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RADIOCHEMICAL ANALYSIS: ACTIVATION ANALYSIS,

INSTRUMENTATION, RADIATION TECHNIQUES, AND

RADIOISOTOPE TECHNIQUES

JULY 1964 TO JUNE 1965

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assur- ing maximum application of the physical and engineering sciences to the advancement of technology

in industry and commerce. Its responsibilities include development and maintenance of the national

standards of measurement, and the provisions of means for making measurements consistent with

those standards; determination of physical constants and properties of materials; development of methods for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices

for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Govern- ment; assistance to industry, business, and consumers in the development and acceptance of commercial standards and simplified trade practice recommendations; administration of programs in cooperation with United States business groups and standards organizations for the development

of international standards of practice; and maintenance of a clearinghouse for the collection and

dissemination of scientific, technical, and engineering information. The scope of the Bureau's

activities is suggested in the following listing of its three Institutes and their organizational units. Institute for Basic Standards. Applied Mathematics. Electricity. Metrology. Mechanics. Heat. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radiation Physics. Radio Standards

Laboratory:* Radio Standards Physics; Radio Standards Engineering. Office of Standard Reference

Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.* Materials Evaluation Laboratory. Office of Standard Refer- ence Materials. Institute for Applied Technology. Building Research. Information Technology. Performance

Test Development. Electronic Instrumentation. Textile and Apparel Technology Center. Technical

Analysis. Office of Weights and Measures. Office of Engineering Standards. Office of Invention and

Innovation. Office of Technical Resources. Clearinghouse for Federal Scientific and Technical

Information.**

*Located at Boulder, Colorado, 80301. Located at 5285 Port Royal Road, Springfield, Virginia, 22171.
NATIONAL BUREAU OF STANDARDS

Technical Note 276 National Bureau of Standards NOV 1 4 1968 ISSUED JANUARY 7, 1966 1S1G7^

U5 753

* Re*

RADIOCHEMICAL ANALYSIS: ACTIVATION ANALYSIS,

INSTRUMENTATION, RADIATION TECHNIQUES, AND

RADIOISOTOPE TECHNIQUES

JULY 1964 TO JUNE 1965

Edited by James R. DeVoe

Radiochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Reset'ch

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

listed or referred to in the open literature.

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

Washington, D.C., 20402 - Price $1
FOREWORD

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

September 1, 1963.5 and became part of the Institute for

Materials Research in the February 1, 1964 reorganization.

It consists at present of seven sections and about 85 technical personnel encompassing some 30 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 highly

important. In addition, however, it has been my experience

that informal, annual summaries of progress describing efforts

of the past year can be very valuable in disseminating information. At the National Bureau of Standards such publications

fit logically into the category of a Technical Note. In

1965 we plan to issue these summaries for all of our sections.

The following is the second annual report on progress of the

Radiochemical Analysis Section.

W. Wayne Meinke, Chief

Analytical Chemistry Division

11
PREFACE

This is the second annual progress report for the Radiochemical Analysis Section of the Analytical Chemistry Division.

It is the intent of this section to develop measurement

techniques for the qualitative, quantitative and structural

analysis of materials through the use of radioisotopes.

Since the Analytical Chemistry Division is interested specifically in the characterization of very pure materials , and

since radiochemical methods are particularly suitable , a large portion of this section's effort is directed towards

the analysis of very small concentrations of the elements.

A basic requirement for the development of such measurement techniques using radioisotopes is the thorough understanding of the chemical, nuclear, and physical principles

which form the foundation of a new analytical measurement

technique. However, 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. Some

misunderstanding has existed as to whether the Analytical

Chemistry Division certifies a particular analytical method.

Under no circumstances is this done. The material is certified for composition and therefore, it should be independent

iii
of the analytical technique that is used. This section is

attempting to make detailed error analyses of the methods

used to assure as high an accuracy as possible.

The section is organized into four groups: activation

analysis, instrumentation, radiation techniques, and radioisotope tracer techniques. The purpose and particular

interests of these groups are reflected in the projects that

will be described in each part of this report. A synopsis

of the activities of each group appears at the beginning of

each part.

Like last year, much effort has been devoted to the

planning and construction of facilities. Toward the end of

1965 the activation analysis group and the radiation techniques group will be moving into the new reactor building at

Gaithersburg. A short description of these facilities is

given in this report. The radioisotope techniques group will

move into the general purpose laboratories with the rest of

the Division in the spring of 1966.

During this year the section has shown marked activity

as is indicated by the fact that there have been 16 talks

presented and 11 papers published by the several groups in

this section.

A roster of the groups in this section is listed in

part 7. Our major staffing requirements have been satisfied

with the exception of one or two areas. Continual efforts

will be made to upgrade the quality of our programs. The

iv
National Bureau of Standards has a program whereby a scientist

from the United States or abroad may work as a guest in our

laboratory for one or two years. It is hoped that by utilizing this procedure in this section we will be able to

perpetuate a stimulating environment.

In order to specify adequately the procedures., it has

been necessary occasionally to identify commercial materials

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

Bureau of Standards, nor does it imply the material or

equipment identified is necessarily the best available for

the purpose.

James R. DeVoe,, Chief

Radiochemical Analysis Section
TABLE OF CONTENTS

PAGE

1. FACILITIES 1

Nuclear Reactor 2

Linear Electron Accelerator 6

Equipment 7

Digital Computers 9

2. INSTRUMENTATION 13

A Frequency Correlation Device Employing a

One-to-One Digital Technique 13

Abstracts of Publications 19

Interface Between Teletypewriter and

Nuclear Instruments 19

Through-port for Reactor 20

3. ACTIVATION ANALYSIS 23

Determination of Trace Amounts of Argon in

Silicon Single Crystals 25

Determination of Chlorine in Polystyrene ... 30

Determination of Hafnium in Zirconium and

Zircaloy Standard Reference Materials ... 35

Determination of the Neutron Flux Spectrum

for the NRL Pneumatic Tube System 4l

Activation Analysis with the NBS Electron

Linear Accelerator 48

Utilization of Computers in Activation

Analysis 4-9

A Gamma-Ray Spectrometer Utilizing a BetaGamma-Gamma Sum Coincidence Technique ... 52

Abstracts of Publications 70

Copper and Zinc in Lung Tissue 70

vi
TABLE OF CONTENTS (con't)

PAGE

NBS Facilities and program 71

Tellurium in Cartridge Brass 72

.4. 'RADIATION TECHNIQUES 74

Standard Reference Material for the Differential Chemical Shift of Iron Compounds in

M<5ssbauer Spectroscopy 75

Standard Reference Material for Iron in

Mftssbauer Spectroscopy 83

Differential Chemical Shift of Iron Compounds . 83

Absolute Velocity Mtissbauer Spectrometer Using

Optical Fringe Counting Technique 84

Design of a Drift-Free M<5ssbauer Spectrometer . 89

Cosine Effect in M<3ssbauer Spectroscopy .... 101

llQm The Sn * Palladium Mossbauer Source .... 105

Abstracts of Publications 107

Drift-Free Mtissbauer Spectrometer 107

A Standard Reference Material for Mossbauer

Spectrometry of Iron and Its Compounds . . 108

Mtissbauer Line Broadening in SnOp Due to

Twinning Effects 7 .

' 109

Standardization of the Differential Chemical

Shift for Fe57 110

A Flip-Flop with Adjustable Bipolar Output . 110

5. RADIOISOTOPE TECHNIQUES . .' Ill

A Method of Trace Element Analysis With Radioisotopic Tracers in a System Using Heterogeneous Equilibrium 112

Differential Controlled Potential Coulometry

Utilizing Radioisotopic Tracers 117

vii
TABLE OF CONTENTS (con't)

PAGE

Some Results on the Substoichiometric Radioisotope Dilution Trace Element Analysis of

Silver and Iron 124

The Effect of Fluoride on the Distribution

Coefficient of Ferric Thiocyanate in

Isobutanol - Water 137

Abstract of Publication 146

Trace Determination of Cobalt by Substoichiometric and Radiometric Methods 146

6. MISCELLANEOUS 149

Sealed Source Fabrication 149

7. PERSONNEL 152

8. ACKNOWLEDGMENTS 153

9. LIST OF REFERENCES 154

Vlll
LIST OF FIGURES

FIGURE NO. PAGE

1. Floor plan - main level, NBS Reactor Building

Gaithersburg, Maryland 4

2. Radiological laboratories and counting room

in reactor building basement, Gaithersburg,

Maryland 5

3. Linac pneumatic tube facility, NBS Radiation

Physics Laboratory, Gaithersburg, Maryland ... 8

4. Frequency correlator logic diagram 17

5. Decay curves of argon-silicon mixture 29

6. Gamma-Ray Spectrum of Polyethylene snap-cap

vial 31

7. Sample placement in NRL rabbit; chlorine in

polystyrene 32

8. Variation of count rate with sample size .... 35

9. Sample placement in NRL rabbit; hafnium in

zirconium 38

10. Determination of integral neutron flux in NRL

pneumatic tube 44

11. Plot of neutron flux vs. height in the polyethylene rabbit 46

12. Diagram of general connections for Pyy

coincidence sum spectrometer 53

13. Electronic apparatus for P-y-y coincidence

sum spectrometer 54

14. P-scintillator-phototube mounting assembly ... 55

15. Detector assembly for P-y-y coincidence sum

spectrometer 56

16. P-detector high voltage wiring system 57

17. Non-inverting preamplifier 58

18. Wiring of p-detector PM tube 59

ix
LIST OF FIGURES (Con't)

FIGURE NO. PAGE

19. Wiring of gamma detector 60

20. Gamma high voltage anode wiring diagram ... 6l

21. Dual-preamplifier wiring diagram 6l

22. Mixer circuits modified 62

25. Comparison of beta spectrum on one phototube

with that gated by the coincident pulse of a second phototube on the same scintillator . . 64

