National

Measurement

laboratory

1 Q7Q Technical Ly,y Highlights

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards NBS-SP 572

National

Measurement

laboratory

1979

Technical Highlights

U.S. DEPARTMENT OF COMMERCE

Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology and Innovation

NATIONAL BUREAU OF STANDARDS

Ernest Ambler, Director

MATIONAL BU&SAV

Or STANDASUS

LIBBART

JUL 8 1980

April 1980
National Bureau of Standards Special Publication 572

Nat. Bur. Stand. (U.S.), Spec. Publ. 572, 126 pages (April 1980)

CODEN: XNBSAV

Issued April 1980

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $4.25 (Add 25 percent for other than U.S. mailing )

.
Foreword

This report of technical highlights of the National Measurement Laboratory is the first in a series of annual re- views. It covers selected scientific accomplishments of

the Laboratory for the calendar year 1979. A general

Laboratory Overview as well as more specific Center and

Program Overviews set the framework for the individual

technical reports. These reports represent the wide range

of Laboratory activities, the breadth of which should be

apparent from the overviews themselves. Future editions

of this report will present different, but nevertheless representative, selections of technical highlights reporting

research activities of the preceding year.
Contents

National Measurement Laboratory Overview 1 Center for Absolute Physical Quantities Overview ... 5 Rydberg Values for X- and y-Rays 7

Richard D. Deslattes and

Ernest G. Kessler, Jr. Laser Cooling of Stored Ions 10

David J. Wineland

Observation of Pure Rotational Transitions in the HBr +

Molecular Ion with Laser Magnetic Resonance .... 11 Richard J. Saykally and

Kenneth M. Evenson

Extension of Absolute Frequency Measurements

to the Visible 13

Donald A. Jennings,

F. Russell Petersen and

Kenneth M. Evenson

Fine Structure Constant Determined to an Accuracy of 1 in 10 7 14

P. Thomas Olsen and

Edwin R. Williams

Superconducting Thermometric Fixed Point Devices . . 16

Robert J. Soulen, Jr. and

James F. Schooley

Improved Laser Test of the Isotrophy of Space 17

John L. Hall and Alain Brillet Center for Radiation Research Overview 21

Radioactivity Standardization 24

Wilfrid B. Mann

Spectra of Highly-Ionized Atoms 25

Joseph Reader

Polarized Low Energy Electron Diffraction (PLEED)

A Technique to Study Surface Magnetism 28

Daniel T. Pierce and

Robert J. Celotta

X-Ray Image Information Theory 29

Joseph W. Motz and

Michael Danos

New Studies of Electric Quadrupole (E2)

Excitation in Probes 31

William R. Dodge

Glueballs and Quark States 34

Sydney Meshkov

Quantum Basis for Visible Region Radiometric

Standards 35

Jon Geist,

Edward Zalenski, and

A . Russell Schaejer

Center for Thermodynamics and

Molecular Science Overview . Bond Energies and Chemical Reactivity

Wing Tsang

Dynamic Thermophysical Measurements

A red Cezairliyan Pressure as a Probe of Molecular Interactions

Vern E. Bean

The Catalytic Methanation Reaction: A Source of Synthetic Natural Gas

John T. Yates, Jr.

David W. Goodman,

Richard D. Kelley, and

Theodore E. Madey

Multiphoton Chemistry

John C. Stephenson

Quantum Calculations of Excimer Molecules

Morris Krauss

New Developments in the Evaluation of Thermochemical Data

Donald D. Wagman,

David Garvin,

Vivian B. Parker,

Richard H. Schunun, and

J. Brian Pedley

Center for Analytical Chemistry Overview

Laser Enhanced Ionization in Flames

John C. Travis,

Gregory C. Turk,

James R. DeVoe, and

Peter K. Schenck

Measurement of Trace Elements in Solar-Cell

Silicon Richard M. Lindstrom,

Ronald F. Fleming, and

Paul J. Paulsen

Quantitative Analysis of Individual Trace-Level

Organic Constituents in Alternate Fuels

Willie E. May and

Stephen N. Chesler

Computer-Controlled Electrochemical System for Electrode Kinetics Research

Lawrence M. Doane and

Richard A. Durst High Resistance Junction Formation in House

Wiring Circuits with Aluminum Conductors . . Dale E. Newbury

39

43

\ 44

i

46

!

47

49

8

52!

53 i

57i

59)

61

63

6-!

6(
Discrimination of Natural From Anthropogenic

Carbonaceous Pollutants Through MiniRadiocarbon Measurements 68

Lloyd A . Currie and

George A. Klouda

Center for Materials Science Overview 75

Erosion of Brittle Materials by Solid Particle Impact . . 79

Sheldon M. Wiederhorn,

Bernard J. Hockey,

A . William Ruff, and

Lewis K. Ives Atomistic Theories of Crack Growth and Fracture ... 81

Edwin R. Fuller, Jr., Robb M. Thomson, and

Brian R. Lawn

i Neutron Diffraction Profile Analysis

and the Structures of Electronic Ceramics 84

- Antonio Santoro and

Robert S. Roth

Recent NBS Advances in Physical Properties

Measurements at Ultra High Pressures 86

Stanley Block,

Ronald G. Munro, and

Gasper J. Piermarini

Piezoelectricity and Pyroelectricity in Poly (vinyl id ene fluoride) 89

Martin G. Broadhurst and

George T. Davis

Electrochemical Noise Measurements: A New

Measurement Technique for Diagnosis and Study

of Localized Corrosion 92

Ugo Bertocci

Migration Models for Polymer Additives 94

Isaac C. Sanchez,

Shu Sing Chang, and

Leslie E. Smith

Office of the Deputy Director for Programs

Overview 99

Load Cell Mass Comparator 102

Randall M. Schoonover

Interagency Cooperation in Mass Spectral Data

Base Dissemination 103

Lewis H. Gevantman

Standard Reference Material for Antiepilepsy

Drugs in Serum 104

Robert Schaffer and

Dennis J. Reeder

Development of Test Procedures to Determine Calorific Value of Refuse and Refuse-Derived Fuels 107

Eugene Domalski and

Joseph G. Berke

Nuclear Fuel Assay Using Resonance Neutrons 108

Roald A. Schrack,

James W. Behrens, and

Charles D. Bowman

Improved Uses of X rays for Nondestructive

Evaluation 110

Masao Kuriyama,

William J. Boettinger and

Harold E. Burdette

High Accuracy Particle Size and Particle-Fluid

Interaction Measurements Using the Particle Doppler Shift Spectrometer 113

David S. Bright and

Robert A. Fletcher

v

National Measurement

Laboratory Overview

John D. Hoffman

Director

Donald R. Johnson

Deputy Director

for Resources &

Operations

Cary C. Gravatt, Jr. Deputy Director

for Programs

Arthur O. McCoubrey

Associate Director for Measurement Services

David T. Goldman

Associate Director for Long Range Planning

Ian R. Bartky * Edmund A. Dimarzio

Lois J. Frolen David T. Goldman

Charles M. Guttman

Ruth A. Haines

Robert E. Huie, III Michael J. Kurylo,

III Rosemary S. Maddock

George P. Lamaze

John Mandel

Arthur O. McCoubrey

Hans J. Oser Robert C. Paule Gaylon S. Ross, Sr. Joe D. Simmons

Gilbert M. Ugiansky

Robert W. Zwanzig

* Staff as of December

31, 1979

John D. Hoffman is studying, by means

of a polarizing optical microscope, a polymer sample which has been crystallized under controlled temperature

conditions while co-workers, Gaylon S. Ross and Lois J. Frolen look on. This

work is part of an ongoing project

directed toward an understanding of the nature of crystals and the crystallization process in industrially important polymers.

NBS was created by Congress in 1901 to meet

the needs of a growing Nation for a unified measurement

system. For more than 70 years the Bureau has contributed to advances in science and technology, the

growth of industry, and efficiency in the marketplace.

In so doing, the Bureau has built a scientific facility and

staff that stand with the world's best.

The basic enabling legislation of 1901, as amended,

establishes these main purposes and functions of the

Bureau:

• The custody, maintenance, and development of the

national standards of measurement, and the provision of means and methods for making measure- ments consistent with those standards, including the

comparison of standards.

• The determination of physical constants and properties of materials, of great importance to scientific or manufacturing interests "and not to be obtained

of sufficient accuracy elsewhere."

• The development of methods for testing materials,

mechanisms, and structures, and the testing of

materials, supplies, and equipment.

• Cooperation with other Government agencies and

with private organizations in the establishment of

standard practices incorporated in codes and specifications.

• Advisory service to Government agencies on scientific and technical problems.

• Invention and development of devices to serve

special needs of the Government.

1
NATIONAL MEASUREMENT LABORATORY

Thus, NBS has both unique and special functions in

relation to the Nation's science and technology, and very

broad responsibilities as well. As part of the U.S. Department of Commerce, the Bureau carries out its mission in a variety of ways.

The cornerstone of the NBS mission is to serve, for

the United States, as the authoritative source of accurate,

compatible, and useful physical measurements and, fur- ther, to ensure their international compatibility. As the

world's leading technological Nation, the United States has a vital interest in ensuring that the world system of

measurement is not only internationally compatible but

is also sufficiently sophisticated to meet U.S. needs.

The National Measurement Laboratory bears a large

part of the responsibility for fulfilling this mission. The

Laboratory was created in 1978 as a result of a major

NBS reorganization. The following major objectives

have been established to guide its programs.

1. Provide the central basis, within the United States of a complete and consistent set of physical and chemical

measurements.

2. Provide reference measurement methods and refer- ence materials so that measurements performed in diverselocations for industrial, commercial, or regulatory applications are consistent.

3. Assure that technological development is not hindered by a lack of accurate physical or chemical information.

4. Apply NML capabilities in the physical sciences

and measurement technology to solving specific problems

of national concern.

5. Provide outstanding comprehension and competence in selected fields of advanced scientific research

with long-term relevance to areas of critical importance

to NBS and the Nation.

To meet these objectives, the laboratory is managed

in a matrix form, with five centers organized generally

along disciplinary lines and a number of offices tha

direct the scientific programs by drawing on the com

petences in the centers and divisions. The centers cove

absolute physical quantities, radiation research, thermc

dynamic and molecular science, analytical chemistry

and materials science.

i The cross-cutting programs are nondestructive evalua

tion, environmental measurements, standard referenc

materials, standard reference data, nuclear technology

recycled materials, and measurement services. Thes

centers and programs will be described in more deta

later. NML has a staff of 1400. Approximately 80 percer

are scientists and engineers, of which approximately 6

percent have earned doctorate degrees. The NML has

total fiscal year 1980 budget obligation of $75.4 millior

of which $45.8 million is directly appropriated func

(STRS money), $29.6 million is in return for technkc

services rendered.

One of the most important events for NBS in recer years was the recognition by the Office of Managemei

and Budget and the Congress of the need to widen th_- Bureau's scientific base (competence building). Th

National Measurement Laboratory has invested almo<

$3 million in new programs in a variety of discipline; In the area of surface science, a program has begun t characterize the structures and reactions of atoms an, molecules adsorbed on surfaces. This area will als investigate other surface phenomena, such as catalysis In electrochemistry, a study has begun of the mechanism

and kinetics of complex electrochemical oxidatior

reduction processes involving organic compounds. Th!

electrochemistry program will also develop methodologie

and detector systems for analyzing selected organic an'

1

organo-metallic compounds.

A small angle neutron scattering system has bee

funded for the NBS reactor. This facility will be use

to study the structure and properties of polymers an magnetic and biological materials. Methods will also t developed to probe bulk microstructures for inhomc

geneities affecting materials performance. Another ne1
NATIONAL MEASUREMENT LABORATORY

ompetence area will investigate fundamental mechan-

:sms of chemical and physical processes induced by inter- ction of ionizing radiation with matter. The results of

his research will be used to develop a sound basis for

i-losimetry measurements and assist those concerned with

he problem of the effects of ionizing radiation on bioogical systems.

I

i A new program involving offsite work by NML sci- -ntists is the use of the National Synchrotron Light Source

t Brookhaven. In a joint effort with the U.S. Naval

ftesearch Laboratory, Bureau scientists will begin real- ime in situ studies on materials transformation, analysis

f crystal structure, and studies of material properties and

phase transitions via electronic bonding properties of

toms and molecules in condensed matter. A new cometence area has also been funded to develop a state-of-

/he-art picosecond pulse laser for studying effects of

ionizing radiation, structure of liquids, surface science,

,:hemical kinetics, and molecular and atomic spectrocopy.

: In addition to these new competence areas, NML has

received Congressional funding for a materials durability

.program. The need for this research was established by

| 1978 NBS-sponsored study on the economic effect of

yietallic corrosion in the United States. Areas to be

emphasized include localized corrosion, elastic plastic

•racture, and wear.

J The center and program overviews will give more

Retails on these new research areas as well as their many

.listing programs. Following each of these overviews,

..^embers of the scientific staff will make brief technical

. presentations describing ongoing research efforts. While

..fjhese presentations cannot give a complete description

. -»f NML activities, they do exhibit the far ranging breadth

tf its research, ranging from the theoretical studies of

uarks to quantitative analysis of trace-level organic

onstituents in alternate fuels.

if

1

Center for

Absolute Physical Quantities

Karl G. Kessler Ralph P. Hudson

John W. Cooper

Mary C. Cutkosky

Ugo Fano

Walter G. Schweitzer Quantum Metrology

Richard D. Deslattes Anthony J. Burek

Kenneth C. Harvey

Raymond W.

Hayward

Albert Henins

Helen K. Holt

Ernest G. Kessler, Jr. Robert E. La Villa Gabriel G. Luther

William C. Sauder Thomas M. Shay*

James J. Snyder

Electrical Measurements and

Standards

Barry N. Taylor

Norman B. Belecki Marvin E. Cage

Giovanni

Costabile** Robert D. Cutkosky

Raymond L. Driscoll** Ronald F. Dziuba

Woodward G. Eicke,

Jr. Bruce F. Field George M. Free

Forest K. Harris**

Louis B. Holdeman

Paul T. Olsen William D. Phillips John Q. Shields June E. Sims

Jaan Toots Thomas E. Wells

Earl S. Williams Edwin R. Williams Thomas J. Witt** Temperature Measurements and

Standards James F. Schooley Vern E. Bean

Theodor H.

Benzinger** William R. Bigge

George W. Burns

Michael J. Di Pirro* Robert E. Edsinger

John P. Evans George T. Furukawa

Sandra C. Greer**

Leslie A. Guildner James T. Hall* James Hines, Jr. Harold J. Hoge**

James C. Houck

Wilbur S. Hurst

Richard W. Hyland

Robert S. Kaeser

Billy W. Mangum

Walter Markus

Harvey Marshak

Kenneth E. McCulloh

Paul H. E. Meijer

Earl R. Pfeiffer Harmon H. Plumb

Martin L. Reilly

John L. Riddle Gregory J. Rosasco

Margaret G. Scroger Robert J. Sou4en, Jr. Donald D. Thornton

Charles R. Tilford Craig T. Van

Degrift

Deborah Van

Vechten*

Bernard E. Welch

Sharrill D. Wood

Length and Mass

Measurements and

Standards Ralph P. Hudson

Richard S. Davis

Marvin Kroll Howard P. Layer James E. Taylor

Time and Frequency James A. Barnes David W. Allan Richard L. Barger

Roger E. Beehler James C. Bergquist*

Joseph V. Cateora George J. Collins John Cooper

Dicky D. Davis

Robert E. Drullinger

Lyman B. Elwell Kenneth M. Evenson Mark Feldman

David J. Glaze James E. Gray

Noboru Hironaka

David A. Howe

Wayne M. Itano* Stephen Jarvis, Jr. Donald A. Jennings James L. Jespersen

George Kamas

Gregory M. Kielian Judah Levine

Lindon L. Lewis

Howard E. Machlan

Robert J. Mahler

Charles H. Manney

Emma MendezQuinones

John B. Milton

Karl B. Persson

F. Russell Petersen Allan S. Risley

Hugh G. Robinson

Earl W. Smith

Samuel R. Stein Charles L. Trembath, Jr. Fred L. Walls Marc A. Weiss

Joseph S. Wells David J. Wineland

Quantum Physics

Gordon H. Dunn

Steven L. Baughcum*

Earl C. Beaty

Peter L. Bender James E. Faller Alan C. Gallagher

Katharine B. Gebbie

Sydney Geltman

John E. M.

Goldsmith*

John L. Hall David G. Hummer

Stephen R. Leone

Jeffrey L. Linsky

Frank Magnotta*

Earl R. Mosburg, Jr. David W. Norcross Arthur V. Phelps

Patricia L. Ruttenberg

Stephen J. Smith

David A. Van Baak*

Richard T. Weppner

*Post Doctoral Staff "Research Associates and Guest Workers

T

Gabriel Luther readies the apparatus for measuring the Newtonian gravitational constant. The change in the period

of a torsion pendulum is caused by the gravitational attraction of two spherical tungsten masses located near the bottom of

the picture.

he NBS Center for Absolute Physical Quantities is responsible for maintaining the consistency and integrity

of the United States' legal physical measurement standards and for assuring that they are compatible with

international standards. The Centers central mission is to carry out this responsibility in accord with the needs

of this Nation. The demands of our highly technical

society for measurement accuracy can no longer be met

entirely with artifact primary standards such as meter

bars and kilogram masses. Consequently, as a result of

research to which NBS has made important contributions,

most basic standards of measurement are or soon may be

based not on artifacts but on properties of nature such as atomic transitions or the speed of light.

As standards have become more sophisticated, the

requirements for improved measurement methods have

intensified and our understanding of the physical world

has deepened. Our physical measurement system is bound together by the laws of physics, a detailed understanding of which is necessary to maintain an accurate,

consistent system, and conversely subjecting these laws

to test. \n order to meet its primary mission, the Center conducts a research program at the forefront of physics.

That program involves measurements of increasingly

high accuracy, made possible by capabilities developed

through the determination of important physical constants and the study of fundamental physical phenomena.

Frequently the measurement expertise of the Center is applied to important problems relating to other national

issues, such as those involved in the development of

advanced energy systems.

The most strongly mission-oriented programs of the

Center are concerned with measurement standards for mass, length, time and frequency, temperature, and electrical quantities. They are multifaceted efforts requiring:

basic research; development of sensors, instrumentation,

and methodology; critical evaluation and documentation

of data; international comparison of standards; and nu- merous measurement services to the public, including

training sessions in measurement procedures, measure- ment assurance programs (MAPs), and instrument

calibrations. Other programs, which concentrate on ex- ploratory research into the basic properties of matter,

focus on areas such as atomic and nuclear spectroscopy,

5
NATIONAL MEASUREMENT LABORATORY

areas that presently offer the best prospects for advancing

our metrological capabilities.

Recently, in response to some difficult measurement

challenges, the Center has: provided frequency standards

stable to 1 part in 1013 for navigation, lunar ranging,

and military targeting; developed relatively simple but

accurate thermometer calibration devices for improving

the reliability of clinical and medical diagnostics throughout the country; harmonized optical x-ray and y-ray

wavelength standards to help resolve a discord in muonic

physics experiments; used state-of-the-art laser technology

to test the validity of Einstein's theory of special relativity; and more accurately measured the gyromagnetic

ratio of the proton and the fine-structure constant. Research to improve our measurement capability and

extend our knowledge of fundamental constants and

processes is underway or planned in a variety of areas. Frequency, the physical quantity that can be measured

with the highest accuracy, can now define our time scale

to within 1 part in 101?

'. However, further improvement

is required for modern global navigation systems and for

measurement of movements of the earth's crust. Researchers in the Time and Frequency Division are exploring new ideas for improved frequency standards and for

methods to coordinate time worldwide at an accuracy of

a few billionths of a second.

Recent work in the Center has established an accurate

wavelength scale for x-ray and gamma-ray measurements.

That work is being extended to both lower energies and

higher energies, with potential applications to atomic and

nuclear spectroscopy. Until now, the main applications of

this new scale have been redeterminations of the scales

for muonic and pionic x-ray spectra. Other work in the

Center has led to a proposed NBS/Naval Research Laboratory research program using the synchrotron light source now under construction at Brookhaven. This effort will emphasize application of the synchrotron source to descriptive materials science while also providing opportunities for further studies in scattering and spectroscopy.

In the temperature measurement area, current work is establishing a firm foundation for the International Practical Temperature Scale. The improvements in that scale

and its extension to lower temperatures have been to a

large extent the result of NBS efforts. The future NBS

program in thermometry will concentrate on the development of methods for dynamic measurements on transient,

non-equilibrium systems such as those encountered in

new energy system.

The laser will continue to be an important tool fo

standards research. Work is underway to improve stable high-accuracy laser systems and to utilize laser systems ii studying atomic and molecular properties and in investi gating the details of molecular interaction processes. / new technique developed by NBS, laser magnetic reso nance, has yielded high resolution information on th

structure of excited molecules. Further development o

this technique will, for the first time, allow studies o absorption properties of molecules in excited states. Pioneering NBS work on the measurement of very hig]

frequencies is leading to the redefinition of the meter ii terms of the speed of light. The technique of frequenc

measurement has now been extended by NBS scientists working in collaboration with scientists at the Nationa

Research Council of Canada, into the visible regioi

where the measured frequency of light is 5 X 1014 hertz

Since the length of a light wave is equal to the speed o

light divided by the frequency of the waves, a very precis

length scale can be established by measuring the fre quency of a stabilized laser source. Because a value fo

the speed of light is agreed upon, accurate frequenc

measurements can be used to define the fundamenta

unit of length, the meter, in terms of the speed of light.

A fundamental phenomenon of physics, the Josephso;

effect, has been used by the Electrical Measurements an< Standards Division to establish a new basis for accurat

voltage measurement. Based on the Josephson effect voltage can be measured in terms of frequency. Smal

devices incorporating this capability are now used in NB!

laboratories to maintain the U.S. legal volt. They replac

banks of standard cells used in the past. Development

now under way will make these devices, called Josephsoi

junctions, more widely applicable, especially in industry

Modern instrumentation capable of high accurac; measurements must not only be automated, but fre quently must also be capable of internal calibration

Studies planned in the Center will investigate basic physi

cal phenomena that can be utilized to develop self-cali brating devices. Such devices, when perfected, will mab

it possible to maintain maximum accuracy in field meas

uring instruments without frequent calibrations by NBS

The thrust of all Center programs is to build a stronger more accurate foundation for physical measurements anc

to improve understanding of the basic phenomena o; which our physical measurement standards are based.

6
4 CENTER FOR ABSOLUTE PHYSICAL QUANTITIES

Rydberg Values

for X- and y-Rays

Richard D. Deslattes and Ernest G.

Kessler, Jr. Quantum Metrology Group

w,e have significantly improved the accuracy with which x-ray and y-ray transitions are known in terms of the Rydberg constant, R

These improved connections have

sharpened critical comparisons be- tween theory and experiment for both

normal and exotic atoms. In the case

of x-ray spectra from normal atoms,

our measurements have helped reveal provocatively simple regularities in the departures of experimental results from

the predictions of relativistic self- consistent field calculations. Our y-ray measurements contribute indirect in- formation on the atomic x-ray problem

just mentioned but, perhaps more importantly, serve to renormalize many

spectra from muonic, pionic, and

kaonic atoms. These measurements

have yielded enhanced tests of quantum electrodynamics in muonic spectra

and improved particle masses in the

case of certain pionic and kaonic x-ray

spectra.

Our strategy in this work followed

the pattern of establishing secondary

spectroscopic standards, well known

in other spectroscopic regions. Although the measurement chain we

developed links the y-ray and x-ray

transitions to the base units of wavelength and frequency quite well, the main scientific interest derives from

their linkage to the Rydberg constant,

Ry.. Specifically, one extracts from

the theory of the hydrogen atom a description of the visible Balmer a

spectroscopic complex. Since the theoretical term differences are proportional to R x , R rx might be said to

set the scale of the H-atom were it otherwise known or R^ may be derived from the scale of the H-atom as

is presently the case. All other atomic

calculations are also homogeneous in R y , hence the manifest importance of

using it as an invariant basis for the establishment of secondary standards.

Today's best knowledge of the Bal- mer « region of atomic hydrogen is derived from linking (by means of Doppler-free spectroscopy) its nonoverlapping components to certain Doppler-free absorption lines in molecular iodine. Of particular interest are those measurements carried out with

respect to I2 absorption features within

the 633 nm HeNe laser gain curve. (Subsequent improvement in precision

of measurement with respect to I2 absorption in another region is not of significance for the present effort in view of limitations which are, for the most part, intrinsic.)

Our work therefore took as a token

for R a visible 633 nm HeNe laser locked to an I_. absorption feature

closely related to that used in the Bal- mer a spectroscopy mentioned above.

We used this source to establish a scale with respect to which the spatial period

of an almost perfect silicon crystal was

determined using x-ray interferometry

along a [110] lattice direction. This

procedure produced a more accurate (~ 0.1 ppm) local standard in the 0.2 nm (2 A) region than is otherwise

available via classical x-ray methods.

Figure 1. Schematic features of the measure ment chain established for linking visible and 7-ray (or x-ray) wavelengths. Part a shows the x-ray/optical interferometer used

to determine a particular lattice spacing

relative to a visible standard (see text). Part b suggests the principle involved in comparing lattice spacings of various speci- mens so as to reduce the effect of x-ray line uncertainty on the comparison. Part c is a diagram of a classical transmission twocrystal instrument for x-ray and 7-ray wavelength measurement. Our realization of this system provides for absolute angle measure- ment with high precision thereby establishing

(through parts b and a) direct connection to certain visible reference wavelengths and

through these to R

Source

( + ) Detector (-)

Figure 1
By utilizing non-dispersive techniques we were able to transfer the above

calibration to other crystals of Si and

Ge having appropriate shapes and ori- entation for x-ray and y-ray diffraction measurements. The non-dispersive (or

slightly-dispersive) character of these

transfer measurements insured that the

results were not degraded due to the

width and shape of x-ray line sources

used. Finally, pairs of crystals cali- brated in this way were used in doublecrystal x-ray and y-ray spectrometers

with accurate means for diffraction angle measurement. These three steps

then relate in a rather direct way the x-ray and/or y-ray lines measured in the last part to the I_. stabilized HeNe

lasers used in the first part and through

them to the fundamental scale unit. Rod. We proceed at this point to comment briefly on the three-step

process. The overall measurement chain is suggested by the three diagrams of

figure 1 . Figure la shows a diagram of

the combined x-ray and optical inter- ferometry of the Si 220 repeat distance. Our procedure yielded (by means of observations at a set of optical lock

points) an effective number of x-ray

intervals transited in a scan of approximately 100 optical half-wavelengths.

For the particular Si specimen used,

the summary of several hundred measurements was d22o(Si, 25 °C) =

192.01707 pm (0.1 ppm). An adja- cent specimen was then used in a geometry such as that illustrated in figure lb to calibrate other specimens

and, indeed, other crystal types. Evidently, the procedure suggested in figure lb gives a null result when

standard and unknown crystals agree, independent of x-ray wavelength. In

the case of a small difference, one can adopt a perturbation type approach in which it becomes rapidly evident that properties of the x-ray line used are

less crucial to the quality of inter- crystal comparisons than they would

be in the case that we had to use fully

dispersive techniques. It is also clear

that the accuracy required of angle

measurement schemes is significantly

less in this approach than it would be

in a fully dispersive area. This step is not a limiting one at the 0.1 ppm level. Most recent work has focused on

the third step, figure lc, wherein pairs of crystals calibrated as just described

are placed in the symmetrical trans- mission geometry shown. The interval between the two diffraction conditions

indicated is twice the Bragg angle, 6, where the wavelength to crystal-spacing ratio, A/d is simply 2sin#. Evidently, accuracy available for A follows

that associated with d, provided measurement precision is adequate and

provided the scale for 0 is established on an absolute basis. Measurement

precision has been obtained by means

of polarization encoded angle inter- ferometry which yields a typical sensitivity of 0.05 milli arcsec for ranges of

± 2.5° and ± 15° in first and second

generation instruments respectively. Perfect crystal resolutions, typically 20

to 50 ppm, provide a basis for adequate pointing precision provided statistically required source strengths

(~ 10 to 100 Ci) are available. Accuracy, on the other hand, requires that the interferometric angle be calibrated

in terms of fractions of the cyclic angle,

2tt. To this end, we have employed

optical polygons and sensitive nulling

autocollimators which furnish indexing

precision well below 1 milli arc-sec. If Angles between all adjacent face pairs of a polygon sum to 2n regardless of i; the polygon's in-plane errors and with

calculable (small) effects from out-of- plane errors. The process of marking

all external angles constitutes a calibration; such calibrations have been established with a precision of about 0.05

ppm but require periodic repetitions to track secular changes in interferometer

geometry. Thus the calibration constant operant during a particular y-ray or x-ray diffraction measurement is inCa

3d-2p Ba5g-4f

Sn

4f-3d

Tl 5g-4f

Pb

5a-4f Ba

4f-3d

Q.

CD

20 -

15 [- T

5 eV

10

JL. 0

UJ

-5

-10

-15

-20 ±M 7^<^M%. V 7. M Wfc

77,

z -7M2> A

77777Z

157.5 ^ 200.6 ^ 347 347.6fi 'V 420.8 'V 434.5 \r I 437.6

E (keV)

Figure 2. Comparison between theory and experiment for a class of outer orbit muonic

x-ray transitions. Hatched bands represent estimates of theoretical uncertainties and data have been renormalized as required by new 7-ray data arising from the measurement chain

of figure 1. In the absence of this revised energy scale, hatched bands would have centered on horizontal dashed line segments shown. The resulting discrepancy pattern would have

been small but significant.

8
ENTER FOR ABSOLUTE PHYSICAL QUANTITIES

terpolated from a "curve of growth"

including prior and subsequent calibration results. We feel that this process may be carried out with errors not

greater than 0.1 ppm.

The above-described procedures

have been by now applied to frequently

used y-ray secondary standards avail- able from the isotopes 198Au and 192Ir

[1] along with those from 170Tm and

169Yb [2]. In separate exercises the same technology has been applied to

the W Kai x-ray line [3], while related procedures were previously applied to Cu Kai and Mo K«i [4]. By also uti- lizing otherwise available high precision results on ratios of K-series x-ray

lines to a y-ray reference [5] together

with our measurement of the y-ray

reference [1], it has been possible to appreciably expand this atomic x-ray

data base [6]. In general, our x-ray

results are significantly poorer owing to the great spectral widths of these lines. On the other hand, both direct and in- direct measurements of these lines have

permitted us to approach the 1 ppm

level. Two examples illustrating applications of this work are shown in figures 2 and 3. In figure 2 we show a comparison between theory and experiment

for a class of muonic x-ray transitions

in moderate-to-high Z elements. This

comparison is a significant test of quantum electrodynamics, especially

the vacuum polarization terms, which

had previously showm significant dis- crepancies [7]. Such a comparison is naturally sensitive to the numerical

values of the y-ray scale [8]. Were we

to neglect the scale change then the

comparison points would lie on the

horizontal segments appearing in figure

2. These results are satisfying in two

ways: evidently we have now agree- ment between theory and experiment

in an area of some significance; and,

' 3 Sb Mo -^-O"'

-cf

Tn- |

Pu

Cu r

10

(E [keV]) v

-6 -

-4 - -2

Th Pu

cC'J

Sb Mo

*Ag

Ru

Figure 3. Comparison of experimental data

as obtained or modified by our work with

relativistic self-consistent field calculations in the relaxed orbital limit. Part a of this figure shows results for Kn, lines while part b shows those for Kq-. Further results presently

being accumulated show similar patterns for

the case of more prominent K/3 lines.

the quality of this comparison is no

longer limited by lack of knowledge of the reference y-ray lines. As a second application area, we

turn to x-ray spectra of normal atoms. Over the past several years, relativistic self-consistent field calculations have

become available [9]. These have been

applied to the case of single inner shell vacancies in the relaxed-orbital limit [10]. When such "term-value" calculations are available with sufficient precision, predictions of emission transi- tions follow from subtraction of terms. Comparisons between theory and ex- periment for the Kd lines of several elements are shown in figure 3 [11].

The systematic discrepancies shown

(which are evidently linear in Z) were

not evident in previous data because

of their lesser precision. Efforts toward

understanding this situation have begun on the theoretical side while, at the

10

(E [keV])%

same time, we have undertaken to broaden the available data base by a systematic study of these discrepancies

as functions of principal and orbital quantum numbers as well as Z. Several avenues suggest themselves

for future work. These will be pursued

as resources permit. Obviously, one

wishes to extend the optically controlled scale to higher energies. This is particularly interesting as it becomes

possible to reckon a y-ray cascade sum- ming to the order of AE = 10 MeV.

In this case, the corresponding mass

decrement, AM is measurable to ~ 1 ppm. This combination of AE and the associated AM is sufficient [12] to establish a value for Faraday's constant which is manifestly independent of chemical procedures. Significant nuclear spectroscopy near 2 MeV awaits

application of the high resolution capabilities of our instrumentation. A precision measurement of the electron- positron annihilation radiation has been

largely developed but remains to be

executed. Finally, the progress to date

in atomic x-ray spectra suggests the

utility and interest of considerably ex- panding their study which we propose

to undertake.

9
NATIONAL MEASUREMENT LABORATORY

References

A more complete description of this work

is in press, Annals of Physics (1980).

[I] Kessler, E. G., Jr., Deslattes, R. D., Henins, A., and Sauder, W. C, Phys. Rev. Lett. 40, 171 (1978).

[2] Kessler, E. G., Jr., Jacobs, L., Schwitz, W., and Deslattes, R. D., Nucl. I n.st r. and Meth. 160, 435

(1979).

[3 1 Kessler, E. G., Jr., Deslattes, R. D., and Henins, A., Phys. Rev. A 19, 215 (1979).

[4] Deslattes, R. D., and Henins, A.,

P/iyy. Rev. Lett. 31, 972 (1973).

[5] Borchert, G. L., Z. Naturforsch. 31a,

102 (1976).

[61 Deslattes, R. D., Kessler, E. G., Jr., Jacobs, L., and Schwitz, W., Phys.

Lett. 71 A, 41 1 (1979).

[7] Rafelski, J., Muller, B., SofTard, G., and Greiner, W., Ann. Phys. 88, 412

(1974); Watson, P. J. S., and Sundaresan, M. K., Canad. J. Phys. 52,

2037 (1974).

[8] Hargrove, C. K., Hincks. E. P., McKee, R. J., Mes, H., Carter, A. L., Dixit, M. S., Kessler, D., Wadden, J. S., Anderson, H. L., and

Zehnder, A., Phys. Rev. Lett. 39,

307 ( 1977); Dubler, T., Kaeser, K., Robert-Tissot, B., Schaller, L. A., Schellenberg, L.. and Schneuwly, H., Nucl. Phys. A 294, 397 (1978).

[9] Desclaux, J. P., Atomic Data and

Nuclear Data Tables 72,311 (1973).

[101 Huang, K-N., Aoyagi, M., Chen,

M. H., Crasemann, B., and Mark, H., Atomic Data and Nuclear Data

Tables 18, 243 (1976).

[II] Deslattes, R. D., Jacobs, L., Kessler,

E. G., Jr., and Schwitz, W., Comparison of Relativistic Atomic SCF

Calculations with Improved Experimental Data (volume of papers done

in honor of Professor Y. Cauchois

upon her retirement).

[12] Deslattes, R. D., and Kessler, E. G.,

Jr., Precision Gamma- and X-ray

Energies (to be published in Proceedings of Sixth International Conference on Atomic Masses (AMCO6) held September 18-20, 1979, in East Lansing, Michigan).

Laser Cooling of

Stored Ions

David J. Wineland,

Time and Frequency Division

wT T e have been able to reduce

the temperature of a sample of Mg+

ions to a value <0.5 K by using

radiation pressure. This technique will allow us to reduce in a fundamental

way the present limits to accuracy in frequency standards and precision

spectroscopy, namely those imposed

by the first and second order Doppler

shifts. When combined with first- order Doppler cancellation schemes,

the method should yield orders of magnitude improvement over present

techniques.

In spite of the great accuracy and

stability achieved by today's frequency

standards, the need for improved

standards continues to grow in both

the pure and applied sciences. The

present uncertainty in the cesium beam

frequency standard which defines the length of the second is about 1 part in 1013

. Interesting tests of various gravitational theories require clocks which

are several orders of magnitude more

accurate. In addition, substantially

better clocks are needed, for example,

in long baseline interferometry and in various navigation systems.

Doppler shifts are the dominant

cause of uncertainty in the cesium

standard and other existing or pro- posed frequency standards. When in- terrogating the frequency of an atomic

transition (which provides the basis

for the standard) we must use some

method to cancel first-order Doppler

shifts. However, our ability to do so is limited, and residual shifts always

occur. More importantly, we are al- ways required to account for the second-order Doppler effect, or relativistic time dilation shift of the moving

atoms. In the Time and Frequency

Division, a group including J. C. Bergquist, R. E. Drullinger, Wayne M.

Itano, F. L. Walls, and D. J. Wineland

has attacked the Doppler shift problem

in a fundamental way—by substantially

lowering the temperature of atoms. The "atoms" in our experiments were actually atomic Mg ions confined

in a room temperature electromagnetic

"trap" which in turn was contained in a high vacuum. (The use of ion traps

has another significant advantage in that the confining forces are benign and do

not cause significant frequency shifts

( < 1 0 ) . The cooling principle can

be understood by first noting that the

ions are harmonically bound to the center of the trap, hence they oscillate back and forth with an amplitude and

velocity governed by the temperature.

We can irradiate the Mg ions with

monochromatic light from a frequencydoubled dye laser whose frequency is tuned lower than the ion rest frequency

but within the Doppler profile. The

ions will preferentially scatter photons

when they are moving toward the laser, and hence Doppler shift it into reso- nance. For each scattering event, the momentum of the ion is changed by

the momentum of the absorbed photon.

After about lO4 scattering events, substantial cooling below room temperature can be achieved. Figure 1 shows

a plot of temperature versus time. The

temperature was monitored by observing the induced currents in the trap

electrodes. This experiment* showed

that, although cooling could be obtained, resolution was limited by noise

in the detection electronics. A more

sensitive measu/e of temperature could

be obtained if we monitored the Doppler width of the optical transition. This was complicated because under

the optimum cooling conditions that were used (small'compact ion cloud,

focused laser beam) cooling rates and

heating rates (as the laser was tuned

through resonance) could be as high

as 1000 K/s. Hence a second, fixed- frequency laser was used to keep the ions cold while the scattered light from

10
CENTER FOR ABSOLUTE PHYSICAL QUANTITIES

Laser on

I- 5.6 min

Ions removed

Laser off

Temperature Time -350 K

Figure 1. Ion temperature versus time when laser cooling is applied for fixed detuning

of about 2 GHz. The ions were initially heated above equilibrium temperature with the

laser. Laser cooling was then applied for a fixed time until a temperature approaching 0 K

was achieved. After the laser is turned off, the ions rethermalize to the ambient temperature

owing to collisions with background gas.

the frequency-swept laser was monitored. Figure 2 shows a resulting trace where the ion temperature was about

2.5 K. Similar traces now indicate that temperatures < 0.5 K can be obtained.

The limit on temperature achievable

by this method is governed by the

effects of recoil. For example, if the

ion comes to rest when the "last" photon is absorbed, it will attain the

recoil energy of the photon upon re- emission. We have studied this problem theoretically and find that, for Mgr

,ions, where the electric dipole transition

(is fully allowed, the limit on temperature is about 1 mK. The cooling limit on less strongly allowed transitions,

however, can be < < 1 ^K. Even with

the presently-attainable temperature, we should be able to reduce the secondorder Doppler shift correction in very

'high resolution studies (e.g., frequency

s:andard) by about three orders of magnitude. The extremely weak re- laxation of cold ions in a trap will pro- duce line-widths that are limited only

by the natural lifetime, implying

Q > 1015

. Used in conjunction with

established techniques (confinement to dimensions less than a wavelength,

2 photon Doppler-free spectroscopy,

saturated absorption, etc.) uncertainties in atomic clocks should be eventually lowered by several orders of magnitude.

cr

ou

co

o

CL

Figure 2. Photon scatter rate versus laser frequency showing the three isotopes of Mgin the trap. The sample is maintained cold during the scan by irradiating the low fre- quency side of the strong line with a second

laser. From left to right the three lines are due to the 1/2)

^

= -3/2)

(3s2S , M) =

1/2 (3p2p , Mj

3/2

transitions in the 24 Mg, ^Mg, and 20 Mg iso- topes. At 300 K the Doppler width is approximately equal to the spacing between

the 24 Mg and 26 Mg isotopes ( = 3 GHz).

Observation of Pure Rotational

Transitions in the HBr+ Molecular Ion with Laser

Magnetic Resonance

Richard J. Saykally 1 and Kenneth M.

Evenson,

Time and Frequency Division

MJL T M olecular ions are extremely

important in the upper atmosphere

and are thought to dominate the chemical dynamics of the more dense

gas clouds found in the interstellar medium. To study ions in these environments by spectroscopic methods, such as radio or far-infrared

astronomy, accurate laboratory spectra

must first be obtained. Although

optical emission spectra have been

observed for a fairly large number of molecular ions, the high-resolution

techniques required to yield the desired

level of accuracy (e.g., microwave

spectroscopy, EPR, rf-optical double

resonance and, recently, Doppler-tuned

laser spectroscopy) have been successfully applied to these species in onlv a few cases. The difficulty is simply that of producing a detectable number of ions by a means compatible with the respective detection systems. We have

recently observed the pure rotational

spectrum of a molecular ion—HBr + —

by the technique of laser magnetic

resonance (LMR). This first detection

of an ion by LMR is significant

because it makes the entire class of paramagnetic ions amenable to study.

Within the last few years, LMR has

emerged as a powerful technique for

the study of pure rotational spectra of

transient species in the gas phase. It exhibits a considerably higher sensitivity than that of competing techniques,

such as microwave spectroscopy and

EPR, while the resolution attainable is roughly the same. The LMR method

itself is intimately related to gas-phase

EPR. The appropriate paramagnetic

energy levels of an absorbing sample

are tuned by a dc magnetic field until their difference frequency equals that

of a fixed-frequency source. The prin11
NATIONAL MEASUREMENT LABORATORY

cipal distinction between these two

methods is: in EPR the relevant transi- tion is between different magnetic sublevels (Mj) of the same angular mo- mentum state (J), typically occurring

in the microwave region for normally

accessible laboratory magnetic fields (2 tesla) ; in LMR the relevant transi- tions are between rotational states (in

molecules) or fine-structure levels (in

atoms), and occur in the far-infrared

( FIR) . In both techniques, the sample

is contained inside a resonant cavity.

The increased sensitivity of LMR is mainly derived from operating at fre- qencies about 100 times higher than

the microwaves normally used in EPR.

and from the fact that absorption coefficients typically vary with the square

or cube of frequency in this region.

Detection limits for OH radicals are roughly as follows: LMR: 1 X

10,; cm " and EPR: 2 X 1012 cnr:!. A schematic diagram of an opticallypumped FIR laser magnetic resonance

spectrometer is shown in figure 1 . In

this system, a tunable CCX laser operating single-mode with a power output

Bolometer

Detector

Fixed

Mirror WAVELENGTH RANGE

FAR-IR: 40-900 Mm

near 50 W transversely pumps the gain

cell of a FIR laser. The FIR radiation

oscillates in the optical cavity defined

by a fixed mirror at one end and a movable mirror attached to a micrometer drive at the other. The detection

volume of the cavity is separated from

the gain cell by a thin polypropylene

window set at the Brewster angle. A

fraction of the FIR laser power is coupled out of the cavity and into a liquid-helium cooled germanium bol- ometer by an adjustable 45° coupling

mirror.

In the apparatus shown in figure 1

the detection volume is centered between the pole faces of a 38 cm electro- magnet. The magnetic field is modulated at frequencies near 5 kHz, and

phase-sensitive detection is employed,

resulting in a first-derivative line-shape.

Until very recently, all of the species observed by LMR had been pro- duced in various types of atomic and

molecular flames reacting inside the detection volume. These flames were

generated by using a concentric flow

tube mounted on the laser system perFAR-INFRARED LASER

1.15m —

Moveable

Mirror

Ins Diaphragm

cGrating

m - M i rror Mirror

2.60 m

Piezoelectric Drive CO LASER

2

Figure 1. Schematic diagram of an optically-pumped laser magnetic resonance spectrometer.

12

A= -79.494 THz

B = 0.23845 THz

5 r

4

-

Micrometer

Drive

- + 280 MHz

J = 9/2

140.3 urn

J = 5/;

j = 3/; :I J = 7/2

- 180.3 [im —p=TJ

Q = 1/2 V2\Q\-g.-cM

79.26 THz + J = 5/2

+ J = 1/: 24 GH.

252.3 urn ±J = 3/2 * (X100)

fi = 3/2

Figure 2. Energy level diagram for the 2II ground state of HBr+

. The small lambda

doubling in the fi = 3/2 state (case A

notation) is multiplied by 100; that in the diamagnetic fi = 1/2 state is to scale.

pendicular to the plane of the diagram.

In the new spectrometer used to observe HBr + , the flame has been re- placed with the positive column of a glow discharge, and the magnetic field

is instead produced by a 5-cm diameter

by 33-cm-long liquid nitrogen cooled

solenoid magnet, which car} generate

fields up to 0.6 T (6 kG). This new^

configuration provides for an increased

detection volume, and the axial symmetry readily accommodates the electrical discharge. With this apparatus, we have succeeded in detecting several short-lived species, including groundstate oxygen atoms and metastable excited electronic states of 02 and CO,

as well as the HBr+ ion. HBr+ was generated in a discharge

through 1 percent HBr in helium at 133 Pa total pressure and near-ambient

temperatures. We observed the two

lowest rotational transitions in the Q = 3/2 spin-component of its 2 II ground electronic state, for which the enegy-levels are shown in figure 2. Bromine occurs in nature with nearly

equal abundances of the 79 and 81
CENTER FOR ABSOLUTE PHYSICAL QUANTITIES

H (Tesla)

.250 .350 .450

79 u

Figure 3. The laser magnetic resonance spectrum of the J = 3/2 -* 5/2 transition of

; HBr T observed at 251.1 M m (CH..OH). This spectrum was recorded on a single scan with

a 0.1 s time constant. The positions of the

eight doublets represent the hyperfine split- ting in the Mj = —3/2 -> —1/2 Zeeman

transition. The doublet splitting is due to

:the lambda doubling,

j

mass isotopes, and the reduced masses

•of these two forms of HBr+ are nearly

identical; thus we observe both isotopes

simultaneously. Since each Br isotope

^has a nuclear spin of 3/2, we find a

pattern of two sets of four lines, additionally split into doublets by the small

, ''lambda-doubling" interaction. The

additional hyperfine splitting from the proton is too small to resolve. This

pattern is shown in figure 3. When completed, the analysis of

these LMR spectra will produce the

first information on the hyperfine

structure of HBr + , as well as precise

values for the rotational constants and

lambda doubling parameters. These,

in turn, can be used to deduce spin

densities, charge distributions, and ac- curate bond lengths for the molecule.

