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NIST TECHNICAL NOTE 1285

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NIST Reactor:

Summary of Activities

July 1989 Through June 1990

NIST Technical Note 1285

NIST Reactor:

Summary of Activities

July 1989 Through June 1990

C. O'Connor, Editor

Reactor Radiation Division

Institute for Materials Science and Engineering

National Institute of Standards and Technology

Gaithersburg, MD 20899

December 1990

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SPATES O* ^/

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards and Technology Technical Note 1285

Natl. Inst. Stand. Technol. Tech. Note 1285, 186 pages (Dec. 1990)

CODEN: NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1990

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

This has been a year of great excitement and progress in the Reactor

Radiation Division. First and foremost, of course, is the fact that the reactor

penetrations for the neutron guides were completed, and the reactor was brought

back on line in April 1990. During the shutdown, several important maintenance

items were completed, including the cooling tower overhaul, refurbishment of the

sealant on the confinement building walls, and the renovation of the thermal shield

cooling system. In addition, the Request for Proposals for a new set of main heat

exchangers was extremely successful, resulting in a number of proposals currently

being evaluated. It is a particular pleasure to note that the Reactor Operations

Group received the Silver Medal of the Department of Commerce in recognition

of their outstanding performance in the cost effective operation of the NIST

reactor as a national resource.

In spite of the very limited operating schedule that resulted from the guide

installation, this was a highly productive year for the Neutron Condensed Matter

Science group. Both in quality and quantity, the published output of the group

continues to increase. Among many possible highlights, I would mention the

successful commissioning of the BT-7 neutron reflectometer, and its use to study

polymer films on various substrates; the successful development of a new method

for the solution of the crystal structures of large molecules of biological

significance based on Maximum Entropy techniques; the first observation of rapid

hydrogen hopping in rare-earth metals at low temperatures (the result of a Kondo

type interaction with the conduction electrons); a wide variety of new results on

the physics and chemistry of high-Tc superconductors; and the first results from

the study of epilayers and multilayers of magnetic semiconductors which show that

these systems display a rich variety of magnetic behaviors.

The Cold Neutron Project continues to progress well, with the first three

neutron guides now installed, and several instruments in the final stage of

fabrication and installation. In spite of a large number of unforeseen problems

arising from funding limitations in the beginning of the fiscal year, scheduling

Droblems with outside vendors, and technical difficulties during the penetration

and guide installation, five instruments will be taking data by the end of 1990.

In addition, several others are in the fabrication and procurement stage, with

installation scheduled for 1991 . A new 3.5-KW refrigerator suitable for operation

of a second generation cold source will be installed late in 1990, and the

conceptual design of this source, which will utilize liquid hydrogen, is now

complete and detailed design is under way. The first meeting of the Program

Advisory Committee was held in late 1989, and their recommendations are being

developed into a user policy for the CNRF. The first two issues of The Neutron

Standard, a facility advisory newsletter, were sent out to over 2500 U.S.

researchers. As a result of a cooperative project with the Department of Energy,

in
the performance of neutron supermirrors has been improved to the point at which

it is now feasible to consider at least partial coating of some of the remaining four

guides, with greatly improved performance as the result. The responses to a Request for Proposals for the actual guide coating are now being evaluated.

As always, the non-RRD programs that rely on the NIST reactor have also

had an impressive year, in spite of the long reactor shutdown. As the CNRF

comes on line, these programs will also increase their efforts—two of the new

instruments will be operated as user facilities by the Nuclear Methods Group of

the Center for Analytical Chemistry, and two others will be operated by the

Neutron Interactions and Dosimetry Group from the Center for Atomic,

Molecular, and Optical Physics.

Finally, it gives me great personal pleasure to thank the entire staff for their

performance during this year. As always, change is disruptive, but everyone has

joined in the effort to fully develop the NIST reactor and CNRF facilities as a

national resource, and have tolerated the disruption to their own research

programs due to the reactor shutdown with grace and patience (in the main !). A special word of thanks is due to Carol O'Connor, the editor of this report,

which has a new format, and required a major effort to produce. She was ably

assisted in this by Linda Clutter, Stephanie Shepperd, Judy Spillman, and Lisa

Thomas.

J. M. Rowe

Reactor Radiation Division

National Institute of Standards and Technology

IV
ABSTRACT

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

for July 1989 through June 1990. 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. REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS ... 1 Rapid Low Temperature "Hopping" of Hydrogen in a Pure

Metal: The ScHx System 1 Elastic Incoherent Neutron Scattering From the a-ScHx System 2

Proton Hopping in a-YHx by Quasielastic Neutron

Scattering 3

Neutron Spectroscopic Comparison of Hexamethyldisiloxane

With Trimethylsilyl Adsorbates on Silica 4

Vibrational Spectroscopy and Location of Ammonium Ions

in Reammoniated Zeolite RHO 5

Inelastic Neutron Scattering Studies of Nonlinear

Optical Materials 6

Neutron Scattering Study of Cs-ND3 Intercalated Graphite 7

Evidence of Random Strain Fields in KCN After Thermal

Cycling 8 An Intermolecular H-0 Potential for Nitromethane 9

AB Initio Phase Determination in a Protein by

Maximum Entropy 11 Fast Fourier Transforms for Macromolecular Crystallography 13

Anion and Cation Disorder in the Ionic Conducting

Pyrochlore System Y2(Zr

xTi1

_x)2 7 13

Protein Symmetry 14

Crystal Structure of C32H46N6 4 .(CH3 )2CO 15

Crystal Structure of 2,2,4,4-Tetranitroadamantane 16

Neutron Difffraction Test of the Method of Reproducible

Preparation of the AlCuFe Icosahedral Quasicrystals 17

Neutron Powder Diffraction Study of the Orthorhombic

Ti2AlNb Phase 18

K1

. X(ND4)X

I: A Crystal Structure Determination 20

K1

. X(NH4) X

I: A Phase Diagram Study by Means of X-Rays 21

Structural Studies in the BaO-Nd2 3-CuO System 22

Polytypic Structures of Sodium Metal Following the

Martensitic Transformation 23

The Crystal Structure of the 60 K Superconductor

La1.6Sr0.4CaCu2°6±5 25

Crystal Structure and Atomic Ordering in Pb2ST2Yl

_ xC^C\i3 Oi+l) (x = 0, 6 = 1.47) 27

Twinning and Oxygen Distribution in Pb2Sr2YCu3 Og+x 29

VII
Incorporation of Gold Into YB&jCi^Oy: Structure and

Tc Enhancement 31

Magnetic Phase Transitions in Sm2Cu04 32

Magnetic Ordering of Nd in (Nd-Ce)2Cu04 34

Neutron Diffraction on Annealed YBa^UgOg 28 36

Temperature Dependence of the Two-Dimensional Magnetic

Correlations of Er in ErBa^UgO-j 37

Field Dependence of the Magnetic Phase Transitions

in Nd2Cu04 39

Two-Dimensional Magnetic Order in Superconducting

DyBa2Cu4 8 41

Bio Sr2Mn06+ : A 2-D Modulated Ferrimagnet 43

A Theoretical Model for the Tunneling-Gap Anisotropy

Observed in Layered Copper-Oxide High-Temperature

Superconductors 44

Neutron Diffraction Studies of Magnetic Semiconductors 46

Magnetoelasticity in Rare-Earth Superlattices and Films 48

Antiferromagnetic Ordering of ErAs Thin Epitaxial

Layers on GaAs .50

Orientation Dependence of the Spin Coupling in Gd/Y Superlattices 51

Propagation of Magnetic Order Through Nonmagnetic

Interlayers in Rare-Earth Superlattices 52

Diffuse X-Ray Scattering From A GaAs/AlAs Superlattice 54

Polarized Neutron Reflectometer and Grazing Angle

Diffractometer 55

Neutron Reflectivity of Y/Gd 57

Interfacial Widths in Homopolymer/Diblock Copolymer

Systems 58

Film Thickness Constraints on Diblock Copolymer

Morphologies 59

Segment Density Distribution of Symmetric Diblock

Copolymers at the Interface Between Two Homopolymers

as Revealed by Neutron Reflectivity 61

X-Ray Reflectivity Studies of Thin Diblock Copolymer Films 63

Neutron Reflectivity Fit Program 65

SANS From Hot Stretched Partially Deuterated Polystyrene 66

Shear Dependence of Critical Fluctuations in Binary

Polymer Mixtures by Small Angle Neutron Scattering 67

Microstructure Effects on LCST Phase Behavior of Polybutadiene

and Polyisoprene Blends Studied by SANS and LS 69

Time-Resolved Small-Angle Neutron Scattering Study on Late

Stage Spinodal Decomposition of Polymer Blends 71

Small Angle Neutron Scattering Characterization of

Processing/Microstructure Relationships in the

Sintering of Crystalline and Glassy Ceramics 73

Small-Angle Neutron Scattering Measurement of the Effect

of Green Density and the Role of MgO Additive on the

Densification of Alumina 75

Microstresses in Ceramic Composites 77

Vlll
A Chord Distribution Description of Microporous Glass 79

Small-Angle Neutron Scattering of Poly(Vinylalcohol) Gels 81

SANS From TPB Based Microemulsions 82

Quasielastic Gamma-Ray Scattering from Pentadecane and

From Polydimethylsiloxane Solutions 83

Texture Study on Tantalum Metal Sheet by Neutron

Diffraction 84

Residual Stress Measurements in Metallurgical Samples 86

Analytical Calculations Using Acceptance Diagrams 86

New Data Acquisition System 88

Theory of the Aharonov-Casher Effect as Observed by

Neutron Interferometry 89

Neutron Activation Autoradiography Program 90

NIST Crystal and Electron Diffraction Data Center 99

Cold Neutron Project 92

Liquid Hydrogen Cold Neutron Source 95

Center for High Resolution Neutron Scattering 96

High Resolution Small Angle Scattering Spectrometer 97

Time-of-Flight Spectrometer Modifications for

Guide-Hall Installation 98

Cold Neutron Depth Profiling 99

High Resolution Time-of-Flight Spectrometer 100

Optimized Design of the Chopper Disks and the Neutron

Guide in a Disk Chopper Neutron Time-of-Flight

Spectrometer 101

Aspects of the Design of the Counter-Rotating Chopper

Pairs for the High Resolution Time-of-Flight

Spectrometer 103

Cold Neutron Crystal Spectrometers 105

Accordion Supermirror Polarizer for Neutrons 106

Supermirror Coatings for Neutron Guides 108

Neutron Focusing Using Converging Guides 112

Multiple Reflections in Curved Neutron Guides 114

6Li-doped Silicate Glass for Thermal Neutron Shielding 116

Fundamental Neutron Physics 117

B. NON-RRD PROGRAMS 118

Neutron Interactions and Dosimetry Group 118

Dosimetry for Material Performance Assessment 118

Personnel Dosimetry 120

Research and Technology 122

Irradiation and Calibration Facilities 124

Nuclear Methods Group: Overview 126

Dissolution Problems With Botanical Reference Materials 129

Measurement of Boron at Silicon Wafer Surfaces by

Neutron Depth Profiling 131

Sol Particle Immunoassays Using Colloidal Gold and

Neutron Activation 133

IX
Characterization of the Mineral Fraction in Botanical

Reference Materials and Its Influence on Homogeneity

and Analytical Results 135

Atmospheric and Environmental Studies by Instrumental and

Neutron-Capture Prompt y-Ray Activation Analysis 137

Effects of Target Shape and Neutron Scattering by Hydrogen

on Neutron-Capture Prompt y-Ray Activation

Analysis (PGAA) 137

Submicrometer Size Distributions of Airborne Particles

Bearing Various Trace Elements 139

Sources of Airborne Particles in and Near Maryland and

Their Wet Deposition 140

Sources of Background in Ge Gamma Detectors ? 141

Chebyshev Expansion of Attenuation Correction Factors

for Cylindrical Samples 143

Measurement of the 93 Nb(n,2n)92mNb Cross Section in a

235U Fission Spectrum 144

C. REACTOR OPERATIONS AND ENGINEERING 146

Reactor Operations and Engineering 146

Neutron Activation Analysis at the Food and

and Drug Administration 147

D. PERSONNEL ROSTER 149

E. PUBLICATIONS 163
A. REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

RAPID LOW TEMPERATURE "HOPPING" OF HYDROGEN

IN A PURE METAL: THE ScHx SYSTEM

N. F. Berk, J. J. Rush, and T. J. Udovic

and

I. S. Anderson (Paul Scherrer Institut, Villigen, Switzerland)

As part of our continuing effort to study

hydrogen dissolved in rare-earth metals, we

performed direct neutron quasielastic scattering

measurements of the localized motion of

hydrogen in scandium [1]. These experiments were carried out between 2 K and 300 K on a

single-crystal slab of scandium prepared from

high purity Ames laboratory scandium metal and

loaded with hydrogen from the gas phase. The

measurements were performed at hydrogen

concentrations between 5 and 20 atomic percent on the IN-5 Spectrometer at the Institut LaueLangevin with an energy resolution of 70 jieV in the range of momentum transfer 1.05 < Q

<2.05 A"1

. Figure 1 shows the quasielastic

linewidth V as a function of temperature for

ScH 16 . Data for ScH005 ScH025 show

behavior consistent with the 0.16 results. All the data show a hopping rate exceeding

1000

© 100-

— TxlO^s'1 over the entire temperature range,

indicative of very rapid motion compared to the

bulk diffusion in these systems, even at low T. The remarkable upturn of T below the minimum

is approximately T1 in the range shown, and is similar in appearance to the observations by

Steinbinder et al. [2] of quasielastic scattering

from hydrogen trapped by oxygen impurities in Nb, which were explained in terms of Kondo's

predictions [3] of nonadiabatic effects of the

coupling of the metal conduction electrons to the

proton, which in that system is constrained by

the trap to hop between a pair of T-sites. We

have shown [1] that the scandium-hydrogen

system studied here presents an analogous

situation-observed for the first time in a pure

metal—in which such coupling is also amplified

by the effective restriction of the proton to a pair

of sites: in this case the nearest neighbor T-sites

• •

10 1

>—i i 1 1 1

1

1

1—i—i—i 1 1 1 > |

1—i—i—i

11 1

1 10 100 1000

TOO

Figure 1. Fitted Lorentzian linewidths (FWHM) for ScHq jg. The solid line

is the fit to the data below 100 K.
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

between metal atoms along the c-axis. In this picture the linewidth is the sum of two contributions: the influence of the interaction of the

proton with the lattice, which falls monotonically

with decreasing T, and the electronically induced

broadening which increases with decreasing T.

The low-temperature linewidth was analyzed

with the "Kondo" formula

T = KA3

(27i:kBT/A)2K+1 /[(2rckBTK)2 +e2

],

where A and e are the tunneling matrix element

and asymmetry energy, respectively, of a model

two-site potential well, and K is the coupling

constant of the weak interaction between the

proton and conduction electrons. The best fit was obtained for a symmetric model, e « 0,

K = 0.039, and A = 0.32 mev. The values for

K and A are comparable to those reported for the

Nb-O-H system, where the nearest neighbor

T-site separation is nearly the same as in the

Sc-H system.

References

[1] I. S. Anderson, N. F. Berk, J. J. Rush, T. J. Udovic,

R. G. Barnes, A. Magerl, andD. Richter, Phys. Rev.

Lett. 65, 1439 (1990).

[2] D. Steinbinder, H. Wipf, A. Magerl, D. Richter, A.

J. Dianoux, and K. Neumaier, Europhysics Lett. 6, 535 (1988).

[3] J. Kondo, Physica B125 , 279 (1984); ibid. B126. 377

(1984); ibid. 305 (1986).

ELASTIC INCOHERENT NEUTRON SCATTERING FROM THE ar-ScH^SYSTEM

N. F. Berk, J. J. Rush, and T. J. Udovic

and

I. S. Anderson (Paul Scherrer Institut, Villigen, Switzerland)

In the previous report [1] we described new

quasielastic neutron scattering results from

a-ScHx

, showing an upturn of the linewidth at low temperatures, which we associate with

nonadiabatic effects of the weak coupling

between the protons and the scandium

conduction electrons, amplified by the effective

restriction of the rapidly moving hydrogens to symmetric near neighbor T-sites between the

metal atoms. Our interpretation of the linewidth

data is consistent with the simultaneously

measured elastic incoherent structure factor

(EISF) shown as function of T in figure 1 for x = 0.16. We assume all of the measured

hydrogens are in two populations, which we

designate as labile (1) and nonlabile (nl), with

relative occupations Cj and cnl , respectively, and

with Cj + c

nl = 1. We attribute the labile defects to configurations in which H-M-H

pairing cannot occur because of the unavailability of nearby protons to form such bridged

parts along the c-axis. The scattering function,

S{@> w) , can be written

S(Q,o>) = eisf(Q)6(o>)

+ (l-eisfCQ))!^), (1)

where L,(w) is the quasielastic function, and

eisf(Q) = 1 -c^sin^Q'd^), - /„2

(2)

2\l/2

Sj. = 1 - z , and with eb = (e

z + A^) 1 Reference 1 , A and e are the parameters of the

labile proton double-well, and dis the T-site

separation.

At T = 300 K, the Q-dependence of the

measured EISF is well described by (2) with

c

1 s1 = 0.89, indicating that most of the scattering

protons at room temperature are labile. With

Sj(T) = 1 for a symmetric well [1], the

measured T-dependence of the EISF is an
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

"i—"—

r

100 150 200 250 300 350

TOO

Figure 1. EISF vs. T for ScHQ 16 . The solid line is a fit to the data using (2) with s ^ = 1. The T-dependence of Cj

is obtained by assuming Boltzmann occupations of labile and nonlabile configurations and a model density of labile

states using two unequally weighted delta functions separated from the nonlabile state by (fitted) energies of 17 meV and 78 meV and with almost all the weight at the higher energy.

empirical measure of c

nl (T), the fraction of

nonlabile configurations. As figure 1 shows, at low temperatures only a small fraction of the

protons are labile, but in our scheme these labile species are responsible for the anomalous

hopping [1]. Preliminary analysis indicates that the occupational density of states for the labile protons are separated from the nonlabile

configurations by energies in the range of 15-80

meV. The lower end of this broad band may

reflect the influence of metastable configurations on the measured EISF at low T, while the upper

portion suggests the magnitude of the T-site

potential asymmetries due to short ranged order

of paired nonlabile protons along the c-axis.

Reference

[1] N. F. Berk, J. J. Rush, T. J. Udovic, and I. S. Anderson, p. 1 of this report.

PROTON HOPPING IN or-YHx BY QUASIELASTIC NEUTRON SCATTERING

J. J. Rush, T. J. Udovic, and N. F. Berk

and

I. S. Anderson (Paul Scherrer Institut, Villigen, Switzerland)

A recent neutron scattering study of

localized hydrogen dynamics in a-ScHx (described on p. 1) has demonstrated that a

fraction of the protons in this system undergo a remarkably fast local "hopping" at low

temperature between near neighbor H sites along

the c-axis in the hep structure. The associated

quasielastic neutron linewidth shows a minimum

at -100 K and then increases at lower

temperatures, leading to an apparent hopping

rate of ~1012

/s at 10 K. The approximate T"

1 dependence of the linewidth is consistent with

nonadiabatic behavior associated with weak

coupling of the quantum two-state system to the

conduction electrons, the first such result observed for hydrogen in a pure metal.

Earlier this year we have extended our

research on local dynamics of hydrogen in the

rare-earth metals to a quasielastic scattering

study of the a-YHx system at H concentrations

of 1% and 10%. Previous quasielastic neutron

scattering studies of this system have primarily

concentrated on very high resolution studies of

slow 3-d (long-range) diffusion of H in yttrium

and on higher hydrogen concentrations. While

the structure of the a-YHx and a-ScHx systems

are similar, diffuse neutron scattering and

neutron vibration spectra suggest a higher degree

of proton ordering in the a-YHx system. In addition, the distance between near-neighbor

tetrahedral sites along the c-axis is 30% greater

for Y than for Sc, which could significantly

affect the details of the potential and local motion and interactions of hydrogen.

Our measurements were performed on the

IN-5 cold neutron time-of-flight spectrometer at the Institut Laue-Langevin using a neutron

wavelength of 6.5 A, which provided an energy

resolution (FWHM) of 50 jieV. Very high

purity single crystal specimens of area 2 x 5 cm

and thicknesses of 2 cm and 0.2 cm,

respectively, (for 1% and 10% H loading) were
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

obtained from Ames Laboratory at Iowa State University. Polycrystalline samples of

equivalent dimensions were used as "blanks"

whose scattering spectra could be subtracted

from the H-loaded crystals to yield the Hscattering contribution.

The data from these experiments are

currently being analyzed, but the results so far again show a minimum in the quasielastic peak

width at -175 K (compared to -100 K for

a-ScHx

) and an apparent turnaround to broader

widths (faster "hopping") at lower temperatures.

The quasielastic intensity decreases rapidly with

T as observed for a-ScHx

, reflecting the

diminishing of the "labile" protons, while the

widths are in general smaller than those observed

for a-ScHx

. These observations are consistent

with the existence of the considerably larger

near-neighbor H-H distance in «-YHx (suggesting a deeper H potential), which

apparently reduces "hopping" rates but still allows coupling of the protons to the conduction

electrons.

NEUTRON SPECTROSCOPIC COMPARISON OF HEXAMETHYLDISILOXANE WITH

TRTMETHYLSILYL ADSORBATES ON SILICA

I

T. J. Udovic, D. A. Neumann, and J. J. Rush

L. C. Sander (Organic Analytical Research Division)

and

S. Anderson (Paul Scherrer Institut, Villigen, Switzerland)

Incoherent inelastic neutron scattering (IINS) was previously used as a spectroscopic probe of

trimethylsilyl ((CH3 )3Si-, TMS) adsorbates

bonded to silica (Si02

) via disiloxane-type

linkages (i.e., (CH3 )3 Si-0-Si<). Above 75

meV, the TMS vibrational spectrum at 4 K (fig.

la) indicate pronounced features due to Si-C-H

and H-C-H bending modes as well as Si-C and

Si-0 stretching modes. Acceptable spectral fits by normal coordinate analysis required the

stiffening of four of the nine Si-C-H bending

force constants and six of the nine H-C-H

bending force constants, assuming steric interactions between the silica surface and the

four H atoms in closest proximity. The

inequality of the Si-C-H bending force constants

is illustrated by the apparent bending-mode

splitting at 86 and 96 meV. This assumption was tested by comparison of the TMS/silica

spectrum with the recently measured vibrational

spectrum of hexamethyldisiloxane

((CH3 )3 SiOSi(CH3 )3

, HMDS) at 10 K (fig. lb)

likewise obtained using the BT-4 triple axis

spectrometer. Except for a slight 1-2 meV shift to lower energies, the HMDS spectrum is very

similar to the TMS/silica spectrum. Hence, the

vibrational splitting in the TMS/silica spectrum

appears not to be induced by a steric interaction

of the methyl groups with the silica surface, but

rather seems to be related to more general

intramolecular interactions within the (CH3)3 SiO- fragment itself. Future experiments will more closely

investigate the origin of this splitting by

determining the vibrational spectra of other

related compounds containing from one to four

bonded methyl groups per Si atom.
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

1 1 1 1 1 1 1

w*->C

o

•

U V* *%¥ V>oo »v

b)

° 00 o°° °o

o0c u

c ? <\^

o3 >

0) * *

_ Z *

u

a)

i i i i i i

OOoOo O0 o ,

1 50 100 150 200

Energy Loss (meV)

Figure 1. HNS spectra of a) trimethylsilyl/SiC^ minus the SiC>2 blank at 4 K and b) hexamethyldisiloxane at 10 K. The

spectra were obtained on the BT-4 triple-axis spectrometer with Cu(220) monochromation and 40' pre- and post-collimation , using both the Be-filter (open circles) and the Be-graphite-Be-filter (closed circles) analyzers.

VIBRATIONAL SPECTROSCOPY AND LOCATION OF AMMONIUM IONS

IN REAMMONIATED ZEOLITE RHO

J. M. Nicol (Reactor Radiation Division and University of Maryland, College Park, MD)

T. J. Udovic, R. R. Cavanagh, and J. J. Rush

and

G. D. Stucky (University of California, Santa Barbara, CA)

In zeolite rho the high flexibility of the

framework and the ability to control the pore

size by nonframework cations offers the

opportunity of introducing a high degree of

catalytic selectivity. This has, for example, been

illustrated by the unique selectivity and activity

exhibited by H-rho in the synthesis of

dimethylamine from methanol and ammonia [1]. Of interest in this reaction is the observation that ion-exchanged ammonium-rho zeolite differs

considerably in catalytic activity to ammoniumrho formed by the reammoniation of H-rho.

Using inelastic neutron scattering techniques, the

vibrational spectroscopy and molecular dynamics

of ammonium ions formed during the

reammoniation of H-rho have been investigated.

The H-rho zeolite used in the present

investigation was generated during a study of the

deammoniation of ion-exchanged ammoniumrho, as described previously [2]. The H-rho

zeolite was loaded consecutively at 293 K with

ammonia to concentrations corresponding to 1

3, 6, and 11 ammonium ions per unit cell. (During reammoniation bound protons react with

the adsorbed ammonia to form ammonium ions.)

Inelastic scattering data in the energy range

5-190 meV were obtained using the BT-4

spectrometer at the NBSR in either the Be-filter

analyzer or triple-axis mode.

The spectral changes that occur in the

energy range 34-190 meV during reammoniation

of H-rho as ammonium ions are formed and
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

protons are lost from the framework have

previously been described [3]. These

observations correlate well with those observed

during deammoniation of ammonium-rho [2]. In the energy range below 20 meV, two

modes due to the hindered rotations of the

ammonium ions are observed at ca. 8 and 16

meV. As the ammonia loading level is increased

both modes are seen to increase in intensity.

Data collected at 10 and 80 K, however, show

very different intensity patterns. In particular, at the highest ammonia loading, additional baseline

intensity is seen below 10 meV at 80 K, which

subsequently disappears on cooling the sample to 10 K. This intensity appears to result from a very broad quasielastic component. Quasielastic

data also indicated the presence of a weakly

bound component, as well as a component with

a higher reorientation barrier.

The observation of two hindered rotational

modes for ammonium ions in reammoniated

zeolite rho reflects the location of ammonium

ions in two different environments. This result correlates well with recent crystallographic data

for ammonium ion location in partially

deammoniated and reammoniated zeolite rho [4]. In both structures the ammonium ions were

found in the same framework sites. Two

crystallographically different ammonium ion

locations were observed, one close to the center of the double 8-rings (D8R) and the other

outside the D8R in the a -cage. The ammonium

ions located in the center of the D8R, were

found to distort the D8R ring and shorten the

unit cell constant considerably more than for

as-synthesized ammonium-rho. The ammonium

ions in the D8R are thus strongly coordinated to the framework oxygens. We suggest that the

higher-energy mode observed at 16 meV, associated with a higher rotational barrier, is due to the ammonium ions located in the D8R site. The ammonium ions located in the a -cage are more weakly bound as their coordination to the

framework oxygens is much less. We can thus

assign the reorientational mode at 8 meV to the more weakly bound ammonium ions located in the a -cage sites.

References

[1] M. Keane, Jr., G. C. Sonnichsen, L. Abrams, D. R. [3] Corbin, T. E. Gier, and R. D. Shannon, Appl. Cat. 32, 361 (1987).

[2] T. J. Udovic, R. R. Cavanagh, J. J. Rush, M. J. [4] Wax, G. D. Stucky, G. A. Jones, and D. R. Corbin,

J. Phys. Chem, 91, 5968 (1987).

J. M. Nicol, T. J. Udovic, R. R. Cavanagh, J. J. Rush, and G. D. Stucky, NBS (U.S.) Tech. Note

1240. R. X. Fisher, W. H. Baur, R. D. Shannon, J. B.

Parise, J. Faber, and E. Prince, Acta Cryst. C45 . 983

(1989).

INELASTIC NEUTRON SCATTERING STUDIES

OF NONLINEAR OPTICAL MATERIALS

J. M. Nicol (Reactor Radiation Division and University of Maryland, College Park, MD)

J. J. Rush

and

G. D. Stucky (University of California, Santa Barbara, CA)

Interest in nonlinear optical materials for

second harmonic generation has grown

considerably in recent years. The potassium

titanyl phosphate and potassium titanyl arsenate family of phases have been shown to have

important nonlinear and electro-optical

properties, which may be modified by

substitution of potassium ions with other cations,

such as ammonium ions, during synthesis.

The vibrational spectroscopy and molecular

dynamics of ammonium ions in a series of

substituted potassium titanyl phosphates and
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

studied a mode due to the torsional vibration of

the ammonium ion is observed in the energy

range 35-50 meV, as well as the symmetric and

antisymmetric bending modes at ca. 180 and ca. 210 meV. Further studies and analysis of the

data are currently in progress to relate the

vibrational spectroscopy and ammonium ion

molecular dynamics with structural information

obtained from neutron diffraction and the

measured nonlinear optical properties of these

materials.

NEUTRON SCATTERING STUDY OF Cs-ND3 INTERCALATED GRAPHITE

D. A. Neumann

Ph. Depondt (Universite Pierre et Marie Curie, Paris, France,

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

and

S. F. Trevino (ARDEC Picatinny Arsenal, NJ and Reactor Radiation Division)

Alkali-ammonia intercalated graphite is an

ideal system for the study of the structure and

dynamics of molecular liquids confined to a

single layer. Earlier studies on graphite

intercalated with K and ND3 have shown a remarkable coupling of the longitudinal phonons

which propagate along the c-axis to a librational

mode of the ammonia molecule which occurs at an energy of about 7 meV [1]. In order to obtain additional information on the nature of

this librational mode, we have recently begun

work on Cs-ND3 intercalated graphite.

The results of this preliminary work show

that as in the K compound, the Cs-ammonia

layer is a liquid at room temperature, but that the liquid structure factor does not show the

prepeak due to clustering of the ammonia

molecules about the alkali that was clearly

evident for the K-ammonia case. This leads us

to conclude that the Cs-ammonia complexes are much less well-defined than are the K-ammonia

ones. We have also observed a liquid to solid

intercalate transition at 190-195 K in the Cs

case. For K-ammonia the same transition occurs

at about 180-185 K.

Inelastic scattering results show that the out- of-plane longitudinal acoustic phonons are coupled with a librational mode of the ammonia

molecule. As in the K-compound this librational

mode occurs at about 7 meV. However, the

splitting of the phonon branch is considerably

larger in the Cs-ammonia system than that for

the corresponding K case indicating that the

rotation-translation coupling is much larger. In addition, no pronounced effect was observed in the inelastic scattering data when the sample was

cooled through the liquid-solid intercalate

transition.

The combination of the less well-defined

complexes and the stronger rotation-translation

coupling seem to be inconsistent with models of

the librational mode which invoke interactions

between the alkali ions and the ammonia

molecules. It should also be pointed out that previous inelastic neutron scattering results on Cs-NH3 powder taken in the solid intercalate

phase show a pronounced peak at about 6 meV

which was identified as librations of the

ammonia molecule about the C3 symmetry axis

[2]. The fact that this energy, in a hydrogenated

compound, is still lower than that observed here

for the deuterated compound raises questions as to the assignment of this mode or points to other

unsuspected interesting effects. Further analysis

is currently in progress in order to more fully

address these and other outstanding questions

concerning the structure and dynamics of alkali- ammonia intercalation compounds.

References

[1] D. A. Neumann, H. Zabcl, Y. Fan, S. A. Solin, and

J. J. Rush, Phys. Rev. B37, 8424 (1988).

[2] Data taken by J. White et al., from the article by C. C. Wilson, Neutron News, JJl), 14 (1990).
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

EVIDENCE OF RANDOM STRAIN FIELDS IN KCN AFTER THERMAL CYCLING

C. Bostoen (University of Antwerp, Wilrijk, Belgium)

and

S. K. Satija

The family of alkali cyanides has attracted a

lot of interest due to its ferroelastic properties

(e.g., [1]). KCN is known to transform to a low

temperature orthorhombic phase. However,

recent light transmittance experiments [1], show

that the sample transforms after repeated cycling

to a monoclinic phase, in agreement with early

specific heat and x-ray measurements [2,3]. It was argued that the free energy of both the

monoclinic and orthorhombic phase are close to each other and that scars of the low temperature

phase are present in the high temperature phase,

which after successive cycles force the crystal to transform to a monoclinic phase. Annealing the

sample at room temperature removes the

memory effect and the cubic phase then

transforms again to the orthorhombic phase.

Recently, a Landau theory has been

proposed to explain certain aspects of the

polymorphism in the alkali cyanides and alkali halide cyanide mixed systems [4]. A

competition between the free energy of the

interactions and the free energy of the entropy

terms exists: for NaCN, the interactions

dominate the entropy terms, resulting in a strong

first order phase transition to an orthorhombic

phase. RbCN is the opposite extreme and closer

to a second order phase transition, transforming

to a monoclinic phase. KCN, finally, acts as a

"bistable borderline case." The scars of the low

temperature phase, present in the system at higher temperatures above Tc

, act as static imperfections and may lead to random strain

fields. These strain fields contribute to the

entropy terms, thus favoring the monoclinic

phase.

We performed inelastic neutron scattering

measurements on KCN and RbCN crystals

(purchased from the Crystal Growth Laboratory,

University of Utah), in order to probe the

possible existence of these random strains. A

central peak was observed in earlier studies on

mixed crystals of KCN/KBr and interpreted in terms of random strain fields [5,6]. The

experiments were done at the BT-4 triple axis

spectrometer with fixed incident neutron energy

of either 5.0 meV or 5.5 meV. Typical

collimations used were 40 '-20 '-20 '-40' giving an energy resolution of the order of 130 jieV.

Figure 1 shows the phonon spectrum from KCN

at 170 K at Q = (1.9,0,2) before (open circles)

the crystal has been subjected to a phase

transformation. The solid lines are a fit to the

data using a model previously used for KCN [7]. No central peak was observed as reported earlier

[7]. First cooling of the crystal below the phase

transition resulted in an orthorhombic phase.

Upon heating the crystal above the phase

transition the phonon spectrum changed

dramatically giving rise to a large central peak

(fig. 1 (closed circles)). A second cooling of the

crystal below the phase transition now gives rise to a monoclinic phase. After annealing the

crystal for 16 hrs at T = 340 K, no central peak was found at T = 170 K, and the crystal

transformed to the orthorhombic phase. This

situation stands in contradistinction with the

RbCN crystal. Here no central peak was found,

although the low temperature phase is monoclinic. In addition, we measured the LA and TA

branches at T = 295 K, 200 K, 150 K, and

135 K. A lineshape analysis has been performed

using a model applied earlier to KCN [7].
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

-t.O -0.6 -0.2 0.2 0.6 Energy Transfer [meV]

-1.0 -0.6 -0.2 0.2 0.6 Energy Transfer [meV]

Figure 1. Scans at Q = (1.9,0,2) of KCN at T = 170 K. Open symbols: before cycling through Tc = 168 K; Closed symbols:

after cycling through Tc. Solid lines are fits with model used in [7].

References

[1] J. Ortiz-Lopez and F. Liity, Phys. Rev. B37, 5452

(1988).

[2] H. Suga, T. Matsuo, and S. Seki, Bull. Chem. Soc. Japan 38 1115 (1965).

[3] A. Cimino, G. S. Parry, and A. R. Ubbelohde, Proc. Roy. Soc. A252, 445 (1959).

[4] K. H. Michel and T. Theuns, Phys. Rev. B40, 5761

(1989).

[5] J. M. Rowe, J. J. Rush, D. J. Hinks, and S. Susman,

Phys. Rev. Lett. 40, 1158 (1979).

[6] K. H. Michel, Phys. Rev. Lett. 57, 2188 (1986).

[7] J. M. Rowe, J. J. Rush, N. J. Chesser, K. H. Michel,

and J. Naudts, Phys. Rev. Lett. 40, 455 (1977).

AN INTERMOLECULAR HO POTENTIAL FOR NITROMETHANE

B. M. Rice (Ballistics Research Laboratory, Aberdeen MD)

and

S. F. Trevino (ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

In past work [1] the crystal structure of solid

nitromethane has been determined by x-ray and

neutron diffraction and the methyl group

rotational energy levels have been measured at 4 K by inelastic neutron scattering. This work

has resulted in a very well determined rotational

potential, V(0), which is presented in figure 1. In this figure the solid horizontal lines indicate

the energy levels corresponding to the CH3

group and the dashed lines to those for the CD3 group. The compound shape of the potential is essential for the explanation of the location of

these measured levels and their pressure

dependence. A simple Lennard-Jones potential

describing the H-0 pairwise intermolecular

interaction had previously been found which well

describes V(6) as presented in figure 1. In this figure, however, the value of 6 = is not
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

arbitrary but corresponds to the equilibrium

orientation of the methyl group about the C-N

bond as determined by the diffraction studies. A

fact not noted in the previous work is that the

minimum of V(0) does not occur at = 0. The

above mentioned H-0 potential, which produces

a V(8) consistent with the spectroscopic

observations, is thus not consistent with the

crystal structure. An extensive search for parameters of a Lennard-Jones, Buckingham, and other standard

forms of pair potentials failed to obtain the

desired V(8). In the mean field approximation

V(6) = E-vOj) (1)

where v(r:) is the pair potential between the

hydrogens and the j oxygen neighbors as a function of r. In order to make the contribution

due to the spatial distribution of pairs (the crystal

structure) explicit one may rewrite eq. (1) in an

approximate form as

where

V(6i) = Z^fcVk

•ik-Sj«[*k-«j<W]

(2)

(3)

is the pair distribution function which contains

the geometry of the problem. In this formulation the vk

are treated as independent parameters

thus removing the constraint of a specific form

for the pair potential. This is, however, not a problem amenable to least squares refinement.

Instead the method of maximum entropy is used

to obtain the following relationship.

vk = v^expjS^jk x [V(e{) - V° (6i)]} (4)

where V(0j) is the desired rotational potential

and V° (0j) is that corresponding to the set of

values of v°k. This expression used in an

iterative manner yields a set of vk which can be

used as a guide for further refinements. Without

going into the details, the result of this procedure is the pair potential of figure 2 shown

as the solid line and compared with the original

Lennard-Jones shown as the dashed line. The

solid line potential of this figure produces a V(0)

indistinguishable from that of figure 1 except

that the minimum is at = 0. This is a rather

novel form of the pair potential which could not be found without the maximum entropy

technique.

Work on H-H and 0-0 potentials is in progress whose aim is to produce reliable

potentials from which simulations of both static and dynamic properties can be made. It is hoped that these simulations will aid in understanding the behavior of nitromethane in both the solid and liquid state.

60 -40 -20 20 40 60

©

Figure 1. The rotational potential of the methyl groups in solid nitromethane. The horizontal lines represent the energy levels.

Figure 2. The H-0 intermolecular pair potential for nitromethane. The dashed line corresponds to the original Lennard-Jones potential and the solid line to that derived in the present work (see text).

Reference

[1] D. Cavagnat, A. Magerl, C. Vettier, I. S. Anderson,

and S. F. Trevino, Phys. Rev. Lett. 54, 193 (1985).

10
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

AB INITIO PHASE DETERMINATION IN A PROTEIN BY MAXIMUM ENTROPY

L. Sjolin (Chalmers University of Technology, Goteborg, Sweden)

E. Prince

and

L. A. Svensson and G. L Gilliland (Center for Advanced Research

in Biotechnology, Rockville, MD)

Previous experiments [1] in the use of

maximum entropy as a phasing tool in macromolecular crystallography have

concentrated on extending phases from a

relatively low resolution starting set to higher

resolution. From the beginning, however, there was a question whether the starting set could be

as small as the two to four reflections whose

phases can be chosen arbitrarily in order to specify one of a set of equivalent origins and one

of the two enantiomorphs. A preliminary test on

a set of x-ray diffraction data collected from

bovine heart creatine kinase [2] gave very

encouraging results, but it was desirable to determine if the structure of a macromolecule

whose structure was known from a high quality

refinement could be reproduced using maximum

entropy for ab initio phase determination.

The data chosen for a test were collected

from crystals of recombinant bovine chymosin

and used in a recently completed determination

and refinement of the structure [3]. The

molecule of this protein contains 321 amino acid

residues, and it crystallizes in space group 1222,

with one molecule per asymmetric unit. In this space group there are three sets of reflections

hkO, hOl, Okl, whose structure factors are constrained to be real. The phases of two

reflections can be chosen to define a unique

origin. There are no reflections whose structure

factors are imaginary, so the enantiomorph must

be determined by a general hkl reflection. The

phases of two strong reflections were assigned

the same phase as they had in the refined

structure to ensure that they were referred to the same origin. Next, 16 more reflections were

chosen, and maximum entropy maps were

calculated giving these reflections 32 sets of

phases according to a so-called "fractional

factorial design," and the entropy of each of

these maps was calculated. From this it could

be estimated which phase for each reflection

produced a higher entropy map. The map with

these phases and its entropy was then calculated,

and the 16 others that had one phase different.

Of the 49 phase sets tried by this procedure, the one that gave the highest entropy was chosen,

and those reflections were then incorporated in the trial map.

This procedure was followed for three more

blocks of 16 reflections, giving assigned phases

for a total of 66 reflections. This map contained no enantiomorph determination, so the next step was to choose a strong hkl reflection and

calculate a maximum entropy map with the phase

of this reflection set at each 20° between 0° and

340°. The results of these calculations are

illustrated in figure la. The entropy is symmetrical about 180°, reflecting the absence

of any enantiomorph determining information,

and has two sharp peaks at 60° and 300°.

Although these calculations contain no

information about the crystal structure, the actual phase of this reflection, calculated from the

refined structure, is 64°. Therefore, 60° was

chosen as the phase of this reflection, and it also was included in the trial map. The same

procedure was then followed for the next

strongest hkl reflection, with extra points in the

vicinity of the peak, with the result shown in figure lb. The entropy now has a unique

maximum at 316°. Using similar procedures,

phases were determined for an additional 93

reflections from the hkO, hOl and Okl sets and

1809 hkl reflections, for a total of 1970

reflections.

Figure 2 illustrates the results of phase

determination. Figure 2a compares a four amino

acid segment of the refined structure with a density map using 948 reflections, while figure

2b is a map that includes the 1970 reflections

whose phases were determined by the maximum

entropy procedure. For comparison, the map in figure 2c is calculated using all data and phases

calculated from the refined structure. 948

reflections is evidently an insufficient number to

11
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

produce connected density, but the similarity

between the 1970 reflection map and the refined

structure map is remarkable. In all three the

molecular diagram is the refined structure,

without any fitting to the map density.

X>u

(0

exo

(-1 cw

60 120 180 240 300 360

Phase Angle (degrees)

B 1

1 1 1

I

•

•

• C

2

>^ •

• U •

<ti u

- • •

" 4J3

• • • •

!h ^<r

1

>>a • • o • • u ~

" —

• cu

1 • •

1 ' 1 1 60 120 180 240 300

Phase Angle (degrees)

36C

Figure 1. The maximum entropy of a fitted electron

density map of recombinant bovine chymosin plotted as a

function of the trial phases of the two strongest acentric

reflections. (A) The prior map contains only centric

reflections, and the strongest acentric reflection has been

added. The plot is symmetrical about ir, reflecting the two

possible enantiomorphs. (B) The prior map contains the

strongest acentric reflection, and the second strongest has been added. The plot is no longer symmetrical.

Figure 2. Portions of three electron density maps, compared with a four amino acid residue segment of the molecule of recombinant bovine chymosin. (A) A map

computed from 948 reflections, with phases determined by

maximum entropy. (B) The map with phases extended by

maximum entropy to 1970 reflections. (C) The map

computed using all data, with phases calculated from the refined structure.

References

[1] E. Prince and L. Sjolin, NIST Tech. Note 1272, C. O'Connor, ed. p. 18 (1989).

[2] G. L. Gilliland, L. Sjolin, and G. Olsson, J. Mol.

Biol. JT70, 791-793 (1983).

[3] G. L. Gilliland, E. L. Winborne, J. Nachman, and A. Wlodawer, Proteins (in press).

12
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

FAST FOURIER TRANSFORMS FOR MACROMOLECULAR CRYSTALLOGRAPHY

M. An and C. Lu (City University of New York, New York, NY)

and

E. Prince

Crystallographic calculations, including

maximum entropy fitting, require repeated

Fourier transforms of electron density maps to compute structure factors and of sets of structure

factors to compute density maps. The fact that the values of the Fourier transforms can be

computed at the points of lattice whose spacing can be chosen to be appropriate to the resolution

of the data makes the problem well adapted to solution by fast Fourier transform (FFT)

techniques, but it requires that efficient FFT

routines be available for a wide range in the

number of grid points per period of the crystal.

At the same time, the space group symmetry of

the crystal can be exploited to reduce still further

the computing time and storage requirements for

the Fourier transforms of the unit cells of

macromolecular crystals [1]. A variety of FFT

techniques [2] and newly developed methods for

utilizing space group symmetry are being used to create a library of programs for crystallographic

computations in the space groups in which

biological macromolecules most commonly

crystallize.

References

[1] L. F. Ten Eyck, Acta Cryst. A29, 183-191 (1973).

[2] R. Tolimieri, M. An, and C. Lu, Algorithms for Discrete Fourier Transforms and Convolution , (Springer-Verlag, New York, 1989).

ANION AND CATION DISORDER IN THE IONIC CONDUCTING

PYROCHLORE SYSTEM Y2 (ZrxTi1_x) 2 7

S. M. Haile and B. J. Wuensch (Massachusetts Institute of Technology, Cambridge, MA)

and

E. Prince

Pyrochlore is a cubic crystal structure with

the composition A

32l~B4

-

l~07 based on an ordered

arrangement of subcells of the fluorite type. If the B ion is much smaller than the A ion, the

ordered structure, which has the A ion at (0,0,0), the B ion at (1/2,1/2,1/2), oxygen

anions in an eightfold and a forty-eightfold

position, and vacancies at another eightfold

position of space group Fd3m, forms. If, however, the B ion is too large, the crystal tends

to take a disordered defect fluorite structure. In the system Y2(Zrx Tij_x)2 7 the disordered

structure has a very substantial ionic

conductivity.

In order to study the nature of the disorder a neutron powder diffraction study was undertaken of samples with x = and x = 0.6. Because of the negative scattering amplitude of

titanium, neutron diffraction is a very sensitive

probe for determining its distribution among the

different cation sites. It is also a sensitive probe

for measuring the extent to which oxygen is redistributed among the sites of the pyrochlore

structure. Table 1 is a summary of the results of

a Rietveld refinement of the structure of the x = composition and of preliminary results of

a study of the x = 0.6 composition. There is insufficient information in the neutron diffraction

data alone for a unique determination of the

distribution of the three species of cation

between the two sites, so this must include x-ray

diffraction data also. The indication is that the

disorder of the anion sites is almost entirely due

to transfer of oxygen ions from the fortyeightfold positions to the vacant sites.

13
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Table 1. Summary of the results of Rietveld refinement of the pyrochlores ¥2^0-7 and Y2

(TiQ ^Lt§ ^2

O7. Space group Fd3m

Y2Ti207 Y2(Ti0.4ZW2 7

Rwp 9.96% 13.26%

RE

7.36% 8.45%

RB

2.17% 11.85%

ao10.0947(1) A 10.2910(4) A

Yin 16c 0H=022=033 0.00160(8) 0.0035(2)

^12 =/3

13 =/323 -0.00048(11) 0.0007(4)

Ti.Zr in 16d %1 =^22=%3 0.00067(17) 0.0046(5)

'3 12 = '3 13 =/323 -0.00023(21) -0.0001(8)

0(1) in 48f

X 0.42082(9) 0.4012(4)

011 0.00157(10) 0.0102(4)

%2~%3 0.00114(9) 0.0065(3)

#23 -0.00051(12) -0.006(4)

0(2) in 8a

/3 ll =/322 =%3 0.00103(12) 0.0060(5)

Reference

[1] P. K. Moon and H. J. Tuller, Solid State Ionics 28-30 , 470 (1986).

PROTEIN SYMMETRY

V. L. Himes and A. D. Mighell

and

J. R. Rodgers (National Research Council, Ottawa, Canada)

As part of our review of symmetry in crystalline materials, we have surveyed the

proteins in the Brookhaven Protein Databank [1]. Using matrix [2] and reduction techniques, our

analyses have revealed that it is not uncommon

in these compounds for the metric symmetry to exceed the reported crystal symmetry.

Of particular note are proteins reported in the monoclinic space group P2j . In a number of

these cases, the metric symmetry is orthorhombic and the lattice can be defined by a C-centered orthorhombic cell. Potential

explanations for this observation include: (1) an

incorrect determination of the crystal system and

Laue symmetry with subsequent refinement in a space group of too low symmetry, and (2) the

overall shape of the molecule is more symmetrical than the internal symmetry or some

other factor associated with the shape of the

molecule. Because of the intense study devoted

to any given protein, it is more likely that the

second explanation accounts for our observation.

Regardless of the reason for the observation, our results have important implications for

14
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

experimental protein crystallography. Standard

laboratory procedure should be modified to always include a direct determination of the

metric symmetry as soon as any unit cell defining the lattice has been determined. With

full knowledge of the highest possible symmetry, one is then able to determine in a logical and

accurate manner the Laue group (2) and the

space group. For those proteins in which it has

been proved that the metric exceeds the crystal

symmetry, the experimentalist must proceed with

caution because a given lattice will have

metrically similar unit cells that are 'not symmetrically equivalent.' Due to indexing/

intensity ambiguities inherent in these cases, great care must be taken in the following

situations: a) when comparing intensities on the same protein but taken by different experimentalists; b) when collecting and combining

data for the same protein but from different

crystals; and c) when measuring data on the same protein with different techniques (e.g.,

neutron and x-ray diffraction). Finally, in these

cases, one should be on guard for such lattice related phenomena as twinning.

References

[1] Brookhaven Protein Databank: J. Mol. Biol. 112 , 535-542 (1978).

[2] V. L. Himes and A. D. Mighell, "A Matrix Approach

to Symmetry," Acta Cryst. A43, 375-384 (1987).

CRYSTAL STRUCTURE OF C32H46N6 4 .(CH3

) 2CO

H. L. Ammon (University of Maryland, College Park, MD)

and

C. S. Choi (ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

The crystal structure of the titled compound

was determined by x-ray diffraction using the

ENRAF-NONIUS CAD-4 diffractometer at the

x-ray laboratory of the Department of

Chemistry, University of Maryland. The

crystals were synthesized and grown by P. R.

Dave of Geo-Centers, Inc., Hopatcong, NJ. The

compound crystallizes in the orthorhombic Pnma

space group with unit cell dimensions of a =

22.782(1), b = 15.950(1), and c = 9.809(1),

containing four formula units of the titled compound including the solvent molecules

(acetone) per unit cell. The structure was solved

with the MITHRIL direct methods link, and was

refined to the final R-factors of R = 0.048 and

R = 0.064.

The compound is a large cyclic-linkaged

molecule consisting of eight smaller molecules;

i.e., two butylcyclotrimethyle-trinitramine rings,

two benzoxyl rings, and four methylene groups

(fig. 1). All the molecules in the crystal are positioned in the mirror plane. The mirror plane

passes through the center of the large molecule,

through the centers of the two benzene rings,

and through the carboxyl atoms as can be seen in

(cii)

6H$

5©

(5HD

Figure 1. ORTEP drawing of the titled compound, viewed

along the b-axis direction. The solvent (acetone) molecule

is also included in the figure.

figure 1. The benzene rings are very slightly

bent along the intersection line with the mirror

plane. The molecules are packed rather loosely.

The acetone molecules occupy the space left between the bulky molecules. All the acetone atoms except the H atoms are located on the

15
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

mirror plane. The intermolecular distances

(nonhydrogeneous) shorter than 3.6 A are C(4) - 0(1) = 3.390(3) between the solute molecules,

and C(19) - 0(4) = 3.597(7) between solute and

solvent molecules. There are no evidences of

hydrogen-bondings.

CRYSTAL STRUCTURE OF 2,2,4,4-TETRANITROADAMANTANE

H. L. Ammon (University of Maryland, College Park, MD)

and

C. S. Choi (ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

The titled compound is primarily of interest

because of the energetic property and high

density. The crystals were synthesized and

grown by P. R. Dave of Geo-Centers, Inc.,

Hopatcong, NJ. The crystal structure was

determined by x-ray diffraction using the

ENRAF-NONIUS CAD-4 diffractometer at the

x-ray laboratory of the Department of

Chemistry, University of Maryland. The

compound crystallizes in the monoclinic P2 1 /m

with unit cell dimensions of a = 7.074(1), b =

13.395(2), c = 7.304(1), and beta = 112.79(1),

containing two formula units per unit cell. The

structure was solved with the MITHRIL direct

methods link and was refined to the final R-factors of R = 0.047 and Rw = 0.069.

The molecule has mirror symmetry with the

C3-C4-C5-C7 unit on the mirror plane as shown

in figure 1 . The bond lengths and angles are all normal, with the exception of the parameters

associated with the dinitromethylene moiety.

Here the two C-N bonds are stretched to 1.547 A and 1.551 A, and the N-C-N angle

diminished to 98.2. Similarly, small N-C-N

angle has been observed in 2,2,dinitroadamantane with C-N distance of 1.555, and

1.560 A and N1-C2-N2 = 98.7. These

distortions may be attributed to the geminal

nitrogroups that, because of the strong electron

demand place on C2, require an enrichment of

the amount of the p character in the exocyclic

orbitals at C. Several short intramolecular

distances occur between the adjacent

nitrogroups; 01... 01* = 2.884(7), 01... 03 =

2.965(4), 02. ..04 = 3.394(4), and 02.. .02' =

3.498(8). There are no intermolecular H-bonds.

The molecules are bound mainly by the Van der

Figure 1. ORTEP drawing of 2,2,4,4- tetranitroadamantane molecule.

Waals' interactions between O and H atoms.

The shortest intermolecular distances occur between O atoms with the distances, 01... 04 =

3.093(3) and 03. ..04 = 3.153(4), and between

O and H atoms with 01... H8 = 2.68(4),

03. ..H5 = 2.66(4), 03. ..H6 = 2.75(3),

04.. .HI = 2.61(3), and 04.. .H3 = 2.77(3).

16
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON DIFFRACTION TEST OF THE METHOD OF REPRODUCIBLE

PREPARATION OF THE AlCuFe ICOSAHEDRAL QUASICRYSTALS

Y. Calvayrac, M. Cornier-Quiquandon, D. Gratias,

and A. Quivy (C.E.C.M./C.N.R.S., Vitry, France)

R. Bellissent (C.E.N. , Saclay, France)

J. W. Cahn (Materials Science and Engineering Laboratory)

and

B. Mozer

Isotopic substitutions are used in crystal structure determinations because this technique

permits the determination of the contribution to neutron scattering from individual atomic

species. Following the original suggestion of

Bellissent, we are attempting to determine each

individual atomic species' contribution to the

neutron diffraction from the AlCuFe icosahedral

quasicrystals whose aperiodic structure in 3-d can be described in terms of a 6-d periodic

structure. We can determine the 6-d spatial

distribution of the pair correlation function for

each element in the quasicrystal and thereby

deduce the 3-d distribution of all the elements.

To be able to separate each atomic species'

contribution, it is necessary to prepare

metallurgical^ identical samples which have

different isotopic composition. At Vitry, a method of preparation of excellent icosahedral

quasicrystal specimens of the AlCuFe alloy

system has been developed. We wanted to determine if the Vitry method of preparation was

capable of preparing metallurgically identical

samples with different isotopic composition. To

test this assumption without using the isotopes,

Vitry prepared by their method two samples of

the same composition under what was claimed to be identical conditions. Then both x-ray and

neutron diffraction patterns were taken of the

samples to note any differences. The two

samples of Al 63Cu24 5Fe12 5

, of roughly 10 g

each, were powdered at Vitry and their x-ray

diffraction patterns taken. Neutron diffraction

patterns were measured on the BT-1 high

resolution powder diffractometer at the NIST

reactor on each of the two powder samples.

Care was taken to insure that the same amount

of powder was illuminated by the incoming

neutron beam and that the vanadium sample

holder was mounted in the same position in the

beam for the two measurements.

The diffraction patterns for each powder

sample at casual perusal seem to be identical. We took a difference of the two patterns and

examined the resultant carefully with regard to the statistics of the measurement both for the

monitor and for the counters. In the smaller q

side of many peaks we found a small additional

contribution to the intensity from one of the

samples. To insure that this difference was a true effect and not a consequence of an error in the multiplicative factor for relative normalization of the two spectra, we varied this multiplicative factor. This adjustment would be

expected to produce peaks or dips in the

difference pattern in the region of each peak but

it was not possible to eliminate the small

additional intensity in the short q extreme wings

of the lines of one sample. Furthermore, it was

not possible to observe differences in the full widths of equivalent peaks for the two samples.

The explanation for the discrepancy in the

two spectra might be given from some earlier work at Vitry on samples cooled from different

temperatures after an anneal of the pure

icosahedral phase. They observed that the shape

of the peaks went from almost a pure Gaussian

to almost a pure Lorentzian shape at 600 °C.

Similar changes result from differences in the

rate of cooling following the icosahedral phase

anneal at a temperature above 660 °C. It is quite likely that the air cool procedure used in Vitry resulted in unequal cooling rates following

the anneal.

We will continue to test preparations until we can say with confidence that the Vitry

method of preparation of the AlCuFe icosahedral

quasicrystals will yield quasicrystals with

identical properties.

17
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON POWDER DIFFRACTION STUDY OF THE

ORTHORHOMBIC Ti2AINb PHASE

L. A. Bendersky and W. J. Boettinger (Metallurgy Division,)

R. G. Rowe (General Electric Corporate Research and Development, Schenectady, NY)

and

B. Mozer

The crystal structure of the orthorhombic

phase based on Ti2AlNb originally discovered

by Banerjee et al. [1] has been refined using the

Rietveld method on neutron powder diffraction

data collected from an extruded and heat treated

alloy fabricated at General Electric.

Neutron powder diffraction was performed at room temperature with the high-resolution five

detector diffractometer, BT-1, at the NIST

reactor [2] using the experimental conditions in table 1. Four 2.5-cm long rods were bundled

together with a common axis about which they were rotated during diffraction in order to minimize any effects from texture. The data were analyzed using the code written by Prince

[3] which allows processing the intensities from

the five detectors simultaneously. The background was represented by a Chebychev polynomial function with up to six coefficients per

detector. Six coefficients were used for the first detector and two coefficients were used for the

remaining four detectors and were refined for

each channel together with the profile and

structural parameters. The scattering lengths

used for the refinement are b(Ti) = - 0.3438

Fermi (10(-12) cm.), b(Al) = 0.3449 Fermi, and

b(Nb) = 0.7054 Fermi.

Rietveld refinement was performed using the

structure suggested by Banerjee et al. [1] as an

initial guess. First level refinement was

performed to establish scale factor, background

parameters, lattice parameters, and spectrometer

zero. Subsequent refinements were performed to establish the atomic positions and the

temperature factor. For the first model, the

Wycoff sites were occupied by a unique atomic

species having individual densities fixed

according to the formula Ti2AlNb. A correction was allowed for possible texture yielding a slight

improvement. This model was not satisfactory

in that the temperature factor for titanium was

larger than acceptable for a simple alloy and

even worse, the temperature factor for aluminum

was negative. In order to improve upon this model, we allowed the three sites to be occupied

by combinations of the three atoms and refined

the occupancies along with all the other

parameters. The best refinement obtained was a model where only aluminum occupied the 4cl

site and titanium and niobium were distributed on the 8g and 4c2 sites. The total density was

fixed according to the chemical formula.