60

24. p-y-y coincidence sum spectrum of Co .... 67

25. Single crystal of sodium nitroprusside,

showing habit and orientation of platelet . . 8l

26. Mossbauer spectrum of Standard Reference

Material for Fe 82

27. Optical system for measuring absolute

velocity 86

28. General layout of Doppler drive using optical

interf erometry 88

29. Mossbauer drive and velocity measuring system. 89

30. Flexure plate 91

31. Mechanical design of the M6ssbauer spectrometer 95

32. Electronics for NBS Mftssbauer spectrometer . . 94

33. The main frame of the Doppler Drive 96

34. Schematic of Doppler Drive 97

35. Detector gate control 99

36. Geometrical relationship between source and

detector for gamma rays emitted at angle (3 . . 102

x
LIST OF FIGURES (Con't)

FIGURE NO. PAGE

37. Normalized counting rate as a function of

the ratio of Doppler velocity V to a spectrometer velocity V 104

38. Least square fit to straight line 5 Standard

Reference Material No. 673 116

39. Apparatus for differential controlledpotential coulometry 121

40. Distribution coefficient versus negative log

of the concentration of sodium fluoride

expressed in mole liter"! l4l

41. Distribution coefficient versus negative log

of the concentration of sodium fluoride

expressed in mole liter"-'- 143

42. Distribution coefficient versus negative log

of the concentration of sodium fluoride

expressed in mole liter-1 144

43. Distribution coefficient versus negative log

of the concentration of sodium fluroide

expressed in mole liter"! 145

44. Exploded view of source assemlby 150

XI
LIST OF TABLES

TABLE NO. PAGE

1. Pneumatic tube system data for NBS

reactor 3

2. Pneumatic tube system data for NBS

reactor neutron fluxes 3

3. Results of chlorine determination .... 34

4. Results of hafnium analysis 40

5. Nuclear data for activation foils .... 43

6. Calculated sensitivities for activation

analysis using (3 ,y ,y sum-coincidence

gamma-radiation detection 68

7. Chemical , physical and crystal properties

of Na

2 [Fe(CN) N0]-2H2

80

8. Determination of cobalt in NBS Standard

Reference Material 671^ 672, 673 of nickel

oxide 117

9. Differential controlled-potential

coulometry of cadmium ideal solutions in

0.1 KNO^ 123

10. Analysis for silver in brass samples . . 133

11. Single substoichiometric radioisotopic

dilution analysis of nickel oxide for

trace amounts of iron . . . 135

12. Determination of trace-iron by single

substoichiometric radioisotopic dilution

analysis on ideal solutions 136

xi1
RADIOCHEMICAL ANALYSIS: ACTIVATION ANALYSIS, INSTRUMENTATION,

RADIATION TECHNIQUES, AND RADIOISOTOPE TECHNIQUES

July 1964 to June 1965

Edited by James R. DeVoe

ABSTRACT

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

the National Bureau of Standards.

Arrangements for the new facilities at the reactor building and Linac are in their final stages of completion.

Additional work is described on the electronics for the handling of data from pulse height analyzers as well as the use

of digital computers for processing these data.

A MiSssbauer spectrometer which uses optical interf erometry

in the Doppler drive, and the mechanical design for the electromechanical drive described in last years report are presented.

A description of chemical shift of iron compounds is given.

The use of a correction factor for certain source detector

geometries are described for Mossbauer spectroscopy. A new

source of Sn " in palladium which produces a very narrow

line width is also described.

For the first time a trace element has been certified in

a Standard Reference Material with the assistance of the

activation analysis technique. Flux measurements for the NRL

reactor are given. A number of special analyses for various

xiii
NBS projects by activation analysis are described (e.g. argon

in silicon, chlorine in polystyrene, hafnium in zirconium).

A beta-gamma sum coincidence gamma ray spectrometer is

described.

A constant potential electrochemical method of analysis

using radioisotopic tracers is presented which has potential

high sensitivity. A method of using radioisotopic tracers in

heterogeneous equilibrium between an aqueous medium of cobalt

to be analyzed and manganese dioxide impregnated filter paper

is described. Some results on the precision of the substoichiometric radioisotope dilution analysis of silver and iron

are presented.

Key words: NBS reactor, NBS Linac, digital computer

,

digital computers frequency comprator, nuclear instrumentation, argon in silicon, chlorine in polystyrene, hafnium in

zirconium, neutron fluxes, gamma-ray spectrometers, copper

and zinc in lung tissue, tellurium in brass, differential

chemical shift, Mtissbauer spectroscopy, M<5ssbauer instrumentation, Sn ^ -Palladium M<9ssbauer sources, iron compounds,

tin compounds, standardization of iron compounds, radioisotope

techniques, controlled potential coulometry, substoichiometric

radioisotope dilution, distribution coefficient of ferric

thiocynate in isobutanol water, radiometric techniques, sealed

source fabrication.

xiv
1. FACILITIES

For the past year the Radiochemical Analysis Section has

utilized the nuclear reactor at the Naval Research Laboratory.

In order to conduct activation analysis utilizing short-lived

radioisotopes the Section leased a radiochemical hood near

the pneumatic tube system at this reactor. The majority of

the samples which can be done by utilizing long-lived radioisotopes was transported to the radiochemical laboratory at

the National Bureau of Standards. Of course this arrangement

is temporary until we are located in our own reactor building

at Gaithersburg.

The Radiochemical Analysis Section is acting as coordinator between the users of the radiochemical laboratory

facilities in the reactor building and the NBS reactor

division. It is expected that there will be many users from

nearby universities and other government agencies. As a result of our contacts with these potential users, we have

found it necessary to expand the overall capacity and

capability of the new reactor laboratory even before occupancy.

It is becoming necessary and advantageous to reduce time

lost in performing tedious computation or information and

data retrieval. Therefore, it is apparent that considerable

effort must be placed on the development of a digital

computer facility. We believe that we have a very satisfactory solution to the problem of providing this facility

economically

.

1
Even though most of the plans for our new Gaithersburg

facilities have been completed, very little of this will be

described in this report. A comprehensive description of

these facilities with appropriate photographs will be

included in next year's progress report.

A. Nuclear Reactor

A heavy water moderated, enriched fuel, tank-type

research reactor, designed to operate at from 10 to 25

megawatts, is being constructed at the new National Bureau

of Standards facility at Gaithersburg, Maryland (about 22

miles from downtown Washington, D.C.). Special features of

the reactor include, (a) split fuel elements for minimization

of fast neutron and gamma fluxes, (b) all heavy water

moderation with graphite only in the outer region of the

thermal column, (c) a large cryostat hole for a cold neutron

facility, and (d) a converter assembly for fast neutron flux

work. An operating cycle of 3 weeks (24 hours a day) with a

2 day shutdown is anticipated. Nine pneumatic tubes (one

and one half inches in diameter - carbon dioxide driven) are

planned for insertion into the reactor as summarized in

table 1. Table 2 gives the expected fluxes and cadmium

ratios for each of the tubes. Each of the nine terminals

will have two sending and receiving stations located in 18

different radiochemical hoods situated in the various radiochemical laboratories in the reactor building (figures 1

and 2). Seven laboratories are of the two-man module type

2
Table 1. Pneuma-tic tube system data for NBS reactor

Specifications

:

Tube size 1-1/2" (i.d.)

Number of terminals 9

Number of stations 18

( s ending-receiving)

Rabbit speed 30-45 f t/s

Pneumatic tube

Gas and pressure . .

.- CCU at atmospheric

pressure or lower

Planned and proposed.

Table 2. Pneumatic tube system data for NBS reactor

neutron fluxes.

Terminal No. or design Approximate flux (X 10 ) n«cm" sec"

IR

IS

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Top of core

Converter thimble - 3.2

Slow Fast Gold cadmium ratio

0.9 0.8 10

1.4 0.6 20

1.0 0.3 >30

0.4 0.02 250

0.1-1.0 — High

Thermal column 0.1

Liquid N~ cooled 0.3

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Highly enriched uranium or lithium deuteride

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and will each have two hoods. In addition, eight people can

use the radiological laboratories with the associated counting

room (figure 2). Four cold laboratories, two counting rooms,

and six offices complete the facilities available in the

reactor building for our use.

Longer periods of irradiation will probably be made byuse of holes at the reactor top or in beam-ports available

on the reactor floor. A thru-port will allow work with millisecond half -life radioactivities. In the latter facility, a

sample will be "shot" at rifle-bullet speed through the

reactor

(G. W. Smith and J. R. DeVoe)

B. Linear Electron Accelerator

The Radiation Physics Division of The National Bureau

of Standards is installing a linear electron accelerator

which will accelerate 500 |iA (average) of electrons up to

150 MeV. Photons, neutrons, and charged particles can be

generated and the activation analysis group has assembled

a pneumatic tube system to perform nuclear activation with

these radiations.

The NBS Linac will produce bremsstrahlung of energies

o up to 130 MeV with dose rates as high as 1CK rads per minute.

By means of secondary targets such as uranium or tungsten,

12 -2 -1

average high energy neutron fluxes of 4 x 10 n-cm sec

can be obtained in a limited volume. Initial work will be

centered in the 45 experimental room using a pneumatic tube
system and radiochemical hood, as shown in figure 3. Suitable

target and cooling-loop systems will be developed and tested

to provide primary gamma-ray and secondary neutron and other

particle beams. It is expected that the system will be

especially attractive for activation analvsis of low atomic

numbered elements at high sensitivity through (y,n) reactions.

(G. W. Smith and J. R. DeVoe)

C. Equipment

In order to accelerate the development of any new

scientific group it is advisable to have certain basic pieces

of equipment that can serve to fulfill a variety of requirements. The following paragraphs give a short description

of some of this equipment.

A high vacuum system will be used for the preparation

of thin counting sources or thin target materials for use

in studying nuclear reactions, for preparation of Mftssbauer

sources or absorbers, for depositing -conductive layers on

electronic systems, for certain preparative stages of solid

state radiation detectors and for various other experiments

requiring high vacuum. The system model No. 7-7087 manufactured by the Ultek Corporation, has pumped most of our

-9 experimental arrangements inside of the bell jar to 10 J

torr in approximately one half hour.

