UNITED STATES EPARTMENT OF OMMERCE

| PUBLICATION

f w

U>rary. toi ^^ ffl(fg_ JAN 2 3 1970

NBS TECHNICAL NOTE 501

Radiochemical

Analysis Section:

Mbssbauer Spectroscopy

Nuclear Chemistry

Nuclear Instrumentation

Radioisotope Techniques

Summary of Activities

July 1968 to June 1969

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' 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 four broad program

areas. 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, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with 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 Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering -—Time and Frequency -'—Astrophysics -—Cryogenics.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops,

produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological

standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation —Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation tb the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition,

and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques,

and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information ' —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

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

- Located at Boulder. Colorado 80302.

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

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 501

ISSUED FEBRUARY 1970

Nat. Bur. Stand. (U.S.), Tech. Note 501, 149 pages (Feb. 1970)

CODEN: NBTNA

Radiochemical Analysis Section:

Mossbauer Spectroscopy Nuclear Chemistry

Nuclear Instrumentation Radioisotope Techniques

Summary of Activities

July 1968 to June 1969

Edited by James R. DeVoe

Radiochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

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

(Order by SD Catalog No. CI 3.46:501 ), 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, 196H 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

inportance also are the numerous small items which are often

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

warrent 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.

11
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.

We plan to issue these summaries for all of our

Sections. The following is the sixth annual report on

progress of the Radiochemical Analysis Section.

W. Wayne Meinke, Chief

Analytical Chemistry Division

111
TABLE OF CONTENTS

PAGE

1. INTRODUCTION 1

2. MOSSBAUER SPECTROSCOPY 2

A. Introduction 2

B. Instrumentation 2

1. Helium Temperature Cryostat for °ljji

Scanning 2

2. Simple Variable Temperature Cryostat for

Temperature Range of 100 to 373 K 3

3. Furnace for Absorber Temperature Studies

from 300 to 1000 K 6

4. Conversion Electron Proportional Counter

for Spectral Measurement with Backscattering

Geometry 6

5. Resonant Detector 12

a. Detection of MSssbauer Events by

Internal Conversion Electrons: Preliminary Theoretical Studies 12

b. Design of Resonant Detector 16

C. SRM for the Isomer Shift of Tin Compounds ... 19

D. Constrained Fitting of Complex MSssbauer Spectra 19

E. Nickel-6l Hyperfine Interactions 28

1. Internal Magnetic Fields in Nickel

Compounds 28

2. Quadrupole Interactions in Planar Compounds. 34

F. MSssbauer Spectroscopy of Iron Doped TiO

?

. ... 35

1. Introduction 35

2. Experimental 36

3. Results 37

4. Theoretical 44

G. Towards a Hartree-Fock Crystal Field Treatment

of the Electrons on Mossbauer Nuclei 49

3. NUCLEAR CHEMISTRY

A. Introduction 52

B. The Discovery of Errors in the Detection of

Trace Components in Gamma Spectral Analysis. . . 52

C. The Significance of Gain Shift 60

D. Quantitative Preparation and Analysis of

Gas Samples 66

1. Argon in Air and in Carbon Dioxide 66

2. Vacuum System Used to Prepare Ar, He

and Li Samples 69

iv
PAGE

E. Photonuclear Yield Analysis 77

P. Indirect Variance Estimation in Chemical

Systems 8l

G. High Sensitivity (Radio)Chemistry 84

NUCLEAR INSTRUMENTATION 89

A. Introduction 89

B. Optical Drive 89

1. Introduction 89

2. Drive Electronics 91

a. Drive Amplifier 91

b. Photocell Trigger Circuit 9^

c. Logic Circuit 96

d. Physical Description 98

3. Frequency Ratio Detector 98

a. Introduction 98

b. Description 103

4. Control Logic for Frequency Synthesizer . . 103

a. Introduction 103

b. Interface Description 103

RADIOISOTOPE TECHNIQUES

A. Introduction 106

B. Error Analysis of Counting System 107

1. Instrumentation 107

2. Calibration 107

C. Application to Standard Reference

Materials (SRM) 109

1. Determination of Tungsten in Molybdenum

Containing Materials Using Toluene-3,

4-Dithiol Ill

a. Experimental Ill

b. Results and Discussion 112

2. Determination of Iron in SRM 1159 and 1160 113

a. Experimental 113

b. Results and Discussion 113

PERSONNEL AND ACTIVITIES 115

A. Personnel Listing 115

B. Publications and Manuscripts 116

C. Talks 117

v
PAGE

7

.

ACKNOWLEDGEMENTS

8. REFERENCES

APPENDIX I. Evaluation of Integrals

Appendix II. Wiring List for Mossbauer Spectrometer . 128

LIST OP FIGURES

FIGURE NO. PAGE

1. Low temperature cryostat 4

la. Source absorber assembly for low temperature

cryostat 4

2. Heater for source-absorber assembly of simple

helium cryostat 5

3. Exploded diagram of high temperature

furnace for absorber 7

4. Photograph of detector for conversion

electrons 8

5. Diagram of detector for conversion

electrons 8

6. Mossbauer spectrum of iron metal layer (2950 A)

evaporated on stainless steel 9

7. Mossbauer spectrum of iron metal layer (600 A)

evaporated on stainless steel 10

8. Backscatter amplitude vs. depth of penetration

into sample 12

9. Energies and relative numbers of conversion

and photoelectrons vs. depth into iron foil . . 16

10. Mossbauer spectrum of stainless steel by

conversion electron detection 17

11. Signal to noise (S/N) ratio as a function

of single channel setting using conversion

electron detection from stainless steel .... 18

vi
PAGE

Tin Mossbauer spectrum of (CH^)pSnFp 20

Nickel Mossbauer spectrum of NiCrpOh at 4 . 2 K . 22

Nickel Mossbauer spectrum of NiBr

?

at 4 . 2 K . . 23

Iron Mossbauer spectrum with one magnetic site 24

Iron Mossbauer spectrum with three magnetic

sites 25

Iron Mossbauer spectrum with two magnetic

and one single line 26

Iron Mossbauer spectrum with two sites of

different electric field gradient and one

single line 27

Nickel Mossbauer spectrum in NiCl ? -6H

p

at 12 K . . 30

Nickel Mossbauer spectrum in NaNiOp at 4 . 2 K . 31

Ni ckel Mossbauer spectrum in NISOk at 4 . 2 K . . 32

Nickel Mossbauer spectrum in Nil p at 4 . 2 K . . 33

Comparison of 1.0 mole % oxidized powder

spectrum with 0.25 mole % growth-doped

spectrum of M. Alam 38

Spectrum of reoxidized iron doped rutile powder

showing a trace of possible interstitial site . 39

Spectrum of reoxidized iron doped rutile powder

at 300 K showing reduction of intensity in the

site with large splitting 40

26. Spectrum of reduced iron doped rutile powder at

300 K 41

27. Temperature dependence of reduced iron doped

rutile powder 42

28. Diagram showing fitted peak intensities and

positions 43

29. Comparison of iron doped single crystal and

powdered rutile 45

Vll
30. Spectra of single crystal rutile as a function

of depth 2|6

31. Temperature vs. splitting of outer doublet for

iron doped reduced form of powdered rutile . . 47

32. Energy level diagram of 3 d levels used for

interpretation of data in Figure 31 48

33- Plot of normalized residuals from the least

squares analysis of 22j\j a> 5^Mn, and background 53

34. Normalized residuals from the analysis of Na,

137cs, 54Mn; correct model (horizontal lines

are drawn at ±2a) 58

24

35. Normalized residuals from the analysis of Na,

137cs, 5^ Mn ; wrong model (horizontal lines are

drawn at ±2a) 59

36. Neutron density wire spectrum fitting residuals

for sample I-3B vs. standard #2 (Co.) 60

37- Peak channel drift as a function of time: long

term stability 62

38. Peak channel drift as a function of time: short

term stability 63

39. System gain as a function of phototube high

voltage 64

40. Peak area as a function of number of channels

analyzed by "LSNORM" 65

41. Gas chromatograph with thermal conductivity

detector 67

42. Glove box setup for lithium sample preparation 68

43. Thermal conductivity cell response to Q>2> ^o

and Ar as a function of sample volume 69

44. Gas chromatogram of pure Ar 70

45. Gas chromatogram of air 70

vm
PAGE

46. Gas chromatogram of early sample taken from

glove box as it was being purged with Ar . . . . 71

47. Gas chromatogram of later sample from glove box 71

48. High vacuum system with dual-trace recorder for

simultaneous monitoring of sample temperature

and pressure 72

49. Recorder response as a function of vacuum

system pressure 73

50. Recorder response as a function of sample oven

temperature 73

51. Topological diagram of high vacuum system ... 74

52. Pressure and temperature response curves

obtained by melting a (C.lg) Li sample in

which Ar at a pressure of 100 torr has been

frozen 75

r q 53. Experimental yields for the reaction Cu(y } n) 80

54. Limits for a./aR vs p„ R 83

55. General scheme for low level radiochemical

separations and identification 86

56. Mass separator experiment 87

57. Block diagram of the optical drive 90

58. Photograph of Dqppler drive electronics .... 92

59. Schematic diagram of drive amplification circuit 93

60. Schematic of the photocell trigger circuit. . . 95

61. Schematic of logic circuit 97

62a. Layout of front panel 99

62b. Layout of chassis . . 99

62c. Layout of rear panel 99

IX
PAGE

63. Amplifier, printed circuit card 100

64. Photocell trigger, printed circuit card .... 101

65. Logic, printed circuit card 102

66. Block diagram of logic for control of

synthesizer 104

67. Photograph of radiation detector assembly . . . 108

68. Photograph of electronic apparatus medium

count-rate capability 109

LIST OF TABLES

TABLE NO.

1. Photoelectron and conversion electron

production in iron metal 15

2. Percent effect vs. thickness of dimethyl

tin difluoride 19

3. Internal magnetic fields measured by nickel

Mossbauer spectroscopy for selected nickel

compounds 29

22

4. Detection limits for Na in various radionuclide mixtures 55

64

5. Effects of model errors on the mixture Cu,

22 Na, and 54 Mn 57

6. Pemtomole chemistry 88

7. Elements that can be analyzed by the substoichiometric radioisotope dilution analysis

technique in the Standard Reference Materials

Program 110

8. Analysis of iron in standard reference

materials 114
RADIOCHEMICAL ANALYSIS: MOSSBAUER EFFECT, NUCLEAR

CHEMISTRY, NUCLEAR INSTRUMENTATION, STATISTICAL ANALYSIS

AND RADIOISOTOPE TECHNIQUES

July 1968 to June 1969

Edited by James R. DeVoe

ABSTRACT

This is the sixth summary of progress of the Radiochemical Analysis Section of the Analytical Chemistry Division

at the National Bureau of Standards

.

The section's effort comprises five major areas:

Mossbauer spectroscopy, nuclear chemistry, nuclear instrumentation, radioisotope tracer techniques and the application

of statistics in nuclear and analytical chemistry.

Low temperature devices for Mossbauer spectroscopy are

a subject of continuing Interest, and this year's effort is

centered around the design of variable temperature devices.

A high temperature device is also described.

Preliminary data on the detection of conversion electrons show the potential for Mossbauer spectroscopy in the

field of chemical bonding in surfaces.

Further exploration of the potential of Mossbauer

spectroscopy for the measure of structural parameters of

nickel compounds have been made

.

Preliminary data are presented on the structure of iron

doped rutile (TiOp) which is of significance to the determination of mineral structures found on the moon. Theoretical predictions of Mossbauer spectra resulting from electric

field gradients due to crystal field interactions are presented. Evaluation of the basic integrals using the selfconsistent Hartree-Fock method is given for ions of the first

row of transition elements.

XI
Precautions to be taken in the analysis of gamma-ray

spectra are presented when model errors exist and when a gain shift is detected.

A system for gas analysis which utilizes gas chromatography is described. Its application to nuclear cross

section studies is also presented. A method for setting

limits on sample heterogeneity using a specific analysis

method provides a useful indicator for evaluation of the

adequacy of standard reference materials

.

An interesting presentation of detection and measurement in the femtogram region is given.

The complete circuitry for the optical interferometric Mossbauer spectrometer is presented in this report.

Quantitative analysis for molybdenum and iron using

radioisotope dilution to a precision that rivals most

other techniques is described.

James R. DeVoe , Chief

Radiochemical Analysis Section

Analytical Chemistry Division

Key Words

:

Standard Reference Materials; cross sections: cryostats;

Mossbauer spectrometry; nickel-6l; rutile; structure

analysis; counting statistics; heterogeneity; proportional

detector; conversion electrons; electric field gradient

tensor; internal magnetic field; self-consistent crystal

field; calculations; interferometric ; Mossbauer spectrometer; gas analysis; radiochemistry ; electronics; radioisotope dilution; molybdenum; iron.

XII
1. INTRODUCTION

The Radiochemical Analysis Section develops measurement

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

techniques require a very interesting and stimulating blend

of knowledge ranging from nuclear and atomic physics through

nuclear chemistry and physical chemistry to descriptive

chemistry

.

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.

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.

In order to specify adequately 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

1
2. MOSSBAUER SPECTROSCOPY

A

.

Introduction

Our efforts remain centered about improving instrumentation, spectral interpretation, and of most importance, the

establishment of suitable standard materials for the purpose

of improving the accuracy of spectral data. The basic design for the constant acceleration interferometer-controlled

Mossbauer spectrometer was completed. Refinements made on

the system have resulted in a spectrometer with exceptionally

fine qualities. A resonant detector was made operational.

Its performance has provided much information on the parameters of design that must be considered to optimize the

overall efficiency of such detectors. Another very significant enhancement of our capability is the availability of

several versatile systems which allow measurement of Mossbauer

spectra at temperatures ranging from 4 to 1000 K.

Another major accomplishment relates to improvements

made on the nonlinear least squares fitting procedures

which incorporate a modified convergence criterion and the

use of basic structural parameters. This work, coupled with

mathematical procedures for calculating electric fields from

single crystal data, has enhanced our capability for interpretation of spectra.

Therefore, the group is now in the excellent position

of having done its "homework", and it now enters a period of

anticipated high productivity. Heading the list are standard

materials in the form of iron metal for velocity calibration

and bench marking isomer shift, a recalibration of sodium

nitroprusside for the isomer shift standard, and a calibration of dimethyl tin difluoride for the isomer shift standard.

B. Instrumentation

1 . Helium Temperature Cryostat for Ni Experiments

The low recoil-free fraction (f) observed in nickel

compounds requires Mossbauer measurements to be made at

2
liquid helium temperatures. Since the attenuation of 67.^

keV gamma radiation by borosilicate glass is small, a simple

helium cryostat was designed for cooled source and absorber,

with rapid sample changing capability. The cryostat is

shown in Figure 1. The glass Dewar (A) is suspended from an

aluminum support plate (B) by a split clamping ring (C) and

Teflon* gasket (D) . The Mossbauer drive is mounted on the

aluminum support plate with an O-ring seal (E), and the sourceabsorber holders (P) are rigidly attached to the drive (G),

such that the drive and source-absorber assembly can be lifted

out of the cryostat. The source and absorber assembly is

shown in Figure 1-a. The 1/8 inch diameter tubing is rigid

enough to prevent vibration and has low thermal conduction.

The source is stationary and the absorber is mounted on a flexure plate (I), and coupled to the drive by a yoke (J)

and 1/8 inch stainless steel tubing (K). Source or absorber

can be changed in 5 minutes. The 67.^ keV gamma-ray attenuation is 15%, and liquid helium consumption is 10 h/liter.

(D. Sweger and J. J. Spijkerman)

2 . Simple Variable Temperature Cryostat for

Temperature Range of 100 to 373 K

The cryostat described in Reference [1], p25, has been

modified for variable temperature use, by inserting a carbon

cloth heater between the heat sink and the sample mounting

block. The 1 inch by 1.5 inch carbon cloth is clamped between

two copper foil electrodes, and insulated by 1/16 inch Teflon*

plates, (see Figure 2). The heater is operated by a 6.3 volt

filament transformer connected to a temperature controller or

variable transformer for manual control. For manual control,

the temperature is stabilized after 30 minutes to within ± 2°C over a period of 6 hours and is measured by a copper

constantan thermocouple.

(P. R. Stampfl and J. J. Spijkerman)

*For disclaimer of equipment and materials see last paragraph

of the Introduction.
Figure la

Figure 1. Low temperature cryostat. Fig. la. Source

absorber assembly for low temperature cryostat
HEATER

SIDE FRONT

XL

CZE

n=i«-

nzi*-

HEATER DETAIL

1/16 in. TEFLON

LEAD

.005 in. COPPER FOIL

CARBON CLOTH

1/16 in. TEFLON

Figure 2. Heater for source-absorber assembly of simple

helium cryostat.

5
3

.

Furnace for Absorber Temperature Studies from 300

to 1000 K

Conventional type furnaces are not very well suited for

Mossbauer work due to their size and difficulty in sample

mounting. By reducing the length of the furnace, the time

to accumulate data can be greatly reduced thereby also

reducing the requirement for long-time temperature stability.

However, the temperature uniformity across the sample is

affected. This can be eliminated if the sample is placed in

contact with the heater, as shown in Figure 3- The sample

is clamped between two BeO disks, 2 mm thick, which are in

direct contact with two carbon cloth heaters. Thin sheets

of nickel plated mica provide a radiation shield, and alumina rings are used for support and temperature insulation.

Including the vacuum jacket, the furnace has a 2 inch outer

diameter and is 1.5 inches long. Temperatures of 1000 K

are obtained and were measured with a chromel-alumel

thermocouple

.

(J. J. Spijkerman)

4

.

