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NIST Q^^f^fTf^i, Neutron Research

Accomplishments and Opportunities

QC

100

U57

no.977

2002

Nisr

National Institute of

Standards and Technology

Technology Administration

U.S. Department of Commerce

March 2002
On the Cover

The two photos dramatize the development of the NIST Center for Neutron Research

over a 12-year period. The lower photo shows the Cold Neutron Guide Hall before

installation of equipment. The upper picture, taken from approximately the same

vantage point, is packed with 7 guide tubes serving 13 instrument stations. See

pages 2-3 for a layout and brief description of the instruments. Pages 4-5 provide

a selected pictorial history of the Center. (Top photo by L. A. Shuman.)
NCNR 2001

NIST

Center for

Neutron

Research

Accomplishments

and

Opportunities

NIST Special Publication 977

J. Michael Rowe, Director

Ronald L. Cappelletti, Editor Linda K. Clutter, Assistant Editor

March 2002

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
DISCLAIMER

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by

the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 977

Natl. Inst. Stand. Technol. Spec. Publ. 977, 76 pages (March 2002) C0DEN;NSPUE2

U.S. GOVERNMENT PRINTING OFFICE - WASHINGTON: 2001

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOR Washington, DC 20402-0001
Contents

Foreword iv

Introduction 1

NIST Center for Neutron Research Layout 2

Chronology of the NCNR in Photographs 4

Research Highlights Measurement of the Free Neutron Lifetime 6

ETS-4: IVlaxweli's Zeolite 8 Measuring Stress Relief in Electron Beam Weld Joints of Superalloys 10 EXPGUI: Smoothing the Path to Powder Crystallography 12

Neutron Diffraction Contributes to Improving the Fatigue Life of Bridges 14

Capillary Neutron Lens for Prompt Gamma Activation Micro-Analysis 16

Giant Anharmonicity and Electron-Phonon Mediated Superconductivity in MgB^at 39 K 1

8

Polaron Formation and Colossal Magnetoresistance in Manganites 20

Ferromagnetism and Spontaneous Vortex Formation in Superconducting ErNi^B^C 22

Structure of Local Spin Excitations in a Geometrically Frustrated Antiferromagnet 24

Pressure-Induced Phase Transition of C^^Ej Micelles 26

Probing the Structure of Aerosol Nanodroplets 28 Nanoporous Ultra-Low Dielectric Constant Materials 30

Neutron Reflectivity Studies of Surfactants at Electrode Surfaces 32

Magnetic Semiconductor Superlattices 34

Pinpointing Chiral Structures With Front/Back Polarized Neutron Reflectometry 36 The Nature of Vibrational Softening in a-Uranium 38

Native and Partially Unfolded Proteins: Neutron Inelastic Scattering and Simulations 40

Atomic Motions in Confined Polymer Films 42

Tuning the Properties of Carbon Nanotubes by Deformation 44

Serving the Scientific and Technological Communities 46 The NCNR User Program 46 The Center for High Resolution Neutron Scattering 46 Summer School on Methods and Applications of Neutron Spectroscopy 47

Collaborations 47

Independent Programs 48

Operations 50

Instrumentation Developments 51 The Advanced Cold Neutron Source 51

Development of an Air-Pad and Floor System for New Neutron Spectrometers 52 Development of an Intercalated Graphite Monochromator 52 NCNR Computer and Software Environment 53 Sample Environment Team 55

Publications 56

Instruments and Contacts 72

Contacts Inside Back Cover

NIST Center for Neutron Research iii
Foreword

La,^.-:^, .. . ,. . ..... .. - - . .. : . _ ASthis is being written, we have just begun a three- month shutdown during which we will, among other

changes, install a next-generation liquid hydrogen cold

source, which will increase cold neutron intensities by

almost a factor of two. This is an excellent example of the necessary operating philosophy for any facility - improvement and change are the only constants. During the past

year, we have had many changes in reactor systems, in instrumentation, in staff, and in science, all of which are aimed at continuing the development of the NIST Center for Neutron Research as a first rate neutron facility.

A new cooling tower, which is specially designed to reduce the visible vapor plume during winter months, as well

as to provide full cooling ability for the next 20 years, has

been constructed. It will be connected into the reactor secondary systems during the shutdown that has just started. A new analysis of major reactor systems has been initiated using the most modem tools as part of preparing the application for a license extension through 2024. Many other

improvements, ranging from instrumentation renewal to

better provision for maintenance to improved electrical

distribution, are under way as part of the preparation for relicensing. In spite of the shutdown that just started, the

reactor has once again performed admirably, operating for 240 days during the fiscal year, meeting our scheduled

operating time.

The three new inelastic scattering instruments, which

were just coming on line last year, are now being fully

incorporated into the user program, with encouraging user demand. The joint National Science Foundation (NSF)/NIST

Center for High Resolution Neutron Scattering is being

expanded to include operation of these instruments in addition to the prior SANS and triple axis instruments. The

thermal neutron upgrade program, which includes two new

triple axis spectrometers, an improved filter analyzer

spectrometer, and other instruments, is advancing well, with

characterization of monochromator crystals now complete,

and contracts awarded for the manufacture of monochromator drums and focusing devices. A new cold neutron triple axis spectrometer is currently entering detailed design phase

as a joint construction project with Johns Hopkins University (partially funded by the NSF). In order to create more

research space in the guide hall, a new building for our

technical support activities is nearing completion. In re- sponse to many recommendations, we have significantly

strengthened our sample environment support and our data

acquisition and analysis computing effort, adding additional

staff and refocusing efforts. The number of research participants continues to grow, surpassing all of our expectations.

Overall, we are moving forward on many fronts to strengthen our research facility operations efforts, in order to

better serve our customers. And, as always, the scientific results are the true measure of success, and the highlights that form the largest

portion of this report comprise an excellent set of examples

of the exciting outputs of the facility. I hope that you enjoy

reading them as much as I did.

iv

i,'
Introduction to the NIST Center for

Neutron Research (NCNR)

Modem technological society is dependent upon

increasingly sophisticated use of materials with

attributes dictated by their sub-microscopic structural and

dynamical properties. A wide range of techniques provides

knowledge of these properties. Scattering probes (for

example: x-rays, light, electrons, neutrons) are arguably the most important. Of these probes, neutrons are perhaps least familiar, but they provide important advantages for many

types of measurements.

Modem sources provide neutrons moving at speeds

comparable to those of atoms at room temperature, thus providing the ability to probe dynamical behavior. Neutrons

are also well matched to probe lengths ranging from the distances between atoms to the size of biological or polymer

macromolecules. Neutrons are sensitive to the magnetic

properties of atoms and molecules, allowing study of the underlying magnetic properties of materials. They also

scatter differently from normal hydrogen atoms than they do

from heavy hydrogen (deuterium), allowing selective study

of individual regions of molecular systems. Finally, neutrons

interact weakly with materials, providing the opportunity to study samples in different environments more easily (at high

pressures, in shear, in reaction vessels, etc.), and making

them a non-destructive probe. These favorable properties are

offset by the weakness of the best neutron sources compared

to x-ray or electron sources, and by the large facilities required to produce neutrons. As a result, major neutron

sources are operated as national user facilities to which

researchers come from all over the United States and abroad

to perform small-scale science using the special measure- ment capabilities provided.

In addition to scattering, neutrons can be captured by

nuclei to probe the atomic composition of materials. The

subsequent characteristic radioactive decays provide "fingerprints" for many atomic nuclei, allowing studies of environmental samples for pollutants (e.g., heavy metals), characterization of Standard Reference Materials, and other measurements. There are important areas in physics that can be

explored by measuring fundamental neutron behavior.

Examples include the lifetime of the free neutron, important

for the theory of astrophysics; the neutron beta decay

process, that provides a stringent test of nuclear theory; and

the effects of various external influences such as gravity or magnetic fields on neutrons. The NCNR's 20-MW NIST Research Reactor provides

a national user community with facilities, including the

nation's only internationally competitive cold neutron

facility, for all of the above types of measurements. There

are about 35 stations in the reactor and its associated beams

that can provide neutrons for experiments. At the present

time 28 of these are in active use, of which 6 provide high

neutron flux positions in the reactor for irradiation, and 22

are beam facilities. The following pages show a schematic

layout of the facility. More complete descriptions of instru- ments can be found at http://www.ncnr.nist.gov.

These facilities are operated both to serve NIST

mission needs and as a national facility, with many different modes of access. Some instrumentation was built years ago, and is not suited to general user access; however, time is available for collaborative research. NIST has recently built new instrumentation, and reserves 1/3 of available time for mission needs with the balance available to general users. In other cases, instrumentation was built and is operated by

Participating Research Teams (PRT); PRT members have

access to 75 % of available time, with the balance available

to general users. Additionally, NIST and the National

Science Foundation operate the Center for High Resolution

Neutron Scattering at the NCNR, with one thermal and five cold neutron instruments. For these facilities, most time is available for general users. While most access is for re- search, with results that are freely available to the general

public, proprietary research can be performed under full cost recovery. Each year, about 1600 researchers (persons who

participated in experiments at the facility, but did not

necessarily come here) from all areas of the country, from

industry, academia, and govemment use the facility for measurements not otherwise possible. The research covers a broad spectrum of disciplines, including chemistry, physics,

biology, materials science, and engineering.

NIST Center for Neutron Research 1
NIST Center for Neutron Research Layout

11

!

i 10 NG-7 Horizontal Sample i Reflectometer allows { reflectivity measurements of !

free surfaces, liquid vapor

) interfaces, as well as polymer ! coatings. }

11 Neutron Interferometry and ! Optics Station with perfect

| silicon interferometer; vibration

| isolation system provides '

exceptional phase stability and jj fringe visibility. I!

12 Spin Polarized Triple Axis j Spectrometer (SPINS) using

cold neutrons with position

j sensitive detector capability for

'

high resolution studies — part of CHRNS.

'

A Cold Neutron Depth 3

Profiling instrument (not shown) for quantitative profiling

of subsurface impurities currently at this site will be

^ moved to another position. Shown is a proposed Triple Axis Cold Neutron Crystal

Spectrometer with double

focusing monochromator and ^

multiple crystal analyzer/

detectors that can be flexibly configured for several energies

simultaneously or for high

throughput at one energy. 6 BT-7 Triple Axis Spectrometer

with fixed incident energy for measurements of excitations and structure.

BT-8 Residual Stress

Diffractometer optimized for depth profiling of residual stress

in large components.

BT-9 Triple Axis Crystal

Spectrometer for measure- ments of excitations and

structure.

Thermal Column A very well- thermalized beam of neutrons used for radiography, tomography, dosimetry and other experiments.

BT-1 Powder Diffractometer

with 32 detectors; incident wavelengths of 0.208 nm,

0.154 nm, and 0.159 nm, with highest resolution of 5d/d = 8x10".

7 BT-2 Triple Axis Crystal

Spectrometer with polarized

beam capability for measure- ment of magnetic dynamics and

structure.

8 BT-4 Filter Analyzer Neutron Spectrometer with cooled Be/

Graphite filter analyzer for chemical spectroscopy.

9 BT-5 Perfect Crystal Diffractometer SANS small angle neutron scattering instrument for microstructure on

the 10" nm length scale, sponsored by the National Science Foundation and NIST,

part of the Center for High

Resolution Neutron Scattering

(CHRNS).

2 Layout 2001
13 Spin Echo Spectrometer

offering neV energy resolution, based upon Julich design,

sponsored by NISI Julich and

ExxonMobil - part of CHRNS.

14 Prompt Gamma Activation Analysis cold neutron fluxes allow detection limit for H of

1 |ig to 10 |ag. Focused beams

are available for profiling.

15 NG-7 30 m SANS for microstructure measurements

sponsored by NIST, ExxonMobil, and the University

of Minnesota.

16 Neutron Pliysics Station a cold neutron beam

150 mm X 60 mm, available for fundamental neutron physics

experiments.

17 Fermi Chiopper TOF Spectrometer a hybrid time-of-flight spectrometer for inelastic scattering with incident wavelengths between 0.23 nm

and 0.61 nm chosen by focusing

pyrolytic graphite crystals. A

simple Fermi chopper pulses

the beam.

18 Disl< Cliopper TOF Spectrometer versatile time-of-flight spectrometer, with beam

pulsing and monochromatization effected by 7 disk choppers. Used for studies of dynamics in condensed matter, including macromolecular systems — part of CHRNS.

19 NG-3 30 m SANS for microstructure measurements

sponsored by the National Science Foundation and NIST - part of CHRNS.

20 Bacl^scattering Spectrometer:

high intensity inelastic scattering instrument with energy

resolution < 1 |aeV, for studies

of motion in molecular and

biological systems — part of CHRNS.

21 8 m SANS for polymer

characterization, sponsored by NIST Polymers Division.

22 Vertical Sample Reflectometer

instrument with polarization analysis capability for measuring reflectivities down to 10"Mo

determine subsurface structure.

23 Vertical Sample Reflectometer

instrument with polarization analysis capability for measuring reflectivities down to 10"^ optimized for biological applications. It will have a

position-sensitive detector for measuring off-specular

reflections.

NiST Center for Neutron Research 3
Chronology of the NCNR in Photographs

1966

Fred Shorten and Chuck Ware at beam tube opening (BT-4) during construction in 1966.

Awaiting criticality, December 7, 1967. Chianti not yet uncorked.

Construction of the Guide Hail of the Cold Neutron Facility begins in 1988. Ray Kammer, Tawfik Raby, Bob Carter, Ernie Ambler, Mike Rowe

(current NCNR director), and Jack Rush (current science group

leader) in the control room as the source goes to 20 MW in 1984.

|

George Baltic and Mike Rinehart in the Guide Hall inserting a cold neutron guide tube, ca. 1990.

John Barker makes adjustments as Susan Krueger and Min Lin get set to record results at the NG-7

30 m SANS, 1991.

4 Chronology of the NCNR in Photographs
Chang-Sun Choi, Sam Treviho and Henry Prask

at a 2-axis diffractometer sited at BT-6, 1973.

Al Cinquepalma at an early version of the

diffractometer at BT-1, 1971.

The Guide Hall begins

to fill up, 1994: NG-7 30 m SANS at left, NG-6 Fermi-chopper

time-of-flight spectrometer, center,

8 m SANS (orange) behind it, and NG-3

30 m SANS at right. Also visible is the chamber

housing the neutron

interferometer at far left, back. See front cover for an early and recent view.

NIST Center for Neutron Research 5
Measurement of the Free Neutron Lifetime

a-Detector

Precision Aperture

B=:5Tesla

~1 l<V "Mirror"

Electrode

"Center"

Electrode

"Gate"

Electrode

FIGURE 1. Schematic drawing of the NIST Penning trap neutron lifetime experiment.

Free neutrons are unstable and decay with a lifetime of

about 15 minutes into a proton, an electron, and an

antineutrino. Accurate determination of this natural lifetime

is important for tests of the Standard Model of Fundamental

Particles and Interactions, as well for understanding certain aspects of cosmology and astrophysics. The neutron lifetime

influences the predictions of the theory of Big Bang nucleosynthesis for the primordial helium abundance in the universe and the number of different types of light neutrinos.

Since the dominant uncertainty for the primordial helium

abundance comes from the accuracy of the neutron decay

rate [1], improved neutron decay rate measurements are needed for sharpening this prediction.

The measurement of the lifetime of free neutrons in a cold neutron beam was one of the first experiments operating in the NCNR Guide Hall. The experiment occupied the NG-6 Physics Station alternatively with other fundamental

physics experiments through December 2000. A statistical precision of one part per thousand was achieved in the final

result. An overall standard uncertainty of about four parts per thousand has been obtained, and that uncertainty may be

eventually reduced by as much as a factor of two by ongoing

developments in characterization of the neutron counting for

this experiment.

Figure 1 shows a schematic diagram of the apparatus.

An axial 5 Tesla magnetic field provides radial containment

of the decay protons, while axial containment is achieved by

the positive electrostatic charge on two end electrodes, called the mirror and the gate. This combination of magnetic and electrostatic fields is called a "Penning Trap." The

neutron detector consists of a well-characterized isotopic

target of ^'Li, viewed by a set of four charged particle detectors. The charged particles from neutron reactions in the isotopic targets are counted with an accuracy of better than one part per thousand. The reaction cross sections for these standard cross sections are known to about 0.25 %.

When a neutron in the beam decays inside the trapping

region, the recoil proton is trapped. Periodically, the gate

electrode is lowered to ground potential to allow the accu- mulated trapped proton(s) to exit the trap and be counted by

the proton detector. A bend in the magnetic field makes it possible to locate the proton detector outside of the incoming neutron beam. The trapped protons have a maximum

energy of less than 800 eV and are undetectable at those low

energies, but by holding the proton detector at a negative

high voltage of the order of -30 kV, the protons may be

detected with a signal well above noise levels. Counting for only a short time (less than 100 |is) after the trap is opened

in each counting/trapping cycle further enhances the signalto-noise ratio. Typical trapping times are of the order of

10 ms, and typical decay event rates are of the order of a few

protons per second.

Determination of the lifetime requires either an

accurately known trap length or a variable trap length with

accurately known differences in length. The latter is more

easily realized physically and is achieved in this apparatus

by the precision machining of the segmented trap structure. Varying the trap length by applying the mirror voltage to

6 Researcli Highlights
M. S. Dewey, D. M. Gilliam, J. S. Nico, and F. E. Wietfeldt

Ionizing Radiation Division National Institute of Standards and Technology

Gaithersburg. MD 20899-8461

W. M. Snow

Indiana University Cyclotron Facility Indiana University Bloomington, IN 47405-7105

G. L. Greene

Los Alamos National Laboratory Los Alamos, NM 87545

different segments does not significantly change the hard-to- define end effects. Figure 2 shows a plot of the measured

proton rate (after normalization to the neutron rate) as a function of trap length; the slope of this line is inversely

proportional to the neutron lifetime. Since the time when some of the present staff of this experiment participated in a closely related experiment [2] at the Institut Laue-Langevin, a large number of changes have

been made to improve the accuracy. Perhaps the most

important factor was simply having much more beam time

1 20

•i 0 & -20x10^

<f 3.5x10 I 3.0 oI 2.5 1 2.0 oO

§ 1.5

1 1 1 1 1 1 \\ I

^ I \

Individual Trap Lengttis -Linear Fit

Fit Residuals

1 1 1 1 1 1 1 1

3 4 5 6 7 8 9

Trap Length (number of electrodes)

10 11

FIGURE 2. A plot of the proton count rate (normalized to the neutron monitor count rate) vs. trap length. The slope of the line is inversely

proportional to the measured neutron lifetime.

910 F — 900

a>E

"H> 890 c

oI

880

870

4. 1 1 1 1 1 1 1 1 . 1 ... .

1

' ' ' ' 1 4

I ;

Individual Data Runs \

e> Extrapolated Neutron Lifetime

+ . . . . 1 .

(statistical uncertainty only) ; — Linear Fit '; ... 1 .... 1 .... 1 .... 1 .... -i

for evaluation of significant systematic effects and accumulation of better statistical precision. Other factors include

Monte Carlo-computed corrections due to magnetic field inhomogeneities, improved beam profiles and proton

alignment, better trap voltage stability and monitoring,

improved definition of the areal mass density of the isotopic

targets, use of a new trap designed to minimize instabilities,

and improved analysis methods.

Figure 3 shows the extrapolation of the measured

lifetime as a function of the backscattering fraction for protons from the detector surface to zero backscattering. The

preliminary lifetime value obtained in this experiment [3] is: 885.3 s± 4.0 s. Continuing efforts to measure the neutron count rate

are underway by both calorimetric and coincidence tech- niques, which should reduce the final uncertainty by half. They will not only improve on the results of this experiment

but will also provide a more accurate and direct cahbration

of the NIST Standard Neutron Source. Also underway at NG-6 is a new experiment to measure the neutron lifetime by an independent method

involving magnetic trapping of ultracold neutrons. This

experiment is being carried out in collaboration with the Physics Department of Harvard University.

References

[1] S. Buries, K. M. Nollett, J. W. Truran. and M. S. Turner, Ptiys. Rev.

Lett. 82 4176, (1999).

[2] J. Byrne et a/., Phys. Rev. Lett. 65. 289 (1990); J. Byrne et al, Europhys. Lett. 33, 187 (1996).

[3] M. S. Dewey et al., manuscript in preparation.

10 15 20

Backscattering Fraction

25 30x1

0"~

FIGURE 3. A linear fit of the measured neutron lifetime at varying values

of the detector backscattering fraction. The extrapolation to zero

backscattering gives the free neutron lifetime.

NIST Center for Neutron Research 7
ETS-4: Maxwell's Zeolite

Scottish physicist James Clerk Maxwell (1831-1879)

discussed a mythical device that could separate molecules by temperature, a concept now known as Maxwell's

demon. Maxwell used this idea to explore how sorting

molecules by size would violate the second law of thermodynamics. If one did have a Maxwell's demon, a more useful

task for it would likely be to separate molecules by size. This

would allow mixtures to be separated into pure substances,

and would facilitate removal of dangerous contaminants

from valuable materials. In some cases, molecular separations can be performed using zeolites and similar materials.

Zeolites are composed of linked tetrahedral SiO^ and AlO^

species, arranged in rings to create a framework of molecule-sized cavities. Related zeolitic materials may use other

tetrahedral atoms to build the framework. Molecules can

access these cavities or pores, under two conditions: if they can pass through the appropriate rings in the framework and

these rings are not blocked by other species in the material,

such as cations. If the ring size in a material matches the needs for a particular chemical separation, the material may

be appropriate. Chemical modifications, such as ion ex- change or changing the framework composition can sometimes influence the pore size. So, occasionally it is possible

to modify a zeolite to a particular separation. However, this does not allow what Maxwell might envision, where the pore size can be adjusted over a wide range to suit the desired process, in effect a Maxwell's demon for size separations. This exact capability has now been demonstrated in material ETS-4 [1]. ETS-4 can be processed in such a way so that the pore sizes can be tuned for particular

separations.

ETS-4 is a titanosilicate material invented by

Engelhard in the 1980s [2]. ETS-4 differs from most zeolitic materials in a number of ways. Chemically, it is an oxide of

Ti and Si, while the majority of zeolitic materials are oxides

of Al and Si. Structurally, the material is composed of both

tetrahedral TiO, and octahedral TiO^ units, as well as SiO,

4 6 4 units. Finally, most zeolitic materials hydrate and dehydrate

reversibly or are destroyed during dehydration. ETS-4, at

least in some ion-exchanged forms, can be induced to dehydrate irreversibly and without destruction of the pores. Dehydration reduces the average pore opening size. Thus it

Room Temp. 150°C 200°C 250°C aocc

FIGURE 1. Size of the eight-ring pore opening in ETS-4 as a function of dehydration temperature.

is possible to tune the zeolite to accept molecules of a

particular size by changing the level of dehydration. The

temperature to which the material is exposed in turn dictates

the amount of dehydration. In 1999, the Advanced Technology Program funded Engelhard to exploit this effect, which

Engelhard calls the Molecular Gate®, to develop methods for separating oxygen from air, a process with useful applications for prevention of pollution as well as medical and other

industrial applications.

The mechanism behind the Molecular Gate® effect was

demonstrated crystallographically using data from the BT-1

neutron powder diffractometer. Pores in ETS-4 must be

accessed via a ring of eight Si and Ti atoms, each linked by

oxygen atoms. This ring is commonly called an eight-ring,

despite the fact that it is actually composed of sixteen atoms.

In Fig. 1, the size of this eight-ring is shown as a function of

dehydration temperature. The actual size of molecules that can be admitted via this opening are dictated by the van der

Waals radii of the O atoms in the eight-ring and the van der

Waals dimensions of the molecule to be admitted. Since

accurate determination of oxygen atom siting is needed, and

since ETS-4 does not form single crystals, neutron powder

TABLE 1. Eight-ring opening as a function of dehydration temperature. Distances are the pore-openings separating van der Waals radii between

opposite 05 (D,), 01 (D^) and 02 (D3) atom pairs (see FIGURE 2).

van der

Waals

opening

(A)

Dehydration Temperature, °C

RT 150 200 250 300

Di 4.27 3.97 3.95 3.94 3.90

D2 4.43 4.02 4.09 4.57 4.57

D3 3.61 3.28 3.29 3.27 2.77

8 Research Highlights
S. M. Kuznicki, V. A. Bell and R. M. Jacubinas

Strategic Technology Group

Engelhard Corporation

Iselin, NJ 08830

S. Nair, H. W. Hillhouse,

C. M. Braunbarth and M. Tsapatsis

Department of Chemical Engineering,

University of Massachusetts

Amherst, MA 01003

B. H. Toby

NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

diffraction measurements are the only way to obtain accurate measurements of the ring openings as a function of dehydration temperature. Measurements of the eight-ring opening

from Rietveld refinements are shown in Table 1 , relative to

the distances diagrammed in Fig. 2. The proof that dehydration indeed does change the pore access is demonstrated by adsorption isotherms, shown

in Fig. 3. The separation shown in the top of this figure,

between O, and N,, shows how precisely the pore sizes can

be adjusted. The difference in van der Waals radius between

oxygen and nitrogen is only 0. 1 A.

References

[1] S. M. Kuznicki, V. A. Bell, S. Nair. H. W. Hillhouse. R. M. Jacubinas,

C. M. Braunbarth. B. H. Toby, and M. Tsapatsis, Nature 412, 720

(2001). FIGURE 2. Atom and pore-opening distance labeling scheme in the [2] s. M. Kuznicki, US Patent 4938939 (1990).

eight-ring opening of ETS-4.

FIGURE 3. Adsorption of selected molecules as a function of dehydration temperature.

NIST Center for Neutron Research 9
Measuring Stress Relief in Electron Beam

Weld Joints of Superalloys

Single-crystal turbine blades are the pinnacle of a decade-long development and refinement of nickelbased superalloys. By eliminating creep along grain boundaries as the main failure mechanism of polycrystals at high

temperatures, the use of single crystal turbine blades has

allowed a rise in temperature in the "hot zone" by more than

100 K, thus substantially increasing thermodynamic effi- ciency of jet engines. The complete turbine blade, including

the mounting base and the cooling channels, is grown from

the melt in a single process and represents a considerable

investment. In order to protect this investment, electron beam welding (see Fig. 1) is used to repair minor structural damage of failed components.

