NBS NOTE

Radiochemical

Analysis Section

Summary of Activities

July 1967 to June 1968

451

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. Today, in addition to serving as the Nation's central measurement laboratory,

the Bureau is a principal focal point in the Federal Government for assuring maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. To this end the Bureau conducts research and

provides central national services in three broad program areas and provides central national services in a fourth. These are: (1) basic measurements and standards,

(2) materials measurements and standards, (3) technological measurements and

standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, and the Center for Radiation Research.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the

United States of a complete and consistent system of physical measurement, coor- dinates that system with the measurement systems of other nations, and furnishes

essential services leading to accurate and uniform physical measurements throughout

the Nation's scientific community, industry, and commerce. The Institute consists

of an Office of Standard Reference Data and a group of divisions organized by the

following areas of science and engineering

Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic Physics—Cryogenics2—Radio Physics2—Radio Engineering2—Astrophysics2—Time

and Frequency.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to methods, standards of measurement, and data needed by industry, commerce,

educational institutions, and government. The Institute also provides advisory and

research services to other government agencies. The Institute consists of an Office of

Standard Reference Materials and a group of divisions organized by the following areas of materials research:

Analytical Chemistry—Polymers—Metallurgy — Inorganic Materials — Physical

Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides for the creation of appropriate opportunities for the use and application of technology within the Federal Government and within the civilian sector of American industry. The primary functions

of the Institute-may be broadly classified as programs relating to technological meas- urements and standards and techniques for the transfer of technology. The Institute

consists of a Clearinghouse for Scientific and Technical Information,3 a Center for

Computer Sciences and Technology, and a group of technical divisions and offices organized by the following fields of technology:

Building Research—Electronic Instrumentation — Technical Analysis — Product

Evaluation—Invention and Innovation— Weights and Measures —Engineering

Standards—Vehicle Systems Research.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement,

and application of radiation to the solution of Bureau mission problems and the

problems of other agencies and institutions. The Center for Radiation Research consists of the following divisions:

Reactor Radiation—Linac Radiation—Applied Radiation—Nuclear Radiation.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D. C. 20234.

2 Located at Boulder, Colorado 80302.

3 Located at 5285 Port Royal Road, Springfield, Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stems, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 451

ISSUED JANUARY 1969

Radiochemical

Analysis Section:

Summary of Activities

July 1967 to June 1968

Edited by James R. DeVoe

Radiochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Research

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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

Price $1.25
FOREWORD

The Analytical Chemistry Division was established as a separate division at the National Bureau of Standards on

September 1, 1963 > and became part of the Institute for

Materials Research in the February 1, 1964 reorganization.

It consists at present of nine sections and about 100 technical personnel encompassing some 45 different analytical

competences from activation analysis and atomic absorption to

vacuum fusion and x-ray spectroscopy. These competences, and

in turn the sections which they comprise, are charged with

research at the forefront of analysis as well as awareness of

the practical sample, be it standard reference material or

service analysis. In addition it is their responsibility to

Inform others of their "efforts.

Formal publication in scientific periodicals is a highly

important output of our laboratories. In addition, however,

it has been our experience that informal, annual summaries of

progress describing efforts of the past year can be very valuable in disseminating information about our programs. A word

is perhaps in order about the philosophy of these yearly

progress reports. In any research program a large amount of

information is obtained and techniques developed which never

find their way into the literature. This includes the

"negative results" which are so disappointing and unspectacular but which can often save others considerable work. Of

importance also are the numerous small items which are often

explored in a few days and which are not important enough to

warrant publication—yet can be of great interest and use to

specialists in a given area. Finally there are the experimental techniques and procedures, the designs and modifications

of equipment, etc., which often require months to perfect and

yet all too often must be covered in only a line or two of a journal article.

li
Thus our progress reports endeavor to present this information which we have struggled to obtain and which we feel

might be of some help to others. Certain areas which it

appears will not be treated fully in regular publications are

considered in some detail here. Other results which are

being written up for publication in the journal literature

are covered in a much more abbreviated form.

At the National Bureau of Standards publications such as

these fit logically into the category of a Technical Note.

In 1968 we plan to issue these summaries for all of our

sections. The following is the fifth annual report on progress of the Radiochemical Analysis Section.

W. Wayne Meinke, Chief

Analytical Chemistry Division

111
PREFACE

The Radiochemical Analysis Section develops measurement

techniques for qualitative, quantitative and structural analysis of materials through the use of radioisotopes. One of

the first successful nuclear techniques for determining

chemical structure is Mossbauer Spectrometry, one of the

major activities in this section.

Understanding of the chemical, nuclear, and physical

principles which form the foundation of a new analytical

measurement technique is basic to the development of measurement techniques using radioisotopes. Consequently, nuclear instrumentation, nuclear chemistry, and statistical

analysis are key activities in the section. These activities also support the Activation Analysis Section as well as

the radiation and radioisotope techniques in this section.

It is essential that the methods developed be applicable in the sense that they can be used successfully for materials of interest to science and industry. Therefore,

these methods are applied to the analysis of NBS Standard

Reference Materials where considerable cross checking of

analytical techniques is required throughout the process of

certification

A group working on radioisotope dilution techniques and

tracer methods has been reactivated after a two year dormancy. Results of this group will be reported next year.

In order to define more accurately the scope of our

work abstracts of publications appear in this report. Brief

descriptions of studies nearing completion are included when

it is our intent to submit this work elsewhere.

A roster of the groups in this section is listed in

part 5 . The National Bureau of Standards has several programs whereby a scientist from the United States or abroad

may work in our laboratories for one or two years. It is

hoped that by utilizing these programs the section

will be able to perpetuate a stimulating environment.

iv
In order to adequately specify the procedures, it has

been necessary occasionally to identify commercial materials

and equipment in this report. In no case does such identification imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the material or

equipment identified is necessarily the best available for

the purpose.

James R. DeVoe, Chief

Radiochemical Analysis Section

Analytical Chemistry Division
TABLE OP CONTENTS

Page

1. INTRODUCTION 1

2. NUCLEAR CHEMISTRY 3

A. Introduction 3

B. Nuclear Reactions 3

1. Mass Spectrometry 3

2. Bremsstrahlung Intensity and Composition . 4

C. Statistics in Nuclear and Analytical Chemistry 9

1. The Peak Height Method for Evaluating

Nuclear Spectra 9

2. Criteria for Reporting Trace Amounts of

Radioactivity 14

3. High Accuracy Calibration and Control in

Single-Channel Gamma Spectrometry 18

4. Counting Statistics and Standard Reference

Materials 20

5. Detection Limits 21

D. Iodine Determination by Static Mass

Spectrometry 22

3. MOSSBAUER SPECTROMETRY 24

A. Introduction 24

B. Instrumentation 25

1. Cryostat for Moving Absorber and

Stationary Source 25

2, Pancake Radiation Detector for Backscattering Mode of Mossbauer Spectrometry . 28

C. Backscattering Mossbauer Spectrometry 29

1. General Description 29

2. Spectra of Steel Samples 30

3. Effective Penetration of Mossbauer

Radiation 38

4. Identification of Corrosion Products on the

Surface of Iron by Scattering Techniques . 44

D. Operator Equivalent Method for the

Determination of Orbital Electric Field

Gradient Tensors 44

VI
Table of Contents (Cont.)

Page

E. The Electric Field Gradient Tensor for

Common Nickel Configurations 51

F. Ni Theory: Nuclear Interactions 61

G. Standard Reference Material for the Chemical

Shift of Tin Compounds 65

H. Fitting of Multiple Line Mossbauer Spectra

Using Constraints 68

1. Introduction 68

2. General Theory 69

3. Uncertainties of the Estimates 75

I. The Application of Mossbauer Spectrometry

to Quantitative Analysis 75

1. Introduction 75

2. Experimental 77

a. Apparatus 77

b. Reagents and Preparation of SnO p -Al ? 0^

Mixtures . . . 77

c. Mossbauer Experiments 78

d. Analysis of Copper-base Alloys .... 78

e. Tin Ores 79

3. Results and Discussion 79

J. Tin Mossbauer Spectrometry Following the

Decay of lx 9sb 90

K. Abstracts of Publications 93

NUCLEAR INSTRUMENTATION 96

A. Introduction 96

B. Overflow Counter 96

1. Introduction 96

2. Description of Logic 97

C. Sequential Scanner to Control Frequency

Synthesizer 99

1. Introduction 99

2. Description of System 101

D. Circuit Change to Improve Reliability of

Interface Units 102

VII
Table of Contents (Cont.)

Page

E. Time of Year (TOY) System 105

1. Time of Year Clock 105

2. Time of Year (TOY) Clock Controller .... 105

P. Maintenance of Teletypes and Paper Tape

Punches 107

G. Interface Between Mass Spectrometer and

Small Electronic Computer 109

5. PERSONNEL AND ACTIVITIES 110

A. Personnel Listing 110

B. Publications Ill

C. List of Talks Ill

6. ACKNOWLEDGMENTS 114

7. REFERENCES 115

APPENDIX I. OMNITAB PROGRAM FOR ANALYSIS OF

MOSSBAUER SPECTRA 117

LIST OF FIGURES

Figure Page

1 Gold foil - Nal spectrum 7

2 Gold foil - Ge(Li) spectrum 8

3 Cadmium-wrapped gold foil - Ge(Li) spectrum . . 8

4

137 Cs photopeak 10

5 Maximum ratio, base line/peak (B/N) vs

peak counts (N) 11

6

5 Mn with 22

Na baseline 12

7 Mossbauer cryostat 25

8 Photograph of pancake proportional detector

source side 26

9 Photograph of pancake proportional detector

back side 27

10 Diagram of pancake proportional detector ... 29

11 Pulse height spectrum of ^'Co source for

pancake detector, operating at 1400 volts,

with 10$, CH^ 90$ A flow gas 31

12 Pulse height spectrum of ^'Co source for

conversion electrons 31

13 Iron block scattering spectrum 32

14 SRM #1174 - white cast iron 33

15 Tool steel (collimated source) 34

viii
List of Figures (Cont.)

Figure Page

16 Tool steel (transmission geometry) 35

17 Photograph of pancake detector used for

backscattering at 45° angle 36

18 Mossbauer spectrum of steel plate taken by

conversion electron detection 37

19 Schematic showing geometry for scattering

experiment 39

20 Amount of backscatter as a function of

thickness of the scatterers 40

21 Scattering spectrum of stainless steel with

overlay of 0.22 mil iron foil 4l

22 Scattering spectrum of stainless steel with

overlay of 0.5 mil iron foil * . . . 42

23 Scattering spectrum of stainless steel with

overlay of 0.72 mil iron foil 43

24 Three common electron configurations for

Ni2+ S using a one-electron Hamiltonian .... 52

25 Two common electron configurations for

Ni2+ a using a many electron Hamiltonian .... 53

26 The effect of magnetic, quadrupole, and

mixed perturbations of the Mossbauer

levels of the °^Ni nucleus 62

27 Quadrupole spectrum as a function of n .... 66

28 Transmission spectrum of BaSnO^ 67

29 Percent effect and attenuation as a function

of sample thickness 68

30 Typical Mossbauer spectrum of SnO„ in AlpO.-, . . 80

31 Plot of height of peak to baseline ratio vs mg of SnOp 83

32 Plot of area of peak to baseline ratio vs

mg of Sn0

2

84

33 Pulse height spectrum of ymSn with lithium

drifted silicon detector 88

34 119 Sb vs BaSnO^ (thickness is 150 mg/cm2

) ... 91

35

119 Sn n O.- vs BaSn0 o (thickness is 150 mg/cm ). . 92

36 Diagram depicting overflow of memory for

pulse height analyzer 97

37 Timing of logic pulses in pulse height

analyzer 98

ix
List of Figures (Cont.)

Figure Page

38 Logic diagram of overflow counter 99

39 Binary scalers for counting overflows 100

40 Block diagram of sequential scanner 102

41 Logic diagram and schematic for interface of

pulse height analyzer and frequency

synthesizer 103

42 Board No. 6 of pulse height analyzer

interface 104

43 Time of year clock 106

44 Time of year clock controller 108

LIST OF TABLES

Table Page

1 Absolute nuclear reaction rates and

relative yields 5

2 Advantages and limitations of the peak

height method 13

3 Trace level reporting 15

4 Minimum upper limits (a=.05) 16

5 Value of upper limits based upon zero counts . 17

6 Observed counting precision 20

7 3d electron contribution to the EFG elements . 52

8 Hamiltonian matrix for the XL»S perturbation

on 3t 58

3

9 Spin-orbit perturbation results for the T,

level of 3p j_ n T, symmetry 59

10 EFG elements for the low lying states of Ni

with T, coordination 60

11 Quadrupole Hamiltonian matrix for the spin

5/2 state 63

12 Partial derivatives for linear approximation . 74

13 Variation of spectral parameters with the

amount of SnO

p

82

14 Comparison of calculated values with the

observed values of H/B 85

15 Analysis of Sn in copper-base alloys 86

16 Effect of geometry on spectral parameters ... 86

17 Effect of Pd filter on spectral parameters . . 87
RADIOCHEMICAL ANALYSIS: MOSSBAUER EFFECT, NUCLEAR

CHEMISTRY, NUCLEAR INSTRUMENTATION, STATISTICAL ANALYSIS

July 1967 to June 1968

Edited by James R. DeVoe

ABSTRACT

This is the fifth summary of progress of the Radiochemical Analysis Section of the Analytical Chemistry Division at the National Bureau of Standards

.

The sections' effort comprises four major areas:

Mossbauer spectrometry, nuclear chemistry, nuclear instrumentation and the application of statistics in nuclear and

analytical chemistry.

Studies in nuclear reactions have centered around measurement by time of flight mass spectrometry of helium-4

7 3 produced from reactions such as Li(y,a) H.

Statistical methods have been applied to the measurement of peaks in radiation spectra, to reporting trace

amounts of radioactivity, to the calibration of Standard

Reference materials, and to the reporting of detection

limits of a system of measurement.

A procedure for cooling a moving absorber with a stationary source for Mossbauer spectrometry is described.

A system for taking spectra using the scattering mode is

described along with a large "pancake" shaped proportional

counter that is used in the system. Theoretical interpretations of Mossbauer spectra of nickel compounds are given.

Computer programs which incorporate constraints such as

field intensity and quadrupole moments have been written.

Several applications of Mossbauer spectrometry, such as

surface corrosion studies are described. Preliminary data

are given on the charge states of Sn after nuclear recoil. A technique for resolving some of the difficulties

in quantitative analysis of chemical structures is also described .

xi
An overflow counter for collecting more counts than the

memory capacity of a pulse height analyzer is described. A

sequential scanner for a frequency synthesizer has adapted

the NBS Optical Interferometric Mossbauer Spectrometer from

a constant velocity to a constant acceleration mode.

James R. DeVoe , Chief

Radiochemical Analysis Section

Analytical Chemistry Division

Key Words

NBS Linac , computers, standard reference materials,

photonuclear reactions, cross sections, flux monitors,

nickel-6l, Mossbauer spectrometry, computer programs for

Mossbauer spectrometry, backscatter geometry, proportional

counter, Mossbauer instrumentation, detection limits for

analysis, counting statistics, corrosion products, electric

field gradient tensors, nuclear recoil, quantitative Mossbauer spectrometry, overflow counter, interferometric Mossbauer spectrometer.

XII
1. INTRODUCTION

The effort of this section has centered around four main

areas, Mbssbauer spectrometry , nuclear chemistry, nuclear instrumentation, and the statistical interpretation of experimental data. An additional project called "Radioisotopic

Methods of Analysis" will use radioisotopic dilution techniques for high precision quantitative analysis for macro

constituents in materials.

Systems of instrumentation continue to be the area of

major effort in Mossbauer spectrometry. This is evidenced by

the continued effort in scattering techniques. This system

provides a capability for measuring bulk samples and should

increase the overall applicability of this spectrometry.

Significant effort has been placed in the area of understanding the fundamental processes of the hyperfine interaction.

Particular attention has centered about measurement of the

sign of the electric field gradient tensor.

Applications of the Mossbauer effect are beginning to

become commonplace. Scattering techniques have been used to

measure corrosion products on surfaces . The structure of

iron in various ferrites used in magnetic tapes or as memory

cores of computers has proven interesting. The structure of

iron in cements and of tin in glasses has been measured.

It would be highly desirable to be able to measure the

amount of various structures in a material. Even though formidable problems exist, it is clear that in many cases a meaningful result can be obtained. It appears more probable

than ever before that the use of calibrated standards will

allow quantitative measurement of structures by Mossbauer

spectrometry

.

While the standards program for "bench marking" chemical

shift has progressed nicely, it is evident that its usage by

scientists must increase. This system of standards has been

endorsed by an ASTM committee, and it is apparent that infor-
mation on these standards must be given greater publicity.

The effort in nuclear chemistry has been related in the

past year mostly to the statistical interpretation of counting data. Considerable progress has been made in this area,

culminating in a manuscript describing techniques for reporting limits of detection of a measurement system. It is anticipated that the impact of this work will have a considerable induction period after which greater efficiency in communication of analytical results is anticipated. It is

expected that greater effort in photonuclear reaction mechanisms will be possible in the coming year.

The effort in nuclear instrumentation has been directed

toward repair and upkeep of a variety of electronic instruments for several sections in the division. Emphasis has been

placed, however, on special systems of instrumentation in

Mossbauer spectrometry. In particular, the sequential scanner

with a frequency synthesizer adapts the optical interferometer

from constant velocity to constant acceleration. A part of our

effort in instrumentation has been for the mass spectrometry

section
2. NUCLEAR CHEMISTRY

A. Introduction

The program has continued in its binary form in which

nuclear reaction studies have served as input for statistical

investigations and statistical problems have been attacked in

order to find better methods for treating nuclear data and to

improve the accuracy of the resulting conclusions. Generally,

both aspects of the program have contributed to the solution

of problems arising in other research groups at NBS

.