We have purchased a 10 kW induction heater, to be used

in a furnace in conjunction with the vacuum system. Other

auxiliary equipment for the vacuum system are a high current
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power supply and a high voltage supply for an electron gun as

yet to be purchased.

A high speed pulse height analyzer system has been

purchased for activation analysis using very short-lived

radioisotopes. The unit, model 24-1 manufactured by Radiation

Instrument Development Laboratories has a total analysis time

of 1 [is per pulse for channels 1 to 100 and an additional

dead time of 0.01 u.s per channel above channel 100 to

channel 256.

(J. R. DeVoe)

D. Digital Computers

It is necessary to utilize electronic digital computation wherever possible in any scientific effort, because

it minimizes errors and time of computation, reduces tedious

computation, and creates an attitude in the user which, in

a sense, promotes greater depth of scientific investigation.

We believe that the proper system is user oriented with a minimum of intermediaries between the user and the machine.

This reduces turn around time for computation and creates

a very important man-machine interaction.

The problem* was to determine the best arrangement for

the specific needs of the section. Time sharing or multiplexing is now beginning to be feasible economically. The

decision that had to be made was whether or not to purchase

a small computer to store data from a series of pulse height

analyzers and/or scalers on a multiplex basis. This seemed
to be attractive because money need be paid for only one

memory as contrasted to that of one for each pulse height

analyzer. In addition, this computer even with fairly

limited storage capacity, could be used to do quite sophisticated computations. For our purposes we felt that the

advantage of owning a computer would not be great since we

could not benefit from progressive developments in computer

technology. In addition, the uses of local small computer

storage is most economical for handling data from many

scalers and the whole process becomes increasingly inconvenient when using pulse height analyzers, especially under

conditions of time coincidence (multi-parameter systems).

Our solution to the problem was made even less difficult

by the availability of an on-line digital computer system.

"We have rented teletype stations which are connected via

direct line through the regular direct dial network to a digital computer on a multiplex basis. This means that for

just the cost of leasing a teletype input station, we had

for practical purposes, the capability of the digital

computer connected to the other end of the communication

lines right in our laboratory. This allows us complete

flexibility in being able to communicate with any digital

computer that is capable of this type of multiplexing.

Although the data transmission rate is slow at present

(approximately 100 bits per second) it will be possible on

the- Federal Telecommunication System ( to be operational

10
in early 1966) to transmit over broadband lines at a rate

of 33^000 bits per second. This means that data storage

can be done from any data collecting device onto computer

compatible magnetic tape and the tape information transmitted

at high speed to a computer center.

At present we have on-line capability to a G. E. 225

multiplexing computer in Phoenix, Arizona. This system using

the G. E. computer was developed by the computer group at

Dartmouth University and they have devised a language called

BASIC (Beginner's All Purpose Symbolic Instruction Code)

which allows an uninitiated user to accomplish simple machine

digital computation within a matter of a few hours. This is

a very important factor for introducing the power of electronic digital computation into a group which is unfamiliar

with it. In addition, the language provides many advantages

of FORTRAN with the exception of input and output format

which is somewhat inflexible to date. The system has proved

to work very well with high reliability in data transmission

and it has quickly made itself an indispensable facility in

our section.

Similar computer systems will be available in the near

future in the Washington area.

We are hoping to be able to use this system for information retrieval, such as nuclide data, cross-sections,

M<5ssbauer differential chemical shift data, and bibliographies

on various subjects such as activation analysis, Mflssbauer

11
Effect } and radioisotope techniques. In theory these data

would be in storage at a computer center , and anyone with

access to a teletype machine could contact this computer to

interrogate the memory on the particular subject of interest,

(J. R. DeVoe)

12
2 . INSTRUMENTATION

Progress has been made on construction of electronic

equipment for the purpose of automating the handling of data

produced by multi-channel analyzers that are used in activation analysis and M<5ssbauer spectroscopy. The major portion

of the system (see abstract in section B) was presented before

the International Symposium on Activation Analysis at Texas

A & M University; the remainder of the system is to be

completed within the next month. A final report of this

system will appear in next year's progress report.

Of particular interest is the design of a frequency

comparator using digital techniques. This device was designed

for the optical interf erometric Mflssbauer Spectrometer s but a

number of other applications are possible.

A. A Frequency Correlation Device Employing a One-to-One

Digital Technique .

1. General Description of the Principle

This digital device utilizes an approach to the problem

of frequency correlation which achieves a very high degree of

precision and yet permits an extremely wide dynamic response.

The characteristics of the device include high stability and

require no critical adjustment of components. The output

of the device provides an error signal to the frequencies.

Of the two signals to be compared^ one is usually considered

to be an independent variable or reference frequency and the

other a dependent variable or tracking frequency. When the

15
correlator is to be used in a frequency discriminating system

the error signal outputs are integrated in bipolar form and

used to control the dependent frequency in a feedback loop

in order to restore the one-to-one correlation between the

two frequencies.

In its simplest form this device is embodied in an

electronic scaler or counter having a numerical capacity of

a scale of four, with the four stable states defined as "-l"j

"-0", "+0", and "+1".

The basic concept requires that the electronic counting

system be so designed that it will count either positively

or negatively , (i.e. add or subtract) signals coming from the

reference frequency and the tracking frequency, respectively.

Sinusoidal signals are converted to pulses having the same

frequency period. The counter must also be incapable of

"overflowing" , (i.e. the addition of too many pulses without

an intervening subtract pulse , thus, entering a number

greater than can be contained in the counter), and it must

stay at the maximum count state without reverting to its zero

state. Employing this system, one now interprets the output

or state of the counter with the lowest count (-1) as being

"too slow" and the maximum count (+1) as being' "too fast".

One more requirement that the counting system must meet

in order to perform satisfactorily in this application is the

capability to suppress any action when two pulses arrive so

close together in time that they cause confusion to the

14
counter. By eliminating both pulses with a coincidence

circuit the state is left unaltered, exactly as though each

count had been accepted.

In operation, when the tracking pulses are precisely

matched in frequency to the reference pulses, the number

contained in the center of the scale of four alternates

between states "-0" and "+0", If the reference pulse rate

increases slightly then pulses are added to the counter at a

faster rate than they are subtracted and the scaler will

alternate between the +1 state and the +0 state, generating

a periodic error signal to slow the dependent frequency.

When the reference pulse rate decreases slightly the pulses

are subtracted too rapidly and the state of the scaler

alternates between the "-1" state and the "-0" state generating an error signal to speed up the dependent frequency.

Very large differences in frequency cause the system to

perform in the same manner due to the non-overflow requirement of the scaler which will produce an error signal until

the relative counting rates permit its return to the center

of its range. If the pulses counted are each periodic in

nature, then the range of the counter need only be a scale

of four where the two middle states ("+0" and "-0") provide

no correction signal. The application of this technique can

be extended to non-periodic or randomly occurring pulses on

either of the inputs. A one-to-one correlation is provided

where the allowable excursion limits (i.e. the degree of

15
randomness) are determined by the number of scaling units

between the two error limits. It is also possible to provide

a progressive or step-wise proportional error signal by

utilizing two or more of the extreme end numbers of the

scaler and decoding these numbers in progressively increasing

steps. Such a system will provide fine corrections for small

errors and larger correction signals when a greater deviation

occurs

2. Description of the"' Circuit

Electronically the requirements for the scaler can be

met either by an appropriately gated scale of four binary

counter or by an "open" ring counter. A description of the

gating circuitry for the binary counter is given below, (see

figure 4)

Incoming pulses are supplied to the add and subtract

lines and are first checked for coincidence by the coincidence

detector. If a coincidence exists, both pulses are discarded

by "inhibit-and" gates (a) and (b) and no further action

takes place. If no coincidence is detected the pulse is then

applied to gate (c) or (d) depending on whether it is to be

added or subtracted. These gates are the non-overflow gates,

"inhibit" gate (c) will block an add pulse if (and only if)

the two bistables are in states (A + B) as determined by

gate (k). This would correspond to the "+l" defined above,

"inhibit" gate (d) will block a subtract pulse if (and only

if) the two bistables are in states (A + B) as determined by

16
ADD

SUB.

-i

'T

(2s

ft

COINCIDENCE DETECTOR

A + B

P=3

?—

v A + B

frB>

JTT

d^r ~3b

A+B

; a+e

OR

AND

B>-

.INHIBIT AND

Figure 4. Frequency correlator logic diagram.

"and" gate (j). This corresponds to the "-1" defined above.

Therefore, no action takes place when the scaler contains its

most positive number and an add pulse is applied. Likewise,

no action takes place when the scaler contains its most

negative number and a subtract pulse is applied. "And"

gates (b) and (m) control steering gates (e) and (f) which

determine if the state of B bistable is to be changed when

a pulse is added or subtracted. "Or" gates (g) and (h)

combine the add and subtract pulses to drive bistables A and

B. The error signals are generated by "and" gates (n) and (p)

corresponding to the "-1" and the "+l" states of the counter.

17
3. Applications

The digital frequency comparator was developed to provide

a velocity feedback control system in the course of designing

a velocity drive for a M<5ssbauer spectrometer. The velocity

of the system is represented by a sinusoidal signal derived

from the output of a Michaelson interferometer such that the

frequency is precisely proportional to the velocity. Therefore ; to control the velocity it is only necessary to compare

this velocity signal to a frequency standard and to provide a

correction signal from the frequency comparator that is incorporated into a feedback loop.

The error signals produced by the scaler are each added

(one positively and one negatively) to an integrator circuity

the output of which is amplified and becomes the actual

control signal in the feedback loop. The velocity of the

system can be varied merely by adjusting the standard

frequency. The systematic velocity resolution of the spectrometer is determined by the charging time constant of the

integrator circuit with respect to the signal frequency.