The weld process creates very high residual stresses

that make the extended weld region prone to cracking.

Cracks are sometimes found parallel to the weld especially

in the c-direction, so that an additional heat treatment must

be applied for stress relief. The standard heat treatment is 2 h

at 1423 K for homogenizing (solution) and a subsequent

aging treatment. In this investigation neutron diffraction is used to compare the stresses before and after heat treatment

in the single crystal side of the weld joint.

[02l|,x

FIGURE 1. Test specimen of a weld joining a Rene N5 single crystal and a

polycrystal. The tliree different regions, polycrystal, weld, and single

crystal, appear in different colors after etching which indicates different chemical compositions. The microstructure shows typical features: 60 %

cuboidal 7 '-precipitates and unimodal size distribution.

Usually a polycrystalline alloy of comparable composition is chosen as a replacement material. The typical grain

size of the polycrystal is approximately one millimeter,

which is about the size of the incident neutron beam. Thus,

stress measurements are not feasible for this side of the weld. Compared to a measurement on a fine-grained polycrystal, the measurement procedure is rather difficult because all small angle grain boundaries (Fig. 2) have to be

brought into reflection and averaged over the rocking angle

CO in the data reduction.

Additionally, both the welding and the heat treatment produce position-dependent chemical gradients that affect the unstressed lattice parameters. Their measurement would

have required cutting the specimen into little pieces for complete stress relief. We chose an alternative approach of

applying mechanical boundary conditions at the surface. The

results obtained from this procedure are shown in Fig. 3. The effect of the heat treatment is dramatic. Both the strong tensile stresses in the heat-affected zone (5 mm from

the weld center) and the strong gradients at the interface between weld and heat affected zones are removed. Stress

relief in the z-direction is especially important because it removes the driving force for crack opening parallel to the weld in the heat-affected zone.

FIGURE 2. 0^-intensity map for the as-welded sample (reciprocal space)

plotted vs. 26 and the rocking angle (O. Small angle grain boundaries as

indicated by the presence of several maxima are characteristic for superalloy single crystals. The existence of two maxima in 2 (? (equivalent

to two cf-spacings) indicates the ^'matrix and the /'-precipitates whose

lattice parameters are close to each other.

10 Research Highlights
T. Gnaupel-Heroltf and H. J. Prask NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

and

^ University of Maryland

College Park, MD 20742

Remarkable features in Fig. 4 are the strong compressive stresses after heat treatment in the middle (4 mm depth)

both for the c-direction and the y-direction. There is evi- dence that solidification partitioning can lead to a different '-microstructure in the weld zone, thus affecting the mean

lattice parameter measured by diffraction. The immediate

results are apparent stresses caused by changes in the unstressed <i-spacing. This effect can be removed with some

confidence only for the measurements close to the surface

( = 1 mm depth) by using mechanical equilibrium condistresses: z-direction

5 10 15

distance from weld middle [mm]

FIGURE 3. Residual stresses before and after the heat treatment for two different depths from the upper surface. AW stands for "As Welded"; AS stands for "Heat Treated." The uncertainties are of the order of the

size of the data points.

tions. Further investigations are under way to clarify this

effect for larger depths.

The stresses parallel to the weld (Fig. 4) are usually the highest and the most dangerous stresses because they reach

the yield limit (around 600 MPa). If the weld direction is oriented such that the weld line is parallel to the main stress

axis - i.e., radially for a rotating blade - then further loading

would exceed the flow stress and cause a dangerous local weakening of the turbine blade. Here, the effect of the heat treatment is most beneficial because almost complete stress

relief is achieved.

The stress relief effect of the heat treatment extends

also to the microstresses between sub-grains by removing

the dislocations created by the plastic deformation in the weld region. While direct observations of dislocations are not available, we can use the full width at half maximum

(FWHM) of the measured integrated strain distributions as a measure for the dislocation density (see Fig. 5). Figure 5 shows clearly that substantial plastic deformation exists only in the weld itself (0. . . 1.0 mm) and within a very narrow zone of about 0.5 ... 1.0 mm in the base material. This effect vanishes completely after the heat treatment. These results demonstrate that a heat treatment after welding

is essential for avoiding detrimental effects to the mechanical properties of the turbine blade.

1000

stresses: y-direction FWHM at maximum resolution

V 200 ^ 0

CO -200

-400

-600

^AW- 1mm

AW- 4mm

^AS - 1mm

: AS - 4mm

5 10 15

distance from weld middle [mm]

20

FIGURE 4. Residual stresses cr^ (parallel to the weld) before and after the heat treatment.

0.008 0.0075 ^ 0.007

'2 0.0065 J

0.006 I

0.0055 ^ 0.005 0.0045

0.004

-2 0 2 4 6 8 10 12 14 16 18

distance from weld middle [mm]

FIGURE 5. FWHM at the best instrument resolution before and after heat

treatment. These results are representative for all measured depths.

NIST Center for Neutron Research 11
EXPGUI: Smoothing the Path to Powder

Crystallography

Back in the dark ages of computational crystallography,

crystallographers were expected to be able to modify

their computer programs to suit the computation at hand.

Most crystallographers could create programs to convert

data between different formats, and felt comfortable reading

through a program's source code when the program documentation did not adequately explain a topic. (There is disagreement on when this ended, some say the 1950s,

others say the 1980s or even the early 2P' century!)

The advent of desktop computing has also introduced a new paradigm of computing — the graphical user interface (GUI). Visual analogies to physical devices (such as switches) make operating a GUI intuitive; a good GUI

implementation significantly reduces learning time.

Simultaneous with the changes in computing, crystallography has evolved from a technique used exclusively by

specialists to one of many tools in the arsenal of chemists,

physicists and engineers. To facilitate this, the NCNR is developing GUI-based software for crystallography. There

are two goals: to increase the utility of the software for the

non-specialist, and to improve the productivity of experts.

This allows novices to concentrate on the science and

techniques of neutron powder crystallography rather than

devote their efforts to learning arcane details of computer

software. The Rietveld refinement technique for crystallographic

analysis was adopted within the NCNR immediately after its

first publication [ 1 ] . The NCNR then produced Rietveld

software specifically adapted to the local instrumentation,

which is still being improved [2, 3]. However, the General

Structure Analysis System (GSAS) package of programs

from Los Alamos is also widely used [4]. GSAS has wide

applicability. It can be used with virtually any type of

neutron or x-ray diffraction data, including both powder and

single crystal data. It supports a wide range of geometric and

compositional restraints and constraints. While many new

features have been added to GSAS on a continual basis, the

original text-based user interface has not changed. Novice

and occasional users often find the many levels of dialogs

cumbersome and difficult to learn. Work on a GUI for GSAS was begun several years

ago, with the modest goal of providing a mechanism for

accessing a small portion of the GSAS features. The project, now called EXPGUI, has grown in scope [5]. Most of the commonly used powder diffraction features are now implemented, allowing a complete Rietveld refinement to be

performed graphically. Features not implemented in the GUI

can still be accessed via the standard GSAS user interface. EXPGUI runs identically on all platforms where GSAS is supported, including both Unix and Windows. In FY2001,

the Advanced Technology Program recognized the value of

this effort and provided funding to accelerate development.

The GUI is designed to display information and

controls in a concise and intuitive fashion. An example

screen, shown in Fig. 1, manipulates structural parameters.

This figure demonstrates how four atoms that share sites in

this material can be selected together, so that their refine- ment options can be changed as a group. Another screen, shown in Fig. 2, applies constraints that link appropriate

parameters in the refinement.

=»; OAHNET.EXP (ntalltlan 2

File Options Powder Xtal Graphs Results Calc Import/Export Help

expnam expedt genles powpref povyplot Istview liveplot ' leBail

[ LS Controls Ptiase Histogram Scaling Profile Constraints MDPref Orient SHPref Orient

Phase:: i Replace i title: Yt Fe Garnet

Add

j a 12 169099 b 12 189099 c 12.189099 Refine Cell p

Phase

j a 90 0000 |) 900000 v 90.0000 Cell damping 0 -

j * name type ref/damp

Y XO OO" 0

fractional coordinates Uult Occupancy Olso

0 125000 0 bOOOOb 0 '250000 ~24 i". 000 0 b 00718

2 FE2

3 AL3

4 AL4

5 FES

FE XO OO FO AL XO uo ro XL XO DO ro EE XO DO 700.000000

0.000000

0.37S000

0.375000

0.000000 0.000000 16 0.57S1 0.00650

0.000000 0.000000 16 0.4249 0.00650

0.000000 0.250000 24 0.6927 0.00404 0.000000 0.250000 24 0.3073 0.00404

6 06 0 XO uo 0 -0.029460 0.053856 0.150670 96 1.0000 0.00816

1

Set refinement options: atoms 2-5 Add New Atoms

Refinement Flags: P X T U r F Damping: X 0-|u 0-.|f 0-| Xfomi Atoms

1 1 1 1

1

FIGURE 1. Sample screen from EXPGUI, showing where atomic parameters are displayed.

Recent EXPGUI development has been aimed at extending the capabilities of the GSAS package. As an example, when EXPGUI was recently given the ability to import atomic coordinates from files, support was added for CIF, the complex standard crystallographic information file format developed by the International Union of Crystallographers. Further, the import routine was written in a general

fashion, so that other formats are easy to add.

12 Research Highlights
B. H. Toby

NIST Center for Neutron Research

National Institute of Standards and Technology

Gailhersburg, MD 20899-8562

File Options Powder Xtal Graphs Results C<dc Import/Export Help expnam expedt genles powpref pov/plot Istview liveplot leBail iLS Controls Phase Histogram Scaling Profile Constraints MDPref Orient Sh^ Atomic [Profile ||L Phase Atom(s) Variable Multiplier Atom(s) Variable Multiplier Delete

1 2^ UlSO X 1.0000 J

j^'^e<fit 1 3 FRAG X -1.0000 2 FRAC x 1.0000

|3 edit 1 45 UlSO X 1.0000

ii ecfit 1 5 FRAC x -1.0000 4 FRAC x 1.0000

New Constraint Delete

FIGURE 2. EXPGUI screen for entering and display of atomic

constraints. In this case, atoms sharing sites are constrained to have the same U.^^ values and have fractional occupancies that total to unity.

Where possible, EXPGUI takes advantage of the graphical nature of modem computers to offer scientists

better access to refinement tools. Figure 3 shows a graphical

display of the observed and computed diffraction pattern.

Through a simple set of mouse operations, the user has

"zoomed in" on a portion of the data, to better see the poor

agreement between observed and computed intensity values,

indicating that the structure is not well modeled.

File Qplions PeaX Gen Help

GARNET cycle 2 Hist 1

FIGURE 3. Graphical display of fit in EXPGUI. This plot has been "zoomed" in to show details. Several features of this plot, for example

the display of reflection indices and the location of the fitted background, were not previously available in GSAS.

This graphical tool also allows the user to see the fitted background and to label the reflection indices — features not previously available within GSAS. Another example is a new tool for background fitting. In most cases, background

fitting is nearly automatic, but for some refinements, fitting

can be quite difficult. GSAS allows a scientist to define the background either through use of a function with refinable terms or as a spline drawn through a set of fixed points. A

new tool in EXPGUI improves GSAS by allowing a user to define where the background curve should occur using a computer mouse. Since many experts agree that fixed

background points are a poor way to treat background

fitting, EXPGUI offers an augmented feature: a background

function can be fit to the user's desired background curve. The terms for this function will be sufficient to obtain an

initial model with good agreement to the data. At the end,

the background terms can be optimized, to obtain the best possible model. This capability was not previously available

in GSAS.

EXPGUI has been widely adopted by Rietveld users. It

is used in most major neutron and synchrotron facilities, as well as dozens, if not hundreds, of universities, companies

and research centers around the world.

References

[1] H, M. Rietfeld, J. Appl. Cryst. 2, 65 (1965).

[2] L. W. Finger and E. Prince, National Bureau of Standards, Report No.

852 (1975).

[3] B. H. Toby, E. Prince, and J. Stalick, unpublished computer program

(2000).

[4] A. C. Larson and R. B. Von Dreel, Los Alamos National Laboratory,

Report LAUR 86-748,(2000).

[5] B, H. Toby, J. Appl. Cryst. 34. 210 (2001).

NIST Center for Neutron Research
Neutron Diffraction Contributes

to Improving the Fatigue Life of Bridges

Welded attachments to large-scale structures such as bridges can limit the serviceability and prevent

effective use of high strength steels. To avoid this, it is desirable to enhance the fatigue resistance of common

attachment details such as transverse stiffeners, cover plates,

gusset plates and other welded details that, at the vv'eld toe,

are often the initiation point for crack growth.

Enhancement of fatigue resistance of welded joints by

plastic deformation of the surface and by improvement of

weld toe characteristics is well established [1,2]. It is known that conventional improvement techniques such as grinding, shot peening, air hammer peening, gas tungsten arc re-melting, and use of improved welding consumables can improve fatigue resistance of welded details [3]. However,

these procedures are time-consuming, and can be inefficient and environmentally unfriendly. Ultrasonic Impact Treat- ment (UIT) offers an alternative means to avoid the negative

aspects of other methods [4]. The post-weld enhancement of welded details by UIT,

developed by Statnikov and co-workers [4], involves

deformation treatment of welds by impacts at ultrasonic

frequency on the weld toe surface. The objectives of the treatment are to introduce beneficial compressive residual

stresses at the treated weld toe zones, and to reduce stress concentration by improving the weld toe profile. UIT is claimed to involve a complex combination of strain hardening, reduction in welding strain, relaxation of residual stress, and a reduction in operating stress concentration, thereby

achieving a deeper cold-worked metal layer. However,

virtually none of the effects ascribed to UIT have been

confirmed by appropriate measurements.

Here we highlight neutron diffraction measurements at the NCNR that have contributed new insights into how UIT

actually affects residual stresses in the weld region around a

full-scale cover plate for a steel girder. In Fig. 1 a diagram is shown of a typical I-beam fatigue-tested at the Center for Advanced Technology for Large Scale Structures (ATLSS)

at Lehigh University. In the initial phase of the NCNRLehigh collaboration, neutron diffraction was used to determine the effect of UIT on a test flange plate by measuring before and after the treatment by UIT. The test plate

consisted of a cover plate having a 12.5 mm (0.5 in.) end

fillet weld attaching it to a base plate. This kind of coverplate detail is often used in steel bridges but has a low level of fatigue strength compared to other welded configurations.

The geometry and dimensions of the welded detail are the same as in the full-scale fatigue test beams of Fig. 1. Both

base plate and cover plate are steel (A572 Gr.50).

ELEVA VON 15/16'

PLAN

-<,(ONLY AT THE END) —<fnpj

s^Cow g 4-8 17 '/I'xl'Jh.

FIGURE 1. Partial details of one of the beam specimens being fatigue- tested at the ATLSS facility. The cover plate location is shown as "W" and

"Y" on the right side.

Neutron diffraction measurements were made in a mesh, mostly near the weld toe, as indicated in Fig. 2. Overall, residual stresses were determined at 30 positions in the weld region, before and after UIT. Gauge volumes of 3 x 3x3 mm"* (interior) and 2x2x2 mm^ (near surface) were

used. The unstressed <i-spacing, d^^, was determined from a small piece of the starting material. The depth dependence of

Z-direction stresses is shown in Fig. 3.

100 mm| 0 mm|

FIGURE 2. Top view of a flat test plate: cover plate (inner rectangular

segment), weld (hatched area), and base plate underneath. Part of measurement mesh is shown by dots.

14 Research Highlights
H. J. Prask, T. Gnaupel-Herold^ , and V. Luzin^ NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8560

and

' University of Maryland ^ State University of New York

College Park, MD 20742 Stony Brook, NY 1 1 794

FIGURE 3. X-direction residual stresses vs. depth at the weld center. Solid symbols are pre-UIT. Uncertainties are typically + 20 MPa.

The near-surface residual stresses determined, nominally 1 mm below the actual surface, are shown in Fig. 4. Both the anisotropy of stresses around the untreated weld

metal, and the additional anisotropy introduced by UIT are

seen. Before UIT, the stresses just beneath the weld toe are comparable and highly tensile in the X and Y directions. These stresses reach and exceed the minimum yield stress (315 MPa) of base plate material, whereas those normal to the surface are generally only ~ 100 MPa tensile. After UIT

the X-direction stresses become compressive, changing in magnitude by about 400 MPa. The F-direction stresses are reduced by = 150 MPa or less but remain in tension except

at one point. The normal stresses (Z-direction) are essentially

unchanged. The UIT effect on Z-direction stresses (Fig. 3) is

significant only near the plate surface, but reaches deeper

than, for example, shot peening. The depth dependences of

the Y- and Z-direction stresses were also obtained.

To our knowledge this is the first determination of the

triaxial stress distribution in the vicinity of a weld treated with UIT. Having established the effects of UIT stress-relief on a test flange plate, ongoing neutron diffraction studies in

this collaboration are now focusing on the mapping of stress gradients within and near weld joints in sections of actual

fatigue-tested I-beams.

J. W. Fisher and Xiaohua Cheng

Department of Civil and Environmental Engineering

Lehigh University Bethlehem, PA 18015-4793

References

[1] J. W. Fisher, K. H. Frank, M. A. Hirt, and B. M. McNamee, Effect of Weldments on the Fatigue Strength of Steel Beams, NCRP Report 102, Highway Research Board, Washington, DC, 1970.

[2] J. W. Fisher, R A. Albrecht, B. T. Yen, D. I. Klingerman, and B. M.

IVIcNamee, Fatigue Strength of Steel Beams with Transverse Stiffeners and Attachments, NCHRP Report 147, Highway Research Board,

Washington, DC, 1974.

[3] J. W. Fisher, H. Hausammann, M. D. Sullivan, and A. W. Pense, Detection and Repair of Fatigue Damage in Welded Highway Bridges,

NCHRP Report 206, Transportation Research Board, Washington, DC,

1979.

[4] Reviewed in Statnikov, Esh, Applications of Operational Ultrasonic Impact Treatment (UIT) Technologies in Production of Welded Joints, Welding in the World 44, 1 1 (2000).

S^^ along the wel(j, near surface

400

I

200

CO

CD a: -200

<

1

<

< > O

. o o

120 100 60 40 X(mm)20 0 -20

S along the w/eld, near surface

-TO 400

Q03 CO

200

0

-200

1 1 1 1

[

1

1 1 1

I r—

120 100 80 60 40 20 X(mr^)0 -20

S along the weld, near surface

CL

CO

CD

400

200

0

-200

f < > ( )

120 100 80 60 40 X(mm)

20 0 -20

FIGURE 4. The residual stresses determined by neutron diffraction for the "near-surface" points. Uncertainties due to counting statistics are

typically about ± 20 MPa. Solid symbols are before UIT.

NIST Center for Neutron Research 15
Capillary Neutron Lens for Prompt Gamma

Activation Micro-Analysis

Glass polycapillary fibers with hundreds of hollow

micrometer-wide channels have been used for guiding

and changing the direction of slow neutrons [1]. The use of a monolithic lens to focus neutron beams for prompt-gamma

activation analysis (PGAA) of small samples has been

explored earlier [2]. In addition, the converging nature of a focused beam can provide magnified images for neutron radiography [3]. We have combined these two uses of the focused beam from a monolithic lens and have implemented

an alignment procedure for PGAA aided by neutron imaging.

The measurements have been performed at the end of

the NG-0 beam line at the NCNR [4]. The average wavelength of the incident beam with a Be filter is about 0.6 nm.

Neutrons are imaged with a camera that consists of a ZnSCAgV^LiF scintillator coupled with a CCD camera with an average pixel size of 50 [am. The gamma-ray spectrometry system [5] consists of a p-type germanium photon

detector with a virtual pulser "loss free" counting module.

The two monolithic lenses used in this study are both

fabricated from a bundle of capillaries that is 50 mm long

and with a hexagonal cross section 10 mm wide (flat-to-flat)

at the entrance and tapered to 3 mm at exit. The distance

from the exit to the focus is 8.8 mm for lens A and 9.8 mm

for lens B. Each lens is installed on x-y-z translation stages

with two directions of tilt motion. The experimental layout is shown schematically in Fig. 1. An optimum focal spot from lens A is recorded with

the imaging detector. Line profiles across the focal spot in

Neutron z. camera

Ge detector

sample

Incident neutrons Neutron lens

both the horizontal and vertical directions are taken to determine the intensity distribution and the FWHM

(H: 87 |jm, V: 100 jam) of the focus (Fig. 2 a). The gain in neutron current density within the 100 |am x 100 fam area is 46 ± 2, and within a 50 |Lim x 50 |am area it is 71 ± 5. Images

and their sizes and intensity as a function of z are shown in

Fig. 2b.

CD

CSJ

B 50

-400 -300 -200 -100 0 100 200 300 400

x(Mm)

FIGURE 1. Schematic experimental layout.

FIGURE 2. (a) Line profiles of the focal spot intensity in the horizontal and vertical directions. Insert: Focused beam image - the focus is near

the center of the hexagonal shadow cast by the lens entrance surrounded by the background of unguided neutrons, (b) Intensity and

base width of the spot along the beam path, and images of the focused beam "foot print." A linear fit to the region between 10 mm and 16 mm

gives a slope of 0.233 radians, or a 13.3° convergence angle.

The focused beam creates a neutron "point source,"

with neutrons departing the focus within a cone of about

13°. Any neutron-absorbing object within the path of the divergent beam will cast a magnified shadow at some further

distance. We make use of this effect in order to align a small

sample, a Gd foil (thickness 25 \im) of size 2 mm x 0.9 mm,

at the focus for PGAA (Fig. 3a). We vary the distance Lj

between the sample and the lens focus by translating the lens

16 Research Highlights
H. H. Chen-Mayer, G. P. Lamaze,

D. F. R. Mildner and R. Zeisler NIST Center for Neutron Research, and

Analytical Chemistry Division Chemical Science and Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8395

W. M. Gibson

X-Ray Optical Systems, Inc. Albany NY 12203

Li = 1mm 4 mm 7 mm 10 mm Without lens

CO Gd foil (25 fjm thick) 2 mm x 0.9 mm

lens

L2 = 19 mm, fixed L f = 9mm

Gd 8.8 ± 0.39 cps

3

2.5 ^ 2 ;^1.5

-s 0.5

0

-0-5

(b) 165 170175180 185190195200205

E(keV)

FIGURE 3. (a) Magnified image of the sample at various values of L^. The image of the sample without the lens is also shown for comparison,

(b) Prompt gamma spectrum for Gd (182 keV) and for In (186 keV)

measured with the lens, and the corresponding image of the sample.

Max gain = 35,4 (±1,5) FWHM = 0,146 (± 0,004) mm

FIGURE 4. (a) Optical image of the Gd piece in comparison with a strand

of human hair (left), and neutron image using lens B. The large solid rectangle is a larger Gd strip marker to help locate the smaller sample more easily, (b) Gd peak intensity produced by a y-scan of the lens.

along the direction of the beam. The magnified image

enables a better alignment of the sample with respect to the focus. From the earlier measurement of the lens focal point

(Fig. 2), we determine the location of the focus with respect

to the large hexagonal shadow cast by the lens entrance. We

then move the lens transversely such that the image of the sample is now superimposed on the image of the focal point.

This guarantees that the focal point is indeed on the sample.

Once the image is centered, the lens is translated to Lj = 0

such that the focus is on the sample, ready for prompt

gamma measurements.

At the center of the same Gd sample described above,

there is an additional indium foil of about 1 mm x 1 mm in cross section, in contact with the Gd strip. The prompt

gamma measurement without the lens gives a peak intensity

for Gd at 182 keV of 2.1 counts per second (cps), but the In peak at 186 keV is not visible even for a 20 min measurement. When the lens is used in a 10 min measurement, not only has the Gd peak intensity increased, but the In peak has

also become visible (Fig. 3b). Since the Gd piece is much

larger than the focal spot size, the gain in sensitivity for Gd

is only a factor of 4. The gain for In is not estimated.

We have also used lens B for an even smaller sample, a Gd piece of 128 pm x 103 |im x 25 |jm in size and 2.59 |ag

in mass (effective mass 1 |ig taking into account self- absorption). The images from an optical microscope and

from neutron measurements are shown in Fig. 4a. Without

the lens, the neutron camera cannot resolve the 100 \im

particle. The gamma intensity with the lens is 17.4 cps with

a 1.6 % la counting statistics after 1 hour of counting, and

with the direct beam 0.506 cps (3.9 % la) after 19.5 hours. The gain, given by the ratio of the two count rates, is 34 ±

1.4. A scan of the lens in the vertical direction yields a peak,

given by the convolution of the sample width with the beam

width, as shown in Fig. 4b. The FWHM of the peak is 148

( ± 1.5) |J.m, coiTectly corresponding to the 100 jim samplewidth and the 100 jim beam-width added in quadrature.

The effort presented here is a step toward providing

quantitative elemental information to traditional neutron

radiography. This new tool will provide a unique capability

in non-destructive analysis for industrial materials. In the

future, the system will be automated to scan, giving a more

efficient probe for microanalysis with neutron beams.

References

[1] M. A. Kumahkov and V. A. Sharov, Nature 357, 390 ( 1992), and H. Chen. R. G. Downing. D. F. R. Mildner, W. M. Gibson, M. A. Kumakhov, I. Yu. Ponomarev, and M. V. Gubarev, Nature 357, 391

(1992).

[2] V. A. Sharov, Q. F. Xiao, I. Yu. Ponomarev, D. R R. Mildner, and H.

H. Chen-Mayer, Rev. Sci. Instrum. 71, 3247 (2000).

[3] K. M. Podurets, D. F. R. Mildner, and V. A. Sharov, Rev. Sci. Instrum.

69, 3541 (1998).

[4] D. F. R. Mildner, H. H. Chen-Mayer, G. R Lamaze, and V. A. Sharov,

Nucl. Insti-um. & Meth. A413, 341 (1998).

[5] R. Zeisler, G. P. Lainaze, H. H. Chen-Mayer, J. Radioanal. Nucl. Chem. 248, 35 (2001).