Conversion Electron Proportional Counter for

Spectral Measurement with Backscattering Geometry

Experiments with the pancake detector described in

Reference [1], p. 28, for detecting conversion electrons

showed that spectra with 16% effect above baseline could

be obtained. A new detector was designed and optimized for

conversion electron detection (see Figures 4 and 5). The

detector has two 0.001 inch wire electrodes, spaced 1/4 inch

apart. The detector operates at 1200 to 1300 V and uses 10$

CH^-90% He flow gas. The effective penetration depth into

the sample for Mossbauer analysis was determined from the

spectra (shown in Figures 6 and 7) of iron evaporated on

stainless steel foils. The contribution from a layer at a

distance (y) from the surface to the Mossbauer backscattering
Top plate

Alumina insulator

Mica heat shield

BeO Sample holder

and temperature

equalizer Sample

mil Aluminum Window

Vacuum can

Carbon Cloth heater

Alumina insulator

Mica heat shield

Bottom plate

<S>

Figure 3. Exploded diagram of high temperature furnace

for absorber.
ho

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O o P M o +3

CD O

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(U H

T3 CD

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in*

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HEATER DETAIL

1/16 in. TEFLON

LEAD

.005 in. COPPER FOIL

CARBON CLOTH

1/16 in. TEFLON

Figure 2. Heater for source-absorber assembly of simple

helium cryostat.

5
3 • Furnace for Absorber Temperature Studies from 300

to 1000 K

Conventional type furnaces are not very well suited for

Mossbauer work due to their size and difficulty in sample

mounting. By reducing the length of the furnace, the time

to accumulate data can be greatly reduced thereby also

reducing the requirement for long-time temperature stability.

However, the temperature uniformity across the sample is

affected. This can be eliminated if the sample is placed in

contact with the heater, as shown in Figure 3. The sample

is clamped between two BeO disks, 2 mm thick, which are in

direct contact with two carbon cloth heaters. Thin sheets

of nickel plated mica provide a radiation shield, and alumina rings are used for support and temperature insulation.

Including the vacuum jacket, the furnace has a 2 inch outer

diameter and is 1.5 inches long. Temperatures of 1000 K

are obtained and were measured with a chromel-alumel

thermocouple

.

(J. J. Spijkerman)

h . Conversion Electron Proportional Counter for

Spectral Measurement with Backscattering Geometry

Experiments with the pancake detector described in

Reference [1], p. 28, for detecting conversion electrons

showed that spectra with 16% effect above baseline could

be obtained. A new detector was designed and optimized for

conversion electron detection (see Figures 4 and 5). The

detector has two 0.001 inch wire electrodes, spaced 1/4 inch

apart. The detector operates at 1200 to 1300 V and uses 10%

CHn-90% He flow gas. The effective penetration depth into

the sample for Mossbauer analysis was determined from the

spectra (shown in Figures 6 and 7) of iron evaporated on

stainless steel foils. The contribution from a layer at a

distance (y) from the surface to the Mossbauer backscattering
Top plate

Alumina insulator

Mica heat shield

BeO Sample holder

and temperature

equalizer Sample

Bottom plate

I mil Aluminum Window

Vacuum can

Carbon Cloth heater

Alumina insulator

Mica heat shield

Figure 3. Exploded diagram of high temperature furnace

for absorber.

7
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CD

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intensity for resonant velocity can be represented by

arc cos y/R /°° _ „ -, rare cos y

dEe n

aJ,a °lH y ' -^—^

J

sin 6 d 9 (1)

E + T

s /4

Where 9 = angle of scattering from the normal to the surface

C = constant

y = absorption coefficient for the incident 14.4 keV

gamma ray

R = the range of an 8 keV electron in iron.

E = Mossbauer gamma-ray energy

n = number of resonant atoms per cm in the absorber.

f = fraction of recoilless absorption in absorber.

a

v

0-, u = cross section for resonant absorption.

T = half width of source.

Since R is very small, the attenuation of the 14.4 keV gamma

radiation can be neglected, and y<R,

T(y) =C'| (l - |) (2)

and the backscattering intensity I(y) is

I(y) = T(Y)dy = C ( 2L. _ L (3)

\2R 3R 2

/

normalizing, such that I(R) = 1

(4)

The backscattering amplitude I(y) as a function of depth into

the sample is shown in Figure o, for R=1200 A. The Mossbauer

backscatter intensity for 600 A iron on stainless steel foil

(Figure 7) was 65% . From Figure 8, this would represent the

o effective penetration depth of approximately 1000 A. However,

2950 A of iron (Figure 6) on stainless steel foil gave 96%,

11
1000 1200 A

DEPTH

Figure 8. Backscatter amplitude vs. depth of penetration

into sample.

with h% of the intensity originating from the stainless steel

foil. This indicates that there is a serious difficulty with

the model for the range of 8 keV electrons in iron. Further

experimental work is in progress.

(J. J. Spijkerman)

5 . Resonant Detector

a . Detection of Mossbauer Events by Internal

Conversion Electrons: Preliminary Theoretical

Studies

The resonance detection should provide unambiguous

recognition of a Mossbauer event characterized by the reemission of the gamma photon one time out of ten, or the

emission of an internal conversion electron and its attendant

12
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intensity for resonant velocity can be represented by

/°° -. /-arc cos y/R

dEe na*a °lh y '-=—^-= / sin 9 d 9

1 E^ + T^ /4 J

7

(1)

Where 9 = angle of scattering from the normal to the surface

C = constant

u = absorption coefficient for the incident lH.k keV

gamma ray

R = the range of an 8 keV electron in iron.

E = Mossbauer gamma-ray energy

n = number of resonant atoms per cm in the absorber.

f = fraction of recoilless absorption in absorber,

a

^

0-. u = cross section for resonant absorption.

T = half width of source

.

Since R is very small, the attenuation of the 14.4 keV gamma

radiation can be neglected, and y<R,

T(y) = 0' |

(l - |) (2)

and the backscattering intensity I(y) is

I(y) = T(Y)dy = C ( Z_ _ £_ } (3)

\ 2R 3R 2

/

normalizing, such that I(R) = 1

^) = 3(|)

2 - 2(^)

3

(4)

The backscattering amplitude I(y) as a function of depth into

the sample is shown in Figure 0, for R=1200 A. The Mossbauer

backscatter intensity for 600 A iron on stainless steel foil

(Figure 7) was 65%. From Figure 8, this would represent the

o effective penetration depth of approximately 1000 A. However

2950 i of iron (Figure 6) on stainless steel foil gave 96%,

11
1000 1200 A

DEPTH

Figure 8. Backscatter amplitude vs. depth of penetration

into sample.

with h% of the intensity originating from the stainless steel

foil. This indicates that there is a serious difficulty with

the model for the range of 8 keV electrons in iron. Further

experimental work is in progress.

(J. J. Spijkerman)

5 . Resonant Detector

a . Detection of Mossbauer Events by Internal

Conversion Electrons: Preliminary Theoretical

Studies

The resonance detection should provide unambiguous

recognition of a Mossbauer event characterized by the reemission of the gamma photon one time out of ten, or the

emission of an internal conversion electron and its attendant

12
x-ray nine times out of ten. Of these three indicators of

Mossbauer events, the photons may be confused with similar

photons entering the detector from the source and absorber,

and thus are hardly unambiguous. The internal conversion

electron, on the other hand, cannot be confused with electrons

originating outside of the counter, since these would never

reach the inside of the counter, but they can be confused

with photoelectrons and Compton electrons created in the

Mossbauer gamma ray and the higher energy gamma radiation

from the source.

The following paragraphs and charts outline some very

preliminary theoretical studies designed to predict the

contributions of the various electron producing processes

.

For this first study, several simplifying assumptions and

approximations have been made

:

1. Only internal conversion (Mossbauer) and photoelectrons have been considered. This is a good approximation

of the lU . 4 keV gamma ray, but not for those of 122 and 136

keV which should be Compton scattered to roughly the same

degree as photoabsorbed.

2. A natural abundance (in Fe-57) thick iron foil

has been assumed as the in-detector absorber, and the effect

of the detector walls has been neglected.

3. Auger electrons and secondary interaction electrons have been neglected.

4. Discrete gamma-ray energies have been assumed,

ignoring natural line broadening and Compton smearing of the

high energy gammas in any external absorber which may be

present

.

5. Only electrons traveling the shortest distance

from the point of origin to the iron surface have been considered. Angular distributions will be included in future

calculations

.

13
6. A "most probable" energy loss, as opposed to a distribution of energy losses, has been assumed for each

electron as a result of deceleration in the foil.

The number of electrons created over a small increment

of thickness at depth 'x 1 into the material is given by

N (x) = a I exp -(a + a.jN^-, x (c)

p po K

p M F v ^

for photoelectrons and

N

M

(x) = a MI o exp " (a

p

+ a M

)NF

x ' a/(] : ] (6)

for internal conversion electrons. Here I is the relative

o intensity of the parent gamma, a and aM

are the photoelectric and Mossbauer cross section, Nn is the number of

3

F

atoms per cm J in the foil, and a is the internal conversion

coefficient. These equations are used independently for each

of the incident gammas, with the appropriate intensities and

cross sections.

The most probable energy loss for an electron of

kinetic energy E, escaping from a depth of 'x' is taken to be

(4 E £y

log -=-^—=- - g

2 + 0.37 ) (7)

i (i-e )

3

2 = 1 - hll keV/(5H keV + E ) 1

K = 5.613 2 x 10 10 keV cm" 1

where I is the average ionization potential for the foil.

The resultant energy carried by the electron escaping from the

foil is simply E - AT , where E. = E - B, and B is the k. p k y electron binding energy.

Table I and Figure 9 show the calculated resultant

energy and relative intensities of Mossbauer and photoelectrons emerging from a specified depth within an iron foil for

the 14.4, 122, and 136 keV gamma rays and cobalt-57.

(J. C. Travis)

14
Table 1. Photo-electron and conversion electron production

in iron metal

.

Relative Intensities

Depth Photo- Mossbauer Resultant Gamma

(A)

electrons conversion energy energy

electrons (keV) (keV)

500 54510.9 498459 6.57612 14.4

1500 51909.6 474673. 5.38439 14.4

2500 49432.5 452021. 4.04075 14.4

3500 47073.6 430451. 2.61123 14.4

4500 44827.3 409910. 1.12123 14.4

25000 83582.7 111.097 122

75000 80628.7 102.703 122

125000 77779.1 93.6968 122

175000 75030.3 84.3441 122

225000 72378.6 74.7475 122

275000 69820.6 64.9623 122

325000 67353. 55.0232 122

375000 64972.6 44.954 122

425000 62676.3 34.7723 122

475000 60461.2 24.4913 122

525000 58324.4 14.1215 122

575000 56263.1 3.67144 122

25000 7218.94 125.358 136

75000 7044.06 117.598 136

125000 6873.^2 109.274 136

175000 6706.91 100.632 136

225000 6544.43 91.7652 136

275000 6385.9 82.7248 136

325000 6231.2 73.5428 136

375000 6080.25 64.2411 136

425000 5932.95 54.8359 136

475000 5789.23 45.3393 136

525000 5648.98 35.761 136

575000 5512.14 26.1088 136

625000 5378.61 16.3892 136

675000 5248.31 6.60758 136

15
1000

500

100

5

50

tx.

2

O

or

Energy of Photoelectrons from 122 keV Gamma

10

100

Number of Internal Conversion Electrons

Energy of Photoelectrons from 136 keV Gamma

Number of Photoelectrons from 122 keV Gamma

Number of Photoelectrons

from 14.4 keV Gamma

Number of Photoelectrons from 136 keV Gamma

Energy of Photoelectrons from

14.4 keV

500 100 5000 10" 5x10" DEPTH INTO FOIL (ANGSTROMS)

10=

1000

500

100

50

10

UJ

5xl0 5

Figure 9. Energies and relative numbers of conversion and

photoelectrons vs depth into iron foil.

b . Design of Resonant Detector

The theoretical study shows that the detection of

conversion electrons with optimum electron collection in

the detector can give a maximum signal/noise ratio of 500,

using 100% enriched iron stainless steel. The conversion

electron counter described on page 6 with a single 50%

enriched 0.001 inch stainless steel foil was used to determine the signal/noise ratio. The spectrum shown in Figure

10 gave a S/N ratio of 3, using only lower level energy

discrimination. Spectra were taken for several single

channel settings to determine the electron energy distribution in the counter for the 8 keV conversion electrons

.

16
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G>

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z:

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or.

ljj a.

en

30'ose oo'ooe 00*052 00*002 00*051

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17
2.0

S/N

Figure 11

3 4

E (volts)

Signal to noise (S/N) ratio as a function of

single channel setting using conversion electron

detection from stainless steel.

The results shown in Figure 11, indicate that the energy

distribution of the electrons escaping from the foil is

shifted toward lower energies. The signal/noise can be

improved by using a multilayer detector, with 1000 A stainless steel foils containing highly enriched iron-57. This

will also increase the counting efficiency, and lower the

photoelectron noise originating from the 122 keV precursor

gamma ray. Please see acknowledgements.

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

18
C . SRM for the Isomer Shift of Tin Compounds

After a long search for a suitable tin Mossbauer

isomer shift standard, a stable compound with a relatively

large quadrupole splitting was obtained. Dimethyl tin

difluoride has a quadrupole splitting of about 4.21 mm/sec

and a narrow line width. Calibration of this material will

be completed in November 1969 . The spectrum for a 100

mg/cm sample is shown in Figure 12. Several plastic

encapsulated samples of various thicknesses supplied by Dr.

Beasley of Austin Science Associates were analyzed with the

results shown in Table 2. The optimum sample thickness

2 would be 100 mg/cm .

Table 2. Percent effect vs. thickness of dimethyl tin

difluoride

.

Sample Line width

Thickness

z 2 mg/cm

50

100

150

Percent effect mm/sec FWHM

Peak 1 Peak 2 Peak 1 Peak 2

4.2

7.5

10.6

3.5

6.3

9.4

.87

.90

.96

.87

.89

.94

D . Constrained Fitting of Complex Mossbauer Spectra

The concepts described earlier [1], for the fitting

of unresolved nickel-6l Mossbauer spectra using the OMNITAB

programming language have been adapted to a nonlinear least

squares, FORTRAN curve fitting program. In addition, the

capability of fitting multisite iron-57 spectra has been

added. The new program which uses the algorithm devised by

D. W. Marquart [2] provided a balance between the GaussNewton method, used in the TN451 [1] and TN404 [3] (PARLORS)

programs, and the method of steepest descent. Inclusion

of the steepest descent method stabilizes the calculation in

19
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20
regions of the parameter space where the first order Taylor

series expansion of the Gauss-Newton method is not accurate.

Although nonconvergence is thus eliminated, caution is still

required in making initial estimates to avoid convergence

to a false minimum.

The program is currently undergoing revision to

improve the input and output and to decrease the running

time. Program decks and documentation will be made available

'! at a later date . The figures included here illustrate the use of the

program in resolving complex spectra. Figures 13 and 14 are

twelve line magnetic nickel-6l spectra with and without quadrupole perturbation, respectively. Magnetic iron-57 spectra

of systems with one and three sites, respectively, are shown

in Figures 15 and 16. Figure 17 shows iron-57 with two magnetic sites and one single line site, and Figure 18 shows

a nonmagnetic iron spectrum fitted with two quadrupole split

sites and an unsplit site.

The number of sites to be fitted is governed by the

number of input estimate cards. Each estimate card (one

for each site) contains four input parameters; 1) centroid

of the pattern, 2) halfwidth, 3) magnetic splitting separation (channels) of the outermost peaks , and 4) quadrupole

splitting. The nature of each site (magnetic, quadrupole

split, etc.) is indicated by the input estimates. For

example, for iron-57, a singlet is fitted if the initial

magnetic field and quadrupole splitting estimates are zero,

a symmetric doublet is fitted if only the magnetic field

estimate is zero, and a six line pattern is fitted otherwise. The six line pattern is constrained to have relative

areas of 3:2:1:1:2:3, with the half widths and amplitudes

modified by a thickness parameter. Quadrupole splitting is

included to first order for a nonzero initial estimate, or

ignored for an initial estimate of zero.

21
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27
The program's input is very similar to the NBS

unconstrained curve fit program, PARLORS [31 and it may be

used in the same way by fitting a singlet "site" to each

line in the spectrum. The "objectivity" inherent in the

unconstrained mode may often be preferable for simple,

resolved spectra. However, the constrained output is given

in terms of the real parameters of interest (i.e. shift,

magnetic field, quadrupole splitting) and their uncertainties,

which is usually more convenient than the positions and uncertainties obtained with the unconstrained fit.

Just as in PARLORS, program CFIT requires no initial

estimates of amplitudes or of the three parabolic background

parameters. The function to be fitted is linear in these

parameters, and they are estimated by a linear fit before the

nonlinear fitting iterations are begun.

On the NBS Univac 1108, the program requires 31 3 000

words of core, with double precision and limits of 50 parameters (9 sites), and 500 data points. Running times, without compilation, have ranged from about 30 seconds for spectra

like Figure 15 to 5 minutes for those like Figure 16 . Changes

in the thickness broadening formulation are expected to speed

the program considerably.

(J. C. Travis)

E . Nickel-6l Hyperfine Interactions

With the He cryostat described in Section 2B1 , it

became possible to measure internal magnetic fields and

quadrupole splitting in many nickel complexes. Of particular

interest was the measurement of internal magnetic fields,

since Mossbauer spectroscopy is at present the only method

available

.

1 . Internal Magnetic Fields in Nickel Compounds

The internal magnetic field can be represented by

H. . = H + H, + H-, . (Q) int c 1 lat v°J

28
where H is the core polarization field, H-, the orbital

c ^ ' 1

contribution and H, , the crystal lattice field, the core

polarization field is about -275 kG, which is considerably

57 lower than that in Fe . The orbital contribution to the

internal field is positive, and is related to the g-factor.