For further details on the operation of this particular

application of the comparator see the part of this report on

M8ssbauer Effect.

I wish to express my appreciation to Mr. Fillmer Ruegg

for transforming these concepts to an operational device.

(R. W. Shideler)

18
B. Abstracts of Publications

The following is an abstract of a manuscript which was

presented at the Texas Conference on activation analysis.

1. Interface Between Teletypewriter and Nuclear

Instruments

Shideler, R. ¥, 3 "interfacing a Teletypewriter to

Nuclear Instruments Used for Computer-coupled

Activation Analysis", Proceedings of International

Conference , "Modern Trends in Activation Analysis

5 Symposium, College Station, Texas, April 19^5^

to be published.

"in performing activation analysis using modern digital

instruments the volumes of numerical data dictate that an

expedient data handling system be used.

"The primary concern of this paper is to disclose the

techniques and actual designs involved in the interfacing of

a teletype to a digital multichannel pulse-height analyzer

and to an automatic sample changer.

"Two separate designs are discussed for interfacing to

the multichannel analyzers. The first design involves only

minor changes to the analyzer.

"The second design provides interfacing with no modification to the standard analyzer, and includes the additional

capability of reading precursor data from external units

including a Time-of-Year clock and programmed data groups.

These additional data are included in the output format

19
along with the analyzer data and serve to identify the data

as well as to instruct the computer in its subsequent analysis.

"Of primary importance in this paper is a discussion of

the merits and advantages to be found in adopting the use of

the teletype machine as an input-output device. As well as

providing typed copy, it will punch and read a paper tape

coded in the new ASCII code. The automatic inclusion of

precursor data and an appropriate readout format brings online computer operation to a reality.

The following is an abstract of a paper which was

presented at the December 1964 meeting of the American

Nuclear Society in San Francisco.

2. Through-port for Reactor

Shideler, R. W. , and Meinke, ¥. W. 5 "An Ultra High

Speed Reactor-irradiation-transport System for

Activation Analysis "

} Trans. Am. Nucl. Soc.

Vol. 7, (1964)*

"Conventional pneumatic tube facilities on many research

reactors deliver samples from positions near a reactor core

to detectors or radiochemical hoods within a few seconds.

These are suitable for work in activation analysis and

nuclear chemistry with radioisotopes of half-lives greater

than a few seconds. The half -life region below a few seconds , on the other hand, is almost completely unexplored from the

This system will be placed in operation at the NBS Nuclear

Reactor after transfer from the University of Michigan

where it was constructed.

20
activation analysis point of view and only a few nuclear

studies are available. Thus facilities for very rapid

irradiation and delivery of samples from a nuclear reactor

are of definite interest.

"We have designed a means of rapid and accurate determination of radioisotopes with half-lives of as little as a

.few milliseconds. The through-port of a conventional research

reactor _, the Ford Nuclear Reactor of the University of

Michigan, was fitted with automatic equipment whereby a capsule containing a sample could be "shot" past the core of

the reactor 3 through the shielding and into a detector for

measurement. The sample moves at speeds as high as

1000 ft/sec, being in the region of the core for the order

of 10 milliseconds. After a delay of another 10 milliseconds

while traversing the shield it enters an orbit ring in which

it spins around a scintillation detector as it slows to rest.

Its decay is measured during this slowing process by a ^1-00 channel pulse-height analyzer operating in the time mode as

a "400 channel" scaler.

"The equipment used to perform these operations consists

of a launching mechanism coupled through the small bore

(1/2") transport tube running past the reactor core to the

receiving mechanism or orbit ring. Mechanically the

launching device, which is remotely operated and entirely

self contained, is comprised of a holdup tank, a firing

valve and a sliding breech loader with which the capsule is

21
introduced into the system. In operation the holdup tank

is charged with helium gas to a predetermined high pressure

from an external source. As this gas is quickly released

by the specially designed firing valve it accelerates the

capsule to the desired velocity. The orbit ring mechanism

is comprised chiefly of an extension of the transport tube

which closes upon itself to form a loop or torus. The sample

coming down the tube from the reactor at a high velocity

enters the orbit ring and follows a closed circular path or

orbit. A 3" x 3" sodium iodide detector is situated inside

the orbit ring and on a common axis to that of the torus

providing a constant geometric relationship between the

sample and the detector regardless of the sample position or

velocity. The transport tube is a single length of thin

walled aluminum tubing which for safety considerations was

entirely contained in a heavy stainless steel tube of somewhat larger diameter open to the air at both ends of the

through-port. This design entirely eliminates the possibility

of pressurizing the hydrostatically-tight through-port tube

which was water filled to maintain adequate shielding.

"Some difficulties were encountered with capsule

container materials but it is possible to use polyethylene

capsules at speeds of up to 500 ft/sec without undue problems.

The system has proven satisfactory in operation and can be

used for activation analysis in the region above a few milliseconds .

22
"Although the orbit ring concept was designed to be used

with very high velocities., it need not be restricted to

through-port systems and should be useful in any fast system

where instantaneous counting is desired. This design should

also prove applicable to pulsing TRIGA reactors s when the

operation of the high-speed transport system is synchronized

with the high-flux,, short-duration pulse."

3. ACTIVATION ANALYSIS

Considerable effort has been placed in the past year

on the design and development of an appropriate activation

analysis system. Although much of this effort is directed

toward the future potential of our capability,, this group

has for the first time contributed to the certification of a Standard Reference Material. This work was reported at the

1965 Texas A & M Conference on Activation Analysis. This

marks the beginning of a program which will result in the

production of standard materials of a high purity which

have been analyzed for many different trace elements.

Continual efforts are being made by many analysts to improve

upon the precision of the activation technique but special

effort will be made by this group to determine its accuracy

for the elements in Standard Reference Materials. Where

possible , this will be done by comparing the activation

method with other trace analytical methods as well as by

conducting a comprehensive analysis of the activation

technique.

23
A majority of our initial systems developed is either

completed or in a final stage of planning. Therefore, after

the NBS reactor becomes usable in the spring of 1966 a maximum effort can be applied to the certification of trace

elements in Standard Reference Materials.

Several interesting developments have occurred in the

past few years which are enhancing the routine applicability

of the technique. The lithium ion drifted germanium and

silicon detectors promise to be of use in a variety of trace

elemental analyses where the detector efficiency and

consequently, the sensitivity need not be high.

The use of special nuclear reactions to accentuate the

sensitivity of particular elements is becoming a useful

technique. It is still necessary, however, to do a chemical

separation after irradiating most samples, but in keeping

with the analysts desire to automate analysis, it appears

worthwhile to investigate the use of a group type of radiochemical separation that is amenable to automation.

During this past year, a number of analyses have been

performed by the activation analysis group for other sections

at NBS. At present we have had to limit the extent of these

analyses to a few particularly interesting samples (e.g.

argon in silicon, chlorine in polystyrene, etc.), some of

which are described on the following pages, but it is our

intention to accelerate this effort. As a step in this

direction we have had so many requests for oxygen analysis

24
that we plan to install an automated system for this analysis

using a Cockroft-Walton generator.

Progress in using the NBS Linac now depends upon when

a beam will be available since most of our own facilities

are installed in the Linac building.

In addition to completing the hardware requirements

for activation analysis , each member of this group has been

involved in a small training project such as the utilization

of a digital computer, performance of coincidence counting

systems, or the measurement of neutron fluxes.

As a result of the above efforts it is hoped that this

group is well on its way towards providing the caliber of

activation analysis that is required for the certification

of Standard Reference Materials.

A. Determination of Trace Amounts of Argon in Silicon

Single Crystals

1. Introduction

The important capability of neutron activation for

nondestructive analysis recently came into focus when we

were asked by Dr. R. D. Deslattes of the Inorganic Materials

Division of NBS to. analyze several extremely pure silicon

crystals for the inert gas argon. These crystals were to

be weighed by a special technique in a density determination

6 having a standard deviation of 2 parts in 10 . During

preliminary work by Deslattes et al, the possibility of an

interstitial argon contaminant was considered, since the

25
pure silicon crystals had been grown in an argon atmosphere.

Electrical resistivity and other analytical techniques would

not detect this impurity. The activation analysis technique

was applied to the silicon to establish a lower limit of

argon contamination.

The neutron activation analysis of argon utilizes the

nuclear reaction Ar (n,y)Ar 1 in which the argon-4l formed

decays with a half-life of 1.82 hours and has a 1.29 MeV

gamma ray (approximately 100$ abundance). An interference

arises, however, due to the reaction Si (n,y)Si , which

gives a nuclide of 2.62 hour half -life and emits a 1.27 MeV

gamma ray (fortunately only of 0.07$ abundance).

2. Experimental

The three silicon crystal samples which were given to

us had already been etched and cleaned. Each sample was

approximately 4 x 6 x 18 mm in size, weighed about 1.2 grams,

and had been cleaved from the same crystal. They were irradiated separately for ten minutes in the pneumatic tube facility

of the Naval Research Laboratory nuclear reactor at a flux

12 of 10 ' neutrons per square centimeter per second,* Each

rabbit contained a sample in a polyethylene snap-cap vial

and an argon standard containing J>2 micrograms of argon.

The polyethylene snap-cap vials, made by Olympic Plastics

Company of Los Angeles can be obtained in several sizes and

* The Naval Research Laboratory reactor is a one megawatt

swimming pool reactor, located in Southeast Washington, D.C

26
and have been found to be relatively free from contamination.,

giving negligible radioactivity under these conditions in

the gamma-ray region of interest. The argon standard was

simply a known volume of air. The argon content of air

(excluding water vapor) is 0.934 + 0.001$ by volume (45th

Edition, Handbook of Chemistry and Physics, Chemical Rubber

Company). Since the small snap-cap vial contained 1.5 milliliters of air, a simple calculation gave the amount of argon

in a vial filled with air.