NIST Center for Neutron Research 17
Giant Anharmonicity and Electron-Phonon

Mediated Superconductivity in MgB2 at 39 K

The recent surprise discovery of superconductivity in MgB, at 39 K has stimulated a great deal of research on

this intercalated graphite-like system (see Fig. 1). Sparked

by this discovery, we set out to unlock the structural secrets and, in particular, to reveal the origin of the high T in MgB^:

an electron-phonon or other exotic mechanism? To answer

this fundamental question, we calculated T , its pressure dependence, and its isotope effect from the electronic band

structure and lattice dynamics of MgB, using density

functional theory within the generalized gradient approximation [1-3].

Figure 2 shows that the features in the calculated

phonon density of states (DOS) are in excellent agreement

with the neutron data (GDOS), giving confidence that the calculations provide a sound description of the physical

properties of the system. The DOS consists of two bands of

phonons, one below 40 meV corresponding primarily to acoustic phonon modes, and one above 50 meV that mostly

involves the boron motions. Inspection of the calculated

phonon-dispersion curves that make up the high-energy band

in the DOS reveals that the in-plane boron phonons (as

depicted in the inset to Fig. 3) are very anharmonic. To

demonstrate this, in Fig. 3 we plot the total energy as boron

atoms move according to one of these zone-center in-plane

phonons with E^^ symmetry. The potential indicates a very

strong anharmonic term. Numerically solving the

FIGURE 1. The crystal structure of MgB^ consisting of B and Mg

hexagonal layers. The in-plane boron modes (shown by arrows) are

strongly coupled to the boron p^^ c bands shown as the green contour and isosurface plots.

Schrodinger equation for this anharmonic potential yields a phonon energy of hoj{E^J - 74.5 meV, a 25 % enhancement

over the harmonic value of 60.3 meV. This value is in good

agreement with recent Raman measurements. The giant

anharmonicity revealed gives the first hint that the in-plane

modes are strongly coupled to the p^^o bonding orbitals of

boron, as shown schematically in Fig.l. This coupling is also evident from the splitting of the boron a bands (red lines)

with the E^^ phonons (see Fig. 3). Note that the other bands

are not affected by the E^^ phonons.

The splitting of the boron a bands, when averaged

over the Fermi surface, gives an electron-phonon (EP)

coupling constant X ~ I. Using this value in the McMillan

expression for with co{E^^) and taking a typical value for the Coulomb repulsion ju* - 0.15, we obtain a T of 39.4 K,

in excellent agreement with experiments. We also solved the Schrodinger equation for the potential shown in Fig. 3 for

different boron masses and obtained co(E^^) = 291.8 M"°"^

and A = 0.6151 M " '^'^ which yields the T -M curve shown in

Fig. 4 and a boron isotope effect a = 0.21, in good agree- ment with the experimental value of 0.26 ± 0.03. Since the pressure dependence of T puts a stringent

test on any theory of superconductivity, we repeated the calculations of phonons and electronic band structures discussed above for isotropic, uniaxial (along c-axis), and

biaxial (in the aZ^-plane) pressures [3]. We find that while

go 25 50 75 100 125 Energy (meV)

FIGURE 2. Generalized (top) and the calculated (bottom) phonon density

of states. The intensities of the peaks do not agree well because in DOS

B and Mg ions contribute equally while in GDOS they are weighted by

neutron cross-sections and inverse masses.

18 Research Highlights
T. Yildirim NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, IVID 20899-8562

co{E^J increases with increasing pressure, the density of

states at the Fermi energy decreases. The EP constant X also shows significant changes with pressure. Inserting all these

competitive effects into the McMillan formula yields the pressure dependence of T shown in Fig. 4. For isotropic pressure T decreases with increasing pressure almost linearly at a rate of = - 1 .0 K/GPa, in excellent agreement with the experimental value of - 1 . 1 K/GPa.

We also predict a cusp in the T -P curve around P = 20 GPa.

0.25 \

{u)^Ay^A^ u\ /\2=9.4 eV/A\ /\,=690.3 eV/A

w(£2g)=74.5 meV

-0.10 -0.05 0.00 0.05 0.10 Boron displacement in E^^ Frozen Phonon (A)

FIGURE 3. Top: Total energy curve as a function of boron displacement

for E^^ mode (shown in the inset), indicating a large anharmonic term in the potential. Bottom: Band structure of the undistorted (left) and

distorted structures (right) by E^^ phonons (u^ = 0.06 A). See Ref. 1 for the animation of the zone center phonons and their coupling with the B a bands.

0. Gulseren NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

and

Department of Materials Science and Engineering

University of Pennsylvania

Philadelphia, PA 19104-6202

r,= r„M-« 7„ = 63,527

10.4 10.6 Boron mass (Mg11.0

Fits 1:7,= 38.8 -^2:7,= 38,8 44.8

Fits

1.0 P+ 0.020 P

1.1 P+ 0.026 1,1 P+0,011 P^

isotropic compression • c-axis compression ab-axes compression

10 20 30

Pressure (GPa)

40

FIGURE 4. Top: Boron mass dependence of the 7 . Bottom: Pressure dependence of T as a function of uniform ad-axes and oaxis

compression, respectively. A similar cusp was recently observed experimentally. Our

calculations indicate that T should increase first and then

c decrease with increasing c-compression, while it should

decrease rapidly with flfo-compression. Hence, when single

crystal samples of MgB,, become available, measurements of

the ab- and c-compression dependence of T should provide

a critical test of our theory.

MgB, may be the ultimate BCS s-wave superconductor, with parameters controlling T fully optimized to yield

the highest possible 7,. However, even if cannot be

increased further, the low cost, hght mass, easy fabrication,

nearly isotropic high conductivity of MgB, , which also has a

large critical current, will no doubt find many important

technological applications in the near future. References

[1] For full details of this work, see the website: http://www.ncnr.nist.gov/

staff/taner/mgb2

.

[2] T. Yildirim, O. Gulseren, J. F. Lynn, C, M, Brown, T. J. Udovic,

Q. Huang, N. Rogado, A. Regan, M. A. Hayward, J. S, Slusky,

T, He, M. K. Haas, P, Khalifah, K. Inumaru, and R. J, Cava, Phys, Rev, Lett. 87, 37001 (2001).

[3] T. Yildirim and O. Gulseren, J. Phys.Chem. Solids (in press, 2002).

NIST Center for Neutron Research 19
Polaron Formation and Colossal

Magnetoresistance in Manganites

The recent discovery of huge magnetoresistance effects

in the manganese oxide class of materials (such as La^ A^MnO, (A = Sr. Ca, Ba)) has rekindled intense interest

in these systems for three reasons. One is that this "colossal magnetoresistance" (CMR) offers potential in a number of

technologies, such as for read/write heads in magnetic

recording media, sensors, and spin-polarized electronics. The second reason is that these materials are interesting from

a fundamental viewpoint. In contrast to traditional isotropic

ferromagnets such as Fe, Co, Ni, and EuO where the spin

system is isolated from the lattice, in the manganites the charge, spin, and lattice degrees of freedom are strongly

coupled together, leading to a delicate balance of interactions that gives rise to a rich variety of physical phenomena

of cun'ent interest in condensed matter science. These

include a metal-insulator transition that is juxtaposed with

ferromagnetism, charge ordering, orbital ordering, polaron

formation, electronic phase separation, and spin and charge

stripes. The third reason is because of the similarity to the high T cuprate oxides. Recent progress in our understanding of the cuprates has provided insights into the manganites, and a deeper understanding of the fundamental properties of the manganites will surely elucidate the shared

concepts underlying both classes of materials. The electrical conductivity in these manganites is

directly connected to the magnetic system through the double exchange mechanism, while the Jahn-Teller distor- tion of the Mn'^^O^ couples the magnetic and lattice systems,

resulting in the electronic, lattice, and magnetic degrees of

freedom being intimately intertwined. The ferromagnetic

transition then is non-conventional, and typically coincides

with a metal-insulator transition to produce CMR. The

present measurements on La„^Ca|, ,MnO,, which is near

optimal doping for CMR, show that polarons form as the

ferro- to paramagnetic transition is traversed, self-trapping the carriers. These polarons then "melt" when a magnetic

field is applied, freeing the carriers and generating the CMR.

The scattering from a lattice polaron arises from the

structural distortion that surrounds a carrier and traps it. Individual polarons generate diffuse scattering around the fundamental Bragg peaks, while polaron correlations will give rise to additional superlattice peaks. We observe both

of these effects [1], and in Fig. 1 we show the four satellite

FIGURE 1. Polaron correlation peaks near the 5 0 0 Bragg peak at 270 K, just above r = 255 K.

peaks at Va Va 0 positions near the 5 0 0 Bragg peak, just

above T (255 K). The intrinsic widths of these peaks

indicate that the correlations are short range {- 15 A) in nature, and this range is found to be only weakly temperature dependent. Polarized neutron measurements show that these peaks originate from lattice distortions rather than

magnetic scattering. Similar direct evidence for polaron

formation and correlations has been observed in the bilayer

manganite La^ .Sr, j,Mn,0^ [2], although the ordering wave

vectors and the ratios of diffuse scattering to coiTelated

scattering are different. A model that can explain both the ordering wave

vector as well as the overall behavior of the observed

intensities of the short-range correlation peaks is the CE

model developed for the half-doped case. The CE model

has an orbitally-ordered Jahn-Teller lattice with charge

stripes in the [110] direction, and an antiferromagnetic

ground state. Such ordering has been observed for x > 0.5

at low temperatures. This behavior contrasts with the present ferromagnetic system, where the orbital/charge

correlations are short range in nature, occur only above T

,

and are not accompanied by any antiferrofnagnetic correlations. The CE "stripes" are also consistent with stripe

formation as seen in other transition metal oxides (nickelates

and cuprates).

Of particular interest is how the polaron ordering is

related to the metal-insulator transition and the quasielastic

20 Research Highlights
C. p. Adams' and J. W. Lynn

NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg. MD 20899-8562

and

' University of Maryland

College Park, MD 20742

Y. M. Mukovskii, A. A. Arsenov, and D. A. Shulyatev Moscow Steel and Alloys Institute Moscow, Russia

"central peak" [3] associated with the spin component of the polaron. As we see in Fig. 2, the temperature dependence of

the intensity of the charge ordering is nearly identical to the quasielastic magnetic peak, and both follow the resistivity

behavior. This is a very strong indication that these properties have a common origin in polaron formation.

A further indication of the strong relationship between

charge ordering and resistivity is established by measure- ments in magnetic fields [1]. In Fig. 3 we see that the

intensity of the charge ordering peaks decreases rapidly with

increasing field. Thus the polarons "melt" when the magnetization develops and the conductivity increases, either by

lowering the temperature (Fig. 2) or increasing the applied

field, producing CMR. Identical field dependence has been

observed both for the central peak in ha^.Ca^M-nO^ [1,3] as well as the polaron peaks in the bilayer system [2].

quantities that control the characteristic temperature for polaron formation, including the role that intrinsic inhomogeneities, stress, and phase separation play in their formation, still remain to be elucidated.

References

[1] CP. Adams. J. W. Lynn, Y. M. Mukovskii, A. A. Arsenov, and D. A.

Shulyatev. Phys. Rev. Lett. 85, 3954 (2000); J. Appl. Phys. 89, 6846

(2001).

[2] L. Vasiliu-Doloc, S. Rosenkranz, R. Osbora, S. K. Sinha, J. W. Lynn,

J. Mesot, O. Seeck, G. Preosti, A. J. Fedro, and J. F. Mitchell, Phys. Rev. Lett. 83,4393 (1999).

[3] J. W. Lynn, R. W. Erwin, J. A. Borchers, Q. Huang, A. Santoro, J. L. Peng, and Z. Y. Li, Phys. Rev. Lett. 76, 4046 (1996); J. Appl. Phys. 81, 5488 (1997).

200

E

3

OO

100

0

• charge order

central peak -- resistivity

T

r

i

220 240 260 280

Temperature (K)

300

15

10 -

5

-

u

1, 7= 270 K

0.0 2.0 4.0 6.0 Magnetic Field (T)

FIGURE 2. Temperature dependence of the intensity of the polaron peak

at 3.75 0.25 0, compared to the magnetic quasielastic central peak, and

the resistivity. The data have been scaled vertically so the peak heights match. The similarity of the data indicates a common physical origin.

The above experimental results reveal that both the spin and charge correlations associated with the polarons in

La^j ^Ca^ jMnOj appear together, and have a very similar

temperature dependence. This coincidence may explain the amplified magnetoresistive effects, as well as the absence of

conventional magnetic critical behavior. The physical

FIGURE 3. Magnetic field dependence of the intensity of the polaron

peak, showing that the polarons melt with increasing field.

NIST Center for Neutron Research 21
Ferromagnetism and Spontaneous Vortex

Formation in Superconducting ErNi^B^C

The competition between magnetic order and superconductivity has had an interesting history. The first examples of true long range magnetic order coexisting with

superconductivity were provided by the ternary Chevrelphase superconductors and related compounds. The magnetic ordering temperatures were all low (= 1 K), indicating

that dipolar interactions dominate the energetics of the magnetic system. Similar behavior is found for the rare

earth ordering in the hole-doped cuprates [1]. The first systems where exchange clearly dominates the magnetic

energetics were provided by the electron-doped cuprates [1], followed by the borocarbides of central interest here [2]. Among these systems, the rare and interesting situation where ferromagnetic interactions are present has attracted

special attention because of the competitive nature between

the superconducting screening and the internally generated

magnetic field. In the Chevrel phase materials this competition gives rise to long wavelength oscillatory magnetic states and/or reentrant superconductivity. Recently, superconducting ErNi.B^C (T = 11 K) appeared to develop a net magnetization below 2.3 K, well below the onset of long range spindensity-wave order at = 6 K. We have carried out a comprehensive study of the magnetic order and vortex

structure in this material, and directly demonstrated that this

transition does indeed correspond to the development of a

net atomic magnetization that coexists with superconductivity, and this results in the spontaneous formation of vortices

in the system [3]. Superconducting ErNi^B^C orders magnetically at 6 K

into a transversely polarized spin density wave structure

[2, 3], with the modulation wave vector 5 along the o axis and the spins along the b direction. A portion of a scan along the a axis is shown in Fig. 1 at three temperatures.

The initial ordering is a simple spin density wave as shown

in the schematic, with an incommensurate modulation of

<5s 0.55 a* (a* = 27i/a). At 2.4 K additional higher-order

peaks become observable as the magnetic structure squares up as indicated schematically at the bottom of Fig. 1 . These

peaks are all odd-order hannonics as expected for a square- wave magnetic structure.

Below the weak ferromagnetic transition (r„^J at WFM

2.3 K. the data (Fig. 1 ) indicate that a new series of peaks

has developed, which are even harmonics of the fundamental wave vector. Polarized neutron measurements unambiguously establish that both the odd-order peaks and the evenorder peaks are magnetic in origin.

The integrated intensities of the odd- and even-order

harmonics are shown in Fig. 2 as a function of T. The evenorder peaks abruptly develop below 2.3 K, concomitant with

the development of the new (weak) ferromagnetism at T^^.

0.2 0.3 0.4 0.5 0.6 0.7 O.l

[hOO]

Sine SDW

Square

SDW

FIGURE 1. Scan along the [hOO] direction at T= 1.3 K, 2.4 K, and 4.58 K. The data have been offset for clarity. Above the weak ferromagnetic

transition at 2.3 K we observe the fundamental peak at = 0.55 along with odd-order harmonics, while below a new set of even harmonics develops

(arrows). Also shown is a schematic of the spin density wave order for T

just below 7^, and the (undistorted) wave when it squares up.

22 Research Highlights
S.-M. Choi and J. W. Lynn D. Lopez, P. L. Gammel, and P. C. Canfieid and S. L. Bud'ko

NIST Center for Neutron Research C. M. Varma Ames Laboratory and Department of Physics and Astronomy

National Institute of Standards and Technology Bell Laboratories Iowa State University Gaithersburg, MD 20899-8562 Lucent Technologies Ames, lA 5001

1

and Murray Hill, NJ 07974

University of Maryland

College Park, MD 20742

T(K)

FIGURE 2. Integrated intensities of the odd (blue circles) and even

(red circles) order harmonics as a function of T.

There is a substantial thermal hysteresis associated with the weak ferromagnetic transition, suggesting that this transition

is first order in nature. The present neutron results demonstrate that a net magnetization develops in ErNi^B^C in the magnetically

ordered state at low temperatures, making this the first such

"ferromagnetic-superconductor" since HoMOgS^, HoMo^SCg,

and ErRh_,B^. For ErNi^B^C the net magnetization is much

smaller than for these earlier systems, which allows coexist- ence with superconductivity over an extended T range. This

presents the intriguing possibility that in an applied field

vortices will form spontaneously when this net atomic

magnetization is present. The small angle scattering data of

the vortex structure (Fig. 3 top) show that the lattice has the expected spacing at lower applied fields. At higher fields, as

the field begins to penetrate, vortices spontaneously form in addition to those expected from the applied field alone,

increasing the vortex density and shifting the vortex peak.

The T dependence of this shift (Fig. 3 bottom) makes it clear

that this behavior is directly related to onset of the net magnetization in the system. It is also interesting to note

that the vortex pinning is enhanced in both magnetic phases,

which may prove useful in high current applications of

superconductors.

0.0032 ' 1,1, 1,1

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 r(K)

FIGURE 3. Intensity of the vortex scattering vs. wave vector Oat 850 Oe

above and below the weak ferromagnetic transition. The shift in the peak

position demonstrates that additional vortices spontaneously form at low

temperatures. The temperature dependence (bottom) shows that this spontaneous vortex formation is directly related to the weak

ferromagnetic transition.

References

[1] J. W. Lynn and S. Skanttiakumar, Handbook on the Physics and

Chemistry of Rare Earths, Chap. 199, Vol. 31, ed. by K. A.

Gschneidner, Jr., L. Eyring, and M. B. Maple, North Holland, Amsterdam (2001) p. 313.

[2] J. W. Lynn, S. Skanthakumar, Q. Huang, S. K. Sinha, Z. Hossain, L. C. Gupta, R. Nagarajan, and C. Godart, Phys. Rev. B55, 6584 (1997).

[3] S.-M. Choi, J. W. Lynn, D. Lopez. R L. Gammel, R C. Canfieid and

S. L. Bud'ko, Phys. Rev. Lett. 87, 107001 (2001).

NIST Center for Neutron Research 23
structure of Local Spin Excitations in a

Geometrically Frustrated Antiferromagnet

For many crystalline magnetic materials, only a long

range ordered spin configuration can satisfy all near neighbor spin interactions. Such systems generally display a

finite temperature transition to a broken symmetry phase

with long-range magnetic order. However, on certain lattices with weak connectivity and a triangular motif, short-range

interactions can be satisfied without long-range order [1]. To explore this possibility we examined magnetic

order and fluctuation in ZnCr,0^. The B-site of this spinel

lattice is solely occupied by spin-3/2 Cr'''^, and this leads to a magnet with dominant nearest neighbor interactions on the

lattice of comer-sharing tetrahedra shown in Fig. 1.

FIGURE 1. Lattice of corner-sharing tetraliedral formed by the

octahedrally coordinated B sites of the spinel AB^O^. Black lines represent a structural unit cell containing eight tetrahedra. Vertices are populated by antiferromagnetically interacting spin-3/2 degrees of freedom in ZnCrp^. Yellow line and six red spins represent a hexagonal

loop of antiferromagnetically aligned spins.

Exchange interactions in this structure are satisfied at the mean field classical level when every tetrahedron carries zero net magnetization. Not only are there many ways that a tetrahedron can have no moment, but there are also many

ways to organize such non-magnetic tetrahedra on the comer-sharing lattice. The result is a "geometrically frus- trated" spin system with many ways to satisfy interactions without long-range order.

Analysis of specific spin models on the B-site spinel

(or pyrochlore) lattice indicates that spin- 1/2 and spin-oo

models have short-range order down to T = 0 [2], while

long-range order is induced by quantum fluctuations for an unknown intermediate range of spin values. Experiments

indicate that spin-3/2 ZnCr^O^ is "close" to the quantum

critical point that separates the low spin quantum-disordered

phase from the intermediate spin long-range ordered phase.

Specifically, the relaxation rate for magnetic excitations, F,

follows a power-law that extrapolates to zero as T ap- proaches 0, indicating quantum criticality [3]. This state of

affairs, however, does not persist to the lowest temperatures.

Instead, at = 12.5 K a first order structural transition from

the cubic cooperative paramagnet to tetragonal Neel order

intervenes.

Figure 2 shows that a gapless continuum of magnetic

scattering above is pushed into a local spin resonance at h(0 ~ 4.5 meV = J with remarkably little dispersion throughout the Brillouin zone. The result is analogous to the spin- Peierls transition of the uniform spin- 1/2 chain, where

quantum critical fluctuations are pushed into a finite energy

singlet-triplet transition through structural dimerization. Our

300

B 100

0

f

J r=i5K

: . \

(1.25 1.25 0) 7=1.5 K

. (1.25 1.25 0)

\\ ' (1.510)

- % .(110)

- H \. i

. TI . 1 . . . 1 . . . 1 . . . 1 . . . 1 . . .

. I . . . I . . I 1 I I 1 1 1 i 1 1 I -2 0 2 4 6 8 10 Ac<;(meV)

FIGURE 2. Constant-0 scans above and below = 12.5 K. In the cooperative paramagnetic phase above magnetic excitations form a continuum. Below the spectrum changes dramatically with a nearly

dispersionless excitation near 4.5 meV appearing abruptly. The red curves are to guide the eye.

24 Research Highlights
S.-H. Lee

NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

and

University of Maryland

College Park, MD 20742

C. Broholm and G. Gasparovic

Department of Physics and Astronomy The Johns Hopkins University Baltimore, MD 21218

T. H. Kim

Francis Bitter Magnet Laboratory Massachusetts Institute of Technology

Cambridge, MA 02139

W. Ratcliff and S.-W. Cheong

Department of Physics and Astronomy

Rutgers University Piscataway, NJ 08854

recent synchrotron x-ray and neutron powder experiments

indicate that deformation of tetrahedra does indeed occur for ZnCr.O^. However, the structural changes push the system to long range order rather than to quantum disorder, as indi0 1 2 3 0 1 2

(OkO) (Okk)

Gated by the magnetic Bragg peaks and spin wave excitations. Our single crystal experiment [4] also enabled unique

insight into the local structure of spin-fluctuations in geometrically frustrated systems. Figure 3 shows the Q-d&pendence of low energy magnetic scattering in two high symmetry planes above and below T^. While the spectrum for spin

fluctuations changes dramatically at the first order phase

transition, the structure factor clearly does not. Also shown

in the figure is the structure factor for six spins of <111>

type kagome hexagons precessing with n phase shift between neighbors. The proposal by O. Tchemyshyov et al.,

[5] that these are the dominant low energy spin fluctuations

for spins on the B-site spinel lattice is clearly borne out by

the data. The present data for ZnCr^O^ show that geometrically

frustrated lattices have composite low energy degrees of

freedom analogous to rigid unit modes in certain open

framework lattice structures. To better understand the unusual type of phase transition that occurs in this system, it must be determined what defines the 4.5 meV energy scale

for hexagon excitation in the ordered phase. Do quantum

fluctuations play a significant role or does the broken

symmetry between exchange interactions within tetrahedra induce the resonance? The answer to this question is now

being pursued through an accurate determination of the complex low temperature lattice and magnetic structure.

References

[1] p. W. Anderson etal., Ptiilos. Mag, 25. 1 (1972); J. Villain, Z. Phys. B

33, 31 (1979).

[2] R. Moessner et al., Phys. Rev. Lett. 80, 2929 (1998); B. Canals et al., Phys. Rev. Lett. 80, 2933 (1998).

[3] S.-H. Lee, C. Broholm, T.H. Kim, W. Ratcliff H, and S.-W. Cheong,

Phys. Rev. Lett. 84. 3718 (2000).

[4] S.-H. Lee et al., unpubli-shed (2002).

[5] O. Tchernyshyov et al., unpublished (2002).

FIGURE 3. Color images of inelastic neutron scattering intensities in the

(hkO) and (hkk) symmetry planes. From top to bottom are shown data from below and above T^, and the structure factor for the hexagon mode

proposed by 0. Tchernyshyov. r= 1 .7 K data were taken integrating over

the energy range 5fi)from 3.0 meV to 6.7 meV. Data in the paramagnetic

phase (7 = 15 K) were taken at fi<w= 1 meV.

NIST Center for Neutron Research 25
i Pressure-Induced Phase Transition of C.-E. iViicelles 12 5

I onionic surfactants form a variety of microstructures

in water, ranging from simple micelles at low surfactant concentrations to complex mesophases, such as hexagonal or lamellar phases at high concentrations. The effect of

pressure on the structure of micelles and microemulsions has

not been extensively studied. Small angle neutron scattering

(SANS) is particularly well suited for such measurements

because the range of length scales probed includes both the

particle size and the interparticle spacing. Pressure effects observed in SANS measurements of

surfactant microstructure are typically interpreted in terms of

geometric packing arguments that focus on the compressible

hydrophobic tails of the surfactants. In this context, increasing pressure has the single effect of decreasing the surfactant

tail volume, thereby increasing the curvature of the oil-water

interface. Conversely, increasing temperature dehydrates the nonionic headgroups, decreasing the headgroup area and

decreasing the film curvature. Thus, temperature and

pressure can be viewed as thermodynamic variables with

inherently different mechanisms and opposite effects for controlling microstructure in nonionic surfactant solutions. Here we report the results of high-pressure SANS

experiments at 20 °C and pressures up to = 300 MPa on a

solution of pentaethylene glycol mono-n-dodecyl ether

(Cj^Ej) in D,0 having a mass fraction of 1 %. The phase

diagram for this system [1] at ambient pressure is shown in

Fig. 1. At this temperature and surfactant concentration, a single-phase micellar solution (L, phase) forms at ambient

pressure, well below the lower critical solution temperature

(LCST) for liquid-liquid equilibrium and far removed from

the H| hexagonal phase at much higher C|,Eg concentrations.