A particularly important product of the work is a manuscript (abstract presented below) which concisely describes

the principles of and standardizes the nomenclature for limits

of detection of a measurement system.

B

.

Nuclear Reactions

1. Mass Spectrometry

Research reported in the previous Progress Report [2]

has continued on a part-time basis collaboration with the

Photonuclear Physics Section. The initial aim of the project

is to demonstrate detectability and measure yields for the

photonuclear production of helium-3,4 from stable isotopes of

lithium. The most significant progress during the past year

has been successful absolute calibration of the sample analysis system and the setting of approximate (experimental) detection limits for helium-4. A calibration curve was determined over a range of several orders of magnitude by means of

multiple expansions of pure He in a system of calibrated

volumes, "packaging" in copper capsules (as described in

Ref. [2]) and puncturing in the "static" Mass Spectrometry

analysis system. Significant nonlinearity set in at about

a pressure of lO

- -5 torr, while background was limiting at

the low pressure end. The system is presently believed reliable down to approximately 10 to 10 atoms of helium.

Two independent measurements of the Li(yjOt) yield were

made by irradiating samples at the Activation Analysis termi-
nal [2] using bremsstrahlung from 90 MeV electrons and copper

foil monitors . The yields obtained relative to that of the

65 reaction, Cu(y s n) , were 0.17*1 and 0.176, respectively. The

"agreement" is fortuitous, for random errors were estimated

to be approximately ±15$ (relative standard deviation).

Before proceeding with additional quantitative studies, however, it has become desirable to gather information on the

beam composition—particularly with respect to bremsstrahlung,

and fast thermal neutrons. Such studies are discussed below.

2 . Bremsstrahlung Intensity and Composition

A considerable effort has gone into the study of bremsstrahlung foil monitoring accuracy and precision; spatial and

intensity comparisons of various available bremsstrahlung

beams; the related absolute yields of radioactivity monitors;

the use of reliability of relative monitor yields as beam

characteristic indices; and a determination of beam composition by means of a number of activation and absorption foils.

Th<

65,

12 The attainable precision using the reactions C(y,n) and

Cu(y,n) has been evaluated by the methods outlined in

Reference [2] and section C of this report. In the absence of

source repositioning the "true" standard deviation is approximately 0.1$, whereas it appears to be limited to about 0.5$

when repositioning is involved.

A program, PYLD, has been written to deal with a number

of aspects involved with absolute beam intensities and fluxes,

absolute stable and radioactivity monitor yields, responses

of the various detectors involved, relative reaction yields,

and predicted yields —all with the associated corrections

and standard error estimates

A comparison of the relative nuclear reaction rates available from different sources of bremsstrahlung at NBS appears

in Table 1. Absolute rates are given for the reaction,

65 Cu(y,n), as determined by detecting the radioactive product,

64

Cu. The relative reaction rates, which are directly pro-
Table 1. Absolute nuclear reaction rates and

relative yields

Electron Reaction Rate

Energy

(MeV) (min ±

) a > v Relative Yield

90 3>'10"-I6 d

.094± .002

60 4><10"-15

• 095± .003

60 5>

2>

<10"

clO"

-13

-12

.074±

.076±

.002

90 .002

The reaction rate, 3

X 10 min is equal to an "equivalent

Synchrotron

Linac-45° Roome Linac-Pneumatic Tube

"Typical" maximum absolute rates for the nuclear reaction,

Cu(Y,n) Cu.

Reaction rates are given to only one significant figure

because significant variations occur in both time and space.

Q The relative yield represents the observed rate for the

reaction ^CuCy^n) divided by that for ^CuCy,]!). Errors

are standard deviations

The reaction rate, 3

X 10'

quantum" flux of about 6*10^ cm -min

e See acknowledgments.

portional to bremsstrahlung flux, indicate that - of the

presently available irradiation positions - the pneumatic

tube position is the most effective for producing a reaction

whose maximum cross section is in the region of the Giant

Resonance. For purposes of isotope production or "uncomplicated" photon activation analysis, it is thus clear that the

best results will occur in this position, using a reasonably

high electron energy. On the other hand, for the study of

excitation functions or for activation with minimal neutron

interference or electron irradiation, the first two locations

may be preferable. They differ from the third location in

that the bremsstrahlung-producing target is considerably

thinner and the transmitted electrons are magnetically "dumped"

rather than being partially absorbed in water.
Since in the reaction, Cu(yj2n) Cu accompanies the

reaction, Cu(y,n) Cu, it has been convenient to examine

the relative yields of the two (Table 1) as a "index" for

bremsstrahlung purity and energy distribution. The relative

yields (61/64) appearing in the last column of Table 1 reinforce the "suspicion" of the pneumatic tube irradiation location, as the 20% decrease in relative yield may have been

64 brought about by a combination of (n,y) production of Cu

and a degraded bremsstrahlung energy distribution, which would

n *. 64 „ also favor Cu.

Further study of the beam composition has been undertaken

by means of the neutron-induced reactions

(1)

27 Al(n, Y ) 28 Al

(2)

197 Au(n, Y ) 198 Au

(3)

27 Al(n,p) 27 Mg

The first reaction serves as an indicator for thermal neutrons, the second for thermal and resonance neutrons, while

the third requires neutrons whose energies exceed about 3 MeV.

Besides those listed above, additional reactions include those

27 24 from the monitors (Cu-foils, as well as Al(n,a) Na , and

Au(y,n) Au) . A combination of spectroscopy (Nal) and decay curve analysis permitted the resolution of all products

197 except those resulting from Au. Here, the Y -energy diff1 qQ

erence is only 14 keV, the Au activity is small compared

that of Au, and the half lives (2.7 days and 6.2 days, respectively) are not convenient for resolution by means of decay. Therefore, one must resort either to isotope separation

or high resolution (Ge) spectroscopy. The latter method was

applied 'in this case, and preliminary information is given in

Figures 1, 2, and 3. The first figure depicts the Nal spectrum

from an irradiated (uncovered) gold foil, while Figure 2 shows

the spectrum obtained with Ge(Li) using the same foil. The
Figure 1. Gold foil - Nal spectrum

peaks observed correspond to gamma rays of energies:

333 keV ( 196 Au), 356 keV ( 196Au), 412 keV ( 198Au), and 426 keV

( Au) . The excess of Au over Au may be appreciated

by remembering that the abundance of the 412 keV gamma ray is

95%, while that of the 426 keV gamma ray is only 6%. A small

(-30%) but significant decrease in the ratio of the 412 keV

peak to those arising from y Au is seen in Figure 3. In this

case the foil was wrapped in "thick" cadmium, and one therefore concludes that significant amounts of both thermal and

resonance neutrons were present in the bremsstrahlung beam.
Figure 2. Gold foil - Ge(Li) spectrum

Figure 3. Cadmium-wrapped gold foil - Ge(Li) spectrum
C . Statistics in Nuclear and Analytics Chemistry

1 . The Peak Height Method for Eva^iating Nuclear Spectra

Peak heights are commonly used for quantitative measurement when output signals are presented in the form of continuous traces, as in mass spectrometry or gas chromatography.

A similar approach has been examined for the analysis of

nuclear spectra—where results appear as digital histograms —as a possible alternative to peak integration (with

base line subtraction) . The method has been thoroughly evaluated for non-overlapping peaks; it has been found to be completely reliable, relatively free from systematic error,

nearly independent of peak shape assumptions , and extremely

simple and rapid. The principal sources of error in the method

include uncertainty in the peak location, gain variation, and

base line (Compton) corrections.

Details of the method may be conveniently developed by

137 reference to Figure 4. The .662 MeV photopeak of Cs is

depicted there for two different channel "densities". The

essence of the method involves taking the number of counts in

the peak channel, corrected for the base line contribution,

as a measure of intensity. At the lower channel density

(left-hand peak in Figure 4) the selected channel is less

likely to lay symmetrically about the "true" peak position.

For both densities the gross number of counts in the peak

channel is characterized by Poisson fluctuations; as a result, the peak channel may not even contain the maximum number of counts. It is in this digital structure of both

coordinates that the nuclear spectral data differ from the

continuous spectra which are observed in many other disciplines. The digital structure both adds limitations to the

use of peak heights and modifies the rules for determining

peak heights, as compared to continuous spectra.

Considering first the coarse structure of the abscissa,

if the peak is not centered in a channel, an error arises
137 Figure 4. Cs photopeak

from this lack of symmetry; the magnitude of this error depends upon the channel density. For a peak whose shape is

approximately Normal, the maximum error due to the symmetry

effect is less than 1% for densities exceeding 8.2 channels/

FWHM. Symmetry changes can be readily corrected for, with an

inaccuracy of less than 1% for a density as low as 4.2 channels/

FWHM. Information (count) loss —compared to counts resulting

from peak integration—between these two limiting densities,

varies from about 55% to 89%.

Numerous approximate methods exist for base line correction. One which has been applied here is especially simple:

one estimates the base line contribution to the peak channel

as the average of the numbers of counts at the minimum (on

the low-energy side) and those in the channel which is equally

distant from the peak on the high energy side. (The rule must

10
be modified in the case of interfering spectra and for Ge(Li)

detectors, but such modifications will not be discussed here)

An estimate of the standard deviation of the net number of

peak counts may then be made on the basis of the counts in

the three channels in question. There are two limitations to

the above procedure, however. (1) If the ratio Base Line/

Peak counts (B/N) is large, but gross counts (N) are small;

one may not be certain that a peak is, in fact, present.

(2) On the other hand, if N is very large, the error estimate (based upon counting statistics) may be vitiated by a

systematic error in the baseline correction. Detailed treatment of these two limitations has led to maximum values of

B/N as a function of the number of peak counts, N, and the

(estimated) relative (systematic) error, f_. , in the base line

due to nonlinearity . Results of the calculations are plotted

in Figure 5. The purpose of the plot is to serve as a guide

10 10 I0

3

N (COUNTS)

10'

Figure 5. Maximum ratio, base line/peak (B/N) vs peak

counts (N)

11
in deciding--f or a given ratio, B/N, and "reasonable" estimate, fR --whether there are sufficient peak counts, N, to

be detectable, and whether there are few enough so that the

Poisson counting error represents the principal source of

uncertainty. Figure 6 displays a "real" base line resulting

|~ Ji from ' Na in the region of .84 MeV ( Mn) taken with Nal

The relative base line error (+12$) is perhaps typical for

non-overlapping peaks with such a detector.

Figure 6

54 22 Mn with Na baseline

In addition to the symmetry error, which may be made

negligible (density >8) or well-corrected (density >4),

there may exist errors due to shifts in gain. A very simple

first-order gain correction may be made by multiplying the

net peak height by the relative change in gain. Assuming

that the peak location may be estimated (by inspection) to

1/2 channel and that the peak is approximately Normal, it can

be shown that gain shifts as large as 10$ or 80$ (depending

12
upon the details of the method) may be thus corrected, with

an inaccuracy in the correction of less than 1% . Such calculated correction accuracy has been confirmed experimentally.

An overall investigation of the accuracy of the peak

height approach has been carried out by means of a factorial

experiment [5] in five dimensions. The four abscissae (independent variables) were: interference level, symmetry

changes, channel density, and gain shifts. The design used

4

was a 1/2x2 balanced factorial. Results of the analysis confirmed that high levels of interference do affect the peak

height estimate and that a factor of two change in channel

density (conversion gain) may be significant (for 1% "counting statistics") while the remaining factors and all interactions among factors are not significant, at least in the

region of hyperspace which was spanned. An additional result of the analysis was an internal variance estimate, which

was found to be quite consistent with that derived from counting statistics

A summary of the advantages and limitations of the peak

height method is presented in Table 2.

Table 2. Advantages and limitations of the

peak height method

ADVANTAGES

1. Simplicity and speed.

2. Relatively insensitive to gain shifts.

3. Accuracy may be somewhat better than than of

"peak area"

4. Little knowledge of other components or "shape"

required

LIMITATIONS

1. Some loss in Poisson precision.

2. Resolution fluctuations must be absent.

3. PWHM > 4 channels.

4. (B/N) < (B/N) . . and f„ estimate (limited required

crit . B

13
The author is grateful to Brian Joiner for suggesting

and outlining the factorial approach. The analytical method

was based upon the Yates algorithm as described in Chapter

12 in Reference [7].

2 . Criteria for Reporting Trace Amounts of Radioactivity [3]

Radiochemical observations which are not clearly "positive" deserve special attention in order to prevent ambiguity

in reporting. Frequently such results are given as zero, or

as an estimated mean (positive or negative) and its standard

deviation. In order to better interpret such borderline results, possibly for legal purposes, it is suggested that they

be considered as "detected" or "not detected" according to

definite statistical criteria for detection and detectability

If "not detected" — the case of particular interest here —

one may either report the associated "detection limit", or

an upper limit may be given. The upper limit should relate

to a one-sided confidence interval, and occasional negative

values should be expected.

By way of illustration, consider an experiment in which

three samples yielded 144, 95, and 115 counts in a given

period of time, where the mean detector background was known

to be 100 counts in that same time interval. The results of

applying to this problem the principles developed previously [3] are given in Table 3- Only in the first case may the

signal be considered "detected", and one may report this conclusion together with the detection limit, L . Both of the

latter two results are reasonable statistically, and they

should be reported as "not detected" together with the corresponding upper limits

When one encounters extremely low levels of activity and

background, however, the method applied above is not adequate,

for it is based upon the assumption of normality. The detector referred to above is one which employs 3-y-y coincidence

14
Table 3. Trace level reportinga,b

Net Counts ±o Detection ^c S Upper Limit (q=.Q5)

44±12 D 16.1 35.6

- 5±10 ND

15 + 11 ND

11.0

32.7

All units are counts

Notation is that of Ref.

and has a measured background of about 1 count per hour.

Hence, the time for the experiment described is 100 hours.

Thus, if one is dealing with somewhat shorter periods — as

may be necessitated by short-lived activity — it may be

necessary to consider in detail the Poisson distribution in

setting confidence intervals and upper limits. The special

situation arising when zero counts are observed is particularly straightforward, and it has been examined in some detail.

Two questions are of primary importance in the extreme

low level (zero count) situation: (1) in planning an experiment, what is the minimum counting time in order to set a given upper limit; (2) having observed zero (or some other

small number of) counts, what is the corresponding upper

limit (or confidence interval, if "detected"). Table 4 lists

the upper limits (a=.05) which would result if zero counts

are observed where the counting "time" is expressed in terms

of the mean number of background counts. The third column

in the table lists the probability of observing zero counts,

under the null hypothesis. Thus, in order to establish an

upper limit equal to the background equivalent activity (BEA)

with the 3-y-y detector (BG=1 cph) one need only count for

1.5 hours. The "chance" of doing so, however, is only 22%

[=exp (-1.5)]. On the other hand, if one wished to establish

15
Table 4. Minimum upper limits (a=.0 5)

\ (

) Mean

M

; irnV a; BG counts (x) W(0) = e~

T

1

6.20 .417 .65

2.76 .818 .44

1.00 1.50 .22

(a) Sample activity/background equivalent activity

a detection limit (as defined in Ref. [4]) equal to the background equivalent activity, a counting period of 16 hours

would be required! In order to make the example more explicit, consider Co whose BEA is 37-5 fCi. If the desired

upper limit were 1 pCi, the time required would be 6.5 min

where the probability of "success" (observing zero counts)

would be 89%. The second facet of the problem may be illustrated by assuming that the result of a 25 min observation

is zero counts. Here, the upper limit may be deduced from

Table 4: the "reduced time" equals 25 min/60 min = 0.417;

hence the upper limit equals 6.20xBEA = 6.20x37.5 fCi=233 fCi

the a priori probability of observing zero counts is

65$.

The foregoing principles have been tested experimentally

and also applied to a problem in activation analysis. In

the "control" experiment, one thousand observations were

found to be consistent with an upper limit estimate based

upon the Poisson distribution. For the "experiment" the

mean rate was adjusted to be just 3 counts/channel. (A

multichannel scaler was used having a "dwell time" of .1

sec/channel). If zero counts were observed in any given

channel the calculated upper limit (a=.05) would likewise

be 3 counts. The expected number of zeroes equals 50, with

16
a 95% confidence interval extending from 37 to 63. The number of zeros observed was 61. The activation analysis problem involved low level activity of moderate half life

(-hours). A number of samples resulting from isotope separation had to be examined within the time limit set by this

half life, and therefore it was necessary to plan the counting interval per sample such that there would be a good

chance of setting an "acceptable" upper limit. The result

of the planning led to an interval of 25 min—as with Co,

above —and the corresponding upper limit was, similarly,

233 f Ci . As the probability of observing zero counts was

only 65%, however, it was necessary to consider the limit

which would result if one were "unlucky" enough to observe

1 count. In such a case, the gross upper limit (as deduced

from the Poisson distribution) becomes 4.74 counts instead

of 3.00 counts. Correcting for background (.42 counts) one

arrives at the net upper limit of 4.32 counts which is

equivalent to 390 fCi.

The principal advantages of applying the "zero-count"

approach are listed in Table 5.

Table 5- Value of upper limits based

upon zero counts

Good chance (225?) of establishing U.L. = BEA

by means of "brief" counting (zero counts).

Usefulness of such limits with "low level"

counters (BEA < 1 pCi) which employ extensive

shielding, spectroscopy, or coincidence.

