nov TECHNICAL NOTi

421

Radiochemical Analysis:

Nuclear Instrumentation,

Radiation Techniques, Nuclear Chemistry

Radioisotope Techniques

July 1966 through June 1967

*<*" 0F ^

4?

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 provides measurement and technical information services

essential to the efficiency and effectiveness of the work of the Nation's scientists and engineers. The

Bureau serves also as a 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

accomplish this mission, the Bureau is organized into three institutes covering broad program areas of research and services: THE INSTITUTE FOR BASIC STANDARDS . . . provides the central basis within the United

States for a complete and consistent system of physical measurements, coordinates that system with the measurement systems of other nations, and furnishes essential services leading to accurate and uniform

physical measurements throughout the Nation's scientific community, industry, and commerce. This

Institute comprises a series of divisions, each serving a classical subject matter area: —Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic Physics—Physical Chemistry—Radiation Physics—Laboratory Astrophysics2—Radio Standards Laboratory,2 which

includes Radio Standards Physics and Radio Standards Engineering—Office of Standard Refer- THE INSTITUTE FOR MATERIALS RESEARCH . . . conducts materials research and provides

associated materials services including mainly reference materials and data on the properties of materials. Beyond its direct interest to the Nation's scientists and engineers, this Institute yields services which are essential to the advancement of technology in industry and commerce. This Institute is or- ganized primarily by technical fields: —Analytical Chemistry—Metallurgy—Reactor Radiations—Polymers—Inorganic Materials—Cryogenics2—Office of Standard Reference Materials. THE INSTITUTE FOR APPLIED TECHNOLOGY . . . provides technical services to promote the use of available technology and to facilitate technological innovation in industry and government. The

principal elements of this Institute are

:

—Building Research—Electronic Instrumentation—Technical Analysis—Center for Computer Sci- ences and Technology—Textile and Apparel Technology Center—Office of Weights and Measures —Office of Engineering Standards Services—Office of Invention and Innovation—Office of Vehicle Systems Research—Clearinghouse for Federal Scientific and Technical Information3—Materials Evaluation Laboratory—NBS/GSA Testing Laboratory.

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

20234.

2 Located at Boulder, Colorado, 80302.

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

Alexander B. Trowbridge, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director ^5^- TECHNICAL NOTE 421

ISSUED NOVEMBER 1967

Radiochemical Analysis

Nuclear Instrumentation,

Radiation Techniques, Nuclear Chemistry

Radioisotope Techniques

July 1966 through June 1967

Edited by James R. DeVoe

Radiochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Research

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

listed or referred to in the open literature.

For sale by the Superintendent of Documents, Government Printing Office Washington, D.C., 20402 - Price 50 cents
FOREWORD

The Analytical Chemistry Division was established as a separate division at the National Bureau of Standards on September 1, 1963, and became part of the Institute for Materials Research in the February 1, 1964, reorganization.

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

competences from activation analysis and atomic absorption to vacuum fusion and x-ray spectroscopy. These competences, and

in turn the sections which they comprise, are charged with research at the forefront of analysis as well as awareness of the practical sample, be it standard reference material or service analysis. In addition it is their responsibility to inform others of their efforts. Formal publication in scientific periodicals is a highly

important output of our laboratories. In addition, however,

it has been our experience that informal, annual summaries of progress describing efforts of the past year can be very valuable in disseminating information about our programs. A word

is perhaps in order about the philosophy of these yearly progress reports. In any research program a large amount of information is obtained and techniques developed which never

find their way into the literature. This includes the "negative results" which are so disappointing and unspectacular but which can often save others considerable work. Of importance also are the numerous small items which are often explored in a few days and which are not important enough to warrant publication—yet can be of great Interest and use to specialists in a given area. Finally there are the experimental techniques and procedures, the designs and modifications

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

yet all too often must be covered in only a line or two of a journal article.
Thus our progress reports endeavor to present this infor- mation which we have struggled to obtain and which we feel might be of some help to others. Certain areas which it appears will not be treated fully in regular publications are considered in some detail here. Other results which are being written up for publication in the journal literature

are covered in a much more abbreviated form.

At the National Bureau of Standards publications such as these fit logically into the category of a Technical Note.

In 1967 we plan to issue these summaries for all of our sections. The following is the fourth annual report on progress of the Radiochemical Analysis Section.

W. Wayne Meinke , Chief Analytical Chemistry Division
PREFACE

The Radiochemical Analysis Section develops measurement

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

chemical structure is Mossbauer Spectroscopy, the major

activity in this section. Understanding of the chemical, nuclear, and physical

principles which form the foundation of a new analytical measurement technique is basic to the development of measurement techniques using radioisotopes. Consequently, nuclear

instrumentation, nuclear chemistry, and statistical analysis

are key activities in the section. These activities are also

in support of the Activation Analysis Section as well as the radiation and radioisotope techniques in this section.

It is essential that the developed methods be practical

in the sense that they can be used successfully on materials

in which science and industry have an interest. Therefore,

these developed techniques are applied to the analysis of NBS Standard Reference Materials where considerable cross checking of analytical techniques is required throughout the process of certification.

The effort in radioisotope dilution techniques for trace analysis has been temporarily suspended pending the attainment of suitable facilities and personnel.

A roster of the groups in this section is listed in part 7- 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 identi-
fication imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for

the purpose. James R. DeVoe, Chief Radiochemical Analysis Section Analytical Chemistry Division
TABLE OF CONTENTS

Page

1. INTRODUCTION 1 2. MOSSBAUER SPECTROSCOPY 3 A. Introduction 3 B. Instrumentation 5 1. New Transducer Design 5 2. Simple Liquid Nitrogen Cryostat .... 8 3. Scattering Experiments 15

4. A Tandem Mossbauer Spectrometer .... 17

5. Absorber Mounting Techniques 18

C. Standard Reference Material for the Chemical Shift of Tin 19

D. Mossbauer Spectroscopy of Nickel-6l .... 25

1. Selection of Parent Isotope 25

2. Source Preparation 28

3. Magnetic Moment of Excited 5/2 State

of Nickel-6l 32

4. Evaluation of Source 32

E. Structural Analytical Applications 36

1. Correlation with Nuclear Resonance ... 36

2. Iron Coordination Chemistry 38

F. Quantitative Analytical Applications .... 39

G. Abstracts of Miscellaneous Publications . . 42

3. NUCLEAR CHEMISTRY 45

A. Introduction 45

B. Nuclear Reaction Studies 45

1. Irradiation Capsules 45

2. LINAC Irradiations - Temperature

and Intensity Data 48

3. Photonuclear Monitors 51

C. Decay Analysis Investigations 54
Table of Contents (Cont.) Page

4. STATISTICS IN NUCLEAR AND ANALYTICAL CHEMISTRY . 58

A. The Precision of Radioactivity

measurements 58

1. Introduction 58

2. Detection of Excess Random Error .... 59

3. Multicomponent Systems - Decay Curves . 59

B. Decision, Detection, and Determination

Limits 6l

1. Introduction and Principles 6l

2. Radioactivity Detection and Procedure Optimization 66

5. NUCLEAR INSTRUMENTATION 69

A. Introduction 69

B. Solid State Detectors 69

C. Solid State Detector Cryostats 69

D. Radiation Spectrum Analyzers and Equipment . 70

E. Pneumatic Tube End Sensor System 71

F. Consultation 72

6. X-RAY FLUORESCENCE BY GAMMA-RAY EXCITATION ... 73

7. PERSONNEL AND ACTIVITIES 75

A. Personnel Listing 75

B. Publications 75

C. List of Talks 76

8. ACKNOWLEDGMENTS 78

9. LIST OF REFERENCES 79 LIST OF FIGURES

Figure Page

1 Cut away of compact transducer 7

2 Mossbauer cryostat 10

3 Detail of source-mount and silver flexure

plate 11

4 Photographs of cyrostat for Mossbauer

drive 12

5 Cooling curves 13 vii
LIST OF FIGURES (Cont.)

6 Cobalt - 57 pulse height spectra taken through assembled and disassembled

cryostat 14

7 Backscattering, iron block, 10 mCi source

5 'Co in Pdj approximately 10 hour count . 16

8 Diagram for Mossbauer spectroscopy using

scattering geometry 17

9 Mossbauer spectrum of beta tin 21

10 Mossbauer spectrum of barium stannate . . 22 11 Mossbauer spectrum of dibutyl tin sulfate 23

12 Mossbauer spectrum of magnesium

hexafluoro stannate 26 13 Decay schemes producing Ni 27 14-a Gamma-ray spectrunvof Co produced by the

(y,p) reaction on Ni

28 14-b Gamma-ray energy spectrum of Ni 15% Cr alloy after LINAC activation 29

15 Phase diagram of Cr-Ni metal extrapolation

done on diagram appearing in reference [7] 30 16 Mossbauer spectrum using Ni 31

17 Mossbauer spectrum of 1.5% 1 Ni-Fe alloy . 33 18 Nuclear energy level diagram of 1.5$ "^Ni-Fe alloy from hyperfine interactions 34 19 Calculated spectrum for 1.5% Ni-Fe

alloy from Clebsch-Gordon coefficients

of table 2. 36 20 Plot of Debye-Waller factor versus characteristic temperature 37 21 Mossbauer spectrum of SnOp in A1„0 ... 2ji 22 Plot of logarithm of A

n

vs mg of SnO . . 42 23 Irradiation capsules 46 24 Gas handling system 47 Three principal analytical regions .... 63 26 Best counting interval 66 27 Minimum detectable activity 67
LIST OF TABLES

rable Page

1 Spectral parameters for potential tin standards 20

2 Relative intensities of transitions in 1.5% 6lNi-Fe absorber 35

3 Sample heating 51

4 6l Cu/ 64 Cu Ratios at 90 MeV . . 52

5 Carbon monitors 53

6 Monitor precision 54

7 Decay-during-observation errors .... 56

8 Effects of wrong a (synthetic decay

curve) 60

9 Decision, detection, determination

levels . 62

10 "Working" expressions for L„, L~, L . . 65

11 Detection of sulfur in protein 68
RADIOCHEMICAL ANALYSIS: NUCLEAR INSTRUMENTATION, RADIATION

TECHNIQUES, NUCLEAR CHEMISTRY, RADIOISOTOPE TECHNIQUES

July 1966 through June 1967 Edited by James R. DeVoe ABSTRACT

This is the fourth summary of progress of the Radio- chemical Analysis Section of the Analytical Chemistry Division at the National Bureau of Standards.

The section's effort comprises four major areas: Mossbauer spectroscopy, nuclear chemistry, nuclear instru- mentation, and the application of statistics in nuclear and analytical chemistry.

A new design of transducer which is more compact than

the previous design has been made so that low temperature experiments can be more easily performed. A method of simultaneously measuring two spectra with a single trans- ducer has been devised. A single line nickel-6l source has been made and the magnetic moment of the excited 5/2 state

has been measured with this source. Efforts have continued

on the interpretation of the spectral parameters and their relation to structure analysis. Studies are continuing on the production of a standard for chemical shift of tin com- pounds . Equipment is in the final state of assembly and initial testing for measuring cross sections of reactions such as

•2 4 (Yj He) or (y, He) by mass spectrometry of the reaction

products

The precision of an analytical method must be carefully

used when expressing a detection limit. In addition, care must be taken to be definitive about the meaning of a limit

of detection. A suggested procedure for uniformity in reporting these often used terms is presented in this report.

The practical operation of Ge(Li) and Si(Li) detectors
has been evaluated. It is found that careful control of. the preamplifier noise and response is required for optimum performance . A study of the precision and accuracy of gamma-ray

excited x-ray fluorescence was made. Using as a basis the analysis of certified Standard Reference Materials, an improvement of a factor of two in precision was measured.

