghamton, NY)

and

E. Prince

Neutron powder diffraction and the Rietveld

[1] method were used to refine the crystal structure of a synthetic tungstate with the

composition NaQ 17WO3 035 ' yH20. Two

samples were used, a protonated sample with y = 0.19 and a deuterated sample with y = 0.23.

This compound crystallizes in a structure that is closely related to hexagonal tungsten bronze

(HTB), which consists of a framework of corner- sharing WOg octahedra, but differs from HTB in that layers perpendicular to the c axis are flat (space group P6/mmm) rather then puckered

(space group P6322, with a doubled c axis). As

is shown in figure 1, the octahedra form six

membered rings, leading to hexagonal channels

running through the crystal parallel to the c axis. The alkali cation and the extra oxygen, some of

which is water, reside in these channels, which

is somewhat surprising, because oxygen and

water apparently diffuse in and out rather easily

although there would seem to be insufficient

space for the atoms and molecules to pass the

cations.

The initial data were collected on the

sample prepared in natural water, but, because

of the high background due to incoherent

scattering from the protons, the data proved to be difficult to interpret. Another sample was

therefore prepared using heavy water, and a second data set was collected. The structure was

successfully refined from these data to R^TM =

6.38% (Re =5.81%), Rg = 9.02%. Table 1 gives the final structural parameters.

After refinement of the deuterated data the

protonated data set also refined satisfactorily,

confirming that the peaks that had been assigned

to deuterium were indeed parts of water

molecules. There is not enough room for

sodium atoms to occupy more than half of the

positions assigned to them. The lack of a doubled unit cell is probably due to disorder

such that there is no correlation from one

channel to another determining which half of the

positions is occupied.

82
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

Figure 1. The arrangement of comer shared WOg

octahedra in hexagonal tungsten bronze.

Table 1. Refined parameters for Na 17WO2 qo^ • 0.23 H^O

Space group P6/mmm, a = 7.329(2) A, C = 3.891(1) A

Atom Site

Reference

[1] H. M. Reitveld, J. Appl. Cryst. 2, 65 (1969).

Occupancy

w 3f 0.5 0.0 0.0 3.8(4) 1.0 01 3g 0.5 0.0 0.5 1.5(1) 1.0 02 61 0.2114(4) 2x 0.0 1.2(1) 1.0 03 2e 0.0 0.0 0.412(4) 2.6(8) 0.5 D 12a 0.071(3) 2x 0.312(7) 1.0 0.095

STRUCTURE OF Mg-EXCHANGED BePO-X

T. M. Nenoff, W.T.A. Harrison, and G. D. Stucky (University of California, Santa Barbara, CA)

and

J. M. Nicol (University of Maryland, College Park, MD and Reactor Radiation Division)

Recently, new analogues of zeolite-X

containing beryllium/zinc and phosphorus as the

framework tetrahedral atoms have been described [1]. Here we report the crystal structure of a magnesium-exchanged beryllophosphate-X

phase (Mg-BePO-X). The preparation of the

BePO-X phase has been described previously

[2]. The new Mg-BePO-X (cubic, a =

23.2423(8) A, space group Fd3) crystallizes in the faujasite-type zeolite-X structure. This

structure consists of cuboctahedral sodalite or pcages linked via double 6-rings or hexagonal

prisms in a diamond like array. The framework

surrounds large 'supercages', which correspond

to about 50% 'empty space' in volume percentage, as described previously [1]. The

framework consists of 1 Be atom, 1 P and 4

atom sites, all on general crystallographic

positions. The framework atoms' unit cell content is BQt^^^^O^^^,^^^ ^^^h charge

compensation provided by the extra-framework

magnesium cations.

For Mg-BePO-X, the framework-atom

geometry is in good accordance with the

expected behavior for these species (J^^,g(Be-0)

83
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

= 1.61 A, d^^^(P-0) = 1.54 A). Two distinct

locations were found for the magnesium cations,

both on the body diagonal of the cubic cell. The

first, at the orgin, is approximately octahedrally

coordinated by 0(3) (i/(Mg-0(3)) = 2.133(4)

A), and is located in the center of the hexagonal

prism joining adjacent P-cages (Sj site). This

site is fully occupied and illustrated in figure 1

The Sji position ('free' 6-ring joining the p- and

supercages), is about 50% occupied with three

Mg-0(2) bond lengths of 2.170(7) A, in pyramidal configuration (fig. 2). Other

magnesium cation sites are probably in the

supercages.

Further studies on other ion-exchanged

species and other new zeolite analogues are now

in progress.

Mg-BePO-X

Figure 1. ORTEP view of the double 6-ring window in magnesium-exchanged beryllophosphate zeolite-X. Be/P

atoms filled circles, O atoms plain, Mg cation speckled.

The Mg(l) cation is octahedrally coordinated (thin lines) by oxygen atoms.

Mg-BePO-X

Figure 2. ORTEP view of the Mg(2) site in Mg-BePO-X.

The pyramidally-coordinated Mg protrudes into the supercage.

References

[I] W.T.A. Harrison, T. E. Gier, K. L. Moran, J. M.

Nicol, H. Eckert and G. D. Stucky, Chem. Mater. 3, 27 (1991) and p. 79.

[2] T. E. Gier and G. D. Stucky, Nature (London) Vol. 349, p 508 (1991).

STRUCTURE OF LITHIUM CHABAZITE

W.A.T. Harrison and G. D. Stucky (University of California, Santa Barbara, CA)

J. E. MacDougall (Air Products Inc., Allentown, PA)

J. M. Nicol (University of Maryland, College Park, MD and Reactor Radiation Division)

The aluminosilicate zeolite chabazite (typical

formular: Ca2Al4Si8024. I2H2O) has a structure based on 8-rings, double 6-rings and 4-rings. It may be visualized as ABC-stacks of double 6-

rings joined through slanted 4-rings, leading to large cavities, interlinked by a 3-d network of 8-

rings. Typical zeolitic cation exchange is

possible, and several structural studies have been

made on chabazites containing various guest

cations and/or waters of hydration.

Li-exchanged chabazite (Li-CHA) is structurally most similar to dehydrated cupric

chabazite [1] in terms of guest cation configuration and framework distortion. The structure of

84
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

Li-CHA was determined by Rietveld refinement

using low-temperature neutron powder data

collected on the high resolution five-detector

diffractometer BT-1 at NIST. The starting

framework model was in space group R3m (No.

166) and the final cell dimensions were a =

9.3354(8) A, a = 93.677(5)°. No evidence for

a monoclinic distortion was observable.

The guest Li"^ cation was located from

difference Fourier syntheses, following

refinement of the framework atoms, and the

usual profile parameters. One confirmed lithium

site was found, on the [lll]-body diagonal:

lithium sits in trigonal coordination (site

symmetry 3m) with three bonds to 0(4) of

1.948(11) A (fig. 1). The structure of cupric

chabazite [1] is most similar to the phase

described here.

The framework of Li-CHA appears to be the

most distorted chabazite determined to date. The

degree of distortion of the TO4 tetrahedron

(where T = Si or Al) leads to a very large T0(2)-T bond angle, linking double-6 rings

(173.6(8)°). The T-0(4)-T bond angle reduces

to 124.0(7)°, reflecting the lithium coordination

of this oxygen (d(T-0) = 1.715(9)A). The

average T-0 bond distance of 1 .645 A is in good

agreement with other studies and in accordance

with the Si:Al ratio in chabazite.

Chabazite — double 6-ring

Figure 1. ORTEP view of the double 6-ring window in Lithium-exchanged Chabazite: silicon/aluminium atoms

filled circles, oxygen atoms plain, lithium cations speckled.

The pyramidally-coordinated Li" cations occupy the outer face of the 6-ring windows.

References

[1] J. J. Pluth, J. V. Smith, and W. J. Mortier, Mater.

Res. Bull. 12, 1001, (1977).

THE LOW TEMPERATURE CRYSTAL STRUCTURE OF ACETONITRILE

S. F. Trevino and C. S. Choi (ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

Acetonitrile (CH3CN) presents a possibly

very interesting candidate molecule for the study

of methyl rotational properties in the solid state. The absence of substantial intramolecular inter- actions contributing to the definition of the

methyl rotational potential leads to an

expectation of a low barrier produced by

intermolecular interactions. A previous

determination of the low temperature structure has been reported [1]. This study was obtained

at 100 K with data of limited quality from

which, in particular, the thermal parameters for

the deuterium atoms were reported only in the

isotropic approximation. It is for the purpose of

verifying the structure and obtaining anisotropic

deuterium thermal parameters that the present

study was undertaken.

The preparation of a good quality powder

sample was accomplished by grinding the sample

in a mortar and pesfle cooled by placing on solid

CO2. The sample was quickly transferred to a cold cryostat to prevent a temperature rise. No

care was taken to follow the procedure suggested

in ref. [2] to ensure the existence of the low

temperature phase. The absence of preferred

orientation was verified by observing a constant

intensity of a strong diffraction peak as a function of sample orientation and the presence

85
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

of a single phase by the success of the

refinement. The diffraction pattern were

obtained from the powder diffractometer BT-1 at a sample temperature of 4.2 K. The 100 K

structure reported by Antson et al. [1], was used

for the initial model of the refinement. With the

anisotropic thermal parameter assigned to each

atom, the final refinement converged to i?B = 2.67, Rp =4.79, R^p = 6.63 for

Rg = 5.32. The molecule is positioned on the

mirror plane with the three nonhydrogeneous

atoms and Dl on the mirror plane. The

molecular axis, C1-C2=N, has a true linear form

within the experimental error (179.6(5)°). The

three C-D bonds of the methyl group are arranged in the normal tetrahedral form with the

deviation less than 2° in bond angles. The bond

angles are given in table 1. The thermal

ellipsoids of the D atoms were about twice larger

than those of the C and N atoms, and their

orientations were consistent with the molecular

libration motion. The rigid body motions (T L

and S) obtained from the atomic thermal motions

by the method of Schomaker and Trueblood are shown in table 2. The largest librational motion

occurs about the molecular axis with the r.m.s.

amplitude of about 6°. This small value

suggests a large rotational barrier.

Table 1. The bond-lengths and bond-angles of CDoCN at 10 K

Bond lensths (A): Bond angles (°): CI -C2 1.460(5) C1-C2-N 179.6(5)

C2-N 1.159(5) D1-C1-C2 109.5(4)

CI -Dl 1.109(6) D2-C1-C2 109.6(3)

CI -D2 1.079(3) D1-C1-D2 108.3(4)

D2-C1-D2' 111.4(5)

Table 2. T(i,j) and L(i,j) elements of the rigid

motions

T(i,i) (a2): Ui.i) (sa. rad.l: 11 0.0083(39) 0.0022(21) 22 0.0122(25) 0.0078(52) 33 0.0095(33) 0.0074(61)

12 0.0000 0.0000

13 0.0000 0.0000

23 -.0031(25) -.0024(24)

References

[1] O. K. Anston, K. J. Tilli, and N. H. Andersen, Acta

Cryst B 43, 296 (1987).

[2] E. L. Pace and L. J. Noe, J. Chem Phys. 49, 5317

(1968).

PHASE DETERMINATION FOR MACROMOLECULAR X-RAY DIFFRACTION

DATA BY MAXIMUM ENTROPY

L. Sjolin (Chalmers University of Technology, Goteborg, Sweden)

and

E. Prince

Because of the interesting results obtained

in a trial of determining the phases of X-ray

diffraction intensities by the use of maximum

entropy [1,2], a further trial was begun with data

taken from crystals of a protein whose structure was unknown. This protein, bovine heart

creatine kinase, or BHCK, has 381 amino acid

residues in its molecule. Pairs of molecules

bond together to form dimers in crystals that belong to space group P422j2, with unit cell dimensions a = 132.4 A, c= 74.9 A.

Consideration of density determines that there is one monomer in the asymmetric unit, and it is reasonable to assume that the two halves of the

dimer are related by a twofold rotation axis. Because the monomer must be asymmetric, the

other twofold axes in the unit cell cannot pass

through it, which makes it probable that the twofold rotation that is a subgroup operation of the

42 screw axis is the symmetry axis of the dimer.

A first step in the trial was to refine the

computer program to utilize fast fourier

transforms [3] that exploit the space group

symmetry [4,5] to reduce memory requirements

and optimize speed of execution. In this space

group the density map meeds to be computed in only one eighth of the unit cell, and only one

86
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

sixteenth of the structure factors need to be

computed.

In P422j2 all hkO and hh^ reflections must

be real, and those of hO£ may be real or imaginary, but may not be complex. The phases

of one hkO reflection with h + k odd and one hh£ reflection with £ odd may be chosen

arbitrarily to define the origin, and that of one

hO£ reflection that has an imaginary structure

factor may be chosen to determine the

enantiomorph. Phase determination proceeded in two steps. First the unit cell was divided into

pixels ~ 2 A on a side, and a subset of the data was selected with d spacings no shorter than

5 A. The strongest reflection from each class was chosen to fix the origin and enantiomorph.

The phases of 240 additional reflections from the

three central classes were chosen, in blocks of

16, by a systematic search for the sign combination that gave the map with the highest

entropy. Reflections with general phases were

added, one at a time, by trying phases in steps of

20° from 0° to 340° and fitting the amplitudes,

again searching for the maximum entropy map.

When phases for reflections in this low

resolution subset had been determined

(R < 3.0%) a new map was computed using

this same subset of reflections but with pixels

1 A on a side. The entire data set (28164

reflections) was then added to the map following

the same procedure as before. Finally, a map

was calculated and analyzed by a program that traces ridge lines in the density, and both density

and ridge line traces were displayed on a computer graphics system.

Because of the configuration of symmetry

elements in the unit cell, it was probable that most of the monomeric molecule would lie in a region from z = to z = 1/2 and either from

X = to X = 1/2 and y = 1/4 to y = 3/4 or from X = 1/4 to X = 3/4 and y = to y = 1/2,

but the choice between these two is determined

by the arbitrary choice of origin. A region of

the unit cell from to 1/2 in all three

dimensions should contain 1/2 of each of two

molecules. When the ridge tracing of this region was displayed, a division between two molecules was clearly visible. A portion of the unit cell was then calculated that contained one molecule,

and the ridge tracing and density were examined

for features of secondary structure commonly

found in proteins. There are several regions that are fit by a-helices, all winding in the same

direction, and also regions fit by p -sheets up to five strands wide. Work on interpretation of the

map is continuing.

Referraices

[1] L. Sjolin, E. Prince, L. A. Svensson and G. L.

Gilliland, Tech Note 1285, C. O'Connor, ed., p. 11 (1990).

[2] L. Sjolin, E. Prince, L. A. Svensson and G. L.

Gilliland, Acta Cryst. A £7, 216-223 (1991).

[3] R. Tolimieri, M. An and C. Lu, Algorthims for Discrete Fourier Transforms and Convolution

(Springer-Verlag, New York, 1989).

[4] L. F. Ten Eyck, Acta Cryst. A 29, 183-191 (1973).

[5] M. An, C. Lu, R. Tolimieri and E. Prince, (see the following report).

EXPLOITING SYMMETRY IN CRYSTALLOGRAPHIC

FAST FOURIER TRANSFORMS

C. Lu (Towson State University, Towson, MD)

M. An and R. Tolimieri (City University of New York, New York, NY)

and

E. Prince

The interpretation of X-ray and neutron

diffraction data requires extensive use of Fourier

transforms. A discrete Fourier transform (DFT)

can be represented as a multiplication of a vector of N elements, often complex, by an N x N

matrix, most of whose elements are complex. A

fast Fourier transform (FFT) is formed by

factoring this dense matrix, by one or a combination of many different techniques [1],

into a product of sparse matices, thus reducing

87
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

the computation from one requiring a number of

computations approximately proportional to N^

to one requiring a number of computations

approximately proportional to N log N, which

increases only slightly faster than linearly as N

increases. In crystallographic computations,

space group symmetry can be used to reduce

execution time and storage requirements still further [2] . Crystallographic computations also

possess other special properties, such as the fact that electron densities are real numbers, so that

their transforms are Hermitian, or the fact that,

when 2 J screw axes are present, data points that are one half of a period apart are complex

conjugates of one another.

Fourier transforms in two or more

dimensions can be performed by sequences of

1-d transforms, (the row-column method) so computer code is needed only for the 1-d

transforms. Because methods that can be

adapted to some of the special conditions found

in crystallography are most efficient if the

number of subdivisions in a period is two or four

times an odd number, transforms are needed for

odd as well as even number of points. A library

of subroutines has been written in Fortran 77 for

all even number of points from 20 to 200 and all odd number of points from 1 1 to 99 that have no

prime factors larger than 17. Additional

subroutines have been written that provide

efficient computation of transforms from real or Hermitian data and data possessing other kinds

of symmetry. Subroutines that exploit the

symmetries of six frequently occurring space

groups have also been written. This list is being

extended to include twenty-one space groups to which more than 80% of observed protein

structures belong.

References

[1] R. Tolimieri, M. An, and C. Lu, Algorthims for Discrete Fourier Transforms and Convolution,

(Springer-Verlag, New York, 1989).

[2] L. F. Ten Eyck Acta Cryst. A 29, 183-191 (1973).

NIST*LATTICE-DETERMINATION AND CRITICAL EVALUATION

OF SYMMETRY

V. L. Karen and A. D. Mighell

NiST*LATTiCE [1] has been completed and

is ready for distribution to the scientific community. The program is written in standard

FORTRAN and can be used in any diffraction or

analytical laboratory. The software is multifunctional and can be used to analyze various

types of lattice relationships. The present

version of the nist*lattice performs several

functions including: 1) the determination of

symmetry and the evaluation of experimental

error; 2) the generation of transformation

matrices relating any two unit cells; 3) the

calculation of the reduced cell of a lattice, and

the calculation and reduction of specified

derivative supercells and/or subcells; and 4) unit

cell transformations. The program has many

appli-cations in the analytical and material

sciences including phase characterization,

materials design, and symmetry determination.

The Symmetry Program Function in NIST*LATTICE is extremely reliable for the

determination of crystal symmetry as it can

calculate groups of symmetry matrices with

respect to any specified cell no matter how

skewed. In sharp contrast to commonly used

techniques, the program allows one to determine

all symmetries and pseudosymmetries, to evaluate experimental error taking into account

all the lattice symmetry operations, and to calculate exacdy where to look for equivalent

hkl's regardless of the reference system [2,4].

During the year, the program has been used

to evaluate the crystal symmetry of compounds

reported in a number of leading journals. It has

been found that it is not uncommon for the

crystal to be assigned the wrong space group.

This is true for small molecules as well as for proteins [3]. To prevent such errors in the

published literature, we have taken several steps.

88
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

First, we have installed the program in the

Technical Office of Acta Crystallographica in Chester, England so that all structures can be

checked for the correctness of symmetry prior to publication. Second, the program along with a Technical Manual [1] will be distributed to the

scientific community so that one can check the

symmetry at any stage of an analysis. Finally,

it is planned to incorporate the code directly into

commercial diffractometers so as to automate,

for the first time, the entire data collection

process in a logical and reliable manner.

Referraices

[1] V. L. Karen and A. D. Mighell, NIST Tech. Note

1290, NIST*LATTICE-A Program to Analyze

Lattice Relationships (Spring 1991).

[2] V. L. Karen and A. D. Mighell, Acta Cryst. A 43,

375-384 (1987).

[3] A. D. Mighell, J. R. Rodgers, and V. L. Karen, J. Mol. Biol, (submitted for publication).

[4] V. L. Karen and A. D. Mighell, J. Appl. Cryst. (1991) in press.

NIST CRYSTAL AND ELECTRON DIFFRACTION DATA CENTER

A. D. Mighell and V. L. Karen

The NIST Crystal and Electron Diffraction

Data Center is concerned with the collection,

evaluation, and dissemination of data on solidstate materials. The Data Center maintains a

central database with chemical, physical, and

crystallographic information on all types of

well-characterized substances. These materials

fall into the following categories: inorganics,

organics, organometallics, metals, intermetallics

and minerals. During this year, the master

database has been significantly augmented with

respect to all categories of materials and now

contains greater than 170,000 entries. From this central database, two distribution databases are produced: nist crystal data [1] and the nist/

sandia/iCDD Electron Diffraction Database [2]. These databases are made available to the

scientific community through computer oriented

modes of dissemination including: magnetic

tape, on-line searching, CD-ROM, and scientific instruments. Recent developments and applications with respect to these distribution modes

are briefly discussed below.

Magnetic Tape. In the Spring of this year, search software and the database were installed

in the Technical Office of Acta Crystallographica

in Chester, England. Using lattice-matching

techniques, each 'new' structure can routinely be

checked against the database to see if the

compound, or related structure, has been

previously studied. As research in the Data

Center has proved that the lattice/element-type

combination uniquely characterizes a material,

this type of check by the journal editors will

prevent unwanted multiple publication of the same compound.

On-Line Searching. The online system for NIST crystal data, known as crystdat [3], consists of the database along with search and

analysis software. Recently, crystdat has

been upgraded to include data on 183,000

crystalline compounds. The system has been

significantly expanded to include over 26 million

searchable parameters. New search tools that are based on Boolean logic have been added.

crystdat is ideal for analytical purposes,

materials research, and statistical studies. For

example, statistical analyses of space group

frequencies for various classes of materials have

been carried out and the results will be published

in 1992.

CD-ROM. A CD-ROM Containing three databases on the same disk is now being distributed.

These databases are: nist crystal data, the

NiST/sandia/iCDD Electron Diffraction Database

and ICDD Powder Diffraction File. This product

is being marketed by the International Centre for Diffraction Data. In August, a workshop was

organized at the picxam International Diffraction

Conference on the combined use of these three

databases for phase characterization using x-ray, neutron, and electron diffraction data. Scientificlnstruments. Throughout 1990/

1991, we have visited a series of manufacturers

89
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

of commercial analytical electron microscopes References and single-crystal x-ray diffractometers. Our

objective is to integrate the database directly into

the instrument to permit identification as soon as

lattice and chemical data are known. To

facilitate use of the database in this mode, a comprehensive manual of relevant papers on

phase characterization has been prepared by the

Data Center. To date, the database along with

search software have been integrated into several

commercial instruments. [3]

[1]

[2]

NIST CRYSTAL DATA (1991). A Database of Chemical, Physical, and Crystallographic data. Compiled and evaluated by the NIST Crystal and

Electron Diffraction Data Center, NIST. NIST/Sandia/ICDD Electron Diffraction Database

(1988). A Database designed for phase characterization by electron diffraction. Prepared by the NIST

Crystal and Electron Diffraction Data Center in collaboration with Sandia National Laboratories and

the International Centre for Diffraction Data, NIST. CRYSTDAT(1991). An International Online Search System for NIST CRYSTAL DATA. Developed

jointly by the Canada Institute of Scientific and

Technical Information (John R. Rodgers) and the NIST Crystal Data Center.

INTERLAYER DIFFUSION IN LANGMUIR-BLODGETT MULTILAYERS

CONTAINING LAYERS OF TWO TYPES

M. D. Foster and T. Vierheller (University of Akron, Akron, OH)

A. Schmidt and W. Knoll

(Max Planck-Institute for Polymer Research, Mainz, Germany)

and

S. Satija and C. Majkrzak

Interdiffusion between layers of LangmuirBlodgett (LB) multilayers has been studied very

little, despite the fact that understanding this phenomenon is critical for the development of

nanoscale devices exploiting the precise layer

thickness and highly ordered structure attainable

by LB deposition. This study compares inter- diffusion in multilayers containing only a

"classical" short chain amphiphilic molecule,

Cadmium Arachidate (CdA) with that in a mixed

multilayer including a bilayer of a rod-like

polyglutamate copolymer with flexible aliphatic

side chains, poly-y-methyl-L-glutamate-co-y-nalkyl-L-glutamate (PG) [1]. Both neutron and

x-ray reflectometry (NR & XR) are used as probes because the two techniques rely on

different types of contrast in the system and thus

provide complementary information.

Multilayers of CdA alone were studied first because their room temperature structure has

been extensively characterized using a variety of

techniques and some characterization data on

their behavior at elevated temperature exists [2]. Our reflectometry measurements were intended

to reproduce and expand on the few qualitative

NR measurements of interdiffusion at 70 °C by

Stroeve et al. [3] and then become the basis for

comparison measurements from a mixed multilayer containing both CdA and PG.

A multilayer containing six layers of

protonated (p-) CdA followed by four layers of

perdeuterated (d-) CdA were deposited on silicon

substrates as shown in figure 1. Use of the

deuterated species provided neutron contrast between the two packets of layers. X-ray

reflectometry, which was sensitive to the

position of the Cd "'""'" ions attached to every

molecule, but not to differences between d-CdA

and p-CdA indicated that the definition of

individual layers in an "as prepared" multilayer was good. NR measurements revealed,

however, that some interchange of protonated

and deuterated molecules between layers had

already taken place even before annealing at elevated temperature, perhaps during deposition

itself. While a very slight change in the NR

curve from this sample occurred upon heating to 70 °C for 15 min, no fiirther change was

90

i
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

d-CdA

p-CdA

Hydrophobic

Silicon Wafer

Figure 1. Schematic structure of the CdA multilayer for which the reflectivity is shown in figure 2.

observed after an additional 2.25 hr at 70 °C

and 5.75 hr at 81 °C. A repetition of the x-ray

measurement after the entire annealing regimen

reproduced the curve seen before annealing,

indicating no disordering of the layered structure had taken place. Our observation of little change with annealing at 70 °C contrasted

sharply with the results of Stroeve et al., who

reported significant change in the NR curve of a similar sample after annealing only 15 min at 70 °C^

A second sample was chilled to 4 °C

immediately after deposition and kept at 4 °C

except during measurements and during

annealing at 84 °C for various lengths of time.

Reflectometry of the "as-deposited" sample again

indicated some interdifftision had already taken

place before annealing. Further measurements,

of which one is shown in figure 2, showed progressive, but subtle changes in the reflectivity

after total annealing at 84 °C for 2, 10, and 33

hr. A third system was made with a single PG

bilayer sandwiched between six layers of p-CdA

and six layers of d-CdA with the intent that the

PG would act as a diffusion barrier. After

annealing at 84 °C this sample's NR curve was

changed significantly as shown in figure 3. Comparison with figure 2 reveals that, in fact, this change was greater than that for the

k(A')

Figure 2. Comparison of the neutron reflectivity curves for the CdA multilayer as deposited (o) (kept at 4 °C until measurement) and annealed 10 hr at 84 °C (•).

multilayer without the PG bilayer annealed in thesame way. Measurements at 70 °C revealed

a similar trend. Modelling of the data is proceeding in order to quantify the degree of

interdifftision in each as-deposited sample and to identify the microstructural changes which occur during annealing.

Figure 3. Neutron reflectivities of a multilayer consisting of six layers of p-CdA, a bilayer of PG, and six layers of d-CdA as deposited (o) and after annealing 10 hr at 84 °C

Polyglutamates provided by K. Mathauer and

Prof. G. Wegner, MPIP. Dr. W. Schrepp of

BASF AG assisted in multilayer preparation.

Research support provided by the Bundesministerium fiir Forschung und Technologie

(BMFT) and the University of Akron.

References

[1] W. Hickel, G. Duda, M. Jurich, T. Krohl, K. Rochford, G. Stegeman, J.D. Swalen, G. Wegner

and W. Knoll, Langmuir 6, 1403 (1990); G. Duda,

A. J. Schouten, T. Amdt, G. Lieser, G. P. Schmidt,

91
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

C. Bubeck and G. Wegner, Thin Solid Films 159, 221 (1988).

[2] e.g. J. P. Rabe, V. Novotny, J. D. Swalen and J. F. Rabolt, Thin SoUd Films 159, 359 (1988).

[3] P. Stroeve, J. F. Rabolt, R. Heleki, G. P. Felcher and S. H. Chen, Mat. Res. Soc. Symp. Proc. 166. 103 (1990).

MICROSTRUCTURE AND EVTERLAYER DIFFUSION IN LANGMUIR-BLODGETT

MULTILAYERS OF A ROD-LIKE COPOLYMER

M. D. Foster and T. Vierheller (University of Akron, Akron, OH)

A. Schmidt and W. Knoll (Max Planck-Institute for Polymer Research, Mainz, Germany)

and

S. Satija and C. Majkrzak

The "as deposited" microstructure and

interlayer diffusion in Langmuir-Blodgett (LB)

multilayers of a rod-like polyglutamate

copolymer with flexible aliphatic side chains,

poly-Y-methyl-L-glutamate-co-y-n-alkyl-L-glutamate [1] (PG) have been studied using neutron

and x-ray reflectometry (NR & XR). Both

protonated (p-) and deuterated (d-) species of PG

were used to provide contrast between layers for

NR. At the same time XR was insensitive to the

distinction between protonated and deuterated

layers, but was sensitive to the slight (ca. 5%)

electron density difference between the PG

backbone and side chains within a layer, thus

providing a complementary sensitivity.

A model of the unannealed microstructure

which described well both the reflectometry data

collected in this study as well as earlier

reflectometry data [2] and results from other

techniques [1] is shown in figure 1. The PG

molecules arranged themselves in bilayers with

the rod-like, hydrophilic backbones localized at the center of the bilayer while the aliphatic,

hydrophobic side chains segregated to the top

and bottom of each bilayer.

Interdiffusion between and within bilayers was studied using three types of multilayers.

The first was constructed to have contrast between adjacent bilayers and could be

characterized by the layer sequence 'ppdd'.

That is, the multilayer consisted of two

protonated layers, comprising a single bilayer,

followed by two deuterated layers, followed by

two protonated layers, etc. A second sample,

characterized by 'pdpd', allowed sensitivity to

Hydrocorbon

Polyglulomole

Hydrocarbon

Substrate

Figure 1. Schematic of the PG bilayer structure, shown for the case of a single bilayer on a hydrophobic substrate.

interchange between any two layers, and a third

sample, 'pddp', was designed to probe

interchange between layers within a bilayer.

Neutron measurements from the ppdd and

pdpd structures after aimealing at 70 °C revealed no significant interchange between bilayers or between layers within a bilayer. Data from the

pdpd structure is shown in figure 2. Likewise,

neutron reflectometry of the pddp sample after annealing at 84 °C revealed no interchange,

though the bilayer spacing did decrease slightly,

suggesting a relaxation of the structure had

occurred. X-ray reflectivity curves of all the

as-deposited sample types exhibited a broad first order Bragg peak resulting from the spacing

between backbones, as shown in figure 3. This

peak's intensity was significantly weakened by

annealing at either 70 or 84 °C, suggesting that a rearrangement of the backbones within the

bilayer had occurred. Development of a detailed

microstructural model which is consistent with

the entire x-ray and neutron reflectivity curves before and after annealing will allow us to describe this rearrangement quantitatively.

92
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

10- 1

10"'

•

10''

Itf'

10"*

•

10"'

10"' -^^- 1 . . . 1 n ... -1 ». . . 1 qP*^, . 0.12 k(A')

Figure 2. Neutron reflectivities of the 'pdpd' multilayer before annealing (o) and after (•) annealing 4 hr at 70 °C.

10'

10"

10

10'

10'

10'

10'

10' 0.08 0.12

k(A"')

Figure 3. X-ray reflectivities of the 'pdpd' multilayer before annealing (o) and after (•) annealing 4 hr at 70 °C.

Polyglutamates provided by K. Mathauer and

Prof. G. Wegner, MPIP. Dr. W. Schrepp of

BASF AG assisted in multilayer preparation.

Research support provided by the Bundesministerium fur Forschung und Technologie

(BMFT) and the University of Akron.

References

[1]

... 1 ... I 1

[2]

[3]

W. Hickel, G. Duda, M. Jurich, T. Krohl, K. Rochford, G. Stegeman, J. D. Swalen, G. Wegner

and W. Knoll, Langemuir 6, 1403 (1990); G. Duda,

A. J. Schouten, T. Amdt, G. Lieser, G. P. Schmidt, C. Bubeck and G. Wegner, Thin Solid Films 159, 221 (1988). M. Foster, H. Knobloch and W. Knoll, unpublished

results. A. Schmidt, G. Reiter, W. Knoll and M. Stamm,

unpublished.

NEUTRON REFLECTIVITY STUDIES OF

MITOCHONDRIAL OUTER MEMBRANES

S. Krueger, J. F. Ankner, S. K. Satija and C. F. Majkrzak

and

D. Gurley and M. Colombini (University of Maryland, College Park, MD)

Biological membranes are assemblies of

lipid and protein molecules arranged as a continuous bilayer 4 to 5 nm thick [1]. Lipid

molecules have hydrophobic (nonpolar) tails and

a hydrophilic (polar) head group. When

surrounded by water, the lipid molecules can form a bilayer, with the hydrophilic head groups

facing outward toward the water and the hydrophobic head groups sandwiched between them.

Thus, the membrane forms a 2-d solvent for

membrane proteins, which are not water-soluble.

A schematic representation of a section of cell membrane is shown in figure 1

Some membrane proteins form hydrophilic

transmembrane channels which allow molecules

of the appropriate size and charge to pass

through. Some of these channel proteins

respond to an electrical potential, changing their

conformation to allow the channel to open or

close. In mitochondria, the outer membrane

contains proteins that form large-diameter

charmels [2], VDAC (voltage-dependent anionselective channels). These channels provide an opening for small molecules necessary for

mitochondrial respiration and metabolism to pass

through the outer membrane. At low potentials,

the channel is in an "open" state. However,

93
CHEMICAL PHYSICS AND CRYSTALLOGRAPHY

when a potential is applied across the membrane,

the channels close. The neutron reflectivity technique [3] is being used to study the VDAC channels in the outer membrane of Neurospora crassa mitochondria. A single bilayer membrane will be

measured in aqueous solvent at low potentials

and at higher potentials. Changes in the neutron

scattering length density profile perpendicular to the bilayer surface will be examined as a function of channel opening and closing. Since

the lipid and the protein components of the

membrane have different neutron scattering

lengths, the contribution from one component

can be enhanced by using a solvent containing an

protein molecule

Figure 1. Schematic representation of a cell membrane

(taken from ref. 1).

appropriate mixture of H2O/D2O. Experiments

performed by two of the authors (D. Gurley and

M. Colombini) indicate that the VDAC channels

are functional in D2O solvent.

It is important to make a sample consisting

of a single, uniform lipid bilayer membrane. If multiple layers and/or nonuniform layers exist, a large percentage of the channel proteins may

not be able to interact freely with the solvent.

Therefore, changes in the membrane due to channel function may not occur in all layers,

making them difficult to detect. Since the

mitochondrial outer membranes must be isolated

from N. crassa, they are only available in very

small quantities. Therefore, a more readilyavailable soybean phospholipid (asolectin) is being used in initial experiments.

Control samples consisting of asolectin

bilayers without the VDAC channel protein are being prepared on single-crystal silicon

substrates containing a thin (-1 nm) oxide layer

using a three-step technique. First, a monolayer

is formed on the substrate by dipping it into a water trough with a monolayer spread on the

water surface. A second monolayer is spread

across the surface of water in a shallow well on

a substrate backing. The substrate is then

sandwiched on top of the backing to form the

bilayer.

Neutron reflectivity profiles from the

control samples will be fit to models for 4 to 5 nm uniform bilayers in order to check the

integrity of the sample preparation method. If the asolectin samples prove to be uniform

bilayers, mitochondrial outer membrane bilayers can be prepared in the same manner. The

neutron reflectivity profile will then be measured

for the outer membrane sample in an electro- chemical cell as a function of applied voltage

across the bilayer membrane.

References

[1] B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts,

and J. D. Watson, Molecular Biology of the Cell . New York: Garland Publishing, Inc. (1983).

[2] M. Colombini, Voltage Gating in the Mitochondrial Channel, VDAC, J. Membrane Biology, UA, 103- 111 (1989).

[3] C. F. Majkrzak and G. P. Felcher, Neutron

Scattering Studies of Surfaces and Interfaces, Materials Research Society Bulletin XV, 65 . (November, 1990).

94
C. MATERIALS SCIENCE

While much of the work performed at the

NBSR could have been identified as Materials

Science, we have chosen to include primarily

those activities which are more traditionally

included in this area in MSEL, such as studies of

polymers and ceramics, residual stress, etc. A

non-traditional area—neutron autoradiography of

paintings—is also included here, reflecting the

origins of the technique, as well as the breadth

of the activities under way. Not all of the work

performed is reflected in this report; for

example, a SANS study of creep cavitation is not

reported, even though it was almost completed

by the time of the report. In assessing this program, it should be noted that the new 30-m

SANS was just coming on line at the end of the

period, and that the results of studies requiring

the new capability will be more fully reflected in future reports.

SANS studies of polymers have always

been a large part of the NIST program,

reflecting the close interaction between the

Polymer Division and the Reactor Radiation

Division. Once again, this program has been

highly productive, and is clearly growing now

that the 8-m SANS is devoted to Polymer

Division work for 75% of available time. In

addition, the burgeoning field of neutron

reflectivity studies as a probe of surfaces and

interfaces has a special role to play in polymer

science, and that is reflected in this year's

accomplishments.

Another area that is receiving renewed

attention and focus is non-destructive residual

stress and texture measurements, where

additional staff and new instrument developments

are aimed at an enhanced capability in this area. Efforts in this area include a program to study

residual stress in the context of fatigue failure in axles, and a new program—under ONR

sponsorship—to characterize residual stresses in weldments. Results in the new area are now

being obtained, and will be reported next year.

SANS FROM PSD/PVME/PSH TERNARY POLYMER BLENDS

B. Hammouda

and

R. Briber and B. Bauer (Polymers Division)

When used along with the Random Phase

Approximation [1] (RPA), the Small-Angle

Neutron Scattering (SANS) technique has proven

to be most valuable for the determination of the

Flory interaction parameter (X) in binary

polymer blends. Phase decomposition diagrams

have been mapped out for a number of binary

mixtures, such as the deuterated polystyrene

(PSD)/poly(vinylmethyl ether) (PVME) system

[2], through a determination of the spinodal line. Temperature and composition dependences of

^PSD/PVME ^^^® ^^^^ ^^^ measured [2]. Following the recent extension of the RPA

formalism to multicomponent systems [3,4], we

have taken SANS data from a ternary polymer

mixture of PSD, PVME and PSH ("hydrogenated" polystyrene). This system involves

three binary-interaction X parameters

(^PSD/PVME' ^PSH/PVME ^"^ ^PSD/PSh)' ^^^ ^^ which are fairly well known (Xpsj^/pvME'

^PSD/PSh)- '^^ ^^^'^ ^^^' -^PSH/PVME' ^^ needed for the interpretation of light scattering

data, whereby until now, one had to either

approximate Xps^/pyME ^y Xps^/pyME ^^

introduce uncertainties (20 °C shifts of the cloud

point for deuterated and nondeuterated

PS/PVME blends have been observed) or be

compelled to use deuterated polystyrene even for

photon scattering!.

For an incompressible ternary mixture of

homopolymers (A, B, C), the RPA gives for the

partial structure factors:

95
MATERIALS SCIENCE

Saa(Q) = Saa°(1 + VbbSbb°)/A

Sab(Q) = - Saa°V^Sbb«/A

where A =

(1 + VaaSaa°)(1 + Vb3Sbb°)-Vab'Saa«Sbb°

and S^°(Q) = N^(t)AVAPA(Q) is a bare

structure factor and N^, (|)^, v^, and Py^(Q) are

the degree of polymerization, volume fraction,

monomeric volume, and Debye function,

respectively. The third component enters only

through:

V^(Q) = l/Scc°(Q) - 2 X^c/Vo

Vab(Q) =

l/Scc°(Q) + X^/v, - X^c/Vo - Xbc/VoThe neutron scattering intensity is given by:

KQ) = (^a/va - ^c/vc)'Saa(Q) +

(^b/vb - ^c/vc)^Sbb(Q) +

2(^a/va - ^c/vc)(^b/vb - ^c/vc)Sab(Q)

where the £'s are the monomeric scattering

lengths for the different components.

A set of three PSD/PVME/PSH samples were prepared with weight average molecular

weights of: 1.95 x 10^, 1.59 x l(r, and 1.90 x

10^ g/mole, respectively with polymer compositions for which the PVME fraction was kept

constant at 50% by weight. It is interesting to see that the reduced SANS data do not follow

the intuitive trend that one expects based on

previous binary blends experiment; i.e., the

scattered intensity at low Q does not necessarily

increase with temperature. In fact, the low Q

intensity can even decrease with temperature

(fig. 1) at first before increasing close to the

spinodal temperature. This can be understood in

the framework of the three component RPA as an interplay of the various terms in the scattering

function.

Results of the fits showed that Xps^/pyME

has the following temperature dependence:

XpsH/PVME/Vo = 10.4x10-4 -0.43/T

(T is in °K). The obtained Xp^pj/pyj^g shows

that the spinodal temperature for the PSH/PMVE

system is 20 °C lower than that of PSD/PVME

in agreement with cloud point measurements.

Figure 2 shows that the temperature variations of

XpsD/PVME ^^ XpsH/PVME ^^^ linear and

parallel to each other which means that deuteration brings about a uniform shift in

temperature of about 20 °C.

3.5 10^

O

II O

3.0 10' -

2.5 10^ H

2.0 10^

1.5 10^ ^

1.0 10^

5.0 10^ h

0.0 10°

Sample

Sample

Sample 3^

A aa

320 340 360 380 420 440 T (K)

Figure 1. Low Q intensity I(Q = 0.085 nm'^) for samples

1, 2 and 3 with increasing temperature.

5.0 10'

0.0 lo" p?w.

•5 .

-5.0 10"'

>° 1.0 lO"* X -1.5 10"*

-2.0 10'*

-2.5 10'*

^^

•v*t.

"^'rs a^ v^ PSH/PVME Samples 2 and 3

Sample 1

.^^a.

PSO/PVME^*^ \^

3.0 10 n J L. J L.

1/T (K)

Rgure 2. Variations of Xpgp/pyj^g (obtained from

sample 1) and Xpgj^/pyj^g (obtained from samples 2 and

3)

References

[1] P. G. de Gennes, J. Chem. Phys. 72, 4756 (1980).

[2] C. C. Han, B. Bauer, J. C. Clark, Y. Muroga, Y.

Matsushita, M. Okada, Q. Tran-Cong, and T. Chang, Poly. 29, 2002 (1988).

96
MATERIALS SCIENCE

[3] A. Z. Akcasu, and M. Tombakoglu, Macromol. 23,

607 (1990).

[4] H. Benoit, M. Benmouna, and W. L. Wu,

Macromol. 23, 1511 (1990).

>

SMALL ANGLE NEUTRON SCATTERING FROM DEUTERATED POLYSTYRENE

IN DIOCTYL PHTHALATE SOLUTION UNDER SHEAR

B. Hammouda

and

A. I. Nakatani, D. Waldow, and C. C. Han (Polymers Division)

High molecular weight polystyrene is known to incur a shear induced phase separation

in dilute solution [1], Small angle neutron

scattering (SANS) has been applied in order to investigate the thermodynamics of high

molecular weight (1.95 x 10^ g/mole) deuterated

polystyrene (PSD) in dioctyl phthalate (DOP)

solution at around 3% concentration and under

steady shear. SANS measurements were made

on the new NIST/EXXON/UofMN 30 m instrument at many shear rates ranging from the

quiescent state (0 s"^) to 1100 s"^ SANS data were corrected for empty cell and beam blocked

background scattering. Scattering from pure

DOP solvent was also rescaled and subtracted.

As evidenced from isointensity contour maps

(fig. 1) and from sector averaging of the data in horizontal and vertical directions, the scattering was isotropic (even for the highest shear rates) so that circular averaging was justified. Figure

2 shows the corrected data for a few runs. The Random Phase Approximation (RPA)

was used to describe [2] the corrected and

reduced SANS data. Fits were made to the

form:

(bp/Vp-b,/v,)2/I(Q) =

l/NpVp<|>pD(Q) + l/v,4>, - 2Xp,/v,

where the usual notation has been used for the

polymer (solvent) scattering length bp (b^),

monomer volume v (v^), volume fraction 4>

(4>3), degree of polymerization N , interaction

parameter X and swollen Debye function

D(Q):

D(Q) = 2 [exp(-U)-l+U]/u2;

U = QVn^"/(2u-I-1)(2u+2)

Q(X)

Figure 1. Isointensity contour maps of the SANS intensity for a shear rate of 323.32s" .

and u is the excluded volume parameter

(u = 0.6 was used here). Results of the fits are reasonable and the extracted interaction

parameter X was found to increase with shear

rate as expected (fig. 3). Beyond a predicted

spinodal value X ^^, the RPA model breaks

down and the extracted X parameters are not to be trusted any longer. It is interesting to note

that 1(0) and X show a steep variation beyond

a "critical" shear rate (around S^ = 57 s'^) which is still below the shear rate corresponding

to the "apparent" spinodal point (S^ = 280 s'^). This use of the RPA is equivalent to assuming that shear affects the monomer/solvent

interation (X) parameter only. This may be

97
MATERIALS SCIENCE

1.0 10' >

t 8.0,0 Z

lU

I— 6.0 10'

,3 .

,3 -

—t

. * DEUTERATED POLYSTYHENE IN OOP

+ Shear Rates

« + + 1100 i>

« • + D 415 S-'

. ^ o 323 »•'

« -t X 182 s'

• * * 57 »"'

+

»ittrm*rt«**.^.^.„_

.02 .04 .. .06 .08 .1 .12

-• 1\

.14 .16 .18 Q (nm" ')

Figure 2. Corrected and reduced SANS data from PSD in DOP solution for some of the measured shear rates. The

intensity is in an arbitrary scale.

unrealistic and effects may exist on

conformations (entropic term in the FloryHuggins equation) through hydrodynamic

interactions. Based on common sense, one would expect shearing (i.e., a form of shaking)

to induce remixing (as observed for polymer

blends). High molecular weight polystyrene

solutions, however, points to the contrary.

showing shear-induced phase separation which

has to be driven by either a reduction of the

entropic term or an increase of the enthalpic

term in the Flory-Huggins equation. Different

approaches based on solutions of the hydrodynamic equations with a shear term included

would be a more appropriate description.

1.2 10 >

X

9.0 10' 3.5 Log{Shear Rate)

Figure 3. Variation of the extracted polymer-solvent

interaction parameter X^^ with shear rate. The horizontal

line corresponds to the predicted "apparent" spinodal line.

Referraices

[1] G. VerStrate and W. Philippoff, J. Polym. Sci., Polym. Lett. Ed. 12, 267 (1974)

[2] P.G. de Gennes, J. Physique 31, 235 (1970)

STRUCTURE FACTOR FOR "DENDRIMER" POLYMER GELS

B. Hammouda

Advances in polymer synthesis are making

possible the polymerization of complicated chain

architectures. Synthesis of highly regular structures such as "stars", "combs",

"dendrimers", etc have now been achieved.

"Dendrimers" [1 ,2] are regular polymer gels that grow through multifunctional polymerization

starting from a central core and branching

outward with a multiplication of the number of

monomeric blocks from one generation to another. Advances in the interpretation of smallangle neutron scattering data, based on the

random phase approximation model, on the other

hand, have made possible the characterization of

rather complicated polymer systems provided

that the "bare" (single-chain) structure factors

are available. In anticipation of the performance

of SANS measurements on dendrimer gels in solution or mixed with other compatible

polymers, the single-chain structure factor for

such a gel has recently been calculated.

The calculation of the various terms

involved are straightforward. There are four

main contributions to the correlations: (1) one

intra-branch self-correlations part, S^f,*, (2) one

intra-branch cross-correlations part between

blocks that originate from the same stem, Sg^^,

(3) one intra-branch cross-correlations part

between blocks that originate from different

stems, Sgt,^, and (4) one inter-branch correlations

part Sjb- The results are:

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MATERIALS SCIENCE

Ssb'(Q) = [(^-l)/(f-l)] P(a,n).

S^bto = 2 {[F(a,n)]2/[fexp(-an)-l]}

{^exp(anN)[exp(anN)exp(an)]/[exp(an)-l]

-f(f^-l)/(f-l)}

Ssb'(Q) = 2 [F(a,n)]2 {(f-l)/[fexp(-2an)-l]}

{A(Q)+B(Q)}

A(Q) = [f2Nexp(-2anN)

-f^exp(-2an)]/[f^exp(-2an)-l]-(f^-f)/(f-l)

B(Q) = 2{f^exp(-anN)[f^exp(-anN)

- fexp(-an)]/[fexp(-an)-l]

- fexp(-an)[f^exp(-2anN)

- f^exp(-2an)]/[f^exp(-2an)-l]

- f^exp(-anN)[exp(anN)-exp(an)]/

[exp(an)-l]+fexp(-an)(f^-f)/(f-l)}/

[fexp(-an)-l]

Sib(Q) =

[F(a,n)]2[f^exp(-anN)-l]^/[fexp(-an)-l]2.

The total structure factor is the sum of all of

these partial structure factors:

S(Q) = Nb[S,b^(Q) + S,bfe) + S,b^(Q)]

+ Nb(Nb-l)Sib(Q).