Considerable saving of time for samples having

negligible activity, and for rapid surveys.

Possible necessity when short lived nuclides

are counted with low background counters.

17
3 . High Accuracy Calibration and Control in SingleChannel Gamma Spectrometry [6]

A single-channel analyzer is especially appropriate for

counting samples having controlled mixtures of radionuclides.

Such is the case, for example, for foil monitors of reactor

or accelerator beams. Since activation monitors for photonuclear reactions must be counted with especially good precision, an investigation has been made of precision and accuracy in the analysis of the photodisintegration products of

copper using the single channel approach.

Selection of discriminator settings was originally attempted by means of an associated multichannel analyzer and

pulser, but serious inconsistencies were encountered. As a result, a simple mathematical approach was adopted, in which

three or four differential observations in the vicinity of

the photopeak are combined with an appropriate analytic expression for the peak shape. If one assumes that a photopeak may be described by the Normal distribution, the number

of counts (N) within a "small" window of a single channel

analyzer may be represented as in Equation 1.

N S exp (-(x-x ) 2 /2a2

) (1)

where x is the centroid of the distribution, x is this

o

' center of the (differential) window, and o is the standard

deviation of the distribution. If two additional differential observations are made, displaced from the first observation by discriminator (base line) charges of +D, -D, respectively, one may calculate the peak centroid as shown in

Equations 2 and 3,

l

n (N/N_) D[2x-x

q

)-D]

(2)

l n (N/N

+ D[-2(x-x

q

)-D]

X

o

= X + I (pTl) (3)

18
where the numbers of counts corresponding to the displacements +D and -D are represented by N and N_ , respectively.

Accuracy of the approach for peak location and "control" using a single channel analyzer was checked by collecting

22 counts from a Na source with both a single- and a multichannel analyzer. Selecting a region extending ±PWHM from

the peak center in each case, approximately 1.52x10 counts

were obtained. The difference between the two results was

Q.5^% j while the standard deviation of the difference was

0.36$.

A computer program has been written for calculating

the peak parameters as outlined above, along with estimates

of the FWHM and peak amplitude and area. If only peak location is desired, it has been found more convenient to use

a nomogram which was constructed on the basis of Equation 3-

A principal application in our laboratory is the counting of 3 -emitters (samples and monitor foils) which result

from photonuclear activation. In order to establish and

check "control" in counting the resulting annihilation

22 quanta, we have used a standard source of Na. Using the

scheme outlined above, we have determined a relative standard deviation for peak location of approximately 0.2%. The

real precision for peak amplitude , however, was found to

depend upon the width of the window, as well as the condition and location of the source. Estimates of reproduci22 bility based on the shape of the Na spectrum indicated

that the optimum window setting was approximately ±2*FWHM.

Experimental estimates of "excess" random error, o , were

deduced by the methods described in the previous report

Reference [2]; they are given below in Table 6.

The results in Table 6 relay the importance of window

width and source positioning, and they show that a reproducibility (standard deviation) of approximately 0.1% is

attainable with the present system.

19
Table 6. Observed counting precision 3.

Position Discrim. Sample v °xs Limits ( .95 C.L. ) Fixed 2*FWHM 22

Na 11 .09$ C - 3 Jg , .19$)

Fixed Integral Cu-5 9 .06$ (0$, .19$)

Fixed FWHM Cu-1 4 .19$ (0$, .75$)

Varied 2*FWHM Cu-8 6 .44$ (.25$, 1.1$)

Poisson counting precision (standard relative deviation)

was equal to 0.1$ for each observation.

"Fixed"=source untouched between observations;

"Varied"=source removed and replaced.

c22

Na is the standard source; Cu's are bremsstrahlung monitors

4 . Counting Statistics and Standard Reference Materials

Standard Reference Materials (SRMs) which are determined

by nuclear methods are characterized by uncertainties depending, at least in part, upon the (approximately) Poisson

distribution of radioactive decay. Generally, the final

estimates (mean and standard error) for a given SRM are

based upon a number of independent observations. Such observations frequently will be characterized by differing

(Poisson) standard deviations and sometimes by an additional

(non-Poisson) component of variance —due to instrument instability, heterogeneity, etc. The statistical treatment of

such sets of observations has been referred to by S. S.

Nargolwalla in a discussion of fast neutron activation analysis (program: NGEN, Ref. [1]); the principles of the treatment have been outlined in a discussion of the precision of

radioactivity measurements (program: XESS, Ref. [2]). In essence, the method of treating the data included first calculating a weighted mean and standard error, based upon "count2 ing statistics". Second, x is evaluated to test for the

presence of additional sources of variation. If the test is

20
negative, the (Poisson) results may be used to set a confidence interval. Otherwise, an iterative calculation is

carried out to set limits for the additional variance.

The foregoing approach was recently applied in deducing

average results for oxygen-in-steel SRM's (#1090, #1091,

#1092) determined by fast neutron activation—for comparison with results obtained by vacuum- and inert gas fusion.

Data sets for these three SRMs were submitted by S. S.

Nargolwalla of the Activation Analysis Section. The direct,

weighted means and standard errors found for the three standards were: 492±12 ppm (#1090), 132.7±1.7 ppm (#1091),

26.6±1.6 ppm (#1092). Only in the case of #1091 was the

2 computed x significant at the 5% level. The resulting 95%

confidence interval for a (Ref. [2]) ranged from .7% to 12.6% X s (R.S.D.), the best estimate being 4.5$. The very wide confidence interval resulted from the facts that there were

relatively few observations and that c was comparable to

.A. o the standard deviation due to counting statistics. The

problem of setting exact confidence intervals when both

sources of variation are important has not yet been solved.

Helpful discussions with S. S. Nargolwalla and B. L.

Joiner are gratefully acknowledged.

5 . Detection Limits

LIMITS FOR QUALITATIVE DETECTION AND

QUANTITATIVE DETERMINATION

by Lloyd A. Currie published in Analytical Chemistry 40_, 1968

The occurrence in the literature of numerous, inconsistent and limited definitions of a detection limit has

led to a re-examination of the questions of signal detection and signal extraction in analytical chemistry and

nuclear chemistry. Three limiting levels have been defined: Lp-the net signal level (instrument response)

above which an observed signal may be reliably recognized

as "detected"; L„-the "true" net signal level which may

21
be a priori expected to lead to detection; and Ln -the

level at which the measurement precision will be satisfactory for quantitative determination. Exact defining

equations as well as series of working formulae are presented both for the general analytical case and for radioactivity. The latter, assumed to be governed by the

Poisson distribution, is treated in such a manner that

accurate limits may be derived for both short- and longlived radionuclides either in the presence or absence of

interference. The principles are illustrated by simple

examples of spectrophotometry and radioactivity, and by

a more complicated example of activation analysis in

which a choice must be made between alternative nuclear

reactions

D. Iodine Determination by Static Mass Spectrometry

In order to assess the feasibility of determining "trace

levels" of iodine isotopes by means of conversion to a suitable volatile compound and "static" mass spectrometry, samples of ethyl iodide (vapor) were prepared and analyzed.

(Ethyl iodide was selected because of its relatively high

vapor pressure and because it should be relatively straightforward to synthesize and purify by gas chromatography.)

Starting with the pure compound, using calibrated volumes

of a vacuum line for multiple expansions, two samples were

prepared in copper capsules for subsequent analysis. Nomi-

-S -9 nal pressures were 1.1x10 torr and 2.4xio torr, respectively . Upon puncturing the capsules, only the one containing

the higher pressure gave a measureable signal with the E.A.I,

quadrupole mass spectrometer (see acknowledgments). Mass

peaks observed corresponded to (CpHj-I) and I ; the former

gave a signal of about 40 mV, and the latter about 20 mV.

Both signals decreased approximately exponentially with a halftime of about 1.5 min. Decomposition of CpHp.1 continued

22
with time, but the I signal reached a steady-state value of

about 1.9 mV. Taking the above results together with the

initial number of molecules placed in the capsule and the

observed background (<.01 mV) , one concludes that about

q 5x10 molecules of ethyl iodide would give a signal equivalent to that of the background. It seems very likely, however, that the true "sensitivity" for the ethyl iodide is

greater by a factor of about 105, considering calibration

factors observed with other gases and those given in the

instruction manual. Probably physical or chemical reaction

of the ethyl iodide with the wall of the capsule took place

during its unfortunately long storage period. Although the

sensitivity of the E.A.I, spectrometer is quite satisfactory,

and the "gas" chemistry approach is feasible, there are significant limitations to the method. One is the fact that the

instrument is not characterized by extremely high resolution;

this would limit the isotope ratios which might be examined.

(One may "trade-off" sensitivity for resolution, however. We

have, for example, been able to measure the natural abundance

of deuterium). A second limitation, which would have to be

explored for the particular molecular species to be used, is

the interference from fragments of the parent molecule or

from impurities

.

(L. A. Currie)

23
3 . MOSSBAUER SPECTROMETRY

A. Introduction

The majority of the studies in this project continues to

emphasize instrumentation and in particular new systems. The

method of backscattering measurements for bulk samples has

been in operation for well over 18 months. More recent developments of a scattering detector and a spectrometer with

interferometric velocity sensing in the constant acceleration mode have provided continuing stimulus in this area.

Increasing emphasis has been placed on studying new Mossbauer nuclides, and this year's efforts provide data which

indicate that nickel compounds can be studied by Mossbauer

Spectrometry

Several new standards are in the final stages of production and calibration. The most important of these is a pure

iron foil whose six peak positions will be determined for

spectrometer calibration.

Continuous modifications are made to the mathematical

methods for resolving complex spectra and a new approach is

described.

Quantitative measurements using Mossbauer spectrometry

have been continued. This report describes some preliminary

attempts on the determination of tin in tin bearing materials

Recent advances in instrumentation may help, but the technique is limited in its ability for quantitative measures of

this sort because of complex solid state properties of the

matrix

Considerable effort has been expended in compiling the

current literature in the field. If Interest is sufficient,

an effort may be made to publish a computer listing of current titles, to supplement the "Mossbauer Effect Data Index"

by A. H. Muir [8].

24
B . Instrumentation

1. Cryostat for Moving Absorber and Stationary Source

The simple liquid nitrogen cryostat described in Reference

[2] has been redesigned to accommodate a moving absorber. This

configuration has the advantage of optimum utilization of the

source-detector geometry without large inverse square law

distortion, cooling of the source, and rapid changing of

sources. Furthermore, the moving absorber configuration

eliminates the problem of iron impurities in the counter window causing interference. In this cryostat, the source is

mounted on a brass tee, which slides into the heat sink, as

shown in Figure 7. The absorber is supported by a 5 mil silver flexure plate connected to the drive pushrod by means of

a yoke. Cooling is provided by a bottom-feeding permanently

DIRECT

INJECTION PORT-I

SI—

j

h-H

7

i i

2

1 ^

'

SIDE VIEW -SECTION A A FRONT VIEW

Figure 7. Mossbauer cryostat

25
Figure 8. Photograph of pancake proportional detector

source side.

26
Figure 9- Photograph of pancake proportional detector

back side.

27
evacuated dewar (A) , which maintains a constant nitrogen

level in contact with the heat sink (E). For a discussion

of other parts indicated see Reference [2]. The heat sink

is insulated with polyurethane foam and an aluminum coated

Mylar radiation shield. The insulation is cast around the

heat sink using Armstrong expando foam; 1/2 inch of foam is

sufficient for nitrogen temperature insulation.

2 . Pancake Radiation Detector for Backscattering Mode

of Mossbauer Spectrometry

The conventional transmission technique used in Mossbauer spectrometry does not lend itself well for the analysis

of bulk materials. To solve this problem, a scattering technique has been developed. This method has the disadvantage

of poor geometry. To improve the geometry factor, a pancake

proportional detector has been developed, see photographs in

Figures 8 and 9. The detector has an effective area of 12

square inches; its detection efficiency is 65% and its resolution is 20% (FWHM) for 6.3 keV x-ray radiation. Prototype

detectors were constructed with wires spaced 1/2 inch apart

in the detector. Although the efficiency of the detector

was good, the pulse height resolution was very poor. The

efficiency of the detector was not a critical function of the

wire spacing, and in the final design only two wires were

used, spaced symmetrically 3/8 inch from the wall. A detector diagram is shown in Figure 10, and is made from aluminum (A) , with aluminized Mylar windows (H) . The 1 mil stainless steel wires (C) are supported by 1/4 inch plexiglass

strips (B) and are connected in parallel to a H.V. supply (E)

at approximately +1400 volts. A conventional charge sensitive

preamplifier and double delay line clipped amplifier are used.

57 The pulse height spectrum of a 4 mCi Co source is shown in

Figure 11. The detector gas is 10% CHk and 90% Ar at atmospheric pressure, with a 0.1 cc/min flow rate (inlet and outlet, D) .

28
WIMMIIMIMIIMIIMMMMMMIIl

%

D

H

J

A

_ Figure 10. Diagram of pancake proportional detector

The detector has also been used for the detection of

conversion electrons. In this case, the Mossbauer absorber

was placed in contact with a 10% CH\-90% He flow gas by putting it in place of the detector window. The pulse height

spectrum is shown in Figure 12, with the detector operated at

+1200 volts.

•The pancake detector increases the detection efficiency

by a factor of about four as compared to the previously used

2 inch diameter Reuter Stokes detector and can be used for

conversion electron detection.

(K. R. Swans on and J. J. Spijkerman)

C . Backscattering Mossbauer Spectrometry

1 . General Description

For Mossbauer analyses of bulk materials, such as gauge

blocks, SRM steel samples, and steel plate, the transmission

geometry is not applicable. A backscattering technique has

been used successfully for this analysis and it represents

a nondestructive technique.

57 For Fe the backscattering technique can detect the

Mossbauer effect by the 14.4 keV gamma radiation, 6.3 keV

29
x-ray radiation or 8 keV conversion electrons. Since the

internal conversion coefficient for the 14.4 keV level is

9.4, detection of the 6.3 keV x-rays or the conversion electrons is very efficient. Both methods have been used.

The maximum geometrical efficiency for the detection of

the backscattered radiation correspond to a 2tt steradian

solid angle (50%). The earlier "through the detector technique", described in Analytical Chemistry 3_8_, 382R(1966)

has this theoretical geometrical efficiency, but the intrinsic

detection of the 14.4 keV gamma radiation by the detector

greatly lowers its ability to provide good spectra in a

short time period. With the commercially available detectors,

and using a 45° angle between the incident and scattered radiation, a geometrical efficiency of 10$ can be obtained. The

spectra of Figures 13 and 14 were obtained using this technique. The cosine effect is large in this configuration, and

Figure 15 shows the spectrum of tool steel with a well collimated source. Figure 16 shows the spectrum of the same material in transmission.

The pancake detector described in Section B-2 greatly

improved the geometrical efficiency. A photograph of this

detector used for scattering is shown in Figure 17- Furthermore, it was possible to insert a 1/2 inch O.D. tube through

the detector, without affecting the resolution (see Figure

10). This eliminated the counting of the 14.4 keV gamma radiation as in the "through the detector technique". A point

source would be highly desirable for this method and the re57 cently developed ^'CoO source [9] will allow this.

The conversion electron detector has 50$ geometrical

efficiency. Figure 18 shows the spectrum of a steel plate

placed inside the pancake detector.

2 . Spectra of Steel Samples

Mossbauer spectra of several Standard Reference Material

steel samples were obtained to determine whether this method

30
Figure 11. 57 Pulse height spectrum of Co source for

pancake detector, operating at 1400 volts,

with 105? CH^ 90$ A flow gas.

Figure 12. Pulse height spectrum of

version electrons

57 Co source for con31
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36
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37
could be used to check uniformity of a particular Standard, Reference Material. Indications are that it may be an excellent method to check reproducability of some types of steel.

Further studies are in progress. Three samples of Standard

Reference Material 117^ were analyzed for the cementite content with the results

1174-(a) 28±6# cementite

(b) 33±6# cementite

(c) 31±6% cementite

(error is standard deviation of a single determination derived from a least squares analysis of the particular spectrum) . The cementite content was obtained from the area

peak ratio of the cementite peaks to that of the ferrite

peaks, and no correction for the respective f-factors was

made. The resolution of the spectra was poor due to cosine

smearing and inverse square law effects. Additional applications of this nature will be studied.

3 . Effective Penetration of Mossbauer Radiation

The penetration depth of the Mossbauer radiation in a solid sample is critical since it is important to determine

whether a surface or a bulk measurement of the sample is

being made

The amplitude of the backscatter Mossbauer signal can be

computed in order to evaluate the penetration depth. Ignoring details of the source and the sample to detector solidangle, the backscatter. intensity at zero velocity for a given

distance, y, into the sample (T(y)) is given as

T(y) = Const, exp {-

oo my

) /"_ f -Vag ny

)

-. , > / dE exp <—: T > • in <j> | I ^ sin 4> J

i ff .,„-. „ S -^y

I d0 sin exp <

it 2 , r

2

E + I )cos sin <J> r s/4 J

38
where a 14

a r /u s/4

1 2

E + r /Ji s/4

Is the corrected cross-section for

resonant absorption and y, ^ an(^ V^ are mass absorption coefficients for 14.4 and 6.3 keV photons, respectively. a is

the cross-section for resonant absorption,5 r is the full

s width at half height of the source and E is the energy of

transition. The angles are defined in Figure 19. The 14.4

keV gamma rays are collimated to be incident at angle <j> with

respect to the sample surface while the 6.3 keV internal con- version x-rays are emitted in directions making an angle 9 with respect to the normal of the sample surface. A similar

calculation has recently been carried out by Hershkowitz and

Walker [10] in the transmission geometry for studying thin

films which are a part of the detector window. The integral

DETECTOR

SOURCE

MOTION

Figure 19 . Schematic showing geometry for

scattering experiment.