After irradiation, the silicon crystal was removed from

the vial, "washed" with compressed nitrogen gas* to remove

the surrounding radioactive argon-4l from the vicinity of the

crystal due to air present in the vial, and placed into a

second, identical snap-cap vial which had not been irradiated,

This sample, and the argon standard, were then counted on a

3" x 3" sodium iodide (thallium activated) crystal with a

400 channel pulse height analyzer. The half lives were

followed for twenty hours, and decay curves were calculated.

3. Results and Discussion

If all the 1.25 - 1.30 MeV gamma radioactivity of the

sample was assumed to be from an argon impurity in the

crystal, the actual count rate found would give a maximum

argon impurity of 4.8 parts per million. However, the

radioactivity of the sample was found to have a 2.78 hour

^Highly purified, water pumped compressed nitrogen was used

27
half -life., which is in substantial agreement with the

silicon-31 half -life of 2.62 hours, but not with the experimentally determined argon-41 half-life of 1.60 hours (literature value, 1.82 hours). To demonstrate this, figure 5 shows

four decay curves. Curve (l) shows the experimentally determined silicon crystal decay curve. Curve (2) is a decay

curve showing 60$ of the total crystal radioactivity as pure

31 silicon radioactivity (Si ) (taken at zero time). Curve (j5)

is a decay curve of pure Ar (which is equivalent to 40% of

the total silicon crystal radioactivity taken at zero time).

Curve (4) shows the sum of the second and third curves, which

is a hypothetical decay curve of 60% silicon radioactivity

(Si ) and 40$ argon radioactivity (Ar ). This fourth

curve definitely shows curvature, and has an average halflife of about 2.4 hours. Thus, we feel it may be safely

stated that if an argon impurity of 2 parts per million were

present in the silicon crystal (or approximately 40$ of the

total silicon crystal activity), it could definitely be seen

in the decay curve. Therefore, there is less than 2 parts

per million of argon in the silicon crystal.

It should be possible by using least squares analysis [1]

or a method of Fourier transform analysis [2] to improve the

estimate. However, only an upper limit of this magnitude

was desired for this analysis.

(D. A. Becker and G. W. Smith)

28
10,000

>

ro<

0.o

<

UJ

a.o

UJ<

UJ

»-z

1,000 -

SYNTHETIC DECAY CURVE

60% PURE Si 31 RADIOACTIVITY

40% PURE Ar 41 RADIOACTIVITY

^EXPERIMENTAL DECAY CURVE

SILICON CRYSTAL

100 -

200 400 600 800

DECAY TIME (MIN.)

1000 1200

Figure 5. Decay curves of argon-silicon mixture

29
B. Determination of Chlorine in Polystyrene

1. Introduction

The neutron activation analysis group received three

samples of polystyrene from the Polymer Chemistry Section at

NBS. The element to be determined was chlorine. The polymer

was obtained by radiation polymerization of pure styrene

monomer with small amounts of carbon tetrachloride present as

a free radical scavenger. Each polymer molecule was believed

to contain one_, and only one, chlorine atom at the position

where the chain reaction was stopped. If this were true, a

chlorine analysis would give a simple and accurate determination of the average molecular weight of this polymer. Activation is particularly suitable here since the commonly

occurring chlorine blank is eliminated.

Since the polymer was pure_, little interference was

foreseen, and a non-destructive method was chosen for the

analysis. The nuclear reaction Cl^ ' (n,,y)Cl (with a halflife of 37.5 minutes) was used., which gave two energetic

gamma rays, one of 1.6 MeV and the other at 2.1 MeV. The

1.6 MeV gamma ray was used for this work because of its

greater abundance.

2 Experimental

Approximately 60 mg samples (weighed to within + 0.1 mg)

of the polymer in the form of a fluffy powder was transferred

from the original glass container to a tared polyethylene

snap-cap vial. As a result of purification and separation

30
by freeze-drying, the fluffy polymer had a high static charge

It was necessary to place the polymer in its closed container

64

near an active p" source (Cu ) for several days to reduce

this charge before appreciable amounts could be successfully

transferred. These snap-cap vials (see page 27) had been

analyzed previously for contamination. A ten-minute

1 2 -2 -1

irradiation at a flux of 10 n-cm ' sec gave negligible

amounts of chlorine-38 . (figure 6)

8000 -

6000

c

3

oo

~o

.o

4000 -

2000 -

Ar 41 .3MeV

1.2 1.5 2.0

Gamma Ray Energy (MeV)

2.8 MeV

Figure 6. Gamma-ray spectrum of polyethylene snap-cap

vial.

51
For a chlorine standard, a reagent grade hydrochloric

acid solution was prepared and then standardized by the

conventional silver chloride gravimetric procedure. Triplicate determinations gave a mean value of 4.224 + 0.014 mg

chlorine per ml solution. Two hundred fifty microliters of

this standard solution was quantitatively transferred to a second snap-cap vial, and the vial heat sealed.* During

irradiation, the vial containing chlorine standard was taped

to the top of the polystyrene sample, as shown in figure 7.

POLYETHYLENE

RABBIT

TOP VIEW

STANDARD HCL SOLUTION

POLYSTYRENE FLUFF

POLYETHYLENE — SNAP-CAP

VIALS

SIDE VIEW Figure 7. Sample placement in NRL rabbit;

chlorine in polystyrene.

The polyethylene snap-cap vials may be easily and quickly

sealed by heating the top edge of the vial in a flame until

soft, then quickly clamping the top together with a pair

of stainless steel tongs. This produces an extremely strong, water tight seal.

32
The neutron flux variation as a function of distance from the

bottom of the polyethylene rabbit was known so that appropriate corrections could be made. Cellulose tissues were

used in the rabbit to keep the vials from changing position.

The irradiations were made in the pneumatic tube facility

of the Naval Research Laboratory Reactor, Washington, D.C.,

for a period of 10.0 minutes at a thermal neutron flux of

12 approximately 10 neutrons per square centimeter per second.

After irradiation, the sample and standards were counted

using a 3" x 3" sodium iodide (thallium activated) crystal

and associated scintillation spectrometer and pulse height

analyzer. The 1.6 MeV peak of chlorine-38 was used for the

analysis by comparing the counts per unit time (after

subtracting background) of the sample and standards under

identical counting conditions.

3. Results and Discussion

The data for the chlorine analyses are given in table 3.

The polymer samples ranged from 0.l4 - 0.87$ chlorine, which

gave count rates of approximately 15,000 to 9°, 000 integrated

peak counts per minute immediately after irradiation. These

count rates were found to be more than adequate for the

analysis, and delays of up to several half -lives could be

tolerated for most samples without undue concern.

A small counting geometry correction was necessary

between the polymer samples and the solution standards, since

they did not fill the same volume of the snap-cap vial. This

33
Table 3. Results of chlorine determination.

Sample No. Percent CI Average percent CI

1 0.43

O.38 0.42+0.02

0.44

2 0.46

0.48 0.47+0.01

0.48

3 0.13

0.15 0.14+0.01

0.13

4 0.22

0.22

5 . 40

0.39

6 0.90

0.84

0.22 + 0.01

0.40 + 0.01

0.87 + 0.05

a + values are standard deviation of the average.

correction was easily determined by successive dilutions in

the snap-cap vials of a small amount of a high specific

activity radioisotope with an appropriate gamma ray energy,

and counting the vial after each dilution. An experimental

curve was thus obtained of the counting efficiency versus

sample height in the snap-cap vial (figure 8) to provide the

appropriate correction. This correction, which was not

linear and was valid for our geometry only, constituted

approximately 2% per millimeter of sample height.

34
23 4 5 6789 10 DISTANCE OF SAMPLE MIDPOINT TO BOTTOM OF VIAL IN MM.

Figure 8. Variation of count rate with sample size.

(D. A. Becker and G. W. Smith)

G . Determination of Hafnium in Zirconium and Zircaloy

Standard Reference Materials

1. Introduction

The Radiochemical Analysis Section was assigned seven

Standard Reference Samples of zirconium and zircaloy for the

analysis of trace amounts of hafnium. The samples (NBS

55
Standard Nos. 1212a, 360a, 1215, 1210, 1211, 1215, and 1214)

were in the form of metal turnings, and had previously been

shown to be homogeneous. In addition to hafnium in the

concentration range of 90-^70 ppm, the samples contained

small amounts of twenty-five other elements.

The trace level determination of hafnium in zirconium

matrix is a particularly difficult analysis by most analytical techniques. Since they both are in group IV B of the

periodic chart, they have great similarity in their chemical

behavior. Therefore, many chemical analytical techniques are

difficult to perform. However, since neutron activation

analysis depends entirely on the nuclear structure of the

isotopes of hafnium and zirconium, it is found that small

amounts of hafnium may be readily determined in the presence

of large amounts of zirconium and most other elements.

The hafnium analysis utilized the nuclear reaction Hf

(n,y) Hf 179m

. The hafnium-179m has a half-life of only 19

seconds, which is shorter than that of most other radioisotopes formed, and therefore makes possible the nondestructive analysis of the sample. The stable isotope, hafnium-178j

has a high isotopic abundance (2'7.1^) and a high cross section

for formation of the hafnium-179m (50 barns). In addition,

the hafnium- 179m decays with a 217 keV y-ray (100^ abundance)

which does not undergo internal conversion. However, this

radioisotope is only useful for activation analysis if some

kind of rapid transfer system is available for quick delivery

36
delivery of the sample from the nuclear reactor to the

counting apparatus.

2. Experimental

A 100 mg sample (weighed to within +0.1 mg) was sandwiched in cellulose tape and placed flat on the bottom of a large size polyethylene snap-cap vial (figure 9). These

vials had previously been found to be relatively free from

radioactivity in the energy region of interest for these

short term irradiations. The vials had tissue forced inside

on top of the samples to eliminate any possibility of sample

movement. A weighed copper foil flux monitor (a disk approximately 4 mm in diameter and 20 mils thick in the range of

20 mg, weighed to a 0.1 mg standard deviation, single determination) was taped to the bottom of the snap-cap vial

exactly in the center.