The observed internal fields of a series of nickel compounds

are listed in Table 3» and some of their spectra are shown in

1 Figures 19 to 22. The internal field is lower for more

! covalent compounds. In Nilp, the orbital contribution cancels

the core polarization field. NaNiO„, which was kindly made

available by Dr. P. P. Bongers of Phillips of Eindhoven,

Netherlands, has nickel in a formal oxidation state of 3. It

is a spin 1/2 compound, and the field is 52 kG, compared to

the 100 kG field found for NiO with spin 1. Further work is

planned to correlate the covalency factor with the internal

magnetic field.

Table 3. Internal Magnetic Fields Measured by Nickel

Mossbauer Spectroscopy for Selected Nickel

Compounds

COMPOUNDS LIGANDS SYM VK ) Neff Ho (kG)

NiO 6

h 520 4.6 98 ± 3

NiFepO^ 6

h 858 96 ± 3

NiCrpO^ 6 T

d 78 38 ± 7

NiSO^ 6 °h 15. 5 5.7 24 ± 3

NiS S 6 °h 200 16 ± 21

KNiF F 6

h 275 4.7 60 ± 8

NaNi0

2

6

h 20 52 ± 2

NiF

2

F 6 °h 80 3-5 99

NiCl

2

CI 6 °h 36 3.3 36 ± 6

NiBr

2

Br 6 °h 65 3.0 75 ± 3

Nil I 6 °h 75 ± 10

NiCl

2

-6H

2

CI 2

4 °h 5. 34 77 ± 9

Ni(HC0

2

) 2

6 °h 15. 7 41 ± 6

29
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2 . Quadrupole Interactions in Planar Compounds

It has been shown [4] in the analysis of the

electric field gradient (EFG), for nickel compounds that the

largest quadrupole splitting can be expected for planar

nickel compounds, which yield a Y„„-- (8/7 )<qr >d . The

spectrum of NapNi(CNK showed a narrow resonance line.

Further studies with the substituted nickel glyoximes

gave a positive AEn for the nickel dimethylglyoxime and

nickel heptoximes; and a negative AEQ for the nickel

diphenylglyoximes which indicates a large ligand contribution to the EFG . Curve fitting of the planar compound

spectra is complicated by the Karayagin-Gol ' danskii effect,

which changes the relative intensity of the triplet. The

intensity is represented by

I(K,L) = -/ Ke

" L cos 9 d(cos 9) (9)

2 2 2 where L = k (<Z > - <X > ), L > 1 for planar compounds

2

K =

2 " c gs 9 for M = ±1/2

5

2

K =

7

^g

s 6 for M = ±3/2

v 1+COS0 _ .. .r-/o K = -r for M = ±5/2

2 Substituting, and expanding -Lcos into a series

expression gives:

Tf + -,/pN_Y**2 . (-l) n 2n+5 T n I(±l/2)-2^ ( 10)

5 -JTi ' (2n+l)(2n+3) L

n

I( + C5/2Y=V*1 . iz±±

n

. 3n+5

T iU

^

n (11) J Z^5 n! (2n+l)(2n+3) L

n

Tf + tr/pN =\T^2 . (-l) n 3n+5 T n K-5/2) ( 12 ) 2v5 -in" (2n+l)(2n+3) L

n

34
for the triplet intensities. Computer curve fitting is in

progress to extract the quadrupole splitting and Karyagin

effect from the nickel glyoxime spectrum.

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

P . Mossbauer Spectroscopy of Iron Doped TiO ^, 1. Introduction

The objective of this investigation is to use Mossbauer

spectroscopy as a nondestructive technique for the deter2+ ? + mination of the Fe /Fe ratio in iron doped rutile single

crystals such as those used in masers . The 0.05 mole % iron

concentration normally used in maser crystals was increased

to about 0.5% in order to obtain good spectra in a reasonable

period of time. An additional departure from normal maser

configuration was the use of powdered samples to supplement

the single crystal data.

The Mossbauer spectra of the powdered specimens yielded

at least two nonequivalent sites for each of the formal

oxidation states. Further experimental studies, including

temperature studies, are currently in progress to attempt

to firmly locate the observed sites in the TiO„ lattice. The

site identification program further includes theoretical

lattice sum calculations of the quadrupole interaction,

including temperature dependence based on both the ideal

TiOp lattice and on various possible configurations including

additional doping materials.

The complex, multisite, multi-oxidation state spectra

obtained in this study have been a primary stimulus in the

development of the constrained curve fitting program discussed elsewhere in this report. The spectral complexity

also led to the use of powdered samples to simplify the

intensity relationships in the spectra. Once the line

position identifications had been firmly established with

the powder spectra, the single crystal spectra were

employed to yield orientation information about the

35
electric field gradient tensor, which added further

evidence for the site identification.

Previous work on the interstitial diffusion of metals

in rutile has been done by Johnson, [5], who used lithium

as a doping material. A strong anisotropy in Li diffusion

has been observed by these workers and substantiated by

lattice potential calculations [6]. Iron diffusion has

also been investigated [7], yielding a diffusion constant

estimate for diffusion parallel to the crystallographic

c-axis

.

The previous application of Mossbauer spectroscopy to

iron containing rutile [8] concerned spin relaxation of

internal magnetic fields, and were evidenced slightly in

some of the work reported here.

2 . Experimental

Both single crystal slices and powders were doped,

by several different methods, with 90+% iron-57 enriched

iron. The process yielding the most uniform doping as

determined by microprobe analysis, was the embedding of

the crystal in a powder mixture of TiO

?

and FepO~ with

subsequent heating. Various specimens were then heated

in different atmospheres (Ar, Ar+Hp, and Op) for varying

lengths of time at temperatures between 900 and 1000 C.

The latter treatment was repeated between Mossbauer experiments for several of the samples, generally with noticeable

effect on the spectrum.

The Mossbauer experiments employed the previously

described NBS spectrometer, a Janis liquid helium Dewar,

and a modified version of the NBS "chicken feeder" liquid

nitrogen Dewar [91. The modification consisted of a sandwich of a heating element (carbon cloth) between layers of

insulator which interrupted the copper heat path between

the sample mount and the liquid nitrogen reservoir. (See

Section 4-B-2 this report). A temperature range of 110 K

36
to room temperature was available with this system. The

power to the heating element was controlled by a variable

transformer and the temperature of the sample holder

was monitored with a copper-constantan thermocouple.

Temperature stability during an experiment was checked

either by frequent thermocouple readings or by monitoring

the thermocouple with a strip chart recorder.

Normal transmission geometry was used for most of the

experiments although backscattering was used on single

crystals to investigate surface effects. It was also hoped

that lower concentrations of iron-57 could be examined with

backscattering geometry, but the expected improvement in

signal-to-background (percent effect) for a given concentration was more than offset by the reduction in overall

counting rate.

The geometrical count rate loss accompanying the low

temperature and backscattering experiments was more than

compensated during later experiments by the use of a Reuter-Stokes* sealed krypton-COp proportional counter,

and the improved NBS counting circuitry.

3 . Results

The spectrum obtained from a melt doped powder containing about 1% Pe, shown in Figure 23, closely resembles

the spectrum obtained by P. Wintersteiner [10] from one of

M. Alam's specimens. Alam's powder sample was doped to

0.25% Re concentration during crystal growth in order to

assure substitutional doping in the Ti sites. Both

samples in Figure 23 were doped with Fe J

. The spectrum

similarity implies that even the sample with 1% concentration (Pigure 23) reveals the presence of only substitutional sites. Of course, the possible existence of

*Por disclaimer of equipment and materials see last

paragraph of Introduction.

37
PS1098

M.flLAM

o.5S5o

VELOCITY (MM/S)

0.70 -0.00 0.70 1.40 2-10

J 1 L

-0.70 •0.00 0.70

VELOCITY

1 .40

(MM/S)

2.80

2.10 2.80

Figure 23

.

Comparison of 1.0 mole % oxidized powder

spectrum with 0.25 mole % growth-doped

spectrum of M. Alam.

interstitial iron ions with extremely low recoil-free

fractions cannot be ruled out

.

The possible existence of such low f-factor interstitial sites is reinforced by the room temperature and

liquid nitrogen temperature spectra of the same sample

after reduction and re-oxidation, (shown in Figures 24 and

25). The widely split site barely resolvable in the low

temperature spectrum, Figure 24, is only slightly visible,

but not fitted, in the room temperature spectrum. Figure

25. Further examination of the nature of this site obviously requires experiments at lower temperatures. These

are planned.

The reduced form of the same sample yielded, at 300 K,

the spectrum shown in Figure 26. A reasonably good fit

to the spectrum was obtained with the constrained program

by assuming the presence of two wide doublets and a singlet. The "singlet" (probably a narrow doublet) may be

3 + attributed to residual Fe J

, and the doublets fall within

38
CO

i/o co

COco

ii UJ2

LU00

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00

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en

ooo2

=)

C£

I—O

UJ

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13 P £ •H o P o a CO

CO

CD CD

\n

H P

•H c

-P •H 3m o rH

Cvi 13 X2 * •H CX CO O CO T3 o o £

03

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CM

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OO'ZOl 09 oz-idi oQ-ooi oToot

AiI^N31NI 3AI1B"13^

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40
(Si COo

r>

CO

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ii UJ

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cc

CD

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CD

z:

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41
?+

a parameter range characteristic of Pe . The leastsquares fit to the temperature dependent series with this

sample are shown in Figure 27. The line positions and

amplitudes are shown graphically in Figure 28. The outer

doublet may be seen to have the type of monotonically

increasing quadrupole splitting with decreasing temperature

2 + that is expected with Fe distorted octahedral systems, but

the inner doublet shows a random behavior with temperature,

with little or no net change in splitting.

The success experienced in the work with powdered

samples has not been equaled to date in the work with single

crystals. This results primarily from the fact that quadrupole doublets are symmetric for ideal powders but exhibit

an orientation dependence of the peak intensity ratio for

-l

300 K EFFECT=

0.0120

258 K EFFECT:

0.0098

200 K EFFECT:

0.0119

165 K EFFECT:

0.0114

1 18 K EFFECT:

0.0151

4.2 K EFFECT:

0.0131

VELOCITY

1 .00

( MM/5

2 .00

-1 .00 0.00 1 .00 2.00

VELOCITY (MM/S)

3 .00 4 .00

3-00 4.00

Figure 27

.

Temperature dependence of reduced iron doped

rutile powder.

42
-1 .00

VELOCITY (MM/S)

0.00 1.00 2.00 3.00

i 4.00

300 K

258 K

200 K

165 K

118 K

4.2 K

-I T -1.00 0.00 1.00 2.00 3.00

VELOCITY (Mfl/S)

4.00

Figure 28. Diagram showing fitted peak intensities and

positions

.

43
single crystals. The additional parameter would provide

useful data in simple spectra, but for the multiple site

spectra encountered in this study, the identification of

the corresponding lines of doublets becomes overly complicated. It is hoped that analysis of the single crystal

data will be made possible by the results of the powder

sample studies. The most consistently occurring doublet

in the single crystal data is shown in Figure 29, spectra

(a) through (f), with powder data, (g) and (h), shown for

comparison. These doublets may be interpreted as substitutional Fe , although they are consistently smaller and

left shifted from the correspondingly identified site in the

powder

.

A particularly interesting sequence of single crystal

results is shown in Figure 30. The crystal used for these

spectra was originally about .8 mm thick, yielding spectrum

(c), a narrow doublet (Fe , substitutional). Because of

the relatively great attenuation of the thick crystal,

backscattering was attempted in the hope of improving the

signal-to-noise ratio. The unexpected result was the completely different spectrum shown in (b), probably due to

2 + Fe . The crystal was then lapped to a thickness of .2 mm,

yielding spectrum (a), which contains both of the previously

observed doublets. These results are taken to imply a

depth dependence of the relative concentrations of the

Fe^ + and Fe states. Further controlled experiments of

this type are underway to investigate this phenomenon further.

4 . Theoretical

Although oxidation state identification may be readily

accomplished by use of the isomer shift, site identification

has been a more difficult matter. A computer program for

the theoretical prediction of quadrupole splittings, written

by Dr. M. J. Bielefeld, (See Section G), is being employed for

this purpose. This program performs lattice sum calculations

44
1 .40

VEL6CITY (MM/S

0.70 -0.00 0.70

PS1000

(a) EFFECT: v ' 0.4551

PS1024

(b) v EFFECT:

0.0020

PS1025

(c) EFFECT: v ' 0.0019

. , PS1028

(d) EFFECT:

0.0033

v PS1005

(e) EFFECT:

0.0040

,. PS 990

(f) EFFECT:

0.0026

, „ PS1085

(g)6 EFFECT:

0.0062

(]n \ PS1098

^ n ' EFFECT:

0.10*3

2.10

-1 .40 -0.70 -0.00 0.70 1.40

VELOCITY (MM/S)

2.10

Figure 29. Comparison of iron doped single crystal and

powdered rutile.

45
-2.00

VELOCITY f MM/S

.00 0.00 1 .00

2.00 .00 0.00

VELOCITY

3.00

Figure 30. Spectra of single crystal rutile as a function

of depth.

of the ligand electric field gradient tensor and the potential

2+ experienced by the d electrons for an "Fe ion in a point

charge lattice. The program, in addition, calculated the

d orbital splittings, and the electric field gradient

tensor due to the above half-full shell Boltzman population

of the d orbitals for any desired temperature . The ligand

and valence contributions are appropriately summed, the

resultant tensor diagonalized, and the quadrupole splitting

calculated. Only a moderately good fit to the experimental

data was obtained by adjusting the low temperature limit,

by suitable choice of nuclear quadrupole moment and/or

46
Sternheimer shielding factor (neither is an accurately

known quantity) and by adjusting the d orbital spacings

by suitable choice of "effective charge" on the ligands

(a reflection of convalency effects).

Another type of fit, a least squares Boltzman fit based

on the assumption of tetragonally distorted octahedral

symmetry, was also obtained, and is shown as the solid line

of Figure 31- The d orbital splitting parameter (ATM)

is illustrated in Figure 32, and the Boltzman function for

the quadrupole splitting, neglecting lattice contributions,

is given by

A(T) = A(0)-

1 - exp(-AT/kT)

1 + 2 exp(- A T/kT)

(13)

where A(0) is the low temperature (T=0) limit of the quadrupole

splitting. The fit shown in Figure 31 was obtained for parameter values of AT=522 cm" and A(0)=2.69 mm/s

.

? 7« r

2.6

en\

5 2.5

2*

> 2.4

S 2.3

_j

LlI > 2.2

2.1 -

— AT

= 522 cm"1, A(T=0)=2.69mm/s

O A, = 459 cm-1

, A2 = 535cm-

,

1 A(T=0)=2.69mm/s

_L _L 100 120 140 160 180 200 220 240 260 280 300

TEMPERATURE ( K)

Figure 31. Temperature vs splitting of outer doublet for

iron doped reduced form of powdered rutile

.

47
The theoretical calculations corresponding most closely

to the Boltzman fit were those using an effective charge

parameter of .75 (i.e. assuming the point ions to bear only

75% of their normally assumed "valence" charges) for which

the two lowest d orbital splittings, A and A of Figure

-1

32, were 459 and 535 cm . Low temperature limit may be

normalized to the least squares fitted value of 2.69 mm/s

by choice of an abnormally low value of Q(l - R) of 0.12.

The results of this process are shown as the open circles

of Figure 31. Neither of the fits is particularly impressive but additional data are being accumulated.

(J. C. Travis and P. R. Stampfl)

2Q_

0, '4h

r

k AT A, A2

\J L_L

D

2h

<V* <v*<vzz

>*

-.5 I. -.5

I. -.5 -.5

-.5 -.5 I.

A(T)^A(0) ' -expHWkT) mU mU' l+2exp(-A AT)

*in units of (4/7)e<r"*>

Figure 32. Energy level diagram of 3 d levels used for

interpretation of data in Figure 31.

48
G . Towards a Hartree-Fock Crystal Field Treatment of the

Electrons on Mossbauer Nuclei

In order to include a crystal field of point charges

in the self-consistency scheme of a Hartree-Fock treatment

of electrons about a Mossbauer nucleus, certain basic

integrals must be evaluated. The following is one analytical

method for this evaluation.

The many-electron problem can be reduced to parts which

| are described in terms of a single electron about the

; Mossbauer nucleus being perturbed by a point charge. A

function which describes the spatial character of the

I electron can be written as

|n,^m> = -i

r-e lm*0 <R (14)

where©,, is a normalized associated legendre function [11]

and (R n is a normalized radial Slater type orbital [12].

For example, | 2 ,1 ,0 > denotes a 2p Q atomic orbital.

-> The interaction between the electron at a position r and the point charge q at a position R can be written

as <n,£,m| | n'

,

I* ,m'>

where = ^

q

_^ (15)

|R - r|

The functions |n,£,n> are centered in a coordinate system in

which the positive Z-axis is directed towards the point

charge. Functions in an arbitary coordinate system can be

readily transformed into this system by a rotation [13].

Since for a point charge the value R-r has no <p dependence

in this coordinate system, the following simplification holds:

«n,*.m|0|n-.t-,m-> = S^.- © £>m

«

njJl |o| 8,. >m

. <V,*' >(16)

where 6 ^is the Kronecker delta function. m,m

49.
In order to separate variables before integration, these

functions are expressed in elliptical coordinates [14] with

the Mossbauer nucleus at one focal point and the point charge

at the other focal point. The necessary relations for this

transformation (r,0,<|) -> y,v,<jO are the following:

cf> = (j> ; ( 0< <j> < 2tt ) ; |r| = R (y+v)/2 ;( 1< y <°° ) ; |8-r| = R (y-v)/2 ;(-1<v<1 ); (17 ) cos 9 = (l+yv)/(y+v) ; sin 9 = /(y z -l)(l-v z )/(y+v);

R

3 sin 8 r

2 dr d6 = -S- (y

2 -v

2

) dy dv

A typical integral, e.g., ls-2s, Is here worked in detail.