In order to normalize S(Q) to unity, one has to divide it by the square of the total number of monomers in the gel: [nNb(f^-l)/(f-l)]^.

The following parameters have been used:

Njj is the number of branches in the dendrimer,

N if the number of generations of monomeric

blocks, f is the multiplication factor between two

subsequent generations, n is the number of monomers in each block forming Gaussian links

of segment length b. Moreover F(a,n) and

P(a,n) are the form and structure factors

respectively for finite chain portions of length n:

F[a,n] = [l-exp(-an)]/a.

P[a,n] = n+2n2{l-[l-exp(-an)]/na]}/(anr and

a = Q^b^/6 is a dimensionless scattering

variable.

DENDRIMER POLYMER GEL

Figure 1. Schematic 2-d representation of a dendrimer

polymer gel.

0.8 -

-^ 0.6 O

0.4 -

0.2

Figure 2. Variation of the structure factor S(Q) with the dimensionless variable Qb for a dendrimer polymer gel with

Ni, = 3,f = 2, n = l, and with N varying from 10 to 40.

Refer^ices

[1] D. A. Tomalia, H. Baker, J. Dewald, M. Hall, G.

Kallos, S. Martin, J. Roeck, J. Ryder and P. Smith,

Macromol. 19, 2466 (1986)

[2] M. C. Moreno-Bondi, G. Orellana, N. J. Turro and D. A. Tomalia, Macromol. 23, 910 (1990)

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MATERIALS SCIENCE

QUASIELASTIC NEUTRON SCATTERING STUDY OF THE DYNAMICS

OF WATER IN CURING CEMENT

R. A. Livingston (Building Materials Division)

and

D. A. Neumann and J. J. Rush

Concrete is one of the world's most

important materials, yet its fundamental

chemistry is not completely understood. It is known that the cement gains its strength through

the reaction of the starting materials with water. During the curing process, which takes several

months to complete, several hydrated compounds

form and then further react with each other.

These phases have been difficult to measure

because they tend to be poorly crystallized

colloids or fibers. The physical state of water in

the material is a critical factor defining the

progress of the reactions, but in the past this could only be measured indirectly through drying

experiments. The lack of direct information on

the state of the water has led to a number of

conflicting models of the cement microstructure

based on differing concepts of water bound in the crystal structure as hydroxyl groups (0H~)

or interlayer water molecules, free water, and

water adsorbed on surfaces or in various sizes of

pores [1]. To resolve this issue and to get a clearer

picture of the kinetics of the cement reactions,

we have undertaken a time-resolved quasielastic

neutron scattering study of the process. This

method is especially suitable because neutron

scattering is very sensitive to hydrogen and

because energy gained or lost in the scattering

process is a direct measure of the dynamic state of the water molecules.

The preliminary data reported here were

obtained using the BT-2 triple axis spectrometer

with a fixed initial energy of 4.85 meV and

collimations of 60 '-40 '-40 '-80' before and after the pyrolytic graphite monochromator and

analyzer crystals, respectively. This configuration yields an energy resolution of 0.22 meV

at the elastic position. The measured background was subtracted and a correction was

applied to account for the changing resolution

volume of the secondary spectrometer.

Figure 1 shows two quasielastic spectra for

cement. The closed circles are data obtained

immediately after mixing the cement, whereas

the open circles are data taken 16 days later. These spectra were fit to a 6 -function plus a

Lx)rentzian, convoluted with a Gaussian

resolution function. The Lorentzian component

of these fits is displayed as a solid line for data

taken immediately after mixing the cement and

as a dashed line for the data taken 16 days later. Two effects are readily apparent. The first is that the integrated intensity of the Lorentzian

with respect to the elastic component has

decreased markedly with time indicating that much of the water has become immobile on the

time scale of these measurements. In addition,

the width of the Lorentzian has decreased by

about a factor of 2, leading to the conclusion that the portion of the water which is still mobile, is not moving as rapidly as it was originally. In order to gain information on the type of

motion involved, measurements of the Qdependence of the width of the Lorentzian

component immediately after mixing have been

made (fig. 2). Here the solid line is a fit to a parabola which is a low Q approximation to the

scattering function expected for translational

diffusion. This procedure yields a diffusion constant of (3. 1 + 0.5) x 10"^m^/s. This is very

similar to that obtained by Teixeira et al. [2] for

bulk water. Thus these preliminary results show

that when originally mixed, the water in cement

behaves essentially like bulk water. However,

the amount of mobile water decreases with time

and the speed with which it diffuses also

decreases.

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MATERIALS SCIENCE

-2 2 Energy (meV)

Figuie 1. Quasielastic neutron scattering spectra for cement

at Q=2.25 A" . The closed circles are data taken immediately upon mixing the cement, while the open circles represent data taken 16 days later. The solid and dashed

line represent the fitted Lorentzian component of this data.

Quasielastic neutron scattering thus gives a

direct characterization of the state of the water in cement during the curing process. This provides

a powerful analytical tool that can be used to study many aspects of cement chemistry. These

include the effects of environmental conditions

such as temperature on the ultimate strength of

the material, a critical factor in the safe

construction of structures. Also, the effects of

additives such as fly ash or silica fume on the

cement curing process can be studied. This kind

of knowledge will be directly applicable to the

restoration of the national infrastructure.

l.J

1 1 1 1 r

_

1.0 - y^ -

0.5 y*

- J D=(3.1±0.5)xlO-^mVs -

0.0 / 1 1 1 1 1

Figure 2. The width of the quasielastic component as a

function of Q obtained immediately after mixing. This

data is indicative of translational diffusion and is consistent with the results obtained for bulk water [1].

Reference

[1] H. F. W. Taylor, Cement Chemistry. Academic

Press, San Diego, pp 252-269 (1990).

[2] J. Tebceira, M.-C. Bellissent-Funel, S. H. Chen, and

A. J. Dianoux, Phys Rev. A 31, 1913 (1985).

SMALL ANGLE NEUTRON SCATTERING STUDY

OF THE MISCIBILITY OF SBR/PB BLENDS

S. Sakurai, H. Hasegawa, and T. Hashimoto (Kyoto University, Kyoto, Japan)

and

C. C. Han (Polymers Division)

Miscibility of different kinds of rubber is an interesting and important problem for rubber

industry. Miscible blends are favored in some

cases, but phase-separated blends with wellcontrolled domain size which could improve

physical properties are favored in other cases. The progress of technology made it possible to synthesize tailor-made polymers with a pre- determined molecular weight, microstructures

and copolymer composition. If one can predict

the miscibility of the blend with given molecular

weights, microstructure and copolymer

compositions of the constituent polymers, it will

be a great advance in designing rubber/rubber

blends with the desired properties.

Because of the plural microstructures

formed in the polymerization reaction, most of

the synthetic polydienes are copolymers made of

only one kind of monomer. Therefore, the

blends of polydienes should be considered as blends of copolymers. In our previous paper

[1], we have reported the small-angle neutron

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MATERIALS SCIENCE

scattering (SANS) analysis to determine the

interaction parameters between various comonomers and to predict phase behavior for

binary copolymer blends. We have also shown

that the theory of random copolymer mixtures

developed by ten Brinke et al. [2] works well for

the blend systems of protonated polybutadiene

(HPB) and perdeuterated polybutadiene (DPB),

and protonated polyisoprene and DPB with

various microstructure, and predicts successfully

the miscibility of the binary blends of polydienes

with different microstructure.

In this study [3] the miscibility of binary

blends of deuterated polybutadiene (DPB) and

poly(styrene-r-butadiene) (SBR) was measured

by the small angle neutron scattering experiment

and the interaction parameter, Xbiend' ^^^

obtained. Using the copolymer theory, the

fundamental segmental interaction parameters

between the 1 ,2-butadiene (v) and the 1,4-

butadiene (B), XvB' the 1 ,2-butadiene (v) and the

styrene (s), Xvs' ^^id the 1 ,4-butadiene (B) and

the styrene (S), Xbs ^^''^ determined from the

measured Xbiend'sUsing these interaction parameters, Xvs' Xbs ^^^ XvB' ^® miscibility of SBR/PB blends

as a function of random copolymer composition,

microstructure composition, blends composition,

and molecular weights can be predicted. The

predictions of the miscibility for several SBR/PB

blends agreed very well with their phase diagram

obtained by light scattering experiments.

References

[1] S. Sakurai, H. Hasegawa, T. Hashimoto, G. Hargis,

I., S. L. Aggarwal, and C. C. Han, Macromol. 23,

451, (1990): H. Hasegawa, S. Sakurai, M.

Takenaka, T. Hashimoto, and C. C. Han, ibid. (1991) in press. S. Sakurai, H. Jinnai, H. Hasegawa, T. Hashimoto, and C.C. Han, ibid. (1991) in press.

[2] G. ten Brinke, F. E. Karasz, and W. J. MacKnight,

Macromol. 16, 1827 (1983).

[3] S. Sakurai, T. Igumitani, H. Hasegawa, T. Hashimoto, and C. C. Han, Macromol. (in press).

THERMODYNAMICS OF HYDROCARBON POLYMER BLENDS BY SANS

N. P. Balsara, D. J. Lohse and L. J. Fetters (Exxon Research and Engineering Company, Annandale, NJ)

R. Krishnamoorti and W. W. Graessley (Princeton University, Princeton, NJ)

and

C. Han (Polymers Division)

The goal of this effort is to understand the

interaction between chemically distinct, saturated

hydrocarbon polymers. In this report, we focus on the interactions between tetramethylene (TM)

and ethylethylene (EE) units. We study these

interactions by conducting small angle neutron

scattering experiments on blends of random

copolymers with varying amounts of EE units. The polymer studies are referred to as hE,(97), hEb(97), hE(88), hE(78), dE^(97),

dEb(97), dE(88) and dE(78). The number in brackets refers to the mole % of EE units in each polymer. The prefix h implies that the

polymer is fully hydrogenated while the prefix d

implies the it is partially deuterated. The weight

average degree of polymerization, N, based on

a C4 monomer unit, for each the polymers are given below. All polymers are nearly

monodisperse with measured polydispersities of

less than 1.06.

Polymer N

hE^(97), dE^(97) 1643

hEb(97), dEb(97) 857

hE(88), dE(88) 1625

hE(78), dE(78) 1304

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MATERIALS SCIENCE

Blends of these materials were studied by

light as well as neutron scattering. The

scattering contrast between the polymer species

in the blends derives primarily from differences

in their deuterium contents. The critical point

(single phase to two phase transition) for blends

of hE(78)/dE(88) (volume fraction, v, ofhE(78) was 0.5) was determined on a home-built lightscattering instrument that detects changes in total scattered intensity in the angular range between

4° and 22°. The sample was heated well above

its estimated critical point for several days and

then cooled in steps ranging between 5 °C and

50 °C. The onset of a phase transition was

inferred from an increase in the measured

scattered intensity with decreasing temperature.

Data obtained from the hE(78)/dE(88) is given in figure 1. Based on these measurements, the

critical temperature, T^., for this mixture was

found to be 133 + 3 °C.

(0o>

u

0) o

40 60 80 t(min)

120 140

Figure 1. Light scattering results for hE(78)/dE(88) mixture

(volume fraction of hE = 0.5) as it is quenched. The

initial and fmal temperatures, respectively, for each data set are given below: 180 °C to 145 °C • 145 °C to 135 °C

• 135 °C to 130 °C.

The temperature dependence of the FloryHuggins X parameter, as measured by small

angle neutron scattering for mixtures of 97% EE

and 88% EE units (E(97) and E(88)) is given in figure 3. According to the Flory-Huggins

theory, the x parameter represents the enthalpic

interactions between monomer units and thus

should be independent of molecular weight of the

polymers and blend composition. The two

blends considered here closely follow this

stipulation. Furthermore, the critical point

estimated from the neutron scattering

experiments on E(78)/E(88) mixtures are in agreement with the light scattering data.

.

' ' ' r^ ..,..

:

nnrrT

1

- t ^ . II • o ^'

-

( 1 t -

:i 1 j

• . • 1 , , 1 . , , 1 , 1 1 1 1 1 1

2.0 10'

2.2 10-^ 2.4 lo' X6 I0-' 2.8 lo' 30 lo' 3.2 lo' 3.4 lo' 1/T (1/K) Figure 2. % versus 1/T for E(97)/E(88) blends. ° hEjj(97)/dE(88) (4)=0.50). « hE(88)/dE^(97) (<|)=0.50). D

hE^(97)/dE(88) (<|) = 0.05). • hE^(97)/dE(88) (<1) = 0.75).

(<t) is the volume fraction of the hydrogenerated polymer).

1.0 10"

1.1 10'

1.6 10'

1.4 10" X

1.2 10

1.0 10"

«.0 10'

6.0 10'

4.0 10'

2.2 10"' 2.4 10"' 2.6 10"^

1/T (1/K)

Figure 3. % versus 1/T for E(97)/E(88) blends. D

hE(78)/dE(88) (<J)=0.50). « hE(88)/dE(78) ((j>=0.25). <> hE(88)/dE(78) ((j)=0.75). ((j) is the volume fraction of the hydrogenerated polymer).

We have demonstrated that small angle

neutron scattering can be used to determine the

interactions between hydrocarbon polymer

blends. Using this data, we can predict the

thermodynamic properties of these blends . Since

the measured x parameter was independent of

molecular weight and composition we can

generate phase diagrams for such TM-EE

copolymer mixtures. One such phase diagram

for a E(97)/E(88) blend with N = 1625

(molecular weights - 90,000) is shown in figure

4.

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MATERIALS SCIENCE

Temp

(°C)

100 r-r

80

60

40 -

20

-20

—'—.--t r

-' ' . , . . 1 [ r-i -r-7~T—r—rr One Phase :

: Binodal Spinodal

N. /

/ V\ ' / / Two phases \ \

'

. /. . 1 •'. 1 1 1 r 1 r 1 '

1 . A 1

Figure 4. Predicted phase diagram for E(97)/E(88) blend

(molecular weight of the two polymers — 90,000). (<}) is the volume fraction of E(97).

0.0 0.2 0.4 0.6 0.8 1.0

9i

PHASE SEPARATION IN DEUTERATED POLY(CARBONATE)/

PROTONATED POLY(METHYLMETHACRYLATE) BLENDS

NEAR GLASS TRANSITION TEMPERATURE

M. Motowoka and T. Hashimoto (Kyoto University, Kyoto, Japan)

and

C. C. Han (Polymers Division)

Poly(carbonate) (PC) has been used

extensively in the area of high performance

engineering blends and alloys. Because of the

relatively high glass transition temperature of

PC, (Tg - 150 °C) the available temperature

range where experiments are normally performed

to study the phase behavior (above Tg) of a polymer blend is limited. There has been

controversial reports about the misibility of

poly(carbonate) with other polymers especially

with poly(methylmethacrylate).

In this study, deuterated poly(carbonate)

(dPC) with Mw = 28 X 10^ and poly(methylmethacrylate) (PMMA) with Mw = 33 x 10 ^

are used. Samples with compositions of

dPC/PMMA at 50/50, 40/60, and 30/70 have

been prepared by fast solvent casting of thin

films. This procedure allows us to freeze in the

miscible distribution of the binary polymer

mixtures into its glassy states. The time

resolved small angle neutron scattering

experiments are then performed after the

temperature of a specimen is jumped from room

temperature which is below its Tg to various

temperatures above its Tg. In figure 1, the time

evolution of scattering intensities, S(q,t), of a 50/50 dPC/PMMA blend are displayed following

J o

• ••

PC/PMM* > so/M wm/wm

T Jump Irom 7(rC to 110*C

137.5 nUn

128.0

114.S

103.0

SI .6 80.0

68.5' 67.0

45.5

34.0

22.5

11.0

0.0 (70^;)

:f!

llf«*ff«f«.a

° ,0 20 30 40 soiior*

Hi a'

Figure 1. The time evolution of scattering intensities,

S(q,t), in the early stage of spinodal decomposition of a 50/50 dPC/PMMA blend following a temperature jump

from 70 °C to 110 °C.

a temperature jump fi-om 70 °C to 1 10 °C. It is clear that spinodal decomposition kinetics at the

very early stage can be studied. By using the

Cahn-Hilliard-Cook theory and following the

procedure [1] we have developed earlier, the

"virtual structure factor", S(q,<»), which is an

analytical extrapolation of the static structure

factor into the unstable two phase region, can be

obtained in this experiment. In figure 2 the

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MATERIALS SCIENCE

virtual structure factor, S(q,<»), for the

dPC/PMMA 50/50 blend at 110 °C obtained

from the kinetics experiment is displayed as a function of q. The Ornstein-Zernike plot of this virtual structure factor which is S'^(q,oo) vs. q^

is shown in figure 3. The q^. which is the

wavelength separating concentration fluctuation

growth from decay in this spinodal

decomposition kinetics is also indicated in the

figure. The negative intercept which is related

to Xs " X with X as the binary interaction

parameter for the blends and Xs ^s the x at spinodal point provides a measure of the x

parameter at 110 °C which is in the phase

separated region and not accessible by any

conventional measurement we know of. In this study, we were able 1) to study the

very early stage of spinodal decomposition

starting from the dimension of the molecular size

of a single polymer chain; 2) to confirm the

Cahn-Hilliard-Cook theory in quantitative

details; 3) to study the influence of the

magnitude of the concentration fluctuation on the

spinodal decomposition kinetics; 4) to obtain

binary interaction parameters as a function of

temperature in the unstable two phase region and

then to calculate the free energy of mixing; 5)

and to study the detailed spinodal decomposition

kinetics in the intermediate and late stages which

will be discussed separately.

lomo"

q* /cm'

Figure 3. The Omstein-Zemike plot of the above virtual structure factor is displayed. The critical wavelength, q^,

which separate the concentration fluctuation growth from

decay is indicated.

Reference

[1] T. Sato and C. C. Han, J. Chem. Phys., 88, 2057

(1988).

-I r

°°°ocoooooiooooooooooooocco

Figure 2. The "virtual structure" factor, S(q,«>), for the dPC/PMMA 50/50 blend at 110 "C obtained from the

kinetics experiment is displayed.

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MATERIALS SCIENCE

NEUTRON REFLECTIVITY STUDY OF FINITE THICKNESS EFFECTS

IN THE THERMODYNAMICS OF WEAKLY SEGREGATING DIBLOCK

COPOLYMER THIN FILMS

M. D. Foster (University of Akron, Akron, OH)

M. Sikka, N. Singh and F. S. Bates (University of Minnesota, Minneapolis, MN)

and

S. Satija and C. F. Majkrzak

Diblock copolymers can order and disorder on a microscopic scale with the variation of

temperature or chain length, N. The entropic

and enthalpic effects which control this transition are small in a weakly segregating system and

therefore the presence of a thin film's symmetry

breaking interfaces is expected to have a

significant effect on the transition character and

diblock microstructure. The microstnicture of

thin (1000 A - 4000 A) films of a nearly

symmetric (f = .55) model diblock copolymer,

poly(ethylenepropylene) - poly(ethylethylene)

(PEP-PEE) spun cast on silicon substrates was

studied using the neutron reflectometer on BT-7.

Room temperature samples of several molecular

weights bracketing that at which the bulk

disorders were considered first. Then, the

microstructures of films of a single molecular

weight, but varying thickness were probed in a temperature range about the corresponding bulk

order-disorder transition (ODT) temperature

(125 °C) [1]. The system studied here differed

from the only other thin film diblock system

studied to date [2], poly(styrene)-

poly(methylmediacrylate) (PS-PMMA), in three

ways: PEP-PEE was much less strongly

segregating than PS-PMMA, neither PEP or PEE had specific attractions to the substrate and

the difference between surface energies of the

two blocks in the copolymer was less for

PEP-PEE.

A primary result of the study is the finding

that the PEE block locates preferentially at both

interfaces for all samples types, regardless of

chain length, deuterium labelling and film

thickness. This is in stark contrast to PS-PMMA on silicon in which the composition

at the polymer/substrate interface is almost pure

PMMA, while the composition at the polymer/

air interface is enriched in PS. The reason for

the symmetric structure of the PEP-PEE film is not yet understood.

A second important result of the work is the observation of a change in the temperature

dependence of the microstructure with film

thickness. For a relatively thick film (t ca. 3900 A) of PEP-PEE having chain length, N,

equal to 1026, the microstructure remains

ordered throughout the film above the bulk ODT

temperature of 125 °C. A reasonably abrupt

change in the character of the reflectivity curve

at about 135 °C signals a disordering of the

film's "interior", while the microstructure near

the two interfaces continues to be symmetrically

ordered even to 175 °C as shown in figure 1. The effect of the interfaces is thus not only to induce interfacial ordering, but also to shift the

ODT temperature of the film interior up some

10 °C. This shift is consistent with the notion

that the presence of die interfaces is dampening

composition fluctuations in the diblock causing it to disorder at higher temperature. Reducing the

thickness of the film further to three times the

bulk domain spacing leads to other qualitative

changes in the disordering behavior which are

still under study.

Disordering of the film with decreasing

chain length at constant temperature was also

observed. The range of chain lengths studied

(N = 210 to 1890 ) covered a variety of

microstructures from nearly homogeneous to strongly segregated and demonstrated that surface ordering also persisted when the film

disordering was caused by decreasing N.

Composition profiles for four chain lengths at 23

+ 1 °C are shown in figure 2. Broadening of

the domain interfaces with decreasing N can be

clearly seen. Particularly interesting is the

microstructure of the N = 790 material, which

is known to disorder at 16 °C in the bulk.

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MATERIALS SCIENCE

0.05 I

1 1 1 1 1 1 r- 0.045

0.04

0.035 b/V 0.03 0.025

0.02 0.015

0.01

500 1000 1500 2000 2500 3000 3500 4000 I (A)

MICROSTRUCTURE CHANGE

WITH DECREASING N

Figure 1. Experimental reflectivity curve (o) for thickest diblock film at 175 °C compared with the simulated

reflectivity curve (solid line) for the model structure shown

in the inset.

M — -1890

r-J — 590

-(A)

Figure 2. Variation of the thin film microstructure with decreasing N.

References

[1] F. S. Bates, J. H. Rosedale, and G. H. Fredrickson,

J. Chem. Phys. 92, 6255, (1990).

[2] e.g., S. H. Anastasiadis, T. P. Russell, S. K. Satija, and C. F. Majkrzak, J. Chem. Phys. 92, 5677

(1990).

ORDER/DISORDER IN THIN DIBLOCK COPOLYMER FILMS

A. Menelle, T. P. Russell, and S. H. Anastasiadis (Almaden Research Center)

and

S. K. Satija and C. F. Majkrzak

The preferential interaction of specific

block of symmetric diblock copolymers with the

air and substrate interfaces has been shown to induce an orientation of the lamellar microdomain morphology parallel to the film surface

for temperatures below the order-disorder

transition temperature, Tj^gj. At higher

temperatures, where the copolymer in the bulk

disordered, the preferential interactions induce an oscillatory variation in the composition which

decays with a characteristic length, $, as a

function of distance from the surface. These

results, far from Tf^gj, were in quantitative

agreement with mean field predictions [3]. In

this study the influence of temperature and film

thickness on the ordering symmetric diblock

copolymers was investigated by neutron

reflectivity.

Symmetric diblock copolymers of polystyrene, PS, and perdeuterated polymethylmethacrylate, d-PMMA, denoted P(S-b-d-MMA),

were spin coated onto Si substrates from toluene

107
MATERIALS SCIENCE

solutions. The thickness of the films was varied

by varying the concentration of the copolymer.

The specimens were annealed at the desired

temperature and quenched to room temperature

where the reflectivity measurements were

performed. The experiments were performed on BT-7 at NIST.

Shown in figure 1 are a series of neutron

reflectivity profiles for a 1.6 urn thick film of

P(S-b-d-MMA) as a function of temperature.

The annealing temperature decreases as one

proceeds from the bottom to top profile. It should be noted that experiments were performed

with increasing and decreasing annealing

temperatures to ensure reproductivity of the

results. At high temperatures the reflectivity

profiles is characterized by a weak maximum

occurring near K2 0~0.02 A'^ Very diffuse

interference are seen at higher k^ q. Such

profiles are characteristic of copolymers in the

disordered state where the concentration profiles

from the air and substrate interfaces are given by

an exponentially damped cosine function. As the

temperature is decreased, the first reflection

becomes sharper and more intense and the inter- ferences at higher k^ q become more pronounced. By modeling the reflectivity profiles it has been found that as one approaches Tj^gj, I

is nearly three time larger than that predicted

from mean filed arguments. Also, 1/$^ varies as (/Tj^gj -1/T), so that by extrapolation T^^j for

the films can be determined. Decreasing the

temperature further, marked changes in the

reflectivity profile occur. Not only does the first order reflection appear but second and third

order reflections at higher K^ q become sharp

and distinct. Such profiles are typical for a fully

order P(S-b-d-MMA) morphology where the

lamellar microdomains are oriented parallel to the film surface.

The values of Tj^gj are shown in figure 2

as a function of the film thickness. For thick

specimens Tp^gj approaches the bulk value of

157 °C. However, as the film thickness is decreased, the temperature at which Tj^gj

occurs increases dramatically. In fact, T^gj

increase in nearly a power law manner with

decreasing film thickness. These results clearly

show that the decaying fields from the air and

substrate surfaces constructively interfere with

one another thereby inducing full ordering in the

P(S-b-d-MMA) at temperatures well above the

bulk value of T»MSTloOr0.000 0.015 0.030 0.045 0.060 0.075

Figure 1. Neutron reflectivity as a function of the neutron momentum for a P(S-b-MMA) at different temperatures.

Temperature increases as one proceeds from the uppermost

to lowermost profile. The thickness of this specimen was

1.6 ^m.

220

210

200 ? 190

170

150

10

Thickness (k)

15 20

xlO-*

Figure 2. The variation in the temperature at which the copolymer films are fully ordered as a function of the film thickness.

Referraices

[1] S. H. Anastasiadis, T. P. Russell, S. K. Satija and

C. F. Majkrzak, J. Chem. Phys. 92, 5677 (1990).

[2] S. H. Anastasiadis, T. P. Russell, S. K. Satija and

C. F. Majkrzak, Phys. Rev. Lett. 62, 1852 (1989).

[3] G. H. Frederickson, Macromolecules 20, 2535

(1987).

108
MATERIALS SCIENCE

NEUTRON REFLECTIVITY STUDIES ON

COPOLYMER/HOMOPOLYMER MIXTURES

A. M. Mayes and T. P. Russell (Almaden Research Center, San Jose, CA)

S. K. Satija and C. F. Majkrzak

Thin films of symmetric diblock

copolymers exhibit a near perfect orientation of

the lamellar microdomains parallel to the surface

[1,2,3]. This arises from the preferential

interaction of the blocks with the substrate or air interfaces. The addition of small amounts of

homopolymers, less than approximately 10 wt%,

has been shown not to perturb the orientation of

the lamellar microdomains [4] . Thus examining

mixtures of a homopolymer labelled with

deuterium with an unlabelled copolymer permits

the investigation of the distribution of the

homopolymers in the lamellar copolymer

microdomains. In this work mixtures of

perdeuterated polystyrene, d-PS, or perdeuterated polymethylmethacrylate, d-PMMA, of

varying molecular weights with an unlabelled,

symmetric diblock copolymer of PS and PMMA,

denoted (PS-b-MMA), were investigated by

neutron reflectivity. Specimens were prepared

by spin coating the solution of the mixtures in toluene onto Si substrates. The films were

annealed at 160 °C for at least 120 hr and

quenched to room temperature.

Reflectivity measurements were performed on BT-7 at NIST up to value of the neutron

momentum, k^Q, of 0.07 A'^ Figure 1 shows

results for a mixture of P(S-b-MMA) with 10

wt% deuterated PMMA homopolymer

(M^ -5.7x10"^). The scattering length density

profile shown in the inset was used to fit the

measured reflectivity profile. Here it is clearly seen that the homopolymer segregates strongly to the center of PMMA microdomains. The system was modelled as alternating layers of PS and

PMMA in which the PMMA layers contained some fraction of dPMMA homopolymer

normally distributed within the domain.

The segregation of the PMMA

homopolymer to the center of the domains

indicates that the loss in entropy by confining the

0.080

Figure 1 . Neutron reflectivity as a function of the neutron momentum for a mixture of 10 wt% d-PMMA(M^ = 5.7 X 10'^) with P(S-b-MMA) (M^ = 9.1 x 10"*). The solid

line is the reflectivity profile calculated using the scattering

length density profile shown in the inset.

homopolymer chains to the domain center is outweighed by that required to stretch the

PMMA blocks to accommodate the homopolymer uniformly. In effect, the homopolymer

chains are sandwiched between layers of the

PMMA copolymer blocks. As molecular weight

of the homopolymer decreases, the balance of

the entropic effects allow better mixing within

the domain. Figure 2 shows the reflectivity results for an analogous mixture in which the

dPMMA molecular weight is reduced to 1.2 x

10"^. As is evident from the loss of strong Bragg

reflections and the scattering length density

profile inset, the low molecular weight

homopolymer is better able to distribute within

the domain, decreasing the contrast between the

PS and PMMA layers. It is notable, however,

that some segregation to the domain center is

still apparent. It is also interesting to note that excess homopolymer appears to concentrate at the substrate surface in both systems.

109
MATERIALS SCIENCE

0.000 0.020 0.040 0.060

k,,0 ( i-' )

0.080

Figure 2. Neutron reflectivity as a function of the neutron momentum for a mixture of 10wt% PMMA(M^= 1 .2x10 ) with P(S-b-MMA) (M^=9.1xl0'*). The solid line is the

reflectivity profile calculated using the scattering length

density profile shown in the inset.

Experiments have also been done on

systems with high molecular weight homopolymers (M^~5xl0^) added to the P(S-bMMA). Interestingly, the analysis does not suggest a stronger confinement to the domain

center in comparison with the 5 x 10'^ mixtures

of the same concentration. The results may

indicate that the addition of high molecular

weight homopolymer interferes with the

copolymer ordering.

References

[1] G. Coulon, T. P. Russell, P. F. Green and V. R.

Deline, Macromolecules 22, 258 (1989).

[2] S. H. Anastasiadis, T. P. Russell, S. K. Satija and

C. F. Majkrzak, Phys. Rev. Lett. 62, 1852 (1989).

[3] S. H. Anastasiadis, T. P. Russell, S. K. Satija and

C. F. Majkrzak, J. Chem Phys. 92, 5677 (1990).

[4] T. P. Russell, A. Menelle, G. S. Smith, W. A. Hamilton and S. K. Satija, Macromolecules (in press).

NEUTRON REFLECTIVITY STUDY OF A POLYMER-SOLID INTERFACE

W.-L. Wu (Polymers Division)

and

S. K. Satija and C. F. Majkrzak

Polychlorotrifluoroethylene (PCTFE) is a semicrystalline polymer whose melting point is about 210 °C [1]. Because there are no hydrogen atoms in this polymer, its specific

neutron scattering cross section is greater than

that of a silicon crystal. Calculation indicates

that a critical angle 0^., for the polymer-Si

interface is -0.1° using a 2.35 A neutron

wavelength. This particular polymer has very

interesting properties and has been used as a model for studying amorphous to crystalline

phase transition.

The experiments were performed at the

BT-7 reflectometer at the NIST reactor. A

graphite monochromator was used to select neutrons of a wavelength X = 2.35 A with a width AX = 0.01. slit collimators reduced the

incident beam divergence to 0.016°. The

detector acceptance angle was set at 0.975°.

single-crystal silicon wafers of 10 cm and

0.6 cm thickness were used as substrates. A

1.5 mm thick PCTFE film was placed in contact with each wafer. The wafer and polymer film were then sandwiched between two aluminum

blocks containing heating elements. A vacuum

of 4 X 10"^ torr was maintained throughout the

experiments.

The raw data from a PCTFE-silicon

interface at 240 °C is given in figure 1. The

reflectivity of a free silicon surface (where the

incident medium is air and not Si) is also

presented in the same figure as a baseline. If the

polymer density distribution at its interface with

the silicon is uniform, the two reflectivity curves of figure 2 would coincide with each other since

the critical angles for these two cases are nearly

the same. The difference shown in figure 1 between the two reflectivity curves indicates that the density of polymer near the interface with

the silicon is far from being uniform. This

roughness could be attributed to some microvoids at the interface. Alternatively, it can be

110
MATERIALS SCIENCE

considered likely that the polymer melt is in complete contact with the silicon substance so

that the interface can be expected to be as flat as the silicon-air surface. The difference in

reflectivity in this case might be attributed to a density fluctuation in the polymer melt near the

interface. If one naively applies a Debye-Waller

factor analysis of the diffuse scattering from a

single rough interface [2,3] then the calculated

magnitude of the roughness is about 60 A RMS.

The reflectivity measurement was also conducted

at 255 °C which is 45 °C above the polymer

melting point, and the result was identical to that

at 240 °C suggesting no improvement in the

interface uniformity by raising the temperature.

A series of measurements was carried out

after cooling the sample to temperatures between

255 °C and 160 °C. At any temperature above

200 °C, no time dependence of the reflectivity was observed. In other words, by keeping the

sample at any temperature between 255 °C and

200 °C, the polymer reached its equilibrium

condition readily and the reflected beam intensity

did not change with time. At 195 °C a long

term drift of the intensity of the reflected beam

in both the specular and the off-specular

components was observed. At even lower temperatures, for example, 166 °C the reflectivity

tended to stabilize within a few minutes. In figure 2 the reflectivity results of 236 °C,

202 °C and 166 °C were presented. The ordi- nate was shifted in order to highlight the

difference among these three curves. These

reflectivity results were reduced from the raw

data by subtracting the off-specular component

and then normalizing to the main beam intensities. Consequendy, one can observe some

interesting features in the reflectivity curves; more noticeable are (1) the shoulder at q =

0.02 A"^ in the melt (236 °C), (2) the

sharpening of this shoulder at 202 °C, (3) the

appearance of a maximum near q = 0.O3 A'\

(4) at 166 °C crystallization process is completed

and all the features mentioned above

disappeared.

A quantitative fit of the reflectivity results

to a polymer segment density profile near the

interface is in progress. The results demonstrate

rather conclusively that the density of molten

polymer near a solid surface is not uniform as in small molecule liquids. This nonuniformity

grows as the temperature passes the melting

point then diminishes as the temperature is lowered further.

>>4

"1 I I I 1 1 1 i r

S88S

Ol I I

I I I I I '?»'o«U«.*

0.0 0.1 0.2 0.3 0.4 0.5 Incident Angle (degree)

Figure 1. The raw neutron reflectivity results from the

silicon-air interface (0) and the polymer-silicon interface ()

kpinsi039.log

0.02 0.04 0.06

Figure 2. The corrected neutron reflectivity results from

the polymer-silicon interface at three different temperatures, • - 236 °C, x - 202 °C, a - 166 °C.

Referraices

[1] F. P. Price, J. Am. Chem. Soc. 74 311 (1952).

[2] L. Nevot and P. Croce, Rev. Phys. Appl. 15 761

(1980).

[3] S. K. Sinha, E. B. Sirota, S. Garoff, and H. B.

Stanley, 38 2297 (1988).

Ill
MATERIALS SCIENCE

SEGMENT DENSITY PROFILE OF GRAFTED POLYMER BRUSHES

NEAR THE SOLID-SOLUTION INTERFACE

S. K. Satija, J. F. Ankner, C. F. Majkrzak

T. Mansfield, D. R. Iyengar, G. Beaucage, T. J. McCarthy, and R. S. Stein

(University of Massachusetts, Amherst, MA)

and

R. J. Composto (University of Pennsylvania, Philadelphia, PA)

Recently, considerable attention has been

focused experimentally and theoretically [1-7] on

understanding polymer brushes and their segment

density profiles. Polymer brushes are polymer

chains terminally attached onto a surface. When

the surface coverage is high enough to allow

overlap between chains, then the polymer chains

are considerably stretched away from the

surface. By using scaling and free-energy

balance arguments, Alexander [7] was able to predict the equilibrium configuration of polymer

brushes as a function of grafting density o and

chain length N. However, he assumed a uniform monomer density for the brush up to a distance h, the height of the brush. Later,

Milner et al. [1] using self consistent field theory, obtained a density profile, for large N

and in the highly stretched limit, which decays

parabolically from the surface. Further numerical calculations by other groups [4,5] show that for small values of N, the parabolic profile is considerably rounded at the end near h = h^^.

The neutron reflectivity technique has been

used with great success to study polymer formations at solution-air [11,12] solution-solid [13]

and solid-solid [14] interfaces. Here we report on neutron reflectivity studies of end carboxylated polystyrene adsorbed on silicon surface in cyclohexane as a solvent. The carboxylated end

is known to adhere well onto the silicon surface.

Our results show the polymer segment density

profile away from the silicon surface to be a parabolic density profile predicted for a grafted

polymer brush by Milner et al. [1]. Two cases were studied: one with hydrogenated polystryene (hps) and C6D22 and the other with

deuterated polystyrene (dps) with C^Hj2. The

grafting density o is such that the polymers were

weakly in the brush regime. For a similar

molecular weight of polymer the brush height for

dPS-C6Hi2 is 50% higher than for hPS-C6Di2,

which may be due to different % parameters for

the two cases. For the longer brush, the

segment density profile is a parabolic profile

predicted for a grafted polymer brush. In the

shorter brush, this parabolic profile is considerably rounded near the end. The

experimentally measured and fitted reflectivity

profiles are shown in figures 1 and 2.

0.0 0.4 o.a Q (nm-')

Figure 1. Open circles show reflectivity from pure CgDj2

near Si surface. Closed circles are the reflectivity from

hps-CgDi2 with the inset showing the fitted polymer

segment density profile. The lines drawn are fits to the

data as described in the text.

0.0 0.4 0,8 1.2 Q (nm-')

Figure 2. Open circles show reflectivity from pure CgHj2

near Si surface. Closed circles are the reflectivity from

dps-CgHi2 with the inset showing the fitted polymer

segment density profile. The lines drawn are fits to the

data as described in the text.

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MATERIALS SCIENCE

Referraices

[1] S. T. Milner, T. A. Witten, and M. E. Gates, [7] Macromol. 21, 2160 (1988). [8]

[2] P. G. de Gennes, Macromol. 14, 1637 (1981).

[3] P. Auroy, L. Auvray, and L. Leger, Phys. Rev.

Lett. 66, 719 (1991). [9]

[4] M. Murat and G. S. Grest, Phys. Rev. Lett 63, 1074

(1989) and M. Murat and G. Grest, Macromol. 32, [10] 4054 (1989).

[5] M. Muthukumar and J. S. Ho, Macromol. 22, 965

(1989). [11]

[6] S. T. Milner, T. A. Witten, and M. E. Gates, Europhys. Lett. 5, 413 (1988).

S. Alexander, J. Phys. (Paris) 38, 983 (1977). X. Sun, E. Bouchard, A. Lapp, B. Famoux, M.

Daoud, and G. Jannink, Europhys. Lett. 6, 207

(1988). L. Dai, J. W. White, J. Kerr, R. K. Thomas, J. Penfold, and M. Aldisi, Synth. Met. 28 D69 (1989).

S. K. Satija, G. F. Majkrzak, T. P. Russell, S. K.

Sinha, E. B. Sirota, and G. J. Hughes,

Macromol. 23, 3860-3864 (1990).

S. H. Anastasidis, T. P. Russell, S. K. Satija, and C.

F. Majkrzak, Phys. Rev. Lett. 1852 (1989).

PHASE BEHAVIOR OF AQUEOUS POLY(VINYL-ALCOHOL)

SOLUTION-A SMALL ANGLE NEUTRON SCATTERE^G STUDY

W.-L Wu (Polymers Division)

H. Kurokawa, M. Shibayama, and S. Nomura

(Kyoto Institute of Technology, Kyoto, Japan)

and

L. D. Coyne (Food and Drug Administration, Rockville, MD)

The physical crosslinking of aqueous

poly(vinyl alcohol) (PVA) solution in the

presence of various anions such as borate,

titanate, cupric and antimonate has been studied

extensively [1]. In this work only the PVAborate pair was included. The borate ion can

attach itself to a PVA chain, and this process is called monocomplexation. Polyelectrolyte

effects are expected from the monocomplexation.

Through the formation of di-diols, the borate ion can also act as a reversible crosslink between

two different chains or different parts of the same chain. The above process has been called

di-complexation. In general, the monocomplexation gives rise to repulsive

polymer-polymer interactions while the di- complexation induces attractive interactions.

The concentrations of both the diol and the di- diol are determined by the chemical equilibrium

of all the species involved. A self-consistent

approach based on a Flory-type mean field theory has been developed by L. Leibler et al. [2] for determining these equilibrium constants. The effective excluded volume interactions

in the PVA-borate solution, as discussed in the

foregoing paragraph, is made of three parts; the

usual polymer-solvent interactions, the polyelectrolyte interactions and the attractive

interaction dues to di-complexation. Adding

suitable amounts of borate salt at a given PVA

concentration, one can increase the formation of

didiol complexes leading to a clear-to-opaque

transition. With further addition of the borate

salt, the polyelectrolyte effect becomes dominant

and this leads to a opaque-to- clear transition [3]. The purposes of this work are to determine (1)

the effective excluded volume parameter, and (2)

the corresponding changes in the radius of

gyration across these transitions. Toward these

goals, small angle neutron scattering (SANS)

measurements were conducted in both the clear

and the opaque regions. PVA solutions at two

different polymer concentrations, one below C*

and one above C*, were included. C* denotes

the polymer overlap concentration. For the

samples with concen-trations above C*, gelation

set in before any of the clear-opaque-clear

transitions. By comparing the results between

samples at these two concentrations, one can

determine the effect of gelation on the changes

of both the effective excluded volume parameter

and the radius of gyration across these

transitions. Deuterated and protonated PVA

with a degree of polymerization (DP) of 1 100

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MATERIALS SCIENCE

were used. The overlap concen-tration, C*, was

determined to be about 1.5% wt. based on

viscosity measurements. The value of C*

showed a modest decrease with increasing boric

acid concentration. Two polymer concentrations used in this work were 1.25% wt. and

2.0% wt. and the NaOH content was held at 0.167 M for all the specimens.

The SANS measurements were conducted

at the NIST reactor with the wavelength set at 9 A.

The values of x obtained from SANS are

listed in table 1; for both polymer concentrations

the value of x reached a maximum at an inter- mediate concentration of boric acid. Concurrently, the samples turned opaque. For all the

clear solutions and gels, the value of x was

always below 0.5, whereas for the opaque

samples the corresponding value was greater

than 0.5.

The change in radius of gyration of the 20

git samples are given in figure 1 . The results clearly show that the chains collapse somewhat

in the opaque region which exists between [b] of

0.01 M to 0.04 M. It is noteworthy that the

polymer concentration is above C* and the clear- opaque-clear transition occurs in a gel state. The present results demonstrate that gelation, at least physical gelation, does not inhibit the

collapse of individual chains. The results from

12.5 gli sample is identical to figure 1; the

collapse of chains coincides with the clear- opaque-clear transition. In summary, the SANS result demonstrated

that the complex polymer-solvent interactions

which exist in borate-PVA solution can be

adequately lumped into a single parameter. In

addition, the constraint of gelation did not inhibit

individual chains from an expansion-collapseexpansion transition as the solution underwent a

clear-opaque-clear transition.

Tabk 1. The effective polymer-solvent interaction para- meter from SANS data, c - clear, - opaque, s - solution, G - gel

POLYMER CONCENTRATION

0.01245 .020

0. 0.461 (CS) 0.440 (CG) .00167 0.466 (CS) 0.444 {CG)

.0050 0.480 (CS) 0.456 (CG)

.00833 0.534 (OS)

Boric Acid (M) .0167 0.530 (OS) 0.521 (OG)

.0333 0.468 (CS)

.0417 0.442 (CS) 0.456 (CG)

.05 0.440 (CS) 0.452 (CG)

C = 20.0 g/l

E2

w

c

CD

100

O IVD • lOCMI

1

'qo

o

A H/D > TSIZS

H/D . lOOra rapMt

1

. a

& a 8 Q

D a

8

0.01 0.02 0.03 b (mole/liter)

Figure 1 . The change in the radius of gyration of PVA

chains as they underwent a clear- opaque-clear transition within a gel state. The transition points are at 0.01 M and

0.04 M of boric acid. Results from three sets of data were

presented in this figure.

References

[1] E. Ahad, J. Appl. Polym. Sci. 18, 1587, (1974).

[2] L. Leibler and E. Pezron, Springer Series in Chem.

Phys., 5185, (1989).

[3] H. Kurokawa, M. Shibayama, T. Ishimaru, W.-L. Wu, S. Nomura, Poly, (accepted for publication).

114
MATERIALS SCIENCE

DIFFUSE SCATTERING IN ICOSAHEDRAL PHASE ALLOYS

J. L. Robertson

Diffuse scattering is observed beneath the

groups of strong peaks in the powder diffraction

patterns of all the rapidly quenched icosahedral

phase alloys studies thus far, including those

thought to be free of phason strain (e.g.,

i-AlLiCu and i-AlRuCu). In the case of

i-AlMnSi such diffuse intensity is found at Q" « 2.8 k-\ 3.5 A^ 5.0 k-\ and 5.8 A"^ (fig. le)

which seems to resemble closely the main

features of the amorphous diffraction pattern

[1,2]. The results of recent work centered on

modeling the structure of i-AlMnSi by a pseudo-random packing of icosahedrally

symmetric atomic clusters [3,4] show that these

features are reproduced in the calculated powder

diffraction pattern without including either the

"glue" atoms (atoms that fill the space between

the clusters) or any substitutional disorder within

the atomic clusters. Here the diffuse intensity is clearly visible in the total intensity calculated

from the model (fig. Id) although it does not seem to be in the undecorated S(Q) (fig. 2).

At least for i-AlMnSi, this must result from

weak peaks with large Q-^ that are part of the

background in the undecorated S(Q) [5] but are heavily weighted in the Al-Al partial intensity

(fig. lb). This has been confirmed by a

calculation of the structure factor for a single

MacKay icosahedron (fig. 3) in which strong

broad peaks appear in the Al-Al partial intensity

at Q"- 3.0 A"^ and 5.0 A"^ which are not in the

Mn-Mn and Mn-Al partials. At these Q

positions, however, the strong Al-Al

contributions appear in symmetry directions

where the "Bragg" peaks due to the model

"lattice" are weak (i.e., large Q-^). Figure 4

shows that the AL-Al structure factor for a single cluster peaks in the threefold symmetry

direction (at Q" = 2.85 A"^) while the strong

quasicrystal peaks at this value of Q are in the

twofold and fivefold symmetry directions; thus

the weaker (and therefore broader) peaks in the

threefold directions are more heavily weighted

and appear as "diffuse" scattering in the

measured powder patterns.

30

26

J 10

eo

- a) Mn-Mn

liUJuJUjJL

40

20

b) Al-Al

^

<jj^aJ^^^\jUju

Figure 1. Partial neutron scattering intensity calculated for

(a) Mn-Mn, (b) Al-Al, and (c) Mn-Al correlations; total neutron intensity (d) and experimental neutron intensity data

(e) on i-AlMnSi [6].

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MATERIALS SCIENCE

Figure 2. The calculated S(Q) for the undecorated model.

Figure 4. Al-Al partial intensity distribution for a single

cluster at Q" = 2.85 A' ; half of reciprocal space is projected onto the plane along a twofold symmetry axis.

Current efforts are directed towards

calculating the "single crystal" diffraction

patterns from the (atomic) decorated model to determine the distribution of the "diffuse"

intensity in reciprocal space.

References

[1] J. L. Robertson, S. C. Moss, and K. G. Kreider,

Phys. Rev. Lett. 60, 2062 (1988).

[2] J. M. Dubois and K. Dehghan, J. Non-Cryst. Solids 93, 179 (1987).

[3] J. L. Robertson, Zeit. fur Phys. 1991 (in press).

[4] J. L. Robertson and S. C. Moss, Phys. Rev. Lett. 66, 3 (1991).

[5] Y. Ma, E. A. Stem, X.-O. Li, and Chr. Janot, Phys. Rev. B 40, 8053 (1990); X.-O. Li, E. A. Stem and

Y. M. — Phys. Rev. B (in press).

[6] Neutron powder diffraction data provided by B. Mozer

(NIST).

Figure 3. Partial neutron intensities for a single AlMnSi

atomic cluster, (a) Mn-Mn, (b)Al-Al and (c) Mn-Al.

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MATERIALS SCIENCE

TEXTURES OF TANTALUM METAL SHEETS BY NEUTRON DIFFRACTION

C. S. Choi and J. Orosz (ARDEC, Picatinny Arsenal, NJ)

and

H. J. Prask

The orientation distributions of five

different samples,TaPA, TaGl, TaG2, TaQ2-Sl

and TaQ2-S4, were studied by neutron

diffraction. The TaPA specimen is a commercial Ta sheet with an unknown

fabrication history. The TaGl and TaG2

specimens were fabricated by uniaxial

compression along the cylinder axis direction,

with 90% reduction in two steps at room

temperature. The TaQ2 specimens were made

by the same forging process except the 90%

reduction was achieved in three steps. TaQ2-Sl

is the section closer to the center of the forged

disk and S4 is the adjacent section at the

periphery side. The pole figure data of 200,

211, 220, 310 and 222 were collected at the

BT-6 neutron diffractometer. The ODF (Orientation Distribution Function) analyses were

performed using the WIMV and harmonic

methods of the program POPLA [1] for the

TaPA and the TaG type samples, and using

WIMV for the TaQ2 samples.