The sample vial with attached flux monitor was placed

in a polyethylene rabbit (figure 9) a^d irradiated for 0.50

minutes in the Naval Research Laboratory nuclear reactor

pneumatic tube system. In this particular rapid transfer

system, the rabbit is delivered from the irradiation terminal

in the reactor to a lead-lined receiving box just outside the

radiochemical laboratory in about: seven seconds. The time of

ejection of the sample from the core is known to within + 1

second.

After delivery of the irradiated rabbit to the laboratory,

the sample was quickly removed from the rabbit and the copper

37
POLYETHYLENE

RABBIT

POLYETHYLENE — SNAP-CAP

VIALS TOP VIEW

/.

TISSUE —*~

WM

'life

pi

IIP

107 mm

COPPER FOIL FLUX MONITOR

SIOE VIEW

Figure 9. Sample placement in NRL rabbit;

hafnium in zirconium.

foil flux monitor separated from the sample. The counting

equipment used was a standard 5" x 3" sodium iodide (thallium

activated) scintillation detector coupled to a RIDL-model

3^--12B 400 channel pulse height analyzer. To expedite readout , a 100 channel quadrant of the analyzer was used, with an

energy spread of 0-0.5 MeV. Rapid readout was achieved using

a Franklin model 1040-D printer, which has readout capabilities on paper tape of approximately 40 channels per

38
second. The readout was adjusted to print 100 channels in

0.10 + 0.01 min. Using this system a recycled 0.40 minute

count plus 0.10 minute readout of 100 channels took exactly

0.50 minute per spectra,, as measured over an eight count

period.

The samples were compared to a pure hafnium oxide standard. Purity of 99.9$ Hf0

2

was verified by NBS spectrograph^ and gravimetric analysis. Varying quantities of a standard solution of this hafnium oxide (1.45 u.g Hf per

1.00 u.1 solution) were used to obtain a calibration curve of

micrograms hafnium versus integrated peak counts per minute.

Known amounts of this standard solution were deposited on

approximately 12 mm diameter disks of filter paper (five

thicknesses) and allowed to dry. These standards were then

enclosed in cellulose tape as were the samples.

3. Results and Discussion

The data for these analyses are shown in table 4. Five

analyses using different portions of sample were run on each

Standard Reference Material. The average values and the

standard deviation of these 'average values are shown.

The data found in table 4 have a small relative standard

deviation, ranging from about 0.4 to 2.0$ of the average.

179m The Hf y isotopic purity was verified by plotting

decay curves of the samples and the pure hafnium standards , using the integrated 217 keV hafnium- 179^ peak. The halflife determined was 0.317 minute and was the same for both

39
Table 4. Results of hafnium analysis.

Standaid

Ref erer

Material

ice

No.

Hafnium

Concentration

(ppm)

Average + S , of

a - d Average

1212a 277

280

280 278 + 1

278

277

360a 92.8

94.0

93.6 94 + 0.5

9^.5

95.7

1215 467

463

452 471 + 7

491

480

1210 87

88

85 91 + 3

97

97

1211 146

140

145 144 + 1

144

mm 144

1213 88.8

93.2

87.6 89 + I

86.0

91.6

1214 189

190

183 192 + 3

200

196

4o
sample and standard, within experimental error. This also

agrees well with the literature value of 0.^2 minute (19

seconds). Under these conditions, no gamma ray interferences

were noted.

The copper foil flux monitors for the samples and standard were allowed to decay until the 5.1 minute copper-66

activity had disappeared, at which time the 0.51 MeV annihilation peak from the positron decay of 12.9 hour copper-64

was counted and corrected for decay. These values were then

used for flux normalization of the hafnium count rates.

Neutron self-shielding corrections were not necessary

since the hafnium was the only significant neutron absorber

present, and the concentration difference between samples and

standards was small.

(D. A. Becker and G. W. Smith)

D. Determination of the Neutron Flux Spectrum for the NRL

Pneumatic Tube System

1. Introduction

Until recently, the pneumatic tube system at the Naval

Research Laboratory had received minimal use, and thus little

was known about the irradiation conditions at the reactor

terminals. Our usage has been limited to the north "rabbit",

and these data apply only to this irradiation position.

Knowledge of the neutron flux spectrum is necessary for

the choice of optimum irradiation conditions. Data on the

cadmium ratios and fast neutron distribution in a variety of

41
irradiation positions allow judicious choice of a position

offering maximum sensitivity with a minimum of interferences.

information about the vertical and horizontal gradients

of the neutron flux within a sample position are important

,

especially if the sample size is varied. Fortunately, in

most cases the variation in flux within the sample position

is quite small. However., the NRL pneumatic tube terminals

are positioned on a graphite ledge just above the reactor

core, so that a significant variation of the thermal neutron

flux would be expected.

2. Experimental

The materials used for the neutron flux measurements

consisted of thin foil disks (1 inch diameter) of the elements gold, nickel, magnesium, copper, and aluminum (see

table 5). The gold and copper foils were used with and

without 20 mil interlocking cadmium covers (foils and

cadmium covers were obtained from Reactor Experiments, Inc.).

For irradiation, the foils were placed in a flat position on

the inside bottom of a polyethylene rabbit and held in that

position by cellulose tissues. The foils were counted on a

3" x 3" Nal(Tl) crystal with an associated pulse height

analyzer. The counting position was calibrated by means of

NBS radioactivity standards (Radioactivity Section, National

Bureau of Standards) for the range of each photon energy.

Thus, after determination of the disintegration rate of these

foils, and knowing the nuclear constants for each element,

42
Table 5. Nuclear data for activation foils.

Product Threshold

Foil Nuclear Reaction Half-life Threshold Ener gy barns

Au Au1 ^ i u 198

( n , y ) Au 2.70d below 0.4 eV

a

96

a

Ni Ni** (n,p)Co 71.3d 2.9 MeV 0.092

Mg Mg24 24

(n,p)Na 15 . Oh 6.3 MeV 0.048

Al Al2 ? 24

(n, )Na 15. Oh 8.1 MeV 0.11

Cu Cu65 (n^YjCu 12,9*1 below 0.4 eVa

3.9

aThe integral neutron flux below 0.4 eV was found by subtracting the radioactivity due to a cadmium covered foil

from the radioactivity due to a bare foil. This gave the

radioactivity below the cadmium cutoff, i.e., o 4 eV.

the integral neutron flux above the energy threshold may be

calculated using the standard neutron activation calculations.

The determination of the flux gradient within the polyethylene rabbit was made by using a thin cylinder of polyethylene to which small (3/16 inch diameter) copper foil

disks had been attached, both circumf erentially and vertically. After irradiation, the 5.1 min. copper-06 in the

foils was allowed to decay and the 0.51 MeV peak of the 12 . 9 h

copper-64 was counted under reproducible conditions and

corrected to the time it was removed from the reactor.

3. Results

The integral neutron spectrum of the north pneumatic

tube terminal is shown in figure 10. With it is shown the

unmoderated prompt neutron uranium-2j55 fission spectrum [3].

43
io 12 P

i l97 (n,?)Au 198

. (jZ35 Fi ss i on spectrum of Prompt Neutrons (Unmoderated ) Calculated from the Formula

of B.E.Watt (Assuming IO

l2 ncm"2 sec

1 Total Flux) n oo Jp N(E)dE when E->o- oo

o

a>

E

o

Figure 10. Determination of integral neutron flux in

NRL pneumatic tube.

44
The thermal neutron flux (i.e. the flux produced by

neutrons of energy below the cadmium- cutoff of approximately

0.4 eV) was calculated by taking the radioactivity of the

gold foil (total radioactivity minus the radioactivity

produced with cadmium shielding) 3 correcting to the disintegration rate at zero time (including a self -shielding

correction) j and using the equation:

-2 -1 where f = thermal neutron flux in n.cm sec

A., = radioactivity of the gold foil in dps

tf-f.^ = thermal neutron cross section of gold-197 in barns

N = number of gold-197 nuclei in the target

A = decay constant of gold-198

t = time of irradiation,, in seconds

12 The result of two determinations gives values of O.83 x 10

—2 —1 12 —2 -1

n.cm '"' sec and 0.84 x 10 n.cm"' sec" for the thermal

neutron flux.

The gold-cadmium ratio [Cd(Au)R] is defined as:

p,/, Np _ Disintegration rate of bare gold foil

Disintegration rate of cadmium covered

gold foil

where both radioactivities are corrected to zero time. The

results of two determinations of the gold-cadmium ratio gave

10.8 and 11.2 for an average gold-cadmium ratio of 11.0. The

copper-cadmium ratio (calculated in the same way only using

a copper foil) gave a copper-cadmium ratio of 63.5.

45
A plot of the neutron flux vs. height in the polyethylene rabbit may be seen in figure 11. These data yield

a flux gradient value of 0.68$ per millimeter of average

sample height in the rabbit. The side-to-side gradient was

0.37$ per millimeter over the 12 mm diameter of the rabbit.

1.00

X

3 0.90

u_ — >. z

o

£ 0.80

UJ

z: •"V

RELATIVE p

o

o

o

• •^

#X*

0.50

1

1

1

1 1 1

1

I 10 20 30 40 50 60 70

DISTANCE FROM BOTTOM OF RABBIT (MM)

80

Figure 11. Flux dropoff versus height for NRL

pneumatic tube.

4. Discussion of Results

The general shape of the neutron flux spectrum determined is quite similar to the theoretical shape as calculated

46
by B. E. Watts [3], as shown in figure 10. As expected, the

slope is slightly greater since considerable moderation is

achieved by means of the cooling water shielding, the graphite in the ledge below the rabbit terminal, and by the polyethylene of the rabbit itself.