Another possible model was used following the

suggestion of Banerjee et al. [1] where different

Wycoff sites were assumed for the atomic

species, their model figure 7(c) of table 4. This

model and modifications of it allowing each site to be occupied by a combination of all species

gave a much poorer refinement.

Table 1. Experimental conditions used to collect the neutron powder diffraction data

Monochromatic Beam

Wavelength

Horizontal Divergences

220 diffraction from Cu with a pyrolytic graphite filter to remove higher order contamination,

0.1553(1) nm

10, 20, and 10 min of arc for the in-pile, monochromator

beam, and diffracted beam collimators, respectively.

18
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

The refinement of our powder neutron

diffraction data of the orthorhombic phase of

Ti2AlNb was able to fix the model of the

structure as one having only aluminum on the

4c 1 site and a mixture of titanium and niobium on the 8g and 4c2 sites and eliminate the other

two models suggested by Banerjee et al. [1]. The

results of our refinement are given in table 2.

Table 2. Results of Rietveld refinement

Space Group Cmcm

Atom Wycoff Positions Temperature Relative X Y Z Factor Occupancy

Ti(8g) 0.2310(12) -0.0959(6) 1/4 2.02(.14) 0.823(4)

Al(4cl) 0.0 0.1633(3) 1/4 0.48 0.5 Nb(4c2) 0.0 -0.3643(3) 1/4 0.62(.10) 0.323(4)

Nb(8g) 0.2310(12) -0.0950(6) 1/4 2.02(.14) 0.177(4)

Ti(4c2) 0.0 -0.3643(3) 1/4 0.62(.10) 0.177(4)

Lattice Constants in Angstroms

a = 6.0893(2) b = 9.5694(4) c = 4.6666(2)

Statistical Factors Rn = 10.53 Rp = = 11.21 Rw = 14.98 Re = 8.11 Chi = 1.845

Note: Figures in parentheses are standard deviations in the decimal figures. The temperature factor

is in Angstroms squared. R factors are defined in A. Santoro, R. S. Roth, and D. Minor. Acta. Cryst. B33 , 3945 (1977).

The authors would like to thank C. S. Choi,

J. Stalick, and A. Santoro for their help and

insight in the use of the Rietveld refinement

program.

References

[1] A. K. Gogia, T. K. Nandi, and V. A. Joshi, Acta

Met. 36, 871.

[2] E. Prince and A. Santoro, Natl. Bur. Stand. (U.S.)

Tech Note No. 1117, F. Shorten, ed. p. 11, (1980).

[3] Ibid, p. 8.

19
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

K^CND^JE: A CRYSTAL STRUCTURE DETERMINATION

C. Bostoen (University of Antwerp, Wilrijk, Belgium)

and

J. K. Stalick and S. K. Satija

This study was motivated by recent incoherent inelastic neutron scattering

experiments [1] and dielectric measurements [2]. These results show that the ammonium ion has a dipole moment; however, its microscopic nature

is unknown. In order to elucidate this question,

neutron powder diffraction measurements were

performed on K1

_X(ND4)X

I mixed crystals. The

mixed crystals were prepared from an aqueous

solution of D2 (99.7+ %) with the appropriate

amounts of KI and ND4

I. The deuterated

ammonium iodide was obtained after successive

exchanges with D20. A Raman scattering

analysis showed that six exchanges gave a

sufficient degree of deuteration. Assuming

Vegard's rule to be valid for this mixed system,

the ND4 +

concentration determined from the

lattice constant obtained from x-ray data is 41 %

.

The diffraction experiments were performed on

the BT-1 high-resolution powder diffractometer

with X = 1.553 A and collimations 20'-20'-10',

before and after monochromator and before the

detectors, respectively. Diffraction patterns were recorded for temperatures T = 4.7 K and

43 K.

For this concentration, the structure is cubic,

space group Fm3m, with the NaCl structure

type. For the Rietveld refinement of the

profiles, we applied a model used earlier for

NH4I: three of four hydrogens are directed

toward the iodide ions, the remaining N-H bond

is pointed along the < 1 1 1 > direction [3]. The

main outcome of the refinement is that the

ammonium ion is slightly distorted from its ideal

tetrahedral shape. For T = 43 K, the bond

angles are 1 15° for the angles between < 1 1 1 >

and iodine bonded hydrogens, and 103° for the

angles between the iodide bonded hydrogens.

For an ideal tetrahedron these angles are all equal to 109.45°. From this deformation one

can extract a dipole moment of 1.3 + 0.2 D for

the 43 K data, in agreement with the dielectric

results [2]. The ammonium concentration was

also allowed to vary during the refinement and

gave x = 0.40, consistent with the x-ray

analysis. The obtained U, V, and W linewidth

parameters were fairly large and evidence a

strain broadening of about 0.3%, comparable to what was obtained in U doped with 0.5% of Ti

[4]. There was no indication that the ND4 +

ion was off center and the N-D distances were in agreement with other ammonium compounds [5]

.

The lattice constants are 7.088 A and 7.090 A

for T = 4.7 K and 43 K, respectively.

References

[1] C. Bostoen, G. Coddens, and W. Wegener, J. Chem.

Phys. 91, 6337 (1989).

[2] I. Fehst, R. Bohmer, W. Ott, A. Loidl, S. Haussuhl,

and C. Bostoen, Phys. Rev. Lett. 64, 3139 (1990).

[3] H. A. Levy and S. W. Peterson, J. Chem. Phys. 21,

366 (1953).

[4] C.S. Choi and H. J. Prask, J. Appl. Cryst. 18, 141

(1985).

[5] See for example, H. J. Prask, C. S. Choi, N. J. Chesser, and G. J. Rosasco, J. Chem. Phys. 88, 5106

(1988).

20
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

K!.X(NH4) X

I: A PHASE DIAGRAM STUDY BY MEANS OF X-RAYS

C. Bostoen (University of Antwerp, Wilrijk, Belgium)

and

S. K. Satija

This study was motivated by recent experimental results. Neutron time-of-flight

results show a very broad distribution of

tunneling lines, related to the different

environments which the ammoniums experience

in the alkali halide lattice [1]. Dielectric

susceptibility measurements indicate that the

ammonium ions have a dipole moment. In

addition, a dynamical freezing process similar to other dipolar glass forming materials was

reported [2], although no glass like anomalies were found in the specific heat [3]. The nature of the dipole moment was determined by neutron

diffraction (see p. 20). However, the phase

diagram of this system was unknown.

Polycrystalline mixed crystals were grown

from an aqueous solution under nitrogen gas

atmosphere in order to reduce the oxidation of

the ammonium salt. At room temperature the

powders contained the pure mixed crystals, with- out any Bragg peaks from the pure compounds,

KI or NH4I. From the lattice constants at room

temperature and the validity of Vegard's rule, one can determine the ammonium concentration.

The concentrations considered are: x = 0.55,

0.72, 0.82, 0.84, 0.88, and 0.97.

A support for a Displex was installed,

making it possible to perform x-ray

measurements from T = 10 K up to 340 K.

Pure NH4

I has a low-temperature CsCl

structure, which transforms to a tetragonal

structure (slightly distorted from the CsCl

structure at low temperatures [1]. The (111)

reflection of the NaCl structure and the (100)

reflection of the CsCl structure were recorded in subsequent steps of AT = 10 K on cooling and

heating. All the samples had the following

similarities: 1) Part of the sample remained in the NaCl structure down to T = 10 K. 2) A

large hysterisis effect was observed: the point

where the transition occurred to a low

temperature ordered phase, was much lower than

the point where the sample was again in the

NaCl structure when heated from T = 10 K to

room temperature or higher (fig. 1). It is known

that the phase transition in pure NH4I has a martensitic character, involving a change of a sixfold to an eightfold configuration and a volume change of 17%. As the concentration of

spherical impurities is increased, Tc

is reduced,

similar to the situation in KCN/KBr [4]. The critical concentration, xc

, where the

phase transition is suppressed, is between 0.72

and 0.82. It would be interesting to test recent

ideas in terms of formations of microdomains

each having their own strain order parameter

[5], on samples which are close to xc

.

&

H

300

1

NaCl

1 1

A

A

1

A

1 A -

200

T

T

100 T

T ' M

O i

1

I 0.7 0.8 0.9 1.0 ammonium cone, (x)

Figure 1. Phase diagram of Kj.^NH^I. NaCl:

orientationally disordered rocksalt structure (:

cooling,*: heating). M: mixed phase of NaCl and CsCl structure.

References

[1] C. Bostoen, G. Coddens, and W. Wegener, J. Chem.

Phys. 21, 6337 (1989).

[2] I. Fehst, R. Bohmer, W. Ott, A. Loidl, S. Haussuhl,

and C. Bostoen, Phys. Rev. Lett. 64, 3139 (1990).

[3] C. Bostoen, K. H. Michel, B. Vaziri, M. Meissner,

and S. Haussuhl, PHONONS 89, S. Hunklingcr el al., eds. Singapore: World Scientific, 1990, 615 p. [4] Sec for example, S. Mertz and A. Loidl, Europhys.

Lett. 4, 583, (1987).

[5] K. Knorr, Phys. Rev. B4J., 3158 (1990).

21
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

STRUCTURAL STUDIES IN THE BaO-Nd2 3-CuO SYSTEM

W. Wong-Ng (Ceramics Division) and J. K. Stalick

The structures of two compounds,

BaNd2Cu05 and BaNd2 4

, have been determined with the Rietveld method using neutron

powder diffraction data collected with the highresolution five-detector diffractometer at NIST.

These studies are in conjunction with phase

equilibrium studies in the BaO(BaC03)-Nd2 3

- CuO system [1], which also contains the superconductor NdBa2Cu3 7

. The material BaNd2Cu05

is the stoichiometric end-member of the solid solution

Ba2+2xNd4_2xCu2

_xO10_2x, with a narrow solid

solution range of 0^ x ^0. 1 . Unlike the smaller

lanthanides (R), which form the green phase

BaR2Cu05 (tetragonal, space group P4m2), for

R = Nd or La the solid solution is brown, with

the tetragonal space group P4/mbm. The La

system has been studied by Kilbanow et al. [2]

and Michel et al. [3]; the extent of solid solution was found to be 0.15 < x < 0.25, with the

stoichiometric composition BaLa2Cu05 absent.

The crystal structure of the La analog has been

studied using x-ray diffraction [3], although the

Ba and La atoms could not be differentiated

owing to the similarity of their scattering factors. The atomic parameters for the La analog were

used as starting parameters for the refinement of

BaNd2Cu05

, and refinement proceeded smoothly

to agreement factors of RB = 2.97, R^^,^ = 5.36,

X - 1.15. Since the scattering lengths for Nd

and Ba are significantly different for neutron

diffraction, we could confirm that the larger Ba

atom exclusively occupies the 2b site, and the

smaller Nd atom exclusively occupies the 4g

site. The BaNd2Cu05 framework is built from

edge- and face-sharing BaO10 and Nd08 polyhedra; the square-planar Cu04 groups are

isolated in the structure. Our results are in agreement with the atomic distribution suggested

by Michel et al. [3] for Baj 2Laj 8Cu 9 4 8

, in which some Ba atoms presumably occupy the

smaller La site. This seems inconsistent with the

inability to prepare the stoichiometric

BaLa2Cu05 compound, as this composition

would not require such site substitution and

should be structurally more stable.

In this same system, phase equilibrium

studies [1-2,4] also showed that for the larger

lanthanides R = La, Nd only one binary phase

BaR2 4is formed. This is in contrast to the

smaller lanthanides, which exhibit additional

binary phases as R becomes progressively

smaller. For example, when R = Y four binary

phases are formed [5], including BaY2 4

. Xray powder diffraction studies of BaY2 4 have

suggested that it is orthorhombic with space

group Pnab (no. 60) [5], or isostructural with

SrY2 4 (CaFe2 4 structure type) with space

group Pnam (no. 62) [6]. Recently, intensity

calculations by Costa et al. [7] for powder x-ray

data indicated that the latter space group is probable, and we have confirmed this for

BaNd2 4 using neutron Rietveld refinement.

The excellent agreement between the calculated

and observed intensities (RB = 5.12, R^,^ =

3.59, x = 1.13) indicates that the space group

Pnam is correct. In this structure the Ba atom is eight-coordinate, located near the center of a

trigonal prism formed by six oxygen atoms; two

,of the rectangular faces are further capped by

oxygen atoms. The two crystallographically

independent Nd atoms exhibit the expected

octahedral coordination by six oxygen atoms.

References

[1] W. Wong-Ng, B. Paretzkin, and E. R. Fuller, Jr. J. Solid State Chem. 85, 117 (1990).

[2] D. Kilbanow, K. Sujata, and T. O. Mason, J. Amer.

Ceram. Soc. 71., C267 (1988).

[3] C. Michel, L. Er-Rakho, and B. Raveau, J. Solid State Chem. 39, 161 (1981).

[4] R. S. Roth, C. J, Rawn, F. Beech, J. D. Whitler, and

J. O. Anderson, "Ceramic Superconductors II," M. F. Yan, ed., pp. 13-26, Amer. Ceram. Soc. (1988).

[5] W. Kwestroo, H. A. M. van Hal, and C. Langereis, Mater. Res. Bull. 9, 1623 (1974).

[6] I. M. Maister and L. M. Lopato, Izv. Akad. Nauk

SSSR, Neorg. Mater. 9, 64 (1973).

[7] G. A. Costa, M. Ferretti, M. L. Fornasini, E. A.

Franceschi, and G. L. Olcese. Powd. Diff. 4, 24

(1989).

22
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

POLYTYPIC STRUCTURES OF SODIUM METAL

FOLLOWING THE MARTENSITIC TRANSFORMATION

R. Berliner (University of Missouri-Columbia, Columbia, MO)

and

H. G. Smith (Oak Ridge National Laboratory, Oak Ridge TN)

and

J. R. D. Copley

and

J. Trivisonno (John Carroll University, Cleveland, OH)

Neutron diffraction studies of sodium metal

performed on the BT-9 spectrometer in 1988

have been the subject of considerable subsequent

analysis. Like lithium metal, sodium undergoes

a martensitic structural phase transformation as

it is cooled [1]. In lithium metal, the transformation is at - 78 K to a multivariant nine layer

R3M rhombohedral lattice [2]. Four c-axis

crystallization axes are found near each BCC

(1 10) direction. The same 9R lattice and multivariant morphology is observed following the

martensitic transformation near 35 K in sodium

metal. Evidence that the low temperature

structure of sodium is more complicated than

lithium is found in the diffraction pattern along

a (10^) line in reciprocal space. The most

prominent diffraction features are those that can be attributed to peaks of the 9R phase, shifted

from their calculated peak positions by the

presence of "double-twin" stacking fault type

layer defects. The 9R diffraction peaks exhibit

shoulders which were initially believed to be the

result of stacking-fault-distorted diffraction from

an HCP phase, but all attempts to model peak

shifting in HCP structures have failed. The

asymmetric nature of the diffraction peak

shoulders was eventually recognized as evidence

that these features were due to another rhombohedral phase, and analysis of the shoulder

position suggests that it has between 21 and 27

layer stacking periodicity. There are, for

example, 13 different 27R stacking patterns and

only one of these produces diffraction patterns

sufficiently like the data to be considered at length. This stacking pattern, illustrated in terms of A, B, and C layers,

/ABCBCBCBC/BCACACACA/CABABABAB/

may also be described in terms of hexagonal (H)

and cubic (K) layer types.

/HKHHHHHHH/HKHHHHHHH/HKHHHHHHH/

This pattern is easily recognized as a superlattice

consisting of an even number of hexagonal

layers separated by single layers in a cubic

environment. It is possible to show that there is an infinite set of these "almost HCP" rhombohedral stackings. Diffraction from these

structures produces better and better approximations to an HCP diffraction pattern as the c- axis lattice parameter becomes longer. This

effect is illustrated in figure 1 where the

calculated (10£) diffraction patterns for the

"almost HCP" structures from 3R (FCC) to 45R

are compared to that for HCP. These structures can be transformed, one into another, by the

injection of special stacking faults. For instance,

the 9R structure is converted to the 15R by the

insertion of sufficient "double-twin" stacking

faults. In like manner the 15R is converted to the21R and the 21R to the 27R. The presence

of all of these structures is suggested by the

analysis of the diffraction patterns of sodium

metal as the specimen is warmed to the martensitic reversion temperature. A model which

incorporates stacking fault broadened and shifted

9R, 15R, and 21R polytype fractions for the

sodium martensite produces a striking resemblance to the major features of the diffraction

data. Figure 2 compares the diffraction data to the polytype mixture model calculation.

Analysis of martensite structures and their relative stability in sodium is continuing. In addition, the investigation of alkali metal alloy

systems that undergo similar martensitic trans23
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

formations and the study of the alkali metals

under hydrostatic pressure is also under way.

Rhcmbohedra I Polytype Diffraction Potterns

..

3-R. C *=£.£ 3

A 1 1

A 1

is-K

A 1

z.ix

A 1

27 K

A 1 . 1

A 1

3?^

A .

I „. 1. . . 1 •

•

4»

, ! ,

Figure 1. Calculated (101) diffraction patterns for "almost HCP" rhombohedral

stacking polytypes compared to the diffraction pattern for HCP.

0.00 0.10 0.20 M 0.40 . 50 0.60 0.70 0.50 0.90 I L

I To?)

1*

(T»7),*K

a-) /Ya

3oK

tTf>8>

'IK

Figure 2. (a) The (101) diffraction pattern for sodium, calculated for a 9R, 15R, and 21R polytype mixture model. (b) The measured (10£)

diffraction pattern for sodium below

the martensitic transformation

temperature.

* • 1 i .00 -9.00 -7 00 -5.00 -3.00 -1.00 1.00 3.00 5.00 7.00 9 00 1 1 00

L References

[1| R. Berliner, O. Fajen, H. G. Smith, and R. L. Hitterman, Phys. Rev. B40, 12086 (1989) and

references therein.

[2] H. G. Smith, Phys. Rev. Lett 58, 1228 (1987).

24
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

THE CRYSTAL STRUCTURE OF THE 60 K SUPERCONDUCTOR Laj 6Sr 4CaCu2 6±&

R. J. Cava, J. J. Krajewski, R. M. Fleming, J. V. Waszczak, W. F. Peck, Jr., and P. Marsh

(AT&T Bell Laboratories, Murray Hill, NJ)

and

A. Santoro

All the known copper oxide-based superconductors contain layers of copper-oxygen

squares, pyramids, or octahedra as their

electronically active structural components. One

structure type (see figure 1) first reported for

La2SrCu2 6 and La^aC^Og [1] has until

recently stood as an enigma because it could not be made superconducting even though at its heart

are the double copper oxide pyramidal layers

common to the highest Tc superconductors. We

have recently shown that the structure type can be made superconducting at 60 K at the

composition Lal 6Sr 4CaCu2 6

[2]. This is a uniquely simple double layer superconductor, as

it becomes superconducting through the introduction of carriers by straightforward atomic

substitution, without the intervention of complex

charge reservoir layers.

The crystal structure of this superconductor

has been determined by neutron diffraction

powder profile analysis in this laboratory, using

the intensities measured with the high-resolution

five-counter diffractometer. The structure was

refined with the Rietveld method [3] adapted to the multicounter diffractometer and modified to include background parameters [4]. Peak shapes were described by Gaussian functions. The

neutron scattering amplitudes used in the

refinements were b(La) = 0.827, b(Ca) =

0.490, b(Sr) = 0.702, b(Cu) = 0.772, and

b(O) = 0.581 x 10 12cm.

Initial positional parameters were taken from

our structural refinement [5] of the nonsuperconducting end-member La: 9Cax iCu2 6_5

, in the tetragonal symmetry spacegroup I4/mmm.

All positional parameters, and isotropic

temperature factors, as well as metal (with the

exclusion of copper) and oxygen site occupancies were allowed to vary simultaneously. The La,

Sr, and Ca contents were constrained to sum to

their chemically determined values. The refinement quickly converged (x = R\v^e < 1-5)

for a model where the Sr is in solid solution on

the nine coordinated La site, the Ca is ordered as

Figure 1. The crystal structure of La2CaCu2Cv and

Lai gSrQ 4CaCu2

0g. The CuO^ pyramidal planes are the active electronic components. The large shaded circles are

large cations nine coordinated (primarily La and Sr) to oxygen and the small shaded circles are small cations (primarily Ca) eight coordinated to oxygen.

expected in the eight-coordinated site between

the bases of the pyramids, with no oxygen in the

Ca layer, and a small amount of oxygen

deficiency is in the oxygen site at the apices of

the copper oxide pyramids. The atomic

parameters for the crystal structure are presented

in table 1 . Also shown for convenience are the

results from our earlier refinement of

Lax 9Ca! jCu2 6. Table 2 presents the metaloxygen bondlengths for Laj 6Sr 4CaCu2 5 94 , again compared to those we found for La1.9Cal.l Cu2°6-

25
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Table la. Refined structural parameters of La^ gSr ^aC^O^ •

Space Group: I4/mmm a = 3.8208(1), c = 19.5993(7), V = 286.13(2)A3

Atom Position x y z B(A2

)

LaCa

La(l)

Sr(l) Ca(l) Cu

0(1)

0(2)

2a 4/mmm

2a 4/mmm

4e 4mm

4e 4mm

4e 4mm

4e 4mm

8g mm

4e 4mm

Rn = 6.27

0.5

*P = 8.41

0.17666(13)

0.17666(13)

0.17666(13)

0.58656(13)

0.08211(11)

0.7036(2)

Rw = 11-36

0.75(16)

0.75(16)

0.79(7)

0.79(7)

0.79(7)

0.76(1)

1.18(7) 1.58(10)

R, 8.60

0.047(8)

0.203(8)

0.353(8)

0.1 0.047(8)

0.5

1.02(1) 0.477(6)

X = L32

Table lb. Refined structural parameters [5] of La^ gCa.\ 1^20^

Space Group: I4/mmm a = 3.8248(1), c = 19.4286(5), V = 284.22(2) A

3

Atom Position x v z B(A2)

LaCa

La(l)

Ca(l) Cu

0(1)

0(2)

2a 4/mmm

2 a 4/mmm

4e 4mm 0.17578(9)

1.0(1)

1.0(1) 0.47(5)

0.014(6)

0.236(6)

0.47(1)

4e 4mm

4e 4mm

0.17578(9)

0.58503(9)

0.47(5)

0.32(4)

0.03(1)

0.5

8g mm

4e 4mm

0.5 0.08230(9)

0.7046(2)

0.74(5)

1.52(8)

1.005(8)

0.506(6)

Rn = 4.58 *p = 5.96 Rw = 8.54 Re = 4.34 X- 1.97

Table 2. Metal-oxygen bondlengths in La

1.9Ca

l.l Cu2°6

and Lai ^Stq ^CaCu^Og *

La

1.9Cal.l Cu2°6 La 1.6Sr0.4CaCu2°5.94

Ca-O(l) x 8 2.493(1) 2.498(1)

La-O(l) x 4 2.638(1) 2.662(2)

La-0(2) x 4 2.762(1) 2.753(1)

La-0(2) x 1 2.324(3) 2.348(7)

Cu-O(l) x 4 1.913(1) 1.912(1)

Cu-0(2) x 1 2.323(3) 2.293(6)

References

[1] N. Nguyen, et al., Mat. Res. Bull. 15, 891 (1980).

[2] R. J. Cava, et al., Nature 345, 602 ( 1990).

[3] H. M. Rietveld, J. Appl. Cryst. 2, 65 (1969).

[4] E. Prince, Natl. Bur. Stand. (U. S.) Tech. Note

1117, F. Shorten, ed., p. 8-9, (1980).

[5] A. Santoro, et al., Mat. Res. Soc. Symp. Proc. 166, 187 (1990).

26
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

CRYSTAL STRUCTURE AND ATOMIC ORDERING IN

Pb2Sr2Y

1_xCaxCu3 8+6 (x = 0, 6 = 1.47)

M. Marezio and R. J. Cava (AT&T Bell Laboratories, Murray Hill, NJ)

J.J. Capponi (Laboratoire de Cristallographie, Grenoble, France)

E. A. Hewat (LETI-CENG, Grenoble, France)

and

A. Santoro and F. Beech

The superconducting series Pb2Sr2Y1

.x CaxCu3 8+8 was first reported by Cava et al.

[1] and almost simultaneously by Subramanian et

al. [2]. The structure for 8 =0 and x = was

found to be orthorhombic, space group Cmmm,

lattice parameters a = 5.393 A « \/2ap, b =

5.431 A ~ N/2ap, and c = 15.773 A (ap being

the simple pervoskite parameter) and two

chemical formulate per unit cell. Subsequently,

Pb2Sr2 Y1

.xCaxCu3 8+a samples were characterized by electron microscopy by Zandbergen

et al. [3].

'

The structure of the prototype compound

Pb2Sr2YCu3 8 can be easily described as built

of (AO) and (B02

) layers arranged as to form

either perovskite or NaC£ blocks in [4]. The

sequence of the (AO) and (B02

) layers for

Pb2Sr2YCu3 8is as follows: (Y) (Cu02)c(SrO)

(PbO)c(Cu) (PbO)c(SrO) (Cu02)c (Y) (see fig.

1.) All oxygen atoms are absent in the (Y) and

the (Cu) layers. The subscripts o and c indicate

whether the cation is at the origin or at the

center of the mesh, respectively.

The structural refinements based on powder

neutron data in [5] revealed that the small

orthorhombic distortion is due to disordered

oxygen displacements in the PbO plane.

Electron diffraction patterns perpendicular to [001] showed that the crystallites are twinned by

the (110) planes, and that, in agreement with

Zandbergen in [3], very weak and well defined

(hkO) diffraction spots with h+k odd, which

violate the C-centering, are always present.

These spots define the unit cell with a = V^a^

b = ^2ap, and indicate that the space group is primitive and the small displacements of the

oxygen atoms in the PbO layers are actually

somewhat ordered [6]. Fu et al. [7] who studied

the isostructural compound Pb2Ba2YCu3 8 by

electron and neutron diffraction, obtained similar

results. By taking into account the reflections

YBa2 Cu3 6 ,

violating the C-centering, they determined from

the neutron diffraction data that the most

probable space group for Pb2Ba2YCu3 8 and

isostructural compounds is P22 : 2

As in the case of

Pb2Sr2YCu3 8 can take up oxygen, too. This

property was first discussed by Cava et al. in their original paper [1] and subsequently by

Gallagher et al. [8], O' Bryan et al. [9], and

Gyory et al. see [10]. No trace of superconductivity has been detected so far in any

oxidized Pb2Sr2YCu3 8+s samples. In order to understand this unique behavior we have carried

out thermogravimetric, x-ray, neutron, and

electron diffraction investigations of some

oxidized samples.

The neutron powder diffraction

measurements were made at room temperature

with the high-resolution five-counter

diffractometer at the NBSR. Neutrons of

wavelength 1 .553 A were used in the experiment

with in-pile, monochromatic beam, and

diffracted beam collimators having horizontal

angular divergencies of 10, 20, and 10 minutes

of arc, respectively. The same sample used for

the determination by neutron diffraction of the

Pb2Sr2YCu3 8 structure [5] was heat-treated at 500 °C in 2 and then cooled rapidly to room

temperature. It was placed in a vanadium can of

about 1-cm diam and the data were collected in the angular range 5°-120° in steps of 0.05°.

The structure was refined with the Rietveld

method [1] adapted to five-detector geometry and

modified to include background parameters [12].

The neutron scattering amplitudes used in the

calculation were b(Pb) = 0.94, b(Sr) = 0.702,

b(Y) = 0.775, b(Cu) = 0.772, and b(O) =

0.581 x 10" 12cm. The peak shape could be

described satisfactorily by Gaussian functions.

The initial atomic parameters of the structure were those of Pb2Sr2YCu3 8

[5] transformed to

27
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

those of the i, x i, x c cell. The refined

positional and thermal parameters are given in tables 1 and 2. It can be seen that the thermal

parameters for the split atoms are reasonable

except along the a-axis. The value of the 05

occupancy factor is 0.47(3) which corresponds to a chemical formula Pb2Sr2YCu3 9 47, in good

agreement with that found by thermo-gravimetry

(°9.40)

structure is shown in figure 1 One unit cell of the Pb2Sr2YCu3 9 47

Table 1. Refined positional, isotropic thermal (A )and occupancy parameters for Pb2

Sr2YCu3J W9.47-

Atom Position X y z B n

Pb 2t mm 1/2 1/2 0.3814(2) 3.5 1 1/2 Sr 2q mm 0.2204(3) 1.5 1 1/2 Y la mmm 0.74(13) 1/4 Cul lc mmm 0.5 3.5 1 1/4 Cu2 2t mm 1/2 1/2 0.1027(2) 0.53(8) 1/2 01 2t mm 1/2 1/2 0.2499(6) 5.4 1 1/2 023 8al 0.165(3) 0.3810(7) 5.1 1/2 03 2r mm 1/2 0.0918(2)

0.0918^

0.96(8) 0.962

1/2 031 2s mm 1/2 1/2 043 4x m 1/2 0.458(1) 6.5 1/4 05 2p mm 1/2 0.217(8) 1/2 0.954

0.117(3)

The thermal paramters of this atom have been refined anisotropically, the value reported in this column represents the corresponding isotropical B value. This value has been constrained to the corresponding value of 03.

These positions correspond to the refinements during which these atoms were split over the positions (Oyz) for 02

and (0 1/2 z) for 04.

Bqj was fixed at 0.95, value given by the previous refinement during which the 05 occupancy factor was fixed at the value

(0.10) obtained by the thermogravimetric analysis.

Table 2. Anisotropical thermal parameters* (A )af Pb, Sr, Cul, 01, 02, and 04 on Pb2

Sr2YCu3 9 47

^f

Atom B 11

5 22 >33

~?\

:i:@®#

: 'ssr 7*

Pb

SrCul

01

02

04

3.0(5)

-0.1(5) 4.3(7)

1.3 5.2(9)

15(2)

5.4(7) 2.0(3)

2.9(6) 1.8(3) 3.9(7) 2.3(5) 11(1) 4.0(5)

7(1) 2.7(6)

2(1) 2(1)

& ©^ ^

£>-

\Ml..

.©.

*For all atoms Bj 2 = Bj 3 = by symmetry.

References

[1]

[2]

13]

[4]

[5] [61

[7]

[81

19)

R. J. Cava, et al., Nature, 336, 211 (1988). M. A. Subramanian, etal., Physica C157, 124 (1989). H. W. Zandbergen, et al., Physica C158 , 155 (1989). A. Santoro, et al., Physica C156 693 (1988).

R. J. Cava, et al., Physica C157, 272 (1989).

E. A. Hewat, et al., Physica CI 57, 509 (1989). W. T. Fu, et al., Physica C159 , 210 (1989).

©<

$

a®

©

©

..©..

r *

4

tf •—!»•} S ®" '®" ®

.•••!"^®

- : 9^®

®

.;© @

v

&-

^-v

IT

*...*•; ©

©

.©- $*

Pb

t Sri Y 0c5 0, Pt4 irt YC«4 b,,,

P. K. Gallagher, et al., Chem. Matls. 1, 277 (1989). H. M. O'Bryan and P. K. Gallagher, Chem. Matls. 1, 526 (1989).

10] E. M. Gyory, ct al., Chem. Matls. 1, 571 (1989).

11] H. M. Rielvcld, J. Appl. Crystal. 2, 65 (1969).

Figure 1. One unit cell of the Pb2

Sr2YCu30^ 47

structure.

[12] E. Prince, Natl. Bur. Stand. (U.S.) Tech. Note 1117,

F. Shorten, ed. p. 8 (1980).

28
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

TWINNING AND OXYGEN DISTRIBUTION IN Pb2Sr2YCu3 8+x

J. J. Capponi and E. A. Hewat (Laboratoire de Cristallographie, CNRS, Grenoble, France)

R. J. Cava and M. Marezio (AT&T Bell Laboratories, Murray Hill, NJ)

and

A. Santoro

The scheme:

...[(Cu06) (Pb0)c (Sr0) o (Cu02)c(Y) (Cu02)c (SrO) (PbO)J (Cu06

) o

...

...[(Cu06) o (BaO)c (Cu02) o (Y)c(Cu02) o (BaO)J (Cu06) o...

is a comparison of the structure of

Pb2Sr2YCu3 8+x (first sequence) and

YBa2Cu3 6+x. The layer (BaO) present in the

1-2-3 compound is replaced by the bilayer

(SrO)(PbO) in the lead compound. The other

structural elements are the same in the two

systems. (In this scheme each layer is repre- sented by a chemical symbol representing the

composition of the layer, and by a subscript o or

c indicating if the cation is at the origin or at the

center, respectively, of the mesh characteristic of

the layer.)

The slice ...(Pb0)c (Cu0x)o (Pb0)c

... in the

structure of Pb2Sr2YCu3 8+x is represented in figure 1. Atom 02, 04, and 05 are split with

respect to their idealized, symmetrical positions.

For x = the 04 and 05 sites are empty. For

x = 1, only sites 04 are occupied with an occupancy of 50%. For x > 1 the sites 05

become gradually filled and the occupancy

reaches a maximum of 50% for x = 2. Sites

02 are always half occupied since this oxygen is not involved in the oxidation process.

The approximate bond distances given in figure 1 are for the composition x « 1.5 (25%

occupancy of the 05 sites).

Geometrical constraints impose a correlation

between the occupation of sites 04 and 02.

More specifically, if an oxygen atom 04 is located on a site above the plane of the copper

atoms, then the two sites 02 within the unit cell in the level above will be empty and those in the

level below will be full, and vice versa. This

situation results in a configuration up-down of

the 04 atoms along the b-axis of the structure. No correlation exists, however, along the a-axis.

Figure 1. Slice (PbO) (CuOx

) (PbO) in the structure of Pb2

Sr2YCu3 8+x.

The up-down configuration of the 04 atoms

results in a (short-range) doubling of the b-axis

and in the formation of square-planar Cu-O-,

chains along this axis. The arrangement of the oxygen atoms along the a-axis, however, can be

of two types, as indicated in figure 2 for the

composition x = 1, and does not impose a multiplicity greater than one in this direction (the

05 sites are empty for x = 1 and have not been

represented for clarity).

29
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Twinning in this system is present at all compositions. The proposed model of the twin

boundary structure for the compound

Pb9 Sr2YCu3 9is represented in figure 3. Full

circles represent the Cu atoms at z = 0, small

hatched circles are Pb atoms at z « + 1/8, full large circles are 04 oxygens atoms whose

location is represented with a + (above plane of

figure) or - (below plane of figure). The circles

outlined with broken lines are 02 oxygen atoms

located at z « 1/8 (dotted) or at z « -1/8 (clear).

This configuration is an extension of the model

of the twin walls proposed for YE^Ci^O-y . The lower part of the figure shows the

structure of the boundary in three dimensions.

Note that the 04 atoms at the wall have an updown configuration to avoid unrealistically short

oxygen-oxygen distances.

Pb A

Figure 2. Possible oxygen distributions in Pb^S^YCugOg.

ca

9£z^&

Q

i_\.;j_

; _...

< £

Figure 3. Model of the twin-boundary structure for x = 1.

30
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

INCORPORATION OF GOLD INTO YBa2Cu3 7

: STRUCTURE

AND Tc ENHANCEMENT

J. K. Stalick and J. J. Rhyne

and

M. Z. Cieplak, G. Xiao, and C. L. Chien (The Johns Hopkins University, Baltimore, MD)

Chemical doping has proven to be a powerful method in unraveling the crucial

aspects of high-Tc superconductors [1-3].

However, noble metals, particularly Au, have

been widely considered to be inert as far as high-Tc superconductors are concerned. The

successful utilization of gold as a buffer layer for

high-Tc films, gold-superconductor composites,

gold contacts, gold-tunneling barriers, etc., reinforces this belief. Also, gold crucibles have

now enjoyed the status of producing some of the

best high Tc single crystals to date. Such single

crystals have been subjected to intense research

in order to capture the fundamental properties

intrinsic to YBa2Cu3 7

. We have found, however, that Au does in fact substitute for Cu when Au powder is annealed with pure 1:2:3 powder according to the formula (YBa2Cu3 7) 1

. xAux

. A solubility

limit of 10 at.% Au (x = 0.1 in the above

formulation) was indicated by gravimetric weight

loss measurements, optical microscopy, and x- ray analysis of the lattice parameters. The 1:2:3

material remains orthorhombic upon Au substitution, with a significant increase in the c-lattice parameter. Neutron Rietveld refinement of data on a sample with x = 0.08, collected on the

high resolution five-detector powder diffractometer at NIST, showed that the Cu-0 plane

assembly is not affected by Au doping.

However, along the c-axis the bond length

between the Cu(l) chain site and the bridging

oxygen 0(1) increases significantly by 1.36%

compared to pure 1:2:3, while the Cu(2)-0(1)

distance decreases by 0.3%. Thus the bridging

0(1) atom is displaced toward Cu(2) while the

Cu(l)-Cu(2) distance increases by 0.4%.

Although the above changes in atomic position

suggest substitution by Au on the Cu(l) site, the

similarities in scattering lengths for Au and Cu

preclude direct determination using neutrons. Therefore, since the x-ray scattering factors of

Cu and Au are very different, the same sample

was analyzed using x-ray data collected on a Scintag diffractometer with a scattered beam

monochromator at NIST. Rietveld refinement of

the x-ray data showed conclusively that Au

occupies exclusively the Cu(l) site. The refined

occupancies of Au(l) = 0.084(4) and Au(2) =

- 0.005(3) are consistent with the 8% as- prepared concentration.

It has been established [3] in 1:2:3 that all the divalent dopants (Zn2+

, Ni2+

) substitute into

the Cu(2) plane sites, whereas all the trivalent

dopants (Al3 +

, Co3 +

, Ga3+) occupy the Cu(l)

sites with a concomitant increase in oxygen

content. The oxygen content of the Ausubstituted 1:2:3 as determined from the neutron

refinement was 7.04, consistent with Au3 +

. For

substitution in the Cu(2) planes, Tcis reduced

drastically, while for substitution in the Cu(l)

chains Tc in general is reduced more gradually

and the material becomes tetragonal. Gold

substitution is unique, however, in that the

orthorhombic distortion increases slightly with

increasing Au content, while Tc also

systematically increases by about 1 .5 K at x =

0.1. This benign effect of Au doping on

superconductivity is a very useful feature. The

presence of Au will change many other

properties such as lattice dynamics and some

normal state properties. Studies of the

superconducting behavior and the properties

affected by Au doping will enable a better understanding of their correlations.

References

[1] J. T. Market, et al., in Physical Properties of High

Temperature Superconductors I . D. M. Ginsberg,

ed., (World Scientific, Singapore, 1989).

[2] C. L. Chien, G. Xiao, M. Z. Cieplak, Oxygen

Disorder Effects in High T Superconductors . J. L. Moran-Lopez and I. K. Schullcr, eds. (Plenum.

New York, 1989).

[3j C. L. Chien, et al., Superconductivity and lis Applications , H. S. Kwok and D. T. Shaw, eds

(Elsevier, New York 1988).

31
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

MAGNETIC PHASE TRANSITIONS IN Sm2Cu04

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

(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 high-Tc superconducting oxides have attracted considerable interest both because of the intrinsic interest

in magnetism and because of the possibility that the magnetic fluctuations are responsible for the

Cooper pairing. It is clear now that the 2-d

sheets of Cu-0 in the (La2_

xSrx)Cu04 and in

/?Ba2 Cu3 6+x (R = rare earth) systems are

intimately involved in both the magnetism and

superconductivity, with magnetic long-range

order occurring at small x and fluctuating

moments existing in the superconducting regime

at larger x. The charge carriers are holes in these systems. Superconductivity has been

discovered in a new class of materials,

R2.xCexCu02.y (R = Pr, Nd or Sm), which are known as electron superconductors since the

charge carriers are electrons rather than holes.

The existence of these materials places important

constraints on any theory of the oxide superconductors. In this regard it is of central concern to elucidate the nature of the magnetism

in this new class of systems.

The magnetic properties of Nd2Cu04 and

Pr2Cu04

, which are parent materials of the

electron superconductors, have been studied

recently by using neutron diffraction techniques

[1-5]. The Cu ions in these compounds order

antiferromagnetically at higher temperatures

( ~ 250 K). Two other spin reorientations of the

Cu sublattice occur at 75 K and 30 K in Nd2Cu04

, and in addition there is a substantial

interaction between the Nd and Cu sublattices

[2]. However, these spin reorientations do not occur in Pr2Cu04 [5]. In the present work we

report neutron diffraction measurements on Sm2Cu04 which is also a parent material of

electron superconductors.

A thin plate-like single crystal weighing 66

mg was grown by mixing Sm03 and CuO

powders. Extra CuO was used as a flux to reduce the melting point of the starting

materials. The crystal was mounted in the (hhf)

scattering plane and the data were taken between

the temperatures of 0.75 and 300 K. The very

high neutron absorption for samarium made the

measurements quite difficult. Measurements were made at higher temperatures to investigate

the ordering of the Cu, and in a

3He cryostat to search for the Sm ordering.

A series of magnetic Bragg peaks were

observed below TN = 280 K, and can be

indexed as (h/2,k/2,l) based on the chemical unit

cell, where h and k are odd integers. Since the

first two Miller indices are half integers, the Cu

magnetic unit cell is double the chemical unit

cell along both the a and b directions. This

indicates that the magnetic spin configuration of

the Cu spins in Sm2Cu04

is the same as the

configuration found in Nd2Cu04 and Pr2Cu04

[1-5]. However, the spin directions are

different. For example, in Pr2Cu04

, and

Nd2Cu04in the high temperature ordered state, the (1/2,1/2,0) peak is found to have negligible

intensity, while in Sm2Cu04 a strong (1/2,1/2,0)

magnetic Bragg peak is observed. The

temperature dependence of that peak is shown in figure 1, and reveals a Neel temperature TN =

280 + 1 K. Our measurements do not indicate

any spin reorientations, in contrast to the

behavior found for the Nd2Cu04material.

Our low-temperature measurements did not show any significant change in the intensities of

(h/2,h/2,l) peaks, or development of new peaks

down to 0.75 K. Specific heat and magnetic

susceptibility measurements show that the Sm

orders antiferromagnetically at 5.95 K [6]. If the Sm ions in Sm2Cu04 order like the Nd ions

in Nd9 Cu04

, then the Sm moment must be less than 0. 1 \i B

. However, it is more likely that the

Sm ions order in a different configuration than

the Nd ions in Nd2Cu04

, and measurements in other scattering planes will be done to investigate

this possibility.

32
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Sm2 Cu04

>

•M

c

CD

C

Hi

(1/2,1/2,0) :

i

I

.... i .... i .... i . ,

Jx - \X

X

1 ^xT33 ? x' 50 100 150 200 250

Temperature (K)

300

Figure 1. Integrated intensity of the (1/2,1/2,0) magnetic Bragg peak as a function of

temperature. The intensity increases with decreasing temperature below the Neel

temperature of 280 K.

References

[1] S. Skanthakumar, H. Zhang, T. W. Clinton, W.-H.

Li, J. W. Lynn, Z. Fisk, and S.-W. Cheong. Physica

C160. 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,

4530 (1990).

[2] J. W. Lynn, I. W. Sumarlin, S. Skanthakumar, W.-H.

Li, R. N. Shelton, J. L. Peng, Z. Fisk, S.-W. Cheong, Phys. Rev. B41, 2569 (1990).

[3] Y. Endoh, M. Matsuda, K. Yamada, K. Kakurai, Y. Hidaka, G. Shirane, and R. J.Birgeneau, Phys. Rev. B40, 7023 (1989).

[4] D. E. Cox, A. I. Goldman, M. A. Subramanian, J. Gopalakrishnan, and A. W. Sleight, Phys. Rev. B40 , 6998 (1989).

[5] P. Allenspach, S.-W. Cheong, A. Dommann, P. Fischer, Z. Fisk, A. Furrer, H. R. Ott, and B. Rupp,

Z. Phys. B77, 185 (1989).

[6] See for example, M. F. Hundley, J. D. Thompson,

S.-W. Cheong, Z. Fisk, and S. B. Oseroff, Physica

C158, 102 (1989). Y. Dalichaouch, B. W. Lee, C.

L. Seaman, J. T. Markert, and M. B. Maple, Phys. Rev. Lett. 64, 599 (1990).

33
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

MAGNETIC ORDERING OF Nd IN (Nd-Ce)2Cu04

J. W. Lynn, I. W. Sumarlin, S. Skanthakumar, and W.-H. Li

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

R. N. Shelton and J. L. Peng (University of California, Davis, CA)

Z. Fisk (Los Alamos National Laboratory, Los Alamos, NM)

S.-W. Cheong (AT&T Bell Laboratories, Murray Hill, NJ)

The microscopic magnetic properties of

superconductors have been of interest for many

years, first for "magnetic superconductor"

systems [1], then in heavy fermion materials [2],

and most recently in the oxide superconductors

[3]. In magnetic-superconductors such as 7?Mo6S8 and ^*Rh4B4 (R = heavy rare-earth

element), the R ions are physically isolated in the

crystal structure from the superconducting

electrons. This feature leads to a major role for

the dipole-dipole interaction as reflected in the

very low magnetic ordering temperatures. The

magnetic and superconducting order parameters

are coupled via the electromagnetic interaction.

The heavy fermion systems, on the other hand,

do not have an "isolation" of the magnetic and

superconducting subsystems, and in fact the

magnetic fluctuations are likely responsible for

the superconducting pairing [2]. The oxide

superconductors provide the interesting situation

of having both heavy rare earths which are often

isolated in the lattice and order at low temperatures, and strongly coupled Cu spins which

reside in the Cu-0 superconducting planes.

We have been investigating the magnetic

properties of the recently discovered Nd2Cu04

class of electron superconductors, which of

course possess both rare earth and Cu spin

subsystems. In the insulating phase the Cu spins

order at relatively high temperatures ( ~ 245 K)

[4-6]. At low temperatures the Nd ions have

also been observed to order magnetically [4,6,7].

We have carried out neutron scattering experiments to characterize the nature of this ordering,

and have found a simple antiferromagnetic

arrangement of spins. For the superconducting

material a sharp N6el transition is observed, but

in the parent insulating compound the transition

appears "smeared" due to substantial coupling

with the ordered Cu sublattice. Hence the rare

earth and Cu sublattices cannot be isolated

electronically. In the superconducting system,

where the Cu spins are disordered, it may be

particularly interesting to determine the effect of

the Nd ordering on the superconducting

properties such as the critical fields, since this could provide an important clue to the origin of

the superconducting pairing depending on

whether the magnetic ordering hinders or enhances the superconductivity.

Unpolarized diffraction data were taken on

polycrystalline samples of Nd2Cu04 and

Ndx 85Ce 15 Cu04

, and a 20-mg single crystal of

Nd2Cu04

. Figure 1 shows the temperature

dependence of the magnetic intensity for the

(V2V23) Bragg peak. For the superconducting

sample (top) a well-defined transition at 1.2 K is observed, which is in good agreement with the

specific heat data [7]. Note that there is a small

amount of rounding of the intensity in the

vicinity of the Neel temperature, which is typical

for second order transitions and likely originates

from critical scattering. The lower portion of

the figure shows the temperature dependence of

the same reflection for the Nd2Cu04 powder

sample. In this case it is difficult to determine

the N£el temperature with accuracy since the

scattering appears to be "smeared" over several

degrees. Such a "smeared" order parameter

might be caused by sample inhomogeneities and

a consequent distribution of N6el temperatures,

but this interpretation is inconsistent with the

sharp transition for the superconducting sample,

and with the behavior observed for the single

crystal sample. We remark that the signalto-noise ratio for the single crystal data is dramatically improved compared to the data

obtained on powders, and we have taken

extensive temperature dependent data on a large

number of Bragg peaks.

To understand these data it is essential to

recall that the Cu spins have already ordered at 245 K in this sample, and that the Cu magnetic

Bragg peaks occur at the same positions as for

the Nd peaks. Hence the intensities of these

peaks represent a coherent addition of the Cu

34
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

and Nd order parameters. At high temperatures

where only the Cu spins are ordered the intensity

is of course directly related to the Cu order

parameter, while at low T the Nd contribution

dominates since the Nd moment becomes much

larger than Cu and there are twice as many Nd

as Cu. At 75 K an abrupt spin reorientation of

the Cu spins occurs, in which the Cu spins rotate by Till. Another spin reorientation occurs at 30

K in which the Cu spins rotate again by rc/2

[4,5]. The single crystal data (not shown)

indicate a continuous increase in the intensity of

the (V£V&3) peak below 30 K. Since the ordered

Cu moments are essentially saturated at these

low temperatures, we believe that the intensity

change below 30 K is due to the polarization of

the Nd sublattice.

Powder a/2.1/2.3]

o

\en +J

c

D

o

CJ

3500

3250

3000 -

2750 -

2500

I I I I I l"I'T a)

%& lilli

1 1 1 1 1 1 7000

1

i—1—1—1—

1—1—1—1—1—1—1—1—r—1—

r

psi I

IT) \W

-P

C

D

O

CJ

6000 -

5000 -

4000

3 4 T

b)

Nd2Cu04

I

1

!

1 1 1

ii

5 iii

Temperature (K)

Figure 1. Magnetic intensity for the (WA3) Bragg peak.

The top portion of the figure shows the data obtained on the Ndj g5Ceg 15C11O4 sample, where a well defined Neel

temperature of ~ 1 .2 K is found. The lower portion of the

figure shows the data for the I^^CuC^ sample, where the

intensity is seen to be spread over a large range of

temperature.

In the present system nearest-neighbor

interactions between the Cu and Nd sublattices

cancel. Next-nearest-neighbor and thirdneighbor interactions, however, do not cancel,

but instead produce a staggered field which

polarizes the Nd sublattice antiferromagnetically

,

and this polarization field has the same symmetry as when the Nd ions order. Hence a net sublattice magnetization of the Nd is induced

at elevated temperatures. The Nd-Cu interaction

may also be the cause of the second spin

reorientation observed at 30 K. In the Ce-doped

superconducting material the Cu spin system is not magnetically ordered, and hence we do not see any effect on the Nd sublattice. However,

when the Nd sublattice orders at low temperatures, we might expect a reverse polarization on

the Cu sublattice. If the Cooper pairing is mediated by magnetic interactions, for example,

then the induced magnetic order of the Cu spins

might quench the superconducting state. We

believe it would be particularly interesting to measure the superconducting properties, such as the critical fields, to determine whether such a polarization enhances or hinders the superconducting pairing.

References

[1] For a general review see Topics in Current Physics . 0. Fischer and M. B. Maple, eds. (Springer, New

York, 1982), Vols. 32 and 34.

[2] See, for example, G. R. Stewart, Rev. Mod. Phys.

56, 775 (1984). Z. Fisk, et al., Science 239, 33

(1988).

[3] For a general review see High Temperature Super- conductivity , J. W. Lynn, ed. (Springer, New York,

1990).

[4] S. Skanfhakumar, H. Zhang, T. W. Clinton, W-H. Li,

J. W. Lynn, Z. Fisk, and S.-W. Cheong, Physica

CI 60, 124 (1989). S. Skanthakumar, H. Zhang, T. W. Clinton, I. W. Sumarlin, W-H. Li, J. W. Lynn,

Z. Fisk, and S-W. Cheong, J. Appl. Phys. 67, 4530

(1990).

[5] Y. Endoh, M. Matsuda, K. Yamada, K. Kakurai, Y. Hidaka, G. Shirane, and R. J. Birgeneau, Phys. Rev. B40 . 7023 (1989).

[6] J. W. Lynn, et al., Phys. Rev. B4J., 2569 (1990).

35
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON DIFFRACTION ON ANNEALED YBa2Cu3 6 28

C. H. Lee, Q. Lin, J. W. Lynn, and S. Green

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

and

A. Santoro

The flBa2Cu3 6+x (R = rare earth)

compounds have a layered perovskite-like

structure in which there are three copper-oxygen

layers stacked along the (unique) tetragonal

c-axis. Two of these layers have oxygen ions

between the copper in both the a and b

crystallographic directions. These are the

so-called "plane" layers, and the oxygen in these

layers cannot be removed. The third layer has

oxygen ions only along the b axis, and is thus

called the "chain" layer. The oxygen concentration can be readily varied in this layer from

full occupancy (x = 1) to complete depletion

(x = 0), and both the magnetic and superconducting properties are very sensitive to the

oxygen concentration x in the "chain" layers. In

the small x regime, we have a tetragonal anti- ferromagnetic insulator; while for x ^ 0.4,

we have an orthorhombic superconductor with

a « b and c « 3a [1]. The magnetic properties in the small x

regime, where the materials are tetragonal and

semiconducting, have proved to be very

interesting. The Cu "plane" layers order

antiferromagnetically with a Neel temperature

TN

as high as 500 K. At lower temperatures the

spins on the Cu "chain" sites have also been

observed to order antiferromagnetically [2]. However, this lower phase transition is not always observed, and clearly depends on more

than just the average oxygen concentration. At

high x it has been found that the superconducting

properties depend on the chemical ordering of

the oxygen ions in the chains as well as the

average oxygen stoichiometry [3-4], and we

suspect that oxygen ordering may be playing an

important role in the magnetic properties at small x as well. Consequently we have initiated diffraction experiments to ascertain the effect of

oxygen ordering on the Cu magnetic phase

transitions.

A fully-oxidized 30-g polycrystalline sample was prepared by the usual solid state reaction

technique, and then was partially reduced by

heating to 600 °C in pure Ar. Oxygen ordering was promoted by cooling very slowly, as opposed to quenching. Thermogravimetric

analysis yielded an average oxygen concentration

of 6.24. The neutron powder diffraction

patterns were taken at 77 K on the BT-1 high

resolution powder diffraction instrument. Profile

refinements of the data were made using two

different models. Model- 1 assumed a tetragonal

structure, with random occupancy of the oxygen

chain sites. This model gave a reasonable fit to the data, although there were some clear

discrepancies between the data and the model.

The weighted R-factor (Rw) was 12.0, which was not particularly good, while the thermal

factor (B) of the chain-layer oxygen was

unreasonably large.

The second model (model-2) allowed for an orthorhombic distortion of the lattice, as well as for an ordering of the oxygen chain ions. The

specific order which we assumed was that the

occupancy of adjacent chains was unequal.

Allowing for the orthorhombic distortion

improved the fit substantially, with the weighted

R-factor decreasing from 12.0 to 9.0. We also

observed a strong tendency to make the oxygen

occupancies different, with a occupancy ratio

which converged to —4.0. However, the

overall improvement in the fit associated with

the oxygen ordering was only marginal, and it is not clear whether this oxygen ordering model is appropriate at these smaller values of x. The

final value of the oxygen concentration we

obtained was x = 0.28. The refined structural parameters for the remainder of the atoms were

identical within experimental error for the two

models.

We have also carried out preliminary

experiments to observe the nature of the

magnetic ordering in this annealed sample. The

plane-ordering peaks were found to be very

weak, as expected at the higher x since it has

already been observed that the ordered moment

decreases rapidly with increasing x [5]. In

36
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

addition, no chain-ordering peaks were observed

at any temperature. Further work is in progress

as a function of x and annealing condition.

References

[1] A review of both theory and experiment that pertains

to the oxide superconductors is given in High

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

[2] See J. W. Lynn, W-H. Li, H. A. Mook, B. C. Sales and Z. Fisk, Phys. Rev. Lett. 60, 2781 (1988); W.-H. Li, J. W. Lynn, and Z. Fisk. Phys. Rev. B41 . 4098 (1990), and references therein.

[3] R. J. Cava, B. Batlogg, C. H. Chen, E. A. Tietman,

S. M. Zahurak, and D. Werder, Phys. Rev. B36 . 5719 (1987).

[4] For a review, see R. Beyers and T. M. Shaw, Solid

State Phys. 42, 135 (1989).

[5] J. M. Tranquada, A. H. Moudden, A. I. Goldman, P. Zolliker, D. E. Cox, G. Shirane, S. K. Sinha, D.

Vaknin, , C. Johnston, M. S. Alvarez, and A. J. Jacobson. Phys. Rev. B38, 2477 (1988).

TEMPERATURE DEPENDENCE OF THE TWO-DIMENSIONAL MAGNETIC

CORRELATIONS OF Er in ErBa2Cu3 7

T. W. Clinton and J. W, Lynn (University of Maryland, College Park, MD

and Reactor Radiation Division)

J. Z. Liu 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 interest both because of the magneticsuperconductor interactions, and because of the

2-d magnetic behavior these materials display

[1-3]. In the RBa2Cu3 7 (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. As could be

expected, the first neutron experiments on ErBa2Cu3 7 observed this 2-d behavior with

powder samples [1], and later with a single

crystal [2], where excellent quantitative

agreement with a 2-d Ising model was obtained.

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 TN =

0.618 K. Above this 3-d N6el temperature a rod

of scattering along (1/2,0, £) characteristic of the

2-d behavior has been observed, showing that the magnetic interactions of the rare-earth ions

are highly anisotropic; below TNthe order

parameter follows the exact Onsager solution for a 2-d Ising antiferromagnet.

Recently, we have made additional measure- ments on a single crystal of ErBa2Cu3 7 to determine the temperature dependence of the 2-d

rod of scattering. Above the ordering

temperature this scattering originates from

critical fluctuations, while below TN

it originates

from low-energy spin-wave excitations. In the

quasielastic approximation this diffuse scattering

is proportional to the wave-vector-dependent

susceptibility. For a 2-d system in the OrnsteinZernike approximation

Kq)diffuse X(q) « (k2 + qx

2 + qyy (1)

where k is the inverse correlation length within

the planes. Thus in a 2-d system we expect to see critical scattering about the reciprocal lattice positions as in the 3-d systems, with the

important difference that in the 2-d case the

critical scattering should be independent of qz

, so one should see a rod of scattering. Therefore by

37
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

observing the rod of scattering for various

temperatures above and below TN , the temperature dependence of both the susceptibility and the

2-d correlation length can be determined.

We have measured the temperature

dependence of the rod intensity at a point on the

rod away from where 3-d magnetic Bragg peaks,

indexed by (1/2,0,^/2), arise. The temperature

dependence of the rod point (1/2,0,0.7) is shown

in figure 1. The rod starts to develop below

about 1 K, and the intensity steadily increases as the temperature is decreased until TN

is reached.

Below TNthe intensity decreases rapidly. The

sharp decrease in the rod intensity below TN

is due to the rapid decrease of the spin wave

population as anticipated for an Ising system.

The full width at half maximum (FWHM) of

the intensity in (1) is equal to twice the inverse

2-d correlation length k. Therefore to observe

the 2-d magnetic correlations in the system we

took scans across the rod at various temperatures. In figure 2 the linewidth (FWHM) of the

rod is shown as a function of temperature.

Because of the Gaussian experimental resolution

the scans across the rod are the convolution of a Gaussian function and the intrinsic Lorentzian

function in (1). Therefore, the data were

deconvoluted using a Gaussian resolution

function and a Lorentzian to produce the

intrinsic Lorentzian linewidth. in figure 2. The

graph indicates that as TNis approached from

above, the 2-d correlation length grows until it diverges at TN, where long-range magnetic order

sets in. Below TN

the measurements are

resolution limited.

o

en

-4-> c

zso

u>

-p

r-i IS) c

CD

o

1300

1200

1100 -

1000 -

900

800h

700

BOO

500

400

300

200

100 I 2

ErBa2 Cu 3 7 (1/2.0,0.7)

0.5 0.6 0.7 0. Temperature (K)

0.9

E nBa2 Cu 3 7

*ra 0.07 •

O 0-06

•

en -u 0.05

r-i § 0.04 . 5 0-03

"D5 0.02 .•• £ 0.01

'

'

1-1 -I ,i i . -i * i i i 0.5 0.54 0.58 0.62 0.66 Temperature (K)

.70 0.74

Figure 2. Temperature dependence of the intrinsic linewidth (FWHM). The linewidth decreases from some

finite initial value down to Tj^, where it reaches a

resolution limited minimum as long range order develops.