The microstructure of the -phase at ambient pressure is known to be a network of branched semi-flexible, cylindrical

micelles with the branch points comprised of three-armed

junctions.

SANS spectra were measured using the NIST highpressure cell and neutrons of wavelength A = 6 A, covering a ^-range of 0.012 A'^ < q < 0.22 A '. The scattering curves

obtained at 3.4 MPa, 241 MPa, and 255 MPa are shown in

Fig. 2. The curves at 3.4 Mpa and 241 MPa are virtually

identical, indicating no significant change in microstructure

with increasing pressure up to 241 MPa. Fitting these curves

using a form factor for cylindrical micelles gives a radius of

(21.0 ± 0.2) A and a length greater than 600 A, independent

of pressure. However, a small increase in pressure from

241 MPa to 255 MPa leads to the appearance of a peak in the scattering intensity at ^ = 0.130 A ', indicative of a

locally ordered system. A similar transition has been re- ported in high-pressure SANS studies of tetradecyldimethylaminoxide (TDMAO) micelles in D^O at pressures up to 300 MPa, but the high-pressure microstructure was never determined [2].

100

80

60

oo

40

20

0 20 40 60 80 100

H^O C^JE^ (% mass fraction) C^^^^

FIGURE 1 . Temperature-composition phase diagram for C^^Ej/water

solutions at ambient pressures [1].

Shown in Fig. 2 is the scattering curve for the H^

hexagonal phase at 49 % Ci^E^ mass fraction at 20 °C, and

ambient pressure. The peak at 9 ~ 0.120 A"\ arising from

the hexagonal lattice of cylindrical micelles, is similar to the peak for the 1 % Ci^E,. mass fraction solution at 255 MPa,

suggesting that this new high-pressure phase may resemble a

slightly compressed state of the H^ hexagonal phase at ambient pressure. The formation of a lamellar phase from

cylindrical micelles is unlikely, since this corresponds to increasing the hydrophobic core volume-to-surface-area ratio per surfactant molecule, or equivalently, decreasing the spontaneous curvature of the surfactant film. The application

26 Research Highlights ; J
D. P. Bossev and M. E. Paulaitis J. N. Israelachvili S. R. Kline Department of Chemical Engineering Department of Chemical Engineering NiST Center for Neutron Research Johns Hopkins University University of California, Santa Barbara National Institute of Standards and Technology

Baltimore, Maryland 21228-2694 Santa Barbara, CA 93106 Gaithersburg, MD 20899-8562

1000 r

100 ^

Micellar {L,, 1% mass fraction C,;Es), = 3.4 MPa

- Micellar (L„ 1% mass fraction C,jE^. p = 241 MPa

-New Phase (1% mass fraction C.^E,). p= 255 MPa

-Hexagonal (H.. 49% mass fraction C.-E^), p= 0.1 MPa ,'

FIGURE 2. Measured SANS spectra at 20 °C for 1 % mass fraction C^^Ej in DjO as a function of pressure and 49 % mass fraction C,2E5 in D^O at ambient pressure.

of pressure would have the opposite effect. We conclude,

therefore, that the observed change in microstructure

corresponds to a pressure-induced Lj-Hj phase transition from a network of branched semi-flexible, cylindrical

micelles to hexagonally ordered bundles of cylindrical

micelles.

To further understand the SANS results, we have

measured the temperature dependence of the Lj-H^ transition pressure and find that the p-T curve follows the p-r freezing curves for liquid /z-alkanes of comparable hydrocarbon chain

length. A^-decane solidifies at a pressure of = 250 MPa at 20 °C, which is close to the pressure for the observed L^-Hj

phase transition. We propose that the Cj,E_ micelle hydrophobic core, equivalent to «-decane, does solidify at these

conditions, such that the micelles lose flexibility, and hence

conformational entropy. An analysis of the geometric

packing constraints for three-arm junctions coexisting with

cylinders shows that when the surfactant tail volume de- creases with increasing pressure, the fraction of surfactant forming junction points also decreases. This indicates that

the formation of three-arm junctions becomes increasingly

unfavorable at higher pressures due to the compression of

the C|,E_ micelle hydrophobic core. Our calculations predict

that no junctions should be present at P > 275 MPa at 20 °C,

which is in good agreement with our observation of a

structural transition between 241 MPa and 255 MPa at this temperature. Consequently, the network of branched threadlike micelles becomes globally unstable. The formation of

hexagonally ordered bundles of cylindrical micelles follows

as the attractive van der Waals forces between the micelles

are not offset by entropic repulsive undulation interactions

that are not present in the now-solidified hydrophobic core. The practical significance of these results is to show

that pressure allows access to regions of the C^E./water

phase diagram that are virtually inaccessible to temperature.

Thus, the use of pressure may offer unique approaches for directing and stabilizing certain surfactant microstructures

that, in turn, could prove useful for creating novel soft materials.

References

[1] R. Strey. R. Schomacker, D. Roux, F. Nallet, U. Olsson. J. Chem. Soc. Faraday Trans. 86, 2253 (1990).

[2] N. Gorski. J. Kalus. D. Schwahn. Langmuir 15. 8080 (1999).

NIST Center for Neutron Research 27
Probing the Structure of Aerosol Nanodroplets

I anodroplet aerosols form readily in the supersonic

I expansions that occur, for example, in turbomachinery, jet exhausts, and volcanic eruptions. Thus,

understanding particle formation and growth when cooling

rates approach 10^ K/s is of broad scientific interest. From a fundamental point of view, particles with radii < 10 nm are important because they lie in the critical

transition zone between large molecular clusters and bulk

material. In multicomponent droplets, there is also strong

theoretical evidence for surface enrichment, i.e., that the surface and interior compositions can differ significantly.

Surface enrichment is important because it affects nucleation, growth and evaporation kinetics, and the heteroge- neous chemistry of aerosol droplets. A major goal of our work is to use small angle neutron scattering to find direct evidence for surface enrichment in nanodroplets.

We produce nanodroplets with radii between 5 nm and

12 nm and number densities in the range of 10" cm to

10'- cm "* by rapidly expanding a dilute vapor mixture of D^O

(or other condensible) and N, in a supersonic nozzle apparatus (Fig. 1) [1]. The volume fraction of droplets is always

less than 10'^ and, thus, the scattering signals are close to the instrument background. Furthermore, the high velocity of

the droplets (400 m/s to 500 m/s) is comparable to the speed

of the neutrons. This leads to a Doppler shift in the momentum of the scattered neutrons [2, 3], and the 2-D scattering

>1 .tte-1 6.0e-2-1.0e-1

A.Oe-2 - 6.De-2 3.0e-2 - 4.0e-2 ZOe-2 - 3.0e-2

FIGURE 2. The observed 2-D scattering pattern for an aerosol with an

average particle velocity Vp= 435 m/s measured using a neutron wavelength of A= 1.0 nm (i/„= 400 m/s). The contour levels correspond to absolute intensities of 0.08, 0.03, 0.008 and 0.003 cm-\ respectively.

^ Detector

Plenum

Flow

\

< \

Nozzle\

Neutron

beam

FIGURE 1. The nozzle is placed in the sample chamber at right angles to the neutron beam.

patterns are anisotropic (Fig. 2) even though spherical

droplets at rest scatter isotropically. This Doppler shift can be used to directly measure the velocity of the particles, and,

in a test case, this velocity was within 2 % of the average

velocity derived from the pressure trace measurements [2]. Rather than simply examine nanodroplets for which

segregation of the components is severe, we use different materials to observe the transition between well-mixed and

fully segregated droplets. Thus, our experiments include

binary mixtures of H^O, D^O, ethanol, and n-butanol, or its

fully deuterated analog d-butanol. Almost degenerate

mixtures, such as D^O - H^O, appear to form well-mixed

droplets [4]. In contrast, binary nanodroplets containing H^O

(or D,0) and a low molar concentration of d-butanol (or

h-butanol) should consist of a water-rich core and an

28 Research Highlights
B. E. Wyslouzil, C. H. Heath,

U. M. Dieregsweiler, and K. A. Streletzky Department of Chemical Engineering Worcester Polyteclinic Institute Worcester, MA 01609

R. Strey, J. Wolk

Institut fur Physikalische Chemie

Universitat zu Koln D-50939 Cologne, Germany

G. Wilemski

Physics Department

University of Missouri - Rolla

Rolla, MO 65409

J. Barker NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

alcohol-rich shell. Our recent SANS experiments with

H,0 - d-butanol nanodroplets clearly demonstrate for the

first time the existence of the alcohol-rich shell. As illustrated in Fig. 3, for these droplets the scattered intensity falls off as q

- in the high q region, as is characteristic for scatter- ing by a shell. Furthermore, the feature in the region

0.4 nm ' < <7 < 0.5 nm ' cannot be reproduced by a well- mixed droplet model that matches the low q data. Also

shown in Fig. 3 is the scattering spectrum from the complementary D^O - h-butanol experiment. As expected, the

scattered intensity is much higher, and the signal intensity

decreases as q'^ in the high q region. In this case, the D,0-rich core dominates the scattering.

In summary, SANS provides us with a powerful new

way to study the properties of nanometer sized liquid

droplets in the environment in which they form. To date, it is

the only technique to directly probe the microstructure of

aerosol nanodroplets. Combined with pressure trace measurements and modeling, SANS provides information

critical to our understanding of droplet formation and growth

in the nanometer size regime.

References

[1] B. E. Wyslouzil. J. L. Cheung, G. Wilemski, and R. Strey, Phys. Rev.

Lett. 79, 431 {\991).

[2] B. E. Wyslouzil, G. Wilemski, J. L. Cheung, R. Strey, and J. Barker, Phys. Rev. E 60, 4330(1999).

[3] G. Wilemski, Phys. Rev. E 61, 557 (2000).

[4] C. H. Heath. K. A. Streletzky, J. Wolk, B. E. Wyslouzil, and R. Strey,

in Nucleation and Atmospheric Aerosols, 2000, ed. by B. N. Hale and M. Kulmala, American Institute of Physics, New York (2000) p. 59.

10"

10"

^ 10"' -

^ 10"'

10"

10"

n 1—I—1—T" D^O-h butanol BbBb H,0

o 2.0 m SDD

3.75 m SDD

d - butanol 2.0 m SDD

10"

q, nm

FIGURE 3. Scattering from a D^O - h-butanol aerosol is compared to that from a H^O - d-butanol aerosol formed under identical conditions in the

nozzle. Both aerosols contain « 6 % molecular fraction butanol. In the high q

region, the intensity falls off as q" for the D^O-rich droplets, but only as q

'^ for the HjO-rich droplets. Our preliminary modeling shows

that the feature in the region 0.4 nm-^ < qf < 0.5 nm-^ of the H^O - d-butanol spectrum cannot be reproduced by a well mixed droplet model. SDD is the sample-to-detector distance.

NIST Center for Neutron Research 29
Nanoporous Ultra-Low Dielectric Constant Materials

The next generation of interlayer dielectric materials for microelectronics must have an ultra-low dielectric constant of less than 2.0 to meet the National Technology

Roadmap for Semiconductors [1]. In addition, the material

must have excellent thermal stability and mechanical

properties [2, 3]. One route is to introduce nanometer sized pores into a material with an initially low dielectric constant

(A' = 2 to 3). The presence of the pores lowers the dielectric constant of the nanoporous film as the dielectric constant of

air is 1 .0. One class of materials that is receiving significant

attention is based on poly(methylsilsesquioxane) (PMSSQ)

which has a dielectric constant between 2.7 and 2.8 (lower

than conventional SiOj and good thermal stabihty up to 500 °C [4, 5]. The dielectric constant is a function of the

level of porosity and detailed knowledge of the morphology

is required for implementation in electronic packaging.

A schematic of the synthesis process is shown in

Fig. 1. The materials are based on an inorganic matrix of

PMSSQ containing a porogen (i.e., a labile pore generating

material) based on a poly(methylmethacrylate) copolymer

(coPMMA) as a template for creating the pores [6, 7]. A

film is prepared by dissolving the PMSSQ matrix precursor

material and the coPMMA porogen in a common solvent

and spin casting onto a silicon substrate. The film is then slowly ramped up in temperature to 450 °C. During this temperature ramp, the matrix film fully cures by about 225 °C, resulting in microphase separation of

the coPMMA polymer. Above about 350 °C the coPMMA

PMMA copolymer

porogen material

PMSSQ matrix

precursor material

combine components

and spin cast

porogen material or pores Cured inorganic

or polymer matrix

^^ol5° oo o oo o"? "

O Oq O O O Q

qO^O O qO

Silicon wafer

polymer degrades leaving nanometer sized pores. Neutron

scattering was used with a deuterated coPMMA polymer to understand the morphological development of the films

during the cure. The use of the deuterated porogen provides

contrast for neutron scattering in the hybrid (polymer

containing) system that is not available with x-ray scattering. In situ studies of the curing process by small angle neutron scattering (SANS) were performed. To provide

sufficient scattering signal, four or more samples on Si

wafers were stacked to increase the scattering volume.

Figure 2 shows a set of SANS data from a PMSSQ

thin film containing 20 % molecular fraction of porogen at various cure temperatures. The scattered intensity varies as a function of curing temperature due to morphology changes

occurring in the sample. For the as-spun materials there is a shoulder at high q from which a radius of gyration {R) of

12 A was obtained by Guinier analysis. After heat treatment

at 225 °C the shoulder in liq) becomes more pronounced

and shifts to about q - 0.06 A '. The shift to smaller q

indicates a coarsening of the microphase separated porogen

domain stmcture. A radius of gyration of 7?^ = 23 A for the domains was obtained for the cured materials. The scatter- ing curves do not change significantly between 225 °C and

300 °C, indicating that the morphology remains fixed until the degradation of the polymer at 450 °C. The dramatic

25 °C

125 °C

225 X

300 °C 450 °C

deuterated toulene

0.010 0.100

FIGURE 1. Schematic showing the generation of nanoporous PMSSQ

films. FIGURE 2. SANS data taken during cure of a nanoporous PMSSQ thin

film.

30 Research Highlights
G. Y. Yang and R. M. Briber E. Huang, H.-C. Kim, K. Shin

Department of Materials and Nuclear Engineering W. Voll(sen, and R. D. IWiller Department of IVIaterials Science and Engineering

University of Maryland IBM Almaden Research Center State University of New York

College Park, MD 20742 San Jose, CA 95120 Stony Brook, NY 11794

decrease in intensity of the SANS curve at 450 °C is due to a

loss of neutron scattering length density contrast because of

the degradation of the porogen and the formation of pores in the film. To provide neutron contrast between the pores and the matrix, the fully cured sample was exposed to deuterated

toluene. These data are also shown in Fig. 2. The scattering

is largely recovered after exposure to deuterated toluene,

indicating that the porous structure is maintained with no

collapse after degradation of the polymer.

Figure 3 shows a set of neutron reflectivity profiles and

scattering length density (SLD) profiles of a nanoporous

PMSSQ thin film prepared with 20 % molecular fraction of

porogen and deposited on a silicon substrate. The film was

exposed to deuterated toluene liquid and vapor to examine

solvent swelling. The film thickness is observed to increase

upon exposure to both liquid and vapor. A higher porosity is observed in a 50 A region next to the silicon surface. This is probably due to porogen surface segregation at the substrate

interface and is in agreement with results from transmission

electron microscopy.

References

[ 1 ] The National Technology Roadmap for Semiconductors: Technology

Needs, SIA, Semiconductor Industry Association, 1997 Edition,

p. 101.

[2] R. D. Miller. J. L. Hedrick, D. Y. Yoon, R. F. Cook, and J. P. Hummel,

Mater. Res. Soc. Bulletin 22, 44 (1997).

[3] C. Jin, J. D. Luttmer, D. M. Smith, and T. A. Ramos, Mater. Res. Soc.

Bulletin 22, 39 (1997).

[4] C. V. Nguyen, R. B. Beyers. C. J. Hawker, J. L. Hedrick, R. L. Jaffe, R. D. Miller, J. F. Remenar, H. W. Rhee, M. F. Toney, M. Trollsas, and D. Y. Yoon, Polymer Preprint 40 (1), 398 (1999).

[5] R. F. Cook, E. G. Liniger, D. R Klaus, E. E. Simonyi. and S, A. Cohen, Mater. Res. Soc. Symp. Proc. 511, 33 (1998).

[6] C. V. Nguyen, K. R. Carter, C. J. Hawker, J. L. Hedrick, R. L. Jaffe, R.D. Miller, J. R Remenar, H. W. Rhee, R M. Rice, M. F Toney, M.

Trollsas, and D. Y. Yoon, Chem. Mater. 11, 3080 (1999).

[7] C. Nguyen. C. J. Hawker, R. D. Miller, E. Huang, and J. L. Hedrick, Macromolecules 33, 4281 (2000).

1

0

-1

0-2

-3 -

-4

-5

CD

CD

ro

C/3 1 10

o film dry

film mih d-toluene liquid .

o film with d-toluene vapor

0.00 0.01 0.02 0.03 0,04 0.05 0.06

6 10

510"^

4

10'

3 lO"

2

10"'

film dry

film with d-toluene liquid

film with d-toluene vapor

500 1000 zA

1500 2000

FIGURE 3. Above, neutron reflectivity data (symbols) and best fit (lines) calculated from the scattering length density profiles shown below for a nanoporous PMSSQ thin film under various conditions.

NiST Center for Neutron Research 31
Neutron Reflectivity Studies of Surfactants at

Electrode Surfaces

Knowledge of the forces that control the assembly of

surfactant molecules at the solid-liquid interface is

vital for traditional fields such as detergency, flotation, oil recovery and tribology [1]. Thin organic films deposited at solid surfaces also find application in the fabrication of

optoelectronic devices, sensors, biosensors, and chemically

modified electrodes [2]. It has long been established that the assembly of surfactants at the solid-liquid interface depends on the charge at the solid surface [3, 4]. For example, the spreading of vesicles into a phospholipid bilayer requires

that the surface of the solid be negatively charged and

hydrophilic.

However, the present understanding of the role played

by charge on the interaction of a surfactant molecule with

the electrified solid surface is far from being complete.

Electrochemistry provides a unique opportunity to study the

effect of the charge on the properties of amphiphilic and

ionic surfactants at the charged solid-liquid interface [5]. When an organic film is deposited on a gold electrode, the charge density at the metal surface may be varied from

about 30 |iC/cm- to about 40 |jC/cm-. This magnitude of

charge generates electric fields on the order of 10'" V/m.

Such a field interacts with polar molecules in the membrane.

By changing the sign of the charge one can use attractive or repulsive forces. In this manner, by turning a knob on a control instrument one can force phase transitions in the film

of organic molecules or force surfactants to desorb or re- adsorb on the surface. We have recently employed electrochemical tech- niques, atomic force microscopy, and neutron reflectivity to study the field driven transformations of thin films formed

by a model anionic surfactant, sodium dodecyl sulfate (SDS), at the surface of a gold electrode [6]. Figure 1 shows

how the surface concentration of SDS at the Au electrode

surface changes with the electrode potential. A convenient

way to interpret these data is to look at the electrode potential as an operational variable that can be easily adjusted

using a control instrument.

Figure 1 shows that the character of SDS adsorption is strongly influenced by the charge on the metal. At suffi- ciently negative potentials SDS molecules are totally

desorbed from the electrode surface. At moderate negative

FIGURE 1. Three dimensional plots of the surface concentration of SDS

as a function of electrode potential measured versus the calomel

reference electrode (SCE) and the logarithm of the bulk SDS

concentration.

charge densities SDS forms a film characterized by a

limiting surface concentration 4.0 x 10 '° mol cm '. When the metal surface is positively charged the surface concentration

of SDS increases to 8.1 x 10 '" mol cm'^. Neutron reflectivity experiments carried out on the NG-7 reflectometer were employed to determine the structure of the film formed by SDS at different charge densities

at the gold surface. Thin layers of chromium (~ 20 A) and

gold (= 80 A) were sputtered onto the crystal quartz substrate. After cleaning, the crystal was mounted on a specially

constructed Teflon® cell [7]. The cell had ports for the counter (gold foil) and reference electrodes (Ag/AgCl,

£ = - 40 mV versus SCE). D,0 (99.9 % molecular fraction) was used as a solvent in reflectivity studies. Figure 2a shows the neutron reflectivity data deter- mined for SDS adsorption at various electrode potentials,

and Fig. 2b shows the scattering length density profiles

calculated from the reflectivity curves. The neutron

reflectivity data are consistent with electrochemical measurements. They show that at very negative potentials the gold

solution interface is free from hydrogenated species. When

32 Research Highlights
I. Burgess, V. Zamlynny, G. Szymanski, and J. Lipkowski J. Majewski and G. Smith S. Satija and R. Ivkov Department of Chemistry and Biochemistry Los Alamos National Laboratory NIST Center for Neutron Research

University of Guelph Los Alamos, NM 87545 National Institute of Standards and

Guelph, ONT N1 G 2W1 , Canada Technology

Gaithersburg, MD 20899-8562

I'l l

I

I I

I

I I I I

0.016 mol/L SDS

550 mV

100 mV

300 mV

0.05 0.1

1

I

1 1 1 1 1 1 1

I

1 1 1

1 1 1 1 1 1 1 1

1

1 -700 mV^^^^^

- N

- r- ra E

300 rnVs,.^

- a

3

E Gold ^"nI o

^ J

/[I

1 Q

SDS <i

100 mV J

550 mV

'

, 1 , , . 1 . . . 1 . . . 1 .

610

510

4

10""

c:

B 210

COo

CO

1 10"' -20 0 20 40 60 80 100 120 140

Length [A]

FIGURE 2. (a) Normalized neutron reflectivity curves for a Au/Cr-coated

quartz substrate in 0.016 mol/L SDS in D^O. (b) Scattering length

density profiles of the interface as determined from the fitting procedure.

the potential increases, the film of hydrogenated species appears at the electrode surface. The thickness of this film

increases, and the scattering length density progressively

decreases, with increasing potential. When combined with

the results of electrochemical measurements and atomic

force imaging, the neutron reflectivity data allow the determination of the structure of the film formed at different charge densities. At small or moderate negative charge

densities SDS molecules form a hemimicellar film that

consists of hemicylindrical stripes, as first observed by

Manne [4]. The packing of SDS molecules in a cross section

of that hemicylinder is shown schematically in Fig. 3 (top).

At positive charge densities the hemimicellar state is trans- formed into the interdigitated bilayer schematically shown in

Fig. 3 (bottom).

The results of this study demonstrate the need for the use of neutron reflectometry to study adsorption and phase

transitions in films of surfactants adsorbed at the solid- solution interface. Specifically, they show that when neutron

reflectivity measurements are combined with electrochemical studies and atomic force microscopy, they provide unique

opportunities to study different stages involved in the

interaction of surfactants with solid surfaces.

44 A

Gold Electrode H20/D20 A

SDS

FIGURES. Models of SDS adsorption at the Au-solution interface: (top) cross section of hemicylindrical aggregates observed at moderately

negative charge densities; (bottom) interdigitated bilayer observed at positive charge densities.

References

[1] A.W. Adamson, Physical Chemisti^ of Surfaces, 5* ed.. John Wiley &

Sons, New York (1990).

[2 E. Sackmann, Science 271, 43 ( 1996).

[3] P. Chandar, P. Somasundaran, N. J. Turro, J. Colloid Interface Sci. 117, 31 (1987).

[4] S. Manne, Progr. Colloid Polym. Sci. 103, 226 (1997).

[5] D. Bizzotto, J. Lipkowski, J. Electroanal. Chem. 409, 33 (1996).

[6] V. Zamlynny, I. Burgess, G. Szymanski, J. Lipkowski, J. Majewski,

G. Smith, S. Satija, and R. Ivkov, Langmuir 16, 9861(2000).

[7] I. Burgess, V. Zamlynny, G. Szymanski, J. Lipkowski, J. Majewski,

G. Smith, S. Satija, and R. Ivkov, Langmuir 17, 3355 (2001).

NIST Center for Neutron Research

I

33
Magnetic Semiconductor Superiattices

Currently a great deal of attention is being focused on

spintronics, a new area of solid-state electronics. In spintronics not only the electric current but also its spin state

is controlled. Spin valves and spin injectors are the first

practical applications of spintronics. Further progress in developing new devices hinges critically on the availability

of suitable materials. Such materials need to be "good"

semiconductors, easy to integrate in typical integrated

circuits, and their electronic properties should exhibit strong

sensitivity to the carrier's spin, ferromagnetism being an

especially desirable property.

EuS is one of the very few natural ferromagnetic (FM)

semiconductors. Since it becomes FM at a low temperature

(r = 16.6 K) it is an unlikely choice for applications.

However, studying the properties of heterostructures made

on its base may give an important insight into fundamental

processes taking place in all classes of materials under

consideration.

GaMnAs is a man-made FM semiconductor. It is an example of a diluted magnetic semiconductor (DMS) in which a fraction of nonmagnetic cations (Ga) is substituted

with magnetic ions (Mn). Such a material can readily be

incorporated into modem GaAs-based semiconductor

devices. Its T is still below room temperature, but this limitation may be lifted in other materials of this class [1]. Interlayer exchange coupling (lEC) in superiattices

(SL), composed of ferromagnetic and nonmagnetic layers, is a crucial element of all spin-valve type devices that utilize the giant magnetoresistance effect. In metallic SLs currently

being used, conduction electrons transfer the interlayer

interactions through nonmagnetic spacers [2]. Here we

address the question whether lEC phenomena are possible in all-semiconductor superiattices, like EuS/PbS and GaMnAs/

GaAs, where the carrier concentrations are many orders of

magnitude lower than in metals. The nonmagnetic spacer in EuS/PbS SLs is a narrow

gap semiconductor with electron concentration of the order

of 10'' cm -' to 10'^ cm l For thin PbS layers (^^pbs< 70 A)

neutron reflectivity spectra, shown in Fig. 1, have revealed a pronounced maximum of magnetic origin at the position

corresponding to the doubled structural SL periodicity, thus

indicating the existence of antiferromagnetic (AFM)

interlayer arrangements [3].