It will be very interesting and informative to observe the effects of the net

positive charge on these properties by

comparing them with properties of cor- responding neutral molecules. More

exciting, however, is the prospect of detecting other similar ions by the same

method. In particular, HC1+ may be

of considerable astrophysical interest. Measurement of LMR spectra of this and other ionic species will provide the accurate rest frequencies required to perform astronomical searches for these molecules in interstellar clouds.

1 Department of Chemistry, University of California, Berkeley, California 94720.

Extension of Absolute Frequency

Measurements to the Visible

Donald A. Jennings, F. Russell Petersen, and Kenneth M. Evenson,

Time and Frequency Division

T. he direct frequency measure- ment of visible light was achieved by

D. A. Jennings, F. R. Petersen, and

K. M. Evenson of the National Bureau

of Standards and K. M. Baird and

G. Hanes of the National Research

Council, Ottawa, Canada. This ex- periment paves the way for frequencybased spectroscopic investigations in the important visible spectral region

which are potentially a thousand times more accurate than measurements of wavelength. In addition, this experiment may eventually lead to a new

definition of the meter.

Visible frequency measurements

were achieved by a laser frequency

synthesis chain, shown in a very simplified way in figure 1. This experiment links the cesium frequency standard directly with the frequencies of 10 hyperfine lines of mI 2 at the fre- quency of the second harmonic of the 260 THz He-Ne laser. The measure- ments were made in seven steps from

cesium to iodine by using only fre- quency counting techniques. Actually,

14 lasers and 6 klystrons were used in the heterodyning experiments, finally

culminating in the 520 THz visible frequency. The measurements of the

first six steps were made at NBS and

the final one at NRC. (The NBS 260

THz laser was transported to Ottawa

for comparison with NRC's 260/520

THz composite laser in the final step.)

The schematic illustrating step 7 is shown in figure 2. The NRC composite laser contained

the iodine cell, the lithium niobate

doubling crystal, and the 260 THz gain

tube. The 260 THz frequency was

stabilized to hyperfine components of the P(62) 17-1 transition in 127T. by

using saturated absorption techniques,

and the 520 THz radiation generated

from the 260 THz by the doubling

13
NATIONAL MEASUREMENT LABORATORY

X 2

+ 63 THz

+ 49 THz

60 THz

X 3

X 3

X 12

X 84

' 2

He -Ne

He -Ne

He -Xe

He -Ne

co

2

H 0

2

HCN

Cs

Frequency, THz

520.0

260.1

196.8

147.9

29.4

10.7

0.890

Figure 1. A block diagram of the laser fre- quency synthesis chain connecting the Cs

frequency standard with the visible.

crystal. The visible frequency measure- ments were made simply by combining

the 260 THz beams from the two lasers on a high-speed photodiode. The beat

frequency was displayed on a spectrum

analyzer and measured with an adjust- able marker oscillator and counter. The

estimated error in the beat frequency

measurement is ± 0.5 MHz, and the

overall error is ± 60 MHz—a result of

the free running nature of two lasers in the frequency synthesis chain. The

frequency of the O - hyperfine component (nearest the center of the sec- ond harmonic output at 520 THz) is

f0 = 520,206,837 ± 60 MHz.

Work is in progress to reduce the large

uncertainty.

The redefinition of the meter in terms of the second is now a real possibility. In fact the CCDM (Consultative Committee for the Definition of

the Meter) at its meeting in June 1979

has proposed that: "The meter is the length equal to the distance travelled in a time interval of 1/299 792 458 of a second by plane

electromagnetic waves in a vacuum."

The era of speed of light measure- ments may cease as the value for the speed of light becomes fixed in the re- definition of the meter. The measurement of this iodine line with frequency measurement technology some 50 years after this line was

first measured by wavelength tech- niques represents a truly exciting advancement in the science of measurement.

NRC

Composite Laser

stabilized on \ 2 at 520 THz

260 THz v

NBS

20Ne Laser Lamb-Dip stabilized

260 THz ^

counter

oscillator t

spectrum

analyser

Figure 2. A block diagram showing how

the last step to the visible in the frequency

synthesis chain was accomplished.

Fine Structure Constant

Determined to an Accuracy of

1 in 107

P. Thomas Olsen and

Edwin R. Williams,

Electrical Measurements and

Standards Division

Avalue for the fine-structure constant accurate to 1 in 10 7 has been

obtained from a new determination of the proton gyromagnetic ratio. This

achievement has helped test one of modern physics' most fundamental

theories: quantum electrodynamics

(QED). QED is believed to provide an

exact description of all atomic

phenomena which do not directly

involve nuclear forces, weak

interactions, or gravitation.

The key to obtaining the new value was our development of a unique magnetic technique to measure accurately

the relative positions of each of 1000

turns of wire on a single-layered precision solenoid. This new method allowed

us to determine directly the proton

gyromagnetic ratio, yp

' (where the prime

indicates that the protons are in a spherical sample of pure H2 0 at 25 °C).

We then obtained an order of magnitude improved value for the fine-struc- ture constant, a, by combining our

result for yp

' with the values of other already well-known constants, such as the speed of light in vacuum, c, and the Rydberg constant for infinite mass, R. The fine-structure constant is the fundamental expansion parameter of QED

and therefore must be accurately known

in order to compare critically the cal- culated values of various QED-dependent quantities with their experimentally

determined values. Indeed, with the new value of a, QED is able to predict the value of the electron g factor, g( ., with an uncertainty of only about

3 in 1010

. This theoretical result may then be compared with the experimental result obtained from the elec- tron magnetic moment anomaly, at

. =

(ge — 2)/2, as obtained in a recent experiment at the University of Wash14
I ENTER FOR ABSOLUTE PHYSICAL QUANTITIES

Laser Interferometer

Figure 1. Solenoid dimensional measurement system used to determine the axial position and radial variations of the wires. The five coils A, A', B, B', and C are attached to a

silica tube T and can be pushed or pulled along the axis of the solenoid. Coils A and A'

locate the axial position of the injected current and coils B, B', and C form a diameter-tovoltage transducer. Mirrors M and M' are part of a linear interferometer. The Pyrex

tube is evacuated.

ngton. The agreement found is deemed

.cceptable in view of the incomplete

lature of the theoretical calculations. The quantity yp

' is an atomic constant

vhich relates the angular frequency,

ip

', with which the proton precesses vhen placed in a magnetic field, B, to he value of the field: yp

' = o> p'/B.

Thus, the experimental determination

')f -/p' requires the measurement of both

he proton precession frequency and

he magnetic field which produces the recession. Determining the precession

requency is relatively straightforward

vhen nuclear magnetic resonance N'MR) techniques are used. More

challenging is the critical problem of naking an accurate dimensional measurement of the precision solenoid so

hat the magnetic field produced by a ,cnown current can be calculated.

T In our experiment, the solenoid was

ingle layered and wound on a hand

. apped, fused silica form. Its critical limensions were measured by a magnetic induction technique which located

. he current in the windings. Coils A

ind A' in figure 1 formed a linear

.differential transformer which located

j he axial position of the current injected

Unto selected turns of the solenoid.

ijZoils A and A' were connected so that

..heir output voltages cancel when cen- tered on the activated turns of the solenoid. A servo system locked the coil assembly to the null point with a precision of better than 0.05 ^m. A mirror

corner cube, M') located in the center

Ibf the coil assembly was part of a

I aser-interferometer system, with the

; reference mirror (M) connected to the \~nd of the solenoid. Thus, with the aid

of a computer-automated system, the

'elative position of successive turns

' :ould be recorded. We chose to acti- vate 10 turns at one time, and then

I move the current injector to successive

1 ) turns until information about all 1000 turns of the solenoid was obtained. Coils B, B', and C formed a radius-to-voltage transducer which

measured the variations in the radius of the injected current. The voltage in- duced in coil C was inversely proportional to the radius of the activated

turns of the solenoid. To detect small

changes we first "bucked out" most of

the voltage in coil C by using the two

coils, B and B'. At the same time that we were bucking out the voltage in coil C, we were also increasing the

sensitivity of this three-coil system to changes in the radius R of the solenoid

because the voltage in coils B and B'

increased when the radius R increased.

This three-coil radius-to-voltage trans- ducer was then calibrated by having a few turns of wire on both ends of the solenoid that were 25 p.m larger at one

end and 25 smaller at the other end.

With this system, the axial position and

the radius variations of the turns of the

solenoid were measured to an accuracy

of 0.05 ^m. No turn deviated from that expected of a perfect solenoid by more

than 2 yu.ni. These improved dimensional measurements along with other refinements

in the experiment provided a 0.21 ppm

(parts per million) value for y,/, which

in turn yielded for the fine-structure constant a" 1 = 137.035963(15) (0.11

ppm). This value of a -1

is (0.33 ±

0.14) ppm less than the value of a~ x derived from measurements of the anomalous magnetic moment of the elec- tron and its current best QED estimate.

The 0.14 ppm uncertainty in this comparison represents a test of QED which

is a factor of ten more precise than all previous tests. The small, 0.33 ppm

remaining discrepancy has the following possible explanations: (1) it could

be purely statistical, about a 2 per- cent probability by chance; (2) the

15
NATIONAL MEASUREMENT LABORATORY'

OED numerical calculations of a,, are not yet complete and in addition, are

sufficiently complex so that they could

easily be in error; (3) one or more of the five fundamental constants required

to obtain a

-1 from yv

', yp

' itself, and/or

the experimental value of ae could have

unknown systematic errors; or (4) there could be something wrong with the basic physics, i.e., quantum electro- dynamics. We are now working to extend this test of QED to yet another

decimal point by carrying out a new

-/,,' experiment with an accuracy of a few parts in 10s

. It is expected that by

the time this new effort is finished, the

theoretical calculations of ae as well as

its experimental determination will also be completed and that an unequivocal

test of a OED prediction to an ac- curacy of a few parts in 10s will be

possible.

Superconducting Thermometric

Fixed Point Devices

Robert J. Soulen, Jr. and James F. Schooley, Temperature Measurements

and Standards Division

The narrow and highly repro- ducible superconductive transitions of 10 materials have been shown to be

useful as reference temperatures

(fixed points) for definition of

temperature scales which are being

developed in the region of 0.01 to 20 K.

Two devices incorporating these 10

superconductors are now available

through the Office of Standard

Reference Materials.

One of the cornerstones of the Inter- national Practical Temperature Scale

(IPTS) —the lingua franca for ther- mometry—is a set of reference temperatures, or fixed points. These well delineated and reproducible points are defined by the change in physical state of selected materials. Their stability is used to hold the temperature scale and to check temperature calibrations. Notable examples are the melting

point of gold, the triple point of water,

and the boiling point of oxygen. When

the IPTS was last formulated (1968),

it was not extended downwards, partly

because few conventional fixed points were available below 13.8 K. Traditional phase transitions (boiling points,

melting points, triple points) of all materials except 3 He and 4 He take

place at higher temperatures. Therefore, fixed points needed for this region

must be found in phase transitions oc- curring within the solid state. A pro- gram was started at the NBS in 1968

with assistance from the NBS Office of Standard Reference Materials 1 to develop cryogenic fixed points based on

the phenomenon of superconductivity.

There now exist ten such points be1 Standard Reference Materials are well characterized materials or simple artifacts with special chemical or physical properties

certified by NBS.

tween 0.015 K and 7.2 K, five of which have been incorporated in a new

provisional temperature scale, EPT-76,

which extends the IPTS-68 down to 0.5 K. The fixed points are provided by

two devices, Standard Reference Material (SRM) 767 and (SRM) 768,

which are produced at NBS and dis- tributed by the Office of Standard

Reference Materials. Each unit contains five very pure, well characterized

materials for which the phase transi- tions into the superconductive state occur over a temperature region of

Temperature

(K)

10

Superconductive

Transition

1.0

0 1

Nb Sn

3 (18.00 K) V Ga (14.30 K)

3 Nb (9.289 K) Pb (7.201 K)

in (3.146 K)

Al (1.175 K) Xn (0.844 K) Cd (0.515 K)

Auln

2 (0.203 K) AuAl

2 (0.150 K)

lr (0.112 K)

Be (0.024 K) W (0.015 K)

V SRM

767

,SRM

768

0.01 -

Figure 1. Superconductive transition temperatures of several materials. The 10 materials included in the Standard Reference

Material units are shown together with

materials at higher temperatures which are being examined as candidates for fixed

points.

16
: CENTER FOR ABSOLUTE PHYSICAL QUANTITIES

about 0.001 K and for which the

reproducibility of the observed transi- tion temperature, designated Tc , is ± 0.001 K or less. The materials

used in the two devices and the as- signed Tc's are shown in figure 1. Observation of the phase change is based on the fact that in the normal

• state above Te the electrons exhibit a very weak magnetic response to an

impressed alternating magnetic field,

- while in the superconducting state the

- electrons respond with perfect diamag-

; netism. This very strong effect is easily

| registered as a change in mutual inductance in two coils surrounding the material. A typical transition is shown in

figure 2, in this case for the element

indium which is used in SRM 767.

Provided that the ambient magnetic

field as well as the measuring field are

less than 1 ,uT (0.01 Gauss), some of

the transitions have been found to reproduce to within ± 0.0001 K.

Referring to figure 1 , we see that the materials included in the SRM units do

not cover the full temperature range over which superconductivity is known

to exist. We are, therefore, conducting

research to produce satisfactory samples of Nb and Nb3 Sn to extend the

region covered by superconducting fixed points to a full three decades in temperature.

ndium

Tc = 3.416

7 K

-3.0 -2.0 -1.0 0Tr (mK)

1.0 2.0 3.0

c Figure 2. Indium phase transition. The change in mutual inductance, AM, in a set of coils

i surrounding the In sample is plotted versus the output of an ac resistance bridge monitoring

a calibrated germanium resistance thermometer. The transition temperature is chosen as

the mid-point of the transition, while the width W of the transition is defined as the temperature interval, centered about T<, corresponding to 80 percent of the change AM.

Improved Laser Test of the

Isotropy of Space

John L. Hall, Quantum Physics

Division, for himself and Alain Brillet,

Joint Institute for Laboratory

Astrophysics

we report the application of

stabilized laser techniques to the precision test of one of the fundamental concepts of Relativity

Theory. These measurements

confirmed the isotropy of space to ± 2.5 parts in 10 13

, and represent a 4000-fold improvement over the best previous test.

One of the most meaningful and

significant human activities might be

called "guessing with inspiration." In- deed, we daily are called upon to invent an "outside reality" with only clues and

shards of evidence for input. But I am thinking here of the kind of scientific guessing, or concept synthesis, that occurs in a field perhaps only a dozen

times a century. This year we celebrate

the 100th birthday of the genius that made three such contributions in a

single year. Einstein's inspired guesses and their consequences must surely have been

troublesome to conventionalists at the

time. In explaining why Michelson and

Morley had failed to see a few tenths of a wavelength shift when their apparatus rotated, Einstein gave up such

reasonable physical ideas as simultaneity, absolute time, and absolute

motion. New and unfamiliar ideas were

introduced such as the equivalence (for

discussion of physical effects) of all intertial reference frames, the physical

equivalence of mass and energy, a kind

of symmetry between time and length,

and certain group properties of frame

transformations. Outrageous and deeply troubling paradoxes were identified,

such as the "twins" paradox. Other

"unphysical" effects were predicted,

such as the shrinking of a physical bar

along the direction of its motion. This

shocking physical theory is still of fundamental interest to us now 74 years

17
NATIONAL MEASUREMENT LABORATORY

later since these remarkable counterintuitive predictions are found to be in perfect agreement with every experiment performed up to now. Indeed, a major industry is based on the conversion of a bit of the binding energy of the uranium nucleus into commercial

amounts of heat energy. Synchrotrons

sweep their frequency as the acceler- ated particles gain mass. Time dilation of the muon's internal decay clock

gives a distance range in the CERN

laboratory some 29 times the distance

that light could travel in one (proper)

decay time.

It is not surprising that experimentalists have found this special theory of

relativity a powerful stimulus to the invention of precision experiments

against which to test the theory. In the 1950's and 60's, high resolution tech- niques based on the Mossbauer effect provided some major advances to confirm our knowledge of the validity of

special relativity (the so-called inter- action of local clocks with distant matter and "anisotropy of inertia" experiments). The other new experimental

tool, lasers, provided a first glimpse in 1964 of the dramatic enhancement of measurement sensitivity that lay ahead.

This first important experiment was

performed in a special isolated laboratory near MIT by Charles Townes,

Ali Javan, and their coworkers. They

pointed out for the first time the gain

in sensitivity possible through the use

of frequency techniques to measure

small length changes. Unfortunately,

their laser experiment was troubled by

systematic noise sources, probably in- ertial forces arising from the periodic

motion, and the attained sensitivity was only somewhat better than that obtained in the 1930's by G. Joos using

Michelson's original techniques. Thus,

a large amount of technical development of laser techniques has been a necessary preliminary to testing relativity theory with lasers. It is probably

fair to say that the first really meaningful laser test of special relativity was

only recently completed. In this measurement, Alain Brillet (from the Laboratoire de l'Horloge Atomique, in Orsay) and I, using appropriate laser techniques, repeated the famous

Michelson-Morley "Isotropy of Space"

experiment of 1887. Impressively

enough, one still finds a null result, in agreement with Einstein's postulate, even with a measurement sensitivity

4000-fold improved over the best pre- vious experiment (the 1964 experiment

of Charles Townes et al.). Our experiment is timely in three ways. First (and trivially), it is appropriate to do an improved relativity

experiment in an Einstein celebration

year. Secondly, laser techniques have

been developed to the point that they

permit a really major advance in sensitivity. Thirdly, it is probably true that

all physical laws are approximate

statements: As the measurements be- come more refined and sensitive, a larger and larger fraction of "outside

reality" needs to be taken into account

in describing the system under study.

Ultimately then, one might expect some

image of the cosmic status in the microscopic physics which we can probe.

Recently, several lines of investigation,

but most notably radio noise ther- mometry done from above the earth's atmosphere, lead to the conclusion that we are probably moving at speed ~

400 km/s relative to the 3 K blackbody

background radiation remaining from

the primordial "big bang". Thus it is interesting to see whether the supposed

small lack of isotropy in physics "in the

large" will have an ultimately observable effect on extremely sensitive lab- oratory experiments which test some

kind of local isotropy. (Honesty compels me to note that the experimental

basis for this third, philosophical point

did not even exist when the experiment was first undertaken. Basically we had

developed new measurement tech- niques that appeared useful for several fundamental experiments, and the NBS

management supported the work on

these completely adequate and reasonable grounds.)

Our experiment was designed to be

highly sensitive, clear in its physical

interpretation, and free of problems

involving spurious effects. Its basic

concept can be understood by considering figure 1. Fundamentally, we were comparing the laser frequency defined

by an etalon of length with the laser frequency defined by an optical fre- quency standard. As depicted in figure

1, a 3.39 /j.m laser was servostabilized

to the standing wave condition in an

isolated and stable passive Fabry-Perot

etalon. The laser, Fabry-Perot inter- ferometer, and associated electronics were mounted on a large granite slab on a table which could be slowly ro- tated around a vertical axis. Information about the length of the interfer- ometer was implicitly carried by the wavelength (frequency) of the portion

of the laser beam which left the table along its rotation axis and which was

directed over to another table to be

optically heterodyned with an "isola- tion" laser. This laser was frequencystabilized relative to another 3.39

laser which was in turn frequencystabilized relative to the saturated absorption peak in CH4 . In following \ Javan and Townes, we had thus pre- pared a highly sensitive readout of the 1 optical length of the Fabry-Perot etalon through the use of frequency measure- ment techniques. To see whether there was a measurable spatial anisotropy in our laboratory, the laser and FabryPerot etalon part of the apparatus was

rotated slowly and we looked for a change synchronized with this rotation.

It is natural—perhaps inevitable when

18
;hnter for absolute physical quantities

Phase

Lock

'Figure 1. Schematic of isotropy-of-space experiment. A He-Ne laser (3.39 ,um) is servostabilized to a transmission fringe of an

isolated and highly stable Fabry-Perot resonator, with provision being made to rotate this whole system. A small portion of the laser beam

:is diverted along the table rotation axis to read out the cavity length via optical heterodyne with an "isolation laser", which is stabilized

relative to a CHi-stabilized reference laser. The beat frequency is shifted and counted under minicomputer control, these frequency measurements being synchronized and stored relative to the table's angular position. After 30 minutes of signal averaging, the data

are Fourier-transformed and printed out, and the experiment is repeated.

lithe sensitivity is sufficiently high—that

: some such synchronous perturbation

jwill be observed. We saw a signal at

jthe table rotation frequency of some

j200 Hz excursion, representing a 2 X

: 10~ 12 length change of our Fabry-Perot

etalon. Its origin is now understood as

a gravitational stretching of the inter- ferometer which occurred because of

residual asymmetry in the mounting of

[the reference etalon when there was a

jslight misalignment of the rotation axis jfrom the vertical. A 10-fold smaller

signal was observed at the double fre- quency of the table rotation, which is the spectral location expected for a ^genuine anisotropy signal. However,

considering that the laboratory axes were rotated relative to the hypothetical "preferred direction in space" by

ithe earth's rotation, a true ccsmic ani- sotropy signal would show a further rrodulation every 12 sidereal hours.

We observed that the spurious signal

at the double rotation frequency was

essentially fixed in the laboratory co- ordinate system, and so averaging over

a few days in 12 or 24 sidereal hour

blocks reduced our spurious signal

essentially to zero. This null result represented a fractional synchronous

length variation below 2.5 parts in 10 1 "', and this limit represents a 4000-fold

improvement over the MIT laser experi- ment. Further, if it is supposed that the

earth's inferred velocity of ~ 400 km/s

is the relevant one to be used in the coordinate frame transformation, this "laserized" Michelson-Morley experiment represents the most demanding

test yet of the Einstein-Lorentz trans- formation.

As mentioned above, the laser heterodyne techniques provided a very high

sensitivity in these experiments. If the spurious background signal could be

reduced or eliminated, a factor of at

least 1000 further improvement would

be possible. In this domain, general

relativistic effects (the earth's gravitation) would cause a vertical spatial

anisotropy of the type we can detect,

Vi (GM/R) (1/c2

) (1/R) ~ 3 X 10- 17

. Unfortunately, such a vertical experiment would be hopelessly contaminated

by gravitational distortion of the FabryPerot etalon itself. A more promising

direction for growth is to redo the Kennedy-Thorndike measurement that shows the equivalence of the transfor- mation of length and time. Extreme

care would be necessary to avoid drifts, since the only modulation in this experiment arises from the earth's motion.

For example, a temperature stability of

50 pK for 1 month and creep rate noise

below 10~ u per day2 would allow a 20-fold improvement on the original

experiment of Kennedy and Thorndike.

19

Center for

Radiation Research

Chris E. Kuyatt

t ne A. Cassatt, h.

Donald Colle 51mer H.

Eisenhower, Jr. rlenry T. Heaton, II lames S. O'Connell Atomic and Plasma

Radiation Wolfgang L. Wiese

Nicolo Acquista

lohn M. Bridges David G. Cooper

Zharles H. Corliss

Jeffrey R. Fuhr

Ronald L. Greene*

Victor Kaufman

Daniel E. Kelleher Arthur K. Kerman

Jules Z. Klose Andrea M.

Malvezzi**

Oscar Manley**

Georgia A. Martin

William C. Martin,

Jr. Tsu-Jve A. Nee

William R. Ott Timothy L. Pittman Donald H. Priester** Joseph Reader

James R. Roberts

Larrv J. Roszman

Phiilip M. Stone**

Jack Sugar

John Weiner

Andrew W. Weiss

Jean-Francois Wyart**

Stephen M.

Younger*

Romuald Zalubas

Nuclear Radiation Randall S. Caswell Robert L. Ayres

Stephen M. Baloga James W. Behrens

Charles D. Bowman

Patricia Burt**

Jacqueline M.

Calhoun

Larry Cardman**

Allan D. Carlson Lucy M. Cavallo

Bert M. Coursey

John J. Coyne

Hall Crannell** John B. Czirr Michael Danos

William R. Dodge

Kenneth C. Duvall

Charles M.

Eisenhauer

Larry Fagg**

Everett G. Fuller Henry M.

Gerstenberg

David M. Gilliam Daniel Golas** James A. Grundl

Evans V. Hayward

Alan T. Hirshfeld Dale D. Hoppes

John M. R. Hutchinson

Kenneth G. W. Inn Ronald G. Johnson

Arthur K. Kerman

George P. Lamaze

John W. Liehtbody,

Jr. Richard Lindgren**

Larry Lucas

Marcia D. Maltese

( Continued)

T

Robert Placious adjusts equipment for characterization of x-ray fluorescent intensifying

screens. Measurements are made, on an

absolute basis, of the total light output and

spectral distribution of the x-ray excited

fluorescence from radiographic intensifying

screens. This unique facility utilizes monoenergetic x-ray production together with highly

sensitive radiometric detection equipment.

These screens are universally used in almost all medical, and in many industrial radiographic

procedures to maintain good image quality

n hile reducing x-ray exposure.

he Center for Radiation Research has a unique

mission. It provides high accuracy radiation measure- ment techniques and quantitative data about atomic

and nuclear radiation processes. These outputs are required for the safe and effective use of radiation in key technologies. Center activities cover the electromagnetic spectrum starting at the infrared, through the

visible, the ultraviolet, and vacuum ultraviolet, to x-ray

and nuclear gamma rays. Particle radiations include

electrons, positions, protons, deuterons and neutrons

below 140 million electron volts. Our facilities include a 140 MeV electron linear accelerator, a 240 MeV electron

storage ring for synchrotron light, many lower energy accelerators, a polarized electron source, plasma devices,

and many sophisticated spectrometers with computer

assisted data acquisition.

During the past year. CRR has directed special atten- tion to several areas of growing national concern. One

of these areas, ionizing radiation health and safety, has

attracted a large amount of public and institutional concern. At the request of the Senate Committee on Commerce, Science and Transportation, the Center is cooperating with the Conference of State Radiation

Control Program Directors. Together, we are reviewing

the need for an intermediate level of calibration services

such as regional facilities to better couple NBS laboratories with State and industrial needs. This review will

contribute to an updated long-range plan and budget for

ionizing radiation measurements.

CRR is working with the State of Illinois to set up a

pilot regional calibration laboratory, with Florida to set up a calibration facility for low-level survey instruments,

and with the University of Michigan on the testing of

personnel dosimetry performance. Special measurements

were made on dosimeters used near the Three-Mile Island

nuclear power plant. Natural matrix radioactivity standards, such as human lung and liver, are being prepared

for environmental monitoring. We are working on dosimetry measurements and standards for radiation steri- lization, a process which replaces the use of carcinogenic

ethylene oxide. Measurements of the response of x-ray

screens are part of our work on the technology for reducing patient exposure. Because of the need for increased

cooperation between the various agencies concerned with

ionizing radiation, the Center is represented on two

21
NATIONAL MEASUREMENT LABORATORY

Wilfrid B. Mann

Xavier K. Maruyama

Leonard C. Maximon

Emmert D. McGarry

Michael M. Meier

Sydney Meshkov

Vernon W. Myers

James R. Noyce

James O'Brien** Gerald Peterson** Mark Reeves** Henry Rosenstock

Francis J. Schima

John C. Schleter Roald A. Schrack

Ivan G. Schroder Robert B. Schwartz

William E. Slater Daniel Sober**

Valentine Spiegel, Jr. William Stapor**

Richard D. Starr* Michael P. Unterweger, Jr. Oren A. Wasson

Elisa Wolynec**

Lungwen Yap**

Radiation Physics

Lewis V. Spencer

Martin J. Berger

L. Randall

Canfield ' Robert Celotta

Barrett E. Cole Randolph Culp**

Joseph Dehmer**

Charles E. Dick

Steve R. Domen

David L. Ederer Margarete Ehrlich Raymond L. Falge,

Jr. Annija D. Galejs

John C. Hamilton*

John H. Hubbell

Lanny R. Hughey

Jimmy C. Humphreys

Paul J. Lamperti

Donald R. Lehman**

Harry Levine** Robert Loevinger

Thomas P. Loftus Toh-Ming Lu**

Thomas B. Lucatorto Robert P. Madden

William L. McLaughlin

Thomas J. Mcllrath

Caroline Mehlman**

Stanley R. Mielczarek

Joseph W. Motz

John Noakes**

Hideo Onuki**

Rathindra Pal**

David R. Penn

Daniel T. Pierce Robert C. Placious Daniel Polansky

John S. Pruitt Ronald W. Rendell* JackC. Rife** Edward B. Saloman

Stephen M. Seltzer Christopher G.

Soares

Julian H. Sparrow

John Sutherland*

Nils Swanson

Peter Takacs**

Bernard J. Waclawski

Harold H. Walker

James T. Weaver, Jr. Radiometric Physics Jack L. Tech

Robert L. Booker,

III Robert J. Bruening

Kenneth L. Eckerle

Joel B. Fowler

Jon C. Geist Warren K. Gladden

Jack J-G. Hsia

Henry J. Kostkowski

Ernest Lewis, Jr. Donald A. McSparron

Fred E. Nicodemus

Charles H. Popenoe

Robert D. Saunders,

Jr. A. Russell Schaefer John B. Shumaker,

Jr. James H. Walker

John F. Ward, III William R. Waters,

Jr. Victor R. Weidner

Charles R. Yokley

Edward F. Zalewski

Radiation Source and Instrumentation Samuel Penner John B. Broberg

Louis Costrell Stephen C. Ebner

David C. Green

Eric R. Lindstrom

Isador J. Livant Anthony B. Marella Joseph Parrington**

George Rakowsky

John E. Rose

Julian K. Whittaker

Mark A. D. Wilson

Post Doctoral Staff ** Research Associates and Guest Workers

recently formed interagency committees on ionizinj

radiation.

CRR is working to improve the state-of-the-art in opti

cal radiation measurements. Better measurements an

needed, for example, in solar and atmospheric monitoring

and by the electro-optics and lighting industries. To make,

available to the lighting industry the new radiometric

scale, which will eventually replace the current scak

based on the candela, 175 lamps are being calibrated or

1 both scales for simultaneous distribution. Progress towarc

the 0. 1 percent radiometry needed in solar monitoring ii being made through quantum-based calibrations of sili con photodiodes, by improved characterization of black

body sources, and through intercomparisons of NB5

radiometry standards. Improved measurements of UVI

radiation are made possible by new source standards anc by accurate measurements of spectroradiometer response

Provision of characterized detector packages on a renta

basis enables users to measure the spectral response o

their detectors and to test the performance of their mea

surement systems. New measurement assurance program:

are being developed for diffuse reflectance and for retro reflectance.

Several of our activities contribute to the Nation':

energy programs. Measurement and calculation of the, spectra of highly-ionized heavy ions are essential to the; diagnostics of fusion-plasma devices. Ionization rate mea, surements now in progress will provide benchmark date for analyzing heavy ion impurities in Tokamak fusioi

devices. Vacuum ultraviolet radiometric calibrations anc portable transfer source standards provide the measure ment base for accurate measurements of radiation fron

fusion plasmas. Radiation phenomena, such as plasm;

line broadening and dielectronic recombination, are beim

investigated as possible new plasma diagnostic tech

niques. Resonance neutron radiography is being devel

oped as a non-destructive method for obtaining accurate values for the total mass of fissionable isotopes. Thi:

method is expected to be useful in calibrating measure ment devices for nuclear fuel rods as well as for waste and scrap from nuclear fuel reprocessing plants. Accu

rate measurements of neutron cross sections are beim

made for both fission and fusion reactions. Integral anc

differential neutron fields continue to be developed fci

22
CENTER FOR RADIATION RESEARCH

: ie calibration of neutron detectors used in fusion re- sarch, nuclear reactors, and personnel dosimetry.

. The Center has a dedicated synchrotron radiation faJlity, SURF-II. This device produces electromagnetic

.idiation in a narrow, intense, highly-polarized beam

jHth a continuous and accurately known spectrum from

fie infrared through the visible and into the extreme

ftraviolet. This tunable, intense radiation is being applied to several new scientific and technical opportuni-

_es. Many of these activities involve collaboration with

ther Centers of NBS or with laboratories outside of

BS. Ultraviolet photodiode detectors are now being calij rated for photon energies up to 250 eV. This capability

was used in a collaborative program with Los Alamos to

•leasure photoemission yields of several promising cathde materials. Several calibrations were carried out on

.oectrometers for space experiments and fusion plasma

iagnostics. A new spectrometer calibration facility, un-

,ter construction with NASA funding, will give more

.ccurate and convenient calibrations. During the past

.ear our new high-flux normal-incidence monochromator

.arae into full operation, and has been used to measure

le angular distribution of photoemitted electrons from

idividual vibrational levels of highly excited molecules.

Hiis monochromator also has been used to study Stark

nixing in high Rydberg autoionizing states of atoms and

Molecules. Further studies on excited state dynamics will

: tilize fluorescence measurements to investigate ion de-

'xcitation processes. Using another monochromator, we

! ave observed high pressure helium microbubbles in thin

'luminum films by measuring the pressure-broadened

'elium resonance in absorption. Scientists from the Na-

' al Research Laboratory are studying insulators by using

: 2flection. photoemission and fluorescence. Scientists

I'om Brookhaven National Laboratory are installing a

Circular dichroism apparatus for measurements of bioholecules. Apparatus is being built in collaboration with

le Surface Science Division to study the photodesorp-

'ion of ions from absorbates at surfaces. We do not have

pace to describe all of the activities at SURF-II. but

'nese examples illustrate the wide range of activities being

'arried out with synchrotron radiation.

High energy photons and electrons are excellent

rjbes of nuclear structure. The coupling of photons and

lectrons to charge, current, and magnetic moments of

nuclear constituents is well known from quantum elec- trodynamics, and the flexibility of electron scattering

allows energy and momentum transfer to the nuclear

system to be varied independently. Using a high-current

140 MeV linear electron accelerator, CRR scientists have

investigated nuclear structure using electron scattering,

photon scattering, and electrodisintegration. Recent elastic photon scattering measurements from 12 C showed a surprisingly large electric quadrupole component. Similar measurements on ieO will be of even greater theoretical interest. Elastic electron scattering has yielded an improved value for the 12 C nuclear radius which has become a world standard. Similar measurements on 4 He

will provide a critical test of quantum electrodynamics.

In electron scattering, the cross section is measured as a function of energy and momentum transfer. In electrodisintegration, however, outgoing particles or induced

radioactivities are integrated over all of the final states of

the scattered electron. An entirely new type of experiment has recently been pioneered by CRR scientists in

which the yield of particles is measured as a function of

the incident energy. From analysis of such measurements

it is possible to locate and measure the strength of the

El and E2 giant resonances that result in proton and «

particle emission. Results on 58Ni, 60Ni and r,2 Ni show

important E2 components in the « channel.

The ultimate experiment would detect both the scat- tered electron and outgoing particle in coincidence. Such

experiments would require a 100 percent duty cycle

(CW) electron accelerator and would be crucial in going

beyond the classical description of nuclear structure and

reactions in terms of elementary neutrons and protons;

that is, in determining the effects of many-body correlations, virtual pions, and excited nuclear states on nuclear

structure. These opportunities are so important that we

have begun, with DOE funding, to develop a prototype

185 MeV room-temperature CW electron accelerator. In addition to demonstrating the feasibility of building a

1 to 2 GeV machine using the same technology, the prototype accelerator will be a powerful tool in its own right

for nuclear physics.

These vignettes cover briefly only a fraction of the

CRR programs. Some of these programs and others are described in more detail on the following pages.

23
NATIONAL MEASUREMENT LABORATORY

Radioactivity

Standardization

Wilfrid B. Mann

Nuclear Radiation Division

-A. adioactivity, like fire, has

potential for both benefit and harm,

and the increasing variety of its applications and public awareness of

its hazards have brought challenges to the NBS Radioactivity Standardization group. We are meeting those

challenges in a variety of ways.

Nearly 2000 samples of some 50

different radionuclides were calibrated or issued as standard reference

materials (SRMs) in 1978. Cooperative programs with nuclearmedicine and environmentalmonitoring quality-control groups

allowed the staff to assure correct instrument calibrations much beyond

their ability to supply direct services.

Such cooperative measurementsassurance programs work to establish the traceability of radioactivity measurements to those of NBS. Why NBS?

Because we are the legal promulgator

of standards of the national radioactivity-measurements system. The term

traceability may not be completely apt,

but it is being used more and more by

regulatory agencies to connote measurements that are demonstrably in agreement with those of the National

Bureau of Standards.

Each year NBS must decide on those

radionuclides for which standards are most urgently needed. Once those deci- sions are made, our work begins. In the

last three years, an efficiency-extrapolation coincidence method employing a pressurized proportional counter and

efficient gamma-ray spectrometers has

been used to establish calibrations for

G7 Ga, f,!, Mo, 00mTc, ulIn, 123

I, 1G0 Yb,

195AUj 201 Xl! and 20Hp5_alJ rad i onu. elides now used in nuclear medicine.

The same system was used to calibrate

14 ~'Pm (used in a pressure measuring

device) and 14 "Ba- 140La (a significant

24

monitor of nuclear-fuel burnup). Measuring the radionuclides released from

the burning of coal required standards

of several naturally occurring radionuclides. For example, a standard of

flyash from the burning of Eastern United States coal was developed in which the sample is highly homogeneous and is certified for -' 2 ' ; Ra and

- 28Ra content. For some applications, preparing the SRM in the proper form and checking

its uniformity offer more of a chal- lenge than the basic calibration. Standards which are prepared to check the

yields from chemical procedures are of

this type. Such natural matrix standards of human lung and liver, soil from Rocky Flats, Irish Sea sediment,

and river and lake sediments are being

developed. Homogenization of these materials is a significant challenge which

is being met partly by borrowing tech- niques from powder-mixing technology.

Ultimately, not only the radioactivity of these materials will be measured, but

the chemical forms will be examined.

For example, we expect to tell users approximately how much of the plutonium in a sample is in the form of

high-fired oxides.

Radioactivity calibrations are unique

in that a value for a standard applies

only for an instant. The radioactive

decay law allows the standard to be

used at a later time if accurate values

for the half life and the associated uncertainty are known. For a number of

years, the half lives of most radionuclides used as standards have been

measured at NBS. The probability per

decay for radiations is also required

if their emission rate is to be deduced

from the activity given for a standard.

Well-calibrated germanium spectrometer systems have been used to measure gamma-ray probability-per-decay

(Py) values for many recent standards.

Such spectrometer systems are used in monitoring at nuclear power plants, or wherever mixtures of radionuclides

must be assayed, and special standards

are prepared for calibrating the effi- ciency of these systems as a function of | r energy. A mixture of 9 radionuclides

with well-established Py's was devel-

oped for this purpose, and commercial

suppliers now submit samples of each

batch of their standards of this mixture

to NBS to establish explicit traceability.

Longer-lived radionuclides, such as

13SBa and ir,2 Eu, which emit gamma i rays at several energies, are also now S available from NBS as standards, and

a mixture of 12r'Sb, ]54 Eu, and 155Eu is being studied as a possible standard.

For some common counting conditions,

large errors can occur if the summing

of coincident radiations is not taken

into account, and computer programs

are now being used to calculate these i . corrections for frequently encountered

radionuclides.

Once a calibration has been established for any gamma-ray-emitting ra- dionuclide, the response of NBS pres- i

!

sure ionization chambers to a known !: k activity is measured relative to that of

a

22,; Ra source producing a comparable

current. This preservation of what

would otherwise be only a temporary , a calibration, in the case of short-lived i radionuclides, eliminates the need for frequent direct recalibrations. Doing so allows the group to offer an efficient calibration service for many radionu- i elides for which the demand does not : justify preparing SRMs.

The preservation of calibrations, and

a comparison of them on an international scale, is also accomplished by i i the routine transmission of calibrated ' ti gamma-ray-emitting solution stand- ards to the Bureau International des !j lj Poids et Mesures (BIPM) and to the International Atomic Energy Agency !>
:enter for radiation research

c(IAEA) for measurment on their ioni zation chambers. NBS calibration tech- niques are also checked by international

- ntercomparisons of the radioactivity

concentration of split samples of the :;ame master solution. Such intercom-

.parisons, arranged by BIPM, have re-

;ently taken place for 139Ce, mCs,

L

:;7 Cs, and 55Fe. In the first two cases

(!:he agreement was quite good, with 22

l aboratories agreeing to within 0.7 per- cent for the mCs, but preliminary re- ports show a much greater spread for

t he more difficult 137Cs and 55Fe.

I International cooperation on applications of radioactivity has been aided

. n the past few years by the International Committee for Radionuclide

Metrology (ICRM), with active participation by NBS. The ICRM spec- trometry group has organized compari-

^ions of gamma-ray emission-rate measurements with germanium spectrometer

iiystems for ir,2 Eu and 133Ba; collected nformation on impurity searching, per- tinent nuclear data, and germanium

detector stability; and initiated, through

TAEA, a study of random pile-up ef- fects in gamma-ray spectrometry. Symposia were held by the low-level group

ho determine the need for specific natural-matrix standards of radioactivity, ;and the group has organized international cooperative projects for their production and distribution,

u Any accurate radioactivity measure- ment requires good technique in addijjtion to good standards, and several itmembers of the NBS group have conl tributed to written standards for the

;use of ionization chambers, germanium

f.and Nal (Tl) detectors and liquid scintillation counters. Much NBS effort Iwent into the writing of the Handbook

of Radioactivity Measurement Procedures, prepared as Report 58 by a group

of experts for the National Council on

Radiation Protection and Measurement. It describes in detail many of the concepts and techniques mentioned

here. Further, the NBS Radioactivity

Standardization group has become

known for its willingness to help those

with radioactivity-measurements problems, through seminars or direct technical assistance. This attitude has

brought them considerable satisfaction,

and a very harried existence!

Spectra of HighlyIonized Atoms

Joseph Reader,

Atomic and Plasma Radiation Division

T_JL he development of controlled

fusion reactors depends greatly on the

ability to observe and understand the physical processes taking place in their very hot plasmas. One of the methods

used to diagnose these plasmas is the observation of line spectra emitted by

the various ionic species present. Such

observations provide information about

the temperature of the plasma, impurities, and spatial and temporal properties. For lighter ionic species much of

the basic atomic data needed to inter- pret these observations are already on

hand. However, for heavier ions, such

as those with Z^>20, relatively few

data are available. The problem is particularly acute for Tokamaks, because

of their use of aperture limiters to prevent stray particles from striking the interior wall of the torus. Atoms

from the limiter, which is usually made

of a refractory metal such as molybdenum or tungsten, are sputtered into the discharge, where they become

highly ionized and significantly affect the plasma conditions. Knowledge of

the spectra of the ions of these heavy

elements is therefore of great importance. By studying spectra of laser- produced plasmas our group has made

progress during the past year toward

providing new data about these highly

ionized atoms. The work was done in collaboration with G. Luther and

N. Acquista.

Data about highly ionized atoms

were obtained in two separate experi- ments. First we focused the light from

a 3 GW Nd/glass laser onto flat metallic targets to produce plasmas of various elements. The spectra were re- corded on a 10.7 m grazing incidence

spectrograph. Typical laser pulses were

15 I in 10 ns. By varying the energy

and duration of the laser pulses we

25
NATIONAL MEASUREMENT LABORATORY

Experimental

Theoretical Pd

37 800 I- Se/

3 5 9 11 Z, -13 15 17 19

were able to distinguish spectra due to

different ionization stages. Analysis of

the data yielded the spectra, electronic

energy levels, and ionization energies

of ions in the zinc and copper isoelec- tronic sequences to Sb21+ and Sb22 + , including the important ions Mo12+ and Mo13+ . As an example of the reFigure 1. Plot of the energy (in cm- 1

) of

the 5s configuration in the Cu isoelectronic sequence. Z(

. is the net charge of the atomic

core seen by the valence electron. Theoretical

values are from Cheng and Kim (Ref. [1]).

1600

1200

e

uoo? 800 >

(3a

UJZ

UJ

400

Laser-produced

Plasma

NBS/LASL)

Laser-produced

Plasma

(NBS)

15 25 35 45

suits, figure 1 shows the energy of the 5s configuration in the Cu sequence, along with the values obtained in fully relativistic calculations [1]. The cal- culations predict the general variation of energy very well, although the ab- ;: solute values are somewhat displaced.

As the scale is greatly expanded here

the absolute differences between the two curves are actually not large; for Mo the difference is only 0.5 percent. In the second experiment we trans- ported a 2.2 m spectrograph and target

chamber to the Los Alamos Scientific Laboratory. There we used a very powerful (100 GW) Nd/glass laser to generate still higher stages of ionization. Spectra were recorded with laser pulses of 30 J in 300 ps. With these observations we extended the Zn and

Cu isoelectronic sequences to W44 + and W4r, + . These results represent the longest isoelectronic sequences ever ob- served as well as the highest stages of ionization recorded with diffraction gratings. Figure 2 shows an isoelec- tronic sequence plot for the 4s-4p resonance lines of Zn-like ions. These experiments also produced

significant results for molybdenum. Besides the resonance lines of Mo12 + and

Moi::+

, two of the most important lines used in Tokamak diagnoses are the resonance lines of the Mg and Na-like

ions, Moo,0+ and Mo31 + . Until re- cently these lines had been observed

only in Tokamaks. Confirmation of

their proposed origins (ion and transi- tion) were thus critically needed. Pre- vious observations of laser-produced Mo

plasmas [2] gave spectra that could not be interpreted unambiguously. In the present work we studied ions in the Mg

and Na sequences from Sr27 + to Pd34 + .