Figure 1. Rod intensity as a function of temperature. The

intensity is seen to peak at Tj^ - 0.62 K where 3-d long range order sets in.

References

[11 J. W. Lynn, W.-H. Li, Q. Li, H. C. Ku, H. D. Yang,

and R. N. Shelton, Phys. Rev. B36, 2374 (1987).

[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] A recent review of the oxide superconductors is given

in Hi^'h Temperature Superconductivity , J. W. Lynn,

ed. (New York: Springer-Verlag, 1990).

38
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

FIELD DEPENDENCE OF THE MAGNETIC PHASE TRANSITIONS IN Nd2 Cu04

S. Skanthakumar and J. W. Lynn

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

and

Z. Fisk (Los Alamos National Laboratory, Los Alamos, NM)

The magnetic properties of the Nd2Cu04

class of electron superconductors have been

studied recently by neutron diffraction techniques

[1-5]. Long range antiferromagnetic order of the

Cu moments develops in Nd2Cu04

at TN = 245

K, with a simple spin configuration in which

nearest-neighbors within the Cu-0 planes are

antiparallel. The coupling between the layers, on the other hand, cancels to a first approximation, and the delicate balance of interlayer

interactions appears to lead to a rich behavior as a function of temperature and field. In

particular, there are two spin reorientations

which occur at 75 and 30 K in this material.

These spin reorientations have not been observed

in the sister compounds Pr2Cu04 and Sm2Cu04

[5]. In addition, there is a substantial interaction

between the Cu and Nd sublattices, and the Nd

sublattice orders [4] at TN = 1.5 K.

Both noncollinear [1] and collinear [2]

magnetic structures have been proposed for this system, and it is not possible to distinguish

between them with the present neutron

diffraction data. In the noncollinear magnetic

structure [1], the spins within the Cu-0 layers are collinear and nearest neighbors are

antiparallel, but the spin direction between layers

is rotated by nil, and hence the spins point in different directions (i.e., they are noncollinear).

The symmetry of the magnetic system in this case is tetragonal. In the fully collinear spin

configuration, the spins within the layers have

the same antiparallel arrangement of nearest- neighbor spins, while the spin directions of

adjacent layers are collinear rather than

noncollinear. The magnetic symmetry in this case is orthorhombic. The two structures are

related in that the noncollinear configuration can be thought of as the coherent addition of two

separate domains of the collinear structure. The

magnetic Bragg intensities calculated for a single

domain are different for the two cases, but for a multidomain sample in which the domain populations are equal the domain-averaged intensities

are identical. Indeed the data obtained by us

[1,3] and by Endoh et al. [2] are in excellent

agreement, and with the present data it is not

possible to distinguish the two spin configurations. It may be possible to prepare a sample

with unequal domain populations by the application of a magnetic field, or by applying stress. However, even in this case such experiments

may not provide an unambiguous answer. For

example, a similar situation has occurred for the

spin structure of pure Nd [6], and this magnetic

structure has been the subject of debate for many

years. Here, we report our preliminary field dependent neutron diffraction measurements for

Nd2Cu04

. The diffraction data were taken with a wavelength of 2.3457 A and a pyrolytic graphite

monochromator and filter at the BT-2 triple-axis

spectrometer. A split-coil superconducting

magnet with a vertical field capability of 7 Tesla was used for these measurements. The crystal was mounted in the (hhl) scattering plane, with

the field applied vertically, that is, in the [hhO]

direction.

The temperature dependence of several

Bragg peak intensities was observed as a function of magnetic field. The data revealed a small but clear shift in the spin reorientation

transition temperatures to higher magnetic fields. The temperature dependence of the (1/2,1/2,5)

Bragg peak intensity in a 7 Tesla field is shown

in figure 1. Since the magnetic Bragg peak

intensities change abruptly at these transitions the

small shift in the transition temperature can be

easily noticed. Both spin reorientation transition

temperatures, which are 75 K and 30 K for zero magnetic field, increase with magnetic field at the rate of about 1.4 and 0.7 Kelvin/Tesla,

respectively. These shifts for a 7 Tesla magnetic

field can be easily noticed in figure 1 . We were

unable to map the complete phase diagram due

to the inaccessibility of obtaining the very high

fields needed. Since the peak intensities at higher temperatures (^ 200 K) are small and

39
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

c

-H

LO

Nd 2 Cu04

1400

1200

if) +J 1000 - c

I]

q 800

600

20 40 60 80 100

Temperature (K)

Figure 1. Temperature dependence of the (1/2,1/2,5) Bragg peak intensity with a 7 Tesla applied magnetic field. The data reveal sharp spin reorientations, which are found to increase with increasing applied field.

change slowly with temperature we were not

able in these initial studies to observe any

significant change in the highest transition

temperature (245 K) phase boundary.

The intensities of some nuclear Bragg peaks were also checked as a function of magnetic field

at 100 K, and those measurements showed no

indication of a measurable induced ferromagnetic

component. Measurements at lower magnetic

fields showed that the domain populations were

strongly affected by modest magnetic fields in the range below 1 Tesla, above which no change was observed in the domain population. Further

experiments on this effect and on the field dependence of the higher order transition

temperature (245 K) will be done using a larger

crystal.

References

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

Physica C160 , 124 (1989).

[2] Y. Endoh, M. Matsuda, K. Yamada, K. Kakurai, Y. [5] Hidaka, G. Shirane, and R. J.Birgeneau, Phys. Rev. B40, 7023 (1989). [6]

[3] S. Skanthakumar, H. Zhang, T. W. Clinton, I. W.

Sumarlin, W.-H. Li, J. W. Lynn, Z. Fisk, and

S.-W. Cheong, J. Appl. Phys. 67, 4530 (1990).

J. W. Lynn, I. W. Sumarlin, S. Skanthakumar, W.-H. Li, R. N. Shelton, J. L. Peng, Z. Fisk, S.-W. Cheong, Phys. Rev. B41., 2569 (1990).

S. Skanthakumar, J. W. Lynn, J. L. Peng, and Z. Y.

Li, (to be published). See, for example, E. M. Forgan, E. P. Gibbons, K. A. McEwen, and D. Fort, Phys. Rev. Lett, 62, 470

(1989), and references therein.

40
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

TWO-DIMENSIONAL MAGNETIC ORDER IN SUPERCONDUCTING DyBa2Cu4 8

H. Zhang, J. W. Lynn, W.-H. Li, and T. W. Clinton

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

and

D. E. Morris (Lawrence Berkeley Laboratory, Berkeley, CA)

The magnetic properties of the oxide

superconductors have been studied because these

materials are ideal systems to investigate the

coupling between the rare-earth magnetic and

superconducting order parameters. We have

been systematically investigating the nature of

the rare-earth ordering in the RB2^Cu3 6+x

systems, and have found [1,2] that they are

prototypical 2-d magnetic systems. Recently, we

reported our experimental work on the

7?Ba2Cu4 8 class of materials, which is similar

to the 1-2-3 system except that the (orthorhombic) c-axis is approximately twice as long,

with two rare-earth ions in the chemical unit cell

[3]. We find unambiguous evidence for 2-d

behavior in both the ErBa2Cu4 8 and

DyBa2Cu4 8 systems. Strikingly, in the case of

DyBa2Cu4 8

, the sample did not order 3-d even

at temperatures far below TN

as shown in figure 1. Here the solid curves are a least- squares fit to the scattering expected for a 2-d

ordered system, with no correlations between

layers, and this is seen to provide an excellent

description of the data. The lack of any 3-d order in the

DyBa2Cu4 8 material is quite interesting. In the

"conventional" 2-d systems such as K2CoF4 [5],

K2NiF4 [6], and more recently ErBa2Cu3 7

[2],

the 2-d and 3-d ordering occur at (essentially)

the same temperature. The reason for the close

proximity of the two transitions is that when the

system develops a static moment in the layers,

then there is an energy ± JA2 between the

layers, where J is the exchange interaction

between planes and A is the average size of a domain in the layer. The - sign is for layers that are properly matched (e.g., antiferromagnetic if J < 0) and + is for layers that are out of phase.

Thus even if the interlayer coupling is very

weak, there is an energy difference 2JA"

between the "correct" and the "wrong" spin

configurations, which can be quite large since A

is large. Hence the layers tend to order along

the c-axis producing 3-d Bragg peaks.

DyBa2 Cu4 0<

en co +J in c ^; o 7

d in -7 O

DyBa2 Cu4 £

Scattering Angle 2G (cleg.

Scattering Angle 29 (deg.)

Figure 1. Magnetic scattering intensity for DyBa2Cu40g at T = 0.05 K. The solid curves are the result of a fit to the theoretical scattering expected for a purely 2-d magnetic system; (left) the scattering profile for the (

xh*h) peak; (right) the scattering profile for the (1/2,3/2) peak.

41
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

In the present DyBa2Cu4 8 system the

nearest neighbor spins within the a-b plane form

a simple antiferromagnetic arrangement as shown

in figure 2. In the next layer up along the c- axis, all the spins are displaced by b/2. For the

closest spins between layers, we will have two

+ spins, and two - spins, and the net interaction

is zero by symmetry [4]. Next neighbor inter- actions also cancel, and, in fact, all the (point)

interactions sum to zero by symmetry. Hence,

this is an example of a fully frustrated spin

system for the interlayer coupling. In addition,

we have an intrinsic interlayer interaction which

is very weak, rendering the effective interlayer

interaction exceedingly weak; in fact DyBa2Cu4 8

is presently the best known

example of a 2-d magnet. In the ErBa2Cu4 8 system this cancellation does not occur because

of the chain-like ordering in the a-b plane, while

in the 1-2-3 systems it does not occur because

adjacent layers are not displaced b/2. Finally,

we note that if the general similarities between

the magnetic structures of the 1-2-3 and 2-4-8

systems hold for the remainder of the

^Ba2Cu4 8 materials, then a similar cancellation

of the net interactions along the c-axis can be

expected since most of the fiBa2Cu3 6+x

compounds which have been investigated so far

[1] exhibit the same in-plane nearest-neighbor

antiferromagnetic spin configuration. A possible

exception to this trend may occur for Gd, where

the single-ion anisotropy is small and an Ising

model is likely not appropriate. The spin

frustration in the 2-4-8 system might then be

alleviated by forming a noncollinear structure. Neutron scattering experiments are planned in the near future to investigate this possibility.

0- e

xQi ©•

e- •e

eFigure 2. 2-d magnetic spin configuration for the Dy

moments within the a-b layers in DyE^Cu^Og. Adjacent

layers along the c-axis are uncorrelated even well below the 2-d ordering temperature.

References

[1] J. W. Lynn, W.-H. Li, Q. Li, H. C. Ku, H. D. Yang,

and R. N. Shelton, Phys. Rev. B36, 2374 (1987).

[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, and T. W. Clinton,

Phys. Rev. B4i, 11229 (1990).

[4] It is easiest to think of a sum of exchange interactions. The dipole-dipole interactions also cancel if the moments point along the c-axis (which we believe to be appropriate for DyBa9 Cu4

Oo), or in fact if they lie anywhere within the a-c plane. They also cancel if they point along the b-axis.

[5] H. Ikeda and K. Hirakawa, Solid State Commun. j_4, 529 (1974).

[6] R. J. Birgeneau, H. J. Guggenheim, and G. Shirane,

Phys. Rev. Bl, 2211 (1970).

42
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Bi2Sr2Mn06+y : A 2-D MODULATED FERRIMAGNET

P. F. Miceli, J. M. Tarascon, K. Remschnig, and G. W. Hull (Bellcore, Red Bank, NJ)

and

W. R. McKinnon (National Research Council, Ottawa, Canada)

This compound is an isomorph of the high

Tc compound containing Cu-both have a

structural modulation parallel to the layers,

giving a large unit cell. By substituting Mn for

Cu, we can use the magnetic ion to directly

probe the relationship between structure and

magnetism (and therefore the electronic

properties as well). Since the magnetic layers are far separated (-12 A), the magnetic

behavior is expected to be two dimensional.

Our preliminary neutron scattering work,

using the BT-9 and BT-2 triple-axis

spectrometers at NIST, on both powders and

single crystals have shown novel magnetic

behavior. The powder diffraction suggests that the magnetically ordered state is a simple

antiferromagnet with the spins along the c-axis.

However, investigation of the crystal shows that a single magnetic domain orders with wavevector perpendicular to the structural modulation

direction when cooled in zero field and a second

magnetic domain, ordered along the structural modulation direction, evolves as a magnetic field

is applied. In addition, weak magnetic satellites are observed, indicating that the

antiferromagnetic sheets have modulated

magnetic moments. These results suggest that the structural modulation induces weak

ferromagnetism and that the field induced

transition arises from the way in which the

magnetic layers stack: antiferromagnetic at zero

field and ferrimagnetic at high field. This work is the first demonstration of a valence modulation of the cations in this general

class of compounds and it is important to understand how the relationship between the

modulated structure and electronic properties

affect the Cu based superconducting isomorphs.

Work in progress is focusing on the detailed nature of the magnetic moment modulation as well as exploring the 2-d aspects of this system.

43
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

A THEORETICAL MODEL FOR THE TUNNELING-GAP

ANISOTROPY OBSERVED IN LAYERED COPPER-OXIDE

HIGH-TEMPERATURE SUPERCONDUCTORS

R. C. Casella

The gap anisotropy and the ratios of these

gaps to the critical temperature observed in single-crystal high-Tc superconductors are explained within a model in which double

interplanar boson exchange takes place between

fermions lying in distinct bands of widths Bc and

Bab satisfying Bc <2o> < Bab , where co is the

cut-off energy of the (unspecified) intermediate

boson. For single-crystal Bi2Sr2CaCu20g where

the observed gap ratio r * Aab /A c « 2 and

2A ab /kTc « 6, I find solutions with co > 0.3 eV

and an extremely narrow width, 60 meV <. Bc

<, 200 meV for the band associated with overlap

along the c-axis, assuming o> < 5 eV.

Theoretical constraints on the parameter values

allow an experimental test. Recent tunneling experiments on singlecrystal Bi2Sr2CaCu2 8 imply an approximate

two-to-one anisotropy of the superconducting

gaps, depending upon whether the current is injected into the ab plane or along the c-axis,

with 2Aab/kTc = 6.2 + 0.3 and 2Ac/kTc = 3.3

± 0.5 [1]. A similar gap anisotropy has been

observed in preferentially oriented films of

YBa2Cu3 7

. Tunneling measurements on

polycrystals of T£, Bi, and Y-123 layered Cu02 compounds yield 2A/kTc = 6. 1 + 0.5, quoted as

a common result, presumably for the larger gap

when one assumes single-crystal gap anisotropy.

These data are considered in terms of a two-band

model involving interplanar high-energy (o > > Wphonon) intermediate bosons (IB)

exchanged between carriers in the two bands,

labeled C and D, of widths Bcand BD, respectively. It is further assumed that the

maximum energy w of the exchanged IB satisfies the inequalities Bc < 2co < BD

. A general

two-band, two-gap model of this type, not

specifically aimed at producing anisotropy with

respect to the crystal c-axis, was considered

earlier by this author [2]. Later, in an attempt

to understand the dependence of the critical temperature Tc (n) on the number n of closely

spaced Cu02 layers characterizing several of the

high-Tc T^ and Bi compounds, an average over

possible multiple gaps was taken for each n. This led to a reasonably good accounting of the

saturation in the increase of Tc (n) with

increasing n [3,4]. In the present analysis the narrow band C is identified in the tight-binding sense with a band whose width results mainly

from orbital overlap along the c-axis, while band

D is associated with orbitals which overlap

mainly in the ab plane. That is, Bc = Bc and

BD = Bab . Detailed consideration of the

energetics of the IB exchange at the single

fermion level leads to an interpretation whereby

coherent double IB exchange takes place between

the carriers in the two bands. The mechanism

involves off-shell propagation of the narrow- band fermion between the emission and

absorption of two IBs connecting the narrow- band fermion with each of two fermions in the

wide band. This interpretation follows

essentially from consideration of energy

conservation together with the assumption that interband transitions of the fermions can be

neglected. As in the original BCS theory where

the phonon coordinates are integrated out leading

to an effective fermionic pair interaction, here

the coordinates of the two IBs connecting the

pair constituents in one band via the

intermediary odd fermion in the other band are

integrated out along with the coordinates of the

third fermion. This unfamiliar pairing

mechanism follows ultimately from the

assumption that the IB energy gj can exceed the

width of the narrower of the two bands. It can be extended to apply reciprocally to both bands.

Phenomenologically it allows gap/Tc ratios much

larger than 3.5 (e.g., 2Aab/kTc > 6) within weak

coupling. Yet it places constraints on the IB

energy as well as on the allowed values of the

gap anisotropy for a given value of 2 A ab/kTc

. While the IB is yet to be identified either

experimentally or by the theory, the model may

provide a clue to the experimentalists as to where to look for the elusive boson. The model

44
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

already excludes <o < 0.3 eV for the Bi

compound measured in reference 1

.

Theoretically, I find

kTc = 1.13Qexp((-l/g)[l+(^ln7?)2

] 1/2

) (1)

and

(l/r)

m(2AD

/kTc

) = 3.54 exp I\ (2)

where

T=(l/g)([l+(-^nR)2

] m-[\+(-^nR/r)2

] m). (3)

Here Q = (a>Bc

/2)

1/2 and R = 2<o/Bc

. o> and

Bc/2 are the cut-off energies in BCS-like

integrals for the wide and narrow bands,

respectively, when o> > Bc

/2, as is assumed,

(g is the coupling parameter.) Starting with

input values of AD, Tc

, and r (i.e., of A c), we

seek values of g and of R such that g < 1 , R

> 1 and such that (2) and (3) are satisfied. One

can then test to see whether the inequality,

Bc

/4kTc > 2, essential to obtain equation (1), is satisfied, rejecting solutions which do not comply. Solutions of these equations, taking as input A D = Aab = 22.5 meV and Tc = 85.4 K

for the Bi compound measured in reference [1] are given in table 1 for r = 2, 2.1, and 2.2. For each value of r, values of R/2, g, o>, Bc

and

are exhibited. If 2Aab /kTc > 6.1 for single-crystal Bi2Sr2CaCu20g, then a statistically

significant upper bound on the gap ratio r of 1.9 would require a revision of the proposed model,

at least in the simplest version considered here.

Experimentally, r = 1.9 ± 0.2. Subsequent

higher statistics experiments should decide the

issue.

Table 1. Computed values for the effective coupling g, intermediate boson cut-off energy u, narrow band width Bp and the parameter x = Bp/4kTc

for the compound Bi^SrCaC^Oo as a function of the ratio cj/Bq = R/2, given that the critical temperature T = 85.4 K and that the in-plane superconducting gap 2Aa(3 =2Ap) = 45.0 meV. Results are presented for three values of the parameter r = Ar^lAhs A UA where 2A is the gap along the c axis. (Data from [1].)

w/B, u[eV] Bc

[meV]

r = 2 40 0.351 2.12 53.0 1.80 50 0.332 2.88 57.6 1.96 60 0.319 3.69 61.5 2.09 70 0.308 4.56 65.1 2.21 80 0.299 5.47

r = 2.1

68.4 2.32

10 0.411 0.51 50.8 1.73 13 0.373 0.77 58.8 2.00 20 0.325 1.50 74.7 2.54 30 0.289 2.81 93,6 3.18 40 0.268 4.40 110 3.73 50 0.254 6.22

r = 2.2

124 4.22

6 0.402 0.36 60.6 2.06

7 0.374 0.48 68.5 2.33 10 0.323 0.91 90.9 3.09

15 0.279 1.88 125 4.26 25 0.239 4.71 188 6.39 30 0.227 6.52 217 7.39

References

[1] G. Briceno and A. Zettl, Solid State Comm. 70, 1055

(1989).

[2] R. C. Casella, Nuovo Cimento D10, 1439 (1988).

[3] R. C. Casella, Solid State Comm. 70, 75 (1989).

[4] R. C. Casella, Appl. Phys. Lett. 55, 908 (1989).

45
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON DIFFRACTION STUDIES OF 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

We investigated new magnetic semiconductor systems fabricated by Molecular Beam

Epitaxy (MBE) growth method: ZnSe/MnSe and

ZnTe/MnTe superlattices and thick (« 1 \im)

epitaxial layers of MnTe. In the superlattices,

the magnetic layers are sandwiched between

non-magnetic spacer layers. Due to differing

lattice parameters between the magnetic and nonmagnetic components, these structures fall in the

category of strained layer superlattices, with the

strain being accommodated by elastic deformation. The MnTe epilayers, on the other hand,

are not strained. Unlike the case of bulk-grown

MnSe and MnTe which crystalize in the NaCl

and NiAs structure respectively, all the

MBE-grown Mn chalcogenide layers are single

crystal zinc-blend (ZB) type-Ill antiferromagnets

(AFM-III). We recall that the only other ZB

AFM-III is the classic case of p-MnS, which can

only be stabilized in polycrystalline form [1].

We performed a systematic study of the

magnetic ordering in ZnSe/MnSe superlattices as a function of both temperature and the magnetic

layer thickness [2]. ZB MnSe can be stabilized

in such superlattices with good crystalline quality

up to thicknesses of around 100 A (about 33

monolayers). In the samples studied, the

magnetic layers contain between 3 and 15 monolayers of MnSe in each superlattice period,

hence covering a crossover region between a 3-d- and 2-d-like effects. All the samples are good quality single crystals with well-defined

interfaces, and at low temperatures all exhibit

long range AFM-III ordering. The magnetic

layer thickness determines the order range in the

perpendicular direction, with no indication of

coupling between the neighboring magnetic

layers. Of three possible AFM-III domains,

only one orientation is observed, indicating the

lifting of a threefold cubic degeneracy usually

c

XIu

>>

V)C

50 75 Temperature (K) 100 125

Figure 1. Temperature dependence of the squared sublattice magnetization (measure of the magnetic order) in MnSe/ZnSe

superlattices. Samples SL 17, 20, 22, 40, and 42 have MnSe layer thickness 10, 15, 7, 5, and 3 monolayers, respectively.

Plotted lines are the squared Brillouin functions for S = 3/2, fitted to the data.

46
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

found in isotropic bulk samples. This

preferential stabilization of one type of domain

is attributed to strain. The antiferromagnetic-paramagnetic phase

transition is continuous, with well defined Neel

temperatures (TN; see fig. 1). Samples with more than 5 monolayers have TN = 107 + 5 K.

However, TN

decreases slightly for thinner

layers. It is not clear at the moment whether

this is due to a true dimensional crossover from

3-d to 2-d, or whether it may be an effect of

interface disorder. It is worthwhile mentioning

that the range of the magnetic order did not change, even for the thinnest 3-monolayer

sample. While thick epilayers of ZB MnSe

cannot be stabilized with good crystalline

quality, MnTe can be easily grown either as thick unstrained epilayers or in strained

ZnTe/MnTe superlattices. This allows a direct

study of magnetic ordering as a function of

strain. The characteristics of ZnTe/MnTe

superlattices are very similar to those of

MnSe/ZnSe, except for a lower N6el

temperature due to a weaker exchange inter- action. However, the case of MnTe epilayers is remarkably different: they exhibit a first order

magnetic phase transition (fig. 2). All three

AFM-III domains were populated, indicating

absence of anisotropies in this sample.

All these studies show that MBE-grown

layered structures involving the Mn-chalcogenides provide a rich variety of systems in which to study fundamental properties of type-Ill

antiferromagnets. Further work involving

different strain configurations and systematic

studies of possible dimensional crossover effects

is now being pursued.

4000

3000

2000

1000 —

i I r I -[ I I -i—i—

|—i—i—r—

. |. , , i

l

i i r —i—1—i—i—i—i

- .

- D - MnTe epilayer

-^%^i

%

« - MnTe/ZnSe SL

1

-

- V,

1 1

.

.

.

. 1

- a X

- a

a

\ —1 1 1 1 1 1

1

i i 1 i i i

, 1 M—i—1—l—t—*-tt- W- +* *-w - . 20 40 60 80 Temperature (K) 100 120

Figure 2. Temperature dependence of the squared sublattice magnetization for MnTe epitaxial layer and the MnTe/ZnTe

superlattice. Superlattice data over entire temperature range were fitted by the squared Brillouin function. In the case of MnTe,

data points below 60 K have similar temperature dependence. At higher T, however, the first-order discontinuity appears in the

data.

References

[1] L. Corliss, N. Elliott, and J. Hastings, Phys. Rev. 104 . 924 (1956).

[2] T. M. Giebultowicz, P. Klosowski, N. Samarth, H. Luo, J. J. Rhyne, and J. K. Furdyna, Phys. Rev. B, August 1990 (in print).

47
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

MAGNETOELASTICITY IN RARE-EARTH SUPERLATTICES AND FILMS

R. W. Erwin, J. A. Borchers, and J. J. Rhyne

and

R. R. Du, C. P. Flynn, and M. B. Salamon (University of Illinois, Urbana, IL)

The magnetic structure has been measured

with neutron scattering and SQUID magnetometry for a number of epitaxial multilayers and

films of the magnetic rare-earths dysprosium and

erbium grown with nonmagnetic yttrium. We

have recently been able to calculate the new

magnetic structures produced by epitaxial strains

in these materials. In particular, we show that the first order ferromagnetic phase transitions are suppressed, although the N6el temperatures

are within 5% of the bulk values. The

calculations of the resulting critical fields are in reasonable agreement with the observed values.

A. Magnetoelasticity and Structure in the

Bulk . The bulk magnetic structures for Dy and

Er have been previously measured using neutron

and x-ray scattering. The spins are aligned

within each basal plane sheet of the hep

structure, and the magnitude or direction is modulated along the c-axis direction with an

effective temperature dependent turn angle, w. Both Dy and Er have a first order phase

transition to a ferromagnetic state, which is primarily driven by magnetoelastic energetics.

For example, in Dy this transition is accompanied by a 0.5% orthorhombic distortion of the

basal plane and a 0.25% expansion of the c-axis

as the moments align.

The bulk magnetoelastic coefficients of Dy

and Er are known from measured aanomalous

strains. HeretanO = <SX >/<SZ > = 0.51 at the

ferromagnetic transition and p « 1 - 2g>/tc is the

averaged nearest neighbor correlation function of

the z spin components.

B. Modifications to Magnetoelasticity in Films and Multilayers . When a magnetic

material is grown with a nonmagnetic material in a film or multilayer, additional elastic terms are added to the free energy. In the case that the

strains in the growth plane are coherent the

additional elastic energy from the nonmagnetic

material can be written as

'2r a LKCpu " C

|ia cua' caa)

{e

p\i

"eo|j^e

ui> " eouJ (1)

The relative amount of nonmagnetic

material is represented by r

a

. The e are the

strains in the free nonmagnetic material

measured with respect to the nonmagnetic strains

in the free magnetic material. In other words

they represent the nonmagnetic lattice mismatch.

This additional elastic energy has both quadratic

and linear strain terms, so that the effective

elastic coefficients for \i, u * a are

c

\iv " c\iv

+ r a(c

|ii> " C|ia c ua' c aa) W

and the effective magnetoelastic coefficients for

[i * a are

*Si =

*V "*" ra i-> GoiAc|ii> "C

(ia c ua' c aa/- w)

C. Critical Fields in Rare-Earth Films and

Multilayers . Here we shall calculate the

magnitude of the critical field necessary to induce the ferromagnetic state. In the case that the layered rare-earth material is grown along

the hep c-axis, the magnetic structure is sufficiently close to the bulk magnetic structure

that the exchange energy can be transferred

directly. If we assume that the elastic constants

are equal in the magnetic and nonmagnetic

materials then the magnetoelastic driving energy

for the ferromagnetic transition, A = f(w = 0)

- f(o>), can be calculated based on the above

equations for the coefficients. For example, if the low-temperature state of the Dy in the

layered material has w = 30°, then in K/atom

A = -1.25-(3.95-91.0r

zGoxy)/(l+rz

). (4)

This magnetoelastic driving energy is plotted in figure la in terms of an equivalent

magnetic field acting on the magnetic moments

48
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

of Dy, for values of the parameter r

z ranging

from zero to infinity as a function of the lattice mismatch e. The conversion of the driving

energy to a magnetic field is based on the

assumption that its value at a spontaneous ferromagnetic transition would correspond to zero

field.A similar calculation of the driving energy

for Er c-axis growth materials is based on <o =

2rc/7 in the low temperature state. Thus the

magnetoelastic driving energy in K/atom

A = -1.56 (0.86 - 553rzeoxy)/(l + rz

). (5)

This expression is plotted in figure lb in terms of an equivalent field on the c-axis

moments of Er. The dependence of the driving

energy on the lattice mismatch is strong

compared to the Dy case. This explains the

large difference in the magnitudes of the critical fields in Dy and Er materials as shown in figure

2. This also explains why critical field data

suggest that a several micron thick film of Er on

yttrium is required before a low-temperature

ferromagnetic transition is obtained. Note also

that for slightly negative values of e, as might

be obtained by growing Er on Lu, the

ferromagnetic transition is expected to occur at a higher temperature than in the bulk.

Presently we are extending this work by

studying Er and Dy rare-earths grown with Lu.

Preliminary results support our calculations for

Er on Lu. However, a DyLu superlattice has

been observed to be ferromagnetic near the N£el

temperature of bulk Dy. It is clearly important

that this new result be incorporated into the

theory outlined here.

^-n 20 : 2 15:

, , . , 1 . , . , I . . . . Dy

(a)

Er

(b)

..!....

-£> 10 - ^ 5

-

- D

/ -

r=0

1 ' O ] £ -5 : o

-10 - . . . ,|

, , , ,

|

, , i,

) 1 -1

strain e

o 1 do-2

)

Figure 1. Calculated critical fields for (a)Dy and (b)Er

c-axis growth layered materials as a function of the lattice mismatch with a nonmagnetic material. The calculation is shown for small and large values of the parameter rz, which represents the ratio of nonmagnetic to magnetic

material. The critical fields in Er materials are suggested to be a much stronger function of the lattice mismatch than in Dy materials. This result in supported by the critical field data shown in figure 2.

c 5

<DE

o2

o

a>c 5

u>D

20 20 40

Internal Field (kOe)

Figure 2. Measured critical fields for Dy(a,b) and Er(c,d)

c-axis growth layered materials. The critical fields in Er

materials are measured to be larger than the critical fields

in Dy extrapolated to the same lattice mismatch.

References

[1] B. R. Cooper, in Magnetic Properties of Rare-Earth Metals . R. J. Elliot, ed. (Plenum, New York, 1972),

p. 17.

[2] W. E. Evenson and S. H. Liu, Phys. Rev. JJ8, 783

49
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

ANTIFERROMAGNETIC ORDERING OF ErAs THIN EPITAXIAL

LAYERS ON GaAs

P. F. Miceli, C. J. Palmstrom, S. J. Allen, and G. W. Hull (Bellcore, Red Bank, NJ)

and

R. W. Erwin, D. A. Neumann, and J. J. Rhyne

Rare-earth arsenides have been recognized

recently for their ability to be epitaxially lattice matched to GaAs, providing a new way to make

buried layer contacts for integrated semiconductor devices. Furthermore, ErAs is expected to order magnetically at low

temperature. Given the recent interest in magnetic thin films which are sensitive to the

microstructural quality, ErAs provides an

excellent system for studying thin film

magnetism since the crystalline perfection can be

rigorously controlled.

A recent transport study [1] has shown that ErAs/GaAs is a semimetal and that spin-disorder

scattering gives rise to a resistivity anomaly at the antiferromagnetic transition. However, they

report a transition temperature of 4.5 K which is significantly higher than the 3.5 K expected from

early neutron scattering [2] as well as specific

heat [3] measurements on bulk ErN and ErSb

powders (ErAs had not been measured but is expected to fall between the nitride and

antimonide).

We have investigated a 1000 A ErAs film on GaAs using the BT-9 triple-axis spectrometer.

Antiferromagnetic ordering with a (111)

wavevector was observed and the temperature

dependence is shown in figure 1 where the

transition occurs at 4.3 + 0.2 K. This direct

measurement of antiferromagnetic ordering,

therefore, confirms the enhanced transition

temperature in the film. For this "thick" film,

the lattice parameters of the ErAs have

completely relaxed to their bulk values, thus, one expects that strain effects may not be an

issue. However, differential thermal expansion

between the film and substrate as well as magnetoelastic effects [3] may be crucial in understanding the magnetic properties of the

film. Future work will focus on the role of

strain by looking at thinner films.

4000

3500

3000

2500

-i 1 1 i 1 1 1 r- T ErAs 1000A film

(1/2 1/2 1/2)

iZ l4 i 4

Temperature [K]

10

Figure 1. Temperature dependence of the intensity of the

(1/2, 1/2, 1/2) antiferromagnetic reflection for a 1000 A

ErAs film.

References

[1] S. J. Allen et al., Phys. Rev. Lett. 62, 2309 (1989).

[2] H. R. Child et al., Phys. Rev. 131, 922 (1963).

[3] M. E. Mullen et al., Phys. Rev. B10, 186 (1974).

50
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

ORIENTATION DEPENDENCE OF THE SPIN COUPLING IN Gd/Y SUPERLATTICES

R. W. Erwin, J. A. Borchers, J. J. Rhyne and C. F. Majkrzak

and

T. F. Tsui and C. P. Flynn (University of Illinois, Urbana, IL)

To determine the symmetry of spin-coupling

through nonmagnetic yttrium interlayers, b-axis

Gd/Y superlattices were grown by Molecular

Beam Epitaxy on (10T0) Y substrates and

examined by neutron techniques. For c-axis

Gd/Y superlattices [1], the Gd layer moments

align or anti-align across the intervening layers

depending in an oscillatory manner on the Y

thickness. The spin interaction is associated

with RKKY coupling mediated by the Y conduction band. Studies of b-axis Dy/Y superlattices

[2] suggest that this spin coupling is not

isotropic. In contrast to c-axis Dy/Y system [3],

the helical Dy spin order is confined to a single

magnetic layer. Because bulk gadolinium is ferromagnetic, the b-axis Gd/Y samples provide

a simpler probe of the orientational dependence

of the RKKY interaction. In figure 1 the scan through the (10T0)

reflection for the sample ^>-[Gd43^/Y52^]^

demonstrates that the Gd moments anti-align across the yttrium. Preliminary measurements at 150 K for a sample with thinner Y interlayers,

b-[Gd60^/Y26A]80' Sive similar results. The

saturation fields for this sample obtained from

bulk magnetization measurements, however, are

significantly smaller than the fields for b-

[Gd43A/Y52A] 85

(i.e. at 50 K, 5 Oe in comparison to 150 Oe respectively). Judging by

these two measurements, it is unlikely that the

antiferromagnetic alignment follows from simple

dipolar coupling between the Gd spins. Instead

the sign of the interaction seems to oscillate

suggesting an RKKY-like interaction, in sharp

disagreement with observations from b-axis

Dy/Y superlattices [2]. Field dependent

diffraction measurements are in progress to determine the relative strength of the spin

coupling in the two Gd/Y samples.

4.0

2.2

Figure 1. Neutron diffraction scans along the b* direction

for b-[Gd43 ^/Y52^]g5 at 80 K. The reflections designated by arrows result from the antiferromagnetic

arrangement of the Gd layer moments across the Y. The

unmarked superlattice satellites are separated from each

other by AQ = 2nn/A where A is the periodicity of the

superlattice. The central peak is the (1010) lattice reflection of the large yttrium substrate.

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).

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

and J. J. Rhyne, J. Phys. Condens. Matter 1, 5997

(1989).

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

Borchers, Shantanu Sinha, R. Du, J. E. Cunningham,

and C. P. Flynn, Phys. Rev. B35, 6808 (1987); M.

B. Salamon, Shantanu Sinha, J. J. Rhyne, J. E. Cunningham, R. W. Erwin, J. A. Borchers, and C.

P. Flynn. Phys. Rev. Lett. 56, 259 (1986).

51
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

PROPAGATION OF MAGNETIC ORDER THROUGH NONMAGNETIC

INTERLAYERS IN RARE-EARTH SUPERLATTICES

1600-

1200

o

X

800-

400-

2.35

J. A. Borchers, R. W. Erwin, and J. J. Rhyne

and

R. S. Beach, T. Matheny, M. B. Salamon, and C. P. Flynn (University of Illinois, Urbana, II.)

Advances in the controlled deposition of

metals have sparked interest in a new class of

materials with tailored structural, magnetic and

electronic properties. Previous studies of c-axis

rare-earth superlattices (i.e., Dy/Y [1], Er/Y [2],

and Gd/Y [3]) have demonstrated that the

complex spin structure in the magnetic rare

earths is propagated through yttrium interlayers.

To determine if nonmagnetic rare earths other

than yttrium can sustain long-range exchange

interactions, we have performed neutron

diffraction experiments on Dy/Lu and

Dy/Y/Sc/Y superlattices grown by Molecular

Beam Epitaxy.

A. Dysprosium/Lutecium. In bulk form

dysprosium exhibits a basal-plane helical spin

structure with a propagation vector along the c*

axis. At Tc the Dy spins align parallel to the a*

direction. Diffraction studies of c-axis Dy/Y

superlattices [1] indicate that the Dy spiral is coherent across intervening layers of yttrium.

The mechanism behind this coupling is a spin

density wave stabilized in the Y conduction band

via the RKKY interaction. The effective

wavevector of the "magnetic order" in the

yttrium is close to Q that maximizes the

generalized susceptibility calculated from the

band structure [4], In addition the low

temperature ferromagnetic state does not develop

in the Dy/Y superlattices. It was suggested [5] that the epitaxy modifies the magneto-elastic

energy and inhibits the orthorhombic distortion

that accompanies the ferromagnetic transition in bulk Dy.

Because the generalized susceptibility [4] for

lutecium has a peak similar to Y, the Dy spiral

should again be coherent through the nonmagnetic Lu layers in Dy/Lu superlattices.

Preliminary studies of the sample [6] [Dy 15 5 /Lu22]75 snow tnat tne sPi ral state never

develops. At all temperatures below TN, the

spins ferromagnetically align within each Dy

layer contrary to predictions from magneto2.15 2.25

Q, (A"1)

Figure 1. Neutron diffraction scans along the [0001] direction for [Dy 15 ^u2l)l5 at 1° K- The central (0002)

lattice reflection is surrounded by superlattice satellites separated from it by AQ = 2n7i7A, where A is the

superlattice periodicity. The peaks designated by arrows

are magnetic reflections that result from the anti-alignment of the ferromagnetic Dy layer moments across the Lu

interlayers.

elastic energy arguments [5]. These Dy layer

moments anti-align across the intervening Lu, as demonstrated in figure 1. Bulk magnetization

measurements suggest that a field of less than 1 kOe is required to induce ferromagnetic order.

The weak antiferromagnetic coupling could thus

result from dipolar coupling of the Dy layer

moments or from the oscillatory RKKY inter- action responsible for the spin order in c-axis

Gd/Y superlattices [3]. Measurements of

samples with varying Lu and Dy thicknesses are

in progress to determine the origin of the anti- ferromagnetic alignment and the mechanism

responsible for the suppression of the helical

spin phase.

52
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

B. Dysprosium/Scandium. Similar to lutecium and yttrium, the generalized

susceptibility of the rare-earth scandium is expected to have a distinct peak [7], which

suggests that the conduction band can sustain

long-range exchange interactions. In rare- earth/Sc alloys, however, the magnetic order

becomes spin-glass like in samples with greater

than 75% Sc [8] (Magnetic order persists in

RE/Y alloys with as much as 95% Y [9]).

To resolve this discrepancy, the sample

[Dy3 5 /Y3 5/Sc! 5 /Y3 5] 2oo was examined using

zero field diffraction techniques. It is noted that

the Y-spacer layers in this sample are required

for coherent growth due to the 6.8% mismatch

between the Dy and Sc lattice. Unlike the

Dy/Lu sample, the Dy spiral does persist in this sample at temperatures as low as 10 K and is coherent through the Y/Sc/Y interlayers.

Because the nominal Sc thickness of 1.5 atomic

layers is comparable to the usual number of

interface layers (3 - 4), it is possible that interdiffused Y is responsible for the Dy spin

coupling. Growth of superlattices with larger Sc

layers is currently under way.

References

[1] R. W. Erwin, J. J. Rhyne, M. B. Salamon, J. A. [5] Borchers, Shantanu Sinha, R. Du, J. E. Cunningham,

and C. P. Flynn, Phys. Rev. B35, 6808 (1987); M.

B. Salamon, Shantanu Sinha, J. J. Rhyne, J. E. [6] Cunningham, R. W. Erwin, J. A. Borchers, and C.

P. Flynn, Phys. Rev. Lett. 56, 259 (1986).

[2] R. W. Erwin, J. J. Rhyne, J. A. Borchers, M. B. Salamon, R. Du, and C. P. Flynn, J. Appl. Phys. 63, [7] 3458 (1988); J. A. Borchers, Ph.D. thesis, University of Illinois, 1990. [8]

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

B. McWhan, Y. Yafet, J. V. Waszczak, and C. [9] Vettier, Phys. Rev. Lett. 56, 2700 (1986).

[4] S. H. Liu, R. P. Gupta, and S. K. Sinha, Phys. Rev. B4> 1100(1971).

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).

In the sample designation [Dy /Lu L, n refers to the number of Dy layers per bilayer, m is the number

of Lu layers per bilayer, and one is the total number

of bilayers in the sample.

R. P. Gupta and A. J. Freeman, Phys. Rev. B13 , 4376 (1976). H. R. Child and W. C. Koehler, Phys. Rev. 174,

562 (1968). H. R. Child, W. C. Koehler, E. O. Wollan, and J. W. Cable, Phys. Rev. 138, A1655 (1965).

53
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

DIFFUSE X-RAY SCATTERING FROM A GaAs/AlAs SUPERLATTICE

D. A. Neumann, C. F. Majkrzak, and S. K. Satija

A. Salvador and H. Morkoc (University of Illinois, Urbana, IL)

H. Homma (Brooklyn College, CUNY, Brooklyn, NY)

and

M. Sanyal and S. Sinha (Brookhaven National Laboratory, Upton, NY)

Neutron and x-ray reflectivity are becoming

standard techniques for measuring the scattering

density profile of many different types of films

and multilayers. These techniques, however,

only yield information perpendicular to the basal

plane of the film thereby making it impossible to ascertain whether the interfacial width

determined from these measurements is due to interdiffusion or roughness. In order to obtain

a complete description of the interface, scans having a component of the scattering vector || to the interface must be performed. We have,

therefore, begun to measure the diffuse in-plane

scattering from a variety of films including

GaAs/AlAs superlattices.

The sample used in this study was grown by

molecular beam epitaxy at the University of

Illinois and has a superlattice period of 120 A.

The data was taken on a standard 2 axis x-ray

diffractometer using CuK^radiation. Scans were

performed by fixing the scattering angle at various satellite reflections and scanning the

sample angle 0. This very closely approximates

an in-plane scan along the 1 10 direction. The

results of one of these rocking curves is shown

in figure 1. The solid line is the sum of a Gaussian peak and a Lorentzian peak which

yielded the best fit to the data. The Gaussian

lineshape represents the specular scattering

while the Lorentzian peak represents the diffuse

scattering presumably from interfacial roughness,

since pure interdiffusion should give monotonic

diffuse scattering. Several features of the

scattering should be noted. At lower values of

Q (i.e., for the first few satellites) the sum of a Gaussian and a Lorentzian was not a good model

for the scattering, presumably due to dynamical

effects. Second, the width of both peaks was

found to be constant in angle, not Q. While this

is not surprising for the Gaussian peak which

reflects the instrumental resolution, one would

expect that the width of the diffuse scattering

should be constant in Q-space. We have no

rock at s=6 sample 2212

<D

1 1 1 1 1 i i

-(-> 3 4 — — Ce

m

_

->-) C

3

° 2 _ o Ay^y L * _o<M tF

'ttP

1 1 i i i 1 1 3.0 4.50x2

6.0

Figure 1. Rocking scan of the 6th satellite of a GaAs/AlAs

superlattice. The solid line is the sum of a Gaussian peak

and a Lorentzian peak which yielded the best fit to the data.

good explanation for why it is not. Third, the

ratio of the diffuse intensity to the specular

intensity is consistent with the Q

2 law typical for

diffuse scattering. Further measurements on this and on other systems are in progress. It is hoped that these measurements will give a more

complete understanding of the microstructure of

the interfaces in a variety of thin-film systems.

54
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

POLARIZED NEUTRON REFLECTOMETER AND

GRAZING ANGLE DIFFRACTOMETER

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

A neutron reflectometer, in which the

incident beam can be polarized and the spin state of the reflected beam analyzed, has recently been

constructed on beam tube BT-7 at the NBSR.

In addition to obtaining reflectivity data, from

which chemical and magnetic density profiles

normal to a surface can be deduced, surface- or

near-surface-sensitive diffracted intensities can be measured from properly oriented single

crystal samples. In this report we briefly

describe the reflectometer itself, i.e., how the

incident beam is polarized and the neutron spin

state selected, what the instrumental resolution

and flux at the sample are, etc. In figure 1 is shown a schematic of the

reflectometer. The polychromatic beam first passes through a pyrolytic graphite (PG) filter to suppress the higher-order wavelengths which

would also be reflected by the vertically

focussing PG monochromator set for a principal

wavelength X of 2.35 A. For collimations of the

order of one degree and one minute of arc preceding and following the monochromator,

respectively, the wavelength resolution AX/A. ~

0.01. Associated with a given wavevector

transfer Q( 1

1 = I Ef - £j I where k*j and lc f are

the initial and final neutron wavevectors,

respectively) is an instrumental resolution AQ

given by

AQ - ^Q * k N q? Or A6

(i)

where A8 is the angular divergence of the beam

incident on the sample. The first term on the

right-hand-side of Equation (1) ranges, for AX/A. = 0.01, from 0.0001 A" 1 at Q = 0.01 A"1 to 0.001 A' 1 at Q = 0.1 A" 1 whereas, for A6 = 1 min. or -0.0003 rads, the second term is -0.0015 A" 1 up to a Q of 0.1 A"1

. Thus, at Q

= 0.01 A"1

, AQ ~ 0.0016 A' 1 and at Q = 0.1

A

-1

, AQ ~ 0.0025 A" 1

. This resolution is adequate in most cases and, at higher values of

Q, can often be further relaxed. Without a cold

source, it is more advantageous to work at 2.35

A rather than, say, 4.0 A since, for a given Qresolution, the neutron flux is substantially

higher. For AX/X ~ 0.01, X = 2.35 A, and

A0 ~ 1 min, the actual flux on the sample is ~5. x 105 neutrons/cm2

-s. Between the pair of slits which define the

angular divergence of the incident beam is a fast neutron mask consisting of two 4-in long slabs of

borated polyethylene spaced -0.040" apart.

The apertures of the incident beam-defining slits can each be opened from about 0.003" to 0.040"

(the distance between slits is of the order of a meter). An adjustable aperture following the

sample more accurately defines the scattering

angle. The 3 He detector (1/2" diam, 4" high),

is surrounded by boron-carbide and borated

polyethylene shielding material. Room

background with the reactor running is about 0.

counts/minute. An identical detector is located

overhead (not shown in fig. 1) for grazing angle

diffraction experiments.

Polarization of the incident beam is accomplished in transmission by one or more

polarizing Fe-Si supermirrors deposited on single

crystal Si substrates. The supermirrors are aligned to reflect out (of the incident beam

direction) neutrons in one spin state. The

deflected neutrons are then captured in the

absorbing material of the masks which define the

slit before the sample. Because the reflectivities of the polarizing supermirrors are not unity for

the unwanted spin state, two or more

supermirrors are used in series. For example,

for a reflectivity of 80%, two supermirrors in series give a polarizing efficiency of -96% or

a flipping ratio of 25. Using three or more

plates results in a higher flipping ratio, but the

transmission of the desired spin state is reduced

to about 50% due to losses which can be

primarily attributed to attenuation of the beam as

it passes through the Si substrates (with path

lengths of several inches at such low angles). Better supermirror coatings such as Fe-Ag and

Co-Ti exist [1] but cannot be used in

55
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

transmission because of the absorption losses

which would occur in the Ag or Co layers.

Nonetheless, by polarizing the beam in transmission, the undeviated beam of the desired

spin state is not appreciably broadened, which is an important advantage. A secondary benefit is that the spectrometer is of simpler design. A

second (or third) transmission polarizing device can be placed in the reflected (or grazing angle

diffracted) beam. To flip from one neutron spin

state to the other, simple flat solenoids, in which

a field is applied orthogonally to both the

vertical guide field and beam propagation

directions, are used (see, e.g., [2]).

In figure 2 is plotted neutron (unpolarized)

reflectivity vs. Q for a diblock copolymer film

as obtained using the reflectometer described

above. Data for two different collimations or instrumental resolutions are shown.

Reflectivities down to about 2. x 10~7 with a signal-to-noise ratio of about 1 can be measured.

A number of reflectometry measurements on

magnetic and polymer films and multilayers have

already been performed on this machine, as has

the first polarized beam, grazing angle

diffraction experiment [3]. Its success is due in large part to the efforts of the expert technical

staff of the Reactor Radiation Division.

"RANSW1SSION SUPERUIRROR POURTCERS

TRANSMISSION SUPERUIRROR POLARIZERS

DETECTOR

/ • t T

1

'

'

' 1 1 1 1 1 1 u 1 V

" l tx •#• . P.3

*

-3 • '\ *

>-> ' *• ' » > •• •* ' '*. •' '. U

d> -5 f. **,',> *v

OS V «y * oM " 7 V-^ \

, 1 '^\ .

.

-9 i i i i i f 0.00

r1 33

5 O

n

0.06 0.12 0.18 Q (A"

1)

Figure 2. Example of the (unpolarized) neutron reflectivity profiles which can be obtained with this reflectometer. The

sample is a multilayer of a diblock copolymer of

polystyrene and polyisoprene made by T. Russell et al. at IBM. Data are plotted for two separate collimations.

Figure 1. Schematic of the polarized beam reflectometer

at the NBSR as described in the text.

References

[1] R. Pynn, in Thin-Film Neutron Optical Devices . C.

F. Majkrzak, ed., SPIE Vol. 983 (SPIE, Bellingham,

WA, 1989) p. 18.

[2] W. G. Williams, Polarized Neutrons . (Oxford

University Press, Oxford, 1988).

[3] J. F. Ankner, C. F. Majkrzak, D. A. Neumann, A. Mathcny, and C. P. Flynn, Physica B (to be

published).

56
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON REFLECTIVITY OF Y/Gd

-0.25

J. F. Ankner and H. Zabel (University of Illinois, Urbana, IL)

Y. Y. Huang and G. P. Felcher (Argonne National Laboratory, Argonne, IL)

D. A. Neumann and C. F. Majkrzak

and

A. Matheny, J. A. Dura, and C. P. Flynn (University of Illinois, Urbana, IL)

Neutron reflectivity is an important tool in the study of surface and interfacial magnetism.

Using polarized-neutron techniques, we have

begun to study the magnetic moment distribution

in the interfacial region of Y/Gd films grown by

molecular beam epitaxy. These experiments,

carried out at the Intense Pulsed Neutron Source

at Argonne National Laboratory, continue work

begun at NIST [1]. The interfacial magnetism of gadolinium has

been a subject of intensive study over the past

several years. These experiments fall into two

categories: work on Gd/vacuum interfaces (free

Gd surfaces) [2] and studies of synthetic Y/Gd

superlattices [3]. Our experiments represent

something of a hybrid of these two fields, since

we use neutron reflectivity to measure the

depth-dependence of the layer- averaged

magnetic structure at one Y/Gd interface.

By measuring the reflectivity of neutrons

polarized parallel and anti-parallel to an applied

magnetic field, one can determine the depth

profile of the magnetization in a sample. A

useful measure of the magnetization is the

neutron spin asymmetry,

D 270 K

a 284 K o 290 K

292 K

A =

where I

T is the reflected intensity of neutrons

polarized parallel to the field applied at the

sample and l

l the intensity for those polarized

anti-parallel. Figure 1 shows the spin asymmetry as a function of temperature for a film

consisting of 97 A Y(0001) on 1000 A Gd(0001)

near the bulk Curie temperature with a 20 G

applied field. Within the uncertainty of the

measurement, all in-plane magnetization in the

sample disappears at T = 293 K. Studies of Gd

using electron diffraction and photo-emission

showed that the surface remains ordered for up

to 20 K above the bulk Curie temperature

0.00 0.01 0.02 0.03 0.04 Q

Figure 1. Spin asymmetry for 97 A Y(0001) on 1000 A

Gd(0001).

(TG

<5? = 293 K). Our results on Y/Gd indicate

that, in this system, any in-plane moment must

be quite small, if it exists at all. We plan, in further work, to perform a more detailed study

of the temperature region around T

G

q and to investigate the depth-dependence of the

interfacial magnetism using both reflectivity and

grazing-angle diffraction.

References

[1] J. F. Ankner, H. Zabel, D. A. Neumann, C. F. Majkrzak, A. Matheny, J. A. Dura, and C. P. Flynn,

in Neutron Scattering for Materials Science , S. M.

Shapiro, S. C. Moss, and J. D. Jorgensen, eds. Matls. Res. Soc. Sym. Proceedings, Vol. 166, (Matls. Res. Soc, Pittsburgh, 1990).

[2] D. Weller, S. F. Alvarado, W. Gudat, K. Schroder,

and M. Campagna, Phys. Rev. Lett. 54, 1555 (1985).

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

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

Vettier, Phys. Rev. Lett. 56, 2700 (1986).

57
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

INTERFACIAL WIDTHS IN HOMOPOLYMER/DIBLOCK COPOLYMER SYSTEMS

T. P. Russell, A. Menelle and S. H. Anastasiadis (IBM Research Division, San Jose, CA)

S. K. Satija

and

W. Hamilton and G. Smith (Los Alamos National Laboratory, Los Alamos, NM)

The interface formed between poly(styrene),

PS and polymethyl methacrylate, PMMA,

homopolymers was found to be 50 A[l,2].

Symmetric, diblock copolymers of PS and

PMMA, denoted P (S-b-MMA) form a lamellar

microphase separated morphology where the

interface between the PS and PMMA microdomains is also 50 A. Yet, placing P(S-bMMA) between the PS and PMMA yielded an

interfacial thickness of 75 A [3] . The quantity

of P(S-b-MMA) placed at the interface

corresponded to a film thickness of L/2 where L

is the repeat period of the lamellar microdomain

morphology in the bulk. This poses two

separate questions which are the subject of this report. First, is the broadening of the interfacial width as a function of the amount of P(S-bMMA) present. Secondly, is there a broadening

of the interface between the PS and PMMA

microdomains of the copolymer as PS and

PMMA homopolymer is added.

The latter question was addressed by

investigating mixtures of P(S-b-MMA) and P(Sb-d-MMA) with PS and PMMA homopolymers

where the homopolymers have the same labeling

as the blocks of the copolymer. Each block of

the copolymer had a molecular weight of -5x1 4 whereas the homopolymer molecular weights were - 1x10 . Mixtures having 4.5% of PS and

PMMA with P(S-b-MMA) were prepared in toluene, cast onto silicon substrates and annealed

for 240 h under vacuum. Reflectivity profiles

displayed interferences arising from the

multilayered morphology. As expected, the

period of the lamellar morphology increased with

increasing concentration of the homopolymer.

In addition, as the homopolymer concentration

increased the width of the interfaces between the

PS and PMMA microdomains was found to increase from the 50 A of the pure copolymer to ~ 55 A for the 4.5% mixture to - 70 A for the

8.3% mixture. The gradual increase in the

interfacial width clearly shows that the

homopolymer molecules penetrate deeply within

the interfacial region. These data also show that the interfacial broadening is a function of the

ratio of the number of copolymer to homopolymer molecules at the interface.

On the opposite end of the composition scale

experiments were performed on trilayer

specimens of PS/P (S-b-MMA)/PMMA where

the MMA segments were perdeuterated

(homopolymer and copolymer). Specimens were

prepared by casting a film of PMMA from

toluene onto a silicon substrate. On a separate

substrate a film of P(S-b-MMA) was prepared

with the desires thickness. This film was then

floated off into a pool of deionized water and

retrieved with the PMMA coated substrate.

Similarly, PS film was prepared on a separate

substrate, floated off onto deionized water and

retrieved with the P(S-b-MMA)/PMMA, bilayer

forming a trilayer. This was annealed at 170 °C

for 240 h under vacuum. The reflectivity profiles could be described

using a bilayer model comprised of a PS and dPMMA layer. The interface formed between the

two layers is given by an error function or hyperbolic tangent whose width increased with

increasing P (S-b-MMA) concentration. At a P(S-b-MMA) thickness of 255 A, i.e., L/2, the

interfacial with was 75 A in agreement with

previous results [3]. However, increasing the

thickness of the P(S-b-MMA) layer above L/2

yielded an interfacial width excess of 250 A.

Measurement of the off-specular scattering, i.e.,

where the diffraction vector is not normal to the

film surface, indicated a substantial amount of

in-plane correlations in the scattering length

density which can be attributed to the information of ordered structures at the interface.

Consequently, a saturation in the number of

copolymers molecules that could be located at the interface was achieved.

The combination of these two sets of

measurements showed conclusively that, at the

molecular weights studied, there is significant

58
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

penetration of the homopolymer into the

interfacial region and that there is a maximum

number of copolymer molecules that can align

themselves at the interface.

The work was supported in part by the

Department of Energy, Office of Basic Energy

Sciences Grant No. DE-FG03-88ER45375.

References

[1] S. H. Anastasiadis, T. P. Russell, S. K. Satija, and C.

F. Majkrzak, J. Chem. Phys. 92, 5677 (1990).

[2] M. L. Fernandez, J. S. Higgins, J. Penfold, R. C. Ward, C. Shackleton, and D. J. Walsh, Poly. 29,

1923 (1988).

[3] T. P. Russell, S. H. Anastasiadis, A. Menelle, G. P. Felcher, and S. K. Satija, Macromol. (submitted for publication).

FILM THICKNESS CONSTRAINTS ON DIBLOCK COPOLYMER MORPHOLOGIES

T. P. Russell, A. Menelle, and S. H. Anastasiadis (IBM Research Division, San Jose, CA)

and

S. K. Satija and C. F. Majkrzak

Secondary ion mass spectrometry studies

[1,2], optical microscopy [3], and neutron

reflectivity studies [4,5] performed on symmetric

diblock copolymers of poly(styrene) and

poly(methylmethacrylate),denotedP(S-b-MMA),

have shown that thin films of the copolymers on

silicon substrates form a multilayered

morphology. It has been shown that the lamellar

microdomain morphology of the P(S-b-MMA)

orients parallel to the surface of the substrate

with PMMA layer adjacent to the silicon

substrate and a PS layer at the air surface. The

widths of the layers adjacent to the air and

silicon interfaces are one-half that seen in the

bulk of the multilayer and, consequently, the

thickness of the specimen at any point in the

specimen is given by (N + 0.5) L where L is the period of the lamellar microdomain

morphology and N is an integer.

This report concerns the effect of film

thickness on the copolymer morphology where

the total thickness of the specimen is less than or

equal to 3L/2, i.e., where N = 1. A series of

neutron reflectively studies were performed on a P(S-b-MMA) copolymer films, where the PS

block is perdueterated, such that the film

thicknesses were L/4, L/2, 3L/4, L, and 3L/2.

The molecular weight of the PS block was 1 .6 x 105 and that of the PMMA block was 1.3 x 105

. In the bulk the lamellar repeat period is 762 A.