The texture of TaPA consisted of three

components: dominant {100}<010> cube

texture and weaker {111}<112> and {111}

< 1 10> texture. Since [1 12] is the TD (Trans- verse Direction) component of (lll)[ll0] and

[lIO] is the TD component of (111)[112], it is conceivable that the TaPA sample was fabricated by cross rolling. The cube texture found

in the sample is considered to be a serious

disadvantage for the deep-drawing application,

because it produces extremely anisotropic

physical properties. The cube texture has been

observed in many FCC metals and alloys after primary recrystallization annealing in certain

temperature ranges. Recently, cube texture has

been found also in BCC metals [2,3,4].

The TaG type specimen textures were

dominated by [111] and [100] fiber axes. The

ODF analyses, using the two different methods,

yielded essentially identical results. The inverse

pole figures of the ND (Normal Direction), RD

(Radial Direction), and TD (Transverse

Direction) of TaGl sample obtained by the

harmonic method are presented in figure 1. The pole figures of the two TaQ2 samples

showed a pseudo threefold rotation symmetry

about the ND and a pseudo twofold symmetry

about the RD, as shown in figure 2. The ODFs

were analyzed with a triclinic sample symmetry.

The ND inverse pole figures of the two TaQ2

sections were totally dominated by 111 poles.

The crystallite orientation distributions of the

TaQ2 were a mixture of fiber texture and sheet texture types, with different components of sheet texture depending on the radial distances. The

SI texture consisted of two components,

<111> fiber and {11 1}< 112> sheet type. S4

contained an additional texture {lll}<ir2>,

and had a higher degree of fiber texture than the

SI sample. Texture development by uniaxial

compressions was found to be quite sensitive to the number of strikes in the forging process. The sample with two strikes (i.e., TaQ2-Sl)

produced an ideal < 1 1 1 > / < 100 > double fiber

texture, with the fiber axes parallel to the

compression direction. On the other hand, the

sample with three strikes (i.e., TaQ2-S4)

produced a hybrid texture consisting of < 1 1 1 >

fiber type and { 1 1 1} < 1 12 > sheet type texture (with additional { 1 1

1 } < T 12 > near periphery of

the compressed disk). The degree of texture was much larger in the three-strike sample, with the

texture index of 4.6 for section-1 and 5.7 for section-4 versus 2.5. Such definitive difference

in the texture formations suggests an entirely

different plastic flow in the two forging

processes. The 90% reduction by the two strike

forging delivers a much larger impact pressure

than the three strike forging. The friction

between the tool and the sample surface becomes

larger with increasing compression, particularly

around the periphery of the sample because of

lubricant break down, which could affect the

strain distribution in the sample. The textures of

the TaQ2 sample have a text-book case radial

symmetry which is consistent with the plastic

flow in the plane-strain compression. The

amount of the radial strain, in the friction-free

117
MATERIALS SCIENCE

plane-strain compression, increases linearly with

the radial distances, and is believed to be the

main cause of the different texture formation in sections #1 and #4 of TaQ2.

TAG1, harmonic IPF

Figure 1. The inverse pole figures of TaGl. The contour

levels are given in 1 mrd (Multiples of the Random

Distribution) intervals. The regions lower than 1 mrd are marked with dots.

Rgure 2. Observed and calculated (100) and (111) pole

figures of TaQ2-Sl (top raw) and TaQ2-S4 (bottom raw). The contour levels are given in 4 mrd steps starting from 1 mrd up to 49 mrd.

References.

[1] J. S. KaUend, U. F. Kocks, A. D. RoUett, and

H.-R. Wenk, Mater. Sci. Eng. (to be published).

[2] J. S. KaUend and C. S. Chang, ICOTOM-8, J. S. KaUend and G. Gottstein, eds. The MetaUugical

Society, Warrendale, PA, (1988) 667-672.

[3] J. B. Clark, R. K. Garrett, T. L. Jungling, R. A. Vandermeer, and C. L.Vold, (to be published).

[4] J. B. Clark and R. K. Garrett, Jt., NSWC TR

88-104 (1989), classified.

RESIDUAL STRESS DETERMINATION IN A STEEL FATIGUE-TEST SPECIMEN

H. J. Prask

C. S. Choi ((ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

and

T. Cordes (John Deere Product Engineering Center, Waterloo, lA)

The Society of Automotive Engineers

Fatigue Design and Evaluation Committee has

been conducting a long-term program aimed at the development of a predictive capability for

fatigue life in which the multiaxial fatigue

behavior of induction-hardened and tempered

SAE 1045 notched shafts is being examined. A

comprehensive, integrated engineering approach

is being applied to the problem: it includes

materials properties characterization, finite element analysis prediction of stress/strain,

residual stress characterization, fatigue-life

measurement, and fatigue lifetime prediction.

Some depth-profiling of residual stresses has

been done by x-ray diffraction with layer

removal [1]; however, the nondestructive

character of neutron diffraction offers a unique

means for both characterizing the residual stress distributions and in testing the x-ray layerremoval approach.

The geometry of the component-like test "axle" is shown in figure 1. An unfatigued and

a fatigued (to ~ half of fatigue life) axle were

studied. Because of induction hardening of the

surface region in these samples, the near-surface

region (0-3 mm depth) is predominantly

tempered martensite, changing to a mixture of

pearl ite/ferrite at -10 mm depth. To determine

the reference dg's for these regions, a third axle was cut by EDM into a 3 mm thick "martensite"

ring which included the surface, and a shaft- centered 20 mm diam pearl ite/ferrite plug, each

1 cm wide.

118
MATERIALS SCIENCE

For all of the measurements the (110)

reflection of steel was employed at a scattering

angle of 80° from the sample. The dg's for the

martensite and pearlite/ferrite structures were

obtained from the ring and plug pieces, respectively, with 2x2 mm apertures and the macroscopic equilibrium conditions for axial stresses. Diffraction elastic constants ("XEC") determined

by Prevey [2] for SAE 1050 steel were used.

360

40-

50-

-200-

f-^100 ^ z

sn

Reference Zero Line 40-25 n

Figure 1. SAE biaxial fatigue test "axle". The 120 mm

length on the left is the "grip" in fatigue test cycling. The

diameters at the left and right ends of the axle are 63.5 and

43 mm, respectively.

Apertures ranging from 1x5 mm^ to 6 x 6 mm^, as detailed in reference 3, were used.

Data were obtained along single radii at positions

"B" (= 10 mm from the reference zero line (fig.

1)) and "D" (50 mm away) in the unfatigued

shaft; at "B" and "D" on the radii parallel to the

bending direction, and at the "B" position at 90°

from the bending direction in the fatigued axle. All d-spacing data were normalized to d(l 10) of

an iron powder which was scanned for each

change of apertures.

A test of the ring-plug do's was made using

the unfatigued axle data at the "D" position.

Using the hardness vs. depth results of Langner

[4] as a measure of the percent martensite, dg's

for each depth examined by neutrons can be

inferred, with the ring-plug dg's at the extremes. The axial stresses at the D position in the intact unfatigued axle as determined from neutron

diffraction balance to 10 MPa. This means that perfect balance would be achieved if the axial

stress determined at each of the six measurement

points was shifted by 10 MPA. Since the

standard deviation on each measured axial stress value is typically -20 MPa, this balance-ofstresses represents an excellent confirmation of

the approach.

Ochsner [5] has made a finite element

analysis (FEA) of an induction hardened steel shaft to obtain the residual stress distribution.

The results are shown in figure 2 along with the

neutron-diffraction-determined stresses at positions "B" and "D" in the unfatigued shaft. The effect of fatigue on the residual stress distributions is illustrated in figure 3 where

results for the two shafts are shown at corresponding positions.

V -400 m

400

-400

1 1 ! 1 T 1 1 1 r

D „

Unfatigued Axle - Radial (D=0. B=[]) .

UrJatigued Axle - Hoop (D=0, B=D)

M^'

Unfatigued Axle - Axial (D=0. B=Q)

-J I 1 L

8 12 Depth (mm)16 20

Figure 2. Neutron diffraction determined residual stresses for the unfatigued axle compared to the finite element

analysis [5]. The dashed line is the FEA result at 5 mm

from the reference zero line; the neutron "B" values are at 10 mm.

In summary, the FEA results are in reasonably good agreement with the stress measurements for the unfatigued axle. However, much less "B"- vs. "D"-position

difference than the FEA predicts is observed.

No FEA results are yet available for the

fatigued-axle case. Experimentally, the

measurements at the "D" position show very

little stress relaxation with fatigue except at 10

119
MATERIALS SCIENCE

r 1 1 1

(J

1

8

>

t f

, , , , ,

jf -400

35 *

•

D, axial (0=«nfaligu«i)

D. radial (0=ualaUgue<i)

•O

•

_•

D, hoop

(O-unlaliiued)

-BOO

-lEOO

4 9 Deptli (mm)

i -400

9,

OepUi (mm)

'

' ' ' s

DepUi (nun)

B. ucial (OauiJatigued)

5 a 8

B. radial

Depth (mm)

-T 1 1 ri

6, hoop

(Oxunfa'Jgued)

Deplh (mm) Depth (mm)

Figure 3. Neutron diffraction determined residual stresses

at the 10 mm ("B") and 50 mm ("D") positions for the

fatigued (•) and unfatigued (O) samples. The triangles

correspond to stresses at a radius 90° to the bending axis.

mm depth for o^^. Except at this depth, there seems to be a tendency for the axial and hoop

stresses to become more tensile with fatigue at the "B" position. At the "B" position, the

stresses measured with neutrons at 90° to the

bending direction are very close in magnitude to the unfatigued stresses, as expected.

An additional, important result of this study

is a confirmation of the validity of x-ray diffraction with layer removal as a means of residual

stress vs. depth determination for a cylindricallysymmetric specimen [1]. A detailed comparison

shows very good agreement for the two

techniques for all residual stress components [3].

Refercnces

[1] P. Prevey, SAE Fatigue Design & Evaluation Committee Minutes, April 1989.

[2] P. Prevey, Lambda Research, Inc., Cincinnati, OH,

private communication.

[3] H. J. Prask and C. S. Choi, in Proc. of ASM Conf. on Practical Applications of Residual Stress Technology, Indianapolis, IN, May 1991, in press.

[4] M. Langner, SAE Fatigue Design & Evaluation Committee Minutes, October 1988.

[5] J. K. Ochsner, John Deere Product Engineering

Center, Waterloo, lA, unpublished.

NEUTRON ACTIVATION AUTORADIOGRAPHY OF PAINTINGS

Y. T. Cheng and J. S. Olin (Smithsonian Institution and Reactor Radiation Division, NIST)

and

W. Leuther and C. O. Fischer (Hahn-Meitner Institute)

Neutron activation autoradiography applies

neutron activation analysis and film autoradiography to the study of oil paintings. The results are based on the pigment selection and painting

techniques utilized by the artist to manifest a unique style. The method helps to gain an

insight into the artist's creative process through

the revelation of characteristic brushstrokes,

underdrawings and alterations. Sensitive x-ray

films are used to record the images processed by

the decay of radioactive components in the

painting following activation. The darkening of

the film is mainly the result of beta-particle

emission. Gamma ray spectroscopy helps in the

interpretation of the autoradiographs by

providing elemental analysis.

Multiple autoradiographic exposures are generated for each painting to isolate the images

created by groups of isotopes with different halflives. Films serve as convenient, high resolution area detectors for the registration of the

painting's radioactivity. However, only twelve

timed exposures can be made in a typical

painting study due to the low efficiency of the

film system. Ten of the exposures are created

during the first two weeks of film exposure with

the period of exposure being about equal to the

time after activation of the painting.

Research was conducted to improve the

radioactivity detection of the autoradiography

process. A more efficient system could provide

better quality autoradiographs and thereby

120
MATERIALS SCIENCE

enhance the opportunity for interpretation. Also,

a more efficient system could reduce the

radiation exposure needed for the activation of

the painting. We are presently activating the

painting for 20 to 30 min at 3 x lO^neutrons/

cm^/sec. This exposes the painting to between

300 and 500 rads of gamma radiation as determined by measurements made using TLD

detectors at the thermal column where the

paintings are activated.

An experiment was carried out to test the

usage of light scintillation screens for film speed

enhancement. Kodak XAR Films, recommended

by the manufacturer for autoradiography use, were used throughout the experiment. The film- screen combination is commonly used in medical

x-ray applications. However, the effect on betaparticle detection is not well known. For the

experiment ten different scintillation screens were used. The screens were selected for their

light emission spectrum, resolution and physical

properties (thickness, light transparency). Gold

foils were activated to simulate a pigment in a

painting. Column A in table 1 shows the film

density readings for various film screen combinations.

Table 1. Film density readings showing effect on speed

enhancement and light output of various scintillation screens.

Scintillation Screen Type Film Density in D

+ A B

Rego Detail 1.27 0.26 Rego Universal 2.35 0.27 Rego Special 2.62 0.25 Rego NPU 2.74 0.42 Rego NPR 2.82 1.00 Rego GR2 2.28 0.23 Rego GR4 2.52 0.33 Rego GR6 2.51 0.25 Kodak Lanex Fast Thin 2.56 0.21 Kodak Lanex Fast Thick 2.70 0.12 No Screen 1.28 D = log T, where x is film transparency.

The results show the efficiency of the

current no-screen film system can be improved

by selecting a proper film screen combination.

It is characteristic that within the normal useful

range of x-ray film the density reading is linearly

proportional to the logarithm of exposure time.

Based on the data presented and extrapolating

from typical plots for Kodak film of density versus log of exposure time, the use of Rego

NPR screen with film can be seen to reduce the

necessary exposure time ten fold or more over

the current no-screen film practice. A more

precise speed enhancement factor will be

determined by generating the specific film

response characteristics with respect to beta

particle exposures. It will also be important to investigate the effect of different beta particle

energies on the speed enhancement factor. Efforts are now being made to obtain a large

size screen on flexible backing fabricated for autoradiography use. Another objective of the experiment was to study the feasibility of converting radioactivity

into visible light through scintillation screens in order to capture the light output by electronic

video means. This was to prepare for the

eventual change to a film-less autoradiography

system.

'R'pically, a gold beta particle fluence of

1x10 /cm^ is required to darken film to a ID

reading. A density of ID allows detection of

differences of 3%. To allow equivalent

differences to be distinguished in a filmless

system requires 1x10-^ photons/cm^ (3% counting statistics). Therefore an electronic light

photon detection system can in principle improve

the present efficiency by as much as three orders

of magnitude and at the same time usher the

autoradiography technique into a new age by

incorporating computer image processing

advancement.

For the experiment the light output from

each scintillator was recorded by placing a film on the back side of the scintillation screen to simulate detection through the screen to an

electronic detector. The density readings from

the various screens are listed in column B of

table 1 . Column B in table 1 indicates the same

rego NPR screen also generates the most light.

Future tests regarding photon detection will use NPR screens of various thicknesses with more

transparent backing.

121
D. NUCLEAR ANALYTICAL CHEMISTRY

NUCLEAR METHODS GROUP: OVERVIEW

R. R. Greenberg (Inorganic Analytical Research Division)

The development and application of nuclear

analytical techniques for elemental compositional

analysis of greater accuracy, higher sensitivity

and better selectivity are the goals of the Nuclear

Methods Group. A high level of competence

has been developed in both instrumental and

radiochemical neutron activation analysis (INAA

and RNAA). In addition, the group has the

capability of using neutron beams as analytical

probes with both prompt gamma activation

analysis (PGAA) and neutron depth profiling

(NDP). NDP determines concentrations of

several important elements (isotopes) vs. depth

profiles within the first few micrometers of a

surface by energy analysis of the prompt

charged-particles emitted during neutron

bombardment. PGAA, on the other hand, measures the total amount of an analyte present

throughout a sample by the analysis of the

prompt gamma-rays emitted during neutron

capture. These techniques (INAA, RNAA,

PGAA, and NDP) provide a powerful combination of complementary tools to address a wide

variety of analytical problems of great

importance in science and technology.

The use of a variety of nuclear methods has

continued to contribute to the Standard Reference

Material (SRM) certification effort; this year's

efforts include measurements performed on a number of SRMs including: Fly Ash, Coke,

Sewage Sludge, two (used) Automobile

Catalysts, Apple Leaves, Peach Leaves, Tomato

Leaves, Bone Ash, Bone Meal, Water, Grass,

Boron in Silicon, two Wear Metals in Oils, and

three Soils. Group members are serving as Technical Champions for several of these new

SRMs, and as such are responsible for scientific decisions made throughout the production and

certification processes of these materials.

Members of the Nuclear Methods Group have

also collaborated with researchers from the

Microanalysis Group (837) in developing a boron depth-profiling SRM.

Optimizing the counting conditions in the

various forms of NAA can generally improve

precision and/or increase sample throughput.

This is especially important when counting lowactivity materials. Much can be gained in

sensitivity by using high-efficiency detectors.

With care in selection of shielding material and

detector, the only important source of gammaray background, other than the sample and the

materials from which the detector shield is constructed, is the interactions of cosmic ray

particles with the shield and the detector itself. As part of our continuing efforts to improve the

NAA method we have performed experiments to apply Cooper's figure of merit for detector

performance to two excellent modern detectors.

The results obtained shows that for the ultimate

sensitivity in quantitating a simple spectrum,

high efficiency overwhelms good resolution, low

background, or any other criterion of detector

quality for a given measurement. In practice,

however, the choice is not so simple, i.e., when

summing corrections are large, when unresolved

multiplets are present, when the total count rate

is high, when background lines (not continuum)

are important, when sample-detectors positioning

uncertainty limits the accuracy, or when requirements of sample shape dictate the counting

configuration. Long-term studies of such

problems are under way.

The Biomonitoring Specimen Bank

Research Project continued to support other

agencies' monitoring programs. These programs

include the EPA Human Liver Project, the

NOAA National Status and Trends (NS&T)

program, the NCI Micronutrient program, the

lAEA/NIST/FDA/USDA Total Diet Study, the

NOAA Alaska Marine Mammal Project, and the

National Marine Mammal Tissue Bank.

Research has centered on specimen banking

protocols and improved analytical methodology.

The group's participation in intercalibration

exercises with the project participants and the

development of QA materials for various marine

analyses has helped to enhance the quality of the

analytical results used in the assessment of the

environmental health of the nation. This year,

122
NUCLEAR ANALYTICAL CHEMISTRY

the group has provided trace element

measurements (for information values) in SRM

1974 (Organics in Mussel Tissue). This material

is the first NIST frozen tissue homogenate

available with concen-trations given for both

trace elements and trace organics. In addition,

as part of the marine mammal projects, we have

prepared a whale liver quality control material

for use by other laboratories involved in marine

mammal tissue measurements. The analytical

experience gained from these measurements will

be used to develop new SRMs for marine

mammal tissues in the future. The joint NIST/FDA/USDA study of trace elements in human diet, sponsored by the

International Atomic Energy Agency, has

completed its sixth year. A total of 275 diets

from different countries have been analyzed to date for minor and trace elements. Preliminary

data evaluation shows some interesting differ- ences, among the countries studied, in the daily

dietary intakes for several elements such as As,

Hg and Se. Ten diets from different regions of

the United States have also been collected, and

thus far, seven have been analyzed. The data

obtained from the U. S. diet composites have

confirmed daily intake results for some of the

elements investigated by the FDA (based on

individual food analysis), as well as established

reliable daily dietary intakes for additional

elements including B, Cr, Cs, Li and Sn. In the case of Li, the observed intake values are considerably lower than earlier published results. In addition, multiple diets from Spain, Italy and

Turkey have been analyzed for B and Li. The

results show considerable variation in the daily

dietary intakes among the three countries.

The collaboration with Brian Clarke of

McMaster University on neutron activation-mass

spectrometry has continued to provide valuable

information on ultratrace levels of boron and

lithium. A wide variety of materials was

investigated this past year, and results were

obtained for boron and lithium concentrations in several different protein components in blood,

various individual foods, and a number of

geological reference materials. These analyses

have resulted in numerous publications as well as a certified value for Boron in the Total Diet

SRM.

The group has taken an active role in the

NIST program on high-temperature superconductivity research, both in the measurement of

impurities in starting materials and final products, and in determination of the

stoichiometry of metallic constituents. The

effort this year has centered on the development

of accurate, rapid measurements of these

materials by both NAA and PGAA. An effort to establish an accurate, monitor activation analysis

technique for further study of high temperature

superconductors at NIST is under way.

Nearly a decade ago a systematic error for

PGAA was observed in which element sensiti- vities (cps/mg) increased with increasing H

concentration. The process responsible for the

observed enhancements is neutron scattering by

hydrogen within the sample. Experimental

studies have shown that PGAA element sensiti- vities are dependent upon the scattering density

(or hydrogen concentration), as well as the

sample shape. Consistent with these results,

Monte Carlo calculations have demonstrated

that, depending on the absorption and scattering

power of the sample, and on the sample size,

shape and orientation with respect to the neutron

beam, scattering within the sample may either

increase or decrease the neutron path length in the material and thus may change the probability

of neutron absorption. Therefore, an analytical

signal, which follows an absorption reaction, is also a function of these variables. Theoretical

work and Monte Carlo calculation experiments

have demonstrated that for spherical samples,

neutron scattering serves to mitigate the effects of neutron self-shielding, and produces no

additional enhancement. PGAA measurements on spheres have confirmed this theory and have

comprised the majority of the measurements

performed in the past year. For most materials

(e.g., paraffin, urea, SRM 1632a, SRM 1577)

results show that element sensitivities for diskshaped samples (of constant H concentration and

diameter [12.7 mm]) vary with thickness over

the range of 2 - 12 mm, but that sensitivities remain constant for spheres of the same

materials (diameters 2-12 mm). The sensitivity

values for the spheres compare well with those

obtained from measurements on nonhydrogenous

standards, indicating that sensitivity

enhancements are eliminated by using spheres.

123
NUCLEAR ANALYTICAL CHEMISTRY

However, for some materials (aqueous solutions

of Gd and Sm), sensitivities were not constant over the same range of sphere diameters. To

insure accurate data interpretation for such

samples that both strongly absorb and strongly

scatter neutrons, standards should be matched as closely as possible to the samples with respect to both the amount of scatterer and the amount of

absorber.

During the past year, a large part of the

group's efforts has been directed at the

exploitation of the analytical applications of cold

neutrons. These efforts are needed to take full advantage of the guided cold-neutron beams now

available at the new Cold Neutron Research

Facility (CNRF). The group's involvement has

been to design and construct state-of-the-art instruments for both PGAA and NDP using cold

neutrons. The group's efforts have been aided

substantially by the ability to use an existing cold

neutron beam at the KFA Jiilich facility. A new

chamber for the cold neutron depth profiling

(CNDP) instrument has been installed at the

NIST Cold Tube West (CTW) location in the

confinement building. The 60-cm diam chamber

has ultra-high vacuum capability, and has been

designed to be adaptable to many different types

of experiments. The final configuration allows

remote scanning of 15 x 15 cm samples and

rotation of both sample and detector angles

relative to the beam. Sample movements are

controlled by a desk top computer. Using the new instrument, the first ^^O depth profiles have

been made in collaboration with the Eastman

Kodak Company. These measurements were made on a cobalt nickel oxide produced using

enriched ^^Oj gas. A permanent, full-time instrument for

prompt-gamma activation analysis (PGAA) has

been constructed as part of the Cold Neutron

Research Facility. This new facility will allow

accurate measurements of low levels of many

elements including hydrogen. Hydrogen

backgrounds of a few tens of micrograms have

already been achieved. This promises to be of

value in numerous applications where

quantitative, nondestructive analysis of small

quantities of hydrogen in materials is necessary.

Further background reduction by one to two

orders of magnitude is probable in die near

future. Sensitivities (counting rate per gram of

analyte) of a number of elements have been

compared using the new cold-neutron instrument

to those obtained with the thermal

Maryland-NIST PGAA instrument. At the same

reactor power, sensitivities for most elements are

a factor of four to six better with the

cold-neutron apparatus as a result of a compact

sample-detector geometry. The first quantitative

measurement with cold neutron PGAA in the

CNRF was the determination of hydrogen in a sample of C^ fullerene "buckyballs" intended

for neutron scattering studies. The sample was

found to contain 0.077 + 0.014 wt % H, which was adequately low for the neutron scattering

measurements. In addition, hydrogen was

sought in a 50-nm borophosphosilicate glass film on one fourth of a 10-cm silicon wafer; an upper

limit of 2 /xg/cm^ was found.

A long-range program to explore and

develop the analytical applications of focused

beams of cold neutrons has been initiated within

the group. The ultimate goal of this research is to produce beams of neutrons which have

intensities several orders of magnitude greater

than previously available. Such beams will

greatly enhance the capabilities of both PGAA

and NDP, and may ultimately lead to a neutron

probe for microanalysis. The neutron microguide appears to be one promising approach to achieve analytically useful focused neutrons. As

a first approach, a device will be developed and

tested using a stack of ultrathin, nickel-coated

single-crystal silicon wafers as focusing

elements. The silicon wafers act as the

transmission medium for the neutrons, which

reflect from the 1000 A nickel surface coatings.

The superposition of the individual outputs

becomes the focal point. One hundred of these

wafers have been obtained for tests of this first focusing element.

The wafers have been characterized with

X-ray reflectivity measurements at a synchrotron

source. Cold-neutron measurements using a polychromatic beam at CTW have shown that these wafers can deflect the neutron trajectories

successfully. More precise measurements have

been performed using monochromatic neutron

beams at BT-7 (NIST) and at ILL, Grenoble.

These measurements, some of which include up

to eleven reflections, have shown a high degree

of reflectivity for the silicon-nickel interface.

124
NUCLEAR ANALYTICAL CHEMISTRY

Another focusing concept involves the use of

capillary optics, a concept which has proven

successful with X-rays, and which allows for the

possibility of two dimensional focusing. In collaboration with scientists from the USSR and

SUNY-Albany, we have begun testing this concept with cold neutrons. We have obtained

a number of capillary fibers, each containing over a thousand individual channels within a

glass diameter of less than one millimeter, and

have observed the transmission (and deflection)

of cold neutrons through an array of such

capillaries bent to radii of curvature as small as 0.4 meters. The strong interaction with industrial

scientists using NDP, PGAA, and NAA has

continued during the year with a growing

number of guest workers, research associates,

and joint publications. For example, working

this year with researchers from Texas

Instruments, we have used NDP to study various

aspects of quality control using CVD (chemical

vapor deposition) reactors. These measurements

will be useful in optimizing process yield and

reliability.

The Nuclear Methods Group is also

collaborating with scientists from the NIST

Physics Laboratory in a new measurement of the

neutron lifetime. The technique used involves

detecting the protons which decay in a neutron

beam passing through a superconducting-magnet

proton trap. Current efforts are concentrating on

lowering the uncertainties due to neutron fluence

rate measurements, one of the largest components of the uncertainty in the measurements we

reported last year. A gamma-alpha coincidence

technique is being used to eliminate the

dependence on detector efficiency. A counting

system with two germanium detectors and one

alpha detector has been installed at neutron guide

NG-6. Preliminary measurements are currently

in progress.

During the coming year the group will

continue to improve the accuracy, sensitivity and

productivity of nuclear methods as applied to elemental composition measurements. Problems

to be addressed include those inherent in sample

preparation, irradiation, radiochemical

separation, counting and data reduction, with the

goal of minimizing and quantifying various sources of random and systematic errors in

analysis by nuclear methods. Maintaining full accuracy at high count rates using current generation data acquisition electronics is the goal

of our count rate dependent studies. Accurate

quantification of gamma-ray self-absorption and

measurement of the shape of the efficiency curve

are required for accuracy in monitor activation

analysis (since matching of sample with primary

standards is not required). In addition to allowing rapid sample turnaround with minimum

sacrifice of accuracy, monitor activation analysis

also provides good quality control when used in parallel with traditional primary standard NAA.

Robert R. Greenberg, Group Leader; Donald A. Becker,

M. James Blackman, Maryanne S. Cassedy, Heather Chen, Rabia Demiralp, R. Gregory Downing, Kathleen A.

Fitzpatrick, Karen M. Garrity, Sun-Tae Hwang, G. Venkatesh Iyengar, George P. Lamaze, John K. Langland,

Richard M. Lindstrom, Kenneth C. Littrell, Elizabeth Mackey, David F. R. Mildner, J. Emlen Myers, Bruce R. Norman, Craig A. Stone, Susan F. Stone, Judy Wilson,

and Rolf Zeisler.

DETERMINATION OF TRACE ELEMENT IMPURITIES IN HIGH TEMPERATURE

SUPERCONDUCTOR STARTING MATERIALS

D. A. Becker (Nuclear Methods Group)

In recent years the entire field of

superconductivity has been revolutionized by the

discovery of Bednorz and Muller of IBM that certain types of metallic oxides become

superconductive at about 35 °K. Since that time

many laboratories and scientists have been

investigating the mechanisms and structures of a wide variety of materials, looking for the next

step towards room temperature superconductivity. Progress has been made, and useful

systems which are superconductive at almost

100 °K have been devised. One of the most

125
NUCLEAR ANALYTICAL CHEMISTRY

promising material systems now being investigated is the yttrium-barium-copper-oxygen

system. This system is being investigated as part of the NIST Superconductivity Program as well as in many other laboratories. As part of

the NIST program, the Nuclear Methods Group

was asked to provide analytical support for the

materials research groups that were involved.

This support involved both compositional

analysis and stoichiometry analyses, and has

been reported elsewhere [1].

It is reasonable to expect that trace element

analyses may provide useful information in trying to understand the mechanisms and

structure of high-temperature superconductors.

It is still not clear what variables are important

in making certain superconductors. The high

temperature superconductivity must be demonstrated in a particular sample to know for sure

that it will work. Tnis report is on the analysis

of starting materials supplied by three different

materials research scientists at NIST, and

included intermediates produced from some of

these starting materials.

A total of 13 starting and intermediate

materials were analyzed for up to 19 elements.

Approximately 100 mg samples of each material were encapsulated in acid cleaned polyethylene

vials and heat sealed. Standards used were

monitor NAA constants normalized to iron foil fluence monitors, with small portions of multielement Standard Reference Material 1633, Coal

Fly Ash used as a control material to confirm the

determined values. Samples and standards were

irradiated for four hours in the NBSR RT-4

pneumatic tube irradiation facility at a reactor power of 19 MW and a neutron fluence rate of

2.7 X lO^^ncm'^s-^

After irradiation, all samples and control

materials were transferred out of the irradiated

vials to standard counting dishes. Transferred

sample weights were obtained and recorded.

Samples were counted on a Gamma-X high

purity germanium detector and associated pulse

height analysis system, starting about 30 days

after irradiation.

There were four groups of samples, from

three different researchers. The most elements

actually detected in any one sample was 13, and

the average number of elements detected in each

sample was eight. The elements determined and

the ranges of impurity concentrations found in the superconductor starting materials were as follows: ytterbium, <0.01 - 1.1 ng/g;

selenium, <0.01 - 0.7 /ig/g; thorium, < 0.003-

0.7 fig/g; chromium, < 0.02-6.1 /xg/g; iridium,

< 0.02-6.2 ng/g; hafnium, <2 ng/g; barium,

< 2-320 /xg/g; strontium, < 1-970 fig/g; cesium,

< 0.001-2.0 /xg/g; terbium, < 0.002-0.32 /xg/g;

scandium, < 0.05-37 ng/g; rubidium, < 0.2-13

ng/g; iron, < 1-67 /xg/g; zinc, < 0.05-410 /ig/g;

tantalum, <3-42 ng/g; cobalt, < 5-670 ng/g;

silver, < 0.01-2.9 /ig/g; europium, < 0.5-1400

ng/g; antimony, 0.02-14 /xg/g. (NOTE: "less- than" values are the detection limits.)

The range of impurities of the various

starting materials submitted was large. For

example, four different copper oxides were

analyzed, with total measured impurities ranging

from 2.51 fig/g to 461 /xg/g. The worst case was a yttrium-barium-copper intermediate, with

almost 1300 /xg/g of measured impurities. On

the other hand, one careful researcher worked

only with highly purified materials, and all components were below 3 /ig/g total measured

impurities. In conclusion, INAA has been shown to be

highly useful analytical technique for the

measurement and evaluation of trace element

impurities in many of the high temperature

superconductor starting materials.

Reference

[1] R. M. Lindstrom, D. A. Becker, R. L. Walters, Jr., E. S. Beary, and P. J. Paulsen, The Chemical

Analysis of Y-Ba-Cu Superconductors (submitted for

publication).

126
NUCLEAR ANALYTICAL CHEMISTRY

DETERMINATION OF FLUORINE IN VEGETATION

REFERENCE MATERIALS BY EVAA

D. A. Becker (Nuclear Methods Group)

This project was undertaken to fulfill a need for an analytical method to determine

fluorine in a variety of materials, but especially

for the determination in biological and food

SRMs. The ability to determine total fluorine in samples without losses due to dissolution is an

important motivation for the development of

such a procedure and INAA provides this. Further, a second method was needed (in

addition to the ion selective electrode method) in

order to certify fluorine in NIST reference

materials.

To review the analysis system, the nuclear

reaction to be used was:

19F + n —20t —>20Ne + p" + hu

E^ = 1633.6 keV

Ty2 = 11-0 s The primary difficulty with this reaction is with the very short half-life of 11.0 s. This

short halflife requires very fast transfer of the

sample to the detector for reasonable sensitivity,

plus it prevents the use of decay time as a method to eliminate or minimize interferences

due to matrix radioactivity. Fortunately, the

1633 keV gamma ray has no significant

interferences, the most important being the

1642.7 keV peak of chlorine which is well

resolved from fluorine. There is a potential

problem, due to the formation of F from

sodium present in the matrix, via a (n, p)

neutron reaction. This potential interference was

investigated, even though it was felt that the very

high ratio of thermal neutrons to fast neutrons of

the RT-4 irradiation facility in the NBSR would

minimize the importance of this interfering

nuclear reaction. Other nuclear reactions that were considered for the determination of fluorine were fast neutron reactions producing fluorine- 1

and oxygen- 19. However, both of these

reactions suffer from fatal flaws, and were deemed not acceptable.

The first requirement was to set up a detector and counting system close to the

irradiation facility in order to minimize the time

between the end of irradiation and the beginning

of the count. It was decided to use conventional

electronics, rather than a newly developed

"loss-free" counting system, which has yet to be

made quantitative in our laboratory. In the

author's experience, conventional electronics can be used without excessive errors as long as electronic dead times are kept below 40% and

appropriate corrections are made.

After setting up, the system was evaluated

using a piece of Teflon FEP as a fluorine

standard. The amount of fluorine was calculated

from the total weight based on the assumption

that the Teflon was a [~CF2~] polymer. The

teflon was calculated to be 75.98% fluorine.

Thus, a Teflon standard weighing 7.545 mg

contained 5733 jig of fluorine. A second

standard of high purity calcium fluoride glass was also used. All samples and standards were

encapsulated in polyethylene foil bags for

irradiation and counting, and these bags were

evaluated for fluorine content. The contribution

of the sodium fast neutron interfering reaction was evaluated by irradiating samples of sodium

chloride and sodium bicarbonate. Finally, a

tentative procedure was evaluated by analyzing

two reference materials, a high fluorine

vegetation and a low fluorine vegetation. Six

samples of each material were weighed into

polyethylene foil bags and heat sealed. The

sample weights varied from 198 mg to 332 mg,

averaging approximately 270 mg. In addition,

several other vegetation reference materials were

also packaged for analysis.

A number of irradiation and counting

combinations were tested and modified in order

to obtain the optimum measurement

characteristics for these vegetation samples.

Even though the high fluorine vegetation has

considerably more fluorine, the samples also had

extremely high aluminum concentrations. Since

the ^^Al produced has a 2.3 m half-life, this interfered with the fluorine determination by

producing a high matrix radioactivity and thus a

127
NUCLEAR ANALYTICAL CHEMISTRY

high counting deadtime. Eventually, optimum

irradiation and counting procedures were

developed for both of these materials.

The decay time required to remove the

sample from the rabbit and then place it reproducibly on the detector ranged from 15 to 24 sec for the samples and standards. A

measurement of the fluorine blank level of

several empty polyethylene foil bags gave no

measurable fluorine peak. In addition, several

measurements of sodium compounds also yielded no measurable fluorine peak, even at a sodium

concentration 30 times that expected in a 270 mg

vegetation sample. In addition to the Teflon

standard, a high purity CaF2 glass was obtained

and used to confirm the reliability of the teflon

as a fluorine standard. Both standards agreed

well, to better than one percent.

Finally, samples of several vegetation

materials were run using the developed

procedures. Both reference materials of primary

interest had abnormally high aluminum contents

as well as fluorine, a result of their being

collected near an aluminum refining plant. The

"low" fluorine vegetation was found to have 78

+ 12 pg/g F, while the "high" fluorine

vegetation had 440 + 58 \iglg F (Is). Both

materials were near their detection limits, due to the high matrix activity, and were counted at 20

cm from the detector. In conclusion, procedures have been

developed for the determination of fluorine in the two types of vegetation reference materials.

For the "low F" material, agreement with the

previously determined values is good. For the

"high F" material, these INAA results are

substantially higher than values obtained using a dissolution method. Work is continuing to resolve this difference.

DETERMINATION OF LITHIUM IN GEOCHEMICAL REFERENCE MATERIALS

BY THERMAL NEUTRON CAPTURE MASS SPECTROMETRY

W. B. Clarke (McMaster University, Hamilton, Ontario, Canada)

and

R. G. Downing (Nuclear Methods Group)

Thirteen geochemical reference standards

have been analyzed for lithium by a technique

originally developed for biological materials [1]. The technique consists of thermal neutron

irradiation, then mass spectroscopy measurement

of "^He. During thermal neutron irradiation of the

samples, tritons are produced mainly by the

reaction ^Li(n,a)^H with the remainder from

%(n,Y)^H. The 2.73 MeV tritons from ^Li are stopped either in the sample, or in the polyethylene liner wall. -^He from decay of "^H (halflife = 12.38 y) escapes rapidly to the gas phase

inside the evacuated lead irradiation capsule and,

after a suitable waiting time, can be measured by

high-sensitivity mass spectrometry.

Early tests of this method carried out with

0.1-0.4 g samples of finely ground clay and

other geological materials showed that about

50% of the ^He was retained by the mineral even when samples were heated to 250 °C over

the decay time. To ensure complete release of

^He, temperatures above 1200 °C would probably be required. To avoid such difficulties 5-10

mg of the reference materials were ground with

acetone in a small agate mortar to a particle size

of 1-5 /im, a thickness less than the range of the

tritons in the geological materials (20-40 fim).

The materials were then dispersed upon a polyethylene (abbreviated PE) disk and after all the acetone had evaporated, the sample was

covered with a second PE disk and sandwiched

between two pieces of glass in a small vacuum

apparatus. Each layer of PE is thicker than the

range of the tritons in PE, i.e. 59 ^m [3]. The

apparatus was placed on a hot plate at about

150 °C, until the PE disks had melted. Although

this modification requires a small sample size, ~ 15 mg, the sensitivity for -^He is such that lithium concentrations of about 0.1 ppm by

weight can easily be measured.

128
NUCLEAR ANALYTICAL CHEMISTRY

Several types of blank samples were

prepared: 1) PE disks were cleaned by washing

in dilute HNO3, distilled water, and acetone,

then weighed and transferred to lead irradiation

tubes. 2) Disks were prepared as above and

melted in the vacuum apparatus before trans- ferring to irradiation tubes. 3) Disks were

prepared as in 2) except that the PE was placed

in a pyrex cylinder, 2 ml of acetone was added,

and then the assembly was treated in the same way as for reference materials. 4) About 5 mg

of spectrosil'*" fused quartz (Li = 1.8 ± 0. 1 ppb) was ground in the mortar and then the procedure

used for the reference materials was followed.

A standard solution containing 231.2 ppb Li was prepared by dissolving an accurately

weighed amount of Li2C03 in distilled water. A

single drop of the standard solution (about 20

mg) was placed on a preweighed PE disk while

it was on the balance pan; the disk plus drop was

weighed immediately, then freeze-dried to remove water. Tests showed that there was

negligible weight loss of the drop during

weighing. Finally, the dry sample was sandwiched against another PE disk in the vacuum

apparatus.

The PE-encapsulated samples were folded

with tweezers and placed in pure lead tubes (o.d. = 0.64 cm, wall thickness = 0.16 cm) which were pinch-sealed at one end. The lead tubes were attached with clamps to a stainless steel manifold on a vacuum line to completely remove

atmospheric He dissolved in the PE, then pinchsealed to form a rugged container for neutron

irradiation.

The irradiations were carried out in the

thermal column for a highly thermal ized neutron

fluence rate of about 3.6 x 10^^ cm'^ s'^

Sixteen lead tubes were placed in each "rabbit"

(a total of 10-12 samples of reference materials,

at least 2 Li standards, and at least 2 blanks).

Irradiation times were from 48 h to 96 h. Although a waiting period of a few days

would have sufficed to ensure adequate amounts

of "^He, the actual waiting period was from 3-6

months. The lead tubes were placed in a mass

spectrometer inlet system and punctured with a pin attached to a bellows. The released gas was

purified through activated charcoal at 186 °C

and admitted to the mass spectrometer. ^He and

^He ion currents were recorded. Analyses of

samples were interspersed with analyses under

identical instrumental conditions of samples of a helium standard and with inlet system blanks [5]. The detection limit of the mass spectrometer for ^He is about 2 x lO'^ atoms. This is equivalent

to 1.3 X 10'^^ g Li, or a concentration of 2.7

ppb in a 5 mg sample assuming a neutron flu- ence rate of 3.6 x 10^^ cm'^ s'^, and irradiation

time of 24 h, and a cooling time of 1 month.

Analyses of the "blank" pieces of PE

showed that there was some lithium in which the

results fell in two distinct groups, 4.7 + 0.4 ng/g

(n = 6) and 20.1 ± 1.1 ng/g (n = 11). This can be explained by the fact that the disks were

cut from two different PE bags. Two disks

weigh about 35 mg, thus the actual lithium

blanks were about 0. 16 ng and 0.70 ng. Most of

the error in the concentration is due to the

uncertainty in the sample mass. No other

difference between blanks prepared in different

ways were noted.

There are only two other known reactions

apart from ^Li(n,Y)'^H which produce •^H in a highly thermalized neutron flux: ^H(n,Y)"^H, and

ternary fission of ^•^^U which produces a very

small amount of ^H [6]. The worst case sample was PCC-1 No. 2, where the correction is only

3.3% of the observed ^He.

The results are given in table 1 along with

literature values for comparison. The agreement

appears to be quite good for concentrations

above about 30 ppm, but for lower concentrations, our results, with the exceptions of PCC-1,

appear to be significantly lower than the

literature values. The results presented show that the ^He technique is applicable to geochemical

materials. The method has two main advantages:

it is remarkably free of interferences or matrix

effects, and does not require dissolution of

samples which should result in lower blank

levels than some other methods.

129
NUCLEAR ANALYTICAL CHEMISTRY

Table 1. Lithium concentrations in ppm for some

geochemical reference materials. Column 3 gives numbers

of separate samples determined by the He method.

Errors are estimates of random and systematic components

at one standard deviation, including sample inhomogeneity.

Column 5 gives recommended and proposed values taken from Govindaraju [2]

Agency Material n He Method

USGS

CCRMP

NRC

ZGI

NIM

AGV-1

BCR-1

DTS-1

G-2

GSP-1

PCC-1

MRG-1

SY-3

BCSS-1

MESS-1

BM

TB

9.5±0.4

12.0 + 0.7 1.36±Q.08

29.7+2.0

30.2±1.3

0.87+0.06

3.10+0.18

90. +7

41.4+2.8

35.6+2.7

79. +7

111. ±10

0.94+0.05

Values

12

12.9

2.1 34

31

1.6

4.2 92

72

111

We wish to thank W. C. Eismont and R. M.

Clarke for assistance with the mass spectrometer

work, O. Mudroch who donated geochemical

reference materials, and the operations staff at the NIST reactor for their cooperation. This

work was supported in part by a grant from the

Natural Sciences and Engineering Research

Council of Canada.

References

[1] W. B. Clarke, M. Koekebakker, R. D. Barr, R. G. Downing, and R. F. Fleming, Appl. Rad. Isolop. 38 (1987) 735.

[2] K. Govindaraju, Geostand. Newslet. 13, 1 (1989).

[3] J. F. Ziegler, J. P. Biersack, and U. Littmark, The

Stopping and Range of Ions in Solids (Pergamon,

New York, 1985).

[4] G. D. Flesch, J. A.R. Anderson, and H. J. Svec,

Intl. J. Mass Spec. Ion Phys. 12, 265 (1973).

[5] W. B. Clarke, W. J. Jenkins, and Z. Top, Int. J. Appl. Rad. Isotop. 27, 515 (1976).

[6] G. Kugler, and W. B. Clarke, Phys. Rev. C 5, 551

(1972).

[7] K. Govindaraju, Geostand. Newslet. 9, 219 (1984).

BULK LITHIUM CONCENTRATION BY THERMAL NEUTRON TRANSMISSION

G. P. Lamaze and R. G. Downing (Nuclear Methods Group)

and

J, Griffin and E. Sadowski (Westinghouse Savannah River Co., Aiken, SC)

The National Institute for Standards and

Technology (NIST) and Westinghouse Savannah

River Site (SRS) have collaborated in a determination of the ^Li content of lithiumaluminum alloy tubes. The method employed

for the Li determination was the measurement of

thermal neutron transmission. Four different

fabrication styles of tubes were measured. Since

the neutron beam only sampled a small volume

of the tube, the tubes were rotated during the

measurement to average over all azimuthal

angles. Blanks (tubes without lithium) were

provided for all but one of the series. For these,

the tube ends which do not contain lithium were

used as blanks. Since the original tube lengths

of about 4.5 m were too long to be handled in the limited space available at the thermal

column, the tubes were quartered (by SRS) and

the transmissions of each section measured

separately. The data for the separate pieces was

then combined for the final analysis. To

demonstrate the ability of the transmission

method to accurately determine lithium content,

two LiF single crystals (natural lithium) were

purchased to be used as "standards". These were 30.00 mm diam disks, approximately 1 mm

thick. This thickness was chosen since by

measuring either one or both disks, the

transmission would be approximately the same as the unknowns. The masses of the disks were

determined by "weighing", after which small

scrapings were sent for mass spectroscopic

analysis to determine the ^Li/^Li ratio. The

purpose of the standards was to demonstrate our

ability to accurately measure the transmission of

samples whose transmission could be readily

130
NUCLEAR ANALYTICAL CHEMISTRY

calculated. It was never intended to use the LiF

results to renormalize the data of the tubes.

Experimental Procedure-All transmission

measurements were made using the central beam

port of the graphite thermal pile of the NIST

reactor. A 14 mm diam aperture was placed at the outer edge of the thermal pile and a 13 mm

diam aperture placed at the edge of the

biological shield. The distance between these

two apertures was 788 mm, giving an angular

divergence of less than 2 degrees. A fission

chamber was placed at the exit aperture to serve

as a run-to-run monitor. The samples (in their

rotator-translator) were 244 mm from the exit

aperture. A second fission chamber, the

transmission detector, was a distance of 622 mm

from the exit aperture. All of the components were aligned optically to an accuracy of about

0.5 mm. All of the data was taken with an 80386 computer which also automatically moved

the sample at the end of each data taking

interval. Preliminary data processing was also

done by this computer at the end of each

interval. The routine was to systematically measure

each series and also to periodically measure the

LiF transmissions. The LiF transmissions were

measured four different ways. For each of the

three blanks, a small slot was cut to hold the LiF

piece in the center of the tube. The transmission was measured by cycling the LiF in and out of

the beam, i.e., by translating the tube by 10 cm

and then back. Additionally, the LiF was also

mounted free-standing in air and cycled in and

out of the beam. In all configurations, the

transmission of one piece of LiF (#1) and both

pieces of LiF (designated B) were measured.

Results—Lithium Fluoride "Standards".

Using the isotopic ratio obtained from the mass

spectroscopic analysis, and ENDF/B-V values

for the cross sections, a calculated value was

obtained for the transmission of both the single

and double thick LiF crystals. The Maxwell ian

temperature chosen for the neutron energy

distribution was 324 K which is equivalent to a neutron energy of 27.9 meV. This value was

based on previous measurements of the neutron

distribution made with a chopper. The statistical uncertainty of the measured value is + 0.1%.