Figure 2. Isoelectronic comparison of the 4s2 1 S0-4s4p !Pj transitions in the Zn iso- electronic sequence.

26
-CENTER FOR RADIATION RESEARCH

"igure 3 shows a plot of two transitions n the Na sequence that are nearly coincident in Mo31-

. From the isoelec- ronic regularities we could identify the observed lines and correlate them with

Tokamak observations. Wavelengths

or Mo are compared in table I. For vome lines the wavelengths agree well. :onfirming the proposed origins of the

Tokamak lines. For other lines the wavelengths do not agree so well, suggesting a possible problem in the Tokamak measurements.

The results are also of interest for

Figure 3. Isoelectronic comparison of 3s-3p

and 3p-3d transitions in the Na isoelectronic sequence.

800

600

400

200

3p 2P 3/2 - 3d 2D 5/2

/Kr

/As

3s 2S 1/2 - 3p 2P3/2

K

1 1 I i 10 15 20 25 20 25

+ 0.2

0.0

S -0.2

<l

-0.4 -

-0.6

Nd W

Mo 20 30 40 50

checking theoretical calculations of the properties of highly stripped ions in high temperature plasmas (energy

levels, wavelengths, transition probabilities, cross sections for excitation and

ionization by electron impact). In the domain of highly stripped ions, where

relativistic effects are extremely important, the wavelengths predicted by the

most accurate fully relativistic calculations agree with our experimental val- ues to within about one percent (see

fig. 4). As the predicted values are sys- tematically low by this amount, the ex- periment reveals a possibly important

effect not included in present theories.

Table 1. Present values of Mo wavelengths compared with Tokamak measurements of E. Hinnov, Phys. Rev. 14,

1533 (1976).

Ion

A(PRESENT)-NM

A(TOKAMAK)-NM

Mo,2 + 34.0909(5) 34.10(5)

Mo13 + 37.3647(5) 37.38(5)

42.3576(5) 42.35(5)

Mo:; " + 11.596(1) 11.70(5)

Mo31 + 12.7814(5) 12.9(1)

17.662(1) 17.7(1)

References

[1] Chen, K. T. and Kim, Y.-K., Energy

Levels, Wavelengths, and Transition

Probabilities for Cu-like Ions, Atomic

Data and Nuclear Data Tables 22,

547-563 ( 1978).

[2] Burkhalter, P. G., Reader, I., and

Cowan, R. D., Spectra of Mo XXX,

XXXI, and XXXII from a laser- produced plasma, J. Opt. Soc. Am.

67, 1521-1529 (1977).

Figure 4. Comparison between observed and

calculated wavelengths in the Cu isoelec- tronic sequence. Calculated values are from

Cheng and Kim (Ref. [1]).

27
NATIONAL MEASUREMENT LABORATORY Polarized Low Energy Electron

Diffraction (PLEED): A

Technique to Study Surface

Magnetism

Daniel T. Pierce and Robert J. Celotta,

Radiation Physics Division

JLAJLt the surface of a magnetic

crystal, the lack of translational invariance perpendicular to the surface and

the reduced number of neighbors of the surface atoms cause the magnetic properties to differ from those in the bulk.

We have developed a new way to measure the magnetization of the outer few

atomic layers of a single crystal surface

based on polarized low energy electron

diffraction (PLEED). [1]

Due to the exchange interaction, the

diffracted intensities of electrons at the surface of a magnetic crystal depend on the relative orientation of the inci- dent electron spin and the magnetization of the surface. The surface magnetization, its temperature dependence,

and the transition temperature at the surface have been described by a number of theories which we now have a straightforward way to test experimentally. The increasing interest in lower

dimensional magnetism and surface

critical phenomena makes the development of our "surface magnetometer"

especially timely.

We also expect the surface magnetometer to have technological applications. In the push to further miniaturize magnetic devices—by using

thinner layers in magnetic storage

media, for example—the magnetic

properties of the outer few layers will become very important. Our technique

provides a research tool to study, for

instance, the magnetic changes at the

surface of a recording head resulting

from corrosion. Magnetic surfaces are

also important in hydrogen storage and

catalysis, such as in the coal gasification process. The surface magnetometer will allow the study of the role of the magnetic electrons in chemisorption on well characterized single crystal surfaces. Electrons are widely used in surface

analytical techniques (e.g., photoemission, LEED, Auger spectroscopy), be- cause their strong interaction with the electrons in the material limits their probing to the outer few atomic layers.

For example, in the Ni(llO) surface

which we studied, electrons with energy

of about 100 eV can on the average

penetrate and return through only two

atomic layers without losing energy and

being removed from the elastically scat- tered beam. Polarized electron scatter- ing provides a way to study the surface

of a well defined semi-infinite single

crystal. The polarized electron probe

is non-destructive and has potentially very high spatial resolution although we

have not exploited it thus far. One can

envision a magnetometer for surface

microstructure which could display

magnetic domains and bubbles.

The measurement of surface magnetization by polarized electron scat- tering is made possible by the polarized

electron gun which we developed. The

electron gun, which is based on photoemission from negative electron affinity

GaAs, provides a constant intensity electron beam like an ordinary LEED

gun, but the spin direction is modulated

at a rapid rate. Since the incident elec- tron beam intensity on the target crystal

is constant, any ac component of the

scattered intensity synchronous with

X

CO

- + .02

—25

1 ^—- + 25 -f-+- HhH (Oe)

- - .02

Figure 1. Measurement of S as a function

of H showing hysteresis in the surface magnetization.

the polarization modulation must re- sult from a spin dependent scattering.

The scattered intensity for incident electron spin parallel to the net spin in the ferro-magnetic surface I j T is in i general different from the scattered in- tensity when the spins are antiparallel

I U . We measure a quantity S, which

is this difference normalized to the spin

averaged scattered intensity,

I f t —I t i

I

T T +1 t i We also correct for the fact that the incident polarization is 43 percent

rather than 100 percent.

Theoretically, S depends on the ex- change potential J(E,#) and the Coulomb potential V(E,0) seen by the elec- tron scattering at an energy E and an

angle 6. It can be shown in the first Born approximation that S is proportional to the average magnetization M

of the outer few layers of the surface.

If multiple scattering is included there may be higher order terms proportional

to M", where n is odd, but in Ni these are very small and may be neglected.

The apparatus [2] consists of the polarized electron gun attached to a[ conventional surface analysis chamber,

j We cleaned a Ni(110) crystal by sue-! cessive Ar ion bombardment and an- nealing cycles. The surface cleanliness I was checked by Auger spectroscopy

using a cylindrical mirror analyzer.

Generally, it is not straightforward to scatter low energy electrons from the surface of a ferromagnet because of the deflection of the electrons in stray

magnetic fields. The stray magnetic

fields have been minimized by making

the Ni crystal the keeper of the small electromagnet that magnetically aligns the domains in the crystal.

We can measure S as a function of magnetic field to obtain a surface hys28
CENTER FOR RADIATION RESEARCH

X-Ray Image

Information Theory

Joseph W. Motz and Michael Danos,

Radiation Physics Division

>:eresis curve S(H). Such a curve is shown in figure 1 for specular scatter7 ng at an angle of incidence of 12°

ind an energy of 125 eV. The maximum S in figure 1 is less than 0.02; this

is a small signal but the noise level in

:his sensitive measurement technique is :ess than 0.001. Reversal of the magnetic field allows us to isolate any possible nonmagnetic contribution to the

spin dependent scattering. We emphasize that figure 1 is a hysteresis curve

epresenting the magnetization of the

Duter two atomic layers. Such a curve

s very sensitive to surface contamination. In particular, segregation of sulfur

:o the surface decreases the magnetization to zero. Future measurements will study the effects of adsorbates on the surface magnetization. We will investigate the existence of magnetic dead

. ayers at the surface (considered unikely from these first measurements)

.and the role of the magnetic d-electrons

n the bonding of adsorbed atoms.

, We also explored the temperature

dependence of the surface magnetizaTemperature, K

400 500 600

-i—i—i—i—i—i—i—i—i—i

n i r

.01

n

0

-.01

-.02 1 1 1 L

Temperature, (T/Tc ) Figure 2. Measurement of the temperature

dependence of the surface magnetization

showing a linear behavior as a function of

:emperature.

tion. There are definite predictions

from theoretical scaling laws relating the surface and bulk behavior in the

critical region near the Curie temperature Te, We were able to measure S(T)

from 0.5 to 0.8 Tc and found that the temperature dependence of the sur- face magnetization shows much less curvature than that of the bulk; in fact,

it is approximately linear. Measurements were made by heating above Tc and cooling in applied magnetic fields of 18, 21, and 26 Oe as represented

by the different points in figure 2. The

cooling was too fast to allow measure- ment near Tc

. In future measurements we will have crystal temperature control and will measure up to Te, thereby

providing a test of theories of surface

critical phenomena. We plan also to extend the measurements to low temperature in order to normalize S(T) to the saturation magnetization. In summary, this very powerful new

technique may be compared to neutron

scattering, which has been so successful in measuring bulk magnetic properties. We expect that analogous information will be obtained about surface

magnetic properties using our new PLEED technique.

References

[1] Celotta, R. J., Pierce, D. T., Wang,

G. -C, Bader, S. D., and Felcher, G.

P., Physical Review Letters 43,

728-731 (1979).

[2] Wang. G. -C, Bunlap, B. I., Celotta, R. J., and Pierce, D. T., Physical Review Letters 42, 1349-1352 (1979).

TM his study analyzes the dependence of image information content on

x-ray exposure in medical diagnostic x-ray examinations. The results indi- cate that with the employment of image

processing techniques and modified

x-ray spectra, it is possible to significantly reduce these exposures without

loss of the visual image information

that is available to the radiologist.

Medical x-ray examinations provide

the major source of radiation exposure

to the population in this country.

Therefore, any reduction in this exposure would have a large impact in re- ducing the adverse effects of radiation

to the general health of the population.

Is it possible to reduce the exposure

and still have the same diagnostic in- formation that appears in visual form

in present-day radiographs? We have

carried out a theoretical study which

concludes that this is indeed possible.

This conclusion seems paradoxical,

inasmuch as an exposure reduction is always accompanied by a loss of image

information. However, our study in- dicates that only a fraction of the image information content in medical

radiographs is visually detected by the

radiologist. Accordingly, it is possible

to reduce the exposure without loss of the visible information obtained previously, providing methods can be

found to display all of the available

image information. In fact, the ex- posure can be reduced to a theoretical

minimum or threshold value without

loss of the visible information on film. Such a lower limit exists because of

the statistical noise introduced by the quantum nature of x-rays. We have estimated the minimum ex- posures required for particular cases in diagnostic radiology from a theoretical relationship between the patient ex- posure and the image information con29
NATIONAL MEASUREMENT LABORATORY

tent. The results for a few of these cases

are shown in figures 1 and 2. Figure 1 shows the minimum exposures at dif- ferent x-ray energies that are theoretically required for the detection with

film of a tissue calcification sphere (0.5

mm diameter) which simulates the

structure involved in the radiographic

detection of breast cancer (mammography) . Present day mammography ex- posures are in the region of 1 roentgen,

primarily because the high film densities produced by such an exposure are necessary to obtain optimum image

Visibility. However, the data in figure 1 indicate that it may be possible to obtain the same visible image information

with exposures that are one to two

orders of magnitude smaller than 1 roentgen, provided that the x-ray spec- trum is confined to an energy band

(±10 keV) in the region of 30 keV.

Figure 2 shows our estimates of the ex- posure reductions that are possible in chest and abdominal x-ray examinations. These data show that for chest

and abdominal x-ray exams with re- spective 1 mm and 0.5 mm diameter

spatial resolutions, the corresponding

exposures may be reduced respectively

from approximately 20 to 4 milliroentFigure 1. Minimum x-ray exposure theoretically required in mammography for the detection of a tissue calcification sphere

(0.5 mm diameter) embedded in 5 cm tissue.

0 20 40

Incident Photon Energy, keV

gens and from approximately 500 to 3 milliroentgens.

The effect of reducing the exposure

in this manner is to produce a very lowdensity, low-contrast radiograph which

provides a poor image on direct visual

inspection. However, with commercially available image processing tech- niques, it is expected that contrast and

density of the image can be suitably en- hanced, essentially restoring image in- formation previously observed. This

procedure assumes that a suitable screen-film combination can be found

to store all of the image information

provided with a given x-ray exposure.

In this discussion, we used as an

example the screen-film system. However, any type of x-ray imaging system

with adequate sensitivity, dynamic

range, and linearity can be used to obtain the described exposure reductions.

An example of such a system is the x-ray image intensifier optically coupled to a TV camera. The video signal passes to an analog-to-digital converter,

and is then stored on a disc. With such

a system, the stored image can be dis- played on a TV monitor with any de-i

sired type of image processing such as contrast or edge enhancement.

The application of these new systems

to radiology requires data on the in- formation content and the correspondI0"4

I0" 3I0"

2

10"' 10°

Exposure, Roentgens

Figure 2. Dependence of the attainable area (cm2

) of the spatial resolution element on

the patient exposure for chest and abdominal (G.I. tract) x-ray examinations for body struc- tures described in Table II of paper by I. W. Motz and M. Danos, Medical Physics 5, 8 (1978).

30
CENTER FOR RADIATION RESEARCH

ing image storage requirements for x-ray pictures. As shown in figure 3, we

have determined the maximum number

of bits per cm2 required for the storage

of x-ray images as a function of the x-ray exposure and the desired spatial

resolution at two x-ray energies (20

and 50 keV) that are in the energy

region used in medical radiology. We

expect that these data will provide the

basis for developing improved radiological systems that will permit either extracting more image information for

a given x-ray exposure or reducing

x-ray exposure for a given image in- formation content.

£u

10 :

1(T

10 c

to -io 2

10 1

10 l

— * s

/ /

(d) :

(e)

\

10" 10

_1 10° 10 1

Exposure, Milliroentgens

10'

Figure 3. Dependence on the number of

bits per cm2 on the x-ray exposure at the image plane for x-ray energies of 20 and

50 keV. and for different spatial resolutions where the area a of the spatial resolution element is equal to (a) 10- 4

, (b) 10-3,

'c) IO- 2

, (d) 10-i. and ( e) 10° cm2 .

New Studies of Electric

Quadruple (EZ) Excitation in

Nuclei Using Electromagnetic

Probes

William R. Dodge,

Nuclear Radiation Division

The (e,p) and (e,a) cross sections

for targets of 38Ni, 60 Ni, and C2 Ni have

been measured in the electron energy

range 16 to 100 MeV. They have been

analyzed using the DVVBA El and E2

virtual photon spectra. Protons are emitted primarily following El absorption but a emission results from a combination of El and E2 absorption.

Since the early days of Fermi, the idea that the effects of the electron's electromagnetic field in electron- nuclear scattering could be related to the real photon cross section has in- trigued nuclear physicists. As early as 1934 Bethe and Peierls had computed

the electric-dipole electrodisintegration cross section of the deuteron and had

compared the result with the photodisintegration cross section. In the next few years electrodisintegration cross

sections for magnetic dipole, electric quadrupole and octopole transitions were calculated. These cross sections were enhanced relative to the electric dipole cross sections. The enhancement was understood on the basis of the strong momentum dependence of these cross sections and of the greater

linear momentum which an electron can deliver to the nucleus compared

to a photon. However, while all of these predictions were based on Dirac

electron theory, they did not include

Coulomb distortion effects (i.e., they

did not allow the incident or scattered electron to change energy and momentum by exchanging longitudinal photons with the nuclear Coulomb field).

Consequently, these early cross section

estimates were grossly inadequate for

all but the lightest target nuclei. Faced with these technical difficulties, electrodisintegration experiments

only played a minor role in mapping

out the properties of the first nuclear

normal oscillation or resonance to be

discovered, the giant electric dipole resonance, GDR. Rather, the GDR

was established and studied through

photonuclear reactions. (As a point

of interest many of the most important early experimental and theoretical

studies of the GDR were done at NBS)

However, during the past seven

years, normal oscillations of the nucleus other than dipole oscillations have been experimentally observed.

Unlike the GDR, these new giant resonances were discovered and studied almost exclusively through inelastic scattering of medium energy hadrons and electrons. Figure 1 shows a pictorial view of

the monopole, dipole, and quadrupole

oscillations of the nucleus (A = 0, 1, 2, respectively). Normal oscillations which do not involve a nucleon spin

flip are labeled AS = 0 while those

which do are labeled AS = 1. Giant

resonances, which are internal collective nuclear motions involving all nucleons, are important because they

involve basic parameters of nuclear matter such as compressibility, polarizability, and the effects of the in-phase

(isoscalar, AT = 0) or out-of-phase

(isovector, AT = 1 ) motions of protons and neutrons. In addition to the

normal oscillations shown in figure 1, the giant isoscalar octopole resonance

has been reported in the literature. While the new multipole resonances (NMR) are commonly called giant

multipole resonances they are in fact

less intense than the GDR. Because of the relatively small intensity of the NMR some method either to suppress

the isovector GDR or enhance the NMR is needed to facilitate the study

of the NMR. In hadron excitation of the isoscalar NMR, the method is to use an isoscalar projectile, thus suppressing excitation of the isovector

31
NATIONAL MEASUREMENT LABORATORY

GDR. In electron scattering or electro- disintegration the method is to enhance

the NMR excitation through the strong

momentum transfer dependence of the

cross section in the manner alluded to

earlier. Fortunately, almost simultaneously

with the discovery of the NMR, a theory which provided a solution of

the Dirac equation in a point Coulomb

field with no kinematic restraints such

as characterized earlier theories became

available. This theory, called a dis- torted wave Born approximation

DWBA theory in the trade—showed

that Coulomb distortion of the electron wave function greatly enhanced the

"virtual photon" intensity spectra,

especially for E2 and higher electric multipoles.

AS = 0 AT = 0 THOMSON

A = 1

A = 2

AS = 0 AT = 0 E20

Figure 1. The giant monopole, dipole, and quadrupole (\ = 0, 1, and 2, respectively)

oscillations of the nucleus. Normal oscillations not involving nucleon spin flips are labeled AS = 0 while those which do are labeled AS = 1. Normal oscillations which involve

in-phase motions of protons and neutrons are labeled AT = 0 while those which involve out-of-phase motions are labeled AT = 1.

The virtual photon spectra are those

functions involving only electron kinematics which relate photo- and electro- disintegration cross sections. Figure 2

shows El and E2 DWBA virtual photon intensity spectra for Ni. The large

difference between El and E2 virtual photon intensity spectra make it relatively easy to separate transitions in- volving El and E2 excitations in an

electrodisintegration experiment.

The upper half of figure 3 illustrates the conventional photoproduction ex- periment, in which bremsstrahlung pro- duced by electrons of energy E0 strikes a radiator of high atomic number. The

real photons—with intensity spectrum,

Ny (E0 ,o>), which of course does not depend on multipolarity of the reaction

which the photons will induce—are absorbed by the target nucleus, which

then emits particle x. The yield of x for

real photons is given by

Y(7,x) (Eo) — (1)

Eo — me N \ 7 (£,„, L

thres

(7,x) (w)do

where o-L

(y,x) is the photodisintegration cross section for the Lth multipole and w is the excitation energy. The lower half of figure 3 shows

schematically an electrodisintegration

experiment in which the electrons strike the target nuclei directly. The electro- production yield, Y u., x) , is given by an

expression similar to Y,yiX ), except that the spectrum of virtual photons seen by the target nucleus now depends on

the multipolarity L of the nuclear transition. In this case

Y (e,x) (E0 ) —

E„

(2)

mc NL

e (E0, oj) thres

cr (tu)dc

(7,x)

32
CENTER FOR RADIATION RESEARCH

E'

10 20 30 40 50

E (MeV)

Figure 2. Virtual pho'on spectra for El and E2 transitions calculated in the distorted wave Born approximation for Ni (Z = 28).

target

E„

Bremsstrahlung

radiator

e- Ec

real photons

product

detector

virtual photons

product

detector

Figure 3. Schematic diagram of a typical photodisintegration experiment (top) and of

a typical electrodisintegration experiment (bottom).

Since Ne L(E0 ,o>) rapidly increases as L increases for a given E0 and u elec- trodisintegration yields are very sensitive to the multipolarity of the transition.Thus armed with a good virtual photon theory the discover}- of the NMR by inelastic hadron scattering

experiments, and the arrival of an

energetic guest worker, E. Wolynec.

from the University of Sao Paulo, we

set about trying to exploit these developments.

For our initial experiments, we se- lected targes of 58Ni, 60Ni, and 62Ni.

Our measurements were made with

electrons in the energy interval be- tween 16 and 100 MeV provided by

the NBS linac and with a magnetic

spectrometer capable of separating the many charged particle groups (such as protons, deuterons, tritons, and alphas)

•vhich can be copiously produced in a nuclear disintegration. At each of

several spectrometer-incident-electron-

:>eam angles we measured proton and

alpha particle energy spectra as a func-

- -2

£

10'

24 32 E 0 or co (MeV)

Figure 4. The 58Ni (e,a) and (e +7, a) excitation functions. The open circles are the results for pure electroproduction and the filled circles are the results for electroproduction plus

photoproduction. The histograms are El and E2 cross sections deduced from these

excitation functions. E0

is the incident electron energy and u is the excitation energy.

33
NATIONAL MEASUREMENT LABORATORY

tion of the incident electron energy, E0. These energy spectra were inte- grated over all particle energies and

then the resulting angular distributions

integrated over all angles to obtain

Y(e , x) and Y( yiP) (E0). The yields

Y( y, p) (Eo) and Y(e,p)(E0) were both

accounted for when the previously

measured cr

( y, p) (o>) was inserted in eqs. (1) and (2) using the electric-dipole

virtual photon spectrum for E0 =16 to 50 MeV. These results confirmed the expectation that photon emission from

the Ni isotopes was a predominately El

process near 20 MeV.

On the other hand, the measurement

of the (y,a) and (e,a) yields showed

that a sizable fraction of the a emission comes from the giant isoscalar quadruple resonance near 16 MeV. Figure

4 shows our 58Ni(e,a) excitation function. The open circles are the results for pure electroproduction and the filled circles are the results for electroproduction plus photoproduction when a 0.217-g/cm2 Ta radiator is inserted in the electron beam. The histograms la- beled El and E2 are the cross sections

which produce the best fit to both

the electroproduction and the electro- production plus photoproduction excitation functions. Note that attempts to

fit the excitation data wih pure El or pure E2 cross sections were not satis- factory.

The striking result of our measure- ments is that the alpha decay mode of

the isoscalar E2 resonance exceeds

statistical estimates by a factor of 4. Said another way the alpha decay channel accounts for 25 percent of the E2

isoscalar-energy-weighted sum rule value. Similar conclusions regarding the decay of the oxygen isoscalar E2 res- onance have been reached by the Heidelberg-Jiilich group on the basis of a

16 0(a,aa") coincidence measurement. Theorists have long speculated

that alpha clusters might be continually

forming and dissolving in the nuclear

surface. A quadrupole oscillation might

preferentially shake off these conjectured alphas. At the present time these

are only speculations and no theoretical explanation of our results is available.

Glueballs and

Quark States

Sydney Meslikov,

Nuclear Radiation Division

A

JL jL systematic enumeration of the

glueball spectrum of QCD and other

SU(n) gauge theories yields J

PC odd . even . and 0 states in addition to the usual quark model

states. The properties of these new

states are discussed.

Quantum Chromodynamics (QCD),

our present theory of the strong inter- actions, describes hadrons as being

composed of colored quarks and anti- quarks. Color is a new degree of free- dom associated with the strong inter- action. All single quarks have color. So

far all observed hadrons are colorless. The strong force between quarks and

antiquarks is mediated by the exchange

of massless colored gluons, just as the

electrical force between electrons and

between electrons and positrons is mediated by the exchange of massless

photons. Convincing manifestations of

the existence of the gluon have been

displayed in recent e + e~ annihilation

experiments at 27 and 31.6 GeV. QCD

implies that glueballs (gluonia) should

exist [1]. These are mesons which are

' composed entirely of gluons; they contain no quarks and have no color. A

current activity of the NBS Elementary

Particle Program is the study of gluonium, i.e., to set criteria for its exis- tence, and to understand how to dis- tinguish gluonic states from the usual

quark states experimentally.

Understanding the formation, structure, and decay of gluonic systems re- quires a full knowledge of the spec- troscopy of "quarkful" mesons, i.e., those described in the usual way as quarks and antiquarks moving with a

relative orbital angular momentum L. Fortunately, the latter is a major component of high energy activity of NBS

as manifested by work on charmonium

and other "onium" spectroscopy (2).

Some of the simplest differences be34
"ENTER FOR RADIATION RESEARCH

:ween the two spectroscopies are simple, i.e... in the quantum numbers JPC

(J=;total angular momentum. P=parity, C=charge conjugation) . The quantum number assignments depend on

whether the glueballs are made of two,

three, or more gluons, and whether the quark systems with which they are compared are simple quark-antiquark

systems, or are multiquark composites.

For example, states with J PC =0~ _

, 0~~, 2'~ occur in the 3 gluon system

but cannot be made with either 2

gluons or the usual qq systems. Other

possible distinguishing factors such as modes of production and decay are

also currently being investigated.

The charmonium quark structure work has continued with emphasis on

the existence and masses of the pseudoscalar cc state nc and its radial excitation n'c . A reanalysis [3] of the

German e _

e _annihilation data which

had led them to claim the existence of

the n,;. at 2.83 GeV led to the conclusion that the claimed state was at most

a 2-3 standard deviation effect. Recent

experimental searches at the Stanford

colliding electron-positron ring have

found no state at 2.83 GeV, but strong

indications of a state at 2.98 GeV

which may well be the ne

. Work is continuing on the properties of the nc

. and n'c .

References

[1] Bjorken, J. D. Electron-Positron Annihilation. Talk given at European

Physical Society High Energy Physics

Conference, Geneva, Switzerland. June 27-July 4, 1979. SLAC-PUB2366.

[2] Fishbane, Paul M., Horn, David and

Meshkov, Sydney, Spin forces in char- monium spectroscopy. Phys. Rev. D19, 288 (1979).

[3] Samios. N". P. and Meshkov, Sydney

CBNL 25926) 1979.

Quantum Basis for Visible

Region Radiometric Standards

Jon Geist, Edward Zalewski, and

A. Russell Schaejer,

Radiometric Physics Division

silicon photodiodes are the detectors most widely used for radiation

measurements in the visible region of the electromagnetic spectrum. When

employed in this capacity, their cali- bration is based on the thermal physics

of either electrical substitution radiometers or the blackbodies. The possibility of basing these calibrations di- rectly on the quantum physics of the

internal photoelectric effect in silicon has been suggested frequently. Recently, we demonstrated a quantumbased calibration that is accurate to within =0.3 percent. This uncertainty

compares favorably with that currently

available from the best thermal based

calibrations, and, for silicon photodiodes, the new procedure is more convenient as well.

In a study of the radiometric properties of several high quality (high

resistivity, shallow junction) silicon photodiodes. we found that the quantum efficiency, after correcting for re400 500 600 700 800

Wavelength (nm)

Figure 1. The absorptance (one minus the

reflectance ) and the internal quantum effi- ciency for five silicon photodiodes.

flectance losses, was within 1 percent

of unity in the red region of the spec- trum. The range of internal quantum

efficiencies for five photodiodes along

with their measured absorptance is shown in figure 1. At wavelengths

longer than 650 nm the internal quantum efficiency is 0.99 or greater. Because it is so close to unity, there are

less stringent requirements on the ac- curacy of the theoretical model used to predict this small difference. Photon energies in the wavelength

region longer than 530 nm are less than twice the band gap. so that for every photon absorbed only one elec- tron-hole pair is created. (Photons

absorbed by the free carriers in the conduction band are a negligible frac- tion of the total absorption.) Photons

that penetrate to the vicinity of the p-n

junction before absorption have a quantum efficiency of one because the minority carrier thus generated is immediately swept by the electric field through the depletion region. It is only

in the regions away from the p-n junction that recombination or trapping

10 F—i

1 1 1 1 1

l i i i i i i i i 400 500 600 700 800

Wavelength (nm)

Figure 2. Variation of the absorption coeffi- cient of silicon in the visible region of the spectrum.

35
NATIONAL MEASUREMENT LABORATORY

can be expected to decrease the quantum efficiency.

Figure 2 shows the variation of the

silicon absorption coefficient with

wavelength. The absorbance is high in the blue region, decreasing by about

two orders of magnitude toward the

red. Therefore, in the blue region the

majority of the photons are absorbed

near the front surface. Here the recombination probability, 1-P, where P is the collection efficiency, is significant

because of the large density of majority

carriers and possible surface effects. One might expect that P(x) would be

a complicated function of distance, x, into the diode; however, we have found

that a model which postulates a layer

of constant recombination probability

followed by a region of zero recombination probability fits our data quite well. An example of the quality of the fit of

this model to the data for one of the

silicon photodiodes is shown in figure 3. Other more physically significant models for the recombination probability

distribution function were also fit to these data. They only affected the shape

100 I 1 I I I

I

I I I 1

J

I I I I I

I I I I

Mil I I I I I I • ' ' '

I ' 350 450 550 650

Wavelength (nm)

Figure 3. The curve obtained from the model of the internal quantum efficiency of

silicon (equation 1) and the data for a

typical photodiode.

of the function at wavelengths shorter

than 400 nm.

In the long wavelength region of the spectrum the photons can penetrate beyond the junction into the bulk region

of the diode before being absorbed.

Because the region between the junction and the rear of the diode has a low concentration of impurities and

defects, the lifetime of the electron-hole

pair is long relative to the diffusion rate back to the junction. Therefore, recombination in the bulk region also has a

very small effect on the quantum effi- ciency.

The simplest mathematical model

which fits our data is of the form

K P+(l-P) exp(-«(A)J)

B

a(A)L2 (1)

Here e is the internal quantum effi- ciency, P is the collection efficiency,

rt(A) is the spectral absorption coeffi- cient of silicon, J is the junction depth,

B is the thickness of the bulk region

and L is the minority carrier diffusion

length. The coefficient K can be inter- preted as the calibration factor of the

electrical substitution radiometer used

to determine the external quantum effi- ciency. From our data the values obtained for K ranged from 0.9987 to 1.0033. These results are consistent

with the estimated accuracy of the

electrical substitution radiometer we

used and the assumption that absorption of a visible photon in silicon pro- duces one electron-hole pair.

Equation 1 can now be used as the

basis of an absolute calibration of a spectrally non-selective radiometer. In order to do this, the internal quantum

efficiency of a silicon photodiode is measured in the blue and near infra- red spectral regions relative to that in the red. These data plus the published

values of « (A) at the appropriate wavelengths can then be used to obtain K

from eq. ( 1 ) . The types of measure- ments involved in this procedure, ex- cept for the current from the photodiode, are all relative as opposed to absolute measurements. That is, the re- flectance, absorption coefficient and relative spectral response all involve a

ratio of similar measurements—no absolute radiometric measurements are

required.

This research has demonstrated the

feasibility of adopting a new measure- ment basis for absolute radiometry

based on quantum physics instead of thermal physics. High quality silicon photodiodes can now be used for meas- urement of visible radiant power with an accuracy comparable to that of standards presently available. It is ex- pected that improvements in the measured values of the absorption coeffi- cient of silicon plus a more complete

knowledge of the physics of these devices will yield more accurate and convenient radiometric standards. The

long term goal of future work in this area would be the development of photodiodes whose radiometric properties could be accurately determined

from a knowledge of their structure and physical properties instead of the

relative radiometric measurements now

used. Possible approaches are elec- tronic test structures built into the photodiode and calculations based on

device fabrication history.

36
1J

Center for Thermodynamics

and Molecular Science

\°eter L. M.

Heydemann

lalph Klein

\lbert E. Ledford,

Jr.

,»urfaee Science

Zedric J. Powell ames J. Carroll

iichard R. Cavanagh*

Villiam L.Clinton**

Villiam F. Egelhoff

,

Jr. oseph Fine ohn W. Gadzuk

Iteven M. Girvin

David W. Goodman

jregory L. Griffin* David K. Hoffman**

ack E. Houston**

"errence J. Jach

iichard D. Kelley •am H. Kim**

"heodore E. Madey

Mian J. Melmed

ioger L. Stockbauer ohn T. Yates, Jr. Tiemical Kinetics Ving Tsang

Walter Braun

Robert L. Brown

Robert F. Hanapson,

Jr. ohn T. Herron

Tadahiro Ishii**

vi. Odo Klais**

vlichael J. Kurylo,

III Mian H. Laufer

Richard I. Martinez

Norman C. Peterson James A. Walker

Francis Westley

Robert E. Huie, III Chemical

Thermodynamics

David Garvin

Stanley Abramowitz

George T. Armstrong

Pierre J. Ausloos Kenneth L. Churney

Jennifer C. Colbert Howard J. DeVoe**

David A. Ditmars

Eugene S. Domalski

Robert N. Goldberg

Thomas L. Jobe, Jr. Duane R. Kirklin Sharon G. Lias Michael Mautner

George W. A. Milne**

Ralph L. Nuttall Vivian B. Parker Edward J. Prosen Richard E. Rebbert

Richard H. Schumm

Frederick P. Schwarz

Lewis W. Sieck

Bert R. Staples

Donald D. Wagman

Jin T. Wang**

Stanley P. Wasik

William J. Webb

Harry D. Youmans**

(Continued)

T,

selected thermophysical properties of metals

are measured with millisecond time resolution by Ared Cezairliyan (foreground) and Archie

"vliiller. The metal sample is heated electrically

'n the metal cylinder shown near the center of

'he photograph. Measurements are made either aider vacuum or in selected inert atmospheres.

The temperature of the sample is measured with

in optical pyrometer seen to the right of the sample container. The large lamp in the foreground is a tungsten filament lamp used for

'he calibration of the optical pyrometer. The

vyrometer, designed specifically for the project,

Is a unique instrument, capable of millisecond

resolution. The laser pulse interferometer which

n easures, with millisecond resolution, the thermal expansion of the sample is on the fable around the sample container.

he Center for Thermodynamics and Molecular

Science (CTMS) provides the scientific community with

the means to measure, interpret, and control accurately

the behavior of complex chemical systems. Examples of

such systems, whether manmade or natural, are ubiquitous: heat engines, lasers, chemical reactors, distillation columns, catalytic processes, the oceans, even biological

systems.

To researchers in this Center, these systems represent

mixtures of chemical substances in one or more phases,

either in equilibrium or in the process of approaching

equilibrium and interacting with both radiation and the

containment surface.

The problems CTMS scientists address are often

solved generically, with the result that the solutions are applicable to a variety of specific practical questions.

For example, the quantification of the behavior of fluids can be applied to the development of standards for pres- sure measurement and to the provision of the values of

those properties of steam needed in the design of electric

power plants. Methods used to calculate the electronic

properties of transient species in the atmosphere are applicable to transient species in chemical lasers. Procedures for measuring of the rates of chemical reactions

relate equally well to atmospheric chemistry or to combustion kinetics. Methods developed to measure the heat

evolved in chemical or physical transformations can be

applied to study biological cell metabolism or to determine the stability of oil in the simulated environment of an internal combustion engine. Accurate spectroscopic

data can be used to calculate the concentration of atmospheric pollutants; such data can also serve as a basis for

calibrating laser frequencies or identifying molecular

species in interstellar clouds.

A brief review of recent work within CTMS will illus- trate the broad applicability of CTMS research and

development.

Researchers involved in surface science are concerned

with developing accurate methods to characterize the

physical and chemical properties of surfaces. Surface

science has been a rapidly developing field over the past

10 years, with important applications in corrosion, catalysis, air pollution, and integrated circuit technology. This

39
NATIONAL MEASUREMENT LABORATOR

Thermophysics

Harold J. Raveche

Francis W. Balfour** Anita Calcatelli** Ared Cezairliyan

Ren-Fang Chang

Jack H. Colwell Theodore D. Doiron*

Nils E. Erickson Vincent J. Fratello* John S. Gallagher Robert W. Gammon

Lester Haar

John R. Hastings

Richard F. Kayser,

Jr. John M. Kincaid

Max Klein James W. Little Rosemary A. Mac Donald

John L. McClure

James B. Mehl

Archie P. Midler Michael R. Moldover

Malcolm S. Morse Raymond D. Mountain

E. Gordon Powell** Jan V. Sengers

Johanna M. Sengers G. Bruce Taggart**

Donald H-N. Tsai Meyer Waxman

Arnold Wexler**

Harold W.

Woolley**

Molecular

Spectroscopy

Merrill M. Hessel Stephen E. Bialkowski* Arthur J. Grimley*

Jon T. Hougen

Marilyn E. Jacox

Paul S. Julienne David S. King

Morris Krauss

Walter J. Lafferty

Frank J. Lovas

Arthur G. Maki, Jr. Frederick H. Mies

David B. Neumann

Hideo Okabe

William B. Olson John C. Stephenson

Walter J. Stevens Lawrence L. Strow**

Richard D. Suenram

Alfons Weber

Richard F. Wormsbecher*

Post Doctoral Staff Research Associates and Guest Workers

1

V"

1

:.

growth has resulted largely from the development of si face characterization techniques and instrumentation

greatly improved specificity and sensitivity, from an

creased understanding of surface properties and pre

esses on a microscopic scale, and from the success! application of modern measurement methods and the

retical concepts to a wide variety of scientific and tea

nical problems.

Yet, in spite of the growth in this area, serious me£

urement-related problems and limitations remain. T

Surface Science Division works to develop quantitati

understanding of the details of the measurement proce

as they relate to surface science and to apply this knou

edge to the development of new and improved measur ment methods and standards. Surface properties such

composition, atomic structure, electronic structure,

atomic motions are being probed through a great varie

of techniques, usually involving the interactions of ene

getic particles (ions, electrons, atoms) or the interactk

of radiation with surfaces. Understanding of these pro

esses on an atomic or molecular scale is critical to tl development of reliable, quantitative measureme

methods.

Researchers are engaged in work on electron stimi

lated desorption of molecules from surfaces, the surfa(

characterization of certain catalytic processes, the appf

cation of infrared spectroscopy to the characterizatiC

of molecules absorbed on surfaces, field ion microscop

ion sputtering, the application of auger spectroscopy 1 the analysis of airborne particulates, and the develo]

ment of theories for describing the interaction of photor

or charged particles with atoms and molecules.

In chemical kinetics, our scientists develop methoc

to measure the rates of chemical reactions and to dete

mine the mechanisms of reacting systems. They also pre vide evaluated kinetics data and measure particular ral constants when data of sufficient accuracy are not avai

able elsewhere. In the chemistry of the atmosphere or i combustion, dozens of reactions often occur simultant

ously, with intermediate products of a transient natur playing a significant role. Frequently, knowledge of th nature of these intermediate products is purely specuk

tive. The possible reactions they undergo must be cor sidered in any quantitative description of a chemic;

40
^CENTER FOR THERMODYNAMICS AND MOLECULAR SCIENCE

process if the behavior of the system over time is to be

dequately characterized,

ii Programs in the Chemical Kinetics Division are con- fined with the intermediates that occur in the atmospheric oxidation of olefins, the upper atmospheric

chemistry of fluorocarbons, the processes that occur in

:. lanetary atmospheres, and the high temperature decomosition of hydrocarbons. Important intermediates and

i leir reactions are being characterized. New methods for

-identifying species are being investigated and new methods for initiating chemical reactions, such as laser in- duced decomposition, are under study. Computer codes

Ipr modeling these systems are also being developed. In- deed, the linking of computer models and laboratory

leasurements will characterize the activities of the

Chemical Kinetics Division in the future. The evaluated

;|: ate constants and related kinetics data provided by this division are used in large scale computer models of

| pper atmospheric chemistry.

In the area of chemical thermodynamics, we provide

accurate chemical thermodynamics data and standards,

scientists develop state-of-the-art instruments for measring thermochemical properties. In support of the NBS

Recycled Materials Program, researchers are designing

he world's largest combustion calorimeter. It will be

j,apable of measuring the heat evolved during the combustion of multi-kilogram-sized samples. Although the

yrincipal purpose of this calorimeter will be to determine

f ,he calorific value of refuse-derived fuel, it will serve as yart of a general research facility for studying combustion processes. At the other extreme, microcalorimeters

apable of measuring the heat evolved during metabolic

•rocesses have already been designed and built. Laboratories throughout the world use microcalorimeters based

>n the NBS designs.

c*

.. . J Other efforts include work in determining equilibrium

constants associated with ionic properties, high accuracy

, >omb calorimetric measurements for certifying heat- constant Standard Reference Materials (SRM's), the

I levelopment of SRM's to serve as standards for the calor imetric assay of radioactive plutonium, and the calculation of the high temperature thermodynamic properties

)f refractory compounds from molecular properties.

Also, Division scientists are studying the heterogeneous

and homogeneous decomposition of halocarbons and ni- trous oxide (contributing to models of the ozone layer in

the stratosphere) and developing a new mass spectrometric technique for characterizing fossil fuels. A major

effort of the Chemical Thermodynamics Division continues to be evaluating and publishing of selected values

of thermodynamic properties under the auspices of the

National Standard Reference Data System. 1 Whereas the Chemical Thermodynamics Division is concerned primarily with the chemical properties of substances, the Thermophysics Division concentrates on the

physical properties of substances. The Dynamic Measurements Program involves the measurement of such properties as thermal diffusivity, thermal expansion, heats of

fusion, and heat capacities under conditions requiring an

extremely rapid response. At present, a millisecond sys- tem is operational and a microsecond facility is under

development. The capability now exists to measure both

equilibrium and nonequilibrium properties at elevated

temperatures.

The equation of state program is concerned with: the

accurate measurement of the temperature, volume, and

pressure relationships in fluids and complex fluid mixtures, the measurement of such properties as dielectric constants near critical points, the development of quantitative theories to explain the behavior of these properties in the vicinity of critical points, the development of new measurement techniques to measure these properties more accurately, and, the development of mathematical

correlation models for the properties of a variety of fluids used in energy generation.

These correlation models have been used, for example,

to calculate accurately the properties of steam over wide

ranges of temperature and pressure. A spherical acoustic

resonator has been designed to accurately measure of

the velocity of sound in gases, from which thermophysical properties can be calculated. A laser light scat- tering instrument has been constructed to measure

transport and equilibrium properties with the greatest

accuracy currently achievable. In the future, properties

of complex mixtures of polar and nonpolar compounds

will be measured and interpreted. Such compounds are

1 See pages 99 to 114 for a discussion of the NSRD program.

41
NATIONAL MEASUREMENT LABORATORY

found, for example, in systems for tertiary oil recovery.

The Thermophysics Division provides methods and

standards for the measurement of pressure. Pressure

measurements in industrial and government laboratories

are made traceable to NBS standards through measure- ment assurance programs developed and maintained by

division scientists. One current program consists of meas- urement assurance services (i.e., transfer standards

consultation, round robin intercomparisons) for aircraft

altimetry measurements. Highly accurate pressure meas- urements also find application in materials properties

research. For example, a new high pressure phase of

CCli has recently been discovered which shows that a substance can have dual melting curves. To meet the demands for static and dynamic properties data, a theoretical capability for prediction is required for systems not accessible with existing measure- ment methods. In response to this need, the Thermophysics Division has a statistical physics program in which models based on molecular properties are being

developed to predict the behavior of bulk matter. The work includes computer simulation of the behavior of properties and development of analytical

theories. Both equilibrium phenomena and such nonequilibrium phenomena as metastability are being studied.

Recent measurements of anomalously high values of solid

heat capacities at high temperatures are being thoroughly

investigated by the computer simulation of the bulk

properties of solids taking into account the effects of

large amplitude, anharmonic lattice vibrations. Theoretical work is also underway to predict the behavior of

systems as they undergo phase transitions and to predict

behavior under conditions of intense pressure and high

temperature.

In molecular spectroscopy, scientists are concerned

with the electronic properties of molecules and their in- teractions with electromagnetic radiation. The laser-high

resolution spectroscopy group devises new measurement

techniques for generating highly accurate spectra and

computer models for interpreting and extrapolating the

values of molecular properties obtained from the spectra.

A laser diode spectrometer for measuring and analyzing infrared spectra of atmospheric constituents is being

developed. The group is also engaged in providing wavelength standards that will be two orders of magnitud more accurate than those presently available. This effoi

is a response to requests from the instrumentation indu;

try, industrial and government laboratories, and stanc ards groups. This group also uses nozzle-beam an single-moded laser techniques to determine the electroni structure of molecules and to improve the resolution anl

sensitivity of molecular electronic spectra measurement:

The microwave spectroscopy group furnishes accurat microwave spectra to astronomers for use in studying th chemistry of interstellar clouds. Researchers determin

the structure of small, unstable molecular intermediate

in atmospheric or combustion processes through analys

of the microwave spectra. NBS scientists have discovere

new compounds such as ethylenimine and dioxirane i these studies.

The laser chemistry and photochemistry groups hav

as their objective the quantitative understanding of th chemistry of molecules when they interact with hig

power laser radiation. The quantum chemistry grou

develops and evaluates methods for calculating from fin principles the electronic, structural, and chemical prop

erties of molecules. Transient species in such systems ? lasers play a critical role in the functioning of the system

but, because of their very small concentration, cannot b

observed experimentally. The quantum chemistry grou

calculates reliable values for the molecular properties q

species encountered in applied problems ranging froi upper atmospheric chemistry to laser development fc defense and energy purposes.

This brief description touches on the activities of th approximately 100 scientists who contribute to the pro

grams of the Center for Thermodynamics and Molecula

Science. Each year, CTMS researchers publish approx 1 mately 300 technical papers and reports and present 25

papers at national and international meetings. They cc operate extensively with scientists in universities, indu:

try, and other Government agencies and participate i national and international standards activities. The technical highlights that follow have been chose

to illustrate in more detail the breadth of the Center

activities. They represent work which is new or partici

larly exciting, but they by no means include all that ha

been significant or new during the past year.