The major advantage concluded from this work is that if rapid analysis requiring portability is desired, the radioisotopic source approach is useful. James R. DeVoe, Chief Radiochemical Analysis Section Analytical Chemistry Division

Key Words

NBS Linac, computers, standard reference materials, photoneutron reaction, cross sections, flux monitors,

nickel-6l, Mossbauer spectroscopy, tin standard for chemical

shift, PARLORS program for Mossbauer spectra, Mossbauer

instrumentation, detection limits for analysis, solid state detectors

1. INTRODUCTION

The effort of this section has centered around three

main areas, Mossbauer spectroscopy, nuclear chemistry, and statistical interpretation of experimental data. Paramount in the efforts of the Mossbauer spectroscopy

group has been studies in improved instrumentation, for example, using scattering from bulk samples and in standardization and measurement of spectral parameters

The NBS spectrometer has been well received in many

laboratories this past year. In addition, several, companies

have elected to manufacture a spectrometer similar to the NBS

design. Being able to take spectra at various low temperatures is most desirable; consequently, much effort is being

devoted to the design of a versatile low cost cryostat. The

first practical Mossbauer source for doing nickel has been discovered by this group during the past two months.

Our efforts in nuclear chemistry and statistical analysis are combined in that they evolve from the efforts of one person. This combination proves particularly advantageous in

that a meaningful set of criteria can be used for describing

such parameters as a limit of detection in activation analysis, for example. Our effort in nuclear chemistry is approaching the data collection stage pertaining to the

cross sections of photonuclear reactions.

Our effort in nuclear instrumentation is being directed

toward special systems, now that most of the basic equipment

for radiochemical and activation analysis is operational.

Future work will involve two projects, one working on the innovation and upkeep of existing sytems and one involving

a more limited effort on special systems. Our experience

with solid state detectors has been somewhat unsatisfactory,

from an operability standpoint. However, developments within the last month or two lead us to increase our hopes once

more
A limited study was made of the gamma-ray excited x-ray fluorescence, and even though we consider it a useful

approach, it doesn't appear to fit into the program developed

in our laboratory.
2. MOSSBAUER SPECTROSCOPY

A. Introduction Mossbauer spectroscopy has demonstrated with the use of only the elements iron and tin, a wide variety of applications related to the analysis of chemical structure. These applications range from corroboration on the symmetry of a molecule with nuclear magnetic resonance with or without

x-ray diffraction, to the effect of surface oxidation or the chemical effects of catalysts. While the areas of application are large, a greater understanding of the technique has lead to a more realistic idea of the region of most potent

applicability. For example, as a means of elucidating

chemical structure, it is very necessary to consider this technique as a component part of data made available by other

methods. When this is recognized, quite often the interpretation of the Mossbauer spectral parameter provides crucial concluding evidence for certain structural effects that prove difficult to detect by other methods. A continuing effort

will be made to improve our understanding of the analysis of chemical structure by Mossbauer spectroscopy.

A most significant part of our effort has been in the area of realizing the practical capability of the technique.

Our laboratory has provided the guidance necessary for assuring that data can be interchanged among laboratories by producing a primary standard for the reference of chemical

shift of iron compounds. Details about this standard, sodium nitroprusside , are available [1]. We have had difficulty in finding the better material to

be used as a secondary standard for the differential chemical

shift of tin compounds. None have all of the desired attributes, so a compromise may be required by making two secondary standards available. Through the necessity of having to calibrate the standard

with high precision and accuracy, much emphasis has been
placed upon the design of the spectrometer. In addition,

our laboratory was one of the first to show the advantages

of handling high count rates with fast preamplifiers, amplifiers, and scalers, and have encouraged the development of fast single channel analyzers. Collection time of a spectrum

can be reduced from days to hours. In our laboratory high accuracy was generated through the use of optical inter- ferometry, and recently, high precision has been provided by simultaneously taking two spectra from the same transducer. Further study on instrumentation has resulted in a new transducer design, a new liquid nitrogen cryostat, a series

of absorber mounting devices; and of great interest is the

use of scattering spectroscopy, since bulk samples can now

be analyzed without tedious sectioning while maintaining

uniform thickness.

To make the technique more applicable, it is necessary

to assure the utility for more elements than iron and tin. There are eight other elements that have been successfully

made practical. However, increasing this number is desirable. Attention must be paid to use of coulomb excitation. Problems which have occurred with this technique center

about the poor selectivity for excitation and the consequent interfering gamma-rays that decrease the signal to noise

ratio. The recent work of Sprouse, et al, [2] indicates

that the reaction recoil which imbeds the Mossbauer excited

nuclei in a non-radioactive matrix may make the technique

more feasible. This technique makes all elements potentially

usable for Mossbauer spectroscopy. Other practical problems

such as spectrum collection time due to low source intensity

and to fractional effect in recoil collections need to be resolved.

In our continuing effort to develop additional practical Mossbauer radioisotopes an unsplit Ni source produced by

a photonuclear reaction has been developed for the first time
Another area in which the practical use of the technique

may be stimulated is the generation of "fingerprint" spectra

that is, spectra that are very characteristic of a given

material, but could imply a mixture of compounds of a given

element in a material. Initial efforts are being made to characterize a number of existing Standard Reference Materials in this way. Results of the more interesting applications have been obtained within the last few weeks, and will

be reported in the future. Some materials measured are: SRM No. Description

103 Chrome refractory containing Fe

? CU 1011 Portland cement containing Pe p 0^ 69a Bauxite containing Fe„0_, We expect to extend this work to the measurement of pure

chemical compounds, see reference [1]. Although many difficulties exist, progress has been made

in making the spectroscopy quantitative for a synthetic

mixture of SnO„ In AlpO-.. It appears that for a given matrix,

limited concentration changes can be measured quantitatively

There is little doubt, however, that greater applicability

can be realized by devising techniques that are less matrix

dependent

B. Instrumentation

1. New Transducer Design

A compact Mossbauer transducer has been developed to supplement the Standard NBS Drift-free Mossbauer Spectrometer [3]. This transducer unit was designed specifically for

use with cryostats, magnets and any other application where

space is limited and close geometry is required. This unit

is also especially well-suited for those experimental

arrangements that make use of lathe beds for the mounting of

the transducer and auxiliary hardware.

The principle of operation of the Mark V transducer is identical to that of its predecessor. It is an electro-
mechanical transducer producing the Doppler motion by driving

a high fidelity loudspeaker with a saw-tooth signal derived

from the analogue output of the address scalers of a multichannel analyzer. A linear velocity transducer coupled to a feed-back circuit assures linearity, while a photocell-slit

system provides a constant reset point thereby minimizing

drift. In the MK V transducer the massive end plates of the Drift-free Transducer have been eliminated and the over-all

length has been reduced by placing the linear velocity transducer inside the loudspeaker magnet. A further reduction in length has been effected by moving the photocellslit system inside the flexure plate.

The compact MK V transducer unit is shown in figure 1. The linear velocity transducer is attached with a sleeve that bolts directly to the speaker magnet shroud. The sleeve is bored concentric with the loudspeaker coil groove and thus

the transducer is permanently aligned. This feature results

in a considerable increase in the ruggedness and portability

of the instrument over that of the previous unit. The slit system has been incorporated into the push-rod

itself, and the photocell and lamp holder assembly is attached to the linear-velocity transducer mounting sleeve.

In addition to reducing the overall dimensions, this photocell-slit arrangement frees the rear of the drive for another source-absorber assembly allowing tandem spectrum

taking [4] (see item 4 of this section).

The very critical areas of the flexure plate sockets and retaining rings are unchanged in both configuration and dimensions from those of the previous transducer. The flexure plates and all associated mounting hardware are completely interchangeable between the two units. The overall diameter of the transducer outer casing is also unchanged. A flange has been provided at the back for mounting the transducer in a cryostat casing or on a suitable
6-60° APART

4 -90° APART3- 120° APART

HOLES TOP AND

BOTTOM 0078" DIAM.

ACCESS SLOT23/4 x 23/8

" 4-90° APART

Figure 1. Cut away of compact transducer approximately

full size mounting frame. This flange contains f eed-throughs for the transducer wiring.

The new transducer performs exceedingly well. No interaction between the speaker magnet and the transducer coil

has been detected. Consequently, considerable savings of space have been realized and it will be possible to use the entire device at a low temperature.

(J. J. Spijkerman, F. C. Ruegg, D. K. Snediker)
2 . Simple Liquid Nitrogen Cryostat

Low temperature measurements have come to be of increasing importance in Mossbauer spectroscopy. In addition to the low temperature requirements of "classical" quadrupole

splitting and magnetic hyperfine interaction studies, the comparatively new fields of protein structure (heme) studies

and the Mossbauer spectroscopy of organo-tin compounds re- quire that spectra be taken at temperatures at least as low

as 77°K (liquid nitrogen). The reason for the low temperature requirement in the case of the latter two areas lies in the fact that most of these absorbers have low effective

Debye temperatures and/or low concentrations of the Mossbauer

nuclide

Furthermore, the trends in the use of new Mossbauer

isotopes indicate an increasing utilization of nuclides having a relatively high gamma-ray energy. Such nuclides require low temperature sources and absorbers in order to obtain desirable spectra. From the foregoing it is obvious

that the increasing application of Mossbauer spectroscopy to

a wide variety of absorbers and nuclides will require the increased application of low temperature techniques.

To this end a cryostat has been constructed to expand our

low temperature capability while meeting a number of specific

design requirements. The cryostat is inexpensive (less than $500.00/unit including dewar) and is extremely flexible. It

is capable of accommodating a wide variety of absorbers in various source-absorber configurations. Furthermore, the cryostat is windowless, and the beam path can be maintained

free from interfering iron for the study of low iron content absorbers, such as heme proteins.

The cryostat is a bottom-feeding permanently evacuated

dewar in order to maintain a constant nitrogen level in a reservoir in contact with a heat sink. The level is main- tained by venting the reservoir only through the feed tube
of the dewar. Hence, so long as the liquid nitrogen level

in the reservoir is above the feed tube outlet no nitrogen

will flow into the reservoir. When the nitrogen level falls below the end of the feed tube the gas in the reservoir

chamber escapes and allows an influx of liquid nitrogen into

the reservoir.

A cryostat operating on these principles has several

important advantages. It is inexpensive: an "off-the-shelf"

dewar is used and the machine work required is simple and within the capability of even the most modest shop. No vacuum system is required. The heat sink design and source- absorber accommodations allow great experimental flexibility.

The bottom-feeding dewar principle has a lower nitrogen con- sumption and a lower ultimate temperature than cryostats

cooled by rods immersed in top-opening dewars.

The cryostat Is shown in detail in figure 2. The dewar

attaches to the cryostat with a stainless steel flange (B). The heat-sink-nitrogen reservoir assembly is fabricated from

a solid block of copper and is attached to the flange by

10 mil stainless steel tubing (C). Thin wall stainless steel

was chosen for this part in order to minimize heat transfer

from the flange to the heat sink while maintaining adequate

strength. The length of the tubing is such that the dewar

feed tube (D) extends approximately 1/2" into the nitrogen

reservoir (E)

The source and abscrber are mounted in the heat sink using flanges (F) and (J). The configuration shown is for a moving source and stationary absorber, both at low temperature. The source is attached to a copper mount which in turn is indium-soldered to a 10 mil thick silver flexure

plate (H) . This assembly is shown in figure 3. The source

assembly is mounted behind flange (F) and is coupled to the Mossbauer transducer via a Mylar push-rod (G). Thin wall

Mylar tubing is used for the push-rod because of its light
DIRECT E INJECTION PORT -I

KM

SIDE VIEW-SECTION A—

FRONT VIEW Figure 2. Mossbauer cryostat : A-dewar, B-mounting flange,

C-10 mil stainless steel tubing, D-liquid

nitrogen feed tube, E-liquid nitrogen reservoir, F,J-flanges, G-Mylar push-rod, H-source mountflexure plate assembly, I-direct injection port, K-absorber
FLEXURE PLATE INDIUM -

I SOLDERED TO MOUNTING

. r- PLATE

Figure 3- Detail of source-mount and silver flexure plate: A-10 mil thick silver flexure plate, B-one of

6 holes to lighten copper heat sink, C-source mounting holes, D-source position on copper source-mount heat sink, E-push rod attachment,

F-source, G-source clamping ring

weight and low heat transfer properties. The absorber (K),

is mounted behind flange (J). In order to increase the cooldown rate of this cryostat for applications involving nuclides

with short half-lives a port giving direct access to the heat

sink chamber has been provided (I). Liquid nitrogen is fed directly into the chamber via this port which is fitted with

a Pyrex tube passing through the insulation.