Copolymer films with thickness of 209 A (L/4),

381 A (L/2), 555 A (3L/4, 726 A (L), and

1299 A (3L/2) were prepared by casting

solutions of the copolymer from toluene onto

silicon substrate and annealed for 240 h at 170 °C under vacuum. In all but one of the

specimens x-ray photoelectron spectroscopy

showed that, over the first 70 A from the air surface, the specimens consisted of pure PS. In the L thick specimen, the surface was comprised

of 54% PS.

Neutron reflectivity measurements on the

L/2 and 3L/2 specimen yielded scattering length

density profiles characteristic of a layered

morphology with PMMA at the silicon interfaced

and PS at the air interface. In the L/2 case a 345 A layer of PS on top of a 210 A layer of

PMMA was found with an interface of 50 A

separating the two layers. In the 3L/2 case four

layers, alternating PS and PMMA from the air surfaces, were found separated by interfacial widths of 50 A. Thus, in these two cases the

morphologies conformed to the expected layered

morphology.

Decreasing the film thickness to L/4 yielded

a reflectivity profile that was well described by

a bilayer of PS and PMMA with thicknesses of

122 and 87 A. Each of these is much less than

that seen for the L/2 specimen. However, the

width of the interfaced between the lamellar

59
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

microdomains was maintained at 50 A. These

results showed clearly that the microphase

separation was maintained and that the

copolymer chains were compressed normal to the

film surface. However, the characteristics of the

interface between the PS and PMMA layers are

vastly different.

Increasing the film thickness to 3L/4 yielded

a reflectivity profile that could be described by

a bilayer of PS and PMMA with layer thick- nesses of 345 and 210 A, respectively. In comparison to the thicknesses of the lamellar

microdomains in the bulk, these are much

thicker. In addition, the width of the interface

between the PS and PMMA layers was found to be 100 A, again larger than the bulk values.

These results show that the PS and PMMA

chains of the copolymer are markedly stretched

and cause a broadening of the interface between

the PS and PMMA layers.

For the L thick specimen the reflectivity results could be well described with a trilayer

model comprised of a 250 A layer of a 54% PS/46% PMMA mixture on top of a 313 A

layer of PS on a 163 A layer of PMMA. The

width of the interface between the lower PS and

PMMA layer was 50 A and that between the

mixed layer and the PS layer was 150 A. The

chain stretching observed in the 3L/4 case could

not be extended to the L case since this would

require too much elastic energy. Thus, the

copolymer chains are forced to mix despite the

fact that the copolymer is well below the orderdisorder transition temperature. The thickness

constraint has caused a suppression of the

microphase separation. In summary, these studies have shown that the film thickness can cause rather dramatic

alterations in the morphology of the diblock

copolymers and, consequently, perturb the

thermodynamics of the copolymer molecules.

The work was supported in part by the

Department of Energy, Office of Basic Energy

Sciences Grant No. DE-FG03-88ER45375.

References

[1] G. Coulon, T. P. Russell, V. R. Deline, and P. F. Green, Macromol. 22, 2581 (1989).

[2] T. P. Russell, G. Coulon, V. R. Deline, and D. C.

Miller, Macromol. 23, 4600 (1990).

[3] G. Coulon, D. Ausserre, and T. P. Russell, J. Phys.

France. 51, 777 (1990).

[4] S. H. Anastasiadis, T. P. Russell, S. K. Satija, and C.

F. Majkrzak, Phys. Rev. Lett. 62, 1852 (1989)

[5] S. H. Anastasiadis, T. P. Russell, S. K. Satija, and

C. F. Majkrazak, J. Chem. Phys. 92, 5677 (1990).

60
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SEGMENT DENSITY DISTRIBUTION OF SYMMETRIC DIBLOCK COPOLYMERS

AT THE INTERFACE BETWEEN TWO HOMOPOLYMERS AS REVEALED

BY NEUTRON REFLECTIVITY

T. P. Russell, S. H. Anastasiadis, A. Menelle (IBM Research Division, San Jose, CA)

G. P. Felcher (Argonne National Laboratory, Argonne, IL)

and

S. K. Satija

The segment density distribution of

symmetric, diblock copolymers of poly(styrene),

PS, and poly(methylmethacrylate), PMMA,

denoted P(S-b-MMA), at the interface between

two homopolymers PS and PMMA were investigated by neutron reflectivity measurements.

Selective labelling of either block of the P(S-bMMA) or of the homopolymers PS or PMMA

with deuterium provided the contrast necessary

to isolate the distribution of the segments of the

components of block copolymer at the interface.

Results from a series of experiments were used

to solve a simultaneous set of linear equations

that yielded the segment density profiles of the

PS and PMMA segments of the homopolymers

and copolymers at the interface.

Typical reflectivity profiles for two (out of

a total of six different combinations) of the

specimens studied are shown in figures 1 and 2

along with scattering density profiles which were

used to calculate the reflectivities shown by solid

lines. Neutron reflectivity experiments were

performed on POSY-II at the IPNS at Argonne

National Laboratory.

The following conclusion could be drawn

from this study [1].

It was found that the addition of the

copolymer to the interface between the homopolymers significantly broadened the interface

between the PS and PMMA homopolymers over

that seen for the pure copolymer or that between

the PS and PMMA homopolymers. It has been

0.05

Figure 1. Experimental neutron reflectivity profile (O) for a trilayer on a silicon substrate. The trilayer is comprised of PS homopolymer at the air surface, a dPS/PMMA diblock copolymer and a PMMA layer adjacent to the substrate. Three different

reflectivity results obtained at 0.27°, 0.78°, and 1.46°, angles of incidence are shown. These have been offset for clarity by

factors of 1, 0.1, and 0.01, respectively. The solid line represents the calculated reflectivity profile using the segment density

profile shown in the inset.

61
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

0.06

Figure 2. The same as in figure 1 but now the trilayer is comprised of a layer of PS at the air surface, a PS/d-PMMA copolymer

at the homopolymer interface and a layer of d-PMMA adjacent to the substrate.

shown that the diblock copolymers are located at the interface between the homopolymers and that the PS segments of the copolymer were located

in the PS phase and the PMMA segments of the

copolymer in the PMMA phase. It was also

found that the junction points of the copolymer were localized to a region around the midpoint

of the interface over a distance comparable to the interfacial width observed for the neat diblock copolymers.

Furthermore, results from a series of

experiments were used to solve a simultaneous

set of linear equation that yielded the segment

density profiles of the two homopolymers, of

each block of the copolymer and of the total copolymer at the interface. Results from this analysis were shown to be consistent with

independent experimental results. This treatment showed that the homopolymers penetrated well

into the interfacial region in contrast to theoretical arguments which would place the

copolymer in the dry brush regime.

Reference

[1] T. P. Russell, S. H. Anastasiadis, A. Menelle, G. P. Felcher, and S. K. Satija, Macromol. (in print).

62
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

X-RAY REFLECTIVITY STUDIES OF THIN DIBLOCK COPOLYMER FILMS

S. K. Satija

E. B. Sirota and G. J. Hughes (Exxon Research & Engineering Co., Annadale, NJ))

S. K. Sinha (Brookhaven National Laboratory, Upton, NY)

and

T. P. Russell (IBM Research Division, San Jose, CA)

The technique of choice for studies of bulk

polymer and polymer surfaces is neutron

scattering because of the large contrasts one can

obtain by substituting hydrogen and deuterium.

Neutron reflectivity has been widely used to polymer surfaces and interfaces [1,2]. Recently,

we have shown by neutron reflectivity measure- ments that annealed thin films of diblock

copolymers of polystyrene and polymethylmethacrylate P(S-b-MMA) form a very highly

oriented multilayer morphology parallel to the

film surface [3]. In neutron measurements a high degree of scattering contrast between

different blocks of copolymers was achieved by

deuterating one of the blocks while leaving the

other block hydrogenated. This enabled us to study details of the interfacial profiles between

PS and PMMA blocks [4]. In this report we show x-ray reflectivity

measurements on films of same PS and PMMA

block copolymers. The type of scattering contrast achievable with neutrons is not possible

with x-rays. However, extremely high Q

resolution and high intensity in x-ray

measurements makes it possible to get unique

information about polymer films which is complementary to neutron reflectivity

measurements.

Figure 1(a) show x-ray reflectivity from a

film of P(s-b-dMMA) (molecular wt. 30 K)

annealed at 170 °C for 24 h. Neutron

reflectivity studies on this and other similar

systems have already been reported [3,4]. X-ray

reflectivity measurements were done at beam

X-10A at the National Synchrotron Light source

at Brookhaven National Laboratory. Neutron

reflectivity from the same sample, which was

reported earlier [3], is shown in figure 1(b).

Figure 1(b) shows three Bragg reflections from

the ordered multilayer morphology of the two

block copolymers. The inset in that figure gives

the scattering density profile perpendicular to the

surface which was used to calculate the solid line

through the data points. However, the x-ray

data in figure 1(a) shows a hint of one Bragg

peak near Q ~ .045 A" 1 and other rich structure. The x-ray scattering contrast between the two

polymer blocks is not strong enough to give

Bragg peaks in the x-ray data. The "high

frequency" oscillations in figure 1(a) arise from

the total thickness of the diblock copolymer film

(1450 A). The resolution in the neutron data was not sufficient to resolve these oscillations.

A further modulation in the "high frequency"

oscillations is also evident in figure 1(a). These

arise due to the fact that the top bilayer of the

polymer film is incomplete giving rise to islands

at the top of the film. These x-ray data can be

well fitted by assuming that scattering density of

the top bilayer is 75% of scattering density of

the rest of the bilayers. This means that the

island size at the top is smaller than the transverse coherence length of the x-ray beam,

which is of the order of 10-15 microns. Both

specular and off-specular data are being used to get detailed information on interfacial structures

in such films.

63
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

0.5

Q(A-!)

10~6

1 0.00 0.08

Figure 1. (top) a) X-ray reflectivity as a function Q from 1450 A thick film of P(S-b-d-MMA) diblock copolymer,

(bottom) b) Neutron reflectivity data from the same film. Note that k

Q = 2Q in figure 1(b).

References

[1] E. Bouchard, B. Farnoux, X.Sun, M. Daoud, and G.

Janink, Europhysics Lett. 2, 315 (1988).

[2] M. L. Fernandez, J. S. Higgins, J. Penfold, R. C. Ward, C. Shackleton, and D. J. Walsh, Poly. 29, 1923 (1988).

[3] S. H. Anastasiadis, T. P. Russell, S. K. Satija, and C.

F. Majkrzak, Phys. Rev. Lett. 62, 1852 (1989).

[4] S. H. Anastasiadis, T. P. Russell, S. K. Satija, and C.

F. Majkrzak, J. Chem. Phys. 22, 5677 (1990).

64
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON REFLECTIVITY FIT PROGRAM

J. F. Ankner

Neutron reflectivity continues to be

important in the study of the interfacial structure of thin films and multilayers of polymer and

both magnetic and nonmagnetic metal and semiconductor systems. We have developed several

least-squares fitting programs to aid in the

extraction of the depth profile of a sample from

its reflectivity.

The reflection and refraction of a neutron

plane wave are described by the Schrodinger

equation and can be solved exactly. We have

adapted the x-ray formalism of Parratt [1] to describe the reflectivity of an arbitrary number

of layers deposited on a flat substrate. This

reflectivity depends on the refractive indices,

thicknesses, and interfacial roughnesses of the

constituent layers, which we can selectively vary

as parameters in a nonlinear least-squares

Levenberg-Marquardt [2] fit routine.

We calculate the effect of interfacial

roughness in two different ways. The program

PvEFFIT employs the static Debye-Waller factor

described by Nevot and Croce [3] to model the

roughness at each interface and folds these

individual reflectivities into the Parratt recursion

formula (see [4] for a FORTRAN source code

listing and description). This static Debye-Waller approach only remains valid as long as the individual layer thicknesses are much

greater than the interfacial roughnesses. Natural

oxide layers at metal and semiconductor surfaces

often violate this condition. The program

BLOCKS explicitly generates interfacial layers

and can therefore be used to model such cases of

extreme interdiffusion.

Figure 1 shows the reflectivity of a nickel

film used to determine the resolution of the BT-7

instrument for a particular slit configuration,

with the data fitted using REFFIT. The best-fit parameters show that a 1043 A Ni film sits atop

a float glass substrate with a surface roughness

of o0Ni = 13.0 A and roughness at the interface

of oNiG = 2.9 A, as measured by a neutron

beam with 60 = 0.28 mradian and 6A./A. =

0.009 (X = 2.35 A).

i I i i i

-s V -

+>S-2 -

fc C0) K

*~o • ¥1iLa jf-4 •Wti i

-6 i i

i i TM

0.00 0.06 0.12 Q (A"

1)

0.18

Figure 1. Neutron reflectivity of nickel film sputtered on float glass substrate. The fitted parameters are: film thickness d — 1043 A, nickel/vacuum roughness 0qm- = 13.0 A, and nickel/glass roughness ctmjq = 2.9 A.

These programs should make the

determination of layer profiles more accurate and

less time-consuming. We have nearly completed

a program (MLAYER) to fit the reflectivity

of artificial superlattices and plan to model

supermirrors in the near future.

References

[1] L. G. Parratt, Phys. Rev. 95, 359 (1954).

[2] W. H. Press, B. P. Flannery, S. Teukolsky, and W. T. Vetterling, Numerical Recipes, Cambridge:

Cambridge University Press. 1986.

[3] L. Nevot and P. Croce, Rev. Phys. Appl. 15, 761

1980).

[4] J. F. Ankner, Ph. D. Thesis, University of Illinois at Urbana-Champaign, 1990 (unpublished).

65
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SANS FROM HOT STRETCHED PARTIALLY DEUTERATED POLYSTYRENE

B. Hammouda

and

J. Schroeder and R. A. Bubeck (Dow Chemical, Midland, MI)

The small angle neutron scattering technique

is applied to hot stretched 5% perdeuterated

polystyrene in order to investigate the residual

macromolecular orientation. The observed

orientation is left over after fast quenching (in

ice water) of the hot stretched sample. Previous

similar investigations [1] focused on the effect of

higher external draw ratios (EDR) on the radius

of gyration. The present measurements [2] also

included varying the hot stretching temperature

and strain rate. Perdeuterated macromolecules are found to follow the external draw ratio affinely for low

EDR's as expected. The extent of chain

anisotropy is found to increase with increasing

EDR, with increasing strain rate, and with

decreasing hot stretching temperature. Local

macromolecular relaxation was observed for

short chain segments. This is shown as an

increase of the eccentricity with the momentum

transfer Q in figure 1

1.00

0.80

5

060

ui 0.40- o

o

UI 0.20

t }t ' t'ii-l

t

*

• •• NIST

ooo MURR

0.02 0.04

Q(A')

0.06 0.08

Figure 1. Variation of the ellipticity of the isointensity contour plots with momentum transfer Q. Data shown

were taken on the MURR-SANS and the NIST-SANS 8-m

instruments. This sample corresponds to an EDR = 1.6.

Most of this work was performed before

B. Hammouda joined NIST.

References

[1] B. Hammouda, R. A. Bubeck, and D. F. R. Mildner,

Polymer 27, 393-397, (1986).

[2] D. Bradford, B. Hammouda, R. Bubeck, J. Schroeder,

C. Glinka, and P. Thiyagarajan, J. Appl. Cryst. 23, 1-5 (1990).

66
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SHEAR DEPENDENCE OF CRITICAL FLUCTUATIONS IN BINARY

POLYMER MIXTURES BY SMALL ANGLE NEUTRON SCATTERING

A. I. Nakatani and C. C. Han (Polymers Division)

Two different polymer blends were used to examine the effect of shear on the critical behavior by small angle neutron scattering. The

two different blends were polystyrene/

poly(vinylmethylether) (PSD/PVME) and

polystyrene/polybutadiene (PSD/PB). The data was analyzed based on simple rotatory diffusion

model and mode-mode coupling theory to relate a critical dimension for concentration

fluctuations to shear rate. Both model

predictions are in good agreement with our

results. PSD of Mw = 4.4 x 105 and Mw/Mn =

1.28 was blended with PVME (Mw = 1.8 x 105 and Mw/Mn = 1.76). The PSD/PVME blend

composition was 20:80 by weight and represents

the critical composition based on the data of Han

et al. [1]. The PSD/PVME blend exhibits LCST

behavior. A second PSD ^Mw = 1.0 x 103 and

Mw/Mn = 1.04) was blended with PB with a nominal Mw = 4.5 x 103

. The sample

composition was 77:23 PSD/PB by weight and is off-critical. The PSD/PB blend displays UCST

behavior.

The shear apparatus and methods used for

data acquisition have been described in detail elsewhere [2]. Sector averages of the scattered

intensity parallel and perpendicular to the flow

(horizontal and vertical) were obtained in + 10°

sectors. The PSD/PVME blend was studied

between 100 °C and 142.5 °C at shear rates between 0.02 s"

1 and 1 .30 s"1

. The optical cloud

point at zero shear was 140 °C. The scattering

profiles for the PSD/PB blend were obtained

between 35 °C and 80 °C and at shear rates ranging between 50 and 200 s . The zero shear

cloud point was determined to be approximately

40 °C by optical methods. For each system in the one phase region at constant temperature, a decrease in scattering intensity at low q was

observed which implies the suppression of large

fluctuations. A cutoff value, qs

, was determined

as the value of q where the zero shear and

sheared structure factors diverge. As the shear

rate increases, qs also increases. Perpendicular

to flow, no changes in the scattering behavior were observed.

For rotatory diffusion of a sphere in solution

the following relation holds [3]:

T = ©

1 _ 8nTiR3 /kBT (1)

where t is a rotational relaxation time, is the

rotational diffusion coefficient, r\ is the solution

viscosity, R is the radius of the sphere, kB

is Boltzmann's constant and T is the temperature.

For the case of concentration fluctuations under

the influence of shear, at a given shear rate y, concentration fluctuations with t > 1 /0 (large

R), will be suppressed. The suppression of these

large fluctuations below qs is related to as follows:

»-i t = 0"* = (8KTi/kBTqs J

) (2)

The viscosity and temperature dependence of the

experimental qs values may be analyzed by

plotting [log qs - (l/3)log n + (l/3)log T]

versus log y. From equation 2, a plot of this form should give a slope of 1/3 and an intercept

value of (l/3)log(8it/kB) = 5.753.

From the mode-mode coupling analysis of

Ferrell [4] and Kawasaki [5], the decay rate, T , of fluctuations with wavenumber q may be

written as a sum of molecular and hydrodynamic

contributions. If we first neglect the molecular

contribution to the relaxation process, the

hydrodynamic term is defined as (kBTq2 /67rr|£)

for q£<l and (kBTq3 /16ii) for q£>l. The

shear rate dependence of the longest wavelength,

qc

_1

, not affected by shear rate is therefore given 1C as:

k l/2 - 1/2 qc = (6iiii VkBT) u-y l,s for qc £ < 1 (3a)

= (16ri/kBT) 1/3 Y

1/3 forqc £>l (3b)

Equating the experimentally determined qs to qc , we would expect in the strong shear limit

(qc £>l), a slope of 1/3 and an intercept of

67
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

(l/3)log(16/kB

) = 5.688 for the types of plots

discussed in the preceding paragraph.

Data for both the PSD/PVME and PSD/PB

blends were combined in the [log qs - (l/3)log t|

+ (l/3)log T] versus log y plot shown in figure

1. The experimentally determined viscosity

values for both blends were utilized in the

analysis. The slope was determined by linear

regression analysis to be 0.332 with a standard

deviation of 0.006. The intercept value was

5.71 with a standard deviation of 0.01. These

values are entirely consistent with the rotatory

diffusion analysis as well as the mode-mode

coupling analysis with the hydrodynamic term

dominating the molecular term. Hashimoto [6]

and Onuki [7] have argued that the molecular

contribution to the relaxation should be the

dominant process and predict that in the strong

shear limit, qc should vary with a y

1/4 power

dependence. Our data does not support this prediction; however, Hashimoto and Onuki have

argued from the standpoint of a blend in the two

phase region while our data is taken in the one

phase region of the phase diagram.

n

noon

6.8

6.6

6.4

6.2

6.0

5.B

I 5.6

bo 5.4 o

5.2

5.0-2.00

PSD/PVME

o T = 140°C

o T = 137.5'C

a T = 135'C

• T =* 130*C

PSD/PB

T = 45'C

• T = 42.5"C

• T = 40"C

-1.00 0.00log y

1.00 2.00

Figure 1. Log q - (l/3)log r\ + (l/3)log T vs log y Open symbols are data for the PSD/PVME blend. Closed symbols are

data for the PSD/PB blend.

References

[1] C. C. Han, B. J. Bauer, J. C. Clark, Y. Muroga,

Y. Matsushita, M. Okada, Q. Tran-Cong, T. Chang,

and I. C. Sanchez, Poly. 29, 2002 (1988).

[2] A. I. Nakatani, H. Kim, and C. C. Han, J. Res. Natl.

Inst. Stand. Technol. 95, 7 (1990).

[3] See, for example, C. Tanford, Physical Chemistry of Macromolecules , (John Wiley and Sons, NY, 1961), 432-451 pp.

[4] R. Ferrell, Phys. Rev. Lett. 24, 1122 (1970).

[5] K. Kawasaki, Phase Transitions and Critical Phenomena, C. Domb and M. S. Green, eds. 5a, New

York: Academic Press; 1976).

[6] T. Hashimoto, T. Takebe, and S. J. Suehiro, J.Chem.

Phys. 88, 5874 (1988).

[7] A. Onuki,J. Chem. Phys. 85. 1122 (1986).

68
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

MICROSTRUCTURE EFFECTS ON LCST PHASE BEHAVIOR OF POLYBUTADIENE

AND POLYISOPRENE BLENDS STUDIED BY SANS AND LS

S. Sakurai, H. Jinnai, H. Hasegawa, and T. Hashimoto (Kyoto University, Kyoto, Japan)

and

C. C. Han (Polymers Division)

It is extremely important to control the state of mixing and the phase-separated structure of

polymer blends if it is expected for the blends to have superior properties than those of the

individual constituent polymers. For this purpose, it is necessary to clarify the

fundamental relationships between the molecular

parameters of constituent polymers and the static and dynamic phase behaviors of polymer

mixtures; e.g., miscibility, phase diagrams, and

mechanism and dynamics of phase transition.

The molecular parameters to be considered are the molecular weight (M), polydispersity

(Mw/Mn

) of polymers, molar volume of monomeric units (v), statistical segment length (b)

related to the flexibility of the polymers, and the

thermodynamic interaction parameter (x)

between two monomers. The other parameters

such as copolymer composition, microstructure

of polydienes, tacticity of vinyl polymers and

isotope effects can be taken into consideration in terms of x-parameter. Our aim is to clarify

these relationships by a systematic scattering

study using well-defined polymer systems.

Polydiene samples were synthesized by

living anionic polymerization initiated by sec- butyllithium, and have narrow molecular weight

distributions. Three HPI samples (sample code:

VIRH) were synthesized and have the microstructures with the content of 3,4-unit in the

range of 7% to 15%. Four DPB samples (sample

code: VBRD) were synthesized and have the

microstructures with the content of 1,2-unit in the range of 12% to 28% . SANS profiles for all HPI/DPB blends studied show increase in intensity at all q's with increasing in temperature. These phenomena are typical for

LCST-type of phase behavior. Data were fitted,

using nonlinear regression fitting routine, with

the scattering theory of deGennes based on the

random phase approximation (RPA). The

effective interaction parameter, xe ff> tne

correlation length, £ and the zero-wavenumber

structure factor, S(q = o) obtained are used to

establish phase diagram of different blends. In order to discuss the microstructure effects on the

blend miscibility, xeff for each HPI/DPB blend

at the same composition was compared and

plotted against T" 1 in figure 1 (for <J>dpb =

0.48). There are 10 sets of data on xeff which

are shown in two figures; figure la shows the

effect of the DPB microstructure on xeff f°r three given HPI's and figure lb shows the effect of the HPI microstructure on xeff for three given

DPB's. xeff shows a linear relationship with

T" (xeff = A + B/T) with a negative slope (B

< 0) for each blend, indicating that all the

HPI/DPB blends studied here show the LCSTtype phase behavior, irrespective of the

microstructure. Moreover, it can be seen that as the content of 1 ,2-unit in DPB increases and/or

the content of 3,4-unit in HPI decreases, the xe ff value decreases, and therefore the blend

becomes more miscible for given molecular

weights and a composition of the blends.

By neglecting a difference between cis-1,4

and trans- 1,4, it can be assumed that HPI is a random copolymer of 3,4-unit (Iv) and 1,4-unit

(I) and that DPB is that of 1,2-unit (Bv) and 1,4-

unit (B). Then xe ff is given by six sets of

fundamental x's according to copolymer theory.

By solving these equations with minimizing

the square residuals, the fundamental x was

obtained as a function of temperature, as shown

in figure 2. All the fundamental x's exhibit the

temperature dependence giving rise to the

UCST-type phase behavior excepting Xiv bv an^

Xj 3 which are likely to be independent of the

temperature. To confirm validity of these

solutions, the predicted xe ff' s f°r a^ ten systems was compared with the experimental data on

Xeff's in figure 3. The predicted xe ff is found to be in good agreement with the experimental xe ff- Although the fundamental x's have the

temperature dependence relevant to the LCSTtype, a subtle balance among the various

fundamental x's gives rise to a temperature

69
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

dependence of xeff relevant to the LCST-type for

the blends studied in this work.

(a)

200 Temperature (-C)

150 100 50

(b)

X

200 Temperature ("C)150 100 50

-

I -

-

o -

2.0

I

-$on= 0.475

: *-X5

^^^^

-

(b)

1 1

2.5 3.0

T" 1 x I0 3 (KH

)

Figure 1 (a). xe

ffPlotted as a function of T1 for the blends of VIRH-7-101 with (1) VBRD-12-52, (2) VBRD-16-61, (3) VBRD-20-103, and (4) VBRD-28-71, for the blends of VIRH-12-192 with (5) VBRD-16-61, (6) VBRD-20-103, and for (7) VBRD-28-71, and for the blends of VIRH-15-136 with (8) VBRD-16-61, (9) VBRD-20-103, and (10) VBRD-28-71. The

composition of each blend is fixed at (^j-jpg =0.475. The first suffix number indicates the vinyl content of the sample and the second suffix number indicates the molecular weight in 10 for the sample, (b). xe

ffplotted as a function of T~ for the blends of VBRD-16-61 with (1) VIRH-15-136, (2) VIRH-12-192, and (3) VIRH-7-101, for the blends of VBRD-20-103 with (4) VIRH15-136, (5) VIRH-12-192, and (6) VIRH-7-101, and for the blends of VBRD-28-71 with (7) VIRH-15-136, (8) VIRH-12-192,

and (9) VIRH-7-101. The composition of each blend is fixed at ^npg =0.475.

Temperature (°C) 50

2.5 3.0 T- 1 x 10- 3 (K" 1

) Figure 2. Fundamental x'sas a function of T .

Temperature (*c) 150 100 SO

J- *«= 0.475

° experimental D

n J Xeff

predicted X c ff

1 L

20 2.5 3.0 T"

1 x I0 3 (K"')

70

Figure 3. Measured (data points) and

predicted values of xe ff (solid lines) plotted against T~ .
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

TIME-RESOLVED SMALL-ANGLE NEUTRON SCATTERING STUDY

ON LATE STAGE SPINODAL DECOMPOSITION OF POLYMER BLENDS

H. Jinnai, H. Hasegawa, and T. Hashimoto (Kyoto University, Kyoto, Japan)

and

C. C. Han (Polymers Division)

Recently there have been a number of studies on the kinetics of phase separation of polymer

blends in early stage spinodal decomposition

(SD) and on self-assembling processes,

dynamics, and pattern formation (morphology) in the late stage SD as a problem of nonequilibrium statistical mechanics of long chain

molecules. Most of the experimental studies

utilized time-resolved light scattering (LS)

methods, and only a limited number of studies

utilized time-resolved small-angle neutron

scattering (SANS) and small-angle x-ray

scattering (SAXS) methods. It has been shown,

efficiently, by time resolved LS, that the

growing global structure can be scaled with a single time-dependent length parameter Am(t). There still remains an important question on

whether the local structure; e.g., interfacial structures including waviness of the interface and

interfacial width, can also be scaled with Am(t). The time-resolved SANS is a powerful method

to investigate this problem as it can approach

much shorter length scale than LS. In this series

of study we investigated the time evolution of

the interfacial structure using time-resolved

SANS and LS on the same blend specimens. In

this report we discuss general trends on the timechange of SANS profiles in detail and the result on the time-evolution of the composition

difference, A$(t), between two coexisting

domains in the late stage SD.

Deuterated polybutadiene (DPB) (Mw =

10.3 x 104

, Mw

/Mn = 1.03

) and protonated

polyisoprene (HPI) (Mw = 13.6 x 104

, Mw

/Mn = I.O4) samples are used in this study. The

volume fraction of DPB of this mixture is very

close to that of the critical mixture, 4>c DPB =

0.52, according to Flory-Huggins theory. This

mixture exhibited the LCST behavior and had

the spinodal temperature Ts = 36. 1 °C by the

SANS analysis of single-phase state. Figure 1 shows the time-evolution of the

structure factor during early-to-late stage SD.

The left and the right half of the figure

correspond to the structure factors obtained by

LS and SANS, respectively. Two sets of

structure factors are vertically shifted for

convenience but are plotted on the common

logarithmic scales to compare the time-evolution

of SANS profile with that of LS. It is quite

obvious that the change of the structure factor at small q's observed by LS is quite large

compared with that at large q's observed by

SANS. The vertical broken line at qR„ = 1 indicates the q value corresponding to 1/R a where R is an average radius of gyration 0?

DPB and HPI. Thus our SANS experiments are concerned with the time-change of the

composition fluctuations at a length scale of R

or at even much shorter sale in early-to-late

stage SD.

SANS profiles IT

e

(q) due to the local composition fluctuations in single-phase state could be quantitatively described by the Random

Phase Approximation. This provides a guiding

principle to determine the time-evolution of the

composition difference A<I>(f) for two phases in the later stage SD. The time-resolved SANS

profiles S(q,t) at the early-stage SD (t < 30

min) show a crossover behavior in 3S(q,t)/dt;

the derivative is positive, nearly zero, and

negative, respectively, at q < q* (regime I), q

aq* (regime II) and q > q* (regime III) but the crossover q* was much larger than the qc predicted by the linearized theory of SD. S(q,t)

at a given t showed the crossover from the

domain scattering Id(q,t) to the internal scattering

IT(q,t) with increasing q through a timedependent crossover wavenumber qs (t). The

wavenumber qs (t) primarily reflects the growth

of the global domain structure Am(t)=2ic/qm

(t), and hence its time-change occurs as a result of

the shift of the large q-tail of the domain

scattering I d

(q,t) toward the smaller q region

with t. I T

(q,t), which depends on the timechange of the local composition <|> A1 (t) and

4> A

-,(t) in domains 1 and 2, allows us to determine A<I>(t). Both qs (t) and A<D(t)

71
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

underwent a characteristic change at the crossover time tcr(* 270 min) from the

intermediate to the late-stage SD as shown in figure 2.

3.5

3.0 >-

h;

COz

111 f2.5

O

O

2.0

1.5

1.0

q (nnr1 ) o

4 5 6 7 8 9 10' V> 7 8 9 10-1 —

x'xXgXX

:

( **

x x

"•« If •

i r LS

X** ^\** x o* * *

•«.*-'

ft-

"i—i—

1 -^o SANS

lf»

No. t (min.

)

1 • 9. A3 2» 121.38 3

* 191.58

4-.« 306.76 5« «2.60

-1 L

8

-2.6 -2.4 -2.2 -2.0

v

" -1.2 LOG (q/nm-1 )

-1.0 -0.8 -0.6

Figure 1. Time-evolution of LS and SANS profiles after the onset of the phase separation.

time / min.

102 -0.5

-0.6-

4 5 6 7 8 9103

,

O -0.8 O

-0.9-

-1.0

I I I

"* O ° s era

1 1 1 1 T"

- °o fry*

^D D tcr = 270 (min.) o

i

II

i i i i i

r

slope=-1

i i i

0.25

O

O

0.3 >

O

2.5

LOG (time/min.)

0.35

Figure 2. Time-change of the crossover wavenumber q and the composition difference between the two coexisting phase A<f>(t) plotted in double logarithmic scale for a HPI/DPB (50/50) blend after T-jump from 23 to 40°C.

72
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SMALL-ANGLE NEUTRON SCATTERING CHARACTERIZATION

OF PROCESSING/MICROSTRUCTURE RELATIONSHIPS

IN THE SINTERING OF CRYSTALLINE AND GLASSY CERAMICS

G. G. Long (Ceramics Division) and S. Krueger

R. A. Gerhardt (Rutgers University, Piscataway, NJ)

and

R. A. Page (Southwest Research Institute, San Antonio, TX)

Knowledge of the microstructure evolution

as a function of thermal processing is important

for the development of process models in ceramics. In particular, such techniques as liquid phase sintering have received renewed

interest, and it will be important to achieve a

detailed understanding of the mechanisms

involved to take full advantage of the possible

benefits of the technique. Since both viscous

flow and diffusion, as well as reactivity, are generally involved, it is useful to first investigate

the porous microstructure evolution as a function

of sintering in two relatively well understood

model systems, crystalline alpha-alumina, in which diffusion mechanisms dominate, and

porous glassy silica, in which viscous flow

mechanisms are dominant.

One important consideration is the surface

free energy, which is the driving force for

sintering. For a glass, the minimum free energy

surface is a sphere. For a crystalline material,

the surface tension is anisotropic, and there are

facets, well-defined flat edges, and sharp

corners. During sintering, the elevated

temperature causes the surfaces to become

rounded because the surface energy is reduced.

Another important consideration is the grain

boundary. In the densification of a crystalline

compact, there is a general tendency to eradicate

both free surfaces and grain boundaries. The

grain boundaries are thought to act as vacancy

sinks, permitting the removal of pores. In the

glassy body, there is no grain boundary, and this

is expected to have consequences on the

microstructure evolution during sintering.

The present study was undertaken to examine processing/microstructure relationships

in a crystalline and in a glassy system in order to gain a quantitative measure of the structural evolution that takes place when different

sintering mechanisms dominate.

In the experiments that form the subject of

this work, samples of pure alumina were

prepared by slip casting, and samples of

microporous silica were prepared by a sol-gel

process. For each system, a collection of nearly

identical samples were prepared and partially

sintered. During sintering, samples were removed one by one from the furnace as they

achieved intermediate densities representative of

sintered microstructure from the early part of

intermediate stage sintering to the late part of

final stage sintering. Microstructure parameters

of these samples were derived from small angle

neutron scattering measurements and multiple

small angle neutron scattering measurements, as required by the sizes and volume fractions of the

pores.

Both the low-Q and the large-Q portions of

the scattering curves were analyzed for the two

scattering systems. The low-Q part of the data was analyzed using SANS or multiple-SANS

(MSANS) theory, and the large-Q part of the curve was studied via Porod analysis. For

silica, the two analyses were qualitatively

different. This was taken as evidence of the

existence of two populations of pores in the

silica system, consistent with a model [1] proposed by Cao, Gerhardt and Wachtman. In

this model, there is a population of small pores

between fine silica particles which exist in clusters or domains. There is also a population

of larger pores between domains, and it is this population which dominates the MSANS [2] data. Results from the low-Q portion of the

scattering curves for silica are shown in figure 1

,

where it can be seen that the interdomain pores

first coarsen, presumably while the domains

densify, and then shrink when domain densification is complete.

Comparing MSANS results and Porod

results for the alumina indicates that there is

73
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

1.0 i i

1 1 1 1 r o polycrystalline alumina

glassy silica

0.1

0.0150 60

J L 70 80

7. TD

90 100

Figure 1. Pore radius as a function of the percent theoretical density for silica and alumina.

only one population of pores in this system. The

SANS results are volume-weighted and the

Porod results are surface-area weighted, but both

results are qualitatively the same. Each set of

results indicates a constant pore radius during the

intermediate stage of sintering, unlike silica for

which coarsening and subsequent shrinkage were

seen, and a rapidly increasing pore radius during

the final stage of sintering. These large sizes were recently confirmed [3] using highresolution SAXS measurements on the same

samples. The SANS results on alumina, also

shown in figure 1 , lend support to a topological

decay model of Rhines and DeHoff [4] in which

the interconnected channel network of pores

decays in a stable manner. The pore channels

become fewer, although of the same diameter, as densification proceeds. At the onset of the final stage, the pore radius coarsens abruptly as the

stable decay breaks down.

Taken together, the silica and alumina

results shown in figure 1 highlight the different

microstructure evolution signatures that exist for

the viscous flow and the diffusion mechanisms of

sintering.

This work was supported in part by the U.S.

Army Research Office under contract no. MIPR

ARO 102-90, and in part by the U.S.

Department of Energy under contract no. DE-FGO5-84ER45063.

References

[1] W. Cao, R. A. Gerhardt, and J. B. Wachtman, Jr.,

J. Am. Ceram. Soc. 71, 1103-1113 (1988).

[2] N. F. Berk and K. A. Hardman-Rhyne, J. Appl.

Cryst. 18, 467-472 (1985).

[3] S. Krueger, G. G. Long, D. R. Black, D. Minor, P. R. Jemian, W. Nieman, and R. A. Page (in preparation).

[4] F. N. Rhines and R. T. DeHoff, Sintering and Heterogeneous Catalysis , Mater. Sci. Res., Jj6, G. C. Kuczynski, A. E. Miller, and G. A. Sargent, eds. (Plenum Press, NY, 1984).

74
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SMALL-ANGLE NEUTRON SCATTERING MEASUREMENT OF THE EFFECT OF

GREEN DENSITY AND THE ROLE OF MgO ADDITIVE ON THE

DENSIFICATION OF ALUMINA

S. Krueger, and G. G. Long (Ceramics Division)

and

R. A. Page (Southwest Research Institute, San Antonio, TX)

Numerous sintering models have been

developed over the years in attempts to reliably

predict the kinetics of sintering and to estimate

sintering rates. Although it has been difficult to find a consensus of opinion regarding many

aspects of sintering mechanisms, there is general

agreement that the driving force for sintering is the surface free energy, and that the microstructure characteristics are determinative of the

specific mechanisms by which the pore and the

solid phases evolve during densification. The

present investigation was undertaken to examine

the microstructure evolution during sintering

with the goal being to gain insight into the

mechanisms operative during thermal processing.

In the experiments that form the subject of

this research, samples of pure alumina and of

alumina with 0.25 wt.% MgO added were pre- pared by slip casting, or alternatively by cold

pressing. For each green state density, a collection of nearly identical samples were

conventionally sintered. During sintering, these

samples were removed one by one from the

furnace as they achieved intermediate densities,

yielding examples of microstructure from the

early part of the intermediate stage of sintering out to the latter part of final stage sintering. The

microstructure parameters of these samples were

derived from multiple small angle neutron

scattering measurements [1,2].

Figure 1 shows the evolution of the pore

size as a function of densification, or percent of

theoretical density (%TD), for four suites of

samples: slipcast alumina, cold pressed alumina,

0.9

0.7

3

•i—

i

T*

CO K 0.5

CDU

o

a 0.3

0.1

1 I

1 1 1

OSlip cast - No MgO

Slip cast - 0.25 wt% MgO

51%

1

52 64 76

% TD

88 100

Figure 1. Effective pore radii as a function of %TD for the four series of samples

75
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

slipcast alumina doped with MgO, and cold

pressed alumina doped with MgO. The numbers

associated with the left-most symbols in figure 1 indicate the average green density of each suite

of samples. It can clearly be seen that the

effective pore radius was constant during the

intermediate stage of sintering for each of the

suites of samples, independent of doping and

green density, and consistent with a stable

topological decay model [3] of porosity during

intermediate stage sintering. However, the

actual value of the effective pore radius depends

inversely on the green density. These results indicate that the connectivity that is established

in the alumina green body determines the pore

size throughout the stable intermediate sintering

stage, and once this size is determined, it is unvarying as long as isolated porosity has not yet

begun to form. At the onset of final stage

sintering, the pore radius apparently coarsens

abruptly. Part of this apparent coarsening may

be due to the pinching- off of pore channels such

that the larger dimension of the newly isolated

pores comes increasingly within the range of

sizes visible to the MSANS technique.

Figure 1 also indicates that for the samples

with 0.25 wt.% MgO, the intermediate sintering

stage remains stable out to significantly higher

density (93%TD) than the pure alumina, which

remains stable out to 87%TD. Consequently,

the pores in the doped sample begin to coarsen

later and thus the grains do not suffer the same

degree of exaggerated growth evident in the

thermal processing of the pure alumina, even

though the average green density was lower in the doped sample.

In summary, it was found that the initial connectivity in the green state plays a dominant

role in establishing the channel diameters during

the intermediate stage of sintering, and

contributes also to determining the onset density

at which the final stage of sintering begins. The

role of MgO as a sintering aid lies, at least in part, in prolonging the stability of intermediate

stage sintering such that the body achieves

greater density before the transition to final stage

sintering after which isolated pores are formed.

This work was supported in part (GGL) by the

U.S. Army Research Office under contract number MIPR ARO 102-90 and in part (RAP)

by the U.S. DoE under grant number

DE-FGO5-84ER45063.

References

[1] N. F. Berk and K. A. Hardman-Rhyne, J. Appl. Cryst.

18, 467-472 (1985) and J. Appl. Cryst. 21, 645-651

(1988).

[2] G. G. Long and S. Krueger, J. Appl. Cryst. 22, 539-545 (1989).

[3] F. N. Rhines and R. T. DeHoff in Sintering and

Heterogeneous Catalysis , Mater. Sci. Res. J_6, G. C. Kuczynski, A. E. Miller, and G. A. Sargent, eds. (Plenum Press, NY, 1984).

76
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

MICROSTRESSES IN CERAMIC COMPOSITES

H. J. Prask

C. Choi (ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

E. Fuller and R. Krause (Ceramics Division)

A. Krawitz (University of Missouri, Columbia, MO)

and

J. Richardson (Argonne National Laboratory, Argonne, IL)

Micro residual stresses occurring between

constituents of a composite, e.g. between

particles or fibers and matrix, are indicators of

the strength of bonding between these

components. X-ray diffraction provides the bestestablished method for determination of residual

stresses and has been used for the study of

ceramic composites. In the x-ray case, however,

surface macro-stresses contribute to peak shifts and complicate the analysis. Neutron diffraction

parallels x-ray diffraction for residual stress determination but because neutrons in the

diffraction-wavelength regime penetrate,

typically, three orders of magnitude deeper than

x-rays, either depth-profiling or total-sample

averaging of stresses is possible. When a sample is totally immersed in the neutron beam

the macro-stress contribution to peak shifts averages to zero. In the present measurements, A12 3 reinforced with SiC(whisker) was studied. Two

different types of whiskers at two different

concentrations were used: TWS 100 (~0.4 \im

diam, ~ 10 urn long) and TWS 400 (~ 1.1 \im

diam, -50 u.m long), each at 20 and 30 vol%.

With each whisker type, processing into the

composite was the same: hot-pressing into a disk

5 cm in diam and -0.25 cm thick, at 6000 psi

at 1700 °C for 30 min. The samples for

neutron study were cut from the center of the

pressed disks, stacked and glued, and ground to cubes -0.9 cm on a side.

The general purpose powder diffractometer

at the IPNS/Argonne National Laboratory was

used to collect data for all reflections in the

range 0.767 < d(hkl) < 2.972 A for four

different directions in the sample coordinate

system. Of the many reflections observed the

(113), (024), (116), and (300) of alumina and

(111), (220), and (311) of the cubic SiC were

selected for analysis based on intensity and

freedom from overlap. Unstressed d-spacings,

d 's, were obtained from the starting materials. Stresses were calculated from the strains in the

standard way, using the elastic isotropic

diffraction elastic constants for both materials.

Representative results are shown in table 1 for one sample.

For each reflection for each constituent it is clear that the pressing direction is unique, with

less average stress than is observed in the

orthogonal directions, which are essentially

equivalent. As one would expect from the

difference in thermal expansion coefficients, and

as has been observed elsewhere, the whiskers are

in a state of high compression, the matrix in tension. The marked variation in stress magnitude for the same stress component for

different reflections indicates that the use of

"bulk" elastic constants is not justified. In table 2 the stress values averaged for the reflections of

each constituent are listed.

It is interesting that in the matrix the

volume fraction seems to be the critical quantity

in that the two 30 vol% samples show stress magnitudes in the alumina that are very similar. In the whiskers, no such clear correlations are seen although the TWS 400 composites look

similar. This correspondence between sample

types is also exhibited in the measured total peak

breadths. One puzzling feature of these results

is that it appears that no reasonable adjustment

of the diffraction elastic constants can be made

which would lead to balancing of microstresses

between the two constituents for all four

samples.

77
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Table 1. Average microstresses in the 20 vol%/TWS 400 sample

°xx °yy

* °zz

Al2 3 (113) 296(5)MPa 299(18) **

221(6)

(024) 314(9) 325(34) 239(12)

(116) 412(4) 416(14) 345(4)

(300) 214(2) 228(8) 141(3)

SiC(lll) -1126(14) -1148(60) -933(17)

(220) -1066(4) -1030(19) -894(6)

(311) -1180(4) -1120(15) -1094(5)

The pressing direction. Standard deviations due to counting statistics only.

Table 2. Averaged residual stress values

xx yy

Ai">C>3 TWS 100, 20vol% 200MPa 197 103 TWS 100, 30vol% 461 460 324

TWS 400, 20vol% 309 317 237

TWS 400, 30vol% 403 464 319

SiC

TWS 100, 20vol%

TWS 100, 30vol%

TWS 400, 20vol%

TWS 400, 30vol%

-1330MPa -1334 -1244

-1148 -1117 -1127 -1124 -1099 -974

-1056 -1037 - 937

78
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

A CHORD DISTRIBUTION DESCRIPTION OF MICROPOROUS GLASS

M. Y. Lin (Exxon Research and Engineering, Annandale, NJ)

S. K. Sinha (Brookhaven National Laboratory, Upton, NY and Exxon, Annandale, NJ)

and

C. J. Glinka

We have used a chord distribution model

for the microstructure of Vycor glass, a widely

used model system for microporous media, to analyze small angle neutron scattering (SANS)

measurements on this material. The chord

distribution derived from the analysis is in good

qualitative agreement with that determined by

other techniques. In addition, we have used the

model to study the process of capillary

condensation, and show how the structure probed

by SANS evolves as liquid condenses in the

pores.

A chord (length) distribution of a random

isotropic two-phase structure is defined as the

distribution of the distance between two adjacent

interfaces along a line at an arbitrary direction.

Depending on the substance between the two

interfaces, it can be either a pore chord

distribution, or a solid chord distribution. These

are denoted as F (1) and Fs (l), where the

subscript p stands for pore and s stands for solid. The two functions are therefore a description of

the porous structure. Levitz et al. have

measured the actual chord distributions for

Vycor glass similar to those we use [1]. Neutron scattering data were obtained at NIST's 8-m SANS spectrometer [2]. The

scattering structure factor S(Q) was measured as a function of the momentum transfer Q. It can be expressed in terms of the chord distribution as

S(Q) ~ 2-fp

{Q) - fs(Q)

+ C.C.

i -fp(QVs(Q)

(l + QH2r> (1)

where C.C. is the complex conjugate of the

previous term, and f

s (Q) are the complex

Fourier transforms of the chord length

distributions F s (l) over the range 1 = to 1 =

~. The factor (1 + Q

2 $2)"'t will dominate when

Q > 2u7£, where £ is about 10 A. It describes

the surface roughness of the solid-pore inter- faces. When the surface is describable by

fractals, ji = (6-Ds )/2 where Ds > 2 is the

surface fractal dimension. For a smooth surface, u =2.

Equation 1 was used in analyzing the SANS

data. The exponent u. = 2.4 is predetermined

using data at large Q in order to minimize the

number of the free parameters. For the chord

distribution we assume a form as

F(l) = l a (Ae bl + Be"

cl

) (2)

The result using equation 1 to fit the

scattering data is shown in figure 1 . The fitted parameters give the average pore size 1 = 84.6

A, the average solid size 1 = 177.6 A, and the

porosity is L/(l + l s

) = 0.32. These values are

in good agreement with the results obtained by

other measurements [2]. Using the fitted parameters, both pore and solid chord

distributions can be constructed and the pore

chord distribution is shown in figure 3. The

pore chord distribution is peaked at 1 = 71 .4 A.

A comparison of the pore chord distribution with

that measured directly with TEM pictures shows

a qualitatively good agreement of the two forms,

although the one shown here decays faster at large chord distances.

We have also studied the process of capillary

condensation inside Vycor's porous structure. The condensing liquid used was a mixture of

C6H14 and C6D14 which acts as a simple liquid

in condensation, and which has the same

scattering density as the solid part of Vycor glass

(silica). This ensures that when condensation

takes place inside the pores, the microstructure seen by the neutrons is still two phases: the

condensing liquid and the solid are one phase,

and the remaining space in the pores occupied by

the much-lower density gaseous molecules is the

other phase. Thus, the chord distribution model

discussed above should still be applicable.

The scattering data were obtained in the same experiment described above. In this case, the molecular pressure of the C6H 14/C6D 14

79
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

mixture is increased from zero (vacuum) to ps

, the saturated vapor pressure of the mixture in bulk, in several steps. For each p, a set of

scattering data was taken. The data are shown

in figure 2. At p = ps

, the entire pore space of

the glass is filled with the liquid, and the sample

becomes one homogeneous phase. This is evident in the scattering data, as it is almost flat, and is more than two orders of magnitude less in peak intensity than the data at p = 0. We were able to analyze the first few data curves using the chord distribution model.

Fitting results similar to that shown in figure 1 are obtained, for p up to a value of 0.57 ps

. The next S(Q) curve at p = 0.71 ps starts to

show noticeable difference in its shape, and the

model can hardly fit the data. This is about

when, presumably, massive condensation starts to take place, as suggested by isotherm

absorption measurements [2]

.

The fact that for small p the scattering data

are well described by the model used for dry

Vycor indicates, that the condensing "liquid"

forms a thin wetting layer on the solid surface.

This changes the chord distributions, but does

not change the overall micromorphology as probed by the neutrons. As the pressure

increases to a certain value, capillary

condensation starts to take place, as the liquid

completely fills some small pore areas.

i

1—I—r-

_i i i i 0.01 0.O2 0.05 0.1 Q (A) Figure 1. SANS intensity data (squares) taken with dry

Vycor and the fit using the chord distribution model.

100000

>- 10000

I—

I

Ul

-ZL 1000

100

% x x*xx

, * t

« S + t + +

t

.005 0.01 0.02

Q (A--*)

Figure 2. SANS data from Vycor as condensation takes place in it. Data were taken at several different pressures

as marked (p = 0) is for dry Vycor).

References

[1] P. E. Levitz, G. Ehret, and J. M. Drake, J. Chem.

Phys. (submitted for publication).

[2] M. Y. Lin, S. K. Sinha, J. S. Huang, B. Abeles, J. W. Johnson, J. M. Drake, and C. J. Glinka, in Neutron Scattering for Materials Science , S. M.

Shapiro, S. C. Moss, and J. D. Jorgensen, eds. (MRS Symp. Proc. 166, Pittsburgh, PA 1990), pp. 449-454.

80
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SMALL-ANGLE NEUTRON SCATTERING OF POLY(VINYLALCOHOL) GELS

W.-L. Wu (Polymers Division)

M. Shibayama, H. Kurokawa, and S. Nomura (Kyoto Institute of Technology Kyoto, Japan)

M. Muthukuma and R. S. Stein (University of Massachusetts, Amherst, MA)

and

S. Roy (Polaroid Corporation, Norwood, MA)

Structure of aqueous poly(vinylalcohol)

(PVA) gels with and without adding borate ions,

hereafter referred as alkaline gels and hydrogel,

respectively, were investigated using small angle

neutron scattering (SANS). The correlation

length £, a measure of the wavelength of

concentration fluctuation, was estimated from

SANS results. The value of £ showed a modest

temperature dependence; it decreased with

increasing temperature. The scattering intensity

within the range of 0.3 < q£ < 10 for the

alkaline gel over a wide temperature range could

be superimposed into one master curve by

employing these reduced variables, I(q)/£

3 and

£ q, where q is the magnitude of the scattering

vector (fig. 1). This indicates that the PVA

clusters can be described adequately by a single

characteristic length over a broad temperature

range embracing the sol-gel transition

temperature, and the internal structure of the

cluster is homogeneous down to a size scale of

0.1$. For the hydrogel, the superposition was

valid only in the region of 0.3 < q£ < 2.0 (fig.

2). In the high q region of 2 < q£ < 10, an

increase in the mass fractal dimension with

lower temperatures is evident; this can be

explained by the local consolidation of the

polymer chains in hydrogels. Such a consolidation is absent in alkaline gels due to the

presence of interchain didiol-borate complexes.

The presence of interchain crosslinks in alkaline

gels while not in hydrogels manifests itself in many other aspects, for example, the

temperature dependence of both the correlation

length £ and the aggregation index. The details

of this work will be presented in a future

publication.

10 Alkaline heating

q£

-i—i 1 1

1

1

10

-4-J C 10

0)

X)

N

a 1 o

"*- E

v_O

10

ooooo 22 C ••35 C

* -- » • 49 C

o o o » « 59 C ..... 7670 C

C 83 C

89 C Hydro heating —i—i— i

1 ql

10

Figure 1 . A mater curve of normalized excess scattering

intensity versus q£ of an alkaline PVA solution. SANS

data of eight different temperatures (26 °C - 75 °C) in a heating cycle were superimposed. The solid line is the Lorentzian form (1 + £ q ) .

Figure 2. A mater curve of normalized excess scattering

intensity versus q£ of an aqueous PVA solution. SANS

data of eight different temperatures (22 °C - 89 °C) in a heating cycle were superimposed. The solid line is the Lorentzian form (1 + £~q"T
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SANS FROM TPB BASED MICROEMULSIONS

B. Hammouda

and

P. L. Lindemuth and R. L. Venable (University of Missouri, Rolla, MO)

Small angle neutron scattering has been used

to investigate tetradecylpyridinium

bromide/pentanol/heptane/heavy water microemulsions in the water-in-oil microemulsion

phase diagram region [1,2] . Heavy water content was increased while the other components

(surfactant, cosurfactant, hydrocarbon) were kept

constant. Using a simple model assuming

spherical micelles interacting with a hard sphere

potential (Percus-Yevick model), aggregate sizes

and packing fractions have been extracted and

found to agree with values determined from the

mixing conditions. These experiments clearly

show the transition from the single particle

(heavy water + Stern layer droplet) scattering

regime at low water concentration to the mixed

single/interdroplet scattering regime when the

intermicellar distance becomes comparable to the

size of the micelles.

The scattered intensity for a droplet system

of arbitrary concentration is given by:

I(Q) = P(Q) + n P(Q) hcm(Q)

in terms of the single droplet structure factor

P(Q), number of particles per droplet n, and the

interdroplet total correlation function hcm(Q)

which can be expressed in terms of the direct

correlation function

Zernicke form:

Ccm(Q) by the Ornsteinhcm(Q) = Ccm(Q)/[l-nCcm(Q)]

The Percus-Yevick form (with hard sphere

potential) for Ccm(Q) is used in figure 2.

MICELLAR CORE ±u L

f$%c£/^~ j- MICELLAR PERIPHERY

"' differ r~ 'IP

STERN LAYER

oaffr

"•"d PENTANOL "~~-© TPB

BF

RADIAL DISTANCE

Figure 1. Scattering length densities in the micellar core,

the Stem layer, and the outside hydrocarbon continuum.

SAMPLE TPB34

0.01 0.02 0.03 0.04 0.05 0.06 0.07 O.OB 0.09 Q (A"') Figure 2. Result of the nonlinear least squares fit to the Ornstein-Zernicke form with Percus-Yevick approximation

(hard sphere potential) for a sample with 12% water volume

fraction.

Most of this work was performed before B. Hammouda joined NIST.

References

[1] B. Lindemuth, B. Hammouda, F. D. Blum, J. R. Duke, and R. L. Venable, PGSE-NMR and SANS

from TPB based Microemulsions. Proceedings of the MRS Fall Meeting in Boston (Dec. 1989).

[2] P. Lindemuth, B. Hammouda, and R. Venable, SANS

from TPB based Microemulsions. J. Phys. Chem. 94 , 8247 (1990).

82
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

QUASIELASTIC GAMMA-RAY SCATTERING FROM PENTADECANE

AND FROM POLYDIMETHYLSILOXANE SOLUTIONS

B. Hammouda

and

G. Schupp (University of Missouri, Columbia, MO)

A Mossbauer instrument was used to study

the dynamics of liquid pentadecane [1] and of

polydimethylsiloxane in benzene solutions [2],

Quasielastic scattering (fig. 1) of gamma-rays

(QEGS) is a purely coherent scattering process

yielding the coherent dynamic structure factor

directly. The instrument used is a gamma-ray

diffractometer which uses an intense Mossbauer

source (Ta-183) cooled to 77 K in which the

energy analyses of the scattered photons are performed by Doppler shifting a Mossbauer

absorber (W foil). In the case of molecular liquid pentadecane,

quasielastic widths ranged from 64 \ieV (at 12 °C) to 192 neV (at 74 °C) at a scattering vector which corresponds to the first maximum

of the structure factor (1.15 A" 1 ). Similar

measurements at five different scattering angles

(at 12 C C) were also taken to investigate the

wavevector dependence of the quasielastic width.

The results observed were in good agreement

with known theories giving the quasielastic width

as: T(Q,T) = [Q

2/S(Q,T)]kbT/M

o

(/> l_l3Km* 8

15

i i i " i —i 1 i i i

10

drodynamic

Region

QELRS

d?

p3S|

i

QELCS 5

Hy

i i i 10

LOG (Q[cm-1D

Figure 1. Diagnostic methods used to investigate the dynamics of liquids: quasielastic light Rayleigh scattering

(QELRS), quasielastic light correlation spectroscopy

(QELCS), quasielastric neutron scattering (QENS) and

quasielastic gamma-ray scattering (QEGS). The computer

simulation region (molecular or Brownian dynamics)

overlaps with QENS and QEGS.

where Q is the scattering vector, S(Q,T) is the

structure factor, kbT the temperature in energy

units and M an effective mass of the scattering

unit. The de Gennes narrowing of the

quasielastic peak at the maximum value of Q for

the structure factor has been observed for liquid

pentadecane.

In the case of macromolecular liquid

polydimethylsiloxane in benzene solutions,

variation of the quasielastic width has been

observed as a function of polymer concentration.

The high energy resolution of the QEGS

technique (8 u-eV) allows the separation of the

scattering signal (see fig. 2) into a narrow component associated with stiff motions along

the polymer chain backbone and a quasielastic

component associated with softer side group

motions. The narrow component disappears

upon dilution in benzene while the intensity of

the quasielastic component grows proportionately. This observation is interpreted as a softening of the backbone normal modes upon

dilution.

-11500 -If, 00 "US 00 -7500 -JB.00 ?600 Jl.OO 115-00 1 M 00 195.00 ENERGY TRANSFER CpcV)

Figure 2. QEGS spectrum from undiluted polydimethyle siloxane taken at room temperature with Q = 0.9 A . Curves A and B give the individual Lorentzian fits for the narrow width and quasielastic

scattering components respectively. Curve C gives the overall fit to the data points indicated hy circles.

83
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

References

Most of this work was performed before B.

Hammouda joined NIST.

[1] G. Schupp, B. Hammouda, and C. M. Hsueh, Phys. Rev. A41, 5610-5615 (1990).

[2] B. Hammouda, G. Schupp, and S. Maglic, J. Chem.

Phys. 93, 5473 (1990).