For the calculated value, there are two sources of uncertainty, the isotopic enrichment and the

temperature of the Maxwellian spectrum. The

uncertainty of the isotopic enrichment was + 0.2%. This contributes + 0.1% to the

uncertainty in the transmission of the single

crystal (LiFl) and ± 0.2% to the uncertainty of

the double thick crystal (LiFB). The uncertainty

of the Maxwellian temperature was estimated to be + 5 K, this adds an uncertainty of + 0.4%

and + 0.7% to the uncertainties of LiFl and

LiFB, respectively. The ratio of the measured to the calculated transmissions is .993 + .005 for

LiFl and 1.002 ± .008 for LiFB. The final one-sigma (lo) uncertainties were obtained by

adding the individual uncertainties in quadrature.

In comparing measured to calculated concentrations, as opposed to transmissions, the ratio of

measured to calculated is 1.014 + .010 for LiFl

and .998 + .008 for LiFB.

Alloy tubes . The alloy tube data was

analyzed in a similar fashion, with the additional

complication of the cylindrical geometry. The

final one-sigma uncertainties ranged from +

0.8% to + 2.8% (for the thinnest samples).

The uncertainty in the thicknesses of the blanks

dominate the uncertainty for the thinnest

samples.

131
NUCLEAR ANALYTICAL CHEMISTRY

INCLUSION OF MARINE MAMMAL TISSUES IN THE

NATIONAL BIOMONITORING SPECIMEN BANK

B. J. Koster and S. A. Wise (Organic Analytical Research Division)

P. R. Becker (National Ocean Service, NOAA, Anchorage, AK)

T. Lillestolen (National Marine Fisheries Service, NOAA, Silver Spring, MD)

and

R. Zeisler and R. Demiralp (Inorganic Analytical Research Division)

Due to increasing interest and concern about

the effects of environmental contaminants on

various marine mammals in the United States and many other parts of the world, the National

Biomonitoring Specimen Bank (NBSB) has been

used for several projects in collaboration with

two branches of the National Oceanic and

Atmospheric Administration (NOAA). The

NBSB, located in the reactor building at NIST,

is the repository for carefully collected and

preserved samples that are representative of the

environment at the time of collection [1]. Subsamples of selected specimens are analyzed

at the time of collection for pollution monitoring,

and the remainder is banked for future analyses.

The majority of the inorganic analyses is accomplished using the NBSR to perform

neutron activation analysis (NAA), a nondestructive multi-element technique [2,3]. In 1987, NOAA and the Minerals

Management Service initiated a project for the

archiving of tissues from selected Alaskan

marine mammals (the Alaskan Marine Mammal

Tissue Archive Project, or AMMTAP). Many

of the native Alaskans still rely on marine

mammals for subsistence. Due to the modern

industrial expansion in petroleum exploration and

mineral mining, environmental monitoring of

marine mammals in these areas is advantageous

both for the population for whom this is a food

source and for assessing the environmental

impact of these anthropogenic activities. Logistics and protocol development for sampling were a challenge due to the Alaskan

environment. Protocols were developed for the

collection of tissues from animals taken by

natives during subsistence hunting [4]. Two documents describing the AMMTAP

program are nearing completion. The first describes the revisions in the sample collection

protocol that were developed as more field experience was obtained. The second is a comprehensive description of the five year

project and includes the sample inventory and

results from organic and inorganic analyses (see

table 1). Baseline data have been gathered from

approximately 20% of the collected tissues. The

tissues are analyzed for trace elements and for

selected organic contaminants. Trace element

analyses were performed using NAA to determine concentrations for about 36 elements.

These results provide a unique and extensive

data base for elemental concentrations in marine

mammal tissues, since other works have

generally reported concentrations for only fi-ve to ten elements.

In 1990, NOAA and the National Marine

Fisheries Service initiated a similar project, the

National Marine Mammal Tissue Bank

(NMMTB), for the analysis and archival of

tissues from marine mammal species found in the United States (excluding Alaska). This

project began as a pilot program for sampling

tissues fi"om animals in the northeast region, and

has been conducted in cooperation with the New

England Aquarium and the Marine Mammal

Stranding Network. Two sampling protocols were developed: (1) for animals from incidental

catches by fishing vessels and (2) for animals

from mass strandings. Both protocols were

implemented in the collection of liver and

blubber tissues from ten animals. A portion of

the samples were prepared for NAA of trace elements and for chromatographic measurements

of selected organic compounds (chlorinated

pesticides and polychlorinated biphenyls). This

pilot program has been very successful and there

are now plans to extend the program to include

samples from the southeast and Gulf coastal areas and, eventually, from the west coast.

132
NUCLEAR ANALYTICAL CHEMISTRY

Table 1. Inventory of marine mammal specimens in the National Biomonitoring Specimen Bank

Program Species Number of

animals^ Northern Fur Seal 15 Alaska Marine Ringed Seal 28

Mammal Bearded Seal 3

Tissue Archive Harbor Seal 3

Spotted Seal 1 (NOAA/MMS) Stellar Sea Lion 1 Belukha Whale 14

National Marine Harbor Porpoise 5 Mammal Tissue Pilot Whale 5 Bank (NOAA/NMFS)

^Samples of blubber, liver and kidney are collected from each animal in the AMMTAP; samples of blubber and liver are collected from each animal in the NMMTB.

References

[1] S. A. Wise and R. Zeisler, Environ. Sci. Technol.

18, 302-307 (1984).

[2] R. Zeisler, S. F. Stone, and R. W. Sanders, Anal

Chem. 60, 2760 (1988).

[3] R. Zeisler, R. R. Greenberg, and S. F. Stone, J. Radioanal. Nucl. Chem. 124, 47 (1988).

[4] P. R. Becker, S. A. Wise, B. J. Koster, and R.

Zeisler, Nat. Inst. Stand. Tech. IR 4529, (1991).

BORON AND LITHIUM IN TOTAL MIXED DIETS OF COUNTRIES

PARTICIPATING IN THE IAEA PROJECT ON DIETARY INTAKE

OF MINOR AND TRACE ELEMENTS

G. V. Iyengar (lAEA-FDA-NIST-USDA Human Nutrition Project, NIST)

W. B. Clarke (McMaster University, Hamilton, Ontario, Canada)

R. G. Downing (lAEA-FDA-NIST-USDA Human Nutrition Project, NIST)

J. T. Tanner (Food and Drug Administration, Washington DC)

and

R. M. Parr (International Atomic Energy Agency, Austria)

A coordinated research program initiated by

the International Atomic Energy Agency (IAEA

Vienna, Austria) in 1985, and supported by the

Food and Drug Administration (FDA), the

National Institute of Standards and Technology

(NIST) and the U.S. Department of Agriculture

(USDA), is currently under way. The purpose

of this project is to obtain reliable comparable

data on the average daily intakes of nutritionally

important minor and trace elements from diets

consumed in a number of developed and

developing countries such as Australia, Brazil,

Canada, China, Iran, Italy, Spain, Sudan,

Sweden, Thailand, Turkey, and the United

States. The program is due to be completed in 1991, by which time approximately 450 diets

will have been analyzed. The analytes of

interest include practically all the minor and

trace elements considered to be of biological

interest. In view of the emerging interest in boron and lithium in animal and human

nutrition, analytical work was extended to include these two elements. As a first step, diets

from China, Italy, Spain, Turkey, and the

United States have been analyzed. Boron and Li

in mixed total human diets were determined by

the method of neutron activation-mass spectrometry (NA-MS). The origin of this technique

[1] and its application to a wide variety of

biomatrices [2] have been previously described.

The food composites from the United States represent FDA's Total Diet Study, and reflect the daily consumption of foods by 25 to 30-yearold males. Presently, five of the ten collections

from different regions of the country have been

analyzed. The average daily intake amounts to 1.5 ± 0.25 (range 1.3-1.8) mg of B, and 37.35

± 14.6 (range 25-62) /xg of Li. For other

countries, presently diets from Spain (n = 20),

Italy (n = 15), and Turkey (n = 6) have been

analyzed and show the following results:

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NUCLEAR ANALYTICAL CHEMISTRY

for Li (jig/d) - Spain 53.5 ± 25.3 (range 10.9-104.7),

Italy 27.8 ± 8.2 (range 16.2-44.6), and

Turkey 39.3 + 10.3 (range 29.3-50.9); and

for B (mg/d) - Spain 2.16 + 0.97 (range 0.69-4.12),

Italy 3.67 ± 1.05 (range 2.11-5.76), and

Turkey 1.41 + 0.34 (range 1-1.9).

Additional diets from these countries are being

analyzed to examine the differences in intake, if any, between rural and urban eating habits. The

daily intake of Li calculated from the present

study is lower than indicated in earlier published

studies, and similar to the trend observed for B.

References

[1] W. B. Clarke, et al., Appl. Radiat. Isot. 38 735

(1987).

[2] G. V. Iyengar, et al., Fres. J. Anal. Chem. 338 562

(1990).

DIETARY INTAKE OF MINOR AND TRACE ELEMENTS IN THE UNITED STATES

G. V. Iyengar (lAEA-NIST-FDA-USDA Human Nutrition Project, NIST)

J. T. Tanner (U.S. Food and Drug Administration, Washington DC)

and

W. Wolf (USDA Human Nutrition Research Center, Beltsville, MD)

A coordinated research program on Human

Daily Dietary Intakes of Nutritionally Important

Trace Elements as measured by Nuclear and

other techniques, initiated by the International

Atomic Energy Agency (IAEA), is currently

under way. Briefly, the purpose of this program

is to obtain reliable analytical data on the

average daily intakes of several important macro

and trace elements in diets consumed in a number of developed and developing countries.

The experimental approach consists of collecting

total mixed diets or duplicate diets from the

participating countries for the determination of

nutrient elements such as Ca, Mg, Cu, Fe, I, Se,

and Zn, as well as the toxicants Al, As, Cd, Hg,

and Pb in view of their biological significance in the diet. The U.S. participation in this program

involves laboratories from three governmental

agencies: the National Institute of Standards and

Technology (NIST), the Food and Drug

Administration (FDA), and the U.S. Department

of Agriculture (USDA).

The analytical data obtained for the U.S.

diet composites are part of the IAEA study

mentioned above. The participating countries

are Australia, Brazil, Canada, China, Iran, Italy,

Spain, Sudan, Sweden, Thailand, Turkey, and

the United States of America, and the work is

still in progress [1].

The Total Diet Study (YDS) developed by

the FDA (2) has been used as the source to obtain foods for preparing U.S. diet composites.

The quantity of foods consumed by the 25-30-

year old male, the highest intake group in the

TDS scheme, has been used as the basis for

preparing the total mixed diet composites.

Foods in the TDS are collected regionally four

times a year by FDA's field personnel, shipped

to the FDA Kansas City laboratory where they

are prepared as ready-to-eat-foods and analyzed.

Portions of prepared foods were packed in routinely used domestic quality ziplock type

plastic bags, identified by a tag, frozen, and

shipped by air in batches under dry ice packing

to the Nuclear Methods Group laboratory for

further processing. The total mixed diet

composites were prepared by pooling appropriate

quantities of the 201 food items as described in the TDS study. The elemental analysis was

carried out mainly by instrumental and

radiochemical neutron activation analysis (INAA

and RNAA). The inorganic constituents

determined are macro elements Ca, CI, K, Mg,

Na, and P, and trace elements Al, As, Au, Br,

Cd, Co, Cr, Cs, Cu, Fe, Hg, I, Mn, Mo, Ni,

Pb, Rb, Sb, Sc, Se, Sn, Sr, V, Zn, and W.

Elements that could not be determined by NAA

134
NUCLEAR ANALYTICAL CHEMISTRY

were assayed by other methods under

collaboration as described [1]. The results are compared with the daily

intakes of the same age group published by the

FDA-TDS scheme (table 1) which is based on

individual analysis of the 201 food items [2,3]- In the TDS scheme, about 8 to 10 aliquots from

each of the 201 different foods are individually

analyzed for several elements. Concentrations

are tabulated and stored in a computer. By

using these values, the daily intake of a given

element is computed by taking appropriate

proportions of the 201 foods that were required

to make up a day's diet [2] for the age-sex group

in question. In contrast to several hundred

separate foods analyzed in the TDS scheme, the

results for USDIETS are obtained by analyzing

a small number of test portions of the total diet

composites. As shown in table 1, there is very

good agreement between USDIETS and FDATDS investigations for the daily intakes of Ca,

K, Mg, Na, and P. CI was not determined in the TDS study. The daily intakes calculated for

the essential trace elements also agree well with

the FDA-TDS results as well as with the

prevalent dietary recommendations [4] as shown

in table 1 . However, the daily intake for iodine

obtained through USDIET composites is about

70% of that of TDS. It should be noted that the

TDS results shown in table 1 are for the years

1982-1989, while the USDIET composites cover

the years 1985-1987. Therefore, the differences seen in the intake of iodine may reflect variations in iodine content of the foods collected

during those years. The daily intake for the

toxic elements As, Cd, Hg, and Pb obtained by

the mixed total diet composite approach also

agrees with the TDS estimates and are well

below the tolerance limits suggested by

nutritional authorities [5]. For several additional

elements such as Au, Br, Cr, Cs, Rb, Sb, Sc,

and Sn, the daily intake data could be obtained

by USDIET composites.

The results from this study have

demonstrated that analysis of mixed total diet

composites can be a valuable supplement to the

data obtained by the TDS scheme. The

comparison shows that for 20 elements which

have been investigated by both approaches, the

results agree well.

Table 1. Comparison of the daily intakes of various minerals from USDIET composites and FDA-TDS based on a daily intake of 3075 g of food applicable to male, 25-30-year old age group (1-3)

DAILY INTAKE

Element Unit USDIETS^

(Conposites)

FDA-TDs'' RDA-ESADDI or

PTDl'^

Ca mg 860±150 858 800

CI 3950±103 . 1700-5100

K 2800±88 2998 1875-5625

Mg 275±11 301 350

Na 2820+130 3040 1100-3300

P 1520±61 1528 800

Co Mg 14+2 11 0.12

Cr <39+7^ - 50-200

Cu 1320+170 1220 1500-3000

Fe 14350+1070 16200 10000

I 280±87® 410 150

Mn 2490±207 2810 2000-5000

Mo 125+7 116.5 75-250

135
NUCLEAR ANALYTICAL CHEMISTRY

Ni 190±38 146 .

Se 110±8 110 70

Zn 16000±1010 16400 15000

V 16±3 18 -

Al 11800±2230 13660^ 60000

As 47±8 45 120

Cd 13±2 15 60-72

Hg 3±1 4 42

Pb 34±18 41 430

Au 0.7±0.5

Br 4680±800

Cs 9±1

Rb 2440±450

Sb 4.0±0.5

Sc 0.40±0.04

Sn 3200+450

Sr 1620±240 1490

U 4.0±1.2

^Calculated from the average concentrations of diet composites representing collections between 1985-87. ^FDA-TDS results for Na, K, Ca, P, Mg, Fe, Zn, Se, I, Cu, and Mn are for all collections between 1982-89. Others are for

all collections between 1982-86. '^Recommended Dietary Allowances (single values) or Estimated Safe and Adequate Daily Dietary Intakes (ranges) (4). PTDI

applies to Al, As, Cd, Hg, Sn, and Pb taken from sources listed in Reference 5. See discussion. ^Reference 6 for USDIETS I-V analyzed for the IAEA project.

From one collection in 1984.

References

[1] G. V. Iyengar, J. T. Tanner, W. R. Wolf, and R. [4] Zeisler, "Preparation of a Mixed Human Diet Material for the Determination of Nutrient Elements,

Selected Toxic Elements and Organic Nutrients: A [5] Preliminary Report," Sci. Total Environ. 6]_ (1987),

pp. 235-252.

[2] J. A. Pennington, "Revision of the Total Diet Study

Food List and Diets," J. Am. Diet. Assoc, 82 [6] (1983), pp. 166-173.

[3] J. Pennington, B. E. Young, "Total Diet Study

Nutritional Elements," 1982-1989, J. Amer. Dietet. Assoc. 91 (1991), pp. 179-183.

Recommended Dietary Allowances, 10th Edition, National Academy of Sciences, Washington DC,

1989. WHO Technical Report Series. Nos. 683 (1982) for Sn, 751 (1987) for Pb, 776 (1989) for Al, As, Cd

and Hg, World Health Organization, Geneva,

Switzerland. M. Dermelj, Z. Slejkovec, A. R. Byrne, P. Stegnar, V. Stibilj, and M. Rossbach, "Iodine in Different

Articles and Standard Reference Materials, Fres. J. Anal. Chem., 338 (1990), pp. 559-561.

136
NUCLEAR ANALYTICAL CHEMISTRY

NEUTRON ACTIVATION ANALYSIS

AT THE FOOD AND DRUG ADMINISTRATION

D. L. Anderson, W. C. Cunningham, S. G. Capar, and T. R. Lindstrom

(U.S. Food and Drug Administration, Washington, DC)

The Food and Drug Administration (FDA)

maintains a neutron activation analysis (NAA)

facility in the reactor building of the National

Institute of Standards and Technology (NIST).

This facility is directed by FDA's Center for

Food Safety and Applied Nutrition and provides

agencywide analytical support for special

investigations and applications research. NAA

complements other analytical techniques used at FDA and serves as a reference technique and

confirmatory quality assurance (QA) tool. Instrumental, neutron-capture prompt-y, and

radiochemical NAA procedures (INAA, PGAA,

and RNAA, respectively) continue as the prime

nuclear analytical approaches. A radioisotopeinduced X-ray emission (RIXE) spectrometer

provides support for FDA programs which

monitor toxic elements in housewares (especially

Pb), and the low-level y-ray counting facility

developed by the Nuclear Methods Group of the

NIST Center for Analytical Chemistry is used to determine the radionuclide content in various

matrices. This combination of analytical

techniques enables diverse multielement and

radiological information to be obtained for foods

and related materials.

FDA's NAA laboratory served as the

reference laboratory in a QA exercise that in- volved several of FDA's major analytical centers. Two well-characterized biological

materials were distributed for multielement

analysis and the results were tabulated at the

NAA laboratory. The findings, which will be

summarized and distributed to participating

laboratories, will be used by the laboratories as a basis for procedural reviews and as a guideline

to further improve accuracy and precision.

There is a need in the analytical

community for infant formula reference materials

(RMs). A preliminary investigation of powdered

infant formula showed that this material had

favorable moisture pickup and homogeneity

characteristics, indicating that the matrix is a good candidate RM. As the 1990-1991 year

closes, a multiagency joint eifort is now under

way to develop a large batch of powdered infant

formula as an RM. FDA, the U.S. Department

of Agriculture, and NIST are combining their

efforts to make this material available with both

element concentration and organic nutrient

concentration information. Pre-release analytical

work is expected to begin next year. Thermal neutron PGAA research concentrated on optimizing the technique for the

analysis of food. Improvements in the Compton

suppression electronics and alignment procedures

have lowered backgrounds significantly. H and

B backgrounds were lowered by more than a

factor of two, and the Compton background was

lowered by a factor of three. A systematic study

of backgrounds and their enhancements due to neutron scattering resulted in a significant

reduction of analytical uncertainty introduced by

background correction.

A collection of 234 Total Diet Study foods

is being analyzed by PGAA. Unlike most

previous studies, these foods are being analyzed

without prior drying to eliminate the possibility

of element losses, especially those of B. Duplicate portions of about 40 foods have been

analyzed for H, B, Na, CI, S, K, Ca, and Cd.

The system improvements discussed above

allowed use of larger test portion sizes and

enabled count times to be shortened by about a

factor of 10 while comparable detection limits were maintained. Current detection limits (15-

to 20-h counts) for 2-g food test portions are given in table 1 . Detection limits for elements

other than H and CI are about a factor of 3

greater when CI is present at levels ^1%.

Findings from this study will be presented at the

8th International Conference, Modern Trends in Activation Analysis (MTAA8) in Vienna,

Austria, in September 1991, and published in the

Journal of Nuclear and Radioanalytical

Chemistry [1]. Two other PGAA articles [2,3]

dealing with food analysis were published during

the past year.

137
NUCLEAR ANALYTICAL CHEMISTRY

Studies continued on PGAA matrix effects associated with thermal neutron scattering [4]. The effects cause enhancements of element

sensitivities and background count rates. This

work and its relevance to analysis of food and

biological material will be presented in two

papers [5,6] at MTAA8.

Table 1. Analytical y^rays, detection limits and

interferences for food analysis by PGAA. "Bkg" denotes

interferences due to activation of apparatus and shielding.

Element keV DL (ue/g) Interferences H 2223 36 bkg

B 477 0.04 Na, CI, bkg

Na 472 69 bkg

CI 787 7 bkg

S 841 155 K

K 770 65 - Ca 6420 330 bkg

Cd 558 0.05 -

DL = detection limit.

The first cold neutron PGAA beam was

extracted at the NIST Cold Neutron Research

Facility on October 29, 1990. The cold neutron

fluence rate was -1.4 X lO^cm'^s'^ (thermal

equivalent). Compared with the current thermal-neutron PGAA system, there were

improved element sensitivities as a result of a much more favorable detector-to-sample

geometry. Signal-to-noise ratios were

significantly better because the cold neutron

beam lacks the y-ray component found in the

thermal neutron beam. Measured sensitivities (counts s'^mg"^) for Standard Reference Material

1566, Oyster Tissue, were better than thermal

PGAA by a factor of -6 for H, B, K, CI, and S

and a factor of ~3 for Cd. Detection limits

would be lowered by a factor of 2-3 if low

backgrounds (comparable to or better than those

for the thermal facility) can be achieved. Future

improvements, e.g., the addition of a bismuth

germanate scintillation detector for Compton

suppression and installation of the new cold

source, will also help reduce the detection limits. The status of cold neutron PGAA will be

presented in a paper at MTAA8 [7].

RIXE experiments were conducted to study

the glazes of 10 ceramicware items collected in the People's Republic of China. By using a ^°^Cd excitation source, Pb was detected in four

items. Other elements observed included Au,

Fe, Nb, Y, Zn, and Zr. More than 100 ceramic

test tiles with a wide range of glazes are being

analyzed by ^°^Cd RIXE.

References

[1] D. L. Anderson, W. C. Cunningham, and G. H.

Alvarez, Multielement Analysis of Foods by Neutron

Capture Prompt-y Activation Analysis, to be

presented at MTAA8 (1991) and published in the J. Nuclear Radioanal. Chem.

[2] D. L. Anderson, W. C. Cunningham, and E. A. Mackey, Determination of Boron in Food and

Biological Reference Materials by Neutron Capture

Prompt-y Activation, Fresenius J. Anal. Chem. 338 . 554-558 (1990).

[3] D. L. Anderson, W. C. Cunningham, and E. A. Mackey, Neutron Capture Prompt-y Activation Analysis of Foods, Biol. Trace Elem. Res. 27. 616-622 (1990).

[4] E. A. Mackey, G. E. Gordon, R. M. Lindstrom, and D. L. Anderson, Elemental Effects of Target Shape

and Neutron Scattering on Element Sensitivities for Neutron-Capture Prompt y-Ray Activation Analysis, Anal. Chem. 63, 288-292 (1991).

[5] E. A. Mackey, G. E. Gordon, R. M. Lindstrom, and

D. L. Anderson, Use of Spherical Targets to Ensure Accuracy in Neutron Capture Prompt y-Ray

Activation Analysis of Hydrogenous Materials, to be

presented at MTAA8(1991) and published in the J. Nuclear Radioanal. Chem.

[6] D. L. Anderson and E. A. Mackey, Neutron

Scattering-Induced Background Enhancement in Prompt y-Ray Activation Analysis, to be presented at MTAA8 ( 1991) and published in the J. Nuclear

Radioanal. Chem.

[7] R. M. Lindstrom, R. Zeisler, D. H. Vincent, R. R. Greenberg, C. A. Stone, D. L. Anderson, and E. A. Mackey, Cold Neutron Prompt-Gamma Activation Analysis at NIST, to be presented at MTAA8 (1991)

and published in the J. Nuclear Radioanal. Chem.

138
NUCLEAR ANALYTICAL CHEMISTRY

ATMOSPHERIC AND ENVIRONMENTAL STUDIES BY INSTRUMENTAL AND

NEUTRON-CAPTURE PROMPT y-RAY ACTIVATION ANALYSIS

G. E. Gordon, J. M. Ondov, and W. B. Walters (University of Maryland, College Park, MD)

Now that the reactor is again operating on

a fairly routine schedule, we are making extensive use of it. We completed a study of the

effects of target shape and neutron scattering by

hydrogen on sensitivities for various elements by

neutron-capture prompt y-ray activation

analysis (PGAA), as discussed by Mackey et

al. in the following section. This work is being done in preparation for the use of cold

neutrons in PGAA. Also, we briefly note

projects in which we have collected large

numbers of samples of airborne particles,

which are being irradiated now.

EFFECTS OF TARGET SHAPE AND NEUTRON SCATTERING BY HYDROGEN ON

NEUTRON-CAPTURE PROMPT y-RAY ACTIVATION ANALYSIS (PGAA)

E. A. Mackey and G. E. Gordon (University of Maryland, College Park, MD)

and

R. M. Lindstrom (Center for Analytical Chemistry)

We have studied the effects of neutron

scattering by H on analytical sensitivities (cps/mg) of neutron capture prompt y-ray

activation analysis (PGAA). Hydrogen causes a

particular problem because of its large free-atom

scattering cross-section, a^^^^ = 20.5 b. For

eleven elements in liquid matrices of constant shape, sensitivities increase linearly with

increasing H concentration over the range of to 11% H. Also, for hydrogenous samples,

sensitivities vary with sample shape [1]. The

magnitude of these effects can be as large as 25%. Because both sample shape and the

amount of H affect sensitivities, further studies were performed to determine a method of analysis which would allow accurate data

interpretation with minimum uncertainty.

In work now published [1], we found that

sensitivities for various elements in hydrogenous

materials in disks, 12.7-mm diam, increased

with disk thickness up to about 2 mm and then

decreased with further increasing thickness. At

thicknesses approaching the disk diameter, sensitivities were approximately the same as those of

non-hydrogenous samples, suggesting that, as the

samples approach a spherical shape, the

scattering effects cancel out. This was

confirmed by the analysis of H in spheres and

disks of paraffin (the former being cast in bullet molds). As we reported in last year's progress

report, we confirmed the paraffin results for

other elements in other materials by irradiating

samples formed with a die for making spherical

samples of urea, tris-(hydroxymethyl)-

aminomethane (THAM), paraffin, SRM 37 Ig

Sulfur; and SRM 1632a Bituminous Coal.

During the past year we analyzed solutions

containing known amounts of individual

elements. The solutions were placed in a series

of spherical quartz globes (made by Jeff

Anderson of the NIST glass shop) of internal

diameters ranging from 5.4 to 11.7 mm. In

addition, as we had done previously for solutions

contained in Teflon bags, we kept size constant while varying the H concentration by use of

three types of solution: pure D2O (0% H), 50%

H2O/50% D2O (5.6% H) and pure H2O (11.2%

H).

Results from these experiments are summarized in table 1 . Each entry in the central three columns represents the average of

sensitivities measured in up to six spherical

globes of different diameters. The fact that the

relative standard deviations of these values is

139
NUCLEAR ANALYTICAL CHEMISTRY

quite small means that the sensitivity is independent of size for these spherical samples,

as expected from the theoretical treatments. Also, except for Gd, the sensitivities for each

element are the same, within uncertainties, for

solutions containing different amounts of H and

for non-hydrogenous standards. However, there

are problems for the Gd solution and for more

concentrated Sm solutions. Note in table 1 that the sensitivity for Gd in pure H2O is significantly greater than for the other solutions.

Furthermore, when we use a more concentrated

Sm solution (1.96 mg/mL vs. 0.56 mg/mL for

table 1), the sensitivity for Sm determinations

increases with volume of the spherical globes, as shown in figure 1. In addition, the sensitivity

for the detection of H itself in the H2O of the

Sm solutions increases by about the same factor. Both Gd and Sm possess isotopes for which

the absorption cross-section curves include very

strong low energy neutron-capture resonances.

A resonance might increase the effective absorption cross section for that element, causing more

self shielding, or cause spectral hardening.

However, any of these effects would result in a decrease in the fluence rate within the sample.

No enhancements were observed for the "1/v"

nuclides. We know of no mechanism by which

the presence of isotopes possessing absorption resonances might increase sensitivities. Many of the enhanced sensitivity effects in

non-spherical targets reported here and our

previous study [1] can be understood, at least qualitatively, in terms of isotropic, elastic neutron scattering [2-4]. However, it does not appear possible for that simple approach to account for several observations:

(1) enhancements for H itself and for Sm that are significantly smaller and for Mn greater

than those for other elements and

(2) several unusual behaviors of solutions

containing Gd and Sm.

Table 1. Element sensitivities measured in aqueous solutions contained in quartz globes

Element element sensitivity in : standards

EY(keV) H2O 50% H2O D2O (0% H)

B (447) 730 ± 9 732 ± 9 715 + 9 703 ± 4

Br (245) 0.298 ± 0.007 — — 0.308 ± 0.005

Cd (558) 219 ± 3 212 ± 2.4 211 + 3 —

CI (516) 2.30 ± 0.03 2.24 + 0.02 2.23 ± 0.03 1.21 + 0.01

CI (787) 1.94 ± 0.04 1.91 ± 0.01 1.90 + 0.03 2.22 ± 0.08 Gd (182) 1034 ± 17 960 ± 14 948 ± 8 —

H (2223) 1.24 ± 0.01 1.18 ± 0.02 — —

K (770) 0.195 ± .010 — — 0.199 ± 0.013 Sm (334) 851 ± 4 860 + 6 — —

Sm (439) 417 + 4 422 ± 5 — —

140
NUCLEAR ANALYTICAL CHEMISTRY

0.0 0.2 0.4 0.6 0.8 1.0

Solution Mass (g) RguTC 1. Samarium (a) and H(b) sensitivities measured in stock solution 14c. The upper curve represents data that were corrected for self-absorption and the lower, data that were not corrected. »

As discussed above, the unusual results for Sm

and Gd suggest a possible role of low energy

neutron resonances, which are quite strong in

both elements, but the most obvious explanations

work in the opposite direction from the

observations. It will probably be necessary to include effects of energy-dependent cross

sections and changes in neutron energy spectra

within samples to achieve a full understanding of

the observations.

A paper covering the new results from this project is being prepared for submission to Anal.

Chem. [5].

References

[1] E. A. Mackey, D. L. Anderson, R. M. Lindstrom, G. E. Gordon, Anal. Chem. 63, 288 (1991).

[2] J. R. D. Copley and C. A. Stone, Nucl. Instrum. Meth. Phys. Res. A281, 593, 1989.

[3] J. R. D. Copley, submitted to Nucl. Instrum. Meth.

Phys. Res., 1991.

[4] J. R. D. Copley and E. A. Mackey, submitted to J. Radioanal. Chem., 1991.

[5] E. A. Mackey, G. E. Gordon, R. M. Lindstrom and D. L. Anderson, Use of Spherical Targets to Insure Accuracy in Neutron-Capture Prompt yRay

Activation Analysis of Hydrogenous Materials, in preparation for submission to Anal. Chem.

SUBMLCROMETER SIZE DISTRIBUTIONS OF AIRBORNE PARTICLES

BEARING VARIOUS TRACE ELEMENTS

J. M. Ondov, F. Divita and A. Cargill (University of Maryland, College Park, MD)

Knowledge of the composition and size

distributions of atmospheric fine particles is important because these parameters govern

respirability, deposition, nucleation of water

vapor and light scattering. Submicrometer

particles are of special interest, as they are

respirable, have long atmospheric residence

times, and are frequently enriched in potentially

toxic species. Trace elements borne by these

particles are also useful for apportioning sources and tracking air masses over long distances.

Despite the importance of this information,

reliable measurements of the distributions of

major, minor and trace constituents of the

submicrometer aerosol have been limited because

of limitations of the equipment available for

reliable collection and analysis of particles in

size cuts ranging far below \-^m diam. As

described in last years' Progress Report, in

1981, Kuhlmey et al. developed an impactor

capable of sizing submicrometer particles at modest pressure drops (i.e., about 250 torr for

0.06-pm minimum size cut) and modest jet

velocities (127 m/sec for 0.09-iim cut) using

micro-orifice technology [1]. The impactor 's low pressure drop and 28-L/min flow rate permit

collection of samples suitable for trace analysis

with a standard air-sampling pump.

In work now published, we tested the

micro-orifice impactor (MOI) by collecting

particles at a rural site in western Maryland near Deep Creek Lake [2]. Most samples were

collected between 1 and 31 Aug., 1983, with

sampling periods ranging from 6 to 24 hr. Cutoff diameters for the stages ranged from

about 1 .0 to 0.08 iim and the cyclone removed

particles down to 3-jim diam.

141
NUCLEAR ANALYTICAL CHEMISTRY

Impactor collection surfaces and the afterfilters ft"om seven out of the 60 sample sets were

analyzed by INAA at the reactor using a combination of 10-min followed by 3-hr irradiations.

At least 26 elements, including Al, As, Br, Ca,

Ce, CI, Co, Cr, Fe, K, La, Mg, Mn, Na, Se,

Sb, V, Ti, and Zn, could be detected on at least three of the five submicrometer stages. Only

because of the high sensitivity of INAA with the

large flux of the reactor was it possible to observe so many elements with a total fine

particle loading of only 180 \ig. Detection on more stages was generally achieved with

loadings of 240 to 400 jig.

We are now performing more extensive

studies of detailed size distributions in a variety

of settings. Field collections with 3 MOIs were

done at suburban Washington sites on the

University campus, at Andrews Air Force Base,

and at Vienna, VA. Samples were collected in the Ft. McHenry motor-vehicle tunnel in

Baltimore, among other things to seek a motorvehicle tracer to replace the Pb and Br, which

have been phased out. Collections were made at urban sites in Philadelphia previously used in studies by this group [3] . These sites are

influenced by several types of industrial sources

(e.g., Sb roaster, pigment plant, oil refineries) as well as common urban sources such as motor

vehicles, oil-fired power plants and municipal

refuse incinerators. We know the chemical

signatures for most of these sources, but will be

interested here in finding out the detailed size

distributions.

Samples from these field studies are now

being analyzed by INAA as quickly as possible.

About 600 MOI samples have been irradiated for

the determination of short-lived nuclides.

References

[1] G. A. Kuhlmey, B. Y. H. Liu and V. A. Marple, Amer. Indust. Hyg. Assn. J. 42, 790 (1981).

[2] J. A. Dodd, J. M. Ondov, G. Tuncel, T. G. Dzubay

and R. K. Stevens, Multimodal Size Spectra of Submicrometer Particles Bearing Various Elements in Rural Air, Environ. Sci. Technol. 25, 890 (1991).

[3] T. G. Dzubay, R. K. Stevens, G. E. Gordon, I. Olmez, A. E. Sheffield and W. J. Courtney,

Environ. Sci.Technol. 22, 46-52 (1988).

SOURCES OF AIRBORNE PARTICLES IN AND NEAR MARYLAND

AND THEIR WET DEPOSITION

M. Han and G. E. Gordon (University of Maryland, College Park, MD)

For several years, first with EPA support

and presently with support of the Maryland

Power Plant Research Project, we have been

determining the concentrations of many elements on airborne particles at various sites in and near Maryland. We resolve the detailed composition

patterns of the airborne particles into contributions originating from various anthropogenic

and natural sources, relying heavily on

concentrations of certain "marker" elements for

certain sources, e.g., V and Ni for oil-fired power plants. As and Se from coal-fired plants,

Zn, Sb, and Cd from refuse incinerators, etc., a technique called "receptor modeling" [1]. Most

of the analyses of these samples have been

performed by instrumental neutron activation

analysis (INAA) at the NIST reactor. Recently, we completed a detailed sampling

program in cooperation with Prof. T. E. Church

and J. Scudlark, who have long operated a

station in Lewes, DE as part of the DOE and,

later, EPA network (MAP3S) of about 10 wet

deposition stations. Our group collected

particulate and gas samples in a filter pack in which the first filter collected particles, followed

by three filters impregnated with KOH and

glycerol to collect acidic gases. This work was

done in two phases. In the first phase, samples

142
NUCLEAR ANALYTICAL CHEMISTRY

were collected every 12 hr without regard to rain

events. However, the sampler was located close

to the Lewes precipitation collector so that results from the two kinds of samples can be

correlated later to obtain information about wet

deposition efficiencies. In the second phase, an

attempt was made to collect air-filter samples for

several hours prior to passage of large frontal systems associated with precipitation. When

precipitation started, the first sampler was shut

off and a second one started. If the precipitation

lasted long enough to collect sufficient material

for analysis, the second sampler was left on until the rain ended, and a third sample was taken for

several hours following the event. The Lewes sampling ended in Feb. 1990

and in Sept. 1990, we began a sampling program

similar to Phase 2 on the grounds of the USDA

Beltsville Agricultural Research Center, where

there is a station that is part of EPA's Dry

Deposition Network.

The base-loaded filters from Lewes have

been analyzed by ion chromatography (IC) and

we recently subjected all particle filters to "shorts" INAA at the NIST reactor. Preliminary

results are listed in table 2 along with comparable data taken earlier by students of this group at the Lewes site and at College Park, MD, about

7 km from the Beltsville site. An additional 30

samples from Lewes have been irradiated for shorts INAA, but the data are not in final form

yet. We are beginning to analyze the samples by

"longs" INAA, e.g., 2-hr irradiations followed

by taking of spectra about 5 days and 3 weeks

after irradiations, which will yield data for an

additional 20 or so elements. When all of the

data are available, they will be subjected to detailed receptor-modeling interpretation. Also,

we will compare the airborne concentrations

before and after precipitation events with

concentrations in rain determined by the Univ.

of Delaware group to obtain information on the

washout efficiency of the precipitation events. At this time, not many conclusions can be drawn

from the limited data set. At Beltsville, we

seem to observe the continuation of trends

observed earlier by our group, e.g., decreasing

concentrations of V, Br and Mn. Vanadium

concentrations have been dropping since the

1970s as oil-fired power plants have had to use

low-sulfur fuel, which is also much lower in V

than the residual oil they used earlier. The

major source of Br has been the combustion of

leaded gasoline, but as that has been phased out,

Br levels have dropped sharply. Likewise, we

think Mn levels have gone down over the years

Table 2. Preliminary concentrations (ng/m ) from Lewes and Beltsville samples*

Ele- Beltsville Sept.90-Dec.90 College Park Nov. 85-Feb.86

ment Cone Std. Dev. N Cone Std. De\ N

Lewes Aug.89-Feb.90 Lewes Feb./Mar. 85

Cone Std. Dev N Cone Std. Dev N

Na 835 ± 260 24 604 ± 50 24 1870± 200 101 750 ± 90 14 Mg 790 ± 230 18 250 ± 60 23 770 ± 130 84 190 ± 60 13 Al 240 ± 45 24 420 ± 30 24 260 + 20 102 430 ± 50 14

S 4300 ± 1000 16 2070 ± 600 24 15100 ± 4000 49 1440 ± 150 14

CI 1080 ± 425 23 770 ± 30 24 1920 ± 300 100 860 ± 115 14

K 360 ± 65 20 167 ± 15 24 340 ± 25 80 200 ± 25

Ca 440 ± 65 22 480 ± 40 24 465 ± 55 83 303 ± 34 14

Ti 86 ± 19 17 38 ± 10 24 61 ± 10 72 39 ±9 14 V 3.9 ± 1.0 21 9.1 ±0.6 24 12 ± 1.0 102 6.4 ± 0.8 14 Mn 5.1 ± 1.1 24 14 ± 2 24 5.4 ± 0.4 97 8 ±0.9 14

Cu 24 ± 7 15 17 ± 3 58 14

Br 4.1 ± 0.6 22 28 ± 3 24 8.9 ± 0.6 100 17 ± 2 14

In 0.013 ± 0.005 6 0.007 ± 0.009 19 0.012 ±0.004

1 1.05 ± 0.19 17 4.7 ± 0.8 24 1.5 ±0.2

*Std. Dev. is standard deviation of the mean value

College Park data from MeCarthy |21

Early Lewes data from Kitto [3]

143
NUCLEAR ANALYTICAL CHEMISTRY

because of the near collapse of the U.S.

ferromanganese industry. Some of same trends

are observed at the Lewes site; however, the

higher V concentrations are probably related to the presence of an oil-fired power plant not too

far away at Vienna, MD. Concentrations of some elements such as Na and CI are strongly

influenced by winds that bring marine aerosol to the site from the nearby Atlantic Ocean and

Delaware Bay. We are surprised by the high

sulfate values, but that will have to be checked

carefully, as S is quite difficult to measure by

INAA. Note that there are quite a number of

missing values for S at both sites. Numbers for

them would lower the average. Part of the

problem of missing values is the fact that the

sampling durations were governed much of the

time by the onset of precipitation; thus, many of

the sampling durations were much smaller than

optimum. Fortunately, we analyzed only half of

each filter, so we may later subject the

remaining halves to further analysis, e.g., IC

analysis for sulfate.

Referraices

[1] G. E. Gordon, Receptor Models, Environ. Sci. Technol. 22, 1132(1988).

[2] L. E. McCarthy, M.S. thesis. Department of Chemistry and Biochemistry, University of Maryland, College Park, 1988.

[3] M. E. Kitto, Ph.D. thesis, Department of Chemistry

and Biochemistry, University of Maryland, College

Park, 1987..

ELEMENTAL CONCENTRATIONS IN AIRBORNE PARTICLES

DEPOSITING IN THE CHESAPEAKE BAY

J. M. Ondov and Z. Y. Wu (University of Maryland, College Park, MD)

Atmospheric deposition is an important source of toxic species including metals such as Cr, Cu, Pb and Zn to some water bodies in the

United States, including the Chesapeake Bay,

which lies downwind of heavily polluted urban

areas running from Norfolk, VA, to Washington, DC, Baltimore, Philadelphia, PA,

and Camden, NJ. Prevailing winds from

southwesterly to northwesterly directions

transport their pollutants, as well as those from

the heavily industrialized Ohio River Valley, into

the Chesapeake Bay airshed. In addition, eight

major coal-fired power stations, i.e.. Crane,

Brandon Shores, Wagner, Morgantown, Potomac

River, Chalk Point, Dickerson, and Possum

Point, may adversely influence the Bay airshed.

To better estimate the atmospheric

component of these substance to their Bay

inventories, the Maryland Department of Natural

Resources and the U.S. EPA Chesapeake Bay

Liaison Office have initiated a three-year field program involving aerial concentration and

precipitation measurements at three sites along

the Chesapeake Bay, e.g., the Wye Research and

Education Center at a northern Bay location, a Bay site near the Chesapeake Bay Biological

Laboratory at Solomon's Island, and a beach site near the Virginia Institute for Marine Science

(VIMS). The work is being conducted by our group in collaboration with the University's

Center for Environmental and Estuarine Studies,

the University of Delaware, the Virginia Institute for Marine Studies, and Old Dominion

University. Aerosol data, consisting of

concentrations of selected minor and trace ele- ments in particles of diameter < 10 /zm are being

collected by our group for use in estimating dry

deposition fluxes to the Bay Airshed.

Particulate samples have been collected

continuously for one year at the Wye site, with

the assistance of Wye technicians, on 47-mm

Teflon fiber filters. Samples are stored under

refrigeration at Wye before delivery to UMCP.

The first batch of 180 samples is now being

analyzed by INAA.

144
E. NEUTRON INTERACTIONS AND DOSIMETRY

J. A. Grundl (Ionizing Radiation Division)

The Neutron Interactions and Dosimetry

Group develops and applies well-characterized

neutron fields and related capabilities for neutron

dosimetry methods evaluation and standardization, for detector development and calibration,

and for nuclear cross section measurements.

Related efforts in cold neutron physics and

metrology provide a basic research component.

Involvement with outside organizations, both in

the federal and private sectors includes many

types of research and technology assistance

projects as well as leadership roles on national

and international standards and radiation policy

making bodies.

DOSIMETRY FOR MATERIAL PERFORMANCE ASSESSMENT

Dosimetry methods for monitoring the

degradation of materials in high fluence neutron

exposures are diverse. This project provides some form of measurement assurance, standardization, or methods development for nearly every

approach to materials dosimetry employed in the

United States. Interlaboratory measurement

cooperation with substantial international

participation are an important feature of this project.

LiF Gamma Dosimeters for Davis-Besse

Reactor Cavity Dosimetry (D. Gilliam) . Nine

LiF gamma dosimeters were irradiated in the

Davis-Besse Nuclear Power Reactor or at the

Three-Mile Island I Reactor and returned to NIST for read out. The welded aluminum

dosimetry packages were cut open in Reactor

Warm Shop. All the LiF "chips" were found to be in good optical condition and free of

radioactive contamination. The optical

absorbance of each of the samples was measured

twice at three different wavelengths. The

samples will be annealed and re-calibrated by

^^Co exposure before results are reported.

NIST/Nuclear Regulatory Commission

Contract Work (E. D. McGarry). This contract provides calibrations for reactor pressure

surveillance dosimetry that is traceable to NIST

standard neutron fission-spectrum fields through

fluence standards, which are radioactive artifacts produced by certified fluence irradiations in the

standard fields. NIST also provides consultation

and dosimetry assistance for NRC metallurgical

test irradiation programs.

Distribution of Neutron Fluence Standards

to Provide Benchmark Field Referencing of

Neutron Dosimetry for Reactor Pressure Vessels

and NRC Metallurgy Test Irradiations (E. D.

McGarry). Radiometric dosimeters (e.g.

activation foils) are benchmarked for customers

by certified fluence irradiation in standard

neutron fields, such as the ^^^Cf spontaneous

fission field or the ^^^U fission field at the NIST

reactor. The customer measures the specific

activity and uses spectrum-averaged cross

sections to evaluate his prediction of the NIST

certified fluence. Seventeen such fluence

standards have been produced this fiscal year. Fluence standards are routinely produced in the form of 1.25-cm diam radioactive disks. A

problem in accurate prediction can arise when

the customer's gamma assay equipment has been

calibrated for neutron sensors of a different size. The problem can be resolved by simultaneously

irradiating both sensors to the same fluence and

deriving suitable adjustment factors.

145
NEUTRON INTERACTIONS AND DOSIMETRY

Davis-Besse Ex-Vessel Cavity Dosimetry

Benchmark Experiment (E. D. McGarry) The

power-reactor industry's benchmark irradiation

at the Davis Besse Reactor is complete and the

dosimeters are being analyzed. Fluence

standards have been shipped to B&W for the

^^Ni(n,p)^^Co, '^^Ti(n,p)4^Sc, and

^^Cu(n,a)^-'Co reactions. Fluence standards are

also being counted by B&W, Lynchburg

Research Center, CEN/SCK, Mol, Belgium, and

Arkansas Tech University, Russelville,

Arkansas.

NIST has supplied, and will analyze, Paired

Uranium Detectors (PUDs) for comparison with

other types of fissionable dosimeters. The PUDs

in Davis Besse were irradiated together with

radiometrics, SSTRs, HAFMs, niobium, and LiF

Chips.

Dosimetry Methods Development for

Reactor Support Structures (E. D McGarry). A

contract for the dosimetry has been let with the

Westinghouse Science and Technology Center to monitor possible radiation damage to structural members at the Trojan Nuclear Power Reactor. The Westinghouse dosimeter-evaluation systems

must be benchmarked prior to obtaining the

Trojan measured results. Fluence standards were prepared in certified fluence irradiation and

shipped in October. The benchmarked

dosimetry will be used to make necessary

adjustments to the calculations at the dosimeter

locations and before extrapolation of radiation

(damage) exposure to the support structures. It

is now necessary to oversee implementation of

the second phase of the contract with

Westinghouse to derive the fast neutron fluence on TROJAN Reactor support structures. NIST

will consult with ORNL to obtain comparisons

of their calculated results with the measured

dosimetry for TROJAN.

NRC Regulatory Guide Preparation (E. D.

McGarry and J. Grundl). NIST's involvement

continues in the development of a draft regulatory guide "Status of a New Regulatory

Guide on Methods and Assumptions for

Determining Pressure Vessel Fluence." The

guide contains advice on up-to-date methodology

for carrying out transport calculations assuming

use of 2-d discrete-ordinates transport codes.

The mechanism for validating calculation is

comparison with measurements. There is also

emphasis on validating transport calculations

against experimental benchmarks and in-situ surveillance dosimetry.

The guide requires validation of the

calculational method against benchmarked

dosimetry results. NIST is preparing a NUREG

for this purpose as part of a calculational

benchmark package.

The Behavior of Np-237 in Ex-Vessel

Cavity Fields ( E. D. McGarrv) . The Np-237

response appears to have some correlation to the

behavior of non-threshold detectors. This

probably is the reason so many experimenters

blame unidentified quantities of Pu-239 for low

C/E ratios. It is worthwhile to examine several

other observations about Np-237 response.

Calculated Response: Although a major

issue is the problem in calculating absolute Np237 accurately, it is also observed that calculated

values also show the trend of Np-237 having a non-zero cross section down to very low

energies.