The entire cryostat, from the flange down, is fitted with Styrofoam insulation from 1/2" to 1" thick. This insulation

is shown in figure 4. The seams in the insulation are caulked with Paiezon putty in order to minimize frost buildup in the beam path and to reduce air currents that might

carry heat into the insulated cavity. The cryostat-dewar

assembly is mounted in a simple angle iron frame that also

mounts the detector.
(a) Transducer connected to cryostat

(b) Styrofoam in place

Figure 4. Photographs of cryostat fo] si c drive

12
The cryostat, as described in the preceding paragraphs,

will hold a 10 liter charge of liquid nitrogen for 30 hours.

It will reach an ultimate temperature of 90°K at the source

mount in the center of the flexure plate. Figure 5 shows cooling rate curves for this cryostat both with and without

the direct injection of liquid nitrogen into the sample

cavity. The temperature was measured using a chromel-alumel

thermocouple mounted under the source-retaining ring. The reference junction was maintained at 77°K. The thermocouple

pair was sufficiently well-matched that when both were im- mersed in liquid nitrogen the resulting EMF was 0.00 volts.

t t 3 4

O COOLING RATE USING PURE CONDUCTION

ABSORBER

LIQUID NITROGEN TEMPERATURE -

ELAPSED TIME, MINUTES Figure 5. Cooling curves

13
The attenuation of the 14.4 keV 57 Co gamma-ray by the insulation was determined by taking gamma-ray spectra with

the cryostat assembled and disassembled (figure 4). These spectra are shown in figure 6. The attenuation, measured

by taking the difference in peak heights, is 30%. Note the comparative lack of scattered radiation in the spectra. The gain change occurs due to a difference in count rate.

1 1 1 1 1 1 1 1 1 1

'1 FULLY ASSEMBLED

I0K

1 DISASSEMBLED

fr ! l' -

en n i

1 1 i h-. | 1 * ~t~

O 1 '

|

1 ,, 27% u

1 1

1 1 -

l 1

1

I 1 A -

\

' M

\ ' r\ - rJ1 i i i i \<~~r i CHANNEL NUMBER X 10"'

Cobalt - 57 pulse height spectra taken through assembled and disassembled cryostat
A sodium nitroprusside Standard Reference Material spec- trum was taken using this cryostat with a stationary low temperature absorber and a moving room temperature source configuration. No detectable peak shift was found, but a maximum line broadening of 1% was observed at liquid nitrogen

temperatures. This 1% broadening is due to the vibration

caused by bubbling nitrogen.

(D. K. Snediker)

3 . Scattering Experiments

For many applications, particularly the Mossbauer spec- trum measurement of bulk matter and surface phenomena, the sample preparation for transmission geometry is either impractical or impossible, and a scattering technique offers

a solution. At present this technique is not often used, and

the results at present are not as satisfactory as trans- mission geometry. Several factors limit the sensitivity of

the scattering technique and determine the ultimate sensitivity. The internal conversion coefficient value of 9 for

14.4 keV radiation from the Fe Mossbauer level makes the detection of this scattered radiation very inefficient, but detection of the resultant x-rays can be used. Besides low efficiency due to the solid angle subtended by the detector, interference of fluorescent x-rays produced by the 122 keV gamma-ray precursor to the 14.4 keV level lowers the signal

to noise ratio. This can be compensated by an increase in source strength, since the count rate for scattering geometry

is considerably lower than transmission, and the system is not count rate limited.

In figure 7 the spectrum of structural steel using the

57 scattering geometry of figure 8 is shown. A 10 mCi Co in palladium source was used with a 10% methane-90% Argon flow detector. The data accumulation time was 10 hours. This geometry reduced the non-Mossbauer scattering considerably.

The count-rate capability could be increased 10 fold, without

15
SPECIAL SERIES, BACKSCATTERING EXPERIMENT

SOURCE: 10 mCi Co-57 IN Pd, MOVING

ABSORBER: IRON BLOCK, AT ANGLE

SETTINGS: DRIVE TWO, VAC 0.10 VAF 8.0 SYMM 6.50 SCAN 5E3 CLOCK: 413.9 ABOUT 10 HOURS

TOY START: 129-10-09-3

TOY STOP: 129-20-166 OVERFLOWS : 3 TEMPT: ROOM ABOUT 2 4.7 C VERTICAL PICTURE TAKEN OF EXPERIMENT

00153 53054 41758 53592 53030 53742 53451 53920 54856 54947

54864 55291 55302 55574 25855 57009 53837 57653 58521 58686 57633 59268 59459 61453 63274 64567 69589 74831 79910 84124

85735 82845 77196 71167 67557 65936 64985 63774 62932 61675

63464 62892 63410 63227 63620 64493 64352 64709 64613 64356

65335 72202 66382 67209 68490 69941 73782 78835 84757 86523

82302 77281 70932 69502 67791 67038 67512 67189 66756 67558

67475 67375 66850 67652 67468 68222 67978 68499 68153 68267 67758 67058 68517 68855 68847 70206 73389 76313 78496 79104

75436 72290 70382 69632 67737 67925 68037 68186 68226 68021

67817 67943 68656 68906 67761 70538 70952 71624 74093 77544

80791 77839 74521 71742 69717 68999 67033 68007 67368 67614 67542 66663 68300 66647 66359 66590 67043 66704 66699 65430 66467 66249 66058 66551 67881 69726 71229 72207 80572 84816

84480 80700 74919 71159 67278 66876 66390 66978 66478 63957 63681 63532 63813 64369 63727 64590 63045 62170 63427 61695 62591 62710 62367 62892 64811 64365 66886 71444 77404 82779

81974 77081 71930 66723 64273 61723 60346 59838 58347 49570 56620 55138 55126 55919 55353 55275 54270 56023 45934 56079 54018 54917 30577 20706 00317 00189 00166 00127 00045 00033

Figure 7- Backscattering, iron block, 10 mCi source Co in

Pd, approximately 10 hour count. Counts in baseline -7x10 , calibration 0.00726 cm/s
SOURCE *

1 •1'

Figure 8. Diagram for Mossbauer spectroscopy using scattering geometry overloading the present detection system. These scattering experiments clearly point out the need

for improved counter design, and the use of coincidence

methods to increase the signal to noise ratio.

(J. J. Spijkerman)

4 . A Tandem Mossbauer Spectrometer

The following is an abstract of a paper presented to the Third Conference on the Applications of Low Energy Gamma- and X-rays

A MULTIPLEX FOR DUAL SPECTRA MOSSBAUER SPECTROMETRY

by F. C. Ruegg

It is often desirable to have the capability of accumulating two Mossbauer spectra simultaneously with both spectra having exactly the same Doppler velocity dependence. This is possible with a multiplexing system which allows accumulation

17
of two spectra simultaneously and storing of each spectrum in

a 200 channel subgroup of a multichannel analyzer. The memory of the analyzer which is operated in the time mode is used on a demand basis by the two detector systems, and the counts are routed to the proper subgroup in complimentary

channel locations, i.e. the channel location in the second

subgroup is the channel number in the first subgroup plus

200. The logic, circuitry and performance are described.

The effect of high relative count rates between the tandem source and magnitude of count rates cause inverted phantom peaks of the high rate spectrum in the low rate spectrum to appear. Experience with this system has indicated

that it is not a serious limitation if the source count rates

are maintained under 10,000 pulses per second.

5 . Absorber Mounting Techniques

It is important to be able to reproduce the Mossbauer

spectral shape, and care must be taken to assure a reproducible thickness of absorbers. A mounting procedure which accomplishes this is described in the following abstract. Since a covering material must be used, it is important to measure the absorption coefficient of this material for the various Mossbauer gamma rays. CRITERIA FOR SELECTION OF ABSORBER MOUNTING MATERIALS

IN MOSSBAUER SPECTROSCOPY

(by D. K. Snediker and L. May, "Nuclear Instruments and Methods" (1967) to be published)

The criteria for the selection of materials for mounting

absorbers include its mechanical and chemical properties and

its attenuation of the y-^ay and x-ray emitted from the Mossbauer source. Attenuation of the 14.4 keV y-^ay and

6.5 keV x-ray associated with Fe was measured for nine commercial materials including plastics and metals. The absorption coefficients and half-thicknesses for each material were measured. The usefulness and limitations of

18
each material are discussed, along with three different

absorber mounting techniques.

C. Standard Reference Material for the Chemical Shift of Tin Work is in progress to develop a Mossbauer chemical shift

and velocity standard for tin-119 which is analogous to sodium nitroprusside (SRM No. 725) for Fe Mossbauer spectroscopy. Such a standard is necessary to standardize chemical shift data for the rapidly expanding area of tin-119 Mossbauer spectroscopy.

The criteria for the tin standard, which were previously discussed in detail [1] are summarized as follows: the spectrum should be a narrow, well-defined singlet or a wellresolved, nearly symmetric doublet with a quadrupole splitging of greater than 2.5 mm/s . The standard should have as high a Mossbauer effect as possible in order to decrease the time necessary to take a spectrum and it should give a spec- trum having a chemical shift that is low with respect to most commonly used sources. The material should also be stable in air and should not be subject to long-term chemical and physical changes that will in any way affect the Mossbauer

spectrum.

Our screening program has narrowed the possible materials

for the tin standard to four; beta- or white tin, barium

stannate, dibutyl tin sulfate, and magnesium hexafluoro

stannate. Table 1 shows the pertinent spectral parameters

for these absorbers, while figures 9, 10 and 11 show the Mossbauer spectra.

The beta-tin was 5 mil thick reagent grade foil and was used as received. Barium stannate spectra were obtained from

samples from two different sources: a commercial chemical

and material made in this laboratory by reacting stoichiometric amounts mately 300°C.
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The dibutyl tin sulfate fcH-(CH

2

) /LsnSO^ was made by the Organic Chemistry Section of the Analytical Chemistry Division by reacting dibutyl tin acetate (Alfa Inorganics) with concentrated HpSCv in ether solution. The compound pre- cipitates immediately upon the addition of H-SOu . The white precipitate is filtered on a fritted glass funnel with suction

and washed several times with ether. After air drying on the funnel the compound is ready to mount as a M'ossbauer absorber. Although dibutyl tin sulfate has been determined to be soluble

in dimethyl formamide, no recrystallization was deemed nec- essary, for the prepared absorber gave a clean, doublet Moss- bauer spectrum (figure 11). None of these materials meet all of the criteria for a tin Mossbauer standard. The choice of a standard will in- volve the careful consideration of the advantages and disad- vantages of each so that the absorber meeting most of the characteristics may be determined. Dibutyl tin sulfate with

its well-resolved, doublet spectrum would appear to be the most desirable of the three, since it can be used for velocity calibration as well as chemical shift calibration. However, its extremely low effect at room temperature virtually

precludes its use as a standard. Dibutyl tin sulfate will

not press into a coherent, self-supporting pellet, and furthermore, doubling the amount of material in the beam

resulted in a broadening of the lines and negligible increase

in percent effect. The National Bureau of Standards will

consider, however, the certification of dibutyl tin sulfate

as a secondary velocity standard to be used in conjunction

with a more suitable material for chemical shift calibration. Beta-tin and barium stannate both give high-effect,

singlet spectra at room temperature. Barium stannate gives

a relatively wide peak, whose width is dependent upon the stoichiometry of the compound. It has been reported how- ever [5], that the position of the peak is independent of an

24
excess of one reactant. The chemical shift of BaSnCU is quite

large relative to sources such as magnesium stannide or pal- ladium-tin, but is zero or nearly zero for barium stannate and stannic oxide sources, respectively. Barium stannate shows

one of the highest effects ever reported in tin Mossbauer

spectroscopy. Beta-tin has a chemical shift that is low relative to the metal-matrix sources; however, it has been observed that its spectrum [6] is an unresolved doublet. The doublet is thought to result from a preferential orientation of grain boundarys during rolling. The splitting of the doublet, and hence the position of the center of gravity of the doublet

(i.e. the chemical shift) is dependent upon the previous

history of the sample. This unfortunate circumstance virtually eliminates beta-tin as a candidate for the tin Mossbauer

standard. Preliminary results indicate that magnesium hexafluoro

stannate is superior to 3-tin or barium stannate in the sense

that it gives a Mossbauer spectrum that is a narrow singlet

(figure 12). For absorbers containing comparable thicknesses

of tin, MgSnFg gives a spectrum that is 30% narrower than BaSnO, with an lQ% reduction in effect. The commercial

MgSnFj- used in these studies is of adequate purity for use as

a standard and no special synthesis is necessary. Future research with regard to the tin standard will focus upon barium stannate and magnesium hexafluoro stannate with perhaps

dibutyl tin sulfate as an auxiliary velocity standard.