10'

10

10

EuS/PbS (60 A/23 A)x15

9 o

e o

• 7=4.3 K,B = 0

. r=35K, B = 0

o 7=4.3 K,B=185G

AFM*

° FM

4 '

030 0 S 5 T

Chem.SL

0.00 0.03 0.06 0.09 0.12

FIGURE 1. Unpolarized neutron reflectivity spectra for EuS/PbS SL with

thin (23 A) PbS spacer. Antiferromagnetic interlayer exchange coupling below r and at zero external field is clearly visible (blue curve). Applying a strong enough magnetic field (185 G in this case) parallel to the SL surface forces all the EuS layer's magnetizations to ferromagnetic

configuration (green curve). Above 7 the system is nonmagnetic, the only Bragg peak comes from the chemical SL periodicity.

For much thicker PbS spacers {d^^^ > 120 A) the only

magnetic peaks visible in the reflectivity profiles, see e.g.. Fig. 2, coincide with the chemical ones, thus leading to the conclusion that the magnetization vectors in adjacent EuS

layers are parallel, which indicates FM lEC.

In the intermediate PbS thickness range

(70 A < Jp^3< 120 A), both AFM and FM peaks are present.

Polarized neutron analysis of these maxima gives evidence

that the magnetization vectors of adjacent EuS layers are not

colinear. Hence, the lEC found in EuS/PbS SLs has an

oscillatory character similar to that occurring in metallic

SLs, although the oscillation period is much longer than the one in metallic systems.

In order to confirm that the free carriers, present in the PbS layer in such a small amount, are the cause of the observed oscillatory lEC, a series of analogous measure- ments have been carried out on EuS/YbSe SLs. The structure and lattice constant of YbSe are the same as those of

PbS. In contrast to PbS, YbSe is a semi-insulator with a negligible carrier concentration. Neutron reflectivity profiles

have shown no evidence of any interlayer couphng in the all

34 Research Highlights
A. Yu. Sipatov, V. Volobuev T. Story, J. Sadowski C. F. Majkrzak

Kharkov State Polytechnical University Institute of Physics, NIST Center for Neutron Research

Kharkov, Ukraine Polish Academy of Sciences National Institute of Standards and Technology

H. Kepa, J. Kutner-Pielaszek, A. Twardowski Warsaw, Poland Gaithersburg, MD 20899-8562

Institute of Experimental Physics, T. M. Giebultowicz Warsaw University Physics Department Warsaw, Poland Oregon State University

Corvallis, OR 97331

10'

10^

10' r

10' r

10'

EuS/PbS (54 A/135 A) X 15 FM

A

7 = 4.2 K, e = 200G

• T=27K

• 7 = 4.2 K, 6 = 0 FM

i

lv.v 3

Chem.SE

0.00 0.02 0.04 0.06 0.08 0.10

600

500

E 400

g 300

o

I 200

100

GaMnAs/GaAs (50 ml/6 ml) x 50

It,,,

mi

T=7.8K

6 = 2G

• + +

• - +

• + -

00.030 0.035 0.040

1, 0.045 0.050 Q.[A

FIGURE 2. The sample with thick (135 A) PbS layers is almost

ferromagnetically coupled. Application of an external magnetic field enhances the FM Bragg peaks and lowers the intensity between them (at the AFM peak position).

FIGURE 3. Polarized neutron reflectivity spectra for GaMnAs/GaAs

superlattice.

investigated samples. That finding, together with the oscilla- tory character of coupling in SLs with PbS spacer, strongly

points to the leading role of PbS free electrons in providing

the necessary lEC mechanism, similar to that discovered in metaUic multilayers.

Ferromagnetic ordering in GaMnAs is carrier (holes)

induced; the Mn atoms, apart from being the magnetic

element in the system, act also as acceptors providing the holes responsible for transferring exchange interactions

between them. The details of the magnetic ordering, in

particular its range, are still being disputed.

To address the latter issue, polarized neutron reflecto- metry has been performed on a number of GaMnAs/GaAs

superlattices. Figure 3 shows an example of the obtained

reflectivity profile in the vicinity of the first SL Bragg peak,

for one of the samples. The very presence of the magnetic

contribution to the structural SL Bragg peak is a strong

confirmation of the FM lEC between consecutive GaMnAs

layers. The absence of any spin-flip scattering shows that the sample is in a one-domain state, i.e., the FM ordering in GaMnAs is long range, and the sample is spontaneously

saturated. The peak in (— ) cross section, and its absence in

the {++), is proof that the magnetization is directed oppositely to the external magnetic guide field, hence the long

range ordering has formed spontaneously, without the influence of the external field. More details can be found

in Reference 4.

References

[1] T. Diet), H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287,1018 (2000).

[2] P. Bruno, Phys. Rev. B52, 411 (1995).

[3] H. Kepa, J. Kutner-Pielaszek, J. Blinowski, A. Twardowski,

C. F Majkrzak, T. Story, P Kacman. R. R. Galazka, K. Ha,

H. J. M. Swagten, W. J. M de Jonge, A. Yu. Sipatov, V. Volobuev,

T. M. Giebultowicz, Europhys. Lett. 56, 54 (2001).

[4] H. Kepa, J. Kutner-Pielaszek, A. Twardowski, C. F. Majkrzak,

J. Sadowski, T. Story, T. M. Giebultowicz, Phys. Rev. B64, 121302

(2001).

NIST Center for Neutron Research 35
Pinpointing Cliirai Structures with Front/Back

Polarized Neutron Reflectometry

Wehave developed a new method of using polarized neutron reflectometry (PNR) to extract the structure of buried magnetic spirals in magnetic films. This technique

improves upon earlier methods by being particularly sensitive to the presence of magnetic twists vis-a-vis structures in which the magnetization direction does not vary appreciably.

Tracking the formation and growth of twists may solve a number of puzzles that hamper the development of magnetic

thin film devices.

In collaboration with IBM scientists, we have applied

the technique to a thin-film exchange-spring magnet and

confirmed that the results may violate the cunent theory

regarding the behavior of such magnets. It has been pre- dicted that exchange-spring magnets, comprised of soft and

hard feiTomagnets in close proximity, are a composite that has a strong moment and does not readily demagnetize [1]. Therefore, exchange-spring magnets should give industry

the ability to make much smaller permanent magnets for use

in the magnetic recording devices, and elsewhere. As a side

effect, when a small external magnetic field is opposed to

that of the magnet, the portion of the soft ferromagnet

farthest from the hard ferromagnet may twist into alignment

with the field. When the field is removed, the soft ferro- magnet untwists. The film provided by IBM consists of the hard ferromagnet Fe..Pt^^ topped by the soft ferromagnet

Figure 1 shows a simplified diagram of the behavior

predicted by cunent theories [1]. A magnetic field of

0.890 T, provided by an electromagnet, is sufficient to align

both the soft and the hard layers of our exchange-spring

magnet, as shown on the left. When a modest reverse field (on the order of 0.025 T) is applied to the exchange spring

magnet, only the top of the soft layer will realign with the magnetic field. The hard layer remains pinned in the

original direction, and a continuous twist is induced in the

soft layer, as the direction of magnetization changes

smoothly between the reverse field direction to the aligning

field direction. Although there are many alternatives to PNR to measure the magnetization, typically they measure only the average orientation of the magnetic spins, and cannot readily

distinguish a spiral from a structure in which all the spins are canted with respect to an external field. PNR can extract the

Soft ferromagnet

Hard ferromagnet

FIGURE 1. Model for field behavior of exchange-spring magnets. On the

left the magnet has been aligned by a large external magnetic field. On

the right a smaller field opposed to the first field causes a twist to form in the soft ferromagnet, while the hard ferromagnet remains aligned.

depth-dependence of the magnetic and chemical structure. We have studied the sample over a wide range of external magnetic fields, and can track the development of the spiral

with field [3]. A PNR experiment begins with neutrons whose

magnetic moments are aligned parallel or opposite (-) to the external magnetic field. When the magnetization of the sample is perpendicular to this magnetic field, the neutron moment processes as it interacts with the sample. When this happens the spin-flip (SF) reflectivities andi? ^ are

strong. If the magnetization of the sample is parallel to the external magnetic field, no precession occurs, but the nonspin-flip (NSF) reflectivities R

^^ and R will differ. The

NSF reflectivities also provide information about the chemical structure of the film. Our new modification of the PNR method greatly

enhances the contrast between colinear and certain noncolinear magnetic structures [4]. We first measure the

reflectivity with neutrons glancing off the front surface of

the material, and then repeat with neutrons glancing off the back surface. The experiment is akin to holding the plane of

the film up to a "magnetic mirror" to see whether the mirror

image is the same as the original structure. In a colinear

structure, all the spins are aligned along a common direction,

and the mirror image is very much like the original structure.

36

0 Research Highlights
K. V. O'Donovan, J. A. Borchers, and C. F. Majkrzak NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

0. Hellwig, E. E. Fullerton IBM Almaden Research Facility San Jose, CA 95120-6001

-G- R--

FIGURE 2. Reflectivity of a Nij^Fe^glFeggPt^j bilayer. The front reflectivity is plotted on the right while the back reflectivity is plotted on the left. The SF

reflectivities R*' and /? * are plotted against the right ordinate axis. The NSF reflectivities R** and R' are plotted against the left ordinate axis.

But the mirror image of a magnetic twist to the right is a magnetic twist to the left. Therefore, if the front and back

reflectivities are significantly different, we can deduce the presence of a spiral. Fitting the data confirms the spiral's

existence.

Figure 2 shows data collected at 0.026 T after aligning

in -0.89 T. Fits to the data are shown as solid lines. The

data from the front reflectivity are shown on the right, and

the data from the back reflectivity are shown on the left. The spin-flip (SF) reflectivities R ^ and R ^ are plotted

against the right-hand axis, which have been shifted relative

to the NSF reflectivities R

** and R plotted against the left

axis. Atq- 0.2 nm ', there is a splitting in the front NSF

reflectivity that is much more pronounced than that of the back reflectivity at the same q. This is a hallmark of the

spiral structure. Figure 3 shows the magnetic structure that gives the

excellent fit to the data plotted in Fig. 2. The location of the

hard/soft interface is marked in Fig. 3. Surprisingly, we

discover the spiral invades the hard ferromagnet even at extremely low fields. Current theory predicts that when this

occurs, the soft ferromagnet will not be able to untwist fully.

Yet, other magnetic studies show that our exchange-spring

magnet does untwist when this field is removed. Thus, our PNR measurements have identified a shortcoming of current

theory.

With this new technique, NIST is now able to better characterize the magnetic properties of thin films, which can improve the capability and reliability of industrial devices

for magnetic recording and sensing.

FIGURE 3. Fitted magnetization of the data presented in Fig. 2. The front

of the sample is at a depth of 0 nm and the back is at a depth of 70 nm. The red curve is a projection of the magnetic structure into the plane of the front surface. References

[1] E. F. Kneller and R. Hawig, IEEE Trans. Magn. 27, 3588 (1991).

[2] O. Hellwig, J. B. Kortright, K. Takano, and E. E. Fullerton, Phys. Rev. B62, 11694 (2000).

[3] K. V. O'Donovan, et al, in preparation.

[4] K. V. O'Donovan, et al., submitted to Applied Physics A.

NIST Center for Neutron Research 37
The Nature of Vibrational Softening in a-Uranium

The standard textbook explanation for phonon softening

with increasing temperature in a single phase is that the interatomic potentials are not perfectly harmonic, but it has

been suggested that phonon softening can also occur if the

potential itself can change with temperature, while remaining harmonic. For example, a large softening of the vibrational properties of a-uranium has been observed [1] that cannot be explained in terms of the anharmonic lattice contribution alone. In the present work a harmonic contribution to the phonon softening is made evident by treating

inelastic neutron scattering spectra as an expansion of the vibrational power spectrum of the atomic motion.

Earlier diffractometry measurements by Lawson et ol.,

[2] suggested that the Debye temperature decreased by 40 %

in going from 300 K to the /?-phase transition temperature at 940 K. In terms of entropy this corresponds to an additional

zlS = -3kjn(0.6) = 7.5 /r^/atom. The usual thermodynamic

argument is that this increase in vibrational entropy compensates for the elastic energy generated by thermal expansion.

However, from heat capacity data, the entropy needed to compensate for the elastic energy is nearly an order of

magnitude too small at 0.16 ^„/atom.

All experiments used uranium powder. High temperature measurements were made using the Fermi-Chopper

Spectrometer (FCS) at the NCNR. Low temperature measurements were performed with the Low Resolution Medium

Energy Chopper Spectrometer (LRMECS) at Argonne

National Laboratory. Figure 1 shows the phonon density of

states obtained from the measured spectra, coirected for the incoherent multiphonon scattering using a procedure

described elsewhere [3]. There is a redistribution of intensity

in the main features at = 8 meV and = 12 meV, with the higher energy peak gaining extra weight with increasing

temperature. These features also show an overall softening

of around one meV for every 200 K increase in temperature.

The Q-summed one phonon scattering function was

used to calculate a quantity proportional to the square of the power spectrum and hence to the average potential energy

per oscillator, <U>. In Fig. 2 we show <U> for a-uranium

at the four highest temperatures scaled so that the lowest

point is at the harmonic energy k^T/2. The effects of

anharmonicity on <U> would be evident as a nonlinearity in

5 10 15 Energy Transfer (meV)

FIGURE 1. The phonon density of states of uranium. Data from 300 K and above were obtained from spectra acquired with FCS. Data from 300 K

and below were measured on LRMECS.

38 Research Highlights
M. E. Manley and B. Fultz

California Institute of Technology

Pasadena, CA 91125

M. E. Manley, R. J. McQueeney,

W. L, Hulls, J. L Smith, and D. J. Thoma

Los Alamos National Laboratory Los Alamos, NM 87545

C. M. Brown

NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

and

University of Maryland

College Park, MD 20742

R. Osborn

Argonne National Laboratory Argonne, IL 60439

J. L. Robertson Oak Ridge National Laboratory Oak Ridge, TN 37831

the plot of <U> vs. temperature. For comparison, attempts were made to calculate the temperature dependence of the

potential energy of Morse and Lennard-Jones potentials with

the appropriate vibrational softening for uranium [1]. The

result shown in Fig. 2 indicates that, in the high temperature

limit, the potential energy has nearly a linear dependence on

temperature, i.e., the phonon softening in ct-uranium occurs while the potential remains primarily harmonic. Evidently

the interatomic force constants are decreasing with increasing temperature. Since the force constants originate from the change of the electronic energy with atom displacements, it must be that thermal excitations of the electronic states are

altering the force constants.

>

50 -

40 -

^ 30

o

CL

20

10 ^

I I I I I I 1 I I I I I II I I I

I

I I I I

I

I I I I

I I I I II

I I

Morse_2

Morse_1

Lennard-Jones

FIGURE 2. Vibrational potential energy of a-uranium (open circles). The

Lennard-Jones, Morse_1 and l\/lorse_2 curves were calculated from

potentials described in the text. The Harmonic curve is the result for a harmonic potential in the classical limit. The phonon density of states of the three solid state phases of uranium, orthorhombic (a), tetragonal and

body centered cubic (7) are compared in the top panel of

Fig. 1 . The /-uranium phonon density of states was

statistically identical at 1113 K and 1213 K. Evidently, the thermal softening mechanism seen in a-phase does not

operate in the /-phase. The y5-phase was not stable over a

wide enough temperature range to obtain a reliable temperature dependence. The phonon softening between

each phase accounted for vibrational entropy changes of

0.15 ± 0.01 Ar/atom and 0.20 ± 0.01 yt/atom for the a to /? and /? to /transitions, respectively. Both of these values

make up only about 35 % to 40 % of the total entropy

changes predicted from latent heat measurements:

(S'^-S")^^^ = 0.37 /t/atom and (S^-s'^)^^^^ = 0.55 /:/atom. The

remaining 60 % of the entropy increases must be electronic

in origin. So not only does the phonon softening disappear in the high temperature /phase, but also it does so with a large

increase in electronic entropy.

Electronic band structure calculations used to predict

phonon frequencies are based on the assumption that thermal

effects can be neglected when compared to volume effects. The actinides, however, show the need for more sophisticated treatments of the role of temperature on interatomic

interactions.

References

[1] M. E Manley, B. Fultz, R. J. McQueeney, C. M. Brown. W. L. Hulls,

J. L. Smith, D. J. Thoma, R. Osborn, J. L. Robertson, Phys. Rev. Lett. 86, 3076(2001).

[2] A. C. Lawson, B. Martinez, J. A. Roberts, B. I. Bennett and

J. W. Richardson, Jr., Phil. Mag. B 80, 53 (2000).

[3] M. E. Manley, R. J. McQueeney, J. L. Robertson, B. Fultz and

D. A. Neumann, Phil. Mag. Lett. 80, 591 (2000).

NIST Center for Neutron Research 39
Native and Partially Unfolded Proteins:

Neutron inelastic Scattering and Simulations

Proteins are long chain-like molecules that are intri- cately folded. As they function they unfold in specific

ways. How proteins can quickly refold without tangling

remains a central mystery in molecular biology. Partially

folded proteins provide an approach to this problem, and are also important in cell functioning. The approach is that

stable partially folded states have been shown to resemble

the kinetic folding intermediate states along the proteinfolding pathway. A molten globule (MG) is a compact

partially folded protein, with a backbone resembling the completely folded protein, but lacking the extensive specific

side-chain packing interactions of the native-like (i.e., properly folded) structure. A full understanding of the mechanism of protein folding requires the knowledge of the

structures, relative energetics, and dynamics of the species

populating the folding pathway. Incoherent inelastic neutron

scattering (INS), which makes use of the large incoherent

cross section of hydrogen nuclei, is a technique well suited

to the study of protein internal molecular motion on the picosecond time scale. Molecular dynamics (MD) simulations, on the other hand, are a potentially valuable tool for

interpreting neutron data on proteins [1,2], and may provide

atomic level description of the motions taking place.

In this study, we have explored the dynamics of bovine

a-lactalbumin. a calcium-binding protein, in the native and

molten globule states, using both techniques. INS data were

collected on the Disk-Chopper Spectrometer at ~ 32 [leV

resolution for native and molten globular bovine

a-lactalbumin. The simulations results were generated from

= 1 ns constant temperature and pressure MD trajectories.

The model systems consisted each of a single protein

monomer, immersed in a large water box

(60 A X 50 A X 80 A). For the native state, the a-lactalbu- min configuration was initiated from the crystal structure. The model molten globule state was generated by partially

unfolding the protein at 500 K at atmospheric pressure until the radius of gyration reached the experimental value

determined for the molten globule state (10 % expansion). A

1 ns trajectory was then generated at 300 K for data analysis

and comparison with experiment.

Figure 1 shows the structure of the a-lactalbumin in the native and molten globule states obtained from the MD

FIGURE 1. a-lactalbumin structure in the native (top) and molten globule

(bottom) state from MD simulations. Note the loss of the j3-domain

structure, with most the a domain (helix) being conserved.

simulations in solution at 300 K. The overall structural change, manifested by the conservation of the helical a-domain and the loss of the sheets in the /^-domain, is consistent with NMR data [3]. We therefore regard this

structure as a reasonable model of a member of the MG

ensemble for comparison with the INS data. In Fig. 2 we report the incoherent structure factors S(Q,co) as a functions of energy transfer hco for selected momentum transfer Q, measured and calculated from the MD simulations trajectories. In both systems the agreement

with experiment is remarkable, assessing the ability of the simulations to reproduce the picosecond dynamics of the

protein. In agreement with previous INS data collected at lower resolution [4], the molten globule has a broader quasielastic spectra compared to the native state, which indicates

that the protein atoms are more mobile in the MG state. The

additional motion is large enough to be detected at the length

scale probed by the DCS spectrometer, i.e., in the range 3 A

to 60 A.To investigate the relationship between the structure of

the protein and the motion of its side chains, we have used

the MD trajectories to follow the motion of these side chains

40 Research Highlights
M, Tarek D. J. Tobias Z. Bu and D. A. Neumann

NIST Center for Neutron Research Department of Chemistry NIST Center for Neutron Research

National Institute of Standards and Technology University of California National Institute of Standards and Technology

Gaithersburg, MD 20899-8562 Irvine, CA 92697-632 Gaithersburg, MD 20899-8562

and

Center for Molecular Modeling and

Department of Chemistry, University of Pennsylvania

Philadelphia. PA 19103

1 ' r

-0.3 -0.1 0.1 0.

1 ' 1 r

-0.3 -0.1 0.1 0.3 E [meV]

FIGURE 2. Incoherent structure factor S(Q,(a) at selected wavevectors 0, measured and calculated from the simulation for the native (top) and

molten globule (bottom) states.

quantitatively. Figure 3 reports the fluctuations per residue

along the backbone for the native and the molten globule

states, as a function of residue. These correspond to the 100 ps root-mean-square amplitudes of motion of the backbone atoms, averaged over the simulation trajectories.

First for the native state, the backbone fluctuations appear to correlate with the secondary structure of the protein, showing much smaller amplitudes in the a-helix and y9-sheet

regions. For the molten globule state, a significant increase

in the protein fluctuation amplitudes is observed, in agree- ment with the broadening of the INS spectra.

FIGURE 3. 100 ps mean squared fluctuations of the backbone atoms computed from the MD simulations of the native state (black) and the molten globule state (red).

Figure 3 shows clearly that most of the additional

motion occurs in the region of the protein that "unfolds,"

i.e., the y?-domain. The results indicate also that the motion

of the side chains is by no means homogeneous. Differences

in amplitudes up to 10 folds are observed between parts of

the proteins, regardless of exposure to the solvent. The MD study of the dynamics of partially folded

states of the protein should in principle be extended to explore a large portion of the conformational space, i.e., be

extended to a large ensemble of non-native conformers. At

any rate, the present results, though limited, combined with

neutron experiments are believed to capture the essence of

the structural and dynamical changes taking place as the protein partially unfolds. They demonstrate the utility of the MD simulations for qualitatively elucidating the dynamical

behavior of native and non-native proteins at the atomic

level.

References

[1] M. Tarek, G. J. Matryna. and D. J. Tobias, J. Amer. Chem. Soc. 122, 10450 (2000).

[2] M. Tarek and D. J. Tobias, Biophys. J. 79, 3244 (2000).

[3] J. Baum et al. Biochemistry, 28, 7 (1989).

[4] Z. Bu et al., J. Mol. Biol. 301, 525 (2000).

NIST Center for Neutron Research 41
Atomic Motions in Confined Polymer Films

aterial properties at surfaces and interfaces often

deviate from their bulk values. An example is viscoelastic properties that are important in many applications such as chemically amplified photoresists, lubricants,

adhesives, etc. A viscosity increase of a liquid polymeric

film, like those used for hard disc lubricants, could have

deleterious consequences. Likewise, a viscosity decrease in

a thin photoresist film could enhance photoacid diffusion

and create image blur. It is clearly important to characterize

the molecular level mobility in thin polymer films. Of

particular interest are changes in the glass transition temperature r , since the glass transition corresponds to a sharp

change in viscosity.

It is well known that atomic thermal motions lead to a decrease in the intensity of the elastically scattered neutrons.

In a harmonic solid the intensity is related to the amplitude

of the motion through the Debye-Waller factor,

I,„JQ) ~ e.xp{-V3Q-<u->), where Q is the scattering vector and <ir> is the mean-square atomic displacement. In a logarithmic plot of /.^, as a function of Q% <u-> is proportional to the slope of a linear fit to the data. This approximation has proven very useful in studying the dynamics of

polymers, both synthetic [1] and biological [2], despite the

fact that their atomic motions are typically anharmonic.

The elastic incoherent neutron scattering intensities can be measured as a function of temperature on the NCNR's High Flux Backscattering Spectrometer (HFBS),

using the "Fixed Window" mode. Polycarbonate (PC) films, seen in Fig. 1, are spin cast onto Si wafers, sectioned into small rectangles, and stacked into an Al sample cell. The

scattering from the Si substrates and Al cell are negligible

because their Bragg peaks are beyond the Q range of the instrument (0.25 A ' < 2 < 1-75 A '); the scattering is dominated by the large incoherent cross-section of the amorphous PC.

Ideally there should be 200 mg of PC per cell to maximize the signal and minimize multiple scattering effects (90 % transmission). For the Si-supported films we are limited by 0.5 mg to 10 mg of PC per cell and a thus obtain a very weak signal (99 % transmission). However, the HFBS

is sufficiently sensitive to observe this signal, as seen in Fig. 2 where the temperature dependence of the elastic scattering is shown. The relative magnitude of the intensity

FIGURE 1. A stack of 1014 A PC films (notice the blue tint) on thin slabs

of Si ready to go into the HFBS spectrometer.

change diminishes with decreasing film thickness, a clear indication of reduced atomic mobility in the thin PC films. Given the HFBS energy resolution of 0.85 peV, these

intensity changes reflect changes in atomic motions at frequencies of 200 MHz or faster. This reduced mobility is qualitatively reflected in

Fig. 3 as strong suppression of <ir>. We emphasize

qualitatively because <ir> arises from a harmonic approximation. Nevertheless, there is a clear reduction in the amplitudes of the thermal motions, both above and below

the calorimetric T^, as the film thickness approaches the root-mean-square (rms) end-to-end length of PC

{R = 160 A).

ee

Q = 0.36 A"' ^75A

--1015A

-e-bulk

- 100 200 300

T(K)

400 500

FIGURE 2. The normalized elastic intensities in the HFBS detector at Q = 0.36 k\ The decrease in intensity at high temperatures reflects the presence of atomic motion in the thin polymer films.

42 Research Highlights
C. L. Soles, J. F. Douglas, and Wen-li Wu

Polymers Division National Institute of Standards and Technology

Gaithersburg, MD 20899-8541

R. M. Dimeo

NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

FIGURE 3. <if> as a function of temperature for the PC thin films. The

inset reveals a stiffening of the harmonic (low temperature) force constant xrwith decreasing film thickness h.