42
CENTER FOR THERMODYNAMICS AND MOLECULAR SCIENCE

Bond Energies and Chemical

Reactivity

Wing Tsang

Chemical Kinetics Division

Bond energies are the essential ingredients for understanding chemical reactivity. Recent studies at NBS

suggest that many long accepted values

I are in error. It is demonstrated that the T new bond energies provide a sound

basis for understanding cis-trans iso- merization and small ring decyclization

processes.

ilf The course of any chemical reaction

: I is marked by breaking and forming

li chemical bonds. For a quantitative

; understanding of chemical kinetics, a

I = knowledge of the appropriate bond

energies is necessary. Chemical kineticists usually define the bond dissociation energy as the enthalpy of reaction

' for the process AB A-fB at 298 K,

r where A and B are molecular fragI ments. Note that this is a thermodynamic quantity and is directly related

to the enthalpies of the molecule and

] fragments. However, since A and B

I are frequently unstable, direct deter-

: minations through heat of combustion

j measurements are not feasible. IndiI rect techniques must therefore be used,

( and for all except the smallest fragj ments, this means kinetic methods,

j Thus, one selects a reaction involving

the unstable species in question and

measures the rate of the reaction in

n both directions. Detailed balancing

' yields an equilibrium constant which

» can then be converted to heats of reacI tion if structural information for all

j species is known. Alternatively, a heat

I of reaction can be calculated from the temperature dependence of the equilibrium constant as measured across a range of temperatures. For all these measurements the instability of the

: molecular fragment introduces severe

1 uncertainties and the heats of formation or bond energies so determined are

necessarily fraught with assumptions.

Nevertheless, for bond energies to play

a role in predictive theories, accuracy

requirements are extremely stringent.

For example, in the cases where the bond energy is directly reflected in the

rates for a particular process, an error of 6 kJs will result in an order of magnitude deviation in rate at 298 K and

a factor of 2 at 1000 K. The history of gas kinetics has been

marked by repeated determinations of bond energies and, as new techniques

and understandings have developed, by

continual revisions of what had been

thought to be well established numbers.

Recent work at NBS has led to new in- sights on such measurements. The validity of this work is suggested by the

self-consistency of the results as derived from a wide variety of measurements. Specifically, the NBS studies have concentrated on the reactions

kf A2 <=> 2A

kb (where A = ethyl, isopropyl and t- butyl radicals) , and have shown that high temperature determinations for kf at 1100 K (carried out at NBS) and a reinterpretation of low temperature

measurements, are completely consistent with existing results on the re- verse reaction kb . These combined

studies cover a range of rate constants

of 35 orders of magnitude. Thus, for these "over-determined" systems we

arrive at a consistent set of thermal

properties for the unstable species A.

Furthermore, the calculated entropies agree with present day ideas on the molecular structure of the species in- volved. The enthalpies are, however,

significantly different from those determined by scattered measurements of

the type described earlier. The NBS results are thought to be more reliable due to the larger data base and consistency of determinations. Generalizing from these results, we arrive at the

values for the heat of formation of simple organic radicals presented in table 1. Also included for comparative

purposes are previously recommended

values. It should be noted that the general trend of the new data is towards

increasing bond energies. In other

v/ords, most aliphatic hydrocarbons are much more stable than previously as- sumed.

An interesting consequence of these

findings, which serves to illustrate the use of bond energies as a predictive

tool, deals with the isomerization of small ring hydrocarbons. This is a subject of great current interest for which

a considerable volume of data exists. The study of such highly strained sys- tems pushes chemical theory to its limits. The simplest way of visualizing

such processes is to postulate a biradical intermediate formed from the

Table 1. Comparison between old and new values for carbon-hydrogen disso- ciation energies.

Previous

recom

New NBS mended

values values

Compound (kJ/mol) (kJ/mol

Ethyl-H 420 412

Isopropyl-H 407 395

N-Propyl-H 420 409

S-Butyl-H 407 398

I-Butyl-H 422 417

T-Butyl-H 404 387

T-Amyl-H 405

Cyclohexyl-H 416 395

Allyl-H 377 369

Isobutenyl-H 373

1-Methylallyl-H 375 355

Propargyl-H 384 393

3-Methylpropargyl-H 385

Benzyl-H 372 352

43
NATIONAL MEASUREMENT LABORATORY

Table 2. Comparison between predicted and experimental activation energies

for ring opening reactions.

Experimental From NBS bond Previous

Compound (kJ/mol) energies (kJ/ mol) results (kJ/mol)

Ethylene (2-membered ring) 275 272 255

Cyclopropane 268 255 238

1,2 Dimethylcyclopropane 259 263 250

1,1,2,2, Tetramethylcyclopropane 228 232 188

1,2 Dimethylcyclobutane 257 251 230

Vinycyclopropane 208 208 108

breaking of the highly strained C-C

bond, followed by appropriate rotations and finally recyclization. The activation energy for the isomerization is therefore: E = AHT (ring opening) -f- RT and AHT (ring opening) = 2DT (C-H)-DT (H-H) + aHt (hydrogenation) where DT is the bond dissociation

energy at temperature T and the heat

of hydrogenation refers to the conversion of the cyclane to the appropriate

aliphatic compound. Since the last two

terms are accurately known, the dependence of the activation energy on

the bond dissociation energy is quite

clear. Such calculations performed

using the older bond dissociation energies have always resulted in values ap- proximately 20 to 40 kJ lower than experimentally measured. As a result, it has been necessary to postulate a 20 to 40 kJ activation barrier for recyclization or rotation. Structural and quantum mechanical considerations are however at variance with the assumption of such a barrier, making this subject quite controversial. With the new

bond energies, we obtain the activation

energy results summarized in table 2. It can be seen that the deficit has been

almost completely removed and in most cases is probably within the ex- perimental uncertainty. Thus, it is now

clear that the biradical mechanism does

provide an accurate quantitative basis for predicting the activation energy for

the isomerization of small ring compounds.

Dynamic Thermophysical

Measurements

A red Cezairliyan, Thermophysics

Division

JL. ^L»esearch has been carried out

to explore, by dynamical means, the thermophysical behavior of solids and

fluids under extreme conditions of rapid heating. In addition to studying

properties of materials of interest in the areas of high temperature sciences and

technologies, we have attempted to:

• clarify the anomalous behavior of

the heat capacity of metals at high

temperatures (fig. 1), • determine the effect of dynamic

heating conditions on vacancy generation and migration,

• study superheating in phase transitions, and

• investigate the non-linear contributions to the transport phenomena

under large thermal and electrical gradients.

Because of limitations in the conventional steady state techniques at high

temperatures (above 2000 K), we have

developed a dynamic technique for

1 conducting the research. For conven-"

ience, our activities may be divided into

1

1000 1500 2000 2500 300I Temperature, K

Figure 1. Heat capacity of molybdenum.

The results indicate considerably higher

values at high temperatures than the classi- cal value of 3R (about 25 J mol- 1 K- 1

). 44
CENTER FOR THERMODYNAMICS AND MOLECULAR SCIENCE

two categories, namely "millisecond"

and "'microsecond" systems—terms in- dicative of the speed of the dynamic

experimentation. In either case, the technique is based on rapid resistive self-heating of a specimen by an electrical current pulse and on measuring

the pertinent experimental quantities

with appropriate time resolution. In the millisecond system, we undertook two types of activities : ( 1 ) measuring selected thermophysical properties of new materials with the facility

developed earlier in this program, and

(2) developing new capabilities to enable the measurement of additional

properties. In the microsecond system, we continued to develop a facility for extending accurate measurements to temperatures above the melting points

of materials and up to 6000 K. Some

of the highlights are given below.

With the "millisecond" system, we

, measured heat capacity, electrical resis-

, tivity, and thermal emittance of a

graphite composite used as a high tem-

. perature material in aerospace applications. Similar measurements, in addition to its melting point, were also performed on palladium. It should be

noted that the melting point of palladium is a reference point on the

International Practical Temperature

Scale and that its value is based on

measurements performed over 40 years

ago. New developments, as additions

to the millisecond system, include a capability to measure thermal expansion simultaneously with other properties. We designed, developed, and

performed a feasibility study of a laser pulse interferometer for this purpose

(fig. 2) . Another addition was a tech- nique for measuring heat of fusion of

metallic materials in experiments of sabsecond duration. To check the

technique, we performed measurements

cn niobium. Other new areas that we

high-speed pyrometer

to the detector

Figure 2. Schematic diagram of the laser pulse interferometer for rapid measurement of thermal expansion.

experiment

trigger timers and

pulse generator

specimen

c __-ri temperature

""""—LI measurement

spark O gap trigger

circuit

standard

resistor rim

tttt

charging

circuit

capacitor

bank

Figure 3. Functional diagram of a dynamic (microsecond-resolution) thermophysical measurements system. 45
NATIONAL MEASUREMENT LABORATORY

embarked on include (in conjunction

with S. Block's group, Center for Materials Science) developing a meas- urement technique under combined

conditions of high temperatures and

high pressures. Research is presently

underway to assess the feasibility of pulse heating specimens in a diamond

anvil high pressure cell. The objective

is to determine the melting temperature

of materials as a function of pressure (up to above 10 GPa).

As an important step in developing an accurate microsecond resolution

thermophysical measurement system

(fig. 3), we designed and constructed a prototype pyrometer which incorporates a digital data acquisition system.

The pyrometer has the capability of measuring the temperature of a rapidly

heating specimen every one microsecond with a resolution of about 0.1 percent.

We have also advanced the state-of- the-art in electrical power measure- ments of high-voltage, high-current dis- charges. We plan to use this very fast system to obtain thermophysical properties data on solids and liquids up to 6000 K and to conduct studies on the

behavior of materials under rapid

dynamic heating conditions.

Pressure as a Probe of Molecular

Interactions

Vern E. Bean, Thermophysics Division

TJLhe. he use of pressure as a variable

parameter is a valuable tool for studying molecular interactions. We have

recently used differential thermocouple

analysis to explore the phase transitions

in carbon tetrachloride (CCI4 ). Our

measurements demonstrate that ( C I, has dual melting curves up to a pres- sure of 100 MPa.

There are four phases in CC14 in the temperature/pressure region investi- gated: liquid, a face-centered cubic

phase (denoted as la), a rhombohedral

phase (lb), and a monoclinic phase

(II). Both la and lb are plastic crystals. The interest is in the relationship

between the liquid, la, and lb. When

CC14 is cooled at atmospheric pressure,

it solidifies into la at about 245 K and

with further cooling transforms to lb. When heated, lb melts around 250 K

without passing back through the la

phase. Should the cooling be limited

so that only la is formed, then la melts

around 245 K. Thus, at atmospheric

pressure, CC14 has two melting points some 5 degrees apart. We have studied

these phase changes as a function of hydrostatic pressure up to 350 MPa

and over the temperature range of 210

to 330 K with the following results:

1 . Between atmospheric pressure and

100 MPa, CCU has dual melting

curves, one for la, and, a few degrees

higher, one for lb, as shown in figure 1. Above 100 MPa, there is only one

melting curve, that of lb. These two

curves diverge with increasing pressure. Above 273 K and 100 MPa, it was not

possible to detect the formation of the

la phase.

2. The Ia-Ib phase boundary is metastable. It is vibration sensitive and

pressure rate dependent. The lb phase

will abruptly and spontaneously form

from the la under a variety of conditions. Ib will also form from the liquid

without passing through the la phase.

23

a».m

a,E

*

350

325

300

275

250

225 i*^

+ = Ross and Andersson

X = Makita and Takagi

Liquid

JL-> Ib

Ia^> Liquid

. A'

200 1

i i i i

1

i : i i

1

'

i i i

1

I i I I

1

i

i I i

1

i i i i i i '

I 0 50 100 150 200 250 300 350

Pressure, MPa

Figure 1. Melting curves for the la and Ib phases and the II-* Ib transition curve for CC14. 46
;enter for thermodynamics and molecular science

5. There is no liquid-Ia-Ib triple point.

4-. Liquid CC14 will superpress (which

: s analogous to supercooling in an

sobaric experiment) forming a band in

Jihe T/P plane where the metastable

; iuid exists. The width of this meta-

. table band increases with pressure. A number of other experiments

[where pressure is an independent vari- able are either underway or are being

'developed. They include:

I. Light scattering measurements on

,iuids at high pressure through use of ?hoton correlation spectroscopy for

"righly viscous liquids, and Brillouin

.cattering for less viscous liquids Such

measurements will yield: (a) the mean

ime for structural relaxation, (b) a measure of the distribution of relaxaion times about the mean, and (c) a neasure of the elastic response of the

iquids to high frequency disturbances.

3ased on these data, researchers will

nitiate a program to develop theories

vhich relate the rearrangement dynam-

!cs to the mechanisms which lead to

'iscosity and connect these mechanisms

vith the ease with which metastable

itates can be realized.

I. Isothermal compressibility measurements in fluids which use the precision

of ultrasonic interferometry. The tech- nique involves making ultrasonic measurements on two samples of the same

fluid in the same pressure vessel. One

sample has a constant ultrasonic path

length, the other a constant mass. From the data, one obtains the speed

of sound in the fluid as a function of pressure and the isothermal compressibility without going through the usual

adiabatic to isothermal conversion.

This capability will be used to: (a)

look for density anomalies in the metastable liquid region of CC14 , (b) obtain

compressibility data on fluids having

nearly spherical molecules (to be used

with change of the index of refraction

with pressure data, obtained by K. Vedam of Pennsylvania State University, in a critical evaluation of equations

of state of fluids), and (c) obtain compressibility data to be used in conjunction with dielectric relaxation measure- ments in an attempt to apply activation

volume concepts to metastability in

fluids.

3. Capacitance measurements on fluids as a function of pressure, temperature,

and frequency for the dielectric relaxation studies mentioned above.

The Catalytic Methanation

Reaction: A Source of Synthetic

Natural Gas

John T. Yates, Jr., David W. Goodman,

Richard D. Kelley, and Theodore E. Madey, Surface Science Division

synthetic methane, CH4 produced

from coal, will become a major component of the Nation's natural gas supply after 1990. Heterogeneous

catalysts will certainly play a signifi- cant role in achieving this objective.

We have been studying the fundamental mechanism of the key catalytic

reaction involved in the synthesis of methane. Using modern surface meas- urement methods, we have determined

the chemical mechanism for the catalytic hydrogenation of carbon monoxide, CO, over nickel catalysts, and

certain mechanisms of catalyst poisoning have been thoroughly investigated.

This understanding of catalytic processes should provide a conceptual basis for optimizing the efficient use of catalysts in what promises to become a

multibillion dollar national enterprise.

All proposed routes from coal to high-BTU methane involve the initial gasification of coal using steam and air or oxygen and yielding coal gas, a mixture of CO, Ho, and CH4 . While this mixture will burn, its BTU content is too low ( 150 to 500 BTU per standard

cubic foot) for economical transport

through the Nation's pipeline system.

Therefore, the coal gas mixture must

be catalytically converted to pure methane (1000 BTU per standard cubic

foot). Although the catalytic synthesis

of methane from CO and H2 has been

studied for the last 50 years [1,2], until recently the mechanism by which the reaction occurs had not been definitely

determined. We have been successful

in focusing modern surface measure- ment techniques on this reaction to learn how it works and how to maintain

efficient catalyst operation.

Carbon monoxide is the most

strongly bound molecule known. The

47
NATIONAL MEASUREMENT LABORATORY

> 1Q -4 ' 1 1 1 1 1 1 1 1 1 L—

c 1.2 1.4 1.6 1.8 2.0 2.2 H 1/T X 103 (K-1)

Figure 1. Arrhenius plot of rate of CH,

synthesis (NCH ) over different Ni catalysts

in various temperature regimes. Ni(100)

present work; PH2 = 96 Torr, P,-,, = 24

Torr (0.85 cm2, Ea = 103 kJ/mol); (a) 25- 50% Ni/Al203 , Ea = 84 kJ/mol; (b) 5%

Ni/Al2 03 , Ea = 105 kJ/mol; (c) 8.8%

Ni/Al203 , Ea = 109 KJ mol.

energy required to dissociate the car- bon-oxygen bond is 1070 kJ/mol -1

. Because of this basic fact, it is necessary to use a catalyst surface to achieve

a suitable chemical pathway leading to: (a) to CO dissociation and (b) to hydrogenation of the fragments to pro- duce CH4 and H2 0. The catalyst must

be able to carry out both reaction steps

(a) and (b) with suitable efficiency

and with the desired selectivity. In other

words, it must produce pure CH4 efficiently without significant production of other undesired products. A

number of transition metal catalysts are

suitable, but Ni is, by a large margin,

the most inexpensive catalyst currently

employed in laboratory and pilot plant

CH4 synthesis studies. We have employed a Ni single crystal

(1 cm2 surface area) as a model catalyst for our investigations of the reaction mechanism [3]. Use of the crystal

permits us to utilize modern surface

spectroscopy techniques for studying

the reaction. However, it was first necessary to show that the Ni crystal

behaved similarly to actual Ni catalysts

having about 106 cm2 surface area per gram. A spectacular result was obtained, as shown in figure 1, where the

rate of CH4 synthesis (turnover number) over the Ni(100) single crystal is plotted against 1/T as an Arrhenius

plot. Two important conclusions are apparent: (a) linear Arrhenius behavior is observed over a very wide dynamic range (5 orders of magnitude),

giving an activation energy of 103

kJ/mol- 1 (~10 percent of the C-0

kJ/mol dissociation energy), and (b)

The agreement of the rate measured for the single, crystal with rates and activation energies measured on Ak>On-supported high-area Ni catalysts is excellent. The single crystal may be used

over a wider temperature range than

the technical catalysts, giving more

confidence that true activation energies

are being determined.

We have directly shown that the

single crystal is a good model catalyst

[3, 4], and that results obtained on the

single crystal are transferable to the

real world.

Auger electron spectroscopy was

then used to study the chemical composition of the surface of the Ni crysta

following methane synthesis. We were

able to show that a fraction of a monolayer of "carbidic" carbon always exists during the catalytic reaction. It origi- nates from CO dissociation on the N:? surface: the rate of dissociation of the CO is closely related to the rate of CH,

production [5]. If either the H2 /CC

ratio or the temperature falls outside ar optimal range, a graphite layer form; on the surface; this graphite layer is £ poison which deactivates the Ni catalyst. Such poisoning is a principa

problem in methane synthesis ovei

technical catalysts. It was also founc

that extremely small traces of gaseous Fe(CO) 5 present in many sources o: unpurified CO produced by coal gasifi- cation will decompose to yield an Fe

covered Ni surface which immediately

produces the graphite poison, shorten- ing the catalyst lifetime. The mechanistic details which wt have inferred are that both CO am

hydrogen dissociate on the Ni surface1 The carbon atoms form an active ! "carbide" layer on the Ni; the carbide

species react with absorbed H to forn

-CHX intermediates. Further hydro , genation of these intermediates result:

in the formation of CH4 product. A

simplified view of the mechanism foi the reaction of CO and H2 over Nj

catalysts to yield CH4 is schematically

48
CENTER FOR THERMODYNAMICS AND MOLECULAR SCIENCE

Multiphoton Chemistry

John C. Stephenson, Molecular

Spectroscopy Division

frustrated in figure 2. Efforts are rnderway to study directly the molecular nature of the postulated CH4 sur- ace intermediates. We are using

leutron energy loss spectroscopy to

, nvestigate these species [6]. Also, we

plan to study the details of sulfur poisoning of Ni single crystal catalysts;

t is known that this will be a major

^echnical problem in the industrial orocess because sulfur is a major impurity of coal. A general conclusion from this work

e s that modern surface measurement

; ools may be applied successfully to xnravel the secrets of catalytic chemisry and to provide new and useful concepts applicable to practical cataytic chemistry.

•References

Chemical and Engineering News, 57,

20-28 (1979). "Synfuels: Uncertain

and Costly Fuel Option," by Ward

Worthy.

; 2] Mills, G. A. and Steffgen, F. W.,

Catalysis Reviews, Science and Engineering, 8, 159 (1973).

J3] Goodman, D. W., Kelley, R. D., Madey, T. E., and Yates, J. T. Jr., Kinetics of hydrogenation of CO over

a single crystal Ni catalyst, J. Catalysis

(in press).

'4] Goodman, D. W., Kelley, R. D., Madey, T. E., and Yates. J. T. Jr.. in, Hydrocarbon Synthesis from Carbon

Monoxide and Hydrogen, E. L. Kugler and F. W. Steffgen, eds., pp.

1-6, Advances in Chemistry Series 178 (Am. Chem. Soc, Washington,

B.C., 1979).

5] Goodman. D. W., Kelley. R. D., Madey, T. E., and White, J. M.,

Measurement of carbide buildup and

removal kinetics on Ni(100), /. Catalysis (in press).

6] Kelley, R. D., Rush, J. J., and Madey,

T. E., Chem. Phys. Letters, 66, 159- 164 (1979).

FJL..undamental und studies of how in- tense infrared laser light causes chemical reactions are described. Timeresolved in situ spectroscopy is used to detect reaction rates, product energy

states, and isotope enrichment factors, as functions of laser wavelength,

fluence, intensity, and gas pressure. The data is compared to a comprehensive model, yielding new conclusions

about the dynamics of excited

molecules.

Radiation from powerful and effi- cient infrared (IR) lasers may directly

induce chemical reactions such as molecular isomerizations and dissociations

[1]. Since the energy required to break

a chemical bond (e.g., 20 000 to 40 000 cm-1

) is substantially greater

than the energy of an IR laser photon

(e.g., the energy of a C02 photon is 1000 cm-1

), a molecule must absorb

many IR laser photons to react.' For

this reason, such chemical reactions are called "infrared multiphoton reactions" or IRMR. This IR laser radiation selectively excites particular

chemical bonds in a molecule, unlike

ordinary thermal reactions in which

energy in the reactant molecules is dis- tributed randomly. Chemists have

hoped that particularly efficient or unique reactions could be effected through laser induced "bond specific

chemistry."

An enormous amount of research on laser-induced chemistry is now

being done at universities, U.S. and

foreign national laboratories, and at several leading corporations. Potential

applications of IRMR include separation of stable and fissionable isotopes.

Our research here at NBS has focused on developing the basic understanding

of how IR laser excitation causes such

reactions. Most experiments in IRMR have

been done with C02 TEA laser pulses

focused into bulbs of gas at high pres- sures and have relied on chemical

analysis of the final stable end products. Such high pressure conditions

may be necessary for potential commercially practical processes. However, it may be difficult to relate the

results of such experiments to the basic questions of how IRMR occurs, This difficulty arises from complicating factors related to collisions between highly excited molecules, radical

chain reactions, reactions on cell walls,

secondary photolysis of initially formed

products, and undefined or inhomogeneous laser fluence (energy unit area)

or intensity (fluence/unit time).

Our experimental approach [2-6]

has been to use a C02 laser pulse

(about 10~ 7 second duration) to cause an IRMR in a very low pressure gas, such as: '"CO., CFoHCl >CF2 (v,J)

+ HCl(v,J) - ET (1)

AH = 209 kJ/mol

'"CO., CH.CFCl >CF2 (vJ) - CFCl(vJ) - I (2)

AH = 419 kJ/mol

We then use a weak visible or uv probe

laser pulse (5 ns duration) to detect in real time before collisions occur (10 ns

resolution) the rate [3. 4] at which

products are formed. This technique

lets us probe primary processes result- ing from the coupling of isolated molecules to the radiation field, before the complicating effects mentioned above

have occurred. We probe a region of constant, known C02 laser fluence to determine the vibrational (V), rotational (J) and translational energy

(ET ) of the products, [2, 6] and the

isotopic enrichment [5] (e.g., 13 C/12 C)

in the products.

49
NATIONAL MEASUREMENT LABORATORY.

We have measured all these factors and determined their dependence on

the intensity, fluence, and wavelength

of the IR laser. Another important

experiment is underway to measure

these parameters under conditions in which the reactants are extremely dilute

(e.g., 1 part in 106

) in a high pressure bath of Ar atoms. Under these cir- cumstances, the reactants undergo

many collisions with bath molecules

during the laser excitation process. 1

5

CD

O

3

T5

Peak intensity

• 880 MW/cm2

x140 MW/cm2

29 MW/cm2

12 MW/cm2

1 10 100 Fluence F (J/cm2)

Figure 1. Intensity dependence in the IRMR

of CF..HC1. Absolute product yields from

the dissociation of CFL,HC1 (9.3 R(32)

COo laser transition of 1086 cm- 1

) for non- modelocked laser pulses for four different

total energies. The pulses all exhibited identical temporal envelopes and are labeled according to the peak intensity. The data

given for each different pulse represent the

yield of CF., observed at various times rD, during the IR laser pulse plotted against the

fluence F(td ) I(t)dt for that time

Any consistent theory of IRMR must

explain product yields, formation rates, isotope enrichment, and the energy

content of the products as a function

of pressure and laser intensity. We are making progress in developing a comprehensive theory.

Some specific results on reaction ( 1

)

are shown in figures 1 and 2. In figure

1, the collision-free reaction yield [4]

is shown as a function, of laser fluence F for COo laser pulses of different peak

intensity, I. One popular theoretical model [7, 8] of IRMR, used to explain

many results on larger reactant molecules such as SF6 , predicts that the reaction yield depends only on F and

is independent of I. This is dramatically not true for CFL.HC1, where for

laser pulses of constant F, the yield

increased by a factor of 400 as the average intensity I increased by only a

and laser pulse. The total pulse energies were 2. 5, 25. and 150 mJ. respectively, for

the 12. 29, 140, and 880 MW/cm? pulses.

100 200 300

Ar pressure (torr)

Figure 2. Rates of production of XCF2 during the dissociation of CF..HC1 by C02 laser pulses, determined for CFoHCl very

dilute in Ar at the indicated Ar pressures

(1 Torr = 133.3 N/m2 ). The pressure dependence was measured for two ex- perimental conditions: (O), C02 laser operating on the R(32) transition at 1086 cm- 1 which is resonant with the CFL,HC1 one- photon absorption, peak laser intensity of 150 MW/cm-; (A), R(32) laser energy reduced by a factor of 10. The curved lines drawn through the data points are the theoretically calculated results, based on the model described in ref. [3]. The isolated point shows the magnitude of the (± 35%)

associated with each experimental datum.

factor of 6. This result shows that foi CF2HC1, coherent excitation of the molecule is important, and that a sig nificant barrier to laser excitation oc

curs in the discrete lower vibration

rotation levels of this molecule. It figure 2, the absolute rate [3] at whicl

CF2HC1 molecules decompose is showr

as a function of Ar pressure, for CO: laser pulses of two different intensities The initial rise in reaction rate wit! increasing Ar pressure is due to col lision-induced rotational relaxation

i.e., CF2HC1 collisions with Ar atom:

result in rotational equilibration withii the absorber pump mode, allowing tht pumping of molecules which were orig

inally in rotational states outside th< power broadened line. The fact tha

there is no downturn in rate at high A

pressure shows that vibrational de

activation of excited CF2 HC1 reactan molecules is not as efficient as theorie

[3, 8] had predicted.

By using a comprehensive model t< understand data like that in figures ] and 2, we can make predictions o draw conclusions about molecular dy

namics that are very difficult to meas

ure directly. Figures 3 and 4 shov

such predictions [3, 8] based on thi model which fits the data for figure 2

In figure 3, the distribution of vibra

tional energy in the laser excited re actant (solid line) is shown at time

(1, 2, and 50 ns) during the lase pulse when the average reactant ha< absorbed 5, 10, and 15 photons, re spectively. For comparison, the dashei

lines show thermal distributions o

the same average energy. Note that th laser-driven distribution always differ from a thermal distribution. Figure < shows the absorption cross section fo a CFL.HC1 reactant to absorb CO

radiation as a function of the amoun

of energy the CF^HCl has absorbed

Note that the absorption cross sectici

50
Renter for thermodynamics and molecular science

ecreases rapidly as the energy in- reases. This decrease is due to very

=kpid intramolecular dephasing rates

" short T2 times) caused by anharrnonic

ouplings. The result is a "smearing

L ut" of oscillator strength over a very

road spectral region.

The field of IRMR is extremely ac-

|ve. Questions related to coherent vs. icoherent excitation. Tx and T2 times.

' nd the feasibility of "bond specific

'hemistry" are still controversial, as

~ re questions regarding the commer-

"ial practicality of IRMR processes, however, these experimental and the-

' retical IRMR studies are creating new

I

l

t

and detailed understanding useful in

all areas of gas phase chemistry and

molecular dynamics.

References

[1] Cantrell, C. D., Freund, S. M.. and

Lyman, J. L., Laser Induced Chemical Reactions and Isotope Separation,

in Laser Handbook, Vol. II (NorthHolland, Amsterdam, 1978).

[2] Stephenson, J. C. and King, D. S..

J. Chem. Phys. 69, 1485-1492 (1978).

[3] Stephenson, J. C, King, D. S., Goodman, M. F.. and Stone. L, /. Chem.

Phys. 70, 4496-4508 (1979).

[4] King, D. S. and Stephenson, J. C,

Chem. Phys. Letters 66, 33-38 (1979).

[5] King, D. S. and Stephenson, J. C.

0 5 10 15

J. Am. Chem. Soc. 100, 7151-7155

(1978).

[6] Stephenson, J. C, Bialkowski, S. E., and King, D. S., Energy Partitioning

in COo Laser Induced Multiphoton

Dissociations: Energy of XCFL

> and X

CFC1 from CF.,CFC1, J. Chem. Phys.

80, (1980).

[7] Bloembergen, N., Yablonoviach, E., Physics Today 31, 23-30 (May 1978).

[8] Goodman, M. F., Stone, L, and

Thiele, E., Laser-Induced Decomposition of Polyatomic Molecules: A

Comparison of Theory with Experiment, in Multiple Photon Excitation

and Dissociation of Polyatomic Molecules, Topics in Current Physics, C. D. Cantrell, ed. ( Springer-Verlag,

New York 1979), and op. cit. therein.

5 10 15

Average vibrational excitation <v>

Figure 4. CF2HC1 absorbtion cross section calculated for various values of energy ab- sorbed (v). These average absorptions were

achieved by varying the time of irradiation

at an intensity of 150 MW/cm2 and a pres- sure of 300 Torr Ar.

Vibrational excitation (v)

i ure 3. Laser excited distributions ( —) compared with thermal distributions ( ):

! a) average energy (v) = 5: (b) average energy (v) = 10; and (c) average energy (v) = 15. The extra curve ( ) in (c) for (v) = 15 is the thermal distribution for a

nclecule arbitrarily truncated at 20 levels. These model predictions are for a CO., laser

ntsnsity of 150 MW and 300 Torr Ar.

51
NATIONAL MEASUREMENT LABORATQR'

Quantum Calculations of

Excimer Molecules

Morris Krauss, Molecular Spectropy

Division

Quantum chemical calculations are being performed to identify and analyze the potential of candidate electronic laser systems for

the Department of Energy and Defense

Advanced Research Projects Agencysponsored programs for generating

high power laser beams. The net gain

of a candidate system must be known.

Experimental determination of this property is difficult for exciplex or excimer systems. Calculation of the spectroscopic behavior of excimer and

continuum-continuum systems has

proved to be an important source of information.

We have calculated the spectroscopic

properties of important molecules in the following classes of laser candidates: Group Vl-rare gas, rare gashalide, Group lib-Group Ha and b

dimers, Group IV-rare gas, and rare gas-rare gas. Ab initio molecule calculation for evaluating energies and spectroscopic

properties can be done for small

molecules containing low Z atoms using

all-electron multi-configuration variational techniques. Considerable experi- ence has been accumulated on dealing

with the calculation of the correlation

energy involved in bonding interactions over the entire range of internuclear

separations. Of particular importance

for excimer systems is the recent development of techniques for calculating van der Waals interactions into dis- tances where the overlap of the charge

distributions is no longer negligible.

We have shown that the modification

of the usual inverse power series expansion by a damping function x(R)>

E(vdw) =

-2 C(/,L) v(/,L,R)R-2 " + l +d

can be applied to all the systems mentioned above.

All the molecules of a class are of

interest; thus the all-electron techniques

have been extended by introducing

pseudo-potentials to replace the core

electrons. Both non-relativistic and

relativistic effective core potentials

(ECP) have been generated and used

as necessary to treat high Z atoms.

Calculations of the electronic properties of ion-pair molecules using ECP

have compared favorably with all- electron results, although more re- search is still needed on the use of the ECP. Using the pseudo potentials, we

have considered molecules containing

elements as heavy as Hg and Pb. The

ECP take the relativistic contraction of

the core into account, but at the present

time the spin-orbit interaction in the valence shell is treated as a perturbation.. The transition moments and energies

have been calculated for the X-A

bound to continuum transition in HgCl.

The photodissociation cross section is illustrated in figure 1 as a function of

the transition energy and temperature.

The laser energy for the B-X transition

is close to the peak energy, and a signi- ficant absorption will result from a buildup in the concentration of X state HgCl in the course of exciting the laser media. This absorption could explain

the low extraction efficiency observed

for certain types of HgCl lasers. Self-absorption in Cd2 is also pre- dicted by ab initio calculations for the excimer transition. From the energy

curves shown in figure 2, we see there

is strong bound-bound, 3IIg-» 3IIu absorption near 470 nm. Absorption was

also predicted earlier for the 335 nm

emission from Hg2 from the reservoir

metastable excited states. The Group Vl-rare gas laser transi- tion is a collision-induced electric dipole transition between electronic

states that correlate to the valence

states of the Group VI atom and thi ground state of the rare gas atom. Wi

have determined that, with the excep

tion of XeO, the transistions are dom

inantly continuum-continuum. Calcu ! lation of the transition line shape v found to require a multichannel coul ' pling among all states arising from th

valence asymptotes. The line-shape fo the Ar-O^D) - Ar-0( ] S) transitioi has been calculated in reasonably!

agreement with an experimental deter; mination.

Pb vapor is now being explored as : Raman scattering media for producin;

highpower backward scattered pulse

from an XeCl laser pulse. Rare ga

buffers are used to enhance the back

ward scattering cross section over thr forward. Collision-induced absorptioi

at either the pump or Stokes frequency

would reduce the efficiency of the proc

ess. A calculation of Pb-Ffe and Pb-Xi

energy curves and transition momenti : reveals that collision-induced absorp j tion is not significant at the frequencie: i of interest. The rare gas halide diatomic anc triatomic excimer spectroscopy owe i great deal to theoretical calculation

Not all of the relevant states have beer observed directly by experiment anc assignments of some transitions an

still not definitive. The relative ener

2

Transition energy (1000 cm -1

)

Figure 1. Photodissociation cross section foi the X2 2 — A2II transition in HgCl as £ function of transition energy and tempe

ature. 52
ENTER FOR THERMODYNAMICS AND MOLECULAR SCIENCE

;tes of the B and C states in the iatomic system are of present interest, ve are now comparing our calculations

r ith the interpretations of the experilental data. We agree with previous

leoretical all-electron calculations that

le B state is lower in energy than the

f for XeF and XeCl.

Theoretical calculations have proved

to be a fast, accurate way to obtain the

spectroscopic data needed in modelling

laser media. We are continuing our

studies of the rare gas halide lasers by

examining photodissociation of bottle- necked states in XeF and the likelihood

of positive gain for the Xe2 Cl system.

4 5 6 7 8 9 Internuclear separation (Bohr)

|

7igure 2. Excited state energy curves for the Cd„ molecule calculated with multi-configura- on self-consistent-field variational techniques.

New Development in the

Evaluation of Thermochemical

Data

D. D. Wagman, David Garvin, Vivian

B. Parker, Richard H. Schumm, and

J. Brian Pedley1

, Chemical Thermodynamics Division

Acomprehensive, up-to-date

compilation of evaluated thermochemical data has been completed by the XBS Chemical Thermodynamics Data

Center. The compilation is called Selected Values of Chemical Thermodynamic Properties. It covers the periodic table except for some of the

actinide elements and includes over 12,000 inorganic and low molecular

weight organic compounds. These

include pure substances in the gaseous,

liquid, and solid phase as well as many

binarv solutions.

The properties tabulated in the compilation are the enthalpy of formation,

AfH

3

, Gibbs energy of formation,

AfG°, entropy, S°, and heat capacity,

Cp

°, all at the temperature 298.15 K

(25 °C). In addition, values are pro- vided for the enthalpy of formation at 0 K. Af H

3 (0). and the enthalpy

difference H° (298) - H° (0). The

values are based almost entirely on

experimental measurements and statis- tical calculations. Approximately 20,-

000 property values are included. Well

over half of these are enthalpies of formation. The next two most numerous entries are for Gibbs energies of formation and entropies.

These tables are designed for use in chemical engineering, chemical and

biochemical research, rocketry, plasma

studies, environmental control, energy

conservation, and other related tech- nological fields. Thermodynamic properties are essential for calculating the

energetics of chemical processes and

optimizing of equilibrium yields. They

are widely used in interpreting physicochemical research. The numerical values in the tables resulted from critical analysis and eval- uation of many different types of

1 University of Sussex, England.

53
NATIONAL MEASUREMENT LABORATORY

physico-chemical measurement. Vari- ous calorimetric reaction enthalpies can

be combined to yield values for the

enthalpy of formation of particular

substances. High temperature equilibrium studies may be combined with

values of the thermal functions for the

individual species in the reaction to yield desired values. Calculating ther- mal functions of gases by statistical- mechanical methods requires a knowledge of molecular structure parameters

(bond lengths and angles) and vibrational frequencies; the calculation for a substance in the condensed phase re- quires values for the enthalpy content

as a function of temperature and enthalpies of phase transitions. Low

temperature (298 to 0 K) Cp

° meas- urements are used to obtain the third

| law S° (298 K). Similarly, data on

solubility, activity and osmotic coeffi- cients, vapor pressures, and enthalpies

of solution and dilution are required

for the proper evaluation of measure- ments on solutions. Enthalpies of combustion, hydrogenation, and halogenation are used in the calculation of thermodynamic properties for organic

molecules.

To do the above evaluations, we

have amassed a reference file of numerical data abstracted from more than

40,000 articles describing over 200,000

measurements made since 1848 on

individual chemical substances. Each

year, an annotated bibliographic refer- ence list of all material added to the Data Center files is published in the

Bulletin of Chemical Thermodynamics.

This list, arranged by chemical formula

of the substance measured, indicates

the type of measurement made as well

as the full bibliographic citatioi Construction of these reference data

tables is an exercise in the art of data

evaluation. Often the properties of a substance can be determined by using

54

^=^LiAi 5 08(c)

II Life) =

LiO(g)

LiOH(1 ,000H 2 O) LiOH(400H 2 O) J

Figure 1. Loops of 5 members or less, containing reaction (a) ; each line between

substances represents one reaction between

lithium compounds; processes (b) and Q appear to contribute large errors.

several different measurement paths.

The interconnections among the paths

may be seen in figure 1 . The results obtained from the various paths usually

do not agree. Choices must be made in establishing the "best" values. There are two fundamental components in this evaluation process. The

first is the heart of the process and must

be done by an expert. It is necessary

to determine, for each measurement, in the light of present knowledge, whether

or not the interpretation of the chemistry was correct and whether the tech- nique was suitable. Reinterpretation,

recalculation, and correction of the data often are necessary. A consistent

set of auxiliary data is used in reducing

the measurements, e.g., physical constants and properties of ubiquitous

substances. Accuracies of the measure- ments must be assigned.

The second component is the combination of the accepted measurements

to produce the final, recommended

values for the substances. The follow- ing criteria are required for this component: (1) The recommended values must reproduce well those measure- ments considered to be reliable. (2)

The values selected must be thermodynamically consistent throughout the. tables and be in reasonable accord with

physico-chemical correlations (similai

substances should have similar properties) . Recently, this second component ha; been automated. More than 1,00(!

measurements involving 600 or mors

substances may be involved in a partic-. ular problem. Computer technique;

are ideal for manipulating such large, masses of data. In addition, a system],

atic fit of the data is obtained with ar analysis of the network. This fit is usec>; as a guide to the strategy of date, evaluation and to indicate the impor-. tance of various substances. Rapic

updating of the recommendation is pos-

:

sible when new and significant meas. urements become available because

| these evaluated measurements ar\

stored in reusable form. The network

solutions combining the measurement: ! are done in an iterative process ir

3 which linear programming is used tc locate the most highly consistent set oj measurements and weight them accordingly, prior to a least squares solution

This two-step approach has been showr

to mimic the decisions made by date analysts when they combine the meas- urements in the traditional "by-hand'

solution. It results in a solution fron

which the user may combine any set o values in the data bank to predict witl confidence the thermochemistry of s process. Automation has been an importan

factor in the development of the date bank. The selected values are stored

in a machine-readable file. Tables car be printed in any system of units
CENTER FOR THERMODYNAMICS AND MOLECULAR SCIENCE

-20 10

Loop Error, Kcal Figure 2. Errors in loops containing up to 5 members, and containing reaction (a) ; shaded

portions are deleted when reactions (b) and (c) are discarded.

(Publication in SI and in "caloric units"

is planned.) A machine-readable cata-

' log of the accepted measurements is

; being developed—both to document

the tables and for direct use in applications. In a cooperative program with

the University of Sussex, England, au- tomated quality control, network solution, and graphic display techniques

have been developed. These simplify

and speed up the routine parts of the

task. These quality controls assure that thermodynamic consistency is main-

: tained. A recent development is "loop analysis". In this analysis, measurements

'are summed over every closed path in hhe loosely connected network. Con-

'servation laws require zero sums, but

'experimental inaccuracies produce errors of closure. An analysis of the loop

errors shows which measurements are highly inconsistent. As an example,

figure 1 shows a network involving

enthalpy measurements for 16 lithium

compounds. It shows all 5 membered

(or fewer) closed paths involving process a (there are 84 of these). The dis- tribution of errors of closure is shown

in figure 2. The mean error is close to zero, indicating that process a probably

is acceptable. But the distribution is distinctly nonnormal. Processes b and

c account for all extreme values and

therefore are of doubtful accuracy. This technique holds the promise of becoming a very rapid technique for preliminary analysis of thermochemical

data and may play an important role in future data evaluation strategies.
4
Center for

Analytical Chemistry

~urt W. Reimann

.ucy B. Hagan

ohn K. Taylor

nstroment

] >eveIopment ames R. DeVoe

"homas C. O'Haver

'illmer C. Ruegg

.obert L. Sams

lonald W. Shideler

.ichard K. Simon,

Jr.* ohn C. Travis

iregory C. Turk

norganic Analytical

Research

'rnest L. Garner

ohn Alienkoff** ohn R. Baldwin

Lynus Barnes. Jr. lllyn S. Beary ames M.

Blackman**

Villiam A. Bowman.

Ill Oavid E. Brown

Lobert W. Burke

. Stephen Carpenter

sabell Casar**

Erie R. Deardorff

larry I. Diamondstone

.ura P. Dunstan

•lichael S. Epstein

ohn D. Fassett

Lonald F. Fleming

homas E. Gills ohn W. Gramlich

Lobert R. Greenberg

ose Guerrero**

higeki Hanamura

oe M. Harris**

ally H. Harrison

Emerson F. Heald

Villiam R. Kelly loward M.

Kingston, Jr. Villiam F. Koch

ames Koskoris

Richard M.

Lindstrom

George J. Lutz Lawrence A. Machlan

E. June Maienthal

George Marinenko

Radu Mavrodineanu

Jerry D. Messman

Klaus D. Mielenz

J. Mitchell** John R. Moody

Pedro A. Morales**

Thomas J. Murphy

John A. Norris

Paul J. Paulsen Theodore C. Rains Robert L. Watters,

Jr. Tadashi

Yamamoto**

Rolf L. Zeisler Organic Analytical Research Harry S. Hertz Robert E. Botto**

Jeanice M. Brown

Stephen N. Chesler Richard G.

Christensen Alex Cohen

Bruce Coxon

Lawrence M.

Doane*

Richard A. Durst Delmo P. Enagonio

Rafael Espinosa- Leniz** Alexander J. Fatiadi Mary L. Fultz Franklin R. Guenther

Diane Hancock

Laurence R. Hilpert Barbara F. Howell

Pamela L. Konash

William A. MacCrehan

Sam A. Margolis

Willie E. May

Reenie M. Parris Dennis J. Reeder

Kristy L. Richie Robert Schaffer Lorna T. Snieaoski William J. Sonnefeld**

Pietr VanDerLijn** Ranee A. Velapoldi

Michael J. Welch**

Edward White. V

Patricia A. White**

Stephen A. Wise

William T. Yap

Gas and Particulate Science Harry L. Rook

Arnold M. Bass

Adville A. Bell John J. Blaha David S. Bright

Barry C. Cadoff

R. Casciani**

Ilan S. Chabay

Lloyd A. Currie William D. Dorko

James W. Elkins Edgar S. Etz Charles Fiori** Robert A. Fletcher Alan Fried Jimmie A. Hodgeson

Ernest E. Hughes

Robert H. Kagann

George A. Klouda

Edwin C. Kuehner

Ryna B. Marinenko

Raymond L. McKenzie

W. R. Miller** Robert L. Myklebust

Dale E. Newbury

Richard Paur**

Peter A. Pella William P. Schmidt

John R. Sieber David S. Simons

Gerald A. Sleater John A. Small

Eric B. Steel W. Thorn**

*Post Doctoral Staff ** Research Associates and Guest Workers

laman spectra from individual micrometer-size particles are

•btained with the Raman microprobe, developed at NBS. The

molecular identity of the constituent chemical species in the ample can be determined. Local environment and crystal-

•nity of the microsample can also be inferred in many cases.

rhis rapid nondestructive technique requires no special sample

reatment or special conditions such as vacuum. It components other microanalytical probe (electron- or ion-beam)

echniques which furnish primarily elemental data. John Blaha

rear) and Edgar Etz identify the composition of airborne

'usts and submicrometer atmospheric aerosols. The mineralisation process of teeth and bone, the characterization of environmental trace organic materials and the study of fluid nd gaseous inclusions in minerals which are related to

'e ilogical processes are also studied.

The Center for Analytical Chemistry maintains

expertise in analytical measurement to support activities in this Nation's industrial, clinical, environmental, and

research laboratories. The results of analytical measure- ments often form the basis for regulatory, medical, and

industrial decisions. It is essential that they be reliable. The Center serves as the reference laboratory for as- suring this reliability. For example, methods of high

accuracy are developed for identifying substances and

for quantifying the chemical composition of materials.

Materials standards of known composition and stability are prepared for aiding in quality control of measure- ment procedures and for determining the reliability of

data.

The National Bureau of Standards uses several means

to transfer to others the Center's analytical methods,

standards, and research results. These means include

publications, seminars, consultation, cooperative programs, and the direct distribution of materials standards

through the NBS Standard Reference Materials Program.