(J. J. Spijkerman, D. K. Snediker)

D. Mossbauer Spectroscopy of Nickel-6l

1. Selection of parent isotope

The operation of the NBS Linac has made it possible to produce radioisotopes for Mossbauer Spectroscopy, and hence

greatly increases the number of elements available for this purpose

25
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the interest in the first series of the transition metals

made Ni a good candidate. The radioactive source can be

made by either the 3 Cu (y,2n) Cu or the 62 Ni (y,p)

6l

Co reaction. The appropriate decay schemes are shown in figure

13. Both possibilities were investigated.

6lCu 3.3h£+,EC

Figure 13. Decay schemes producing

Cu production of the source via Cu gives a single

line Mossbauer source, but the intensity of the 67.2 keV radiation was too low, and there was too much interference

from higher energy radiation. Using the (y 3 p) reaction,

0.5 mCi of Co could be produced in a 30 minute LINAC irradiation at 100 MeV, with no noticeable interference as shown in figure l4-a. Moreover, the nickel matrix in which

the Co is produced would give a magnetically split source.

To obtain a single line nickel source, several nickel alloys

were investigated.
Figure l4-a. Gamma-ray spectrum of Co produced by the

(y,p) reaction on °^N1

2 . Source Preparation

Two methods were available. One could chemically

separate the Co and diffuse it into a suitable source matrix, or one could irradiate a nonmagnetic alloy which produced little interfering radiation from the other elements. possibility of irradiation of a nickel compound for a s,ource. For the first method, 1 gram of Ni(OH)„ was irradiated for

30 minutes at 60 MeV. The Co was separated by ion exchange

in the form of CoClp, which was evaporated on a copper disk, and annealed in a hydrogen furnace. The gamma-ray energy spectrum of this source is shown in figure 14-b. A small Mossbauer effect was observed, but no useful data could be extracted. The 1.5 hour half life made this procedure

impractical for routine operation.

The second method restricted the number of matrices

available due to induced interfering radioactivity. Only

Be, Al, Si, V, Cr, Mn and Rh or separated isotopes of some

28
Figure l4-b. Gamma-ray energy spectrum of Ni 15% Cr alloy after LINAC activation

elements can be used for Ni alloy sources. Furthermore, a cubic crystal structure would be the most desirable for a single line source. The Debye-Waller factor requirement

eliminates Be. For the preliminary source evaluation, NiAl,

NiV., and NiSi

? were activated, but no Mossbauer effect was

observed. In the case of NiAl, magnetic ordering could be present. In NiV-,, the nickel content is low, NiSi has semi- conductor properties, and during the radioactive decay high

nickel oxidation states could be formed.

To increase the nickel content of the source, solid metal

solutions were investigated. The NiCr system shown in figure

15 offered good source possibilities [7]. At 15% Cr , the internal magnetic field collapses, and the alloy is cubic.

The Mossbauer spectrum of this source is shown in figure 16, served line width is 0.097 mm/s with a fraction effect of

0.1 at 80°K.
3 13 23 WEIOHT PMCEKT NICKEL

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done on diagram appearing in reference [7].
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31
The magnetic moment of the ground state of Ni has been measured in Ni(COK by Drain [8] using nuclear magnetic

resonance. For the 3/2 spin ground state, the magnetic

moment is -0.74868 ± 0.0004 nm. Prom the nickel Mossbauer

experiments of Wegener and Obenshain [9], two possible values

for the magnetic moment ratio ye/yg were reported,

ye/yg = -0.558 ± 0.023 and ye/yg = 2.543 ± 0.029- To determine the ratio of the moments, a 1.5% Ni-Fe alloy was made and the resultant Mossbauer spectrum is shown in figure

17. As an aid to analyzing this spectrum, the hyperfine

interactions are shown in figure 18. The relative intensities can be obtained from the square of the Clebsch-Gordon

coefficients C . . (J,M,m,m'). These are listed in table 2. The hyperfine interaction which best fits the spectrum

of figure 17, for the 5/2 to 3/2 transition is shown in figure 18, and a calculated spectrum is shown in figure 19 using the moment ratio of -0.558 ± 0.023 from the work of Wegener and Obenshain and an internal field of 235 ± 1 kG from the work of Streever, et al [20]. Using the same transition probabilities, a value of the moment ratio

(-0.568 ± 0.055) and of the internal magnetic field

(241 ± 7 kG) were obtained (errors are standard deviations

of the determination). Therefore, high confidence can be given to the assignment of a positive magnetic moment for the excited state.

4. Evaluation of the Source

This source for Ni Mossbauer spectroscopy can be used

for further studies of magnetic and chemical properties of nickel compounds. A plot of the expected Debye-Waller

factor as a function of the Debye temperature which is shown

in figure 20 indicates that for most compounds a temperature

of 4°K is required to obtain a satisfactory Mossbauer effect (characteristic temperature 100-200°K) . However, since a

32
Figure 17. Mossbauer spectrum of 1.5% Ni-Pe alloy.

33
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:: i I

5 g 4 II 4 £z i i i i

TRANSITIONS Figure 18. Nuclear energy level diagram of 1.5% Ni-Fe

alloy from hyperfine interactions.

34
Table 2. Relative intensities of transitions in 1.5%

IntensTran 3 .5/2 Angular Mo Am = +1 Coefficient

C 3/2,l (5/2 ' 5/2;3/2 ' +1)= 1 ity

3/2 - 10 3/2 > 3/2 Am = C 3/2 (5/2 3 3/2;3/2,0) = 2/5 4 1/2 - 3/2 Am = 1 cl\. (5/2,3/2;l/2,l) = 3/5 6 3/2 -> 1/2 Am = -1 C

2 /2 _ 1 (5/2,l/2;3/2 J -l) = 1/10 1 1/2 - 1/2 Am = C

2

n (5/2,l/2;l/2,0) = 3/5 6 "3/2,0'

,2

( "3/2,

l .2"3/2, -l'

-1/2 * 1/2 Am = +1 C

2

- 1/2 - -1/2 Am = -1 C

2 _ (5/2,-l/2;+l/2,-l) = 3/10 3 -1/2 -+ -1/2 Am = -3/2 > -1/2 Am = 1

3/2,0'

^2

-1/2 - -3/2 Am = -1 C^/2 _ 1 (5/2,-3/2;-l/2,-l) = 3/5 6

1 (5/2,-5/2;-3/2,-l) =1 10

35
Figure 19. Calculated spectrum for 1.5% Ni-Fe alloy from Clebsch-Gordon coefficients of table 2. functional effect of 0.1 was obtained at 80°K a very high effective characteristic temperature is indicated and little advantage will be derived from operating at 4°K.

(J. J. Spijkerman, D. K. Snediker)

E. Structural Analytical Applications

1. Correlation with Nuclear Magnetic Resonance

It is important in any spectroscopy for the scientist to attempt to relate the measured parameters with those of other spectroscopies. Attempts to do this have not been very suc- cessful because experimental parameters are often very different. An exception to this may be the correlation of chemical shift in Mossbauer spectroscopy with coupling

36
CHARACTERISTIC TEMPERATURE (e )°K Figure 20. Plot of Debye-Waller factor versus characteristic

temperature

coefficients in nuclear magnetic resonance. More work needs

to be done to prove the relationship to be widely applicable,

but the following is an abstract of a manuscript which demonstrates the interrelationship for a homologous series of methylstannes

37
ON THE RELATIONSHIP BETWEEN MOSSBAUER SPECTROSCOPY

AND THE NUCLEAR MAGNETIC RESONANCE OF ORGANOTIN COMPOUNDS

by J. J. Spijkerman and L. May, published in the Journal of Chemical Physics.

A relationship between the Mossbauer spectroscopy chem- ical shift and the nuclear magnetic resonance coupling co- efficient has been obtained for organotin compounds. By partitioning the total s-electron density at the tin nucleus

in terms of the ligand contributions, the MS chemical shift

can be calculated from the NMR coupling coefficient. This model was tested for the methylstannes by a least square fit of the Mossbauer and NMR data. The fit was within the experimental error. The calculations also provided the ligand contribution to the total s-electron density at the tin nucleus, which explained the absence of a quadrupole

splitting.

2 . Iron Coordination Chemistry

It is very important in the field of structural chemistry

to continually renew attempts to be as quantitative as possible with respect to the correlation of the Mossbauer

parameters with known chemical structural properties of the material. Much has been accomplished in this area, and the abstract below briefly summarizes the latest techniques:

A STUDY OF IRON COORDINATION CHEMISTRY BY MOSSBAUER SPECTROSCOPY

by J. J. Spijkerman and L. May published in Mossbauer Effect Methodology, Vol. 2, 85-93 (1966) Although a great amount of data has been collected on the

57 chemical shift of Fe compounds, comparatively little attention has been focused on the relation of the chemical shift

to the nature of the bonding. Walker, Wertheim and Jaccarino

first showed a relationship between the total s-electron

density at the nucleus and the chemical shift, which provided

the 4s contribution to the chemical bond. They used the
ionic ferric and ferrous sulfate to correlate the chemical

shift with the total s-electron density. Danon was able to provide additional data to the WWJ plot from the results of calculations applying the principle of electro-negativity

equalization to a few complexes. Since that time, Gray and co-workers have made molecular orbital calculations on some important ferric and ferrous complexes. Using their results

and a redetermination of the chemical shifts relative to the standard (sodium nitroprusside ) , we have reevaluated the WWJ

plot. The implications of the revised plot will be discussed. Studies of the quadrupole interaction in iron compounds

have been focused on a more complete understanding of the cause of asymmetric peak intensities. Three principal phe- nomena are known: crystal orientation, Goldsankii effect,

and relaxation phenomena. These effects, particularly the relaxation phenomena, can be used to interpret the structure

of iron complexes.

Of the many methods used for the solution of physiochemical problems in coordination chemistry, Mossbauer

spectroscopy provides unique information as well as confir- mation of the results of other techniques. The relationships between the various approaches will be evaluated.

F. Quantitative Analytical Applications Mossbauer spectroscopy has proved to be an important

technique for elucidating the structure of a material. It would be significant if this technique could be extended to the quantitative analysis of the material under study.

An investigation was carried out to evaluate the parameters pertinent for quantitative applications. A SnC^-AlpO^

system was chosen for this Initial study in order to determine what variables require close control. Synthetic mixtures of Sn0

2 and A1

2 were made at different concentration ratios. The following parameters were

39
varied within the framework of an analysis of variance procedure. Of course, it is necessary to determine what measure

of quantitativeness is to be used if a meaningful calibration

curve is to be produced. The analyst has a choice between

peak height, peak width (at half maximum) or peak area. The peak area has certain advantages. Among these are (1) the area is less sensitive to instrumental vibration, (2) it is virtually independent of the line shape of the source, and

(3) the area is adversely affected less rapidly than the height with increasing sample thickness. For these reasons

it appears that the peak area (A) is the best indicator. A meaningful experimental indicator is the area normalized for

the height of the baseline (B).