At temperatures below 200 K, <ir> is small and the harmonic approximation is reasonable. In this regime the inverse of the slope of <u-> vs. Tis proportional to an

elastic force constant k. The inset of Fig. 3 displays a dramatic stiffening of k with confinement. This implies that

the atoms or molecules in PC become strongly caged by

their environment in the thinner films. Similar <ir> measurements on thin films of polymethyl methacrylate (PMMA; /?^^ = 915 A) and polyvinyl

chloride (PVC; = 437 A) are shown in Figs. 4(a) and

4(b). There is a suppression of <u-> with decreasing film

thickness above the bulk that is qualitatively consistent

with the PC films. However, below the T for bulk PMMA g the suppression is less pronounced, and not observed in the PVC films. Careful inspection of Figs. 3 and 4 suggests a

relation between the magnitude of <u-> at T in the bulk and

the degree of <u-> suppression in the thin glassy films. Bulk PC has the largest <m'> at T^, indicating extensive mobility. In fact, PC is well known for its segmental

mobility in the glassy state, which is in part related to its superior impact resistance. Molecular mobility helps dissi- pate strain energy. (PC is used in bullet-proof glass.) This

extensive mobility is also susceptible to thin film confinement, as also seen through the stiffening of the elastic constants from 1.9 N/m in bulk PC to 5.8 N/m in the 75 A

film. At the other extreme, bulk PVC has the smallest <u->

at r , indicating a lack of motions to be affected by confine- ment. This is also consistent with the strong intermolecular

0 n

O.U : (a) PMMA /

^2.0

ACM 5^ 1.0

bulk

80 A

0.0 =^^^^^"^17^ bulk

.1... . 1 1 1 1 1 1 1 1 ,. 1 1 inn onn lUU ZUU onn /inn cnn oOv 4UU OUU

7(K)

_1.2 PVC /

""^0.8

A

0.4

- bulk

; _ 220 A

0.0 100 200 300 400

7(K)

FIGURE 4. <u^>as a function of thickness and temperature for similar PMMA (a) and PVC (b) films.

caging at low temperatures; both bulk PVC and the 220 A

film have large elastic force constants of 4.6 N/m. This lack

of mobility reflects the observation that PVC is a brittle polymer that needs high levels of plasticization for practical

use. Only when PVC is taken above the bulk T^, where

long-range modons dominate, do the effects of confinement

become evident. In short, we have seen two different responses to thin film confinement. While there is a general suppression of <«-> and the fast dynamics, polymers with greater mobility

are more susceptible to confinement, especially below the calorimetric T^. In contrast, polymers with strongly local- ized motions appear bulk-like, even in highly confined

glassy films. More details of these studies can be found in our upcoming publications [3-5].

References

[1] B. Frick, D. Richter, Science 267, 1939 (1995).

[2] G. Zaccai, Science 288, 1604 (2000).

[3] C. Soles, J. Douglas, W. Wu, R. Dimeo, submitted to Phys Rev E.

[4] C. Soles, J. Douglas, W. Wu, H. Peng, D. Gidley, in preparation.

[5] C, Soles, E. Lin, J. Lenhart. R. Jones, W. Wu, D. Goldfarb, M.

Angelopoulos, J. Vac. Sci. Tech. B, in press.

NiST Center for Neutron Research 43
Tuning the Properties of Carbon Nanotubes

by Deformation

Carbon nanotubes continue to surprise scientists with

their novel properties. Recently we have discovered

many intimate relationships between structural deformation

and the properties of single-walled nanotubes (SWNT) that could be important in technological applications [1, 3]. These observations were made using state-of-the-art first- principles total energy calculations within the pseudopotential and the generalized gradient approximations

(GGA). Here we present a brief review of our findings.

SWNTs are basically rolled graphite sheets which are characterized by two integers {njn) defining the rolling vector of graphite. Therefore, the electronic properties of

SWNTs, to first order, can be deduced from that of graphite

by mapping the band structure from a 2D hexagonal lattice onto a cylinder. Such an analysis indicates that {n,n) "armchair" nanotubes are always metallic and (77,0) zigzag

nanotubes are metallic only if 77 is an integer multiple of

three. From our first-principles calculations we find that these electronic properties can be significantly modified by

radial strain, which distorts the circular cross section to an

elliptical one. Figure 1 shows that the energy gap of an insulating

SWNT decreases and eventually vanishes at an insulator- metal transition with increasing applied elliptical strain. The

density of states at the Fermi level N{E^), of a metallized

SWNT increases with increasing deformation. More interestingly, the elliptical deformation necessary to induce

metallicity is found to be in the elastic range. Therefore, all

strain induced changes in electronic and also in mechanical

properties are reversible. Another significant effect of the elliptical distortion of

SWNTs is the change in the uniformity of their charge

distribution. This, in turn, imposes changes in the chemical

reactivity and hence on the interaction of the tube surface with foreign atoms and molecules. To demonstrate this, we

calculate the binding energies of H and a simple metal atom,

Al, on SWNTs as functions of nanotube radius (R), type, and

elliptical deformation. We find that the binding energy, E^,

is positive (i.e., stable) and follows a remarkably simple l/R

scaling for (77,0) SWNTs for both H and Al, even though H

prefers to sit on top of C atoms and Al favors the hollow

sites (i.e., top of hexagons). Increasing binding energy with

£

FIGURE 1 . Variation of tlie energy band gap E^^^(a) and density of tlie states at the Fermi energy (b) as a function of elliptical deformation.

increasing curvature (i.e., decreasing R) suggests that can

be modified by elliptical deformation.

Figure 2 shows the variation of the binding energy E^

of a single hydrogen atom adsorbed on the sharp and flat edges of the (8,0) surface with elliptical distortion. With

increasing distortion, the binding energy of the sharp site

increases, while it decreases for the flat site, creating an

energy difference of = 1 . 1 eV at the distortion value £-0.3.

This value is 44 % of the binding energy of H on the undeformed SWNT.

The binding energy of Al exhibits a behavior similar to

that of H, despite H and Al favoring different sites on the

(8,0) tube. Elliptical deformation increases the binding

energy of both the flat and sharp sites for Al, given an energy difference of 0.5 eV at the deformation £ = 0.3. It is

44 Research Highlights
T. Yildirim NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

2.0 I ' . ' 0.00 0.10 0.20 0.30

FIGURE 2. Binding energies of the sharp (top) and the flat sites (bottom) versus elliptical deformation for a hydrogen adsorbed on a (8,0) SWNT.

remarkable that Al, which is not bound to the graphite, can

be adsorbed at the sharp site of the distorted SWNT with a binding energy of 1.8 eV. Hence, we conclude that not only

band gap engineering but also chemical reactions taking

place on the surface of a SWNT can be engineered through

radial deformation.

Explanation of this remarkable change of the binding

energy with elliptical deformation is sought in the electronic

structure of SWNTs. Figure 3 shows the response of the first conduction band to elliptical distortion. It is clear that the

distortion disturbs the uniformity of the charge distribution of the SWNT and pushes the chemically most active electrons from the flat to the sharp site of the nanotube, increasing the adsorption energy of the sharp site significantly.

Surprisingly the effect of distortion on metallic armchair

(n,n) nanotubes is found to be very small due to the metallic

bands of the tubes [2]. This could be important in selective

functionalization of nanotubes. See Ref. 1 for animation of

this effect. We believe that the tunable electronic and chemical

properties reported here can have important implications for metal coverage and for selective adsorption and desorption

of atoms and molecules on carbon nanotubes. This could

allow the fine tuning of the properties of SWNTs via

reversible deformation and can ultimately lead to a wide

variety of technological applications such as variable metalinsulator junctions, quantum wells, catalysts, hydrogen

storage devices, magnetic tubes (by absorbing magnetic

ions), etc.

0. Gulseren NIST Center for Neutron Research

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

and

Department of Materials Science and Engineering

University of Pennsylvania

Philadelphia, PA 19104-6202

FIGURE 3. Contour plot of the first conduction band of an undeformed

(top) and elliptically deformed (bottom) (8,0) SWNT. See Ref. 1 for animation of this effect.

References

[1] See http://www.ncnr.nist.gov/staff/taner/nanotube.

[2] O. Gulseren, T, Yildirim, and S. Ciraci, Phys. Rev. Lett. 87, 1 16802

(2001).

[3] C. Kilic, S. Ciraci, O. Gulseren, and T. Yildirim, Phys. Rev. B62,

R16345 (2000).

NiST Center for Neutron Research 45
Serving the Scientific and Teciinoiogical

Communities

The role of the NCNR as a national user facility has

expanded significantly over the past few years, as the

last of the instruments envisioned in the original cold neutron project has become operational. The Disk-Chopper

Spectrometer, the Filter-Analyzer Neutron Spectrometer, the High Flux Backscattering Spectrometer, and the Neutron

Spin-Echo spectrometer now permit U.S. scientists to carry out neutron spectroscopy with greatly improved resolution

and sensitivity. The thermal perfect-crystal small angle neutron scattering instrument (USANS) has expanded the length scale available by this technique to 10"* nm. User

experiments show a steadily increasing diversification in subject area and technique, enabled by the new instruments.

We anticipate that the trend will continue over the next few

proposal cycles.

User participation over the past 15 years shows continuing growth (see Fig. 1). The NCNR currendy accommodates more than half of all neutron users in the U.S. While the Spallation Neutron Source is being built at Oak Ridge, the NCNR continues to be the nation's premier facility for providing neutrons to the U.S. research community.

The NCNR User Program

Researchers may obtain use of NCNR neutron beam

instruments in several ways, the most direct being through

the formal proposal system. Approximately twice a year, a

2000

1500

1000

98 '00

FIGURE 1. Research Participants at the NCNR.

Call for Proposals is issued. After a thorough review

process by external referees and by the NCNR Program

Advisory Committee (PAC), approved proposals are allo- cated beam time. The PAC is a panel of distinguished

sciendsts with expertise across a broad range of neutron methods and scientific disciplines. It is the body primarily

responsible for proposal review and recommending user

policies for the NCNR, working closely with the Center's

Director and staff. Its current membership includes Sanat

Kumar (Penn State University, chair), Robert Briber (University of Maryland), Michael Crawford (DuPont), Dieter

Schneider (Brookhaven National Laboratory), Kenneth

Herwig (Oak Ridge National Laboratory), Yumi Ijiri (Oberlin College), Michael Kent (Sandia National Laboratories, John Tranquada (Brookhaven National Laboratory),

and Andrew Allen (NIST).

At the recent PAC meeting in January 2001, the PAC

considered 121 proposals for SANS and reflectometry, in addition to 49 for inelastic neutron scattering. Although we

expect that both categories will see increased user demand in future proposal rounds, the latter area is likely to see more

growth, since the new inelastic scattering spectrometers have

added capabilities previously unavailable at U.S. neutron

facilities. The Center for High Resolution

Neutron Scattering

The National Science Foundation (NSF) through the Center supports several NCNR instruments for High Resolution Neutron Scattering (CHRNS), a very important component of the user program. Until the present, CHRNS instru- ments included a 30 m SANS machine, the SPINS tripleaxis spectrometer, and USANS. Approximately 40 % of the instrument time allocated by the PAC went to experiments

carried out on CHRNS instruments. During the past year,

the NSF approved a proposal to provide support for a

substantial enlargement of CHRNS, enabling us to offer more beam time to users on the three new high-resolution

inelastic scattering instruments. In the near future, another

SANS diffractometer, the 8 m machine on neutron guide

NG- 1 , which is presently used primarily for NIST programmatic research, will be upgraded to a more powerful 9 m

46 Serving the Scientific and Technological Communities
instrument with a new detector, and made available to users through CHRNS. The expanded suite of CHRNS instru- ments will provide enhanced technical support and accessibility to the user community.

Summer School on Methods and

Applications of Neutron Spectroscopy

The NCNR's seventh annual Summer School on

neutron scattering, held from June 18-22, 2001, focused on

high-resolution neutron spectroscopy. Most of the 32

participants were graduate and postdoctoral students; we

also had an undergraduate student and a professor.

In a radical departure from the format of previous

NCNR summer schools, plenary lectures were presented on

the first morning only. After lunch the students were divided

into four groups of eight, and each group performed an

experiment using a different instrument (in one case a pair of

instruments). Students learned the scientific motivation for

the experiment, how the instrument works, and how to set up

measurements. The instruments collected data overnight and

on the following morning the data were analyzed. After

lunch on each of the second, third and fourth days the groups rotated to new experiments. After lunch on the final

day, eight of the students gave short presentations describing an experiment.

Students used the following instruments: the Spin

Polarized Inelastic Neutron Spectrometer, the Disk Chopper

Spectrometer, the High-Flux Backscattering Spectrometer,

the Neutron Spin Echo spectrometer, and the Filter Analyzer

Neutron Spectrometer. Together these instruments probe

motions in materials over six decades in time (from

approximately 10"^ s to 10 '^ s) and two orders of magnitude

in distance (0.1 nm to 10 nm). Test samples included a geometrically frustrated antiferromagnet. a transition metal

compound with organic ligands, a "symmetric top"

molecule, and a system of spherical micelles. The

experiments illustrated coherent scattering, incoherent

scattering, magnetic scattering, and tunneling. In evaluating

the school more than 80 % of the participants described it as

"excellent," and generally high ratings were received in all

areas. The lectures and experiment handouts have been

placed on the Web (www.ncnr.nist.gov/programs/

Photography by L. A. Shuman FIGURE 2. Seventh annual Summer School on Neutron Scattering

participants analyze results with NCNR's Seung-Hun Lee (bottom left).

spectroscopy/SS01/SS01_materials.html.) A short report

about the summer school will appear in "Neutron News." As

in the past, the summer school was jointly sponsored with

the National Science Foundation, which provided financial assistance to many of the university participants.

Collaborations

Direct collaborations on specific experiments remain a common way for users to pursue their ideas using NCNR

facilities, accounting for approximately half of the number

of instrument-days. The thermal-neutron triple-axis spec- trometers are mainly scheduled in this way. Most of the time reserved for NIST on these and all other NCNR

instruments is devoted to experiments that are collaborations

with non-NIST users. Collaborative research involving

external users and NIST scientists often produces results that could be not obtained otherwise.

Another mode of access to the NCNR is through

Participating Research Teams (PRTs). In this case, groups of

researchers from various institutions join forces to build and

operate an instrument. Typically, 50 % to 75 % of the time on the instrument is then reserved for the PRT, and the remaining time is allocated to general user proposals. For

example, a PRT involving ExxonMobil, the University of

Minnesota, and NIST cooperates on the NG-7 30 m SANS

instrument. Similar arrangements involving other PRTs

NIST Center for Neutron Research 47
apply for the horizontal-sample reflectometer, the highresolution powder diffractometer, the filter-analyzer spectrometer, and the neutron spin-echo spectrometer.

Independent Programs

There are a number of programs of long standing

located at the NCNR that involve other parts of NIST,

universities, industrial laboratories, or other government

agencies.

The Polymers Division of the Materials Science and

Engineering Laboratory has two major program elements at the NCNR. In the first, the purpose is to help the U.S.

microelectronics industry in addressing their most pressing

materials measurement and standards issues. In today's

integrated circuits and packages the feature size on a chip is ever shrinking, approaching 250 nm, while the size of a polymer molecule is typically 5 nm to 10 nm. As feature

size shrinks, the structure and properties of interfaces play an increasingly important role in controlling the properties

of the polymer layers used in interconnects and packages.

NIST scientists use both neutron reflectivity and other neutron scattering methods to characterize polymer/metal

interfaces with regard to local chain mobility, moisture

absorption, glass transition temperature and crystalline

structure. In the second program element, the objective is to understand underlying principles of phase behavior and

phase separation kinetics of polymer blends, both in the bulk

and on surfaces, in order to help control morphology and

structure during processing. SANS and reflectivity measure- ments in equilibrium, in transient conditions, and under

external fields, provide essential information for general

understanding as well as for specific application of polymer

blend/alloy systems. Customers include material producers

and users, ranging from chemical, rubber, tire, and automotive companies, to small molding and compounding companies. The focus of research on polymeric materials includes

commodity, engineering and specialty plastic resins, elas- tomers, coatings, adhesives, films, foams, and fibers. The ExxonMobil Research and Engineering Company is a member of the Participating Research Team (PRT)

that operates, maintains, and conducts research at the NG-7

30 m SANS instrument and the recently conmiissioned NG5 Neutron Spin Echo Spectrometer. The mission is to use

those instruments, as well as other neutron scattering

techniques, in activities that complement research at ExxonMobil's main laboratories as well as at laboratories of

its affiliates around the world. The aim of these activities is

to deepen understanding of the nature of ExxonMobil's

products and processes, so as to improve customer service

and to improve the return on shareholders' investment.

Accordingly, and taking full advantage of the unique

properties of neutrons, most of the experiments use SANS or other neutron techniques to study the structure and dynamics

of hydrocarbon materials, especially in the fields of poly- mers, complex fluids, and petroleum mixtures.

ExxonMobil regards its participation in the NCNR and

collaborations with NIST and other PRT members not only

as an excellent investment for the company, but also as a good way to contribute to the scientific health of the Nation.

The Nuclear Methods Group (Analytical Chemistry

Division, Chemical Science and Technology Laboratory) has

as its principal goals the development and application of

nuclear analytical techniques for the determination of

elemental compositions with greater accuracy, higher

sensitivity and better selectivity. A high level of competence

has been developed in both instrumental and radiochemical

neutron activation analysis fINAA and RNAA). In addition,

the group has pioneered the use of cold neutron beams as

analytical probes with both prompt gamma activation

analysis (PGAA) and neutron depth profiling (NDP).

PGAA measures the total amount of a particular analyte

present throughout a sample by the analysis of the prompt

gamma rays emitted during neutron capture. NDP, on the other hand, determines concentrations of several important

elements (isotopes) as a function of depth within the first few micrometers of a surface by energy analysis of the prompt charged particles emitted during neutron bombardment. 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, and are used to help

certify a large number of NIST Standard Reference Materials.

48 Serving the Scientific and Technological Communities
During the past several years, a large part of the Group's efforts has been directed toward the exploitation of

the analytical applications of the guided cold-neutron beams

available at the NIST Center for Neutron Research. The

Group's involvement has been to design and construct state- of-the-art cold neutron instruments for both PGAA and NDP

and provide facilities and measurements for outside users, while retaining and utilizing our existing expertise in INAA

and RNAA.

The Center for Food Safety and Applied Nutrition,

U.S. Food and Drug Administration (FDA), directs and

maintains a neutron activation analysis (NAA) facility at the NCNR. This facility provides agency-wide analytical

support for special investigations and applications research,

complementing other analytical techniques used at FDA

with instrumental, neutron-capture prompt-gamma, and

radiochemical NAA procedures, radioisotope x-ray fluores- cence spectrometry (RXRFS), and low-level gamma-ray

detection. This combination of analytical techniques enables

diverse multi-element and radiological information to be

obtained for foods and related materials. The NAA facility

supports agency quality assurance programs by developing

in-house reference materials, by characterizing food-related

reference materials with NIST and other agencies, and by

verifying analyses for FDA's Total Diet Study Program.

Other studies include the development of RXRFS methods

for screening foodware for the presence of Pb, Cd and other

potentially toxic elements, use of instrumental NAA to investigate bromate residues in bread products, and use of

prompt-gamma NAA to investigate boron nutrition and its

relation to bone strength.

The Neutron Interactions and Dosimetry Group

(Physics Laboratory) provides measurement services,

standards, and fundamental research in support of NIST's

mission as it relates to neutron technology and neutron

physics. The national and industrial interests served include

scientific instrument calibration, electric power production,

radiation protection, defense nuclear energy systems,

radiation therapy, neutron radiography, and magnetic

resonance imaging.

The Group's activities may be represented as three major activities. The first is Fundamental Neutron Physics

including magnetic trapping of ultracold neutrons, operation

of a neutron interferometry and optics facility, development

of neutron spin filters based on laser polarization of -^He, measurement of the beta decay lifetime of the neutron, and

investigations of other coupling constants and symmetries of

the weak interaction. This project involves a large number

of collaborators from universities and national laboratories. The second is Standard Neutron Fields and Applications utilizing both thermal and fast neutron fields for materials dosimetry in nuclear reactor applications and for personnel dosimetry in radiation protection. These neutron

fields include thermal neutron beams, "white" and monochromatic cold neutron beams, a thermal-neutron-induced

-^^''U fission neutron field, and --^-Cf fission neutron fields,

both moderated and unmoderated. The third is Neutron

Cross Section Standards including experimental advancement of the accuracy of neutron cross section standards, as well as evaluation, compilation and dissemination of these

standards. Several universities have also established long term

programs at the NCNR. The University of Maryland is heavily involved in the use of the NCNR, and maintains

several researchers at the facility. Johns Hopkins University participates in research programs in solid-state physics

and in instrument development at the NCNR. The University of Pennsylvania is working to help develop biological

applications of neutron scattering. It is also participating in the second stage construction of the filter analyzer neutron

spectrometer, along with the University of California at Santa Barbara, DuPont, Hughes, and Allied Signal. The

University of Minnesota participates in two PRTs, the NG-7 30 m SANS and the NG-7 reflectometer. The University of Massachusetts also participates in the latter PRT.

NIST Center for Neutron Research 49
Operations

The NIST neutron source (NBSR) operated for 240

full power (20 MW) days during the past year, or 100 % of the scheduled time. Routinely, the NBSR is scheduled to operate a seven-week cycle, seven times a year. Each operating cycle includes 38 days on-line and 11 days

shutdown for refueling, routine maintenance, and surveil- lance tests. (Figure 1 shows the operations staff adjusting

the power-regulating rod.) This year, four days in March

were required for maintenance on the Thermal Column

Cooling System. The NBSR was shutdown on August 26,

2001, for a planned maintenance period of three months.

During this time the cold source will be replaced, building

transformers and switchgear will be replaced to provide

additional electrical capacity, and the new cooling tower will undergo final hookup and testing. (Figure 2 shows the new

cooling tower in the final stages of construction. For

comparison the old cooling tower is seen on the left.) A comparison has been made of thermal-hydraulic

characteristics of the NBSR based on two calculations. The

first calculation used correlations and simple models avail- able when the licensed power level was raised to 20 MW in 1985. The second used newer correlations that better represent research neutron source characteristics. The

analysis shows that the heat transfer coefficient, onset of

flow instability, and critical heat flux of the original calculation are all well within the operating conditions predicted by

the new calculation. In addition, the NBSR also has an ample margin before the onset of nucleate boiling.

The above results are for extreme yet still conceivable

conditions of operation. The Energy Sciences and Technology Department at the Brookliaven National Laboratory has

been tasked to perform transient analysis with state-of-the art methods. Monte Carlo calculations will be used to accurately predict parameters that are used in the transient

analysis.

Work is also continuing in the preparation of an updated license renewal submittal to the NRC. In conjunction with this effort, an in-service inspection and ultrasonic

testing of the primary cooling system are planned for the summer/fall shutdown.

FIGURE 1 . Bill Mueller and Greg Heller of the operations staff adjust the power-regulating rod.

FIGURE 2. The new plume-abated cooling tower (right) nears completion.

It will replace the lower capacity and less efficient system that had no

abatement ability (left).

50 Operations
Instrumentation Developments

The Advanced Cold Neutron Source

After six years of safe and reliable operation, the liquid

hydrogen cold source cryostat at the NIST reactor is being

replaced by one with a new advanced design. Based on

extensive operating experience and sophisticated Monte

Carlo calculations of the cold source and NIST reactor core, the new design improves the neutron coupling between the reactor fuel and moderator and the cold source cryostat. The

advanced source, (called Unit 2). will differ from the

original in many key respects, as can be seen in Fig. 1. The

most important change is the addition of 60 liters of D,0,

partially surrounding the moderator chamber, reducing the

FIGURE 1. Comparison between the original (top) and new designs for

tiie liquid hydrogen cold source cryostat at NIST. Heavy water volume is indicated in blue, liquid hydrogen in orange, and hydrogen vapor in yellow.

moderator void volume and increasing the thermal neutron

flux in the cryostat region by about 40 %.

The new cryostat is shaped like an ellipsoidal shell, rather than a spherical one. This results in a smaller volume

so that more D^O can be introduced in the cryostat assembly.

The center of the inner ellipsoid is offset, so the liquid

hydrogen shell is 30 mm thick near the reactor core. This

change results in an additional 5 % to 10 % increase in flux.

FIGURE 2. False color representation of stresses from finite element

calculations of the helium containment jacket with an internal pressure

of 82 atmospheres.

Finally, in contrast to the original design, the inner ellipsoid

will be evacuated through a small port, removing most of

the hydrogen vapor. This change will add another 15 % to 20 % gain. Based on these changes, users can expect an

overall intensity increase of 50 % to 80 % between 2 A and

20 A with this design.

The complex geometry of the advanced source posed

many engineering challenges. The moderator chamber is surrounded by a vacuum vessel, which is surrounded by a helium containment vessel, strong enough to withstand the design basis accidental detonation of liquid hydrogen and

solid oxygen. These vessels determine the extent of the D,0

NIST Center for Neutron Research 51
volume, so they were designed to partially surround the moderator chamber, allowing an opening just large enough

to fully illuminate the neutron guides. This "hour-glass"

shape created regions of very high mechanical stress, analyzed and ultimately mitigated with the aid of finite element analysis (see Fig. 2). During the three-month period

beginning at the end of August 2001, we are removing the old cryostat and installing Unit 2. Development of an Air-Pad and Floor

System for New Neutron

Spectrometers

The NCNR is currently developing several new

neutron triple axis spectrometers with a key design feature

that allows interchangeability of the back-end detector/

analyzer systems. Exchanging the back-end system on these instruments requires a universal connection and positioning

system between it and the sample position of the instrument.

This interface must be capable of achieving an angular

accuracy of 0. 1 degrees, an angular precision of

0.01 degrees and a vertical positional accuracy of less than

a millimeter.

The design approach adopted uses a commercial air- pad system with a precision-leveled floor. After considering

several different types of floors we chose to use epoxy as our

FIGURE 3. Michael Murbach and Colin Wrenn conduct tests on the prototype floor system for a triple axis spectrometer.

sub-floor with precision cut and anodized aluminum plates

as our contact surface. Epoxy will self-level, fill in holes,

and mask over any imperfections in the concrete floor and

results in a highly smooth and level surface to build on. The

epoxy is also low cost, relatively easy to use, and provides

great flexibility for repair or maintenance. The aluminum

plates provide protection to the epoxy from cryogenic spills,

increased wear resistance, and an easily removable/reusable

floor. We have developed and tested a prototype single air- pad system that has successfully demonstrated the construction techniques for the floor system and the ability of the air pad positioning system to meet the accuracy and repeatability requirements for a 1 ton test load. Currently, a three-pad

prototype system is under development that will allow us to

test control system concepts needed in a final design.