< 1,0,0|S|2,0,0> - |^(i-)*(i:)ij/jGT-^pxp[-(i-+ £)r] (18)

Where £ and £ are the orbital exponents of the bra and ket

functions, respectively. After a change to eliptical

coordinates and with the energy and distances in atomic

units this expression becomes

<l,0,0|o|2,0,0>=r

^^(Rc)^(Rr)^/dy/dv(y+vfexp[-|(R^+|^)(y+v)]

(19)

<1,0,0|0|2,0,0> = 3J7S § (RC)*(RC')2 [A

2

B + 2A

1

B

1

+A

Q

B

2

] (20)

where Ammand B are easilyJ evaluated [15] and are defined

below.

50
For ease of notation the following symbols are defined

for the integral <n, I ,m| | n" 3 l' ,m'> :

Aj -f aV J exp [- 1 (|i + ^-) v ]-, (21)

-/'dvv^exp [-1 B. (St + |tl)v];

[2n+l]/2

C = (RC)

[2n'+l]/2

D = (RC')

Accordingly, the twenty-eight integrals of interest for a metal ion in the first transition series are listed as

Appendix I in this report.

A word of caution should be mentioned concerning these

integrals. Their numerical evaluation for specific values

of R, q, C and C

* can be readily performed on a digital

computer provided the arithmetic is done in double precision.

(Michael J. Bielefeld)

51
3. NUCLEAR CHEMISTRY

A. Introduction

The program has continued in its dual nature in which

nuclear reaction studies have been supported by statistical

investigations in order to improve the accuracy of the

resulting conclusions by finding superior methods for

treating the experimental data. Generally, both aspects

of the program have contributed to the solution of problems

arising in other research groups at NBS. Examples are the

gain shift problem described in Section C and the estimation

of variance in Section P.

B

.

The Discovery of Errors in the Detection of Trace

Components in Gamma Spectral Analysis

The following is a condensation of a talk given at

the 1968 International Conference on Modern Trends in

Activation Analysis, held at the National Bureau of

Standards in October, 1968.

I. Introduction

Least squares solutions to over-determined systems in

physical science generally provide parameter estimates, their

standard errors, and various measures of the quality of the

fit of the set of observations to the model. Such an approach

has been applied for a number of years to the estimation of

individual radionuclide contributions to an observed composite

gamma-ray spectrum [16]. Perhaps the most popular indicators

for misfit of the observations to the assumed model are the

resulting series of normalized residuals and the statistic,

X

2

, which is derived from this residual set. Too large a value for x

2 is indicative of trouble, while a plot of the

residuals may often pinpoint the source of trouble. Errors

which are frequently uncovered in the process include missing components, shifts in peak position (gain, zero), and

gross, isolated mistakes (dropped channels).

52
o

o

o.

o

o

.. • ••

CO

uJ

re:

oo

o

(NJI

%-• •• -r -v- -•• %." —VB*

20 ~

i 40

CHANNEL

60~ ~80~ too

Figure 33. Plot of normalized residuals from the least

squares analysis of 22 Na, 54Mn, and background

Channel zero is the timing channel, and the

enlarged points refer to channels containing

systematic errors.

That errors may not always be directly located by the

residuals, however, is illustrated in Figure 33. This figure

resulted from the least squares analysis of a mixture of

22 54 /22 background, Na, and Mn. The negative ( Na) Deak and

5I1 positive ( Mn) peak in the residual plot clearly indicate

the presence of trouble, but they do not directly point to

its origin. In fact, the (unintentional) mistakes occurred

54 in the standard spectrum for Mn in channels 62, 65, and 67.

53
A more subtle kind of error-detection problem may arise

when one or more components are present at "trace" levels - i.e., if they are scarcely detectable. In order to examine

this problem we must first discuss the question of detection

limits as related to the least squares approach.

II. Detection Limits in Overdetermined Systems

The general problem of limits for decision, detection,

and determination has been discussed previously for the

one component situation [17]. For the particular case of

radioactivity (Poisson distribution) the "critical level"

was seen to be,

L = k c (22)

C a o

and the "detection limit",

h

D "

L

C + V* (23 )

where: a .a represent the standard deviation when the

o D

component of interest is absent, and present at its detection limit, respectively, and k

a ,k represent the values of

the standardized normal variate corresponding to the

selected limits for the errors of the first and second

kinds, respectively. In the limit of large numbers of

counts (>100), a ^a , and 1^21^. Thus, for example, if

the two types of error are each set at 5% > k

a =k

3 = 1 • ^5

,

and the relative standard deviation at the critical level

equals .608, while that at the detection limit equals .304.

(Note that Lc is to be used for making a posteriori decisions

regarding detection, whereas L

p

is used to assess a priori

detectability )

.

Equations (22) and (23) have been applied to the measurement process in which estimation is carried out by means of

least squares. The method of solution is iterative in nature

54
in that a must first be calculated from a model containing

o the component of interest at level zero; then the level is

adjusted until equation (23) is satisfied. The results of

such calculations are presented in Table 4. The reference

levels for the standards and background used in deriving the

detection limits in Table 4 were as follows:

Background 28000 total counts

22 Na 69000 total counts

6i|

Cu 4400 total counts

54 Mn 44000 total counts

The detection limit is thus seen to depend both upon the

levels of interfering components (and background) and the

magnitude of the correlation coefficient. If the model

54 64 includes Mn and Cu, even at level zero, the detection

22 limit for Na exceeds its value in the one component

model. Variation with level of interference is rather

complex; when one interfering component overwhelms all

others, the variation approaches the "square root" relationship discussed in reference [17].

22

ts for Na in various radionuclide

Correlation

, coefficient

Component-J Level-J (Na,J)

Table 4. Detection li

mixtures

.

a

Number

of

component s Lp( 22 Na) b

1 .00012°

.0053

2 .0147

.0062

.0104

.0260

64^:cu -.92

^Mn -.50

-^Mn 1. -.59

54 Mn 10. -.61

•0291 6 ^Cu Ln (.284) -.93

54Mn u

1. -.62

Number of observations equals 6.

c Levels refer to the respective standards as unity. Background = 0.

55
III. Model Errors

When one or more of the components in a mixture lies

below its detection limit there exists a significant chance

of missing such a component (by definition) . It therefore

becomes extremely important to consider the effects of such

a nondetected component upon the estimates of the remaining

components. If the model is correct - i.e., includes the

trace component, "A" - the remaining estimates are unbiased

and no difficulties arise. On the other hand, if the model

does not include "A", either because of ignorance or because

of model simplification, the results are necessarily biased.

The signs and magnitudes of the normalized biases are dependent upon the actual magnitude of "A" and the respective

correlation coefficients. More seriously, it can be shown

that, neither x

2 nor the magnitudes of the residuals may

indicate the presence of trouble if "A" lies below its

detection limit.

Errors of the type just discussed may have especial

significance in the evaluation of several trace components

in the same sample. For example, in the Standard Reference

Materials program at the National Bureau of Standards , we

are concerned with certifying specific materials (including

high-purity materials) with respect to their trace constituents. It is quite likely if an incomplete model is used

that, besides the systematic errors induced in macroconstituents, trace components may be seen when they are

absent and/or missed when they are present. The probability

of such mistakes can become quite large when large (positive

or negative) correlation coefficients are involved, as the

following example demonstrates.

Table 5 is based upon least squares analyses of a gamma64 22 S4

ray spectrum constructed from Cu, Na, and Mn at levels

L r (.1428), 0, and 1, respectively. (Levels are given in

fill terms of the "standards" as stated earlier) . Since Cu

56
/T 1

1 O O

Table 5. Effects of model errors on the mixture Cu, Na

,

and 5%n.

22 Na Errors (p=-.93) Mn Errors (p=+.46)

Model Systematic Random (a) Lq Systematic Random (a)

Correct

(Includes

64 Cu ) .0088 .0145 .0082

Incorrect

(Excludes

64cu) +.0137 .0032 .0052 -.0063 .0072

was present at its critical level it will be missed with

a probability of 50%. It may be seen that the effect of

54 the incomplete model on the macro-constituent, Mn, is

a slight reduction in the standard error estimate and the

introduction of a serious, negative, systematic error.

22 The effect on the trace constituent Na - which is, in

fact absent - is profound! A rather large decrease in the

standard error estimate (and critical level) is accompanied

by a large positive systematic error which causes the

absent component to be "detected". It was observed that

neither the magnitude of the residuals (extreme=-l . 4 ) nor

the value of x

2 (P = '72) were indicative of misfit. A

miniature Monte Carlo experiment was performed with the

same system and it was observed: that x

2 exceeded its .975

22 limit more often than one time out of six; that Na was

detected every time; and that the most serious error in

22 the Na estimate was equal to 6.05 times its standard

error - an event which would be expected, by chance, with

_7

a probability of 2x10

The conclusions are, perhaps, obvious. When dealing with

trace analysis one must be extremely cautious in using too

simple a model, as the systematic errors can be enormous,

while the popular fit-statistics may appear perfectly

satisfactory. The probability of such pitfalls can be

57
assessed rigorously, but this subject will be discussed

elsewhere. A principal caution to be taken, however, is

to examine the correlation coefficients of any trace components which are possible but which are being considered

for exclusion. If these coefficients exceed ^±0.4 it would

be wise either to include such components in the model or

to "orthogonalize " by means of chemistry, coincidence

methods, mass separation, etc.

* * # *

The study of the effects of missing components has been

extended to the case where there are many degrees of freedom

(many channels) in order to learn whether, in this case also,

components which are "detectable" but excluded from the

model are apt to yield discernable patterns in the residuals.

Re

54

22 137 Results from a mixture of three radionuclides Na , Cs,

Mn - depicted in Figures 34 and 35 > display the normalized

residuals resulting from analysis to two sodium iodide spectra

.b000+01<

5000+01<

* * *

»

* «» * « * *** * *»*

+ »*'*• * *** » » «» «*» *** .»+.* *

.5000 + 01'

-00000 .50000+05 .t 000 +0 5 .b0000+02 .80000+05 .10000+0 3

24

Figure 34. Normalized residuals from the analysis of Na

,

137cs, 5%n; correct model (horizontal lines

are drawn at ±2 a)

.

58
i , 6000.UI.

• ••

.20UU0.02 , 10001*02 .60000*02

24 Figure 35. Normalized residuals from the analysis of Na,

137Cs, 5^Mn; wrong model (horizontal lines are

drawn at ±2o . )

of a mixture of the three radionuclides. The energy scale in

each case runs from zero to approximately 0.94 MeV. Figure

34 is the result of using the correct model, whereas Figure 35

54 resulted from the omission of Mn from the model. The wrongmodel pattern is by no means marked even though the actual

54 level of the Mn exceeded its detection limit by 50%. Quantitative evaluation of the normalized residuals lead to the

conclusion that the chance of being alerted to this error

in the model is only about 0.50. As was found in the earlier

study involving only a few channels, the bias in the remaining

22 components may be quite serious. In this case, the Na

result contained a significant positive bias, while that for

137 Cs was slightly negative.

(L. A. Currie)

59
C. The Significance of Gain Shift

A temporary discrepancy between the results of two

different analytical methods for the determination of ^0.1%

Co in Al neutron density wires, led to the application of

least squares spectrum fitting of neutron activated wires.

Although the discrepancy was later discovered to be due

to an error in the Co standard, the ensuing investigation

revealed some significant aspects of alternative methods

for fitting gamma-ray spectra. Least squares fitting of

sample #I-3B to Co-standard #2, for example, resulted in

the residuals shown in Figure 36. (Nal spectral observations were supplied by D. Becker and T. Gills of the

Activation Analysis Section. ) The residual pattern is

characteristic of gain shift, which amounted to about .kh%

for this particular case. Least squares fitting of the

two cobalt activation standards to one another led to a

ratio of 1.036 ±.002 (± standard error). Here the gain

• » • * • • • • • • *

. 21)000 *0 2 .6 ji:no*-J2 .10000*03

Figure 36. Neutron density wire spectrum fitting residuals

for sample I-3B vs standard #2 (Co.).

60
shift was somewhat smaller, amounting to about 0.2$. The

significance of the shift in gain, however, may be seen,

not only in its distortion of the residuals, but also by

the fact that the mass ratio for the standards was known to

be 1.022 Q

. Alternative use of the peak-height method of

analysis [18] which is relatively insensitive to gain shifts,

led to a result of 1.019 ± .009. One especially significant

point which results from this investigation is that small

gain shifts as well as trace components may remain undetected

while leading to bias in the results of least squares analyses.

Short and long term stabilities of the gamma-ray detection system used in the previous studies were determined as

follows: System gain was set at approximately lOkeV/channel

.

A Cs source was positioned over the Nal crystal to give

a count rate of ^2x10 cpm. A series of 10 minute (live time)

counts were then taken, spanning a period of 14 days. Peak

position height, resolution, and their respective uncertainties; peak area, and the fit (x

2 / v )> where v=degrees of

freedom of the points to an assumed normal-shaped curve,

were all determined by means of a computer program,

"LSNORM", described elsewhere (See Section 3, ref 9). Long

and short term gain drifts as measured by the peak channel

position are presented in Figures 37 and 38 respectively.

The measurements comprising Figure 38 were taken during

the interval of time denoted by the dashed box of Figure 37.

The estimated long term drift rate is 0.1%/day. Part of

the drift observed in Figure 38 may be attributed to a 0.5°C

increase in temperature which occurred during the 10 hour

measurement span. The overall temperature range for the 14

day period was only 0.6°C.

The long term drift rate is small and readily compensated

for by a daily adjustment of the phototube high voltage.

The possibility of gain stabilization by electronic means

61
66.0

<

I

<

UJ

0-

65.5 —

65.0

TIME (days)

Figure 37. Peak channel drift as a function of time: lon^

term stability.

may be explored at some future time. To see how precisely

the high voltage potentiometer might be adjusted a series of

measurements was made in which the high voltage was varied

and peak position for a Cs source determined. The results

of this experiment, presented in Figure 39 > show that the

adjustment precision is approximately .03%. High voltage

pot hysteresis was also found to be negligible.

137 A Cs source, that gave an integrated counting rate

r of ^2x10 , cpm was found to decrease the gain by about 0.h%

relative to the same source measured with poorer geometry

(count rate of only 2.5x10 cpm. At more typical count rates,

4

say 2x10 cpm, this effect is negligible.

62
66.0

TIME (hours)

Figure 38. Peak channel drift as a function of time: short

term stability.

Possible shapes for the background used to reduce input

gross counts in "LSNORM" are: (1) BG=0; (2) BG=constant

;

(3) BG fit to a straight line connecting the (equally spaced)

minima on each side of the peak channel. In Figure 40 is

plotted peak area as a function of the number of channels

used in the analysis. Results based on background options

#1 and #3 are given along with the computed (x

2 /v) fit.

The results are disturbing, not only in their internal

disparity but in the apparently acceptable fit for several

63
72.5

809 810 HIGH VOLTAGE

Figure 39. System gain as a function of phototube high

voltage

.

of the data. It is thus clear that this relatively popular

approach for determining spectral parameters can be quite

misleading. The reason, of course, is that the mathematical

model is only an approximation, whose answers are more

seriously biased for some parameters (peak area) than for

others (centroid energy). Our concern was the latter, but

the details have been given to stress the difference between

model consistency and "truth" in this particular area.

64
enc

O

o

<

<

<

a.

yuis 1 1

1 1

If (8.55)

\

(19.54)

85K < f(2.ll) —

i (.79)

1J) (.79)

80 K

75K

Fw

<j>(.86)

(J>(2.34)

70 K hf 1 1 1 5 10 15

NUMBER OF CHANNELS ANALYZED

20

Figure 40. Peak area as a function of number of channels

analyzed by "LSNORM" . The closed circles

represent results based on the assumption:

BG=0; open circles have had background subtraction from gross data as described in test.

Numbers in parenthesis are \

2 /x> values .

(L. A. Currie and H. L. Steinberg)

65
D . Quantitative Preparation and Analysis of Gas Samples

The need for dealing with small, known amounts of rare

gases - both pure and in mixtures - has arisen in a number

of projects. Mixtures of argon in air and carbon dioxide

have been analyzed by means of gas chromatography in work

related to the atmospheres existing in a glove box and the

NBS reactor, respectively. Pure samples and mixtures of

helium and argon have been prepared for calibration of a static mass spectrometer used for nuclear reaction studies.

In this latter case multiple expansions among calibrated

volumes permitted the preparation of samples containing 10

15 to 10 atoms of rare gas. Discussed below are some aspects

of the measurement systems involved in these preparations.

1 . Argon in Air and in Carbon Dioxide

The gas chromatograph, employing a thermal conductivity

cell detector is shown in Figure 4l. This system will,

hereafter, be referred to as the GC-TC. The GC is of the

dual column type for good base-line stability. For the

experiments discussed, sample constituents were separated

on a 10 ft long molecular sieve column; oven and TC Temperatures were set at 50°C and 60°C, respectively; a TC bridge

current of 230 mA was used; and the carrier gas was helium

at a flow rate of about 50 ml/min

.

A glove box, Figure 42, has been modified to permit

the establishment within it, of high purity, inert atmospheres

Li samples relatively free of nitride and oxide "skins" have

been prepared in this box. The atmosphere within the box can

be sampled through a septum-port located between the two

gloved ports.

The response function of the GC-TC for

?

, N

?

and Ar

as a function of injected sample volume in \ii is plotted

in Figure 43. Typical pure Ar and air spectra are shown in

Figures 44 and 45 respectively. As can be seen 0„ and

Ar have similar retention times. Nevertheless, with Figure

66
Figure 4l. Gas chromatograph with thermal conductivity

detector

.

43 in hand, the percentage of air in Ar can be deduced from

the sample volume and the height of either

? + Ar or Np peaks;

spectra of two 45.5 y& samples drawn from the glove box at

different times during a flushing operation are presented in

Figures 46 and 47.