The gold-cadmium ratio of 11 agrees with NRL's estimated

value. This is a relatively high gold-cadmium ratio compared

to the in-core ratio of 2 or 3. Thus, the pneumatic tube

facility would be the preferred position for thermal neutron

activation. The in-core irradiation positions would be

preferred for fast neutron activation and high-sensitivity

thermal neutron activation if fast neutron reactions do not

cause interference. The numerical difference between the

gold-cadmium ratio and the copper-cadmium ratio determined

with the same neutron flux spectrum illustrates the ambiguity

of using the term "cadmium ratio" indiscriminately, without

reference to the element with which it was determined.

The plot of the vertical neutron flux drop off is a straight line, so that, if necessary, a suitable correction

may be made for various sample sizes. However, it is better

to control the irradiation conditions so that all samples and

standards are at the same average level in the polyethylene

rabbit. A numerical correction for side-to-side flux variation is difficult, since the rabbit turns while moving.

Therefore, efforts must be made to keep the sample as close

to the center line of the rabbit as possible. However, by

47
affixing the flux monitor to the base of the sample container,

the need for side-to-side correction may be eliminated for

samples of small diameter.

(D. A. Becker and G. W. Smith)

E. Activation Analysis with the NBS Electron Linear

Accelerator

A forty kilowatt microwave linear electron accelerator

(Linac) is being installed in Building 245 at the NBS

Gaithersburg, Maryland facility. By utilizing suitable

radiators to intercept the electron beam, large quantities

of high energy bremsstrahlung can be generated. These high

energy photons can induce photonuclear reactions in many

elements, giving rise to radioactive products. The thermal

neutron activation analysis of all of the elements has been

reviewed. A number of elements , including carbon, nitrogen,

fluorine, niobium and lead, can be determined by activation

analysis more accurately, more rapidly, or to a greater

sensitivity with photon irradiation than with thermal neutron

activation. We have made estimates of sensitivities that can

be obtained using the Linac and it appears that it may be

possible to go below 1 u.g in the case of lead, and down to

_2 perhaps 10 u.g for the other mentioned elements. The

possibility of using chemical separation techniques, based

on existing methods, are being reviewed.

In addition to the possibility of using the photonuclear

reactions, metastable isomers of elements such as germanium,

48
zirconium,, lead, bromine, tungsten, gold, barium, cadmium,

strontium, and indium can be produced by irradiation with

photons below the threshold for a photonuclear reaction. By

using gamma rays of energy below 6 MeV only the radioactivities from these isomers will be produced. Although cross

sections for production of these isomers are not well known,

they are believed to be small. Therefore, the method would

not be very sensitive, but should be very specific for the

elements indicated.

The design and installation of associated equipment is

underway. These include fume hood, heat exchangers, beam

trap and pneumatic tube transfer system for utilization of

short-lived radioactivities. Several concepts in radiation

targets are being examined and a suitable one for our

experiments is being designed.

(G. J. Lutz)

F. Utilization of Computers in Activation Analysis

An important aspect of the activation analysis program

is the ability to conveniently handle the large quantities

of data that the multi-channel analyzers are capable of

producing. Even more important is the ability of the analyst

to treat the data in an ever increasing comprehensive manner

through the use of digital computers. In addition, this

capability allows the analyst to perform a wide variety of

computations that will result in a better analysis.

49
1. Computer Simulation of Activation Analysis

The first computer technique used is based on the

OPTIMIZE method of Isenhour and Morrison [4]. One may, in

some instances, by judicious selection of irradiation and

decay times, be able to count the gamma photopeak of the

radioisotope of interest relatively free from interference

from other radioactivities. This program determines these

optimum times of irradiation and decay for activation

analysis of any element in a mixture. The original paper

should be consulted for details. We have utilized this

program in several analyses.

a. Aluminum in Steel Samples . It was desired to

determine aluminum in NBS Standard Reference Material l4e by

27 28 the reaction Al (n,y)Al . The qualitative and semi-quantitative analyses were available for this metal, as well as the

thermal neutron flux, minimum acceptable radioactivity, and

minimum and maximum times of irradiation and decay. These

data were put into the computer. The computer output

included a measure of the maximum signal to noise ratio

pO

which ranges from to 1.0 for the Al ' photopeak. In this

sample the ratio was O.365. It was verified that the major

56 interference was due to Mn , produced from the (h,y) reaction

55

on Mn^ . When the manganese data were excluded from the

computations, the ratio was 0.99°. Thus the analyst has

several options in doing this determination. He may do a quick separation of manganese after irradiation. He may

50
follow the decay in the energy range of interest and resolve

the components in this manner or he may do a spectrum

stripping of the interfering nuclide.

b. Silicon .in phosphate, rock . It was desired to

determine silicon in NBS Standard Reference Material 120a

. . 2Q, v 29 (phosphate rock) by inducing the reaction Si ^(n^pjAl y

utilizing reactor neutrons. A computer analysis indicated

that a number of radioactivities would interfere with the

29 detection of Al , and therefore, a rather complete chemical

separation would be required.

2. Analysis of Mult i component Gamma-Ray Spectra . A

number of least squares and simultaneous linear equations

methods for calculating the contribution of individual radionuclides in a multicomponent pulse-height spectrum have been

described [5]. One of these techniques described by

Nicholson, et. al. [6] has the distinct advantage of

allowing the reduction of the spectrum into a set of channel

groups for the computation. The proper selection of groups

of channels reduces errors introduced by gain change or

zero drift. This program also permits the analysis of time

dependent spectra.

The main disadvantage of the technique is that it nullifies the inherent resolution in a Nal(Tl) detection system.

The method has the effect of reducing substantially the number

of channels of data. This results in a loss of precision in

the estimation of relative amounts of each component present.

51
However j it Is felt that for routine analysis of practical samples, the minimizing of errors due to gain change or

zero drift, which may occur during counting and may amount

to a substantial fraction of a channel or more, is of great

enough Importance that the loss of resolution can be tolerated.

This technique is currently being tested with synthetic

samples and will be included in the activation analysis

capabilities of the group.

( G . J . Lut z

)

Or. A Gamma-Ray Spectrometer Utilizing a Beta-Gamma-Gamma

Sum Coincidence Technique

In activation analysis the number of radioisotopes

produced by irradiation of the sample is often large. In

order to perform a successful qualitative and quantitative

experiment, the analyst must be able to measure accurately

the radiation of the desired radionuclide. This may be

accomplished by chemically separating the radioisotopes of

a given element and then by using the gamma-ray spectrometer

to measure the radiation from the desired radioisotope.

The complexity of the radiochemical separation can be reduced

and, in many cases, the sensitivity of the technique increased by the use of coincidence counting techniques. If

the desired radioisotope decays by beta followed by at least

two gamma rays in cascade, and the sum of the gamma rays is

analyzed in the spectrometer, a very high degree of discrim52
ination can be obtained. This system is closely patterned

to one that has been developed by R. W. Perkins and R. E.

Connally [7,B] of the Battelle Research Laboratories, Hanford

Washington.

1. Detector Systems

The detection system that is used is shown diagrammatically in figure 12 and pictorially in figure 13. Two Nal(Tl)

single crystals 8" diameter x 4" thick are placed as close

as possible to a plastic scintillator-phototube array which

i—

-

i—

c——J—H>-

Cs^A

NO! CRYSTAL

BETA OETECTORS a.eac ^)Q(^

-\>-

NOl CRYSTAL

PHOTO TUBES

L.H> J

Ml -VOLTAGE SUPPLY

I ^_J

ANODE 6 1 -^ £

a

t=L CONNECTION DYNODE a ANODE A-2. A-3 NOT SHOWN

INVERTING PREAMP

id

\—

I \

I

""U

n

COINC IN hiSB COiMCC

1

1

I OYNOPES B

1 U

u

L_

~\-

I ANOOES A _J

RIOL 34-I2B PULSE -HEIGHT ANALYZER

I J

Figure 12. Diagram of connections for fiyy

coincidence sum spectrometer.

53
Figure 1J5. Electronic apparatus for P-y-y coincidence sum spectrometer.

5^
is shown in figures 14 and 15 . The sample is placed in the

recess of the plastic scintillator and a plastic scintillator

cap is placed over this and is held on with a seal of optical

coupling. Two ~5/k- inch phototubes are optically coupled to

the scintillator inside a stainless steel tube. The phototubes are then held in place by means of black plastic tape

which provides a light tight seal.

The signal from each phototube is passed through a

non-inverting preamplifier to the input of a four channel

mixer. A schematic of the high voltage divider and preamplifier assembly is shown in figures 16 and 17. These preamplifiers are plug-in units which have been designed on circuit

boards. A schematic of the wiring in the beta detector is

shown in figure 18. A tenth dynode wire is available on

these tubes but is not used for this application. The anode

signal is then used to generate the appropriate coincidence

logic.

PLASTIC SCINTILLATOR

S. S. TUBE 3/4 DIA.-

OPTICAL COUPLING WITH

SILICONE GREASE

CAP SAMPLE BLACK PLASTIC TAPE PLUGS, MOUNTED

TO PHOTOTUBE

Figure 14. (3 - scintillator-phototube mounting assembly,

55
Figure 15. Detector assembly for (3-7-7

coincidence sum spectrometer.

A wiring diagram of the gamma detector system with its

high voltage divider is shown in figures 19 and 20. Each

detector is comprised of 3 photomultipliers coupled to a single 8x4 inch Nal scintillator. Each of the 3 anodes

for each detector are joined together and the sum of the

anodes from detector A and the sum of the anodes from

detector B are connected to a common input of an inverting

preamplifier and then to the analyzer for pulse height

analysis, (figure 21) It is necessary to match cable

capacitances between the three anodes of A and B. This can

56
4-250K —w\

BA-I

TO ANODE

450K-IOT —vv\

IOOK

2W

IOOK-2W —v\a

.Ol/xf

2kv

--.Ol/xf

" 2kv

L

OFF

I

V

250K M 250K H —W\ • 'W\/ •- H.V.