The diversity of materials standards produced in the

Center reflects the ubiquity of analytical measurements

in this society. These standards include samples used in

the metals industries (e.g., steel, aluminum, copper),

chemical industries, and clinical and environmental laboratories. To carry out the research underlying each of the

several hundred gas standards, the Center must maintain

a level of expertise not achieved in any other single analytical laboratory in the world. Moreover, standards be- come increasingly complex, and technical requirements

for their development become more demanding. These

factors create needs for new methods of measurement.

The Center works to achieve a balance between provision

of currently needed standards and research in new analytical methods.

The technical activities of the Center focus on sample

and compound types (analytes) —organic, inorganic,

gas, and particles. For each analyte, researchers in several disciplines take part in an integrated measurements

standards program.

The Center has developed more than 80 gas standards. Though initially stimulated by the need for reliable

measurement of air pollutants, such standards are becoming increasingly important in occupational health and

safety, technology applications (e.g., evaluation of en57
NATIONAL MEASUREMENT LABORATORY

gine performance), and atmospheric monitoring and

research. Requirements for increasing accuracy, measurements of trace amounts of constituents, and determination of a greater variety of gases and toxic vapors

necessitate research and development in the area of

highly sensitive spectroscopic methods—those involving

the use of lasers as well as mass spectroscopic techniques.

Organic compound analysis is often called a new frontier in analytical chemistry in recognition of two facts: hundreds of thousands of organic compounds occur in nature or are produced commercially; yet methods of

measurement of complex mixtures are just now beginning

to emerge. Moreover, the basis for standardization of

methods is in a very early stage of development. The

importance of methods for the detection and quantitative

determination of organic molecules is evident in the

potential applications: measurement of health-related

constituents, including therapeutic drugs, in body fluids: measurement of nutrients and contaminants in foods;

measurement of carcinogenic compounds in water, in air, on particles emitted with pollutant gases by factories and

in synthetic fuels. Though the Center has been involved

in organic analysis for a relatively short time, it has made

a number of significant contributions: development of

the first standard for trace organic analysis ( antiepilepsy

drugs in serum); development of quantitative methods

of measurement of polynuclear aromatic hydrocarbons

in recycled oil, and development of definitive methods

for clinical chemistry. Research plans include the development of sophisticated chromatographic separation

schemes and the linking of separation systems with novel

spectroscopic, electrochemical, and mass spectrometric

quantitative measurement systems.

The largest research activity in the Center is in inorganic analysis. Studies in this area have led to hundreds

of materials standards, ranging from trace metals in food

materials and blood to samples used at all stages in the

production of metals—steels, cast iron, aluminum, and

copper. In addition, high precision and high accuracy mass spectrometry on isotopic composition has led to the

redetermination of atomic weights accepted as international standards. In the future, efforts will focus more on

measurements of high technology materials and very

complex matrices such as hazardous wastes. These en- deavors will require the development of very sensitive

techniques, with emphasis on improving the accuracy o multielement methods involving the use of lasers, neutro

radiation, electroanalytical techniques, and mass spec trometry.

Research in microanalysis is an important activity i| the Center. Work in this area has emphasized under

standing the fundamentals of electron, photon, and io:< interactions with matter so that more quantitative infor- mation may be derived from microprobes. Most existin' electron microprobes now use one of a series of quanti

tative x-ray emission data reduction programs developer

in the Center. The most recent development, a compute

program called FRAME-«, has yielded promising im' provements in the ability to determine quantitativ

elemental compositional information on micron size par

tides. These basic studies have aided in research t develop improved techniques in particle analysis, incluc

ing a major effort to standardize the analysis of asbestc

in air and water. The field of analytical chemistry is undergoing a rapi

growth and an increase in scientific complexity. Th

"evolution" is stimulated by the growing application c analytical chemistry to areas of public concern. Alsc

advances m analytical chemistry are helping to promot

scientific advancement in other fields. As analytical mea:, urements become more sensitive, they are increasing

valuable in areas such as materials science, climate re search, health research, and nutrition research. Thus, th Center for Analytical Chemistry faces a dual challenge

to keep pace with the growing demand for standards t support current analytical applications and to take pai

in the advance of analytical chemistry itself.
3ENTER FOR ANALYTICAL CHEMISTRY

laser Enhanced Ionization n Flames

John C. Travis, Gregory C. Turk,

'ames R. DeVoe, and

°eter K. Schenck 1 instrument Development Group and

Chemical Stability and Corrosion

Division (CMS)

FJLrom its unception in the summer

.f 1976 [1], until today, laser nhanced ionization (LEI) has held

he promise of unprecedented

<nsitivity and accuracy in atomic ame spectrometry . It is characterized

n a recently awarded U.S. patent

No. 4-148,586) [2] as part of a

eneral phenomenon called the opto-galvanic effect" (OGE).

Jnderstanding the underly ing

lechanisms of LEI has been a

Significant step in establishing a solid oundation for the atomic spectronetric method.

Laser enhanced ionization occurs

/hen a tunable dye laser is used to

electively populate a particular excited

tate of an atomic species present in a ame. Since the laser-prepared excited toms have a lower effective ionization

otential than they did prior to excitaon, the rate of ionization of these toms by thermal collisions increases

ignificantly. This ionization enhancenent can be easily monitored by drawig a current through the flame and

y observing changes in the current

nat are synchronous with pulsed or hopped laser illumination. As shown

i figure 1, the laser-induced ac comonent of the flame current is separated

'om the dc background current by an

LC filter. It is then amplified and

ignal averaged by gated integration.

The analytical application of LEI

omes from the well established techique of using a flame as an atom

reservoir". It requires a liquid solu- on of the sample to be "nebulized"

nto the oxidant supply as a fine mist,

arried through the fuel/oxidant mixing

number, and finally into the flame.

energy population statistical weight

92

"1 9i

optica processes

Figure 1. Energy level diagram showing the radiational and collisional processes used to model laser enhanced ionization.

The flame rapidly evaporates the sol- vent, while most trace metals present

in the solution are left as free atoms.

Quantitation is achieved by comparing

spectrometric signals from unknown

sample solutions to those from "cali- brated" solutions.

Calibration curves exist for a number of elements detected by LEI. They

yield "limits of detection" (LOD) often

less than 1 part per billion (ng/mL)

for aqueous solutions, as shown in table 1 [3]. These data show that the technique compares favorably with the others listed. Conspicuously absent

from the table are any elements having

high (> 7.5 eV) ionization potentials.

The role of the ionization potential, as

well as other pertinent parameters, can now be explained on the basis of a model using the simplified energy level/

rate constant scheme of figure 2. Several processes with relatively slow

rates are neglected, for simplicity,

including the "recombination" rates from the ionization level to the neutral

levels. Recombination is insignificant

because the applied electric field lowers

the electron density and thus the recombination rate. For example, consider the population of level 2 (n L.) to be negligible in the absence of laser irradiation. Then

the LEI signal current is taken to be

proportional to k2 in2 during the pulse,

with level 2 being totally populated by

the laser (see fig. 2). Assuming a square laser pulse which maintains a

fraction R(< [g2 /(gi + g2 )] of the

neutral atom density (n, + n L>) in level 2 during the pulse, the LEI current should go as k Lj R (n, + n L.), where R

is determined by the rate constants

of figure 2 and the laser spectral ir- radiance, Ev

. Since the total number

density, n T , is given by nT = nx +

n L

. + n ; , the final enhanced ionization

rate becomes

(dn,/dT) f=a L-i R (nT — n : ) a signal current. (1)

The effect of the ionization potential

59
NATIONAL MEASUREMENT LABORATORY

Table 1. Comparison of detection limits" (ng/ml).

Laser

Element LEI 1 ' FAAC FAE'1 FAFe FAFf

Act i1

1 0 1 4

LJCX 11 SO

Ri — 70000 T. J Ca 0.1 1 1.1 20 0.08

Cr 2 2 2 5 1 Cu 100 1 0.1 0.5 1 Fe 2 4 5 8 30

Ga 0.07 50 10 10 0.9 In 0.006 30 0.4 100 0.2

K 1 3e 0.05 e Li 0.001 1 0.02 0.5 Mg 0.1 0.1 5 0.1 0.2

Mn 0.3 0.8 1 1 0.4

Na 0.05 0.8 0.1 0.1 Ni 8 5 20 3 2

Pb 0.6 10 100 10 13

Sn 2 20" 100« 50e

T) 0.09 20 20 8 4

a Detection limits for comparison taken from S. J. Weeks, H. Haraguchi, and J. D. Winefordner,

Anal. Chem. 50, 360 (1978), unless otherwise noted,

b Ref. 3, laser enhanced ionization in flames,

c Flame atomic absorption,

il Flame atomic emission.

e Flame atomic flourescence, conventional light sources,

f Laser induced flame atomic flourescence.

g Taken from J. D. Winefordner, J. J. Fitzgerald, and N. Omenetto, Appl. Spectrocs. 29, 369 (1975).

li Taken from V. A. Fassel and R. N. Knisely, Anal. Chem. 46, 1110A (1974).

comes from the excited state rate constant k2i ^«exp [— (Ei — E2 )/kT], (2)

where k is Boltzmann's constant, and

T the flame temperature. Equations

(1) and (2) indicate an inverse ex- ponential dependence of signal current on ionization potential. However,

semi-log plots have yielded slopes dif- fering from a realistic 1/kT value by

> 50 percent, and, therefore, an improvement of the model was required.

Re-examining equation ( 1) reveals

that a possible explanation to the problem might arise from the depletion of

the neutral atom pool (nT — n0 by

ionization during the laser pulse. Thus,

at any time after the beginning of the pulse (for which nt 0), although the LEI current is favored in k2i for low

ionization potential elements, it is penalized by the factor (nT — ni) representing depletion of neutrals. Since

real laser pulses are not "square", but

have a finite risetime, the population

fraction R is building up in time as the neutral atom density nT — ni is diminishing and thus, a complex solution is expected for the maximum value of dnydt.

Such a solution has been obtained

for a laser pulse shape approximating

that used in these experiments. The

resulting expression agrees with experimental sensitivities for 23 transi- tions within 17 elements using an air/

acetelyne flame [4]. This expression, as recently reported in Analytical Chemistry [4], can be used to predict sensitivities for observed transitions and

elements, utilizing conventional spec- troscopic cross sections. Two additional areas of study are in

j process: (1) the relative insensitivity ol high ionization potential atoms, and (2) the characterization of the mechanism

of electron/ ion collection. The first problem is being approached throug

sequential—or "stepwise"—excitation

to high energy levels using two tunable

lasers, and satisfactory results have al- ready been published [5]. The seconc

area of study is being approached botl experimentally and theoretically. The

experimental work involves measuring

a number of variables, including signa

strengths, flame potentials, and flame background currents as a function o

other variables, such as electrode ge

j ometry, applied potential, flame com

position, and aspirated solution com

position. The theoretical studies wili involve obtaining numerical solution: of Poisson's equation in a flame sub1

; jected to an applied field, with realistii electrode configurations and flame con

ditions. The outlook is promising for LE

in flames to become established as ; major method for quantitative spec

troscopic analysis.

References

[1] Green, R. B., Keller, R. A., Schenct

P. K., Travis, J. C, and Luther, C

G, Opto-Galvanic detection of specie

in flames, J. Am. Chem. Soc. 9<{ 8517-8518 (1976).

[21 Green, R. B., Keller, R. A., Luthe

G. G., Schenck, P. K., and Travi

J. C, Apparatus for Galvanic Df

tection of Optical Absorptions, U>! Patent No. 4,148,586, April 10, 197!

60
:ENTER FOR ANALYTICAL CHEMISTRY

?] Turk, G. C, Travis, J. C, DeVoe,

J. R., and O'Haver, T. C, Laser

j enhanced ionization spectrometry in analytical flames, Anal. Chem. 51,

1890-1896 (1979).

|t] Travis, J. C, Schenck, P. K., Turk,

G. C, and Mallard, W. G., Effect of

selective laser excitation on the ionization of atomic species in flames, Anal. Chem. 51, 1516-1520 (1979).

[5] Turk, G. C, Mallard, W. G., Schenck,

P. K., and Smyth, K. C., Improved

sensitivity for laser enhanced ionization spectrometry in flames by step- wise excitation, Anal. Chem. 51, 2408

(1979).

flashlamp

dye laser M

mil energy

probe

oscilloscope

o

trigger

photodiode

chart recorder

trigger signa

out boxcar

I averager

.'-IGURE 2. Block diagram of the laser enhanced ionization apparatus.

Measurement of Trace Elements

in Solar-Cell Silicon

Richard M. Lindstrom, Ronald

F. Fleming, and Paul J. Paulsen,

Inorganic Analytical Research Division

Large quantities of pure silicon

will be required in the 1980's as solar photovotaic electricity becomes an

important energy source. Accurate

measurements of trace impurities help

to define the optimum balance between

purity and cost.

The Low-Cost Solar Array project,

managed by Jet Propulsion Laboratory

(JPL) for the Department of Energy,

has as its goal the generation of electrical power from the sun at a cost comparable to that charged by electric utilities. Reaching this target requires a massive increase in the scale of operations of the solar-cell industry. This in- crease is expected to cause a dramatic

drop in the price of solar cells, their structural supports, and the power-conditioning equipment required for a large

distributed activity.

It now appears that the targets set

at the beginning of the project in 1975

can be met by more than one combination of methods. The most mature

technology is that of flat-plate, singlecrystal silicon arrays, which were

developed primarily for space applications. For such applications, the quantity of cells produced was small and the

cost was relatively high. The cells were made by slicing disks, one by one, from

large single-crystal ingots of semiconductor-grade silicon, the purest material produced in multi-ton quantities

in commerce today.

For generation of electrical power,

much greater quantities of silicon will be required in the 1980's. Current es- timates are that photovolatics will re- quire tens of thousands to hundreds of thousands of tons of silicon to be pro- duced per year by 1990.

This quantity of silicon must be

produced without an unacceptable degradation of cell quality, at a price no

61
NATIONAL MEASUREMENT LABORATORY

greater than one-fifth the current price

of semiconductor grade silicon. It is the quality of this "solar grade" silicon that our work addresses through trace element characterization.

Workers at Westinghouse, Dow Corning, and Monsanto are studying the

effects upon solar-cell performance

caused by adding specific impurities to

silicon. These studies will define "solar grade" silicon. Measuring cell effi- ciency as a function of concentration

requires that the concentration be

known; however, for some impurity

elefnents, the concentration at which

the cell degradation begins is less than

1 part in \0'] (1 ppb). Such low concentrations cannot be measured by

most methods of trace analysis. In those cases where sufficiently sensitive analytical techniques are unavailable,

the concentration in a doped crystal

must be inferred from that in the melt

from which it is grown. This requires a knowledge of the liquid-solid distri- bution coefficient, which is difficult to measure because of the great tendency

of crystallizing silicon to exclude certain impurities from its crystal lattice. Related difficulties must be faced by

chemical engineers who design large- scale production processes for solar

silicon. Since impurity concentrations

of interest are very low chemical analysis performed by most service laboratories will have low accuracy. Therefore, an engineer will have difficulty in constructing an impurity mass balance

that would assure with high confidence

that some sensitive contaminant will not concentrate in the final product.

To this set of problems analytical

chemists at NBS have applied two sensitive methods of trace analysis that have been shown to be reliable in the past by repeated agreement in the certification of Standard Reference Materials. These two techniques are neutron

activation analysis (NAA) and isotope

dilution spark-source mass spectrometry (IDSSMS). In NAA, packaging

and handling methods have been established and detection limits have been

caluculated and verified by measure- ment of blank silicon samples. Nearly

all elements of interest can be deter- mined quantitatively at concentrations

where changes in electrical properties

begin to be visible. Two notable exceptions are titanium and vanadium, which we are still unable to measure at subppb concentrations in silicon. In IDSSMS, the laboratory blank sample

and instrument sensitivity are roughly

equally important limits to the minimum quantity of impurity that can be

observed, which is generally in the region of a few nanograms for the ele- ments of primary interest. The results of our measurements,

when compared with those made by

other investigators, show the difficulty of the measurement problem. For

forty percent of the samples compared,

the discrepancy is greater than a factor of two. The problem is most extreme

for copper (see table 1 ) . Our measure

ments in some cases differ by three orders of magnitude from those re ported by other analysts. Systematic

errors in our NAA determinations are not likely since excellent agreement was found in an analysis of the same

sample by both IDSSMS and NAA

Measurements of concentrations of other elements in samples also show

large discrepancies when NBS meas

urements were compared with others. Experience in geochemical trace ele ment measurements has shown how

discrepancies of this kind can be re- solved. The method used is an analysis of a common lot of materials in differ- ent laboratories by different techniques.

If disagreement in results exists, the process is repeated, with improvements

in methods, until agreement is reached

To aid in such comparisons of anal- yses, a silicon Research Material is in the process of being prepared.

Table 1. Comparison of copper analyses (10 15 atoms/ cm 1 ). Sample W-007-Cu W-017-Cu W-051-Cu/Ti W-056-Cu

g I Mass Spec 1.80 32 4 70

$ { NAA 1.65 25 NA

o

'

NAA 0.050 \ 11.2 | 0.002 ±0.002 45.3 ± 0.8 :0.01 ,nn ±0.5 m 0.065 }- u- ul 10.9

Z IDSSMS — — 42.2

62
'Renter for analytical chemistry

Quantitative Analysis of

Individual Trace-Level Organic

Constituents in Alternate Fuels

Willie E. May and Stephen N. Chesler,

Organic Analytical Research Division

Increasing energy demand in the

3 Jnited States, coupled with increasing

•eliance on foreign sources of petro-

. eum, has caused national concern that

las resulted in a program for developing new sources of energy. The conversion of coal to gaseous or liquid

uels and the utilization of oil shale and

( iar sands appears to be some of the promising new energy sources. However, a serious though largely unknown

complication of developing these alter- nate fuels is their potentially deleteious effect on man and the environ- ment. Of particular concern are the lumerous compounds (for example,

jhose identified on EPA's priority pollutant list) which result from processing alternate fuels, and which may

'. ventually be transported into the en- 'ironment as gaseous or liquid effluents. To evaluate the potential environmental

Iffects, the alternate fuels base source,

ts industrial processing, and its resultint effluents must be analyzed. Results

»f these analyses cannot be assessed

dequately without knowing the accuracy of the analytical techniques and

aiata obtained.

j We have in the past conducted

everal collaborative studies aimed at

f ssessing the accuracy of data obtained

om various trace organic analytical

lethodologies. The results of a recent omparison of the determination of ydrocarbons, and N- heterocylics by

veral laboratories under contract to

;ae Department of Energy, are given

i Table 1. The variance among reults reported by these laboratories

ocument the current state of measurement in this area. One well-doculented method of increasing the accuacy of analytical measurments involves

tie uses of quality assurance standards,

r Standard Reference Materials

SRM's). However, until recently,

RM's for environmental trace organic

Table 1. Interlaboratory comparison of shale oil analysis (mg/kg).

Compound NBS 2

a 3 4 5 6 7 8 9

pyrene 107 155 360 150 168 212 141 620 185

flouranthene 61 102 220 80 108 104 112 380 115

benzo(a) pyrene 22 117 3.3 —

benzo (e) pyrene 21 — 1.3 —

phenol 394 392 180 399 — 310

o-cresol 338 350 150 381 2400 328

Numbers indicate another laboratory's determination.

Table 2. Shale analysis (mg/kg, 95 percent confidence level).

HPLC extraction with

quantitation by

Acid/Base No

extraction with extraction with quan- quantitation by titation by

Compound LC GC GC/MS GC GC/MS GC/MS

pyrene 108± 16 101 ± 4 102± 9 94±10 104±8

fluoranthene 53± 6 55 ± 6 62 ± 6 75± 5 58±5

benzo(a) pyrene 20± 3 21± 5 24 == 2

benzo (a) pyrene 20± 6 22r= 5 phenol 383± 50 387 ±26 416± 28 334d= 63

o-cresol 330± 34 334 ±86 350± 16 322d=45

analyses did not exist because they

require for certification a determination

by a minimum of two totally independent methods. During the past year, we

have developed the requisite methods

for certifying a Shale Oil SRM.

The methodology used in the certifica- tion of shale oil was developed over an

18 month period. Initially, we focused our efforts on the evaluation of tech- niques for extracting the compounds of

interest from shale oil matrix. A classi- cal acid-base extraction scheme and a

high performance liquid chromatographic fractionation procedure, which was developed in this laboratory, served

as independent methods for isolating the analytes from the sample. Next, we

investigated the use of gas chromatographic (GC), gas chromatographicmass spectrometric (GC/MS), and

high performance liquid chromatographic (HPLC) methods for identifying and quantifying the compounds of

interest in the various fractions obtained from the isolation procedures.

Direct analysis by GC/MS was also used when possible.

These data, given in table 2, show

that the overall agreement between the various methods of extraction and

quantitation was good up to the 95 percent confidence level. This excellent agreement between these results compared to those obtained in the interlaboratory collaborative study pro- vided the basis for establishing state- of-the-art methods for trace organic

analyses.

The specimen of shale oil being cer63
NATIONAL MEASUREMENT LABORATORY

tified is from a 150-ton retort for in-situ simulated combustion operated by the

Laramie Energy Technology Center,

Laramie, Wyoming. The shale is from

the Mahogany Zone of the Colorado

Green River formation. The stock of

shale oil being certified was obtained

from Dr. Bruce R. Clark at Oak Ridge

National Laboratory, Oak Ridge, Tennessee. It has undergone centrifugation

to separate water (~ 40 percent) and

sludge from the oil prior to shipment to NBS.

This SRM, having the concentrations

of phenol, o-cresol, fluoranthene, pyrene, benzo(e)pyrene, and benzo(a)-

pyrene certified, will be issued in early

1980. Its use should greatly improve

the accuracy of related environmental

measurements. Accurate measure- ments will assist scientists in correctly

relating health effects to levels of indi- vidual pollutants, engineers in correctly

assessing effectiveness of control technologies, regulatory agencies in relating

levels of pollution emissions with ambient air and water quality, and the Federal Government in correctly making policy decisions on cost-benefit trade offs concerning environmental

protection, energy conservation, and

economic health. This work represents efforts by the

staff of the NBS Organic Analytical

Division who have expertise in gas chromatography, liquid chromatography, and mass spectrometry.

Computer-Controlled

Electrochemical System for

Electrode Kinetics Research

Lawrence M. Doane and Richard

A. Durst, Organic Analytical Research

Division

M. odern electrochemical

research often requires measurements

of fast electrode kinetics and transient

intermediates. Since commercial elec- trochemical equipment generally do

not meet these rate requirements, a

state-of-the-art, computer-controlled

system capable of data acquisition at a

rate of 100 kHz is in the final stages

of development. A specially designed

potentiostat, with dynamic and nearly

complete compensation of the cell re- sistance, will be incorporated into this system. Fourier transform experiments can be implemented, since this system can sample two inputs simultaneously

at a rate of 50 kHz. The system has

been designed for maximum data acquisition throughput and versatility,

and simplified control.

The recent emphasis on finding energy-efficient processes has stimulated

a resurgent interest in electrochemistry

and electrochemically related fields. Since maximizing the efficiency of elec- trochemical processes will involve the elucidation of electrode reaction mechanisms it is necessary that fundamental

investigations of these processes and

their associated elecrode kinetics be

undertaken. Commercially available

electrochemical instruments are generally not versatile enough to be useful

in many of these types of investigations. We are therefore constructing

a computer-controlled electrochemical

system (CCES) that can generate complex waveforms, apply these waveforms

to an electrochemical cell, and then

acquire data at a high rate. A system

similar to this is used in single-drop

square wave polargraphy and will serve

as a model for evaluation of the CCES.

In the single-drop experiment, the potential range of interest is scanned at

the end of the mercury drop lifetime, so that several polarographic problems can be minimized, e.g., electrode charging current and solution agitation. In

addition, the potential is applied as a square wave while sampling the result- ing current at the end of each potential

change and adding together the two

sampled currents. Thus, this method

gives an increased current signal over

that obtained from the linear ramp

experiment. This system is obviously

applicable for trace metal analysis, and

such an analysis will be used to evaluate the CCES.

We have designed the CCES around

the relatively low-cost central processing units (CPU) that are now commercially available. The CPU is a Zilog Z80A microprocessor, operated

at a clock rate of 4 MHz, in which the

fastest instruction cycle takes place in

1 ju.s. Memory consists of 64 kilobytes

(K) of random-access memory (RAM)

and 240 K dual-drive disk memory. A

block diagram of CCES is shown in figure 1. The CCES acquires the elec-r trochemical signals via two modes

mode A uses fast commercial potentiostats (an electronic instrument used to control the potential of an electrochem

ical cell) from which analog signals are converted to digital format in the com

FLOPPY

disk

CPU

POTENTIOSTAT

CELL

Figure 1. Block diagram of the computecontrolled electrochemical system (CCES)

64
E ITER FOR ANALYTICAL CHEMISTRY

titer; mode B uses a laboratoryDnstructed potentiostat which digitizes

ae electrochemical signals before en- Ting them into the computer. There

| a five-fold difference in signal acquition rate between the two modes: 20

s for mode B versus 100 for mode

In mode A, the analog electrochemial signals are digitized by two analogs-digital converter boards (ADC)

esiding in the computer. The converion can occur at a maximum rate of 0 kHz (i.e., one data point every 33

•s). however, execution of the pro-

;ram to input and store the data

'equires an additional 30 to 60 tts,

J epending on whether or not an analog

ignal is being sent to the potentiostat.

"herefore, the overall throughput could

'<e restricted to no less than 100 tts. Although the two data words are stored

equentially, the two converters operate

n a master slave relationship so that

'be two electrochemical signals (current

nd potential) are sampled simultaneously.

We have built a fast potentiostat in

: vhich two ADC's and a digital inter- lace have been included to minimize

he software required to operate the

ICES (fig. 2). In this scheme, the :omputer loads a group of parallel regFROM

COMPUTER ii CONTROL WORD

OUTPUT CONTROLLER

1 TIME CONTROLLER LOW BYTE HIGH BYTE

VOLTAGE REGISTER

TO DAC

F:gure 2. Block diagram of data flow through interface.

isters with the parameters of the elec- trochemical signal to be outputted by

the potentiostat. When this function

is finished, the computer reverts to an

input sequence that sends the digitized

data to the computer upon signal from

the ADC's. One input sequence can

occur during the conversion of the next analog signal; however, when two

signals are converted simultaneously,

the second datum input to memory will add an additional 10 lis to the overall throughput, i.e., 20 /xs are required to perform one complete two-data input.

The registers can be reloaded during an outputting sequence so that the out- put can be dynamically altered in a manner dependent on the format of the program.

Currently, we are developing soft- ware for this system. We have chosen

to write our program in FORTRAN

and use assembly language subroutines

to input the data. This allows us to operate CCES at the maximum possible

computer-controlled speed. In addition

to the gain in speed, the RAM memory

space which a BASIC interpreter would

normally occupy is now available for data storage. However, a direct memory

access (DMA) controller is being designed to directly input the data to memory (bypassing the CPU). By

using the DMA, the CCES speed will depend only on the ADC conversion

rate, freeing the CPU for other tasks. Following an evaluation of the

CCES, its application will be broadened to include: (a) electrode kinetics experiments via Fourier transform of AC voltammetry, a technique providing high-precision measurements of faradaic admittance vs potential or fre- quency; (b) rapid scanning detectors

for high performance liquid chromatography, where the dynamic resistance

compensation and rate of data acquisition will permit the real-time identification of eluting species; (c) signal

enhancement by ensemble averaging

techniques employing newly developed

pulse and relaxation methods, e.g., the

square wave experiment described

above. There also exists excellent opportunities in data retrieval and pattern

recognition, two operations which have

been underutilized in electrochemistry.

Such operations would be useful for

the identification of specific compounds

in complex mixtures (deconvolution)

and in the elucidation of electrode

kinetics and mechanisms by comparison of acquired data with theoretical curves generated by digital simulation

of coupled chemical-eletrochemical

reactions.

65
NATIONAL MEASUREMENT LABORATORY

High Resistance

Junction Formation in House

Wiring Circuits With

Aluminum Conductors

Dale E. Newbury, Gas and Particulate

Science Division

TJLhe.he mechanism of high resistance

junction formation in aluminum-wire

iron screw connections is shown to be

the generation of iron-aluminum

infermetallic compounds during arcing.

In the period 1968-1972, many new

homes built in the United States were

equipped with aluminum wiring and

conventional brass-coated iron junctions. In laboratory experiments, these aluminum-iron junctions have been

observed to undergo a glow-arcing

phenomenon accompanied by severe overheating which could lead to the

initiation of a fire [1]. The origin of

the overheating, which involves two

normally satisfactory electrical conductors, has not been previously explained.

Electron probe microanalysis of the

microstructures of circuit components

which have undergone such failures has revealed the probable mechanism

of the glow phenomenon [2]. Previous laboratory investigation of

the glow phenomenon revealed that glowing began when the aluminum

wire-iron screw junction became mechanically loose [1]. As a starting point

for this study, conducted in cooperation with the NBS Center for Consumer Product Technology (CCPT)

a loose connection between an aluminum wire and an iron screw was simulated in a circuit carrying 15 amperes

of current. When the components were

separated by a small gap, an arc was

created that led to the glow phenomenon. Scanning electron microscopy and

x-ray microanalysis of the regions

damaged by the arc revealed evidence

of significant material transport across

the arc and of high temperatures at the

surface of the components (see figs.

1 and 2) . Iron and a minor amount of

~Jk

0.1mm

Figure 1. SEM image of an arc-damage crater.

1 m FeH

1

j Figure 2. X-ray spectrum of a region of a crater.

66
:opper were transported to the surface

of the alurninurn wire, and aluminum

eras transported to the iron screw apparently in the vapor phase. The zinc

remponent of the brass was not deected and was probably lost by evaporation. Temperatures in excess of

1500

C were suggested by the observation of solidification structures of

learly pure iron at the periphery of

•he crater on the screw. This combination of high temperaxres and intimate mixing of the metals

suggested the possible formation of

ntermetallic compounds.

A section

Through the damage crater revealed the

taresence of such

a compound. FesAL

£s determined by x-ray microanalysis.

The existence of the intermetallic com- i sound at the current-transmitting inter- face is of special significance since its -esistivity is generally a factor of 10 to 100 times greater than that of either of he pure metals. In order to confirm that this phenomenon occurs in actual junction boxes

•sired with aluminum,

a second series

:>: experiments was carried out in cooperation with CCPT. Typical household

iunction boxes wired with aluminum

vere subjected to high" current cycling I ontil glow failure was induced. In the

process large portions of the assembly

-eached temperatures of 400 to 500

r

C

ll]- Samples for metallographic examnation were prepared from a sec- tion through the wire screw contact; i typical section through a contact is phown in figure 3. A thick reaction

?cne spanning the entire region of coniz: between the wire and screw can

r>e seen in the optical micrograph ( figJre3). An electron microscope image

3f this region. Figure 4. reveals that he zone is composed of two layers.

X-ray microanalysis with the VBS

Theoretical matrix correction procedure

FRAME

C indicates (1) that the layer 1
NATIONAL MEASUREMENT LABORATORY

adjacent to the aluminum corresponds

to the intermetallic compound FeAl3 and (2) that the outermost layer consists of Fe2Al3 with 0.5 percent copper. Examination of samples taken at var- ious stages in the glow process suggests

that initially intermetallic compounds

are either iron-rich or aluminum-rich

depending on the major constituent

present. With continued reaction, inter- metallic compounds containing roughly

the same quantities of each element are formed [3]. All of the intermetallic

compounds containing aluminum and

iron have resistivities 20-200 times

greater than aluminum or iron [4]. The observation of the development

of thick intermetallic layers in the current carrying region during the glow

failure suggests the following chain of events: ( 1 ) an arc occurs in an initially

loose connection, causing local heating

and material transport in the vapor

phase; (2) on both the wire and the screw, the metallic elements react in the high temperature environment,

forming intermetallic compounds; (3)

the high resistivity of the intermetallic

compounds leads to sharply increased

PR heating at the interface; and (4)

the heat produced leads to enhanced

material transport, first as a vapor and

perhaps eventually as a liquid, which

then promotes rapid reaction to form

even more intermetallic material, re- sulting in an unstable, runaway process. Knowledge of this basic mechanism for glow failure of electrical junctions provides important information which could lead to the design of

safer circuit connections.

References

[1] Meese, W. J. and Beausoliel, R. W.,

Exploratory Study of Glowing Electrical Connections, NBS Building Sci- ence Series 103 (NTIS, Washington,

D.C. 1977).

[2] Newbury, D. E., Investigation of the Mechanism of the Formation of Resistive Junctions in Aluminum Wire,

Report of Analysis 78-96, Center for Analytical Chemistry, Gas and Particulate Science Division, National

Bureau of Standards (1978).

[3] Hansen, M., Constitution of Binary

Alloys, (McGraw-Hill, New York,

1958) pp. 90-95.

[4] Vol, A. E., Handbook of Binary

Metallic Systems, Vol. 1 (U.S. Dept.

of Commerce Washington, D.C,

1966) pp. 136-149.

Discrimination of Natural From

Anthropogenic Carbonaceous

Pollutants Through MiniRadiocarbon Measurements

Lloyd A . Currie and George A . Klouda,

Gas and Particulate Science Division

adiocarbon dating rests on the

fact that living matter is in isotopic

equilibrium with cosmic ray-produced

14 C; death breaks the photosynthetic

chain, leading to reduction of the

isotopic ratio 14C/ 12C in accordance

with the physical half-life of 5730

years. We have taken advantage of

B this natural phenomenon to develop a method for quantitatively discriminating between biogenic ("living")

and fossil ("dead") carbonaceous

contaminants in the environment [1]

(see fig. 1).

The importance of distinguishing

biogenic from fossil carbon arises from

the effects of both natural and anthropogenic carbonaceous species on

health, stratospheric ozone, and cli- mate. In order to plan responsible control strategies, it is therefore vital to determine the relative contribution of man's activities to atmospheric hydrocarbons, halocarbons, carbon monoxide, and primary and secondary car-^ bonaceous particles. Although natural! emissions of hydrocarbons far exceed

i- those originating from man s use ol

fossil fuel [2], for example, there re- main very significant questions con-; cerning the sources of hydrocarbons

contributing to rural pollution episodes

as well as the influence of natural forest emissions on urban air pollution [3],

Longer-lived tropospheric species, such

as methane, may lead to depletion ol stratospheric ozone; and trace gases ; and particles may influence the earth's i radiation balance directly through

"greenhouse" and albedo effects or in- directly through cloud nucleation [4],

It is worth noting that radiocarbon is the only unique biogenic/fossil dis- j criminator. Chemical composition and;

68
2ENTER FOR ANALYTICAL CHEMISTRY

.table isotope measurements ( 13 C/ 12 C)

are frequently helpful but the former

's indirect and subject to interference

md the latter exhibits large natural

• ariations. Because of the small atmospheric

:oncentrations of many of the species

of interest (table 1 ) , it has been neces-

;ary to advance the state of the measurement art by a factor of a thousand.

That is, measurements must be pracicable for samples containing just a

'ew milligrams of carbon—rather than

laving gram-size samples which are

ypical of conventional Radiocarbon

fable 1. Tropospheric concentrations

(mg-C/nr).

Range

Air

quality

standard

COo

:h4 :o*

-ic

i

180.

0.76

0.06

<10"3 xl, c,H(; a i0"3

Articles <10"3

"Dissolved 2

'rain)

Organic

larbon

0.03 —

20. 17. 2.00 0.14

10"3 —

0.10 0.052

10(mg/l) —

5 N-S asymmetry.

Dating. 1 This requisite sensitivity was

achieved through the design of a high

purity, miniature gas proportional counter which is linked with a two- parameter low-level counting system,

described in the following section.

Measurement System

Following collection and separation

according to size or chemical characteristics, environmental samples

containing 5 to 15 mg carbon are converted quantitatively to CCX. for radiocarbon measurement. Prior to counting of the 14 C, the sample-COL

> must

be rigorously purified—a few ppm

of electronegative impurities interfere with counting—and cryogenically trans- ferred to a high-purity quartz counting

tube. Measurement of the radiocarbon

takes place over a period of several days using a special low background

system incorporating massive shielding,

electronic anticoincidence cancellation

of cosmic ray muons, and two-dimen1 The required sample size is given by the

fact that 10 mg of contemporary carbon

yields a net counting rate of about 0.15 cpm

which results in a 10 percent uncertainty

(Poisson relative standard deviation) after about one day of counting. The corresponding air sample size would be 100. m3 for a substance whose concentration is 0.1 mg- C/nr1 [1]. RADIOCARBON

14 Production: Cosmic Rays (p) —*. n N

(atmosphere)

14/ CO, (Biosphere)

a

Dating: A = A e~t/r [10 g, 1%]

Pollutant Source: A = f

• A 0 + (l-f) • 0 [10 mg, 15%]

Figure 1. Radiocarbon production by cosmic rays, and its application to dating and

pollutant source identification, (f equals the fraction of contemporary or living carbon in m environmental sample; A0 represents the equilibrium "C/^'C isotope ratio in living

natter. Typical sample sizes and Poisson Standard deviations are given in brackets.)

sional spectroscopy according to pulse

amplitude and risetime. Special fea- tures of the system, which includes an

on-line computer, provide for rejection

of spurious electronic pulses and longterm monitoring of stability. As a result of the attention given to materials'

radio-purity plus active (timing, spec- troscopy) and passive background re- duction, our residual background is comparable to the signal (~ 0.1 cpm).

This is equivalent to a "noise" attenuation (with reference to the unshielded

counter) of more than a factor of two

hundred.

Figure 2 is a photograph of our 15

mL quartz counter and the internal

shield of high purity copper. This

counter is used for samples containing

up to 30 mg-carbon; a second counter having an internal volume of 5 mL and

operated up to 4 atmospheres C02 is used for samples having <10 mgcarbon. (The residual background rate

for the 5 mL counter is remarkable: a recent measurement in the underground counting laboratory at the University of Bern yielded a mean interval of 25 minutes between counts.) A dis- play of the two dimensional spectrum,

for an air particulate sample collected

in Salt Lake City, is given in figure 3. Valid pulses lie below the dashed line, and eight (rejected) spurious events

are seen above. Further details concerning the sample preparation and

purification system, and a brief description of the counting system may be

found in References 5 and 6, respectively

Experimental Evidence of Man's

Activities Our research, whose objective is to use radiocarbon to discern man's per- turbations of environmental carbonaceous matter, has been directed toward

three classes of samples: atmospheric

69
NATIONAL MEASUREMENT LABORATORY

Figure 2. Photograph of the small quartz counter (15 mL volume) and associated shield. The shield, along with the inner wall of the counter, is constructed of OFHC copper.

Table 2. Radiocarbon in aerosols and sediment.

Mass- Percent Carbon contemporary Supporting

(mg) [ 14 C/ 12C] (±1 S.D.) data

Ambient particles

Los Angeles

Salt Lake City

Utah Desert Impact particles

«3fxm)h

Portland, OR.

7.8

5.3 10.2

5.1 - 10.7

Sediment c Puget Sound, WA. ( FA 60. (PAH 14.

23 ± 15

28 ± 13

88 ± 16

62. - 107.

86 ± 21

19 ± 21

Organic

(insoluble)

composition

Inorganic

composition

Isotopic

(13C/12C) composition

" Ref. 5. » Ref. 8.

' Ref. 9 (FA Fatty acid fraction, PAH = Polycyclic aromatic hydrocarbon fraction).

gases, atmospheric particles, and or- ganic matter in "recording" media,

such as sediment and ice cores. Study

of the first two categories yields infor- mation on the significance of current

activities, while the third provides the

historical record and a link to the global carbon cycle. Work with atmospheric gases is still at an early stage; we have just begun sampling atmospheric methane, and suitable apparatus

for the collection of mg-quantities ofjj non-methane hydrocarbons and carbon

monoxide is under development. In cooperation with University groups, however, we have carried out a number

of measurements of radiocarbon concentrations in atmospheric particles

and in sediment. The results are summarized in Table 2. The first point to be noted in Table 2

is that, with the exception of one sample, all samples consisted of only about

10 mg of carbon. This is the amount

which was available with normal, daily

operation of a HiVol air sampler or with the chemical processing of about

600 grams of surface sediment. The

only two previous studies of radiocarbon in urban aerosols involved a thousand times as much carbon and re quired a week of sampling [7]. (Sim

ilarly, conventional radiocarbon meas

urements of PAH in sediment would

have required extraction of a metric ton of material.) Aside from the in-i creased ease of sampling and time res-: olution, the ability to measure very1 small samples is essential in terms of selectivity. That is, miniradiocarborr

counting makes it possible to derive

reliable information on individual

chemical or size fractions—especially

fine particles which may be free from

local contaminants, such as resuspended waste or spores. The ambient particles (table 2).

studied in collaboration with the Uni

70
CENTER FOR ANALYTICAL CHEMISTRY

60

aQ

<

»a

E

3

C

flj C

cn£

U

40

20

2 I I ^112 1 2 1 t II

I 2 I 12 11 1

I 12 3 I 2 2 II

2 I I I S 2 II - 3 3 2 11 X I 5 1 I I I I I I S 2 13 2 1 _ *3 2I3IS22 X . ?

3

? * X<i,2 »l (! I

Channel number (E)

Figure 3. Two-parameter proportional-counter spectrum for the Salt Lake City sample.

Abscissa represents pulse energy (E); ordinate, pulse shape (ADP). Spurious pulses appear

above the 45° dashed line.

versity of Utah Research Institute and

the University of Utah [5], confirmed

what is generally expected; that is, the urban particles consisted largely of

fossil carbon while the rural (desert)

particles indicated a biogenic source. Supporting studies on the composition

of the insoluble carbonaceous component (soot) also pointed to an anthropogenic (fossil) source for the urban

samples. Pyrolysis-gas chromatography-mass spectrometry revealed several cyclic and substituted aromatic

compounds which have also been observed in vehicular exhaust.

Collaborative work with the Oregon

Graduate Center was designed to assess

the impact of vegetative burning (by

man) on urban (Portland) air particulate pollution [8]. The results of that

study, indicated in table 2 and summarized in figure 4, demonstrated that on high impact days, contemporary

carbon from vegetative burn sources such as field and slash burning and

space heating with wood contributed

between 27 and 51 percent of the fine particulate mass, and up to 35 percent

of the total suspended particulate material collected in the Portland and

Eugene, Oregon, airsheds. The radiocarbon analyses of filters selected for high impact from residential wood

combustion suggested that this source

is currently significant and could be- come one of the most significant sea- sonal sources of respirable particulates

in the future, unless steps are taken to improve combustion efficiency and/or

emission control. Chemical Mass Bal-

(Fossil)

1.00-

>-u

(13 ImOa

E

<uco c

0 o

"5 £ 0.75 c o 1 g 0.50

(0 O)— 0.25

o

rec

. 0)

BCD

I I I

© ©/

/

o

0 0.25 0.50 0.75 1.00

with

Expected fraction of contemporary carbon

(pure source)

Figure 4. Comparison of observed

expected vegetative burn impact (fine particles). Dashed line represents the idealized

situation for a pure source. The numbered

box [3] indicates the median and l<r is shown only on #4 for clarity. Abscissa lo- cations (A, B, C, D) are derived from the atmospheric radiocarbon input function and

the ages (t) of the material burned.

ance, based on the inorganic composition of the particles, tended to corroborate the radiocarbon data, but it could

not provide the same degree of unique

source identification. (Note that the "anthropogenic" carbon in the Port- land study was contemporary—i.e., biogenic, whereas it was fossil in the Los Angeles and Salt Lake City study.)

One of the most interesting areas of investigation is the record of man's past

impacts on the environment. This was

the stimulus for' the last study [9] in- cluded in table 2, the measurement of

the chemical and isotopic record in organic matter in a sediment core from

Puget Sound, taken midway between

urban North Seattle and the rural Olympic Peninsula. The initial results, which apply to the top 1 0 cm sediment

71
NATIONAL MEASUREMENT LABORATORY

layer or material deposited during the

last two to three decades, show that

different organic species are far from

isotopic equilibrium. The high radiocarbon content (86±21 percent con- temporary) of the fatty acid fraction,

indicating a biogenic origin, is consistent with the premise that this frac- tion originated from recently-living

planktonic matter. The Polycyclic

Aromatic Hydrocarbon (PAH) frac- tion on the other hand, was found to contain little or no radiocarbon, indi- cating that it was primarily of fossil origin, presumably associated with

man's use of fossil fuel. Stable isotope

measurements ( 13 C/ 12 C) of the PAH

did not clearly discriminate between

coal and petroleum, but they did rule out waste oil as a primary source. These findings support the contention

[10] that PAH in the surface sedi- ment comes mostly from fossil fuel combustion rather than from natural

forest fires. (PAH, which contains

known carcinogens and mutagens, is a common product of incomplete combustion, and it is generally associated

with the particulate emissions.) Further studies, at increased depths in the

sediments, should yield the historical record for both fossil and vegetative

combustion.

Future Directions

Our immediate plans call for ex- panding the miniradiocarbon counting

system to accommodate up to twenty or more simultaneous counting channels,

such that sample throughput need not be limited by the relatively long counting times associated with individual

measurements. Information on the global methane cycle and assessment

of the fossil contribution to urban and

rural non-methane hydrocarbons will follow from our current work on at- mospheric gas sampling.

Within the past few months, in co- operation with the University of Rochester, we have begun also to explore a new measurement technique which

promises to extend our sensitivity by

an additional factor of 102 to 103

. Through the use of Accelerator Mass

Spectrometry (AMS), in which indi- vidual radioactive atoms (rather than

their disintegrations) are counted in a nuclear accelerator, we have suc- ceeded in measuring the radiocarbon

concentration in just 40 of a con- temporary radiocarbon sample [6, 11].

The relationship between decay counting (11c) and atom counting (AMS)

as well as the main features of our ex- ploratory experiments are sketched in figure 5. Considerable work remains

to be done, particularly in the areas

of ion source sample preparation and

fast and reproducible beam switching,

before AMS will be established as a

reliable and accurate technique; and

machine availability and cost make it currently somewhat less attractive than

11c if 5 to 10 mg of environmental

carbon samples are available. However, its incredible sensitivity—literally a million-fold better than conventional

radiocarbon dating—promises to make

practicable hitherto impossible experi- ments. We can anticipate the use of radiocarbon to determine the origin of the rarer atmospheric species (e.g., halocarbons), and we should be able

to greatly increase our knowledge of environmental sources and transformations through the examination of indi- vidual organic compounds found in ocean sediment, polar ice cores, and

atmospheric particles.