TEXTURE STUDY ON TANTALUM METAL SHEET BY NEUTRON DIFFRACTION

C. S. Choi (ARDEC, Picatinny Arsenal, NJ),

and

H. J. Prask

Tantalum is a high-density metal with a high

melting point; it has a high ductility and

moderate tensile properties. Recently, this material has attracted considerable attention

among the defense-related, research community

as a prime candidate for shaped-charge liners

and explosively-formed projectiles. The texture measurements are conducted commonly by x-ray

diffraction methods because of the easy

availability and the faster measurement time

compared to neutron diffraction. However, the

penetration power of the x-ray is so weak

(approximately 1/10000 of that of neutron) that

it is essentially impossible to see the subsurface

texture below a few microns depth in the heavy

metals, and the surface texture is often different

from that of the interior. Therefore, neutron

diffraction was used in this study. A sample

specimen with approximate dimensions of 0.64

x 1.0 x 1.9 cm3

, was cut from a long strip, 0.64

x 1.9 x 20 cm3

, of Ta metal for a texture study.

The sample was mounted on the four-circle

neutron diffractometer with the long axis

oriented approximately parallel to the sample

rotation axis. The intensity distributions of five

reflections, 200, 211, 220, 310, and 222, were

measured over an entire orientation hemisphere

ranging from to 360° in phi-angle and 90° to

in chi-angle, each with 5° step intervals.

The raw data were corrected for background neutrons and converted to the pole

densities in the multiples of random distribution

(mrd) units. The observed pole density data were smoothed by the Gaussian-filtering method

with a 5° width. The experimental pole figures are shown in figure 1. The "12 o'clock

direction" of the pole figures correspond

approximately to the normal direction of the

sample, the "3 o'clock direction" to the rolling

direction, and the center to the transverse

direction. The contour levels are shown in mrd

units in the legend of each figure. The textures are clearly the sheet-type, but have no clear

mirror symmetry about the RD or TD. This

suggests that the sample has been either annealed

after the rolling or processed with

multidirectional rolling or forges.

100 Stereographic Projection

Contours at .50 1.00 1.50 2.00 2.50

Figure 1. Observed pole figures.

The Orientation Distribution Function (ODF)

analysis, based on the harmonic method, was

performed using the program LATEX developed

at Los Alamos National Laboratory. The

coefficients of the series were determined up to

1 = 22, using the five experimental pole figures

with diad symmetry. The recalculated pole

84
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

figures from the resultant ODF are shown in figure 2. The calculated pole figure is quite

similar to the corresponding experimental one, which indicates that the ODF represents the true texture reasonably well. The inverse pole

figures of the two sample directions, SOP1

(rolling direction) and SOP2 (normal direction), were calculated from the ODF, and are shown in figure 3. The rolling direction is dominated by

{100} poles. It also contains two smaller peaks

of {110} and {112} poles. There are only two

maxima in the normal direction, one centered at {100} and the other at {111}, with approximately equal intensity. This study suggests that the texture of the Ta sheet has three-slip

systems: the cube texture (010) [100] with the

volume ratio of 50% or more, and (111)[110]

and (111)[112] with equally less volume ratio. The occurrence of (001)[1 10] is possible but

probably less significant because of the small

amount of [110] aligned toward the RD. The

cube texture found in the sample is considered to be quite important because of the fact that it produces extremely anisotropic physical

properties. The cube texture has been observed

mostly in many FCC metals and alloys after primary recrystallization annealing in certain

temperature ranges. Recently, the cube texture has been found also in BCC metals; i.e.,

laminated steels.

SOPl Stereographic Projection

Contours at .50 .75 1.00 1.25 1.50 1.75 2.00 2.25

SOP2 Stereographic Projection

Contours at .50 1.00 1.50 2.00 2.50

Figure 3. Inverse pole figures calculated from ODF,

where SOPl and SOP2 corresponds to the rolling direction and the normal direction, respectively.

(100) Stereographic Projection

Contours at .50 i.00 1.50 2.00 2.50

Figure 2. Recalculated pole figures from ODF.

85
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

RESIDUAL STRESS MEASUREMENTS IN METALLURGICAL SAMPLES

H. J. Prask

C. S. Choi (ARDEC, Picatinny Arsenal, NJ and Reactor Radiation Division)

and

T. Cordes (John Deere Product Engineering Center, Waterloo, IA)

Last year a feasibility study was made for

the SAE Fatigue Design and Evaluation (FD&E)

Committee to determine the applicability of the

neutron diffraction technique to residual stress determination as part of a fatigue lifetime

prediction project. Specifically, the project is an

integrated engineering approach for design

analysis and validation of components for

vehicles in which test spindles will be tested for

fatigue life, characterized for materials

properties-including residual stress, and

modeled by finite element techniques. Depthprofiling of residual stresses had been done by x- ray diffraction with layer removal, about which

the FD&E Committee felt some uncertainty.

The nondestructive neutron diffraction residual

stress determination, in which (o n

- o ee) was

measured as a function of depth at two positions on a test spindle, showed the applicability of the

technique.

This year these measurements have been

continued. Values of d-spacings have been

obtained as a function of depth for two positions on the original spindle and three positions on a fatigued spindle. In progress is the determination of devalues for the fully inductionhardened outer 3 mm, and the unaffected 10-mm

radius core region of the 20-mm radius spindle.

On completion, the full triaxial residual stress distributions will be determined.

ANALYTICAL CALCULATIONS USING ACCEPTANCE DIAGRAMS

J. R. D. Copley

This year we have used the acceptance

diagram technique [1,2] to perform calculations

which have been of direct benefit to group

members charged with the design of neutron

scattering instruments. Acceptance diagrams

show the y-p distribution of neutrons in a beam,

where y is one of the neutron's spatial

coordinates normal to the beam direction and p

is the corresponding angle between the neutron's

direction and the beam direction. Using these

diagrams we can compute integrated intensities

and related properties.

Vertically curved monochromators . We

have investigated the performance of vertically

curved monochromators, both at beam tubes and

guide tubes. Our work complements the analytical treatment of Currat [3], in some cases

offering important additional insights. In figure

1 we show an example of a calculation of the

integrated intensity per unit height of sample, for

samples of various heights, as a function of the

effective monochromator radius

R = R/sin B' (1)

where R is the radius of curvature of the

monochromator and Bis the mean Bragg

reflection angle. For small samples the intensity

is not only constant but spatially uniform, over

a range of R which decreases with increasing

sample height.

We have also performed calculations of the

behavior of vertically curved monochromators at the BT-4 spectrometer, taking account of

structural components which limit the height of

the beam. The results, shown in figure 2, are in good agreement both with independent Monte

Carlo calculations [4] and with an extensive set of neutron measurements [5]

.

86
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

2.5 (R*/Lo) Figure 1. Dimensionless intensities per unit height of

sample, as a function of the ratio (R /Lq), where R is explained in the text and Lq is the distance from the source

to the monochromator. The distance from monochromator

to sample is Lq/2. The heights of the source and

monochromator are identical, and the ten curves, starting

from the top, are for ratios of sample height to source

height of 0.0, 0.05, 0.1, 0.25, 0.5, 0.625, 0.75, 1.0, 1.5, and 2.0 .

g&fe^

0.0 2.5 5.0 R* (m)

7.5 10.0

Figure 2. Gain, i.e., the ratio of intensity on a 1 cm

sample using a curved monochromator of effective radius R to intensity on the same sample using a flat monochromator, plotted as a function of the quantity R . The

solid line is the analytical calculation, the large open

squares are the Monte Carlo results, and the small symbols

are the neutron measurements with a copper monochromator; each type of symbol corresponds to a

different value of 0g, in equal steps from 30° to 75°.

Design of the CNRF time-of-flight spectrometer . The acceptance diagram technique has

recently proved invaluable in two important

aspects of the design of the chopper system for

the CNRF time-of-flight spectrometer.

(1) We have calculated 2-d phase space

volumes transmitted by a separated counterrotating pair of choppers, as a function of time

and wavelength. The setup is illustrated in figure 3, and typical results are described

elsewhere in this report [6].

'S/////S/////WS/S//SS/W, aM!WB«WM""»-

//S////MM//,

vvw/s/M>w.

/M/MM/m/mmm/MiL

'/////////M///////////////M,

Figure 3. A scale drawing of the setup for one of the

calculations of the transmission properties of a separated

pair of choppers embedded in a guide. The separation

between the disks is 54.6 mm.

(2) A simple system of two choppers or chopper pairs, phased to transmit neutrons of

wavelength X , will also transmit neutrons of

wavelength

X- = XQ + j (h/m) (TR/L14), (2)

where the order number j is an integer, TR

is the

time between chopper bursts, L 14 is the distance

between the mid-planes of the choppers or chopper pairs, h is Planck's constant and m is the mass of the neutron. To suppress these

unwanted bursts, one or more additional

choppers must be included in the design of the

spectrometer. The problem of deciding where to place these intermediate order removal choppers

has been tackled using the acceptance diagram

technique. For given intermediate chopper

positions, and a given order number j, the task

is equivalent to the problem of determining the

intensity transmitted by a set of four noncollinear

apertures, given their separations, widths and

lateral displacements, illustrated in figure 4. This problem appears to be very simple, but in fact it is not easy to write an algorithm for its general solution without recourse to the concept

of acceptance diagrams.

87
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

L> l:._

[^

_-^.

1

r2W 4

'34

~T

Figure 4. A set of four noncollinear apertures. The parameters of the problem are indicated. Without loss of generality the

first and fourth apertures are placed on a common axis.

References

[1] J. R. D. Copley, NIST Tech. Note 1272, C. [5] O'Connor, ed. p. 202 (1989).

[2] J. R. D. Copley, Nucl. Instr. Meth. A287, 363 (1990). [6]

[3] R. Currat, Nucl. Instr. Meth. 107, 21 (1973).

[4] The Monte Carlo calculations were performed by D.

A. Neumann.

The neutron measurements were made by S. F. Trevino.

J. R. D. Copley, "Aspects of the design of the counterrotating chopper pairs for the high resolution time-offlight spectrometer," p. 103 in this report.

NEW DATA ACQUISITION SYSTEM

B. R. Schmid (Reactor Radiation Division and University Of Maryland, College Park, MD)

and

G. Greene and J. J. Rhyne

The shutdown of the NIST research reactor during the neutron guide installation was used to update the instrument control system in the

reactor hall by replacing the old PDP 11/44

computer with a pair of n VAXes and a new

operating system. This afforded the opportunity

to develop a new data-acquisition system using

many of the enhancements of VMS version 5. We completely redesigned the system around

CAMAC modules and intelligent motor

controllers. We run four instruments from one CAMAC crate without interfering with each

other. CAMAC communication is achieved

through a Q-bus interface. The stepping motor

controllers come with a nonvolatile memory for

the motion control variables and a seven digit

LED position display. The internal controller

logic in combination with the incremental

encoders on the motor shafts make the motors

move very precisely. One RS232 line is used to communicate with up to 5 units each housing

two controllers.

The new instrument control program (ICP)

has a significantly improved user interface and

combines several of the old programs into one. The entire code (including the CAMAC,

temperature and motor controller routines) is written in VAX-Fortran. All instrument specific

parameters are concentrated in an ASCII

configuration file, unique to each instrument, so

that only one software version for all six

instruments is needed. ICP makes extensive use of the VMS screen management utilities and is menu driven to a degree. Its features include

two scanning modes; e.g., constant increment

and constant Q/E with predefined buffers, an
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

enhanced temperature control with sample

feedback as well as the possibility to scan magnetic fields. The data files are in pure

ASCII format and, therefore, easily transferable

from one computer to the other using various

devices and protocols.

THEORY OF THE AHARONOV-CASHER EFFECT AS OBSERVED

BY NEUTRON INTERFEROMETRY

R. C. Casella

Recently, Cimmino et al. [1] have measured

the Aharonov-Casher [2] (AC) phase shift <j>AC

via neutron interferometry. Derived originally on the basis of a duality principle [2], the AC

effect has also been discussed in terms of a

spatially extended model of the neutron.

Physically, however, at the low momentum

transfers inherent in such experiments [1], the

neutron can clearly be treated as a point particle

with a dipole moment \i proportional to its intrinsic spin. Moreover, I show that to obtain

(J>AC one needs only the minimal prescription of

electromagnetism, according to which the nonrelativistic (NR) Hamiltonian H(P,x) follows

from the NR classical energy functional E(p,x)

by replacing the kinematic momentum p by

P - eA/c where Pk = dL/dxk

is the canonical

momentum. <j> AC then follows directly from the

Lorentz-invariant stationary-phase Feynman path

integral over the classical action, assuming a unique axis of spin quantization exists over the

entire spatial trajectory (classical orbit) in the

absence of ambient magnetic fields. Moreover,

the electric field E produces no acceleration of

the neutron if one neglects terms 0(E~). To

OQE), the local aspects {dynamics) are shown to be effectively 1 dimensional (7-d)--again

assuming the full 3-d Hamiltonian depends upon

only one component of the neutron spin everywhere on the loop-while, in the conventional

geometric arrangement [1,2], <f> AC depends only on the linear charge density contained in a 2-d

loop. By arriving at the AC result in such a

direct fashion, the present analysis makes

especially clear the nonlocal nature of the effect as well as its associated topological invariance in a 2-d sense. Further experimental tests of

topological invariance in more general situations

are under consideration. A detailed analysis of

the conventional arrangement follows.

I consider a neutron with instantaneous

velocity v = (v

x ,0,0) traveling in an electric

field E(x,y) = (EX ,E 0). In its instantaneous

inertial rest frame, the four-momentum (E',p')

is given by (c = 1, \i = - 1.913 u- N)

E' = m - n«B' = m + u.sinh u o z E

p' = O, with tanh u = vx

. In the original

frame (where B = O), neglecting gravity and

putting back c , p = (mvx ,0,0) and

E = mc2 + xh mvx

2 + u.(vx

/c)o

z E

to lowest order in v/c. Replacing the kinematic

momentum p in the NR energy

ENR = Px

2 /2m + ^Px °z Ey/mc

by letting p

- P - eA/c, one obtains the NR

(1-d) Hamiltonian (e = O),

H = Px

2/2m + nPx oz E

y /mc

where Px = dL/dx is the canonical momentum,

L is the 1-d Lagrangian and

E

y = E

y (x,y) | y

_. const .

By Hamilton's equation,

x = 3 H/8PX = (1/m) (P

x + u ° zE

y

/c).

Taking the inverse Legendre transform,

L = xPx

- H, it follows that

89
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

L = Vimx1

- \io 7 Ev x/c + (u.Evr/2mcz

.

From the Euler-Lagrange equation x = V2 d/dx

(u.E /mc)2. For uniform fields x and y vanish

identically. In any event they are of second

order in E and can be neglected in experiments

such as that of Cimmino et al. [1]. Letting tx be

the length of a straight-line segment of path

(which, neglecting gravity, is the distance

between reflections in the experiment of Ref. 1),

Xj, the distance along the path, and <j>i5 the pathintegral phase change due to the electric field,

then to OCE1 ),

(J). = 1/M

1 f V J i,EM dt

= IHL J

(-uozEyiXi/c) d

= mj (-nazEyi/c)dXi

At each reflection, we may rotate the coordinate

frame about z (axis of quantization) such that the

new x-axis lies along the path. Then, in a

circuit

E4>i = l/M f

lem dt

= - u-o

z/hc J

(ds x E) -z

= - 4tcAu.o

z/Mc,

where A is the path-enclosed charge per unit

length along z and the sense of the contours is left-handed along z. Defining 4>AC righthandedly, 4>AC = 4rcAu.o

z/Mc.

Thus, the AC phase shift depends only on

the enclosed linear charge density A and is therefore topological in a 2-d sense. As noted

above, the phase shift produced by static electric

fields in neutron interferometers under more

general conditions is currently being investigated

with a view towards suggesting new experiments

to test for topological invariance.

References

[1] A. Cimmino, G. Opat, A. Klein, H. Kaiser, S. Werner, M. Arif, and R. Clothier, Phys. Rev. Lett. 63, 380 (1989) and S. A. Werner (private

communication).

[2] Y. Aharonov and A. Casher, Phys. Rev. Lett. 53, 319

(1984).

NEUTRON ACTIVATION AUTORADIOGRAPHY PROGRAM

Y.-T. Cheng and J. 01in (Smithsonian Institution, Washington, DC and the Reactor Radiation Division)

The technique of neutron activation

autoradiography to obtain images of underpainting, brushstroke, and changes in the

composition of paintings has been demonstrated

to be a highly valuable complement to x-ray

radiography and other techniques of painting

examination. Gamma-ray spectroscopy can also

be conducted and provides the general composition information of the painting. It is desirable,

however, to identify the specific element or elements that comprise the different features in the painting. Localized gamma-ray spectroscopy

has been found useful for this purpose [1],

An alternative method of identifying what

elements are present employs changes in the film

density resulting from the radioactive decay.

The autoradiography films used in a painting

study act as area detectors and register the

radioactive decay characteristics of all the

activated elements in the painting that emit

charged particles. Therefore, a careful study of

the changes in density at a given area by

digitizing the series of autoradiography films

should yield information for the composition of

the corresponding area on the painting. Certain

anticipated difficulties are imprecision in the

90
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

density measurements, mixtures of multiple

elements with similar half-lives and inconsistency

in film handling and processing. A feasibility

study was initiated by establishing a procedure

that compares the local density changes in an

autoradiography series to reference film

responses for individual elements and

combination of elements. Reference film

responses were obtained using a set of selected

metal foils. The intrinsic characteristic of the

film used was deduced from the analysis of the

above results. This information was then applied

in calculating the film responses for aluminium

and nonmetallic elements antimony, arsenic,

barium, manganese and sodium.

The first application of this procedure is shown in figure 1. The autoradiographic

responses of an edge area on the painting

"Pastoral Study" (R. A. Ryder, National

Museum of American Art) is compared to that of

gold. (There is no corresponding exposure 6 for

the paintings.) The gold was likely transferred

to the painting from the gilded painting frame.

Figure 2 compares the film responses of another

area on the painting to the combined responses

of antimony and barium and shows that these

two elements do account for the densities of the

films in this area. However, as there is no

obvious opaque white pigment containing only

barium, it is likely that further refining of the

density relationships will be needed. Figure 3

shows the comparison of the responses from a blue-white sky area on the painting "Lord Ullin's

Daughter" (R. A. Ryder, National Museum of

American Art) to that of iron and zinc. As iron

could be present as the blue pigment, Prussian

blue which has been identified in Ryder's

paintings, and zinc as zinc white, this identification can be considered complete.

A data bank is being collected with these

reference film responses. Other selected

elements will be added in the future. With the

anticipated increased use of image processing for

the study of autoradiographs of paintings, this information can help in distinguishing areas of

restoration in a painting and identifying the

pigments the artist used. The locations of

specific elements in a painting can be mapped

and shown visually.

The Smithsonian Institution's Conservation

Analytical Laboratory (CAL) has received grants

from the Samuel H. Kress Foundation and the

Smithson Society to extend the present auto- radiography facility to handle larger paintings

and to increase the number of paintings which

can be studied. New shielding will be constructed to accommodate paintings larger than 4

ft by 4 ft and up to 7 ft by 8 ft in size. Figure

4 shows the shielding layout. This design

replaces one reported earlier which would have

been a permanent installation. The shielding

will be removed after its use and painting

activation will be carried out during reactor start up. In order to accommodate a larger number of

paintings NIST has provided a second laboratory

which CAL will equip with environmental

control and security devices.

b -i

>-

1- 4

in

LJ , n

r d2 1—

*1 v i^'^ 1 1 H -h 4K- 12 3 4 5 SLQULNIIAL

6 / 8 9 10 11 UM EXPOSURE NO.

PASTORAL STUDY GOLD STANDARD Figure 1. Pastoral study (edge area).

I 2 3 4 5 6 7 8 9 10 11 ; 2 SEQUEN1IAI FILM EXPOSURE \0.

PASTORAI STUDY fj BARIUM STANDARD ^ ANTIMONY S1AN3AR.1

Figure 2. Pastoral study (yellow-white area).

91
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Reactor Thermal Column

123456789 10 SEQUENTIAL FILM EXPOSURE NO.

LORD UIUN'S DAUGHTER Q ZINC STANDARD IRON STANDARD

Figure 3. Lord Ullin's daughter (sky area). Figure 4. Proposed shielding layout for large painting

activation.

Reference

[1] C. Fischer, C. Laurenze, W. Leuther, and K.

Slusallek, "Neutronenautoradiographie," in Per Mann

With Goldhelm.

NIST CRYSTAL AND ELECTRON DIFFRACTION DATA CENTER

A. D. Mighell and V. L. Himes

The NIST Crystal Data Center is concerned

with the collection, evaluation and dissemination

of data on solid-state 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 the year, the master database

has been updated and now contains greater than

150,000 entries. From this centra! database, two

distribution databases are produced: nist

CRYSTAL DATA [1] and the NIST/Sandia/ICDD

Electron Diffraction Database [2]. These data are made available to the

scientific community through computer modes of

dissemination. A new CD-ROM product with

documentation has been produced that contains

three related databases on the same disc: nist

crystal data, the Electron Diffraction

Database, and the icdd Powder Diffraction File. With this product, many types of searches can be

carried out at one's desk using a PC computer.

A second, and highly versatile method of

dissemination, is the on-line mode. This method

is discussed separately below as it has been

significantly enhanced during the year. On-line Search System. The on-line system

for NIST CRYSTAL DATA is known as CRYSTDAT

[3]. This system consists of the database along

with search and analysis software, crystdat is built and maintained via a long-term cooperative

agreement between the National Research

Council of Canada and nist.

Recently, we have upgraded crystdat to include all 150,000 entries in the master

database. Other data oriented improvements

include: (1) cross-references to other databases;

(2) all key data items directly searchable

(indexed); (3) additional derived data. Two important new search tools have been

recently created~a generic chemical formula

search and an interplanar spacing search. These

tools, designed in response to requests from

industrial users, will be applicable to research in analysis and materials design.

The new formula search permits one to search on generic classes and types of formulas.

Both the elements present and subscripts in the

92
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

formula are subject to algebraic manipulation in the search 'find' command. Thus, for example,

all formulas with atoms in specified ratios can be

found regardless of element type. The inter- planar search permits one to find all materials

having any specified combination of d-spacings

and will be used in research in such areas as thin

films. This command, especially when used

with the lattice matching search function, will

permit one to establish many types of inter- and

intralattice relationships. As all data parameters

are indexed, the system can rapidly execute the

searches. Resultant answer sets from individual

searches can be intersected using Boolean logic.

The crystdat System has many industrial

applications in the materials sciences. Thus it can be used in the following: 1) in research in materials design (e.g., super conductors, lasers,

thin films, optical materials); 2) in materials

characterization and analysis (e.g., in neutron

profile analysis, it can be used to identify the

presence of minor impurities); 3) and in phase

identification by neutron, x-ray and electron

diffraction (e.g., from a few elements and

d-spacings, most materials can be uniquely

identified).

References

[1] NIST CRYSTAL DATA (1990). A Database of

chemical, physical and crystallographic data (~ 150,000 entries). Compiled and evaluated by the NIST Crystal Data Center, NIST,

Gaithersburg, MD 20899.

[2] NIST/Sandia/ICDD Electron Diffraction Database

(1988). A Database designed for phase characteri- zation by electron diffraction. Prepared by the NIST

Crystal Data Center in collaboration with Sandia

National Laboratories and the International Centre for Diffraction Data, NIST, Gaithersburg, MD 20899.

[3] CRYSTDAT (1990). An International On-line Search System for NIST CRYSTAL DATA. Developed

jointly by the Canada Institute of Scientific and

Technical Information and the NIST Crystal Data

Center, NIST, Gaithersburg, MD 20899.

COLD NEUTRON PROJECT

G. Baltic, J. R. D. Copley, D. H. Fravel, C. J. Glinka, J. A. Gotaas, G. Greene, W. C. Knill,

J. G. LaRock, H. P. Layer, C. F. Majkrzak, C. O'Connor, D. Pierce, H. J. Prask, M. J. Rinehart,

J. M. Rowe, S. K. Satija, I. G. Schroder, T. J. Udovic, R. E. Williams, and R. H. Williams

On June 5, 1989, the NBSR was shutdown

to begin guide installation. As planned, the

guides, fabricated by Cilas-Alcatel of France, were in-hand for the first three beam-lines (NG

5, 6, 7). The initial phase of installation entailed

the very difficult job of drilling 0.50-m diam

holes through 7.5 m of reinforced concrete in the

reactor-building wall that had stored hot plugs.

This and the placing and grouting of 0.50 m

diam stainless steel liners were completed in August, 1989.

With technicians from Cilas-Alcatel,

installation of guide segments then began. As of

January 1990, the special isolation shutters and

neutron guides through the reactor-building wall were in place and leak-checked. By late March,

installation of the three guide segments and

associated shielding from reactor-building wall to cold source was completed, along with shielding

associated with the NG-0 port which ends in the

reactor hall. Follow-on leak and surveillance

tests of the reactor-hall confinement were

completed, after which the reactor was restarted on April 8, 1990.

Following reactor startup and resumption of

"thermal" neutron research, installation of the

remaining sections of neutron guides NG 5-7 in the guide hall continued. It is worth noting that these initial three guides will be 58 Ni-coated over

their entire lengths to maximize intensity. Major

new developments in Ni-based supermirrorsdescribed on p. 116 in this report-will be

incorporated in neutron guides NG 1, 2 and 4,

installation of which is planned for 1991-2.

Neutron instruments are being developed as quickly as possible, and are described in some

detail in other sections of this report. Overall,

it is planned that at least six instruments will be

operational by early 1991. These will include an 8-m SANS to be installed and operated by the

93
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NIST Polymers Division; a diffuse scattering

time-of-fiight spectrometer; the 30-m NIST/

Exxon/U. Minn. SANS, which will be among

the world's best in resolution, intensity, and

versatility; the cold neutron depth-profiling

facility, which is expected to increase

measurement sensitivity by a factor of twenty

over the existing thermal instrument; the new

cold neutron prompt-gamma activation analysis

station, which is expected to be the world's best

full-time instrument of this type; and the

neutron-lifetime experiment which will be

installed at the fundamental neutron physics

station. The planned layout for all guides is shown in figure 1

The CNRF, which is a response to a

critical national need for a cold neutron

measurement capability, will be a National User

Facility; to this end, the experimental stations

installed will be available to the scientific and

engineering community on the basis of scientific merit of proposals or in Participating Research

Teams. A Program Advisory Committee (PAC)

will play a critical role in advising NIST on the

utilization of the facility.

On October 24, 1989, the PAC for the

CNRF met for its first session. The PAC

members are Chairman S. K. Sinha, Brookhaven

National Laboratory and Exxon Research and

Development Corp.; D. L. Price, Argonne

National Laboratory; E. A. Schweikert, Texas

A&M University; and H. Yu, University of

Wisconsin. In the first meeting the committee

made the following principal recommendations:

-- Utilize calendar 1990 for "shake-down" and

development. ~ Send out the first call for proposals toward the

end of 1990, with two (possibly three)

submission periods a year thereafter. — Have several external reviewers evaluate the

proposals by mail with the PAC allotting

instrument time based on the reviews.

-- Expand the PAC. - Form a CNRF User Committee in 1990.

Action is being taken to develop the details

of all aspects of the user program with the

recommendations of the PAC as the starting

framework. Bill Kamitakahara, who is newly

added to the staff, will be the scientific coodinator for the program.

Relating to instrument development, the

following major collaborations have been

initiated in this period. The University of

Minnesota has agreed to participate in the

development and use of two high resolution

instruments: the 30-m NIST/Exxon/U. Minn.

SANS and the cold neutron reflectometer. Their

utilization is expected to focus on the elucidation

of the structure and microstructure of polymers

and other materials as well as surface and

interfacial structure and interactions. According

to the agreement, the University of Minnesota

will provide a share of the development costs of

the instruments, and will continue contributions

HIGH RESOLUTION

TIME-OF-FLIGHT SPECTROMETER

HALL

INTERFEROMETER

30 METER SANS

Figure 1. Cold Neutron Research Facility layout showing instrumentation complex.

94
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

to maintain state-of-the-art capabilities following

instrument installation. IBM, through its Almaden Research Center, has entered into a similar agreement for development and

utilization of the cold neutron reflectometer.

Their interests are also expected to focus on

polymers and surface interactions. In March, the second issue of our

newsletter, the Neutron Standard, was sent to approximately 2500 scientists and engineers.

This issue highlighted the installation of the

guides and the development of the instruments.

In the next issue we will include a detailed

description of instruments and a request for proposals.

Detailed planning for a workshop on

condensed matter spectroscopy with cold

neutrons, tentatively scheduled for mid- 199 1

, has

begun.

LIQUID HYDROGEN COLD NEUTRON SOURCE

J. Gotaas, P. Kopetka, J. M. Rowe, and R. E. Williams

The flux of long-wavelength (> 4A)

neutrons at the CNRF can be increased by

replacing the D2ice moderator with a liquid

hydrogen (LH2

) moderator. Although deuterium

has a lower absorption cross section than

hydrogen, H2 has a larger scattering cross

section, and is more efficient at slowing

neutrons. Therefore, LH2

will be used both to minimize the inventory of liquid hydrogen in the

reactor (deuterium and hydrogen are chemically

equivalent), and to avoid generating gaseous

radioactive tritium.

Monte Carlo calculations are being

performed to predict the cold-neutron flux as a function of source geometry using the MCNP

code developed at the Los Alamos National

Laboratory [LANL]. MCNP allows accurate

representations of complex volumes and

surfaces. We have obtained neutron scattering

kernels for LH2

, developed at LANL in support

of calculations on LANSCE moderators. In

addition, we have developed cold-neutron

scattering cross sections for D2

ice and bismuth

to compare MCNP results with previous NBSR

flux measurements (see "Cold Source Spectrum

and Flux Measurements" [1]).

We are planning to install a hydrogen

source fabricated from 6061-T6 aluminum, with

triple containment of the hydrogen for all parts

of the system. Cooling will be provided by a new 3.5 KW helium gas refrigerator providing

14 K gas at up to 150 g/s. This refrigerator is currently being fabricated, and installation will begin late in 1990. Based on current

calculations, the new source should provide an

increase in intensity of long wavelength neutrons

of at least two.

A safety analysis report is being prepared

for submission to the Nuclear Regulatory

Commission this fall, as the design is finalized.

It is expected the LH2 source will be approved

and ready for installation when the second set of

neutron guides is installed.

Reference

[1] C. J. Glinka, T. J. Udovic, J. M. Rowe, and J. J. Rush. NBS Tech. Note 1257, C. O'Connor, ed. p. 134 (1989).

95
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

CENTER FOR HIGH RESOLUTION NEUTRON SCATTERING

J. J. Rhyne, B. Hammouda, and S. Krueger

The National Science Foundation (NSF)

has provided funding for a national user facility,

the Center for High Resolution Neutron

Scattering (CHRNS), as part of the NIST Cold

Neutron Research Facility. Two instruments are under development for CHRNS--a 30-m SANS

(see fig. 1) and a Spin Polarized INelastic

Spectrometer (SPINS, to be jointed developed by

NIST and NSF). These instruments will be

operated as a user facility open to qualified

research workers from universities, national

laboratories, and industry. Proposals for beam

time will be evaluated by the CNRF program

committee.

The 30-m SANS is to occupy an end-guide

position in the Guide Hall and will provide the

highest resolution available in the United States (minimum Q of 0.0005 A"1 with a sample flux of

approximately 106 n/sec for a Qmin = 0.01 A"1 on a 2 cm diam sample) and will incorporate a high counting rate 2-d detector. The detector will

move inside a 15-m evacuated flight path to

within 1.5 m of the sample permitting a maximum Q of 0.6 A"

1 without the use of a second area detector. It is also planned to provide a polarized neutron capability using a polarizing filter or multilayer reflection crystals.

The neutron monochromator is to be a

variable pitch velocity selector providing wavelength tunability over the range 4 to 25 A with a variable wavelength spread, AX/A., from about

5% to 25%.

Two sample tables will be incorporated- one with a multiple sample changer and a second

independent table for cryostats, magnets,

furnaces, etc. As of July 1990 the CHRNS SANS

instrument is over 90% designed with more than

30% of the parts and fabrication complete. It is expected to be operational in the summer of

1991. Preliminary design work is complete on

the SPINS instrument with detailed design work

and major parts procurement to occur in the last quarter of calendar 1990.

BERYLLIUM/BISMUTH

FILTER

VELOCITY SELECTOR

PRESAMPLE FLIGHT PATH SECTIONS

SAMPLE CHAMBER

STAGE

SCATTERING FLIGHT PATH

AREA

DETECTOR

PRINTER MAIN MACINTOSH | CONSOLE

CONNECTION TO ETHERNET

MICROVAX COMPUTERELECTRONICS RACKS

Figure 1. The 30-m NSF-SANS instrument.

96
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

HIGH RESOLUTION SMALL ANGLE SCATTERING SPECTROMETER

C. J. Glinka, J. J. Moyer, A. E. Heald, C. H. Chen, S. T. Krueger, and B. Hammouda

S. K. Sinha (Brookhaven National Laboratory, Upton, NY

and Exxon Research and Engineering Co., Annandale, NJ)

and

M. Lin (Exxon Research and Engineering Company, Annandale, NJ)

Construction of the 30-m long, highresolution SANS spectrometer developed by the

participating research team of the Exxon

Research and Engineering Co., the University of

Minnesota, and NIST is now essentially

complete. Operational testing will take place in

the Fall (1990) with the first scheduled

experiments expected to begin in early 1991.

A line drawing of the instrument is shown

in figure 1. The 15-m long, post-sample flight

path is designed to house two large positionsensitive area detectors; one that can be

positioned within the main cylindrical section of

the flight path at any distance from 4 m to 15 m

from the sample, and a second detector that moves along a circular arc at a fixed distance of

2 m from the sample. Together the two

detectors will span a Q-range from roughly

0.001 to 1.0 A"1

. The carriage for the primary

detector includes a motor driven mechanism for

selecting and positioning any of four beam stops

in front of the detector to match the size of the

incident beam.

As indicated in the cross-sectional view

(A-A) in figure 1, the instrument's 15-m long

pre-sample flight path has provision for the

insertion of 1 .5-m long neutron guide sections to allow the effective source to-sample distance to be varied from 5 m to 15 m to match the beam

collimation to the sample-to-detector distance.

The beam collimation and number of guide

elements can be easily changed externally while

the flight path is under vacuum. The section of the incident flight path

closest to the sample will house a doubly curved,

grazing incidence mirror, 2.4-m long, to focus

the beam onto the detector. The mirror consists

of eight, 30-cm long x 5-cm high, concave

cylindrical segments that will be aligned tangent

to an arc 1700 m in radius. Each segment will

Velocity Selector

Filler

Figure 1. NIST/Exxon/University of Minnesota SANS spectrometer.

97
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

be slightly bent to conform approximately to this 1700-m arc to achieve focusing in the horizontal

plane. In principle, such a mirror can provide

up to a fourfold increase in flux on the sample,

for a given minimum Q-value, over what can be

obtained with pinhole collimation. The

cylindrical quartz mirror segments have been

obtained and tested both optically and with

neutrons and do meet their design specifications

for figure and finish. These elements will be

coated with 58Ni and then assembled and aligned

for actual performance testing.

Proceeding in parallel with the construction

of the instrument has been the development of

the data acquisition system and software. Data

will initially be stored in CAMAC-based histogramming memory modules under the control of

a u-VAX 3400 computer. This scheme provides

both the capability to perform time resolved

measurements, by switching among a set of

memory modules, and the capability to carry out

on-line data reduction and analysis since the

main cpu is essentially freed from the task of

acquiring data. The data currently being

collected, as well as previously acquired data, can be displayed in a variety of ways on a color

Macintosh computer to the jiVAX using image

processing software that has been customized for use with SANS data from a 2-d detector. A

menu driven user interface has been written for

the nVAX to aid users in both acquiring and

displaying data. Once in operation, the members of the PRT

will control 75% of the available beam time with

the remaining 25% to be allocated by the

Program Advisory Committee for the CNRF.

The first call for experimental proposals is planned for early 1991.

TEVIE-OF-FLIGHT SPECTROMETER MODIFICATIONS

FOR GUIDE-HALL INSTALLATION

T. J. Udovic, I. G. Schroder, and J. G. LaRock

The NBSR Time-of-Flight (TOF) spectrometer is most-suited for quasielastic and lowenergy inelastic neutron scattering studies.

Monochromated neutrons (X= 2.3-6.1 A) are produced by double Bragg reflection from two

pyrolytic graphite (002) crystals. Using the

(004) reflection extends the wavelength range to 1.15 A. Double reflection allows for a fixed

sample position independent of the incident

neutron energy and reduces the fast-neutron

background contributions from the primary beam

by shifting the monochromated beam axis away

from the primary beam axis. After monochromation, fast-neutron and higher-order

contamination is removed using either a pyrolytic graphite filter (typically for X= 2 A3,

2.35, and 1.71 A) or a liquid-nitrogen-cooled Be

filter (for X> 4.08 A). The beam is subsequently pulsed with a curved-slit Fermi chopper

before scattering off the sample. The 98 3 He

detectors are radially located 2.286 m (i.e., 90

in) from the sample encompassing an angular

scattering range of -20° < 26 <120°. The

spectrometer is controlled with a dedicated

CAMAC-interfaced U.VAX computer. A TOF

interface encodes the time and position of each

detected neutron for subsequent recording by the

CAMAC histogram memory. Contents of the

histogram memory are periodically transferred to the u-VAX computer and added to data from

previous transfers. This updated data can then

be analyzed with the appropriate TOF software

routines available.

The TOF spectrometer was previously

situated at the CTW-beam position in the

Reactor Hall and is being relocated in the Guide

Hall in a non-end position on the NG-6 guide

tube. Installation is scheduled for the end of this year. Necessary modifications have been made

to the spectrometer design in order to reutilize

the instrument at its new location. Among other

things, because of the non-end guide position,

the first monochromator will now be held

translationally stationary within a 15-cm guide

98
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

cut. Wavelength selection will then occur by the

linear motion of the second monochromator

using the first monochromator as a pivot point.

The distance between the second monochromator

and the sample position has been minimized.

The flight path has been rotated from right-side

scattering angles to left-side scattering angles and

the angular scattering range has been altered to 0°< 20 <140°.

Additional upgrade modifications are also

being made at this time to (i) increase the

neutron flux at the sample position for all wavelengths, (ii) mitigate the effects of parasitic

scattering (from cryostat shielding, etc.) on the

sample spectrum, and (iii) upgrade the data

acquisition instrumentation. The double

monochromators will be increased in size to match the large 15 x 6-cm guide cross section. In addition, unlike the previous arrangement, the

second monochromator will be vertically

focussing onto the sample position. Vertical

focussing is optimized when the vertical mosaic

spread is minimized. Yet, for an isotropic

mosaic spread, this minimized fwhm in the

horizontal direction leads to an unnecessarily

small wavelength spread and a subsequent

decreased intensity on the sample. Hence, to gain maximum benefit from vertical focussing

while still maintaining a desirably broad

horizontal mosaic spread, both monochromators

will possess an anisotropic mosaic distribution,

25' FWHM in the vertical direction and 40 '-50' FWHM in the horizontal direction. This is being accomplished by using two layers of

PG(002) crystals, each layer possessing a 25'

mosaic, but staggered horizontally with 20'-25'

angular offset. Parasitic scattering will be

mitigated by the installation of an oscillating

radial collimator centered on the sample position

and located between the sample and the

detectors. New data acquisition systems are currently being investigated for the new highresolution TOF spectrometer. If an acceptable one is found or developed, it will be tested on

the existing TOF spectrometer to ascertain its potential use for both TOF spectrometers.

COLD NEUTRON DEPTH PROFILING

G. P. Lamaze, R. G. Downing, J. K. Langland, and K. C. Littrell (Inorganic Analytical Research Division)

The Cold Neutron Depth Profiling (CNDP)

facility at Cold Tube West (CTW) is nearly

ready for routine operation. The sapphire filter

(13.5-cm thick) and the collimation have been

installed. Figure 1 shows the expected neutron

energy profile.

o.i

1.0E-2--

1.0E-3

1.0E-4-'

1.0E-5

0.1

Neutron Energy Spectrum

m-H 1 1I

No filters

1 3.5 cm of sapphire 24.3 cm of sapphire

1.0 10.0 100.0 Energy (meV)

Figure 1. The neutron energy profile for the unfiltered beam and two different sapphire thicknesses. The 13.5-cm thickness has been selected for CTW.

99
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

A rotating shutter external to the reactor has been installed, aligned, and tested. Pancake

type uranium fission chambers have been

installed for monitoring the neutron fluence. A

new 60-cm diam chamber has been installed. This chamber is capable of ultra high vacuum (UHV) at pressures of 5 x 10"8 Torr, which have

been achieved without baking the chamber. The

chamber has been designed to be adaptable to many different types of experiments by changing

the component configuration. The main

configuration provides for remote controlled

scanning of 15 cm x 15 cm samples while

varying both sample and detector angle with

respect to the neutron beam. The scanner and

rotator are undergoing final tests prior to

installation in the chamber. They are controlled

by a PC compatible computer. The software for

the instrument control has been written. All of

the necessary electronics for signal processing

are now in place. The analog-to-digital (ADC)

converters have been installed in a Vax based

Genie system. The Genie work station is in

place at CTW and is connected to the main

Genie at the thermal NDP facility at BT-3.

HIGH RESOLUTION TIME-OF-FLIGHT SPECTROMETER

J. R. D. Copley

A schematic layout of the high resolution

time-of-flight spectrometer is shown in figure 1. The instrument will be located on guide NG-1,

which will have supermirror-coated top and

bottom inside surfaces in order to increase

intensity at the sample, by a factor of at least two as compared with 58Ni-coated surfaces.

A total of seven disk choppers, in four

evacuated housings, will be used to produce

pulses of monochromatic neutrons at the sample.

The maximum speed of the choppers will be

20,000 rpm, and the outside diameter of the

disks will be 580 mm. The disks for the pulsing

and monochromating choppers will each be fitted with three slots of different widths, allowing a choice of three distinct energy resolution widths

at a given wavelength and chopper speed.

Downstream from the fast neutron filter the

inside dimensions of the guide will be 30-mm

wide, 100-mm high. The guide will be partitioned into five channels in order to improve

intensity in the medium- and high-resolution

modes of operation [1,2].

///////////////;//////-?

Figure 1. A schematic view of the layout of the proposed high resolution time-of-flight spectrometer. The fast neutron

filter will be located in a separate room through which the guide passes on its way to the guide hall.

100
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

The distance from sample to detectors will

be 4 m, and the total active area of the detectors

will be of the order of 10 m

2

. Furthermore, the

detectors will be relatively thin in the direction

traveled by neutrons from the sample ( ~ 12 mm

or less) to achieve the required timing resolution.

We anticipate that the full width at half

maximum of the energy resolution, at 20,000

rpm with 6 A neutrons, will be ~ 12, —25, and — 50 u.eV in the high, medium and low

resolution modes, respectively.

Specific aspects of the design of the

chopper system are discussed in this report [3,4].

References

[1] J. R. D. Copley, Nucl. Instr. Meth. A291 . 519

1990).

[2] J. R. D. Copley, "Optimized design of the chopper

disks and the neutron guide in a disk chopper neutron

time-of-flight spectrometer," p. 101 in this report.

[3] J. R. D. Copley, "Aspects of the design of the counter-rotating chopper pairs for the high resolution

time-of-flight spectrometer," p. 103 in this report.

[4] J. R. D. Copley, "Analytical calculations using

acceptance diagrams," p. 86 in this report.

OPTIMIZED DESIGN OF THE CHOPPER DISKS AND THE NEUTRON GUIDE

IN A DISK CHOPPER NEUTRON TIME-OF-FLIGHT SPECTROMETER

J. R. D. Copley

In last year's Summary of Activities [1] we

discussed the possibility of using a channeled

guide to increase the intensity on the sample in a counter-rotating disk chopper spectrometer.

Since that time we have thoroughly investigated

this problem and our principal findings are presented in the following paragraphs. Full

details of most aspects of this work may be

found in [2].

Consider the arrangement illustrated in figure 1. The first pair of counter-rotating

choppers pulses the beam and the second pair

(subscript 4) selects a narrow range of wavelengths from the pulsed beam. A neutron guide

is placed before, between, and after the chopper

pairs in order to increase intensity at the sample.

In the present context we may (and shall) ignore

the additional intermediate choppers which are found in a real instrument.

DETECTOR ARRAY

NEUTRON

GUIDE

Figure 1. Schematic diagram showing the principal components of a counter-rotating disk chopper spectrometer; the intermediate choppers have been omitted for clarity. The disks of the CNRF spectrometer will each be fitted with three single slots rather than with a pair of slots as shown in the figure.

101
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

The intensity per pulse at the sample

position, Is

, for the setup illustrated in figure 1, may be written as follows [2]:

Is = AqOV^2/WG) Pl (2- Pl )p 4(2-p4),

where

(1)

A = * (A) (20c)2 (2HG) (2/v2

) (h/mL14), (2)

Wj and W4 are the half-widths of the slots in the

choppers (fig. 1), WG

is the half-width of the

guide, and pk(k = 1 or 4) is the ratio (WG/Wk) or unity, whichever is the smaller. In equation

2, $ (A.) is the differential current density of

neutrons of wavelength X in the incident beam,

C

is the (wavelength-dependent) critical angle

of the guide material (assumed identical for all surfaces), HG

is the half-height of the guide (and

of the slots in the choppers), v is the tangential

velocity of the choppers, L14 is the distance

between the chopper pairs, h is Planck's

constant, and m is the mass of the neutron. The resolution of the chopper system,

measured by its contribution to the standard

deviation of the time distribution at the detector

bank, is given by [2]

'D

2 = a2 o2 + p

2

'4 ' (3)

where ok(k = 1 or 4) is obtained from

ok

2 = (1/6) (Wk

/v)2

(2 - 2 Pk + Pk

2

), (4)

the dimensionless parameter

a = [L4S + LSD (A. s/aj) ] / L14

and

P = 1 + a.

(5)

(6)

In eq. (5), L4S is the distance from the

second chopper pair to the sample (fig. 1), LSD

is the distance from the sample to the detectors,

and A.j and \s

are the neutron's wavelengths

before and after scattering, respectively.

The intensity Is

is maximized at constant

resolution o Dwhen [2]

{WGIWX ) = 2a

and

(a + y)

(W4/Wl} = a( P 1 *>,

where

.2 2

Y - a

z + p'

(7)

(8)

(9)

This is illustrated in figure 2, which is a plot of

the reduced intensity

Js = Is/[A (o D

v)3

] (10)

for the case that a = 0.5 . For a given value of (A. S

/A. T ), such as unity,

the optimum width ratios (Wq/W^ and (W^W^)

only depend on the distance ratios (L4S/L14) and

(LSD/L 14). This implies that the optimum values

of the width ratios, for an instrument of the type

shown in figure 1, are independent of A. and v. A second implication is that if 2Wj and 2W4 are changed, as is possible with a counter-rotating

disk chopper spectrometer, the guide width 2WG

should also be changed. This is most practically

achieved using a channeled guide, which is not unlike a very long reflecting Soller collimator,

o

-2

logaCWg/Wj)

Figure 2. Contours of constant reduced intensity J<j for a = 0.5. The interval between contours is 0.5 and the value of Jg at the peak is 6.746

102
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

except that the widths of the channels are not

identical.

We have shown that a heavy price can be

paid in lost intensity if a simple guide of fixed

width, as opposed to a channeled guide, is used

in conjunction with counter-rotating chopper

pairs fitted with slots of different widths [2]. In order to avoid such losses we shall use a channeled guide in the CNRF high resolution

time-of-flight spectrometer [3]. The optimization of slot widths and guide

widths in a real instrument is more complicated

than the analysis presented above and in reference 2. One reason is that the half-widths

Wj and W4 vary with z, whereas WG

is a fixed

quantity. The channeled nature of the guide also

implies behavior which cannot be ignored. Both

of these effects were included in the calculations

which were used to determine final dimensions

for the slots and the channeled guide in the

CNRF time-of-flight spectrometer.

References

[1] J. R. D. Copley, NIST Tech. Note 1272, C. O'Connor, ed. p. 207 (1989).

[2] J. R. D. Copley, Nucl. Instr. Meth. A291 . 519 (1990).

[3] J. R. D. Copley, "High resolution time-of-flight

spectrometer," p. 100 in this report.

ASPECTS OF THE DESIGN OF THE COUNTER-ROTATING CHOPPER PAIRS

FOR THE HIGH RESOLUTION TIME-OF-FLIGHT SPECTROMETER

J. R. D. Copley

The CNRF high resolution time-of-flight

spectrometer [1] will use seven disk choppers to provide a pulsed, monochromatic beam of

neutrons at the sample position. The conventional description of this type of instrument

(e.g., Ref. 2) states that the first pair of counterrotating choppers pulses the beam, the final pair

selects a narrow band of wavelengths from the

pulsed beam, and the intermediate choppers remove unwanted orders and reduce the burst

repetition frequency in order to alleviate

problems of "frame overlap". In what follows

we shall see that this type of statement

oversimplifies the behavior of the first and final chopper pairs in a real instrument.

One of the advantages of using counterrotating pairs of choppers, rather than single

choppers, is that the effective chopping speed is doubled; it follows that the transmitted intensity

is doubled for a given choice of burst time [3,4].

A second advantage is that each disk can be

fitted with a small number of slots of different

widths in such a way that different slot widths can be selected by suitably changing the relative

phase of the choppers; this in turn implies a choice of resolution widths without changing the

chopper speed or incident wavelength [4]. The disks in the first and final chopper pairs

of the CNRF high resolution time-of-flight

spectrometer will each have slots of three

different widths. One of the challenges in designing these disks has been to determine

where to place the slots in order to eliminate

unwanted bursts of neutrons. If the separation

between the disks can be neglected, the problem

becomes that of finding a solution to a set of

inequalities of the form j aj > b, where a and b

only involve angular slot positions and the

dimensions of the slots and the guide. This

problem is by no means trivial because the

inequalities are nonlinear. We have nevertheless

devised a procedure to search for solutions (if they exist), as well as a convenient method to evaluate their stability to engineering and

electronic phasing tolerances. In the course of a more detailed analysis it has became clear that we cannot neglect the

separation between the members of a counterrotating chopper pair in the new CNRF time-offlight spectrometer. Though small (~ 55 mir.), this separation drastically modifies the

transmission properties of the chopper pair, and

a different approach is required to the problem

of where to place the various slots on a disk.

An additional consequence is that we have had to abandon the idea of using two closely spaced

slots in each disk in order to increase intensity at constant resolution [3]. The transmission function T, for a separated

pair of counter-rotating chopper disks, is a

103
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

function not only of the time t that neutrons

cross the mid-plane between the disks, but of

their wavelength X. This is most easily

understood by thinking about two disks, each of

which is fitted with a single very narrow slot,

spinning in opposite directions with angular

velocity to, separated by a distance 2s, and

illuminated by a very narrow highly collimated

beam of neutrons. Such a system transmits

neutrons of a single wavelength X , which

depends on the relative phasing of the disks, at times (tQ + mTR), where m is any integer and

the chopper period TR = 2tc/g>. [Its transmission is also non-zero for wavelengths

(A.Q + nXR) at times (t + nTR

/2 + mTR), where n is any integer and XR

is the wavelength

of neutrons which take one complete chopper

period to travel the distance between the disks,

but we shall ignore this periodicity in X since A. R

is typically of order 200 A or more.] The

transmission function for this simplest of systems

may therefore be represented in a "transmission

diagram", which is a plot oft vs. X, by a series

of points at wavelength X and times separated

byTR

. If each disk has N very narrow slots there

will be N

2

distinct paths for neutrons to pass

through the system. In (t,X) space the situation

is represented by a set of N

2 points within a

"unit cell" which has dimensions (TR, A. R). These points are the basis of a centered

rectangular lattice (plane group pi); if the disks

are identical an additional mirror symmetry

operation is introduced and the plane group

becomes cm. Negative wavelengths correspond

to neutrons which travel in the opposite direction

through the system. A change in the phasing of

the disks corresponds to a shift of the pattern of

points with respect to the origin of the

transmission diagram.

In a realistic situation both the slots and the

channel between the disks have significant width,

and the incident beam has considerable

divergence; the transmission function is a set of

peaks of finite width in both t and X, each of

which is in part represented by a closed loop

such that T is only non-zero within these loops.

This concept is illustrated in figure 1, which

shows (a) the arrangement of the slots in the first two disks of the CNRF time-of flight spectrometer and (b) a portion of the corresponding

transmission diagram, calculated using the

acceptance diagram technique [5], for the low

resolution mode of operation.

We have made exhaustive use of this type of

diagram in order to select positions for the slots in the first and final chopper pairs of the new

spectrometer. The basic idea is to ensure that all neutrons transmitted by the first chopper pair,

other than those in the intended bursts, cannot

get through the final chopper pair because they

have wavelengths for which the final chopper

pair's transmission function is zero at all times.

The inverse condition, which may or may not impose additional constraints, must also be met. Our choice of slot positions satisfies both

criteria, in all three resolution modes, for X <, 12 A and (X - X ) z 66 A.

3000

50 100 150 Wavelength (A) 200

Figure 1. (a) The arrangement of slots in the first pair of disks of the CNRF high resolution time-of-flight spectrometer, as "viewed" by the incident neutrons. The disk nearer the source rotates clockwise, (b) A portion of the transmission diagram for

this pair of disks. The incident wavelength Xq is 12 A, indicated by the arrow. The period TR

is 3000 \xs and the separation

is 54.6 mm: A Ris therefore — 217 A.

104
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

References

[1] J. R. D. Copley, "High resolution time-of-flight

spectrometer," p. 100 in this report.

[2] J. R. D. Copley, NIST Tech. Note 1272, C. O'Connor, ed. pp. 201, 207 (1989).

[3] J. R. D. Copley, Nucl. Instr. Meth. A273 , 67 (1988).

[4] J. R. D. Copley, Nucl. Instr. Meth. A291 , 519

(1990).

[5] J. R. D. Copley, "Analytical calculations using

acceptance diagrams," p. 86 in this report.

COLD NEUTRON CRYSTAL SPECTROMETERS

J. Ankner, C. Majkrzak, D. Neumann, S. Satija, and S. Trevino

Five cold neutron crystal spectrometers- some of which share common design

components—are currently under development at the CNRF. Two of these, a reflectometer and a

triple-axis spectrometer, will be installed on the

first three guides in 1991. The spin-polarized

inelastic neutron scattering spectrometer

(SPINS), the backscattering spectrometer, and

the spin-echo instrument will be installed on the

final four guides. Progress in the last year

includes the following.

The cold neutron triple-axis spectrometer and

the reflectometer are each in the final detailed

design stage. Many components of each

instrument have been fabricated or purchased.

For the backscattering spectrometer,

preparations are under way for tests of the

phase-space transform of neutrons using mosaic

graphite crystals mounted on a rapidly moving

chopper. Computer simulations indicate that success with this device could increase the

intensity of neutrons on the sample by a factor of

five or more.

Progress on the SPINS spectrometer includes

the development of neutron supermirrors

discussed on p. 108 in this report. Development

of the wavelength-dependent spin-flipper is continuing in a NIST/Brookhaven National

Laboratory collaboration.

105
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

ACCORDION SUPERMIRROR POLARIZER FOR NEUTRONS

C. F. Majkrzak

and

L. Passell (Brookhaven National Laboratory, Upton, NY)

Various polarizing mirror and supermirror

Soller arrays and benders for neutrons have been

proposed and constructed [1-3]. These devices

effectively produce polarized neutron beams with

large cross sections via reflection of one of the

two neutron spin states. Nonetheless, all of

these devices, in one way or another, broaden

the angular divergence of the beam. In a transmission polarizing device, wherein the

undeviated or nonreflected beam of neutrons of one spin state is utilized, this broadening is avoided. Furthermore, if a transmission device

is used to polarize, the beam which is utilized is not deviated so that the overall instrumental

design can often be simplified. Mezei [4] has

proposed placing a polarizing supermirror within

a guide tube section at the proper angle to function as a transmission polarizer. Another,

similar, transmission geometry which we have

been developing consists of trans- parent, single

crystal plates of Si, on which polarizing

supermirrors have been deposited, arranged in an antiparallel or accordion array as shown in figure 1(a). The problem with a conventional

parallel or Soller array, depicted schematically in figure 1(b), is that multiple reflections of

neutrons of the unwanted spin state cannot be

entirely avoided for a beam with a finite angular

divergence. Consequently, in the parallel

arrangement, neutrons in the unwanted spin state are in effect deviated from the incident or

transmitted beam direction only if they make an odd number of reflections from the super-mirror

coatings. That fraction of the neutron beam of

the undesired spin state which makes an even number of reflections emerges parallel to the

transmitted or undeviated beam of the desired

spin state, thereby reducing the polarizing

efficiency. One way the multiple reflections

could be eliminated would be to properly stagger

the parallel plates along the beam direction.

Unfortunately, for the relatively small angles

usually involved, the required overlap is negligible and the resultant configuration is nearly the same length as a single long plate

which, for large beam cross sections, can be

impractical.

Another solution is the accordion

arrangement of figure 1(a) in which it is possible

to insure, for a given wavelength spread and

angular divergence, that neutrons of the wrong

spin state undergo a single reflection if the angle

of inclination and critical angles of the polarizing

supermirror are properly selected. In many

cases it is, of course, necessary to subsequently

absorb the neutrons of the wrong spin state, which were intentionally deviated by the

polarizer, in a nonreflecting Soller collimator.

(b)

(a)

Figure 1. (a) antiparallel or accordion transmission

polarizer array for neutrons consisting of polarizing

supermirror films deposited on transparent single crystal Si plates as described in the text; (b) conventional parallel or

Soller array of supermiiTor polarizers illustrating detrimental double reflection.

106
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Now in transmission, the polarizing

efficiency is determined by the reflectivity of the

supermirror for the neutrons of the spin state being eliminated. Thus, if that reflectivity is only, say, 80%, then the polarizing efficiency

cannot be better than ~ 80% . However, even if the reflectivity were only 80%, for example,

placing two such transmission polarizers in series

would result in a polarizing efficiency of about

96%. Currently we are using Fe-Si supermirrors which are not yet as good as the Fe-Ag

or Co-Ti supermirrors made by O. Scharpf and

coworkers [1,5]. This is necessary because the

absorption losses in the latter supermirror

materials are prohibitively high in transmission

(the transmission losses in the Si single-crystal

substrate plates, on the other hand, amount to at most -20% for practical thicknesses and critical angles and neutron wavelengths as short as 2 A

passing through two plates in series). Fe-W

supermirrors are also being tried, but, due to rougher interfaces, their reflectivities are, so far,

lower than those of the Fe-Si. In figure 2 is shown a more detailed

schematic of the accordion transmission

polarizer, which we intend to construct, where

each pleat of the accordion is composed of a pair

of partially overlapping Si plates with the

supermirror coating deposited on opposite sides.

This particular topology allows the supermirror

films on adjacent substrates to make direct contact at each apex, thereby preventing passage

of the beam through the edges of the Si plates

without being incident on the supermirror film at some point (refraction of the beam in the Si

substrate has a negligible effect on the

performance of the device). It may in fact be

possible to continuously adjust the angle of

inclination of the plates relative to the incident

beam accurately by sliding spacers (located

above and below the beam) between plates along

the beam direction. Because the spacing

between 100-mm long plates would actually be

only about 1 mm, there is a substantial

geometrical advantage in that a small change in angle requires a relatively large translation of the

shim along the beam direction. The transmission

polarizer could then be adjusted for a specific

neutron wavelength mechanically.