39
y /in

T(y)dy gives the intensity of 6.3 keV

u x-rays detected as a function of maximum penetration depth,

rrr into the surface. The results of the calculation are

shown in Figure 20. To check the validity of the assumptions,. spectra of a stainless steel plate were taken, with 0.22, 0.50

and 0.72 mil thick iron foil placed over the stainless steel

plate

Figure 20. Amount of backscatter as a function of

thickness of the scatterers.

Spectra shown in Figures 21, 22 and 23 show 77-8±2$,

93.0±2#, 98±2$ reflection intensity of the stainless steel

peak through the 0.22, 0.50 and 0.72 mil iron foil, respectively. The uncertainty reported is approximately two standard deviations. The standard deviation was estimated from

the last iteration of a non-linear fitting procedure based

on a Taylor series expansion about the estimated values.

Agreement with the theoretical calculation for these limited

data is satisfactory.

40
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4 3
4 . Identification of Corrosion Products on the Surface

of Iron by Scattering Techniques

The detection and identification of a compound formation

on a steel surface was made using a backscatter Mossbauer

effect technique. The compound was identified as 3-FeOOH

from Mossbauer effect parameters of iron oxide and oxyhydroxide compounds. An estimate of the compound thickness

is given based on a calculation of the backscatter amplitude

versus distance into the sample. This technique can be used

to identify corrosion products on the surface of iron bearing materials. (To be published in the "Journal of Applied

Physics Letters").

Special appreciation is expressed to Dr. J. H. Terrell

of Mithras, Inc. for working on the mathematical expression

and for providing some of the calibrated samples

(K. R. Swanson and J. J. Spijkerman)

D . Operator Equivalent Method for the Determination o f

Orbital Electric Field Gradient Tensors

The method of operator equivalents is a time-honored

technique for converting real space operators to angular

momentum operators compatible with Dirac notation. The advantage of Dirac notation is that common quantum mechanical

integrals may be evaluated by the application of the following relationships :

<LM' |0|LM)E

J

y*

M , Vmv ch CI ) LM' LM'

if i 1 j

<LM' |LM> = 6 MMr , 6,, = { (2)

1 if i = j

ij

L |LM) = M|LM> (3)

L

2 |LM> = L(L+1) |LM)

L + |LM> = (L±iL )|LM> = /(L+M) (L±M+1) I L M±l>

x y

44
where the operator is a general polynomial in the operators

L , L , L , and L . Combining (2) and (3) yields:

<LM' |L

z |LM> = M S

ffl , (4)

<LM' |L 2 |LM> = L(L+1) 6 MMf

<LM'|L

± |LM> = /(L+M)(L±M+1) <5 M

, (M±1

The elements of the electric field gradient (EPG) tensor

at the origin due to a charge ' q' located at a point ' x,y,z'

are given by the various second partial derivatives of the

potential due to the charge

.

v

xx 4 = 4 ( 2 V 2

) qP^M* = /* 2 ^

2 -2

sx

2 ax2 /x +

y

+z te=>

V

yy

= q(3y

2

-r

2 )/r 5 (5)

V

zz

= q(3z 2

-r

2 )/r 5

V = V = 3qxy/r 5

xy yx

^ H J/

V = V = 3qxz/r 5 xz zx M

V = V = 3qyz/r 5

yz zy

J4J

The more commonly written spherical polar form, e.g.

V = c zz tutingV q r

J (3 cos 6 - 1) , is readily obtained by substiLj e-i

x - r sin cos $ (6)

Iy = r sin sin <f> z = r cos

into ( 5) •

45
When the charge in question is distributed over a wave

function, V , the elements of the EFG tensor are given by

the expectation values of the point charge expressions.

v

ab ./"^V dT a > b x > y. z

T

V = q(3 a - r

d )/v° (7)

olgL

V

ab

= q(3 a b)/r b

In many cases of interest, the wave function is expressed

as a linear combination of angular momentum eigenfunctions

having the form

* " N t c m W C8)

M=-L

L

M LM

n = ( e ic M r)2x-l/2

M M=-L

Thus, evaluation of each EFG tensor element may require the

2 performance of as many as L integrations if all of the C,

are non-zero

V

*b

= |N|2

„.?_L „?.L

C"' °nfh«' V

ab *LMdT (9)

For such a case, the desirability of expressing the V . in

terms compatible with Dirac notation is clear.

Stevens [11] and others have shown that, within a manifold

of states for which L is constant, [a]

x = k'L (10)

x

y = k'L

y

z = k'L

z

"

2 , ,"2

r = k'L

a "R The method is not valid for hybrid orbitals such as sp , since two values of L are involved.

46
subject to the constraint that the commutation properties

of the real space operators be imposed upon their angular

momentum counterparts, e.g.,

/\ /\. x yJ = l/2k' (L L + L L ) (11)

x y y x

With these substitutions, the EFG operators become

V

aa

= k (3 L

2 - L2

), k = k'q<r"5

>, a,b = x,y,z (12)

S\ /S /\ /S. V

ab " 3 k < L

a

L

b + Lb

L

a )/2 -

_5 The spherically symmetric r may be lumped with the constant

without loss of generality. The final form of the operators

is obtained with the substitutions

L

x

= (L

+

+ L_)/2 (13)

I L

y

= (L

+

+ L_)/2i

to yield

V

xx

= k 3(L^ + L

+L_ + L_L

+

+ l/)/4 - l/ (14)

= k -3(L^ - L

+L_ - L_L

+ V = + l/)/4 - i/

yy

[3 i\ - £

2

]

v

zz

= k| 3 l; - L'

V = 3k(L^ - L )/4i

xy + -

V = 3k(L^L + L L + L L_,_ + L L )/4

xz"T + Z -Z Z + Z -

S\ S\ S\ /\ /s /\ /\ /\ Vyz+z = 3k(L,L - L L - L L, - L L )/4i -z z+ z -

47
In order to apply (14) easily, it is convenient to have an

additional set of relationships which may be obtained from

(3):

) 1/2

L |LM±2) (15)

2 |LM> = (L+M-l)(L+M)(L±M+l)(L±M+2)

L^L_|LM> = (L±M)(L+M+1) LM> = (L -L L ) I LM>

± +

z z

'

l + l |lm> = m (l+m)(l±m+i) 1/2 |l M±l>

L

z L + |LM> = (Mil) (L+M)(L±M+1) 1/2 |L Mil)

The simplest applications of (-14) involve pure hydrogenic orbitals such as d

n

, d+, , d +? . For instance, the

element V of EPG tensor for d~ is given by

yy o 6

V = <20|V | 20> flf-s yy ' yy

1 do)

= -3k(<20 |LT I 20) -<20|L^L | 20) - <20|l lJ 20) +

+

' + — — +

<20 |L_| 20) )/4 -K( <20 |L | 20)

= -3k( -6 -6 + 0)/4 - k(6) = 3k

In a similar manner, it may be shown that V = 3k , V

XX zz -6k, and all of the others are zero.

The constant 'k' may best be evaluated by performing

one (preferably the simplest) of the integrations. The

constant may be found most easily for the d manifold using

V

zz

= q r~ 3 (3 cos

2

9 - 1) and d

Q =/^(3 cos

2

9 - 1).

48
2tt tt <20|V

zz

|20> =-^J J

(3 cos

2

6 - l) 3 sined6dcf)<q r" 3

> (17)

o o

= -(5/8) <q r~

3 )/* (27 cos

6 9-27 cos 9+9 cos2 9-l)

d(cos 0)

=(4/7)<q r~ 3> = -6k

or

k = -(2/21)<q r" 3> (18)

The evaluation if 'k' in (18) applies for both one electron ' d

f orbitals or many electron 'D' orbitals, since they

have the same angular dependence. The difference lies in

the radial part of the wave function used for the evaluation

of <r" 3>

.

In general, the orbitals of interest may be linear combi'

nations of hydrogenic orbitals due to the presence of crystal

fields, spin-orbit coupling, or other interactions. It is

useful to demonstrate the use of the operator equivalent

method for such a case with the "stationary" ' d' orbitals

which are ' d' manifold eigenfunctions in the presence of an

octahedral, tetrahedral, or square planar field.

d 2 = d n = I 20) (19)

z '

d

x

2

-y

2 = < d 22 " d

2 -2 )/(i/2_) = (

l 2 2> " |2 -

2) ) / ( l/2)

d

xy

= (d 22

+ d

2 _ 2 )//2 = (|2 2)+ | 2 -2))//2

d xz

= (d 21

- d

2 _ 1

)/(i/2) = (|2 1>- | 2 -l>)/(i/2)

d = (d

2]_ + d2 _ 1 )//2 = (|2 1>+ |2 -l>)//2

^9
As a random example, the EPG element V for the orbital

-A. -A. d is given by

xz b a

VvY =|l/(i/2)| 2 «2 l|- <2 -l|)V„([2 1> -|2 -1>) (20)

= 1/2 <2 l| V 2 1) - <2 l| V | 2 -1)

|_ XX XX 1

Since V is a sum of five operators, (20) may be thought

J\. A

of as a sum of twenty terms. However, many terms, such as

i"2i <2 1 1 L I 2 1>, may be seen to be zero by inspection, since

the 'M' values in the bra and ket must match after the

application of the operator. The non-zero terms in (20) are

V

xx

= (k/2) <2 l|h(L+L_ + L_L

+ )/4 - L

2 |2 1) - (21)

-<2l|3L 2 /4|2 -1>- <2 -1|3L 2 /4|2 1) + <2 -l|

[

3(L

+L_ + L_L

+ )/4 - L

2 |2 -1)

|3(6 + 4)/4 - 6 - 3(6)/4 - 3(6)7-4 + 3(4+6)/4 - 6

J

= -6(k/2) = (2/7) <q r~

3

>

In many actual cases, neither the "rotating" orbitals,

d„ , d

+

, d _ , nor the stationary orbitals given in (19) are

the eigenfunctions of the Hamiltonian. In such cases, the

tabulated values of the EFG tensor elements for the common

'd T orbitals are of no value, and the operator equivalent

method is extremely useful in the necessary evaluation.

The operator equivalent method is also convenient for

manifolds other than 'd', and for cases in which many

electron basis functions should be used. An example of the

50
f\ 1 latter is included in the section " Ni Theory: EFG Tensor"

for an eight-electron problem in which F wave functions

(L = 3) are used as a basis set.

In concluding, it is worthwhile to note that the EFG

tensor obtained by the method of operator equivalents, as with any other method, is at best a 'first guess'. Before

relating the calculations to experimental results, it is

necessary to put the EFG tensor Into 'standard form', in

which the tensor is diagonal, and the elements are ordered

such that |V |>|V |>|V j .

1 zz ' yy xx ] E . The Electric Field Gradient Tensor for Common Nickel

Configurations

The interpretation of nickel spectra exhibiting a quadrupole interaction requires an understanding of the origin

of the electric field gradient tensor. Of the two contributions to the EFG tensor, ligand and 'd' electron, only the

f) 1 latter requires special consideration for Ni . The study

presented here concerns only the most common oxidation state

+ 2 of nickel, Ni , and the most common nickel complex configurations, octahedral and tetrahedral. The effect of distortions is considered only qualitatively.

The one electron approximation technique for predicting

57 the EFG tensor has proven quite satisfactory for Fe Mossbauer spectrometry, and may be extended to nickel by inclu-

+ 2 sion of the additional electrons . Ni has a total of eight

+ 2 + 3 3d electrons, whereas Fe and Fe have six and five, respectively. The one electron approximation yields expected

+ 2 EFG elements of zero for Ni in both octahedral and tetrao hedral symmetries, and V = 8/7 (q/r > (where q is negative),

n = for square planar symmetry. The orbital populations

are illustrated in Figure 24 and the EFG contributions are

given in Table 7. The presence of a small distortion from

tetrahedral symmetry to slightly lift the t„ degeneracy

^g

would be expected to produce a non-zero value for V .

51
Electron configuration in Ni z*(3d8

) Figure 24. Three common electron configurations

for Ni+2, using a one-electron

Hamiltonian.

Table 7- 3d electron contribution to the EFG elements.

Orbital

d 2

z d 2 2 x -y

d

xy

xz

d

yz

V /<qr" 3 >d V /<qr 3

yy

* )d V /<qr

zz M

-2/7 -2/7 + 4/7

+ 2/7 + 2/7 -4/7

+ 2/7 + 2/7 -4/7

+ 2/7 -4/7 + 2/7

-4/7 + 2/7 + 2/7

The eight electron problem has been solved for the two

simpler symmetries for the purposes of comparison with one

electron results and the inclusion of spin-orbit coupling.

o The crystal field theory-energy-level-diagrams for d in

octahedral and tetrahedral symmetries with spin-orbit coupling are well documented and are reproduced in Figure 25.

However, energy level diagrams alone are not sufficient for

52
8 A2 (3)

3 T,(9)

k F(2l) /

3 F(2I)

3 t,(9)

l T2 (9)

(5)

3 T, (9)

(3)

(I)

3 A2 (3)

(no first order s-o coupling)

Figure 25. Two common electron configurations for

Ni + 2

} using a many electron Hamiltonian

the computation of the EFG tensor, and the needed wave functions are less well documented. It is often simpler to perform the calculation than to conduct the necessary literature

search. In the present case, the wave functions for octahedral and tetrahedral symmetries in the absence of spin-orbit

coupling were easily located in Reference [12]. The calculation of the wave functions with the inclusion of the spinorbit perturbation is sketched below, as well as the operator

equivalent method (Section D) for obtaining the EFG elements

once the wave functions are known.

P

o The results of d calculations may be applied to the d

problem by recognizing that two "holes" in the ' d' manifold

is equivalent to eight electrons, since ten ' d' electrons

constitute a full shell. The energy level diagram must be

inverted, however, since the holes act like positive electrons. The change from an octahedral to a tetrahedral crystal field results in another inversion of the energy level

53
diagram, as well as a change in the magnitude of the crystal

field splitting. The free ion ground state is known from

atomic spectroscopy to be F, where the superscript is the

spin multiplicity, 2S+1, and the letter F denotes a total

angular momentum of L=3 according to spectroscopic notation.

The total orbital degeneracy of the free ion ground state is

2L+1=7, to give a total degeneracy of 3

X 7 = 21. The crystal

field lifts only orbital degeneracy and yields for the present symmetries two triply degenerate levels (T, and T

p

) and

one singly degenerate level (A„).

The orbital eigenfunctions are given in Reference [22],

p. 393) as:

A

2

0> = ^| |32) -|3 -2)

T 1) = —

1 2 + t

- /5|3-3> - /3| 3D]

T,0> = | 30>

T

x -1) - -^

T

2 1} " 272

T

2

0> = 2^

|{-/5|3 3> - /3|3 -1>|

| |/3|3 3) + /5|3 -1>}

|{|3 2) + 1 3 -2>|

T, ,_1> + ^l 7^' 3 " 3> + ^l 3 1>

l

(1)

where the numbers in the kets on the right denote the values

of L and 1YL , respectively. Spin-orbit coupling may be included rigorously by adding the spin-orbit interaction term

to the crystal field term of the Hamiltonian and diagonalizing the 21 x 21 matrix obtained with the set of product basis

functions, LNL SM„ . A much simpler way is to consider the

effect of a spin-orbit perturbation on the lowest crystal

54
field level alone. This approach is justified by first order

perturbation theory for cases in which the crystal field

splitting is much larger than the spin-orbit splitting.

For the case of an octahedral crystal field, the ground

state orbitals are:

|A

2 0,S Ms

> 5 |A

2

0> |S M

s

> = t|{|3 2) -|3 -2>| |S M

s

> = 1,

(2)

M

s

= -1,0.1.

The spin-orbit term of the Hamiltonian may be expressed as:

XL-S = X L S + 1/2(L S + L S ) (3)

where the operators S , S , S, , and S obey the same rules

*; ' . z.'..;.+. - ^p

with respect to the kets |S 1VL> as do the operators L , etc.,

with respect to the kets | L IVL > . (See Equation 3, Section

D) . Adopting the simplified notation

|ML,M

s >s |L ML

> |S M

s

> (4)

(since L = 3 and S = 1 throughout the problem), it may easily

be verified that

L

Z

S

Z |ML

,M

S

> = M

L

M

S

|ML,M

S

) (5)

L

± S_|M

L ,S>=r(L-M

L )(L

± M

L+l)(S ± M

L )(S_M

L+l)l 1/2 |ML ±l,M

s +l>

The matrix elements of the spin-orbit Hamiltonian over the

ground state orbitals are then given by:

55
A<A 9 0,1M' |L-S|A 9 0,lMq '2 >

) _ (6) S ' - 1"2 3 S

= 1/2A <2,M

S |L-S|2,MS

> - < 2 ,M

S

| L • S

| -2 ,M

S

)

-<-2,M<Jl-S|2,Mq

> + <-2,MQ |L-S|-2,MQ

>

t

= 1/2A|2M

S

-0 -0 -2M =

In other words, the low lying level for octahedral symmetry

experiences no spin-orbit coupling, to first order.

The spin-orbit problem need not be repeated for the

higher lying energy levels before computing the EFG tensor

elements because kT is much smaller than the crystal field

splitting in general.- Thus the relative Boltzmann population of the higher levels is negligibly small. The EFG

tensor for the system is then the Boltzmann average of the

EFG tensors for the three states of the low lying level,

and, since the states are degenerate, the Boltzmann weighting

factors are equal.