References

[1] Currie, L. A., Noakes, J., and

Breiter, D., Measurement of Small

Radiocarbon Samples: Power of Alternative Methods for Tracing

Accelerator Mass Spectrometry

A = -dN/ r XN = N/ T

tic : n = A'e'A t AMS : n' = A»f'«T

Exploratory Experiment

(Rochester, July 1979)

OBJECTIVES • Environmental sample preparation/dilution

[particulate SRM] • Electromagnetic isotope enrichment/

implantation [C02 • Minimum sample size [new radiocarbon

dating SRM]

PRELIMINARY RESULT

(dating SRM)

40 ,ag - C ^ 14C/12C = (1.41 ± 0.04) X 10-12

Figure 5. Accelerator Mass Spectrometry (AMS) Exploratory Experiment at the Microgram

Level. (lie represents low-level counting; n, number of counts; A, disintegration rate; ef detection efficiency; At, lie counting time; t = the mean life of radiocarbon; N,

the number of atoms of radiocarbon. 72
CENTER FOR ANALYTICAL CHEMISTRY

Atmospheric Hydrocarbons, 9th

Internatl. Radiocarbon Conf., R. Berger and H. Suess, eds., Univ. of

Calif., Los Angeles and San Diego, June 1976; Currie, L. A. and Murphy,

R. B., Origin and Residence Times

of Atmospheric Pollutants: Application of C14

, in Methods and Standards for Environmental Measurement, W. H. Kirchhoff, ed., NBS

Special Pub. 464, 439-447 (U.S.

Government Printing Office, Washington, D.C. 1977).

[2] Covert, D. A., Charlson, R. J., Rasmussen, R., and Harrison, H,

Atmospheric chemistry and air quality, Review of Geophysics and Space

Physics, 13, 765-771 (1975).

[3] Maugh, T. H, II., Air pollution:

where do hydrocarbons come from?

Science, 189, 277-278 (1975).

[4] Geophysics Study Committee,

Energy and Climate, NRC Geophysics Research Board, National

Academy of Sciences. Washington,

D.C. (1977).

[5] Currie, L. A., Kunen, S. M.,

Voorhees, K. J., Murphy, R. B., and

Koch, W. F., Analysis of Carbonaceous Particulates and Characterization of Their Sources by LowLevel Radiocarbon Counting and

Pyrolysis/Gas Chromatography/

Mass Spectrometry, in Proc. of the Conference on Carbonaceous Particles in the Atmosphere, T. Novakov, ed., Lawrence Berkeley Laboratory, Berkeley, California (1978).

[6] Currie, L. A., Environmental Radiocarbon Measurements, in Proceedings of First Conference on Radiocarbon Dating with Accelerators, H.

Gove, ed., pp. 372-390 (University

of Rochester, New York, 1978).

[7] Clayton, G. D., Arnold, J. R., and

Patty, F. A., Determination of sources of particulate atmospheric

carbon, Science 122, 751-753

(1955); Lodge, J. P., Jr., Bien, G. S., and Suess, H. E., Carbon-14 content

of urban airborne particulate matter,

Intl. J. Air Poll. 2 309-312 (1960).

[8] Cooper, J. A., Currie, L. A., and

Klouda, G. A., Application of car- bon- 1 measurements to impact as- sessment of contemporary carbon

sources on urban air quality, (to be

published in Environmental Science

& Technology, 1979).

[9] Swanson, L, Fairhall, A., and Currie,

L. A., Carbon Isotope Analysis of Sedimentary Polycyclic Aromatic

Hydrocarbons (submitted to Nature,

1979).

[10] Hites, R. A., Laflamme, R. E. and

Farrington, J. W., Sedimentary polycyclic aromatic hydrocarbons: the

historical record, Science 198, 828-

381 (1977).

[11] Currie, L. A., Klouda, G. A., Elmore,

D., and Gove, H, Accelerator mass

spectrometry and electromagnetic

isotope separation for the determination of natural radiocarbon at the microgram level (to be submitted to Science, 1979).

Center for

Materials Science

John B. Wachtman,

Jr. Darrell H. Reneker

John G. Abbott

Harvey J. Anderson

David B. Blanchard

John W. Cahn

Donald G. Fletcher Fred Goraick

Melvin Linzer John D. McKinley,

I Jn

IStephen J. Norton

'Stephen L Parks Samuel J. Schneider,

Jr. Bruce W. Steiner

| Robb M. Thomson

Claudia F. Vosbeck

1 Melvin R. Wallace M. Katheryn

Wharton

Gerry N. Wixson

Chemical Stability and Corrosion

! Thomas D. Coyle

L'go Bertocci

• Tames R. Bethin William R. Blair David W. Bonnell

Frederick E. Brinckman, Jr. denrietta J. Brown

Zhwan-Kang

Chiang

' John J. Comeford

Richard G. Crumley

Arthur L. Cummings

Klan L. Dragoo

Edward Escalante Klan D. Franklin

Richard S. Gates

Villiam F. Gerhold

.udwig H. Grabner

ohn W. Hastie

rVilliam S. Horton

tephen M. Hsu

iaioshi Ito** A arren P. Iverson

Jo-Anne A. Jackson Kenneth L. Jewett Rolf B. Johannesen Jerome Kruger

Chia-Soon Ku

David S. Lashmore

Jasper L. Mullen

Fielding Ogburn

Gregory J. Olson*

Edwin J. Parks

Patrick T-S. Pei Ernest R. Plante Edison N. Pugh

Joseph J. Ritter Benjamin T. Sanderson

Peter K. Schenck

Joseph H. Simmons**

Jacob Smit

Stephan J. Weeks

John P. Young

Fracture and

Deformation

Richard P. Reed

John M. Arvidson

Mark W. Austin

Betty A. Beck

Elliot L. Brown

Bruce W. Christ Leon Chuck

Roland deWit

Ronald C. Dobbyn

James G. Early. Jr. John W. Elmer

Richard J. Fields Stephen W. Freiman

Edwin R. Fuller, Jr. Arthur H. Heuer

George E. Hicho

Bernard J. Hockey

Charles G. Interrante Maurice B. Kasen

Ralph F. Krause, Jr. Ronald D. Kriz*

Hassell M. Ledbetter Samuel R. Low, III Clyde L. McDaniel

Harry L McHenry

Terry A. Michalske* Robert S. Polvani

David T. Read

D. Ellis Roberts Raymond E. Schramm

Thomas R. Shives John H. Smith

Nancv J. Tighe

Ralph L. Tobler Thomas A. Whipple

Sheldon M.

Wiederhorn

William A. Willard Polymer Science and

Standards Ronald K. Eby

Joseph M. Antonucci

Harold Argentar**

John D. Barnes Gary M.

Beaudreau**

Barry Bernstein Leonard H. Bolz Rayphel L. Bowen**

Gerhard M. Brauer Martin G.

Broadhurst

Daniel W. Brown

Walter E. Brown**

Anthony J. Bur

James M. Cassel Shu-Sing Chang

Tzeng Chen**

Lawrence C. Chow**

Edward S. Clark James P. Colson

John M. Crissman

George T. Davis

Ronald E. Dehl

Aime S. Dereggi

Brian Dickens James G. Dillon** Seymour Edelman

Bruno M. Fanconi

Barry L. Farmer

Lewis J. Fetters Roland E. Florin Joseph H. Flynn

Edmond L. Graminski Warren H. Grant

(Continued)

TAhe

Siew of the counter arm of the five-counter diffractometer. Antonio Santoro and Kay Hardman are investigating a

r jterial at low temperature; the sample is enclosed in the

-ryostat shown standing in the central part of the picture. The

leutron beams diffracted by the powdered material are inter- ested and measured by the counters in the horizontal plane

V the instrument. There are five such counters, placed 20°

ipart, so that different regions of the pattern can be measured

hiultaneously. Visible in the picture are also the catcher of

'he primary beam (on the left), and shielding walls, necessary

o "educe background.

.he National Bureau of Standards has had a continuing involvement in materials research and development since its founding in 1901. Today, more than ever, NBS is committed to a strong program in the materials

sciences, and has focused most of such research in the

Center for Materials Science.

This Center's program is geared towards providing

technical support to private organizations and government agencies to enhance more effective use of materials.

Further, we believe that a strong base of materials sci- ence at NBS is an essential foundation for U.S. industry

in developing new products, increasing productivity and

assuring future economic competitiveness. Accordingly,

the Center conducts major research activities in poly- mers, metals, ceramics, and glasses focus on areas where

measurement services and data are especially needed.

Our materials research program addresses major

phases of the materials cycle, from processing through

design, manufacture, and assembly, to use and finally to recycle or discard. The Center's research program covers

three major aspects of materials science: transformation

science, structure/ property science and durability science.

Transformation science is the basic science which underlies materials processing research, for it deals with the

changes in state or microstructure of materials. It seeks

to predict the microstructure which results from variables

of composition, temperature gradients, pressure, and

mechanical stresses. Those same variables are of importance in industrial processing. Structure/ property

science addresses how to measure structures and how

chemical and physical properties depend upon structure. Durability science deals with the changes which cause

deterioration in service, such as corrosion, wear and

fracture.

Center activities are formulated partly on the basis of

materials' trends and problems, such as those identified

by the National Academy of Science. We find it especially effective, however, to plan our efforts through di- rect contact with the users of our services, frequently by

working in cooperation with our many "clients" in industry and government. Thus, our assessment of the needs

for materials' standards is aided by having members of our staff participate in more than 120 engineering standards committees concerned with measurements needed

for effective use of materials. We are helped both in pro75
NATIONAL MEASUREMENT LABORATORY

Robert E. Green Thomas M.

Gregory**

Stephen S. Hass**

Charles C-C. Han

Gerald R. Harris** William P. Harris Harley L. Heaton**

Ture Heline**

Elliot A. Kearsley James M. Kenney

Freddy A. Khoury

David E. Kranbuehl

Samuel J. Kryder

William V. Loebenstein David J. Lohse*

Robert E. Lowry

Curtis P. Mabie**

Cyrus G. Malmberg**

Gordon M. Martin Robert M. Marvin**

Arthur A. Maryott**

Mathai Mathew**

John R. Maurey

Jacob MazLir Frank L. McCrackin

Donald Mclntyre

Gregory B. McKenna

John E. McKinney

Daniel L. Menis**

Dennis B. Minor

Dwarika N. Misra**

Frederick I. Mopsik

George C. Paffenbarger** Robert W. Penn Anton Peterlin Joseph C. Phillips Walter J. Pummer

Darrell H. Reneker

Steven C. Roth

Robert J. Rubin

Nelson W. Rupp**

Isaac C. Sanchez

Charles P. Saylor**

George A. Senich Jack C. Smith

Leslie E. Smith

Jeffrey W. Stansbury

Shozo Takagi**

John A. Tesk

Elizabeth E. Toth

Ming S. Tung**

David L. VanderHart

Peter H. Verdier Gerald L. Vogel**

Herman L. Wagner

Francis W. Wang

Richard M.

Waterstrat** James J. Weeks, Jr. Eric P. Whitenton**

Richard P. Whitlock**

William K. Wilson**

Lawrence A. Wood**

Wen-Li Wu

Hyuk Yu

Louis J. Zapas

Metallurgy Robert Mehrabian

David B. Ballard Cletus J. Bechtoldt Lawrence H. Bennett

Francis S. Biancaniello

Peter J. Blau William J. Boettinger

Harry C. Burnett, Jr. Daniel B. Butrymowicz

Roger B. Clough

Gabrielle G. Cohen

Sam R. Coriell John R. Cuthill

Billy J. Evans

Steven E. Fick Anna C. Fraker Stephen C. Hardy M. Ashraf Imam**

Lewis K. Ives Daniel J. Kahan

Masao Kuriyama

Kenneth C. Ludema

John R. Manning

Thaddeus B. Massalski George J. Mattamal

Archie J. McAlister Simon C. Moss

Joanne L. Murray

Lucien A. Nedzi**

Robert L. Parker Michael E. Read

Robert C. Reno

Steven T. Ridder Arthur W. Ruff, Jr. Robert J. Schaefer Robert F. Sekerka

Robert D. Shull John A. Simmons

Gregory B. Stephenson

Pei Sung**

Lydon J. Swartzendruber Ann C. Van Orden

Richard E. Watson Morgan L. Williams

Ceramics, Glass, and Solid State Science Hans P. R. Frederikse Michelle C. Austin Michael I. Bell Rebecca A. Bird Douglas H.

Blackburn

Stanley Block

William S. Brower,

Jr. Simon J. Carmel

Mario J. Cellarosi Given W. Cleek**

Martin I, Cohen

Lawrence P. Cook

David J. Cronin

Marilyn J. Dodge

Louis P. Dominquez**

Eloise H. Evans** Edward N. Farabaugh

Albert Feldman

Thomas A. Hahn

Wolfgang K. Haller Vicky L. Himes

William R. Hosier Camden R. Hubbard

Arnold H. Kahn

Michael J. Lynch**

Floyd A. Mauer

Howard F. McMurdie**

Alan D. Mighell

Marlene E. Morris**

Ronald G. Munro

Taki Negas

Craig D. Olson

Helen M. Ondik

Boris Paretzkin** Harry S. Parker

Alvin Perloff Gasper J. Piermarini Carl R. Robbins

Robert S. Roth

David M. Sanders Richard D. Spal

Judith K. Stalick Jon L. Waring

Roy M. Waxier

John T. Wenzel

Grady S. White

Reactor Radiation Robert S. Carter Norman F. Berk

Russell C. Casella Yu-Tarng Cheng

Donald A. Garrett Charles J. Glinka

V K"' w-r-fiman*

Jeffrey W. Lynn

Bernard Mozer

John H. Nicklas Edward Prince Tawfik M. Raby

James J. Rhyne

J. Michael Rowe

John J. Rush

Antonio Santoro James F. Torrence

Alexander Wlodawer

Post Doctoral Staff

* Research Associates and Guest Workers

gram design and execution by 22 industry-supported

Research Associates who work with our staff. In addition,

we have attracted 54 guest workers from industry and

universities. We also work closely with other government

agencies. In fact, the Center currently provides technical

assistance to approximately 60 different organizations

within the Federal Government. About 40 percent of our materials program is carried out to provide direcl support to other government agencies. In addition, we

sponsor or co-sponsor national or international confer

ences, symposia, and workshops.

The technical activities of the Center are carried oui

by six divisions: Chemical Stability and Corrosion; Frac

ture and Deformation; Polymers Science and Standards

Metallurgy; Ceramics, Glass and Solid State Science; and

Reactor Radiation. These divisions vary considerably ir their balance between theory and experiment, betwee

direct standards work and research, and in their orienta

tion toward industrial needs and the needs of othe

components of the scientific and technological co

munity.

For instance, to meet the needs of the metals process

ing and manufacturing industries for data required to develop durable, high performance alloys, the Cente

carries out theoretical, experimental, and data-analysi

activities on alloy phase stability. The needs for accurat phase diagram information on new alloy systems hav

been addressed frequently in recent years, and a join

effort with the American Society of Metals (ASM) or phase diagrams is now underway. In this joint progran

the Center provides overall guidance to the program t assure reliability of the data evaluations, to maintai

coordination among participating phase diagram center (most of which are in the private sector), and to develo

techniques for the graphic presentation of phase diagrar

data. ASM will serve as a direct interface with the meta

industry so that needs and priorities can be properly dc termined. An international council involving 70 men

bers from nine countries will advise us on this effort. Tw

significant features of this joint effort are the publicatio

of a new bulletin for rapid information transfer and th availability to industry of evaluated data directly froi ASM.

One unique feature of our structure/property scienc

work is the 10-megawatt research reactor located at oi

76
;enter for materials science

•aithersburg site. The low energy thermal neutrons

nitted by the reactor are used in the structural analysis

f complex and sophisticated materials. Researchers in- olved in such analyses come from a variety of government agencies, universities, and industrial organizations,

forking with our staff, these investigators have studied

tany materials and material transformations—among

lem cements, magnetic materials, and battery materials,

'lost recently, they have carried out fundamental studies

f the chemistry of ceramics by neutron diffraction profile

nalysis, which promises to be a powerful new tool for

"rystal structure analysis.

We are constructing a special facility at the reactor

abr small angle neutron scattering, to be used, for ex- ample, in the study of polymer structures. Next year,

re will develop an additional facility at Brookhaven

aboratory for the application of synchrotron radiation

p materials. This application, we feel, is to be the next

eneration of x-ray techniques for structural studies of

rystalline materials.

Similarly, we are broadening our research on the

:ructure of noncrystalline materials through new studies n glassy materials which are formed by deposition from

jie vapor phase. Under construction is a film deposition

pparatus which allows the simultaneous codeposition rom separately controlled molecular beam sources. Deposition rates will be monitored during observation of optical properties of the forming films. In addition, the

eposition facility will be coupled to a diagnostic facility,

llowing in vacuo transfer and analysis by several elec- ron and particle emissive techniques. When operative,

.te facility will be used for the production of reflective

nd antireflective optical coatings and flat configuration

aveguides, and in such fundamental studies as molecular cluster formation in glassy films, the determination of

accommodation coefficients in multi-component amor-

'hous structures, and in the simulation of amorphous

>xide films found on corrosion-resistant metals.

Additional work on structure/ property relationships

avolves investigations to define the molecular mechanisms in polymers associated with deformation and fracure. For polyethylene, these mechanisms are elucidated

hrough comparisons between Fourier transform infrared

FTIR) spectra of undeformed and fractured specimens.

>mall variations in the concentrations of carbon-carbon

double bonds, methyl groups, and carbonyl groups result from rupture of carbon-carbon backbone bonds and subsequent free radical reactions. These concentration

changes can be detected in difference spectra obtained

through subtraction of the digital FTIR data. The detectability of small concentration variations is enhanced

by the superior sensitivity of the FTIR method over conventional dispersive infrared spectroscopy. In addition,

the FTIR method has potential application in the study

of polymer matrix composites, a group of new materials

of interest to Center scientists. These materials are finding increasing use as structural parts in the transportation

industry because they can be fabricated into lightweight

structural components.

The anticipated durability of materials and products

in service is a central consideration in their selection and

use. Thus, a major aspect of the Center's materials program concerns corrosion abatement as well as other as- pects of materials durability including wear, which is the

primary cause of materials degradation in operating machinery; fracture, which is the cause of catastrophic

failure of structural components; and the migration of

plastic stabilizers, which is one of the principal causes of

failure in plastics.

One example of our research in measurement and cor- rosion science is localized corrosion wherein a metal is attacked at specific sites instead of being dissolved generally, leading to pitting, stress corrosion, and crevice corrosion. When it happens, failure can result even when

most of the metal is relatively unattacked. In fact, local- ized corrosion accounts for the majority of corrosion

failures. This form of corrosion is the most difficult to

detect, since the total amount of metal dissolved prior to

failure may be small. We consider localized corrosion to be a major technical challenge and are working on several approaches to help understand and control it. In particular, Center scientists are developing measurement

methods to determine the susceptibility of a variety of

metals to localized corrosion. This information is essential for developing standard methods of assessing the

suitability of a metal for service in a given environment

and for the developing of new, corrosion-resistant alloys.

Another and a very important aspect of materials

durability is fracture control. Improved fracture control

methodologies depend upon predictive models. In fact,

77
predictive models coupled with nondestructive evaluation

techniques are becoming increasingly important in developing materials performance codes and standards, in- cluding those for metals. For example, in 1976, Alyeska

Pipeline Service Company requested waivers from the

Department of Transportation (DOT) for a number of

buried girth welds. The welds contained radiographically

detected defects which appeared to be not in conformance

with DOT regulation. The waiver was based on a fracture

mechanics analysis. In anticipation of the waiver, DOT requested assist- ance from NBS in evaluating the fracture mechanics

analysis and the nondestructive evaluation methods used

to detect and determine the dimensions of specific girth

weld defects. In large part because of the analysis provided by Center scientists, DOT did eventually conclude

that under some circumstances, exemptions from existing

standards could be granted if warranted on the basis of

fracture mechanics test results. Thus, more petitions for

exemptions can be anticipated. The Center is working

with DOT now to establish a technical framework for

ruling on these waiver requests—a framework which

would minimize the need for subjective judgments and

thus obviate a tendency toward overly conservative

design.

The breadth, scope, and accomplishments of the Centers technical program are comprehensively reflected in the more than 420 articles by Center scientists published

in 1979 in numerous professional journals, books and

conference proceedings. The following brief reports highlight some current work of particular interest.

78
CENTER FOR MATERIALS SCIENCE

Erosion of Brittle Materials

by Solid Particle Impact

Sheldon M. Wiederhorn, Bernard J. Hockey, A. William Ruff and Lewis K.

Ives, Fracture and Deformation

Division

n an effort to provide a better

; understanding of the micro-mechanics

of erosion and other impact-related

processes, the detailed nature of

j damage produced in ceramics and

i metals by impingement with solid

particles has been investigated.

Results obtained by scanning and

transmission electron microscopy

| clearly specify the elastic-plastic nature of the impact. This has led to

j the development of models which

I incorporate the concept of elastic- plastic fracture in describing the

erosive wear of brittle materials.

] Erosion is a major cause of material

degradation in both civilian and military applications. It is caused by the

high velocity impact of solid particles or liquid droplets against solids. Although each impact causes only a minute amount of damage, the cumulative effect of millions of impacts can

result in appreciable loss of material

from structural components. Further- more, even the minute damage caused

j by the impact of a single particle often

i can significantly reduce the strength of

brittle materials. In many military applications, erosion shortens the lifetime and increases

1 the cost of replacement of mechanical

components. The operation of gas tur- bines in dusty terrains, for example,

1 reduces turbine lifetime to one-tenth

jl the normal value. As a result of this shortened lifetime, the cost of replacing turbines in helicopters used in Southeast Asia amounted to $150 million in 1970 alone. Other examples

of component degradation include:

the erosion of radomes by water, ice,

and dust; the damage of optical components such as laser windows by rain

and dust; the damage of turbine and

rocket components by combustion

products; and the erosive thinning of helicopter rotor tips by dust. Because of their outstanding erosion

behavior, ceramic materials are often

used in applications where erosion re- sistance is required. Thus, castable re- fractories have largely replaced metals

in certain high wear areas of oil refin- eries and are being considered for use

for similar applications in coal gasifica- tion plants. In another application,

ceramics have been usefully employed

as nozzle vanes in experimental radial- flow gas turbines, where they have exhibited a pronounced superiority over

the standard metallic vanes in resisting

erosion. Ceramics are also used in a number of applications where other

properties are important, but where

erosion is a factor in limiting reliability. In windscreens and radomes for ex- ample, ceramics are used because of

their transparency to electromagnetic

radiation, but must also be able to

resist erosion to maintain their integrity. Similarly, in turbine nozzles and

vanes, ceramics must withstand both

high temperatures and erosion from

combustion products and dust. The few examples given above

underline the need for improved erosion resistant ceramics, the development of which could, in principle, be

assisted through a better understanding of the erosion process. Studies

have been conducted primarily through

the use of the scanning and transmission electron microscopes. Erosion rate measurements have also been made in a variety of chemical environments,

and at temperatures ranging from room

temperature to 1000 °C. The combination of techniques employed on this project has led to the development of
NATIONAL MEASUREMENT LABORATOFT

a deeper understanding of the erosion

process, and to the development of new

elastic/plastic models of erosion that are particularly applicable to brittle materials. These models are based on

the observation that all erosion of

ceramic materials, no matter how brittle, is accompanied by plastic deformation.The earliest indication of plastic deformation during erosion of hard brittle materials, such as A12 03 , SiC and Si, came from examination of individual

impact sites by light microscopy. Residual plastic impressions were found to be associated with cracks that formed

as a result of the impact. The damage

resulting from impact is, in fact, very

similar to that produced during hardness testing using Vickers or Knoop

diamond indentors. For soft materials

such as LiF or MgO, etch-pit tech- niques have been used to show that dislocation slip-bands are associated

with impact damage or hardness impressions. These observations naturally suggest that similar deformation

accounts for impact impressions in hard materials such as Si, A12 03 and

SiC. However, direct evidence for

plastic flow in these materials was difficult to obtain, because the dislocations produced by impact are closely

confined to the impact site. This

close proximity of the dislocations to the plastic deformation was observed

for all materials studied (Ge, Si, A1203 , SiC, MgO), but the extent of deformation and the morphology of the cracks

associated with the deformation was

dependent on the crystal structure. MgO, an ionic crystal, exhibited the most extensive deformation at impact

sites; cracks that formed (on 110

planes) in this crystal were always

confined within the zone of plastic

deformation. Quite the opposite behavior was observed for Si and Ge,

which are covalent materials and more

80

brittle than MgO. For these materials,

plastic deformation was confined to the near vicinity of the impact site, while

cracks propagated relatively large dis- tances into the crystal. The behavior

of SiC and AI0O3 was somewhat inter- mediate, the extent of deformation

being approximately twice the size of

the residual impact impressions. Cracks

in SiC and A12 03 propagated beyond

the deformation zone and were arrested

in materials that was free of dislocations. A qualitative comparison of this type of erosion damage with that oc- curring in metals indicates a great similarity for both types of materials with

regard to the density and extent of

dislocation formation at the impact site

( figure 2 ) . For both types of materials, the density of dislocations at the contact site is so high that the crystal structure is severely strained; fruitful examination by transmission electron microscopy is not feasible. Dense

tangles of dislocations result in complete scattering of the transmitted elec- tron beam. At the edge of the impact

site, the dislocation density drops

abruptly to the density characteristic of the undamaged crystal. Ceramics and

metals differ in appearance because

metals are able to accommodate the mechanical strains resulting from the impacting particle, whereas ceramic

materials crack when subjected to high

strains. As a consequence, crack for- mation is normally observed during the erosion of ceramics. In fact, it is this crack formation that normally accounts

for material loss during the erosion of ceramics.

Investigations of impact damage by

light microscopy and by scanning j ^ electron microscopy are useful for revealing the relative extents of frac- ture and plastic flow. Plastic deforma- i tion in ceramic materials is enhanced

when particles strike target surfaces j [. at low angles of impingement. Scan-
:enter for materials science

ning electron micrographs of the damage surface suggest extensive plowing

as a consequence of the particle impact (figure 3). At low impact angles, fracture during contact is greatly re- duced. The damage sites are often

free of cracks, and plastic deformation

appears to be the primary mode of material removal. These observations are confirmed by transmission electron microscopy studies, which show that crack-free plastic indentations are fre- quently obtained in brittle materials

^for small angles of impact.

1 Based on scanning and transmission

electron microscopy data obtained on

ceramic materials, several models of 'erosion have been developed recently

J which incorporate concepts of elastic-

'plastic fracture into erosive wear. The

formation and growth of plastic zones

during the impact process determine

the types of cracks that are formed and

hence, the amount of strength degradation and material loss from brittle materials during erosion. These theories are currently being tested in the Center

for Materials Science in order to develop a body of knowledge that can be

used to model the lifetime of ceramic

components in environments where

erosion is important. Studies suggest

that the hardness, H, and the critical stress intensity factor, Kc, are the two

materials properties that control erosion: a strong dependence of erosion

rate on Kc, but a weak dependence on

H, is observed. Although the available

experimental data supports the view

that Kc and H (measured using dynamic loading) are independent of temperature, additional research is needed to confirm this conclusion. Consequently, future research in the Center

for Materials Science will concentrate on the dependence of erosion on Kc and H, and on the effect of temperature on these parameters.

Atomistic Theories of Crack

Growth and Fracture

Edwin R. Fuller, Jr., Robb M. Thomson, and Brian R. Lawn 1 Astructural component that

fails by a brittle fracture mode generally involves the growth of an intrinsic flaw, or crack under sustained

load until it attains some critical size. The desire to understand this crack

growth and fracture from a fundamental level has provided considerable

impetus to the atomistic modeling of crack tips and crack-tip processes. While these models are usually overly

simplistic in relation to real structures and materials, they demonstrate the connection between microscopic processes in the crack-tip region and

macroscopic fracture parameters; they

thereby provide a qualitative framework for considering the complicated

phenomena of crack growth and fracture. Only through this fundamental

understanding can more realistic models be developed and more quantitative calculations be performed that permit an assessment of crack growth

behavior, and accordingly, of component reliability from more fundamental principles.

Our studies in the Center for Materials Science have been to develop

atomistic models of a cracked solid,

which minimize the mathematical complications that necessitate elaborate

computer calculations, yet which contain the essential elements of the frac- ture process and do not preclude general conclusions. Reconsidering an

earlier model of Thomson et al [1], we

were able to decouple effectively the nonlinear interatomic interactions in the crack-tip region from the linear

elastic interactions in the bulk of the

solid [2,3]. This stratagem provides a mechanics model within which the in- fluence of the crack-tip interaction can

Figure 3. Impact crater in dense A10 0,, 3-5 (im grain size, caused by 150 fim SiC particles

'i at a velocity of 90 m/s and an impingement angle of 15°.

1 On study leave from University of New South

Wales, Australia.

81
NATIONAL MEASUREMENT LABORATORY

be explored. It was into this framework

that we were able to incorporate, again

in a simplified manner, the molecular

interaction of a gaseous species with the crack-tip atoms to obtain a theory of chemically assisted crack propagation [4,5]. In the following section we describe

the lattice model of a crack we have

considered, and indicate how the atomisticity of the problem leads to a number of mechanically stable crack configurations ("lattice trapping"), from

which the crack can advance by

activation. The final section discusses

how a molecular chemical reaction can

be incorporated into this mechanics

model to give a qualitative model of chemically assisted bond rupture.

Intrinsic Bond Rupture. Our atomistic model of a crack in a quasi-onedimensional solid is shown in figure 1.

It consists of two semi-infinite chains

of atoms that maintain their rigidity by means of an interatomic interaction. This interaction is modeled as a flex- ural spring element with force constant

/?. The chains of atoms are bonded to- gether by a series of cohesive interactions between the chains, modeled as stretchable spring elements between

the atom pairs. To form a crack, we

suppose that the first n stretchable ele- ments are ruptured, or stretched beyond their assumed finite range of

interaction. We assume further that the remaining cohesive spring elements are

P

Figure 1. Atomistic model of a crack in a quasi-one-dimensional solid.

in the linear regime of their force- displacement response with spring constant a. The one exception is the cracktip interaction which, by the necessity

that it must rupture to advance the

crack, must be allowed to be in the nonlinear regime of its response. Transverse opening forces P are applied to the end atoms of each chain to force open the crack. The total potential energy of this crack system is given by [3] U = — 2Pu„ + /JS°° [uj+ i — 2Uj

j=i

+ u^x]2 + nUan + U„(2un ) + Vi [2 Uj ]-, (1)

j =n+l

where the first term on the right hand

side is the potential energy of the loading system; the second term is the

strain energy stored in the flexural bonds; and the remaining terms are the contributions from the cohesive bonds

across the crack. These cohesive contributions are, respectively, the total bond energy from the n ruptured bonds,

the bond energy of the nonlinearly

strained bond at the crack tip, and the

linear-elastic strain energy of the re- maining cohesive bonds. From a frac- ture mechanics viewpoint, this potential provides both the driving force for

fracture and the resistance force to

fracture. The crack driving force is derived from the stored elastic strain energy which is released upon crack

advance; whereas, the resistance to fracture is contained in the cohesive

bond energy of the crack-tip bonds that must be ruptured to advance the crack. In contrast to a Griffith continuum

analysis, where the fracture resistance

is provided by the average cohesive

energy required to form the new ele- ment of surface, the atomistic treat- ment reveals a fine structure in this process through the discrete nature of the bond rupture event. To illustrate this atomistic influence, we examine

the variation of the system potential

energy as the crack-tip bond ruptures.

We require, however, that for each

crack-tip bond displacement the re- maining atoms of the solid are in an

equilibrium configuration with respect

to this displacement. Necessary conditions for these equilibrium configurations are

[3U/?(2uj)] = 0, forj^n, (2)

which correspond to an infinite set of

fourth-order difference equations. Analytical solutions of these equations are j obtainable 131 in the functional form

Uj(P, n, u„). Substitution of these equilibrium-displacement solutions into Eq.

(1) accordingly reduces the system potential energy to an explicit function of ; both the applied fracture mechanics

parameters (load and crack length) and

the crack-tip bond displacement

8n = 2u„:

U(P,n,8„) = nUBB - (PV6/J)

(2n :; + 3n2 t + n) — KSn + Ub (8n) + Vi tt (_L=J_)8 2, (3)1

where is an elastic coefficient defined

by 1) = (2/3/«), and K = (P/£>

(n-j-t) is an effective one dimensional

"stress intensity factor". This expression is particularly useful

for demonstrating the origin of the atomistic energy barrier to intrinsic bond rupture and crack advance. Fo-r

cusing our attention on the n

th bond,

the first two terms on the right hand

side of Eq. (3) are constant for a given

applied force and will not be con-j

sidered further. The next term, — K8„,

corresponds to the reduction in potential energy that results from the applied

fracture driving force as the crack-tip

bond ruptures. The two remaining

terms represent the resistance of the system to the rupture of the crack-tip

bond. The first of these resistance

82
i, CENTER FOR MATERIALS SCIENCE

.terms. Ub (8n), corresponds simply to the interatomic interaction between the

: £ — 1 term, Vi "C^,—^ «' * s more su^^e

: crack-tip atoms; whereas the second

• and corresponds to the harmonic re-

- storing potential exerted on the cracktip atoms by the remainder of the

(linear elastic) structure. It is the bal- ance between these last three terms that

i determines the atomistic energy barrier, las illustrated graphically in figure 2. -;The individual terms are plotted in

r figure 2a and their sum is shown in -figure 2b. Clearly, the stress intensity

, factor term, — K8U , biases the bond

- rupture to make the final state the more

: stable configuration in this example.

Chemically-Assisted Bond Rupture.

' An important part of the above construction was that the interatomic potential between the crack-tip atoms was

effectively decoupled from the interactions of the remainder of the solid. Accordingly, this construction provides a mechanics model whereby an arbitrary

I bond-rupture reaction can be studied in jan equivalent linear elastic continuum

I t_\

of crack-tip stiffness a (

- ). An apI propriate crack-tip reaction to consider

iis a bimolecular chemical reaction. It

1 provides a qualitative model for understanding stress corrosion cracking in a

H

gaseous environment.

We begin by reviewing the energy

' surface for a bimolecular chemical

j reaction of two free molecules (similar

I to Lawn and Wilshaw [6]. Consider

the reaction depicted in figure 3 where

; a gaseous molecular A2 interacts with

ithe crack-tip bond —B—B— to form

;the product molecule A—B—. Several

< possible potential energy surfaces for

. this four-atom complex are depicted in

figure 4 as a function of the B—B bond

Figure 2. System potential energy as an explicit function of the crack-tip bond dis- placement, 5

n

: (a) Individual contributions; and (b) Sum of these contributions, illus- trating the stable initial and final config- urations (i) and (f) respectively, and the activation state (/*) through which the sys- tem passes to rupture the crack-tip bond. AUT is the activation energy barrier to this intrinsic bond rupture, and aU- is the energy barrier to the activated healing of

this bond.

Figure 3. Schematic of a bimolecular chemical reaction to rupture the crack-tip bond. A gaseous A., molecule reacts with the crack-tip bond — B — B— to produce the

'"terminal" surface species A — B —

(exothermic)

Figure 4. Possible potential energy surfaces

for the bimolecular chemical reaction illus- trated in figure 3. The chemical reaction reduces the bond-rupture energy or thermo- dynamic surface [curves (2) or (3)] compared to the nonreacted surface [curve (1)].

displacement. We assume that the

A—A atoms arrange themselves in the most energetically favorable position

for each value of the B—B bond dis- placement. Comparison of curve (1)

with curves (2) or (3) show that the presence of the A2 molecule makes it easier to dissociate the B atoms. Generally this reaction occurs with an

activation energy barrier that could result, for example, from the repulsive

interaction of the polar component of the AB molecule. If the reaction is en- dothermic, the final state of 2AB is metastable with respect to that of A2 -f- BB, and the reverse is true for exothermic reactions. The procedure for inserting this chemical reaction, via a modified co- hesive force law, into the constraint of a lattice crack is the same as that used

in the construction of figure 2b. The

initial and final equilibrium configurations now correspond to nonreacted

and reacted (adsorption) states of the bond, respectively. The unstable configuration relates to the activated complex of the reaction. The exact in- fluence of the chemical reaction on the rupture of the crack-tip bond depends on the details of the chemical reaction

and its activation barrier. Various possibilities have been considered in a re- cent manuscript [7]. The main effect of the chemical reaction is to further

bias the intrinsic activation energy

barrier and to allow crack growth to occur at a lower applied stress. Another important conclusion is that the thermo-dynamic surface energy, which

determines the thermodynamic condition for crack propagation, is also lowered to that of the chemically re- acted surface.

References

[1] Thomson, R. M.. Hsieh, C, and

Rana, V., Lattice Trapping of Frac83
NATIONAL MEASUREMENT LABORATORY

ture Cracks, J. Appl, Phys. 42, 3154-

3160 (1971 ). [2] Fuller, E. R., Jr. and Thomson, R. M.,

Nonlinear Lattice Theory of Fracture,

in Fracture 1977, D. M. R. Taplin,

ed. Vol. 3, pp 387-394 (Univ. of Waterloo Press, Canada, 1977).

[3] Fuller, E. R., Jr. and Thomson, R. M.,

Lattice Theories of Fracture, in Fracture Mechanics of Ceramics, R. C.

Bradt, D. P. H. Hasselman and F. F. Lange, eds. Vol. 4, pp 507-548 (Ple- num Publishing Corporation, 1978).

[4] Fuller, E. R., Jr. and Thomson, R. M.,

Theory of Chemically Assisted Fracture, in Proceedings of the Third

International Conference on Mechanical Behavior of Materials, K. J. Miller and R. F. Smith, eds. Vol. 2, pp

485-494 (Pergamon Press).

[5] Fuller, E. R., Jr. and Thomson, R. M.,

Atmosphere Assisted Fracture: II. Atomic Models of Crack Growth",

(J. Matl. Sci., in press).

[6] Lawn, B. R. and Wilshaw, T. R., Fracture of Brittle Solids, Section 7.5 (Cambridge University Press, Cambridge, 1975).

[7] Fuller, E. R., Jr., Lawn, B. R. and

Thomson, R. M., Atomic Modelling

of Crack-Tip Chemistry (to be sub- mitted to Acta. Met).

Neutron Diffraction Profile

Analysis and the Structures of

Electronic Ceramics

Antonio Santoro and Robert S. Roth,

Reactor Radiation Division

In recent years it has become evi- dent that certain crystalline phases

allow cations and anions to move

through the crystal structure of which

they are part much more quickly than

had previously been believed possible.

Such materials, which include, for example, sodium /^-alumina and cubic

stabilized zirconia, are often called "super-ionic conductors." In an effort to better understand the mechanisms

involved in super-ionic conductivity,

the phase equilibria, crystal chemistry,

and crystal structure of these materials

are being examined in laboratories

throughout the world. In fact, only a thorough understanding of the possible

mechanisms of conduction in different types of conductors will enable a more

accurate prediction of properties and

allow the researchers to tailor-make

solid ionic conductors for specific applications in batteries, fuel cells, etc.

The two most promising classes of possible ionic conductors have 0~2 and

Li+ ions as the mobile species. Conventional oxygen ion conductors have

the fluorite-type structure with oxygen

vacancies, and the conduction involves some type of hopping mechanism

through the vacancies. However, it has

been found that a new group of compounds based on the scheelite- and

BaMnF4 -type structures may accom- modate oxygen ions in interstitial positions which then become the vehicle for the ionic conduction. On the other

hand, the Li+ containing materials

have structures with channels suffi- ciently large to permit the motion of the conducting ions. A problem associated with the study

of compounds of the type mentioned

is related to the fact that heavy and

light atoms are present simultaneously

in the structures. It follows that x ray

diffraction cannot lead to an accurate knowledge of the atomic distribution,

and neutron methods are the only

alternative. In addition, many of the materials of interest cannot be pre- pared as single crystals of sufficiently

large size for single-crystal work and

have to be analyzed by powder dif- fraction. Because of the random orientation of the crystallites in a powder,

Figure 1. View from the top of the counter arm of the five-counter diffractometer. The

counters are placed 20° apart and simultaneously measure different regions of the pattern. The sample is located at the geometrical center of the diffractometer. 84
CENTER FOR MATERIALS SCIENCE

the three-dimensionally diffracted intensities are collapsed into a monodimen-

"sional distribution, which results in a

loss of information about the spatial

properties of the crystal, and, if over-

- lapping of the diffracted lines occurs, about the intensities associated with

each set of crystallographic planes. The

Most information can be partially re- covered if the powder patterns are

: measured with diffractometers of high

resolution, and if, instead of using in- tegrated intensities in structural refine- ments, one fits the entire observed pro-

'•file with one calculated in terms of a

c plausible model of the structure ("Rietveld Method," or "Profile Analysis").

Not surprisingly, the application of

• the method of profile analysis in the

5 U.S. and in foreign laboratories has

become increasingly important in the study of a wide class of polycrystalline

- materials, and numerous improvements

of both the experimental techniques and

data refinement methods have been

•proposed or applied. In the past few

years, scientists and technicians at the NBS reactor have established the fore-

' front capability in the U.S. in the

| methods and applications of neutron

diffraction-profile analysis and have de- veloped a new multidetector high resolution diffractometer for profile analysis studies. With this facility, illustrated in figure

1, five counters are used simultaneously. The resulting gain in intensity

may be used to reach an instrumental

resolution such that the natural broadening due to the physical conditions

of the sample becomes observable and

constitutes the predominant factor in determining the width of the diffracted

lines. At the NBS reactor, studies of possi- ble ionic conductors are underway as part of a collaborative effort between

scientists of the Reactor Radiation Division and the Ceramics, Glass, and

Solid State Science Division. Compounds with both O

-2 and Li+ ions

have been, or are being, examined with

powder neutron diffraction. The struc- tures of CeNb04 , CeTaO,, and NdTa04 have been well refined so that reasonable conjectures can be made about the

relationship between the stiochiometric

and the oxygen-rich phases obtainable

by oxidation of some of these compounds. Similarly, the structures of LiNb3 Os , M-LiTa308 , and Ta2W08 have been accurately measured as part

of an attempt to determine the still unknown positions of the Li+ ions in

the structure of H-LiTa3 Os . As an

example of the results obtained in these

studies, the calculated and observed

profiles for the structure of LiNb3 Os are compared in figure 2. More recently, it has become ap- parent that the structures of many ionic

conductors are modulated. Modulation,

for example, has been clearly established in CeNbO.i.0K, and there are good

reasons to believe that it is also present

in H-LiTa:1 Os . Little is known about

modulated structures, and even their definition is still being debated. We

may define modulation from a strictly

crystallographic point of view in the following way:

We may say that a structure is "modulated" when it has one or more structural "features" having a periodicity

which is "inconsistent" with, or "in- commensurate" with respect to, the

periodicity of the bulk of the structure. When this condition occurs, the ratio of the two periods is not a rational number (perhaps, we should say that this ratio is not "a simple" rational number). The "features" which cause the

modulation may be: 1) positional in nature (i.e., shifts from the positions

that one or more atomic species would

have if the structure were unmodulS 4

i? 3

"3I 2 c1 1

2 o10Li12 14 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64

Diffraction angle (degrees)

Figure 2. Calculated profile (continuous line) and observed data (dots) in the powder

pattern of LiNb,Os

. The close agreement between theoretical prediction and experimental

c'ata, shown, in the diagram, indicates that the refined structure is highly accurate. 85
NATIONAL MEASUREMENT LABORATORY

ated), 2) substitutional (i.e., one or more sets of positions, which would

be crystallographically equivalent if the

structure were unmodulated, have vari- able occupancies), or 3) a combination

of the two. The diffraction patterns of such

structures contain reflections corre- sponding to the periodicity of the bulk

of the structure ("main" reflections)

and reflections which are inconsistent

with, or incommensurate with respect

to. those of the first type ("satellite"

reflections). These definitions are necessarily provisional and may well be

modified when more information is gained about modulated structures. The solution of structures of this sort requires the development of new tech- niques for the interpretation and processing of the experimental data. Along

this line, we are at present developing

methods for indexing powder patterns

in which the presence of satellite re- flections is suspected or ascertained. At

the same time, least-squares fitting pro- grams are being, or have been, modified so that the intensities of satellites can be taken into account, and so that background and thermal motions of the atoms are properly treated. This

last development should be particularly useful in the study of nonstoichiometric

phases, in which one or more sets of equivalent positions are partially occupied.

We hope the NBS program to combine advanced profile refinement

methods, high-resolution neutron dif- fraction, and new approaches to deal- ing with and predicting incommensurate features in complex diffraction patterns will lead to a significant improvement of our understanding of

structure and properties of ionic conductors and broad classes of other crystalline materials.

Recent NBS Advances in Physical

Properties Measurements at Ultra High Pressures

Stanley Block, Ronald G. Munro, and

Gasper J. Piermarini

PJL^ res:essure is beginning to take its rightful place beside temperature as an

important research variable in the quest

to understand the nature and behavior

of matter. Two developments at NBS

are chiefly responsible for the recent blossoming of high pressure science:

(1) the capability to generate very large

static pressure and (2) the capability to measure it. As a result of these two

milestones, the discovery of new phases

and properties of materials, new in- sights into the physics of condensed

matter systems, the synthesis of new

materials, and the opportunity to test experimentally theories which pre- viously could be considered little more

than interesting conjectures have all become possible. A steady flow of re- finements and new applications are now

being developed in our laboratory.

Other developments can be found in the current scientific literature.

In the past, the direct generation of pressures by applying a known force

to a known area required the costly

construction of a large apparatus and

a considerable effort to operate it successfully. The development of the dia- mond anvil pressure cell (DAPC) at the National Bureau of Standards sur- mounted these difficulties. The NBS

pressure device obtains large pressures by using small areas, The substance

to be studied is placed between two

diamonds (see figure 1). When the dia- monds are squeezed together with a moderate force, a large pressure is transmitted to the substance because

of the small area presented by the faces

of the diamonds. The resulting apparatus (see figure 2) is compact, small

enough to be held in one hand, relatively inexpensive, easy to operate, and

readily adaptable to existing measure- ment equipment. Furthermore, the dia- monds provide optical and x-ray access

to the stressed sample. The result has

Plate

Hemisphere

Figure 1. Schematic cross-section of the Diamond-Anvil assembly of the NBS pres- sure cell. Two gem diamonds roughly 1/3

of a carat in size act as opposed anvils. The

sample is contained in a small hole (ap- proximately 0.5 mm in diameter) in the gasket located between the anvil faces. The

diamonds also act as windows for 180° and EPI optical access.

been an increase in the number of ex- periments that can be performed at high

pressure. Previously, work was res- tricted primarily to compressibility

studies, pressure-volume-temperature

relations, and electrical resistance measurements. The DAPC adds microscopic observations, infrared spectros- copy, x-ray studies, and, most recently,

viscosity measurements.