Figure 2. Accordion transmission polarizer detail showing

pairs of Si plates with supermirror coatings on opposite

sides forming individual pleats as described in the text.

References

[1]

[2]

[3]

[4]

[5]

O. Scharpf, Physica B 156 &157. 639 (1989).

W. H. Kraan, J. B. van Tricht, and M. Th. Rekveldt,

Physica B 156 & 157 . 657 (1989). H. Mook, private communication.

F. Mezei, in Thin-Film Neutron Optical Devices . SPIE Proc. Vol. 983, C. F. Majkrzak, ed. (SPIE,

Bellingham, WA, 1989) p. 10. R. Pynn, in Thin-Film Neutron Optical Devices , SPIE

Proc. Vol. 983, C. F. Majkrzak, ed. (SPIE,

Bellingham, WA, 1989) p. 18.

107
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

SUPERMIRROR COATINGS FOR NEUTRON GUIDES

C. F. Majkrzak, D. A. Neumann, J. R. D. Copley

and

D. F. R. Mildner (Analytical Chemistry Division)

During the past 2 years significant progress

has been made in the development of

nonpolarizing, supermirror guide coatings for neutrons. Reflectivities of the order of 95% or

better at glancing angles up to three times the

critical cutoff for total reflection from an

ordinary Ni surface have been achieved. The

implications of this advance go beyond the factor

of nine enhancement in flux which would be

obtained by coating the surfaces of a guide

installed on a cold source with such a multilayered thin-film structure. It is also now

possible that thermal neutron guides; i.e., guides

for shorter wavelength neutrons, can be

constructed more advantageously given the

increased angular acceptance afforded by these

supermirrors.

Although high reflectivities for Fe-Ag

polarizing supermirrors up to a critical angle

twice that of Ni have been achieved by O.

Scharpf and coworkers some time ago [1], the

nonpolarizing supermirrors composed of Ni and

Ti layers (the combination of which gives one of

the highest scattering powers for elements

consisting of naturally occurring isotopic

mixtures) possess a relatively large amount of

interfacial roughness as has been clearly revealed

by electron microscopy [2]. Not only does the

interfacial roughness reduce the specular

scattering power of each bilayer, but the diffuse

scattering arising directly from the roughness

makes possible additional nonspecular scattering

through simultaneous or multiple scattering

processes as illustrated schematically in figure 1

In this figure the points along the x* axis in reciprocal space correspond to the fundamental

and higher harmonic reflections for a multilayer

with a single periodicity of about 63 A or

reciprocal lattice vector of 0.1 A"1

. In this picture the incident and primary diffracted

wavevectors have a length of 2.67 A"

1 and define an Ewald sphere (circle) as indicated by the

dashed curve. Superimposed on the multilayer

specular reflection points are bars parallel to the

y* axis which qualitatively represent diffuse

scattering due to roughness along the boundaries

between layers of dissimilar materials. The

intersection of a diffuse scattering ridge with the

Ewald sphere means that simultaneous or

multiple scattering events can occur when the

multilayer is oriented to reflect from, for

example, the first order reflection as shown in the figure.

The larger the interfacial roughness, the

greater the direct and indirect diffuse scattering

losses. In certain cases it is therefore necessary

to consider whether the superposition of

additional bilayers to compensate a reduction in bilayer reflecting power caused by interfacial roughness is counterproductive. The possibility

of multiple and simultaneous scattering occurring

increases with decreasing wavelength since the

radius of the Ewald sphere becomes larger and

the arc of the circle approaches the multilayer

growth axis more closely.

Well-defined peaks due to the secondary

scattering mechanism described above can be

observed by fixing the incident wavevector

relative to the sample and Q, as shown in figure 1 , and rotating the scattered wavevector to a position such as indicated by the primed final wavevector. In practice this can be accomplished simply by scanning the spectrometer

detector which is equivalent to following the

trajectory defined by the Ewald circle in reciprocal space. Alternatively, scans can be

performed transverse to the longitudinal or growth axis along the diffuse ridge until the

perimeter of the circle is intersected at which

point a peak will occur (such a scan is very

nearly a sample rocking curve sufficiently close

to the x* axis and can be performed by rotating

the sample relative to a fixed incident beam

direction and detector or scattering angle).

For relatively rough interfaces these multiple

scattering effects can be pronounced in trans- verse scans as is shown in figure 2 for a Ni-Ti

supermirror in which a cusp shaped interface had

been observed by transmission electron

microscopy [2]. In the case of a supermirror,

108
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

multiple periodicities are present, so that a continuous ridge of scattering extends from the

origin out to a cutoff along the longitudinal or growth axis. As for a simple multilayer,

however, if diffuse scattering is present, similar

multiple scattering peaks should appear.

Interference effects due to the nonspecular

scattering of x-rays from Si-Ge multilayers have

also been observed and analyzed [3] by

generalizing the treatment discussed by Sinha

et al. [4] of scattering from single roughened

surfaces. The problems pertaining to surface

and interfacial roughness, steps, terraces,

conformal and nonconformal in-plane correlations, and interdiffusion are of interest not only for technological reasons such as the one

presently described, but are of importance in the

understanding of fundamental physical

phenomena in lower dimensional systems.

Efforts to understand the diffuse scattering from

interfaces in a more general, quantitative way

are in progress (see, for example, "Diffuse

X-ray Scattering from a GaAs/AlAs Superlattice," by D. A. Neumann et al., p. 55 in this report).

Two commercial enterprises with experience

in thin film coatings, the Ovonics Co. of Troy,

MI and the Optoline Co. of Andover, MA, were

awarded Small Business Innovation Research

(SBIR) Phase I contracts with the Department of

Energy (DOE) to develop supermirrors and solve

the interfacial roughness problem. This support

supplemented funds initially invested by NIST

through the Department of Commerce. J. Wood

and his colleagues at Ovonics have been

experimenting with a carbon buffer layer

in-between the alternating polycrystalline Ni and

Ti layers to minimize the roughness, whereas

J. Bradshaw et al. at Optoline have been

attempting to make amorphous Ni(x)C(l-x)-

Ti(y)Mn(l-y) alloy layers based on predictions

of molecular dynamics simulations of sputtered

film growth [5]. Neutron reflectivity and diffuse

scattering measurements are performed at the

NBSR on these new products in order to determine what progress is actually being made.

For example, a reduction in the interfacial roughness is clearly evident by making a simple

qualitative comparison of the transverse scans shown in figures 3 and 4 for Ni-C-Ti and Ni(x)

C(l-x)-Ti(y)Mn(l-y) supermirrors, respectively,

with that shown in figure 2 above. In figure 5

is shown the reflectivity profile for a Ni-C-Ti

supermirror from Ovonics as measured at the

NBSR. Superimposed on this plot of the

supermirror reflectivity is the profile for an ordinary 1000-A Ni film. In both cases the

substrate material is ordinary float glass and the

neutron wavelength is 4 A. The reflectivity is at least 95% out to about three times the critical cutoff for Ni. Similar reflectivity results were

obtained on this particular supermirror by W.

Yelon who generously agreed to perform

measurements at the University of Missouri

Research Reactor during the period of time when

the NBSR was temporarily shut down for guide

tube installation [6]. In a separate but parallel development,

J. Hayter and H. Mook have derived an optimal

algorithm for generating a supermirror layer

thickness sequence [7] which enhances the

performance of the improved supermirrors even

further. This can be seen by comparing figures

6 and 7 which show neutron reflectivity profiles

for a Ni-C-Ti supermirror composed of discrete

sets of bilayers of different thicknesses (where in this case Ni-C is considered one layer and Ti-C

the other layer of a bilayer) [8] (fig. 6) and one made up of layers whose thicknesses were

determined according to the algorithm derived by

Hayter (fig. 7).

Recently both Ovonics and Optoline were awarded Phase II SBIR DOE Contracts to develop high-quality neutron supermirror

coatings on a larger scale. J. Hayter has also

been instrumental in securing additional funding

from the DOE through ORNL to support the

corresponding research at the NBSR. We are hoping that it will now be possible to coat several of the guide tubes at the new NIST Cold

Neutron Research Facility (CNRF) with these

improved, high-performance supermirrors.

109
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

Q=0.1 K=Z67 A

-1

1

0.6

-0.2-

-0.6-

-1.0

1 1 1—-r

- K;

< —*—i i—( i—« >—< ^P-* F-« 4 i-« - Q

t

1

1 r

\

\

K, \*

ar^ii (b it o—

-1.0 -0.6 J 1 I I

' » -0.2 0.2 X* (A"»)

0.6 1.0

Figure 1. Reciprocal space schematic for multiple or simultaneous scattering effects arising from diffuse

scattering ridges in multilayered structures with rough inter- faces intersecting the Ewald sphere as described in the text.

.3 4-

CD \wC

2 2 o

c o

soo

i

1 r

/"""\ ,•

'

/ \

i *

.i" h0m\

0l_

0.0

r

'"' KJ \

J L 0.5 1.0

9 (29=1.5)

1.5

Figure 2. Transverse scan intensity versus sample angle

with fixed detector or scattering angle for a Ni-Ti supermirror with rough interfaces.

0.5 1.0

9 (29=13)

1.5

Figure 3. Transverse scan for a Ni-C-Ti supermirror.

CDw

-4-> cs2 2

o

cjfl o

1 i i i r~

iiiii

i

l||ff

l|,"

i

i

i

i i

Ifffl

0.0 0.5 1.0

9 (20=1.5)

1.5

Figure 4. Transverse scan for a Ni(x)C(l-x)-Ti(y)Mn(l-y)

supermirror.

110
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

1000

0.00 0.02 0.04 O06 Q (A'

1 )

0.08 0.10

Ni-C-Ti

1200, 1

980

I 720

Z 480

240

«*°*V*ie&cptA^to

0.020 0.032 0.044 0.058 Q (A"

1 )

0.068 0.080

Figure 5. Neutron reflectivity as a function of wavevector

transfer for a Ni-C-Ti supermirror superimposed on the

reflectivity profile of a 1000-A thick Ni film. This supermirror has an effective critical angle about three times

that of the Ni film and a nearly uniform reflectivity of

better than 95%. This supermirror is composed of discrete

sets of bilayers of different thicknesses as described in the

text.

Ni-C-Ti

1200

9eop***A «w>«V

I 720

480

240

<A*

0.020 0.032 ^^^^^fii^At^i^^^^if^eM ^ 0.044 0.056 Q (A"

1 )

0.080

Figure 6. Reflectivity profile for a Ni-C-Ti supermirror

composed of discrete sets of bilayers with different thicknesses.

References

[1] R. Pynn, in Thin-Film Neutron Optical Devices . C. F. Majkrzak, ed. SPIE Proc. Vol. 983, (SPIE,

Bellingham, WA, 1989) p. 18.

[2] J. E. Keem, J. Wood, N. Grupido, K. Hart, S. Nutt, D. G. Reichel, and W. B. Yelon, in Thin-Film Neutron Optical Devices , C. F. Majkrzak, ed. SPIE

Proc. Vol. 983, (SPIE, Bellingham, WA, 1989) p. 38.

[3] A. Bruson, C. Dufour, B. George, M. Vergnat, G. Marchal, and Ph. Mangin, Solid State Comm. 71 , 1045(1989).

[4] S. K. Sinha, E. B. Sirota, S. Garoff, and H. B. Stanley, Phys. Rev. B38, 2297 (1988).

[5] I. K. Schuller, MRS Bulletin XIII, 23 (Nov. 1988).

[6] W. B. Yelon and J. Wood, private communication.

[7] J. Hayter and H. Mook, J. Appl. Cryst. 22, 35 (1989).

[8] C. F. Majkrzak and L. Passell, Brookhaven National Laboratory, Report No. 34343 (Upton, NY, 1984) and Acta Cryst. A41, 41 (1985).

Figure 7. Reflectivity profile for a Ni-C-Ti supermirror

composed of layers with thicknesses determined according

to the optimized algorithm derived by Hayter as described

in the text.

Ill
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

NEUTRON FOCUSING USING CONVERGING GUIDES

D. F. R. Mildner (Inorganic Analytical Research Division)

The neutron current density can be increased

by using a converging guide; this can be

particularly useful as the last section of a long

straight guide [1]. This increased density is obtained at the expense of increased divergence

of the beam, which is unimportant in neutron

absorption techniques. A diagrammatic representation of neutron trajectories within the guide

in each of the two transverse directions of the

guide is useful for deriving analytical expressions in the small angle approximation which

describe the transmission properties of the guide.

We define a 1-d gain G for the converging

focusing guide as the ratio of the current density

of neutrons (transmitted acceptance per unit

dimension) leaving the converging guide system

to the current density of neutrons (acceptance

area per unit dimension) incident on the system.

Using the acceptance technique, Anderson [2] has indicated that the converging guide gain as a function of wavelength shows discontinuities in the first derivative. Analytic expressions for the

functional form of the gain for the focusing

guide with limited acceptance have been derived

[3] as a function of the converging guide

dimensions, the incoming beam divergence, and

the critical angle of the converging guide surface

material. The results can be reduced to a set of

nine basic formulae (some of which have a

reflection index n) with three independent

parameters: 1) the convergence factor H/H', 2)

the ratio m of the critical angles C and 8 of the

converging and straight guides, and 3) the

neutron wavelengh A. which effectively links the

other two parameters. These expressions are in agreement with measurements on straightconverging guide systems [4]. This method can

also be applied to a reflectivity which is describable by a series of step functions.

A graphical method has been found for

determining the appropriate equation for the current density gain for the particular guide

configuration and relative surface coatings, as a function of wavelength. Shown in figure 1 is a diagram giving the various regions in (0,6C) space for the most convergent geometric

configurations of the measurements of Copley

glass-supermlrror

glass-nickel

glass-glass

io e(mrad>

Figure 1. A schematic diagram in (8,8 ) space showing

the regions for the configuration of the converging guide measurements of Copley and Majkrzak [4]. The line 8 =

4.43 mrad corresponds to X =4,1 A for float glass.

and Majkrzak [4]. This is determined solely by

the geometry of the converging guide (the length

L, and the entrance and exit heights, H and H'),

and at this point the figure is independent of the

coatings of the guide. The abscissae are normalized to the particular reflecting material of

the straight guide, so that a given wavelength

112
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

corresponds to a particular value of 6. Straight

lines through the origin, 8C = m8, indicate the

loci of the different combinations of guide

coatings. The intersection of this line with the

wavelength line determines the region and hence

the equation for the current density gain of the

guide combination.

We have calculated the expected gain for the

cold neutron flux at the exit of a 50 x 50 mm2 guide on the cold source at the NIST reactor using a 2-d converging guide with supermirror

coating having a critical angle three times that of

natural nickel. The 2-d gains G

2 are shown in

figure 2. At short wavelengths the gain is unity.

It then rises at longer wavelengths, until the

divergence angle becomes too great to be

reflected within the converging guide, at which

point the gain levels off or even drops. Except

for certain cases, the distribution of neutron

trajectories at the exit is neither uniform in position nor in angle.

10

'eg 8

(0c

o

w

c

o

E

o

6 -

' '

i '

H' = 10 mm

• /20 mm " y^ 22 mm

/s 25 mm

j^y 30 mm . ^f^

/

- ^/Zy / 50 mm

,

i .

-

2 4 6 8 10

wavelength (Angstroms)

12

Figure 2. Two-dimensional current density gains obtained on a 50 x 50 mm straight guide of Ni using a 500 mm lon< converging guide with exit dimension H' and with a coating of critical angle three times that of natural nickel.

References

[1] M. Rossbach, O. Scharpf, W. Kaiser, W. Graf, A. Schirmer, W. Faber, J. Duppich, and R. Zeisler, Nucl. Instrum. & Meth. B35, 181-190 (1988).

[2] I. S. Anderson, SPIE Proc. 983, 84-92 (1989).

[3] D. F. R. Mildner, Nucl. Instrum. & Meth. (accepted

for publication).

[4] J. R. D. Copley and C. F. Majkrzak, SPIE Proc. 983,

93-104 (1989).

113
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

MULTIPLE REFLECTIONS IN CURVED NEUTRON GUIDES

D. F. R. Mildner (Nuclear Methods Group)

There is increased use of neutron optical

elements which depend on the principle of total external reflection of cold neutrons. For

example, curved devices in neutron optics are used quite extensively as beam benders, particularly for filtering a beam of cold neutrons from

the fast neutron component, in lieu of a crystal

filter. In particular, our concept for focusing

cold neutrons is to use stacked curved

microguides which superimpose their outputs at the focal point. The method of acceptance

diagrams is used to obtain analytic expressions

for the intensity and spatial and angular

distributions of the neutrons which are transmitted along a curved guide [1], provided

that the length of the guide is greater than the

minimum length necessary to eliminate direct

radiation. The diagram describes the path

traversed in position-angle space in the plane of

curvature as the neutron proceeds along the

curved guide.

The acceptance area for curved guides is parabolic in shape, and shows the asymmetry in the spatial distribution of the transmitted

neutrons. The transmitted intensity curve as a function of wavelength X is given by

i(X)~4/mkc (\/xcy for X <. Xr

-4/3HXc [(mc)3

- {{XIXcf - 1}

3/2

],

where Xcis the wavelength characteristic for the

curved guide and depends both on the

dimensions of the guide and on the surface

coating of the guide. H is the width of the

curved guide with a radius of curvature R.

Then the characteristic wavelength XQ

is given

by \/(2H/R)/y c

, where yc

is the critical angle

per unit wavelength for the guide surface

material. These equations are valid only if the

guide is longer than the line of direct sight,

A8HR).

The effect of less than perfect reflectivity

appears as a factor of the n

th power of the

reflectivity of the neutron density in the phasespace regions corresponding to trajectories which

undergo n reflections. Provided that the

reflectivity is close to unity, the reflectivity for

all neutrons averaged over all regions in phase

space is given by the average reflectivity to the

power n, where n is the number of reflections

averaged over all possible reflections.

For the curved guide, the mean number of

reflections depends on the neutron wavelength

and the length of the guide [2]. The mean

number of reflections per length of direct sight

is given by

n = 3/2 (XJX) for X <. Xr

for X z Xr

which make only garland reflections from the outer surface, and by

n = 3A.

C (2X2

- X

2)/2[X3

- (X2- X

2

) 3'2]

which make both garland and zig-zag reflections.

The mean number of each type of reflection in

this case is given by

gar

zz

= 3XC

3 /2[X3

- (X2- X

2

)312} =

2\il2A 3xc (V - x/)/[x' - (x' - vrz

]

for X > X,

X > Xc

, and

X > Xn

114
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

For X > Xc ,n approaches 2X/XC

, which is the

value of the straight guide of length \/(8HR). Of

course, the total number of reflections is the

addition of the number in the plane of the curve,

o

g!-Ho

e

a

o

eW3

100

75

50

25 -

and the number in the plane perpendicular to the curve, which is given by the straight guide of the same length.

0.5 1 1.5 2 2^

wavelength (A, / Xc)

Figure 1. The relative transmission efficiency of the curved guide in units of

the characteristic wavelength.

10co

o 4

2 3 oE

CO

<DE

1

tti—i——

r

i i l i i—

i

0.5 1 1.5 2

wavelength "kfk

2.5

Figure 2. The mean number of reflections n for a curved guide of characteristic wavelength X

c ,having only garland reflections

for X^.c ,and both garland and zig-zag reflections for X > X

c

.

References

[1] D. F. R. Mildner, Nucl. Instrum. & Meth. A290. 189- 196 (1990).

[2] D. F. R. Mildner, Nucl. Instrum. & Meth. A292 . 693- 699 (1990).

115
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

6Li-DOPED SILICATE GLASS FOR THERMAL NEUTRON SHIELDING

C. A. Stone and R. Zeisler (Center for Analytical Chemistry)

and

D. H. Blackburn, D. A. Kauffman, and D. C. Cranmer (Ceramics Division)

Choosing a thermal neutron shielding

material for neutron scattering instruments or prompt gamma-ray activation analysis (PGAA)

instruments is often difficult. The ideal neutron

shield is one which attenuates the neutron beam

rapidly without creating additional background.

But the materials which are most attractive for

attenuating the thermal neutrons, such as boron

and cadmium, produce gamma-rays in the

neutron capture process. Although this gammaray background might be tolerated near some

neutron scattering instruments, it quickly

degrades the quality of a PGAA instrument.

The nuclide 6Li is more attractive than other

nuclides as a thermal neutron shield because it has a large absorption cross section, 941 barns,

and the primary reaction is the (n, a) charged

particle reaction. There is only a small prompt

gamma-ray branch (0.0004% of the total).

Enriched Li can thus provide rapid attenuation

of the neutron beam without producing

significant background problems.

A disadvantage of using 6Li has been the

lack of a suitable host matrix. Fused 6Li2C03

is one common form but it is prone to attack by

moisture and has poor structural properties.

Sintered 6LiF is a better material for shielding

since fluorine has a low absorption cross section

and will not contribute substantially to the

gamma-rav background. Structural properties of

sintered LiF are also better than those of

lithium carbonate, as it is not prone to attack by

moisture and is less brittle. This material,

though, is not easily formed into irregular shapes

and cannot be easily machined.

We have developed a

6Li silicate glass for use as a thermal neutron shield. There are only

two components to the glass: Si02 and 6Li20.

Lithium silicate glasses tend to be susceptible to crystallization at higher Li2 concentrations.

Glasses have been successfully prepared with

6Li2 concentrations of up to 18% wt with no

evidence of crystallization. This glass provides

a thermal neutron attenuation of 76% per mm of

glass. To further increase the concentration of

Li2in the glass, alkaline earth or other divalent

oxides can be added to the melt. We have been

successful in creating a

6Li glass containing as much as 22% wt percent by adding 8% wt

A12 3

. The thermal neutron attenuation for this alumina-doped lithium silicate glass is 82% per

mm.

Long-term irradiation tests have been

performed on the 6Li silicate glass. A one by

one cm piece of glass, with a thickness of one mm, was cut from a larger sample of glass and

then polished. This glass was irradiated in the

1PH1 pneumatic beam tube of the MITR-2

reactor at the Massachusetts Institute of

Technology. The neutron current density in this beam tube is 8 x 10 12 neutrons/cm2

s and the

cadmium ratio is 400. To irradiate the sample,

the glass was wrapped in Al foil and placed in a polyethylene rabbit that was packed with

polystyrene packing material. Short irradiations,

lasting from a few seconds to 15 min, showed

that the temperature of the rabbit was stable after 15 min and thus heating by the glass was not a problem. Following these short irradiations, the

sample was irradiated for 10 hrs. Two weeks

after the end of irradiation, the glass was

unpacked and inspected. Some localized

charring of the polystyrene was observed within

ca. 2 mm of the glass sample. The glass showed

no detectable change in appearance other than a discoloration from color centers produced by

charged particle recoil. Much of the usefulness of this glass is in external beam environments, where the neutron

current densities are lower, on the order of 109 neutrons/cm2

s, and the beams have much lower

gamma-ray and fast neutron components. The

irradiation studies described above were

equivalent to an external beam irradiation of at least seven years. This lithium glass is more

applicable as a thermal neutron shield than other

shielding materials: it can be cast into a variety

of shapes and sizes, and it can be machined

using ordinary glass machining techniques into

shapes with close tolerances.

116
REACTOR RADIATION DIVISION AND COLLABORATIVE PROGRAMS

FUNDAMENTAL NEUTRON PHYSICS

G. L. Greene (Quantum Metrology Division)

Two instrumental positions at the CNRF

have been designated as experimental stations for

investigations in an area of research which has

come to be known as "Fundamental Neutron

Physics." This work includes measurements of

basic neutron properties, tests of fundamental

symmetries of nature as well as investigations

involving advanced methods in neutron optics.

An additional program in advanced methods for

neutron flux determination will have implications

in neutron dosimetry, analytical chemistry, and

nuclear physics. One instrumental position (on

NG-7) will be on a monochromated beam,

devoted to neutron interferometry. The other

position (on NG-6) will be a full flux end

position. It will initially be devoted to studies of

weak interaction physics.

Design and construction work on the

neutron interferometer position has been

focussed primarily on the implementation of a sophisticated seismic and vibrational isolation

system. Neutron interferometers are

exceptionally sensitive to external noise sources. The seismic isolation will consist of a cascaded

series of passive isolation elements which will allow the use of an entirely new class of neutron

interferometers employing separated perfect

crystals as optical elements. This work is a collaboration with the University of Missouri,

the University of Vienna (Austria), and the

University of Melbourne (Australia).

The end position will initially be employed

for an experiment to determine the neutron

lifetime. This quantity has important

implications for the study of the theory of weak

interactions, for astrophysics and cosmology.

The neutron life apparatus has already been used

in a initial measurement at the Institut Laue

Langevin in Grenoble. It will be installed at NG-6 in October 1990. This work is a collaboration between NIST, the University of

Sussex (England), the Central Bureau for

Nuclear Measurements (Belgium), and the

Scottish Universities Research Reactor. It is supported in part by the Department of Energy

and the Science and Engineering Research

Council (U.K.).

117
B. NON-RRD PROGRAMS

NEUTRON INTERACTIONS AND DOSIMETRY GROUP

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 reactor cross-section measurements.

Involvement with outside organizations, both in the Federal and private sectors includes many

types of research and technology assistance

programs as well as leadership roles on national

and international standards and radiation policy

making bodies.

A selection of accomplishments for FY90

with emphasis on NIST reactor related activities are outlined below in titled paragraphs grouped

under four projects.

DOSIMETRY FOR MATERIAL PERFORMANCE ASSESSMENT

E. D. McGarry, C. M. Eisenhauer, J. A. Grundl, and D. M. Gilliam

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.

NIST/Nuclear Regulatory Commission

(NRC) Contract . By law, Title 10 of the Code

of Federal Regulations, Appendix G and H

require that a program, involving irradiation of

metallurgy test specimens and dosimeters, be

carried out for the surveillance of pressure vessel

embrittlement damage. NIST continues to participate in Pressure Vessel Surveillance

Dosimetry to benchmark dosimetry measure- ments and calculations and continues as general

dosimetry consultant to the NRC.

Whereas the contract of former years has

concerned itself with materials dosimetry tasks

related to pressure vessel exposures to fluence

greater than 1 Mev [<J>t(E > 1 MeV)], current

dosimetry concerns have expanded to include

damage in steel by neutrons in the energy range

0.01 -to- 1 MeV and, for different reasons, by

thermal and epithermal neutrons. Concern for

this energy range comes about because of the

gas-cooled advance reactor design and growing

discontent with either the 4>t(E > 1 MeV) or the

DPA Model (Displacement Per Atom) of

embrittlement in ferritic steels. New Heavy

Section Steel irradiation programs are under way

to compare pressure vessel steel damage in spectra with enhanced neutrons in the highepithermal and "just below 1 MeV" ranges to damage in conventional light-water reactor

spectra. Because the test efforts represent design

and startup of new irradiation experiments, NIST

is involved in various test reactors. Separate concern for the very low-energy neutrons comes

from a realization that experimental data exist to suggest that more damage is observed at lower

fluence when the thermal -to-fast neutron ratios are a factor of 50 higher than in PWR reactors. NRC Regulatory Guide Preparation . Major

involvement continues in the development of a draft regulatory guide "Status of a New

118
NON-RRD PROGRAMS

Regulatory Guide on Methods and Assumptions

for Determining Pressure Vessel Fluence," to provide guidance for improving and benchmarking neutron transport calculations. A nearfinal version is now being reviewed by the

Nuclear Regulatory Commission prior to its issuance as a Draft Regulatory Guide for review

by the nuclear industry. The primary object of

the guide is to achieve better consistency in determining and reporting neutron fluences and

associated uncertainties.

The guide contains up-to-date methodology

for carrying out absolute transport calculations,

assuming a widespread use of two-dimensional

discrete-ordinates Sn transport codes. The

mechanism for validating calculation is comparison with accurate measurements.

Discussion of neutron fluence measurement

methods emphasizes more measurements, quality assurance of dosimetry materials, and better

reporting. There is also emphasis on validating

transport calculations against experimental

benchmarks and confirming power cycle

calculations against in-situ surveillance

dosimetry.

ASTM Benchmark Standard . A draft of this ASTM Standard Guide for Benchmark Referencing of Neutron Dosimetry for Reactor

Pressure Vessel Surveillance was prepared by

NIST and was proposed for ballot at the June

1989 E 10.05 meeting in Scottsdale, Arizona. A

revised draft was then sent to ASTM in January

1990 in preparation for an E10.05 ballot at ASTM and Radiation Effects Symposium

meeting in Nashville.

NIST/Westinghouse Cooperative Agreement. Dosimetry Measurement Assurance

Program: Measurement assurance activities for

Westinghouse radiometric (activation foil) and

Solid State Track Recorder (SSTR) dosimetry

continues. NIST is especially interested in

problems associated with obtaining reliable masses for SSTR fissionable deposits in the

nano-gram to pico-gram range.

SSTR Ultralight Mass Determination: The

trend in the nuclear power industry to implement

SSTR dosimetry in ex-vessel cavities is increasing. The bulk of the interest is in work

taking place in Westinghouse PWRs and in the

B&W Owner's Group's Davis Besse Dosimetry

Benchmark Experiment. NIST provides known

fluences for mass determinations of the ultra- lightweight SSTRs. The irradiations are carried

out in the 235U Cavity Fission Source in the

reactor thermal column, to an accuracy of about

2.5% (la).

Cooperative Research on a New Dosimeter:

The dosimeter consists of a fissile layer placed

adjacent to a polymer material that becomes

conducting as a result of implantation of

energetic heavy ions such as those from neutroninduced fission. The results of preliminary

calibration experiments carried out on a prototype detector in the Cavity Fission Source were reported at the 7th ASTM/Euratom

Symposium, Strasbourg, France, August 1990.

Dosimetry Methods Development for

Reactor Support Structures . A priority effort in the Heavy Section Steel Technology (HSST)

Program is evaluation of low-temperature, lowfluence-rate embrittlement on reactor vessel

support structures. This issue arose from

findings of higher than expected embrittlement in the High Flux Isotope Reactor (HFIR), at Oak

Ridge, as compared with the reactor shield tank

from the Shippingport Reactor. The HFIR and

Shippingport reactors have different fast fluences

(E > 1 MeV) but HFIR has a factor of 50 more

thermal fluence. The physics of interest here is that there is thought to be a thermal damage

mechanism tied up with the excitation energy

required to displace iron atoms after the thermal

capture of a neutron by any nearby nucleus (iron or impurity atom). This process is similar to the

Szilard-Chalmers reaction.

Materials-damage interests in reactor support

structures have not been of concern in the past

because of their low exposure to fast neutrons. Now, there is emphasis to obtain dosimetry

measurements and neutron transport calculations

for the total neutron spectrum near these critical support structures. NIST is responsible for

consultation and contracting for the dosimetry

measurements, and for benchmarking the

measurements against NIST standard neutron

fields in two operating reactors. The

benchmarked dosimetry will be used to make

necessary adjustments to the calculations at the

dosimeter locations near the support structures. Calculations will be used to extrapolate the

radiation (damage) exposure into the support

structures.

119
NON-RRD PROGRAMS

The two plants selected for specific-plant

evaluation of support structures are the TROJAN

Reactor (Oregon) and the Turkey Point Reactor

(Florida). Current experimental efforts focus on

the TROJAN plant where, under NIST advisement, dosimetry will be installed in the ex-vessel

cavity. The purpose is to make measurements as close as possible to the cantilever-beam type

support structure buried in the concrete of the

biological shield. These measurements will

normalize neutron transport calculations used to predict cumulative exposures at critical flaw

depths in the support structure. A contract for the dosimetry has been let with the Westinghouse Center for Science and

Technology (formerly Westinghouse Research)

and the irradiation (to last about 1 year) are under way.

Dosimetry Development for Test Reactors

Metallurgy Irradiations . NIST has assumed a dosimetry consultants role for the new,

combined Materials Engineering Associates and

Oak Ridge National Laboratory run Heavy

Section Steel Technology (HSST) Program at the

University of Michigan Research Reactor. The

current interest in this particular reactor comes

about because of the near absence of available

test reactors with sufficient fluence rate (5 x

10 11 n/sec) to carry out HSST steel experiments.

The 2-MW Reactor has room in its pool for

several rather large experiments and there is interest in looking at tailored spectrum

experiments where the epithermal fluence is

enhanced by several orders of magnitude over

the fluence greater than 1 MeV.

Energy Response of Innovative ElectronicHardware Dosimetry . The Nuclear Effects

Directorate (NED) at the Aberdeen Proving

Ground has been evaluating a new personnel

dosimetry system for battlefield use by the

Army. Special NIST neutron-field facilities were used to establish the neutron energy

sensitivity of this semiconductor device.

Multiple irradiations were performed for NED in a thermal neutron beam, in the 2-keV scandium

filtered beam, in the 24-keV iron filtered beam,

and at the 144-kev silicon filtered beam, all at the NIST Reactor, and at several Van de Graaff

beams, and finally at the NIST 252Cf Fission

Neutron Irradiation Facility.

LiF Gamma Dosimeter for Oconee Reactor

Cavity Dosimetry . The results of a test of "LiF

Chips" as gamma dosimeters in the cavity

surrounding a reactor pressure were analyzed

and reported. The results appear to contradict

previously reported neutron sensitivity data and

suggest that the dosimeter package may be better

than expected as a gamma monitor for mixed

gamma and neutron radiation fields. However,

the ratio of gamma dose to neutron fluence was

found to be too low in the Oconee reactor cavity

for accurate gamma determination of gamma

dose, until better neutron sensitivity data is obtained and until the possibility of fading of the

color center development at temperatures in the

range of 50 °C is ruled out.

PERSONNEL DOSIMETRY

R. B. Schwartz, E. Boswell, and C. M. Eisenhauer

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.

Characterization of Neutron Exposure

Fields . As a result of TEPC measurements, the

Armed forces Radiobiology Research Institute (AFRRI) recently obtained very surprising

results in experiments which examined the

relative biological efficiency (RBE) for lethality

in mice exposed to reactor radiations. In brief,

the experiments indicated that a slight (5%)

120
NON-RRD PROGRAMS

addition of neutron dose, into a pure gamma-ray

field, decreased the lethal dose for the mice by

almost 40%. The neutron and gamma-ray dose

components were determined by conventional

AFRRI ion chamber dosimetry and fission

chamber measurements made this year; it now

appears that the neutron kermas measured with

ionization chambers are too low. In the most

extreme case the neutron kerma inferred from

both TEPC and fission chamber measurements is seven to nine times greater than that measured

with ionization chambers. The larger neutron

doses combined with the observed lethality of

mice imply a neutron RBE fairly constant with

neutron energy, as expected.

Summary of Threshold Neutron Detector

Measurements at AFRRI. A summary of

threshold neutron detector chamber measure- ments made over a period of about 6 years are complete. In addition to discussion of

measurements in two of AFRRI's conventional

fields, the summary documents measurements

with a

237Np fission chamber, which is now

used as a neutron monitor in AFRRI exposure rooms.

Instrument Calibration Service. Approximately 40 neutron radiation protection instruments were calibrated this year. As usual, the

majority of the calibrations were done for commercial nuclear power plants, but our customers

also included institutions as diverse as Redstone

Arsenal and Baylor College of Medicine.

Performance Tests of "Bubble Dosimeters."

We are continuing our measurements of the

properties of "bubble", or superheated drop,

neutron detectors. In collaboration with J. B. Hunt of the

National Physical Laboratory (NPL), we have

concluded a major study of linearity, and

response as a function of neutron energy, for

several of these devices. Most of the data were

taken at NPL, with additional low-energy data

points taken at the NIST Reactor filtered beams.

The "bubble dosimeter" (supplied by Bubble

Technology, Inc. (BTI), Chalk River, Canada)

has high sensitivity, a good dose equivalent

response as a function of energy, and is linear

up to the point where the bubbles can no longer

be accurately counted. "Good . . . response . .

., " in this context, means within plus-or-minus

a factor of two over the four decades of neutron

energy from 2 keV to 20 MeV. The "pen"

dosimeter (Apfel Enterprises, New Haven,

Conn.) is easier to read than the bubble dosimeter, but has intrinsically lower sensitivity and a useful response only between ~ 150 keV and 20

MeV. The performance of the Bubble Detector

Spectrometer, from BTI, was disappointing.

These devices are not reliable enough to be used

as intended for neutron spectrum unfolding. The

results of this study are to be published in Radiation Protection Dosimetry, under the title, "Measurement of the Energy Response of

Superheated Drop Neutron Detectors". A major drawback of all devices based on

nucleation of superheated drops is the rather severe dependence of sensitivity on temperature - ~ 5%/°C. We have recently completed tests of

prototype temperature compensated bubble

dosimeters from BTI. While the performance of

the devices was a bit erratic, they generally

performed satisfactorily (within +10%) over the

range from ~ 10 °C to 30 or 35 °C. The data

for one of the tubes, in terms of bubbles per

millirem as a function of temperature, are

plotted on figure 1. The error bars (one

standard deviation) are based solely on the

number of bubbles. In most cases, we had ~

100 bubbles (~ 40 mrem, from moderated

Response as Function of Temperature

Tube Number 1

. _ T T < • T • • l> J. .

: j. j. . . ;

s

20 30 40

Temperature (Degrees C)

Figure 1. Response (bubbles/mrem) as a function of temperature for a temperature-compensated bubble

dosimeter. The error bars (one standard divation) arc based on the number of bubbles.

californium), so the uncertainty is ~ 10%. (The

point at 20 °C was run twice: as the initial datum point, and again near the end of the test, to check reproducibility.) This test serves as a

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"proof of principle", and we will soon be testing

the BTI production models.

We have also made preliminary tests of the

Apfel Spectrometer. This device utilizes the fact that the threshold for nucleation is a function of

pressure. It uses a single superheated drop

detector, and a computer controlled system for

stepping through a range of pressures, recording

the count rate at each pressure, and then inverting the resultant data matrix to obtain the neutron spectrum. It is a very clever concept, but

is not quite ready for prime time.

ICRU Publication on Practical Determination

of Dose Equivalents . As was noted in last year's progress report, the International Commission on Radiation Units and Measurements

(ICRU), in ICRU Report 39, recommended a new system for determining dose equivalents

resulting from exposure to external radiation sources. These new "operational quantities"

have several advantages, and our report committee is preparing the ICRU report on the practical use of these quantities; i.e., instrument design,

calibration techniques, etc. Our report, entitled

"Measurement of Dose Equivalents from External Photon and Electron Radiations," has been

completed and submitted to the main ICRU

commission for its approval. (For reasons which

are totally unclear, neutron radiation has been

split off and will be the subject of a separate,

future, document.)

The main problem for our report committee

has been the choice of a phantom for dosimeter

calibration; the problem arises from the state- ment in ICRU 39 that "... a suitable phantom

is the ICRU sphere.". In fact, the ICRU sphere

is totally unsuitable, and much effort went into

trying to determine a truly suitable phantom and

calibration system which would not do violence

to any of the ideas in ICRU 39. In the end, we

decided to skate around that statement in ICRU

39 and recommend a simple lucite slab phantom

(as is generally used anyway), together with a formulation which ties the results to the basic

principles of ICRU 39. Unless this recommendation is shot down by diehards on the Commission, health physicists in the future will not have

to wrestle with spherical phantoms.

ISO Document on Instrument Calibration . The (it-is-hoped) final version of ISO document

CD 10647, "Procedures for Calibrating and

Determining the Energy Response of Neutron

Measuring Devices Used for Radiation Protection", has been submitted to the ISO secretariat,

and from there gone to the member organizations

(e.g., ANSI, for the US) for voting. A special

meeting of the ISO working group responsible

for this document is to be held at NIST during

the first week in October, in order to consider

the comments received as part of the voting

procedure. Unless serious objections are raised

by member organizations, this standard should

finally be on the books

Data Acquisition System for AFRRI Dosimetry Group . NIST has provided a PC-based data

acquisition system and software to replace the

two HP-85 systems previously used for Ionization Chamber and Fission Chamber data acquisition by the Armed Forces Radiobiology Research

Institute Dosimetry Group. The single PC

system can acquire and display data from both

the ionization chambers and fission chambers

either together or singly. New set-up screens

replace the former sequential set-up data entry so

that default selections are clearly presented and

easily selected or changed.

RESEARCH AND TECHNOLOGY

(all group members)

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.

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Neutron Lifetime - Absolute Neutron Counting . The result of the first phase of the Univ. of

Sussex/NIST/Geel collaboration to measure the

free neutron lifetime was reported in Phys. Rev.

Letters . The goal for this first phase of the

project, as stated in our proposal to the DoE,

was to achieve an overall uncertainty of ± 0.5%

at this juncture; and the reported value of 893.6-

+ 5.3s comes very close to this goal.

Two additional papers were presented documenting the work of the Central Bureau for

Nuclear Measurements (Geel, Belgium), Univ.

of Sussex, and NIST to make characterized

deposits of B-10 and Li-6. These papers were

presented at the International Nuclear Target

Development Society meeting in Sante Fe (9/90)

and will be published in a special issue of Nucl.

Inst. Meth. in Phys. Res . The NIST alpha-gamma coincidence apparatus has been tested with an isotopic alpha-gamma

source and is ready for neutron beam runs as soon as a beam is available. This device is meant to achieve absolute neutron counting by means of the alpha-gamma coincidence method,

without reference to isotopic target mass assay. Nevertheless, a continued collaboration with

Geel in further refinement of B-10 deposits is planned, as well as continued collaboration with

Harvard Univ. and Los Alamos National Laboratory in calorimetric neutron counting.

Benchmark Measurements for Critical ity

Counting . Two series of careful measurements

of neutron leakage from a water-moderated Cf252 source have been completed and compared

with rigorous Monte Carlo calculations

performed by the Computational Methods

Development Group at Los Alamos Scientific

Laboratory. The first results from this program

were for the case of a 4-in diam sphere of water. For this case very detailed calculations by the

Monte Carlo Neutron Photon (MCNP) code

required about 50 h of supercomputer time. The

comparisons of measured and calculated fission

rates outside the sphere showed some interesting

discrepancies. The thermal ization was

overpredicted in the calculations relative to the

measurements. A number of auxiliary tests for

unexpected absorption of thermal neutrons in the

water, steel spherical shell, or fission chamber

components failed to reveal any contamination

that could explain the discrepancies. These

results will be reported in the American Nuclear

Society Transactions . Winter, 1990. A second

series of measurements with a 3-in diam sphere

appear to show the same discrepancy in thermal ization. The discrepancies for U-235 and

Pu-239 fission rates are of the order of 4% to 7%, well beyond the combination of the overall

uncertainty of the experiment and the statistical error of the calculation. However, the known

errors in the cross section data and the

thermalization model employed may be sufficient

to explain the discrepancies. A complete

uncertainty analysis is not yet available for the

calculation, including the uncertainties of the cross sections and the S(a,P) thermalization

model.

Neutron Fluence Rate Measurements at CNRF Guide Positions . Several measurements

of both fast and thermal neutron fluence rates were made at various positions on cold neutron

guide NG6. All of these measurements were made at a time when the cold source was not

operational and filled with warm helium gas

rather than heavy water ice. On April 8, 1990, when NG-6 had been built

only a few feet beyond the wall of the reactor building, a series of thermal and fast fluence rate measurements were made at the end of the guide

as it stood at that date. These measurements were made at reactor power levels of 20 kW,

50 kW, 200 kW, 500 kW, and 1 MW. Since

the average thermal neutron speed was not known, the thermal neutron results were

reported as if the neutron speed were 2200 m/s.

The fluence rate derived in this way is also

known as the "capture flux." The energy

spectrum of the fast neutrons was also unknown,

and the fast neutron data was interpreted as if the fast spectrum were an unmoderated fission

spectrum from the thermal neutron fission of U235. The fluence rate determined in this way is termed the "fission-equivalent fluence rate." The guide was evacuated during these

measurements.

On May 8, 1990, with the guide built to full length, the fast and thermal fluence rate measurements were repeated at 1 MW at the end

of the guide. The guide was filled with helium,

but somewhat contaminated with air during these

measurements.

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On May 17, 1990, the thermal fluence rate measurements were made once more with the

guide filled with pure helium. These measure- ments were made at two positions: at the end of

the guide, and inside the guide at an access point

about 3 meters outside the reactor building wall.

Collaboration With U.S. Naval Academy . Monte Carlo calculations with the MCNP code

have been used at NIST to determine the neutron

spectrum from a 14-MeV neutron generator at the U.S. Naval Academy (USNA). Comparisons

of dose equivalent with similar calculations by

USNA showed a significant (40%) discrepancy.

Further investigations revealed that the MCNP

code at USNA did not duplicate sample

problems distributed with the computer code

package. It is not clear whether the discrepant

values are due to the particular version of the

code used at USNA or due to adaptation to their

computer.

Collaboration With Institut de Protection et de Surete Nucleaire (IPSN). Because of our

experience in calculations of anisotropic emission

from small neutron sources NIST was asked to make calculations for a source used by IPSN.

We checked their calculations of neutron

spectra from a 14-MeV neutron source moderated by iron, uranium-238, and heavy

water. This collaboration arose out of

participation on ISO committee TC85,

subcommittee 2, "Radiation Protection."

Neutron Penetration in Slabs of Finite Extent. Monte Carlo calculations of neutron

transmission through slabs have been performed

to help the U.S. Navy estimate shielding of

personnel on submarines carrying nuclear

missiles. These calculations demonstrate that neutron and gamma-ray penetration is insensitive

to the position of a slab between a localized

(point) source and detector. Furthermore, an

angular parameter can be specified such that the

relative contribution of neutrons or photons

within that angle is also insensitive to the slab

position. This is equivalent to quantifying the

increase in scattered particles as one moves from

a narrow-beam to a broad-beam configuration.

A paper on the subject has been accepted for

publication in Nuclear Science and Engineering . An oral paper has also been accepted for

delivery at the Winter Meeting of the American

Nuclear Society.

Radiation Shielding Calculations for the

NIST Cold Neutron Facility (CNRF). Calculations of the necessary shielding for two shutters

at the CNRF and for shielding around neutron

guides were made. The latter calculations

approximated the neutrons scattered from the

walls of a guide as a line source. All

calculations predicted dose equivalent rates which were conservative as determined by

measurements with shielding in place.

Staff Member Elected to NCRP. C. Eisenhauer was elected to the National Commission on Radiation Protection and Units.

IRRADIATION AND CALIBRATION FACILITIES

J. A. Grundl, E. D. McGarry, E. Boswell, and D. M. Gilliam

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 multi-purpose

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.

Neutron Source Strength Calibrations in FY90. Fifteen neutron source-strength calibrations were accomplished this year by the MnS04 Bath

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Facility. Eight of the calibrations were accomplished under Standards Publications (SP-250)

contracts for the customers identified in Table 1

The remaining calibrations were for in-house

neutron sources used with the Cf-252

Fission-Neutron Standard Field and the D20-

moderated (and bare) Cf-252 source-range

calibration of personnel monitoring equipment.

There was an on-site refurbishment of the

manipulator arm by Sergeant Industries. This

included some hands-on maintenance training of

NIST personnel.

New Reference Neutron Field to Support

NIST/NRC Dosimetry Contract Work. To

supplement calibration and benchmarking

irradiations carried out in NIST standard fission

spectrum facilities, there is a need for a higher

fluence rate and larger volume irradiation facility to accept dosimeters in steel capsules. The

University of Michigan Reactor has expressed an

interest in cooperative research to develop such

a facility in the water experimental area at their reactor. Fission Neutron Irradiations Operations . The storage cave for radioactive sources, and

standard neutron field assembly pieces is essentially complete. The extensive heavy-work

required to build this facility was carried out

entirely with staff personnel.

Table 1. Customers for FY90 neutron source emission rate calibrations

Laboratory/Company No. of Sources Type of Source

Oak Ridge National Laboratory

Battelle Pacific Northwest Laboratory

Sandia Laboratory

NAVALEX Laboratory

Naval Research Laboratory

Brookhaven National Laboratory

2 Cf-252

1 Cf-252 2 Pu-238 Be

1 Pu-239 Be

1 Am-Be

1 Cf-252

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NUCLEAR METHODS GROUP: OVERVIEW

R. R. Greenberg (Center for Analytical Chemistry)

The development and application of nuclear

analytical techniques for 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 for both prompt gamma

activation analysis (PGAA) and neutron depth

profiling (NDP). NDP determines concentrations of several important elements (isotopes) versus 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 multielement determinations

performed on a number of SRMs including:

Total Diet, Frozen Mussel Tissue, ' Apple

Leaves, Peach Leaves, Bone Ash, Bone Meal,

Water, 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. RNAA measurements are currently under

way to determine Hg in Bovine Serum. This

will allow the certification of an additional,

highly-important element in this existing SRM.

In collaboration with members from other groups

in the Inorganic Analytical Research Division, a dissolution study of the two new leaf materials

has been completed which identifies problems

potentially leading to analytical errors during the

certification analyses. In addition, a study to

determine the feasibility of preparing a dietary

slurry SRM, instead of the traditional type of

dried material, is under investigation. Members

of the Nuclear Methods Group have also collaborated with researchers from the Microanalysis

Group (553) in developing a boron depth-profiling SRM.

Recent applications of NAA in the life sciences by the group include nonclassical detection and determination of tags associated with

biological macromolecules such as proteins,

enzymes, and antibodies. In one application, an

activatable tag has been attached to an antibody

used in a medical diagnostic test. The INAA

determination of the activatable tag is then used

as the detection step of the diagnostic analysis.

Research is continuing on the polyacrylamide gel

electrophoresis neutron activation analysis

technique (PAGE-NAA). The activation and

subsequent determination of phosphorus in phosphoproteins has already been demonstrated,

and research in this area was included as part of

the Ph.D. requirements for one group member

who has received her doctorate this past year. Research to extend this technique to the

determination of other elements in specific

macromolecules is under way. Such advanced

uses of NAA in the life sciences may open a wide variety of applications for nuclear

analytical chemistry.

The Biomonitoring Specimen Bank Research

Project has continued its research support for

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

IAEA/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. Of major

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importance this year has been the analysis of

samples from the Alaska Marine Mammal

Project. Both essential and pollutant elements

have been studied to assess the effects of

anthropogenic inputs to the environment.

The joint NIST/FDA/USDA study of trace elements in human diet, sponsored by the Inter- national Atomic Energy Agency, has completed

its sixth year. A total of 200 diets from differ- ent countries have been analyzed to date for

minor and trace elements. 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. 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.

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 elemental sensitivities (counts//tg-s) increased with increasing

hydrogen concentration. Sensitivities in hydrogenous samples have been observed to increase by

up to approximately 25% relative to samples

with little H. Until now, this error has been

handled by matching H concentrations in samples

and standards without fundamental understanding

of the problem. Substantial progress has recently been made in understanding the causes of this sensitivity enhancement, and in developing a method of analysis in which these effects are

eliminated. Theoretical and experimental studies

have shown that the elemental sensitivities in PGAA depend upon the scattering density (or

hydrogen concentration), as well as the sample

shape. Monte Carlo calculations have demonstrated that scattering can increase or decrease

the neutron path length within the sample and

thus may change the probability of neutron

absorption. Thus the analytical signal is a function of the sample scattering density, the

physical dimensions of the sample, and the

sample orientation with respect to the beam.

Theoretical studies have demonstrated that for

spherical samples, neutron scattering only serves

to mitigate the effects of neutron self shielding,

and produces no additional enhancement.

Experimental studies have shown that elemental

sensitivities for disk-shaped samples with constant hydrogen concentration and constant diameter (12.7 mm) vary with thickness over the range

of 2-12 mm. However, elemental sensitivities remain constant when spherical samples of the same materials (diameters 2-12 mm) are used.

Research on the recoil-nucleus time-of-flight

NDP technique has focused on the problem of

detecting (with good energy resolution) the low- energy heavy nuclei which are emitted along

with the light nuclei used in conventional NDP.

These heavy nuclei can be used to provide very

high resolution depth profiling capabilities in the

sample region very close to the surface. Initial measurements appear to have validated this approach, and efforts are under way to determine how closely the actual depth resolution

measured approaches the calculated values. This

work is a continuing collaboration with Emile

Schweikert at the Chemistry Department of

Texas A & M University.

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 soon

to be available at the new Cold Neutron

Research Facility (CNRF). The group's involve127
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ment includes the design and construction of

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, and the new facility will be the

closest instrument to the reactor cold source. 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 will allow remote scanning of

15 x 15 cm samples, and will also allow rotation

of both sample and detector angles relative to the

beam.

The cold neutron prompt gamma activation

analysis (CNPGAA) instrument is currently

under construction and will be ready for measurements this fall. Over the past year, several new mechanically-strong, radiation-stable,

synthetic glasses containing Li-6 have been

developed to efficiently absorb neutrons with

minimal gamma-ray emission. These glasses

will be used in the beam shutter and in the

sample chamber. The conceptual design of the

beam line is now complete, and it will include a cold beryllium/bismuth filter to make this the

finest facility in the world for PGAA. In addition to greatly enhancing the sensitivities for

all elements compared to existing thermal

neutron PGAA instruments, collaborative

measurements with Jiilich have indicated that the

cold neutron facility should allow the determination of a few micrograms of hydrogen in a wide variety of materials.

A novel technique of PGAA analysis of gas

samples has been developed using both the

currently available thermal neutron beam at the

NBSR and the ELLA cold neutron facility at KFA, Jiilich. Feasibility studies involving noble

gases and gas mixtures indicate that sensitivities

in the sub-ppm range for a number of elements

should be achievable using the new NIST cold source PGAA instrument which will be operational later this year. The development of a nuclear technique for gas analysis will establish an independent analytical method for reference

material certification.

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 the most promising approach

to achieve analytically useful focused neutrons. As a first approach, a device will be developed

and tested using a stack of ultrathin, nickelcoated single-crystal silicon wafers as focusing

elements. The silicon wafers act as the trans- mission 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. Ultimately, this focusing element will

be combined with a real-time, high-resolution

neutron imaging detector to measure the neutron

distribution and intensity. On the theoretical

side of this project, the acceptance diagram

technique has been adapted for both curved

neutron guides and focusing elements, and this technique can be used for predicting the number

of reflections within these neutron optical

devices.

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. In collaboration with researchers at Eastman Kodak, NDP has been used to identify

unambiguously, and to quantify the surface

boron on "as received" single crystal silicon

wafers. NDP requires no special sample preparation prior to analysis, in contrast to commonly

used analytical techniques. By carefully controlling a variety of sample processing procedures

prior to NDP analysis, the source of boron has

been identified as ambient atmospheric boron

compounds which react with the exposed surface

of the silicon wafer and become incorporated

into the native oxide layer.

The Nuclear Methods Group is also collaborating with scientists from the NIST Center for

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Atomic Molecular and Optical Physics, the NIST

Center for Radiation Research, and the

University of Sussex in a new measurement of

the neutron lifetime. The technique used

involves detecting the protons which decay in a neutron beam passing through a superconductingmagnet proton trap. A measurement of the

neutron lifetime has been made with a cold

neutron beam at the Institut Laue-Langevin in Grenoble, France. A value of 893.6 + 5.3 s was determined, and this has been reported in a paper in Physical Review Letters. Additional

measurements will be made at the new NIST

CNRF later this year. During the coming year the group will continue to improve the accuracy, sensitivity and

productivity of nuclear methods as applied to elemental 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 done). The development of monitor activation analysis is part of the high-temperature

superconductivity effort to achieve rapid sample

turnaround with minimum sacrifice of accuracy;

however, the approach also provides good

quality control when used in parallel with tradi- tional primary standard NAA.

Robert R. Greenberg, Group Leader; Donald A. Becker, Maryanne S. Cassedy, M. James Blackman, Rabia Demiralp, R. Gregory Downing, Kathleen A. Fitzpatrick, Ronald F. Fleming, Karen Gilliland-Garrity, Brent L. Grazman, G. Venkatesh

Iyengar, George P. Lamaze, John K. Langland, Richard M. Lindstrom, Elizabeth Mackey, David F. R. Mildner, J. Emlen

Myers, Bruce R. Norman, Craig A. Stone, Susan F. Stone, Dana K. Weiss, and Rolf Zeisler.

DISSOLUTION PROBLEMS WITH BOTANICAL REFERENCE MATERIALS

R. R. Greenberg, H. M. Kingston, R. L. Watters, Jr., and K. W. Pratt (Inorganic Analytical Research Division)

For many analytical techniques, the first step

in analyzing solid samples is chemical dissolution or fusion. Losses in this stage of the

analysis will affect the accuracy of the final results obtained. There are a number of possible

loss mechanisms during sample decomposition

including gaseous evolution, absorption or adsorption onto surfaces, precipitation and

persistence of undissolved material. It is well

known that botanical materials may contain

various soil and/or mineral fractions, and thus

may be difficult to dissolve [ 1 -4] . Consequently,

prior to analyzing the candidate Apple and Peach

Leaves SRMs (SRMs 1515 and 1547) for certifi- cation, a dissolution study was undertaken to ensure that all elements of interest could be

solubilized by the dissolution techniques intended

to be used as part of the complete analytical

procedure. The method chosen to study such

losses involved irradiating the samples, performing the decomposition, filtering the clear solutions, and finally measuring residual

radioactivity in the vessels and on the filters. Both botanical SRMs were irradiated in the

NIST Research Reactor to form radioactive

nuclides of a number of different elements. The

irradiated samples were then decomposed using

a variety of procedures. The sample decompositions were performed by four different

chemists from NIST to minimize individual bias

and to study the effects of minor differences in the dissolution procedures. The dissolution

procedures produced clear solutions, which were

then passed through filters to separate any

residual particulate matter. The filters,

solutions, and vessels used for decomposition, as

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well as neutron-irradiated samples of

unprocessed material, were then subjected to gamma-ray spectrometry to determine the

fraction of each element remaining in undissolved, precipitated, or absorbed forms.

Various sample decomposition methods were

investigated including: open-vessel dissolution

with nitric, perchloric, and hydrofluoric acids,

using several different types of dissolution

vessels and procedures; closed-vessel microwave

dissolution with nitric and hydrofluoric acids;

closed-vessel microwave digestion with nitric and hydrofluoric acids followed by open-vessel

dissolution with perchloric acid, and lithium

metaborate fusion in platinum crucibles. It was

originally expected that most, if not all, of the

sample decomposition methods investigated

would produce final solutions with negligible

fractions of insoluble materials. In addition, it was believed that losses of metals to the walls of

the Teflon and platinum vessels would be very

small. The results of these studies, however, did not support these expectations.

Large fractions of some elements were

observed on undissolved particles after some of

the dissolutions, despite the fact that all solutions

appeared clear prior to filtration. The microwave-only dissolutions produced very large

fractions (i.e., 10% of the total amount present)

of particulate Cr, Fe, Hf, Sc, and rare earths. Much of this material was probably a mixture of

undissolved material and rare-earth fluorides that precipitated during the dissolution. Smaller

amounts of most of these elements, with the

exception of Cr, were observed on particulate

material when perchloric acid was used after the

initial decomposition with nitric and hydrofluoric

acids. However, still significant amounts (i.e., percent level) of some elements were observed.

In addition, sample-to-sample differences in the

amount of material remaining on particles were

observed, apparently due to minor differences in actual dissolution procedure followed. Two

samples of the Peach Leaves, which were

subjected to essentially the same dissolution

procedure, contained very different (insoluble)

fractions of some elements. It was noted during

this study that one of these samples was

processed on a slightly cooler area of the hot

plate, and that the precipitated perchlorate salts were yellow at the end of dissolution instead of

white (as was observed for the other sample)

indicating the presence of some residual organic

material. Since the oxidizing power of

perchloric acid is highly dependent on temperature, small differences in experimental conditions

appear to have important effects on the amount

of insoluble residue.