A = A

n B

A typical spectrum of SnO

? in A1„0^ is shown in figure

21. All spectra are processed by machine digital computation

which fits a Lorentzian peak shape plus a parabolic back- ground shape to the experimental data by an iterative least- squares procedure [1]. Experimental

The NBS drift-free Mossbauer spectrometer was used in these experiments. The cell for mounting the powdered samples was especially constructed to accommodate various sample thicknesses. A complete description of the cell appears elsewhere [10 ]

Stock solutions of tin and aluminum were prepared by dissolving analytical reagent grade tin metal in concen- trated HC1 and Al (N0^ ) _ . 9H 9 in distilled water. The aluminum solution was standardized by titration with EDTA. The hydrous oxides were coprecipitated according to Kolthoff

and Sandell [11]. The mixtures were then heated above 1200°C

in order to ensure complete conversion to the nonhydroscopic

40
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3 i g

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41
oxides

7 to 86%.

A plot of the logarithm of A vs mg of SnO

? for each of two sample thicknesses is shown in figure 22. Interpretation of this curve is now in progress, and a manuscript on this work will be submitted for publication.

(P. A. Pella, J. R. DeVoe)

AMOUNT Sn02 ,mg Figure 22. Plot of logarithm of A vs mg of SnO„

G. Abstracts of Miscellaneous Publications

The abstracts to follow constitute those that are review

papers or are on subjects discussed in previous progress

reports

42
APPLICATIONS OF MOSSBAUER SPECTROSCOPY TO STRUCTURE ANALYSIS

by J. J. Spijkerman, published in the Proceedings of the Second Symposium on Low Energy X- and Gamma Sources and Applications, March 1967. Further studies have been conducted on the tin-palladium

alloy system for preparing Sn Mossbauer sources. Pal- ladium-tin Mossbauer sources have certain advantages over

other Sn sources now in use. Palladium-tin sources have

given percent effects of the order of 20% at room temperature

as compared to 7% for Mg

2 119 Sn at 77°K and 10% for 119 SnO

at room temperature. Furthermore, the family of tin-palladw„ (0.15cm/s). Recent

research, however, has indicated that tin-palladium solid

solution sources broaden somewhat at low temperatures due to

the precipitation of the intermetallic compound Pd^Sn. Further investigation suggested that Pd„Sn should give a single, narrow line at all temperatures if the composition

is carefully controlled to provide the proper stoichiometry

In this study, particular attention has been directed to the synthesis and Mossbauer properties of Pd^Sn. A new method

allows more precise control of stoichiometry and eliminates

the need for a fast-response, controlled atmosphere furnace.

The room temperature absorption as well as the full width at half maximum at room and liquid nitrogen temperatures have

been determined. The effect of annealing on the line width

of solid solution sources has also been investigated. MOSSBAUER SPECTROSCOPY

by J. J. Spijkerman and L. May to be published in Chemistry

Journal, 1967.

A review of Mossbauer spectroscopy is presented for the high school senior and college freshmen chemistry students.

The fundamental concepts of nuclear resonance fluorescence

43
are described in particular for its application to chemistry.

Specific examples are given of how Mossbauer spectroscopy

solved analytical and structural problems. A list of selected references are included for further study by the readers.
3. NUCLEAR CHEMISTRY Introduction

The project in nuclear chemistry was initiated in the

fall of 1965, and much of the activity during the ensuing

year involved direct consultation with groups in this section

and the Activation Analysis Section. Also during this time, necessary equipment was designed, constructed and purchased

for the accomplishment of a comprehensive program in the evaluation of photonuclear reactions.

Most of the equipment needed for handling the irradiated

samples and their reaction products has been completed. A major effort must be forthcoming to produce a high intensity,

high purity, thin target bremsstrahlung terminal. Much of

this particular work is being performed in collaboration with

scientists in the Radiation Physics Division.

B. Nuclear Reaction Studies

1 . Irradiation Capsules Investigation has begun on the photonuclear yields of stable, gaseous nuclides using the NBS 140 MeV electron linear

accelerator. Yield measurements are to be made by means of "static" mass spectrometry, in conjunction with radioactivity

bremsstrahlung intensity monitors. In order to calibrate the

mass spectrometer, and in order to trap reaction gases and deliver them to the mass spectrometer, a suitable gas-tight

container was needed. It was also desirable that the con- tainer be small enough to fit within an aluminum "rabbit", having an internal volume of about 2.5cm , which is available for remote irradiation by means of a pneumatic transfer

system [8]. In the search for a material which would be suitable for ultra high vacuum mass spectrometry and which

could be sealed and opened without large appendages, it was

found that high purity copper was most satisfactory. Copper

capsules were therefore prepared with a thin, sealed end and

a slightly narrowed neck for cold-welding. Evacuation or

45
metering of calibration gases has been accomplished by- attaching the capsules to the vacuum system with a compression port. Cold-welding then gives a gas-tight seal, and

the capsule is ready for irradiation. Figure 23 shows a sealed capsule as well as one which had been partially

filled with lithium, irradiated, and subsequently sectioned.

The internal volume of the capsule is approximately 1 cm . Figure 24 shows a portion of the gas-handling system used to prepare standard gas mixtures for calibration of the mass spectrometer

Figure 23. Irradiation Capsules
'f

ii

V

1 -

1

Ml9Hlli^HI^BHf'

" 1 MH HilM

":'... \,: :<

Figure 24. Gas handling system Following irradiation or calibration-sample-preparation,

the capsules are attached to the ultra high vacuum system of

the mass spectrometer. A port containing a bellows system is used for attaching and, following evacuation, opening the

capsule. Expansion into the fixed static volume of the mass spectrometer and measurement of the signal from the electron

47
multiplier for the masses of interest completes the operation.

2 . Linac Irradiations - Temperature and Intensity Data

A number of "exploratory" irradiations have been

carried out since the availability of high intensity brems- strahlung from the Linac in mid-1966. Although the ultimate

goal — determination of photonuclear reaction cross sections

and mechanisms above the giant resonance — must await a source of high intensity, high purity, thin-target bremsstrahlung, several important preliminary experiments have been carried out using the pneumatic transfer system and its "thick" target. The experiments have been of three types:

(1) feasibility of measuring (y, He) yields by bremsstrahlung production followed by neutron activation; (2) feasi- bility of measuring (y,a), (y, He), etc. yields by means of mass spectrometry; (3) studies of available intensities, intensity profiles, and sample temperatures within the rabbit

Relative intensity information has been obtained by examining the radioactive products from the reactions:

12,-,/ \ 11~ 65,-, / % 64,, , 63/-, / n \ 6l„ C(y,n) C, ^Cu(y,n) Cu, and Cu(y,2n) Cu. Although Cu may be produced by thermal neutrons on Cu, or fast neutrons on Cu, and C may be produced by fast neutrons on C, the last product, Cu, may be produced

only by bremsstrahlung. Consistency in the observed ratios

of the various products suggests that, under the present circumstances, secondary neutrons are not producing significant effects. For example, the first sample irradiated

included carbon (polystyrene) and copper monitors at each

end of the 30 mm long rabbit. The relative beam intensity

(rabbit top/rabbit bottom) based upon C, was 1.44 ± .03, while that based upon Cu was 1.43 ± .02. (The errors represent standard errors of activity ratios.) Additional

investigations of intensity variations with position led to

48
similar ratios for the beam intensity, the largest observed

ratio being 3.3. In order to get a little more information

on the intensity profile cartesian coordinate graph paper

was included with two samples, together with the usual polystyrene and copper monitors. As will be discussed below,

the graph paper appeared to be a perfectly satisfactory

relative monitor, and it offered the convenience of simple

cutting into sections of known size. Relatively small masses

of graph paper are counted, because of the sectioning process and because of the thickness of the paper. The high

bremsstrahlung intensity, however, leads to ample counting

rates, even from masses less than 1 mg. Counting of "squares"

was found to lead to greater accuracy, for relatively small

pieces, than "ordinary" weighing, since the paper used was

2 found to have a ratio of 1 mm per 0.1 mg. Radial intensity variations, examined with graph paper,

were found to be less than about 10%, over the rabbit diameter of about 12 mm. (The rabbits were spinning in a stream

of cooling air during irradiation.) Longitudinal variations

were observed with both the paper and the foil monitors. For example, a sample irradiated at 90 MeV gave the following

position (mm) 7 12 17 24 relative intensity : 1 1.2±.l 2.0±.l 2.0+.1 2.10±.05

Intensity variations, such as the above, may be most serious

in the measurement of photonuclear cross sections, where random errors are increased by factors of 10 to 100 in the unfolding of yield curves. Even for 1% monitoring accuracy,

as might be required for activation analysis, the position

of the monitor must differ from that of the sample by no more than about 0.2 mm. The use of copper monitors also permitted a direct, experimental comparison of the brems- strahlung intensity available from the Linac, as compared to

49
that from the NBS synchrotron. With both machines operating

at 90 MeV, the intensity of giant resonance bremsstrahlung

available with the Linac pneumatic tube facility exceeded the

h intensity from the synchrotron by a factor of about 1.3 x 10 , for the most intense beam used. Heating of the sample within the rabbit became a problem

at the higher energies and intensities because the incident electrons were not stopped by the target assembly. Besides yielding practical limits for beam currents with the present irradiation facility, within-rabbit heating information provides very important in planning for the future high intensity, high purity bremsstrahlung terminal. For a number of irradiations at 90 MeV, graph paper served unintentionally

as a crude temperature monitor. Decomposition of cellulose

is known to set in at about 260-270°C, and a series of experiments showed that paper turns yellow slightly below 300°C and brown in the neighborhood of 330°C. Charring takes place by about 450°C, and paper becomes quite black above 500°C. Estimates of beam power incident upon the rabbit, power absorbed by the sample, and temperature of the sample are given in table 3. Temperature-sensitive crayons were available when the

last sample was irradiated; this allowed lowering the estimate from <250°C to <100°C. A principal conclusion from the above observations is that the air-cooled rabbit will allow irradiations with thin target, high purity bremsstrahlung

(dumped electrons) making use of the full power of the Linac.

50
Table 3- Sample heating

Incident Power

(kW) Absorbed

Power

(W/g) 100S amp 1

B-2 Heating Evidence

paper carbonized

Temperature

Estimate

5 >500°C (a)

1 20 B-3 paper slightly

yellowed _260°C (b)

2 40 Li-1 paper completely

yellowed ~320°C

5 100 Li

paper charred ~480°C

0.5 10 B3A paper unaffected „ (c) <250°C V ; a. Beam dump temperature: 220°C

b. Beam dump temperature: 60°C

c. Temperature-sensitive crayon: sample temperature <100°C Photonuclear Monitors

A fairly extensive investigation is underway with respect

to high accuracy bremsstrahlung monitoring by means of radioactive products. As mentioned earlier, precision of -.!%-. 2%

is desirable, because of unfolding, and systematic errors

should be kept below 1-2%, if possible. Therefore, attempts

have been made to determine and to minimize the true experimental random error in monitoring, and to detect systematic

errors arising from monitor-foil or beam impurities and interfering nuclear reactions.

One means of monitoring beam composition is through the

use of radioisotope ratios, such as Cu/ Cu. The ratio

depends upon irradiation time, bremsstrahlung energy distribution, and the presence of neutrons. By correcting observed

ratios to zero irradiation time, we have looked for possible

effects due to the latter two factors. Typical data appear

in table 4. The significant decrease in the ratio when the

51
Accelerator

Synchrotron

Synchrotron

Linac (b) Linac (b)

a. Corrected to zero irradiation time, (counting statistics) are given.

b. Original location, end of drift tube.

c. New location, magnet room (Cd-wrapped terminal).