The EFG tensors for the three states may be found by

using the method of operator equivalents discussed in Section

D. Thus,3 the element V for the state |A~0,1 1> is given

zz i 2

5 to by:

V

zz " < A

2

' 1 1 'l \z IV' 1 1} (7)

= 1/2 <2,l|V 3 |2,1> -<-2,l|V |2,1)-<2,1|V 1-2,1)+ ' zz ' ' ' ' zz ' 5 ' zz

<-2,l|v

zz |-2,l>}

= l/2|3(2) 2 -3x4-0-0+3(2) 2 -3x4|=

Since the orbital portion of the wave function is the same

for all three states, the same result obtains for |ApO,l -1>

56
and |ApO,l 0). In a similar manner, the other EFG elements

may also be shown to be zero, and the elements of the total

EFG tensor are thus zero as well, in agreement with the oneelectron result.

The spin-orbit coupling problem is considerably lengthier

for the case of tetrahedral symmetry, since the low lying

3 T- level is nine-fold degenerate. The Hamiltonian matrix,

obtained in a manner similar to Equation (6), is given in

Table 8, with the states arranged so as to make "blocking"

of the matrix convenient. Upon diagonalizing the five submatrices, the eigenvalues and eigenvectors indicated in Table

9 are determined.

The EFG elements for the system are given by the Boltzmann average of the EFG elements for the nine states . Before

taking the Boltzmann average, however, it is convenient to

compute the EFG elements for each of the spin-orbit energy

levels, since the Boltzmann factor is the same for each state

within an energy level. The EFG elements for each state may

be easily obtained by the method of operator equivalents,

and the resulting diagonal elements are shown in Table 10;

the off-diagonal elements were all zero. It is obvious

from the table that the total values of the diagonal EFG

elements for each of the three energy levels are all zero.

Thus the Boltzmann average is zero also for each element,

in accord again with the one electron result.

It is interesting to note that the individual states

within the three-fold and five-fold degenerate levels have

non-zero EFG elements. This means that a small distortion

to lift the degeneracies would be expected to yield a nonzero Boltzmann average, again consistent with the one electron result. Such is not the case for octahedral symmetry,

since the individual states have zero EFG elements

.

(John C. Travis)

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hO S >

Cm •H £ T3

CO £ >5

•H Eh X3

,-< * * *

59
Table 10. EFG elements for the low lying states of Nl with

T, coordination

d

State V

xx

/ < qr

3 )F V / < qr"

yy

H

" 3 )P V

zz /< qr 3 )F

al

bl -1/15 -1/15 2/15

b2 -1/15 -1/15 2/15

b3 2/15 2/15 -4/15

cl 2/15 2/15 -4/15

1

c2 2/15 2/15 -4/15

c3 -1/15 -1/15 2/15

c4 -1/15 -1/15 2/15

c5 -2/15 -2/15 4/15

60
P. Ni Theory: Nuclear Interactions

The calculation of the nuclear interactions for Ni is

straightforward and proceeds in exactly the same manner as

the corresponding calculations for J

' Fe with the single exception that the excited and ground nuclear states have

spins 5/2 and 3/2, respectively. The familiar interaction

Hamiltonians are recorded here for correlation with Figure

26 which shows the energy level diagrams

HM = -g3 HI . . M

& n z (1)

/\ _ /\ H

Q

= A(3I^ " I

2

), A = eQV

zz /40

/\ /\ HM

= HM

cos

/\ >\ H^ = H

Q

(3 cos

2

6 - l)/2

where g is the magnetogyric (gyromagnetic) ratio, 3 is the

nuclear magneton, Q is the nuclear quadrupole moment, and V

is an element of the electric field gradient (EFG) tensor

(discussed in Section E) . The primed Hamiltonians are perturbation Hamiltonians, with being the angle between the

principal quantization axes of the zeroeth order and the perturbing interactions. The quadrupolar Hamiltonians in (1)

and Figure 26 are for the special case in which the EFG

elements V and V are equal,

xx yy H

The determination of the relative magnitudes of the magnetic and quadrupolar interactions in the ground and excited

states is best done experimentally. The determination of

the magnetic moment ratio was discussed in NBS Technical Note

421 [2]. The latest value determined at NBS for this ratio

is u /u = -0 . 56±0 .082 . * As for the quadrupolar interactions,

*The uncertainty reported is approximately two standard deviations. The standard deviation was estimated from the last

iteration of a non-linear fitting procedure on a Taylor series

expansion about the estimated values.

61
CM I CM CM

IO ItO

— * yk M y » v o \ t \ t \ AI \ +

IICM XX

+sX

II ii« » \

01

N TcM <"J *N io — I— to m | i i

CMIOi

K)'

IOto

CO

CMT CM 7 f *

CMV.

ro

CM\

IT)

CM

ro

coCo

•HP

05 £2m^

-PM

CD

CDX CO •H 3 S CD H

T3 o C 3

cti

C

•»iH

CD S

rHrH a

t3 -P

Cti 3 <M a1 o

•> CO O rH

•H CD P >

CD CD C rH

bD

cti

m 6 CD3

Cm cti O .Q

CO -P CO O :o

CD S

Cm

<m CD

cd x:P

CD

EhO

CM

CDM

hO

•H

62
rH|C\J

LT\|CM

CD -P

Cd P

CO

CM\me

•H

ft

CO

CD £

-P

PhO

Oh

•H

Sh -P

cdSC

cd •Hco

-pH

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cd

ffi

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Cd a?

CD H

cd EH

PO|CM I

lt\|cm 1 1 CM

LPi|CM

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lh|cm

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m|cM

LfMCM

63
preliminary results confirm the indication fron NMR work [13]

that the ground state quadrupole moment is essentially zero

for purposes of Mossbauer spectroscopy.

The indicated intensities shown in the line spectra in

Figure 26 are for a powdered sample, and are obtained from

the squares of appropriate Clebsch-Gordon coefficients, as

discussed in Technical Note 421 [2]. The intensity calculations are much more complicated for the Hamiltonians H,

and Hp, as well as for the quadrupolar Hamiltonian for the

case in which V 7: V , but may be performed by the method

xx yy

J of Reference [1-4] once the eigenvectors are known.

For the general to quadrupolar interaction case of V / V , ^ ^ xx yy

the Hamiltonian becomes

H

A

= A 3>i\ - i

2 +'nd+ + i

2

)/2J,

n = ^

-W- ( 2 ) zz

The Hamiltonian matrix for the spin 5/2 excited state is

shown in Table 11. The reason for the arbitrary ordering of

rows and columns will become evident. The elements of value

/10ri A tend to mix the ±1/2 states with the ±5/2 states,

while the elements of value /l8n A tend to mix ±3/2 with ±1/2

states. According to second order perturbation theory, the

latter mixing has the most influence on the energy levels by

a factor of roughly ( /18/10T( 18A/6A) , or approximately 5-4.

Therefore, a crude approximation to the n-dependence of the

energy levels may be obtained by ignoring the smaller mixing

terms and diagonalizing only the blocked out sub-matrices of

Table 11. This procedure yields the approximate eigenvalues

and eigenvectors

64
E(±5/2) = 10 A, |±5/2>' =|±5/2>

E(±3/2) = A(-5 + 3/I+2^"2

), |±3/2>' = N

+

[| 1/2) - A + |±3/2>]

E(±l/2) = A(-5-3/l+27T2

), |±l/2>' = N_[|±l/2> - A_|±3/2)]

x±= /F|!tl

} N± = u a2] -1/2

The accuracy of the approximation may be checked by noting

that the three lines approach a very nearly symmetrical

arrangement for n = l, as expected.

The behavior of the quadrupole spectrum as a function of

asymmetry parameter is illustrated in Figure 27 for a more

accurate, although less convenient, set of eigenvalues than

given in (3). As of the present, only small quadrupolar interactions have been observed (see Ref. [15]), and no asymmetry

parameters effects have been observed.

(J. C. Travis and J. J. Spijkerman)

G. Standard Reference Material for the Chemical Shift

of Tin Compounds

Work in progress to develop a Mossbauer chemical shift

standard for Sn, described in Technical Note 421 [2] has

been primarily focused on BaSnO^. This material is stable

in air, and exhibits no long term chemical and physical

charges which affect the Mossbauer spectrum. The spectrum

shown in Figure 28 shows a narrow, well defined singlet,

with a large Mossbauer effect . Several attempts to obtain

single crystals of this material failed. Encapsulation of

BaSnO^ in plastic did not affect the spectrum, and a 2%

uniform distribution could be obtained. Several absorbers

2 2 with BaSnO^ concentration from 5.0 mg/cm up to 250 mg/cm

were analyzed, using a 3 mCi Pd„Sn source, for percent effect,

line width and y radiation attenuation. The percent effect

and attenuation are shown in Figure 29 . These results in65
V = I.O

Figure 27. Quadrupole spectrum as a function of n

2 dicate that a 100 mg/cm concentration is most desirable,

for a maximum Mossbauer resonance absorption without excessive attenuation. Extrapolation to zero thickness gave

a line width of 0.79±^% mm/sec and 0.85±4$ mm/sec* for a

*The uncertainty reported is approximately two standard deviations. The standard deviation was estimated from the last

iteration of a non-lineary fitting procedure on a Taylor series

expansion about the estimated values.

66
CO

(£> CO

O)rii UJ

UJ

COcr

CD

CM

o

Q_

z>

ct: i—

o

UJ

Q_

co

00**>0! 00*301 00*OoT~ 00*86 00*96 00* »6

UISN31NI 3AI1013y

67

00*36 00*06
!00

t 60 -

UJo

UJ

Q.

Figure 29.

100 150 200

SAMPLE THICKNESS (mg /cm2

)

250

Percent effect and attenuation as a

function of sample thickness

2 100 mg/cm absorber. Further work is in progress to determine uniformity and reproducibility of absorbers prepared

by plastic encapsulation.

(J. J. Spijkerman)

H . Fitting of Multiple Line Mossbauer Spectra Using

Constraints

1 . Introduction

Most of the commonly used programs for computer processing of Mossbauer spectra, such as NBS's own PARLORS [2], fit

the sum of an appropriate number of "independent" Lorentzian

curves to the data. In these programs the positions, heights,

and half-widths of the Lorentzian 's are varied independently

until a best fit is obtained. The lack of prejudice of such

programs with respect to the relative positions and intensi68
ties of such lines is desirable in most cases. However, at

other times, the extraction of useful experimental results

requires the use of known theoretical relations between the

relative positions, heights and half-widths. Such theoretical relations may be employed as constraints to reduce the

number of variables and thereby provide better estimates of

the remaining parameters

.

Spectra for which a constrained fit program is necessary

are characterized by the presence of one or more sets of unresolved lines, low signal-to-noise ratios, or both. The

programs described in the following sections were written

because all multiple-line nickel spectra fall in the first

category, and many in the second as well. Work is currently

underway on a program for iron spectra with more than one

site present

.

2 . General Theory

In the analysis of a spectrum, it is first necessary to

assume a theoretical model for the spectrum in question.

Such a model will ordinarily depend on the velocity (v) and

on a set of (e) of n (unknown) physical parameters (FL) as

indicated in (1)

.

Y = f(v,e), e=(R

1

, r

2

, ,R

n

) (1)

The parameters reflect the height of the base line, the

curvature and position of the parabola, the isomer shift, the

magnetic field, etc. When the function is observed at N

discrete points in velocity, its .value at the i'th velocity

may then be theoretically described by

Y

± = f(vi ,9) + e

±

, 9= (R1} R

s ,...,R

n

), i = 1, 2,...N (2)

where the e. are random variations due to counting statistics.

l

The problem then is to obtain estimates for the unknown

69
parameters (0), and the method of nonlinear least-squares is

usually selected as a means of obtaining these estimates,

(see, for example, Draper and Smith, [16]).

If "good" initial estimates, 8 , of the unknown parameters are available, the following method iteratively improves

the initial estimates until the true least-squares estimates

of the parameters have been obtained, and it usually works

quite well. The mathematical model is first approximated

by the first order terms of a Taylor series expansion about

the initial estimates, the expansion being carried out in

terms of the unknown parameters as in (3).

n / x 9f (v. ,8)

Y. - f(v

1 ,e<°>) V ( VRk

(o)

-

) -^—

k=l k

,-o(0)

This may be rewritten as

AY. (o) _ nE AR

k=l

(o)

k F

(o)

k,i

where

and

AY.

(o)

= Y. - f(v.,8 (0)

),

^ (o)

= R, - R1

( ° )

J k k k *

k,i

(o) _ 3f(v. ,8)

3Rk ,-n(0)

70
Equation (4) is then fitted to the experimental data,

which in the present case consist of roughly 200 data

points, and the n unknowns, AR, , are found by the usual

( \ method of linear least squares. The AR, ^°' are then treated

as corrections to the initial guesses, R, , and the refined

parameter estimates R, = R^ + AR, are computed.

This process is repeated until a desired convergence criterion is met

.

If the initial parameter estimates are not close enough,

the linear approximation is no longer valid and a fit is not

obtained. Alternatives to the procedure described above,

which have wider regions of convergence, are discussed by

Draper and Smith [16].

The computer programs discussed below, for various types

of nickel spectra, were written in OMNITAB, a user-oriented

programming package developed at NBS which facilitates communication with the electronic computer. A listing of one

of the programs is included below, inasmuch as the instructions are basically English sentences and are largely selfexplanatory. The OMNITAB package has been written in standard FORTRAN IV and is available from the NBS Statistical

Engineering Laboratory upon request.

Case I - Magnetic Interaction

The equation for the magnetic nickel spectrum (see Ref.

[2] , p. 32) is given by

p 12 (i)

Y. = B + Lv. + GvT - A T. T

v y

where the T . are the individual Lorentzians,

J

T

(i) = J J 1 + D(P - v ) 2

71
with the (known) relative peak amplitudes, A., given by

(Ref. [2], p. 35)

[A ] = [10, 3, 1, 6, 6, 3, 3, 6, 6, 1, 4, 10].

J

The peak positions, P., depend on the ground and excited

J state magnetic splittings, a and 3, and the isomer shift, 6

P . = a. a + b.3 + 6

J J J

[a.] [ "I "I -i -I -1 -I I I I 3 3 3 "I L

2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2j

[b ] p-5 -3-1-3-11-113135

I 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2

The variable parameters to be fitted are listed below

B: Base line height (counts).

L: Linear term of parabolic background.

G: Quadratic term of parabolic background.

D: (1/H 2

), where H is the half width at half

maximum

A: Overall amplitude factor

a: Splitting of ground level magnetic substates

( channels

3: Splitting of excited level magnetic substates

( channels

6: Chemical shift (channels)

The program was originally used to establish the ratio 3/a

in the high field alloy (1.5%) Ni - Pe . In later applications, the ratio has been fixed at the value of .338 to lower

the number of fitted variables from eight to seven.

72
The first three terms of (5) are common to all types of

nickel spectra and have the obvious partial derivatives

8Y. 3Y. 3Y.

l _ , _i_ _i 2

3B ' 3L V

i 9G " V

i

The remaining partials are summarized in Table 12, along

with those for the other cases

.

Case II - Quadrupole Splitting with Zero Asymmetry Parameter

The quadrupole spectrum for the special case of n = is

given by

Y. = B + Lv. + Gv? - A Y S^

fa. J

where the Lorentzians are given by

S

( ^} = /p

X „ vo , P, = S< Y

:

(

P, - v, \2 » j' &

j

r j = 1,2,3

1 + - -J * H [g ] = [-8, -2, 10]

J

The* variable H is the half width at half maximum. The

other new parameter, y is given by

Y

= eQV

zz/40

where eQ is the nuclear quadrupole moment and V" zz

is the

principal component of the electric field gradient tensor.

The pertinent partials again are listed in Table 12, and

the program for this equation is listed in the appendix.

73
^-s rC •H hD > 3O

•H 1 TD 1 M

>H

m 1 Pm 1 .G CD <ro 1 ^^ 1 -P

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•H

Q \ a1 -P CM ^—> W

cd \ •H S ^^ > C

•H •H •H X •H > 1 1 o >H Q 1 1 (D u CD CD •m 1 TD CO a fin P-. O a ^^ v—

£

cd •H TD

•H TD -P

CM

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cd

<d

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Cm hO o t3 P

CO IW

w

cd +

CD

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cd T3 \ > CD •ID •H •H T3 II < M >H <o •H CD

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EH

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(Dw

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p4

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cd hO

w

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PM ^

Eh •H

H ^

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>H

a :_> 1 id

O rH •H TD ^^

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Q

cd cd rH

Q v_^ < w w O

CH •• <CAJ CM CM

CD CD -=T II II CO H H H £1 rH •H -id

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74
"Case III - Magnetic With Quadrupolar Perturbation

For a small, axially symmetric quadrupolar perturbation

on a magnetic spectrum, only the line positions are affected

significantly. Equations (5) through (7) are still valid,

but the positions are now given by

PI = P . + C . Y

J J J

[ Cj ] = [10, -2, -8, -2, -8, -8, -8, -8, -2, -8, -2, 10]

where the P are given by (8). The partials are listed in

Table 12.

3. Uncertainties of the Estimates

In the less well defined spectra no attempt has been

made to approximate the uncertainties of the estimates, but

in some of the better defined spectra, where the parameters

are rather precisely determined, the Taylor series expansion

has been used to provide rough approximations to the uncertainties . When the Taylor series approximation is very good

in the neighborhood of the final solution, the approximate

uncertainties should also be very nearly correct.