No acid dissolution procedure studied was

adequate for Cr. The procedure that produced

the smallest amounts of particulate Cr involved an extended reflux in HC104

. Although this study was not designed to accurately determine

losses due to volatility, a comparison of the

combined amount of Cr remaining in the solution

and on the filter for each material, with the

amount present in the corresponding unprocessed

material, indicated that approximately 60% of

the Cr in both the Apple and Peach Leaves was

volatilized during this dissolution. Significant

fractions of Hf were also present in particulate

form after all the acid dissolutions investigated,

apparently indicating the presence of some Hfcontaining mineral fraction or fractions, which were unaffected by the acid treatment.

Significant fractions of five elements (Au,

Cr, Hf, Sb and Sc) remained in some of the

Teflon dissolution vessels after the dissolved

solutions were transferred and the vessels were washed (four times). Since bulk TFE Teflon is formed by pressing together small particles of

material, it was expected that this type of Teflon

would trap more material than the other two

types of Teflon. Surprisingly, the FEP Teflon

seemed to retain greater amounts of the four

elements determined than did the PFA or TFE

Teflons. Up to 6% of the Cr was found in the

FEP vessels compared to < 0.5% of the Cr in the other types of Teflon.

Very large fractions (i.e., 20%-60%) of the

Co, Fe, Cu, Zn, As, and Sb present in the

Leaves remained in the Pt crucibles used for the

LiB02fusions. These results, along with the

amount remaining on undissolved particles, are

listed in table 1 . Although insoluble forms of Cr

were small, and only 1-3% of the Cr was lost to the platinum crucibles, up to 30% was lost from

the Apple Leaves by volatility.

The results of this study indicate the

importance of fully testing sample decomposition

methods to be used in the certification process of

reference materials. No two analytical tech130
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niques can truly be considered independent

unless the possibility of common dissolution

losses can be eliminated. It is possible that some

analytical methods like Inductively Coupled

Plasma spectrometry may effectively recover

analytical signal from small particles suspended

in sample solutions. However, accurate results can only be obtained if these particles do not

stick to the walls of the containers used to store the samples during the period between dissolution and analysis, and if the particles are uniformly suspended at the time of sample

introduction into the analytical instrument. This

study further points to the value of using a nondestructive method of analysis as one of the

analytical techniques used in the certification

process of this type of botanical reference

material.

Table 1. Percent loss from Apple and Peach Leaves during

lithium metaborate fusion in platinum crucibles

Peach Leaves Apple Leaves

Crucible Filter Crucible Filter

Br 3 0.05 4 1.8 Co 36 0.4 50 0.1 Cr 3 0.7 ~1 „ . ** <0.1 Fe 34 <0.09 49 0.05 Cu 29 <1 36 <1

Zn 21 0.03 18 <0.02

As 35 <1 27 <1

Sb -40 <1 -60 <2

Cr volatility losses sl0%

Cr volatility losses -30%

References

[1] K. Heydo, Damsgaard and B. Rietz, Anal. Chem. 52 , 1045-9 (1980).

[2] M. Ihnat, Comm. in Soil Sci. Plant Anal. 13, 11 , 969- 79 (1982).

[3] A. R. Byrne and L. Bendik, Talanta. 35, 161-6

(1985).

[4] National Bureau of Standards (U.S.), Certificates of

Analysis, Standard Reference Materials 1570, 1571,

1572, 1573, 1575 (Spinach, Orchard Leaves, Citrus Leaves, Tomato Leaves, and Pine Needles).

MEASUREMENT OF BORON AT SILICON WAFER SURFACES BY NEUTRON

DEPTH PROFILING

J. P. Lavine, T. Z. Hossain, J. B. Russell, and G. P. Zenner

(Eastman Kodak Company, Rochester, NY)

and

R. G. Downing and G. P. Lamaze (Inorganic Analytical Research Division)

Low levels of boron have been detected at the surface of high purity single crystal silicon

wafers using spreading resistance [1] or by

secondary-ion mass spectroscopy (SIMS) [2-6].

The presence of boron is observed after a vacuum anneal [1], after various types of wafer

cleaning procedures [2,3] and after silicon

molecular beam epitaxy [4] . A high temperature

processing step is used to diffuse the boron into

the silicon prior to analysis. Slusser and

MacDowell [2] have deposited a polysilicon

layer to permit the use of SIMS without a high

temperature drive. However, even this

introduces an additional processing step that may

complicate the isolation of the true source of a surface contaminant such as boron. Neutron

depth profiling (NDP) with the nuclear reaction n -I- 10B -

4He +7Li provides a direct

quantitative method to investigate the presence of

boron at the surface of a silicon wafer without

intermediate processing. Using NDP,

measurements were performed on as-received Si wafers, on wafers with an oxide, on a polysilicon layer, and on an epitaxial silicon layer.

The results indicate that a prime source of boron

contamination is air, providing direct support for

131
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the suggestions of Iyer, Delage, and Scilla [4] and of Kubiak et al. [5] that some boron contamination occurs upon air exposure of the wafers.

Also these measurements support Casel et al. [6] experimental results that indicate air is a source of boron contamination.

Figure 1 displays the total front surface

boron for 100-mm diam n-type Czochralski Si

wafers from three different vendors. The wafers were removed from the plastic sealed as-received

boxes at the time of measurement for vendors B

and C. The wafer box from vendor A had been

opened earlier. The measured 10B concentration

is converted to a total boron surface dose by

assuming normal isotopic abundance. The

highest boron concentration is seen for vendor

C. The unpolished back surface of this wafer

has nearly the same concentration of boron as the front side. The vendor-to-vendor variation seen from the figure indicates that wafer processing is probably responsible for some portion of

the observed boron. Recent work supports this suggestion [6].

Silicon Wafer

Figure 1. Total boron determinations by NDP for five- silicon wafers and nominal system background.

Boron was also found on the surface of a wafer which had been covered with a silicon

epitaxial layer. The substrate underwent intrinsic getting before the deposition of a 10-u.m

arsenic-doped silicon layer. Figure 1 shows that the total surface boron is similar to that of the

wafers from vendors A and B. Boron was also

measured on the surface of and throughout a 250-A oxide layer, grown during an intrinsic gathering procedure. As indicated in the figure

the least amount of boron appears on this wafer.

NDP measurements indicate that boron

appears at the surfaces of all wafers when the

surface has been exposed to the atmosphere.

While there is usually a low level of boron

distributed throughout a silicon wafer, ordinary

heat treatments cannot produce ~ 10

atoms/cm2 by up diffusion from the bulk of the

wafer. Even if all the boron in an n-type wafer

diffused to the surface during the heat treat- ments, only 2.5 x 10 11 atoms/cm2 would be at each surface for a bulk boron concentration of 1 x 1013 atoms/cm3

. The measured boron levels exceed this by about an order of magnitude.

Thus, the simplest explanation is that there is a common external source for the observed boron.

Exposure to air is common to all the samples

and thus can explain the surface peaks. The

amount of surface boron is not correlated to the

duration of exposure to air. Perhaps the native

oxide that forms on silicon provides a limited

number of sites for incorporation of boron

atoms. Casel et al. [6] showed that the presence

of the natives oxide is essential for boron to be

found in the quantities seen by the NDP

measurements of the native Si02 layer on the

surface of the wafer. Since the formation of

borated glass is well known, it is quite possible

that a companion chemical reaction produces

oxides of boron. This hypothesis is supported

by the presence of boron in the air as both a gas

phase and as an aerosol [7]. Gas-phase boron

has a nominal concentration of 10-100 ng/m3

, and is probably in the chemical form B(OH)3

. In summary, NDP was used to show the

existence of surface boron contamination of

processed silicon wafers. It is suggested that boron adsorbs or chemisorbs from air onto the

wafer surface and likely is incorporated into the

native oxide. As suggested by the data in the

figure, the wafer preparation processes may also

deposit boron during chemical or mechanical

cleaning and polishing. However, the surface

boron seen after polysilicon or epitaxial silicon

deposition requires another boron source—the air

itself. The present direct measurements of boron

with thermal neutrons do not require heat treat- ment for sample preparation. Thus unequivocal

support is provided for the earlier suggestion of

Iyer, Delage, and Scilla [4] and Kubiak et al. [5] that the atmosphere is the source of some of the

observed boron surface contamination of silicon

surfaces.

132
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We thank Dr. Gilbert Declerck, Interuniversitair Micro-Elektronica Centrum, Leuven,

Belgium; Dr. David L. Anderson, the United

States Food and Drug Administration; and Dr.

Richard S. Hockett, Charles Evans and Associates for helpful discussions.

References

[1] M. Lieher, M. Reiner, R. A. Wachnik, and G. S.

Scilla, J. Appl. Phys. 61, 4619,(1987).

[2] G. J. Slusser, and L. MacDowell, J. Vac. Sci. Technol. A5, 1649 (1987).

[3] H. T. Yang and P. M. Mooney, J. Appl. Phys. 58,

1854 (1985).

[4] S. S. Iyer, S. L. Delange, and G. Scilla, J. Appl.

Phys. Lett. 52, 486 (1988).

[5] R. A. A. Kubiak, W. Y. Leong, M. G. Dowsett, D.

S. McPhail, R. Houghton, and E. H. Parker, CI J. Vac. Sci. Technol. A4, 1905 (1986).

[6] A. Casel, E. Kasper, H. Kibbel, and E. L. Sasse, J. Vac. Sci. Technol. B5, 1650 (1987).

[7] M. E. Kitto, Ph.D. dissertation, University of Maryland, 1987.

SOL PARTICLE IMMUNOASSAYS USING COLLOIDAL GOLD

AND NEUTRON ACTIVATION

R. Zeisler and S. F. Stone (Inorganic Research Division)

R. P. Viscidi (Johns Hopkins University School of Medicine, Baltimore, MD)

and

E. H. Cerny (Fondation pour la Recherche Medicale, Geneva, Switzerland)

We are developing an ultrasensitive immunoassay using colloidal gold and neutron activation.

The method is described in detail elsewhere [1] and is briefly summarized here. Individual wells

of microtiter strips were coated with the specific

samples to be measured, and the antibody re- agent labeled with colloidal gold added and

allowed to incubate. The excess unbound anti- gen was washed off and the gold labeled conjugate bound to the adsorbed analyte was measured

via NAA. Samples for irradiation consisted of

the individual wells of the microtiter strips

containing the gold labeled reagent bound to the

analyte and appropriate standards. Reactor

irradiations were conducted in the NBSR pneumatic tube facilities at fluence rates of 2.0 and

7.7 x 10 17n x m"2 x s"

1 and length of irradiation

ranged from 10 to 20 minutes per sample set (approximately 24 wells). Gamma counting

utilized a sample changer with a germanium

detector of about 4% absolute detector efficiency

for the 411-keV line. The counting times per

sample were between 300 to 1800 seconds. A

comparison of the measured gamma activities of

the Au 41 1-keV peak from the standards and

the samples was used to calculate the amounts of

gold in the samples.

The initial studies were aimed at determining

the sensitivity of immunoassays using colloidal

gold labeled antibodies and neutron activation for detection of solid phase human immunoglobulin

(IgG). Quadruplicate sample wells were prepared

from logarithmic dilutions of purified human

IgG. Sample wells containing bovine serum

albumin (BSA) and uncoated wells served as controls to determine the gold blank due to the

unspecific binding of the gold labeled antibodies

and gold in the reagents and microtiter wells.

Assays were performed with two concentrations

of gold labeled antihuman IgG(l ng/mL or 0.2

u.g/mL) and with a 0.2-u.g/mL antihuman IgG

horseradish peroxidase conjugate.

The results from the gold labeled and en- zyme labeled antibodies are compared in figure

1 for the 0.2-jig/mL antigen concentrations.

Figure la depicts the amounts of gold determined in each well with its associated uncertainty due to the counting statistics. The line in figure la represents the mean value at each

dilution point. The mean blank concentration

(0.29 + 0.12 ng) calculated from wells with

133
NON-RRD PROGRAMS

BSA was subtracted from the individual values.

Similar results were obtained in assays with 1 u-g/mL gold labeled antihuman IgG, although the

amount of gold found ranges from 1.2 to 140

ng. The lowest concentration was again similar

to the blank in this particular assay (0.8 + 0.4

ng). Figure lb depicts the optical density

plotted with its uncertainty from three absorbance measurements. The sensitivities from

each of the assays show similar linear ranges of

between 0.001- and 0.033-jig/mL IgG.

However, the relative change (between blank and

saturation) in response to each dilution step is greater for the neutron activation assay than for

the enzyme assay. Also, in the enzyme assay, the signal for the 0.0033-u.g/mL dilution is not

significantly different from the upper limit of the

blank, while in the neutron activation assay, the

signal remains discernible from the blank until the 0.00033 [ig/mL dilution. These results demonstrate that an immunoassay for IgG using

neutron activation of colloidal gold labeled

antibody is at least comparable in performance,

if not superior, to a conventional enzyme immunoassay.

This colloidal gold label and its detection is obviously adaptable to a number of standard

immunoassay procedures that use absorbance

measurements of conjugates or radioactive

labels. The irradiation and measurement

conditions in the preliminary studies were chosen

to make the assay applicable to clinical investigations and could be modified to lower detection

limits. Gamma counting in connection with a sample changer and well detector or auto- radiography would provide the capability for

high throughput. The method currently allows

detection of 10" 16 mol of analyte in routine

operation with possible extension to 10"20 mol.

For research applications, it would be desirable

to approach the latter limit of detection.

Currently, the limits caused either by the blank

gold contained in substrates and reagents or by

nonspecific binding of gold labeled reagent to the

solid phase can certainly be reduced with technical improvements in the procedure.

100

1a)

-4.0 -3.0 -2.0 -1.0 0.0

log IgG (ng/mL)

1.0

-4.0 •2.0 -1.0 0.0 log IgG (ng/mL)

1.0 1b)

Figure 1. Immunoassay of human IgG. a) Assay with

gold labeled antihuman IgG and subsequent NAA

determination of gold. Plot depicts individual values of

four replicates and their analytical uncertainty corrected for blank contribution. The solid line is a fit to the respective mean values, b) Assay with antihuman IgG horseradish peroxidase conjugate. The optical density (@ 405 nm) is plotted with its uncertainty from three measurements.

Reference

[1] R. Zeisler, S. F. Stone, R. P. Viscidi, E. H. Cerny,

Clin. Chim. Acta (submitted for publication).

134
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CHARACTERIZATION OF THE MINERAL FRACTION IN

BOTANICAL REFERENCE MATERIALS AND ITS INFLUENCE

ON HOMOGENEITY AND ANALYTICAL RESULTS

R. M. Lindstrom, D. A. Becker, and K. M. Garrity (Center for Analytical Chemistry)

and

A. R. Byrne and B. Smodis (Institut "Jozef Stefan," Ljubljana, Yugoslavia)

NIST has produced a series of naturalproduct leaf Standard Reference Materials

(SRMs) which have proven valuable in developing and validating methods of analysis. Since

these materials are prepared from field- grown

leaves, some mineral matter (herein called grit)

is generally present. The presence of this grit

leads to two consequences that must be taken

into account in the use of these materials. First, some trace constituents are present in a phase

that may not be solubilized by the method used

to decompose the bulk cellulose matrix.

Incomplete dissolution may lead to error in any

analytical method which requires the sample to be in solution. A second analytical consequence

of the presence of a minor phase occurs when a

trace constituent is present mostly in a small

number of high-concentration grains. The

statistics of sampling may lead to heterogeneity

which cannot be resolved by any analytical

method. Both of these effects have been reported

in the literature to be observable in leaf SRMs.

To verify that these experimental difficulties are indeed related to the presence of a mineral

phase, we performed heavy-liquid separations in carbon tetrachloride on all the available NIST

leaf SRMs to concentrate the coarser, heavier

particles from the material. The resulting grit was weighed, examined both optically and with an electron microscope, and analyzed by neutron

Table 1. Quantity of Grit in SRMs

activation. The weight fraction of grit from each

of seven botanical SRMs is given in table 1

Two new SRMs (So be issued in 1990), SRM

1515 Apple Leaves and SRM 1547 Peach

Leaves, have been prepared with a jet mill which

directs two high-velocity streams of the material

in air against each other, resulting in a very

finely ground leaf material. A cyclone classifier in this process discriminates against coarse grit,

and therefore the content of minerals was expected to be less than in the first-generation materials. In fact, the expected improvement was

observed. The CC14 separation showed very

little heavy phase present in the jet-milled leaf

materials, and that was of small particle size. Excellent homogeneity is observed in both by

activation analysis, with at least six elements

homogeneous to better than 1.5% (Is) at the

100-mg sample size [1]. Grit from four separations were analyzed by

instrumental neutron activation analysis (INAA)

for a number of elements. The contribution of

the grit to the total content of each element is given in table 2. Duplicate analyses were made

for the Apple Leaves grit, as indicated by pairs

of numbers in the right column of the table. Gold, cobalt, zinc, and possibly uranium are

significantly different in the two portions, which

may indicate that these elements are contaminants. The rare earths (but not scandium or

SRM wt. % grit

1570

1571 1572

1573 1575

1515 1547

Spinach

Orchard Leaves

Citrus Leaves Tomato Leaves

Pine Needles Apple Leaves

Peach Leaves

1.5 0.6 0.6

0.9 0.2

0.1

0.1

135
NON-RRD PROGRAMS

thorium) appear to be 97% -98% concentrated in the leafy matrix. Sodium (but not potassium or rubidium) has a large contribution from the grit.

Even though the amount of grit in this SRM is an order of magnitude less than in the older

materials, the contribution of this fraction to the

total trace element content is still 5% or greater

for sixteen of the elements measured.

These results indicate that the two new

botanical Standard Reference Materials, Peach

Leaves and Apple Leaves, are substantially

improved materials for evaluation of analytical

methodology and confirmation of quality assess- ment procedures. As with all materials,

however, the existence of resistant mineral

phases must be taken into account. The analysis

of any botanical samples by solution-based

analytical techniques must ensure that the

mineral is dissolved as well as the bulk matrix.

Commonly, a small amount of HF is added to complete the solution, but Greenberg et al. [2] have shown that incomplete solubilization can occur even when well-accepted, vigorous

regimens are used. Whenever a new technique

is applied to real samples, even if it has been

shown to be reliable for the analysis of these

SRMs, caution must be observed that any

sampling inhomogeneities be considered and

appropriately dealt with.

Table 2. Percentage of total element in grit phase

SRM

1572 1570 1515

Element Citrus Spinach Apple

Leaves Leaves

Ag 4.5 Al 39 8.3 As 34 >40 6.7 Au 44, 5.2 Br 26 0.2 Ca 3 17, 17 Ce 3 1.7, 1.4 Co 33 2.0 11, 6

Cr 30 12 13, 12 Eu >2 1.7, 1.0 Fe 22 11 7.8,8.6

Hf 8,5

K <0.1 0.9,0.7 La 3 16 1.7,1.3 Lu 2.1,1.3

Mg 0.3 0.1 Mn 15 1.9 Na 9, 8 Nd 1.5, 1.1 Rb >1 >5 0.5,1.0

Sb >5 >40 14 Sc 22 10 6.6, 6.6 Sm 2 1.4, 1.0 Sr 2 10, 8

Si 99 Tb 1.0, 1.0 Th 110 5.7, 5.1 Ti 140 U 5, 3 V 25 12 Yb 3.6, 1.1 Zn 40, 1.4

References

[1] D. A. Becker, NIST Tech. Note 1272 . 238 (1989); D.

A. Becker. Biolog. Trace Elem. Res. (in press).

[2] R. R. Greenberg, H. M. Kingston, R. L. Watters, Jr; and K. W. Pratt, Fresenius Z. Anal. Chem. (in

press).

136
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ATMOSPHERIC AND ENVIRONMENTAL STUDIES BY INSTRUMENTAL AND

NEUTRON-CAPTURE PROMPT 7-RAYACTIVATION ANALYSIS

G. E. Gordon, J. M. Ondov, and W. B. Walters (University of Maryland, College Park, MD)

We have made rather little use of the reactor

this year because of the long shutdown. The

major new work involved studies 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,

which will be irradiated soon. It had been

several years since we had upgraded our y-ray

spectrometry facilities, so we have been doing

that in preparation for the new round of projects.

EFFECTS OF TARGET SHAPE AND NEUTRON SCATTERING BY HYDROGEN ON

NEUTRON-CAPTURE PROMPT 7-RAYACTIVATION ANALYSIS (PGAA)

E. A. Mackey and G. E. Gordon (University of Maryland, College Park, MD)

and

R. A. Lindstrom (Center for Analytical Chemistry)

We are studying the effects of neutron scat- tering by H on analytical sensitivities (cps/mg)

of neutron capture prompt y-rny activation

analysis (PGAA). Hydrogen is a particular

problem because of its large free-atom scattering

cross-section, oscat = 20.5 b. For 11 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% . If H concentration were the only parameter influencing sensitivities, accurate data interpretation could be

achieved through a simple calibration of sensitivity vs. H concentration. 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.

Theoretical work by R. F. Fleming [2] indicates that the use of spherical samples will

greatly reduce the magnitude of the effects of

scattering on analytical sensitivities. He argues

that for a spherical sample, neutron scattering

will only serve to mitigate the effects of neutron

self-shielding, so that no enhancement would be

observed. According to this work, the cor- rection factor, defined as the ratio of the observed reaction rate to the reaction rate which

would be observed in the absence of scattering,

is bounded by the self-absorption correction

factor [3] and unity. PGAA was performed on

a series of spheres and disks for each of several

hydrogenous materials to test this theory.

A series of disks, 12.7-mm diam, ranging

in thickness from 1 to 13 mm and a series of

spheres ranging in diam from 5 to 13 mm were

prepared from each of the following substances: urea, tris-(hydroxymethyl)-aminomethane

(THAM), paraffin, SRM 37 lg Sulfur; and SRM

1632a Bituminous Coal. Disks were formed

using a standard 12.7-mm diam die and hydraulic press; spheres were formed using dies designed and fabricated by instrument maker, Dick

Turner. Hydrogen sensitivities were measureJ

in all samples except SRM 37 lg. Sensitivities

for B, Si and S, were measured in SRM 1632a.

and sulfur sensitivity was measured in SRM

37 lg. All data were corrected for neutron self- shielding, temporal variations in the neutron

fluence rate, the effects of pulse pile-up, and for the presence of H and B in the background.

137
NON-RRD PROGRAMS

Results for H sensitivities in paraffin disks

and spheres are shown in figure 1 . For spherical samples, H sensitivities remain constant within the errors shown. These errors include

uncertainties associated with counting statistics and correction factors. Sensitivities measured in disks showed increasing sensitivities with decreasing thickness from the thickest disks down

to thicknesses of about 2 mm. Sensitivities for

thinner disks showed decreases with decreasing

thickness. Similar results were obtained for H

sensitivities in urea, THAM, and SRM 1632a

(not shown).

Boron sensitivities measured in SRM 1632a

disks and spheres are shown in figure 2. These

data show the same qualitative trends as those of

the H data.

Sensitivities from disks and spheres of SRM

37 lg, pure sulfur, are identical within the uncertainties associated with counting statistics. This was expected since these samples contain no H

and should scatter fewer neutrons (o

scat =

0.975 b for S).

These findings are consistent with the theoretical work of R. F. Fleming [2], and the

Monte Carlo calculations of Copley and Stone

[4] . Copley and Stone have shown that neutron

scattering by H can alter the distance traveled by

neutrons within the sample and that, depending

upon the size and shape of the sample, can cause

either an increase or a decrease in the probability

for neutron absorption. For the types of samples

routinely analyzed at this facility, i.e., samples

which are smaller than the neutron beam and

which are thinner than they are long, sensitivity

increases (rather than decreases) are commonly

observed. Accuracy in the analysis of hydroge- nous materials has been achieved by carefully

matching standards to samples or by calibration

techniques [5]. These experiments demonstrate

that scattering effects will be successfully eliminated through the use of spherically shaped

samples.

Results of this study were presented at the

1989 winter meeting of the American Nuclear

Society. A manuscript describing this work has

been submitted to Analytical Chemistry.

0.21

500 1000 1500 Sample Mass (mg)

Figure 1. Hydrogen sensitivities (cps/mass) in paraffin disks (0) and spheres (•).

875-

I

&

3. 825 b

1

|I

n 775 -

725

1000 1500 Sample Mass (mg)

2000

Figure 2. Boron sensitivities (cps/mg) in SRM 1632a,

Bituminous Coal, disks (0) and spheres (•).

References

[1] E. A. Mackey, D. L. Anderson, R. M. Lindstrom, G.

E. Gordon, American Nuclear Society Transactions,

60, 14, 1989; E. A. Mackey, G. E. Gordon, R. M.

Lindstrom, D. L. Anderson. Anal. Chem. (submitted

for publication).

[2] R. F. Fleming, unpublished work.

[3] R. F. Fleming, J. Appl. Radiat. Isot. 33, 1263,

(1982).

[4] J. R. D. Copley and C. A. Stone, Nucl. Instrum. Meth. Phys. Res. A281 , 593, (1989).

[5] D. L. Anderson, W. C. Cunningham, E. A. Mackey,

Biol. Trace Element Research (in press).

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SUBMICROMETER 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 l-^im 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-nm minimum size

cut) and modest jet velocities (127 m/sec for

0.09-jim 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.

We tested the micro-orifice impactor (MOI)

by field collections of particles at a rural site in western Maryland near Deep Creek Lake. Most

samples were collected between August 1 and

31, 1983, with sampling periods ranging from 6

to 24 h. Cutoff diameters for the stages ranged

from about 1.0 to 0.08 \im and the cyclone

removed particles down to 3-u.m diam.

Impactor collection surfaces and the afterfilters from seven out of the 60 sample sets were

analyzed by INAA at the reactor using a combination of 10 min followed by 3-h 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 or 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 u.g. Detection on more

stages was generally achieved with loadings of

240 to 400 u.g.

A paper describing the experimental work

was published during the past year [2] . We have

applied a deconvolution procedure to the raw

size spectra to remove the effects of the nonsharp collection-efficiency curves to obtain the

inherent size distributions and will soon publish

the results. Recently, the EPA Extramural Grant

Program awarded us a grant for more extensive

studies of detailed size distributions in a variety

of settings. Field collections with three MOIs

are being done now at Washington suburban

sites on the University campus, at Andrews Air

Force Base, and at Vienna, VA. Plans are being

made to collect samples in the Ft. McHenry

motor-vehicle tunnel in Baltimore, among other

things to seek a motor-vehicle tracer to replace

the Pb and Br, which are nearly phased out. Urban sites in Philadelphia previously used in studies by this group [3] will also be studied.

From the previous work, we know that the 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.

References

[1] G. A. Kuhlmey, B. Y. H. Liu, and V. A. Marple, Amer. Indust. Hyg. Assn. J. 42, 790 (1981).

[2] J. M. Ondov, J. A. Dodd, and G. Tuncel, Aerosol Sci. Technol. 13, 229 (1990).

[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).

139
NON-RRD PROGRAMS

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 per- formed by INAA at the NIST reactor. Results of a study conducted at Deep Creek

Lake in western Maryland, performed in cooperation with EPA have been published [2]. Recently we have performed more detailed

studies in cooperation with Prof. T. E. Church

and J. Scudlark, of the University of Delaware,

who have long operated a station in Lewes, DE

as part of the DOE and, later, EPA network

(MAP3S) of about ten 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 were collected every 12 hours

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. We will soon begin a

sampling similar to Phase 2 on the grounds of

the USDA Beltsville Agricultural Research

Center, where there is already a station that is part of EPA's Dry Deposition Network. The

base-loaded filters from Lewes have been ana- lyzed by ion chromatography (IC) and we will

shortly begin to analyze the one hundred particle

filters by INAA at the NIST reactor. We expect

to collect and analyze another 40 to 50 from the

Beltsville experiment.

References

[1] G. E. Gordon, Receptor Models, Environ. Sci. Technol. 22, 1132-1142 (1988).

[2] T. L. Vossler, C. W. Lewis, R. K. Stevens, T. G. Dzubay, G. E. Gordon, S. G. Tuncel, G. M.

Russwurm, and G.J. Keeler, Composition and Origin of Summertime Air Pollutants at Deep Creek Lake,

McHenry, MD, Atmos. Environ. 23, 1535-1547

(1989).

[3] M. E. Kitto, Ph.D. thesis, Department of Chemistry

and Biochemistry, University of Maryland, College

Park, MD, 1988.

[4] L. E. McCarthy, M. S. thesis, Department of Chemistry and Biochemistry, University of Maryland,

College Park, MD, 1988.

140
NON-RRD PROGRAMS

SOURCES OF BACKGROUND IN Ge GAMMA DETECTORS

R. M. Lindstrom and J. K. Langland (Center for Analytical Chemistry)

D. J. Lindstrom (NASA Johnson Space Center, Houston, TX)

and

L. A. Slaback (Occupational Health and Safety Division)

Three sources of gamma radiation contribute to the background of a shielded gamma-ray

detector: natural radioactivity of the detector

and its surroundings, man-made contamination,

and cosmic radiation. As discussed in last year's

report, a detector system is in use which shows

no detectable contribution from long-lived radioactivity in the detector itself and very little from

contamination. The cosmic contribution was shown to be dominant and evaluated quantitatively in the present work.

Two broad peaks in the background result from inelastic scattering of cosmic-ray neutrons

(0.02 cm'V1 ) in germanium. The counting rate of the fast-neutron and positron peaks did not change when heavy neutron shielding (7.5%

lithium in polyethylene, 20-cm thick by 120-cm

square) was laid on top of the shield, from

which we conclude that the neutrons were generated in the lead itself. Fast neutrons also excite

the lower nuclear levels of lead and structural

iron to produce additional gamma-ray peaks in the spectrum.

The shield for the low-background "Ty"

detector was at first lined with material of lower

atomic number to absorb the lead fluorescent x- rays excited by background radiation. Neither

cadmium, tin, copper, nor plastic (hydrocarbon

or fluorocarbon) was desirable in the end as a

shield liner, since all these increased the background continuum or introduced characteristic

peaks into the background spectrum; in conse- quence no x-ray absorber was used. The shield was finally lined with sheets of 1.6-mm Teflon

TFE to provide an easily cleanable surface.

Excitation peaks from F make even this amount of lining material undesirable.

The background of the Ty detector in the

final shield configuration showed only 511-keV

positron annihilation radiation, products of

cosmic-ray neutron interactions with the germanium of the detector itself, excitation peaks of

19F and 206- 207Pb, radon daughters, 60Co,

thorium daughters, and 40K, in that order of

importance (Table 1). All peaks in the background were identified. Only the positron peak was higher than 5 c/hr. Except for 60Co gamma

rays, no peaks are detected which can be attrib- uted with certainty to long-lived radioactivity in the materials of which the detector or the shield are constructed.

A comparison among four detector: shield

combinations helped to elucidate the sources of

the background observed. The four counting

systems are the Ty detector in the lowbackground shield in the Reactor Building; the same detector in a lead shield in the underground

Radiation Counting Laboratory at the Johnson

Space Center; a second low-background detector,

named RCL5, with a J-shaped cryostat in the same shield (now with a shadow shield beneath

and a graded liner to absorb x-rays) in the RCL;

and a third large, standard detector called HP2

in a commercial cylindrical shield in the low-level laboratory at NIST. All the detectors were

comparable in efficiency

The results of the comparison are given in table 2; since the detectors are of similar effi- ciency, the ratios of count rates are quantitative measures of the effects of the different materials

and the laboratory environments. The Cd-Cu

liners in the RCL5 and HP2 shields suppress the

Pb x-rays by a factor of five. The importance of

the cosmic-ray contributions are most clearly

shown in the relative intensity of the positron

annihilation peak, the

206 - 207pb excitation peaks,

and the level of the background continuum at 1 MeV. These three components were reduced

fivefold in both detectors in the underground

laboratory. The lower levels of positron and lead

peaks in the HP2 background compared to the

Ty detector above ground may be due to the

lesser buildup of the cosmic cascade in the HP2

shield, which was only a third as massive as the other. The benefits of low-background construction and selected lead were clearly evident in the much greater intensity of radon (and uranium).

thorium, and potassium peaks in background of

141
NON-RRD PROGRAMS

the HP2 detector. The continuum background,

however, was not greatly different from the

low-background Ty detector.

Achieving a low background in close

proximity to a reactor, or to laboratories in

which substantial quantities of radioactivity are handled, might appear to have small chance of

success. However, we have found that, with

care in selecting materials and in locating the

laboratory, the achievable background can be

remarkably good. Since these experiments have

shown that the most important source of residual

background in the low-background detector

described is cosmic radiation, we conclude that the greatest additional improvement will require

either burial of the counting room 10 meters or more deep to reduce the cosmic-ray flux, or

active (anticoincidence) shielding.

Table 1. Gamma-ray peaks in residual background. The number of significant figures given reflects the uncertainty of counting

statistics

Energy, Nuclide Count rate, Energy Nuclide Count rate, keV c/s keV c/s

53.4 73 *Ge 0.0004 511.0 B annihilation 207*pb

0.029

66.7 73 *Ge 0.003 569.7 0.0008

72.8 Pb Ka2 0.009 583.1 208T1 0.0003

75.0

84.8

87.3

109.9

PbKol

Pb KB1'

PbKB2'

19*F

0.015

0.011 0.003 0.002

596.1 609.3 693.4

803.3

74Ge(n,n')

214Bi 72Ge(n,n')

206*Pb

0.010

0.001 0.007

0.001

122.1 57*Fe 0.0003 846.8 56*Fe 0.0006

139.7 75mGe 0.002 911.2 228Ac 0.0003

159.7 77mGe 0.001 1063.6 207*pb 0.0003

174.9 71 *Ge 0.0003 1120.3 214Bi 0.0003

197 71*Ge +

19*F 0.006 1173.2 60Co 0.0006

238.6 212pb 0.0005 1332.5 60Co 0.0006

295.2 214pb 0.0004 1460.8 40K 0.0003

352.0 214Pb 0.001

Table 2. Comparison of backgrounds of different detectors and locations. Uncertainties are given as + 1 s. d.

Detector identification Location

Cryostat configuration

Background counting rates:

Lead x-ray B annihilation Lead excitation 1-MeV continuum

Radon

Thorium

40xr

Ty Ty RCL5 HP2

NIST RCL RCL NIST

Upright J-shape Upright

= 1 0.83 ± 0.02 <0.2 0.16 ± 0.03 = 1 0.23 ± 0.01 0.23 + 0.01 0.57 ± 0.01 = 1 0.18 + 0.09 0.18 + 0.05 0.63 + 0.10 = 1 0.20 ± 0.01 0.22 ± 0.02 1.16 ± 0.03

= 1 1.5 ± 0.6 0.8 + 0.3 3.5 ± 0.7 = 1 0.8 + 0.3 0.5 ± 0.2 5. ±2

= 1 1.6 ± 0.5 0.7 ± 0.2 9.9 ± 1.6

142
NON-RRD PROGRAMS

CHEBYSHEV EXPANSION OF ATTENUATION CORRECTION FACTORS

FOR CYLINDRICAL SAMPLES

D. F. R. Mildner (Center for Analytical Chemistry)

and

J. M. Carpenter (Argonne National Laboratory, Argonne, IL)

The attenuation correction factor A(0) for

cylindrical samples for all scattering angles 6

can be expressed in terms of a cosine series of

functions Zmn(0), which are independent of the

scattering angle for m or n equal to zero, using

Chebyshev expansion coefficients c

s (m,n) [1]. The accuracy of these expansion coefficients has

been improved [2]. An increased order of

expansion allows the method to be useful over a

greater range of attenuation. It is shown that many of these coefficients are exactly zero, others are rational numbers, and others are

rational fractional fractions of it.

The attenuation factor for scattering of x-rays and neutrons in cylindrical samples can be

expressed for all scattering angles in terms of a Fourier cosine series. The factors may be

calculated by numerical integration [3], and

tables are available of values with an accuracy of

10~4

[4]. With more accurate values for the

expansion coefficients, the limited range of

applicability of the Chebyshev expansion method

can be extended to greater values of attenuation

and with increased accuracy.

The attenuation correction factor is given by

A(6)=-L( [ exp(-^exp(-uY)ptfp</4>=£ E

^^

rP^ E c

s(m,n)cos(sd)

~»2J I .t^r^n mini trk TlR' m=0 n=0 5=0

with the summations extending over a range

dependent on the required accuracy. This ex- pansion is an absolutely convergent series, and

in practice we may compute all terms up to

(m + n) = N for the determination of A. The

error (precision) is less than the (N + l)th order

term, so that for all 6 the error is less than

(2MR)N + 1 Z0>N + 1 /(N+1)!

For s > 0, the Chebyshev expansion coefficients are given by

cs (m,n) =

1 2rc 2n

ic~2

Jpdp J dZcossUpcosZ+(l-phmH)

l/i ] m

j d^coss^[- pcos^+(l-phin2 ^)

lA

] n

Integrals for the coefficients c

s (m,n) may be

evaluated using a power series expansion for the

complete elliptical integrals, which for lowerorder coefficients are expressed as p moments of

combinations of complete integrals of the first and second kinds. When s has the same parity as

m (or n), the i (or i|j) integral gives an expression involving n multiplied by a polynomial in

P- We have computed values of the coefficients

c

s (m,n) for s ^ 9 and for (m + n) * 10 to six decimal places with an accuracy of at least 10"4

,

143
NON-RRD PROGRAMS

and have shown that many of these coefficients are identically equal to zero. We have also

found some interesting properties of these expansion coefficients. The summation of coefficients

c

s (m,n) may be expressed in terms of specific

fractions of Z0m+n. Also the difference between the summation of the coefficients c

s (m,n)

for even s and the summation of c

s (m,n) for odd

s is exactly Z m+n. These sum laws provide a check on the accuracy of the computation of the

coefficients. Various general properties have

been found for these coefficients. When both m

and n are even (odd), these summations are

rational numbers, and the coefficients themselves

are rational numbers for even (odd) s. When m

and n are mixed indices, these summations are

rational fractions of it"1

, and the coefficients

themselves are also rational fractions of n~

l for

alls.

These values are sufficiently accurate for /*R

< 1.0, which is the usual range for neutron

diffraction. Extra precision may be obtained by

using the Chebyshev expansion as a correction to an approximation exp(-2^7) to A. For example,

if i/R is set to unity, the results are reasonable

for /xR < 2.5. An approximate analytic expression for the correction factor which is adequate

in most cases of neutron diffraction and inelastic

scattering can be expressed in terms of the first two Chebyshev expansion coefficients [3]. An

analytic expression for A(0) given by Rouse et

al. [5] can use asymptotic values determined

from these two coefficients. We show that this

is equivalent to taking T/R = 0.8488.

References

[1] J. M. Carpenter, Rev. Sci. Instrum. 40, 555-556

(1969).

[2] D. F. R. Mildner and J. M. Carpenter, J. Appl. Cryst. 23, 378-386 (1990).

[3] V. F. Sears, J. Appl. Cryst. 17, 226-230 (1984).

[4] V. F. Sears, Atomic Energy of Canada Report AECL8176 (1983).

[5] K. D. Rouse, M. J. Cooper, E. J. York, and A.

Chakera, Acta Cryst. A26, 682-691 (1970).

MEASUREMENT OF THE 93Nb(n,2n)92mNb CROSS SECTION

IN A

235U FISSION SPECTRUM

G. P. Lamaze (Inorganic Analytical Research Division)

and

T. G. Williamson (University of Virginia, Charlottesville, VA)

When niobium is used as a fast neutron

monitor, the isotopes 93mNb and 92mNb are

93, \92mi produced. The reaction yj Nb(n,2nfzmNb

produces an isotope with a 10.15 day half life, and has a threshold energy of about 9 MeV.

This reaction is of interest as a monitor for

fusion studies. In the course of measurements of

the 93 Nb(n,n')93mNb reaction rate, spectrumaveraged cross sections for the 93 Nb(n,2n)92mNb

reaction in a U fission spectrum have been

determined.

Five niobium foils (1.27-mm diam and

ranging in thickness from .004-mm to . 125-mm

thick) from two commercial suppliers were

irradiated in the cavity fission source [1] of the

NIST reactor. Calibration of this facility has

been described by McGarry et al [2].. In this

facility, two 235U metal fission source disks are placed outside a cadmium box coaxial with the

detector stack. This assembly is placed in the

center of a 0.3-m diam cavity in the graphite

thermal column. Three of the foils were irradi- ated at one time and the other two included in separate irradiations. Each foil was exposed for - 100 h. The neutron fluences were determined

by fluence transfer with nickel foil irradiations

[3]. In the fluence transfer method, the fluence

is, by a series of ratio measurements, traceable

to NBS-1, the national standard Ra-Be neutron

source, which has been absolutely calibrated

144
NON-RRD PROGRAMS

with the manganous sulfate bath technique. No

cross sections enter directly into the fluence

determination. Corrections have been made for

deviations from a pure fission field due to scattering. These corrections were made with a Monte Carlo technique and the correction varies

from 1.7 + 1.0 to 2.9 ± 1.0% depending on the

number and thickness of other foils irradiated

with the niobium. The one standard deviation

uncertainty in the fluence measurement is 2. 1 %

The foils were counted with germanium

detectors and the analysis done with the 934-keV

gamma ray. The detectors were calibrated with

standard point sources and have an uncertainty in the detector response efficiency of 3% at this energy. A gamma-ray abundance of 0.9915 and

half life of 10.15 days was used in the analysis.

The activity of each foil was measured several

times over a 2 week decay period. All

uncertainties were combined in quadrature to give an uncertainty of 4% for each individual

determination of the cross section.

The average determination of the spectrumaveraged cross section from the five foils is 0.433 mb with a one standard deviation from the

average of 2.6%. Combining all uncertainties

gives a final result of 0.433 ± 0.017. Using the

calculated cross section of Strohmaier [4],

ENDF/B-V [5], and NIST [1] evaluated fission

spectra, a calculated value of the

Nb(n,2n)92mNb spectrum-averaged cross

section has been obtained. Table 1 lists the

present results compared with some previous

results and the calculated cross section. In summary, a spectrum-averaged cross

section of 0.433 ± 0.017 mb has been measured

for the reaction 93 Nb(n,2n)92mNb in a

235U

fission spectrum. The measurement compares

favorably with a calculated value using the

Strohmaier calculation and either ENDF/B-V or

the NIST evaluated 235U fission neutron

spectrum. The uncertainty is about half that of

previously reported measurements.

Table 1. Nb(n,2n) Nb spectrum averaged cross

section

Value

(mb)

Reference Comments

0.433 ± 0.017 This work °U fiss. spec. 0.432 ± 0.033 Ref. 6 Reactor Spec. 0.402 ± 0.034 Ref. 6 Fission Plate 0.47 ± 0.03 Ref. 7

235U fiss. spec. 0.44 Ref. 4 and 5 Calculated 0.43 Ref. 4 and 1 Calculated

References

[1] J. A. Grundland C. M. Eisenhauer, "Compendium of benchmark neutron fields for reactor dosimetry,"

NBSIR 85-3151, Jan. 1986.

[2] E. D. McGarry, C. M. Eisenhauer, D. M. Gilliam,

J. Grundl, and G. P. Lamaze, "The U.S. U-235

fission spectrum standard neutron field revisited," in Proceedings of the Fifth ASTM-Euratom Symposium

on Reactor Dosimetry, Geesthact, West Germany,

Sept. 1984.

[3] G. P. Lamaze, E. D. McGarry, and F. J. Schima, in Proceedings of the Intn'l. Conf. on Nuclear Data for Science and Technology, Antwerp, Belgium, Sept. 1982.

[4] B. Strohmaier, Ann. Nucl. Energy, 16 (1989) p. 461.

[5] B. A. Magumo, R. R. Kinsey, and F. M. Scheffel, Guidebook for the ENDF/B-V Nuclear Data Files, EPRI NP-2510 (1982).

[6] H. Kimura, K. Kobayashi, and T. Shibata, J. Nucl.

Sc. and Tech. 8(2} 59-69 (1971).

[7] A. Fabry, Rep. BLG-465 (1972).

145
C. REACTOR OPERATIONS AND ENGINEERING

REACTOR OPERATIONS AND ENGINEERING

T. M. Raby, J. F. Torrence, J. H. Ring, and N. A. Bickford (Reactor Operations and Services)

and

J. H. Nicklas and R. S. Conway (Reactor Engineering)

Installation of the first three of seven guide

tubes for the cold neutron research facility

allowed time for only two operating cycles

during the past year. The reactor was on-line

the entire available operating time of slightly more than 2 months and was heavily utilized for

both neutron beam and irradiation programs. The

shutdown period was used by both reactor operations and engineering to perform design

modifications and improvements, major

maintenance, and equipment repair. Highlights

of significant activities are listed below, all of

which were performed in-house by the operating

and engineering staffs. Confinement Building and Penetrations . Seven penetrations through 24-ft thick

confinement building walls were drilled to accommodate the new guide tubes. Four

penetrations were temporarily resealed for future

tubes. Confinement isolation valves were

installed in the remaining three penetrations

currently in use. A complete control system for

the operation of these valves was designed and

installed. This included connection to the

reactor safety system that would cause automatic

closure and isolation of the confinement building

upon a major scram signal. Both the operation

and safety function of these valves, as well as

their effect on confinement leak rate, were tested

and the valves met all aspects of the technical

specifications requirements on the first try. This

is a significant milestone because of the complex

design and the stringent requirements.

The confinement building was completely

refurbished. The experimental and operating

floors were repainted with special sealant type

paint. The building roof was replaced. Most of

the asbestos used as insulation was removed and

replaced with nonasbestos insulation.

Heavy Water. Fuel, and Shim Arms . Both

heavy water and enriched fuel are in extremely

short supply in the United States. Because fuel

utilization, which is already the best in the

country, continues to improve further through

innovative management and design, there is enough fuel on hand to last to the end of the

decade. Negotiations with the Department of

Energy resulted in a commitment of 36 tons of

heavy water for future use. Three sets of shim arm blades, which are unique to the facility, are currently being assembled and should provide

replacement for the next 15 years. Extensive maintenance was performed on all four shim arm drives. A test stand was designed

and built to test the drives prior to installation. The stand provides for rapid and superior testing

compared to that done in-place, which is cumbersome and time consuming. For some

time, shim No. 2 would not return to the same

position following a scram. After months of

testing, the problem was traced to the inner

bearing of the shim arm shaft. Finding the

actual cause was in itself a significant

accomplishment because the problem was elusive

and nonrepetitive. Both the bearing and the

heavy water seal were replaced with the core in place. This is the first time in the history of the

facility that something of this critical nature was

done and it could not have been performed better

with minimal exposure and flawless operation.

Main Heat Exchangers . The extended

shutdown was used to locate a very small leak

(on the order of 0.0001 gpm) in one of the main

heat exchangers. After extensive searching and

testing, several tubes were plugged. Two new

leaks subsequently developed and additional

tubes were plugged. This heat exchanger has

been in service for more than 15 years and every

effort is being made to extend its use until replacement heat exchangers are obtained.

Comprehensive specifications for new heat

146
REACTOR OPERATIONS AND ENGINEERING

exchangers were completed and a request for

bids has gone out. Thermal Shield Cooling System . An

improved cooling system which incorporates a new high efficiency heat exchanger and an

automatic temperature control system was

designed and installed. This system minimizes

temperature variations under both operating and

shutdown conditions, and should help to reduce

leaks caused by sudden temperature changes.

Our program of leak sealing using a method

developed by the National Nuclear Corporation

of England is now well established, and will

continue to be followed.

Cooling Tower. A major overhaul of the

cooling tower was initiated and is near

completion. This work included the replacement

of nearly 1000 carbon steel bolts with stainless

steel bolts. Staffing . Extensive training was provided

to the staff of 16 licensed senior operators in many technical areas as part of the new Nuclear

Regulatory Commission (NRC) required requalification program. Two new operators will take

their senior licensing examination scheduled for

the end of the year by the NRC. One senior

operator, who is a graduate electrical engineer,

will then be transferred to Engineering to augment the small staff. The efforts and

performance of the operating staff over the years

received high recognition from the Department

of Commerce with the unprecedented award of

the Silver Medal to the entire group.

NEUTRON ACTIVATION ANALYSIS AT THE FOOD AND DRUG

ADMINISTRATION

D. L. Anderson and W. C. Cunningham

(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 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

analytical approaches. In addition, a radioisotope-induced x-ray emission (RIXE)

spectrometer was assembled this year to provide

support for FDA programs which monitor toxic

elements in housewares, and the low-level y-ray

counting facility developed by the Nuclear

Methods Group of the NIST's Center for

Analytical Chemistry was used to determine the

radionuclide content in various matrices. This

combination of nuclear techniques enables

diverse multielement and radiological

information to be obtained for foods and related

materials.

A variety of quality control-related NAA

studies were undertaken in 1989-90. Foods

collected as part of FDA's Total Diet Study and

IUPAC (International Union of Pure and Applied

Chemistry) check samples were analyzed for

aluminum. Multielement analysis of 13

proposed food reference materials prepared by

Agriculture Canada was completed, and work

continued on the development of methodology

for sodium determination by INAA. The

method for sodium will be collaboratively

studied for adoption as an official AOAC

(Association of Official Analytical Chemists)

method. Preliminary work has also begun on

the development of an in-house FDA infant

formula reference material.

PGAA efforts during the past year were

primarily devoted to the optimization of boron

determination in foods and biological materials.

Matrix effects resulting from the scattering of

147
REACTOR OPERATIONS AND ENGINEERING

thermal neutrons by the analytical portion were

accurately determined. These scattering

phenomena and the resulting enhancements of

element sensitivity and background count rate are detailed elsewhere [1-3]. Boron

concentrations in 31 reference materials were

determined and reported at the Fourth

International Symposium on Biological and

Environmental Reference Materials [2]. Among

the findings are boron concentrations of three

proposed NIST Reference Materials, SRM 1548

Total Diet (2.61 ± 0.25 ug/g), SRM 1515

Apple Leaves (28.8 ± 0.6 ug/g), and SRM

1547 Peach Leaves (28.6 ± 0.6 ug/g). In other

boron studies, PGAA and inductively coupled

plasma atomic emission spectrometry (ICP-AES)

results were compared from analyses of Total

Diet food composites (in conjunction with the

United States Department of Agriculture and

NIST) and orange juices. These studies showed

that an average boron loss of approximately 20%

occurs during the digestion step in analysis by

ICP-AES. Previous experiments [1] had shown

that large boron losses may also occur during

freeze-drying, especially for wet, low-pH

samples.

The availability of specialized nuclear

counting facilities had made possible the

initiation of preliminary RIXE experiments

designed to identify lead (Pb) in ceramic glazes.

Using a

109Cd excitation source, Pb- and nonPb-glazed ceramics may be distinguished from

one another. The possibility of implementing

field use of such a unit is being explored. Other

excitation sources (e.g., 55Fe and 24I Am) are being considered for the identification of other

potentially toxic elements such as Cd.

References

[1] D. L. Anderson, W. C. Cunningham, and E. A. Mackey, Biol. Trace Elem. Res. (1990) (in press).

[2] D. L. Anderson, W. C. Cunningham, and E. A. Mackey, Fresenius J. Anal. Chem. 338, 554-558

(1990).

[3] E. A. Mackey, G. E. Gordon, R. M. Lindstrom, and

D. L. Anderson, Anal. Chem. (1990) (in press).

148
PERSONNEL ROSTER

D. PERSONNEL ROSTER

REACTOR RADIATION DIVISION

J. M. Rowe, Chief

T. M. Raby, Deputy

H. J. Prask, Sci. Asst. E. C. Maxwell, Admin. Officer

S. L. Neal, Admin. Asst.

REACTOR OPERATIONS

& ENGINEERING

T. M. Raby, Chief

J. F. Torrence, Deputy

J. H. Nicklas, Chief Eng.

A. Wise, Sec'y

C. M. Sweeney, Sec'y

COLD NEUTRON

PROJECT

J. M. Rowe, Manager

I. G. Schroder, Tech. Leader

C. O'Connor, User Coord.

L. M. Thomas, Sec'y

NEUTRON CONDENSED

MATTER SCIENCE

J. J. Rush, Leader

J. K. Spillman, Sec'y

L. K. Clutter, Sec'y

149
PERSONNEL ROSTER

RESEARCH AND ENGINEERING STAFF

J. F. Ankner

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

J. A. Gotaas

o Condensed matter physics o Neutron reflectometry and grazing-angle diffraction o Instrument development

o Condensed matter theory o SANS theory for microstructure analysis o Computer software for graphics and data analysis

o Thin-film analysis o Artificially modulated materials o Magnetism

o Theory of neutron scattering from light-atom defects in metals o Group theory analyses of neutron scattering from condensed matter o Elementary particle theory, especially as related to reactor generated experiments

o Electronic engineering o Nuclear instrumentation

o Time-of-flight spectrometer development

o Neutron instrumentation conceptual design o Condensed matter physics

o Neutron scattering instrumentation o Microcomputer interfacing o Nuclear and engineering physics

o Magnetic materials o Phase transformations o Spin echo techniques o Cryogenics

o SANS microstructure of metals and porous media

o Magnetic materials o Cold neutron instrument development

o Magnetism and phase transformations o Cold neutron sources o SANS instrumentation and research

P. H. Graham o Engineering design

G. C. Greene o System and user software for cold neutron instrumentation o Spectrometer and data acquisition systems interfaces

B. Hammouda o SANS from polymer systems o Quasielastic scattering from polymer solutions o SANS instrument development

150
PERSONNEL ROSTER

V. L. Himes

P. A. Kopetka

S. Krueger

J. G. LaRock

H. P. Layer

J. W. Lynn

C. F. Majkrzak

A. D. Mighell

B. Mozer

D. A. Neumann

J. H. Nicklas

D. J. Pierce

H. J. Prask

o Crystal database development

o X-ray crystallography

o Mechanical engineering o Cold source design o Electro-mechanical systems

o Small angle neutron scattering instrumentation o Microstructure of materials o Biological problems

o Mechanical engineering o Neutron instrumentation design

o Electronics and data processing o Advanced instrumentation o Fundamental physics

o Condensed matter physics o Magnetic materials o Neutron scattering methods

o Condensed matter physics o Polarized neutron scattering o Polarizing and monochromating devices

o Crystallographic database development

o Single crystal diffraction o Theory of crystal lattices

o Structure and microstructure of metallic glasses o Dynamics of liquids o NDE of alloys

o Condensed matter physics o Two-dimensional materials o Neutron and x-ray scattering instrumentation

o Mechanical engineering o Reactor fuel design o Reactor engineering support

o Mechanical engineering o Neutron instrumentation design

o Residual stress measurement

o Neutron NDE of hardware

o Neutron NDE instrumentation

151
PERSONNEL ROSTER

E. Prince

T. M. Raby

Structural properties of alloys, catalysts and minerals

Advanced crystallographic refinement methods

Software for materials structure analyses

o Reactor operations o Nuclear engineering o Reactor standards

J. J. Rhyne o Properties and transformations of high technology magnetic materials o Structure of amorphous solids o Data acquisition and analysis systems

J. M. Rowe o Orientationally disordered solids o Hydrogen in metals o Cold neutron research and instrumentation

J. J. Rush o Catalysts and molecular materials o Hydrogen in metals o Two-dimensional systems o Inelastic scattering methods

A. Santoro

S. K. Satija

I. G. Schroder

o Structure of electronic and structured ceramics o Theory of crystal lattices o Powder diffraction methods

o Low-dimensional molecular systems o Fractal aspects of microporous media

o Neutron reflectometry

o Cold neutron instrumentation development

o Nuclear and engineering physics o Optical devices for neutron transport

J. K. Stalick

J. B. Sturrock

M. Suthar

J. F. Torrence

o Neutron and x-ray diffraction o Inorganic chemistry o Crystal database development

o Design engineering o Nuclear systems and components

o Design engineering o Nuclear systems and components

o Reactor supervision o Reactor maintenance o Nuclear engineering

T. J. Udovic o Neutron time-of-flight instrumentation o Properties of catalysts o Spectroscopy of surfaces

152
PERSONNEL ROSTER

R. E. Williams o Cold neutron source development

o Nuclear engineering

TECHNICAL STAFF

Reactor Operations

Richard D. Beasley

Myron H. Bell

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 I. Wright

Neutron Research

George M. Baltic

David Clem

Donald H. Fravel

Thomas Green

Wayne C. Knill

Michael J. Rinehart

David Roeseler

William H. Rymes

Dan Swearingen

Thuan Thai

Robert H. Williams

Reactor Engineering

James A. Beatty

Richard P. Hayes

153
PERSONNEL ROSTER

NON-RRD NIST STAFF LOCATED AT REACTOR

Occupational Health

and Safety Division (354)

D. Brown

C. Campbell

J. Clark

G. Deardorff

I. Jensen

T. Mengers

F. Moore

J. Shubiak

L. Slaback

Ionizing Radiation

Division (536)

E. Boswell

C. Eisenhauer

D. Gilliam

J. Grundl

E. McGarry

R. Schwartz

Inorganic Analytical

Research Division (551)

D. Becker

G. Downing

K. Fitzpatrick

K. Garrity

R. Greenberg

G. Iyengar

H. Kingston

G. Lamaze

J. Langland

R. Lindstrom

K. Littrell D. Mildner

B. Norman

K. Pratt C. Stone

S. Stone

R. Watters, Jr. R. Zeisler

GUEST WORKERS AND COLLABORATORS

MATERIALS SCIENCE AND ENGINEERING

LABORATORY (400)

J. Cahn

Ceramics Division (420)

D. Black

D. Blackburn

D. Cranmer

E. Fuller

W. Haller

D. Kauffman

R. Krause

G. Long

S. Malghan

R. Roth

W. Wong-Ng

Polymers Division (440)

B. Bauer

R. Briber

F. Brinkman

L. Coyne

T. Davis

C. Han

A. Nakatani

W. Wu

Metallurgy Division (450) L. Bendersky

F. Biancaniello

W. Boettinger

R. Fields

F. Gayle

C. Johnson

D. Lashmore

R. Schaefer

Surface Science Division (541)

R. Cavanagh

154
PERSONNEL ROSTER

Organic Analytical

Division (552)

B. Koster

L. Sander

S. Wise

Gas and Particulate

Science Division (553)

W. Dorko

C. Poston

D. Simmons

J. Small

S. Wight

Center for Advanced Research

Biotech. (581)

G. Gilliland

L. Svensson

Federal Bureau of Investigation

R. Aaron

R. Bilko

R. Halberstam

J. Havekost

K. Lundy

E. Peele

R. Rebbert

J. Riley

P. Sanderson

Food and Drug Administration

D. Anderson

W. Cunningham

J. Tanner

Geological Survey

M. Mrose

Center for Atomic, Molecular,

and Optical Physics (570)

M. Arif

R. Deslattes

S. Dewey

G. Greene

NASA

D. Lindstrom

National Institutes of Health

M. Gellert

R. Nossal

Semiconductor Electronics

Division (727)

J. Ehrstein

J. Suehle

Thermophysics Division (774)

H. Hanley

J. Straty

Armed Forces Institute of Pathology

D. Nagarajan

Army Armament Research, Development,

and Engineering Center

C. Choi

S. Trevino

National Oceanic and

Atmospheric Administration

G. Lauenstein

Naval Research Laboratory

D. Collins

K. Grabowski

P. Skeath

C. Roland

Naval Surface Weapons Center

G. Riel

M. Spano

Pittsburgh Energy Tech. Ctr. (DOE)

G. Olsen

Army Ballistics Research Lab

B. Rice

L. Vande Kieft

Smithsonian Institution

R. Bishop

M. Blackman

Y. Cheng

R. Cunningham

E. Myers

J. Olin

G. Stein

155
PERSONNEL ROSTER

Argonne National Laboratory

T. Brun

J. Carpenter

G. Felcher

Y. Huang

J. Liu

J. Richardson

P. Thiyagarajan

K. Vandervoort

Atom. Inst. (Vienna)

G. Westphal

Boris Kidric Inst., Belgrade, Yugoslavia

M. Davidovic

D. Djordjevic

Brookhaven National Laboratory

H. Chou

R. DiNardo

L. Passell

A. Saxena

M. Sanyal

G. Shirane

S. Sinha

CECM/CNRS Grenoble, France

M. Bessiere

Y. Calvayrac

J. Capponi

M. Cornier-Quiquandon

D. Gratias E. Hewat

S. Lefebvre

M. Marezio

A. Quivy

CEN - Saclay, France

R. Bellissent

Central Bur. Nucl. Meas. (Belgium)

J. Pauwels

Energy Research Foundation

(The Netherlands)

J. Woittiez

Foundation pour la Recherche

Medical e (Geneva, Switzerland)

E. H. Cerny

Hahn-Meitner Institute

(Berlin)

D. Behne

Harwell Lab (U.K.)