6lCu/611 Cu< a > 2 62 + .06

2 64 ± .04

2 03 ± .08

2 03 ± • 09

1 94 ± .02 Standard errors

Linac supplied the bremsstrahlung is probably due to the fact that a thicker bremsstrahlung target was used, thus favoring

lower energy quanta and, hence, the lower energy reaction

64 64 product, Cu. Additional Cu due to neutrons cannot be ruled out, however, at this time. No significant differences

are observed in the ratios at the Linac, even though the final value resulted from an irradiation in a cadmium- wrapped terminal. One must conclude, at least, that thermal neutrons do not presently produce detectable differences in

64 the Cu production. The consistency between beam profiles indicated by C and those indicated by Cu, discussed

earlier, also suggests no detectable effects due to neutrons. Monitor impurities or unexpected reactions may lead to decay curves which do not represent just the component sought. For example, although graph paper and "old" polystyrene

(extruded) gave decay curves which were almost entirely due

to C, a "new" supply of polystyrene (impacted) gave very poor fits indicating additional decaying species, which must

52
have been due to impurities. Both the pure and the impure

polystyrene, however, were characterized by a small, but statistically significant, long-lived component. The longlived component is very likely not due to an impurity, but

12 7 rather to the interfering reaction, C(y,on) Be. These

observations are summarized in table 5. The quantity,

FIT[13], should not exceed about 1.5 for these samples.

7Be was not detected in the graph paper simply because of

the lower counting rate. Table 5. Carbon monitors Monitor 7 Be/ 11

C

(

' a ^

) FIT Graph paper not detected 1.2 Old polystyrene (1.2 ± .1) x 10 1.2 New polystyrene (2.7 ± .5) x 10" 9-9

(a) Corrected to zero irradiation time. Standard

errors (counting statistics) are given

The "true" monitoring precision has been estimated

using the methods outlined in section 4-A of this report.

Many of the observations were obtained with a multichannel

analyzer having limited stability. This resulted in a nontrivial- excess random error. Except for the last set of observations, "counting statistics" have also been limited,

and therefore the limits for the excess variation were rather

wide. More recently a single-channel analyzer was employed

with careful control of peak location, and approximately

10 counts were collected per observation. In connection

with the control program, it was necessary to determine the

peak parameters for the single channel analyzer. A very

simple method was devised for this purpose: three observations with fixed window width, but different baseline

53
settings were combined to calculate peak location, width,

and amplitude. A computer program, NORMAL, was written for the calculation; it assumes a normal peak shape and solves

the three simultaneous equations. In table 6 are given the observed limits for the excess relative standard deviations

for two different multichannel analyzers and for the single channel analyzer. In all three systems the precision was estimated from decay curves of Cu, Cu, in which the annihilation radiation was detected by a sodium iodide

crystal. An unexpected observation arose when the precision

of ~ .1% became possible. Namely, the decay curve no longer

gave a satisfactory fit to the model Cu- Cu (plus Cu when appropriate). Rather there was evidence of a longer- lived component, which appears to be Co - again from a Alt

64

1% of the initial Cu activity, it quickly became evident when the measurement precision was -.1%, and it was deter- mined from the three-component decay curve with a relative standard deviation of 1%

Table 6. Monitor precision

Detection System

Multichannel analyzer-1 Multichannel analyzer-2

Single channel analyzer

(a) Limits are given at the 95% level of confidence.

(L. A. Currie)

C. Decay Analysis Investigations

The analysis of radioactivity decay data has been im- proved in a number of particular respects, since the original preparation of the computer program, "CLSQ-1" [9].

54

Degrees of Freedom

Excess Random (a Error (Limits)

43 0.9% (0.6%-1.2%)

15 1.2% (0.8%-2.3%)

9 0.06% (058-0.2?)
Discussion of the separate modifications follows. Each set

of modified instructions, except the first two, is available

on paper tape which temporarily alters the original program

to do the desired job. The first two modifications have been

built into a revised version of the program.

The first change allows the output of the counting rate

at any desired time. Frequently, during the monitoring of nuclear reactions, one needs to examine several monitor foils

in order to gain information on the spatial intensity distribution of the irradiating beam. If each such foil must be observed several times for the analysis of decay components,

considerable effort may be required. In the course of studying photonuclear reaction monitors, it has been observed that

the bremsstrahlung intensity did vary over the sample volume,

but that the ratio of monitor radionuclides (Cu-6l, 62, 64) was essentially constant. Therefore, CLSQ-1 was modified to allow the complete analysis of a single monitor foil, and based on the results of the analysis to calculate, for any specified observation time, the counting rate and its standard error for that same monitor foil. In this way, several

monitor foils may be normalized to the principal one, requiring only one or two observations of each, taken at any convenient time. Considerable time and effort has been saved

by this modification; reliable estimates of intensity variations have resulted. (Note that if complete decay data are required for all foils, one must either sacrifice counting

precision, or multiple-intercalibrated detection systems must

be available .

The second, incorporated change corrects for decay during

counting. In the original version of the program, each rate

for any time interval was assumed to be equal to the sum of

the rates of the individual components at a time corresponding to the mid-point of the interval. Such an assumption is valid only if there be negligible decay during the counting

55
interval. The approximation has been replaced by the exact expression

-A. At AN/

At

= Z R° (1-e J )/\ (3-1) where AN and At represent the net number of counts and length

of the counting interval, respectively; A. represent the decay constant for component -j ; and R° represents the count- ing-rate of component -j at the beginning of the time interval in question. The importance of replacing the approximate ; expression by the exact one is illustrated in table 7, where

an exact, constructed decay curve has been analyzed by both methods. All results are seen to be seriously in error when calculated by means of the usual, approximate equation. Such errors are generally much smaller, however, if the counting schedule is such that each counting interval is small com- pared to the half-lives of all important components. The example presented in table 7 represents a situation in which

the counting intervals ranged from one to two times the relevant half-lives.

Table 7. Decay-during- observation errors Component Half-life (m) Initial Rate (count Exact Eq. Approx.

s/m)

Eq. Star dard Error

1 770 389 437 2.

2 200 941 1145 5.

3 9.8 74300 80921 77

The exact treatment for decay discussed above has been extended to the problem of decay analysis including one or more "unknown" components. That is, the analysis has been formulated according to methods for non-linear least squares,

56
including the missing half-lives as parameters to be estimated. Three different forms of Taylor expansion have been

studied: (1) expansion in terms of 6t-, /? , (2) expansion in

terms of 6A, (3) expansion in terms of R° x 61nt ,„1 The

first approach represents the "preferred" method of treating

non-linear least squares problems; the second, the method

previously used for the specific problem of "half-life"

iteration; and the third, the method selected by this laboratory — chosen because it requires no initial guesses for

the amplitudes (initial rates) for the unknown components.

Except for slight differences in rates of convergence, all

three methods appear to give essentially similar results.

One limitation of all methods is that they require an initial

guess for the half-life. If that guess is unfortunate, the procedure rapidly diverges leading to meaningless solutions.

Means for avoiding the divergence, especially when several

half-lives are to be estimated, is presently under study.

The fourth modification allows for the collapse of a multicomponent model into a smaller number of components by

using known (or assumed) relationships among the activities

of the various components. The change from a three-component

problem to a one-component problem, for example, is given in equations (3-2) and (3-3) -At -At -At

Three-component: R = R° e + R° e + R° e

J (3-2)

— A -, "t — A ,-j t — A^"C One-component: R = R° e + K

2 e + K e

5 (3-3) Where R° ' s represent initial rates, A's represent decay constants, and K's are constants representing the fixed relationships referred to above. Such model simplification may

lead to much more precise estimates because of the smaller

number of "unknowns". One set of experimental data, so analyzed, reduced the standard error estimate by about a factor of ten.

(L. A. Currie)

57
4. STATISTICS IN NUCLEAR AND ANALYTICAL CHEMISTRY

A. The Precision of Radioactivity Measurements

1. Introduction

The precision accorded to measurements of radioactivity

is usually deduced from "counting statistics." That is, the standard deviation associated with a given number of counts

is assumed to be equal to the square root of the number of counts, as predicted by the Poisson distribution. The pre- cision of nonradioactivity measurements, on the other hand,

is normally determined from the observed distribution of results, using the statistical estimator,

and x are the individual and mean observations, respectively; n is the number of observations. When "counting statistics" do give

a valid measure, x, of the variance, then one has two meas2 —

ures of the same variance whose ratio, s /x, should be dis- tributed about unity, according to the x /v distribution [21] Thus, one may test the validity of the assumption of "count2 —

ing statistics" by comparing s /x with appropriate

2 critical values for x / v - If consistency is not indicated, one must consider the possibility of extra sources of variability. Such excess random error can be of great significance for it must be known in order to (1) properly estimate the (weighted) mean,

(2) to estimate the standard error correctly, (3) to detect outliers and sources of variability, and (4) to estimate the limiting precision when the standard deviation due to the decay process becomes negligible. It is assumed that the relative standard deviation for the excess variability is constant. Thus the variance of a reported value is of the

2 form [(true mean)/( counting time)] + a . A computer
program, "XESS" has been written for the purpose of estimating the excess random error and its limits for a series of observations of radioactivity. The process is complicated by _ 2 the fact that x and s require knowledge of the individual

weights; but these, in turn, are based on the "counting

statistics" as well as the excess random error, o . Therexs fore, the solution is iterative in nature, trial values of

2 2 o being input and iterated until s /o takes on the "ex- pected" value, 1, and the upper and lower limits, as allowed

2 by the distribution of x /v • 2

Detection of Excess Random Error

As the preceding example indicates, the limits for o may be rather broad. This follows in a straightforward way

2 from the limits of X / v and from the compounding of errors in

2 2 estimating s /o . One finds that it becomes very difficult

to detect random errors very much smaller than about twice

the counting standard deviation. In order to reliably detect

a variation whose standard deviation is 50% of that due to counting, 460 observations are required! Similarly, the limits which one may place on the excess standard deviation

estimate are equivalent to a range of ± 30% if one has twenty observations and if the standard deviation due to counting is quite trivial.

3

Multicomponent Systems - Decay Curves Determination of excess random error when one must analyze a multicomponent system by means of a weighted least squares procedure is still more complicated, and again, the weights require an initial estimate for a . In order to illustrate the significance of such excess random errors in

the analysis of radioactivity decay curves, a synthetic

decay curve was constructed with added random errors. Two components were present, one having a half-life of 138 minutes and an initial counting rate of 100 cpm, the other,

h a half-life of 750 minutes and 10 cpm; ten, 100-minute

59
observations were assumed. As shown in table 8, the wrong

choice for a had only a slight effect on the error in the initial rate estimate for component-1 (138 min), but it had

.a profound effect on the standard error estimate, a-,, and on

2 the value of x /v. Thus, one at the very least would have an incorrect estimate of the standard error, and, at worst, might falsely assume that the "fit" was consistent or incon- sistent with the assumed model. Table 8. Effects of wrong a (synthetic decay curve)

(1) 138 min : 100 cpm

(2) 750 min : 10

4 cpm°

Ten, 100 min observations

(a ~ 0.12%) Correct Assumed

°xs (%) 6R° (cpm) a °1 (cpm) 1.67 c„Wab 1.97 0.88

3 C 1.00° 9-31° 0.10 C

3 3 7.62 9.52 0.86

o

c 7-92 c 1.68 c 8.8l c

(b) 95% confidence limits: 0.52, 1.48

(c) Designates results from incorrect assumption

In a recent series of experiments on photonuclear reac- tion monitors, it became desirable to combine information

from several independent sets of decay curves. The results

of the separate decay curve analyses, as given by the program, "CLSQ-1" [13] , included values of the "FIT" for each

decayJ curve,' and for each of several values of a . This

xs matrix of FIT-values was then combined according to the

2 addition law for x > using the program, "CHISQR". The final
results gave closer limits a . since they were based upon

the number of degrees of freedom in all of the constituent

decay curves. The limits for a for a single one of the decay curves ("Cu-5") were 0% and 4.4% based upon three

degrees of freedom. The composite sample, however, was derived from four decay curves, and a total of eleven degrees

of freedom. Here, the limits for a were narrowed to 0.6%

and 2.3%- (The relative standard deviation due to counting

only, was approximately 1% for each observation in the four

decay curves . ) (L. A. Currie)

B. Decision, Detection and Determination Limits

1 . Introduction and Principles

The detection capabilities of analytical measurement

processes generally, and radioactivity measurements specifically, are rated according to many different criteria and given many different names by various authors. When applied

to a particular process, such varying definitions lead to a wide range of "detection limits". For example, one commonlyused definition equates the limit of detection to the standard deviation of the background, while another, equates it to 1000 dpm. Applying such a pair of definitions to a specific

problem involving the detection of a gamma emitter leads, in the first case, to a limit 0.63 pCi, and in the second,

450 pCi —some 700 times higher!