Development of an Intercalated

Graphite Monochromator

As part of an ongoing collaboration between NIST and

Harvard University to measure the neutron lifetime using

magnetically trapped ultracold neutrons (UCNs), a new

monochromator has been developed. In the lifetime experiment, ultracold neutrons (UCN: neutrons with energies = 100 neV) are produced in a superfluid *He bath inside the trapping apparatus using the "superthermal process." A

beam of cold neutrons is introduced into the helium, in which neutrons with wavelength near 0.89 nm scatter to UCN energies by creation of a single phonon. Only neutrons within a nanow wavelength band (± 0.01 nm for our

current magnetic trap) participate in this single-phonon

production mechanism. Since the wavelength band for UCN production occupies only a small fraction of the neutron spectrum produced by the cold source (see Fig. 4), a significant signal-to-background improvement is possible

with appropriate wavelength filtering of neutrons that enter

the apparatus. Bragg reflection from a crystalline monochromator with a lattice-plane spacing d can be used to

select a narrow wavelength band for neutrons with wavelengths up to Id. Unfortunately, widely used materials for neutron monochromatization, such as pyrolytic graphite,

have insufficiently large d spacing for reflecting 0.89 nm

neutrons. One attractive class of materials for producing

52 Instrumentation Developments
long wavelength monochromators is intercalated graphite.

Intercalation is the process of inserting foreign atoms or molecules between the layers of the graphite, and results in a

crystal with a larger d spacing. Intercalated graphites are formed in "stages," where the stage number defines the number of planes of graphite between layers of intercalant atoms.

Neutron Wavelength [nm]

FIGURE 4. Neutron wavelength spectra were measured using a time-offlight technique. Only 1 % of the neutrons emerging from guide NG-6

(red) lie within the wavelength band (between the orange vertical bars)

available for production of UCN. The spectra of neutrons transmitted

(blue) and reflected (green) by the monochromator are also shown.

We have designed, fabricated, and tested a new

0.89 nm monochromator using stage-2 potassium-intercalated graphite. Nine pieces of intercalated graphite are tiled

to form the monochromator, which has a total size of

6 cm X 15 cm. Heating potassium and graphite to two

different temperatures in an evacuated glass cell produces

each tile. Performance of the assembled monochromator

was determined using time-of-flight techniques (see Fig. 4). The monochromator reflects more than 80 % of the neutrons

incident upon it in the wavelength band relevant for UCN

production. However, only 3 % of all incident neutrons are

reflected. When combined with additional techniques that

filter out neutrons from higher order Bragg reflections, the monochromator results in an improvement of the ratio of

useful to total neutrons of almost two orders of magnitude

while sacrificing few trapped neutrons. During the summer

of 2001, this new monochromator was used in conjunction

with the trapping apparatus in a temporary setup. Presently, we are permanently installing the monochromator at the fundamental physics station at NG-6. This installation will place the monochromator much closer to the end of the neutron guide and will result in an increase by an additional

factor of two in the number of trapped UCNs.

NCNR Computer and Software

Environment

The NCNR's computerized data acquisition and

reduction environment encompasses all stages of research: experiment preparation, data collection and analysis of

results. NCNR provides site-specific tools that augment the

tools in researchers' own scientific arsenal. Some specific areas of recent activity are described below:

• NCNR is providing simulation tools that model the behavior of our instruments, to evaluate the feasibility of

proposed experiments. They can be used by prospective

users, especially those who aren't already famihar with neutron scattering experiments, to explore "what-if' sce- narios with different sample and instrument configurations.

Currently, we have simulations of small angle scattering

experiments, with reflectivity and stress measurement

simulations being developed.

• Researchers who plan to use NCNR instrumentation

must submit their proposals for review. We have created a web-based proposal submission system that tracks user proposals from submission to review and ranking, to the

final instrument time allocation.

• All instruments available at NCNR are computercontrolled. The control software running on these computers

must satisfy two seemingly contradictory needs: simplicity,

for straightforward use, and sophistication, for power uses. New, more complex instruments, such as the spin echo

spectrometer, require carefully designed graphical user

interfaces (GUIs) that hide unnecessary detail, while still providing access to all functions, if needed. Since the NCNR

instruments run around the clock, the data acquisition

functions must be capable of automation and unattended

operation. To this end, the experiment control software is

scriptable, in addition to the GUI interface. The scriptable,

command-based interface fulfills "power-user" needs, while

NIST Center for Neutron Research 53
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FIGURE 5. A screenshot of several NCNR applications. Clockwise from top left corner: the web-based simulator of a SANS experiment, peak analysis

application, Time-of-flight detector selection, and a Spin Echo instrument control panel.

a simplified GUI interface tends to be easier to learn and

use.

• Rapid post-acquisition data reduction is important to guide the ongoing experiments, and, later, to aid in the processing of data to obtain the scientific results. Many data

reduction procedures are common to all the instruments. The

Data Visualization and Analysis Environment (DAVE) is a

set of tools commonly used by NCNR scientific community;

these tools include programs to manipulate and compare

data, extract parameters of theoretical models by data fitting,

etc.

• NCNR maintains a sophisticated computer infrastructure including a modem network installation, and an ensemble of nearly 400 computers and other IT equipment,

for scientific and office computing, ranging from email

servers to supercomputer-style parallel computing clusters.

• NCNR participates in inter-laboratory collaborations

developing software for specialized domains. Perhaps the best known of these is the GSAS software for crystallographic data reduction. Another ongoing NCNR collaboration is in international scattering data exchange format,

NeXuS.

54 instrumentation Developments
FIGURE 6. Superconducting magnet/dilution refrigerator as it is being

lifted onto the DCS spectrometer. The adjustable lifting frame compensates for the lifting point shifting in case liquid cryogens are added.

Another experiment required the development of a system capable of going to temperatures as high as 675 K

with a high voltage applied to the sample. The design goal was to be able to achieve a 5 kV potential difference across

the sample. After continued development work, we were

able to apply a potential difference of 8 kV to the sample

while at the maximum temperature of 675 K. In addition to the human safety features designed in from the beginning, a

circuit was developed which prevents the continued arcing

that can occur if the sample breaks down during the course

of the experiment.

Sample Environment Team

The sample environment team expanded by two new

members this year, bringing the total number to four employees that are actively working to ensure that the needs of

researchers working at the NCNR are met. This team works

to improve the infrastructure sample environment equipment

and to support neutrons scattering instruments at the NCNR.

A principal objective is to provide safe and easy to use equipment to the research staff. Equipment development

activities are done in collaboration with the engineering and

technical staff, with input from both staff and guest scientists. As an example, a new superconducting magnet/

dilution refrigerator system needed modifications for use on

the Disk Chopper Spectrometer (DCS) for an experiment.

The impracticality of separating the cryostat from its support

pumps and electronics led to the decision to lift the entire system onto the instrument for use. A new adjustable lifting

frame was design and manufactured to lift the various

components of the magnet (see Fig. 6).

NIST Center for Neutron Research 55
Publications

Abdurashitov, J. N., Gavrin, V. N., Kalikhov, A.V., Matushko, V. L., Shikhin. A. A., Yants, V. E., Zaborskaia, O. S., Adams, J. M.,

Nico, J. S., Thompson, A. K., "A High Resolution Low

Background Fast Neutron Spectrometer," in Nuclear Instruments and Methods, in press. Adams, C. P., Lee, Y. S., Lee, S.-H., Lynn, J. W., "Triple-Axis Research at NIST," Neutron News 12 (2), 15 (2001). Adams, C. R, Lynn, J. W., Mukovskii, Y. M., Arsenov, A. A., Shulyatev, D. A., "Charge Ordering and Polaron Formation in the Magnetoresistive Oxide La„^Ca,| ^MnOj," Phys. Rev. Lett. 85

(18), 3954 (2000). Adams, J. M., "Results From the NIST Round Robin Test of Fissionable Dosimeters in a Reactor Leakage Spectrum," REACTOR

DOSIMETRY, in Proceedings Radiation Metrology and

Assessment, ASTM STP 1398, edited by J. Williams, D. Vehar,

F. Ruddy, D. Gilham, (ASTM. West Conshohocken, PA, Feb. 2001), p. 295. Adams, J. M., Greenhouse, L. A., "Nuclear Science at the National

Institute of Standards and Technology: A Historical Overview,"

Transactions of the American Nuclear Society, November 2000,

(Washington, DC), 83, 492 (2000).

Akpalu. Y. A, Karim, A., Satija, S., Balsara. N. P.. "Suppression of

Lateral Phase Separation in Thin Polyolefin Blend Films,"

Macromol. 34 (6), 1720 (2001).

Allen. A. J., Ilavsky, J., Long, G. G., Wallace, J. S., Bemdt, C. C,

Hennan, H., "Microstructural Characterization of Yttria- Stabilized Zirconia Plasma-Sprayed Deposits Using Multiple

Small-Angle Neutron Scattering," Acta Mater. 49 (9), 1661

(2001).

Allen, A. J., Long, G. G., Boukari, H., Ilavsky, J., Kulkarni, A., Sampath, S., Herman, H., Goland, A. N., "Microstructural

Characterization Studies to Relate the Properties of Thermal

Spray Coating to Feedstock and Spray Conditions," Surf. Coat.

Techn.. in press.

Allman. B. E., McMahon, P. J., Nugent, K. A., Paganin. D., Jacobson, D. L., Arif, M., Werner, S. A., "Phase Radiography with Neutrons," Nature 408, 158 (2000). Amos, T. G., Sleight, A. W., "Negative Thermal Expansion in Orthorhombic NbOPO^," J. Solid State Chem.. 160, 230 (2001).

Anderson. C. R., Andersen, K. H., Stirling, W. G., Sokol, P. E., Dimeo,

R. M., "Dynamics of Superfluid ""He Confined in Xerogel

Glass," Phys. Rev. B, in press.

Anderson, D. L., Capar, S. G., "Identification of Lead in Metal Food

Can Seams Using "Co RXRFS," J. Radioanal. Nucl. Chem. 249

(1), 77 (2001).

Anderson, D. L., Cunningham, W. C, "Revalidation and Long-Term

Stability of National Institute of Standards and Technology

Standard Reference Materials," J. AOAC International, 83 (5), 1121 (2000).

»

Anderson, D. L., Cunningham, W. C, Capar, S. G., Baratta, E. J., Mackill, P., "Nuclear Methods for Food Analysis at the U. S. Food and Drug Administration," J. Radioanal. Nucl. Chem. 249

(1), 29 (2001).

Bao, W., Aeppli, G., Lynn, J. W., Pagliuso, P G., Sarrao, J. L., Hundley, M. F., Thompson, I. D., Fisk, Z., "Anisotropic Thiee- Dimensional Magnetic Fluctuations in Heavy Fermion

CeRhln.," Phys. Rev. Lett., in press.

Bao, W., Christianson, A. D., Pagliuso, P. G., Sarrao, J. L., Thompson,

J. D., Lacerda, A. H., Lynn, J. W., "Effect of La-Doping on Magnetic Structure in Heavy Fermion CeRhln^," Physica B, in

j press.

j Bao, W., Pagliuso, P. G., Saixao, J. L., Thompson, J. D., Fisk, Z., Lynn,

j J. W., "Magnetic Structure of Heavy Fermion Ce,RhIng," Phys. | Rev. B64 (2), 0401 (2001).

"

j Bao, W.. Pagliuso, P. G., Sarrao, J. L., Thompson, J. D., Fisk, Z., Lynn, I

J. W., Erwin, R. W., "Incommensurate Magnetic Structure of ; CeRhln^," Phys. Rev. B63 (21), 9901 (2001).

j Bateman, F. B., Boukharouba, N., Brient, C. E., Carlson, A. D.,

j Grimes, S. M., Haight, R. C, Massey, T. N., Wasson, O. A.,

] "New Measurements of the H(n,n)H Angular Distribution,"

j REACTOR DOSIMETRY. Radiation Metrology and Assessment,

j ASTM STP 1398. edited by J. Williams, D. Vehar, R Ruddy, D.

Gilliam, (American Society for Testing Materials, West

Conshohocken, PA, Feb. 2001), p. 431.

i| Bauer, B. J.. Amis, E. J., "Characterization of Dendritically Branched

Polymers by SANS, SAXS, and TEM," Tomahs/Frechet:

Dendritic Polymers. 2000, in press.

Bauer, B. J., Lin, E. K., Lee, H. J.. Wang, H., Wu. W.-L.. "Structure and Property Characterization of Low-k Dielectric Porous Thin

Films," J. Electron. Mater. 30 (4), 304 (2001).

Becker, P. R., Mackey, E. A., Demiralp, R., Epstein, M. S., Donais, M. K., Goessler, W. A., Porter, B. J., Wise, S. A., "Heavy Metals and Other Trace Elements Measured in Tissues of Marine

Mammals Archived by the Alaska Marine Manmial Tissue : Archival Project from the Bering Sea and Cook Inlet, Alaska," in Proceedings of The AMAP Heavy Metals Workshop, in press.

| Becker, R R., Pugh, P R., Schantz, M. M., Mackey, E. A., Oflaz, I R. D., Epstein, M. S., Donais, M. K., Porter, B. J., Wise, S. A., i Mahoney, B. A., "Persistent Chlorinated Compounds and 1 Elements in Tissues of Cook Inlet Beluga Whales Banked by the

j Alaska Marine Mammal Tissue Archival Project," NISTIR 6702,

(Gaithersburg, MD), p. 67. I

Baklanov, M. R., Kondoh, E., Lin, E. K., Gidley, D. W, Lee, H.-J., Mogilnikov, K. P., Sun, J. N., "Comparative Study of Porous ' SOG Films with Different Non-Destructiv6 Instrumentation," in Proceedings of the National Interconnect Technology Confer-

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Bellitto, C, DiMarco, M. G., Branford, W. R., Green, M. A., Neumann, D. A., "Cation Distribution in Ga-Doped ; Li, jj,Mn,0^," Solid State Ionics 140 ( 1 ), 77 (2001 ). |.

56 Publications
Bianco-Peled, H., Don, Y., Satija, S., Schneider, J., Sung, L., Tirrell, M., "Structural Study of Langmuir Monolayers Containing

Lipidated Poly (Ethylene Glycol) and Peptides," Langmuir, in press. Blackman, M. J.. Redford, S., "Northern Syrian Luster and Fruitware: Petrographic and Chemical Implications for Productions and

Distribution," Antiquity, in press.

Bode, P., de Nadai Femandes, E. A., Greenberg, R., "INAA as a Primary Method in Metrology for Chemical Measurements," in Proceedings Metrolsiil. '99, 2nd South American Congress of

Metrology (Foz do Iguacu, Brazil, 2000), p. 89.

Borchers, J. A., Ijiri, Y., Lind, D. M., Ivanov, P G.. Erwin, R. W., Qasba, A., Lee, S.-H., O'Donovan, K. V., Dender. D. C,

"Detection of Field-Dependent Antiferromagnetic Domains in Exchange-Biased FejO^/NiO Superlattices," Appl. Phys. Lett. 77

(25), 4187 (2000).

Bordallo. H. N.. Chapon, L. C. Cook., J. C, Copley, J. R. D., Goremychkin, E., Kern, S., Lee, S.-H., Yildirim, T., Manson,

J. L., ""Spin Excitations in 3D Molecular Magnets Probed by

Neutron Scattering," Appl. Phys. A, in press.

Borgoul-Maciejczyk, P., Ondov, J. M., "Heterogenous Nature of

Metallic Components in Baltimore Particulate Matter," Environ.

Sci. Techno!., in press.

Bossev, B. P, Kline, S. R., Israelachvilli. J. N., Paulaitis, M. E., "Pressure-Induced Freezing of the Hydrophobic Core Leads to a LlAEHl Phase Transition for C^.E^ Micelles in D^O," Langmuir,

in press.

Bossev, D. P., Ferdinand, S., Lesemann, M., Paulaitis, M. E., "The

Effect of Kilobar Pressures on Microstructure in C|,Ej Nonionic

Microemulsions," Langmuir, in press.

Bossev, D. P., Ferdinand, S., Paulaitis, M. E., "Effect of Pressure on

Microstructure in Water-in-Oil and Oil-in-Water Microemulsions for C^jEj n-Octane/D,0 Mixtures," Langmuir, in press.

Bossev, D., Kline, S., Paulaitis, M. E., "Pressure-Induced Aggregation

of C|,E.D,,j Wormlike Micelles," Langmuir, in press.

Bourdarot, F., Lynn, I. W., Baggio-Saitovitch, E., Huang, Q., Sanchez,

D. R., Skanthakumar, S., "Magnetic Behavior of the Rare Earth Compounds RNiBC by Neutron Scattering," Phys. Rev. B64, 104410 (2001). Brome, C. R., Butterworth, J. S., Dzhosyuk, S. N., Mattoni, C. E. H., McKinsey, D. N., Doyle, J. M., Huffman, P R., Dewey, M. S., Wietfeldt, F. E., Golub, R., Lamoreaux, S. K., Coakley, K. I., "Magnetic Trapping of Ultracold Neutrons," Phys. Rev. C 63

(5), 5502 (2001). Brown, A. D. B., Rennie, A. R., "Monodisperse Colloidal Plates Under

Shear," Phys. Rev. E 62 (1), 851 (2000).

Bu, Z., Chowdhuri, Z., Callaway, D. I. E., "Coexistence of Fluctuating

Secondary Strucmre and Long-Range Tertiary Interaction in Highly Denatured Alpha-Lactalbumin - A Neutron Scattering

Study," Protein Sci., in press.

Bu, Z., Cook, J., Callaway, D. J. E., "Dynamics Regimes and Correlated Structural Dynamics in Native and Denatured AlphaLactalbumin," J. Mol. Biol., in press.

Buckley, C. E., Birnbaum, H. K., Lin, J. S., Spooner, S., Bellmann, D., Staron, P., Udovic, T. J., Hollar, E., "Characterization of H

Defects in the Aluminium-Hydrogen System Using Small-Angle

Scattering Techniques," J. Appl. Crystallogr. 34, 119 (2001).

Burauer, S., Belkoura, L., Strey, R., Co, C. C. Kaler, E. W., "Size and

Polydispersity of Polymer Particles Obtained by Cryo-TEM,

SANS and DLS," Elektronenmiki-oscopie 21, 20 (2001).

Burgess, I., Zamlynny, V., Szymanski, G., Lipkowski, J., Majewski, J., Smith, G., Satija, S., Ivkov, R., "Electrochemical and Neutron

Reflectivity Characterization of Dodecyl Sulfate Adsorption and

Aggregation at the Gold-Water Interface," Langmuir 17, 3355

(2001).

Butler, P. D.. Porcar, L., WaiT. G. G., Hamilton, W. A., comment on

"Evidence of a Sponge-to-Lamellar Phase Transition Under

Shear by X-Ray Scattering Experiments in Couette Cell," Phys. Rev. Lett., in press.

Caffrey, P. F, Ondov, J. M., "Dry Deposition of Elemental Aerosol

Constituents to Lake Michigan Surface Waters, by Size and by

Source," Environ. Sci. Technol., in press.

Caffrey, P. F, Ondov, I. M., "Dry Removal of Elemental Aerosol

Particulate Constituents in Near Shore Gradient of Southern Lake Michigan," Environ. Sci. Tech., in press.

Caliskan, G., Kisliuk, A., Tsai, A. M., Soles, C. L., Sokolov, A. P,

"Protein Dynamics in Viscous Solvents," J. Biophys., in press.

Cambell, B. I., Billings, S., Lynn, I. W., Sinha, S. K., "The Structure of

Jahn-Teller Polarons in the Colossal Magnetoresistive Manganites," in press.

Campbell, B. J., Osborn, R., Argyriou, D. N., Vasiliu-Doloc, L., Mitchell, I. F, Sinha, S. K., Ruett, L. U., Islam, Z., Lynn, J. W., "The Structure of Nanoscale Polaron Conelations in a Colossal Magnetoresistive Manganite," Phys. Rev. B, in press,

Cappelletti, R. L., "The National Institute of Standards and Technology

Center for Neutron Research (NCNR)," Neutron News, 12 (2), 10 (2001).

Cappelletti, R. L., Clutter, L. K., edited by, "NIST Center for Neutron

Research Accomplishments and Opportunities, NCNR 2000,"

NIST SP962 (2001).

Cappelletti, R. L., Clutter, L. K., edited by, "NIST Center for Neutron

Research, Programs and Accomplishments," NISTIR 6598.

Cappelletti, R. L., Glinka, C. I., Krueger, S., Lindstrom, R. A., Lynn,

J. W., Prask, H. J., Prince, E., Rush, J. I., Rowe, I. M., Satija,

S. K., Toby, B. H., Tsai, A., Udovic, T. I., "Materials Research With Neutrons at NIST," I. Res. Nad. Inst. Stand. 106 (1), 187

(2001).

Carpenter, I. M., Iverson, E. B., Mildner, D. F. R., "Time Focusing of Pulsed-Source Crystal Analyzer Spectrometers II Practical Expressions," Nucl. lustrum. Meth. A, in press.

NIST Center for Neutron Research 57
Cava, R. J., Yamaura, K., Loureiro, S.. Huang, Q., Erwin, R. W., Lynn,

J. W., Young, D., "Magnets, Mischief and Metals in Cobalt Analogs of the Superconducting Cuprates," Physica C 341, 351

(2000) .

Chen, S. H., Liao, C, Fratini, E., Baglioni, R, Mallamace. F., "Interaction, Critical, Percolation and Kinetic Glass Transitions in Pluronic L-64 Micellai' Solutions," Colloid Surface, 183, 95

(2001) .

Chen, W. C, Heuser, B. J., King, J. S., "SANS Investigation of

Deuteride (Hydride) Precipitation and Decomposition Morphology in Single Crystal Pd," J. Appl. Crystallogr. 33, 442 (2000).

Chen, Y. Y, Honda, Z., Zheludev, A., Broholm, C, Katsumata, K.. Shapiro, S. M., "Field-Induced Three- and Two-Dimensional

Freezing in a Quantum Spin Liquid," Phys. Rev. Lett. 86 (8), 1618 (2001). Chen, Y. Y, Lodge, T. P., Bates, F. S., "Entropically Driven Phase

Separation of Highly Branched Linear Polyolefin Blends,"

J. Polym. Sci. Pol. Phys. 22, 2965 (2000).

Cheng, Y. T., Mildner, D. F. R., "A Sensitive High Resolution Neutron Imaging Detector," Nucl. Instrum. Meth. A 454 (2), 452 (2000).

Cheng, Y. T., Mildner, D. F R., Chen-Mayer, H. H., Sharov. V. A., Glinka, C, "An Imaging Plate Detector for Small-Angle Neutron

Scattering," J. Appl. Crystallogr. 33, 1253 (2000). Chen-Mayer, H. H., Lamaze, G. P, Mildner, D. F. R., Zeissler, R., Gibson, W. M., "Neutron Imaging and Prompt Gamma Activation Analysis Using a Monolithic Capillary Neutron Lens," in Proceedings of the 3rd International Congress on Analytic

Sciences, Analytical Science, in press. Chen-Mayer, H. H., Lamaze, G. P., Satija, S. K., "Characterization of BPSG Films Using Neutron Depth Profiling and Neutron/X-Ray

Reflectometry," in Proceedings of 2000 International Confer- ence on Characterization and Metrology for ULSI Technology,

edited by D. G. Seiler, A. C. Diebold, W. M. Bullies, T. J. Shaffner, R. McDonald, E. J. Walters (AIP Press, Woodbury,

NY), 550. 407 (2001).

Choi, S. M., Barker, J. G., Glinka, C. I., Cheong, Y. T, Gammel, P L., "Focusing Cold Neutrons with Multiple Biconcave Lenses for Small-Angle Neutron Scattering," J. Appl. Crystallogr. 33 (792), 2000.

Choi, S. M., Briber, R. M., Bauer, B. J., Liu, D.-W., Gauthier, M., "Small Angle Neutron Scattering of Blends of Ai^borescent Polystyrenes," Macromol. 33 (17), 6495 (2000).

Choi, S. M., Lynn, J. W., Lopez, D., Gammel, P L., Varma. C. M..

"Direct Observation of Spontaneous Weak-Ferromagnetism in the Superconductor ErNi,B,C," Phys. Rev. Lett. 86, 7121

(2001).

Christianson, R. J., Leheny, R. L., Birgeneau, R. J., Erwin, R. W.,

"Critical Dynamics of a Spin-5/2 Two-Dimensional Isotropic

Antiferromagnet," Phys. Rev. B63 ( 14), 0401 (2001 ).

Co, C. C, De Vries, R., Kaler E. W., "Small-Angle Neutron Scattering

Study of Monomer Partitioning," Macromol. 34, 3224 (2001). Cohen, R. E., Gulseren, O., "Thermal Equation of State of Tantalum,"

Phys. Rev. B63, 224101 (2001).

Copley, J. R. D., "NIST Recommended Practice Guide; The Fundamentals of Neutron Powder Diffraction," NIST SP 960-2, in press.

Cosgrove, T, Howe, A., Thomsen, L., Armes, S. P., Billingham, N., "The Structure of Hydrophilic-Hydrophobic Block Copolymers

and Their Interactions With Salt and a Surfactant," Langmuir, in press. DeVries, R., Co, C. C, Kaler, E. W., "Influence of Monomer Partitioning, Termination, and Diffusion Limitations on Polymerization

Kinetics," Macromol. 34, 3233 (2001). DeWall, J., Dimeo, R. M., Sokol, P. E., "Diffusion and Phase Transitions of Molecular Hydrogen in Zeolites," Phys. Rev. B, in press. Dewey, M. S., Kessler, E. G.. "Precision Measurement of Fundamental

Constants Using GAMS4," J. Res. Natl. Inst. Stan. 105 (I), II (2000). Dimeo, R. M., Chowdhuri, Z., Meyer, A., Gehring, P. M., Neumann,

D. A., "The NIST High Flux Backscattering Spectrometer,"

Appl. Phys. A, in press. Dimeo, R. M., Neumann, D. A., "Low Temperature Dynamics of Confined Methyl Iodide." Phys. Rev. B63 (1), 4301 (2001). Dimeo, R. M.. Neumann, D. A., "The Effects of Confinement on

Rotational Tunneling," Appl. Phys. A, in press.