Cross Sections : We continue to monitor

results from NIST benchmark experiments that consist of Np-237 and other fission isotope

responding to various diameter water-moderated

spheres around a Cf-252 point neutron source. The results to date gives C/E ratios near unity

indicating there is no problem with the Np237

cross section in the energy ranges of these

spectra.

Dosimetry Support for New NRC/HSST

Irradiations at University of Michigan (E. D.

McGarry). NIST has assumed a dosimetry

consultants role for the combined MEA and

ORNL operations for the Heavy Section Steel

Technology (HSST) Program at the University

of Michigan Reactor. The current interest is a

result of two circumstances.

There has been a serious decrease in the

number of available test reactors with sufficient

fluence rate (5 x 10^^ n/sec) to efficiently do

HSST steel experiments, and the 2 MW

University of Michigan Reactor has space in their pool for several rather large experiments.

Specifically, tailored spectrum experiments will

be undertaken, in which the epithermal fluence

is enhanced by several orders of magnitude over

the fluence greater than 1 MeV.

146
NEUTRON INTERACTIONS AND DOSIMETRY

RADIATION PROTECTION DOSIMETRY

Standard neutron fields are used to calibrate

radiation protection instrumentation and to investigate and test new types of dose measuring

techniques. Responsibilities in national and

international dosimetry methods research focuses on tissue dose modeling, and tissue equivalent

proportional counter (TEPC) measurements, and

the development of written standards.

Calibration Services (E. Boswell and R. Schwartz). Approximately 35 neutron instruments were calibrated this year. Although the

majority were conventional radiation protection

instruments, calibrations were also done for

neutron flux monitors used at radiation therapy

installations, and fission chambers for use at reactor sites. ICRU Publication on Practical Determination of Dose Equivalent (R. Schwartz). As far as can be determined we are through with our ICRU report, entitled "Measurements of Dose

Equivalent from External Photon and Electron

Radiations". The report was approved by the

main commission last winter, revised a few

times since then, and should be ready to go. ISO Document on Instrument Calibration

(R. Schwartz). The final draft of the standard

"Procedures for Calibrating and Determining the

Response of Neutron Measuring Devices Used

for Radiation Protection" is being sent off to the

member countries for comment before final publication as an ISO standard. We are already

preparing a revised version of the standard to take into account some new work which has

recently been published.

Measurement of Neutron and Gamma-Ray

Kerma Rates at AFRRI (C. Eisenhauer. J. Grundl. R. Schwartz). Two sets of on-site

measurements were performed at AFRRI to resolve a discrepancy between ratios of neutron

and gamma-ray kerma rates determined with

tissue-equivalent ionization chambers (TEIC) and

tissue equivalent proportional counters (TEPC).

The ratio obtained in one particular field with

TEIC only led to anomalous results for the

relative susceptibility of mice to neutron and

gamma radiation. Neutron reaction rates obtained with NIST fission chambers corro- borated both concurrent and earlier

measurements of neutron kerma rates with

TEPC. It was concluded that TEIC do not respond properly in fields with a high neutron to gamma ratio. Using the consistent ratio from

TEPC and fission chambers, the neutron quality

factor for mice no longer appeared anomalous.

RESEARCH AND TECHNOLOGY ASSISTANCE

Research and technology assistance are strongly coupled in neutron dosimetry. A

multiplicity of institutional involvements, drawn

to the group by the availability of unique

irradiation facilities and measurement

capabilities, encourages a variety of attractive

projects and unavoidable responsibilities.

Benchmark Measurements for Critical ity

Safety (D. Gilliam) . The transport of ^^^Cf

neutrons through spherical water moderators has

been studied by observing fission rates of four

isotopes in the leakage spectrum outside the

moderator. Both measurements and Monte

Carlo calculations with highly detailed modelling

of the experiments have been compared and

reported for two different water radii [1]. The

calculations tended to over-predict the thermal

flux, especially for the thicker moderator.

Reference

[1] D. M. Gilliam and J. F. Briesmeister, "Benchmark

Measurements and Calculations of Neutron Leakage

from Water Moderators," Proceedings. International Topical Meeting on Advances in Mathematics,

Computations, and Reactor Physics, Pittsburgh,

April, 1991.

Neutron Fluence Rate Measurements at the

CNRF (D. Gilliam). The fluence rate was

measured at four wavelengths for the beam

coming from the velocity selector and going to

147
NEUTRON INTERACTIONS AND DOSIMETRY

the 30 m SANS instrument on the NG7 beam.

The ^-^^Pu fission rate was found to drop by

about a factor of 2 for each increase of

wavelength of 2 angstroms.

At the NG6 end-station, fluence rates for

both fast neutrons and cold neutrons were measured for several different filter configurations.

Calculations of Slab Penetration (C.

Eisenhauer) . Results of a basic study of slab

transmission and reflection by neutrons and

gamma rays were published in Nuclear Science

and Engineering. Abstract is as follows:

Using Monte Carlo calculations, it is shown

that the transmission of scattered neutrons or gamma rays from a point source through a plane

slab of infinite extent to a point detector depends on the orientation of the slab but varies very

little with the slab position. This is also true for

reflected radiation if the source is replaced by its image source and the results are interpreted in terms of a transmission problem. It is also

shown that the transition from a slab of small

extent (narrow beam conditions) to a slab of

infinite extent (broad beam conditions) can be

characterized by a simple function of the singlescatter angle. This function, too, can be applied

to reflected radiation by invoking the image

source. Typical results are presented for

polyethylene and iron.

Upgrading Neutron Transport Calculational

Capability and Converting All Radiation

Transport and Related Codes to the Cray

Computer (C. Eisenhauer"). The 2-d discrete

ordinates code, DORT, has been acquired and some sample problems have been run on the

Cray. All neutron and photon transport codes,

including ANISN and MCNP, are being

converted from the NIST CYBER 205 computer

to the new NIST Cray Y-MP Computer. The

DETAN code for dosimetry analysis and

miscellaneous smaller codes are already on the

Cray. The MCNP and YSPEC codes will be

operational by the end of FY 91.

Calculations of Transmission of Neutrons

Through Iron Shells (C. Eisenhauer). There is a long-standing question regarding the accuracy

of the inelastic cross section in iron used to calculate fast neutron gradients in thick steel. This is a critical issue for predicting the useful

service life of power reactor pressure vessels.

In preparation for a benchmark program of iron

shell transmission measurements at NIST, a

series of calculations of the transmission of

neutrons from a point fission neutron source through spherical shells of iron have been

performed. The endpoint of these benchmark

measurements will be to assess the adequacy of

recently revised inelastic cross sections for iron.

Two-Component Room-Return Model (C.

Eisenhauer). Accurate calibration of neutron

personnel instruments requires a correction for

the effect of neutrons reflected from the walls of

the calibration room. The current model for

making this correction, derived from reflection

in a spherical cavity, predicts a constant fluence

of room-return neutrons over the volume of the room. Actual calibration rooms are usually

rectangular parallelepipeds for which the room

return fluence will vary with position. A

modified room-return model has been devised

which treats the first reflection as if it were from

a plane slab, and the second and higher orders of

reflection according to the original spherically

symmetric model. The model for first reflection

establishes a variation with position. The twocomponent model agrees well with Monte Carlo

calculations and with experiment.

NISTAVestinghouseCooperative Agreement

for Measurement Assurance (E. D. McGarry).

Measurement assurance activities for

Westinghouse radiometric (activation foil) and

Solid State Track Recorder (SSTR) dosimetry

continues. NIST is especially interested in problems with reliable masses for SSTR

fissionable deposits that are in the nano-gram to pico-gram range, and which were developed by

Westinghouse Industrial Technology Center.

The SSTR dosimeters provide for the registration

of fission products tracks in mica disks place next to thin deposits of select fissionable

material. The track density for uniformly thin

fissionable deposits is proportional to fast neutron fluence. The absolute accuracy of the

technique has been validated at NIST through

controlled irradiation experiments in standard

neutron fields. SSTR Ultralight Mass Determination (E. D.

McGarry). Mass determinations require NIST

provide known fluences to the ultra-lightweight

SSTRs and Westinghouse measure their response

and deduce masses. The irradiations are carried

148
NEUTRON INTERACTIONS AND DOSIMETRY

out in the 235U Cavity Fission Source at the

NIST Reactor, to an accuracy of about 2.5%

(lo).

High T^ Research (S. F. Trevino. F. J. Owens, and E. D. McGarry). The structure of

a variety of high critical temperature (Ri-Tq)

superconducting materials are being developed

and studied at NIST. One material containing

[B-Pb-Sr-CuO] may have properties that are dependent upon the state of the CuO. The

current density that this materials can sustain is

a function of the mobility of the microscopic

magnetic flux vortices. Speculation that atomic

defects may effectively trap the vortices is being

tested.Use has been made of the NIST ^^^U

neutron fission spectrum irradiation facility to introduce controlled amounts of displaced atoms

(i.e., defects) in the material by performing

certified fluence irradiations, which are free of

lower-energy neutrons that cause complicating

neutron capture transmutations.

IRRADIATION AND CALIBRATION FACILITIES

Well-characterized neutron fields built and

maintained as permanent irradiation facilities,

provide certified fluences of pure fission

neutrons, sub-MeV distributions, monoenergetic

keV beams, and thermal neutrons. Passive and

active detectors are exposed in these neutron

fields for response calibrations, for cross section

measurements, and for the investigation of new

measurement techniques. A multipurpose fission

rate measurement capability is centered around

the NIST "go anywhere" double fission

chambers and the NIST set of fissionable isotope mass standards (FIMS). The Manganous Sulfate

Bath is the primary neutron source strength

calibration facility for the United States. Absolute neutron fluences for all fission-neutrondriven standard neutron fields at NIST are derived from source strength calibrations at this

facility.

Design and Construction of the Materials

Dosimetry Reference Facility (MDRF) (J. Grundl, C. Eisenhauer. and E. D. McGarry).

A new, high-intensity reference neutron field for reactor dosimetry, designed and constructed by

NIST, will be operated at the University of

Michigan 2MW Research Reactor. Certified

fast-neutron fluence irradiations and related

validation experiments, traceable to NIST, are to be performed in support of neutron dosimetry

for the nuclear power and metallurgical-test

communities.

The development of this benchmark facility

is a natural extension of the long-term NIST

program to develop standard neutron fields for

measurement assurance applications in materials

dosimetry, and the testing of new dosimetry

detectors and techniques. The primary user of

this irradiation facility will be industry and

government organizations that have major

responsibilities to assess the integrity of high-risk

nuclear power reactor components and more

generally to establish the data base that describes

radiation damage in steel. The MDRF complements existing fission

neutron standard fields at NIST by providing a tenfold increase in fast-neutron fluence, a much

larger irradiation volume with modest flux

gradients, and a neutron spectrum rich in intermediate-energy neutrons. For the latter, the

MDRF offers two spectrum options by operating

with or without a boron- 10 filter. A number of

detector response characteristics and methods of

dosimetry interpretation can be investigated with

this choice.

Essentially, the MDRF is a 15.2 cm diam

steel cylinder with an axial bore mounted in the

pool near the face of the Michigan research

reactor. Irradiation thimbles, inserted from the

top of the pool, will locate samples mounted

inside at the core midplane and can also

accommodate active instruments such as NIST

double fission chambers. Two separate thimbles, one with and one without a boron-10 liner,

allows a convenient change from a neutron

spectrum that goes down to the cadmium cut-off

at 0.4 ev to one which cuts off at about 5 keV.

149
NEUTRON INTERACTIONS AND DOSIMETRY

Estimated neutron fluence rates with the MDRF

close to the reactor are as follows:

without boron-10:

with boron-10:

4E U cm'^s"^

4EU

>lMeV; 1.7E12>0.4ev

>lMeV; l.lE12>5Kev

Certification of the neutron fluence for

dosimetry calibration and performance validation

irradiations will be established on the basis of

neutron fluence transfer from the NIST Fission

Neutron Irradiation Facilities. It is anticipated

that the neutron fluence above 1 MeV will be

specified to an uncertainty better than 10%.

Specification of the total neutron fluence will be more uncertain because of a much greater

dependence on calculation.

A three-group summary of the neutron

energy distributions in MDRF, based on scoping

calculations, is as follows:

0.4ev 0.1 MeV 1 MeV 18 MeV

w/o BIO: 0.45 0.33 0.22

w/BlO: 0.18 0.48 0.34

Main features of the neutron spectra are the

mild departure from a fission spectrum in the

MeV energy range, the relaxation into a 1/E

spectrum below 1 keV for the MDRF without the

boron-10 filter and the cut-off in the keV range

with the filter. A thorough characterization of the neutron

spectrum will come from a combined interpretation of 2-d neutron transport calculations and

benchmarked reaction rate measurements with

threshold and low-energy detectors. A special

feature of the spectrum tailored with a boron-10

filter will be the determination of the boron-10

thickness by means of a neutron transmission

measurement in the 2 keV filtered beam at NIST. The effective nuclear thickness obtained

in this way removes all or most of the

uncertainty in boron-10 enrichment, powder

density in the thimble, and the absolute cross

section scale for neutron absorption.

Activation detector calibrations planned for

the MDRF will serve on-going and planned steel irradiations by Materials Engineering Associates

and by ORNL at the U. Michigan Reactor,

pressure vessel dosimetry contracts at the

Westinghouse Nuclear Technology Division, and

surveillance capsule backfitting and cavity

dosimetry projects at B&W. Expected dosimetry

measurement assurance requirements proposed

by the Nuclear Regulatory Commission may be

conveniently and inexpensively served by the

MDRF.

Continued development and calibration of

Solid State Track Recorders (SSTRs), a dosimetry detector of great promise for reactor pressure vessel damage monitoring, will be

substantially enhanced by the availability of the

MDRF. A new generation of fission dosimeters

in the form of paired uranium detectors (PUDS),

sintered wires, and encapsulated beads can also

be investigated and validated in the two

component neutron fields of the MDRF.

Examples of attractive research

opportunities are: (1) the iron inelastic scattering cross section discrepancy, crucial for the

pressure vessel embrittlement uproar; (2)

establishing Nb(n,n') activation as a practical

main-line reactor dosimetry detector; and (3)

determining proper corrections for subthreshold

fission in Np, U234, and U236.

Calculations for the MDRF (C.

Eisenhauer). The design of the MDRF required

scoping calculations to explore the sensitivity of

fluence intensity and spectrum to variations in design parameters. Accordingly, four types of

1-d neutron transport calculations were

performed with the ANISN discrete ordinates

code to simulate the actual cylindrical

configuration. The first was a study of the

spectrum at the center of an iron slab in water

near the core of the reactor as a function of iron

thickness. The second was a comparison of

spectra in slab and cylinder configurations. The

third calculation used the output spectrum from

the slab calculation as input to the exterior

cylindrical surface of the detector configuration.

Finally, we studied the effect of a cylindrical

sleeve of ^^B on the fluence spectrum at the

center of the cylindrical array. Results were

adequate to decide upon a final configuration and

proceed with the final design.

150
NEUTRON INTERACTIONS AND DOSIMETRY

COLD NEUTRON PHYSICS AND METROLOGY

Research activities in low energy neutron

physics involves experiments in neutron beta

decay and weak interaction physics, and the

development of advanced techniques in neutron

optics and interferometry. Strong collaborations,

both foreign and domestic, exist with national

laboratories, universities and independent

research laboratories.

NEUTRON BETA DECAY

M. S. Dewey, D. M. Gilliam, G. L. Greene, and W. M. Snow

(Ionizing Radiation Division)

and

G. P. Lamaze (Nuclear Methods Group)

The free neutron is a beta unstable particle

which decays with a mean lifetime of —900 sec. Because neutron decay is the simplest nuclear

beta decay, measurements of the parameters

which characterize neutron decay can be directly

related to the fundamental theory describing

weak nuclear interactions. An accurate knowledge of the neutron lifetime is also important in a variety of astrophysical contexts which range

from the production rate of neutrinos in the sun

to the details of nuleosynthesis during the Big

Bang. In addition, detailed measurements of

correlations between t^e particles (protons and

electrons) created in neutron decay can provide

a precise test of fundamental symmetry principles. The NIST program in neutron beta decay

has several aspects and is a collaborative effort between several institutions. A current major

effort involves the accurate determination of the

neutron lifetime. This project evolved from a

collaboration between NIST, the University of

Sussex (U.K.), the Central Bureau for Nuclear

Measurements (E.E.C., Geel, Belgium) and the

Scottish Universities Research and Reactor

Center (Glasgow). The current program

centered at the NIST Cold Neutron Research

Facility follows on work carried out at the

Institut Laue Langevin in Grenoble. A second

experiment will use the lifetime apparatus to determine energy spectrum of the decay protons

created from neutron decay. A third project

involves a determination of the correlation

between the momenta of the decay electrons and

protons from the decay of polarized neutrons. The observation of a nonzero triple correlation

between the spin vector of the decaying neutron

and the momenta of the decay proton and

electron would imply a violation of time reversal

symmetry (T). This T violation experiment is a collaboration between NIST, Los Alamos

National Laboratory, and Harvard University.

The neutron decay detector to be employed

for the neutron lifetime experiment has been

extensively rebuilt in the last year with the

completion of an all new precision electrode

assembly, new mechanical supports, new data

collection electronics and new high voltage

electronics. The improved apparatus is currently

ready to accept beam at neutron guide six (NG6). With minor additions to the control

electronics this apparatus will be employed for

the determination of the decay proton spectrum.

The apparatus to be used for the T violation

experimental will consist of two major

components. One compound is a neutron decay

detector capable of detecting both the electrons

and proton emitted in neutron decay. This

detector has been developed as a Los Alamos,

NIST, Harvard collaboration and will be

installed for preliminary testing in late 1991.

The other component in the T violation

apparatus is a novel neutron polarizer based on

optically pumped ^He. This polarizer is under

development at Harvard University. A prototype

will be tested at CNRF in late 1991 /early 1992.

151
NEUTRON INTERACTIONS AND DOSIMETRY

ABSOLUTE DETERMINATION OF NEUTRON FLUX

M. S. Dewey, D. M. Gilliam, G. L. Greene, and W. M. Snow (Ionizing Radiation Division)

T. Chupp and J. Richardson (Harvard University, Cambridge, MA)

H. Robertson and J. Wilkerson (Los Alamos National Laboratory, Los Alamos, NM)

J. Pauwels (Central Bureau for Nuclear Measurements, Geel, Belgium)

and

R. Scott (Scottish Universities Research and Reactor Center, Glasgow, UK)

The present state of the art in the absolute

determination of thermal and cold neutron fluxes

allows measurements with an accuracy of ~ 1/2

of one percent. In addition to the obvious gains

in the accuracy of neutron detectors and

dosimeters for low energy neutrons, there are a

variety of other motivations for a program which

would improve this level of precision.

In particle physics, recent measurements of

the beta decay lifetime of the neutron have

required, as a calibration, a determination of a neutron capture flux to scale an observed neutron

decay rate. The uncertainty in this calibration

contributed approximately one half of the total error in the neutron lifetime determination. In chemistry, the accuracy of the analytical method

of prompt neutron activation analysis is ultimately limited by the knowledge of the

neutron flux at the activation position. In nuclear physics, the accuracy of several

important neutron capture cross sections may be

improved if better techniques for neutron flux

determination were available. In nuclear

metrology the activity of standard neutron

sources could also be determined with improved

precision.

Our program for improving the technology

for absolute neutron flux determination involves

several related measurements and the

collaboration of a number of institutions. Among these institutions are Los Alamos

National Laboratory, The Central Bureau for

Nuclear Measurements of the EEC, Harvard

University, and the Scottish Universities

Research and Reactor Center (Glasgow). Two

specialized neutron detectors have been

constructed and are presenfly undergoing initial testing. Both of these detectors have the

characteristic that they completely absorb

(nearly) all neutrons which enter the detector.

For this reason these new instruments are known

as "black" detectors.

The first of these new black detectors is calibrated by detecting, in coincidence, both the

alpha particles and gamma rays produced in the

capture of thermal neutrons on ^°B. Such a coincidence measurement allows the

determination of the absolute efficiency of both

the alpha and gamma detectors. By alternately

employing "thin" and "thick" ^% targets, it is possible to configure the detector so that it stops

essentially all neutrons and detects emitted

gamma rays with well known efficiency. This

detector was installed on neutron guide 6 (NG-6)

in July of 1991.

The second "black" detector is based on a quite different, calorimetric calibration scheme.

In the calorimeter, a neutron beam is totally

absorbed in a ^Li rich target. Neutron capture on ^Li leads to the production of alpha particles

and tritons with a well known total energy

release. These particles have a very short range

in the target and essentially all of their kinetic

energy is deposited as heat in the target. At

cryogenic temperatures, (~ 5 K in the present

apparatus) the heat capacity of the target is sufficiently small that this heat is easily detected.

This neutron calorimeter can be calibrated by

comparing the nuclear heating with Joule heating

from well characterized current and voltage

sources. The neutron calorimeter project is a

collaborative effort between NIST, Harvard

University, and Los Alamos National

Laboratory. It is anticipated that this detector

will have its initial tests in late 1991,

The most accurate traditional method for

the determination of neutron fluxes employs well

characterized depositions of ^^B and ^Li. These

materials are selected for the large and well

known (n,a) cross sections. If such a target is placed in a neutron beam and is "viewed" with an alpha particle detector of known solid angle,

the incident neutron flux can be deduced from

the alpha count rate and the product of the areal

152
NEUTRON INTERACTIONS AND DOSIMETRY

density of the deposit, the cross section and the

detector solid angle. At present the limitation to the accuracy of this method lies in the ability to accurately determine the areal density of the

deposits. Thus, another aspect of this flux

determination program involves better

preparation and characterization of such deposits.

This work is being carried out as part of a cooperative program with the Central Bureau for

Nuclear Measurements of the European

Community. New evaporation systems and

other apparatus have been acquired and

fabricated at NIST and CBNM (Geel, Belgium).

NEUTRON EVTERFEROMETRY

M. Arif, M. S. Dewey, and G. L. Greene

Werner and H. Kaiser (University of Missouri, Columbia, MO)

A. Zeilinger (University of Innsbruck, Austria)

and

A. Klein and G. Opat (University of Melbourne, Australia)

The quantum mechanical description of

particle dynamics implies that the neutron has a wavelike as well as a particle like character.

This wavelike character in turn implies that neutrons can exhibit interference phenomena

analogous to that familiarly encountered in

optical interferometry. The neutron

interferometer facility under construction at the

guide hall of NIST's Cold Neutron Research

Facility will be employed to investigate a variety

of phenomena using interferometric techniques.

While much of the phenomenology of neutron

interferometry is similar to optical

interferometry, there are several essential

differences which make neutron investigations more challenging technically, as well as potentially more interesting scientifically. Cold

and thermal neutrons have wavelengths which

are typically three orders of magnitude less than

the wavelength of light typically used in optical

experiments. This and other factors imply that neutron interferometers are significantly more

sensitive to environmental perturbations than

optical systems. While this poses serious

experimental challenges, it also promises novel

scientific opportunities.

The optical elements to be employed in the

NIST neutron interferometer will be fabricated

from "perfect" crystal silicon crystals. The use

of perfect crystal optical components implies that the wavelengths of the neutrons to be used will be on the order of the atomic spacing in the

crystal lattice. As this is typically three orders

of magnitude less than the wavelength of visible

light, such a neutron interferometer will be far more sensitive to vibration than optical devices.

Much of the work over the last year was devoted

to the design and construction of an

environmental isolation system to reduce seismic

and acoustic interferences.

A schematic of the vibration isolation under

construction is show in figure 1 . To attain the

requisite degree of vibration reduction the system

employs two stages of isolation. The first stage

is a large reinforced concrete slab supported by

airsprings with a characteristic frequency of

approximately 2 hertz. The second stage is a granite table also on airsprings with a somewhat

lower resonant frequency. In combination, these

two stages of isolation provide a noise rejection

which is multiplicative. Both of these airspring

systems employ a long term positional servo system which was developed at NIST and is now

a commercial product. The secondary airspring

system will also be equipped with a high band

width magnetic actuator which may ultimately be

used to "lock" the upper table to an inertial test mass. Much of our efforts this year have been

devoted to development and construction of this system. The neutron interferometer facility will

employ monochromatic neutrons. These

neutrons will be extracted from NG-7 using a novel in-guide monochromater. This

monochromator (fig. 2) provides control of two

153
NEUTRON INTERACTIONS AND DOSIMETRY

P

~"*' •"'•>" ln«rr.no..,.,- 'J

Block Hou*«

Figure 1. Schematic of the NIST neutron interferometer setup.

axes of motion of the monochromating crystal

using a quadralateral flexure. The neutrons

reflected off the monochromating crystal

(pyrolytic graphite in the first instance) exit the

guide through a single crystal silicon "window"

coated with a thin layer of ^^Ni. This window

transmits, with high efficiency, the reflected neutrons. However, the unreflected beam

continues with the window acting as a normal

neutron guide surface. This procedure allows

the insertion of a monochromator without the

need for a break in the guide. This will reduce

background as well as reducing beam attenuation

from windows and guide gap losses. The design

and operation of the neutron interferometry

facility is a collaborative effort between NIST

and the University of Missouri (Columbia). This

work is supported by NIST and the National

Science Foundation. Additional scientific collaboration involves faculty and students from

the University of Vienna and the University of

Innsbruck (Austria) as well as the University of

Melbourne (Australia).

Removabl

silicon window

FlBxture for

tilt odjuatment

Neutnon golds

Honocnromotor

Flexible coupler

Vacuum cf-iamber

fc—IJ—jU^Motor for

^ ^^^ angle adjustment

Motor for tilt adj ustment

Figure 2. Diagram of the monochromator assembly

which would allow rotation as well as tilting of a monochromator inside the guide.

154
NEUTRON INTERACTIONS AND DOSIMETRY

THE ELASTIC DEFORMATION OF A MONOLITHIC PERFECT

CRYSTAL INTERFEROMETER: IMPLICATIONS FOR GRAVITATIONAL PHASE

SHIFT EXPERIMENTS

H. P. Layer

and

G. L. Greene (Ionizing Radiation Division)

Colella, Overhauser and Werner [1] (COW)

first observed the quantum mechanical phase

shift induced by a gravitational potential on a massive particle (neutron). Their experiment

employed monolithic, perfect crystal neutron

interferometer whose orientation was varied with

respect to the local gravitational field (fig. 1). This and subsequent [2,1] experiments remain

unique in that a distinctly quantum mechanical

observable is influenced by gravity. The COW

experiment may be viewed [2] as an equivalence

principle test taken to a strong quantum limit, or

alternatively, as an essentially quantum method

for the determination of g^,, the local acceleration

of gravity for free neutrons. The principle of the COW experiment is exhibited in figure 1 . An input beam of thermal

neutrons is incident on the first plate of an LaueLaue-Laue monolithic interferometer. This

interferometer is fabricated from a "perfect"

single crystal of silicon having no dislocations

within its volume. In analogy with optics, an

intensity modulation at counter C2 or C3 will

appear if a phase shift is introduced between

paths ACD and ABD. In the COW type experiment, this phase shift is introduced by rotating

the interferometer about line AB.

This gravitational shift must be corrected

for several additional effects [3] the Sagnac

phase shift, which results from the Coriolis

acceleration due to the rotation of the earth, 2)

corrections [3,4] which result ft-om the Borrman

effect, and 3) dynamical diffraction corrections

which arise because the diffraction is taking

place in a non-inertial frame. Finally, there is the instrumentally induced phase shift which

results from the elastic deformation of the

interferometer which depends on the rotation

angle of the interferometer. In the experiment of

Werner, Kaiser, Arif, and Clothier, the phase

shift due to elastic deformation was measured by

repeating the COW experiment with x-rays.

He DETECTORS

INPUT BEAM

Figure 1. Experimental geometry to measure

neutron phase shift.

From such measurements it was observed that the phase shift due to elastic deformation,

'^^bend' ^^^ proportional to sine}). In view of the basic nature of the theory

which is tested in the COW experiment (nonrelativistic quantum mechanics and Newtonian

gravity), it is disconcerting to note that the two

most accurate COW experiments [2,3] yield

results which disagree with theory by several

times the quoted error. The final results of

Werner, Kaiser, Arif, and Clothier can be

summarized by an experimental and theoretical

value for Ap y. These were (in radians):

experiment: Apgj.^^ = (58.72 + 0.03)sin(t),

theory: Apg^^^ = (59.19)sin(t).

The discrepancy of Apj^eor-exp = (^-47

+0.03)sin<l) is well outside experimental error. The method used to experimentally measure

^^bend ^ssumes that the neutron and x-ray beams

had the same lateral extent. That is, they

"interrogated" the same regions of the silicon

monolith, and, thus, were influenced by the

gravitationally induced deformation in the same

way. It is our hypothesis that this was not the

case, and 1) the neutron diffraction occurred over an area of greater lateral extent than the x- rays, and 2) the gravitationally induced

155
NEUTRON INTERACTIONS AND DOSIMETRY

deformation caused the plates to be curved in the

lateral direction. The result is that the x-ray

diagnostic did not accurately measure ^R^^^nd ^^^ neutrons.

In this paper we calculate the elastic deformation of the monolithic interferometer

used in the COW experiments using the finite element method. We will analyze the recent measurement of Werner, Kaiser, Arif, and

Clothier [3] in which considerable attention was

paid to the deformation problem. However, our

conclusion will also be applicable to the original

COW experiments [1,2].

The model used in the finite element

calculation is made by subdividing the

monolithic interferometer into smaller regions,

each of which is in the shape of an irregular 8- node brick. Boundary conditions are established

by rotating the model into the appropriate

orientation, fixing the positions of the support

nodes and imposing a gravitational force at the

centroid of each brick. Handbook values for the

elastic constants and density of silicon were used

and isotropy was assumed. The program

calculates the stress and displacement at each

node. Considerable effort was taken to duplicate

the dimensions of the silicon interferometer and

to design the mesh (the placement and size of the

nodes and elements) to insure accurate results. The finite element method analysis has

correctly reproduced the angular dependence of

the x-ray gravitationally induced phase shift,

figure 2, and, in addition, has identified local variations of the deformation which affects the

neutrons differently than the x-rays, figure 3. The magnitude of the calculated result agrees

well with the measured discrepancy.

By remeasuring the gravitationally induced

x-ray path length change over a larger region

which more accurately represents the neutron

diffraction region, one can expect a further

resolution of this problem.

JOO -

50 - ^,<<^ — -r<rT

50 o ^ ^'^S^^^^^^ D SUPPORT SEPARATION - 40.75lim ^^^'"^ ICOMPUTeOI

- 6 O SUPPORT SePARATlON • 4?. TSmm

(COMPUTED} JOO ,1,1,1, 1

EXPERIMENTAL DATA

1 1 1 1 1 1 1

Rgure 2. Plot of the calculated and observed X-ray

gravitationally induced phase shift as a function of

interferometer rotation about the axis of the input beam.

This comparison was used to estabUsh the accuracy of the

finite element analysis.

Figure 3. Calculated shape of the deformed interferometer showing tilt and curvature of the plates. The scale of deformation is greatly exaggerated.

References

[1] R. Colella, A. W. Overhauser and S. A. Werner,

Phys. Rev. Lett. 34, 1472 (1975).

[2] J. L. Stauderman, S. A. Werner, R. Colella, and A. W. Overhauser, Phys. Rev. A 21, 1419 (1978).

[3] S. A. Werner, H. Kaiser, M. Arif, and R. Clothier,

Physica B 151, 23 (1988).

[4] S. A. Werner, J. L. Staudenmann, R. Colella, and

A. W. Overhauser, Neutron Interferometry , Bonse

and Ranch, eds. (Clarendon, Oxford, 1979) p. 209.

156
NEUTRON INTERACTIONS AND DOSIMETRY

THEORETICAL ANALYSIS OF FUNDAMENTAL TESTS OF QUANTUM

MECHANICS VIA NEUTRON INTERFEROMETRY

R. C. Casella

The discoveries in 1957 that the laws of

nature are not invariant under mirror reflection

(P) and in 1964 under the combined operation

CP (where C denotes exchanging matter with

antimatter) led some physicists to suspect the

fundamental CPT theorem, which follows from

little more than causality, i.e., local coupling

between particles and their fields. Because of its deep origins, most physicists bet on CPT,

whence yet another heretofore sacrosanct microscopic symmetry, time reversal (T) would

have fallen. An analysis by the author in the

late sixties of the (rather crude) data then

available showed that the latter was indeed true

[1]. To this day, CPT (and local coupling)

remain valid as they come under ever-closer

experimental scrutiny. Paradoxically, quantum

mechanics manifests a nonlocal aspect in that the

results of certain experiments depend only on the

so-called "topological charge" contained

between classically distinct orbits traversed by

one and the same particle, but are otherwise

independent of the detailed orbits. These

nonlocal effects and their topological nature, first discussed in the context of neutron

interferometry by Aharanov and Casher (AC)

[2], have been observed in pioneering

experiments by a team led by S. A. Werner at University of Missouri, at Columbia, MO and

G. Opat and A. Klein of the University of

Melborne, Australia [3]. The interpretation of

these experiments has been the subject of some

controversy as has the AC effect, itself. Is the

AC effect topological? If not, in what sense is the action of static electric fields on the passage

of neutrons through an interferometer nonlocal?

Does the neutron experience an acceleration? Is the effect really the dual of the (magnetic) phase

shift induced by an isolated coil on charged

particle, i.e., the effect introduced earlier by

Aharanov and Bohm (AB)? Is there an experimental way to tell? In the previous progress report [4] I showed how to apply the path integral technique

(introduced by Dirac and developed by

Feynman) to arrive at an especially clear way of

viewing these experiments. I subsequently was

led to suggest a new experiment to demonstrate

that while the AC effect is indeed topological in a two-dimensional context, the phase shifts introduced by static electric fields are in general

not topological, but are nonetheless nonlocal in that the measured phase shift results from the

transport along classically distinct paths of the

quantized spin of a single neutron. Such an

experiment, which is illustrated in figure 1, could be carried out at the interferometer

currently under construction at the CNRF at NIST. Although the electric-field-induced phase

shifts are nonlocal in the sense discussed, the

coupling is always local. In the AB effect the

charge couples locally to the magnetic vector

potential A. In the AC effect and in the

experimental configuration I have suggested the

magnetic moment of the neutron couples locally

to the magnetic field B ' = - v/c x E that exists

in the neutron's rest frame. Thus, no conflict

with the CPT theorem exists and the latter remains best tested via K° (and possibly B°)

experiments. Answers to the other questions

have also been obtained and the results published

in a Physical Review Letter [5]. Subsequently,

I have begun to examine the theoretical

underpinnings and physical implications of yet

another novel neutron-interferometer experiment,

which has been proposed by S. A. Werner [6].

157
NEUTRON INTERACTIONS AND DOSIMETRY

Figure 1. Arrangement of the capacitor plates in a neutron

interferometer (a) as in reference [3] where the axis of spin

quantization is normal to the orbital plane and (b) in the proposed experimental setup where the axis lies in the

orbital plane. In (a) the electric field E lies within the

orbital plane whereas in (b) it does not. For simpUcity, the

crystal mirrors have been omitted from the diagrams.

[Here, z labels the invariant axis of spin quantization, and

X]^ the path variable along the fu-st leg of the trajectory; yj^ (not shown) is such that (x, , y, , z) is a right-handed triad. In (a) the axes are rotated about z at each mirror such that Xj denotes the path variable (see text). In (b) Xi denotes

the path variable along the active legs. Along the legs

where E is null the path variable is -z, but the field-induced phase-change integrals vanish there. The physics, of

course, is independent of the arbitrary labels and the

quantization axis acquires a sense physically only in the presence of a polarizing biasing field.]

References

[1]

[2]

[3]

[4]

[5]

[6]

R. C. Casella, Phys. Rev. Lett. 21, 1128 (1968);

ibid. 22, 554 (1969). Y. Ahamov and A. Caher, Phys. Rev. Lett. 53, 319

(1984). A. Cimmino, G. Opat, A. Klein, H. Kaiser, S. Werner, M. Arif, and R. Clothier, Phys. Rev. Lett. 63, 380 (1989). R. C. Casella, NIST Tech. Note No. 1285, C. O'Connor, ed. p. 89 (1990). R. C. CaseUa, Phys. Rev. Lett. 65, 2217 (1990).

S. A. Werner (private communication).

158
F. FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

A significant part of on-going research

programs includes the design and construction of new instruments. Associated with this is development of neutron experimental and dataanalysis techniques to utilize available flux in the

most efficient manner possible. In this section,

new instrumentation and methods—primarily

related to the CNRF—are described. Also

included are a summary of reactor operations

and engineering activities for the year, and some

developments in the area of health physics

relevant to NBSR-based programs.

COLD NEUTRON PROJECT

G. M. Baltic, J.R.D. Copley, D. H. Fravel, C. J. Glinka, J. A. Gotaas,

G. C. Greene, W. A. Kamitakahara, W. C. Knill, J. G. LaRock, H. P. Layer, C. J. Majkrzak,

C. O'Connor, D. J. Pierce, H. J. Prask, M. J. Rinehart, J. M. Rowe, S. K. Satija, I. G. Schroder,

T. Thai, T. J. Udovic, and R. H. Williams

In the period from October 1990 through

May 1991 four instruments of the CNRF became

operational: the 8-m SANS spectrometer, the

cold neutron depth-profiling station, the cold

neutron prompt-gamma activation analysis

spectrometer, and the 30-m NIST/Exxon/U. of

MN SANS spectrometer. The neutron lifetime

determination apparatus at the fundamental

neutron physics station, the CHRNS 30-m SANS

spectrometer (part of the NSF/NIST Center for

High Resolution Neutron Scattering), and the

medium resolution time-of-flight spectrometer

are all being installed. All of the completed

stations have already been used for NIST,

collaborative, and independent guest researcher

experiments, the details of which are given

elsewhere in this report. Design of the new,

hydrogen cold source is nearing completion, and

a new higher capacity He refrigerator has been

delivered to Reactor Radiation Division (RRD).

The CNRF, which is a response to a critical national need for a cold neutron measurement

capability, is a National Facility; to this end, the

experimental stations installed are available to the scientific and engineering community on the

basis of scientific merit of proposals or in Participating Research Teams. The Program

Advisory Committee (PAC) has a critical role in advising NIST on the utilization of the facility. In February 1991 the PAC for the CNRF met at NIST for the second time along with the newly

constituted Executive Committee (EC) of the

CNRF Researchers' Group. The PAC members

are S. K. Sinha, Exxon Research and

Development Corp.; D. L. Price, Argonne

National Laboratory; E. A. Schweikert, Texas

A&M University; and H. Yu, University of

Wisconsin. EC members are Chairman T. P. Russell, IBM, San Jose; R. Cappelletti, Ohio

University; T. Hossain, Texas Instruments; and

J. Weertman, Northwestern University. In this meeting, organized primarily by Bill Kamitakahara—the NIST scientific coordinator

for the program, final recommendations and

timelines were proposed for the CNRF

researcher program. Based on the recommendations made, details of the program have been

developed with the following actions taken:

- The first Call for Proposals was sent out to researchers at U.S. university, industrial, and

government laboratories in mid-June.

- The deadline for proposal submission was

September 1, followed by mail review, and

beam time allocation by the PAC.

- The first cycle of instrument time for the

CNRF as a National Facility will begin in November 1991.

Two workshops were held during this reporting period. In May, the University of

Minnesota's Center for Interfacial Engineering

(an NSF Engineering Research Center collaboration involving some 30 affiliate companies)

conducted with NIST staff a workshop to familiarize CIE researchers with neutron techniques for interfacial applications. Ten CIE

affiliate companies participated. In June, over

159
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

150 scientists from academia, industry, and

government laboratories attended the Workshop

on Applications of Cold Neutron Spectroscopy in

Chemistry, Biology, and Physics, conducted by

RRD. The purpose of the meeting was to explore some of the exciting new opportunities

which will be made available to U.S. researchers

when six new high-resolution inelastic scattering

spectrometers are completed at the CNRF. The

attendees represented 55 U.S. universities,

industrial, and government laboratories, and

included 13 representatives from foreign

countries. Presentations made by invited

lecturers gave overviews of the principles and

practice of neutron spectroscopy, and the wide

range of science that has been done and is being

done with cold neutron spectrometers.

Following the workshop, a one-day

Researchers' Group meeting was held. About

65 scientists were in attendance, of whom 47 were from institutions other than NIST. The

formal part of the meeting consisted of a status report on the neutron beam facilities and detailed

technical presentations on experimental stations

by NIST scientists. Many important issues were

discussed which should help NIST to better meet

the needs of the multidisciplinary science and

engineering community which will perform

research at the CNRF.

Relating to instrument development, the

following collaboration was initiated in this period. The Department of Physics and

Astronomy at the Johns Hopkins University has

agreed to participate in the development of a cold neutron inelastic scattering spectrometer for

materials research in the areas of condensed

matter physics, and magnetic materials

characterization, including heavy fermion

systems. Johns Hopkins joins the NSF, Exxon,

the University of Minnesota, IBM, Sandia

National Lab, and NIST as PRT members for

CNRF instruments.

In April the third issue of the Neutron

Standard, the RRD and CNRF newsletter, was

printed and sent to approximately 2500 scientists and engineers from specialties which the CNRF

could benefit. Over 1300 positive responses

expressing interest have been returned by

Neutron Standards recipients.

THE CHRNS-SANS INSTRUMENT AT NIST

B. Hammouda, C. Glinka, S. Krueger, C. H. Chen, J. Moyer, and W. E. Dickerson

The 30 m Center for High Resolution

Neutron Scattering (CHRNS)-Small Angle

Neutron Scattering (SANS) instrument has many

similarities with the NIST/EXXON/ University

of Minnesota (UofMN) 30 m SANS that has

been in operation since April 1991. The

CHRNS-SANS, however, uses a single high data

rate detector of the ILL-type in a slightly

different vacuum flight path that enables the

detector to approach within 1 .2 m of the sample.

This results in an instrument that is simpler to construct and use, and which has a Q-range that

is nearly as wide (0.01 to 6 nm"^ compared to 0.01 to 10 nm-i for the NIST/EXXON/UofMN

SANS). A schematic of the CHRNS-SANS

instrument is attached (fig. 1), and a brief

description of its characteristics is included in

table 1. Construction of the 30 m CHRNSSANS instrument is now about 90% complete.

It will operate on a dedicated neutron guide,

NG-3, that is scheduled for installation this winter (1991-1992). The CHRNS-SANS instrument will be completed and operational by the

end of 1991.

A set of user friendly SANS software

packages have been developed at NIST for data

acquisition and instrument control on the two

new 30 m SANS instruments as well as on the

old 8 m SANS instrument. These consist of:

(1) a screen management system that provides a menu driven user environment for data

acquisition, (2) a set of computer programs that control the data acquisition hardware which uses CAMAC modules with Direct Memory Access

160
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

to a Vax-3400 computer, (3) a software package

that controls the various instrument motors

through an RS232 interface, (4) a graphics

software that allows for online imaging of the

data, and (5) a set of data treatment programs

for the reduction and preliminary analysis of the

data on the Vax-3400 computer. The ability for the users to take reduced data back home on IBM-PC and Macintosh diskettes is being

provided.

CHRNS 30 METER SANS INSTRUMENT

20 DETECTOfl

PRE-SAM=LE FLOHT PATH

16 M \

POST-SAMPLE FLIGHT PATW

13 M \

Figure 1. Schematic of the CHRNS-SANS instrument

Table 1 . Main characteristics of the CHRNS-SANS instrument

Source Size: Wavelength Range:

(velocity selector) Wavelength Resolution: (continuously tunable) Q-Range:

Sample size: Expected Flux at sample:

6 cm X 6 cm

0.4 nm - 2.0 nm

7% - 30%

0.01 nm"^ - 6 nm'^

0.5 - 2.5 cm

o ft -0 1 10 to 10 cm s" depending on sample-detector distance and wavelength

THE 30 METER NIST/EXXON/UMVERSITY OF MINNESOTA SMALL ANGLE

SCATTERING INSTRUMENT

C. J. Glinka, J. G. Barker, B. Hammouda, S. T. Krueger, J. J. Moyer,

A. E. Heald, W. Dickerson, and C. H. Chen

and

M. Y. Lin (Exxon Research and Engineering Co., Annandale, NJ)

The CNRF's second SANS instrument, a 30 m long, high resolution instrument developed

jointly by NIST, Exxon Research and Engineering Company, and the University of Minnesota

is now in operation in the guide hall at NG-7.

The instrument was officially commissioned by

representatives of the three institutions at a brief

ceremony held at the CNRF on May 10, 1991.

This instrument extends the measurement

range of the 8 m SANS instrument it

complements by nearly a factor of five and, in addition, offers a wide range of measurement

conditions whereby the resolution and beam

intensity can be varied to meet the requirements

of an experiment. A rotating helical channel

velocity selector with variable speed and pitch

provides the capability to vary both the

wavelength and wavelength resolution of the

incident neutron beam over a wide range. Incorporated in the instrument's 15 mpre-sample

161
FACILITIES, OPERATIONS, AND TECHNIOUE DEVELOPMENT

flight path are eight 1.5 m long neutron guide

sections that can be easily shifted in or out of the

beam to vary the beam divergence and flux on

the sample by changing the effective source-to- sample distance from 3.0 to 15 m in 1.5 m

increments.

To provide an exceptionally wide measure- ment range, the instrument's 15 m post-sample vacuum flight path has been constructed to house

two large 2-d detectors; a primary detector that moves axially within the cylindrical portion of

the flight path, and a second detector that moves

along a circular are (over a 40° range) at a fixed

distance of 2 m from the sample. A layout of

the instrument is shown in figure 1 and its characteristics are summarized in table 1 . A

comparison of the neutron intensity at the sample

(for a 1.5 cm diam sample and wavelength

resolution AX/X = 25%) versus minimum

accessible Q-value for the 8 m and 30 m SANS

instruments is shown in figure 2. The instrument is presently operating with

only its primary detector; the wide-angle detector

will be installed in 1992. Meanwhile, development work continues on two novel components

for this instrument, which if successful will

significantly expand its measurement capabilities.

These are a Fe/Si supermirror array for polarizing the incident beam, and a double curved,

grazing incidence mirror (discussed on p. 169 in

this report) to increase the flux on the sample.

Table 1. 30 m SANS: Instrument characteristics and performance

Source:

Monochromator:

Wavelength Range:

Wavelength Resolution:

Source-to-Sample Distance:

Sample-to-Detector Distance:

Collimation:

Sample Size:

Q-range:

Size Regime:

Detectors:

Ancillary Equipment:

neutron guide (NG-7), 5x5 cm^

mechanical velocity selector with variable speed and pitch

0.5 to 2.0 nm

10% to 30% AX/X (FWHM)

3.0 to 15 m in 1.5 m steps via

insertion of neutron guide segments

primary detector - 3.5 to 15 m

wide-angle detector -2m (fixed),

angular range from 0° to 40°

circular pinhole collimation

0.5 to 2.5 cm diam

primary detector - 0.01 to 2.5 nm'^

wide-angle detector* - 0.5 to 10.0 nm'^

0.5 to 500 nm

65 X 65 cm^ -^He position-sensitive

proportional counters (1 cm^ resolution)

- automatic multi-specimen sample changer

with temperature control from -10 to 200 C

- electromagnet (0 to 1.5 Tesla)

- couette type shearing cell Neutrons on

Sample vs. Q^i„ Qmin(nm- ') 1/ (n/s)

0.01 2.5 X 10- 0.02 3x lO'^ 0.04 3x 10^

0.10 1 x 10^

I

available in late 1992

^measured with AX/X = .25, 1.5 cm diam sample

162
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

30 METER SANS INSTRUMENT

2D DElECTOn^

iig^r^

POST-SAf*>L£ RJWr PATH

15 M \

Figure 1. NIST/Exxon/University of Minnesota SANS spectrometer.

Intensity at sample

1000

Figure 2. Comparison of neutrons/sec on a 1.5 cm diam sample versus minimum accessible Q-value for the CNRF's 8 m

and 30 m (NIST/Exxon/U Minnesota) SANS instruments (for Ak/X = 0.25).

MULTIPLE DISK CHOPPER TEME-OF-FLIGHT SPECTROMETER

J. R. D. Copley

The chopper system for this instrument

[1] comprises four vacuum housings, seven

motor/bearing assemblies, seven chopper disks,

and a PC-based control system. Delivery is anticipated in 1992. The design of the guide is almost complete, as is the design of masks

which will limit the number of guide channels

through which neutrons are permitted to pass. The detailed design of the sample chamber and

detector flight path is expected to begin very

soon. Prototype detectors and detector

amplifiers have been delivered and are being

evaluated; we expect to place an order for a large number of detectors and amplifiers within

the new few months. The instrument computer

has also been ordered. Our work on the

transmission properties of a separated pair of

counter-rotating choppers [2] has been

published [3], and an overall review of the

project was presented at the International

Conference on Neutron Scattering in Oxford in

August 1991.

163
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

References

[1] J. R. D. Copley, NIST Tech. Note 1285, C. O'Connor, ed. p.lOO (1990).

[2] J. R. D. Copley, NIST Tech. Note 1285, C. O'Connor, ed. p. 103 (1990).