In the search for a unique, statistically-sound approach

to the problem, it has been found necessary to define three

levels of meaning. Each of the three refers to the net instrument response (signal); the first two relate to the problem of detection (qualitative analysis) and the last relates to the problem of determination (quantitative

analysis). The meanings, as summarized in table 9, follow.

Lp is applied to an experimental observation to yield the decision: "detected" or "not detected";

61
Table 9- Decision, detection, determination levels Quantity Role Application Type of Analysis

L^ Decision Level Observation Qualitative

L„ Detection Limit Measurement Qualitative

Process

L

Q Determination Measurement Quantitative

Limit Process

("true") net signal which may be expected to lead to the de- cision, "detected"; Ln is the minimum ("true") net signal

Q which may be expected to be characterized by a relative standard deviation as small as some preselected value. L

r and Ln are directly related to the principles of statistical inference, or hypothesis-testing, and they, therefore, are charac- terized by associated errors of first (a) and second (3) kinds [10]. Defining equations for the three levels are:

L

C

= Vo (1) L

D

= L

c + k

g a D

(2) L

Q

= VQ (3) where a and a^ represent the standard deviation of the net

o D signal when its "true" level is zero and L~ , respectively.

k and k are statistical constants taking on the value 1.645,

a 3 if one assumes normality and takes a = 3 = 0.05- k„ equals

the reciprocal of the preselected, minimum relative standard deviation which characterizes "quantitative" analysis; a reasonable value for k

Q is 10. The foregoing principles are graphically displayed in figure 25- If one expresses a and a_ in terms of the standard

deviation of the blank, and if the above specific values are used for the k's, relatively simple, "working" formulas may

be given for the limits. In order to simplify a*, some

62
REGION I

UNRELIABLE DETECTION

REGION H

DETECTION: QUALITATIVE ANALYSIS

REGION IE

DETERMINATION: QUANTITATIVE ANALYSIS

L cL D

LQ Figure 25. Three principal analytical regions

assumption must be made regarding the variation of the standard deviation with the net signal level. Two such assumptions will be used: (1) for analytical methods, in general,

one may assume that the variation is negligible, and that

(2) for radioactivity, one may assume the Poisson

a D distribution, in which case = ^L

'D

' °o ' The res ulting

"working" formulas appear in table 10. The quantities a R and uR represent the standard deviation and the ("true") mean value of the blank, respectively. Units for the "general analytical" case are those of the measuring instrument, whereas those for "radiochemical" analysis are simply

counts . Conversion of the above limits to the related physical

quantity of interest may be carried out as expressed in equation (4), Ln = K mn (4) D '"D where rru is the desired physical quantity (e.g. minimum

detectable mass) and K is a generalized calibration constant.

For an analytical balance, K may be the dimensionless number,

63
L

Q Type of Decision Detection Determination

Analysis Level Limit Limit

4.65 -, Radiochemica i 2.3ir~^ 2.71+4.65^

50J1+ i+^|

(a) Errors of first and second kinds = 0.05; R.S.D. (Determination Limit) = 10%

(b) "Paired" observations of sample and blank assumed

one, whereas for activation analysis, K must take into account the particle flux, reaction cross section, irradiation time, delay time, counting time, detection efficiency,

and product half-life. Besides the selection of the "best" procedure from among

several discrete possibilities, one may utilize the foregoing

principles and equations to select the best value for various continuously-variable parameters. For example, one may wish

to determine what counting interval will lead to the minimum

detectable activity for a given radionuclide. In this case, equations (2) and (4) take the form

2.71 + 4.65 ^

where t is the counting interval and A Is the decay constant.

The value of At which minimizes A~ depends upon both the half-life and the background, and therefore no general answer

may be given. A specific example has therefore been studied,

and the results appear in figures 26 and 27- Here the halflife was chosen to be 100 minutes, and various background

rates were considered. Figure 26 illustrates the existence

64
Figure 26. Best counting interval

of minima in the curves of minimum detectable activity vs. counting interval. The "best" times, t , are indicated by arrows in figure 26, and they are plotted vs. background in figure 27. As both figures show, the best counting interval

decreases with increasing background and approaches an asymptote of l8l minutes as indicated by the dotted lines.

A similar study has been made of the best "window" width for

the analysis of gamma ray-spectra. As with counting intervals, the best window width depends upon the magnitude (and shape) of the background spectrum in a rather complicated

way, so that, again, no general choice may be stated. Once

again, however, the best window setting approaches an asymptotic value with increasing background. This value is
Figure 27. Minimum detectable activity

approximately 1.2 times the full width at half-maximum. Calculations of the quantities Lp, L^, and L

Q have been

carried out using the computer program, "LIMIT". The program

continuously-variable parameters could be plotted, and the minima in the resulting curves could be determined by an iterative process.

66
2 . Radioactivity Detection and Procedure Optimization

The general equations developed in the preceding section

have been applied to the selection of the "best" activationdetection procedure for detection of a given element. Inter- ference has been allowed for by explicitly expanding the concept of the "blank" to include both background and interfering radionuclides, and the calibration constant, K, has been formulated to take into account both long- and shortlived radionuclides. One illustration, derived from work in our laboratory [11], is the detection of sulfur in protein.

In table 11 are listed the detection limits (g) for sulfur

using these alternative detectors with a neutron generator

for activation. "Reasonable" values for the flux, background,

efficiencies, and the various times have been assumed in the calculation of L and K, but the details will not be given

here. One sees, using the neutron generator, that as little

as 120 pg of sulfur may be detected, and that the "low level b" and "liquid scintillation" detectors are comparable

Table 11. Detection of sulfur in protein

Neutron Generator:. Detector Detection Limit (g)

32 S(n,p) 32 P(l4.3d) L - L - e 1.2 x ID"

4 -4 Liq. Scint. 2.1 x 10 Nal (bremss.) 0.17 Reactor: I/F I(3%) a L.L.

2.4 x 10" 11 0.10

67
and about one thousand times more sensitive than the brems- strahlung detector. Reactor activation, on the other hand,

is far more sensitive in the absence of interference. Activation of protein phosphorus by thermal neutrons, however,

increases the limit of (sulfur) detection by a factor of about 10 . Thus, the choice of activating source is critically dependent upon the possible presence of interference.

(L. A. Currie)
5. NUCLEAR INSTRUMENTATION

A. Introduction Approximately ninety percent of the last year's effort

was expended in modification and/or repair of existing electronic equipment, such as amplifiers, preamplifiers, and data handling equipment. The remainder of the time was spent in developing a capability of radiation detection with a variety

of solid state detectors. Since these systems are for use of both the Radiochemical Analysis and Activation Analysis

sections considerable versatility is desired.

The same applies to the associated electronic equipment

for these solid state detectors. We have some of the very

best as well as the utilitarian equipment for survey types of analyses

B. Solid State Detectors

Several solid state detectors are now in use; they are a

30 cm Ge(Li) 5-sided drift detector and two planar Ge(Li) detectors, of approximately 4 cm in volume. Another detector 60 mm x 2 mm deep Si(Li) has been used for high resolution low energy gamma-ray spectroscopy and another Si(Li)

2 detector 300 mm x 5 mm deep is temporarily out of service.

Each of the above detectors is a commercial unit. A large planar Ge(Li) detector 10 cm is being prepared by Dr°. J. Coleman of the Electron Devices Section (Instrumentation

Division) of the National Bureau of Standards.

C. Solid State Detector Cryostats

A pair of cryostats of the "chicken feeder" type were

designed and then fabricated by the NBS shop division. The

first of these will house the large planar (Ge(Li) detector

2 and the other is to be used with the 300 mm x 5 mm Si(Li)

detector. The cryostat design is patterned after a com- mercially available unit and is relatively inexpensive to produce. The design makes as much use as possible of commercially available components to reduce the machine shop

69
time required for construction. When fitted with a 10 liter dewar, the cryostat will hold a charge of liquid nitrogen for

a period of just under seven days. Reducing the thickness of the inner stainless steel tube to .008" wall thickness might increase this time between charges.

A vacuum system and manifold have been designed and built

to permit as many as four cryostats to be operated on a single vacuum pump. This will be used for detectors that are not particularly affected by loss of vacuum or low temperature.

D. Radiation Spectrum Analyzers and Equipment

In order to make effective use of the solid state detectors put in operation over the past year, considerable upgrading of the laboratories' equipment was necessary. Two new

1024 channel analyzers were procured, each including peak and zero stabilization as well as digital integration capabilities. The analyzers were supplied with teletype readouts but will be provided with high speed paper tape punch equipment

in the near future.

To meet the need for very high resolution capabilities

the 4096 channel analyzer was extended to perform a full 4096 channel analysis by adding a new analogue to digital con- verter. The performance of the new ADC system met all expec- tations except for the accuracy of the digital "dead time" correction operation. Subsequent modifications by the factory have improved this. Several other additions and modifications to the 4096 channel analyzer include a unit to provide for automatic programming of the input format and

the addition of an X, Y point plotter.

The need for linear amplifier systems with the solid

state detectors has been accommodated by the addition of five new amplifier-preamplifier systems of various makes. Each system has some special merit such as pole-zero cancellation

and one system includes a post amplifier and stretcher unit.

In the use of solid state detector systems our experience

70
has revealed the critical nature of the performance of the preamplifier. The preamplifier requires careful selection

and matching of input field effect transistors to assure

superior or even acceptable performance. By selecting input

FET's for the lowest possible noise measured in the system

being used, the system resolutions was improved far beyond

the manufacturer's specifications.

A specially designed FET input stage was enclosed in the

2 cryostat along with the 60 mm x 2 mm deep Si (Li) detector.

Care was taken to optimize the temperatures of both the detector and the FET in order to achieve the lowest possible

noise. This system was prepared for high resolution at low energies for the photon excited x-ray fluorescence work de- scribed elsewhere in this publication.

Other work by the instrumentation group on pulse height

analyzer systems includes the completion of the interfacing

of the RIDL Models 3^-27 and 3^-26 analyzers to teletype data handling units. Each of these systems had to be handled as special cases due to departures in their design from the RIDL 3^-12, 12B on which our design of the original interface was based. Some further work has been done on the interface units improving reliability in the readout mode.

E. Pneumatic Tube End Sensor System

The end sensor system designed to sense the presence of

a rabbit in the irradiation located in the Reactor's pneumatic tube system has been designed and built. Two transducer stations service each pair of pneumatic tubes and are situated on either side of the base of the reactor at floor

level of the beam port floor. These stations are connected

to a source of highly regulated C0

? and are electrically

connected to a panel located by the rabbit sending and receiving stations where an electronic timer independently

monitors each of four tube systems.
F. Consultation

A process control system to be used with a three state

mass spectrometer is being designed around a small digital

computer. This system will control, not only the programming

of the instrument, but will gather and manipulate the data as well. This work is being performed for W. R. Shields of the mass spectrometry section (310.06), Analytical Chemistry

Division.
6. X-RAY FLUORESCENCE BY GAMMA-RAY EXCITATION

A one year study of this technique was made in order to determine the answer to two questions. Is the lithium

drifted silicon detector competitive with a proportional

counter for low energy gamma- and x-ray detection (e.g. efficiency and resolution)? What advantages can be realized

by using the radioisotopic source in lieu of the x-ray tube?