DiNoia, T P, Kirby, C. F, Van Zanten, J. H., McHugh, M. A., "SANS

Study of Polymer-Supercritical Fluid Solutions: Transitions from

Liquid to Supercritical Fluid Solvent Quality," J. Am. Chem.

Soc. 33 (17), 6321 (2000).

Divita, E, Ten7, J., Kidwell, C. A., Ondov, J. M., Pandis, S., "Characterization of Primary Aerosol Emissions from a Municipal

Incinerator Via Ground-Level Sampling with a Micro-Orifice

Impactor." Atmos. Environ., in press.

Douglas, J. F, Kent, M. S., Satija, S. K., Karim, A., "Swelling of Grafted Polymer 'Brushes,' " in Proceedings Encyclopedia of

Materials: Science and Technology, in press. Downing, R. G., Iyengar, G. V, "Methodological Issues in the Analytical Determination of Boron," Environ. Health Perspect.,

in press.

Dubsky, J., Prask, H. J., Matejicek, J., Gnaupel-Herold, T. "Stresses in Plasma Sprayed Cr,0, Coatings Measured by Neutron Diffraction," in Proceedings of the International Conference on

Neutron Scattering (Munich, Germany, September 2001), in press.

Eid, K., Portner, D., Borchers, J. A., Loloee, R., Tsai, M., O'Donovan,

K. v., Pratt, W. P, Bass, J., "Mean-Free Path Effects in CPP

Magnetoresistance of Magnetic Multilayers," Phys. Rev B, in press.

Elliot, J., Meuse, V., Sihn, V., Krueger, S., Woodward, J. T, Petralli- Mallow, T. P., Plant, A. L., "Biomedical Membranes on Metal

Supports: Opportunities and Challenges," in Proceedings of the Biomolecular Films: Design, Function, and Application, edited by J. Rusling (New York, Marcel Dekker, Inc.), in press.

Esker, A. R., Grull, H., Satija, S. K., Han, C. C, "Probing Polymer

Dynamics with Ultra-Thin Membranes by Neutron Reflectivity"

Polymer Preprints 221, (2001 ).

Esker, A. R.. Grull, H., Wegner, G.. Satija. S. K., Han, C. C, "Isotopic

Selectivity in Ultra Thin Langmuir-Blodgett Membranes of a Cross-Linked Cellulose Derivative," Langmuir 17, 4688 (2001). Evmenenko, G., Bauer, B. I., Kleppinger, R., Forier, B., Dehaven, W., Amis, E. J., Miscehnko, N., Reynaers, H., "The Influence of Molecular Architecture and Solvent Type on the Size of

Structure of Poly (Benzyl Ether) Dendrimers by SANS," Chem.

Phys. 202 (6), 891 (2001). 58 Publications
Farrow. R. F. C, Marks. R. R, Toney, M. R. Hannibal, K., David, S., Kellock, A.. Borchers, J. A.. O'Donovan, K. V., Smitii, D. J., "Spontaneous Chemical Ordering and Exchange Bias in Epitaxial Mn^ .,Pdp^/Fe (001) Bilayers Prepared at Room

Temperature," Appl. Phys. Lett., in press.

Feeney, M., Brown, C. M., Tsai, A.. Neumann. D. A., Debenedetti,

P. G., "Incoherent Quasi-Elastic Neutron Scattering From

Fructose-Water Solutions," J. Phys. Chem. 105 (32). 7799

(2001).

Ferdinand, S., Lesemann, M., Paulaitis, M. E.. "Effect of Pressure on

Microstructure of CpE. n-Octane-in-D,0 Microemulsions,"

Langmuir, 16, 10106 (2000).

FitzGerald, S. A., Thomas. J. J.. Neumann, D. A., Livingston, R. A., "A Neutron Scattering Study of the Role of Diffusion in the Hydration of Tricalcium Silicate." Cement and Concrete Res., in press.

Fratini, E., Chen, S.-H. Baglioni. P., Cook, J., Copley, J., "Dynamic

Scaling of Quasi-Elastic Neutron Scattering Spectra From

Interfacial Water," Phys. Rev. E, in press.

Freeman. K. S.. Beck Tan. N. C. Trevino. S. R, Kline, S., McGown,

L. B.. Kiserow. D. J., "Size and Polydispersity Determinations of AOT/Bile Salt Reversed Micelles Obtained by Small-Angle Neutron Scattering," Langmuir 17 (13), 3912 (2001).

Fultz, B., Witham, C. K., Udovic, T. J., "Distributions of Hydrogen

and Strains in LaNi, and LaNi^ ^jSn^ ,.," J. Alloys Compnds., in press.

Garvey, C. J.. Parker, I. H.. Simon. G. P, Whittaker, A. K., Knott. R. B., "An Experimental Study by NMR and SANS of the Ambient Hydration of Paper," in Proceedings of the 12th Fundamental Research Symposium, (Pulp and Paper Fundamental Research Society, Oxford), in press.

Gehring. P. M., Park. S.-E., Shirane. G.. "Dynamical Effects of the Nanometer-Sized Polarized Domains in Pb(Zn|^jNb,^j)Oj" Phys. Rev. B63 (22), 4109 (2001).

Geissler, E., Horkay R, Hecht, A. M., "Structure of Polymer Solutions and Gels Containing Fillers," Macromol. Symp. 171, 171

(2001).

Geissler, E., Hecht, A. M.. "Treatment of Incoherent Signal Subtraction" in Proceedings of the Frontiers in SAXS and SANS, ESRFILL-CEA Workshop (Grenoble, France, February 12-13, 1999),

in press.

Geissler, E., Horkay, R. Hecht, A. M., Deschamps, P., "Neutron

Scattering From Elastomer Gels," Kautschuk, Gummi, and

Kunstoffe, in press.

Geissler, E., Horkay, F, McKenna, G. B.. Deschamp, P., "Osmotic and

Mechanical Observations of Swollen Polyisoprene Rubber and

its Microscopic Structure," in Proceedings of Worldwide Amazon

Rubber Conference (Manaus, Brazil, October 31 - November 4, 1999), in press.

Gentile, T. R., Jones, G. L., Thompson, A. K., Rizi, R. R., Dimitrov,

1. E.. Roberts, D., Gefter, W., Schnall, M. D., Leigh, J. S., "A

Compact Compressor for Application of Metastability-Exchange

Optical Pumping of He to Human Lung Imaging," Magn.

Reson. Med., in press.

NIST Center for Neutron Research

Gentile. T. R., Rich, D. R., Thompson, A. K., Bailey, C„ Gericke, M., Hussey, D., Neff, B., Snow, W. M., Jones, G. L., Wildman, E., "The NIST-Indiana-Hamilton Polarized ^He Spin Filter Program," in Proceedings of the Fundamental Physics with Pulsed Neutron Beams Conference, edited by C. R. Gould, G. L. Green, F. Plasil, W. M. Snow (World Scientific, Singapore,

2001), p. 230.

Gibson. W. M.. Chen-Mayer, H. H.. Mildner. D. R R., Prask, H. J., Schultz, A. J., Youngman, R., Gnaupel-Herold, T., Miller. M. E.,

Vitt, R., "Polycapillary Optics Based Neutron Focusing for Small Sample Neutron Crystallography," in Proceedings of the Adv. In X-Ray Analysis 45, in press.

Giebultowicz, T. M., Kepa, H., Blinowski, J., Kacman, P., "Neutron

Scattering Studies of Interlayer Correlations in Magnetic

Semiconductor Superlattices," Physics E, in press.

Gilra, N., Cohen C, Briber, R. M., Bauer, B. J., Hedden, R.. Panagiotopoulos, A. Z., "A SANS Study in Strained and

Unstrained End-Linked Elastomers," Macromol., 34 (22), 7773

(2001).

Gilliam, D. M., "NIST Neutron Physics and Dosimetry," in Proceedings of the Transactions of the American Nuclear Society, November 12-16, 2000, (Washington, DC, 2000), p. 496.

Glagolenko, 1. Y., Carney, K. P., Kern, S., Goremychkin, E. A., Udovic, T. J., Copley, J. R. D., Cook, J. C, "Quantitative

Analysis of UH^ in U Metal and UO, Matrices by Neutron

Vibrational Spectroscopy," Appl. Phys. A, in press.

Glinka, C. J., "Review of Methods of X-Ray and Neutron Scattering in Polymer Science," by Ryong-Joon Roe, Physics Today 54 (3), 60

(2001).

Gnaupel-Herold, T, "Single Crystal Elastic Constants From Powder

Measurements," in Proceedings ofAccuracy in Powder

Diffraction. Ill (NIST Special Publication), in press.

Gnaupel-Herold. T, Haeffner. D. R.. Prask, H. J., Matejicek, J., "Stress Determination by Whole Pattern Analysis-Application to Synchrotron HEXRD Measurements on Thermal Sprayed

Coatings," in Proceedings of The Sixth International Conference on Residual Stresses, (lOM, London, 2000), p. 751.

Gnaupel-Herold, T, Matejicek, J., Prask, H. J.. "Mechanical Properties of Plasma Sprayed Coatings - Measured by Diffraction," in Proceedings of the 9th International Metallurgical Conference Metal 2000, (Ostrava, CZ), paper no. 508 (CD-ROM).

Gnaupel-Herold, T, Prask, H., Fisher, J., Cheng, X., "Residual Stress Modification by Means of Ultrasonic Impact Treatment," in Proceedings of the Annual Conference of Society for Experimental Mechanics. (Portland, OR, June 2001), in press.

Gnaupel-Herold, T, Prask, H. J., Magiera, J., "Effect of Grinding

Strategy on Accumulation of Damage in Rails: Neutron

Diffraction Investigation of Residual Stresses in Transverse and

Oblique Cut Rail Slices," in Proceedings of The Sixth International Conference on Residual Stresses (lOM, London, 2000),

p. 879.

Green, M. A., Waldron, J. E. L., Neumann, D. A., "Charge and Spin

Ordering in Monoclinic Nb^O,,," J. Am. Chem. Soc. 123 (24), 5833 (2001).

Greenberg, C. C, Foster, M. D., Turner, C. M., Corona-Galvan, S., Cloutet, E., Quirk, R. R, Butler, R D.. Hawker, C, "Effective

Interaction Parameter Between Branched and Linear PS,"

J. Polym. Sci. 39, 2549 (2001).

59
Greenberg, C. C, Teale, D. M., Foster, M. D., Turner, C. M., CoronaGalvan, S., Cloutet, E., Quirk, R. P., Majkrzak, C. R, Demaree,

D., "Surface Segregation in Blends of Linear and Regularly Star Branched Polystyrene," Macromol., in press.

Greenberg, C. C, Foster, M. D., Turner, S., Corona-Galva, E., Cloutet,

R., Quirk. P, Butler, R D., Hammouda, B.. Hawker, C., "Effective Interaction Parameter Between Branched and Linear PS," J. Polym. Sci. 39 (21), 2549 (2001). Crohn, F., Bauer, B. J., Akpalu, Y. A.. Jackson, C. L., Amis, E. J., "Dendrimers as Templates for the Formation of Inorganic

Nanoclusters," Macromol. 33 (16), 6042 (2000). Crohn, F., Bauer, B. J., Amis, E. J., "Organic-Inorganic Hybrid

Particles by Dendrimer Nanotemplating," in Proceedings of the Materials Research Society Meeting (April 2000), p. 628.

Gruell. H., Sung, L.. Karim, A.. Douglas, J. F, Satija, S. K., Hayashi, M., Jinnai, H., Hashimoto, T., Han, C. C, "Surface Segregation

of a Symmetrical Segregating Polymer Blend Film Above and Below the Critical Point of Phase Separation," Phys. Rev. Lett., in press.

Gulseren, O., Yildirim, T. Ciraci, S.. "Tunable Adsorption on Carbon

Nanotubes," Phys. Rev. Lett. 87 (11), 116802 (2001).

Gulseren, O.. Yildirim, T, Ciraci, S., Kilic, C, "First-Principles Study

of the Electronic Structure of Carbon Nanotubes: Variable and

Reversible Band Gap Engineering," Phys. Rev. B, in press. Guo, L., Colby, R. H., Lin. M. Y, Dado, G. P, "Micellar Structure Changes in Aqueous Mixtures of Nonionic Surfactants,"

J. Rheol. 45, 1223 (2001). Hagemann, I. S., Huang, Q., Gao, X. P. A., Ramirez, A. P., Cava, R. J., "Geometric Magnetic Frustration in Ba,Sn,ZnGa^CrgO„: A TwoDimensional Spinel Based Kagome Lattice," Phys. Rev. Lett. 86

(5). 894 (2001). Hammouda, B., "SANS From Polymers Under Pressure," Neutron News 12, 20 (2001). Han, C. C, Hobble, E. K., Nakatani, A. I., Karim, A., "Critical Temperature Shift and Mixing in Polymer Blends Under Simple

Shear Flow," J. Phys. See. Japan., 70, 317 (2001). Han, C. C. Schmidt, G., Nakatani, A. I., Butler, R, "A Small Angle

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Loureiro, S. M., Felser, C, Huang, Q., Cava, R. J., "Refinement of the Crystal Structures of Strontium Cobalt Oxychlorides by Neutron Powder Diffraction," Chem. Mat. 12 (10), 3181 (2000).

Luchette, P. A., Vetman, T. N., Prosser, R. S., Hancock, R. E. W., Nieh,

M.-R, Glinka, C. J., Kiueger, S., Katsara, S. J., "Moi-phology of

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Lynch, M. L., Wireko, F, Tarek, M., Klein, M., "Intermolecular

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Mildner, D. F. R., "Time Focusing of Pulsed-Source Crystal Analyzer

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Nair, S., Tsapatsis, M., Toby, B. H., Kuznicki. S. M., "A Study of Heat-Treatment Induced Framework Contraction in Sr-ETS-4 by

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Nakatani, A. I., Chen, W., Schmidt, R. G., Gordon, G. V., Han, C. C,

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Nakatani, A. I., Ivkov, R., Papanek, P., Jackson, C. L., Yang, H.,

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Nieh, M.-P, Glinka, C. J., Krueger, S., Prosser, R. S., Katsaras, J., "A SANS Study on the Effect of Lanthanide Ions and Charged

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Nieh. M.-P, Kumar, S., Ho, D., Briber, R. , ""Neutron Scattering of Polymer Conformations in Energetic Neutral Pores of the Vycor," Macromol., in press.

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O'Donovan, K. V., Borchers, J. A., Majkrzak, C. F, Hellwig, O., Fullerton, E. E., ""Extracting Buried Twists With Polarized Neutron Reflectometry," Appl. Phys. Lett. A, in press. O'Donovan, K. V., Borchers, J. A., Majkrzak, C. F, Heilwig, O., Fullerton, E. E., "Pinpointing Chical Structures Using Front/ Back Polarized Neutron Reflectivity," Phys. Rev. Lett., in press. Ohwada, K., Hirota, K., Rehrig, P W., Gehring, P M., Noheda, B.,

Fujii, Y, EaglePark, S.-E., Shirane, G., "Neutron Diffraction Study of the Irreversible R-MA-MC Phase Transition in Single

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Olson, D. H., Toby, B. H., Reisner, B. A., "Neutron Diffraction Study

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Papanek, P., Fischer, J. E., Sauvajol, J. L., Dianoux, A. J., McNeillis,

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Papanek, P., Kamitakahara, W. A., Zhou, P., Fischer, J. E., "Neutron

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Paul, R. L., ""Development of a Procedure for Measuring Nitrogen by

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Paul, R. L., "Determination of Nitrogen in Some Reference Materials by Cold Neutron Prompt Gamma-Ray Activation Analysis,"

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Paul, R. L., ""Nitrogen in Some Reference Materials by Cold Neutron Prompt Gamma-Ray Activation Analysis," Analyst, in press.

Paul, R. L., ""Nondestructive Measurement of Hydrogen and Other Elements by Cold Neutron Capture Prompt Gamma-Ray

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Paul, R. L., ""Reference Minerals for Microanalysis of Light Elements," Geostandards Newsletter, in press.

Paul, R. L., Chen-Mayer, H. H., Lindstrom, R. M., "'Characterization of Hydrogen in Concrete by Cold Neutron Prompt Gamma-Ray

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Paul, R. L., Simons, D. S., "Neutron Activation Analysis for Calibration of Phosphorus Implantation Dose," in Proceedings of the Characterization and Metrology for ULSI Technology, edited by

D. G. Seller, A. C. Diebold, W. M. Bullies, T. J. Shaffner, R. McDonald, E. J. Walters (American Institute of Physics,

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Pivovar, A. M., Ward, M. D., Yildirim. T, Neumann, D. A., "Vibrational Mode Analysis of Isomorphous Hydrogen-Bonded

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Pochan, D. J., Lin, E. K., Wu, W.-L., Satija, S. K., Cheng, S. Z. D., "Themial Expansion and Glass Transition Behavior of Thin

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Pochan, D. J., Pakstis, L., Nowak, A. P, Deming, T. J., "Self- Assembled Polypeptide Hydrogels; Morphology and Cytotoxicity," BiomacromoL, in press.

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Porcar, L., Hamilton, W. A., Butler, P. D., Warr, G. G., "A New Vapor

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Prask, H. J., Brand, P C, Gnaupel-Herold, T, Hicho, G, E., "Determination of Residual Stresses Using Neutron Diffraction: Tank-Car

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Prask, H. J., Gnaupel-Herold, T, Fisher, J. W., Cheng, X., Stuart. J. T,

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Reidy, R. F., Allen, A. J., Kiueger, S., "Small Angle Neutron Scattering Characterization of Colloidal and Fractal Aerogels," J. Non

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Rich, D. R., Fan, S., Gentile, T. R., Hussey, D., Jones, G. L., Neff, B., Snow, W. M., Thompson, A. K.. "Polarized 'He Neutron Spin

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Rosenkranz, S., Osbom, R., Vasiliu-Doloc, L., Lynn, J. W., Sinha,

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Salamat, G., De Vries. R., Kaler. E. W„ Satija, S., Sung, L., "Undulations in Salt-Free Charged Lamellar Phases Detected by Small- Angle Neutron Scattering and Neutron Reflectivity," Langmuir

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Sankar, S., Dender, D., Borchers, J. A., Smith, D. J., Erwin, R. W.,

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Santoro, A., "Neutron Crystallography at NBS/NIST" ACA Newsletter, in press.

Sayer, E. V., Yener, A. K., Joel, E. C, Blackman, M. J., Ozbal, H., "Stable Lead Isotope Studies of Black Sea Anatohan Ore

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Schmidt, G.. Nakatani, A. I., Butler, P. D., Ferreiro, V., Karim, A., Han. C. C., ""Polymer-Clay Nanocomposite Materials, Solution and Bulk Properties," in Proceedings of the Materials Research

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Schmidt, G., Nakatani, A. I., Butler, P D., Karim, A., Han, C. C., "Shear Orientation of Viscoelastic Polymer-Clay Solutions," in Proceedings of the Xlllth International Congress on Rheology

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Schober, H.. Losert, C, Mezei, F., Cook. J. C, ""Chopper Cascades: An

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Sharpless. K. E.. Colbert. J. C. Greenberg. R. G.. Schantz, M. M., Welch, M. J.. ""Recent Developments in Good-Matrix Reference

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Shin, K., Zhang, Y, White. H., Rafailovich, M. H., Sokolov, J., Peiffer, D., "Organoclay Nanoplatelets at the AirAVater Interface," Mat.

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Shin. K.. Hu. X.. Zheng, X.. Rafailovich. M. H.. Sokolov, J., Zaitsev,

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Structure of Hydrogen-Bonded Polymer Blends," J. Polym. Sci. Part B: Poly. Phys., in press. Zimmerman, M. H., Baskin, D. M., Faber, K. T, Fuller, E. R., Allen, A. J., Keane, D. T, "Fracture of Textured Iron Titanate," Acta. Mater. 49, 3231 (2001). Zimmerman, M. V, "X-Ray Scattering Studies of Orbital and Charge

Ordering in Prj ^Ca^MnO,," Phys. Rev. B, in press.

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Publications
NIST Center for Neutron Research 71
Instruments and Contacts

High resolution powder diffractometer {BT-1)

J. K. Stalick, (301) 975-6223, judy.stalick@nist.gov

B. H. Toby, (301) 975-4297, brian.toby@nist.gov

DARTS, Residual stress and texture diffractometer (BT-8)

H. J. Prask, (301) 975-6226, hank@nist.gov

T Gnaeupel-Herold, (301) 975-5380, thomas.gnaeupel-lierold@nist.i

30-m SANS instrument {NG-7)

C. J. Glinka, (301) 975-6242, cglinka@nist.gov

J. B. Barker, (301 ) 975-6732, barker@ nist.gov

B. S. Greenwald, (301) 975-5795, bsgreen@nist.gov

30-m SANS instrument (NG-3) (NIST/NSF-CHRNS)

B. Hammouda, (301) 975-3961, hammouda@nist.gov

S. R. Kline, (301) 975-6243, steven.kline@nist.gov

D. Ho, (301) 975-6422, derek.ho@nist.gov

T K. Misra, (301) 975-6650, tmisra@nist.gov

8-m SANS instrument (NG-1)

D. Ho, (301) 975-6422, derek.ho@nist.gov

0. J. Glinka, (301) 975-6242, cglinka@nist.gov J.G. Barker, (301) 975-6732, john.barker@nist.gov

USANS, Perfect crystal SANS (BT-5) (NIST/NSF-CHRNS)

' J. B. Barker, (301)975-6732, barker@nist.gov

C. J. Glinka, (301) 975-6242, cglinka@nist.gov

Cold neutron reflectometer-vertical sample-polarized beam option

(NG-1)

C. F. Majkrzak, (301) 975-5251, cmajkrzak@nist.gov

J. A. Dura, (301) 975-6251, jdura@nist.gov

Cold neutron reflectometer-horizontal sample (NG-7)

S. K. Satija, (301) 975-5250, satija@nist,gov

K. W. Shin, (301) 975-8829, kwshin@nist.gov

Triple-axis polarized-beam spectrometer (BT-2)

J. W, Lynn, (301) 975-6246, jeff.lynn@nist.gov

Triple-axis fixed incident energy spectrometer (BT-7)

J. W. Lynn, (301) 975-6246, jeff.lynn@nist.gov

Triple-axis spectrometer (BT-9)

R. W. Erwin, (301) 975-6245, rerwin@nist.gov

P. M. Gehring, (301) 975-3946, pgehring@nist.gov

SPINS, Spin-polarized triple-axis spectrometer (NG-5)

(NIST/NSF-CHRNS)

8. Park, (301 ) 975-8369, sungil. park® nist.gov P.M. Gehring, (301)975-3946, pgehring@nist.gov

S.-H. Lee, (301) 975-4257, seung-hun.lee@nist.gov

FANS, Filter-analyzer neutron spectrometer (BT-4)

T. J. Udovic, (301) 975-6241, udovic@nist.gov

C. M. Brown, (301) 975-5134, craig.brown@nist.gov

D. A. Neumann, (301) 975-5252, dan@nist.gov

FCS, Fermi-chopper time-of-flight spectrometer (NG-6)

C. M. Brown, (301) 975-5134, craig.brown@nist.gov

T J. Udovic, (301) 975-6241, udovic@nist.gov

DCS, Disk-chopper time-of-flight spectrometer (NG-4)

(NIST/NSF-CHRNS)

J. R. D.Copley, (301) 975-5133, jcopley@nist.gov

J. 0. Cook, (301) 975-6403, jeremy.cook@nist.gov

HFBS, High-flux backscattering spectrometer (NG-2)

(NIST/NSF-CHRNS)

R. M. Dimeo, (301) 975-8135, robert.dimeo@nist.gov

Z. Chowdhuri, (301) 975-4404, zema.chowdhuri@nist.gov

D. A. Neumann, (301) 975-5252, dan@nist.gov

NSE, Neutron spin echo spectrometer (NG-5) (NIST/NSF-CHRNS)

N. S. Rosov, (301) 975-5254, nrosov@nist.gov

Prompt-gamma neutron activation analysis (NG-7)

R. M. Lindstrom, (301) 975-6281, dick.lindstrom@nist.gov

R. L. Paul, (301) 975-6287, rpaul@nist.gov

Other activation analysis facilities

R. R. Greenberg, (301) 975-6285, rgreenberg@nist.gov

Cold neutron depth profiling (NG-0)

G. Lamaze, (301) 975-6202, lamaze@nist.gov

Instrument development station (NG-0)

D. F. R. Mildner, (301) 975-6366, mildner@nist.gov

H. H. Chen, (301) 975-3782, chenmayer@nist.gov

Neutron interferometer (NG-7)

M. Arif, (301) 975-6303, muhammad.arif@nist.gov

Fundamental neutron physics station (NG-6)

M. S. Dewey, (301) 975-4843, mdewey@nist.gov

Theory and modelling

N. F. Berk, (301) 975-6224, nfb@nist.gov

T. Yildirim, (301) 975-6228, taner@nist.gov

Sample environment

D. 0. Dender, (301) 975-6225, dender@nist.gov

72 Instruments and Contacts
NIST Center for

Neutron Research

For copies of or information on this report, contact:

Ronald L. Cappelletti

(301) 975-6221

ron.cappelletti@nist.gov

For additional information on the facility, contact:

J. Michael Rowe

(301)975-6210

mike.rowe@nist.gov

John J. Rush

(301)975-6231

john.rush@nist.gov

To obtain guidelines for preparing proposals to conduct research at the facility, contact:

William Kamitakahara

(301)975-6878

william.kamitakahara@nist.gov

Location of all contacts:

NIST Center for Neutron Research

Building 235, Room E151 , Mail Stop 8562

National Institute of Standards and Technology

Gaithersburg, MD 20899-8562

Details are also available on the NCNR Web site: ww/w.ncnr.nist.gov.
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