A (visual) comparison of darkening rates for freshly

exposed Li surfaces vs composition of the glove box atmosphere

has shown that a proportion of Ar:air of at least 20:1 is

required to avoid rapid nitride formation.

67
Figure 42. Glove box setup for lithium sample preparation.

The carbon dioxide blanket of the National Bureau of

Standards reactor was suspected of containing impermissibly

high content of Ar, (i.e. Ar present in the blanket as a

result of a leak from the surrounding atmosphere will react

with thermal neutrons to form the 1.8 h activity - Ar .

)

Our group was asked to determine the level of Ar in the

COp "blanket" before and after several attempts were

made to eliminate the leak.

The GC-TC was calibrated for a range of air-CO„ mixtures

from 0-100%, using the operating conditions already given.

The system was not observed to respond to C0„ so that the

resulting spectra obtained are in accord with those of air

for equivalent volumes. All C0„ blanket samples measured

indicated air percentages of from 15% to 30%.

68
80

VOLUME {.(Ml)

120

Figure 43. Thermal conductivity cell response to

?

, N

p and Ar as a function of sample volume.

2 . Vacuum System Used to Prepare Ar , He and Li Samples

A portion of the vacuum system in which pure and mixed

samples of Ar and He were prepared is shown in Figure 48.

It is similar to Figure 24 in Ref. 9 except for the following

A lmV/full scale, dual trace, strip chart recorder has been

incorporated into the system. Simultaneous monitoring of

pressure, (as measured with a Pirani gauge), and sample

temperature, (as measured with a chromel-alumel thermocouple)

as functions of time are now possible. Calibration curves

for each of these parameters are shown in Figures 49 and

50. (Note that 1 torr=133-32 N/m2

.

)

69
u

•H

«j

OhO

IhbOO

P

a

e

o

ino

ta

tt)

•H

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*- _____ <

TM"===: it ~:::::::::::\

S5--I

if --

. 1 . ------L-------.-J

<> O O O O

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o O O O O <>

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1h <

a,

O

h

bfl O

e

o

O

K3cd

(D

hO

•H

70
1 1 1 1 1 I—1—1—i 1 I 1 1 I 1 1

1—1—|—|—l—I—r—t—

| " " J.09fU!F> J

i < „__-U i I — ._1_J_ ..1 !,,,_ 1 ...Ll 1 - «=1B»r3*^- O O fM r O <> <=> o O O Of O o <f> jy is j a n "t o «si '- -\

i

i

i j

1 1

i ' *\l - .. Us 01 ,2<^

z 'V

i—i—i—i 1 •[ —i— —i—i—i—

i

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-11 1 1 "L

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Cd • rH x

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Jh >H

hO faO OP S

cd o

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CD

hO

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-p- -r "p"^" - ~i Q O O O O O «> (5 O O * < O <l> (0 Is o n '* O <J

, I^ofl lJOlUIII -A9

+ _ :£:*: f

I I< 1 I IV

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1 ! 1

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o P

>>x> •H

i-H £

!m CD cd > TS

CD O CD rH fat) <tn faO ^ o 3

e a

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cd U faO U Cm c

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!h £

x: cd O r-\ p

a •H

co S • cd cd CO M

Ci3 CO cd <

vr> =t

CDU

3

hO

•H

Ph 71
Figure 48. High vacuum system with dual-trace recorder for

simultaneous monitoring of sample temperature

and pressure.

For clarity of discussion a topological diagram of the

vacuum system is given in Figure 51. In it V represents

that part of the vacuum line to which the sample is connected,

but does not include the sample volume per se . The sample

can be closed off from V by a stopcock, Zj,, also not shown.

Included in V is a precise aneroid-type pressure gauge with

a measurement capability of from 1 to 800 torr. V is that

part of the vacuum line to which a vacuum pump is connected.

The Pirani gauge samples P .

72
PRESSURE (torr) Figure h9 . Recorder response as a function of vacuum system

pressure

.

liCW

1 1

1

~ 80 >

E -

u

coO

0_

CO

UJ

a: 40

•s

m.p. (Li)

-

1 1 , 1 100 200

TEMPERATURE (°C) 300 400

Figure 50. Recorder response as a function of sample oven

temperature

.

73
TO PUMP

Figure 51. Topological diagram of high vacuum system.

Critical factors in the preparation of picomole

quantities of gas samples are:

(1) degassing of samples before filling, and potential

behavior of these filled samples in the mass spectrometer.

A sample into which a known quantity of gas has been introduced is said to be filled. Degassing is accomplished by

heating the copper capsule (with or without Li) to about

300°C under high vacuum. To test the efficacy of this procedure and to, in effect, determine the probable behavior

of the filled samples in the mass spectrometer several

Cu-capsules, containing *\» . lg Li each, were degassed and

then filled with He and/or Ar at pressures of about 100 torr.

Each sample was then cooled and later evacuated. With Z. closed but Z

1 open the sample temperature was gradually

increased to 300°C. A typical run, as observed by the

recorder, is shown in Figure 52. Note that the breakpoint in the pressure curve came at the Li melting point

(T=179°C).

74

'3
1

1 too 1 "K^^ ""*. W7 " ^""v^

00

7V

>w_>, tJV IN.^

"V \aais

N

N

flU '"' x\

s v 9J

\

\ TDJ \^ \\ ~IW? ' V\\s

i J 1 —e—

1

TOO

90

80

70

UO

30

40

1 30

v\

1 \\ \|

\**r4

- i ,H— ^, ty. ^^^^^^ —-*— x

1—«

TIME

Figure 52. Pressure and temperature response curves obtained by melting

a (O.lg) LI sample In which Ar at a pressure of 100 torr has been "frozen"

75
(2) Determination of expansion ratios, r, of the form:

r=V /V +V ) . Neither the aneroid-type nor the pirani

& lD \^ pressure gauges are satisfactory at the pressures required,

_o _o

(i.e. 10 to 10 torr), nevertheless, a method of obtaining

low, but accurately determined pressures was devised. Using

the aneroid-type gauge in its sensitive range the ratio, r,

was determined for two V volumes V and V , differing

s S]_ S2

byJ the addition of two bulb's to V . These bulbs can be

s closed off from the remainder of V by stopcocks. Ideal

gas behavior and isothermal conditions are assumed.

The calibration of the volume ratio, r, was performed

as follows. (Note that no sample need be present for this

calibration.) Volume V +V +V was filled with either He or

s e g

Ar gas to a pressure of about 800 torr (=P ) as measured

by the aneroid-type pressure gauge. Z

1 was then closed and

Vegand V evacuated. Z n was then closed and V expanded

3 s F

into V +V . After equilibrium Z, was closed, Z

? opened

and V expanded into V +V . The final pressure P is then

e eg ^ g

measured and the two pressures P and P are related as

s g

P , /P = r = (V./V . )(V /V ) (?4) e+g s o

v s s+e e e+g k^^)

with V at some measured pressure P' Z„ is closed and V

g g 2 s and V are evacuated. Then Zp is opened, V expanded into

V +V , and the final pressure P' recorded. P and P

g e

5 v g+e g g+e

are related as

p '/P ' e r = V /V roc > g

7 e+g a e+g

7 g (25)

Equations 24 and 25 may be combined to determine r as a

function of r and r , namely

o a

V/V^ Er = r(l + —=kp) (26) s s+e o r -1' \<^^j

76
Quantitative filling of samples then proceeds as follows.

First the sample is degassed, & s the entire volume V=V +V +V

s g e having been evacuated in this process. The sample is then

isolated by closing Zu. A quantity of pure gas is introduced

into V and the gas pressure P , determined by means of the

aneroid-type gauge. Z ?is then closed and by m successive

evacuations of V (with Z-, closed and Z^ opened) followed

by equilibrations of V and V (with Z-. opened and Z- closed)

n

P is reduced to some final pressure P

f

, by a factor r . With Z-, closed and a sample capsule present Zu is opened and

the capsule filled to a pressure P' readily determined from

P„, V and the capsule volume.

f ' s ^ Precise mixtures of Ar and He are obtained in V

(and then the sample) by confining each, in one of the bulb

appendages of V , at a known pressure and then opening each

to V after V had been evacuated,

s s (H. L. Steinberg and L.A. Currie)

E. Photonuclear Yield Analysis

Copper discs have been irradiated with bremsstrahlung from

the NBS synchrotron. Yields were obtained, at 20 endpoint energies, X., from 19.5 MeV to 166 MeV for the following reactions:

^CuCyA); 63 Cu ( Y ,n); 63 Cu(y,2n); and 63 Cu(y,3n). This

experiment was the basis for one of the authors (HLS) M.S.

Thesis [19].

Several methods exist for unfolding yield data to produce

either a histograph-shaped excitation function or integrated

cross sections [20]. As discussed in Reference 9 existing

methods of analysis magnify yield uncertainties 10- to 100-

fold. Alternate methods of analysis have been explored in

preparation for future experimentation by this group. In

particular we seek techniques that will give definitive or

limiting information about the "tail" region, (i.e. the region

E £ 30 MeV) .

77
All techniques used, both old and new, involve a least

squares (or simultaneous equation) reduction of yield data.

In one technique explored the shapes of the (y,n) and (y,2n)

excitation functions in the giant resonance (GR) region,

(E,, E 30 MeV) are either taken bodily, from previous results,

or are assumed Lorentzian [21]. Above 30 MeV, where the

literature is sparse, the following shapes have been used:

(1) c(E) = 0;

(2) a(E) = an , ,; D(y,n)

E

2 r2 (3) a(E) = Lorentz Shape:

E

2 T

2 +(E 2 -E 2

) 2

(4

)

a = Const . ; and

(5) a combination of (2) and (3).

If the GR is assumed Lorentz-shaped the yield equation

can be written as

30 X.

Y( V = a /

N(X

i' E)a

L

(E > E

o' r)dE + a /

1

N(X.,E)T(E)dE (27)

E th

3 °

= Vi( Wr) + V2 ( V

By means of a linear, weighted, least squares program,

written for a time-sharing computer, the amplitudes, a and

a , , of assumed GR and tail shapes may be determined. Our

63cu(y,n) yields with error bars representing ±1 standard

error are plotted in Figure 53. For comparison a composite

curve comprised of the actual a(E) values of Sund, et al . [22],

to 25 MeV and Fultz, et al . [23], from 26 to 28 MeV, is given.

If we assume no tail (shape number 1), the dashed part of

the yield curve results. Although this curve is consistent

78
with our data below about 40 MeV, our yields appear to depart

from this curve above HO MeV. This suggests the existence

of non-negligible tail. Our data, when fit with this shape,

(i.e. Sund + Fultz + tail shape number 1), result in the

normalized residuals depicted in Curve A of Figure 53. If a "quasideuteron"-type tail (shape number 2) is used, the

residuals plotted as curve B result. It is evident that by

assuming a "quasideuteron" tail the fit is improved considerably

This does not necessarily imply the correctness of this shape

so much as the need for additional cross section above 30 MeV

to satisfy our high energy yield data. A similar need for

additional cross section above 30 MeV also resulted from

65 analysis of our ^Cu(Y,n) yield data.

If E and V are not known a Taylor expansion can be

performed about these quantities, thus

Y(X.) (n)

= c I,(X. ,E (n)

,r

(n)

) + c (3I-/3E )5E (n)

i olio' o 1 o o

s (n) (28)

+ a

o Ol

1 /9D6r ln;

+ a

t

I

2

(X

±

)

A nonlinear, iterative, weighted least squares option

exists for the program discussed. Input guesses E

/ \ o and T for E and r are made. Each successive least

O / , V squares iteration results in new estimates: E =

E

(n) (n) (n+1) . r (n) (n) (n+1)

a

° d fn+1)

o o

' ' o t

If either, or both, test ratios \6 E

^ n) / E

^ | ,

| 6T (n) /r (n)

| are less than some preset value, (i.e. the value .001 has

been used to date), another iteration is performed using the

updated values for E and r. In addition to computing a , the optional parameters E , r, and a, , and their respective

uncertainties, yields based on these "least squares",

parameters are formed at each X.. Residuals, normalized

residuals, average cross sections for each interval,

A = X..-.-X. and the integrated cross section at each X. l+l i

to i

79
EO

_lUJ

>-

::

:

_i<

LU

cc

I

-4

—

1 1 A

/A .

1 1 i i i 1 1 y. A / X^/ —

— / V v/x / *

1 III 1 1 1

B

1 1

—

20 50 100

ENDPOIIMT ENERGY (MeV)

200

Figure 53. Experimental yields ± 1 standard error for the

reaction °3cu(y,n) for comparison: the solid

curve, based on the excitation function for this

reaction due to Sund [22] to 25 MeV and, from

25 to 28 MeV, Fultz [23]. If cr(E) is assumed

zero above 28 MeV the dashed curve results

.

Residuals obtained when our yield data (to 166

MeV) were fit with the above shape are plotted

in Curve A. If a "quasideuteron" tail is

assumed above 30 MeV least-squares fit to our

data gives the residuals of Curve B.

80
are determined. Goodness of fit quantities: x

2 /v and the

serial correlation coefficient for the set of data are also

part of the program output

.

(L. A. Currie and H. L. Steinberg)

F . Indirect Variance Estimation in Chemical Systems

A method for assessing the variability of a chemical

component has been studied for the case where the component

cannot be directly measured, but where information is derived

from observations on a second, combining component. The method

is based upon a simple physical model incorporating stoichiometry and the observation of the variability of the second

related component and/or the ratio of the two. A principal

result of the investigation is an expression giving limits

for the relative variance of component-A, <|>j?:

l*A/B " *Bl 1 *l< 1/B + *B (29)

where ^aad an^ <J> R represent the relative standard deviations

of the ratio of components-A and -B and of component-B,

respectively.

The problem arose because of the need to estimate the

heterogeneity in the calcium content of chemical microstandards

,

but in which it was not possible to carry out direct observations of the calcium variability [24]. The problem was

solved by examining the variability of the ratio, Ca/S, and

that of the sulfur (-S0~), to which the calcium was bound.

Derivation of the inequality [24] was based upon the

Taylor expansion for error-propagation: [25]

*A/B " *A + *B " 2 P AB*A*B

(30)

where p. R represents the correlation coefficient between

the variations of components-A and -B. The goal of the

indirect approach is to estimate, or set limits for,

81
(f>. - which is presumed immeasurable - based upon observation of (j)„ and/or <{>. , Ronly. (We assume no information

regarding the correlation coefficient.)

In order to derive limits for <J>. using Eq . (30) a

stoichiometric model was assumed, in which the two chemical

species were assumed to have a common source of variation

(fluctuations in the amount of the chemically bound substance)

plus independent sources (fluctuations characteristic of

excess amounts of the components). Based upon this model,

one can establish limits for the ratio, <J>./(}> , as given in

.H. ID Equation ( 31 )

,

'AB ± V*B - 1/PAB ( 3D

assuming the mixture of components to be stoichiometric, on

the average. Alternative models which might be proposed

include: (a) fixed amount of chemically-bound A and B plus

varying excess amounts of either or both, and (b) a fixed

amount of B, but varying fractions available for chemical

binding. Both of these latter models can be considered

special cases of the original model, in which p. =0. The

first alternative eliminates the last term from Equation

(30), and the second, the last two terms.

Equations (30) and (31) permit the setting of limits

for the heterogeneity in component-A as a function of p„ R

based upon knowledge of <f> R and/or $ a /tv ^he limits are

represented graphically in Figure 5^> which also indicates

some concomitant restrictions on <j> R and/or 4>a/ r

- If one has available chemical or physical information

which may be used to restrict the range of p then, of course

the curves in Figure 5^ may be used to derive <J>. limits.

which are correspondingly narrower based upon this additional

constraint

.

82
[.-o]

[+. *A/e] °0

AB

r^/B-o]

Figure 54. Limits for o./o^. vs p no A d Ad

Extending the study to the case where cf> R and <J>«/R

(but not P AR ) are known leads to the inequality given in

Equation 29. One conclusion, which will be dealt with in

<f> A/B more detail elsewhere is that if the ratio, is

-1/2 ^R less than 5 , <K is quite well approximated (within

l /? about 10$) by <|> R , whereas, if the ratio exceeds 5 , B

* * Ar A/B

This study has been applied to Ca variation in chemical

microbeads, as previously stated, in which the variability

of sulfur and of the ratio Ca/S were employed to deduce

limits for that due to Ca, and in which no knowledge of

the correlation coefficient, o n Q , was assumed. The lower

limits for <j> p as reported in ref. [24]. was found to be

o a zero, while the upper limit (Equation 29) was 7%.

(L. A. Currie)

83
G . High Sensitivity (Radio) Chemistry

Unique opportunities and problems associated with the

detection or determination of fewer than one billion atoms

of radioactive or stable nuclides have recently presented

themselves as a result of (1) a study for the ICRU on

The Measurement of Low Level Radioactivity [26], and

(2) research in nuclear chemistry and activation analysis

in which the probability of nuclear reaction, leading to

a given residual nucleus is extremely small. The small

reaction probability generally results from a small nuclear

cross section in the case of nuclear chemistry, or small

(trace) amounts or concentrations of target nuclei in the

case of activation analysis.

In the absence of interference or contamination a number of the methods used in this laboratory - - for

example, direct measurement of long-lived radionuclides,

and measurement of stable nuclides by means of nuclear

activation or static mass spectrometry - - permit the

o detection of femtomole (^6x10 molecules) quantities. The

fundamental limitations in this case include detector

background, detection efficiency and chemical or physical

yield, and statistical fluctuations in the signal, itself.