The results of this work which have been published [17]

are given below:

The application of gamma-excited x-ray fluorescence in

the determination of medium weight elements has been studied,

using a solid state detector for non-dispersive analysis [12,

13, 14]. The pertinent components of the spectrometer are an

24l Am source and a lithium-drifted silicon detector, with a low-noise preamplifier and multi-channel analyzer. The resolution of the system is 1.2 and 1.3 keV (FWHM) for 8 to

45 keV photons, respectively. Three different techniques of sample mounting were investigated. For single elements in the range from Cu to Dy the estimated detection limits are 10-100 yg and 0.01-1 mg/ml , depending upon the sample mounting procedure. The corresponding K -peak intensities are

4 3 a4 r 2

2 x 10 - 3 x 10 cpm per mg and 4 x 10 - 6 x 10 cpm per

mg/ml, respectively. For testing the application of the method two series of NBS Standard Reference Materials were

analyzed for Sn and Mo as minor constituents within a concen- tration range from about 0.04 to 9%. The best results were obtained by direct counting of the solid samples. Using

this technique, a non-destrucive analysis for Mo and Sn can

be performed in a relatively short time with a precision of

a few percent, a detection limit of about 100 ppm and a

3 4 sensitivity of 6 x 10 J to 1.4 x 10 cpm per percent concentration.

It may be concluded that in two areas the radioisotopic

source proves to be superior to the x-ray tube. The radio73
isotopic source stability provides about a factor of 2 better precision than the tube. An instrument can be made portable

when using the radioisotopic source.

(M. Hollstein, J. DeVoe)

74
7. PERSONNEL AND ACTIVITIES*

Personnel Listing Radiochemical Analysis Section

J. R. DeVoe, Section Chief (1)

M. K. Oland, Secretary (1/6) Instrumentation

R. W. Shideler (1) Project Leader

W. N. Bettum - terminated (1/4)

P. N. Thomas - terminated (1/3) Mossbauer Spectroscopy

J. J. Spijkerman (1) Project Leader

P. Black (1/2)

W. L. O'Neal (1)

P. A. Pella (3/4)

F. C. Ruegg (1)

D. K. Snediker (1)

M. G. Hollstein, Karlsruhe, Germany (One year

appointment) (1)

L. May - Guest Worker

D. K. Dorrell (1/4)

W. 0. McSharry (1/4) Nuclear Chemistry

L. A. Currie (1) Project Leader

G. Tassey (1) Publications

L. A. Currie, "Systematic Errors in 'Recovery' and 'Detection Efficiency' as Related to Radiochemical Analysis*' Proc. 11th Annual Bio Assay and Analytical Chemistry Meeting

October 1965. CONF-651008 (TID-4500) January 1967.

F. C. Ruegg, J. J. Spijkerman and J. R. DeVoe - "A Mossbauer Spectrometer for the Structural Analysis of Materials", Proceedings of Symposium on Radioisotope Instruments

in Industry and Geophysics, International Atomic Energy

Agency, Warsaw, p. 325, October 1965.

J. R. DeVoe and J. J. Spijkerman - "Mossbauer Spectroscopy: Application to Aerospace" - Radioisotopes for Aerospace, Part 2, Systems & Applications, 254-259 (1966).

J. J. Spijkerman, F. C. Ruegg and L. May - "The Use of Mossbauer Spectroscopy in Iron Coordination Chemistry" - Mossbauer Effect Methodology, Vol. 2, 85-93 (1966). *Number in parenthesis indicates fraction of year on roster.

75
J. J. Spijkerman, A. L. Landgrebe and F. C. Ruegg - "Mossbauer Effect in Etylenediamine tetraacetatoiron III Complexes" - ACS 150 National Meeting, Atlantic City, New Jersey, September 1965.

R. H. Herber and J. J. Spijkerman - "Narrow-Line Source

for 11 °Sn Mossbauer Spectroscopy" - Journal of Chemical

Physics, Vol. 43, Number 11, 4057-4059, December 1965.

D. K. Snediker - "The Mossbauer Effect of Sn 119 in Palladium-Rich Palladium-Tin Solid Solution, Mossbauer Effect Methodology, Vol. 2, 161-170 (1966).

J. J. Spijkerman, L. Hall and J. Lambert - "Preparation

and Coordination Studies of the Complex Acid, Dihydrogen

Diethylene-triaminepentaacetatoferrate (III)" - Dihydrate and some of its M+l salts, to be published in J.A.C.S.

J. J. Spijkerman, L. Hall and J. Lambert - "Coordination

Studies by Mossbauer Spectroscopy and Magnetic Susceptibility

of Some M+l Salts of Complex Acid Hydrogen Aquoethylenediamine- traacetatoferrate (III), "to be published in J.A.C.S.

J. J. Spijkerman and L. May - "On the Relationship Between Mossbauer Spectroscopy and the Nuclear Magnetic Resonance of Organotin" - Journal of Chemical Physics, Vol.

46, No. 8, 3272, April 1967.

C. List of Talks

M. Hollstein, "Determination of Medium Weight Elements

by Gamma- Excited X-ray Fluorescence", presented at 2nd Symposium on Low Energy X- and Gamma-Ray Sources, Austin, Texas, April 1967.

J. J. Spijkerman, "Chemical Applications of Mossbauer

Spectroscopy", Kansas State University, April 1967.

F. C. Ruegg, "A Multiplex for Dual Spectra Mossbauer

Spectrometry", presented at 2nd Symposium on Low Energy

X- and Gamma-ray Sources, Austin, Texas, April 1967.

J. J. Spijkerman, "Application of Mossbauer Spectroscopy

to Structural Analysis", presented at 2nd Symposium on Low Energy X- and Gamma-ray Sources, Austin, Texas, April 1967.

J. R. DeVoe , "Mossbauer Spectroscopy and Chemical

Analysis", ACS Meeting, Miami, Florida, April 1967.

76
L. A. Currie, "The Limit of Precision in Nuclear and Analytical Chemistry", 153rd National American Chemical

Society Meeting, Symposium on Nuclear Methods of Chemical

Analysis, April 1967 • P. A. Pella, "Differential Controlled-Potential

Coulometry Utilizing Radioactive Tracers", 153rd National American Chemical Society Meeting, Symposium on Nuclear

Methods of Chemical Analysis, April 1967

P. A. Pella, "Application of Mossbauer Spectroscopy to Quantitative Analysis", Maryland Section ACS, Goucher

College, Baltimore, Maryland, May 1967.

J. J. Spijkerman, "Instrumentation for Routine and High Precision Mossbauer Spectroscopy", International Con- ference on Electron-Nuclear Hyperfine Interactions in Spectroscopy, Wellington, New Zealand, October 1966.

J. J. Spijkerman, "A Study of Iron Coordination Chemistry by Mossbauer Spectroscopy", International Conference on Electron-Nuclear Hyperfine Interactions in Spectroscopy,

Wellington, New Zealand, October 1966.

J. J. Spijkerman, "A New Tin-Mossbauer Source", Inter- national Conference on Electron-Nuclear Hyperfine Interactions in Spectroscopy, Wellington, New Zealand, October

1966.

J. R. DeVoe, "Mossbauer Spectroscopy", Xavier University,

Cincinnati, Ohio.

G. W. Smith, D. A. Becker, G. J. Lutz, L. A. Currie

and J. R. DeVoe, "Determination of Trace Elements in Standard

Reference Materials by Neutron Activation Analysis", NBS-IMR Symposium on Trace Characterization - Chemical and Physical, October 1966.

L. A. Currie, "Detection Limits and Experimental Design

in Activation Analysis", Eastern Analytical Symposium, November 1966.

J. R. DeVoe, "Nuclear Methods Session", Chairman, NBS-IMR Symposium on Trace Characterization - Chemical and Physical, October 1966.

J. J. Spijkerman, "Chemical Application of Mossbauer

Spectroscopy", Washington Chapter Society of Applied

Spectroscopy, November 1966.
J. R. DeVoe, "Application of Mossbauer Spectroscopy to Analytical Chemistry", Washington Chapter American Chemical

Society, November 1966.

M. Hollstein, "Range and Range DistributiongOf Fission

Fragment in the Thermal Neutron Fission of Pu , Argonne National Laboratory, October 1966.

». ACKNOWLEDGMENTS

We wish to express our thanks to Dr. C. 0. Muehlhause

and his staff for their cooperation in making arrangements

for our use of the new facilities in the NBS Reactor Building.

Further, the assistance of Dr. J. E. Leiss and staff of the Radiation Physics Division for arrangements to use the Linac facilities is greatly appreciated. We also wish to thank

Dr. R. Schaffer, 310.07, for making some of the dibutyl-tin

sulfate

The assistance of Dr. B. Christ of the Metallurgy Division (NBS) who made it possible for numerous revisions

and improvement in the computer program that resolves Mossbauer spectra is greatfully acknowledged.

We wish to thank the Statistical Engineering Laboratory

for assistance in statistical analysis of experimental data. This section acknowledges Mrs. R. S. Maddock for her assistance in preparing this report. Special appreciation is expressed to Mrs. M. Oland who typed the entire report.
9. LIST OP REFERENCES

[1] Spijkerman, J. J., Snediker, D. K., Ruegg, F. C. and

DeVoe, J. R. , "Mossbauer Spectroscopy Standard for Chemical Shift of Iron Compounds", Miscellaneous Publication 260_, 13 (1967).

[2] Sprouse, G. D., Kalvius, G. M. , Hanna, S.S., "Mossbauer

Effect by Recoil Implantation Through Vacuum", Phys.

Rev. Letters 18, 1041 (1967).

[3] DeVoe, J. R. , editor, NBS Technical Note 276 (1965).

[4] Ruegg, F. C. , "A Multiplex for Dual Spectra Mossbauer

Spectrometry", Second Symposium on Low Energy X- and Gamma-Radiation, April 1967.

[5] Herber, F. H., Department of Chemistry, Rutgers Uni- versity, New Brunswick, New Jersey.

[6] Gonser, U. , personal communication.

[7] Hansen, M. , "Constitution of Binary Alloys", McGrawHill, New York (1958), p. 592.

[8] Drain, I. E., Phys. Letters 11, 114 (1964).

[9] Wegner, M. H. F. and Obershain, F. E., Phyzik 163 , 17 (1961).

[10] Snediker, D. K. and May, L., "Criteria for Selection

of Absorber Mounting Materials in Mo'ssbauer Spectroscopy", Nuclear Instr. & Methods (1967) to be published.

[11] Kolthoff, I.M. and Sandell, E. B., "Textbook of Quantitative Inorganic Analysis", The Macmillan Company,

New York, N. Y., 3rd ed., 1952, p. 318.

[12] Lundgren, F. A. and Lutz, G. J., "Photo Activation

Target Assembly for the NBS Linear Electron Accelerator", 13th Annual Meeting of the American Nuclear

Society, San Diego, June 1967-

[13] DeVoe, J. R. , editor, NBS Technical Note 404 (1966).

[14] Dixon, W. J. and Massey, F. J., "Introduction to Statistical Analysis", Chap. 7, McGraw-Hill, New York

(1957).

[15] DeVoe, J. R. , editor, concurrent report, Activation

Analysis Section, NBS Technical Note 428 (1967).
[16] Bowman, H. R. , Hyde, E.K., Thompson, S. G. and Jared,

R. C, Science 151, 562 (1966).

[17] Elad, E. and Nakamura, M. , Nucl. Instr. Methods 4l_, 161 (1966).

[18] Hyde, E. K. , Bowman, H. R. and Sisson, D. H., U. S. Atomic Energy Commission Report UCRL - 16845, Lawrence Radiation Laboratory, Berkeley, California (1966).

[19] Hollstein, M. G. and DeVoe, J. R. , Proceedings of the Second Symposium on Low Energy X- and Gamma-Radiation,

April 1967.

[20] Streever, R. L. , Bennett, L. H., LaForce, R. C, and

Day, G. F., J. Appl. Phys . 34, 1050 (1963)

[21] Hald, A. , Statistical Theory with Engineering Applications, John Wiley & Sons, New York, N.Y., 1952, p. 727.
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