Quantitative use of pressure in re- search requires both a means to generate the pressure and a means to measure it. Pressure is force per unit area, and hence if a known force is applied

to a known area, the pressure is easily

computed. This direct method of pres- sure determination was the basis of early pressure devices, but severe problems in accuracy develop above about

3.0 GPa. Consequently, secondary

pressure standards are required for most of the work at high pressure. Fortunately, a very useful, rapid.

i

0 :.:

i

v.

i

- .

-

:

86
ENTER FOR MATERIALS SCIENCE

ranslating

diamond

Mount

late

Diamond

Anvils

[Siting Diamond

Mount lemisphere

basket

Pressure

Plate 'oke Pressure Plate Yoke Bearing

igl're 2. The mechanism for generating force consists of a spring loaded lever-arm

'psembly shown in cross section above. Belleville spring washers are compressed by the

: mple rotation of a screw. The force generated is transmitted through the lever arms to

ie pressure plate yoke which loads the piston containing one of the anvils.

Convenient, and versatile pressure scale

as developed at NBS using the fluores- ;nce spectra of ruby. The famous red

Dior of ruby is produced by chromium

npurities in the host material, alumi- num oxide, and the red fluorescence

-:nes known as the sharp R-lines occur

t well defined wavelengths. The presume dependence of the R^-line was

alibrated at NBS as a pressure scale. 'Vim the original calibration, pressures p to 30 GPa can be determined acurately. With increased use of the uby method, work to extend the scale 100 GPa has been reported by

athers recently. The temperature dependence of the scale has also been

xamined in other laboratories over the ange in 4.2 K to 358 K; and it has

een found that the rate at which the avelength of the R^-line changes with

bressure is nearly constant over the

ntire range. As a result of its conenience and its large range of utility, ie ruby fluorescence technique is the

tost widely used pressure scale presntly available for pressures above 1 PPa.

The impact of the ruby scale on high

pressure research with the diamond

anvil cell is indicated in figure 3. The

curve is the result of a survey we made

of the scientific literature from 1960

through 1978. After the introduction

of the DAPC in 1959, activity in- creased slowly for a few years, but

without a quantitative knowledge of the pressure, interest was limited. However,

after the introduction of the ruby pres- sure scale in 1972 and its calibration

in 1975, a dramatic increase occurred

in research activity which has not yet

reached its peak.

Among the recent advances associated with the increasing activity have

been the applications of x-ray energy

dispersive and single crystal techniques.

We have, for example, carried out with

F. Mauer of NBS a detailed study of

factors affecting intensity and resolution in energy dispersive powder dif- fraction in order to optimize the ex- perimental conditions for obtaining x- ray diffraction data with the diamond

anvil cell. The factors studied included

size and intensity distribution of the

TM 4

01

o<

2 -

Research Activity with the Diamond Anvil Cell

Ruby Scale

Calibrated at NBS

Ruby Pressure Sensor

Introduced by NBS

70 Year

Figure 3. The chart shows the intensity of research activity with the diamond cell relative to the year 1972—the year that the ruby pressure sensor was introduced by NBS.

The chart is based on results from a recent

literature survey.

white radiation source, absorption in the diamonds, beam divergence and

particle size of the sample, as well as collimator and slit system design. Improvements in equipment and tech- niques have led to obtaining better data in less time, and thus have en- hanced NBS capabilities for high pres- sure crystallographic studies. As examples, we discuss our results on CuCl, CdS, and ice VII. The behavior of CuCl under pressure has re- ceived much attention recently,

prompted by the reports that it undergoes a transition to a metallic state at about 4.0 GPa. Further, anomalies in compressibility, resistivity and magnetic susceptibility in the pressure range

of 4.0 GPa and in the temperature

range 4.2 K to 300 K have been re- ported, and the existence of superconductivity has been postulated for

the anomalous diamagnetic behavior.

The pressure dependence of the

electrical resistance of CuCl to pressures up 14.0 GPa was measured using our recently developed two-probe

method adapted to the diamond anvil

87
NATIONAL MEASUREMENT LABORATOf:

high pressure cell. The results of our

electrical measurements, which were made on four samples of CuCl pre- pared by different methods, do not show any evidence for a "metallic'

1 Ila Phase even after repeated cycling. We

also considered the possibility of a photo-conduction effect in the CuCl

samples. The electrical measurements

were performed in the absence of light, in dim red light, and in white light,

and no significant photo-effect was observed. In additional experiments done

in collaboration with A. Jayaraman,

et al., at Bell Telephone Laboratories

with a piston cylinder device, no large

resistance drop was observed in the range of 0.1 to 5.0 GPa, which is in agreement with our diamond cell results. We have also performed x-ray measurements on CuCl. We used both film techniques with ungasketed CuCl samples and gasketed samples with our newly developed energy dispersive

method mentioned previously. Our re- sults confirm the transitions from zinc- blende to tetragonal structures at 4.4

GPa and from tetragonal to the NaCl

type structure at about 8.0 GPa. Our

x-ray pattern of the black opaque

phase taken in an ungasketed diamond

anvil consists of NaCl-type lines. We

therefore believe that in the NaCl

phase of CuCl, a disproportionation re- action sets in, in the presence of large

shear stress. In summary, we found no

evidence for a dielectric-transition in CuCl.

In collaboration with G. E. Walrafen of Howard University, we have

carried out high pressure Raman and

x-ray diffraction experiments on ice VII. The group at Howard University

did Raman studies in a diamond anvil

cell. They determined that the O-H

vibrational frequencies of ice VII shifts downward with pressure and they ex- pected that the shift would correlate

with 0-0 distances. We carried out x- ray powder diffraction studies using

wavelength dispersive film and energy

dispersive counter techniques. We

found that the cell dimensions changed

from 33.55 nm at 2.36 GPa to 30.35 nm

at 20.2 GPa indicating a AV/V, of 26

percent and a shortening of the O-O

bond distance from 29.06 nm to 26.38 nm. There is a sharp break in the compressibility at approximately

15.86 GPa with very little change in A„ (30.52 to 30.35 nm). This probably

indicates that the repulsive forces be- come highly significant.

At the request of Watervleit Arsenal we are studying the electrical and

structural properties of CdS as a function of pressure and temperature. The

semiconductor-conductor transition has

been measured electrically, by energy

dispersive x-ray diffraction, and observed microscopically. The transition

is dependent on particle size purity.

Single crystals transform at 2.65 GPa,

while powders transform at 3.25 GPa.

Interestingly, the slope AP/APtl. a iis is negative. At 100°C, the transition oc- curs at 2.49 GPa and at —44°C, the

transition is at 3.24 GPa. All measure- ments were performed in a 4:1 methanol-ethanol solution and were there- fore hydrostatic.

We also made x-ray diffraction measurements above and below the CdS

transition. The transition is from the zincblende structure where each atom

is tetrahedrally coordinated to the so- dium chloride structure where each

atom is octahedrally coordinated. Pre- liminary results indicate that the low

pressure phase has a volume compression of — 3.1 percent to the transi- tion and an additional — 19.3 percent

upon transforming to the high pressure form.

The compressibility of S,N4 was

studied with J. K. Stalick of NBS to a pressure of 4.2 GPa through use of a

I m

1.5 2.0 2.5 3.0 3.5 4.0 4.5 Pressure, GPa

Figure 4. Viscosity of n-butyl chloride as function pressure at 20 °C. The equilibriun

freezing pressure is 2.3 GPa: above thii pressure, the liquid is metastable (super

pressed ).

new type of diamond anvil pressun

cell designed for single-crystal x-ra^ studies. A single crystal of S4 N4 wa:

first mounted on a glass fiber and tht unit cell determined by centering 15 re<. flections on the Syntex diffractometeri The cell parameters of the monoclinio

unit cell (a = 87.91(1), b = 71.73!

(13), c = 87.06(1) nm;/? = 92.47(1)°

numbers in parentheses represent un

certainty in the values) agree witl those obtained from a single crysta

placed within the pressure cell with n( external pressure applied. Unit eel data were also determined at pressure

of 1.3, 3.0, 3.7, and 4.2 GPa. At 4'J GPa, the cell parameters decrease to

a = 81.56(21), b = 66.67(5), c =

81.62(8) nm; « = 89.93(7), p =

91.74(16), y = 89.90(15)°. The vol ume of the unit cell decreases fron 5.481(2)X105 nm3 to 4.437(13) X 10

nm3

, a reduction of 19 percent, whif. the a and c axial lengths become equa

within the accuracy of the measure

ments. The monoclinic metric symme

try is retained to 4.2 GPa.

88
•

: 'ENTER FOR MATERIALS SCIENCE

Since the diamond anvil cell places

;vere restrictions on the acquisition

f data, especially with respect to

ngle, a theoretical investigation is eing conducted to examine the con- equences of these limitations for radial

istribution function analysis. The

nethod of investigation is to form a omparison of the reliability of various

omputational procedures and to contruct the comparison as a function of he degree of data limitation. An exact

et of data having an exactly known

adial distribution function is used so

hat the absolute error can be deter- nined in each case. The recently developed diamond anil cell falling-sphere viscometer has

>een utilized to measure the pressure lependence of the viscosity of stable

.nd metastable n-butyl chloride (see

igure 4). This substance was chosen as

" uitable for a cooperative intra-NBS

investigation of metastable states of

-natter. The equilibrium freezing pres-

{ ure (2.3 GPa) and the glass transition Pressure (4.2 GPa) were determined at oom temperature by separate measure- nents to identify the region of metatability (2.3 GPa < p > 4.2 GPa)

• see figure 5). The viscosity was then

obtained by starting at a pressure of

ill. 94 GPa. This transport property was

i ound to vary smoothly through the

transition from the stable to the metastable state.

> The viscosity results for n-butyl

Chloride were also used in conjunction

vith the critical point viscosity model,

7 =A(1-P/Pe)-"

(A-here -q is the viscosity, PK

is the glass

transition pressure, and A and v are pther parameters. Use of the model

r akes it possible to obtain a reasonable

estimate of the glass transition pressure if the viscosity as a function of oressure is known for sufficiently high

p

_ essures. Values of Pg have thus been

SO I i i i i i i i i—

0 2.0 4.0 6.0 8.0 Pressure, GPa

Figure 5. Line width of the ruby Rj fluores- cence relative to the one atmosphere value,

as a function of pressure at 20 °C when

n-butyl chloride is used as the pressure transiting medium. Glass transition pres- sure = 4.2 GPa.

determined for n-butyl chloride, 4:1 methanol-ethanol, and isopropyl alcohol, and the results have been used to confirm the ruby line-broadening method of determining glass transition pressures. For the noted liquids, the model

yields, respectively, 4.6, 10.2, and 5.1 GPa for Pg , and the ruby method yields

4.2, 10.4, and 4.5 GPa.

In addition to confirming the line- broadening method of finding Pr

, the

viscosity model might prove to be useful for describing the properties of

lubricants. Since lubricants are used

under conditions of widely varying

stresses, the value of the exponent in the viscosity model could be important

for evaluating the ability of a lubricant

to respond to the changes in the stress. The unusual magnitude of the critical exponent also presents a challenge to theoreticians in this field. The temperature dependence of the

glass transition in 4:1 methanol-ethanol was determined by the ruby line broadening method. At — 48 °C the glass transition is 5.65 GPa yielding APg/

AT = 0.68 GPa per degrees. This

value agrees with values obtained at lower pressures for other alcohols.

Piezoelectricity and

Pyroelectricity in

Poly(vinylidene fluoride)

Martin G. Broadhurst and George T. Davis, Polymer Science and Standards

Division

l n the past ten years polymer trans- ducers made of po!y(vinylidene

fluoride) (PVDF) have been discovered

and accepted as reliable, practical and

unique measurement devices. NBS

was involved in early developments in

this field and demonstrated a variety of applications such as intrusion and

fire detectors, laser beam profile

measurement devices, absolute radiometers, hydrophones, etc. NBS work

has stimulated commercial development

of transducer devices and their application to measurement problems

throughout government.

During this period, we were particularly involved in establishing a broad

description of the basis of piezoelectricity and related phenomena in PVDF. This knowledge was needed to guide optimization of the effects and

define reasonable limits on how much

could be expected from these devices. In the following, we present a general

description of the molecular structure of PVDF, show how a simple piezoelectric model was developed and

successfully tested, and describe the important consequences of this development.

Microscopic Details of PVDF

PVDF crystallizes from the melt into spherulitic structures. Figure 1 is a photomicrograph of a melt-crystallized

PVDF film taken between crossed

polaroids. The volume fraction of crystalline material is typically about

50 percent, depending on thermal his- tory. Most of the uncrystallized molecules are in a metastable supercooled

liquid phase. The glass transition temperature for this liquid phase is around —50 °C.

We assume, as shown in figure 2, that the spherulites consist of stacks of lamellae which grow outward from the center of the spherulite. These lamellae

89
NATIONAL MEASUREMENT LABORATORi

are typically 10-20 nm thick, depending on crystallization conditions. The

molecular chains are approximately

normal to the large lamellae surfaces and to the radii of the spherulites.

Much of the liquid material is probably

located between the crystalline lamellae. That is, a typical region (figure 2)

consists of parallel layers of alternating

crystal and liquid material, each layer

of the order of 10-20 nm thick. Of

course, on a larger scale one expects

the usual stacking faults, grain boundaries, and other course defects typically

found in a polycrystalline solid. Since

the molecular lengths are of the order

of 100 times the lamellae thickness,

each molecule must pass many times

through one or more of the crystal

layers and is free to assume flexible and irregular configurations in the

liquid layers.

PVDF is inherently polar (figure 3). The hydrogen atoms are positively

charged and the fluorine atoms negatively charged with respect to the car- bon atoms in the polymer. In the

liquid phase, the molecules continually

change shape due to rotations about

carbon-carbon bonds.

The net dipole moment of a group

of molecules in a liquid region will be

zero in the absence of an applied

electric field because of the random

orientations of individual dipoles. In the crystal phase, certain regular

molecular conformations are energetically favored for crystal packing.

Viewed down the molecular chain, four

such conformations are shown in figure

4. All four crystal structures are long

lived (whether stable or metastable) at room temperature, and three have

polar unit cells. That is, a crystal of polar a, (3, or y phase has a net dipole

moment. Crystal phase transformations can occur through mechanical stretching, high temperature annealing or application of large electric fields. In addition to phase changes, changes in orientation of the crystal moment can

occur in large electric fields. The field induced changes in orientation and

phase occur by rotations about carbonFigure 1. Photomicrograph of between crossed polaroids.

90

a rapidly cooled melt of commercial unoriented PVDF

carbon covalent bonds, or by rotatioi of molecular segments about their lorn axes, or both. The driving force is tht decrease in the applied field-molecula

dipole interaction energy which is ex tremely large. A typical moleculai

segment in the crystal has a dipole

moment of roughly 2 X 10~ 28 Cm (70

Debye). The energy of interaction be

tween it and a field of 108Vm_1

several times the thermal energy room temperature and comparable t( the energies needed to rotate molecule

in the crystal phase. Thus PVDF i truely ferroelectric, but with the pola

alignment caused by VanderWaal

forces rather than dipole-dipole force and with a Curie temperature wel

above the melting range (170-200 °C

of the various crystal phases.

Model for Piezoelectricity

The preceding established the exist ence in PVDF of polar crystals pref

erentially aligned by an applied fielc so that a PVDF film has a net electric moment. The charge on the film dut

to aligned dipoles is the product of the polarization (dipole moment per uni

volume) and area. Q = N/x0 (ec + 2) <Cos0o >/3L, (1)

Crystalline Lamella

i

Non-crystalline

Component

Figure 2. A schematic diagram of a sphere

lite and a detail of a section emphasizing the lamellar structure of the radiating branches
"center for materials science

-/here N is the number and /j. 0 the

; acuum dipole moment of the dipoles, the angle between the dipoles and

-he normal to the film, /s the film thickless and £C the permittivity of the

Irystal. All of these quantities are neasurable. As a beginning, we asked what the

j fleet of temperature and pressure will

>e. assuming the dipoles remain orienationally frozen—the simplest possible

ase. This question does not preclude

•>ther possible effects, but we will at

ast take into account a very basic

Effect before looking for more subtle nechanisms. Most workers in this field

: iave begun with special mechanisms

: uch as special space charge distribu-

-ions, changes in the molecular conormations or orientations, etc., without

irst eliminating thermal expansion and

impressibility effects. This neglect is 'iue in part to the relative unimportance

)f volume expansion in inorganic

Mezoelectrics. Taking the temperature and pressure

derivatives of the surface charge on

he PVDF film and assuming no change

in fio or the mean dipole orientation, we

)btain the hydrostatic piezoelectric conitant dp and pyroelectric coefficient p v .

dp = Pofr J_(, _1) _i_J_<£2y fluorine

d( lnvc.)

Py — —Po«C

(2)

4-(«c-D +4- <t> 2 [y j Z o + (2T«C )- 1 ] (Ink)

(lnvc) where

Po = *(€c+?)Nf| 0 J„ (<*» 0 ) <cos0o >/3v(

.

,

(3)

(4)

vc, /?c and «t

. are the crystal volume and

its pressure and temperature coefficients of expansion, and y are the amplitude of thermally induced dipole libra- tions and associated Gruneisen constant, $ the crystal volume fraction and

J0 ( </><>) a zeroth order Bessel function

of the first kind.

Independent experimental values were used to evaluate the right hand

side of eq. (2) and eq. (3) without

adjustable parameters and the results are shown in figures 5 and 6 together

with data obtained at NBS.

Figure 3. Molecular model of PVDF in the all-trans conformation found in the p crystal phase. The backbone is composed of carbon atoms, the spheres represent fluorine atoms and

the hydrogen atoms, if shown, would appear at the ends of the "sticks" which represent

zl emical bonds.

phase

polar « phase

o

v

-y phase

Jf o

fi phase

Figure 4. Projection of four crystal struc- tures of PVDF viewed along the molecular

axes. Dipole moments are shown by arrows.

91
These results show that without

changing the number of dipoles, vac- uum dipole moment or dipole orientation, essentially all of the observed

piezoelectric and pyroelectric response

of PVDF is accounted for. Additional

mechanisms connected with transport

of space charge, stress dependent re3 4 5 6 P0 ( MC/cm2)

Figure 5. Piezoelectric coefficient of PVDF

film for a change in hydrostatic pressure

versus polarization. Circles represent measured data points; the solid line represents values to be expected using eq. (2) with the appropriate values of material properties.

01 23456789 10 P0 (/X/cm2)

Figure 6. Pyroelectric coefficient of PVDF

film versus polarization. Open circles are NBS data points, solid circles are from the

literature; the solid line represents values to be expected using eq. (3) with the appropriate values of material properties.

versible crystallization, molecular rotations, etc., appear unjustified by the

data.

Significance of this Development

Based on the above mechanism of

piezoelectricity in PVDF, we can make

some very strong statements about the optimization and limitations of this material.

( 1 ) Present PVDF has about half the

theoretical maximum polarization, so

that improvements in performance are expected to be small. (2) To find a "better" polymer than

PVDF, one needs to have a dipole

with a smaller volume and larger

moment than the compact (CF2-CH2) group.

(3) Increasing the crystal to amorphous volume fraction can increase P0 , but will decrease sample compressibility and expansion coefficient with

small net gain. Some increase in compressibility can result from incorporating small voids in the specimen.

(4) Use of the polymer is limited to temperatures below the melting point.

Increased stability can be expected

from increasing the crystal lamellar

thickness and thus reducing the ratio of fold surface area to crystal volume.

(5) Moisture and moderate amounts

of ionic impurities have negligible effect on the performance of PVDF.

(6) Piezoelectricity and pyroelectricity

in PVDF are directly related to the compressibility and expansion coeffi- cient so that both effects will exist together.

(7) The phase having the largest dipole

moment will be the most active phase.

These conclusions give confidence in the dependability of PVDF transducers

used within the stated limitations and

provide firm guidance in the optimization of transducer processing and

polymer modification efforts.

NATIONAL MEASUREMENT LABORATORY

Electrochemical Noise

Measurements: A New

Measurement Technique for

Diagnosis and Study of

Localized Corrosion

Ugo Bertocci, Chemical Stability

and Corrosion Division

FJLluct luct uations in the electron potential

and/or current of corroding electrodes are giving information on the breakdown and repair of protective films and are being studied as a possible means of detecting the output of localized corrosion. Iron and aluminum

in solutions that can cause pitting were

examined. A large increase of noise

signal was detected when pitting

occurred.

The detection and analysis of the fluctuations in current and potential of electrodes, which are generally called electrochemical noise, are receiving in- creasing attention in electrochemistry

as a means for the study of some

aspects of electrode kinetics [1] and

are also being applied to the field of corrosion, where the method has the potential to be a useful tool, particularly for the detection of localized corrosion

[2].In the area of electrode kinetics, one of the advantages is that there is no need to apply an external signal to' the system under study. The analysis

of the experimental results, however,

still requires further development of the mathematical models necessary for interpreting the results. As far as local- ized corrosion is concerned, the examination of the characteristics of the deviations from the average values of the electrode potential and current can

give insight in the processes of formation of protective films and their breakdown and might be able to signal the onset of pitting when the average cor rosion rate is still low.

A number of techniques can be employed to characterize the electrochemical noise. My main effort has been

directed toward looking at the signals

92
JtENTER FOR MATERIALS SCIENCE

Frequency, Hz

! -igure 1. Current spectra recorded in poten-

, iostatic conditions on different resistive oads.

n the frequency domain, through use

if a spectrum analyzer. In order to ninimize interference, some instru- ments are battery operated, and electro- nagnetically shielded as well as pro-

' :ected from mechanical vibrations. The

evel of instrumental noise often limits

^.:he electrochemical systems that can be

examined, thus, reduction of such noise

. s of paramount importance in these

> studies. For the purpose of studying corro- sion processes, it is often convenient

to control the potential of the electrode

jnder study. This is accomplished by

f neans of a potentiostat. Commercial

i nstruments, however, tend to intro- duce noise of unacceptable levels into

,:he circuits, preventing the study of the

signals of interest. For this reason, I developed a special low-noise potenLtiostat with the cooperation of the Spe-

:ial Analytical Instrumentation Group,

Center for Analytical Chemistry. The

{.instrument is designed for the detection

bf fluctuations in the frequency range

between 0.1 and 2000 Hz. The internal

'noise is of the order of 3-10" 8 V/ V Hz.

Figure 1 shows the noise current pro- educed by the potentiostat on different

I resistive loads. With such an instal- lment, current fluctuations in the nA

range, caused by changes in the electrode characteristics, can be detected

above the instrumental noise. Noise measurements are now being

made on various electrodes in order to gather phenomenological information

about the noise levels generated by dif- ferent electrochemical processes and

to test the range of usefulness of the apparatus we have developed. For in- stance, I studied, as examples of lownoise processes [3], the deposition of copper and nickel. Because it is an electrode system of

interest in corrosion science, I ex- amined the Fe electrode in a neutral

borate solution with a small amount of cholride added. The voltage noise

under galvanostatic conditions was much larger when the electrode was

polarized anodically. As shown by the spectra in figure 2, the noise levels in- creased by more than a factor of ten when the electrode was above the pitting potential. Correspondingly, observation of the voltage signal as a function of time revealed sudden bursts of

10-4 |

! 1 1 ,

1

0 1 2 3 4 5 Frequency, Hz

Figure 2. Voltage spectra iron in borate

solution with chloride added, fa) anodic +1050 mV versus SCE; (b) anodic +750

mV versus SCE; (c) open circuit —280 mV

versus SCE. Spectra are an average of 16

spectra.

oscillations which were probably re- lated to the formation of pits.

Another system I examined was Al

in a boric acid: sodium metaborate

mixture with and without addition of

a small amount of NaCl. In the chloride-containing solution, which is known to cause pitting if the potential

is raised approximately above —700

mV vs SCE (Saturated Calomel Electrode) [4], the spectra obtained below

and at the pitting potential are shown

in figure 3, together with the voltage

spectrum.

By dividing the voltage by the current at each frequency, a plot of the absolute impedance \L\ of the electrode

as a function of the frequency can be

constructed. Figure 4 shows such a plot.

Here again, the onset of pitting is marked by a fundamental change of

the impedance at low frequency. When

pitting does not occur, the random

fluctuations in the current density are

the system response to the voltage

noise input, and the electrode behaves

1 10 102 103

Frequency, Hz

Figure 3. Voltage and current spectra for aluminum in borate solution with chloride added below and at the pitting potential. Current density (a) —650 mV versus SCE;

(b) 800 mV versus SCE. Dashed line is voltage.

93
105

104

£ 103 u

c: NT 102

10

a "a

10-1 1 10 102 103 Frequency, Hz

Figure 4. Plot of the absolute impedance |Z| for aluminum in borate solution. — —330 mV versus SCE (no chloride); A = —800

mV versus SCE (0.05 M NaCl); = -650

mV versus SCE (0.05 m NaCl).

like a capacitor of about 0.3 ^F/cm2. Above the pitting potential, the noise current increases more than two orders

of magnitude, and the impedance plot cannot be explained except as being

caused by fluctuations of the electrode

current, independent of the applied

voltage signal.

The detection of the current noise

is a good indication of the beginning

of pitting, and the large fluctuations

observed are probably due to hydrogen

evolution from the pits [5]. I found

the onset of the large current noise to be reproducible within 20 mV.

The noise current, which can be ac- counted for by oscillations of the order

of a few percent of the average dc current, is the sum of several contributions; however, it can be shown that the two major ones are caused by fluc- tuations of the electrode resistance

from the breaking of the protective film and the fluctuations of the double layer

capacitance caused either by changes

in surface area or in the thickness of the insulating film. At the moment, it

is not possible to decide whether the

noise is due more to the faradaic or to the capacitative current. However, the increase in impedance at higher fre- quency indicates that the capacitative

term is probably negligible above about

100 Hz. These results suggest that a dynamic breaking and repairing of protective films on the metal surface are responsible for the signals observed.

This work should thus provide new

insights into the basic processes that control corrosion and, in so doing, also

offer a new and more effective tool for

detecting, characterizing, and measuring various forms of localized corrosion

such as pitting.

References

1 1 J Tyagai V. A., Elektrokhimiya 3, 133 1 (1967); Blanc G., Gabrielli C, Keddam M., Electrochim. Acta 20, 687

(1975); Blanc G., Epelboin I., Gabrielli C, and Keddam M., J. Electroanal. Chem. 62, 59 (1975).

[2] Iverson W. P., J. Electrochem. Soc.

115, 617 (1968); Okamoto G., Sugita T., Nishiyama S., and Tachibana

K., Boshoku Gijutsu 23, 439, 445

(1974).

[3] Bertocci U., 7th International Congress on Metallic Corrosion (Rio De

Janeiro 1978). p. 2010 Abraco, Rio

de Janeiro (1979). Bertocci, U., /. Electrochem. Soc. (submitted for publication)

[4] Pourbaix M., Corrosion 26, 431

(1970); Galvele J. R., and de DeMicheli S. M., Corr. Sci. 10, 795

(1970); Nisancioglu K., and Holtan

H., Corr. Sci. 18. 835 (1978).

[5] Bergeron C. B., and Givens R. B.,

/. Electrchem. Soc. 124, 1845 (1977);

Corrosion Res. Conf., NACE, Atlanta (1979).

NATIONAL MEASUREMENT LABORATORY

Migration Models For Polymer

Additives

Isaac C. Sanchez, Shu Sing Chang,

and Leslie E. Smith, Polymer Science

and Standards Division

An important problem in food

packaging is the absorption into food

of chemicals present in the packaging

material. For the past two years, the Polymer Science and Standards

Division has been studying the migration of additives from polymeric

packaging materials to food simulating

solvents. This study has been carried out under the aegis of an interagency

agreement with the Bureau of Foods

of the U.S. Food and Drug Administration. Our primary goal in this research program has been to develop

a migration model which can be used

as a guide for the food packaging

industry and serve as a sound scientific basis for FDA regulatory decisions.

Initially our research has focused on

a class of polymers called polyolefins.

These polymers, of which polyethylene

is the prototype, represent the majority

of plastics used in food packaging applications. Most polyolefins are semicrystalline, with the glass transition temperature of the amorphous component below room temperature. For

such polymers migrations of additives

at room temperature often follow

simple Fickian kinetics. We have found a "migration deci- sion tree" (MDT), as shown below,

to be a useful device in focusing atten- tion on the specific technical questions

that must be answered when evaluating

migration. The basis for the MDT is that migration can be limited by any

of four physical factors:

1 ) initial loading of additive

2) solubility of additive in food (simulating solvent)

3) equilibrium partioning of the additive between food and package

4) diffusion.

Non-Diffusion Limited Migration

The maximum permissible amount

of migration of a particular additive is

94
Migration Decision Tree

1 M 0 < M*

i

Acceptable Migration

(load limited)

S < C

Acceptable Migration

(solubility limited)

M <NT

Acceptable Migration

(partition limited)

£

Acceptable Migration

(diffusion limited)

Given: Packaging plastic (mass M p

) containing M 0 gram of additive. Compare: Migration of additive into a food

simulating solvent of mass M sto the max- imum permissible dose or concentration C*: C* = M*/M s I M 0 > M*

1

Determine Solubility (S) of additive in simulating solvent IS > C

Determine: Is migration limited by partition- ing of additive between solvent and plastic 7

Residual concentration of additive in plastic must be greater than C = (M0

- M*)/M

p Since the partition coefficient K is greater than S, the ratio C*/C is compared with S

S < C*/C I s > c*/c

InK =ln S+X°° + 1 I

Estimate or determine:

Additive polymer interaction parameter X

00

Equilibrium extraction. M

1 + M p/KM S I M oo >M'

Estimate or determine: Diffisuion coefficient (D) of additive in polymeric film

1

Estimate: Maximum migration for

a package thickness, £,,and a shelf Jife, t: M t = 4M 0 (DI/tt£ 2

) Vz IM, > M*

1

Unacceptable Migration
NATIONAL MEASUREMENT LABORATORY

determined from its toxicology. This

amount is presumed known and is usually expressed as a maximum allowable

concentration of the additive in the food (C*).

It is obvious that any level of migration is acceptable if the initial loading

of additive to the plastic package is small enough so even if all the additive migrates to the food, its concentration level will never exceed C*. This

is the first branch of the MDT.

It is also obvious that if the solubility

(S) of the additive in the food simulating solvent is limited and S<C*,

then migration will always be within

acceptable limits. This is the second

branch of the MDT.

If the migration is neither load nor

solubility limited, then it may be limited by partitioning. At very long

times, the additive will equilibrate or

partition itself between the food simulating solvent and plastic package. A

residual amount of additive is always

retained by the package. An example

of partitioning is shown in figure 1 for the migration of radiolabeled (carbon14) n-octadecane from low density

polyethylene to a 50/50 ethanol/water

mixture. Notice that more than 80 per- cent of the original amount of additive

is retained by the polyethylene. (In

100

10-1 It

10-2

10-2 10-1 100 101 1Q2 1Q3 1Q4

Time, hours Figure 1. Migration of n-CVH,s from LDPE

into 50/50 ethanol/water mixtures at 60 °C.

this example the ratio of solvent to polymer is about 100.)

At equilibrium, the ratio of additive

concentration in the solvent to its concentration in the plastic is called the

partition coefficient, K. We have shown

[1] that if the additive has only limited

solubility in the food simulating sol- vent, then the relationship between the

partition coefficient and the solubility

is given by:

In K = In S + x ^ + 1 (1)

where x°° is a parameter (a reduced

chemical potential) which characterizes

the thermodynamic interaction at infinite dilution of the additive with the

plastic packaging material. The parameter is also related to the volume

fraction activity coefficient (y°°) at

infinite dilution by [2]

In y°° = x

00 + 1 (2)

Combining eqs. (1) and (2), we obtain the simple result K/S (3)

Experimental values of x

00 are conveniently measured by inverse gas chromatography [3]. For non-polar

additives and non-polar polymers, x°°

is invariably positive and usually falls in the range:

0 < x M < 2

Positive x°° values are indicative of an unfavorable thermodynamic interaction

between additive and polymer. Values

of x

00 larger than 2 can occur when:

1 ) the additive/polymer combination

is polar/non-polar and/or

2) the additive is large in size. Positive x

00 values imply through

eq. (1) that K > S.

In figure 2 measured values of K and

S are compared for the migration of radiolabeled n-octadecane from high

density polyethylene to various etha- nol/PFO mixtures.

Direct experimental determinations

of K or x

00 are difficult and time con- suming. However, K can be easily esti- mated by eq. ( 1 ) from a known solubility and a theoretically estimated xx - Theoretical methods of estimating x x

are available [2], but even this calculation can be avoided by assuming that

X

00 has a worst case value of 2 (or

larger) . Using the definition of the

partition coefficient, the amount of additive that will migrate at infinite time

(M x ) can be calculated as shown in the third branch of the MDT.

Equation ( 1 ) is based on the as- sumption that the food simulating sol- vent is not absorbed by the plastic

package. If the solvent is absorbed,

then an appropriate correction can be

made [2]. Fortunately, in most cases where partitioning is an important consideration, solvent absorption is not. For example, polyolefins do not appreciably absorb water or alcohol and

it is usually aqueous and alcoholic food

stuffs for which partitioning is important.

Diffusion Limited Migration

If migration is not load, solubility, or partition limited and the maximum

101

70 80 90 100

Ethanol Wt. %

Figure 2. Partition coefficients of n-GsHas between PDPE and ethanol/water mixtures and solubilities of n-GsHss in ethanol/water

at 60 °C. 96
CENTER FOR MATERIALS SCIENCE

-helf-life of the packaged food is much

.horter than the time required to tchieve equilibrium partitioning, migration may be diffusion limited. For

simple Fickian diffusion from a plastic ;heet of thickness / to an infinite reservoir of solvent, the amount of migration

it time t can be estimated from the ollowing equation [4]: Mt /M0 = 4(Dt/7r/2 )'- (4)

Ahere D is the diffusion coefficient of he additive in the plastic sheet. For

-ickian diffusion, Mt /M0

is linear in

'i A for M0 /Mt values up to about V2. For longer times eq. (4) overestimates

:he amount of migration. As shown in he fourth and final branch of the VlDT, if the maximum shelf-life of he packaged food is known, then the naximum amount of diffusion limited

nigration can be calculated. (For a

inite solvent reservoir, M0 is replaced

oy M*> in eq. (4) )

Most of our migration experiments

:rom polyethylene (PE) have exhibited

;mall departures from ideal Fickian

oehavior. The most common deviation

s illustrated in figure 3 which shows

:he migration of the normal alkane

n-C3 oH0 c from PE to heptane and to ethanol. The migration curves tend to be slightly sigmodal in character rather than completely concave to the time

axis as is characteristic of Fickian dif- fusion. This positive deviation, which

is an unexpected acceleration of migration, is thought to be caused by the absorption and swelling action of the food simulating solvent. To test this conjecture, migration experiments were

performed on high density PE containing radiolabeled n-octadecane in contact with unlabeled n-octadecane as

solvent. In one experiment the PE was

saturated with the labeled C1S (about

5 percent by weight) . In this kind of experiment solvent absorption effects

10° 101 Time, hours

Figure 3. Migration of n-C-Hes from LDPE

into n-heptane and ethanol at 60 °C. Dashed

lines have a slope of one half.

480 720

t1/2, s1/2

960 1200

cient in low density PE that is about

30 times larger in heptane than in ethanol. Use of eq. (4) for migration assumes

that diffusion in the plastic package is rate controlling. This is not always the

case. Our experiments have shown that when the solubility of the additive is very low in the food simulating solvent

(K<<1), migration often follows a t rather than a V

A law. This behavior can be seen in figure 1 where the initial slope of the migration curve is approximately unity. Under conditions of low

solubility and solvent stirring, diffusion of the additive through the relatively

stagnant boundary layer surrounding

the polymer sheet can become rate controlling. We have shown that a linear

t law is consistent with boundary layer

limited diffusion. We are currently

working on a more general theoretical

model that will bridge the gap between

polymer limited and boundary layer

limited migration.

Figure 4. Migration of labeled n-GsHse from

HDPE into unlabeled n-CisHM at 30 °C. References

are completely absent because the PE

is already fully swollen. This experiment was compared with one in which

the PE was only partially swollen with

labeled C1S (1 percent by weight). The

results are shown in figure 4. As can

be clearly seen, simple Fickian kinetics are observed for the fully swollen

sample whereas a positive deviation is observed for the partially swollen

sample.

Although solvent absorption would

appear to cause migration to deviate

from ideal Fickian behavior, this deviation is relatively unimportant compared to the effect that solvent absorption has on the absolute value of the

diffusion coefficient. In figure 3 n-C32 HG6 has an apparent diffusion coeffi-

[1] Smith, L. E., Sanchez, I. C. Chang,

S. S., McCrackin, F. L., and Senich, G. A., Models for the Migration of Additives in Polyolefins, NBSIR-79-

1779.

[2] Sanchez, I. C, Equilibrium Distribution of a Minor Constituent between

a Polymer and Its Environment, ACS

Symposium Series Vol. 95, R. K. Eby,

ed. (American Chemical Society,

Washington, DC, 1979).

[3] Senich, G. A. and Sanchez, I. C,

ACS Organic Coatings and Plastics Chemistry, Vol. 41% 345-349. (American Chemical Society, Washington,

DC, 1979).

[4] Crank, J. and Park, G. S., Diffusion

in Polymers, Capt. 1. (Academic

Press, New York, NY, 1968).

pffice of the

Deputy Director

or Programs

C. Gravatt, Jr.

| ndestructive

iluation 'old Berger

es B. Beal**

rge Birnbaum

nard Mordfin

bert G. Sutter

i

I

. ironmental

I asurements

I Ham H. Kirchhoff

I tie T. McClendon

[ les R. McNesby

Liter L.

£ielinski, Jr.

I ndard Reference

terials \orge A. Uriano

: bert L. Alvarez semond K. Bell** ^aul Cali

'Keith Kirby

.onora T. Klein

bert E.

vlichaelis. Jr. nley D. Rasberry

llliam P. Reed

Shhard W. Seward

Ines I. Shultz**

5 ndard Reference

\vid R. hide, Jr. |:rman P. ivozinsky ms H. Gevantman

rithia A. Goldman

rla G. Messina

tijoyce B. Molino

rtrude B. Sherwood

Robert C. Thompson

Howard J. White, Jr. Measurements for Nuclear Technology H. Thomas Yolken

Recycled Materials Harvey Yakowitz

Donald A. Becker

Joseph G. Berke

Directorate for Measurement Services Arthur O. McCoubrey

Weights and Measures

Albert D. Tholen

Carroll S. Brickenkamp

Stephen Hasko

Joe H. Kim

Henry V. Oppermann

Richard N. Smith

O'to K. Warnlof

Harold F. Wollin Measurement

Services Brian C. Belanger

Kenneth W. Edinger Lee J. Kieffer Kathryn O. Leedy

Jack L. Vogt

Domestic and

International Measurement

Standards David E. Edgerly

Helmut M.

Altschuler Samuel E. Chappell

* 'Research Associates and Guest Workers

ward White is calibrating the high

olution organic mass spectrometer,

is instrument is capable of achieving

tic resolutions of better than 70.000

AM). The gas chromatographic

et feature of this instrument permits

titifying individual organic compounds

complex mixtures such as in shale oil

t e elemental compositions of the

•lecules and fragment ions are obtained,

e field desorption capability of this

iss spectrometer is used in the analysis

peptides with molecular weights of

'ater than 1,000. The clinically imrtant peptide, angiotension I, is being

ihiated as a potential Standard

ference Material. This instrument has

id applications in studying samples of

erest to several of NML's programs.

seven program offices have been established within

NML to focus diverse technical activities on specific

problems and goals. These offices fund, coordinate, and

direct efforts in all NML and several NEL (National

Engineering Laboratory) technical centers. Thus, they

draw on expertise in many scientific disciplines to meet

national needs for measurement services and data. The

offices can be readily separated into two distinct categories: 1 ) those based on traditional outputs fundamental

to the NBS mission and related to general, ongoing

national needs; and 2) those that deal with topical areas and direct NBS expertise in measurement science toward

solving specific current problems.

Into the first category fall offices managing the traditional NBS outputs: Measurement Services, Standard

Reference Materials, and Standard Reference Data.

They provide the means for tracing measurements to the

national reference standards. Further, they provide

quality assurance for the application of NBS measure- ment technology in the field. Dating back to the founding

of NBS, these programs have promoted the growth of

organizations which improve measurement, data, and

standardization services throughout the United States and abroad.

The Measurement Service activities—weights and

measures, calibration coordination, and coordination of

voluntary standards—are specifically concerned with the

practical application of fundamental standards in the

real world of measurement. The weights and measures

effort, which predates NBS, provides the fundamental

standards for the exchange of goods and services in the

marketplace. Although NBS does not have statutory

authority to regulate weights and measures within a State,

it does provide State weights and measures offices with

the necessary technical support to insure fair and equitable measurements throughout the Nation. It also works

to assure that State weights and measurement activities are traceable to national standards and are capable of

dealing with the complex measurement problems of the

modern marketplace. In the area of calibration co- ordination, Measurement Services provide leadership and

services to laboratories in industry, State and local

governments, and other Federal agencies through NBS

calibration and measurement assurance programs. A new

99
NATIONAL MEASUREMENT LABORATORY

program has been established to improve the management and use of precision test equipment in the Federal

Government. The Measurement Services staff also co- ordinate the NML participation in domestic voluntary

standards and plays a major role in managing key inter- national standards responsibilities. This office is specifically charged with coordinating NBS participation in

the International Organization for Legal Metrology, the

Treaty of the Metre organizations, the International

Organization for Standardization, and the International

Electrotechnical Commission and in preparing policy

positions for NBS delegates to these organizations.

The Office of Standard Reference Materials produces

and certifies materials standards for the quality assurance

of most of the physical and chemical measurements of the

United States—and the world. This program presently

provides more than a thousand different Standard Refer- ence Materials (SRM's), ranging from over 100 different

steel reference materials, through such biomatrix SRM's

as trace elements in spinach and bovine liver, to such

physical SRM's as Mossbauer Spectroscopy standards

and superconductive fixed point devices. The range of

SRM's provided serves traditional measurement activity as well as measurement needs related to new technologies.

Considerable effort is now going into the production of

new environmental and clinical SRM's—a reflection of

national concerns in these areas. Presently, the Environmental Protection Agency requires that the calibration

standards used in atmospheric measurements be traceable

to NBS SRM's, whenever possible.

The third traditional output program is the Office of

Standard Reference Data (OSRD) . The need for coordination and support at NBS of an international effort to compile and evaluate scientific and technical data has

been specifically recognized by Congress in the Standard

Reference Data Act of 1968. One-third of the Federal

data collection effort is funded through OSRD, and

another third is directly managed or coordinated through

this office. In addition, OSRD and its constituent data

centers in this country cooperate with other data centers throughout the world to avoid duplication of effort and

to aid the promulgation of standard reference data. Data

efforts in NML range from the large, ongoing data centers

in chemical kinetics and thermodynamics to shorter term

100

data evaluation efforts in high resolution infrared spectra.

A highly successful vehicle for disseminating reference

data has been the Journal of Chemical and Physical^

Reference Data.

The "topical" program offices of NML presently focus

j the scientific and technical resources of NBS on national : problems in the recycling of waste materials, nuclear

technology, nondestructive testing, and the environment.

The Office of Recycled Materials is presently engaged

in activities related to urban solid waste and reycled oil. Due primarily to environmental considerations, the problem of the disposal of urban waste is becoming acute for many cities across the Nation. The best solution to the disposal problem is to use waste as a resource, both for raw materials and for fuel. To be a resource standards

are needed. Standards, however, are particularly difficult to set for a substance as heterogeneous as urban waste. A current major focus for this office's Resource Recovery

Program is the development of standards for refuse- derived fuel. In the Office of Measurements for Nuclear Technology

current activities focuses on the need for accountability jii throughout the nuclear fuel cycle. This program is con-i|iio

cerned with all aspects of the handling and measurements jlif of nuclear fuels, from the determination of volume and

flow of nuclear material through the determination of the- : precise isotopic composition of nuclear fuels by nondestructive evaluation, to the measurement of more accu-jilli rate values of the life-times of radionuclides. Future plans include standards for nuclear waste disposal.

The Office of Nondestructive Evaluation (NDE) has: j

as its objective the improvement of the reliability and

durability of materials and structures by means, of im-#i

proved NDE procedures and standards. The program's

role is to help industry develop and bring to use methods

for accurate and reproducible NDE measurements. This

includes technical investigations of standards (both physical calibration standards and procedural documents), \%

characterization of instruments, development of imp

proved techniques and the assessment of the meaning of 1

the NDE on material performance.

The Office of Environmental Measurements manages 1

; the oldest topical program in NML, the air pollution

program. It has evolved since 1972 from a single effort it
E OF DEPUTY DIRECTOR FOR PROGRAMS

Resource

Recovery

Climate

Toxic

Substances

seed

Particulates

Safeguards

^ Water

+

growth

research

&

development maintenance

,he 1. Schematic development of nationally focussed NML programs.

the problems of standardization in homogeneous air

llution, to its present major thrust in the development

standards for water pollution and particulates. Areas

i intense effort involve the measurement and characteration of environmental asbestos and the development of

:lM's for trace organic compounds in water. A recent

roject resulted in a significant revision in the mechanism

I ozone-alkene reactions used in photochemical smog

:3dels.