[3] J. R. D. Copley, Nucl. Instr. Meth. A 303, 332

(1991).

COLD NEUTRON DEPTH PROFILING

G. P. Lamaze, R. G. Downing, J. K. Langland (Nuclear Methods Group)

and

S. T. Hwang (KSRI, Taedok Science Town, Taejon, Korea)

The Cold Neutron Depth Profiling (CNDP)

facility at Cold Tube West (CTW) became

operational on October 31, 1990. The neutron

beam is filtered by 13.5 cm of single crystal

sapphire which has the effect of reducing the

slow neutron fluence rate by 1/3, but the fast neutron fluence rate by a factor of about 500.

Collimators are located both within the biological

shield and in the external-to-the-shield rotating

shutter. The shutter is two cylinders, whose

beam tubes fully align in the beam open

configuration and are nonaligned in the beam

closed configuration to reduce streaming. The

collimators in these shutters can be accessed in the beam closed configuration with the reactor at

full power. The diameter and intensity of the

neutron beam can then be modified at any time

to suit the needs of a particular experiment. The

measured neutron fluence rate (capture flux) at the target position with the 16 mm diam

collimator is 1.2 x 10^ cm'V^ A pancake

fission chamber mounted on the entrance port of

the NDP chamber provides a run-to-run monitor.

The target chamber was obtained from a commercial vendor using a design developed at NIST. The entire chamber is stainless steel and

uses copper gaskets at all but three sealing

surfaces: the beam entrance and exit windows

and the opening used for changing samples.

This last surface can use either viton o-rings or copper gaskets. In practice, the desire to quickly change samples usually outweighs the

need for ultra high vacuum, but that capability

does exist. The chamber itself is a 61 cm diam

cylinder with access ports in the top and bottom

plates as well as through the side walls. All

flanges conform to standard Conflat flange

specifications. The standardized flanges will

allow new features to be easily added in the

future. Some of these include in-situ cleaning of

samples, time-of-flight measurements, heating

and cooling of the sample, and cooling of the

surface barrier detectors.

The beam enters and exits through 10-cm

diam ports sealed with thin aluminum windows.

These can be replaced with sapphire windows if a metal gasket seal is required. The chamber is evacuated with a 1 80 1/s magnetic bearing turbo

molecular pump. This pump was chosen to reduce microphonic effects on the charged

particle detectors. The detectors are surface

barrier detectors in a ring mount. A rotary base

positions the charged particle detectors about the

axis of the sample. Detectors can be placed at any angle and detectors can be mounted every 10

degrees. Currenfly, there exists electronics to operate four detectors simultaneously.

Samples (up to 15 cm in diam) are mounted

on a set of motor driven positioners. A rotary

base selects the angle of the sample with respect

to the beam. The ability to rotate the detectors

and sample independenfly allows the detector to be positioned at any angle with respect to the

sample without putting the detector in the beam.

Mounted on top of the sample rotator are x and

y positioners. These have 15 cm of travel each,

allowing a full scan of 15 cm diam samples. All

four positioning devices are controlled by a PC

compatible microcomputer. A program has been

written in BASIC to enable unattended sample

164
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

scans. The signals from the surface barrier

detectors are processed in a standard fashion and

the amplified signals are sent to Analog-toDigital converters interfaced with a multiuser

minicomputer. This computer can

simultaneously process data from both the

thermal and cold NDP facilities. Spectra from

these computers can then be transferred to a

variety of other computers for data reduction,

plotting, etc. Figure 1 shows a sample spectrum taken at the cold NDP facility. The sample of CobaltNickel Oxide (^^O enriched) was prepared by

Eastman Kodak. To our knowledge, this is the

first nondestructive determination of near-surface

oxygen made by NDP anywhere in the world.

The ability to obtain oxygen profiles is a

significant addition to the elements that are now

analyzed routinely at NIST: boron, lithium,

nitrogen, and helium.

^

3.0

2.5-

S 2.0

^ 1.5

S

1-°

0.5:

0.0

Oxygen- 17

?

^i-%^::^^i^-

Oxide Surface

Boron- 10

Suriacc Contaminant

Tff.^x>^vife»?:Sn'ffi-^

1.0 0.8 0.6 0.4 0.2 0.0 Depth (micrometers)

1 T Rgure 1. Neutron Depth Profile of O sample in metal

oxide.

PROMPT-GAMMA ACTIVATION ANALYSIS WITH COLD NEUTRONS

R. M. Lindstrom (Inorganic Analytical Research Division)

A permanent, full-time instrument for

prompt-gamma activation analysis (PGAA) is being constructed as part of the Cold Neutron

Research Facility. As a result of a compact

sample-detector geometry, the sensitivity

(counting rate per gram of analyte) is a factor of

four better than the existing Maryland-NIST

instrument. Hydrogen backgrounds of a few

tens of micrograms have already been achieved,

which promises to be of value in numerous

applications where quantitative nondestructive

analysis of small quantities of hydrogen in materials is necessary. Further reduction by one

to two orders of magnitude is probable in the near future. Experience gained in more than a decade's

operation of the Maryland-NIST thermal

instrument, and with cold neutrons at the

German research center in Julich, has been

incorporated in the design of the instrument at the CNRF, to give high gamma detection

efficiency, low background (particularly for

hydrogen), and facile operation. The instrument

is installed on neutron guide NG-7 in the CNRF

hall. A filter of 152 mm of single-crystal Bi and

127 mm of Be, both at liquid nitrogen

temperature, is installed in the guide 3.1 m

upstream from the PGAA sample position.

The apparatus is simple: the lower 50 x 45

mm of the 50 x 110 mm NG-7 guide is extracted

into air through a window of magnesium 0.25

mm thick. (The upper 50 x 50 mm beam

continues past the PGAA station another 2 m to the velocity selector of the 30-m SANS

instrument.) A translating shutter of Li glass 15

mm thick just behind the window admits the

neutrons to the sample. The 10-mm thick glass

slab which forms the bottom of the continuing

guide is capped with 15 mm of Li glass to avoid background from neutron scattering and

capture. The neutron fluence rate was measured

with 25 nm Au to be 1.5 x 10^ cm'^^s"^ (thermal

equivalent: using o = Ojj^ = 98.65 b) at a reactor power of 20 MW.

The 1-m section of neutron guide adjacent

to the PGAA station is made of boron-free

165
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

silicate glass in order to avoid generating large

amounts of ^^B capture gamma radiation from

neutron leakage due to imperfections and

misalignment in the guide. The outside of this guide section will be covered with a material

high in ^Li and low in H, such as ^LiF-graphite

paint. A plate of fused ^Li2C03 with a 20-mm

aperture, placed just behind the shutter,

collimates the neutron beam to a size not much

larger than a typical sample. A beam stop of

^Li glass is placed behind the sample. Because

of the purity of the neutron beam, shielding

requirements are modest and scatter-capture

background is low. Secondary fast neutrons

generated in the collimator and beam stop by

reactions of the fast tritons from ^Li(n, a )^H have

not yet been troublesome. Hydrogen-containing

materials have been avoided to the maximum

extent possible in the vicinity of the sample and

detector. Samples are held in the beam in envelopes of 25-iim Teflon FEP between the

prongs of a supporting fork. The volume

between the neutron collimator and the beam

stop assembly can be flooded with He at atmospheric pressure in a Teflon tent to reduce

air scatter. A Ge gamma-ray detector (27% relative

efficiency, 1.7 keV resolution) views the sample

through a ^Li-glass window along an axis at right angles to the neutron beam. The field of

view of the detector is collimated so as to view

the sample. Environmental gamma radiation is reduced by shielding the detector with at least 100 mm of lead in all directions. The lead in turn is shielded from stray neutrons (which

produce 7-MeV Pb capture gamma rays) with

sheets of Boral. The detector shield rests on a

table, the top of which is adjustable vertically

and parallel to the beam with leadscrews. The

table rolls on a track perpendicular to the beam

to adjust the counting distance, which can be as close as 200 mm from the sample to the front

face of the detector. Experiments are

continually under way to optimize the shielding

in the vicinity of the sample-detector assembly.

Gamma-ray spectra are acquired with a 16384-channel analog-digital converter coupled

to a multichannel pulse height analyzer (ND556

AIM). The AIM is controlled over the buildingwide Ethernet by Nuclear Data acquisition and

display software on a VAXstation 3100.

Several measurements have been made in

the short time that this cold-neutron instrument

has been in operation. Sensitivities of a number

of elements were compared with those of the

thermal Maryland-NIST PGAA instrument. At

the same reactor power, sensitivities for most

elements are a factor of four to six better with

the cold-neutron apparatus than with the thermal

instrument.

The first measurement with cold neutron

PGAA in the CNRF was the determination of

hydrogen in a sample of C^ fullerene

"buckyballs" intended for neutron scattering

studies. Cold-neutron PGAA found a hydrogen

concentration of 0.92 + 0.09 wt %, which was

too high for satisfactory scattering

measurements. After repurification of the

material, hydrogen was measured again. A

600-mg sample, contained in the aluminum

sample holder intended for the scattering

measurements, was irradiated in the PGAA beam

for 100 minutes live time while surrounded with

a flowing atmosphere of He contained in a Teflon tent. A clear H peak was visible at more

than ten times the intensity of a blank sample of

spectroscopic grade graphite. The sample was

found to contain 0.077 + 0.014 wt % H, which was adequately low for the neutron scattering

measurements. In other work, hydrogen was

measured in a Pr2Cu04 superconductor analog;

a 1-hr irradiation gave a concentration of [H] =

0.017 ± 0.010 wt %. Hydrogen was sought in a 50-nm borophosphosilicate glass film on a quarter of a 10-cm silicon wafer; an upper limit

of 2 ng/cm^ was found.

Future enhancements to the counting

system will include a Compton suppressor, an

automatic sample changer, and a second detector

system which will permit y-y coincidence

measurements. The difficulties associated with

working adjacent to the upper guide—restricted

space and Si capture background—may be

ameliorated in the future by deflecting the PGAA

beam away from the guide. Additional improvement in sensitivity is possible since neutron

optics may be used to focus cold neutrons onto

a small area. Gains of an order of magnitude in

fluence rate may be obtainable.

166
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

PRELIMINARY DESIGN OF THE LIQUID HYDROGEN

COLD NEUTRON SOURCE

P. Kopetka, J. M. Rowe, and R. E. Williams

A liquid hydrogen cold neutron source has

been designed to replace the heavy ice cold

neutron moderator chamber now in use. While

a fmal choice of the LH2 chamber geometry has

yet to be made, a 3 to 5 liter vessel will be

positioned as close as possible to the reactor

core. Major features of the system are independent of this choice, however, and a preliminary safety analysis report has been

prepared and distributed locally. The design

philosophy is to install a simple, passively-safe

system, with multiple barriers preventing oxygen

from mixing with hydrogen.

Source Geometry. Until very recently,

the choice for the LH2 moderator was two rows

of vertical tubes, 30-cm tall and with 1.6-cm

radii, as shown in figure 1. With seven tubes

facing the neutron guides, the source presents an

area of more than 900 cm^. A large area is needed to fully illuminate the guides because

their point of intersection is located about 25 cm

in front of the source. Monte Carlo neutron

transport calculations using MCNP indicate gains

of a factor of 2 to 3 over the existing source for

wavelengths greater than 4 A. Approximately

2.5 L of LH2 at 20 K and 1.5 atmospheres will

fill the tubes, assuming a density of 0.056

g/cm^, corresponding to a 20% void fraction.

Liquid hydrogen would be delivered to the

bottoms of the tubes through 13 lines from a

liquid manifold, and the vapor from the tops of

the tubes returned via 13 lines to a gas manifold.

The manifolds are connected to the hydrogen

condenser, completing a thermosiphon. The

bismuth tip surrounding the existing source will

be removed, allowing the new source to be

located much closer to the core than the heavy

ice. The tubes would be mounted in an

insulating vacuum, which will be surrounded by

a He-filled containment. The cryostat shell will

be cooled by a layer of D2O.

Recent MCNP results have shown a chamber in the shape of either a spherical shell or a cylindrical annulus will produce substantially higher yields of long wavelength neutrons

than the tube source. The voids in the centers of

these sources act as cold-neutron flux traps, as demonstrated by the performance of the new

NC-O

Figure 1. Plan view of a liquid hydrogen cold neutron source consisting of 13

vertical tubes.

167
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

cylindrical LH2 cold source at the Orph^e

reactor in Saciay, France [1]. In our case, the

chamber would be larger than the Saciay source, and would have a reentrant hole facing the

guides. The inside chamber may have very thin

walls because the void is maintained by

hydrogen vapor trapped inside. Most of the heat

is generated in the outer wall. Liquid is introduced at the top of the irmer (cool) wall,

flowing downward by gravity to the chamber

bottom, thus establishing a naturally circulating

loop, with vapor exiting at the top. An effort is now underway to model the boiling heat transfer

in these sources, and to optimize the shape parameters for the most favorable neutronics. A

shell type source would require 4 to 5 L of LH2

It would be mounted in an insulating vacuum

identical to that described above.

System Description. A schematic diagram

of the system is shown in figure 2. The choice

of moderator geometry has very little impact on

the rest of the system, which is designed to be as safe and simple as possible. The hydrogen

system is a closed loop thermosiphon with no moving parts. There will be a minimum of gas

handling because once the system is charged

with hydrogen, it need not be reopened except to repair a major failure. A series of large ballast tanks, always open to the LH2 chamber, insure

low system pressures in the event of refrigerator

failure and vaporization of the entire LH2

inventory. The ballast volume will be chosen so

the maximum pressure will be 3 to 5 atmospheres. A large fraction of the H2 inventory

will be liquified while the cold source is operating, as the cold operating pressure will be

0.6 to 1.5 atmospheres.

A naturally circulating loop will be

established as the hydrogen vapor produced in the moderator chamber rises to the condenser, is liquified, and flows by gravity back to the

chamber. Such thermosiphons, requiring no pumps or blowers, are being used at Saciay and

the ILL reactor in Grenoble. The condenser,

located outside of the reactor biological shield,

will be cooled by He gas at 18 K provided by

the 3.5-kW refrigerator now installed and in its final phase of acceptance tests. All parts of the hydrogen system will be

located to minimize the possibility of accidental

ruptures. The moderator chamber will be in core and out of harm's way. The condenser will

Hydrogen Condenser

r<#>^ L»—

(

Figure 2. Cold source schematic. The moderator shown consists of vertical tubes. A different choice of moderator chamber changes only the interior of the cryostat.

168
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

be located at the face of the reactor shield, out of reach of the crane, behind a steel shroud.

The ballast tanks will be mounted in the cavities on the ceiling, above the path of the crane. Every precaution will be taken to prevent

the mixing of oxygen and hydrogen. The H2

used in the system will be checked to assure a

purity of 99.9995% before it is introduced to the

system, and checked periodically for the

presence of O2, as will the He containment

volumes. There are three barriers preventing the

introduction of air into the LH2 chamber: the

hydrogen volume boundary, the insulating

vacuum, and the He containment volume, maintained at 2 psi above atmospheric pressure. System pressures are constantly monitored for

evidence of leaks. The existing heavy ice source

has such a triple containment scheme. The

warm hydrogen components are also surrounded

by He containment volumes so leaks may be

detected before mixing with air can occur. Safety Analysis. Although the cold source

is designed to minimize the probability of mixing

hydrogen and oxygen, it is necessary to analyze

the effects of accidents resulting from system

failures. There has been a great deal of research on hydrogen deflagration and detonation, making

possible such an accident analysis. Results

indicate:

(1) Overpressure due to LH2-solid O2

reactions will not exceed the working

pressure of the hydrogen system.

(2) Detonation of the Hj inventory in room C100 will not result in structural damage in an unconfmed vapor phase explosion.

(3) A catastrophic rupture of the lines to the

cryostat vacuum chamber, followed by a release of H2 into this volume containing

condensed O2 and subsequent detonation,

will not exceed the bursting pressure of the vacuum chamber.

(4) The detonation of a stoichiometric air- hydrogen mixture in the moderator will not rupture the hydrogen container.

No credible accident can result in damage to the

reactor or breach the confinement boundary.

The Safety Analysis Report will be

completed this fall, as decisions are made

regarding the moderator chamber geometry.

The new cold source is expected to be ready for

installation at the same time as the installation of

the remaining neutron guides.

Reference

[1] B. Famoux, Mesures de Qualification de la Source

Froide Annulaire, private communication, Dec.

1990.

DEVELOPMENT OF A FOCUSING MIRROR FOR SANS

J. G. Barker, C. J. Glinka, C. F. Majkrzak, and S. K. Satija

The high resolution small angle scattering

spectrometer will use a doubly curved grazing

incidence mirror described in ref. [1]. Before

coating the cylindrical mirror sections, scattering

measurements were made on additional test mirror surfaces to determine whether the amount

of nonspecular scattering produced by the mirror

surface roughness is tolerable. Measurements were made on one of the cylindrical quartz

sections to be actually used in the focussing

mirror construction, on nickel coated and

uncoated sections of a Borkron glass, and on a

nickel coated float glass. Optical profilometry

results indicate that all the surfaces have similar

mean roughness, o = 0.4-0.6 nm.

The measurements were made on the 8 m

SANS instrument in a configuration that allowed

the scattering (specular plus nonspecular) from

the mirror specimens to be measured over nearly

six orders of magnitude in intensity. The

scattering was found to be concentrated

perpendicular to the mirror surface. A plot of

the scattering in this direction is shown in figure

1 with the scattered intensity normalized by both

the beam intensity incident on the mirror and

detector solid angle. The uncoated mirrors were

tested at a lower incident scattering vector

(q^ = 0.1 nm"^), to achieve complete reflection.

The scattering roughly follows a power-law:

I « q'^. Table 1 lists the surface roughness

169
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

and scattering at q = 0.15 nm"^ for each mirror.

The mirrors have similiar mean roughness but

exhibit an order magnitude difference in scatter- ing at small angles. This indicates that the

distribution of the surface roughness is as important as the mean roughness in determining

the magnitude of nonspecular scattering at small

angles. Previous X-ray measurements on mirror

surfaces indicate that the surface roughness

correlation function can be approximated by an

exponential distribution [2]. According to the

Kirchoff approximation [2] , the nonspecular scat- tering at angles large compared to the angle of

incidence follows the observed law: I^^ « q'^.

The background from the uncoated cylindrical mirror is considered satisfactory for

the current application. Before coating the

mirrors, further testing is being conducted to improve the coatings. The 30-m SANS spectrometer will extend the present measurements to smaller angles designed for the focussing mirror

( q = 0.01 nm"^).

. 1 . 1 , , '-r , , . 1 . , , , ,~r

:

• Float Glass (Nickel)

r- 2

Borkron (Nickel) Borkron (uncoated)

;

0)

•

» Concave Quartz :

J 1 ^

-;

0.2 0.4 0.6 0.8 1 q (nm')

Figure 1. Plot of the measured nonspecular scattering as a

function of 9 for different mirror surfaces.

Table 1. Mean surface roughness and measured background scattering at q = 0.15 nm"

Mirror

Ni Coated Float Glass Ni Coated Borkron

Uncoated Borkron

Uncoated Concave Quartz

0.45 nm

0.55 0.55

Si0.15 nm

0.15 0.10

0.10

-1 !„. (q=0.15 nm'l)

0.9 ster'^ 1.45 0.54

0.12

References

[1] C. F. Majkrzak, C. J. Glinka, and S. K. Satija, SPI E 983 Thin-Film Neutron Optical Devices 1 29 ( 1 988)

[2] B. Aschenbach, Rep. Prog. Phys. 48, 579 (1985).

170
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

TRANSMISSION PROPERTIES OF CURVED-STRAIGHT

NEUTRON GUIDE SYSTEMS

J. R. D. Copley

and

D. F. R. Mildner (Nuclear Methods Group)

Curved neutron guides are often used to reduce fast neutron and gamma ray background.

They have the inherent disadvantage that the

transmitted neutron intensity distribution is spatially asymmetric; this asymmetry, which

depends in part on the dimensions of the guide,

is particularly pronounced at short wavelengths.

The idea of adding a straight section of guide at die end of a curved guide has been suggested [1] as a method to reduce this asymmetry and to make the beam more uniform. We have

examined [2] the behavior of neutron trajectories

within curved-straight neutron guide systems,

using both ray-tracing and analytical approaches

to the problem. Trajectories which correspond

to neutrons transmitted by the curved guide [3] are used as input for the straight guide

calculations, and distributions are determined as

a function of the length of the straight guide.

We have described the construction of the

acceptance diagram at the exit of a straight guide

of arbitrary length L following a curved guide

which is at least as long as the line-of-sight

length, L^. The acceptance diagram at the exit of the curved guide is bounded by a parabola

which depends on the critical angle 6^ of the

guide surface (and therefore on the neutron

wavelength), and on the characteristic angle ij/^ of the curved guide (and therefore on the guide

dimensions). In the absence of the straight

guide, the acceptance diagram at a distance L

from the curved guide has shifted in phase

space, and is bounded by a similar parabola,

though its vertex is shifted. Using the method of

rotation of acceptance diagrams [4], we have

constructed the phase diagram at the exit of a

straight guide of arbitrary length.

The characteristic angle or wavelength of

the curved guide determines properties such as the spatial intensity distribution function N(z),

which represents the intensity at position z

within the guide, integrated over the angular

variable \|r. This function changes as neutrons

propagate down the straight guide. Examples

are shown in figure 1 . At the entrance of the

straight guide N(z) is a parabolic function but its form becomes more complicated as L is increased. The intensity tends to slosh from one

side of the guide to the other as neutrons

proceed down the guide, like water in a bath

tub, and this transverse wave dampens out with

increasing L. At sufficiently large distances it tends to the constant value Nq. We find that neutrons of a given wavelength X move through

the straight guide in such a way that the

distribution is both better centered and more

uniform at some locations than at others further

downstream.

T" ' ' T 1.75

b)

2.0O

2,25

2.50

325

4.00

—

1 H

5.00

0.00

0.25 H i h

0.50

-I 1 H

0.75

1.00

H \ 1-

1J25

H 1 1-^N

1.50

1.75

2.00

2.25

2.50

3.25

4.00

H H

5.00

(z/H) (z/H)

Figure 1. N(z)/2e^ for $ e^/>|r^equal to (a) 0.8 and (b)

1 .2, for values of (i|f /i|; ), indicated on the plots, from to

The straight guide length L is characterized

by the angle tItq = H/L where H is the width of

the guide. We use two parameters to characterize the spatial distribution N(z): the normalized

first moment < z> /H, or effective center of the

distribution, and the normalized standard

171
FACILITIES, OPERATIONS, AND TECHNIOUE DEVELOPMENT

deviation o^/Nq which is a measure of the

uniformity of tiie beam. Values of these

quantities are shown in figure 2 as a function of

^c^^o (which is proportional to L), for different

values of ^ = dj\\f^ (which is proportional to X).

For increasing straight guide length L, the

first moment oscillates about zero with

decreasing amplitude, indicating that the intensity

distribution is becoming better centered. The

asymmetry and non-uniformity introduced by the

curved guide, though more pronounced at wavelengths less than the characteristic

wavelength of the guide, dampen out more

rapidly as a function of straight guide length than

in the case of neutrons of longer wavelength.

The distance between successive nodes becomes

shorter at longer wavelengths. There is a

correlation between the positions of the nodes of

<z>/H and the positions of the minima in

Oj^/Nq. The standard deviation dampens more

rapidly for shorter wavelengths, though the

distance between successive externa is greater.

References

[1] C. J. Carlile, M. W. Johnson and W. G. Williams, "Neutron Guides on Pulsed Neutron Sources", report RL-79-084 (Rutherford Laboratory, 1979).

[2] J. R. D. Copley and D. F. R. Mildner, Nucl. Sci. &

Engr., in press (1992).

[3] D. F. R. Mildner, Nucl. Instrum. & Meth. A 290

189(1990).

[4] J. R. D. Copley, Nucl. Instrum. & Meth A 287

363(1990).

Figure 2. (a) The normalized first moment <z>/H, and

(b) the normalized standard deviation Oj^/Nq, plotted as functions of i|r /i|rQ, for three different values of I =Q /\\r .

112
FACILITIES, OPERATIONS, AND TECHNIOUE DEVELOPMENT

TRANSMISSION OF NEUTRON BENDERS

D. F. R. Mildner (Nuclear Methods Group)

B. Hammouda

A. Schirmer (Institut fiir Festkorperforschung, Jiilich, Germany),

and

A. A. van Well and V. 0. de Haan (Interfacultair Reactor Instituut, Delft, Netherlands)

Beam benders and curved guides can be

used on neutron guide systems for steering the

beam over short distances, and for increasing the

number of desirable end positions; they can also

be used as devices on neutron optical systems,

such as stacked microguides for neutron

focusing. Analytic expressions are available for

the transmission of the bender with no reflectivity losses and the average number of both

garland and zigzag reflections for trajectories

through a curved guide as a function of wavelength [1]. Expressions are obtained for cases where the concave and convex surfaces of the

guide have the same and different coatings

respectively. We assume complete illumination

of the guide, and that the coefficient of reflection

is unity for all surfaces up to their critical angle.

Since the grazing angle at the concave surface

for zigzag reflections is always greater than that

at the convex surface, the convex surface may

have a smaller value of the critical angle than

that of the concave surface.

Approximate analytic expressions for

determining the transmission of neutron benders

which take into account reflectivity losses using

the mean number of reflections agree well [2] with computer simulation results which track the

neutron trajectories direcfly, provided that the

reflectivity R is high. The transmission for a

particular trajectory must be modified by a factor

R" where n is the number of reflections for that trajectory. In the computer simulation all possible successful trajectories at a particular

wavelength must be sampled randomly, so that the simulation should perform an average

< R" > , where <....> indicates an average over all trajectories at a given wavelength. The

average number of reflections applied as a

reflectivity correction R < n > to the transmission curve for the bender agrees well with the

simulation results. This approximation is valid

provided that the reflectivity is close to unity.

The plots show, however, an increasing

deviation at longer wavelengths.

Figure 1 shows the values of the mean

number of reflections <n>j^ for successful

trajectories through a bender of length L^ (the

length necessary to eliminate lines of direct

sight) obtained from the simulation for

reflectivities R = 0.96 and 0.92. The deviation

is small for garland reflections only, or X^X^.

For a given reflectivity R, the average number

of reflections is approximated by <n>j^ ~ <n> -(1 -R)(<n^> - <n>2). We find that

this equation gives very good agreement with the

data from the simulation for R = 0.96 and 0.92

for X > V. We also find that <i?> - <n>^

may be replaced by <n>^/3. This is exact at X = X^, and asymptotically as X -» 0°, and is a good approximation elsewhere since the

coefficient of the most significant term is zero.

a> JOE

3

Ca

O)

(0 4 6 8 wavelength /A

Figure 1. Values of the mean number of reflections <n>^ for successful trajectories through a bender of

width H and length L^ = /(8Hp) obtained from a computer

simulation, a) assuming no reflectivity losses (open circles), and b) accounting a reflectivity R = 0.96 (closed circles) and 0.92 (closed diamonds). Also shown by the solid lines are the mean number of reflections a) <n> assuming no

reflectivity losses, and b) <n>j^ <n> - (1 - R)

(<n^> - <n>^) for reflectivities R = 0.96 and 0.92.

173
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

The transmission of a neutron bender or curved

guide with any value of reflectivity can be

determined exactly [3] using exponential integral

functions, Ej,(x). The functional form differs

above and below the characteristic wavelength of

the guide, and we have deter-mined various

properties of the transmission as a function of

wavelength. The transmission through the

bender of length L^, including reflectivity losses

is given by

T(X) = 2 (X/X/ E4(y),

where y = (A,j,/X) | log R | , and

X<X,

T(X) =

2{XJX) [E4(x) - {E4(2x) - (x*/x)-3E4(2x*)}/4

+ {Eo(2x) - (x*/x)Eo(2x*)}/4],

where

x*= iiogRi [(xa,) + {aa/-i}''^]

and x = I log R I . This is shown in figure 2 for

various values of reflectivities. If the bender has

a length L, then the values of the functions y, x, and X should be multiplied by a factor (L/L^).

wavelength XlX

Figure 2. The transmission through a neutron bender of

width H and length L^ = /(SHp) as a function of wavelength A. for various values of the reflectivity R of the

surface.

The mean number of reflections has also

been determined for any value of reflectivity in terms of exponential integral functions. This

may be written as <n>R = |d(log<R">)/d(logR)|, and is

evaluated to give:

< n > R = {XJk) E3(y)/E4(y), X < X,

where y = (X^/A.) | log R | , and

<n>R =

[EgCx) - {E3(2x) - (x*/x)-2E3(2x*)]/2

+ {E.i(2x) - (x*/x)2e i(2x*)}/2]

[E4(x) - {E4(2x) - (x*/x)-3E4(2x*)}/4

+ {Eo(2x) - (x*/x)Eo(2x*)}/4]

X>X,

where

X* = I log R I [{\IX^) + {{klX.f- l}l^2j

and X = I log R I

These expressions become the same as those for

the straight guide in the limit of large

wavelengths. From the results, a simple

approximate expression for the transmission can be obtained which has been shown useful for

short benders with high reflectivity. The

calculations have also been applied to the

transmission for a beam bender which has

different coatings on the inner and outer surfaces

of the guide.

References

[1] A. A. van Well, V.O. de Haan and D. F. R. Mildner, Nucl. Instrum. & Meth. (accepted for publication).

[2] A. Schirmer and D. F. R. Mildner, Meas. Sci. &

Tech. (accepted for publication).

[3] D. F. R. Mildner and B. Hammouda, J. Appl. Cryst. (accepted for publication).

174
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

STUDIES ON SILICON WAFER MICROGUIDES FOR NEUTRON FOCUSING

H. Chen, R. G. Downing, D. F. R. Mildner, and R. M. Lindstrom (Nuclear Methods Group)

C. F. Majkrzak (Reactor Radiation Division)

S. M. Heald (Brookhaven National Laboratory, Upton, NY)

and

U. Griining and A. Magerl (Institut Laue-Langevin, Grenoble, France)

The concept of using bent silicon wafers as a focusing element for analytical measurements

using neutron absorption techniques has been

tested at CT-W. We have obtained preliminary

results using nickel-coated silicon wafers as neutron microguides.

We have conducted studies of the neutron

reflection properties of the 200 \im thin single

crystal silicon wafers coated on both sides with

(natural isotopic) nickel. These wafers will be

used to construct a microguide, where the

neutrons are transmitted through the silicon and

reflected by the nickel. A large number of these

wafers, stacked and bent to a particular radius of

curvature with an appropriate variation in the

length, can be formed into a neutron lens to focus a large area beam onto a high-intensity line

with a width in the millimeter range. These

wafers have been characterized with x-ray

reflectivity measurements at beam line X-1 1 B of

the National Synchrontron Light Source, with

x-ray wavelength of 1 .74 A. As shown in figure

1 , a fit with a Fresnel model gives a thickness of

the nickel coating of 700 A, and a surface

roughness of 20 A. For the wavelengths of cold

neutrons (A.>4 A) such roughness is negligible

in terms of its effect on the reflectivity below the

critical angle. However, x-ray photographs of

the reflected beam show significant broadening,

indicating long-range waviness of the wafers,

which is well known for thin unsupported layers. In our application, the wafers are to be tightly

clamped down and, more importantly, to be bent

by exerting a force on one edge. Therefore, the

waviness should be minimized.

The neutron measurements have been

performed in conjunction with the development

of the new neutron camera as a detector, which

enables real-time position sensitive neutron

counting. The wafers are stacked and oriented

along the direction of the neutron beam.

x-ray reflectivity train a silicon wafer coated wllh 700 A nickel

Angl« (mrsd)

Figure 1. X-ray reflectivity measurements and fits for a 200 mm diam silicon wafer coated with 700 A nickel. Data

taken at NSLS beam line X-1 IB.

Pictures are taken at various angles of orientation. The guide length formed by the wafers is 2 inches (the diameter of the wafers). The

neutron beam has a wavelength range from 2 to 9 A. Those neutrons incident with an angle

smaller than the critical angle of the total external reflection are reflected from the wafer

surfaces at an angle of 26 from the incident

beam, 6 being the angle between the incident

beam and the wafer surface. The remaining

neutrons penetrate without being reflected. As

shown in figure 2, two fringes are seen on the

video monitor, a main peak as the straight

through beam and a side peak as the reflected beam. As the incident angle 6 is varied by A 6

by rotating the wafer holder, the reflected beam

shifts by 2A6. This effect is shown in figure 3,

where the horizontal profiles are obtained by

integrating over all vertical pixel readings,

displaying the intensity of the direct and

reflected beams as a function of horizontal

position at various incident angles. With an

175
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

]%rc2. ^htInbean(l4ltfiana^k^lI1i^}gide

inn^pd iH^ fhe FEW iHlnii caiBa Bo(h d^ciod Ijl^lD and direct (right) beams are imaged.

Neutron tramlsslon from a Si microgulde

800

<0

700

600

500

a; 400

3

C

Eo

300

200

100

I I I II I

" • •

I T

2 4 6 8 10 12 14

horizontal distance (mm)

Figure 3. Integrated relative intensity (arbitrary units) of

neutron transmission and reflection profile from a silicon microguide at various incident angles.

improved neutron sensor, further fine structure within the main peak and the reflected peak is revealed. We do not yet understand the nature of this fine structure. Similar experiments have

been performed on a single wafer and equivalent

results are found.

Ultimately, we wish to focus a white

neutron beam for material research. However,

to understand quantitatively the transmission of

neutrons through nickel-coated silicon wafers

and to compare with the results of theoretical

studies, it is meaningful to perform measure- ments with a monochromatic beam. We have

mounted a stack of nickel coated silicon wafers

parallel to the direction of the neutron beam at BT-7 (wavelength 2.35 A). One of the measure- ments performed is a profile of the transmitted

beam using three different entrance-slit widths:

1/4-, 2-, and 6 times the wafer thickness of

200 nm. The integral of the profile normalized

to the direct beam gives an estimate of the

transmission efficiencies, which are 0.68, 0.86,

and 0.85 for the three settings, respectively.

We have also carried out cold neutron

transmission measurements through the 200 \im

wafers using neutrons of wavelength 7 A on the

instrument S3 at the Institut Laue-Langevin,

Grenoble, France. Using a well coUimated

incident beam, rocking curve measurements have

been conducted on straight wafers placed end-toend and side-by-side, and on curved wafers.

When the straight wafers are placed end-to-end,

we observe the expected broad angular response,

with a width dependent on the critical angle of

the silicon-nickel interface (7^= 1 -53 mrad-A'^),

essentially independent of the number of

reflections necessary for transmission. The

measurements show that within the entire

angular range, the measured reflection

coefficient for the internal silicon-nickel interface

has the value of 0.988 + 0.005. Some measure- ments include up to 11 orders of reflection. The

transmission measurements suggest a total removal cross section through the silicon

somewhat higher than expected. The silicon

wafers are undoped and prompt gamma acti- vation analysis shows that the boron to silicon

ratio is less than 10'^ so that this cannot be

accounted by the bulk boron concentration. In addition, transmission measurements have been

performed on a straight wafer as a function of

wavelength at different incident angles to the

incident beam, on the instrument EROS at the

Laboratoire L6on Brillouin, Saclay, France. As

the inclination angle is increased, we observe a

cut off in transmission which increases with

angle, and therefore with wavelength, which

agrees well with expectation.

These studies provide information on the

properties of the microguide materials for the

future construction of focused neutron elements.

176
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

DEFLECTION OF NEUTRON BEAMS USING CAPILLARY OPTICS

H. Chen, R. G. Downing, and D. F. R. Mildner (Nuclear Methods Group)

I. Yu. Ponomarev and M. V. Gubarev (Kurchatov Institute for Atomic Physics, Moscow, USSR)

M. A. Kumakhov (Institute for Roentgen Optical Systems, Moscow, USSR)

W. M. Gibson, Q. Xiao, and R. Benenson (SUNY, Albany, NY)

and

D. M. Gibson (X-ray Optical Systems, Inc., Voorheesville, NY)

Low energy neutrons have optical

properties analogous to those of photons. A

recent development [1] in x-ray focusing using

capillary optics, a device called the Kumakhov

lens, has prompted us to experiment with this new approach. In principle, the capillary lens

transports neutrons in the same way as the

silicon wafers, except that the former provides a guide which can focus in two dimensions rather

than one. Technology has enabled thousands of

channels to be formed within a glass fiber of

sub-millimeter diameter. These fibers are termed polycapillary fibers. The lens is formed

by bending and orienting the fibers in such a way that the beams emerging from each fiber

point to a small spot and so we achieve focusing.

We have observed the transmission of

neutrons through bent capillaries. Each fiber has

a hexagonal cross section, containing over one

thousand capillaries, each capillary having an

internal diameter of 6 jim, and an external

diameter of 10 [im. We cut 30 fibers each to a length of 200 mm. These 30 fibers were laid horizontally, side by side, on a steel ruler. We

placed small bar magnets across the top of the

fibers to hold them flat on the ruler at one end.

A positioning bar driven by a translational stage

at the other end enabled us to curve the fibers up

and out of the straight-through beam. Raising one end by a distance s over a length i (from

the pivot point to the lifting point), the induced

radius of curvature of R the fibers is given by R

= ?'^/2s. We raised the end of the capillaries

from the straight through beam by a distance of

9 mm, and the pivot point was 150 mm from the

end, giving a radius of curvature of 1.25 m.

The incoming neutron beam was defined by slits of cadmium and lithium carbonate, with the same width as the fibers. A polaroid neutron camera recorded both the direct and the guided

neutron beams at different distances from the

exit of the fibers. As shown in figure 1, the

results demonstrate that the trajectories of

neutrons are bent by their passage through

curved fibers. Another measurement has been performed

by mounting the fibers vertically, and taking

pictures with a specially modified CID camera at the exit, for various bending distances s. As s is increased, so that the radius of curvature R is decreased and therefore the characteristic

wavelength A.^ is increased, the relative intensity

of neutrons whose trajectories can be bent by the

capillaries is decreased (fig. 2).

This measurement has been repeated on

four different types of capillaries: two

polycapillary fibers, one made of lead glass, the

other containing boron; and two single capillary

fibers, one coated with plastic protecting layer

and the other without. The two polycapillary

fibers appear to conduct neutrons equally well

and much better than the single capillary fibers. The latter show the characteristic 'crescent' shaped output image, indicating the lower

transmission efficiency caused by incomplete

illumination due to the increased channel

diameters. These measurements demonstrate

clearly the ability to transport cold neutrons

through narrow capillaries and to guide their

trajectories, and will lead eventually to the

focusing of neutron beams using this technique.

177
FACILITIES, OPERATIONS, AND TECHNIOUE DEVELOPMENT

Figure 1. Photographs of neutron beam intensities through the capillaries. The bottom images, indicated by an arrow, are from

the direct beam. The images (inclining) are from the guided beam, which is deflected through a small angle of 120 mrad. As

the cameras is moved farther away, the distance between the direct beam and the guided beam becomes larger, as seen from left to right. The two photographs were taken with different entrance slit widths.

R

/rvs\ 3.3S 1.5v 1.13 0,85 0,67 0.56 0.49 0.42 Q.ZS

^cC-^) ^' 1.35 1-92 2.35 £.71 3J3 332 3.59 3.83 4.07

i

t

Figure 2. Images of neutron intensities transmitted through polycapillary fibers at various bending radii.

Reference

[1] Science 252, 208 (1991).

178
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

CORRECTEVG TIME-OF-FLIGHT NEUTRON DIFFRACTION DATA

FOR CHOPPED DELAYED NEUTRON BACKGROUNDS

J. M. Carpenter and J. W. Richardson, Jr. (Argonne National Laboratory, Argonne, IL)

and

D. F. R. Mildner (Nuclear Methods Group)

Neutron choppers in pulsed neutron source beams serve to reduce the background caused by

delayed neutrons in pulsed source instruments.

We have analyzed the effect of a drum chopper

placed in an incident pulsed beam which contains

delayed neutrons and computed its influence on

the detector counting rate. Expressions [1] are found for the time and wavelength dependence of

the counting rates for both prompt and delayed

neutrons in both the monitor and the scattered

neutron detectors.

The transmission probability of a drum

chopper has been derived as a function of the

time, the neutron wavelength and the geometric

parameters [2]. Three separate cases have been

found depending on the chopper slot width

relative to the incident neutron beam width. In addition, each of these cases depends on the

neutron wavelength, the frequency of rotation of

the chopper and the diameter of the chopper.

The development of these expressions is in the

black chopper, first order, parallel beam

approximation. These expressions have been

used to calculate the open time functions of the

drum choppers operated in time-of-flight

diffractometers for the suppression of delayed

neutron backgrounds. They can also be used for

the calculation of the prompt and delayed

neutron counting rates in pulsed source

diffractometers.

An exact method is suggested for

determining the delayed neutron counting rate, using the prompt and delayed neutrons with the

chopper operating in random phase with respect

to the source. In addition, we have examined

three approximate methods by which the delayed

neutron response might be determined in terms

of the prompt response; these appeal to the

calculated pulse shape functions and might be

refined iteratively. These approximate methods

serve as a check on the exact random phase

chopper method. Measurements have also been

taken which confirm the analysis of this random-phase chopper method.

References

[1] J. M. Carpenter, D. F. R. Mildner, and J. W.

Richardson, Jr., Nucl. Instrum. & Meth., A301 . 305-314 (1991).

[2] J. M. Carpenter and D. F. R. Mildner, Nucl.

Instrum. & Meth., A301 . 315-327 (1991).

NEUTRON REFLECTIVITY FIT PROGRAMS

J. F. Ankner

We report here the latest results of our ongoing development of data modeling software

for the analysis of neutron specular reflectivity

experiments. In addition to programs for nonlinear least squares refinement of block profiles

using the Nevot-Croce formalism (REFFIT) and

discrete micro-layers (BLOCKS), we have

during the past year implemented programs for

multilayer systems (MLAYER) and exotic

interfacial profiles (TAILS) [1].

Layered structures are of both fundamental

and practical interest. The program MLAYER

simulates the specular reflectivity of multilayer

materials and performs least-squares fits to measured profiles. Figure 1 shows the specular

reflectivity of 10 bilayers of NiC/TiMn sputtered

onto a float glass substrate. This particular

multilayer structure is a prototype material for

the construction of neutron supermirror guides

because of the large contrast between the NiC

and TiMn layers. At bulk densities and

179
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

concentrations, <2^Nic ~ 7 x 10'"^ A'^ and

eicTiMn = -1 X 10 n-\ As can be seen from

the inset to figure 1, the scattering densities of

the individual layers do not approach these

values—indeed, the TiMn layers exhibit a net

positive density. This behavior indicates that the

sample possesses a great deal of interdiffusion

between its constituent layers. Reduction or

elimination of this interdiffusion is a key step in

the development of these films for use as neutron

guide materials.

The study of polymer films continues to be

a major subfield of materials science. Over the

past several years, an important question has

been how a polymer, attached at one end to a

fixed surface, uncoils into a solvent. Figure 2

shows the reflectivity (closed circles) of such a "polymer brush," consisting of a deuterated

polystyrene chain with a carboxyl group at one

end attached to a silicon substrate, trailing off

into a protonated solvent (cyclohexane, C6Hj2)

[2]. We have used the program TAILS and fit- ted the data to a parabolic density profile 155 A

thick with an 1 1 A depletion layer at the inter- face with the native silicon oxide layer. The

inset to figure 2 shows the volume fraction of

the polymer in the solution as a function of depth

for the profile used to generate the fitted curve. This model fits the data better than either a parabolic profile without a depletion layer, a

rectangular profile (fitted using the BLOCKS

program), or a power-law tail, and is consistent

with theoretical predictions for a polymer brush

in a good solvent. The radius of gyration of the

polystyrene is 35 A, meaning that the polymer is stretched to about twice its free-floating length in

this system.

We have briefly described two examples of

the type of modeling that can now be done using

the MLAVER and TAILS programs. Currenfly,

we are working on magnetic multilayers, diblock

copolymer films and multilayers, iron oxidation

in an electrolytic cell, and protein function in biological membranes. Future improvements to the software include the incorporation of

independent magnetic density profiles, modeling

the spin-flip specular scattering of nonferromagnetic layers, different functional forms for

polymer and magnetic tails, and interfacial profiles other than the standard error function

and hyperbolic tangent shapes.

S

"5

0.00

J I ' I I I

I I ' ' I 0.04 0.08 ' 0.12 0.16 0.20

Figure 1. Fit to NiC/TiMn multilayer using MLAYER

program. The inset shows the scattering density profile used to generate the solid line.

0.8 Q (nm-^)

Figure 2. Fit to deuterated polystyrene polymer tail in protonated cyclohexane solvent. The open circles show the

reflectivity of the solvent without the polymer, while the closed circles show the same in the presence ofpolystyrene.

The inset shows the volume fraction of the polymer in the

solution as a function of depth for the density profile that was used to generate the solid line.

Refemices

[1] J. F. Ankner, in Proceedings of the Second

International Conference on Surface X-ray and

Neutron Scattering, Springer-Verlag (in press)

[2] S. K. Satija, J. F. Ankner, C. F. Majkrzak, T. Mansfield, D. R. Iyengar, G. Beaucage, T. J. McCarthy, R. Stein, and R. J. Composto (to be

published).

180
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

PROFILE REFINEMENT IN GRAZING ANGLE DIFFRACTION

J. F. Ankner

The excitation of a diffracted beam under

conditions of total external reflection is known as grazing-angle diffraction. The incident beam

strikes the sample at an angle below the critical angle for total external reflection, thereby

confining the intensity to an exponentially

damped layer at the surface, while the sample is oriented such that the in-plane components of the

incident and exit wavevectors satisfy a Bragg

condition (fig. 1(b)). By means of this geometry, one can use x-rays and neutrons to study in- plane near-surface structures. Grazing-angle

diffraction has most often been employed to determine surface structures using the high

intensity available at x-ray synchrotron sources. Some experiments have attempted to study

structure beneath the surface, but not in a very

quantitative fashion. We have developed a formalism whereby one can apply the wellknown techniques of specular reflectivity

modeling to the determination of depth-resolved

in-plane structures in grazing-angle diffraction.

Depth-resolved measurements of this kind are

especially important for neutron grazing-angle

diffraction because of the intrinsically large

scattering depths (typically 50-100 A) of

neutrons in matter. The specular reflectivity of neutrons or

x-rays from a refractive boundary is described

by the 1-d wave equation. A neutron plane wave k ^ incident onto the surface of a material

will excite both reflected and refracted beams

within that material. One can describe an

arbitrary refractive-index gradient by means of

a discrete sequence of steps of constant

refractive index. At the interface between two

different media, the neutron wave function is (see figure 1(a))

^^^""Itie^l • r + rj e^sl • ? 1^^ layer (1)

where ki and k,, are the transmitted and 1 si specularly reflected wavevectors in refracting

layer 1 and tj and q their amplitudes. According

to Snell's Law, only the wavevector components

normal to the surface change in specular

scattering.

= k + k = k

^k l,-4nNi b, n±.

(2)

where Nj is the number density of scatterers and

bj their nuclear scattering length, in general a complex quantity. The subscripts refer to wavevector components parallel and perpendicular to the sample surface. One obtains the

(3) (b)

Figure 1. (a) A beam incident on the interface (k; /.t; /) excites reflected 0^c/-7>r/_/) and refracted (k/.ti) waves, with a beam

reflected from the next lowest interface (k./.n) also possibly being present, (b) In grazing-angle diffraction, the sample is aligned such that a diffracted beam (I/^qq) is excited simultaneously with the specularly reflected beam.

181
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

amplitudes tj and Tj by applying as boundary

conditions the continuity of T and V^T • nj. and

recalling that k ^j = k j - 2(k j

• n_L)n_L,

t Q^x\-l(i\-l/2 + rj_jg-ikj.l-ldi.i/2 ^

tje-ikxldi/2 + riei^^^ldi/2, (3)

(t^_^e^n-ldi-m . ri-ie-a^xl-ldl-l/2)k^i.i =

(t^e-*xldi/2 . rieik^ldi/2)k^„ (4)

where di is the thickness of refracting layer 1. Solving the above equations for the ratios rj.j/tj.^ and rj/tj one obtains the well-known Parratt recursion formula [1],

R/-I «/-l (5)

where

^l

2 n a, -1,

' n

«/ = J^ii^i /2»

and

i-^J " J TIT'

(6)

(7)

(8)

This recursion relation forms the basis of

the modeling schemes commonly employed in

reflectivity analysis, where one measures the

specularly reflected intensity Ir^l^. Grazing-angle diffraction from ideally

imperfect crystals can be described by the

distorted wave approximation, which splits the

scattering problem into two parts: a kinematical

solution of the in-plane Bragg diffraction and a dynamical treatment of the specular reflectivity.