(J. C. Travis and B. L. Joiner)

I . The Application of Mossbauer Spectrometry to Quantitative

Analysis

1 . Introduction

During the past year Mossbauer spectrometry was investigated for use as a tool for quantitative analysis . This

application could be important because the selectivity of

this technique is such that no nuclei other than those of interest in the sample can be resonantly excited by the incident radiation and in this respect the method would be completely specific. Initial studies consisted of evaluating

75
the effects of variables such as drift in detector response,

sample thickness, sample concentration, and time stability

of the single channel analyzer, on the spectral parameters.

Using Pd_ Sn as a source in conjunction with synthetic

samples of SnO„ in an AlpCU matrix, the effects of these

variables were systematically evaluated.

In the transmission mode, the magnitude of resonant absorption is related to concentration by the expression

I

_ K") - 1(0) . f L.e

-TA/2 j ( I(°°) s o A/2 (1)

e is called the Mossbauer fraction of resonant radiation

absorbed, I(°°) and I(o) refer to the transmitted intensities

for non-resonant and resonant absorption, respectively. The

effective absorber thickness is given by the expression

T

A = f.a a.n.t. where f and f. are the Debye-Waller factors

for the source and absorber, respectively, a is the cross

section for nuclear resonance absorption, and a. is the

isotopic abundance for n„ atoms per unit volume with a sample

of thickness, t n 3

. J is the zero-order Bessel function with

A o an imaginary argument

Because f

fl depends on the lattice dynamic characteristics

of the sample, an absolute determination of n

fl based on

equation 1 would be difficult even If the remaining parameters, i.e. f , a , a., and t. could be measured with high

S O r\. ri. accuracy. Therefore, the simplest approach for quantitative

analysis would be the dissolution of the sample and incorporating the analate in a reproducible matrix. A series of

standards in an identical matrix would then serve as the

basis for a calibration curve

To demonstrate this technique, a tin system was chosen.

A stannic oxide sample was selected for study because of

76
previous experience with this compound [17] and the availability of a good single line tin source, i.e. Pd^ Sn [18].

An AlpO.., matrix was chosen because the photoelectric absorption of the 23.8 keV Mossbauer gamma photons is small and

homogeneous samples are easily prepared. For application

to real samples a series of standard reference materials

(copper-base alloys) was analyzed for tin.

Preliminary studies on a series of tin-containing ore

samples were undertaken in order to investigate the possibility for nondestructive analysis of the sample. Additional

studies consisted of the evaluation of the source-sampledetector geometry, use of filters, a high resolution detector, and a resonant detector in an effort to optimize the

ratio of the resonant absorption intensity to the total transmitted intensity.

2 . Experimental

a. Apparatus

The NBS drift-free Mossbauer spectrometer was used in

these experiments [19]- The aluminum sample holder was especially constructed for mounting powdered samples of varying thickness from 0.5 to 2.5 mm [20]. The area of the

cell was 2.84 cm . The moving absorber geometry was used

throughout this work. The detector consisted of a Nal(Tl)

crystal (2.5 cm in diameter x 0.2 cm thick).

b

.

Reagents and Preparation of SnO ^-Al^O^ Mixtures

.

Stock solutions of tin and aluminum were prepared

by dissolving reagent grade tin metal in concentrated HC1

and A1(N0_)_ .9H

?

in distilled water. The aluminum solution was standardized by complexometric titration with the

disodium salt of EDTA. The hydrous oxides were coprecipitated according to Kolthoff and Sandell [21], placed in Pt

crucibles, and heated in an electric furnace to about 1200 °C

for complete conversion to the nonhygroscopic oxides. The

SnO ? content of these mixtures varied from 1 to 86%. In

77
constructing the calibration curves, the tin contents of

three of the mixtures were verified by controlled-potential

coulometry to an accuracy of better than 1%

cMossbauer Experiments

Procedure A - Two values of sample thickness (0.5 and

2.5 mm), 5 values of sample composition from 7% to 86% of

Sn0

2

, and 5 single channel analyzer (SCA) widths were tested. The lower level discriminator of the SCA was set at

about 20 keV and the upper level was varied in five steps

from 30 to 60 keV. This permitted an energy range from 10

to 40 keV to be detected. A weighed sample of the mixture

was placed in the cell and mounted. The spectra were accumulated until a total of approximately 9*10 counts/channel

appeared in the baseline. This required from 2 to 6 hours

per spectrum. All spectra were fitted as a singlet.

Procedure B - A 300-mg sample of mixture was placed in

the 0.5 mm cell and mounted. The source-to-sample distance

was 4.6 cm. This geometry was convenient since the sample

could be mounted and removed without moving the detector,

hence constant geometry is insured. The single channel was

set to pass radiation from 20 keV to 30 keV. The spectra

were accumulated until a baseline of 8x10 counts/channel

appeared and required from 1.5 to 2 hours per spectrum.

The uncertainty in the computer-estimated height of the

Lorentzian function varied from 0.8% to 3% (relative standard deviation) whereas the uncertainty in the area varied

from 2% to 7%.

d. Analysis of Copper-Base Alloys

Prom 2 to 10-gram samples were weighed and dissolved

in 4:1 HNO^. After digestion on a hotplate for 24 hours, the

metastannic acid precipitates were filtered and washed with

hot . 1M HNCU . The precipitates were then placed in beakers

and enough aluminum solution added (see Section 2-b) so that

about 1.8 g of AlpO.., would be present in the final mixture.

78
Prom this point on the procedure of Kolthoff and Sandell was

followed. The tin content of these mixtures was obtained

from comparison with a calibration curve (Section 2-c, Procedure B) .e . Tin Ores

Mossbauer spectra of seven ore samples were obtained.

The tin content of these ores ranged from 0.3% to 0.9% as

determined by emission spectroscopy. An approximately 1-gram

sample was placed in the 2.5 mm cell and mounted. The sourcesample-detector geometry and SCA settings used are described

in Section 2-c, Procedure B. A 1.5 mil Pd foil was placed in

back of the sample holder (side facing the detector). Spectra

were accumulated until a difference of at least ^xio counts

appeared between the baseline counts and the counts in the

peak channel. This required only a few hours for three of

the samples (Nos. 9088, 9085, 9093) but 2 1/2 days for the

remaining four (Nos. 9100, 9095, 9097, 9098).

3. Results and Discussion

In Figure 30 is a typical spectrum of SnO„ in AlpCU.

The height (H) and the full-width at half-height (D are

computed from the fitting of a Lorentzian function for the

absorption peak and a parabola for the baseline to the experimental points by an iterative least-squares procedure [22],

For the Lorentzian, the peak area (A) is then related to the

peak height by the equation

A = 1/2 ttRT

Experimentally, the transmission of nonresonant radiation

(I(°°)) through the sample is accompanied by background radiation (I') from the source which appears in the window of the

SCA. Therefore, the total transmitted intensity (I(°°) + I')

represents the baseline (B) of the Mossbauer spectrum.

The background radiation is comprised of other sources

79
-4.00 -2.00 0.00 2.00

VELOCITY (mm /s)

4.00

Figure 30. Typical Mossbauer spectrum of

Sn0

2

in AlpO~

of radiation even though pulse-height discrimination is used.

The pulse-height spectrum of Pd~ Sn taken with a Nal(Tl)

crystal consists of a broad single peak from approximately

18 to 35 keV. The energy of the gamma photons responsible

for the resonance absorption is 23.8 keV. A number of x-rays

are produced in this source, the most important being the 21,

25 and 28 keV x-rays. These cannot be resolved with the above

detector, and therefore contribute to I'. In addition,

photons of higher energy (180 to 1170 keV) have been identified in this source. These can be scattered into the detector by means of Compton processes and also contribute to I'.

In addition to the attenuation of the incident photons

due to resonant absorption, there is an attenuation of the

incident photons by the sample due primarily to the photoelectric absorption process. Since photoelectric absorption

should affect the resonant and nonresonant portions of the

80
incident radiation in the same way, the absolute magnitude

of the ratio of peak area or peak height to the baseline

would not be affected. For this reason the ratios of A/B

and H/B are used in these experiments.

An experiment using a modified Youden square was designed to test the effect of varying the window width, the

amount of SnOp, and the sample thickness on A/B and H/B

(Procedure A) . An analysis of variance did not give any

evidence of an effect with variation in SCA over the indicated range. In addition, when either the amount of SnO p or thickness was increased, no effect on the portion of

pulse-height spectrum passed by the SCA was detected other

than a decrease in height because of the attenuation of the

incident radiation. The increase in sample thickness from

0.5 to 2.5 mm resulted in a large increase in self-absorption

by SnOp. A significant broadening of the spectral line was

also observed. Line broadening as a function of sample

thickness has been discussed elsewhere [23]. Indeed, these

authors have shown that this effect can be reduced by

judicious selection of an "effective" sample thickness.

The results from this study showed that the reproducibility at the larger sample thickness was much poorer than

anticipated. This suggested that a more rigid control over

the source-sample-detector geometry was necessary.

Additional measurements were made (Procedure B) under

carefully fixed geometry conditions. Figures 31 and 32

show the variation of H/B and A/B, respectively with increasing amount of SnOp. The data are given in Table 13-

As seen from Figure 31, H/B has nearly reached a maximum

value at 50 mg of SnOp. Above this value the peak continues

to broaden, hence A/B increases as shown in Figure 32 over

a much larger range of concentration. Of the variables

studied, the critical one is the source-sample-detector

geometry

.

81
Table 13. Variation of spectral parameters with the amount

of Sn0

2

-

SAMPLE THICKNESS: . 5 mm

SnOping A/B (Mean) a (A/B) s b H/B (Mean) a (H/B) s b

3.50 0.1994 0.010 5 0.0116 0.0006

9.04 0.4298 0.0082 0.0314 0.0016

16.28 0.6937 0.0015 0.0496 0.0015

28.82 1.064 0.017 0.0726 0.0015

38.22 1.286 0.028 0.0831 0.0017

52.84 1.534 0.053 0.0943 0.0016

56.85 1.648 0.032 0.1004 0.0012

76.35 1.961 0.040 0.1086 0.0006

97.11 2.249 0.034 0.1165 0.0006

121.9 2.422 0.086 0.1181 0.0005

3. Average of 3 replicates

Computed standard deviation of single measurement based

on 2 degrees of freedom

82
10 20 30 40 50 60

Sn 2 , mg

70

Figure 31. Plot of height of peak to

baseline ratio vs mg of Sn0

?

A least-squares fit of Equation 1 to the experimental

data was performed. If I' is assumed constant, Equation 1

can be rewritten in the form

H/B =_ I(co) - I(o)

rc^y + i'

K«0 f.

= f (-

" Km/2

I o (Km/2) (2)

where f = i(»)

s + I'

, K includes the constants V, (volume)

f . , a 3 a

A

, t.j and m is the mg of SnO

? (n.V). The Bessel

function J (iKm/2) = I (Km/2) see [24]. At low concentrations (e.g. low nq

) the function, I (Km/2) is approximately

one; therefore, Equation 3 can be rewritten as the familiar

exponential relationship (similar to Beer's law)

H/B = f (l-e- K

' m/2

) (3)

where f and K' are empirical constants that approach the

theoretical constants f and K, respectively as m approaches

0.

83
Figure 32. Plot of area of peak to baseline

ratio vs mg of SnO„

It can be seen from Table 14 that both Equations 2 and 3

both fit the data fairly well. This indicates that the presence of the Bessel function does not change the exponential

form of Equation 3 appreciably over the concentration range

studied. Thus either Equation 3 or Equation 2 can be used

In this range although Equation 2 is preferable on theoretical grounds

In Table 15 the results of the analysis of tin in the

copper-base alloys are given.

The negative bias of the results from the certificate

values can be attributed to losses in the separation of

metastannic acid from the alloys. In addition there is non84
Table 14. Comparison of calculated values with the observed

values of H/B

Sn0

2 mg Observed Calc. a Deviations ,#

c Calc. b Deviations ,%'

3-50

9.04

16.28

28.82

38.22

52.84

56.84

76.35

97.11

121.9

.0116

.0314

.0496

.0726

.0831 • .0943

.1004

.0186

.1165

.1181

.0133

.0311

.0495

.0719

.0835

,0961

.0987

.1083

.1148

.1199

-14.7

0.9

0.0

0.9

- 0.5

- 1.9

1.7

0.3

1.5

- 1-5

.0125

.0297

.0481

.0716

.0840

.0974

.1001

.1094

.1146

.1177

-7.2

5.8

3.1

1.5

-1.1

-3.2

0.3

-0.7

1.7

0.3

aH/B = f l_ e Km/2

i

Q (Km/2) ; f = 0.1580; k = 0.02595

-K'ra/2 ; f = 0.1204; K' = 0.03131 H/B = f 1-e'

deviation, % = Observed-Calc. x 100%

Observed

agreement between the tin assay values calculated from H/B

and A/B spectral parameters

.

In order to maximize the ratios of H/B and A/B it is

necessary to reduce I'. The source-sample-detector geometry

was investigated for one sample concentration. Table 16 shows

an increase in H/B and A/B as the source-to-detector distance was increased while keeping the source-to-sample distance constant. The decrease in I' is believed to be due to

a reduction in the detected x-rays produced by fluorescence

in the samples. Of course, this improvement was realized at

the expense of a reduction in transmitted intensity, necessitating a much longer time to reach the same statistical precision in the baseline.

85
Table 15. Analysis of Sn in copper-base alloys

Cert.

SRM# #Sn(H/B) a #Sn(A/B) b Value

% %D ev.(H/B) c %D ev.(A/B) d

37e 0.850 0.856 1.00 -15.0 -14.4

124d 3.82 4.30 4.56 -15.2 - 5-7

52c 6.66 6.95 7.85 -15.2 -11.5

184 4.91 5-97 6.38 -23.0 - 6.4

C-1107 O.896 1.07 1.04 -13.8 + 2.9

Sn determined using H/B as spectral parameter

Sn determined using A/B as spectral parameter

Deviation of value calculated using H/B as spectral parameter

from the certificate value

Deviation of value calculated using A/B as spectral parameter

from the certificate value

Table 16. Effect of geometry on spectral parameters

Source-toDetector

Dist . , cm A/B H/B

Time

Min

Baseline ,- Countsxio 5

Transmission

Rate, cpm

2.0 1 .182 0.0743 57-0 8.751 1.54x10

0.81X10 11 0.23xl0 4

4.6 1 .286 0.0831 97.4 7.897

9.1 1 .445 0.0947 345.8 7.852

NOTE : Sample Content, 38.2 mg SnOp

Sample Thickness, 0.5 mm

Source-to-Sample distance, 1.0 cm

86
Another way of decreasing I' is to reduce the 25 kev" Sn

x-rays by using a palladium foil which has a K-absorption

edge at 24.3 keV. There is some reduction in the intensity

of these x-rays due to the presence of palladium in the

source itself. However, as shown in Table 17, there is a 25%

increase in H/B when a 1.5 mil Pd foil was placed between the

sample and detector. This increase is independent of the

sample thickness over the range investigated. The same results were obtained when the foil was placed between the

source and the sample

.

Table 17. Effect of Pd filter on spectral parameters

NO Pd Foil 1.5 mil Pd foil

Sample SnO p Thickness, mm mg A/B H/B A/B H/B

0.5 38.2 1.286 0.0831 1.654 0.1065

1.5 97.7 2.265 0.1166 2.795 0.1454

2.5 154.7 2.917 0.1294 3.566 0.1614

2.5 154.7 3.6l6b 0.l6llb

b 1.5 mil Pd foil placed between source and sample

Increasing the resolution of the detector also reduces

I'. In Figure 33 is a pulse-height spectrum of Pd_ m

Sn

taken with a Li-drifted silicon detector (sensitive area

p 80 mm ; sensitive depth 2 mm) which has a resolution between

1.2 and 1.3 keV FWHM for photon energies between 8 and

45 keV [2]. With this detector, the single channel can be

set to pass only 23.8 and 25 keV photons. In conjunction

with a Pd foil filter, the 25 keV Sn x-ray can virtually be

eliminated and therefore permit only 23.8 gamma photons to

87
IOK

keV

Figure 33- Pulse height spectrum of Sn

with lithium drifted silicon detector

be detected. Again since the sensitive area is only 80 mm , the counting efficiency is such that long counting times are

required for the Mossbauer experiments. The tin source used

has high energy gamma emitters as contaminants which produce

Compton-scattering of the proper energy in the SCA to contribute to I'. Of course, this can be reduced by the production of a tin source with a minimum of contaminants.

Preliminary studies on a resonance detector for tin were

initiated. The main advantage of such a detector is that it

would virtually detect only the recoil-free Mossbauer gamma

radiation from the source (via conversion electrons) and

88
hence eliminate the complications that arise in existing detection systems. Such a detection system would be of tantamount importance in quantitative analytical applications.

Counting efficiency studies using a scintillation gel

with tritiated glucose showed that the highest count rate was

obtained with the most rigid gel. In addition, when various

amounts of SnOp were added to the rigid gel no significant

decrease in count rate was observed which indicates that this

compound is transparent to the light produced by the scintillator. It was also found that deareation of the scintillator

solution prior to forming the gel had no effect on the count

rate . First experiments with the detector using the E.M.I.

tube consisted of observing the phototube noise with increasing applied voltage. Again using the E.M.I, tube J.J.

Spijkerman investigated the optimum selection of the SCA

settings in order to observe a Mossbauer emission spectrum

due to the conversion electrons produced by the internal

conversion of the 23.8 keV photons in the SnO„ detector.