5 The topical programs, unlike the traditional "output"

rograms, are planned for limited duration. Figure 1 .es an idealized time development chart for topical

Lograms and shows the approximate status of various

jograms. The time axis can be quite different for the

|fferent programs. For example, Recycled Oil is ex- cted to have a lifetime of 5 to 10 years, while NDE will

•obably last considerably longer. Nevertheless, the chart

strates the philosophy of the topical programs as they

ply NBS expertise to national problems. When the

BS contribution is complete, the program is terminated,

le ongoing effort represents the transfer of the products

the topical program to output programs and, possibly,

me ongoing activity carried out within a technical

nter. The types of products transferred to output

ograms would include SRM's, data compilations, and

libration services.

Time

101
NATIONAL MEASUREMENT LABORATORY

Load Cell Mass Comparator

Randall M. Schoonover, Length and

Mass Measurements and Standards

Division for Office of Weights and

Measures

TJL he calibration of mass standards

upwards from 25 kg in value is presently performed on mechanical balances

used as mass comparators. The slow

balance response time and the handling

precautions required to protect the knife-edge bearings make this a time

consuming process. In addition, a large

amount of laboratory space must be

dedicated to a permanent installation. Economies of time and laboratory

space would be effected if the mass

comparisons could be accomplished

using a strain gauge load cell, provided

that its precision could be improved at least tenfold over its inherent repeatability. I have constructed a simple

comparator based on the use of a load

cell, which demonstrates that the tenfold improvement is readily attainable.

The problem in using a load cell to compare a mass standard with an un- known object of the same nominal

value is that hysteresis in the cell and

loading errors are too large to allow one to exchange loads in the most

direct way, i.e., completely unloading

the cell and reloading with a different mass. Use of a load cell in this manner

is limited to a precision of 0.005 percent. However, the hysteresis and loading errors would be greatly reduced if most of the tension on the load cell could be maintained during the inter- change of loads. I was able to do this in a very straightforward way, as described below.

About 90 percent of the tension on

the load cell is maintained during the load interchanges by use of a coil spring. Essentially, this is accomplished by suspending the cell coaxially

inside the coil spring, allowing the load

to compress the spring, and then adjusting the surrounding rigid frame to pre102

vent the release of more than 10 percent

of the spring tension.

Figure 1 shows the device in detail. The lifting eye carries the rigid frame

and is itself suspended from a building

structural member, "A" frame, etc., through a hydraulic cylinder.

A coil spring, with a predetermined

spring constant and length, rests on the bed plate centered about a passageway.

On the upper end of the spring rests a

floating plate with its passageway similarly centered. Horizontal motion of the floating plate is restricted by verti- cal rods that are part of the frame.

The rods, however, do not impede

vertical motion of the spring and plate.

Resting on top of the floating plate are a thrust bearing and ball-and-socket

assembly. The ball-and-socket permits

crude vertical alignment of the cell during initial assembly. The bearing

allows the spring to rotate during compression and extension and also re- moves torque from the cell during the loading cycle.

Adjustable shunt stops are set to interdict vertical motion of the floating

plate just before full loading is reached,

thus providing a rigid suspension of the

cell. When the load is removed, slight vertical motion of the spring and cell

is permitted before the constant load

Ball & Socket^

Thrust Bearing

Shunt Stop

Constant Load

Stop

-Lifting EyeFrame

Floating

Plate

Flexure

Universal Load Cell-- -

' Spring

Bed Plate

OVLoad Point

;

ill

i a

stop runs against the bed plate. This

stop is adjusted to provide about 90

percent of full load to the cell when

the mass load is removed.

Universal flexure joints provide for

a repeatable load axis. Without these, the minor mechanical misalignment

that occurs during the load/unload

cycle would degrade cell performance.

In addition to the components shown

in the schematic drawing, a hydraulic

lifting cylinder is required, as is a weight transport system. Dollies and

tracks provide excellent weight handling (up to 1000 kg) for the existing

device. Also, an electronic device is required to indicate load cell output

signal. However, a single such device

will suffice for loads from 25 kg to 25 000 kg.

I have constructed a realization of

this device at the 225-kg (500-lb)

level [1]. Using the device, calibration

of 500-lb State mass standards was

accomplished to a precision of 0.0002

percent. In addition, the calibration

effort required only a few hours as compared to a few days for the same

measurements using an equal-arm bal- ance.

I believe that the same approach

will be applicable for load cells operating in the range between 25 kg and

5 000 kg—and possibly higher. In particular, at the 25 kg level such a device

operating at a precision of 0.001 per- cent or better would be an important

contribution to industrial metrology.

Reference

[1] Schoonover, R. M., A high precision load cell mass comparator, /. Res. NBS, 84, 347 '(1979).

i J

• if

i HI

i

f

*

ft

tin

Ell

St

pi

i|i

! ilil

Figure 1. parative.

Schematic of load cell com-
OFFICE OF DEPUTY DIRECTOR FOR PROGRAMS

Interagency Cooperation in

Mass Spectral Data Base

Dissemination

Lewis H. Gevantman, Office of

Standard Reference Data

: F^M^ or over three years the Office of Standard Reference Data (OSRD) has

>een involved in a joint data dissemitation project with the Environmental

Protection Agency (EPA) and the National Institutes of Health (NIH).

This project deals with the collection,

evaluation, and dissemination of a nass spectral data file containing over

: H,000 chemical compounds. The

•ompounds profile relate largely to wealth and environmental concerns, out the file is sufficiently diverse to nclude information on chemical substances of interest to other commercial

ind academic areas.

Prior to OSRD involvement, the IH EPA Mass Spectral Data file was

accessible to users via an on-line computer system as a commercially avail- able service [1]. The entry of the OSRD

: nto this project has broadened the 'nethods for disseminating the data and

olaced a greater emphasis upon the

quality of the spectra, both contained

-n and being added to the file. The

'iltimate intent is for the activity to -provide a depository of the highest

juality spectra and to make the high

Quality data available in a number of

' ormats so as to achieve the widest

dissemination tailored to user require- ments. Finally, it seeks to influence

'Jhe community of mass spectral data

'isers and producers to continue to improve the quality of mass spectral data

eported in the open literature. The OSRD, in collaboration with

he EPA and the NIH, has made the

lata base available in two forms: magnetic tape and book. The magnetic

ape is available under a lease agree- nent [2] and is intended for use by

organizations who engage in a large

limber of spectral analyses in the

:ourse of their normal operation. The

iata book [3] is intended to reach the

scientific and technical audience which

is involved in mass spectral measure- ment but does not require the degree of use implied in an automated system.

Each entry in the tape file contains

a spectrum, quality index, compound

name, molecular weight and formula,

and the Chemical Abstracts Service

(CAS) Registry Number. The entire data base is written on a single 2400-

foot reel of magnetic tape in IBM

unformatted Fortran(G) and is avail- able in 9-track, 800 or 1600 bpi. Updates of data are provided on a regular

(annual) basis. The book form is published as four

volumes plus an index volume. The

spectra are indexed in four ways: by

chemical substance name and syno- nyms, formula, molecular weight, and

CAS Registry Number. Supplements

are being produced periodically, and a complete revised set of indexes is published as each supplement appears. Every entry is formatted as shown

in figure 1. Each spectrum is accompanied by the CAS Registry Number,

the molecular formula which is ordered

by the Hill convention [4] and derived

by computer from the CAS-supplied

connection table record. The molecular

weight is derived by computer from the same records using the integral atomic masses C = 12, H = 1, O = 16, etc. The structure diagrams, a unique fea- ture of the book, are also computer

derived from the connection table record for each substance.

The "Quality Index" (01) employed

in selecting the data is based upon a

set of criteria established by F. W.

McLafferty [5]. Originally the overall index was made up of seven individual

factors dealing with such items as evaluation of the spectrum by an ex- perienced mass spectroscopist, ionization conditions, impurity peaks observed, illogical neutral losses, isotopic

abundance, number of peaks, and the comprehensiveness of the data. Additional factors such as compound purity

and instrument calibration procedures

have been added to the OI to select the spectra in the book and tape.

Molecular Weight Molecular Formula CAS Registry Number

173 Substance Name - Naphthalene, 1-nitroStructural Formula.

C10 H7 NO2 NO,

86-57-7

Mass Spectrum

10080

60

40 * 20

.r 0

'35 § 100 c 80 ~ 6040

20

0

- , .J,IU ,l„ll ,.J., n

, llj f 10 30 50 70 90 110 130 150

170 190 210 230 250 270 290

Mass/Charge

Figure 1. Typical mass spectrum.

103
NATIONAL MEASUREMENT LABORATORY

Standard Reference Material For

Antiepilepsy Drugs in Serum

Robert Schaffer and Dennis J. Reeder,

Organic Analytical Research Division

Further efforts to refine and extend

the use of quality criteria have been

pursued in the OSRD. Several technical

discussions have been held under the auspices of the Joint Committee for Atomic and Molecular Physical Data

(JCAMP) concerning the identification and promotion of measurement specifications which would lead to the improvement of mass spectra in the open literature. A preliminary manuscript based upon the McLafferty cri- teria but extending and clarifying the required steps is now being prepared.

Further refinement of the paper based on critiques by acknowledged experts

will lead to a final document which is planned for publication in 1980. Acceptance of these criteria by the Inter- national Union for Pure and Applied

Chemistry (IUPAC) is also being contemplated. In employing the two new mechanisms for disseminating the mass spectral

data the OSRD has attempted to effect the widest distribution possible. The

leasing of the tapes has resulted in a number of arrangements, some related

to primary (in-house) use of the tape

and others to more extended use. The

latter has involved lease by large cor- porations for distribution within their subsidiaries, lease by data services for dissemination via on-line service, lease by foreign nations for distribution of the data within the specified borders of the country, and finally lease by instru- ment manufacturers for incorporation

of the data in an automated search file associated with experimental mass spec- trometer systems. In this way, a wide

and divergent group of users has been

embraced.

A final step taken by the OSRD to improve the data quality and guarantee

the acquisition of mass spectral data

is a more recent agreement reached

with the United Kingdom Mass Spec104

tral Data Centre (MSDC), now based

at the University of Nottingham. Originally this group, based at Aldermaston,

contributed a large portion of the

spectra contained in the file. After some hiatus, this group is again active and seeks to collect, evaluate, and submit for inclusion in the data file the

better spectra now found in the litera- ture. The Centre also plans to solicit spectra from industrial, academic, and

miscellaneous sources for ultimate in- sertion into the data file, provided the data meet the quality requirements

discussed above. It is believed that the renewed involvement of the MSDC

should lead to the achievement of improved spectra and a system which

conforms fully to the OSRD goals of improving the dissemination of scientifically reliable data.

Reference

[1] Heller, S. R., Milne, G. W. A., and

Feldman, R. J., A computer-based

chemical information system, Science,

195, 253-259 (1977).

Heller, S. R., and Milne, G. W. A., The NIHI EPA Chemical Information

System, M. M. Milne and W. J. Howe,

eds., ACS Symposium No. 84, Chapt.

10. (American Chemical Society,

Washington, D.C., 1978).

[2] For further information regarding the

leasing of this magnetic tape, please

contact: Office of Standard Reference

Data. National Bureau of Standards,

A323 Physics Building, Washington,

D.C., 20234.

[3] Heller, S. R., and Milne G. W. A., EPA/NIH Mass Spectral Data Base, NSRDS-NBS 63, 5 volumes, 4685 pp. 1978. PB290661 NTIS, Springfield).

[4] Hill, E. A., /. Amer. Chem. Soc, 22,

478-494 (1900).

[5] Speck, D. D., Venkataraghavan, R., and McLafferty, F. W., J. Org. Mass

Spectrometry 13, 209-213 (1978).

PJL hysicians hV! have depended on the laboratory testing of blood from epilepsy patients to establish that their serum levels of anticonvulsant drugs

are within the medically beneficial concentration ranges. Patients might other- wise suffer seizures or toxic effects, depending on whether the drug levels are below or above the therapeutic

ranges. A large-scale comparison of laboratory results from testing identical serum samples containing the com- monly prescribed drugs revealed unacceptably large differences among laboratories and methods [1], For each of the drugs tested for, the laboratories

reported results ranging from not pres- ent to present in toxic amounts. To help overcome this problem,

NBS was asked to provide a Standard

Reference Material (SRM) consisting

of three serum samples, each to contain

four antiepilepsy drugs at different concentration levels. The SRM would

fill a basic role for the achievement of accurate analysis, and help ensure the

reliability of analyses for these drugs.

Table 1 lists the drugs and actual certi- fied concentrations which typify medically designated levels. The SRM was

to be supplied as freeze-dried material

which would be reconstituted by the user by adding an exactly specified

volume of water.

In order to certify an antiepilepsy

drug SRM, a series of problems had

to be considered: (1) analytical criteria and specifications had to be developed

for selecting a special supply of each

of the four drugs. These supplies would

be employed in preparing the SRM

and, of greater importance, they would

serve as the primary standards for the certifying analyses that would be per- formed; (2) the serum used as the matrix for the SRM had to be compatible with the many methods in cur-
)=FICE OF DEPUTY DIRECTOR FOR PROGRAMS

Table 1. Concentration Level, jug/mL

Drug Toxic Therapeutic Subtherapeutic Blank

Phenytoin

Diphenylhydantoin)

Ethosuximide

Phenobarbital

'Primidone

60.7 ± 0.9 a 174.7 ± 0.6

103.6 ± 0.3 18.6 ±0.7

16.7 ±0.3

75.9 ±0.5

21.6 ± 0.2

8.1 ±0.2

4.2 ± 0.1 11.8 ± 0.4

5.3 ±0.2

3.6 ±0.1

I The uncertainties represent one standard error for the above certified values. The imprecisions observed

^oth within and between the liquid and gas chromatographic anlyses used for this certificate are included

~n the standard errors.

-ent use and. to the degree possible,

vith prospective methods. This re- quired serum that would be free from

:hemical and biological (e.g., imrauno-

.ogically active) interferences; and

:3) two, independent, highly accurate

: analytical methods had to be developed

i or determining the drug levels in the SRM preparations, because NBS ceriification would not be based on the ;veighed-in concentrations. The two

methods would have to yield sufficiently accurate and precise data that the ssignment of values could be unambiguous. This required use of methods

or achieving accuracy and precision

lot ordinarily utilized in clinical labo-

; atory situations. For selecting supplies of each of the hugs, a variety of chromatographic

md spectroscopic techniques were applied to samples whose properties generally conformed with those given in he literature for the drugs. The purity

bf the selected supply of each drug was

ietermined by liquid chromatography

LC), the compositions of fractions

)eing characterized by supplemental

:hromatography and spectroscopy.

The specifications we set for compatibility of the serum matrix with a

/ariety of antiepilepsy drug assay nethods precluded the use of any but

Timan serum. (Serum from a bovine

source, for example, might exhibit

cross-reacting antibodies to bovine components in radioimmunoassay reagents.)

The large volume of serum needed

required the pooling of sera. Since serum from different donors could be

expected to contain different interfer- ing substances due to individual dietary

habits and drug utilization, we employed several treatments to obtain an

interference-free serum. First, hepatitisfree plasma from several donors was

clotted by adding calcium chloride, and

the serum obtained by centrifuging.

The lipoproteins were removed and the serum was heated to deactivate some

of the enzymes. The serum was again

centrifuged and then dialyzed to remove

electrolytes. After re-equilibriation with

0.15 mol/L sodium chloride at pH 6.5

to 7.0. the serum was passed through

a bed of charcoal and a 0.22-^m poresize membrane filter into a sterile container.

We tested the processed serum by

observing the clarity of the product

after portions were freeze-dried and

reconstituted. Unprocessed and processed serum was also tested by reversedphase LC and gas chromatography (as

described below) for evidence of inter- fering substances. The difference between the original and the processed serum as detected by reversed-phase LC

is illustrated in figure 1. All of the serum as detected by reversed-phase LC

then divided. A portion would serve

as the serum blank and the remainder,

in three separate portions, as the matrix

for the drugs. Appropriate quantities

of the four drugs were added to the

three portions. The four pools were

then dispensed into vials with a mechanized pipettor which was precalibrated for accurate delivery. The vials of serum were freeze-dried in a pro- longed single operation to minimize any

irregularity in residual moisture level, and then, when under nitrogen, they were stoppered.

A random selection of vials containing blank serum and three levels of drug-spiked serum were reconstituted

with weighed quantities of water. The

homogeneity of each kind was deter- mined by differential refractometry.

With the refractive index of serum

about 0.01 greater than that of water,

detection of variations of 0.1 percent

required refractive index measurements

better than 0.000 0 1 . This was accomplished by comparing the serum samples

to a reference aqueous solution containing about 100 g/L of potassium

chloride, using a differential refractometer with a sensitivity about 0.000 001.

The vial-to-vial variations in the reconstituted sera were less than the variance

found by chemical analysis.

Quantitative analysis of drugs in serum was performed by reversedphase LC, with a fixed-wavelength

detector at 254 nm and, for primidone,

a variable-wavelength detector set at 210 nm. Peak areas were determined

by use of an electronic integrator.

Standard samples containing each of the four drugs in 20 percent methanol80 percent water were run alternately

with serum samples. Calibration data were thus obtained concurrently with

the sample analyses and were applied

105
t -» Human Serum t -» Processed

human serum matrix

Figure 1. Reversed-phase liquid chromat- ograms of unprocessed, pooled human

serum (left panel) and processed serum

matrix (right panel).

to give the quantitative results. Figure 2

illustrates a typical chromatogram, re- corded at 254 nm. Our gas chromatographic results were obtained with a special column

packing, GP 2 percent SP-2510 DA

on 100/120 Supelcoport, to separate

the anticonvulsant drugs in their underivatized forms. Flame ionization detection was used, and the output was

interfaced with a computing integrator.

The compounds used as internal standards in the analyses for the four drugs

are listed in table 2. Uniform quantities of the internal standards were combined with 1 mL of the reconstituted

SRM sample and, in separate containers, combined with the primary standard drugs taken in quantities about

2 percent lower and about 2 percent

higher than the expected (from the weighed-in) quantities of the drugs in

NATIONAL MEASUREMENT LABORATORY

ethosuximide

internal standard (i.s.)

phenobarbital

_L J_

10 20 30

Time, minutes

40

Figure 2. Reversed-phase liquid chromat- ogram of antiepilepsy drugs in processed

human matrix (detection at 254 nm).

the SRM sample. Then 1 .0 mL volumes

of the serum blank was pipetted into the tubes containing the mixtures of primary and internal standards. Usually four SRM samples and two of the bracketing standard mixtures were

analyzed as a group, as follows: Each was acidified to pH 1 to 2 and

then extracted with lOmL of highpurity chloroform. After centrifugation,

the aqueous layer was discarded and

the chloroform was decanted into a

Table 2. Drugs and internal standards used in GC analysis

Drug Internal standard

Phenytoin 4-(p-Methylphenyl)-5-phenylhydantoin

(Diphenylhydantoin)

Ethosuximide a-Methyl-«-propylsuccinimide

Phenobarbital 5-Ethyl-5-(p-methylphenyl)-barbituric acid

Primidone 4-Methylprimidone

diphenylhydanto

i.s.

2 3 4 5 6 7 3 Time, minutes

Figure 3. Gas chromatogram of antiepi- lepsy drugs.

15 mL conical centrifuge tube and evaporated at 20 °C under vacuum. The

concentrate was dissolved in 50 yuL of chloroform and vortex-mixed. One microliter of this solution was then in- jected into the gas chromatograph.

Figure 3 illustrates the separations

achieved by gas chromatography.

Standard reference material (SRM)

900 is now being used in clinical laboratories throughout the nation. A

new comparison of laboratory results

is now in progress, and preliminary

findings indicate that this SRM is having a dramatic effect on the quality of . the clinical tests for antiepilepsy drugs

in human serum.

Reference

[1] Pippinger, K. F., Penry, J. K., White,

B. G., Daly D. D., and Buddington,

R., Interlaboratory Variability in Determination of Plasma Antiepileptic

Drug Concentrations, Arch. Neurol.

33, 351-355 (1976).

106
Development of Test Procedures

to Determine Calorific Value of

Refuse and Refuse-Derived

Fuels

Eugene S. Domalski, Chemical Thermodynamics Division and Joseph G.

Berke, Resource Recovery Program

oV«X ver the past two years, we

have been working on a joint Department of Energy/Environmental Protec-

:ion Agency project to develop inforI nation for a product specification

m refuse-derived-fuels (RDF). More

! -ecently, we have been working with

:he Office of Recycled Materials to ;ontinue the RDF work in direct re- sponse to the mandate of the Resource Recovery and Conservation Act

"RRCA) PL94-580.

The research problems stem from

:he fact that the calorific value of municipal solid waste (MSW) or refusederived-fuels (RDF) is currently deter- nined in a bomb calorimeter by using

•gram-size samples which have been

subjected to a high degree of processing.

The processing includes grinding, shredding, milling, riffling, and pelletizing of he waste sample. Engineers who are

'experts in incinerator technology and

solid waste processing believe that after ;uch treatment the waste sample no

longer resembles or represents the original waste. They claim that significant

:hemical changes take place as a result

Df the size reduction (from gross sizes down to below one millimeter) which

is necessary in the preparation of a lomogeneous waste sample for convenional calorific value determination.

Similarly, such engineers have stated

:hat the calorific value determined for the

highly processed waste sample may

lave little or no correlation with the original heterogenerous waste. Because

pf the enormous heterogeneity of

: ~efuse, a suitable procedure has not emerged as yet which may be used

with confidence to select and extract p

truly representative sample. A suit- able test procedure for the calorific

v'alue of solid waste from which a

j "eliable thermal efficiency could be

:alculated would supply a mechanism

1 1 which engineers, public works adL

ministrators, and private owners could

evaluate with confidence whether their large incinerators or refuse-fired boilers are in compliance with contract per- formance specifications. Such a test procedure can be validated from measurements carried out in a multi-kilogram capacity flow calorimeter on large

samples of refuse which have undergone little or no processing. Both the business and industrial communities

have expressed the need for a test procedure. In addition both American

Society of Mechanical Engineers and

American Society for Testing and Materials have also lent support to the NBS program.

NBS Activities During the initial phases of the pro- gram at NBS, the tests were carried out in a 2.5 gram bomb calorimeter.

However, due to the concerns of processing the material cited above, a larger

sample size calorimeter was required.

NBS met the challenge by building a 25-gram sample size bomb calorimeter.

Processing of samples, although still required, was less severe than for the 2.5-gram unit; in addition, samples of

as received-pellets of commercially

produced RDF could also be combusted. The specifics of this work are available in NBS publications: NBS

IR 78-1494 and NBSIR 80-1968.

Much of this work is being coordinated with the ASTM committee E-38

on Resource Recovery, which is currently working to establish standard test methods to characterize refuse-derivedfuels and enhance their potential for becoming an article of commerce. The

development of markets and sales of refuse-derived-fuels will be linked

strongly to appropriate characterization

procedures. In particular, the calorific value, which provides a measure of the energy content of RDF, will have

greater impact than any other material

property on market development be- cause burning will be an important

mode of reclaiming energy from processed solid waste for at least the next decade. Thus, a test procedure for determining the calorific value will better equip commercial laboratories to

certify accurately the energy content

of RDF of various compositions. In

addition, the determination of the calorific value of RDF at various stages of processing will identify any stages

which impart a different chemical character to the solid waste. Since many of the current incinerators for waste-to-energy systems burn

unprocessed garbage as received, both

ASME and ASTM expressed the need

to determine the calorific value of the unprocessed garbage. To do so requires

using large samples of garbage to ensure some sort of representative "grab"

sample. NBS, in cooperation with

DOE, is designing and plans to build

and operate a multi-kilogram sample

size flow calorimeter to handle unprocessed or minimally processed refuse. To date, pilot units of the flow

system using 25-gram samples have

been built and tested. Currently, tests are being conducted to determine

proper operating parameters of the

small units that can be scaled up. Calibration work with known materials

is going on to correlate between the 25-gram bomb and the 25-gram flow

units. Over the next two years, NBS,

with assistance from DOE will construct and operate the multi-kilogram

calorimeter and conduct a variety of experiments on fuels and various

fuel mixes as related to resource recovery and waste-to-energy systems. The

data will include heating values, moisture, ash, and elemental analysis. Although the principal thrust of this research is on the combustion calorime107
NATIONAL MEASUREMENT LABORATORY

Nuclear Fuel Assay Using

Resonance Neutrons

Roald A . Schrack, James W. Behrens,

and Charles D. Bowman, Nuclear

Radiation Division

try of large samples, a variety of other

problem areas will be examined which

have application to the conversion of waste into energy and the processing of municipal solid waste into refusederived-fuels. The problem areas are

analysis and characterization of stack

effluents, temperature measurement and

materials durability in hostile environments, sample composition and characterization, combustion and pyrolysis

kinetics, corrosion, and statistical studies in waste stream sampling. Various

measurement techniques will be used

in the research, including calorimetry,

gas-liquid chromatography, spectrophotometry, infrared spectrometry, and

thermometry.

T

Collimator

he Center for Radiation Research

is developing a technique that will produce images similar to the familiar x-ray image, that will show the dis- tribution of any isotope desired. Using

neutrons generated by the NBS linear accelerator, this technique—resonance

neutron radiography—provides a unique tool for determining both

distribution and content of any sample.

The technique is being developed as a reference standard measurement

system for the NBS Nuclear

Technology Program.

This program seeks to provide accurate and timely techniques to keep

track of nuclear materials. The reso- nance neutron radiography technique is used to measure the concentration and

uniformity of distribution of fresh fuel

Detector

Source

maging

System

Figure 1. Schematic diagram of Resonance Neutron Radiography system. Neutrons are generated in a source by using the electron beam of the NBS linac. The neutrons are then

collimated into a fan-shaped beam. The sample to be examined is passed through the beam

to produce an image in a computer controlled system.

108

rods and to measure nuclear material in scrap and waste from a nuclear plant.

The physical phenomenon that is utilized in the technique is the selective absorption and scattering of neutrons by the nucleus. Every isotope has a

different set of energies for which it interacts with neutrons. These energies

have been carefully cataloged so that the observations of a characteristic absorption pattern can be used to identify the presence of an element—and

in particular the amount and isotope of the element. Previous research at NBS

has determined to better than 1 percent

accuracy the values of these interaction

cross-sections for a wide variety of materials. The values previously deter- mined at the Bureau were submitted to the national clearing house maintained

at the Brookhaven National Laboratory on Long Island. We drew on these

files in the analysis of our data. Resonance neutron radiography thus

represents a new application and use

for information that has already been

accumulated. The procedure used is shown in figure 1. Neutrons generated

by the NBS linear accelerator are col- limated down to a thin fan-shaped

beam. A sample of material to be

analyzed is placed in this beam. The

number and location of neutrons that pass through the sample are determined

with a position-sensitive neutron detector placed behind the sample. This

information is accumulated in a computer as the sample is scanned. When

the sample is completely scanned, the accumulated data are analyzed to pro- duce a picture of the distribution of the desired isotope in the sample. The

total amount of the isotope in the sample can then also be determined.

The ability to discern details in the distribution of the desired isotope de- pends on the resolution of the detector system. Commercially available detec-
CFFICE OF DEPUTY DIRECTOR FOR PROGRAMS

r'tors have a resolution limi t of about

5 mm; details smaller than this could

}

: not be observed. Fuel pellets used in nuclear reactors are 10 mm in diamePter, so the available resolution in commercial detectors does not permit

a meaningful measurement of nuclear

fuel. To overcome this deficiency, a high-resolution detector system was

•developed by NBS in collaboration with

('the Oak Ridge National Laboratory7. This detector, shown in figure 2, has

: a resolution of about 1 mm and has

I'been used in the analysis of nuclear

ffuel.In addition to its value in the

[ Nuclear Technology Program, reso- nance neutron radiography promises to

['be of value in other fields, such as i-nondestructive evaluation. The examination of manufactured items for hidrden defects is frequently done with

I low-energy neutrons as well as x-rays. Hidden defects in welds can be defected before the equipment is used,

['preventing costly equipment failure, an example of this type of use, a ample was made with an intentionally

'defective joint. Figure 3 shows the

'result of a resonance neutron radiography of this sample when the distribu- tion of silver in the brazing material was made. No defect was observable

with normal x-ray imaging techniques.

The defect was only marginally detectable using low-energy neutron imaging

techniques.

Research is continuing on the reso- nance neutron radiographic technique

to improve accuracy and resolution and

to reduce the time required to produce an image. Although the technique is low restricted to laboratories with high- energy linear accelerators, methods are being worked on to allow the technique

to be used on a broader basis and thus

become more available as a tool for technology.

Figure 2. High-resolution position sensitive neutron detector. The detector region is the center small-diameter tube. Special amplifiers are enclosed in the large-diameter cylinders

at each end. The valve allows the gas used in the ionization chamber to be changed to determine the effects of different mixtures and pressures. The detector is presently operating with a mixture of 3.1 X 105 pascals of 3He, 7.9 X 103 pascals of xenon and 0.4 pascals of carbon dioxide. (105 pascal == 1 atmosphere).

Figure 3. Computer-generated distribution of silver in a defective silver solder joint as determined by a resonance neutron radio- graphic scan.

109
NATIONAL MEASUREMENT LABORATORY

Improved Uses of X-rays for

Nondestructive Evaluation

Masao Kuriyama, William J. Boettinger

and Harold E. Burdette, Metallurgy

Division

TJL. he current need in industry for

quality control and early detection of flaws in materials cannot easily be

satisfied by simple refinements of existing NDE technology. Fundamental

principles underlying the existing

traditional techniques should be

re-investigated from a fresh point of view. Here we will report the results of such efforts in two different aspects

for NDE techniques: one is the resolution improvement of radiography

and the other is the measurement of residual stresses within materials.

A. X-ray Image Magnification

Industrial applications of radiography presently demand the early real- time detection of small flaws such as

tight cracks in materials. The live images of flaws or structures are re- corded (or viewed) usually on a detector (an electro-optical imaging sys- tem) with similar size as the image

itself. In order to detect small objects, an improvement in the resolution of real-time imaging systems is required,

hopefully without loss of image brightness. However,, attempts to improve

the resolution usually result in a signifi- cant loss of intensity. Thus, the detectability (or intensity sensitivity) of small signal changes caused by flaws

will be lost because of lack of brightness. Any approach in the improvement of resolution for real-time imaging

always encounters two opposing problems: If one improves the resolution,

then the brightness generally decreases,

and vice versa. Present commercial

image intensifiers have resolution limits determined by the multistage configuration and the fiber diameter in fiber optic plates. They may be about 50

/xm, down to 10 /mi at best. We have developed [1, 2] a tech- nique which enables us to magnify an

x-ray beam containing structure infor- mation (radiography or topographic

images) before it reaches a detector. This magnification capability can be

used either to improve the overall resolution of existing x-ray radiographic

techniques which are limited by the detector or to permit the use of less complex (and high gain) imaging sys- tems to attain resolutions already possible. In our demonstration an x-ray magnifier displayed a detailed image

of 1000 mesh gold grid (25-p.m spacings) with good contrast on a real-time

basis on an ordinary image intensifier of 100-/xm resolution. In order to magnify an x-ray beam which contains

structure information (such as one

containing radiographic images), the beam is successively diffracted from

two silicon crystals. The first diffrac- tion magnifies the beam horizontally

and the second in a perpendicular

direction. This magnification technique makes

use of Bragg diffraction from a nearly perfect silicon crystal free from

dislocations. This type of Bragg dif- fraction, often called dynamical diffraction, has unique features which are

essential for making x-ray image magnification possible. When an x-ray beam diffracts from the surface of a crystal and the diffracting planes are not parallel to the crystal surface, the

diffraction is termed asymmetric. There

are three aspects of asymmetric dynamical diffraction which are important for

the x-ray magnifier: beam magnification, reflectivity, and the acceptance

angle for the diffraction. Figure 1 shows schematically the asymmetric

diffraction geometry. An incident parallel monochromatic beam of x-rays

is magnified in one dimension by a

factor m as shown in figure 1, where

0, n and 0Out are the angles between the

crystal surface and the incoming and

outgoing beams, respectively. Obviously, high magnifications are obtained

when <9 in is very small. The "quantum"

efficiency of an x-ray magnifier is de- termined by reflectivity under Bragg

conditions. The ratio of the diffracted

total flux (photon/second) to the inci- dent total flux is called the reflectivity

and is a function of the deviation from

the Bragg condition. For a thick perfect crystal with negligible absorption,

this ratio is unity for a range of angles

centered about the Bragg condition and

falls to zero rapidly for larger deviation from the Bragg condition. Because

the reflectivity is unity regardless of the

diffracting plane, the intensity (photon/

second cm2

) of a parallel beam magnified in one dimension by asymmetric

diffraction from a perfect crystal is decreased by a factor (1/m) only be- cause of the magnification of the beam

area. Since refractive indices for any

materials cannot be larger than one for the ordinary x-ray energy range, the unity reflectivity is the highest effi- ciency—that is, no loss—which one ! can expect in x-ray optics. As shown

i A # 0 ut I'll

l din / L

sm 0 out d out = md in ; m = —j0>S

j A0O ut= A<? in = VTri vlrT

Figure 1. Asymmetric Bragg diffraction showing one-dimensional magnification of an

x-ray beam. The cross-section of the in- coming beam, d in , is magnified by a factor, m, to produce the outgoing beam dout. The

extremely narrow divergence of the outgoing

diffracted beam created by dynamical diffrac- tion guarantees the faithful magnification of x-ray images present in the incoming beam.

110
CFFICE OF DEPUTY DIRECTOR FOR PROGRAMS

-in figure 1, the maximum divergence

:of the outgoing beam, A0ont , is related

to the quantity ws , which is determined

by the crystal property of silicon, and

;iis of the order of 6 arc second. This

j small value of 0ollt guarantees the one-

.to-one correspondence of the details

! .between the unmagnified images and

c the magnified images. Since the reci- Lprocity law holds for x-ray magnifiers,

.demagnification needed for lithography

ban, in principle, be achieved by these

; magnifiers.

B, X-ray Residual Stress Evaluation in

the Interior of Materials

| One of the nondestructive methods

currently used for measurements of residual stresses in industrial materials

..nvolves the application of x-ray analysis for stress measurement. X-ray

.diffraction phenomena are used to determine macroscopic residual stresses

I n engineering components. The methodology of analysis and its basic principles are well known. Briefly, when

-naterials are under stress, x rays are

diffracted with a Biagg angle which

s slightly different from the Bragg

ingle expected in unstressed materials.

The change in the Bragg angle is re- ated to the alteration of atomic inter-

:>lanar spacings when the crystal lattice

jf the materials is strained. As a measirement system, the essential part of

ihis x-ray technique is the accurate n:asurement of lattice constants (or

heir changes) in materials by use of

veil refined Bragg diffractometry. The

;urface residual stress can be evaluated rom the strains measured by x-ray

iiffractometry using a set of assumpicn regarding material elasticity and

ic mogeniety. Although the accuracy

'btained by this technique is, in principle, superior to any other methods,

n obvious shortcoming lies in its incapability of detecting strains (or

stresses) in the interior of bulk materials. Currently, improved quality control

of industrial components demands

quantitative information concerning the

stress distribution near cracks and re- sidual stress distributions after different types of cold working and heat treating.

These demands naturally lead to the

necessity of measuring residual strains

in the interior of materials. Ordinary

Bragg diffractometry will not be sufficient to respond to these demands.

There is an entirely different approach

to the x-ray evaluation of residual

strains although it uses the principles of x-ray diffraction. This approach in- volves the use of energy dispersive solid

state detectors with high energy x-ray photons. The use of solid state detectors has been introduced very ef- fectively in the fields of scanning elec- tron microscopy (microanalysis) and

x-ray fluorescence analysis of materials

in the past ten years. It is called energy-dispersive spectroscopy. Unlike

these successful applications, this tech- nique has not been applied to the re- sidual stress (strain) measurements of

materials. Extremely high resolution

and accuracy are required for strain measurements in comparison with the other applications. If one could improve the resolution sufficiently and overcome other important technical

problems, this new system will be ideal

for residual stress measurement in in- dustry; the equipment will be compact

and the results will be displayed visually and almost instantaneously, and

analysis of the data will be made on

site by a computer. Also this system,

in general, works better for high energy

photons. High energy photons also make practical the use of the trans- mission geometry, since the x-ray absorption coefficient of materials is approximately inversely proportional to the third power of the energy of x rays. The use of an energy dispersive sys- tem for possible residual stress evaluation was tested in 1973 by Leonard

[3], who concluded, however, that this system did not have sufficient accuracy

for this purpose. We have proven that

his conclusion was premature, and that

it is still too soon to dismiss the possible industrial use of this technique

for the evaluation of residual stresses. In so doing, we have proposed [4, 5] a method for determining a strain ten- sor in predetermined volumes in the

interior of a material, as shown in figure 2. In energy dispersive spectros- copy, one is concerned with the energy

spectral profiles and peak positions at different energy values, instead of Bragg angles which play a key role in Bragg diffractometry. Lattice constant

will be measured in energy dispersive

spectroscopy by the energies of dif- fracted photons:

J/A , / 6.199 \ 1 M x d(A) =1 ^—-I -Fry. r- d) v smO > E(kev)

Here, d is an atomic interplanar spacing related to lattice constants, E is the energy of diffracted photons measured

from peak positions of the spectra, and

the coefficient including half the scat- tering angle 9 is constant for a given

scattering geometry. For our purposes,

the scattering angle should be smaller

than 30° to ensure complete trans- mission geometry.

One may claim that the energy resolution of current solid state detectors is not adequate for the evaluation of

residual strains. Fortunately, it has

been known that each diffraction profile in the energy spectrum obtained

from a solid state detector is very close

to a Gaussian. If the mathematical

shape is known for a spectral profile,

then the claim mentioned above can be

111
NATIONAL MEASUREMENT LABORATORY

Sample

Detector

Rotation

Translation

Figure 2. An example of the geometrical

arrangements for the evaluation of residual

strains in a predetermined volume inside materials. The sample is rotated three times around an x-ray beam to give six independent sets of measurements with two detectors. The mapping of strains throughout the volume of the sample is achieved by translations.

circumvented: Mathematically fitting the observed peaks with Gaussian functions, one can determine the centroids

(or peaks) of the profiles far more

accurately than the AE/E value of detectors would indicate. As shown in figure 3, one can improve by a factor

of 100 the accuracy of determining the centroid positions.

Figure 3 shows an example of the observed spectral profile which has

been fitted by a Gaussian curve with

a linear background. The accuracy of determining the centroid (or peak) position has been found to depend simply

upon counting statistics. Reproducibility of those positions has also been

tested; as long as the same predetermined volume is viewed, the centroid

position remains identical within the

statistical error. As a reference to possible instrumental instability, Ag Ka

line spectrum in each run has been

fitted with a Gaussian curve. (White

radiation was obtained using a Ag tar- get tube.) The Ag Ka line result indi- cates that the instability, if any, is not

significant enough to jeopardize the

800 820 840 860 880 900

Channel Number

Figure 3. An example of the observed

diffraction profile fitted with a Gaussian

curve with linear background.

accuracy in the determination of dif- fraction peak positions. Our result demonstrates that the strains of 5 X

10~ 5 can be detected in the predetermined volume of 0.5 X 0.5 X 0.5 mm:! with reliable reproducibility, if suffi- cient counting statistics are established,

while strains of 3 X 10~ 4 can easily be

detected even with moderate counting

statistics. In conclusion, an energy dispersive

diffraction system has demonstrated its capability as a useful tool for the determination of residual stresses inside materials, when the curve fitting tech- nique is used simultaneously. The pres- ent demonstration has been carried out

with low energy photons. The use of high energy photons certainly makes

more practical the use of these energy

dispersive systems for residual stress evaluation. More penetration and in- creased energy resolution of detectors

will improve the resultant accuracy and

detectability of strains. Since the en- ergy dispersive system can be made

simple and compact, without any deli- cate moving parts, the system is quite

ideal for an industrial use on site, particularly when data are handled by

a mini-computer. By no means has an energy dispersive system such as used

here reached the ultimate resolution; further improvements of x-ray optics,

and detectors should be made, in particular, with high energy photons

11

i

III

i

i

8 Reference

[1] Kuriyama, M., Boettinger, W. J., and

Burdette, H. E., Proceedings of Sym

posium on Real-Time Radiologic'

Imaging, National Bureau of Standards, May (1978)

[2] Boettinger, W. J., Burdette, H. E., and Kuriyama, M., Rev. Sci. Instr., 50 26-30 (1979).

[3] Leonard, L., Franklin Institute Re

search Lab (Philadelphia) Report

F-C3454 luly (1973).

[4] Kuriyama, M., Boettinger, W. J., and

Burdette, H. E., American Society for Nondestructive Testing, National Fall Conference, Denver, CO, 49 October

(1978).

[5] Kuriyama, M., Boettinger, W. J., and

Burdette, H. E., Symposium on Ac

curacy in Power Diffraction, Nationa Bureau of Standards, lune (1979).

112
FF1CE OF DEPUTY DIRECTOR FOR PROGRAMS

ligh Accuracy Particle Size and

•article-Fluid Interaction

Measurements Using the Particle

)oppler Shift Spectrometer

favid S. Bright and Robert A. Fletcher,

}as and Particulate Science Division

T T e have increased the accuracy

nd precision of the Particle Doppler

hift Spectrometer (PDSS) measurejent technique through improvement

f its light optics and its data reduction

nd analysis systems. With the PDSS,

e can now determine the diameter of

pherical aerosol particles in the 5- to

5-fj.m range with an accuracy of

.05 ixm. This high accuracy made it ossible for us to study particle/fluid

iteractions that have been undetectable

i y other techniques, though such

I nteractions have been theoretically

iferred.

• The basis of particle diameter measirement by the PDSS is a modification

if the laser velocimetry technique. The

•ample cell, a vertical tube, is inter- acted by a horizontal laser beam.

3 ight is scattered by both the stationary

indows of the sample chamber and

y any particles falling through the

iser beam. Scattered light at 6° above

orizontal is detected and measured

ith a photomultiplier having well col- mated collection optics. The light :attered by the settling particles is •oppler shifted with respect to the

icident light; the magnitude of the equency shift is proportional to the

sttling velocity of the particle. The

'oppler shifted frequency is measured

m mixing the light from the particle

nd from the windows (unshifted light) id analyzing the resulting signals,

/ith a Fast Fourier Transform anar zer, the amplitudes of the scattered

ght signals are stored according to equency. From the frequency of the

:attered light, we determine the set- ing velocity of the particle; from the

ittling velocity, we can calculate the

article's diameter by using Stokes's

LW.

In addition to the particle diameter

information contained in the frequency

of the scattered light, we are able to

also obtain particle size information

from the amplitude of the scattered

light. The intensity of light scattered from a spherical particle is a complex

function of the particle size, angle of observation relative to the incident

light, the indices of refraction of the

particle and the suspending media, and

the wavelength of the light. The in- tensity of scattered light as a function

of particle diameter may be calculated

from Lorentz-Mie theory. The calculation for the parameters of our experiment (6° angle of observation, known

wavelength, and indices of refraction)

give the curve shown in figure 1, which

contains many maxima and minima

features. The calculated curve is ex- perimentally equivalent to summing the

scattering intensities from a uniform

distribution of particles having diameters covering the range of the Mie

calculations. Such a summation of

intensities is also shown in figure 1 and clearly demonstrates the features

of the calculated Mie curves. The

duplication of the calculated Mie curve

features in the experimentally obtained

curve allows us to use the features in the experiment curve as accurate size calibration markers. For a given feature, the Mie calculation gives us an

accurate value for the particle diameter,

from which we can calculate the particle settling velocity on the basis of Stoke's law. The settling velocity can

also be determined experimentally from

the Doppler shift frequency for that feature of the curve. Through sophisticated data analysis, we are able to compare the settling velocities calculated from the Mie theory with the

settling velocities experimentally deter- mined from the Doppler shift such that we can detect velocity differences as

0 Doppler frequency F, Hz 2000

0 5 10 16 18

Diameter,

Figure 1. Relative scattered intensity as

a function of diameter calculated from

Lorentz-Mie theory (lower axis) and of Doppler frequency obtained experimentally

(upper axis). ... is calculated data, — is experimental data.

small as a few ^m/s, or equivalent to diameter differences of only 0.05 /xm. This sensitivity allowed us to verify

the Stoke's equation for particles in the 5- to 15-um diameter range. In order to accomplish this verification, it was necessary to demonstrate that we

were not perturbing the settling velocities in some way. Thermal convection

adds large velocity increments to the

settling velocity and removes most or

all size dependency, which may easily

be seen in the loss of the features of

the scattering curves. Though we find no evidence of thermal convection, we

do find under certain sampling conditions a particle-induced fluid flow

phenomena.

When a particle gravitationally settles through a fluid (air in our experiments) the drag force of the fluid results in a small momentum transfer

to the fluid causing a small fluid flow

in the direction of the settling particle.

This flow then serves to increase the

settling velocity of neighboring particles

because the fluid is no longer stationary, but is moving with the settling

particles.

113
We demonstrated this effect by comparing the following two experiments:

In the first experiment, we measured

the settling velocity of particles when

they uniformly filled the settling chamber and compared it to the calculated

velocity to verify that no velocity shift occurred. We use this sample geometry

for particle size measurements. In the second experiment we measured the

settling velocity of a 4-mm diameter

column of settling particles in a 9-mm

diameter settling chamber. The air sheath around the column of particles was void of particles. With this experimental condition we observe an in- crease in the settling velocity, which

is particle concentration dependent, as shown in figure 2. The velocity increase

represents the velocity of the induced

air flow, which adds to the settling

velocity.

The induced fluid flow is manifested

in the second experiment because a return flow path exists—the particlevoid region of the settling chamber.

In the first experiment, no velocity

shift is observed because the return

flow is uniform over the entire volume

and effectively cancels the drag effect on the particles.

The induced fluid flows we observed

are on the order of 10 to 100 /xm/s.

An order of magnitude calculation, bal- ancing the fluid viscous diffusion force

and the Stoke's drag force, gives fluid velocities which are in good agreement

with these values. The sensitivity of the particle settling

velocity to the induced flows of such

low magnitude can be measured by the PDSS due to the accuracy we can

obtain by using the Mie scattering curve features as size calibration markers. We know of no other technique

that can determine particle diameter in the 5- to 15-^m diameter range to an

uncertainty of 0.05 or that has the

sensitivity to detect particle-fluid inter- actions of the low magnitudes we have

described above.

.02

0

.00 500 1000

Particle Concentration (cm -3)

Figure 2. Fluid velocity as a function of

particle concentration. 114

U.S. GOVERNMENT PRINTING OFICE : 1980 O-311-046 (133)