Although extremely powerful means exist for detecting

pure materials, reagent and/or sample contamination frequently fix the actual detection limits at levels far in

excess of those which characterize the instrument capabilities. To cite an extreme case, the sea-level flux of

secondary neutrons originating from cosmic ray interactions

in the atmosphere is sufficient to produce a steady state

concentration of 32 P in HC1 of about 7000 atoms/kg. (Note

that the conventional 6 lb bottle of concentrated hydrochloric acid contains about 1 kg of HC1 . ) Beta decay of

the 14. 3 day J P in turns leads to a linear increase of

op

(stable) J S which is equivalent to an impurity concentration

84
of 7-^ ppT/10 years. Although such an impurity level of

stable sulfur is trivial, even for trace analyses, the

| impurity level of radioactive phosphorus might cause

difficulty in specially low-level radiochemical procedures

in which quite large amounts of HC1 are employed as a reagent [27]

.

In addition to chemical (or radiochemical) Interference,

two forms of contamination are particular noxious [28],

The first, isotopic contamination, is illustrated by the

85 production of Kr in nuclear fission such that the specific

radioactivity of atmospheric krypton has been raised to about

140 pCl/mmole. This contamination has posed a significant

On

problem for the detection of cosmic ray produced Kr

,

whose atom concentration in atmospheric krypton is one

85 million times smaller than that of Kr [29]. A still

more subtle form of contamination is isonuclidic . Here,

the reagents or samples are contaminated with the very

nuclide sought, and therefore, neither physical nor chemical

methods may be applied for separation. H serves as one of

the most characteristic examples. In an international comparison of low level tritium samples, "Ostlund reported

measurements in which mine water used in the preparation

of the lowest concentration HTO samples was shown to contain

a tritium concentration (T/H) of less than 10~ [30].

Due to the proximity of nuclear reactors, however, one of

the participating laboratories found levels In the blank

solution some ten to twenty times higher.

When one is faced with forms of interference, other than

isonuclidic, the detection limit can often be substantially

reduced [31] by chemical or physical separations according

to Z (atomic number) or A (mass number) or both. Figure 55

illustrates a general scheme for sample purification in

order to concentrate the radionuclide of interest as well

as to separate it from interfering radionuclides. The

85
BULK

GROUP

Z

(Chem.)

A

(Mass)

Spinach

Ash

(non-destructive)

T

Insol. Carbonates

Ce-141, 144

ZA

(Generic)

Ce-144 Pr-144

ZA

(Decay

Char.)

7(Ce-l44)

Figure 55. General scheme for low level radiochemical

separations and identification.

four stages of separation shown vertically (bulk, chemical

group, chemical element (Z), and mass (A) which may serve

as increasingly specific interference-reduction alternatives

to non-destructive analysis, which is represented by the

dashed, horizontal line. Two modes of "ZA" discrimination

are represented by horizontal, solid lines. Both modes

depend upon nuclear characteristics: the first, upon the

nuclear decay scheme; and the second, upon the unique

parent-daughter chemical relationships resulting from

radioactive decay. The particular radiochemical analysis

represented in the figure, Ce-l44 in spinach, has been more

fully discussed in the literature [32].

Mass separation is highly desirable when one encounters

isotopic contamination. Figure 56 illustrates schematically

the approach taken in this laboratory for the physical separation of 1-131 from 1-125 which was present in the original

sample as a diluting radioisotope [33].

86
Mass Profile (1--125 tr<

122 ~ .001

123 .002

124 .008

125 1.000

126 .009

127 .002

128 - .002

Sample

n—Activation I

Sample*

Z (-Br2 ) |~

Agl (10 mg)

A (M.S.)

J

1-131 (Al)

Br - decontamination ~ 10-7

Level too low for7-spectral resolution (ZA)

Figure 56. Mass separator experiment.

In order to give prespective to the enormous sensitivity

which may be achieved by a judicious combination of the

separation/identification methods depicted in Figure 55,

summary of a characteristic series of experiments involving

the detection of extremely small amounts of material (10 to

10 atoms) has been collected in table 6. The experiments

have been listed in order of decreasing element concentration,

and the principal means (in addition to bulk concentration)

for purification have been included. The first two experiments refer to nuclear reaction studies underway at NBS; the

effects of cosmic rays on HC1 have already been discussed;

and the last three experiments represent large scale studies

related to cosmic ray interactions in the vicinity of the

earth. Note that the concentration of Ar which may be

detected in CCIl as shown in last row of Table 6 is less

than one atom per 80 moles (CI)! It should be clear from

the table that in the most highly sensitive experiments

even so rare a process as cosmic ray contamination of the

sample or the reagents indeed may not be negligible.

(L. A. Currie)

87
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h. NUCLEAR INSTRUMENTATION

A

.

Introduction

This activity has been transferred from this section as

of July 1, 1969. A new group called the "Technical Support

Group" will be formed with staff from this section and section

no. 8. To some degree this arrangement has been operating

already. For example, Mr. Shideler has been detailed from

this section to section 6 for the past two years.

The new Technical Support Group will be staff support

to the Chief of the Analytical Chemistry Division. Its

members are: B. Bowman, F. A. Lundgren, J. Matwey, T.M.

McCleod, F.C. Ruegg and R. W. Shideler. Further information

on the function of this group will appear in a new progress

report next year.

B. Optical Drive

1 . Introduction

A Mossbauer Spectrometer of the constant acceleration

type which uses an interferometer to sense and control the

velocity with high accuracy and precision has been built

at the National Bureau of Standards. The spectrometer

consists of five component parts: (1) the drive electronics,

(2) the optical interferometer, (3) the frequency ratio

detector, (4) frequency synthesizer and its control logic,

and (5) the drive transducer, [9].

In figure 57 a block diagram of the system is shown.

The spectrometer's velocity input signal is derived from

the multichannel scaler's address analog [39] which is

a sawtooth voltage waveform vs time. The drive electronics

amplifies this signal and compares it with two feedback

signals, one from the linear velocity transducer and one

from the optical system. The feedback from the optical

system is derived from the fringe frequency compared to

the frequency of the digitally controlled frequency synthesizer. When the two frequencies are unequal a correction

signal is applied to the integrator which makes a continually

89
DATA INPUT

MULTICHANNEL

ANALYZER

DIGITAL ADDRESS

LINES

+ + ++ +

ADDRESS

RESET

ANALOG

OF ADDRESS

DIGITAL CONTROL

FOR FREQUENCY

SYNTHESIZER

FREQUENCY

SYNTHESIZER

OPTICAL

INTERFER- OMETER

FREQUENCY

OUT

FRINGE PULSE TRAIN

FREQUENCY

RATIO

DETECTOR

GAMMA RAY

DETECTOR

SYSTEM

INPUT

DRIVE

ELECTRONICS

DRIVE MOTOR

FRINGE

DETECTOR

OUT

NTEGRATOR

LINEAR VELOCITY TRANSDUCER

FEEDBACK

SIGNAL

INTERFEROMETER

FEEDBACK

SIGNAL

Figure 57. Block diagram of the optical drive.

larger correction until the correct velocity is achieved.

The fringes or the gammas coming from the source can now be

counted. The fringe count will be proportional to the velocity

in each channel and the velocity can then be calculated from

the fundamental values of the wave length of the interferometer and time with a high degree of accuracy.

90
2 . Drive Electronics

The drive electronics are an improved version of the

electronics as published before [9, ^0, 4l], and the new

circuits will be described in detail here. The electronics

can be divided into three parts: (1) drive amplifier,

(2) photocell trigger circuit and (3) logic circuit. The

drive electronics are designed to control our new transducer

[9]. The electronics package is designed to be assembled

into a triple width Nuclear Instrument Module, and it derives

all of its operating voltages from a NIM standard bin and

power supply. (See figure 58).

The drive electronics will operate in a nonoptical

Mossbauer spectrometer system by eliminating all of the

drive components which are related to the optics and the

optical feedback path.

a. Drive Amplifier

The schematic of the drive amplifier is shown in

Figure 59. The analog of the address scaler is brought

in on Pin 21 of the drive amplifier circuit board. It

is summed with a DC (direct current) level from potentiometer, R20, so that it will be symmetrical with respect

to ground. The ramp is then summed with the correction

signal produced by the optical interferometer, and its

amplitude is adjusted by R22. The ramp signal is now

compared with the output of the linear velocity transducer

by operational amplifier number one. Amplifier nos . 2 and

3 amplify the signal and control the frequency response

so that it can be applied to the symmetrical power amplifier.

The power amplifier consists of four transistors: MD985 a

dual package and STC 1080 and 2N3171 power transistors

mounted on the chasis for the purpose of heat dissipation.

The power amplifier has a voltage gain of 1.

91
"•> - Itfefcw

Figure 58. Photograph of Doppler drive electronics.

92
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The purpose of the photocell trigger circuit is to

shape the signal arriving from the photocell position sensor

mounted on the transducer, to generate a fast rise-time

pulse to control some of the logic circuits, and to generate an address scaler reset within the multichannel scaler.

The circuit also has a bistable multivibrator to generate

the input signal when the spectrometer is operating in

the constant velocity mode.

A schematic of the photocell trigger circuit is

shown in Figure 60. The photocell signal enters the circuit

board on pin 5. When the slit is between the lamp and the

photocell the light illuminates the photocell and its

resistance diminishes. This applies a slowly varying

positive voltage to the anode of tunnel diode, Dl, through

an input sensitivity adjustment, Rl . When the peak point

of the tunnel diode is reached it rapidly changes state,

and when it does, transistor Tl is driven into saturation

with a rise time O.lys. This pulse is now inverted by

transistor T2 and both of them are capacitor coupled to

switch #1 which is used to select either the positive

or negative 10V pulse for the address reset output to the

multichannel scaler. The positive pulse is also used to

trigger the bistable for the constant velocity generator,

and it is brought out for use on the logic board on pin 1

.

The constant velocity generator consists of a bistable (T3,

T4) and a DC level shifter circuit (T5, R16-R19). T5 inverts

the output of one side of the bistable and shifts its level

to +12V. This signal is now summed with the complementary

output of the bistable through potentiometer R19, and by

adjusting R19 the voltage can be made symmetrical about any

DC level contained within ±12V with a constant 12V range.

94
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c . Logic Circuit

The logic circuit was designed and built to interface the data signals to the multichannel scaler, to provide

a safety interlock circuit, and to gate the data signals for

an integral number of sweeps. The circuit is shown in figure

61.

The data input signal (either a pulse from the

detector system or a pulse from the fringe discriminator)

enters the board on pin 5. It is coupled to a monostable

multivibrator by capacitor C3 and coupled into the data gate

by capacitor CI. The monostable shapes the pulse so that

they are suitable for controlling the delayed coincidence

gate on our multichannel pulse-height analyzer. This feature

allows one to accurately set the single channel analyzer

on the peak in the spectrum of interest for MSssbauer studies

.

The data gate generates pulses to the time mode data input

of the multichannel analyzer only under the following conditions: (1) the drive is running, (2) the detector gate

switch is held in the enable position until the control

bistable (T7, T8) is set. The control bistable is set by

the first photocell trigger pulse that occurs while the

detector gate switch is held in the enable position. When

the detector gate switch is held in the disable position,

the control bistable will be reset by the next photocell

trigger pulse, i.e. the data input signals are only counted

for an integral number of cycles of the Mossbauer drive.

The interlock circuit protects the drive against

photocell lamp burnout and it also interlocks the spectrometer

to the analyzer so that the analyzer must be in the time

mode store program before the power amplifier of the drive

will operate. The interlock circuit consists of an "and"

gate and a relay. The relay controls the DC bias to the

power amplifier so that all of the conditions for an "and"

condition must be present before the relay will be activated.

96
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The conditions for an "and" are: (1) the analyzer must be

in the time mode store program, (2) the photocell lamp must

not be burned out and (3) the power switch on the front panel

must be on. The interlock circuit also must be in a "go"

position for the gate to be enabled, so that if the lamp

burns out during a spectrum accumulation the storage of bad

data will not occur. The condition of the gate is indicated

by a front panel lamp which is on when all of the data gate

conditions are satisfied.

d . Physical Description

(1

)

Layouts

The physical layout of components on the chasis

is illustrated in Figure 62 a,b,c. Figure 62a shows a layout

of the front panel. Figure 62b shows the layout of the chasis

and Figure 62c shows the layout of the back rear panel.

The next 3 figures, 63-5 , show the layout of the

printed circuit cards.

( 2

)

Wiring List See Appendix II

The wiring list is one which is redundant

,

but because of its redundancy it is a self check of almost

all of the wiring. The unit is built with a one point ground

system to avoid ground loop problems.

(3) A parts list can be made available

upon request

.

3 • Frequency Ratio Detector

a. Introduction

It was decided to use the information generated by

the interferometer in a feedback loop to control the drive's

velocity. Because the fringe frequency contains the velocity

information, it was necessary to compare the fringe frequency

with a known frequency and to provide a signal which is

suitable for use in a negative feedback loop. The frequency

ratio detector was designed and built to fulfill this need.

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b . Description

The logic of this device is described elsewhere l39l

,

and it is only necessary to note some of the modifications

which were required for our particular problem. The fringe

detection system does not determine the direction of the

velocity vector, and therefore the only information obtainable

from the fringe frequency (Pf) is the magnitude of the velocity

or Ff a |v| . Because the sign of the velocity changes, it is

necessary to change the sign of the correction signal, and

this is accomplished by interchanging the two inputs to the

frequency ratio detector. The inputs are interchanged at the

time of transition of the "100s" bistable in the multichannel

analyzer, and the controlling signal enters the frequency

ratio detector through the velocity sense input

.

The output of the frequency ratio detector is integrated,

and then summed with the analog of the address scaler to produce the negative feedback control of the spectrometer's

velocity

.

k . Control Logic for Frequency Synthesizer

a

.

Introduction

Because the frequency ratio detector needs a source of

frequency with which to compare the fringe frequency, it

was necessary to design and build a digitally programmable

frequency source. This was done by interfacing a remotely

programmable frequency synthesizer to the digital address

scaler

.

b

.

Interface Description

In Figure 66 a block diagram of the control logic is

shown. The binary coded decimal (BCD) address scaler information is decoded into decimal by the digital decoder. The

decimal information is then put into a circuit which either

transfers the numerical information unchanged or takes its 9

T s complement. For example if the number were 28 it would be

transferred as a 71 in the 9's complement mode, or as a 28

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in the unchanged mode. It is necessary to complement the

channel address number in channels 0-99 so that the synthesizer's frequency is a decreasing function vs time, as is

the fringe frequency. The address number should be passed

unchanged in channels 100-199 so that the synthesizer's

frequency is an increasing function vs time, as is the

fringe frequency. The circuit which complements the address

information is controlled by the analyzer's 100 bistable.

The numerical information is then transferred to the synthesizer's remotely programmable digital units by the synthesizer

driver

.

Performance of this spectrometer will be described in

detail in a manuscript to be submitted for publication and

in next years progress report

.

(F. C. Ruegg and J. J. Spijkerman)

105
5. RADIOISOTOPE TECHNIQUES

A . Introduction

Radioisotope techniques have been used in many analytical

techniques to provide data on the adequacy of separation procedures and to generate quantitative results . Quite often

the results provide valuable information which is difficult

to obtain in other ways . A considerable enhancement to the

value can be realized if the precision of such tracer experiments could be improved. It is in this area that our attention is being focussed.

Reproducibility of the radiation detection process is

being evaluated along with the chemical techniques to determine which variables are limiting. In particular, for

macro-constituent elemental quantitative analysis it is

important to utilize reproducible chemical reactions in the

separation process. The substoichiometric approach may be a good one for improving this precision. This may be the case

particularly when interfering chemical reactions are likely.

In substoichiometric radioisotope dilution analysis

(SRDA) (4l-2), the need to isolate the element of interest

quantitatively (a requirement of most of the other analytical techniques) is eliminated and the determination of the

specific activities, before and after dilution by the

analyzed element, can be replaced by activities in the

relation

:

X = Y

'1 - 1

L

A

2

Where X = the amount of element to be determined; Y = the

known amount of element in the standard solution; A-. = the

radioactivity of the standard solution and A„ = the activity

of the unknown solution. The activities A. and A„ are

usually obtained as counting rates with a suitable electronic counter, thus enabling one to associate a minimum

relative error with the counting rate which would be

106
affected only by counting statistics if experimental conditions are kept constant (i.e., constant geometry maintained

and repeated measurements made [43]). In most cases, the

error associated with Y is known and very small, therefore,

one should be able to do chemical analysis (determine X)

very accurately and precisely with the substoichiometric

radioisotope dilution technique.

The first part of this report describes the counting

system used for determining the activities A, and A

?

mentioned earlier and the precautions taken to yield the

minimum error obtainable. The latter part of this report

describes applications of the SRDA technique to several

elements

.

B . Error Analysis of Counting System

1

.

Instrumentation

The instrument used for our work is very simple and

economical, which is a great advantage to low budget institutions interested in high precision quantitative analysis.

It consists of a 2in. x 2in. Nal(TI) well-type scintillation

detector, mounted very rigidly so there would be no vibration,

enclosed in a four inch thick shield built with lead bricks

and a single channel pulse height analyzer. The use of this

quality of shielding is not critical at the count rates used.

The system is used for other counting purposes in addition

to this one. A plastic liner which was made to fit firmly

into the detector serves a dual purpose, to hold the counting

vessels (vials which are screened for uniformity) in a constant geometry and to prevent any contamination of the detector. Photographs of the detector and its accessories are

shown in Figures 67 and 68. The cost of this system is less

than thirty-five hundred dollars

.

2

.

Calibration

Counting the same volume of sample solution each time

allows one to maintain the constant geometry with the system

107
Figure 67. Photograph of radiation detector assembly.

previously described, so that the error in the counting rates

(A-. and A

?

) will be due mainly to counting statistics.

Cobalt-60 was chosen as the standard isotope to calibrate and

determine the performance of this counting system. The controls are adjusted on the analyzer to exclude bremsstrahlung

1Q8
Figure 68. Photograph of electronic apparatus medium

count-rate capability.

so that only the photopeak of interest is counted. This

reduces the sensitivity of the system to source-position

errors. Counting an aliquot of Co repeatedly and collecting

at least 4 x 10 counts, resulted in a reproducibility of

0.04% relative standard deviation of a single measurement.