Additional studies consisted of using an RCA photomultiplier tube with much less dark current instead of the

E.M.I, tube in an effort to decrease the tube noise. Also,

in an effort to obtain a pulse-height spectrum of conversion

electrons a number of radionuclides were investigated in a plastic scintillator using an anti-coincidence detection

system. All spectra obtained thus far seem to indicate that

conversion electron spectra are very similar in shape to

beta particle spectra. This seems to preclude any capability for pulse-height discrimination. Since the photosensitive surface of these photomultiplier tubes are sensitive to gamma radiation, another source-detector geometry

where the source is not directly in line with the phototube

should be used. A number of these- factors are currently

being studied.

(P. A. Pella)

89
J . Tin Mossbauer Spectrometry Following the Decay of

119 Sb

119 The Sb radioisotope has not been reported as a source

for Sn Mossbauer spectroscopy. The 38 hour half life and

the difficulty of production of this radioisotope compared

~\ 1 O m

to the 245 day reactor-produced Sn source makes the Sb

119

source undesirable for routine work. However, the Sb

radioisotope makes it possible to study the tin compounds

119 formed by the decay of Sb , and the possible formation of

"charge states" during its decay, by electron capture. Sb is

particularly attractive for this study since Sb and Sn have

57 57

no common oxidation states, as is the case for the Co- Fe

119 decay. Furthermore, the lifetime of the Sn excited state

_o

is 1.8xio sec, an order of magnitude shorter than that of

57 Fe.

The Sb is produced by the Sb(y2n) Sb nuclear

reaction, using 100 MeV bremsstrahlung from the NBS Linac

Reagent grade Sb metal was irradiated for 8 hours, and used

2 as a source with a 150 mg/cm BaSnO^ absorber at 80°K. The

spectrum is shown in Figure 34, with a chemical shift of

1.65±0.08 mm/sec, and a line width of 1.28±0.l6 mm/sec*

119 Figure 35 shows the spectrum of a Sb p Cv, prepared

2 from the irradiated metal, with a 150 mg/cm BaSnO^ absorber.

The chemical shift is -0.02±0.10 mm/sec and the line width

119 is 1.90±0.1o mm/sec. This would indicate that Sb decays

+ 4 to a Sn oxidation state, since the spectral parameters are

similar to those of SnO p [17]- Further work is in progress.

(P. A. Pella and J. J. Spijkerman)

*The uncertainty reported is approximately two standard deviations. The standard deviation was estimated from the last

iteration of a non-lineary fitting procedure on a Taylor series

expansion about the estimated values.

90
o

o

CO

o

CO

• o •—v (n\

z: o z: o ^^

• o

1 >-

h-

•—

i

o

u m

CO _i

UJ o

1 >

o

to

.CM

I

-CO

I

09* tOt 08" 00 1 00*001 03*66 0**86 09" LS 08'

A1ISN31NI 3AIld"13U

96

91
09*101 08*001 00*001 02*66 Ofr'86 09*1.6

A1ISN31NI 3AIlb13y

08*96 ipiiui'96'

92
K. Abstracts of Publications

MOSSBAUER SPECTROSCOPY OF 6lNi

presented by J. J. Spijkerman to the Faraday Society,

London, England, December 1967.

The Mossbauer effect of Ni has been observed using

a single line source prepared by a 30 minute LINAC

irradiation. Of the many sources evaluated, the 85%

Ni-Cr alloy gave the best result, with a f-f actor of

0.1 and a linewidth of 0.097 cm/s at 80°K. The source

was prepared by 100 MeV bremsstrahlung irradiation of

the alloy, by the reaction "' Ni(y,p) Co. The pulse

height spectrum showed a single peak at 67.2 keV, with

no interfering radiation. The 1.5 hours half life of

the source required repeated irradiations, but no annealing was required.

Several alloys and compounds of nickel were examined,

and showed partially resolved hyperfine interactions.

The magnetic moment of the 67.2 keV, 5/2 spin, first

excited state is + 0.425±0.043 ran.* The magnetic field

at the Ni nucleus in antiferromagnetic NiO is 96±10

kilogauss. Quadrupole splitting was observed, but no

value of the quadrupole moment of the 5/2 state could

be determined. A tentative chemical shift-electron configuration diagram has been worked out, and gives a value

of AR/R = 2.5X10" 4

.

SOME ASPECTS OF THE INTERPRETATION OF THE MOSSBAUER

CHEMICAL SHIFT IN TIN CHEMISTRY

by J. J. Spijkerman, published in the American Chemical

Society, Advances in Chemistry Series No. 68, p. 105,

(1967).

The Mossbauer spectra of a large number of organic

* The uncertainty reported is approximately two standard deviations. The standard deviation was estimated from the last

iteration of a non-lineary fitting procedure on a Taylor series

expansion about the estimated values.

93
and inorganic tin compounds have been published. Prom

these spectra, the oxidation state of tin in most compounds can be determined.

To interpret the Mbssbauer spectra properly, the relationship between the chemical shift and the electron

density at the Sn nucleus must be established. This requires that the magnitude and sign of AR/R (a factor

that relates the change in the effective charge radius

of the tin nucleus on passing from the excited to the

ground state) must be known, in most instances, this

factor can be determined from nuclear shell model calculation, but, for tin, it must be evaluated experimentally. The discussion includes the results of the

various experiments that were designed to obtain this

factor so that the chemical shift observed for tin compounds could be interpreted correctly.

MQSSBAUER EXPERIMENTS WITH NICKEL-61

by J. C. Love, G. Czjek, J. J. Spijkerman and D. K.

Snediker, published in U.S. Atomic Energy Commission

Report No. ORNL-P-3251 , Proceedings of Conference on

Hyperfine Interactions, Pacific Grove, California

(1967).

Using Mossbauer effect, we have measured the ratio

r -1 of the magnetic moments of Ni nuclei in the first excited state and the ground state: u ( 5/2)/u( 3/2 ) = (-0.559 ±0.0 12) .* Hyperfine fields at nickel nuclei in

iron (24l±7 kilogauss) and in a Ni (.8530 Cr (15/0 alloy

(-15 kilogauss) are determined. Very approximate values

for isomer shifts of some nickel alloys, NiPp , and NiO

are given.

*The uncertainty reported is approximately two standard deviations. The standard deviation was estimated from the last

iteration of a non-lineary fitting procedure on a Taylor series

expansion about the estimated values.

94
DETERMINATION OF SURFACE COMPOUND FORMATION

BY BACKSCATTER MOSSBAUER SPECTROSCOPY

presented by H. Terrell, R. H. Forsyth and J. J.

Spijkerman at the Abstracts Symposium "Fundamental

Corrosion Research in Progress" held March 12-21,

Cleveland, Ohio.

The detection and identification of a surface compound formation on a steel surface was made using a backscatter Mossbauer effect technique. The compound was

identified as 3-FeOOH from Mossbauer effect parameters

of iron oxide and oxy-hydroxide compounds . An estimate

of the compound thickness is given based on a calculation

of the backscatter amplitude versus distance into the

sample. This technique can be used to identify corrosion

products on the surface of iron bearing materials

.

95
4. NUCLEAR INSTRUMENTATION

A

Introduction

This group is now comprised of two projects. One project is closely tied to other projects in the section and

provides assistance on design and fabrication of equipment

of specific use on a single task. In addition, a maintenance

service is provided for all who wish it within or outside of

the section. The other project in the nuclear instrumentation group devotes its efforts to special long term design

and development problems. For example, this effort is now

focused on a major data handling and computation system for

the Mass Spectrometry Section, 310.06.

B

Overflow Counter

1. Introduction

In the taking of Mossbauer data, a large number of counts

must be accumulated in order that the Mossbauer spectrum can

be defined with high statistical precision. The spectra

5 accumulated are contained in the 10 memories of our multichannel analyzers but the counts in the base line of the

spectra are frequently on the order of 10 counts/channel.

For example, in Figure 36, a Mossbauer spectrum could be as

shown with A overflows where A is frequently on the order of

100-200. Because of the large number of overflows, it is

difficult to keep track of them accurately. Before the counter was designed and built, the method of computing the number

of overflows was to measure approximately the count rate and

then multiply it by the elapsed time. When this method was

used and there were a large number of overflows ( >100), one

could not hope to calculate the correct number of overflows

to better than ±2.

A counter has been designed which is gated on by one of

the channels, channel 100, and this counts all of the overflows in either channel number 100, or in the second sub-group

(200-399), channel 300.

96
z+icr -i

Z + 40,000 -

Z + 30,000

Z + 20,000

z+o

200

Figure 36. Diagram depicting overflow of memory

for pulse height analyzer

2 . Description of Logic

In Figures 37 and. 38 are shown the logic signals which

control the overflow counter and the logic diagram of the

counter, respectively. The signals in Figure 37 represent

the timing of the memory cycle of our multichannel analyzer

and the timing of signals which affect the overflow counter.

The first thing that occurs is the pulse from the time base

oscillator (T.B.O.) which triggers the memory cycle. During

the memory cycle the sequence of events is as follows:

(1) Reset temporary store bistable (T.S.B.) if it had

stored a count in the previous memory cycle.

(2) If the T.S.B. had stored a count, add a count to

the contents of the data scaler.

(3) ADD-1 acts as a delay in the time mode so that the

T.S.B. count could be added to the data scaler.

97
5 ms \

GATE BISTABLE OVERFLOW COUNTER

TIME BASE OSC.

| TEMPORARY STORE BISTABLE

ADD COUNT TO DATA SCALER

Ul.5^LS-* ADD-I i

1

•-2.5/XS-* WRITE

•-l.5yu.S-* E.O.P.

^ i

CHANNEL

>-4^tS *

ADVANCE

DELAY

•—2.5/XS

»

1_

I 1

; TEMPORARY STORE BISTABLE r~i 1 I

READ

1 ADD COUNT TO DATA SCALER

POSSIBLE OVERFLOW COUNT

Figure 37. Timing of logic pulses in pulse height analyzer

(4) WRITE enters contents of data scaler into memory.

(5) END OF PROGRAM (E.O.P.) acts as a delay in the time

mode; at end of delay it produces the channel advance pulse

(6) DELAY acts as a delay in time mode.

(7) READ retrieves the contents of the new channel out

of the memory into the data scaler.

The timing of the gate bistable in the overflow counting

is shown in Figure 37. The gate bistable is set by the transition of the 100 bistable from the 100 to the 100 state. The

bistable is triggered by the channel advance pulse when the

address is in channel 99 (299). When the gate bistable is in

98
ALL DIODES DR435

ALL CAP .001 /IF ALL RESISTORS 5.IK

100

0"

100

T.B T ©-

pi cari CARRY OUT I05 BOARD

0"

")|-*-M—

BINARY SCALER

Figure 38. Logic diagram of overflow counter

its "set" state it allows pulses to reach the binary counter

and it remains in its set state until the next T.B.O. pulse.

It can be seen in Figure 37 that the T.S.B. can add a count to the contents of channel 100 while the gate bistable

is being reset. It is therefore necessary to delay the turn

off of the gate to insure that a T.S.B. generated overflow

would be counted. The type of gate being used has a time

constant for shut off of 5 ys , which meets the requirement

that T.S .B .-generated overflow pulses will not be cancelled.

The binary scaler is shown in Figure 39 • Each bistable

is read out by a lamp on the front panel which is appropriately labeled with its binary designation. Also on the front

panel is a switch to reset the scaler to zero.

(F. C. Ruegg)

C . Sequential Scanner to Control Frequency Synthesizer

1 Introduction

A special scanner has been designed to increment the

99
100
frequency of General Radio frequency synthesizer in synchronization with the address advance of a R.I.D.L. 34-12B

multichannel analyzer operating in the time mode . The synthesizer is an accurate source of frequency and it can be

used with instruments which are frequency controlled. If

the output of such an instrument is a frequency or if it can

be converted to a frequency, then it can be counted by the

analyzer obtaining a spectrum of output counts as a function

of synthesizer frequency.

An example of an application for such a system is to control the velocity of the NBS optical Mossbauer spectrometer.

The optical spectrometer uses a Michelson type interferometer

to sense the velocity. The output of the fringe sensor is a frequency which is directly proportional to the velocity.

This frequency is compared with the output of the synthesizer

and the velocity of the spectrometer is corrected to make the

two frequencies equal. Therefore, if the synthesizer's frequency is changed, the velocity of the spectrometer is correspondingly changed, i.e. the velocity is directly dependent

on the synthesizer's output frequency. The output of the

Mossbauer spectrometer detector system is counted by the

analyzer resulting in a spectrum of y-ray counts as a function of velocity (Mossbauer spectrum).

2 . Description of System

A block diagram of the system is shown in Figure 40. The

time base oscillator is used to increment the scaler and the

multichannel address simultaneously. The number stored in

the decades of the scaler is decoded from B.C.D. into decimal

for its front panel display (see Figure 39)- The ten line

decimal information from the first two decades is brought

out to the interface and transformed into the signals which

are necessary to control the remotely programmable decades

of the synthesizer, Figure 41.

(F. C. Ruegg)

101
TIME 8ASE GENERATOR

TIME BASE OUT

MULTICHANNEL ANALYZER

FREQUENCY

SYNTHESIZER

Figure 40 -lack diagram of sequential scanner

D. Circuit Change to Improve Reliability of Interface Units

On the whole, the reliability of the interface units [2]

has been quite good, but a failure mechanism occurred which

required circuit modification. After the teletypes had been

used for a substantial period of time, their commutators began to produce noise. The noise caused the gate control bistable to be set; this inhibits the datum in a channel from

being printed and replaces it with spaces. To stop the gate

control bistable from being triggered by the noise it was

necessary to modify the circuit. The modified circuit is

shown in Figure 42. Transistor Q6 is used to sense the signal coming from the commutator and its output triggers Q7

and Q8. To cancel the noise Q7 and Q8 have been converted

into a monostable action with a direct coupled input. The

monstable action has the effect of keeping the output of Q7

and Q8 constant for t seconds after the input signal from

102
SCALER

FEEDBACK

2' 2

i ,

4 4

i »

2 2

. k

T 1

i kVSAr- 20V

FREQUENCY

SYNTHESIZER REMOTELY

PROGRAMABLE

DECADE

Tl - TIO 2N404

Figure 4l. Logic diagram and schematic for interface of

pulse height analyzer and frequency synthesizer

103
Ql 2NI384

Q2-Q6ZN404

Q7 2NI307

Q8 2NI304

ALL DIODES DR435

18 ©

®

®

IOK

-I<H

hH<3-

.620

-20V

10 K<

8.2 K

GATE

CONTROL

2

DATA

GATES

Q2

h«HO-

+ 20V ^H<3-

Q3

>

Q5

®

®

24)

22)

© ® (S) (1)

BOARD 6

Figure 42. Board No. 6 of pulse height analyzer interface

104
Q6 changes state. Because the noise pulses have a duration

of 1 millisecond, they are cancelled if x is greater than

one millisecond.

(P. C. Ruegg and R. W. Shideler)

E. Time of Year (TOY) System

1. Time of Year Clock

To provide time of year information to the interfaceanalyzer data readout system, a long term clock had to be

obtained. Most of the clocks known to be available commercially which are capable of electronic readout with sufficcient resolution had drawbacks. Decimal days were judged to

be an inconvenient time form, and electronic scalers had a tendency to fail to change state in the digits that remained

unchanged for long periods. Also, all of the clocks available are very expensive. A new design was made in order to

overcome these deficiencies.

The design centers around an electromechanical digit

counter having individual contacts for electrical readout and

capable of very fast reset. Eight of these counters provide

999 days, 23 hours, and 59.9 minutes. A synchronous motor

with a cam-operated switch provides impulses at six second

(0.1 min) intervals driving the least significant digit

(Figure 43). Each digit provides a "carry" after the nine,

and relay logic performs the resets at 60 minutes and 24

hours . Provision is made to set the clock to any permitted

state, and a warning light and circuit shut off the clock in

the event of a power failure. Provisions were made in the

clock to allow time manipulations as described below. An

anticipated addition will increase the readout resolution to

. 01 minutes

.

(R. W. Shideler)

2. Time of Year (TOY) Clock Controller

Complete timing information for a given spectrum is only

obtained if one knows the start time, the stop time, and the

105
"6: © © _

o

o

rHO

>»

cmO

6

•H

Ehm

(D

hO

•H

[ft

S8> > S

» 5 CO OJ z S ? + + - 0©i

106
live time of the analyzer during the run. It is therefore

necessary to synchronize either the clock with the analyzer

or vice versa. A control circuit (Figure 44) was designed

which sets the clock ahead 0.1 minute when the analyzer is

manually set to "store" and prints this time only while holding the analyzer from storing. When the time pulse corresponding to the clock reading occurs, the analyzer is started

synchronously while the clock resumes normal timing. At the

end of the store cycle the analyzer automatically starts the

readout cycle. The clock is stopped and held until the time

has been read out and then restarted catching up by one time

pulse if necessary. The logic for these operations is performed by a system of relays which is contained in the clock

chassis . (R. W. Shideler and P. C. Ruegg)

P. Maintenance of Teletypes and Paper Tape Punches

The Teletype (Model 33-C-ASR) is used an an input-output

device for our digital multichannel pulse-height analyzers

and also for the purpose of providing a data link with a computer.

A system of preventive maintenance was established which,

through a scheduled examination of the equipment, will provide an opportunity to detect defective parts and thereby

minimize on-line failures. While the machines are being examined, dust and dirt are removed, and new lubricant is

applied.

It was observed that many of the failures which developed

were caused by the improper replacement of paper tape, which

resulted in jamming of the punch mechanism.

One other frequent problem is the rapid accumulation of

dirt on the "dash pot" plunger, which causes the carraige to

stick. The frequency of occurrence of this problem can be

reduced considerably by frequent cleaning of the plunger.