A. Allen

Institut Laue-Langevin

Grenoble, France

A. Magerl

M. Thomas

C. Vettier

G. Zaccai

Institut "Jozef Stefan" (Yugoslavia)

A. Byrne

B. Smodis

Instituto se Pesquisas

Energeticas Nucleares (Sao Paulo, Brazil)

M. Saiki

International Centre for Diffraction Data

M. Morris

H. McMurdie

KFA Jiilich, Germany

H. Grimm

R. Hempelmann

D. Richter

M. Rossbach

M. Stoepler

Korea Atomic Energy Research Institute

C.-H. Lee

Korean Inst, of Sci. & Tech.

H. Kim

Korean Standards Research Inst. S.-T. Hwang

Paul Scherrer Inst. (Switzerland)

I. Anderson

O. Elsenhans

National Research Council (Canada)

J. Rodgers

W. McKinnon

156
PERSONNEL ROSTER

Lawrence Berkeley Lab.

D. Morris

R. Sid

Los Alamos National Laboratory

T. Bowles

S. W. Cheong

J. Eckert

Z. Fisk

W. Hamilton

H. Robertson

R. Robinson

G. Smith

T. Sosnick

J. Trewhella

J. Wilkerson

M. Yethiraj

Oak Ridge National Laboratory

J. Cable

J. Fernandez-Baca

J. Hayter

H. Mook

R. Nicklow

B. Sales

H. Smith

Riso National Laboratory

J. Bohr

Sandia National Laboratory

B. J. Olivier

Chinese Academy of Sciences

Q. Lin

College of William and Mary

E. Schoe

R. Wright

Colorado School of Mines

N. Wada

Columbia University

G. Luke

Y. Uremura

Florida International U.

W. Jones

Georgetown University

W. Clinton

Goethe University

W. Baur

A. Bieniok

Harvard University

T. Chupp

J. Richardson

E. Wasserman

Iowa State University

A. Goldman

R. Barnes

Alfred University

R. Snyder

R. Zhow

Auburn University

B. Taterchuck

Chalmers University of Technology

L. Sjolin

City University of New York

M. An

C. Lu

City University of New York (Brooklyn)

H. Homma

Istanbul Tech. University

R. Demiralp

John Carroll University

J. Trivisonno

Johns Hopkins University

C. L. Chien

M. Cieplak

D. Musser

F. Streitz

R. Viscidi

T. Wilson

G. Xiao

157
PERSONNEL ROSTER

Kyoto Institute of Technology

H. Kurokawa

M. Shibayama

Q. Tran-Cong

Kyoto University (Japan)

H. Hasegawa

T. Hashimoto

H. Jinnai

S. Sakurai

Lehigh University

A. Klein

L. Sperling

S. Yang

J. Yoo

M.I.T.

R. Birgeneau

M. Dresselhaus

S. Haile

J. Speck

B. Wuensch

McMaster University

W. Clarke

R. BanMiami University (Ohio)

J. Cantrell

Oklahoma State University

B. Ackerson

Penn State University

L. Pilione

J. Yehoda

Princeton University

W. Graessley

Purdue University

A. Overhauser

Rochester Institute of Technology

M. Kotlarchyk

Rutgers University

R. Gerhardt

San Jose State University

P. Englert

Scottish Universities Research Reactors

R. Scott

Temple University

J. Crow

C.-S. Jee

A. Kebede

T. Mihalisin

Michigan State University

T. Pinnavaia

S. Solin

Texas A&M University

B. Grazman

E. Schweikert

Nagoya University

Y. Matsushita

Y. Mogi

K. Mori

Y. Muroga

M. Nagasawa

I. Nodo

R. Saguchi

Y. Takahashi

Northwestern University

J. Barker

P. Jemian

J. Weertman

University of Antwerp (Belgium)

K. Michel

C. Bostoen

University of California (Davis)

P. Klavins

J. Liu

R. Shelton

University of California (Irvine)

V. Guinn

158
PERSONNEL ROSTER

University of California A. Salvador

(Santa Barbara) F. Tsui

S. Cox H. Zabel

D. Klingensmith

E. Mader University of Illinois (Chicago)

R. Odette M. Said

G. Stucky J. Kouvel

University of California (San Diego) University of Maryland

J. Arnold H. Ammon

A. Berkowitz A. Cargill

J. Ferreira C.-H. Chen

B. Lee 0. Chitale B. Maple T. Clinton

J. Neumeier F. Divita

K. Nishiizumi G. Gordon

K. Yang D. Gretz

M. Han

University College (London) W. Li

F. Beech Z. Li

J. Lynn

University of Delaware E. Mackey

J. Beamish J. Nicol

W. Dubner J. Ondov

J. Peng

University of Exeter (U.K.) T. Quinn

R. Slade B. Schmid

S. Skanthakumar

University of Florida W. Sumarlin

S. Nagler J. Valette-Silver

W. Walters

University of Guelph (Canada) J. Weber

F. Hallett H. Zhang

University of Houston University of Massachusetts

W.-K. Chu A. Hanyu

S. Moss L. Jong

L. Reinhard R. Stein

J. Robertson

University of Milan (Italy)

University of Illinois (Urbana) M. Zocchi

R. Beach

R. Du University of Michigan

J. Dura D. Vincent

C. Flynn R. Fleming

J. Kordas

A. Matheny University of Minnesota

G. Moore F. Bates H. Morkog M. Foster

P. Reimer M. Sikka

M. Salamon

159
PERSONNEL ROSTER

University of Missouri (Columbia)

R. Berliner

H. Kaiser

A. Krawitz

G. Schupp

S. Werner

D. Worcester

University of Missouri (Rolla)

P. Lindemuth

R. Venable

University of Vermont

T. Flanagan

University of Virginia

T. Williamson

University of Washington

J. Billman

B. Heckel

E. Kaler

W. Zoller

University of NSW (Australia) B. Hunter

University of Nancy (France)

C. Du Four

P. Mangin

G. Marchal

M. Vergnat

University of New Hampshire

N. Blakemore

R. Blakemore

Y. Gorby

University of North Carolina

N. Parikh

M. Swanson

University of Notre Dame

J. Furdyna

T. Giebultowicz

P. Klosowski

H. Luo

N. Samarth

University of Pennsylvania

P. Davis

R. Egami

University of Pittsburg

W. Goldberg

Universite Pierre et Marie Curie (France)

Ph. Depondt

University of Sussex (U.K.)

J. Byrne

P. Dawber

J. Spain

University of West Virginia

R. Kannan

M. Seehra

Utah State University

T. Isenhour

Yale University

J. Flanagan

AT&T Bell Laboratories

R. Cava

S. Cheong

G. Fredrickson

J. Krajewski

P. Marsh

J. Mitchell

D. Murphy

W. Peck

'

W. Rhodes

J. Riley

J. Rosedale

L. Schneemeyer

S. Sunshine

J. Waszazak

S. Yegnasubramanian

Aeroject Electrosystems

R. Bowman

Aerospace Corporation

J. Knudsen

Allied Signal Corporation

T. Barden

G. Fish

C. Parker

160
PERSONNEL ROSTER

Atomic Energy of Canada, LTD

W. Lennard

Battelle Pacific Northwest

Laboratories

F. Brauer

G. Dudder

G. Endres

M. Failey

Bellcore

S. Allen

G. Hull

P. Miceli

C. Palmstrom

K. Remschnig

J. Tarascon

Exxon

B. Abeles

L. Fetters

J. Huang

G. Hughes

M. Kim

K. Liang

M. Lin

D. Lohse

R. Overfield

E. Sirota

GE Corporate R&D

R. Rowe

GE Plastics

J. Schroeder

Charles Evans & Assoc.

R. Fleming

CIDTECH

J. Carbone

M. Wadsworth

John Deere Product Eng. Ctr. T. Cordes

Directed Technologies, Inc. N. Chesser

Dow Chemical

R. Bubeck

E. I. DuPont de Nemours Co.

J. Parise

M. Subramanian

Eastman Kodak

M. Landry

J. Lavine

J. O'Reilly

J. Russell

C. Swanson

J. Tritten

H. Yang

G. Zenner

GEN Corp.

I. Hargis

GTE Laboratories

G. Hamill

W. R. Grace

R. Wormsbecker

Hughes, Malibu

R. Wilson

IBM (San Jose)

S. Anastasiadis

G. Hadziioannou

A. Menelle

T. P. Russell

Imperial Chemical Industries

J. Howard

H. Stanley

Intel Corporation

J. Cox

Mobil Oil

A. Moini

Optoline Corp.

J. Bradshaw

Polaroid Corp.

S. Roy

161
PERSONNEL ROSTER

Southwest Research Inst. R. Page

SRI, Inc. S. Johnson

Texaco Research Center

E. Sheu

Texas Instrument

T. Hossain

162
E. PUBLICATIONS

Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F, "The Morphology of Symmetric

Diblock Copolymers as Revealed by Neutron Reflectivity," J. Chem. Phys. 92, 5677 (1990).

Anderson, I. S.; Berk, N. F.; Rush, J. J.; Udovic, T. J.; Barnes, R. G.; Richter, D.; Magerl, A.,

"Rapid Low Temperature 'Hopping' of Hydrogen in a Pure Metal: The ScHx System," Phys. Rev.

Lett. 65, 1439 (1990).

Ammon, H.; Choi, L.; Damvarapu, C. S.; Iyer, R. S.; Alster, S., "Stucture of Cubane-1, 4- diammonium Bis-(trinitromethide)," Acta Crystallogr. C 46, 295 (1990).

Ankner, J. F.; Zabel, H.; Neumann, D. A.; Majkrzak, C. F.; Matheny, A.; Dura, J. A.; Flynn. C. P., "Neutron Reflectivity Study of Gd-Y Interface," in Neutron Scattering for Material Science , edited

by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166

(Materials Research Society, Pittsburgh, 1990), p. 109.

Ankner, J. F.; Zabel, H.; Neumann, D. A.; Majkrzak, C. F.; Dura, J. A.; Flynn, C. P., "GrazingAngle Neutron Diffraction," J. Phys. (Paris) 50, C7-189 (1989).

Bashir-Hashemi, A.; Ammon, H. L.; Choi, C. S., "Chemistry and Structure of Phenylcubanes," J. Org.

Chem. 55, 416 (1990).

Baur, W. H.; Bieniok, A.; Shaaon, R. D.; Prince, E., "Neutron Powder Diffraction Study and

Thermogravimetry of Sodium Cesium Zeolite Rho," Z. Kristallogr. 187, 253 (1989).

Beach, R. S.; Borchers, J.; Salamon, M. B.; Du, R.; Flynn, C. P., "Observation of Intermediate Spin

States in Er/Y Superlattices," J. Appl. Phys. 67, 5710 (1990).

Berk, N. F.; Glinka, C. J.; Haller, W.; Sander, L. C, "Analysis of SANS from Controlled Pore

Glasses" in Neutron Scattering for Materials Science , edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166 (Materials Research Society, Pittsburgh,

1990), p. 409.

Berk, N. F.; Rush, J. J.; Udovic, T. J.; Anderson, I. S., "Anomalous Hydrogen Dynamics in Rare

Earth Metals," J. Less-Common Met. (in press).

Biswas, R.; Bouchard, A. M.; Kamitakahara, W. A.; Grest, G. S.; Soukoulis, C. M., "Vibrational

Localization and Vibrational Spectra in Amorphous Silicon," in Atomic Scale Calculations in Materials Science, edited by J. Tersoff, D. Vanderbilt, and V. Vitek, Mater. Res. Soc. Symp.

Proc, Vol. 141 (Materials Research Society, Pittsburgh, 1989), p. 213.

Boolchand, P.; Enzweiler, R. N.; Cappelletti, R. L.; Kamitakahara, W. A.; Cai, Y.; Thorpe, M. F.,

"Vibrational Thresholds in Covalent Networks," Solid State Ionics 39, 81 (1990).

Bowman, R. C; Maeland, A. J.; Venturini, E. L.; Rush, J. J.; Cantrell, J. S., "Studies of the Structure

and Properties of Amorphous a-Zr76Feo4Hx

, " Z. Phys. Chem. 163, 373 (1989).

163
PUBLICATIONS

Bowman, R. C; Torgeson, D. R.; Barnes, R. G.; Maeland, A. J.; Rush, J. J., "Relationship of

Hydrogen Site Occupancy to Diffusion Behavior in Crystalline and Amorphous Zr2PdHx

," Z. Phys.

Chem. 163, 203 (1989).

Bradford, D.; Hammouda, B.; Bubeck, R. A.; Schroeder, J. R.; Glinka, C. J.; Thiyagarajan, P.,

"Deformations of Polystyrene Chains Upon Hot Stretching," J. Appl. Crystallogr. 23, 1 (1990).

Carstanjen, H.-D.; Huang, X. Y.; Kieninger, W.; Kirchheim, R.; Rush, J. J.; Udovic, T. J.; Glinka,

C, "A Neutron Scattering Study of Deuterium Trapping by Dislocations," Z. Phys. Chem. Neue

Folge 163, 203 (1989).

Cava, R. J.; Marezio, M.; Krajewski, J. J.; Peck, W. F.; Santoro, A.; Beech, F., "Neutron Powder

Diffraction Study of Pb2Sr2YCu308

, the Prototype of a New Family of Superconductors," Physica

C 157, 272 (1989).

Cava, R. J.; Santoro, A.; Krajewski, J. J.; Fleming, R. M.; Waszczak, J. V.; Peck, W. F.; Marsh, P.,

"The Crystal Structure of the 60K Superconductor Laj 6Sr 4CaCu2 6+d," Physica C (in press).

Casella, R. C, "Comparison of Interplanar-Boson-Exchange Models of High-Temperature

Superconductivity-Possible Experimental Tests," Appl. Phys. Lett. 55 (9), 908 (1989).

Chaiken, A.; Dresselhaus, M. S.; Orlando, T. P.; Dresselhaus, G.; Tedrow, P. M., Neumann, D. A.; Kamitakahara, W. A., "Anisotropic Superconductivity in C4KHg," Phys. Rev. B 41, 71 (1990).

Choi, C. S.; Prask, H. J.; Ostertag, C. P., "Texture Study of Magnetically Aligned YBajCiigOy - Type

Materials by Neutron Diffraction," J. Appl. Crystallogr. 22, 465 (1989).

Cieplak, M. Z.; Xiao, G.; Chien, C. L., Stalick, J. K.; Rhyne, J. J., "Unexpected Effects of Gold on

the Structure, Superconductivity, and Normal-State of YBajQ^O-y, " Appl. Phys. Lett. 57, 934

(1990).

Cieplak, M. Z.; Xiao, G.; Bakhshai, A.; Artymowicz, D.; Bryden, W.; Chien, C. L.; Stalick, J. K.; Rhyne, J. J., "Incorporation of Gold Into YBa Cu O: 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^UgO-^"

Physica C (in press).

Collins, D. M.; Prince, E., "Exponential Density: Exact Fitting of Structure Moduli by Entropy

Maximization," Proc. of the International School on Crystallogr. Computing (in press).

Copley, J. R. D., "The Effects of Chopper Jitter on the Time-Dependent Intensity Transmitted by

Multiple-Slot Multiple Disk Chopper Systems," in Advanced Neutron Sources 1988. edited by D.

K. Hyer, The Inst. Phys. Conf. Series, Vol. 97 (Institute of Physics, Bristol and New York, 1989),

p. 327-339.

Copley, J. R. D.; Mildner, D. F. R., "Production of Intense Clean Neutron Beams for Neutron

Absorption Experiments," Trans. Am. Nucl. Soc. 6_i, 105 (1990).

Copley, J. R. D., "An Analytical Method to Characterize the Performance of Multiple Section StraightSided Neutron Guide Systems," Nucl. Instrum. & Methods in Phys. Res. A 287, 363-373 (1990).

164
PUBLICATIONS

Copley, J. R. D., "On the Use of Acceptance Diagrams to Calculate the Performance of Multiple Section

Straight-Sided Neutron Guide Systems," in Advanced Neutron Sources 1988 , edited by D. K. Hyer,

The Inst. Phys. Conf. Series, Vol. 97 (Inst, of Physics, Bristol and New York, 1989), p. 821- 828.

Copley, J. R. D., "Acceptance Diagram Analysis of the Performance of Vertically Curved Neutron

Monochromators," Nucl. Instrum. & Methods in Phys. Res. A (in press).

Copley, J. R. D., "Optimized Design of the Chopper Disks and the Neutron Guide in a Disk Chopper

Neutron Time-of-Flight Spectrometer," Nuclear Instruments & Methods in Phys. Res. A 291 , 519-532 (1990).

Erwin, R. W., "Domain Size and Spin Waves in a Rentrant Spin Glass," J. Appl. Phys. 67, 5229 (1990).

Erwin, R. W.; Rhyne, J. J.; Borchers, J. A.; Salamon, M. B.; Du, R.; Flynn, C. P., "Magnetoelasticity in Rare-Earth Multilayers and Films," in Neutron Scattering for Material Science . edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol.

166, (Materials Research Society, Pittsburgh, 1990), p. 133.

Erwin, R.; Rhyne, J. J.; Borchers, J.; Du, R.; Salamon, M.; Flynn, C. P., "Magnetoelasticity in RareEarth Superlattices and Films," Physica B J59, 1 1 1 (1989).

Fan, Y.; Neumann, D. A.; Solin, S. A.; Kim, H.; Pinnavaia, T.J., "Elastic and Inelastic Neutron

Scattering Study of Hydrogenated and Deuterated Trimethylammonium Pillared Vermiculite Clays,"

J. Chem. Phys. (in press).

Felcher, G.P.; Schneider, D.; Farnoux, B.; Majkrzak, C; Spooner, S.; Timmins, P.; White J.; Wignall,

G., "Small-Angle Scattering and Reflectometry," in Proceedings of the Oak Ridge National

Laboratory/ Brookhaven National Laboratory Workshop on Neutron Scattering Instrumentation at High-Flux Reactors . Report CONF-89063 1 1 , (1990), p. 39-47.

Fernandez-Baca, J. A.; Nicklow, R. M.; Tun, Z.; Rhyne, J. J., "Magnetic Excitations in Thulium

Metals," J. Appl. Phys. 67, 5283 (1990).

Fernandez-Baca, J. A.; Hennion, M.; Hennion, B.; Rhyne, J. J.; Fish, G. E., "Spin Dynamics of

Amorphous Fe90.xNixZr10

," J. Appl. Phys. 67, 5223 (1990).

Fischer, R.; Baur, W.; Shannon, R.; Parise, J.; Faber, J.; Prince, E., "New, Different Forms of

Ammonium Loaded, and Partly Deammoniated Zeolite Rho Studied by Neutron Powder

Diffraction," Acta Crystallogr. C 45, 983 (1989).

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.; Yadavalli, S.; Zhu, X.; Zabel, H.; Cunningham, J. E.; Erwin, R. W.;

and Rhyne, J. J.; in Proceedings of the MRS Conference on Advanced Materials, Tokyo, 1988

edited by Yamamoto Ryoichi, J. Phys. (Paris) Collog. (in press).

Forano, C; Slade, R. C; Krogh, E.; Andersen, I. G.; Prince, E., "Neutron Diffraction Determination

of Full Structures of Anhydrous Li-X and Li-Y Zeolytes," J. Solid State Chem. 82, 95 (1989).

165
PUBLICATIONS

Giebultowicz, T. M.; Klosowski, P.; Rhyne, J. J.; Udovic, T. J.; Furdyna, J. K.; Giriat, W.,

"Magnetic Exchange Interactions in Co-Based II-VI Diluted Magnetic Semiconductors: Zn: xCoxS," Phys. Rev. B 41, 504 (1990).

Giebultowicz, T. M.; Klosowski, P.; Rhyne, J. J.; Samarth, N.; Luo, H.; Furdyna, J. K., "Neutron

Diffraction Studies of C'd^Mn^Se Epilayers and ZSe/MnSe Multilayers," in Neutron Scattering

for Material Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc.

Symp. Proc, Vol. 166 (Materials Research Society, Pittsburgh, 1990), p. 115.

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

"Antiferromagnetic Phase Transition in Cd^Mn^ex Epilayers," Phys. Rev. B 42, 2582 (1990).

Giebultowicz, T. M.; Rhyne, J. J.; Furdyna, J. K.; Klosowski, P., "Inelastic Neutron Scattering Studies

of II-VI Diluted Magnetic Semiconductors," J. Appl. Phys. 67, 5096 (1990).

Glinka, C. J.; Sander, L. C; Wise, S. A.; Berk, N. F., "Characterization of Chemically Modified Pore

Surfaces by Small Angle Neutron Scattering," in Neutron Scattering for Materials Science, edited

by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp., Vol. 166 (Materials

Research Society, Pittsburgh, 1990), p. 415.

Hammouda, B.; Schupp G.; Maglic, S., "QEGS from Polydimethylsiloxane in Benzene Solutions," J. Chem. Phys. 93, 5473 (1990).

Hammouda, B., "Treatment of Asymmetric SANS Data," J. Polym. Sci., Polym. Phys. Ed. B 27, 1971

(1989).

Hammouda, B.; Yelon, W. B.; Lind, A. C; Hansen, F., "SANS From a BPAC/DMS Block Copolymer

Under Uniaxial External Stretch," Macromol. 22, 418 (1989).

Hempelmann, R.; Richter, D.; Rush, J. J.; Rowe, J. M., "Hydrogen Site Distribution in the Alloy

System Nb100xVxH Studied by Means of Neutron Vibrational Spectroscopy," J. Less-Common

Met. (in press).

Himes, V. L.; Mighell, A. D.; Stalick, J. K.; Young, S. L.; Carr, M. J.; Lyman, C. E.; Anderson, R.; Holomany, M. A.; Jenkins, R., Elemental and Interplanar Spacing Index for Phase Identification

by Electron or X-Ray Diffraction, (U.S. Dept. of Commerce, NIST and the International Centre

for Diffraction Data, 1989), 880 pages.

Kamitakahara, W. A., "Low Frequency Vibratonal Spectra of Amorphous Solids," in Phonons 89 . edited

by S. Hunklinger, W. Ludwig, and G. Weiss, (World Scientific, New Jersey, 1990), p. 387.

Kamitakahara, W. A.; Biswas, R.; Bouchard, A. M.; Gompf, F.; Suck, J. B., "Vibrational Spectra for

Hydrogenated Amorphous Semiconductors," in Neutron Scattering for Material Science, edited by

S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166

(Materials Research Society, Pittsburgh, 1990), p. 361.

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).

Krueger, S.; Chen, S.-H.; Hofrichter, J.; Nossal R., "Small Angle Neutron Scattering Studies of HbA

in Concentrated Solutions," Biophys. J. (in press).

166
PUBLICATIONS

Krueger, S.; Lynn, J. W.; Russell, J. T.; Nossal, R., "Small-Angle Neutron Scattering Method for In Situ Studies of the Dense Cores of Biological Cells and Vesicles: Application to Isolated

Neurosecretory Vesicles," J. Appl. Crystallogr. 22, 546 (1989).

Krueger, S.; Long, G. G.; Page, R. A., "Multiple Small Angle Neutron Scattering Characterization of

the Densification of Alumina," in Neutron Scattering for Materials Science, edited by S. M. Shapiro,

S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166 (Materials Research

Society, Pittsburgh, 1990), p. 61.

Krueger, S.; Olson, G. J.; Rhyne, J. J.; Blakemore, R. P.; Gorby, Y. A.; Blakemore, N., "Small Angle

Neutron and X-Ray Scattering From Magnetite Crystals in Magnetoactic Bacteria," J. Magn. Magn.

Mater. 82, 17 (1989).

Krueger, S.; Olson, G. J.; Rhyne, J. J.; Blakemore, R. P.; Gorby, Y. A.; Blakemore, N., "Small Angle

Neutron Scattering From Bacterial Magnetite," J. Appl. Phys. 67, 4475 (1990).

Krueger, S.; Zaccai, G.; Wlodawer, A.; Langowski, J.; O'Dea, M.; Maxwell, A., Gellert, M.,

"Neutron and Light-Scattering Studies of DNA Gyrase and Its Complex With DNA," J. Mol. Biol.

211,211 (1990).

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).

Li, W. H.; Lynn J. W.; Skanthakumar, S.; Clinton, T. W.; Kebede, A.; Jee, C. S.; Crow, J. E.; Mihalisin, T., "Magnetic Order of Pr in PrBa^UgO-?," Phys. Rev. B 40, 5300 (1989).

Li, W. H.; Lynn, J. W.; Stanley, H. B., Udovic, T. J.; Shelton, R. N.; Klavins, P., "Crystal Field

Splittings in the Cubic Magnetic Superconductors HoPd2Sn and ErPd2Sn," Phys. Rev. B 39, 41 19

(1989).

Li, W. H.; Lynn, J. W.; Fish, Z., "Magnetic Order of the Cu Planes and Chains in NdBa Cu3 6+x," Phys. Rev. B 41, 4098 (1990).

Lin, M.; Sinha, S. K.; Huang, J. S.; Abeles, B.; Johnson, J. K.; Drake, J. M.; Glinka, C. J., "A

SANS Study of Capillary Condensation in Porous Media," in Neutron Scattering for Materials

Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc,

Vol. 166 (Materials Research Society, Pittsburgh, 1990), p. 449.

Lindemuth, P.; Hammouda, B.; Duck, J. R.; Blum, F.; Venable, R., "PGSE-NMR and SANS From

TPB Based Microemulsions," in Macromolecular Liquids , edited by C. R. Safinya, S. Safran, and

P. A. Pincus, Mater. Res. Soc. Symp. Proc, Vol. 177 (Materials Research Society, Pittsburgh,

1990), p. 117.

Lindemuth, P.; Hammouda, B.: Venable, R., "SANS from TPB based Microemulsions," J. Phys. Chem.

J. Phys. Chem. 94, 8247 (1990).

Long, G. G.; Krueger, S., "Multiple Small Angle Neutron Scattering Characterization of the

Densification of Ceramics: Application to Microporous Silica," J. Appl. Crystallogr. 22, 539

(1989).

167
PUBLICATIONS

Long, G. G.; Krueger, S.; Black, D. R.; Cline, J. P.; Jemian; P. R. Gerhardt, R. A., "Small Angle

Neutron Scattering and Small Angle X-Ray Scattering From Bulk Microporous Silica," in Neutron

Scattering for Materials Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater.

Res. Soc. Symp. Proc, Vol. 166 (Materials Research Society Pittsburgh, 1990 p. 421.

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. (in press).

Lynn, J. W., "Magnetic Order and Spin Fluctuations in Oxide Superconductors," Physica B 163, 69

(1990).

Lynn, J. W.; Clinton, T. W.; Li, W. H.; Erwin, R. W.; Liu, J. Z.; Vandervoort, D.; Shelton, R. N.,

"2D and 3D Magnetic Behavior of Er in ErBa^UgOy," Phys. Rev. Lett. 63, 2606 (1990).

Lynn, J. W.; Clinton, T. W.; Li, W. H.; Erwin, R. W.; Liu, J. Z.; Shelton, R. N.; Klavins, P., "TwoDimensional Magnetic Order of Er in ErBa^^O?," J. Appl. Phys. 67, 4533 (1990).

Lynn, J. W.; Li, W. H.; Trevino, S. F.; Fish, Z., "Pressure Dependence of the Cu Magnetic Order in RBa2Cu3 6+x," Phys. Rev. B 40, 5172 (1989).

Lynn, J. W.; Sumarlin, I. W.; Skanthakumar, S.; Li, W. H.; Shelton, R. N.; Peng, J. L.; Fisk, Z.; Cheong, S. W., "Magnetic Ordering of Nd in (Nd,Ce)2Cu04

," Phys. Rev. B 41_, 2569 (1990).

Majkrzak, C. F.; Kwo, J.; Hong, M.; Yafet, Y.; Gibbs, Doon; Chien, C. L.; Bohr, J., "Magnetic Rare

Earth Superlattices," Adv. Phys. (in press).

Majkrzak, C. F.; Felcher, G. P., "Neutron Scattering Studies of Surfacers and Interfaces," MRS Bulletin

(in press).

Majkrzak, C. F.; Satija, S.; Neumann, D. A.; Rush, J. J.; Lashmore, D.; Johnson, C; Bradshaw, J; Passell, L.; Dinardo, R., "Determination of Hydrogen Density Profiles in Thin Films and

Multilayers by Neutron Reflection," in Neutron Scattering for Materials Science, edited by S. M.

Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp., Vol. 166, (Materials Research

Society, Pittsburgh, 1990), p. 127.

Mangin, P.; Boumazouza, D.; George, B.; Rhyne, J. J.; Erwin, R. W., "Static and Dynamic Properties

of the (¥exCTUx)15Pl5 Cl0 Reentrant Spin Glass Amorphous Alloy," Phys. Rev. B 40, 11123

(1989).

Marezio, M.; Santoro, A.; Capponi, J. J.; Hewat, E. A.; Cava, R. J.; Beech, F., "Crystal Structure,

Atomic Ordering and Charge Localization in Pb2Sr2YCu3 8+z(M=1.47)," Physica C 169, 401

(1990).

McKinnon, W. R.; Tselepis, E.; LaPage, Y.; McAlister, S. P.; Pleziter, C; Tarascon, J. M.; Miceli,

P. F.; Ramesh, R.; Hull, G. W.; Waszczak, J. V.; Rhyne, J. J.; Neumann, D. A., "Structure and

Magnetism of Bi2(Sr,Ca)2 Mn06+y," Phys. Rev. B 41, 4489 (1990).

168
PUBLICATIONS

Miceli, P. F.; Tarascon, J. M.; Bagley, B. G.; Greene L. H.; Barboux, P.; Hull, G. W.; Giroud, M.;

Rhyne, J. J.; Neumann, D. A., "Magnetic Properties of Some High Tc Superconducting

Compounds," in High Temperature Superconductors , edited by T. Akachi, J. A. Cogordan, and

A. A. Valladares, Prog, in High Temp. Superconductivity, Vol. 20, (World Scientific Publishing,

Singapore, 1989), p. 89.

Miceli, P. F.; Tarascon, J. M.; Greene, L. H.; Rhyne, J. J.; Neumann, D. A., "Magnetic Ordering In YBa2Cu3

.xMx 6+y," Physica C 162, 1267 (1989).

Mighell, A. D.; Himes, V. L., "A New Method for Phase Identification for Electron Microscopists,"

J. Electron Microscopy Technique. 16, 155 (1990).

Moon, R.M.; Fernandez-Baca, J. A.; Hewat A.; Jorgensen, J.D.; Kamitakahara, W.; Powell, B.M.;

Yelon, W.B., "Powder/Liquids Diffraction," in Proceedings of the Oak Ridge National Laboratory/

Brookhaven National Laboratory Workshop on Neutron Scattering Instrumentation at High-Flux

Reactors . Report CONF-89063 1 1 , (1990), p. 31-37.

Mozer, B.; Bendersky, L. A.; Boettinger, W. J.; Rowe, R. G.; "Neutron Powder Diffraction Study of

the Orthorhombic Ti2AlNb Phase," Scripta Metallurgia (to be published).

Neumann, D. A.; Nicol, J. M.; Rush, J. J.; Wada, N.; Fan, Y.; Kim, H.; Solin, S. A.; Pinnavaia, T.

J.; Trevino, S. F., "Neutron Scattering Study of Layered Silicates Pillared With Alkylammonium

Ions," in Neutron Scattering for Materials Science, edited by S. M. Shapiro, S. C. Moss, and

J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166 (Materials Research Society, Pittsburgh,

1990), p. 397.

Neumeier, J . J.; Bjornholm, T.; Maple, M. B.; Rhyne, J. J.; Gotaas, J. A., "Neutron Diffraction Study

of Pr Valence and Oxygen Ordering in the Y1

.xPrxBa2Cu3 7^1 System," Physica C 166, 191

(1990).

Nicol, J. M.; Udovic, T. J.; Rush, J. J.; Cox, S. D.; Stuckey, G. D., "Inelastic Neutron Scattering

Studies of Nonlinear Optical Materials: p- Nitroaniline Adsorbed in ALPO-5," in Neutron Scattering

for Materials Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166 (Materials Research Society, Pittsburgh, 1990), p. 367.

O'Connor, C. L., editor, "NIST Reactor: Summary of Activities July 1988 Through June 1989,"

National Institute of Standards and Technology Tech. Note 1272, Dec. 1989.

Passell, L.; Carpenter, J. M.; Copley, J. R. D.; Dorner, B.; Ito, Y., "Time-of-Flight," in Proceedings

of the Oak Ridge National Laboratory/ Brookhaven National Laboratory Workshop on Neutron

Scattering Instrumentation at High- Flux Reactors . Report CONF-89063 1 1 , (1990), p. 15-23.

Parise, J. B.; Torardi, C. C; Subramanian, M. A.; Gopalakrishnan, J.; Sleight, A. W.; Prince, E.,

"Superconducting Tl2 Ba2 CuO6+d : A High Resolution Neutron Powder and Single Crystal XRay Diffraction Investigation," Physics C 159, 239 (1989).

Prask, H. J. "The Reactor and Cold Neutron Facility at NIST," Neutron News I (1), 9 (1990).

Prask, H.; Choi, C. S., "Residual Stress Measurements by Means of Neutron Diffraction," in Neutron

Scattering for Materials Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater.

Res. Soc. Symp. Proc, Vol. 166 (Materials Research Society, Pittsburgh, 1990), p. 293.

169
PUBLICATIONS

Prince, E., "Mathematical Aspects of Rietveld Refinement," Proceedings of the International Workshop

on the Rietveld Method, Petten, The Netherland, Oxford University Press (in press).

Prince, E.; Evans, H. T., "The Crystal Structure of Ammonium Paradodecatunstat Tetrahydte by

Neutron Diffraction," Acta Crystallogr. C ( in press).

Prince, E.; Sjolin, L., "Maximum Entropy as a Phasing Tool in Macromolecular Crystallography,"

Chemometrics and Intelligent Laboratory Systems (in press).

Rhyne, J. J.; Erwin, R. W.; Borchers, J.; Salamon, M. B.; Du, R.; Flynn, C. P.; "Magentic

Multilayers," Phys. Scr. T29, 31 (1989).

Rhyne, J. J.; Majkrzak, C. F., "The NIST Cold Neutron Research Facility and Magnetic Neutron

Scattering," J. Appl. Phys. 67, 5693 (1990).

Rowe, J. M.; Rorer, D. C, "Existing Cold Sources at U. S. Reactors." Proceedings of International

Workshop on Cold Neutron Research Sources (in press).

Rush, J. J.; Flanagan, T. B.; Craft, A. P.; Clewley, J. D.; Sakamoto, Y., "The Optic Mode Frequencies

and Solubilities of Hydrogen in Ordered and Disordered Pd3 Mn," J. Phys.: Condens. Matter 1, 5095 (1989).

Russell, T.P.; Anastasiadis, S. H.; Satija, S. K.; Majkrzak, C. F., "Temperature Dependence of the

Morphology of Thin Diblock Copolymer Films as Revealed by Neutron Reflectivity," in Neutron

Scattering for Materials Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater.

Res. Soc. Symp. Proc, Vol. 166 (Materials Research Society, Pittsburgh, 1990), p. 145.

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, " Phy. Scr. (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).

Santoro, A., "Models for Oxygen Ordering and Diffusion in Ba2YCu3 Ox and Ba2YCu3

.xMxO [M=Fe,

Co, Al, Ga]," Proc. 7th CIMTEC Meeting on Superconductors (in press).

Santoro, A., "Defective Structures of Ba2YCu3 Ox and B^YG^ M Oz , (M = Fe, Co, Al, Ga,...),"

in Chemistry of Superconducting Materials , edited by T. A. Vanderah, Noyes publications (in

press).

Santoro, A.; Beech, F.; Cava, R. J., "Neutron Powder Diffraction Study of the Structures of

La: 9Cax !Cu2 6 and Laj 9Srj iCu2 6+w," in Neutron Scattering for Materials Science, edited by

S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc, Vol. 166

(Materials Research Society, Pittsburgh, 1990), p. 187.

Santoro, A., "Crystallography" in High Temperature Superconductivity , edited by J. W. Lynn,

(Springer-Verlag, New York, 1990), p. 84.

170
PUBLICATIONS

Satija, S. K.; Majkrzak, C. F.; Anastasiadis, S. H.; Russell, T. P., "The Morphology of Symmetric

Dieblock Copolymers as Revealed by Neutron Reflectivity" in Neutron Scattering for Materials

Science, edited by S. M. Shapiro, S. C. Moss, and J. D. Jorgensen, Mater. Res. Soc. Symp. Proc,

Vol. 166 (Materials Research Society, Pittsburgh, 1990), p. 139.

Satija, S. K.; Majkrzak, C. F.; Russell, T. P.; Sinha, S. K.; Sirota, E. B.; Hughes, C. J., "Neutron

Reflectivity Study of Block Copolymers Adsorbed From Solution," Macromol. 23, 3860 (1990).

Schupp, G.; Hammouda, B.; Hsueh, C. M., "QEGS From Pentadecane," Phys. Rev. A 41, 5610 (1990).

Sjolin, L.; Svensson, L.; Prince, E.; Sundell, S., "Phase Inprovement in the Structure Interpretation of

Fragment TR2C From Bull Testis Calmodulin Using Combined Entropy Maximization and Solvent

Flattening," Acta Crystallogr. B 46, 209 (1990).

Sjolin, L.; Prince, E.; Svensson, L. A.; Gilliland, G. L., "Maximum Entropy Phase Determination for X-Ray Diffraction Data From Crystals of Native Proteins," Science (in press).

Skanthakumar, S.; Li, W. H.; Lynn, J. W.; Kebede, A.; Crow, J. E.; Mihalisin T., "Magnetic

Properties of Pr in PrBa^^Oy Physica B 163, 239 (1990).

Skanthankumar, S; Lynn, J. W., "On the Nature of the Magentic Order and Structure of Nd2Cu04

," Physica C (to be published).

Skanthakumar, S.; Zhang, H.; Clinton, T. W.; Li, W. H.; Lynn, J. W.; Fisk, Z.; Cheong, S-W.,

"Magnetic Phase Transitions and Structural Distortion in Nd2Cu04

," Physica C 160, 124 (1989).

Skanthakumar, S.; Zhang, H.; Clinton, T. W.; Sumarlin, I. W.; Li, W. H.; Lynn, J. W.; Fisk, Z.; Cheong, S. W., "Magnetic Phase Transitions in Nd2Cu04

," J. Appl. Phys. 67 4530 (1990).

Slade, R. C. T.; Ramanan, A.; West, B. C; Prince, E., "The Structure of V9 Mo6O40 Determined by

Powder Neutron Diffraction," J. of Solid State Chem. 82, 65 (1989).

Stalick, J. K.; Wong-Ng, W., "Neutron Diffraction Study of the "Brown Phase" BaNd2Cu05

," Mater.

Lett. 9, 401 (1990).

Straty, G. C; Hartley, H. J. M.; Glinka, C. J., "Shearing Apparatus for Neutron Scattering Studies on

Fluids: Preliminary Results for Colloidal Suspensions," J. Stat. Phys. (in press).

Trevino, S. F., "Inelastic Neutron Scattering in Molecular Crystals" in Phonons 89 . edited by S. Hunklinger, W. Ludwig, and G. Weiss, (World Scientific, New Jersey, 1990), p. 7.

Tsai, D. H.; Trevino, S. F., "Investigation of Energy Transport in the Detonation Wave and Energy

Sharing in Rapid Chemical Reactions," in Chemistry and Physics of Energetic Materials , edited by

Surya N. Bulusu, Mathematica and Physical Sciences, Vol. 309 (1989), p. 228.

Udovic, T. J.; Rush, J. J.; Anderson, I. S.; Barnes, R. G., "Hydrogen Vibrational Modes and

Anisotropic Potential in a-ScHx

," Phys. Rev. B 41, 3460 (1990).

Vergnat, M.; Houssaini, S.; DuFour, C; Brunson, A.; Marchal, G.; Mangin, Ph.; Erwin, R. W.;

Rhyne, J. J.; Vettier, C, "Evidence of Hydrogen Modulation in Si/Si:H Amorphous Multilayers,"

Phys. Rev. B 40, 1418 (1989).

171
PUBLICATIONS

Vergnat, M.; DuFour, C; Brunson, A.; Houssaini, S.; Marchal, G.; Mangin, P.; Rhyne, J. J.; Erwin,

R. W.; Vettier, C, "Multilayers and Low Angle Neutron Scattering," J. Phys. (Paris) 50, C7-202

(1989).

Wada, N.; Kamitakahara, W. A., "Inelastic Neutron and Raman Scattering Studies of Muscovite and

Vermiculite Layered Silicates," Phys. Rev. B (in press).

Werner, S. A.; Overhauser, A. W.; Giebultowicz, T. M., "Neutron Diffraction Structure in Potassium

Near the [Oil] and [022] Bragg Points," Phys. Rev. B 41, 12535 (1990)

Yang, K. N.; Ferreira, J. M.; Lee, B. W.; Maple, M. B.; Li, W. H.; Lynn, J. W.; Erwin, R. W.,

"Antiferromagnetic Ordering in Superconducting and Oxygen-Deficient Non-Superconducting

RBa2Cu3 7.d Compounds (R = Nd and Sm)," Phys. Rev. B 40, 10963 (1989).

Yethiraj, M.; Robinson, R. A.; Rhyne, J. J.; Gotaas, J. A.; Buschow, K. H. J., "Magnetic and

Crystallographic Properties of UNiSn," J. Magn. Magn. Mater. 79, 355 (1989).

Yoo, J. N.; Sperling, L. H.; Glinka, C. J.; Klein A., "Characterization of Film Formation From

Polystyrene Latex Particles Via SANS," Macromol. 23, 3962 (1990).

Yu, S. C; Lynn, J. W., "X-Ray Diffraction Studies of Ni-Cr-Based Amorphous Alloys," Jpn. J. Appl.

Phys. 29, 902 (1990).

Zabel, H.; Magerl, A.; Rush, J. J.; Misenheimer, M. E., "Diffusion and Melting in Two Dimensions:

A Quasi-Elastic Neutron Scattering Study of Alkali-Metals in Graphite," Phys. Rev. B 40, 7616

(1989).

Zhang, H.; Lynn, J. W.; Li, W. H.; Clinton, T. W.; Morris, D. E., "Two- and Three-Dimensional

Magnetic Order of the Rare-Earth Ions in RBa^^Og," Phys. Rev. B 4J_, 11229 (1990).

INDEPENDENT PROGRAMS

Abderrahim H. A., McGarry, E. D., Spiegel, V., "Assessment of the Fast Neutron Sensitivity of

Thermoluminescent Gamma Dosimeters," 7th ASTM/Euratom Symposium on Reactor Dosimetry,

Strasbourg, France, 27-31 August, 1990.

Bartko, J., Ruddy, F. H., Schoch, K. F., Congedo, T. V., McGarry, E. D., "A Novel On-Line Neutron

Dosimeter Based on a Conducting Polymer, " 7th ASTM/Euratom Symposium on Reactor Dosimetry,

Strasbourg, France, 27-30 August, 1990.

Becker, D. A., "INAA Homogeneity Measurements on the New NIST Leaf Reference Materials,"

National Institute of Standards and Technology Tech. Note 1272, U.S. Dept. of Commerce,

Washington, D.C. pp. 238 (1989).

Becker, P. R., Wise, S. A., Zeisler, R., Alaskan Marine Mammal Tissue Archival Project: "Acquisition

and Curation of Alaskan Marine Mammal Tissues for Determining Levels of Contaminants

Associated With Offshore Oil and Gas Development," National Atmospheric and Oceanic

Administration, Oceans Assessment Division, Report (1989), p. 98.

172
PUBLICATIONS

Becker, P.R., Koster, B.J., Wise, S.A., and Zeisler, R., "Alaskan Marine Mammal Tissue Archival

Project," Biol. Trace Elem. Res. 26, 329 (1990).

Becker, D. A., "Standards and Reference Materials for Mercury Analysis by NAA," Trans. Am. Nucl.

Soc. 60, 48 (1989).

Blackman, M.J., Vidale, M., "Defining the Inter-Site Variability of Stoneware Bangles at Mohenjo-Daro and Harappa by Neutron Activation Analysis," Proceedings of the South Asian

Archaeological Conference, Paris (in press).

Bode, P., Hoffmann, E. L., Lindstrom, R. M., Parry, S. J., Rosenberg, R. J., Practical Aspects of

Operating a Neutron Activation Analysis Laboratory (TECDOC-564), Internat. Atomic Energy

Agency, Vienna (in press).

Byrne, J., Dawber, P. G., SpainJ. A., Williams, A. P., Dewey, M. S., Gilliam, D. M., Greene, G. L.,

Lamaze, G. L., Scott, R. D., Pauwels, J., Eykens, R., Lamberty, A., "Measurement of the Neutron

Lifetime by Counting Trapped Protons," Phys. Rev. Lett. 65, 289 (1990).

Byrne, J., Dawber, P. G., Spain, J. A., Dewey, M. S., Gilliam, D. M., Greene, G. L., Lamaze, G. L.,

Williams, A. P., Pauwels, J., Eykens, R., VanGestel, J., Lamberty, A., Scott, R. D.,

"Determination of the Neutron Lifetime by Counting Trapped Protons," Nucl. Instrum. & Meth. A

284. 116(1989).

Carew, J. F., Todosow, M., McGarry, E. D., Grundl, J. A., Kam, F.B.K., Maerker, R. E., Stallmann,

F. W., "Status of a New Regulatory Guide on Methods and Assumptions for Determining Pressure

Vessel Fluence," 7th ASTM/Euratom Symposium, and presented oral paper on the status of the new

draft regulatory guide.

Caroli, S., Iyengar, G. V., Muntau, H., (Eds). Bioelements: Health Aspects, Annali dellTstituto

Superiore di Sanita, 25, (1989).

Cohen, A., White, E., Coxon, V. B., Christensen, R. G., Welsh, M. J., Paule, R. C, Becker, D. A.,

"Certification of Bilirubin SRM 916a," Fres. Z. Anal. Chem. (in press).

Copley, J.R.D., Stone, C.A., "Neutron Scattering and Its Effect on Reaction Rates in Neutron

Absorption Measurements," Nucl. Instr. Meth., A 281, 593 (1989).

Eisenhauer, C. M., "Study of Slab Transmission and Reflection," publication in Nuclear Science and

Engineering .

Eisenhauer, C. M., Review of Scattering Corrections for Calibration of Neutron Instruments, Radiation

Protection Dosimetry 4, 253, (1989).

Eykens, R., Lamberty, A., "Measurements of the Neutron Lifetime by Counting Trapped Protons,"

Phys. Rev. Lett. 65, 282 (1990).

Gilliam, D. M., Greene, G. L., Lamaze, G. P., "Absolute Neutron Counting Basedon B-10 AlphaGamma Coincidence Methods," Nucl. Instr. and Meth. in Phvsics Research . A 284, 220 (1989).

173
PUBLICATIONS

Greenberg, R. R., "Recent Advances in Radiochemical Neutron Activation Analysis," Trans. Am. Nucl.

Soc. 60, 9 (1989).

Greenberg, R.R., Kingston, H.M., Zeisler, R., Woittiez, J.R.W., "Neutron Activation Analysis of

Biological Samples With a Pre-Irradiation Separation," Biol. Trace Elem. Res. 26, 17 (1989).

Greenberg, R. R., "Determination of Mercury in Biological Materials by Radiochemical Neutron

Activation Analysis," Trans. Am. Nucl. Soc. 60, 47 (1989).

Greenberg, R. R., Kingston, H. M., Watters, R. L., Jr., Pratt, K. W., "Dissolution Problems With

Botanical Reference Materials," Fresenius Z. Anal. Chem. (in press).

Greenberg, R. R., "Analysis of Atmospheric Particulate Samples Via Instrumental Neutron Activation

Analysis,

" Monitoring Methods for Toxics in the Atmosphere , in Zielinski and Dorko, edited by,

ASTM STP 1052, 175 (1990).

Iyengar, G. V., "Multidisciplinary Approaches in Biological Trace Element Research," ACS Symposium

Series, No. 445, (in press).

Iyengar, G. V., "Milestones in Biological Trace Element Research," Sci. Total Environ, (in press).

Iyengar, G. V., Kumpulainen, J., Okamoto, K., Morita, M., Hirai, S., and Nomoto, S., "Recent Trends

in Analytical Approaches for Trace Element Determinations in Biomedical Investigations," in Essential and Toxic Trace Elements in Health and Disease, A. S. Prasad, edited by, Alan R. Liss,

New York, 1990 (in press).

Iyengar, G. V., "Analytical Methodology, Chapter 3 in FAOAVHO/IAEA Report on Trace Elements in Human Nutrition, Geneva/Rome, (in press).

King, S. Q., Gilliam, D. M., Comparison of the Calculated to Measured Values of the Integrated

Gamma Dose in the Cavity of a Full-Scale Operating Power Reactor, ASTM-STP (in press).

Lindstrom, R. M., "Analytical Standards and Silicon Analysis," in Analytical Chemistry in Semiconductor

Manufacturing: Techniques, Role of Nuclear Methods and Need for Quality Control"

(IAEA-TECDOC-512), Internat. Atomic Energy Agency, Vienna, (1989), p. 35.

Lindstrom, R. M., Greenberg, R. R., "Quality Control," in Bode, P., Hoffmann, E. L., Lindstrom,

R. M., Parry, S. J., and Rosenberg, R. J., Practical Aspects of Operating a Neutron Activation

Analysis Laboratory (TECDOC-564), Internat. Atomic Energy Agency, Vienna (in press).

Lindstrom, R. M., Lutz, G. J., Norman, B. R., "High-Sensitivity Determination of Iodine Isotopic

Ratios by Thermal and Fast Neutron Activation," J. Trace Microprobe Tech. 8 (in press).

Lindstrom, R. M., Byrne, A. R., Becker, D. A., Smodis, B., Garrity, K. M., "Characterization of

the Mineral Fraction in Botanical Reference Materials and Its Influence on Homogeneity and

Analytical Results," Fresenius Z. Anal. Chem. (in press).

Lindstrom, D. J., Zolensky, M. E., Lindstrom, R. M., "Procedures for Instrumental Neutron

Activation Analysis of Individual Cosmic Dust Particles," Lunar and Planetary Science XX, Lunar

and Planet. Inst., Houston (1989), p. 574.

174
PUBLICATIONS

Lindstrom, R. M., "Neutron Beams in Analytical Chemistry," Trans. Am. Nucl. Soc. 61, 132 (1990).

Lindstrom, R. M., Lindstrom, D. J., Slaback, L. A., Langland, J. K., "A Low-Background Gamma

Ray Assay Laboratory for Activation Analysis," Nucl. Inst. Methods, (in press).

Mackey, E. A., Anderson, D. L., Gordon, G. E., Lindstrom, R. M., "Effects of Neutron Scattering

by Hydrogen on Prompt Gamma Activation Analysis," Trans. Am. Nucl. Soc. 60, 14 (1989).

McGarry, E. D., Grundl, J. A., Carew, J. R., Todosow, M. Kam, F. B. K., Stallmann, F. W., Status of New Regulatory Guide on Methods and Assumptions for Determining Pressure Vessel Fluence,

7th ASTM/Euratom Symposium on Reactor Dosimetry, Strasbourg, France, 27-30 August, 1990.

McGarry, E. D., Grundl, J. A., Carew, J. R., M. Todosow, Kam, F.B.K., Stallmann, Becker, D. A.,

"Homogeneity and Evaluation of the New NIST Leaf SRMs," J. Micro. Tech. (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, (in press).

Mildner, D. F. R., "Comments on 'Design Optimization of a Small-Angle Neutron Scattering

Spectrometer," Nucl. Instrum. & Meth. A 290, 259 (1990).

Mildner, D. F. R., "The Neutron Microguide as a Probe for Materials Analysis," Nucl. Instrum. &

Meth. (in press).

Mildner, D. F. R., "Neutron Focusing Using Microguides," Nucl. Instrum. & Meth. (in press).

Mildner, D. F. R., "Multiple Reflections Within Neutron Optical Devices," Nucl. Instrum. & Meth. A

292 . 693 (1990).

Mildner, D. F. R., "Acceptance Diagrams for Curved Neutron Guides," Nucl. Instrum. & Meth. A 290 , 189 (1990).

Myers, J. E., Boone, J. L., Redman, C. L., "Archaeological and Historical Approaches to Complex

Societies: The Islamic States of Medieval Morocco.American Anthropologist , 92 (1) 47-63 (1990).

Myers, J. E., "Book review of The Collapse of Complex Societies , by Joseph Tainter and The Collapse

of Ancient States and Civilizations by Norman Yoffee and George L. Cowgill," American

Anthropologist. 91 (4) 1065-66 (1989).

Parr, R. M., DeMaeyer, E. M., Iyengar, G. V. et al. "Minor and Trace Elements in Human Milk From

Guatemala, Hungary, Nigeria, Philippines, Sweden and Zaire: Results from a WHO/IAEA Joint

Project," Biol. Trace Elem. Res. (in press).

175
PUBLICATIONS

Pauwels, J., Eykens, R., Lamberty, A., Gestel, J. Van, Tagziria, H., Scott, R. D., Byrne, J., Dawber,

P. G., Gilliam, D. ML, "The Preparation and Characterization of 6Li and 10B Reference Deposits

for the Measurement of the Neutron Lifetime," Nucl. Inst, and Meth. in Physics Research, (in

press).

Rossbach, M.,Zeisler, R.,Woittiez, J.R.W., "The Use of Compton Suppression Spectrometers for Trace

Element Studies in Biological Materials," Biol. Trace Elem. Res. 26, 63 (1990).

Rothman, M.J., Blackman, M. J., "Instrumental Neutron Activation Analysis of Sealing Clays From

Tepe Gawra, Nineveh, Arpachiyah, and Tell Brak, Northern Mesopotamia," MASCA Journal, (in

press).

Ruddy, F. H., McGarry, E. D., Benchmark Referencing of Solid State Track Recorder Neutron

Dosimeters in Standard Neutron Fields, 7th ASTM/Euratom Symposium on Reactor Dosimetry,

Strasbourg, France, 27-30 August, 1990.

Schantz, M. M., Benner, B. A., Chesler, S. N., Koster, B. J., Hehn, K., E., Stone, S. F., Kelly, W.

R., Zeisler, R., Wise, S. A., "Preparation and Analysis of a Marine Sediment Reference Material

for the Determination of Trace Organic Constituents," Fresenius Z. Anal. Chem. (in press).

Scholfield, D. J., Iyengar, G. V. and Reiser, S., "Long Term Stability of Mitochondrial Superoxide

Dismutase activity in USDIETS I-V," Fres. J. Anal. Chem. (in press).

Schwartz, R. B., Hunt, J. B., "Measurement of the Energy Response of Superheated Drop Neutron

Detectors," in Radiation Protection Dosimetry, (in press).

Stone, S. F., Zeisler, R., Gordon, G. E., "Application of Polyaery1 amide Gel Electrophoresis/Neutron

Activation Analysis for Protein Determination," Biol. Trace Elem. Res., 26-27 . 85 (1990).

Stone, S. F., Zeisler, R., Gordon, G.E., Viscidi, R.P., Cerny, E.H., "Neutron Activation Analysis

Combined With Biological Isolation Techniques for Protein Determination, " ACS Symposium Series,

No. 445, Biological Trace Element Research: Multidisciplinary Perspectives, (in press).

Stone, S. F., Koster, B. J., Zeisler, R., "Instrumental Neutron Activation Analysis of SRM 1941

Organics in Marine Sediment, Element Content and Homogeneity," Biol. Trace Elem. Res., 26-27 . 579 (1990).

Stone, S. F., "Quantitative Analysis With Autoradiography" Trans. Am. Nucl. Soc. (in press).

Stone, C. A., "Neutron Scattering and Its Effect on Reaction Rates in Neutron Absorption Experiments,"

Trans. Am. Nucl. Soc. 60, (1989).

Tagziria, H., Pauwels, J., Verdonk, J., Gestel, J. Van, Eykens, R., Gilliam, D. M., Scott, R. D.,

Byrne, J., Dawber,P. G., "Problems Related to the Determination of Mass Densities of Evaporated

Reference Deposits, Nucl. Instr. and Meth. in Physics Research , (in press).

Tanner, J. T., Iyengar, G. V., Wolf, W. R., "Organic Nutrient Content of U.S. Food and Drug

Administration's Total Diet and Its Possible Use as a Standard Reference Material," Fres. J. Anal.

Chem. (in press).

176
PUBLICATIONS

Williamson, T. G., Lamaze, G. L., "Measurement of the 93 Nb(n,2n)92Nb Cross Section in a

235 U

Fission Spectrum," Proceedings of the 7th ASTM-Euratom Symposium on Reactor Dosimetry

(Strasbourg, France, Aug. 26-31, 1990) (in press).

Wise, S. A., Koster, B. J., Parris, R. M., Schantz, M. M., Stone, S. F., Zeisler, R., "Experiences in Environmental Specimen Banking," Intern. J. Environ. Anal. Chem. 37, 91 (1989).

Wolf, W. R., Iyengar, G. V., Tanner, J. T., "Mixed Diet Reference Materials for Nutrient Analysis

of Foods: Preparation of SRM 1548 Total Diet," Fres. J. Anal. Chem. (in press).

Zeisler, R., "Recent Advances in Instrumental Neutron Activation Analysis in the Life Sciences," Trans.

Am. Nucl. Soc. 60, 5 (1989).

Zoetelief, J., Eisenhauer, C. M., and Coyne, J. J., "Calculations on Displacement Corrections for

In-Phantom Measurements with Ionization Chambers for Mammography," Phys. Med. Biol, (in

press).

Zolensky, M. E., Lindstrom, D. J., Thomas, K. L., Lindstrom, R. M., Lindstrom, M. M., "Trace

Element Compositions of Six 'Chondritic' Stratospheric Dust Particles," Lunar and Planetary Science

XX, Lunar and Planet. Inst., Houston (1989), p. 1255.

177

NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NIST/TN 1285

2. Performing Organ. Report NoJ 3. Publication Date

December 1990

4. TITLE AND SUBTITLE

NIST Reactor: Summary of Activities July 1989 Through June 1990

5. AUTHOR(S)

C. O'Connor, editor

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

QAITHERSBURG, MD 20899

7. Contract/Grant No.

8 , Type of Report & Period Covered

Tech Note, 7/89 - 6/90

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Same as item #6 above.

10. SUPPLEMENTARY NOTES

~J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

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

the period for July 1989 through June 1990. 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 (Six to twelve entries; alphabetical order; capitalize 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. AVAILABILITY

PH Unlimited

f | For Official Distribution. Do Not Release to NTIS

fxl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

186

15. Price

•fr U.S. GOVERNMENT PRINTING OFFICE: 1991 - 281-557 '20766 USCOMM-DC S043-P8C



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