Further study is underway, utilizing high speed electronic

equipment (scaler, timer, etc.) which may improve our precision by a factor of ten.

C . Application to Standard Reference Materials (SRM)

Our research efforts have been directed to those elements

in which this technique would be superior to other existing

quantitative analytical techniques (e.g. spectrophotometry,

109
gravimetry) and thus play a vital part in the SRM certification program at the National Bureau of Standards. At the

same time a high degree of flexibility is maintained so that

the method developed could be adapted easily for both routine

analyses and special cases.

Before an element is considered a candidate for analysis

by the SRDA technique, it must have a long-lived radioisotope

obtainable with a high specific activity. Table 7 lists those

elements where use of the SRDA technique would be advantageous

not only to the Bureau's SRM program, but to industrial laboratories as well, and a significant contribution to the state

of the art is accomplished simultaneously with the development

of these methods.

Table 7. Elements that can be analyzed by the substoichiometric radioisotope dilution analysis technique

in the Standard Reference Materials Program.

Antimony Niobium

Cadmium Silver

Chromium Tin

Cobalt Titanium

Iron Tungsten

Mercury Zinc

Previous reports [3-^1] described the use of the SRDA

technique for trace elemental analysis (Co, Cd, Ag, etc.)

in several Standard Reference Materials. In this report we

will describe the SRDA methods developed for tungsten and

iron and discuss the results obtained when SRM 480 and SRM

1159 and 1160 were analyzed for these elements using this

technique

.

110
1 . Determination of Tungsten in Molybdenum Containing

Materials Using Toluene-3 ,4-Dithiol

There are several reagents which produce colored complexes

with tungsten (44) thus providing the basis for numerous

spectrophotometric methods for the determination of tungsten

in various materials. However, these reagents also produce

colored complexes with molybdenum, and since most tungsten

containing materials also contain molybdenum, an interference

problem exists with these methods. Most colorimetric methods

which result in the formation of stoichiometric complexes

can be adapted to radioisotope dilution techniques.

Toluene-3 ,4-dithiol forms colored complexes with several

elements [45] including tungsten and molybdenum in various

materials. Several workers [46-50] have developed colorimetric

methods for determining tungsten and molybdenum but the success

of these methods is very dependent upon the control of the

problems (quantitative separation of each element, interferences, color stability, etc.) inherent in photometric methods.

Thus arising from the need of a method in our laboratory for

tungsten determinations in the presence of molybdenum, we

developed the following radioisotope dilution procedure for

tungsten using "dithiol"

.

a . Experimental

The analyzed samples were special cuts from NBS W-Mo

Alloy (SRM-480). Each sample (^5 mg) was etched in a freshly

prepared mixture of 10 ml concentrated HNO 10 ml cone.

H

2

SO i[ and 10 ml H

2

for 15 minutes. The samples were rinsed,

dried and weighed. The weight loss from this etching step

is very small (< .5%) . Each sample was dissolved in approximately 10 drops HF plus 3 drops HNO heated until NO was

driven off, diluted to 5 ml in a volumetric flask with 1 M

HC1 and then transferred immediately to a polyethylene bottle.

Aliquots of these samples were taken for analysis . Prior to

tungsten analysis, a molybdenum separation was carried out

111
as follows: 8.5 ml HC1 (sp. gr . 1.06), a known amount of W

-i O -i carrier (Y) W and 4 drops of 10% NH

2

0H.HC1 were added to

the sample and mixed. Then 0.15 ml of 0.5% dithiol solution

was added. After 5 min the Mo-dithiol complex was extracted

into CCIk (four times using 5 ml portions). The aqueous

phase (~10 ml) was evaporated very carefully to 2 ml and

25 ml hot titanous sulfate and 20 ml hot cone. HC1 were

added. To the mixed solution 0.20 ml of the dithiol solution

was added, mixed and allowed to stand for 90 minutes, after

which the W-dithiol complex was extracted into 10 ml CHCl^,

and 0.50 ml aliquots of both phases were taken for counting.

A control (i.e., all ingredients added except the sample)

is subjected to the same procedure, providing A-. . If no molybdenum is present, the Mo-dithiol complex

formation can be omitted, and the remaining part of the procedure would be carried out as described. The amount of

tungsten present is calculated using equation 32.

b . Results and Discussion

The results obtained by this radioisotope dilution

method are reported in a note to be published in the journal,

Analytical Chemistry. The following is an abstract of

this article

:

RADIOISOTOPE DILUTION ANALYSIS OF TUNGSTEN AS A MAJOR

CONSTITUENT IN MOLYBDENUM CONTAINING MATERIALS

USING TOLUENE-3,4-DITHIOL

ABSTRACT

A radioisotope dilution procedure was developed for

tungsten determination in molybdenum containing materials

using a substoichiometric amount of the complexing reagent,

toluene-3, 4-di thiol. Using this approach, an equal amount

of tungsten is isolated before and after dilution of the

radioisotope, thus avoiding serious interferences by eliminating the need for quantitative separation. This tech112
nique was used for tungsten determination in NBS tungstenmolybdenum, where the average concentration found was

79 . 31% with a pooled estimate of the standard deviation

(for a single determination) of 0.43%. The precision

obtained is very good for this technique

.

2. Determination of Iron in SRM 1159 and 1160

Iron is one of the elements which is very often certified at varying concentrations in (SRM) steel samples at

the Bureau of Standards. A radioisotope dilution procedure

was developed for iron which utilizes solvent extraction [51].

This procedure offers an attractive alternative to existing

chemical methods for the determination of iron because it

provides a rapid analysis with very good precision and can

be adapted to iron concentrations at all levels . Iron was

determined in SRM 1159 where it is the major constituent and

in SRM ll60 where it is a minor constituent in a nickel matrix

a. Experimental —±—

Samples of these SRM steels weighing one gram

were dissolved in concentrated HC1-HN0 mixtures (minimum

amount) and diluted to 100 ml in volumetric flasks with H 0.

Aliquots (1 ml - 3 ml) of the samples were placed in separatory funnels along with a known amount of Pe +3 (Y)

,

59 Fe

,

3 ml of 30% H

2 2

and mixed well. 0.75ml of 2% cupferron was

added to each, mixed, and the Fe-cupferron complex extracted

with 10 ml CHCl^. The volume of the aqueous phase is adjusted

to 10 ml with water before extraction. A control (A ) was

subjected to the same procedure. Aliquots (0.5 ml) of both

phases were taken for counting and the amount of iron present

was calculated using equation 32.

b

.

Results and Discussion

The results of the iron analysis are shown in Table

8. The precision obtained with this technique is excellent.

Precautions must be taken to insure that all of the iron

113
present is in the ferric state. For this reason hydrogen

peroxide is added in the procedure

.

Iron is an element that is of biological interest.

Thus, this technique can be used for iron determinations in

varied matrices with rapidity and very low cost.

Table 8. Analysis of Iron in Standard Reference Materials

SRM % F e Average

1159 51.21 51.19i0.008 1

51.19

51.19

51.18

51.20

51.19

51.17

51.19

1160 14.51 I4.50t0.008 1

14.49

14.50

14.51

14.51

14.50

14.50

t s 1 Errors are given as : at the 95% confidence level [52]

'n

(Lottie T. McClendon)

114
J. C

p. A

M. J

W. L

L. J

P. P

6. 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 (5/6)

W. E. Critchley (1/6)

Mossbauer Spectrometry

J. J. Spijkerman (1) Project Leader

Travis (1)

Pella (1/4) One year leave of absence

Bielefeld (3/4) Post Doctoral Program

O'Neal (1/2)

J. Romanowski (1/4)

P. Stampfl, Guest Worker, NRC Resident

Research Associate, work

supported by NASA

Nuclear Chemistry

L. A. Currie (1) Project Leader

H. L. Steinberg (3/4)

Nuclear Instrumentation

P. C. Ruegg (1) Project Leader

M. E. Stalbird (1/2) transferred to Sec. 501.01

P. A. Lundgren (1/2) transferred to Sec. 310.08

T. M. McLeod (1/4)

Special Project in Nuclear Instrumentation

R. W. Shideler (1) Project Leader, on detail

to Section 310.06

Radioisotope Techniques

L. T. McClendon (1)

115
PUBLICATIONS AND MANUSCRIPTS, JULY 1968 TO JUNE I969

1. DeVoe, J. R., Editor, LaFleur, P. D., Asst. Editor

"Modern Trends in Activation Analysis" Proceedings

of the 1968 International Conference on Modern

Trends in Activation Analysis, October 7-11, 1968.

National Bureau of Standards Special Publication

312, Vols. I and II, 1334 pp., June 1969. Superintendent of Documents, U. S. Government Printing

Office, Washington, D. C. 20402.

2. DeVoe, J. R. Editor, NBS Technical Note 451, 1-119,

(January 1969). Superintendent of Documents, U. S.

Government Printing Office, Washington, D. C. 20402

3. Pella, P. A. and DeVoe, J.R., Snediker, D.K., May, L.

Problems in Using Spectrometry for Quantitative

Analysis: Applications To Tin, Anal. Chem. 4l , 46-50 (1969).

4. Spijkerman, J.J. r-, Mossbauer Spectroscopy of Ni , "The Mossbauer Effect"

Butterworth and Co. Ltd., London, England (1968).

5. Terrell, J. H. and Spijkerman, J.J.

Determination of Surface Compound Formations by

Backscatter Spectroscopy, Appl. Phys . Letters

13, 11-13 (1968).

6. Travis, J.C. and Spijkerman, J.J.

Mossbauer Spectroscopy Using ^Ni, Mossbauer

Effect Methodology, J. J. Gruverman, ed., Vol. 4,

Plenum Press, N.Y., (1968), pp. 237-259-

7. Currie, L.A.

The Discovery of Errors in the Detection of Trace

Component in Gamma Spectral Analysis, Proceedings

of the 1968 International Conference in Modern

Trends in Activation Analysis, Vol. 11, p. 1215,

June (1969).

8. Qureshi, I.H., McClendon, L.T., LaPleur, P.D.

Radiochemical Separations for Activation Analysis

Using Bis ( 2-ethyl-hexyl ) Orthophosphoric Acid,

Proceedings of the 1968 International Conference

in Modern Trends in Activation Analysis, Vol 1,

p. 666, June (1969).

116
TALKS, JULY 1968 TO JUNE I969

DeVoe, J. R. - General Chairman, International Conference

on Modern Trends in Activation Analysis held at NBS

October 7-11, 1968.

DeVoe, J. R., "Activities of Radiochemical Analysis

Section", Mound Laboratory, Monsanto Chemical Society

Meeting, September 8-13, 1968, Atlantic City, N. J.

DeVoe, J. R. , Chairman of Session in Symposium on Mossbauer Spectroscopy at American Chemical Society Meeting

September 8-13, 1968, Atlantic City, N.J.

DeVoe, J. R., Chairman of Session on General Activation

Analysis at American Chemical Society Meeting, Sept.

8-13, 1968, Atlantic City, N.J.

DeVoe, J.R., Participated in panel discussion on Radioactivity Standards at American Chemical Society Meeting,

September 8-13, 1968, Atlantic City, N.J.

DeVoe, J.R., "Quantitative Measurement of Chemical

Structures by Mossbauer Spectrometry" at American

Chemical Society Meeting, September 8-13, 1968,

Atlantic City, N.J.

DeVoe, J.R., "Nuclear Techniques in Analytical Chemistry"

American Chemical Society, Rochester, New York Chapter.

March 1969.

DeVoe, J.R., "Opportunities at NBS in Materials Characterization", Eastern Kodak Corporation, Rochester,

New York, March 1969.

DeVoe, J.R., "Nuclear Techniques in Analytical Chemistry"

University of West Florida, Pensacola, Florida, April

1969.

DeVoe, J.R., "Radiochemical Analysis" University of

Florida, -Gainesville , Florida, April 1969.

DeVoe, J.R., "Standard Reference Materials for Chemical

Shifts and Velocity Calibration in Mossbauer Spectroscopy", Society of Applied Spectroscopy, Chicago,

Illinois, May 1969.

Currie , L.A., "The Discovery of Errors in the Detection

of Trace Components in Gamma Spectral Analysis",

International Conference on Modern Trends in Activation

Analysis, October 7-11, 1968, NBS, Gaithersburg, Md

.

117
u 13. Currie, L. A., Chairman of two sessions on "General

Tracer Applications" and "General Radiation Chemistry",

American Chemical Society Meeting, September 8-13,

Atlantic City, N. J.

14. Qureshi, I. H., and McClendon, L.T., "Radiochemical Separations for Activation Analysis Using Bis ( 2-ethyl-hexyl

Orthophosphoric Acid" International Conference on Modern

Trends in Activation Analysis held at NBS, October 1968.

15. McClendon, L.T., "The Application of Bis ( 2-ethyl-hexyl)

Orthophosphoric Acid to Radiochemical Separations"

American Chemical Society Meeting, September 8-13,

1968, Atlantic City, N.J.

16. Spijkerman, J.J., "Symposium on Mossbauer Spectroscopy".

Organized symposium and chaired one session at American

Chemical Society Meeting, September 8-13, 1968,

Atlantic City, N.J.

17. Spijkerman, J.J., "Resonant Detectors for Mossbauer

Spectroscopy", American Chemical Society Meeting,

September 8-13, 1968, Atlantic City, N.J.

18. Spijkerman, J.J., "Applications of Mossbauer Spectroscopy to Nondestructive Testing", Symposium on

Nondestructive Testing, O'Hara International Airport, Chicago, Illinous , September 23, 1968.

19. Spijkerman, J.J., "Analysis of Corrosion Products by

Mossbauer Effect Spectroscopy", American Society

for Metals, Los Angeles, California, March 1969.

20. Spijkerman, J.J., "Magnetic Interactions in Nickel

Compounds", Society of Applied Spectroscopy,

Chicago, Illinois, May 1969.

21. Spijkerman, J.J., "Application of Mossbauer Spectroscopy

to Coordination Chemistry", Mossbauer Spectroscopy

Institute, Catholic University, June 1969.

22. Spijkerman, J.J., "Instrumentation in Mossbauer Spectroscopy", Mossbauer Spectroscopy Institute, Catholic

University, June 1969.

23. Travis, J.C., "Mossbauer Spectroscopy with Ni Quadrupole Interactions", American Chemical Society Meeting,

September 8-13, 1969, Atlantic City, N.J.

24. Travis, J.C., "The Electric Field Gradient Tensor",

Mossbauer Spectroscopy Institute, Catholic University,

June, 1969.

118
7 . ACKNOWLEDGEMENTS

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

and in the LINAC Building. Of even greater importance is

the excellent assistance we received from Dr. G. Lutz of

the Activation Analysis section (310.08) in producing the

cobalt-6l Mossbauer source.

We wish to thank Dr. Brian Joiner of the Statistical

Engineering Laboratory for repeated guidance in the experimental design and interpretation of data.

We wish to express our appreciation to the U. S. Atomic

Energy Commission, Division of Isotope Development, for their

financial assistance in the development of the resonant

detector

.

Special appreciation is expressed to Mrs. W. Critchley

who has assumed the almost overwhelming burden of section

secretary and in spite of it all still managed to type this

entire report

.

119
REFERENCES

[1] DeVoe, J. R., Editor, NBS Technical Note i* 5 1 1-119 (January 1969). Superintendent of Documents, U S Government Printing Office, Washington, D. C. 20402.

[2] Marquart, D. W., J. Soc . Appl. Math. 11 431-42| (1963).

[3] DeVoe, J. R., Editor, NBS Technical Note 404, Superin- tendent of Documents, U. S. Government Printing Office Washington, D. C. 20402.

"^

[4] Travis J. C. and Spijkerman, J. J. Mossbauer Spectroscopy Using blNi, "Mossbauer Effect Methodology" J J Gruverman, Ed., Vol 4. Plenum Press, N. Y. (1968)'pp"

[5] Johnson. 0. W. , Phys. Rev. 136 A284 (1964).

[6] Kingsbury, P. J., Doctoral Thesis, Univ. of Utah, 1968.

[7] Wittke, J. P., j. Electrochem. Soc. 113, 193 (1966).

[8] Alam, M Chandra, S. and Hoy, G. R. , Phys. Letters 22 , 26 (1966 ) .

[9] DeVoe, J. R., Editor, NBS Technical Note 421, Supt of Doc, U.S. Government Printing Office, Washington, D. C

[10] Wintersteiner, P., NASA, private communication.

[11] Eyring H Walter, J., and Kimball, G., "Quantum Chem- istry , John Wiley and Sons, New York, I960, p. 59.

[12] Ibid, p. 162.

[13] Rose M. E., "Elementary Theory of Angular Momentum", John Wiley and Sons, New York, 1957, p. 660.

[14] Eyring, op, cit . , p. 367.

[15] Eyring, op. cit ., p. 389.

[16] Nicholson, W. E. , Schlosser, J. E., and Brauer P P

Nucl. Inst. Meth. 25, 45 (1963); Pasternack /b .

,

and Luizzi, A., Biometrices 2_4, 353 (1968); Parr, R. M. and Lucas, H. P., Jr., Proc . Ninth Scintillation'semi-

conductor Symposium, IEEE Trans. Nucl. Sci. NS-11 349

•

(1964; Trombka, J. I., NAS-NS-3107, 183 (1963)

120
[17] Currie, L. A., Anal. Chem. 40_, 586 (1968). See also

reference given in abstract contained in text.

[18] Currie , L. A., "Peak Height Method for Evaluating Nuclear

Spectra:, 155th National ACS Meeting, April 1968., San

Francisco, Calif.

[19] "The (y,xn ) Reactions on Copper-65 and Copper-63 from

19.3 MeV to 166 MeV", M. S. Thesis of Harold Steinberg,

Maryland University, (1966).