One can modify the conventional treatment of

grazing-angle diffraction from a monolithic slab

to study the effect of buried layers of different

composition and/or structure on the diffracted

intensity [2,3],

1(0) = Io\^ F0i^m^^-di) *

I

JQlMidi , tfiix))^-— tfixoG) I ^ (9)

the 1 summation runs over the buried slabs with

structure factors Fj, reciprocal lattice vectors Gj

containing Mj layers of thickness dj with (J =

^Qj - Icj. The illuminating amplitudes and all of

the phase information about the specular process

are included in the transmittivity factors, which

are given by equations (3)-(8),

'I =

/-I

4-\~^l-lJ^l-\

1 - F1-1,1

7-1.

(10)

This relationship suggests that any

measurement of depth-resolved grazing-angle

diffraction must begin with a thorough specular

reflectivity characterization in order to obtain the

tj as fixed parameters. One could then study,

for example, surface-induced ordering, strain

profiles, or magnetization variation with depth,

as well as multilayer effects.

References

[1] L. G. Parratt, Phys. Rev. 95, 359 (1954).

[2] J. F. Ankner, C. F. Majkrzak, D. A. Neumann, A. Matheny, and C. P. Flynn, Physica B (in press).

[3] J. F. Ankner, in Proceeding of the Second

International Conference on Surface X-ray and Neutron Scattering, Springer-Verlag (in press).

182
FACILITIES, OPERATIONS, AND TECHNIOUE DEVELOPMENT

RESOLUTION CONSIDERATIONS FOR POLARIZED

TRIPLE-AXIS SPECTROMETRY

N. Rosov and J. W. Lynn

(University of Maryland, College Park, MD and Reactor Radiation Division)

and

R. W. Erwin

The polarized beam triple-axis technique is known to be a very powerful method for

identifying the origin of both magnetic and

nuclear cross sections [1]. For ^example, with

the incident neutron polarization Pparallel to the

momentum transfer Q, P||Q, ferromagnetic spin waves can only be observed for spin-flip

scattering, and in fact for the (-+) configuration

the spin waves can only be observed for neutron

energy loss scattering, while for the (+-)

configuration spin waves can only be observed in neutron energy gain. For PxQ, on the other

hand, the (-F--) and (-+) cross sections are equal

in strength for energy gain and energy loss, and

one-quarter the intensity of the P||Q situation.

Such selection rules are often utilized to separate

and identify cross sections. However, when the

relative orientation of P and Q varies substantially over the extent of the instrumental

resolution function, substantial distortions of the

expected scattering intensities can occur, in addition to the usual resolution effects applicable

to unpolarized neutron scattering. The

corrections are largest in cases when the

resolution is very coarse, as would be expected.

The resolution effects are also large when

measurements are made at small wave vectors,

as is necessary for amorphous materials, and this

is the case of particular interest here.

The angular dependence of the polarized

neutron cross section for. spin wave creation

(minus sign) and destruction (plus sign) is given

by [2]

« 1

-1- (Q-M)2 T 2 (P-Q)(Q-M) (1)

where Q and M are unit vectors in the directions

of Q and the magnetization M. In the horizontal

field configuration Q-M = 1 and P- Q = ± 1

and equation 1 reduces to 2 + 2P, so that for

spin wave creation we have the maximum value

of 4 with P = -1 [(-+) configuration] and for P = -f-1 [(-I--) configuration]. For spin wave

destruction we have 4 for P = -1-1 [(-I--)] and

for P = 1 [(--I-)]. On the other hand for the

vertical field configuration we have Q*M a and P-Q = 0, and the cross section for energy

gain and energy loss is unity and independent of

P. Hence for either the (--!-) or (-f--) configuration we will see equal intensities of spin waves

in energy gain and energy loss.

If the instrumental resolution allows the

direction of Q to deviate substantially from M,

then we must take this angular variation into

account when convoluting the resolution with the

appropriate cross section. In the horizontal

configuration Q-M = cos6, and equation 1 becomes

,2 _ « 1 + (1 T 2P)cos^e

dd

(10)

so that we obtain 1 -I- 3cos^6 for {-+) and (-!--) in energy loss and energy gain, respectively, and

1 -cos^6 for {+-) and (--!-) in loss and gain. In the vertical field configuration we have

^ « 1 + (1 T 2P)sm^

dd

(11)

where we have defined 4> = k/2 + 6. Then we

obtain 1 -I- Ssin^cf) for (--!-) and (-1--) in energy

loss and energy gain, respectively, and 1-sin 4> for (+-) and (-+) in loss and gain.

These angular variations are important for

the case of amorphous systems, in which the

spin waves are only well defined around near Q = and the typical coarse vertical resolution

therefore can be a substantial fraction of Q. To

illustrate the nature of the effects we chose the

particular case of a ferromagnet with dispersion

183
FACILITIES, OPERATIONS, AND TECHNIOUE DEVELOPMENT

relation E^^ = Dq^. Typical experimental

parameters for these types of measurements are

E. = 14.8 meV, and collimation (FWHM) of

10'-10'-10'-20'. We took a convenient value of

D = 70 meV-A^. The kinematical details of

these types of scans are given elsewhere [3]. Figure 1 shows the convolution of the

polarized spin wave cross sections with the

instrumental resolution function [4] for the

vertical field situation, PlQ, and (- + ) spin-flip

scattering. The vertical resolution in this case was chosen to be 0.1 A'^ For q = 0.05 A"^

the ratio of the energy loss (E > 0) to energy

gain (E < 0) peak is 2.1, while the ideal case where P-Q = this ratio is 0.9, and is given by

the kf-^cotO^ resolution factor for the analyzer

[4] (and a very small contribution from the

detailed balance factor). With increasing q the

ratio becomes closer to the ideal, and for

q = 0.1 A~^ the ratio has decreased to 1.2.

2:-

v>c

-1.2 -0.4 0.4 Energy (meV)

0.8 1.2

Figure 1. Spin-flip (spin wave) scattering as a function of

q. The ideal ratio of energy loss to energy gain is —0.9,

while the effect of the vertical resolution (Q^ = 0.1 A" ) is to make this ratio greater than unity. The instrumental

distortion of the intensity of the peaks is seen to decrease

with increasing q, as the effect of the vertical resolution becomes less important.

The effect of the vertical resolution is shown in figure 2, where the ratio of the energy

loss to energy gain peak at q = 0.1 A'^ is plotted as a function of the vertical component of

the resolution. For tight vertical resolution we

get the ideal ratio given by k^^cotO^, and then

this ratio rapidly increases with increasing Q^.

These calculations are in 'semi-quantitative'

agreement with our recent measurements on the

amorphous Fcg^Bj^ system [5], while for

complete agreement we must include the energydependent reflectivity of the analyzer crystal,

which has not been accurately measured yet.

1.5

1.4

"^ 1.3

tuS

uj. 1.2 gI 1.1

n* 1.0

0.9 -»

0.8

10'-10'-10'-20'

E| = 14.8 meV

D = 50 meV-A^

q = 0.lA"'

0.05 0.1 0.15 Q^ (A"', FWHM)

Figure 2. Ratio of the energy loss to energy gain intensities as a function of the component of vertical resolution. The

ideal ratio is 0.89, given by the k/- cot9 a factor.

References

[1] R. M. Moon, T. Riste, and W. C. Koehler, Phys. Rev. 181, 920 (1969).

[2] S. W. Lovesey, Theory of Neutron Scattering from

Condensed Matter . Vol. 2 (Oxford University Press, Oxford, 1984) p. 180; W. G. V^illiams, Polarized Neutrons (Oxford University Press, Oxford, 1988)

p. 79.

[3] A review is given by J. W. Lynn and J. J. Rhyne,

in: Spin Waves and Magnetic Excitations , A. S. Borovik-Romanov and S. K. Sinha, eds. (NorthHolland, Amsterdam, 1988), Chap. 4, p. 177.

[4] M. J. Cooper and R. Nathans, Acta. Cryst. 23, 357

(1967).

[5] J. W. Lynn, N. Rosov, Q. Lin, C-H. Lee, and G.

Fish, p. 6 of this report.

184
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

SCATTERING EFFECTS IN SPHERES

J. R. D. Copley and J. M. Rowe

In a neutron beam experiment the rate of

any given absorption can be significantly

modified depending on the overall absorption

cross section of the sample, the nature and

magnitude of its scattering cross section, and its shape and size. This phenomenon has recently

been investigated analytically [1], using an

adaptation of Sears' [2] formalism for the treatment of slow neutron multiple scattering. In our previous work we also described Monte

Carlo calculations for slab samples of various

shapes and sizes, always assuming the scattering was both elastic and isotropic [1]. Following a suggestion of Fleming [3], we

have now extended our calculations to the special case of spherical samples [4]. We have made

calculations for pure absorbers, pure scatterers,

and samples which both absorb and scatter neutrons. The calculated quantity is either

[(j)(r)/(j)J, which is the average flux at points

distant r from the center of the sphere, relative

to the external flux 4>o, or else the "self-shielding

factor" f , which is the volume-averaged value of

[<l>(r)/4)o]: R

where V is the volume of the sphere and R is its radius.

We have compared the Monte Carlo

calculations with independent numerical

calculations using an iterative technique to solve

the neutron transport equation. The Monte

Carlo calculations assumed a monodirectional

incident beam of neutrons whereas the iterative calculations were performed for a sphere in an

isotropic field, and in every comparison the

agreement was very satisfactory. This strongly

suggests that the calculations are correct, and

that the calculated quantities, defined above, are independent of the directional nature of the

neutron field. This last result is intuitively

appealing and has been rigorously derived [4]. We have also succeeded in proving and

demonstrating that f = 1 for a purely scattering

sphere, or indeed for a purely scattering sample

of arbitrary shape placed in an isotropic field of

neutrons [4], In figures 1 and 2 we show results from our Monte Carlo calculations for spheres which both

scatter and absorb neutrons. Figure 1 shows the

flux ratio [<l)(r)/(j)Q] as a function of (r/R) for

several choices of x^ = 2S^R and Xg = 2SsR,

where 2^ and S^ are respectively the

macroscopic absorption and scattering cross

sections within the sphere. The good agreement

between Monte Carlo and iterative numerical

calculations is apparent. Also shown are the

results of diffusion theory calculations which

work well except at points close to the surface of

the sphere and when S;^ is large.

o.ol 0.00 0.25 0.50 p=r/R

0.75 1.00

Figure 1. Flux distributions within spheres, calculated by

Monte Carlo (points) and by an iterative numerical tech- nique (solid lines). Results obtained using diffusion theory

are shown as dotted lines. The ordinate is identical to

[*(r)/<l>o].

Figure 2 shows the dependence of the self- shielding factor f on x^ and Xg. An increase in the scattering cross section clearly decreases f if S^ is held constant, but an important conclusion

185
CO

X!<

X

1.0

0.8

0.6

0.4

0.2

0.0

FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

is that f is less sensitive to Sg than in other

commonly chosen sample geometries. This

conclusion is consistent with the experimental

prompt gamma neutron activation analysis results reported by Mackey and coworkers [5].

References

[1] J. R. D. Copley and C. A. Stone, Nucl. Inst. Meth.

A 281, 593 (1989).

[2] V. F. Sears, Adv. Phys. 24, 1 (1975).

[3] R. F. Fleming, private communication.

[4] J. R. D. Copley, Nucl. Inst. Meth. A 307, 389

(1991).

[5] E. A. Mackey, Ph.D. Thesis, University of Maryland

(1991); E. A. Mackey, G. E. Gordon, R. M.

Lindstrom and D. L. Anderson, Anal. Chem. 63, 288

(1991); see also p. 139 of this report.

I

\

1fe^.xs=0

Xs=10-

^>v^ ^

**"

10

x^=2S^R

Figure 2. The self-shielding factor f(x a , Xo), plotted as a

function of x^, for integer values of Xg between and 10.

REACTOR OPERATIONS AND ENGINEERING

T. M. Raby, J. H. Nicklas, and J. F. Torrence

Reactor operation during the past year has

been uneventful and essentially routine. The

reactor operated approximately 90% of all available reactor time. The small amount of lost time was mainly caused by minor heat exchanger

leaks and outside electrical power interruptions.

Fuel utilization continues to be the best in the

United States and will improve further in the

coming years. Reactor utilization is at its peak.

Highlights of significant items are outlined below.

Guide Tubes . The first three cold neutron

guide tubes became fully operational. The

isolation valve for a fourth tube was installed and

successfully tested clearing the way for installation

of the guides.

Main Heat Exchangers . Minor leaks

continued to develop in one of the main primary

heat exchangers. The size of the leak was so

small, on the order of one drop a minute, that it was virtually impossible to find among the 1200

tubes of the exchanger. This has caused extra days of shutdown plus added exposure to personnel. In April 1991, the leaky heat

exchanger was removed from service. Operations

continued utilizing the remaining heat exchanger at reduced power. Analysis indicated that a

maximum of 15 MW of power could be achieved

under almost all weather conditions. This turned

out to be not only the maximum but the optimum

for operation with a single heat exchanger.

Operations in this mode will continue for ±e

foreseeable future. The leaky heat exchanger is being monitored and maintained and could be

returned to service if needed. Negotiations for

new replacement heat exchangers of a different

plate-type design have been completed. Special

emphasis on quality and reliability have been

incorporated in the specifications. Fabrication is scheduled to begin in late summer of 1991.

New Advanced Fuel . The fifth generation of

advanced fuel elements was introduced into the

reactor in April 1991. This will further improve

fuel utilization thereby extending the operating

cycle. The new elements are the heaviest loaded

in the history of the facility and contain more than

twice the uranium loading of the original elements.

This loading is the highest that can be used at the

NBSR.

Control Room Upgrade. Plans are being

prepared for a complete upgrade of the reactor control room once funding becomes available.

186
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

Meantime, several segments and components have

been modernized and upgraded.

Staffing . Two new operators received NRC

senior licenses with high marks. The staff of 16

operators, the smallest for any comparable reactor,

all hold senior licenses. Two senior nuclear

engineers are spending a one-year sabbatical at the

Division and will be assisting in the reactor modernization plan.

PRACTICAL APPLICATIONS OF NUCLEAR RESEARCH REACTORS

B. Hammouda

Most people associate nuclear energy with

weapons programs and electrical power

production. While it is true that these two

applications are the most important ones, there

is a host of other very beneficial applications of

nuclear energy that are not as well known.

Some of these are described in a report put

together by this author for a series of lectures. These include: neutron activation analysis,

radioisotopes applications, neutron radiography,

neutron gauging, neutron depth profiling,

radiation effects and applied neutron scattering.

Neutron activation is an analytical

technique used to detect and quantify minor and

trace levels of elements for a wide variety of

applications encompassing many fields of

physical, natural, and life sciences. It allows a precise "fingerprinting" of objects based on their

radioactive decay following neutron activation.

This technique has found various biomedical,

geological, archeological, forensic science, art history, industrial, and environmental

applications. It is used, for example, to trace ancient artifacts to their place of origin therefore

allowing archeologists to infer routes of trade

and population migrations. It is also routinely

used by some law enforcement authorities to detect traces of gunshot on the hands of crime

suspects.

The need to use radioisotopes has been

growing ever since their discovery, i.e., since

the advent of particle accelerators and nuclear

reactors. Radioisotopes produced using nuclear

reactors are beta and/or gamma emitters and can be produced in substantial quantities. They

found wide spread applications in medical,

industrial, agricultural, and environmental fields. Cobalt-60 applications, for example, are well

known for their wide use in radiation therapy,

for the "radurization" of foods and for the

sterilization of medical supplies. Another

example is the attachment of Mo-99 to monoclonal antibodies to seek and destroy cancer

cells through beta emission.

Three kinds of applications (neutron

radiography, neutron gauging and neutron depth

profiling) are sometimes referred to as "neutron

interrogation". Neutron radiography is another

very much unknown application (compared to Xray radiography, for example). Yet, this technique is very useful for visualizing contrasts between various elements as well as in following

real time dynamics of hydrogen-containing

fluids. Neutron gauging is used in mining

operations to produce indirect imaging of

geological layers, in industrial bulk process

streams for the grading of ores, and in commercial applications to determine the

hydrogen and nitrogen contents of various food

stuffs (nuts, grains, etc). Neutron depth

profiling is a recently-developed technique for the characterization of the density profile of some prompt charged particle emitters (boron for example) following neutron absorption close to a sample's surface.

The radiations produced in reactors (fast

neutrons, gamma rays) have strong "effects" on

materials. Note that the word "damage" is sometime used in this context with a negative

connotation, whereas these effects can be useful

such as in track etching (a form of dating over a range of ten billion years), in the transmutation

doping of silicon to produce semiconductors, in the coloration of gemstones through the

introduction of color centers, and through

radiation hardening of various hard working

187
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

mechanical components (drill bits for example).

Microporosity produced by fission tracks in

uranium doped glass is essential in High

Performance Liquid Chromatography. Track

etching is also used to produce the best plastic

filters available for chemical and pharmaceutical

applications. Also, fast neutron irradiation of

memory units in bulk can reduce the probability

of occurrence of random bit resets in computers.

The field of neutron scattering uses thermal

or cold neutrons as a diagnostic probe to investigate the structure, phase transitions and

dynamics of a wide variety of materials. There

are some very practical aspects of neutron

scattering such as in the monitoring of

microstresses in engineering materials, the

estimation of porosity in coals, etc. Most of the practical applications of

nuclear research reactors have started as limited

research projects. Some of them have matured

enough that they have become viable business

ventures operated as a "service" to earn much

needed funds in the national laboratory and

university based reactors. However, as yet, no

research reactor has been built by an industrial

firm for profit making only, through the

exploitation of such applications.

RADIOISOTOPE PRODUCTION RADIATION EFFECTS

NEUTRON RADIOGRAPHY

NEUTRON SCATTERING

NEUTRON ACTIVATION ANALYSIS NEUTRON PHYSICS

Figure 1. Practical applications of nuclear research reactors.

References

[1] Annual Reports of the University of Missouri Research Reactor (1983-1989)

[2] NIST Reactor: Summary of Activities July 1986

Through June 1990, NIST Tech Notes Nos. 1240,

1257, 1272, 1285 (1986-1990).

188
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

ACTIVATION DETERMINATION FOR NBSR IRRADIATED FUEL ELEMENT

HARDWARE (NONFUEL SECTIONS) AND SPENT SHIM ARMS ASSEMBLIES

D. R. Brown (Reactor Health Physics)

In November 1990 a waste shipment

containing irradiated fuel element hardware (nonfuel sections) and spent shim arm assemblies was

packaged and shipped to Barnwell, SC, for

processing and burial. Regulations required

specific nuclide activity determinations for the

entire shipment. The resulting project involved

confirmation of elemental composition of

materials used for element and shim arm

fabrication, neutron activation measurements and

calculations, regional core and vessel neutron

flux imaging as well as direct dose rate measurements.

Material Composition. Elemental

composition of fabrication materials was

determined via several methods and comparisons. Initially, data from ASTM standards and

material batch certification of micro-alloying

constituents and residual trace contaminates, were used to estimated activity. This, however,

did not support results from direct measurements. Samples of material were machined from

a clean, disassembled fuel element head piece for

neutron activation analysis. Major elements in

this analysis were in good agreement with

published standards.

Activity Calculations and Measurements.

Direct measurements were made of each nonfuel section (4) of a selected element with a known core history. Measurements were made

using a portable germanium detector and

multichannel analyzer set up at a considerable

distance. The predominate nuclides in this measurement were ^°Co and ^^Zn. Activity was

determined using free air gamma dose rate measurements and tabulated ly dose rate conversion factors for ^^Co and ^^Zn.

These computed activities were then used

to ascertain the average neutron fluence for each

element region/section. Using fuel element

histories and assuming a 'flat' neutron flux

distribution, radially across the core, fluence was

calculated from measured activity and known

nuclear properties of materials irradiated. The

results correlated well with data available in NBSR-9 and flux mapping data available from

irradiation facility (rabbit tubes and verticle

thimble(G2)) experimental measurements. The

neutron fluences were then used to calculate the

remaining unmeasurable activation products.

L - MATERIAL COMPOSITION AND ACTIVATION DATA

6061 Aluminum SOX Stainless Steel* Cadmium

Elemental Composition

Al-99% + Cu-.36% Fe-.33% Fe- 84% Cr- 22% Ni- 11%

Cr-.09% Zn-.06% Ti-.03% Co-.22% Mo-.22% Sb-.003% Natural Cd Metal

Co-.0003% Hf-.0002%

Activation Products

60co 65zn 59Fe 60co 51Cr 59Ni 109cd 109mAg

46sc 18lHf 55Fe 63Ni 58co 99tc 113mcd llOm^g

185Hf 45ca 51cr 124sb 59Fe 55Fe 115mcd ^^«Ag

* - CONSTITUENTS ADD UP TO >100% DUE TO APPROXIMATIONS IN CALCULATIONS.

189
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

Reference Element - ;yNBS-492 Date in Core - 10/6/88 Date to Pool - 2/18/88 Date last irradiated - 2/2/88 MWHrs 101 ,896 (7 cycles)

Table 1. Fuel element section dose rate measurements and

activity calculations - 2/7/90

Section Dose Rate Dose Rate Nuclide

@ 8.5 m @ 14.2 m

*

Activity \iCi

mR/hr mR/hr

Head 8.0

Long Box 1.6

Mid-Fuel 1.6

Bottom 1.3

2.7

0.6

0.6

0.6

60Co

65Zn

60Co

65Zn

60Co

65Zn

60,Co

65 Zn

337

7

44

91

51

112

40

70

* - ALSO DISTANCE FOR GAMMA SPEC

MEASUREMENT

Table 2. Reference fuel dement activity —decay corrected to 11/15/90

Nuclide

60, 65

59

5559

63

5158

124

Co

Zn

Fe

Fe

Ni

Ni

Cr

Co

46, Sb

181 185

Sc

99.

Hf

Hf

Tc

•^-life Activity

2mCi

5.27 Y 445.9 244 D 124.8 44.5 D 5.7E-5

2.74 Yl 351.7 7.5E4 Y 0.1 100 Y 16.9 25 D 3.1E-7

71 D 2.8E-3 60 D 1.4E-5 84 D 5.9E-5 42 D 3.9E-6 70 D 3.9E-4 2.13E5 Y 1.2E-7

Total Activity « 1940 mCi

This same procedure was followed for the

shim arms with some variation. The shim arms

do not remain in the same flux region throughout

a reactor cycle, hence, corrections must be

made. Self shielding is also a factor to be

considered when evaluating the shim arms.

Total activity of shipment of 128 unfuelled

fuel elements and 4 shim arm assemblies was

approximately 400 curies, as of the ship date in

November 1990. The dominate nuclide in terms

of total activity was ^^Fe.

190
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

NEUTRON ACTIVATION DATABASE

L. A. Slaback (Occupational Health and Safety Division)

A personal computer based database in a spreadsheet format was prepared for the

evaluation of the activation characteristics of

experimental samples. Cross-section data from

the IAEA [1] was used. The fast neutron

reaction data was weighted assuming a fission

spectrum. Provision for production via

epithermal reactions and radioactive daughters is included. The figure 1 displays the results of a

calculation for all elements for typical irradiation

conditions at the NBSR experimental facilities.

8of-i

ItTI-206 [_jl'b iX)6 Neutron Induced Activity: All Elements

Atomic Number -vs- Specific Activity

-iHg-I OOm. l Ig COS— rn-204 -Qgg-19?mV<9-eoaf3H9-'»^

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jDtt 101 W1 ( .Cte'l OCm.Da 137 m

I \ L» 137 - C« 130 1—|Co | 137m'.C« fST^PffRjlCa 1 4a^C« 1 4 1 p--'<'

I ^ B« 1 3Q| ^ D« 133 I ^ « 133tfT I | «te>05m. 0« WV La-UO

54rr ^jX«-l g6m I .Xa 1P7m I .Xo 136m i. Xe 1371 c« i O' "' »-P^)(«J^3&l

r!l-l28 , , -4Te 123, To 13H ^1 1 •^f^l t3m-t-^^J53m,bw 1

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1

|l p« 66. Wi 66. Hi 63 \ |l O» 66 eu-64

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-JN-16 -O^

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,e Ji-i-8 r i B« 18-QH-a

Activity (microCurie/gram) -Bt«- H-3

1 E 7rVom2/mc

1 1 1 inn—I I iiiiiii—I I ifiifii—I I iihiii—I I iiinii—rrmrm—i iiniiir i i iiiiiii—i i iiiiiii i iiiiiiii i i iiiiiii i iiiiiiii 1E-10 1E-09 1E-08 1E-07 lE-06 lE-05 0,0001 001 001 0,1 1 10TTD lE-11

Notes related to the figure: Each radioactive nuclide computed by the database is displayed, with the ordinate being the atomic number. The conditions for the calculation are: 110 neutrons cm' sec" (fast or thermal), one week irradiation, one day decay

(which suppresses the very short lived materials), and one gram of the natural element. The cadmium ratio is taken as zero, e.g., the epithermal cross-section is suppressed. For display purposes results of less than 10" microcuries per gram are shown as

that value. All points are labeled except for several xenon, cadmium, and tellerium nuclides. Where the labels are grouped the

left-to-right order of the labels correspond to the unlabeled points on a left to right order. Those reactions produced by fast neutrons are identified by a T in front of the nuclide label. The symbols +,*,s,t appearing after a nuclide name indicate the presence of a radioactive daughter. The differing indicators relate to different calculational treatments due to the presence of that daughter. Several radioactive daughters that result only from the decay of an induced parent are included in these results, primarily when the daughter is longer lived than the parent.

Referraice

[1] IAEA, Handbook on Nuclear Activation Data, Tech.

Rept. Series No. 273, IAEA, 1987

191
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

RESPONSE TESTS FOR A NBSR SECONDARY SYSTEM ON-LINE H^ MONITOR

T. F, Mengers (Occupational Health and Safety Division)

This report is a summary of the development of a continuous flow liquid scintillation counting system to monitor H^ concentrations in the NBSR secondary cooling loop.

Before operating our on-line H^ monitor we

tested three different brands of "environmentally

safe" cocktails: Berthold Flow-5; Packard

Instagel; and Beckman Ready-Safe. The

environmentally safe scintillants can be released

to the sanitary sewage system if the H concentrations are sufficiently low. These tests were

the basis for our choice of scintillant and ratios. We desired high sample to scintillant ratios to increase response (by having more sample in the

counting volume) and minimize scintillant consumption. Irregularities were observed in the

response curves (cpm/ml of sample vs. ratio)

corresponding with inhomogeneous mixing,

opacity, or gelling at higher sample-to-scintillant

ratios. This indicated a maximum acceptable

sample to scintillant ratio of 1:4. Benchtop tests were used to evaluate total flow rates and minimum response times. We

then connected the monitor to the secondary

system and began operational tests. Figure 1 indicates the system response to a high H

concentration shortly after start-up following a tube leak repair. The dots indicate the

secondary H"^ concentrations measured by liquid

scintillation analysis of secondary basin grab

samples. The graph illustrates some of the

failure modes associated with the on line

monitor.

The intermittent dips in the count rate are presumed to be due to small air bubbles

introduced into the system. The sudden drop is due to plugging of the capillary tube on the

sample inlet. Plugging is caused not only by

scintillant gelling, but also by fouling due to impure secondary water and corrosion on

fittings. The frequency of plugging is greatly

reduced if we purge the capillary lines every

other day using a 10 ml syringe and the priming

valve on the HOB pump. We also perform a

periodic "fast flow" purge by fiiUy opening the

LSC and On—Line Monitor Comparisons

Figure 1 . Tritium concentrations in the secondary colling

system after plugging a leak in the heat exchanger.

sample throttling valve and running the two

pumps at the maximum flow rates. This does

not appear to significantly disrupt the sample to

scintillant ratios or affect the count rate records.

These operations can be performed in less than

10 min.

The balance of scintillant and sample flow

rates is critical to system calibration since this determines the amount of sample in the fixed

volume of the counting chamber. Set-up factors

that influence this balance include: the elevations

of the scintillant supply and effluent containers

relative to the pumps; and the sample supply line pressure. The scintillant supply must be below

the pump inlet. The effluent tank must be above

the monitor and pumps to prevent a siphoning

action which disrupts the flow rates. Also, the

sample supply lines must have a vent and over- flow to allow atmospheric pressure on the pump

inlet, otherwise the pump allows unmetered

flow-through which disrupts calibration.

Figure 2 illustrates the response of the

monitor to a bolus of H^ introduce into the

secondary system at the N^^ monitor prefilter.

We added 16.81 +1- 0.85 mCi (47 ml) to the

system.

The rise to a peak began at roughly 12 min.

The bolus actually passed the tap off for the H^

monitor within 2 min and it took an additional

10 min for the sample to be drawn into the

192
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

Bolus Test H-3 Monitor Response

100 200 300

150 250

Tim« After Bolus (minutes)

Figure 2. Continuous H-3 monitor response after adding a bolus of 16.8 m Ci of H to the secondary system.

counter. Results for a sequence of samples

taken at the top of the basin were used to verily

estimates of the secondary sample volume.

After adjusting for a blow down rate of 110

GPM the calculated secondary system volume

averaged to be 106,094 +/-5957 gal (1 a). This

agrees well with estimates of 100,000 gal based on piping dimensions measurements.

The monitor required a substantial amount

of time to return to the equilibrium levels which

were obtained by complete mixing at the basin in

less than 20 min. Presumably this is due to the

slow sampling flow rate and relatively large

volume of the throttling valve and vent tubing.

There may also be some residual contamination

of the sample line tubing. The system exhibited

similar clearance delays following benchtop tests before connection to the secondary system.

Our current configuration uses a 1 :4 sample

to scintillant ratio with a total flow 0.75 ml/min.

We attempted initially to run at half that flow

rate but we obtained inconsistent results. The

current ratio results in 0.2 ml of sample in the

counting chamber at all times. We have

obtained 56.3 cps for a secondary concentration

of 1.5E-2 uCi/ml and 0.25 CPS (roughly twice

background) for secondary concentrations as low

as 4.4E-5 uCi/ml. Therefore the overall

counting efficiency is approximately 40%.

Our annual scintillant consumption is around 320 liters per year which costs roughly

$3200. This cost is justified by the potential for

faster response time for detecting a major heavy

water leak.

REACTOR BUILD^G AREA MONITORING

USING THERMOLUMINESCENT DOSIMETERS

T. Mengers (Occupational Health and Safety Division)

Starting in the last quarter of 1989, TLD

monitors have been placed in twenty-eight

locations throughout and around the facility. The

monitoring stations include: eleven locations

around the walls of the guide hall; five outside

surrounding the guide hall; five in the offices of

the A and E wings facing the guide hall; four in

the offices along the front of building 235; one by the control room door; one in the pump

house; and one in the high bay. The primary

goal of this monitoring is to assess the impact of

the new guide hall experimental stations on

unrestricted area doses.

During the last quarter of 1989 and most of

the first quarter of 1990 the reactor was in an

extended outage for neutron guide installation.

The TLD data collected during this time serves

as a baseline.

Comparisons of the first two graphs in figure 1 (Baseline) with the second two graphs in

figure 2 reveals several things. None of the

measurements in the unrestricted areas outside

the guide hall or in the office wings indicated

significant increases due to guide hall operations.

The neutron dose equivalents at some the

monitors (BR2, BR3, and BR4) on the guide hall wall increase by a factor between two to eight,

but this remains a minimal part of the total dose

equivalent. These increases are related with the

proximity to and use of the prompt gamma

facility on NG7. Changes in the experiment

shielding have reduces or removed even these

small doses. The gamma dose rates at these

193
FACILITIES, OPERATIONS, AND TECHNIQUE DEVELOPMENT

locations increased by as much as a factor of

four.

Elevated dose rates at around BR8 are due

to radwaste shipment work in or near the guide

hall truck bay. The maximum daily averaged

dose equivalent measured was 1.01 mrem/day.

This dose rate is less than 20 % of the

regulatory limits for areas outside the building. In addition to the measurements made

around the guide hall, four offices along the

front of building 235 were monitored as control

points. The maximum dose equivalent rate (gamma plus neutron) was 0. 166 mRem/day and

does not indicate any correlation with reactor

operations. These locations do display a slightly

increasing trend in neutron dose equivalents with

respect to the proximity to building 245. This

trend was also seen in the TLDs located outside

the guide hall. The trend was indicated

consistently each monitoring period although the

peak levels did fluctuate. We hypothesize these

neutron doses are due to the Californium sources

in the time-of-flight facility. This Californium

facility has since been moved to a more shielded

location.

The calibration factors used to interpret the

neutron response of all the TLDs are appropriate

for the purely thermal neutron sources in the

guide hall. The reported environmental results should not be used as a measure of the harder

cosmic spectrum neutron dose.

The TLD monitoring program will continue

with its present plan until all guide hall experimental stations are operational and doses

are demonstrated to be acceptably low.

GAMMA DOSES AROUND THE GUIDE HALL

Periods Sept. 1989 to July 1990

NEUTRON DOSES AROUND THE GUIDE HALL

Periods SepL 1989 to July 1990

0.001 0.01 0.1 Gamma Dose (mRem/day)

0.001 0.01 0.1 Neutron Dose (mRem/day)

Figure 1. Results of the locations around the guide hall for the first four periods of data collection.

GAMMA DOSES AROUND THE GUIDE HALL

Periods July 1990 to July 1991

0.001 0.01 0.1 Gamma Dose (mRem/day)

NEUTRON DOSES AROUND THE GUIDE HALL

Periods July 1990 to July 1991

Dose Increases

are due to the NQ7 Prompt Gamms

Operations

0.001 0.01 0.1 Neutron Dose (mRem/day)

-f—I i I i II

Figure 2. Results of the four periods from July 1990 to July 1991.

194
G. PERSONNEL ROSTER

REACTOR RADIATION DIVISION

J. M. Rowe, Chief

T. M. Raby, Deputy

H. J. Prask, Sci. Asst.

J. M. Hill, Sec. E. C. Maxwell, Admin. Officer

S. L. Neal, ^^dmin. Asst.

REACTOR OPIiRATIONS 1 COLD NIiUTRON 1 NEUTRON CONDENSED

& ENGINEERING PROJECT MATTER SCIENCE

T. M. Raby, Chief J. M. Rowe, Manager J. J. Rush, Leader

J. F. Torrence, Deputy I. G. Schroder, Tech. Leader J. K. Spilhnan, Sec.

J. H. Nicklas, Chief Eng. C. O'Connor, User Coord. L. K. Clutter, Sec. D. D. Wilkinson, Sec. L. M. Poole, Sec. C. M. Sweeney, Sec.

-

195
PERSONNEL ROSTER

RESEARCH AND ENGINEERING STAFF

J. F. Ankner

J. G. Barker

N. F. Berk

J. A. Borchers

R. C. Casella

R. S. Conway

J. R. D. Copley

W. E. Dickerson

R. W. Erwin

C. J. Glinka

P. H. Graham

G. C. Greene

B. Hammouda

V. L. Karen

Condensed matter physics

Neutron reflectometry and grazing-angle diffraction

Instrument development

SANS instrumentation and research

o Condensed matter theory o SANS theory for microstructure analysis

Computer software for graphics and data analysis

Thin-film analysis

Artificially modulated materials o Magnetism

o Condensed matter theory, including high T^

Group theory analyses of neutron scattering from condensed matter

Fundamental physics, especially as related to reactor experiments

o Electronic engineering o Nuclear instrumentation

o Time-of-flight spectrometer development

Neutron instrumentation conceptual design o Condensed matter physics

o Neutron scattering instrumentation

Microcomputer interfacing

Nuclear and engineering physics

Magnetic materials

Phase transformations

Spin echo techniques o Cryogenics

o SANS microstructure of metals and porous media

Magnetic materials

Cold neutron instrument development

Engineering design

System and user software for cold neutron instrumentation o Spectrometer and data acquisition systems interfaces

o SANS from polymer systems

Quasielastic scattering from polymer solutions

SANS instrument development

Crystal database development

X-ray crystallography

196
PERSONNEL ROSTER

P. A. Kopetka

S. J. Krueger

Mechanical engineering

Cold source design o Electro-mechanical systems

Small angle neutron scattering instrumentation

Microstructure of materials

Biological problems

J. G. LaRock

H. P. Layer

Mechanical engineering

Neutron instrumentation design

Electronics and data processing

Advanced instrumentation

Fundamental physics

J. W. Lynn

C. F. Majkrzak

A. D. Mighell

B. Mozer

D. A. Neumann

J. H. Nicklas

D. J. Pierce

H. J. Prask

Condensed matter physics

Magnetic materials

Neutron scattering methods

Condensed matter physics

Polarized neutron scattering and instrumentation development

o Neutron reflectivity measurements

Crystallographic database development

Single crystal diffraction

Theory of crystal lattices

o Structure and microstructure of metallic glasses

Dynamics of liquids

NDE of alloys

Condensed matter physics

Two-dimensional materials

Neutron and x-ray scattering instrumentation

Mechanical engineering

Reactor fuel design

Reactor engineering support

Mechanical engineering

Neutron instrumentation design

Residual stress measurement

Neutron NDE of hardware

Neutron NDE instrumentation

E. Prince o Structural properties of alloys, catalysts and minerals

Advanced crystallographic refinement methods

Software for materials structure analyses

197
PERSONNEL ROSTER

T. M. Raby o Reactor operations o Nuclear engineering

Reactor standards

J. L. Robertson

J. M. Rowe

Quasicrystals

Structure of amorphous solids

Structural properities of alloys

Orientationally disordered solids

Hydrogen in metals

Cold neutron research and instrumentation

J. J. Rush o Catalysts and molecular materials

Hydrogen in metals

Two-dimensional systems

Inelastic scattering methods

A. Santoro Structure of electronic and structured ceramics

Theory of crystal lattices Powder diffraction methods

S. K. Satija

I. G. Schroder

J. K. Stalick

J. B. Sturrock

M. Suthar

J. F. Torrence

T. J. Udovic

R. E. Williams

Low-dimensional molecular systems

Fractal aspects of microporous media

o Neutron reflectometry

Cold neutron instrumentation development o Nuclear and engineering physics

Optical devices for neutron transport

Neutron and x-ray diffraction

Inorganic chemistry

Crystal database development

o Design engineering o Nuclear systems and components

Design engineering

Nuclear systems and components

o Reactor supervision o Reactor maintenance

o Nuclear engineering

Neutron time-of-flight instrumentation

Properties of catalysts o Spectroscopy of surfaces

Cold neutron source development

o Nuclear engineering

198
TECHNICAL STAFF

PERSONNEL ROSTER

Reactor Operations

Richard D. Beasley

Nathan A. Bickford

Mark G. Cassells Forrest C. Clark

Howard W. Dilks

Daniel J. Flynn

Larry T. Lindstrom

Michael J. McDonald

William W. Mueller

Thomas J. Myers

John H. Ring

Ricky P. Sprow

Robert F. Stiber

Attila L. Toth

Daniel P. Wilkison

Kevin L Wright

Neutron Research

George M. Baltic

Douglas Bostian

David Clem

Donald H. Fravel

Thomas A. Green

Wayne C. Knill

Michael J. Rinehart

Lou Robeson

William H. Rymes

Dan E. Swearingen

Thuan T. Thai

Robert H. Williams

Reactor Engineering

James A. Beatty

Richard P. Hayes

Lynn A. Shuman

199
PERSONNEL ROSTER

NON-RRD NIST STAFF LOCATED AT REACTOR

Occupational Health

and Safety Division (354)

D. Brown

C. Campbell

J. Clark

S. Dominic

G. Deardorff

I. Jensen

T. Mengers

F. Moore

J. Shubiak

L. Slaback

Ionizing Radiation

Division (846)

M. Arif

E. Boswell

M. S. Dewey

C. Eisenhauer

D. Gilliam

G. L. Greene

J. Grundl

E. McGarry

R. Schwartz

W. M. Snow

GUEST WORKERS AND COLLABORATORS

Inorganic Analytical

Research Division (834)

D. A. Becker

H. Chen

R. Demiralp

R. G. Downing

K. A. Fitzpatrick

K. M. Garrity

R. R. Greenberg

G. V. Iyengar

H. Kingston

G. P. Lamaze

J. K. Langland

R. M. Lindstrom

K. C. Littrell E. A. Mackey

D.F.R. Mildner

B. R. Norman

K. Pratt R. Watters, Jr. R. Zeisler

Semiconductor Electronics

Division (812)

J. Ehrstein

Organic Analytical Research

Division (835)

W. Dorko

B. Koster

L. Sander

S. A. Wise

Biotechnology Division (831)

G. Gilliland L. Svensson

Surface and Microanalysis

Science Division (837)

R. Cavanagh

C. Poston

D. Simmons

J. Small

Thermophysics Division (838)

H. Hanley

J. Straty

Quantum Metrology Division (845)

R. Deslattes

MATERIALS SCIENCE AND

ENGINEERING LABORATORY (850)

J. Cahn

Ceramics Division (852)

D. Black

D. Blackburn

B. P. Burton

D. Cranmer

E. Fuller

W. Haller

D. Kauffman

R. Krause

G. Long

200
PERSONNEL ROSTER

Ceramics Division (cont.)

S. Malghan

C. J. Rawn

R. Roth

W. Wong-Ng

Polymers Division (854)

B. Bauer

R. Briber

F. Brinkman

T. Davis

C. Han

A. Nakatani

A. Takano

W. Wu

Metallurgy Division (855) L. Bendersky

F. Biancaniello

W. Boettinger

R. Fields

F. Gayle

C. Johnson

D. Lashmore

R. Schaefer

Federal Bureau of Investigation

R. Aaron

K, Bryan

R. Bilko

R. Halberstam

K. Hargadon

J. Havekost

J. Inga

K. Lundy

E. Peele

R. Rebbert

J. Riley

P. Sanderson

D. Thompson

Food and Drug Administration

D. L. Anderson

S. G. Capar

L. Coyne

W. Cunningham

T. Lindstrom

J. Tanner

Geological Survey

M. Mrose

Army Research, Development,

and Engineering Center

C. Choi

J. Orosz

F. J. Owens

S. F. Trevino

NASA

D. J. Lindstrom

National Institutes of Health

M. Curran

R. Nossal

Army Ballistics Research Lab

B. Rice

L. Vandekieft

DARPA/NMRO

G. Lasche

National Oceanic and

Atmospheric Administration

P. R. Becker

G. Lauenstein

T. Lillestolen

Naval Research Laboratory

D. Collins

C. Roland

P. Skeath

National Science Foundation

K. Kim

201
PERSONNEL ROSTER

Naval Surface Weapons Center

S. F. Cheng

A. E. Clark

J. Cullen

G. Riel

M. Spano

Smithsonian Institution

R, Bishop

M. Blackman

Y. Cheng

R. Cunningham

A. Moore

E. Myers

J. Olin

G. Stein

U. S. Department of Agriculture

W. Wolf

Argonne National Laboratory

J. M. Carpenter

W. Dosier

J. W. Richardson, Jr. P. Thiyagarajan

C.E.C.M./C.N.R.S. (com.)

D. Gratias E. Hewat

S. Lefebvre

M. Marezio

A. Quivy

CEN - Saclay, France

R. Bellissent

Central Bur. Nucl. Meas. (Belgium)

J. Pauwels

Chinese Acad, of Sciences

Q. Lin

ETH Zurich

W. Buhrer

Hahn-Meitner Institute (Berlin)

D. Behne

C. 0. Fischer

W. Leuther

S. Stone

Atom. Inst. (Vienna)

G. Westphal

Boris Kidric Inst., Belgrade, Yugoslavia

D. Djordjevic

Brookhaven National Laboratory

J. Axe

H. Chou

P. Gehring

S. Heald

L. Passell L. Rebelesky

M. Sanyal

A. Saxena

G. Shirane

J. Tranquada

CECM/CNRS Grenoble, France

M. Bessiere

P. Borolet

Y. Calvayrac

C. Chaillout

M. Cornier

Harwell Lab (U.K.)

A. Allen

IAEA

R. M. Parr

Inst, for Roentgen Opt. Sys.

M. A. Kumakhov

Institut Laue-Langevin

Grenoble, France

I. Anderson

B. Farago

B. Frick

U. Griining

A, Magerl

M. Thomas

C. Vettier

G. Zaccai

Institut "Jozef Stefan" (Yugoslavia)

A. Byrne

B. Smodis

202
PERSONNEL ROSTER

International Centre for Diffraction Data

H. McMurdie

IRI Delft, Netherlands

A. van Well

V. O. deHaan

KFA Jiilich, Germany

R. Hempelmann

O. Jucknischke

D. Richter

M. Rossbach

A. Schirmer

G. Schonfeld

M. Steepler

V. Urban

L. Winner

A. Zirkel

Max-Planck Institute W. Knoll

S. Schmidt

National Research Council (Canada)

W. McKinnon

J. Rodgers

Oak Ridge National Laboratory

J. Fernandez-Baca

J. Hayter

H. Mook

R. Nicklow

H. Smith

M. Yethiraj

Ris0 National Laboratory

J. Bohr

Korea Atomic Energy Research Institute C.-H. Lee

Korean Standards Research Institute S.-T. Hwang

Lawrence Berkeley Lab.

D. Morris

R. Sid

Los Alamos National Laboratory

T. Bowles

J. Eckert

S. Elliott Z. Fisk

W. Hamilton

R. Hjelm

A. Lawson

H. Robertson

R. Robinson

G. Smith

T. Sosnick

J. Trewhella

J. Wilkerson

Sandia National Laboratory

C. S. Ashley

B. J. Olivier

D. Schaefer

T. Thurston

Alfred University

R. Snyder

Boston University School of Medicine

K. Zaner

Chalmers University of Technology

L. Sjolin

City University of New York (Brooklyn)

H. Homma

College of William and Mary

E. Schoe

R. Wright

Colorado School of Mines

N. Wada

Paul Scherrer Inst. (Switzerland)

O. Elsenhans

National Research Council (Canada)

J. Rodgers

W. McKinnon

Columbia University

G. Luke

B. Sternlieb

Y. Uremura

203
PERSONNEL ROSTER

Florida State University

S. Berry

G. Cao

J. Crow

J. W. O'Reilly

P. Pernambuco-Wise

Kurchatov Institute, Moscow

M. Gubarev

I. Ponomarev

Korean Advanced Inst, of Science & Technology

Y.-B Ban

Fujian Institute of Research

Q. Huang

Georgetown University

W. Clinton

Harvard University

T. Chupp

J. Richardson

E. Wasserman

Kyoto Institute of Technology

H. Kurokawa

M. Shibayama

Q. Tran-Cong

Kyoto University (Japan)

H. Hasegawa

T. Hashimoto

H. Jinnai

S. Sakurai

International Centre for Diffraction Data

M. Morris

Kyushu University

K. Shutto

Iowa State University

R. Barnes

A. Goldman

S. Martin

John Carroll University

J. Trivisonno

Johns Hopkins University

W. Bao

C. Broholm

A Cavrin

C. L. Chien

J. Childress

M. Cieplak

H. Kerch

S. Lee

S. Ma

D. Musser

D. Reich

F. Streitz

T. Wilson

Kansas State University

T. Isenhour

Lancashire Polytechnic

J. Gottas

Lehigh University

A. Klein

L. Sperling

S. Yang

J. Yoo

Linfield University

P. Davis

M.I.T.

R. Birgeneau

A. Cassanho

M. Greven

C. Hermans

J. Hill T. Hyuga

M. A. Kastner

B. Wuensch

McMaster University

R. Barr

W. Clarke

Kent State University

S. Kumar

J. Mang

Michigan State University

T. Pinnavaia

S. Solin

204
PERSONNEL ROSTER

Nagoya University

Y. Matsushita

Y. Mogi

I. Nodo

Y. Takahashi

North Carolina State University

A. Kingon

Northwestern University

G. Fougere

P. Jemian

J. Weertman

Ohio University

R. Cappelletti

Oklahoma State University

B. Ackerson

Penn State University

A. Hariharan

J. S. Lannin

L. Pilione F. Li

L. Pilione

P. Sokol

Princeton University

W. Graessley

R. Krishnamoorti

Stanford University

M. Fair

A. Cast

Stevens Institute of Technology

S. Dagli

A. Patel

State University of New York at Albany

R. Benenson

W. Gibson

Q. Xiao

State University of New York at Binghamton

K. P. Reis

M. Suzuki

M. D. Whittingham

State University of New York (Stoney Brook)

J. Parise

Temple University

C.-S. Jee

A. Kebede

Texas A&M University

W. Saslow

E. Schweikert

Tohoku University

Y. Endoh

Purdue University

A. Overhauser

Tokyo Science University

C. Lu

Rochester Institute of Technology

M. Kotlarchyk

Ruhr Universitat

A. Schreyer

H. Zabel

Towson State University

C. Lu

University of Akron

M. D. Foster T. Vierheller

Rutgers University

R. Gerhardt

University of Antwerp (Belgium)

K. Michel

San Jose State University

P. Englert

C. Stone

Scottish Universities Research Reactors

R. Scott

University of California (Davis)

Y. X. Jia P. Klavins

J. Liu

R. Shelton

205
PERSONNEL ROSTER

University of California

(Santa Barbara)

S. Cox

T. E. Gier

W. Harrison

D. Klingensmith

E. Mader

T. Nenoff

R. Odette

G. Stucky

University of California (San Diego)

J. Arnold

A. Berkowitz

N. Elango

B. Lee

B. Maple

J. Neumeier

K. Nishiizumi

L. F. Ten Eyck

University of California (Santa Cruz)

D. Belanger

W. Murray

University of Delaware

W. Dubner

A. Full

W. Kaler

S. Kline

R. Koehler

M. Long

N. Wagner

University of Exeter (U.K.)