Other failures were of a nonrecurrent nature, which does

not reflect adversely on the equipment reliability.

107
CDo

in PCOO

J*oou

0)

<UO

CDe

•H

Eh

(D

hO

•H

108
The Teletype High Speed Tape Punch Set (BRPE) produces

a punched tape at a high rate of speed, and its mechanical

adjustments were found to be quite critical. The problem

which developed was the need to accurately adjust the tension

of the punch solenoid spring. This problem was overcome by

the purchase of a very delicate gram scale enabling a precise

adjustment of the tension. The only recurrent problem was

the punch block becoming jammed due to improper replacement

of new tape.

Since there are twelve Model 33 Teletypes and two High

Speed Punches (BRPE) a substantial supply of parts have been

ordered and stocked, so that the amount of equipment down

time could be reduced.

(M. E. Stalbird)

G. Interface Between Mass Spectrometer and Small Electronic

»

Computer

Design and fabrication of a system which causes a small

electronic computer to control a mass spectrometer has been

completed for the Mass Spectrometry Section. The system

which is described in the progress report of that section is

now operable.

(R. W. Shideler)

109
5. PERSONNEL AND ACTIVITIES*

A. Personnel Listing

Radiochemical Analysis Section

J. R. DeVoe, Section Chief (1)

L. A. Currie, Assistant Section Chief (1)

M. K. Oland, Secretary (1)

Instrumentation

P. C. Ruegg (1) Project Leader

M. E. Stalbird (1)

P. A. Lundgren (1/2)

R. W. Shideler (1) Project Leader

Mossbauer Spectrometry

J. J. Spijkerman (1) Project Leader

J. C. Travis (5/6)

P. A. Pella (3/4) One year leave of absence

K. R. Swanson (5/12) Bethlehem Steel Corporation,

(NBS Research Associate Industrial Cooperation

Program)

W. L. O'Neal (1)

L. J. Romanowski (1/4)

Nuclear Chemistry

L. A. Currie (1) Project Leader

G. Tassey (1/4)

M. C. Scheid (1/4) Summer Student

J. L. Burton (1/4) Summer Student

B. Publications

J. R. DeVoe and J. J. Spijkerman - "Mossbauer Spectrometry", Anal. Chem. _4C_, 472-489 (1968).

S. S. Nargolwalla, M. Crambes and J. R. DeVoe - "Techniques for Evaluation of Systematic Errors in the Activation

Analysis for Oxygen With 14 MeV Neutrons, Anal. Chem. 40 , 666-671 (1968).

L. A. Currie - "Limits for Qualitative Detection and

Quantitative Determination; Application to Radiochemistry "

Anal. Chem. 40_, 586-593 (1968).

P. A. Pella, A. R. Landgrebe, J. R. DeVoe and W. C. Purdy

"Differential Controlled-Potential Coulometry Utilizing Subtoichiometric Radioisotope Dilution", Anal. Chem. 39, 1781

(1967).

^Number in parenthesis indicates fraction of year on roster.

110
L. May and D. K. Snediker - "Criteria for Selection of

Absorber Mounting Materials in Mossbauer Spectroscopy",

Nuc. Instr. and Methods 55, 183-188 (1967).

A. R. Landgrebe, L. T. McClendon, J. R. DeVoe, P. A. Pella,

and W. C. Purdy - "The Application of Substoichiometric Radioisotopic Dilution Principles to Controlled-Potential Coulometry

and Solvent Extraction", Anal. Chim. Acta 39, 151-159 (1967).

F. C. Ruegg - "Multiplex for Dual-Spectrum Mossbauer Spectrometry", Proc . Second Symposium on Low Energy X- and Gamma

Sources and Applications, Austin, Texas, 157-17^ (1967).

M. G. Hollstein and J. R. DeVoe - "Determination of Medium

Weight Elements by Gamma-Excited X-Ray Fluorescence", Proc.

Second Symposium on Low Energy X- and Gamma Sources and Applications, Austin, Texas, U.S. Atomic Energy Comm. ORNL 11C-10,

Vol. 1, 583-602 (1967).

J. J. Spijkerman - "Applications of Mossbauer Spectroscopy

to Structure Analysis", Proc. Second Symposium on Low Energy

X- and Gamma Sources and Applications, Austin, Texas, U.S.

Atomic Energy Comm. ORNL 11C-10 , Vol. 1, 85-100 (1967).

J. J. Spijkerman, D. K. Snediker, F. C. Ruegg and J. R.

DeVoe - "Mossbauer Spectroscopy Standard for the Chemical

Shift of Iron Compounds", NBS Misc. Publ. 260-13, July 1967.

C. I. Wynter, P. Hambright, C. H. Cheek and J. J. Spijkerman - "Mossbauer Spectroscopy of Some Iron Porphyrins", Nature

216 , 105 (1967).

J. C. Love, G. Dzjzek, J. J. Spijkerman and D. K. Snediker

U.S. Atomic Energy Commission Report No. ORNL-P-3251 (1967).

R. R. Ruch and J. R. DeVoe - "Radiochemical Separations,

of Copper by Amalgam Exchange', Anal. Chem. 39., 1333-35 (1967).

G. W. Smith, D. A. Becker, G. J. Lutz, L. A. Currie and

J. R. DeVoe - "Determination of Trace Elements in Standard

Reference Materials by Neutron Activation Analysis", Anal.

Chim. Acta 38, 333-3^0 (1967).

C. List of Talks

J. R. DeVoe - "The Future of Mossbauer Spectrometry in

Chemistry" - Louisiana State University, New Orleans, May

1968.

Ill
J. R. DeVoe - "Modern Trends in Activation Analysis",

Local Chapter of American Chemical Society, Louisiana State

University, New Orleans, Louisiana, May 1968.

J. J. Spijkerman - "Aspects of Laboratory and Commercial

Instrumentation; Mossbauer Spectra of Exotic Nuclei", a panel

discussion Central Regional Meeting, American Chemical Society

May 1968.

J. J. Spijkerman - "The NBS Standard Reference Materials

for the Chemical Shift of Iron and Tin Compounds", ASTM

Committee Meeting, Chicago, Illinois, May 1968.

L. A. Currie - "Peak Height Method for Evaluating Nuclear

Spectra", National ACS Meeting, San Francisco, California,

April 1968.

L. A. Currie - Chairman of General Sessions on "Hot Atom

and Radiation Chemistry" and "Activation Analysis", Division

of Nuclear Chemistry and Technology, 155th National Meeting

of the American Chemical Society, April 1968.

J. R. DeVoe - "Fluorine Analysis with 14 MeV Neutrons",

National ACS Meeting, San Francisco, California, April 1968.

J. C. Travis - "The Mossbauer Effect in Nickel", Symposium, Mossbauer Effect Methodology, Chicago, Illinois,

January 1968.

J. J. Spijkerman - "Mossbauer Spectroscopy of Ni",

The Faraday Society, London, England, December 1967-

57

J. J. Spijkerman - "Spin Relaxation in Fe Mossbauer

Spectroscopy", Oxford University, Oxford, England, December

1967.

J. R. DeVoe - "Standard Reference Materials for Trace

Analysis of Biological Materials", Bio-Assay Conference,

Oak Ridge, Tennessee, November 1967.

F. A. Lundgren - "Non-reactor Methods of Activation

Analysis", Conference Analytical Chemistry in Nuclear Technology, Gatlinburg, Tennessee, October 1967.

J. J. Spijkerman - "The NBS Standard Reference Materials

for Chemical Shift", ASTM Committee Meeting, Boston, Massachusetts , 1967

L. A. Currie - "High Accuracy Calibration and Control

in Single-Channel Gamma Spectrometry", Pacific Conference

on Chemistry and Spectroscopy sponsored by the Society of

Applied Spectroscopy, Anaheim, California, October 1967.

112
L. A. Currie - "Criteria for Reporting Trace Amounts of

Radioactivity", 11th Conference on Analytical Chemistry in

Nuclear Technology sponsored by Oak Ridge National Laboratory,

Gatlinburg, Tennessee, October 1967.

113
6. ACKNOWLEDGMENTS

We wish to express our thanks to Dr. C. 0. Muehlhause

and his staff for their cooperation in making arrangements

for our use of the facilities in the NBS Reactor Building.

Further, the assistance of Dr. J. E. Leiss and staff of the

Radiation Physics Division for arrangements to use the LINAC

facilities is greatly appreciated. Use of the mass spectrometer of the Photonuclear Physics Section and the assistance of H. M. Gerstenberg are gratefully acknowledged.

Cooperation with R. B. Schwartz of the Radiation Physics Civision in assisting with the 45° room irradiations is gratefully acknowledged.

The assistance of Mr. J. D. Waggoner and Mrs. Gloria

Holmes for their part in modifying various parts of our

computer programs is greatly appreciated.

We wish to thank the Statistical Engineering Laboratory

and in particular, Dr. Brian Joiner, who has provided excellent consultation and who set up the OMNITAB routine for

resolving Mossbauer spectra.

Special appreciation is expressed to Mrs. M. Oland who

has assumed the almost overwhelming burden of section secretary and in spite of it all still managed to type this

entire report

114
7. REFERENCES

DeVoe, J. R. , editor, NBS Technical Note 428 (1967).

2. DeVoe, J. R. , editor, NBS Technical Note 421 (1967).

3. Extended abstract of a paper presented at the 11th Conference on Analytical Chemistry in Nuclear Technology,

Gatlinburg, Tennessee, October 1967-

4. Currie, L. A., Anal. Chem. 40_, 586 (1968).

5. Extended abstract of a paper presented at The Pacific

Conference on Chemistry and Spectroscopy, Anaheim,

California, October 1967.

6. Extended abstract of a paper presented at the 155th

National Meeting of the American Chemical Society, San

Francisco, California, April 1968.

7. Natrella, M. G. , "Experimental Statistics", National

Bureau of Standards Handbook No. 91, U.S. Gov't. Printing Office (1959) .

8. Muir, A. H., Jr., Aido, K. S., Coogan, H. M. , "Mossbauer

Effect Data Index (1958-1965)", Interscience , New York,

1966.

9. Muller, James G. and Hang Nam, 0. K., "Mossbauer Effect

Methodology", Vol. 4, Plenum Press, New York, N. Y.,

(to be published).

.0. Hershkowitz, N. and Walker, J. C, Phys. Rev. 156 , 391

(1967).

.1. Stevens, K. W. H., Proc. Phys. Soc. A6J^, 209 (1952).

.2. Griffith, J. S., "The Theory of Transition-Metal Ions",

(Cambridge University Press, Cambridge, England, 1961).

Locher, P. R. , and Geschwind, S., Phys. Rev. Letters 11,

333 (1963).

Collins, R. L., and Travis, J. C, "Mossbauer Effect

Methodology", Vol. 3, Plenum Press, N. Y., 1967, pp. 123-

162.

Travis, J. C, and Spijkerman, J. J., "Mossbauer Effect

Methodology", Vol. 4, Plenum Press, New York (to be published) .

115
16. Draper, N. R. , and Smith, H., "Applied Regression Analysis", John Wiley, New York, N.Y. (1966).

17. Herber, R. H., and Spijkerman, J. J., J. Chem. Phys. 42 , 4312 (1965).

18. Snediker, D. K., "Mossbauer Effect Methodology", Vol. 2,

Plenum Press, New York, 1966, pp. 161-170.

19. Ruegg, P. C, Spijkerman, J. J., and DeVoe , J. R. , Rev.

Sci. Instr. 36_, 356 (1965).

20. May, L. , and Snediker, D. K., Nucl. Instr. Methods 55.,

183 (1967).

21. Kolthoff, I. M. , and Sandell, E. B., "Textbook of Quantitative Inorganic Analysis", The Macmillan Company, New

York, N. Y., 3rd ed., 1952, p. 318.

22. DeVoe, J. R., editor, NBS Technical Note 404 (1966),

p. 108-115.

23. Marguelies, S., and Ehrman, J. R. , Nucl. Instr. Methods

12, 131 (1961).

24. Abramowitz, M. and Stegun, I. A., eds . , "Handbook of

Mathematical Functions", AMS 55, U.S. Gov't. Printing

Office, Washington, D.C., 1966, p. 378.

116
APPENDIX I

OMNITAB MOSSBAUER SPECTRA FOR TRAVIS BY B.L. JOINER 3/13/68

TITLE1 PARABOLIC BACKGROUND, PURE QUADRAPOLE SPLITTING

TITLE2 AMMONIUM-9-MOLYBDONICKELATE FITTED 3/15/68

$

$ 1 $ 3 1 1.0

$ Y = B + L-X + Q"X" 5C 2 - A-SUM1

$ J=ll

$ 1 1G-SUB-J « GAMMA + DELTA1 SQUARED

$ 1 1.0 + 1 1 $ 1 1 H 1 $ 1 $ (G-SUB-J = 10,-2,-8) 1

$

$

$$

$

$

$ 00599 07217

$ (NH4)6NIM09032

HEAD 2/ CHANNELS

HEAD 5/ OBSERVATIONS

HEAD 6/ OBS-PARAB

HEAD 16/ FITTED

HEAD 17/ DEVIATIONS

FORMAT A (1P9E13.5)

DIMENSION 200 BY 25

SET 1 SMALL G(J)

10 -2 -8

GENERATE 7.0(1.0) 200.0 IN 2 THIS IS X = CHANNELS

SUB 100.0 2 2 MULT 2 -1.0 2 MULT 2 2 3 X-SQUARE

SET ROW 2 OF COL 4 STARTING VALUES FOR B,L,Q, A, GAMMA, DELTA,

H

4683.5 1.1358 .0652 1355.17 .5511 1.05936 9.35807

RESET NRMAX TO

SET Y IN COL 5 05350 05951 05543 06475

06400 05211 06234 04972 03981 05039 04911 05493 05760 05055

04707 05132 04490 05010 04918 04668 05716 04212 04343 03674

04980 04495 05822 05288 05208 05270 04965 04618 05130 04192

117
05434 05188 04548 04998 04183 04965 04691 05417 05619 04386

04881 04665 04961 02864 05594 04856 04973 05118 05856 04420

04704 04736 05283 04368 05965 05241 04833 05400 05166 04813

05091 04836 04537 04097 04352 05407 03702 04635 04851 04306

03747 03825 02859 03823 04625 03661 03903 02072 04157 03511

02682 02844 02828 02391 02048 02176 01989 02236 01006 02114

01841 01347 01930 02312 01802 02848 03511 04009 03163 04602

04933 03788 03958 03501 03642 05195 04373 05271 04501 04621

04864 04729 03954 05478 04832 03775 04159 05582 03649 04510

04447 05606 04901 05854 04950 04,369 04372 05084 04638 03984

05241 05145 05705 03893 05533 04910 04609 04549 04252 04021

04469 05057 04670 05259 04146 05257 04415 04310 05262 04311

04089 04739 04698 04424 05304 05516 05304 04707 06454 04985

05031 05357 04845 05360 04732 06107 04831 04258 05350 05131

04301 04918 05244 04294 05518 04516

PLOT 5 VS 2 ADD 0.0 1.0 7 DEFINE 1.E20 1,4

10/ MULT -1,1" G(J) BY "6,4" GAMMA INTO 8 11/ INCREM 10 -1,0" "0,0"

12/ ADD "7,4- DELTA TO 8 IN 8 13/ SUB 2 8 8 14/ DIV 8 -8,4- 8 (P(J)-X)/H

15/ MULT 8 8 9 T(J)

16/ ADD 1.0 9 10

17/ DIV 1.0 10 10

18/ MULT 10 10 11

19/ MULT 8 11 2.0 12 PARTIAL (DELTA)

20/ MULT "1,1" G(J) BY 2.0 20

21/ INCREM 20 "1,0" 0.0

22/ MULT 20 8 11 13 PARTIAL (GAMMA)

23/ DIV 9 -8,4- 20

24/ MULT 20 -2.0 11 14

25/ SUB 11 15 15

30/ ERASE 12 13 14 15

31/ EXECUTE 10 25 3 32/ RESTORE 10 "1,1" "6,4" 8 33/ RESTORE 20 -1,1- 2.0 20

34/ MULT 15 "5,4 16

35/ MULT 14 "5,4" 14

36/ MULT 13 "5,4" 13

36.5/ DIV 13 -8,4- 13

37/ MULT 12 "5,4- 12

37.5/ DIV 12 "8,4" 12

40/ SUB -2,4- 5 6 41/ MULT "3,4" 2 -1.0 6 42/ MULT -4,4- 3 -1.0 6 43/ SUB 16 6 17 44/ PLOT 16 6 VS 2

118
OMNI TAB MOSSBAUER SPECTRA FOR TRAVIS BY B.L. JOINER 3/13/68

45/

60/

61/

62/

63/

64/

65/

66/

79/

80/

PLOT 17

SFIT 17

MTRANS

ABRI DGE

DEFINE

ADD 18

MTRANS

ABRIDGE

MULT i{ IFGT I

7 1,4

A

.0 41,4

A

199

F

VS

7

22 20,

200

1,4

420, 1 199

1" 1 1,20-

15

200

13

, 1

199, 1 1

::::: 00002

12 14 18 19

20

IS GREATER THAN : 200, 1

: STOP

EXECUTE 30 80 TIMES

60/ FIT

EXECUTE

17 7 30 80

2 3 15 13 12 14 18 19

FIXED 1 PRINT 2 PUNCH 2 SUB 19

FIT 20

STOP

6 16 17 5

6 16 17

17 20

7 7 2 3 15 13 12 14 18 19

119

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