[20] The two most widely used methods of unfolding data are

discussed by Penfold and Leiss, Phys. Ref., 114 , 1332

(1959); and Cook B. C, Nuc . Instr. and Meth . , 2_4,

256 (1963).

[21] The energy dependence of the giant resonance may be

approximated by a Lorentz-shaped resonance line (Evans

Hayward, Photonuclear Reactions , Scottish Universities,

Summer School , 1964 ) and as the (y,n) channel is the

principal one below 20 MeV it is not unreasonable to

attribute such a shape to a(y n\ to 20 MeV, at least.

This assumption is reasonably ' corroborated, at least

for previous low-resolution experiments, for the (y,n)

reactions on 65cu and 63cu.

Sund, Baker, Kull, and Walton, Phys. Rev. 176 , 1366 (1968)

Fultz, Bramblett, Caldwell, and Harvey, Phys. Rev., 133

B1149 (1964.

Freeman, D. H., Currie, L. A., Kuehner, E. C., Dixon,

H. D. and Paulson, R. A., Anal. Chem. (In press).

Cameron, J. M., Statistics "Fundamental Formulas of

Physics" Menzel, D. H. , Editor, Dover Press, New

York, 1966.

[26] The Measurement of Low Level Radioactivity, report in

the final stages of preparation by ICRU Task Group IA

.

[27] For Measurements of Cosmic-ray Produced Nuclides in

Chemical Reagents, See: Rama and Honda, J. Geophys

.

Res. 66, 3533 (1961).

[22]

[23]

[24]

[25]

121
[28] For an Extensive Survey of Radioactive Contamination,

see: J. R. DeVoe, "Radioactive Contamination of

Materials Used in Scientific Research," NAS-NRC

Publication No. 895 (1961), National Academy of

Sciences, National Research Council, Washington,

D. C.

O -1 [29] Attempts to measure cosmic ray provided Kr have been

been discussed for Aegerter, Houtermans, Oeschger

and Rama, "Earth and Planetary Science Letters, l a 256 (1966).

[30] Ostlund, Brown, Bainbridge, Tellus 16, 131 (1964).

[31] For a discussion of the quantitative effect of interference upon detection limits, See: Currie , L. A.

Anal. Chem. 4_0, 586 (1968).

[32] Currie, France, and Steinberg, Int. J. Appl. Rad.

Isotope, 16, 1 (1955).

[33] DeVoe, J. R., Reed, W. P., Currie, L. A., LaFleur, P. D.

unpublished data.

[34] O'Connell, Currie, L. A., Gerstenberg, unpublished.

[35] Currie and Rodriquez-Pasques , unpublished.

[36] Oeschger, Alder, Loosli, Langway, and Renaud , Proc

.

6th Internat . Conf. Radiocarbon and Tritium Dating,

CONF - 650652 (1965), Pullman, Washington.

[37] Lai, Goldberg, and Koide, Science 131, 332 (i960).

[38] Davis, R., Phys. Rev. 97, 766 (1955).

[39] Ruegg, F. C.j Spijkerman, J. J. and DeVoe, J. R.

,

Review of Scientific Instr. 36, No. 3, 356 (1965).

[40] DeVoe, J. R., Editor, NBS Technical Note 248, Supt . of

Doc., U. S. Government Printing Office, Washington,

D. C. 20402 (1963-64).

[41] DeVoe, J. R., Editor, NBS Technical Note 276, Supt. of

Doc., U. S. Government Printing Office, Washington,

D. C. 20402.

[42] Ruzicka, J. and Stary, J. Talanta 8, 228-34 (1961).

[43] Weiler, H., Int. J. Appl. Rad. Isotope 12, 49-52 (1961).

122
[44] Sandell, E. B., "Colorimetric Determination of Trace

Metals", 3rd Ed., Interscience , New York (1959).

[45] Hammence, J. H., Analyst 65, 153 (1940).

[46] Bagshame, B. and Truman, R. J., Analyst 7_2, 189 (1947)

[47] Greenberg, P., Anal. Chem. 29, 896 (1957).

[48] Piper, C. S. and Beckwith, R. S., J. Soc . Chem. Ind

.

67, 374 (1948).

[49] Allen, S. H. and Hamilton, M. B., Anal. Chem. Acta.

7, 483 (1952).

[50] Hobart, E. W. and Hurley, E. P., Anal. Chem. Acta. 2_J_,

142-152 (1962).

[51] Morrison, G. H., and Prieser, H., "Solvent Extraction

in Analytical Chemistry", John Wiley and Sons, Inc.,

New York (1957)

•

[52] Youden, W. J., "Statistical Methods for Chemists"

John Wiley and Sons, Inc., London, (1961).

123
Appendix I. Evaluation of Integrals

Evaluation of the Integrals <n, I ,m| | n'

,

I

,m'>

= 8 -

A, = /°°dy y

J

" exp[- ^(^ + §C)p]

J i 2 n n

B. = / dv v

J

" exp[- i(^i + B|l)v]

j -i ^ 2

N n n

[2n+l]/2

C = (R?)

[2n'+l]/2

D = (R^)

<1,0,0 o|

/N <1,0,0 0|

/s <1,0,0 o|

^ <1,0,0 o|

/N <1,0,0 o|

•s. <1,0,0 o|

1,0, 0> = \ I

CD ^ A

i

B

o +Ao Bi

]

2 >°> 0> = M R CD[A

2

B 0+ 2A lBl+ A B

2

]

3 5 0,0> = 32

^

3Q |

CD[A

3

B + 3A

2

B

1

+3A

1

B

2 + A B

3

]

2,1,0> = j^ a CD[A

2

B

1

+A

1 (B

2

+B )+A B

1

]

3,1,0> =

3 2Wi0R

CD ^ A

3

B

i

+A

2

(2B

2 + BQ HA-^B^B^+AqB^

3 ' 2 ' 0> = SW^ R CD[A

3

(3B

2

-B ) + 3A

2

(B

3

+B

1

)

+3A

1 (B

2

+B )+A (-B

3

+3B )].

124
<2,0,0 | | 2,0,0> = lh R CD[A

3

B + 3A

2

B 1+ 3A lB2 + A B

3

]

1 Q L2,0,0|O|3,0,0> = 155 ^ 2t/ 5 g

CD[A

4

B +4A

3

B

1

+6A

2

B2+4A

1

B

3

+A B

4

]

:2,0,0|0|2,1,0> = y|g- | CD[A

3

B

1

+A

2 (2B

2

+B )+A

1 (B +2B1

)

+a b

2

].

q 2,0,0|O|3,l,0> = 5ie

^

15 g

CD[A i| B

1

+A

3

(3B

2

+B

Q

)+3A

2

(B

3

+B

1

)

+A

1

(B

4

+3B

2 )+AQ

B

3

].

<2 5 s 0|0|3,2,0> = 31 ^ Qi| |

CD[A

4 (3B2

-B )+A

3

(6B

3

+2B

1

)

+A

2

(3B

4

+6B

2

+3B

q

)+A

1 (2B

3

+6B

1

)

+A (-B

4

+3B

2 )].

q <3,o,o|o|3,o,o> =

2 4o78i a r

cdu^o+sa^+ioa^+ioa^

+ 5A

1

B

4

+A

Q

B

5

]..

<3,0,0|6|2,1 } 0> = 51 q^15

':§ CDLA^B^ A

3

( 3B

2

+B

Q

)+A

2

( 3B

3 + 3B

1

)

+A

1 (B

1| +3B

2 )+A B

3

].

<3,0,0|o|3,l,0> =

2i|Q ^ 8l ^ | CD[A

5

B

1

+A 1| (4B

2

+B )+A

3

(6B

3

+4B

1

)

+a

2

(4b

4

+6b

2 )+a

1

(b

5

+^ib

3

)+a b

4

]

125
<3,0 5 0|0|3,2,0> =

it8 ^

8l2 | CD[A

5

(3B

2

-B )+A

4

(9B

3

+B

1

)

+ A

3

(9B lj +8B

2

+3B )+A

2

(3B

5

+8B

3

+9B

1

)

+A

1 (B

1| +9B

2 )+A (-B

5

+3B

3 )].

1 Q <2,1,0|0|2,1,0> = 2% r CD[A

3

B

2

+A

2 (B

3

+2B

1 )+A1 (2B2

+B )+A B

1

]

<2,l,0|O|3,l,0> = -i_ ^ 5YM75 R CD[A

4

B

2

+A

3

(2B

3

+2B

1 )+A

2

(B i| + 4B

2

+B )

+A

1 (2B_+2B1 )+A B

2 ].

<2 ,1,0 |

| 3,2 3 0> = 10368/3 R CD[A

4

(3B

3 - Bl ) + A3

(3B 4+ 6B

2

-B )

+A

2 (6B-+6B1 )+A1 (-B 1 .+6B2

+3B )

+A

Q

(-B +3B

1 )].

<3,1,0.|0|3,1,0> = 8o . 8l ^ | CD[A

5

B

2

+A

4 (3B

3

+2B

1

)

+A

3

(3B ij + 6B

2

+B )+A

2

(B

5

+6B

3

+3B

1

)

+A

1 (2B

2| +3B

2 )+A

() B

3

].

<3,1,0|0|3,2,0> =

1|8 ^T

|1

. | CD[A

5

(3B

3

-B

1

) + A

4

(6B

4

+5B

2

-B )

+A

3

(3B

5

+12B

3

+5B

1 )+A2

(5B lj +12B

2

+3B )

+ A

1 (-B

5

+5B

3

+6B

1 )+AQ

(-B

1| +3B

2 )]

126
<3,2,0|0|3,2,0> = 192 , 8l 2 | CD[A

5

(9B

4

-6B

2

+B )

+A i| (9B

5

+l8B

3

-7B

1

)

+A (18b^+28B

2

-6b

q

)

+A n (-6B [-+28B_ + l8B n ) 2 D 5 1

+A

1 (-7B i| + l8B

2 + 9B )

+A (B

5

-6B

3

+9B

1 )].

f2,l,±l|0|2,l,±l> = 5X2 § CD[(A

3

-A

1 )(B -B2 )+(A2

-A )(B

1

-B )]

2,l 9 ±l|0|3,l,±l> = 103 ^ 8v/ 5 ICD[(A

4

-A

2

)(B

Q

-B

2

)

+ 2(A

3

-A

1 )(B1

-B

3

) + (A

2

-A )(B

2

-B i| )]

1 q 2 } 1 3 ±1|0|3 S 2 3 ±1> = Iq|m ^CD[(A i| -A )(B

1

-B

3

) + (A

3

-A

1 )(B -B

4 )]

1 q 3,1,±1|0|3,1,±1> =

1 g : 8l 2 gCD[(A

5

-A

3

)(B -B

2

) + 3(A lj -A

2

)(B

1

-B

3

+ 3(A

3

-A

1 )(B2

-B

]| ) + (A

2

-A )(B

3

-B

5

)].

<3,l,±l|0|3,2 J ±l> = l6

^

8l . |CD[(A

5

-A1 )(B 1

-B

3

) + (Ai| -A

2 )(B -B 1| )

<3 } 2 } ±1|0|3 5 2,±1> =

<3 i 2 5 ±2|0|3,2,±2> =

+(A

3

-A

1 )(B1

-B

5

)+(A

4

-A

Q

)(B

2

-B

4 )].

32 .3 X

2 I

CD[(A

5

-A

1 )(B

2

-B ij ) + (A

4

-A

2

)(B

1

-B

5

)

+ (A

3

-A

1 )(B -B

1| ) + (A

4

-A )(B

1

-B

3

)]

Ig^ep-^ CD[(A

5

-2A 3+ A

1 )(B -2B 2+B

4

) +(A

4

-2A

2

+A

Q

)(B

1

-2^ +B

5

)].

127
Appendix II

Pin #

2, 14, 18

3

5

6, 24

12

13

15

16

17

21

22

23

Wiring List for MSssbauer Spectrometer

Board #1

Signal

Detection pulses

to analyzer

-12V

Ground

Data Signal

+ 12V

Photocell Lamp

Burnout interlock

To

S.C. #2 R

Pin #3

Power Plug

Pin #17

Chasis Grnd . Buss

Center Pole

Switch §3 F

Power Plug

Pin #16

S.C. #1

Pin #2

To delay coincidence S.C. #2

input of analyzer Pin #2 R

Relay controlled

-12V = -12R

Detector gate

Lamp Driver

Relay Controlled

+12V = +12R

Bd. #3

Pin #3

Lamp F

Bd #3

Pin #17

Analyzer Time Store S.C. #2 R

Interlock input

Control Bistable

gate line (enable)

Position Photocell

Trigger Pulse

Control Bistable

gate line (inhibit)

Rn #1

Switch #4 P

Bottom Pole

Bd #2

Pin #1

Switch #4 F

Top Pole

128
Mossbauer Photocell Trigger

Board #2

Pin # Signal

Positive photocell

Trigger pulse

Address reset

3 +12V

4 Photocell

Bias +12V

5 Photocell

Signal

7 Ground

11 -12V

19 Positive

Square Wave

21 Negative

Square Wave

Wire Or

Coax To

W Bd #1

Pin #22

w Switch #2 F

Center Pole

w Bd #1

Pin #6

w S.C. #1 R

Pin #8

w S.C. #1 R

Pin #9

w Ground Buss

w Bd #1

Pin #14

c Pot #3 F

C. W. Pole

c Pot #3 F

C.C.W. Pole

129
Mossbauer Amplifier

Board #3

Wire or

Pin § Signal Coax To

1 +12V W Power Plug

Pin #16

2,6,9, Ground W Ground Bus

16, 22

3 -12V W Power Plug

Pin #17

4 Input to C

Spare Op . amp

5 Output of C BNC F

Spare op. amp

7 L.V. Feedback C S.C. #1 R

Signal Center Pole

12 C Base of

STC 1080

13 -12V R W emitter of

STC 1080

and Bd #1,

Pin #13

15 Power amp C Collectors

output of STC 1080

and 2N3171 & BNC R

Power out

17 +12V R W emitter of

2N3171 and

Bd #1, Pin #16

19 C Base of

2N3171

20 Symmetry C Pot #2

Control Center Pole

130

F
Pin §

23

24

Wire or

Signal Coax To

Address Analog C S.C . #2 R

Input Pin #6

Input Signal C Pot #1 P Pin #3

Attenuator BNC P Input

Square wave C Pot #3 F

131
Signal Connector #1 (S.C.#1) Mounted on Rear

Wire or

Pin # Signal Coax To

2 Photocell lamp W Bd #1

burnout interlock Pin #7

3 Photocell lamp W Switch #1 F

Power Bottom Pole

8 Photocell W Bd #2

Bias +12V Pin #5

9 Photocell W Bd #2

Signal Pin #5

11 Ground W Ground Buss

13 L.V. Feedback Center Bd #3

Signal Conductor Pin #7

of coax

14 L.V. Feedback Shield of Ground

Signal above coax at Bd #3

132
Signal Connector Cable #1 to Drive Motor

All Coax

Pin # Signal To

2 Lamp interlock Lamp

3 Lamp power Lamp

8 Photocell Bias Photocell

9 Photocell signal Photocell

11 Ground Ground

13 Linear Velocity LV

Transducer Feedback

Signal

14 Linear Velocity LV

133
S.C.#2 Mounted on Rear

Wire or

Pin # Signal Cable To

1 Analyzer Store W Bd #1

interlock Pin #17

2 To delay coincidence W Bd #1

input of analyzer Pin #12

3 Time Mode Detector W Bd #1

input of analyzer Pin #1

4 To output of Single W Switch #3

Channel Analyzer Bottom PoleP

6 Analog of Address C Bd #3

Scaler Pin #21

8 Address Reset W Bottom Pole

Sw #2

10 To fringe pulse Switch #3 F

train Top Pole

11 Ground W Ground Buss

134
Signal Connector Cable #2

All coax with BNC coaxial connector on other end of each

connection.

Pin # Signal To Connector

Analyzer store

Interlock

Multichannel

Analyzer

P-2 Pin J

External Live Timer

Delay Coincidence Multichannel

Pulses Analyzer

Delay Coincidence

3 Data Signal Multichannel

Analyzer

Time Mode Detector

Input

k Output of Singal

Channel Analyzer

Single

Channel Analyzer

Output

6 Analog of Address

Scaler

Multichannel

Analyzer

Ramp Out

8 Address Reset Multichannel

Analyzer

Address Reset

Input

Fringe count

Pulse train

Frequency

Ratio Detector

Fringe Pulses

Out

11 Ground Shield of

All Coax Cables

135
Pot #1

Pot #2

Pot #3

Front Panel

Velocity Amplitude Control

use coax 20 K

Pin #3

10 turn

Pin #2

Pin #1 BcT #3

Pin 8

/TnBNC®

v_y Input

vBd #3

Pin 2 3

Constant Acceleration Symmetry Control

use coax on wiper 100 K 10 turn

Bd #3

Pin #3 + 12V

XPin #2 J ^ —

r Pin #1

r

-12V

Pin 20

Constant Velocity Symmetry Control

Use coax 10 K one turn

CW Bd #2

Pin 19

CCW

vBd #3

Pin 24

.Bd #2

Pin 21

Switch #1 Power Switch

on

/

S.C. #1

Tin #3

-12V

off

Switch #2 Address Reset

S.C. #2

, Pin #8

/ Bd #2

Pin #2

136
Switch #3 Mossbauer or Calibrate

Mossbauer

, JS.C. #2

Pin #4

Bd #1

Tin #5

, S.C. #2

Pin #10

Calibrate

Switch #4 Control Bistable Gate

Store Bd #1

Tin #21

-ll' Bd #1

Pin #23

Inhibit

Lamp Indicator for Detector Gate

12V fe§ *Bd #1

Tin #15

BNC Output of Spare Op. Amp.

Use Coax

© ^Bd #3

Pin #5

137
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