R. Slade

University of Guelph (Canada)

F. Hallett

University of Houston

W.-K. Chu

S. Moss

J. Robertson

University of Illinois (Urbana)

R. Beach

L. Chen

M. Chow

R. Du

C. Flynn

University of Illinois (cont.)

A. Matheny

G. Moore

H. MorkoQ

C. Rueb

M. Said

M. Salamon

F. Tsui

C. Zukowski

University of Maryland

H. Ammon

P. Brand

C.-H. Chen

T. Clinton

T. Colley

M. Colombini

F. Divita

G. Gordon

S. Green

V. Guinn

D. Gurley

M. Han

W. Li

Z. Lin

J. Lynn

J. Nicol

J. Ondov

J. Peng

T. Quinn

N. Rosov

S. Skanthakumar

W. Sumarlin

J. Valette-Silver

W. Walters

Z. Wu

H. Zhang

B. Zimmerman

University of Massachusetts

A. Birnholz

H. Birnholz

R. Composto

A. Hanyu

L. Jong

T. Mansfield

R. Stein

University of Melbourne

A. Klein

G. Opat

206
PERSONNEL ROSTER

University of Michigan

A. Z. Akcasu

R. F. Fleming

J. S. King

D. H. Vincent

University of Milan (Italy)

M. Zocchi

University of Minnesota

F. Bates

A. Karim

J. Rosedale

M. Sikka

N. Singh

University of Missouri (Columbia)

R. Berliner

H. Kaiser

A. Krawitz

J. Rhyne

G. Schupp

P. Schmidt

Y. Tang

S. Werner

D. Worcester

University of Missouri (RoUa)

P. Lindemuth

University of NSW (Australia)

B. Hunter

University of Nancy (France)

C. Du Four

P. Mangin

G. Marchal

M. Vergnat

University of North Carolina

N. Parikh

M. Swanson

University of Notre Dame

J. Furdyna

T. Giebultowicz

P. Klosowski

H. Luo

N. Samarth

University of Oslo

P. Karen

A. Kjekshus

University of Paris (SUD)

J. N. Daou

P. Vajda

University of Pennsylvania

N. Coustel

R. Egami

J. Fischer

R. Hu

B. Latshaw

N. Maliszewskyj

J. P. McCauley

O. Zhou

University of Pittsburg

W. Goldberg

X. Wu

Universite Pierre et Marie Curie (France)

Ph. Depondt

University of Rhode Island

M. Crow

S. Pickart

University of Southern California

Z. Amis

University of Sussex (UK)

J. Byrne

P. Dawber

J. Spain

University of Vermont

T. Flanagan

University of Vienna

K. Remschnig

University of Virginia

T. Williamson

University of Washington

B. Heckel

W. ZoUer

207
PERSONNEL ROSTER

University of West Virginia

R. Kannan

M. Seehra

University of Wien

O. Blaschko

University College (London)

F. Beech

Bellcore

S. Allen

P. Barboux

M. Giroud

L. Greene

G. Hull

P. Miceli

C. Palmstr0m

J. Tarascon

Yale University

J. Flanagan

Allied Signal Corporation

G. Fish

C. Parker

Charles Evans & Assoc.

R. Fleming

CIDTECH

J. Carbone

M. Wadsworth

Aerojet Electrosystems

R. Bowman, Jr.

Directed Technologies, Inc. N. Chesser

Aerospace Corporation

J. Knudsen

Dow Chemical

R. Bubeck

AT&T Bell Laboratories

R. Cava

S. Cheong

R. Fleming

G. Fredrickson

R. Kleiman

J. Krajewski

P. Marsh

J. Mitchell

D. Murphy

W. Peck

W. Rhodes

J. Riley

L. Schneemeyer

S. Sunshine

J. Waszazak

S. Yegnasubramanian

Atomic Energy of Canada, LTD

Z. Tun

Aware, Inc. M. An

R. Tolimieri

E. I. DuPont de Nemours Co.

M. Crawford

R. Harlow

Eastman Kodak

M. Landry

J. Lavine

J. O'Reilly

J. Russell

C. Swanson

J. Tritten

H. Yang

Exxon

B. Abeles

N. P. Balsara

D. M. Cox

K. M. Creegan

J. Dunsmuir

L. Fetters

J. Huang

G. Hughes

M. Kim

H. King, Jr. K. Liang

M. Lin

D. Lohse

D. Peiffer

208
PERSONNEL ROSTER

Exxon (cont.)

S. Sinha

E. Sirota

H. Stasiewski

GE Corporate R&D

R. Rowe

GE Plastics

J. Schroeder

Texas Instrument

T. Hossain

Westinghous Savannah River Co.

J. Griffin E. Sadowski

X-Ray Optical Systems

D. M. Gibson

W. R. Grace

R. Wormsbecker

IBM (San Jose)

R. Farrow

G. Hadziioannou

A. Mayes

A. Menelle

T. P. Russell

Imperial Chemical Industries

H. Stanley

Intel Corporation

J. Cox

John Deere Product Eng. Ctr. T. Cordes

Mobil Oil

A. Moini

Morris Research

D. Morris

Optoline Corp.

J. Bradshaw

Polaroid Corp.

S. Roy

Southwest Research Inst. R. Page

SRI, Inc.

S. Johnson

Texaco Research Center

E. Sheu

209
H. PUBLICATIONS

An, M.; Lu, C; Prince, E.; Tolimier, R., "Fast Fourier Transforms for Space Group Containing

Rotation Axes of Order Three and Higher," Acta Crystallogr., Sec. A (in press).

An, M.; Lu, C; Prince, E.; Tolimieri, R., "Fast Fourier Transform Algorithms for Real and Symmetric

Data," Acta Crystallogr., Sec. A (in press).

Anderson, I. S.; Berk, N. F.; Rush, J. J.; Udovic, T. J.; Barnes, R. G.; Magerl, A.; Richter, D., "Reply

to the Comment by Kadaro and Kiefl," Phys. Rev. Lett. 66, 2415 (1991).

Ankner, J. F., "Profile Refinement in Reflectivity and Grazing Angle Diffraction," in Proceedings of the

Second International Conference on Surface X-Ray and Neutron Scattering , edited by H. Zabel

and L K. Robinson, (Springer-Verlag, New York, 1991) in press.

Ankner, J. F.; Majkrzak, C. F.; Neumann, D. A.; Matheny A.; Flynn, C. P., "Polarized Neutrons

Grazing Angle Diffraction," Physica B (in press).

Ankner, J. F.; Majkrzak, C. F.; Neumann, D. A.; Matheny, A.; Flynn, C. P., "Reflectivity and Grazing

Angle Diffraction of Polarized Neutrons," in Magnetic Thin Films. Multilayers and Surfaces , edited by S. P. Parkin, Mater. Res. Soc. Symp. Proc, Vol. 231, (Materials Research Society,

Pittsburgh, 1991) in press.

Bashir-Hashemi, A.; Ammon, H. L.; Choi, C. S., "Chemistry and Structure of Phenylcubanes," J. Org.

Chem. 55, 416 (1990).

Beach, R. S.; Borchers, J. A.; Erwin, R. W.; Matheny, A.; Flynn, C. P.; Rhyne, J. J.; Salamon, M.

B., "Effects of Epitaxial Strain in Er/Lu Thin Films," J. Appl. Phys. 69, 4535 (1991).

Beach, R. S.; Borchers, J. A.; Erwin, R. W.; Flynn, C. P.; Matheny, A.; Rhyne, J. J.; Salamon, M.

B., "Magnetic Order in Dy/Lu Superlattices," J. Magn. Magn. Mater, (in press).

Berk, N. F., "Scattering Properties of the Leveled Wave Model of Random Morphologies," Phys. Rev.

A. (in press).

Berk, N. F.; Anderson, L S.; Rush, J. J.; Udovic, T. J.; Barnes, R. G.; Magerl, A., "Rapid Low

Temperature "Hopping of Hydrogen in a Pure Metal: The ScH^^ System," Phys. Rev. Lett. 65,

1439 (1990).

Berk, N. F.; Rush, J. J.; Udovic, T. J.; Anderson, L S., "Anomalous Hydrogen Dynamics in Rare Earth

Materials," International Conference on Hydrogen-Metal Systems, J. Less-Common Met. (in

press).

Borchers, J. A.; Salamon, M. B.; Erwin, R. W.; Rhyne, J. J.; Du, R. R.; Flynn, C. P., "Structural and

Magnetic Properties of Er Thin Films and Er/Y Superlattices: Magnetoelastic Effects," Phys.

Rev. B43, 3123 (1991).

Borchers, J. A.; Salamon, M. B.; Erwin, R. W.; Rhyne, J. J.; Nieuwenhuys, G. J.; Du, R. R.; Flynn,

C. P.; Beach, R. S., "Structural and Magnetic Properties of Er Thin Films and Er/Y

Superlattices: Modification of the Commensurate Spin States," Phys. Rev. B (in press).

210
PUBLICATIONS

Cannelli, G.; Cantelli, R.; Cordero, F.; Trequattrini, F.; Anderson, I. S.; Rush, J. J., "H Tunneling and

Trapping by Y by Anelastic Relaxation Measurements," Phys. Rev. Lett, (in press).

Cao, G.; Yuen, T.; Pernambuco-Wise, P.; Crow, J. E.; O'Reilly, J. W.; Kuric, M. V.; Guetin, R. P.; Rosov, N.; Lynn, J. W., "Thermodynamic and Magnetic Properties of the Tetravalent Rare Earth

Perovskite System BaPrOg," J. Appl. Phys. (in press).

Cappelletti, R. L.; Copley, J.R.D.; Kamitakahara, W. A.; Li, F.; Lannin, J. S.; Ramage, D.,

"Neutron Measurements of Intramolecular Vibrational Modes in C^q," Phys. Rev. Lett. 66, 3264

(1991).

Casella, R. C, "Nonlocal Phase Shifts Induced by Static Electric Fields in Neutron Interferometers When

the Path-Enclosed Charge Vanishes," Phys. Rev. Lett. 65, 2217 (1990).

Casella, R. C, "A Theoretical Model for the Tunneling-Gap Anisotropy Observed in Layered CopperOxide High-Temperature Superconductors," Solid State Commun. 78, 377 (1991).

Cava, R. J.; Santoro, A.; Krajewski, J. J.; Fleming, R. M.; Waszczak, J. V.; Peck Jr., W. F.; Marsh,

P., "The Crystal Structure of the 60 K Superconductor La^ ^Sr^ 4CaCu205^.," Physica C (in

press).

Cieplak, M. Z.; Xiao, G.; Bakhshai, A; Artymowicz, D.; Bryden, W.; Chien, C. L.; Stalick, J. K.; Rhyne, J. J., "Incorporation of Gold Into YBaoCu307: Structure and T^ Enhancement," Phys.

Rev. B (in press).

Clinton, T. W.; Lynn, J. W., "Magnetic Ordering of Er in Powder and Single Crystals of ErBa',Cu307,"

Physica C 174, 487 (1991).

Clinton, T. W.; Lynn, J. W.; Liu, J. Z.; Jia, Y. X.; Shelton, R. N., "Magnetic Order of Dy in DyBa2Cu307," J. Appl. Phys. (in press).

Collins, D. M.; Prince, E., "Exponential Density: Exact Fitting of Structure Moduli by Entropy

Maximization," in Crystallographic Computing V: From Chemistry to Biology, edited by D.

Moras, A. D. Podjarny, and J. C. Thierry (Oxford University Press, 1991), 308.

Copley, J. R. D., "Acceptance Diagram Analysis of the Performance of Vertically Curved Neutron

Monochromators," Nucl. Instrum. Methods Phys. Res. A 30i, 191 (1991).

Copley, J. R. D., "Monte Carlo Calculation of Multiple Scattering Effects in Thermal Neutron Scattering

Experiments: Modification to Spherical Geometry," Comput. Phys. Commun. (in press).

Copley, J. R. D., "Transmission Properties of a Counter-Rotating Pair of Disk Choppers," Nucl.

Instrum. Methods Phys. Res. A 303, 332 (1991).

Copley, J. R. D.; Mildner, D. F. R., "Simulation and Analysis of the Transmission Properties of CurvedStraight Neutron Guide System," Nucl. Sci. Eng. (in press).

Copley, J. R. D., "An Analytical Method to Characterize the Performance of Multiple Section StraightSided Neutron Guide Systems," Nucl. Instrum. Methods Phys. Res. A 287, 363 (1990).

211
PUBLICATIONS

Copley, J. R. D., "Scattering Effects Within an Absorbing Sphere Immersed in a Field of Neutrons,"

Nucl. Instrum. Methods Phys. Res. A (in press).

Dane, P. R.; Ferraro, M.; Ammon, H. L.; Choi, C. S., "Synthesis of 2, 2, 4, 4-Tetranitroadamantane,

J. Org. Chem. 55, 4459 (1990).

Depondt, Philippe; Neumann, D. A.; Trevino, S. F., "Neutron Scattering Study of Cs-Ammonia

Intercalated Graphite," Mater. Sci. Forum (in press).

Fan, Y.; Solin, S. A.; Kim, H.; Pinnavaia, T. J.; Neumann, D. A., "Elastic and Inelastic Neutron

Scattering Study of Hydrogenated and Deuterated Trimethyl ammonium Pillared Vermiculite

Clays," J. Chem. Phys. (in press).

Flynn, C. P.; Borchers, J. A.; Demers, R. T.; Du, R.-R.; Dura, J. A.; Klein, M. V.; Kong, S. H.;

Salamon, M. B.; Tsui, T. F.; Adavalli, S.; Zhu, X.; Zabel, H.; Cunningham, J. E.; Erwin, R. W.; Rhyne, J. J., "Growth and Properties of Superlattices with Antiferromagnetic Rare Earth,"

in Proceedings of the MRS International Meeting on Advanced Materials: in Multilayers, edited

by Y. Ryoichi, (Materials Research Society, Pittsburgh, 1990), 275.

Giebultowicz, T. "Neutron Diffraction Studies of Magnetic Semiconductor Superlattices and Thin Films,"

XX^^ International School of Physics Semiconducting Compounds, JASZOWIEC, Poland,

May 26-June 1, 1991.

Giebultowicz, T. M.; Klosowski, P.; Rhyne, J. J.; Samarth, N.; Luo, H.; Furdyna, J. K.,

"Incommensurate Antiferromagnetic Order in Strained Layer MnSe/ZnTe Superlattices," Physica

B (in press).

Gehring, P. M.; Tranquada, J. M.; Shirane, G.; Copley, J.R.D.; Erwin, R.; Sato, M.; Shamoto, S.,

"Magnetic Correlations and Energy Gap in Superconducting YBa2Cu30gg With T^ = 53K,"

Phys. Rev. B 44, 2814 (1991).

Hammouda, B.; Schupp G.; Maglic, S., "QEGS From Polydimethylsiloxane in Benzene Solutions," J. Chem. Phys. 93, 5473 (1990).

Hammouda, B.; Krueger, S.; Glinka, C, "Small Angle Neutron Scattering at Natl. Inst, of Stand.

Technol." J. Res. 1991 (in press).

Hammouda, B.; Briber, R.; Bauer, B., "SANS From PSD/PVME/PSH Ternary Polymer Blend," Polym.

Comm. (in press).

Hammouda, B.; Nakatani, A.; Waldow, D.; Han, C, "SANS From PSD in DOP Solution Under Shear,"

Macromol. (in press).

Hammouda, B., "Structure Factor for Dendrimer Polymer Gels," J. Polym. Sci., Polym. Phys. (in

press).

Hammouda, B., "SANS from Homogeneous Polymer Systems: A Unified Overview," Adv. Polym.

Sci. (in press).

212
PUBLICATIONS

Harrison, W. T. A.; Gier, T. E.; Moran, K. L.; Nicol, J. M.; Eckert, J.; Stucky, G. D., "Structure and Properties of New Zeolite X-Type Zincophosphate and Berylliophosphate Molecular Sieves,"

Chem. Mater. 3, 27 (1991).

Hempelmann, R.; Richter, D.; Rush, J. J.; Rowe, J. M., "Hydrogen Site Distribution in the Alloy

System Nhjoo^V^^H Studied by Means of Neutron Vibrational Spectroscopy," J. Less-Common

Met. (in press).

Kamitakahara, W. A.; Cappelletti, R. L.; Boolchand, P.; Halfpap, B.; Gompf, P.; Neumann, D. A.; Mutka, H., "Vibrational Densities of States and Network Rigidity in Chalcogenide Glasses,"

Phys. Rev. B 44, 94(1991).

Karen, V. L.; Mighell, A. D., "Converse-Transformation Analysis," J. Appl. Cryst. (in press).

Karen, V. L.; Mighell, A. D., "NIST*LATTICE—A Program to Analyze Lattice Relationships,"

(Version of Spring 91). NIST Tech. Note 1290. Natl. Inst. Stand. Technol., U.S. Department

of Commerce (1991).

Kirchheim, R.; Kieninger, W.; Huang, X. Y.; Filipek, S. M.; Rush, J. J.; Udovic, T. J., "Hydrogen in Amorphous Ni-Zr and Ni-Ti Alloys," J. Less-Common Met. (in press).

Klosowski, P.; Giebultowicz, T. M.; Rhyne, J. J.; Samarth, N.; Luo, H.; Furdyna, J. K.,

"Antiferromagnetic Ordering in MnSe/ZnSe Multilayers," J. Appl. Phys. 69, 6109 (1991).

Klosowski, P.; Giebultowicz, T. M.; Samarth, N.; Luo, H.; Furdyna, J. K.; Rhyne, J. J., "Investigation

of ZnMnTe Weakly Diluted Magnetic Semiconductor," Physica B (in press).

Klosowski, P.; Giebultowicz, T. M.; Samarth, N.; Luo, H.; Furdyna, J. K.; Rhyne, J. J., "Magnetic

Critical Phenomena in fee Antiferromagnets—Role of Strain and Dimensionality," Proc. Int. Conf. Magn., Edinburgh, 1991 (in press).

Krueger, S.; Long. G. G.; Black, D.; Minor, D.; Jemian, P. R.; Nieman, G. W.; Page, R. A.,

"Evolution of the Pore Size Distribution in Final Stage Sintering of Alumina Measured by SmallAngle X-Ray Scattering," J. Am. Ceramic Soc. (in press).

Krueger, S.; Chen, S.-H.; Hofrichter, J.; Nossal R., "Small Angle Neutron Scattering Studies of HbA

in Concentrated Solutions," Biophys. J. 58, 745 (1991).

Krueger, S.; Long, G. G.; Page, R. A., "Characterization of the Densification of Alumina by Multiple

Small-Angle Neutron Scattering," Acta Crystallogr. A 74, 282 (1991).

Layer, H. P.; Greene, J. L., "Elastic Deformation of a Monolithic Perfect Crystal Interferometer:

Implications for Gravitational Phase Shift Experiments," Phys. Lett. A 155, 450 (1991).

Lewicki, A.; Schindler, A. I.; Furdyna, J. K.; Giebultowicz, T. M., "Magnetic Properties of Co-Based

Diluted Magnetic Semiconductors," in Diluted Magnetic Semiconductors , edited by M. Jain,

(World Scientific), in press.

Lindemuth, P.; Hammouda, B.; Venable, R., "SANS fromTPB Based Microemulsions," J. Phys. Chem.

(in press).

213
PUBLICATIONS

Long, G. G.; Krueger, S.; Jemian, P. R.; Black, D. R.; Burdette, H. E.; Cline, J. P.; Gerhardt, R. A.,

"Small Angle Scattering Determination of the Microstructure of Porous Silica Precursor Bodies,"

J. Appl. Crystallogr. 23, 535 (1990).

Long, G. G.; Krueger, S.; Gerhardt, R. A.; Page, R. A., "Small Angle Neutron Scattering

Characterization of Processing/Microstructure Relationships in the Sintering of Crystalline and

Glassy Ceramics," J. Mat. Res. (in press).

Lynn, J. W.; Sumarlin, L W.; Neumann, D. A.; Rush, J. J.; Peng, J. L.; Li, Z. Y., "Phonon Density

of States and Superconductivity in Nd^ gjCeg ;i5Cu04," Phys. Rev. Lett. 66, 919 (1991).

Lynn, J. W., "Two-Dimensional Behavior of the Rare Earth Ordering in Oxide Superconductors," J. Alloys and Compounds (in press).

Majkrzak, C. P., "Polarized Neutron Reflectometry," Physica B 173, 75 (1991).

Majkrzak, C. F.; Felcher, G. P., "Neutron Scattering Studies of Surfaces and Interfaces," MRS Bulletin,

2CV(11), 65 (1990).

Majkrzak, C. P.; Kwo, J.; Hong, M.; Yafet, Y.; Gibbs, Doon; Chien, C. L.; Bohr, J., "Magnetic Rare

Earth Superlattices," Adv. Phys. (in press).

Marezio, M.; Santoro, A.; Capponi, J. J.; Hewat, E. A.; Cava, R. J.; Beech, F., "Crystal Structure,

Atomic Ordering and Charge Localization if Pr2Sr2Y^_j^Caj^Cu30g^3 (x = O, 6 = 1.47),"

Physica C 169, 40 (1990).

McKinnon, W. R.; Tselepis, E.; Tarascon, J. M.; Miceli, P. F.; Remschnig, K.; Hull, G. W.; Neumann,

D. A.; Rhyne, J. J., "Weak Ferromagnetism and Canting in BiPbCa2MnOg and BiPbSr2MnOg,"

Phys. Rev. B 41, 4489 (1990).

Menelle, A.; Russell, T. P.; Anastasiadis; S. H.; Satija, S. K.; Majkrzak, C. F., "The Ordering of Thin

Diblock Copolymer Films," Phys. Rev. Lett, (in press).

Mighell, A. D.; Karen, V. L., "NIST CRYSTAL DATA - A Data Base of Chemical, Physical and

Crystallographic Data and the NIST/Sandia/ICDD Electron Diffraction Database" 1990 Versions.

The databases and search software are made available to the scientific community on CD-ROM,

on magnetic tape, via Scientific Instruments and the CRYSTDAT on-line system.

Mildner, D.F.R.; Hammouda, B., "The Transmission of Curved Neutron Guides With Non-Perfect

Reflectivity," J. Appl. Crystal, (in press).

Mozer, B.; Bendersky, L. A.; Boettinger, W. J.; Rowe, R. G., "Neutron Powder Diffraction Study of

the Orthorhombic Ti2AlNb Phase," Scripta Metallur. 24, 2363 (1990).

Neumann, D. A., "Inelastic and Quasielastic Neutron Scattering Studies of Layer Silicates," in Neutron

Scattering for Clay Mineral Investigation , edited by F. Ross (Clay Minerals Society,

Bloomington), in press.

Nicol, J. M., "Chemisorbed Hydrogen and Hydrogenous Molecules," Spectrochimica Acta (in press).

214
PUBLICATIONS

Prask, H. J., "Neutron Probes Tackle Industrial Problems," Advanced Materials and Processes 140, No.

3, 26 (September 1991).

Prask, H. J.; Choi, C. S., "Residual Stress Characterization in Technological Samples," in Proceedings

of Practical Applications of Residual Stress Technology Conference, edited by C. Ruud,

(Published by ASM International) in press.

Prask, H. J.; Choi, C. S., "Residual Stress Measurement in Armament-Related Components," in Proceedings of NATO Advanced Workshop on Measurement of Residual and Applied Stress Using Neutron Diffraction, edited by M. Hutchings, (Oxford, 1991) in press.

Prince, E., "Mathematical Aspects of Rietveld Refinement," in Proceedings of the International Workshop

on the Rietveld Method , edited by R. A. Young, (Oxford University Press, Oxford, England) in press.

Rhyne, J. J.; Erwin, R. W.; Borchers, J. A.; Salamon, M. B.; Tsui, F; Du, R.; Flynn, C. P., "Magnetic

Rare Earth Artifical Metallic Superlattices," in NATO Advance Study Institute on the Science and

Technology of Nanostructured Magnetic Materials , edited by G. Prinz and G. Hadjipanayis,

(Plenum Press, 1990), in press.

Rice, B. M.; Trevino, S. P., "An Intermolecular H-0 Potential for Methyl Rotations in Solid

Nitromethane," J. Chem. Phys. (in press).

Robertson, J. L., "Random Cluster Models for Icosahedral Phase Alloys," in Proceedings of the

International Workshop on Methods of Structural Analysis of Modulated Structures and

Ouasicrystals . (World Scientific Publishing), in press.

Rowe, J. M.; Prask, H. J., "Status of Research Reactor Instrumentation in the USA," Physica B 174, 421

(1991).

Rowe, J. M.; Rorer, D. C, "Existing Cold Sources at U.S. Reactors," in International Workshop on

Cold Neutron Research Sources , compiled by G. J. Russell and CD. West (Los Alamos National

Laboratory, Los Alamos, New Mexico, 1990), LA-12146-C Conference, 121.

Russell, T. P.; Menelle, A.; Anastasiadis, S. H.; Satija, S. K.; Majkrzak, C. P., "Unconventional

Morphologies of Symmetric, Diblock Copolymers Due to Film Thickness Constraints,"

Macromol. (in press).

Russell, T. P.; Menelle, A.; Hamilton, W. A.; Smith, G. S.; Satija, S. K.; Majkrzak, C. F., "The Width

of Homopolymer Interfaces in the Presence of Symmetric Diblock Copolymers," Macromol. (in

press).

Salamon, M. B.; Beach, R. S.; Borchers, J. A.; Erwin, R. W.; Flynn, C. P.; Matheny, A.; Rhyne, J. J., "Magnetism of Rare-Earth/Y and Rare-Earth/Lu Superlattices and Films," Physica Scripta (in

press).

Salamon, M. B.; Borchers, J.; Du, R.; Flynn, C. P.; Tsui, F.; Beach, R. S.; Erwin, R. W.; Rhyne, J. J., "Magnetic Properties of Epitaxial Metallic Superlattice," Physica Scripta T35, 163 (1991).

215
PUBLICATIONS

Samarth, N.; Klosowski, P.; Luo, H.; Giebultowicz, T. M.; Furdyna, J. K.; Rhyne, J. J.; Larson, B.; Otsuka, N., "Antiferromagnetism in MnSe/ZnSe Strained-Layer Superlattices," Phys. Rev. B 44,

4701 (1991).

Santoro, A.; Beech, F.; Cava, R. J., "Models for Oxygen Ordering and Diffusion in Ba2YC30^ and

Ba2YCu3.xM^Oy [M=Fe, Co, Al, Ga]," in Proceedings of the 7th CIMTEC Meeting on

Superconductors (in press).

Santoro, A., "Defective Structures of Ba2YCu30x and Ba2YCu3.yMy02, (M = Fe,Co,Al,Ga,...)," in Chemistry of Superconducting Materials , edited by T. A. Vanderah (Noyes Publications), in press.

Santoro, A., "Crystal Chemistry of Superconductors and Related Compounds," in Chemistry of

Superconducting Materials , edited by T. A. Vanderah, (Noyes Publications), in press.

Sanyal, M. K.; Sinha, S. K.; Gibaud, A.; Satija, S. K.;Majkrzak, C. F.; Homma, H., "Specular and

Diffuse Scattering Studies of Multilayer Interfaces," in Proceedings of the 2nd International

Conference on Surface X-Ray and Neutron Scattering , edited by H. Zabel and I. K. Robinson,

(Springer-Verlag, New York, 1991), in press.

Schroeder, J.; Hammouda, B.; Bubeck, R. A.; Chang, J. W., "SANS From Hot Stretched Polystyrene

Revisited," J. Polym. Sci. (in press).

Schupp, G.; Hammouda, B.; Hsueh, C, "Quasielastic Gamma-Ray Scattering From Pentadecane, " Phys.

Rev. A 4i, 5610 (1990).

Sinha, S. K.; Sanyal M. K.; Gibaud, A.; Satija, S. K.; Majkrzak, C. F.; Homma, H., "The

Characterization of Interface Roughness and Other Defects in Multilayers by X-Ray Scattering,"

NATO Advanced Study International Conference Proceeding, (Greece, June 1990), in press.

Sjolin, L.; Prince, E.; Svensson, L. A.; Gilliland, G. L., "Ab Initio Phase Determination for X-Ray

Diffraction Data From Crystals of a Native Protein," Acta Crystallogr. A 47, 216 (1991).

Skanthakumar, S.; Lynn, J. W.; Peng J. L., "Antiferromagnetic Ordering of Cu in Sm2Cu04," J. Appl.

Phys. 49, 4866 (1991).

Skanthakumar, S.; Lynn, J. W., "On the Crystal and Spin Structures Nd2Cu04," Physica C 170, 175

(1990).

Skanthakumar, S.; Lynn, J. W., "Magnetic Order of Cu in Nd2.xCej^Cu04," J. Magn. Magn. Mater,

(in press).

Stalick, J. K.; Gotaas, J. A.; Cheng, S. F.; Cullen, J.; Clark, A. E., "Crystallographic and Magnetic

Structure of Pr2Fe27N2 g," Mater. Lett, (in press).

Straty, G. C; Hanley, H. J. M.; Glinka, C. J., "Shearing Apparatus for Neutron Scattering Studies on

Fluids: Preliminary Results for Colloidal Suspensions," J. Stat. Phys. 62, 1015 (1991).

Tsui, F.; Flynn, C. P.; Salamon, M. B.; Borchers, J. A.; Erwin, R. W.; Rhyne, J. J., "Layer Thickness

Dependence of Anisotropic Coupling in Gd/Y Superlattices," J. Magn. Magn. Mater, (in press).

216
PUBLICATIONS

Tsui, F.; Flynn, C. P.; Salamon, M. B.; Erwin, R. W.; Borchers, J.; Rhyne, J. J., "Anisotropic

Magnetic Behavior in Dy/Y Films and Superlattices," Phys. Rev. B 43, 13320 (1991).

Wada, N.; Kamitakahara, W. A., "Inelastic Neutron and Raman Scattering Studies of Muscovite and

Vermiculite Layered Silicates," Phys. Rev. B 43, 286 (1991).

Wu, W. L.; Satija, S. K.; Majkrzak, C. F., "Neutron Reflectivity Study at a Polymer-Solid Interface,"

Polymer Communications (in press).

Yoo, J. N.; Sperling, L. H.; Glinka, C. J.; Klein, A. "Characterization of Film Formation From

Polystyrene Latex Particles via SANS 2. High Molecular Weight," Macromol. 24, 2868 (1991).

Yu, S. C; Lynn, J. W.; Rhyne, J. J.; Fish, G. E., "Low Temperature Magnetization and Magnetic

Excitations in Amorphous Fe78B^3Si9," J. Magn. Magn. Mater. 97, 286 (1991).

Zhang, Huai; Lynn, J. W.; Morris, D. E., "Two-Dimensional Bilayer Magnetic Order of Dy Ions in Dy2Ba4Cu70^5," J. Magn. Magn. Mater, (in press).

INDEPENDENT PROGRAMS

Anderson, D. L.; Cunningham, W. C; Mackey, E. A., "Determination of Boron in Food and Biological

Reference Materials by Neutron Capture Prompt-y Activation," Fresernius, J. Anal. Chem. 338 . 554 (1990).

Anderson, D. L.; Cunningham, W. C; Mackey, E. A., "Neutron Capture Prompt-yActivation Analysis

of Foods," Biol. Trace Elem. Res. 27, 616 (1990).

Anderson, D. L.; Cunningham, W. C; Mackey, E. A., "Neutron Capture Prompt-y Activation Analysis

of Foods," Biol. Trace Elem. Res. 26, 613 (1990).

Bauer, B. J.; Briber, R. M.; Dickens, B., "Studies of Grafted Interpenetrating Polymer Networks," Proc. Am. Chem. Soc, Div. Polym. Mater: Sci. Eng. 65, New York, 1990.

Bauer, B. J.; Briber, R. M., "The Effect of Crosslink Density on Phase Separation in Interpenetrating

Polymer Networks," Advances in IPN's, Vol. 4, in press.

Becker, D. A., "A Look at the ASTM Reactor," Trans. ANS 62, 223 (1990).

Borner, H. G.; Jolie, J.; Robinson, S. J.; Kessler, E. G.; Ulbig, S.; Casten, R. F.; Dewey, M. S.; Deslattes, R. D.; Lieb, K.; Krusche, B.; Cizewski, J., "Gamma-Ray Induced Doppler Broadening

and the Determination of Lifetimes of Excited Nuclear States," in Capture Gamma-Ray

Spectroscopy, edited by R. Hoff (in press).

Bowman, R. C, Jr.; Knudsen, J. F.; Downing, R. G., "Neutron Depth Profiling of Boron Implanted

Semiconductors," Mater. Res. Soc. Symp. Proc, Vol. 166, 331 (1990).

217
PUBLICATIONS

Briber, R. M.; Bauer, B. J., "Small Angle Neutron Scattering of Blinds of Crosslinked and Linear

Polystyrene," Macromol. 24, 1899 (1991).

Briber, R. M.; Bauer, B. J., "Phase Transitions in Complex Polymer Systems," Physics News 1991 (in

press).

Byrne, J.; Dawber, P. G.; Spain, J. A.; William, A. P.; Dewey, M. S.; Gilliam, D. M.; Greene, G. L.; Lamaze, G. P.; Scott, R. D.; Pauwels, J.; Dykens, R.; Lamberty, A., "Measurement of the

Neutron Lifetime by Counting Trapped Protons," Phys. Rev. Lett. 65, 289 (1990).

Carpenter, J. M.; Mildner, D.F.R.; Richardson, J.W., Jr., "Correcting Time-of-Flight Neutron

Diffraction Data for Chopped Delayed Neutron Backgrounds: The Action of a Drum Chopper

in a Pulsed Neutron Beam," Nucl. Instrum. Meth. A301 . 305 (1991).

Carpenter, J. M.; Mildner, D.F.R., "Transmission of a Drum Chopper in a Thermal Neutron Beam,"

Nucl. Instrum. Meth. A301 . 315 (1991).

Clarke, W. B.; Downing, R. G., "Determination of Lithium in 13 Geochemical Reference Materials by

Thermal-Neutron Capture and Helium-3 Mass Spectrometry," Geostandards Newsletter, Vol. XV,

No. 1, 121 (1991).

Cohen, A.; White, E.; Coxon, B.; Christensen, R. G.; Welch, M. J.; Paule, R. C; Becker, D. A.,

"Certification of Bilirubin SRM 916a," Fresenius J. Anal. Chem. 338, 426 (1990).

Copley, J.R.D.; Mildner, D.F.R., "Simulation and Analysis of the Transmission Properties of CurvedStraight Neutron Guide Systems," Nucl. Sci. Eng. (accepted).

Dewey, M. S.; Kessler, Jr., E. G.; Fischback E., "A Test of Special Relativity by a Determination of

the Lorentz Limiting Veclocity: Does E = mc2," Phys. Rev. D, Rapid Comm. (in press).

Dodd, J. A.; Ondov, J. M.; Tuncel, G.; Dzubay, T. G.; Stevens, R. K., "Multimodal Size Spectra of

Submicrometer Particles Bearing Various Elements in Rural Air," Environ. Sci. Technol. 25, 890

(1991).

Downing, R. G.; Lavine, J. P.; Hossain, T. Z.; Russell, J. B.; Zenner, G. P., "The Measurement of

Boron at Silicon Wafer Surfaces by Neutron Depth Profiling," J. Appl. Phys. 67, 3652 (1990).

Eisenhauer, C. M., "Study of Slab Transmission and Reflection," Nucl. Sci. Eng. 107, 67 (1991).

Fischbach, E.; Hughes, R., "A New Test of Quantum Mechanics, Is Planck's Constant Unique?," Phys.

Rev. Lett. 66, 256 (1991).

Gilliam, D. M.; Briesmeister, Judith F., "Benchmark Measurement and Calculations of Neutron Leakage

from Water Moderators," in Proceedings International Meeting in Advances in Mathematics,

Computations, and Reactor Physics . (Pittsburg, PA, 1991), 9.1, 4.1.

Greenberg, R. R.; Kingston, H. M.; Watters, R. L., Jr.; Pratt, K. W., "Dissolution Problems With

Botanical Reference Materials," Fresenius. Anal. Chem. 338, 394 (1990).

218
PUBLICATIONS

Hasegawa, H.; Tanaka, H.; Hashimoto, T.; Han, C. C, "SANS and SAX Study of Block

Copolymer/Homopolymer Mixtures," J. Appl. Crystallogr. (in press).

Hasegawa, H.; Sakurai, S.; Tekenaka, M.; Hashimoto, T.; Han, C. C, "Small-Angle Neutron Scattering

and Light Scattering Study on the Miscibility of Protonated Polyisoprene/Deuterated

Polybutadiene Blends. Macromol. 24, 1813 (1991).

Jinnai, H.; Hasegawa, H.; Hashimoto, T.; Han, C. C, Time-Resolved Small-Angle Neutron Scattering

in Later Stage Spinodal Decomposition of Polymer Blend," Macromol. 24, 282 (1991).

Kessler, E. G.; Dewey, M. S.; Deslattes, R. W.; Borner, H. G., "Precise Absolute Gamma-Ray

Wavelength Measurements," in Capture Gamma-Ray Spectroscopy, edited by R. Hoff, (in press).

Kitto, M. E.; Faller, S. H.; Anderson, D. L.; McCarthy, L. E., "Ariborne Chernobyl Radioactivity in College Park, Maryland, Radiochim. Acta (in press).

Lamaze, G. P.; Downing, R. G.; Langland, J. K.; Hwang, S. T., "The New Cold Neutron Depth

Profiling Instrument at NIST," J. Radioanaly. Nucl. Chem. (in press).

Lamaze, G. P.; Langland, J. K.; Downing, R. G.; Hwang, S. T., "Neutron Depth Profiling: Overview

and the NIST Facilities," J. of Res. Natl. Inst. Stands. Technol. (in press).

Lavine, J. P.; Hossain; T. Z.; Downing, R. G.; Carducci, T. R.; Mehra, M.; Cumbo, M. J., "Neutron

Depth Profiling of Boron Distribution," Proc. Electrochem. Soc. 90, 1041 (1990).

Lindstrom, R. M.; Byrne, A. R.; Becker, D. A.; Smodis, B.; Garrity, K. M., "Characterization of the

Mineral Franction in Botanical Reference Materials and Its Influence on Homogeneity and

Analytical Results," Fresenius J. Anal. Chem. 338, 569 (1990).

Lindstrom, R. M.; Norman, B. R., "Determination of Iodine and Iodine-129 in Several New Botanical

Standard Reference Materials by Neutron Activation Analysis," Trans. Am. Nucl. Soc. (in press).

Lindstrom, R. M.; Greenberg, R. R., "Quality Control," in Practical Aspects of Operating a Neutron

Activation Analysis Labortory, edited by P. Bode, E. L. Hoffmann, R. M. Lindstrom, S.J. Parry

and R. J. Rosenberg (Internat. Atomic Energy Agency, Vienna, 1990), IAEA TECDOC-564.

Lindstrom, R. M.; Lindstrom, D. J.; Slayback, L. A.; Langland, J. K., "A Lo\y-Background Gamma

Ray Assay Laboratory for Activation Analysis," Nucl. Inst. Methods A 299, 425 (1990).

Mackey, E. A.; Gordon, G. E.; Lindstrom, R. M.; Anderson, D. L., "Effects of Target Shape and

Neutron Scattering on Element Senstitivities for Neutron-Capture Prompt y-Ray Activation

Analysis," Anal. Chem. 63, 288 (1991).

Mackey, Elizabeth, "Enhancement of Neutron-Capture Pompt y-Ray Activation Ananlysis Capabilities,

Ph.D., 1991 (Theses - G. E. Gordon, supervisor).

Mackey, E. A.; Anderson, D. L.; Lindstrom, R. M.; Gordon, G. E., "The Effects of Target Shape and

Neutron Scattering on Element Sensitivities of Neutron Capture Prompt-y Ray Activation

Analysis," Anal. Chem. 63, 288 (1991).

219
PUBLICATIONS

Mildner, D.F.R., "Neutron Focusing Using Microguides," Nucl. Instrum. Meth. A297. 38 (1990).

Mildner, D.F.R., Caq)enter, J. M., "Improvements to the Chebyshev Expansion of Attenuation

Correction Factors for Cylindrical Samples," J. Appl. Cryst. 23, 378 (1990).

Mildner, D.F.R., "The Neutron Microguide as a Probe for Materials Analysis," Nucl. Instrum. Meth.

A 299, 416 (1990).

Mildner, D.F.R., "Neutron Intensity Gain for Converging Guide Systems," Nucl. Instrum. Meth. A

30L 395 (1991).

Mildner, D.F.R., "Neutron Focusing Using Converging Guides," Proc. 7th. Intl. Symp. Capture GammaRay Spectroscopy and Related Topics (in press),

Mildner, D.F.R., Carpenter, J. M., "Improvements to the Chebyshev Expansion of Attenuation

Correction Factors for Cylindrical Samples," J. Appl. Cryst. 23, 378 (1990).

Mildner, D.F.R., Hammouda, B., "The Transmission of Curved Neutron Guides with Non-Perfect

Reflectivity," J. Appl. Cryst., in press.

Parikh, N. R.; Frey, E. C; Hofsass, H.C.; Swanson, M. L.; Downing, R. G.; Hossain, T. Z.; Chu, W.

K., "Neutron Depth Profiling by Coincidence Spectrometry," Nucl. Instrum. Meth. B 45, 70

(1990).

Pauwels, J.; Sakurai, A.; Jinnai, H.; Hasegawa, H.; Hashimoto, T.; Han, C. C, "Microstructure Effects on LCST Phase Behavior of Deuterated Polybutadiene and Protonated Polyisoprene Blends

Studied by Small-Angle Neutron Scattering," Macromolecules, in press.

Schirmer, A.; Mildner, D.F.R., "Calculation of the Transmission of Neutron Benders," Meas. Sci. Tech. 2 (in press).

Schumacher, H.; Kunn, A.; Menzel, H. G.; Coyne, J. J.; Schwartz, R. B., "The Dose Equivalent

Response of Tissue-Equivalent Proportional Counters to Low Energy Neutrons," Radiat. Prot. Dosim. 31, 383 (1990).

Schwartz, R. B.; Hunt, J. B., "Measurement of the Energy Response of Superheated Drop Neutron

Detectors," Radiat. Prot. Dosim. 34, 377 (1990).

Stone, C. A.; Mildner, D.F.R.; Zeisler, R; Cranmer, D. C, "Capture Gamma-Ray Spectroscopy using

Cold Neutron Beams," Proc. 7th Intl. Symp. Capture Gamma-Ray Spectroscopy and Related

Topics (in press).

Stone, S. F.; Zeisler, R.; Gordon, G. E.; Viscidi, R. P.; Cerney, E. H., "Neutron Activation Analysis

Combined with Biological Isolation Techniques for Portein Determination," in Multi-Disciplinary

Approaches to Trace Element Research Investivations ." K. S. Subramanian, editors, G. V.

Iyengar and K. Okamota, ACS Symp. Series No. 445 (American Chemical Society, Washington,

DC, 1990) Ch. 21.

Stone, S. F.; Zeisler, R.; Gordon, G. E., "Application of Polyacrylamide Gel Electrophoresis/Neutron

Activation Analysis for Protein Determination," Biol. Trace Elem. Res 26, 85 (1990).

220
PUBLICATIONS

van Well, A. A.; de Haan, V. O.; Mildner, D.F.R., "The Average Number of Reflections in a Curved

Neutron Guide," Nucl. Instrum. Meth (in press).

Wastney, M. E.; Gokmen, I. G.; Aamodt, R. L.; Rumble, W. F.; Henkin, R. I.; Gordon, G. E.,

"Kinetic Analysis of Zinc Metabolism in Humans Following Simultaneous Administration of ^^Zn

and "^^Zn," Am. J. Physiol. 260, R134 (1991).

Williamson, T. G.; Lamaze, G. P., "Measurement of the ^•^Nb(n,2n)^^Nb Cross Section in a ^-^^U Fission

Spectrum," Proceedings of the 7th ASTM-Euratom Symposium on Reactor Dosimetry. (Strasbourg, France, August 1990) in press.

Williamson, T. G.; Lamaze, G. P.; Gilliam, D. M.; Eisenhauer, C. M., "Iron and Cadmium Capture

Gamma Ray Photofission Measurements," Nucl. Sci. Eng. 104. 46 (1990).

Wu, W. L., "Elastic Scattering of Polymer Networks," Macromol. 24, 1145 (1991).

Wu, W. L.; Kurokawa, H.; Roy, S.; Stein, R. S., "Small-Angle Neutron Scattering of Poly(vinylAlcohol) Gels," Macromol. 24, 4328 (1991).

Wu, W. L.; Kurokawa, H.; Coyne, L. D.; Shibayama, M.; Nomura, S., "Phase Behavior of Aqueous

Poly(vinyl-Alcohol) Solution," (ACS Polymer Preprints, New York, August 1991).

Zeitzoff, P. M.; Hossain, T. Z.; Boisvert, D.; Downing, R. G., "Measurement and Control of the Boron

and Phosphorus Concentration inn LPCVD Borophosphosilicate Glass (BPSG)," J. Electrochem.

Soc. 137, 3917 (1990).

•w- U.S. GOVERNMENT PRINTING OFFICE: 1992—3 11 -8 91/1+0511

221

NIST-114A

(REV. 3-90) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1 . PUBUCATION OR REPORT NUMBER

NIST/TN-1292

PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE January 1992

4. TITLE AND SUBTITLE NIST Reactor: Summary of Activities July 1990 Through June 1991

5. AUTHOR(S)

C. O'Connor, Editor

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

QAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Tech Note. 7/90 - 6/91

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Same as Item #6 above,

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOQRAPHY OR

UTERATURE SURVEY, MENTION IT HERE.)

This report summarizes all those programs which use the NIST Reactor. It covers the

period for July 1990 through June 1991. The programs range from the use of neutron

beams to study the structure and dynamics of materials through nuclear physics and

neutron standards to sample irradiations for activation analysis, isotope production,

neutron radiography, and nondestructive evaluation.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

activation analysis; cold neutrons, crystal structure, diffraction, isotopes; molecular

dynamics; neutron; neutron radiography; nondestructive evaluation; nuclear reactor;

radiation.

13. AVAILABIUTY UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). X

XX

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

231

IS. PRICE

ELECTRONIC FORM

b

NIST.Technical Publications

Periodical

Journal of Research of the National Institute of Standards and Technology— Reports NIST

research and development in those disciplines of the physical and engineering sciences in which

the Institute is active. These include physics, chemistry, engineering, mathematics, and computer

sciences.

Papers cover a broad range of subjects, with major emphasis on measurement methodology and

the basic technology underlying standardization. Also included from time to time are survey

articles on topics closely related to the Institute's technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs — Mmor contributions to the technical literature on various subjects related to the

Institute's scientific and technical activities. Handbooks — Recommended codes of engineering and industrial practice (including safety codes)

developed in cooperation with interested industries, professional organizations, and regulatory

bodies.

Special Publications — Include proceedings of conferences sponsored by NIST, NIST annual

reports, and other special publications appropriate to this grouping such as wall charts, pocket

cards, and bibliographies.

Applied Mathematics Series — Mathematical tables, manuals, and studies of special interest to

physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others

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properties of materials, compiled from the world's literature and critically evaluated. Developed

unaer a worldwide program coordinated by NIST under the authority of the National Standard

Data Act (Public Law 90-396). NOTE: The Journal of Physical and Chemical Reference Data

(JPCRD) is published bimonthly for NIST by the American Chemical Society (ACS) and the

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Building Science Series — Disseminates technical information developed at the Institute on building

materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the

durability and safety characteristics of building elements and systems.

Technical Notes — Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive

in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NIST under the sponsorship of other government agencies.

Voluntary Product Standards — Developed under procedures published by the Department of

Conunerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish

nationally recognized requirements for products, and provide all concerned interests with a basis

for common understanding of the characteristics of the products. NIST administers this program

as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series — Practical information, based on NIST research and experience,

covering areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order the above NIST publications from: Superintendent of Documents, Government Printing Office,

Washington, DC 20402.

Order the following NIST publications -FIPS and NISTIRs—from the National Technical Information

Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB) — Publications in this series

collectively constitute the Federal Information Processing Standards Register. The Register serves

as the official source of information in the Federal Government regarding standards issued by

NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717 (38 FR 12315,

dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)-A special series of interim or final reports on work

performed by NIST for outside sponsors (both government and non-government). In general,

initial distribution is handled by the sponsor; public distribution is by the National Technical

Information Service, Springfield, VA 22161, in paper copy or microfiche form.
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