IN

BA I TO MIXER

NON-INVERTING PREAMR

NOTE. ALL RESISTORS WIRE WOUND IW

UNLESS OTHERWISE SPECIFIED

Figure 16 . (3-detector high voltage wiring system,

be done by first balancing all of the anodes of detectors

A and B using a common preamplifier by adjusting the peak

137 position of the 0.66 MeV Cs to the same channel within

+0.5 channels i and then adjusting Cs and Cs (approximately

50-100 pF) to give no change in peak position when the A and

B switches are alternately off or on.

In order to derive logic pulses from the gamma detectors

a tenth dynode signal is taken from each phototube. The

three dynodes, each for detectors A and B are joined at the

input to an inverting preamplifier and are then connected to

the coincidence logic.

57
-I2V 8.2K -I5V 100X2 WV AW <j> OUTPUT

Figure 17. Non-inverting preamplifier.

2. Coincidence logic is performed in two modified

4-channel mixer circuits made by RIDL for use as a 4-detector

input device for a pulse height analyzer. Figure 22 shows

the representation of the function of the circuits as modified. The numbers adjacent to the inverters, gates i and

monostables refer to the transistor numbers in the standard

RIDL mixer logic circuit. No modification to the amplifierdiscriminators was made. All coincidence logic is conducted

with 2 |is pulses produced by the discriminator. The dynode

53
3

4

8

R, = 0.39M I/2W

R2-R|| = 0.2M I/2W

C, = 260 pf 500 V

C 2

= 60pf500V

JUNCTION PLUG

Figure 18. Wiring of P-detector photomultiplier tube

59
r

2M

_T

DYNODE

OUT

'2?*-

O.OI /a F 2kV

<?

I.OI/xF

-1 I—

2kV

.470K

<«<

2

<

3

<*

<«

<

s

<ro

FOCUS

R 2

= I.5MA POT

R =470kiil/2W

<k )T0 ANODE

HV

Figure 19. Wiring of gamma detector,

signals from gamma detectors A and B are put into coincidence.

The output of this coincidence is put into another coincidence

with the output of any of three beta coincidences.

Since each beta scintillator has two phototubes attached

to itj the output of each of these tubes is placed in coincidence with the other. This means that random noise pulses

60
r

4-250 kSl -AM

HV

T0 c^- \ZS

G3AI 1

ANODE

HV

TO GA2

ANODE$

HV

TO

GA3

ANODE 9-

450 kil CW I0T —AAA,

looks,

2W

OFF

I00k.fi. •AAAi 2W

I00k.fi. ^WV

2W

I00k.fi. -AM

2W

A-^WVM 250k.fi. H

-• AAA,

ETC.

ETC.

O.OI^iF H I 2kv "X

0.01/i.F 2kv TL

0.01 /£F

•f

2kv

2KV

O.OIflF

—I I 0.01 u F 2 KV

0.01/U.F H I 2KV

IM.fi. 2W

<?

ANODES A OUT

note: all resistors are iw wire wound 1% unless

otherwise specified

Figure 20. Gamma high voltage anode wiring diagram,

ANODES A

ANODES B

/-

TO 34-I2B

PHA AMPL. INPUT

INVERTING PREAMPLIFIER

Figure 21. Dual-preamplifier wiring diagram,

61
T3

CD •H

•Hno

CQ -P•HO

!h •HOu

CDXHa

OJ

CM

CD

HO

•H

62
in each phototube will be discriminated, at the coincidence

gate since only events in the scintillator will activate

the phototubes- at the same time (excluding the random noise

coincidences). The beta coincidence output is placed in

coincidence with the differentiated leading edge of the live

time pulse from the pulse height analyzer to produce a

routing pulse. Using this method prevents production of a

routing pulse after a pulse is accepted in the analyzer.

The output from this coincidence gate provides the routing

signal to the analyzer for storage of the gamma anode pulse

in the appropriate 100 channel subgroup. The same process

occurs for each of the three beta channels. In order to

prevent simultaneous routing of pulses from two or three

beta channels, a logic system is used to produce a blocking

pulse which prevents the pulse height analyzer from accepting

a pulse from the anodes of the gamma detectors. This is a three way coincidence gate using two transistors to drive a

5 |is mono stable which produces the blocking pulse to the

pulse height analyzer (figure 23). Thus, this system allows

the simultaneous measurement of three samples without

electronic cross talk between the samples.

The output of each beta channel coincidence is put in

coincidence with the gamma-gamma coincidence output. This

output generates a 5 P-S enable pulse which is inverted before

entering the pulse height analyzer. The enable pulse is

connected to the delay coincidence gate in the analyzer.

63
10000

1000

o

o

100

10

oL

W/0 COINCIDENCE

W/ COINCIDENCE

~1A

COMPARISON OF BETA SPECTRUM ON

ONE PHOTOTUBE WITH THAT GATED

BY THE COINCIDENT PULSE OF A SECOND PHOTOTUBE ON THE SAME

SCINTILLATOR.

50

CHANNEL NO

Figure 2j5. Comparison of beta spectrum on one phototube

with that gated by the coincident pulse of

a second phototube on the same scintillator.

Although it should operate as well in the prompt coincidence

mode, this delay assures that a blocking pulse can close

the gate to the analogue to digital converter of the analyzer

in plenty of time before the anode pulse arrives.

64
3. Equipment List

1. Plastic scintillators - Pilot B - Nuclear

Enterprises Ltd.., Winnepeg, Canada.

2. Phototubes 3/4" diameter , 10 stage , RCA special

model. Development type C70136E, six required.

3. Sodium iodide detectors, 8 inch diameter x

4 inches thick, used with three RCA 8054,

3 inch diameter, phototubes on each crystal,

Harshaw Chemical Company, two required.

4. High voltage power supply - Model N4035 Hamner

Electronics

.

5. Four channel mixer, two required, Radiation

Instrument Development Company (RIDL) Model

30-17:, modified to requirements by K. Miller

and J. R. DeVoe (NBS)

.

6. Block logic circuit. Designed by J. R. DeVoe

(NBS).

7. Preamplifier, non-inverting. Designed by

R. E. Connally, six required.

8. Preamplifier, inverting. RIDL Model 10-8

two required.

9. Voltage divider, designed by R. E. Connally,

one for beta PMT ' s and one for gamma PMT ' s

.

10. Dual preamplifier, designed by R. E. Connally.

11. Pulse height analyzer, RIDL Model 54-12b.

12. Silicone grease, type C-2-0057 Dow Corning

Corporation, used for optical coupling of the

plastic scintillators to the photomultiplier

tubes

.

4. Performance

The system has been checked out using a calibrated

60

source of Co . Figure 23 shows the beta spectrum of one

phototube in comparison with the same spectrum with the

analyzer gated by the coincident output of the two phototubes

65
on the same scintillator. It can be observed that the efficiency for removal of noise pulses in the detectors is high.

It has been determined that phototube noise pulses at a rate

up to 10,000 pulses sec" will result in a negligible

increase in the coincident background counting rate.

The resolution of each of the two sodium iodide detectors

is 8.3$ for the 0.662 MeV photopeak of Cs 1^7. A study of

gain shift with time indicates that the .662 MeV peak varies

within + 0.5 channel maximum over a period of 5 days with a calibration of 30 keV channel using 100 channels.

The background between 0.5 and 3 MeV is 16 counts per

hour in P-y-y coincidence mode. Background of each sodium

iodide detector within the same energy range is approximately

350 cpm.

60 Using the Co sample mounted in a plastic scintillator

measurements were made of counting efficiency. Overall

counting efficiency for the 2.50 MeV sum peak is approximately 3.8$ using a source with a count rate of 10,000

d • min" . A representative (3-y-y coincidence spectrum of

60 Co is illustrated in figure 24.

Several experiments were performed by removing the

coincidence requirement from the second beta' phototube.

Results show that the f3-y-y coincidence counting efficiency

was the same as the coincidence counting efficiency for Co

However, the background count rate increased by approximately

25$. This means that the overall net detection efficiency is

66
improved by 25% when using the £-p coincidence to eliminate

electronic noise pulses.

i.o 1.17 1.33 ENERGY (MeV)

2.0 2.50 3.0

Figure 24. f3-y-<y coincidence sum spectrum of Co 60

60

For Go radiation cross talk was observed to occur

between samples. Cross talk between samples is 0.1% of the

overall radioactivity of the sample in the adjacent mounting

position. It is less than .01% for the separated detectors

(e.g. 1 and 3)

.

This spectrometer allows a high degree of discrimination

from all radioisotopes not decaying by a beta with two or

more coincident gamma rays with the exception of the positron

emitter which could be expected to interfere in the energy

region of annihilation.

67
Table 6. Calculated sensitivities for activation analysis

using P-y-y sum-coincidence gamma-radiation

detection.

a,b

Isotope

t l/2

Sensitivity ' (1.0 c/h) Isotope t l/2Sensitivity ' (1.0 c/h)

M 24

Na

42 K

15h

12h

0. l6pg

. l8ng

Sm153

Gd13 ^ 47h

l8h

0.092pg

5.5pg

46

Co 60

84d

5.25y

0.13pg

24pg

Tb l60

Ho 166

72d

27h

6.5pg

0.8pg

Ga72 l4h 2.2pg Yb 175 lOlh 2pg

Ge 77 llh 37pg Lu177 6.8d 2.7pg

Br 36h 0.37pg Hf l8l 45d 10pg

Mo^9 66h 48pg Ta

182

115 d 8pg

Ru103

Pd 10?

40d

l4h

47pg

2.2pg

W

187

Re 186

24h

89h

0.22pg

20pg

Ag110mSb 122

Te 127

Cs 134

253d

2.8d

9.4h

2y

O.lOng

. 92ng

46ng

2ng

Re 188

Os

1 ^

Ir 192

Pt

1 ^

17h

3In

74d

l8h

1. 6pg

37pg

0.25pg

27pg

, i4o La

n 1^-3 Ce -^ 40h

33h

0.92pg

12pg

Au1 ^

2.7d 3.3pg

Calculated from data contained in: Senftle, F. E.,

Champion, W. R. , Suppl. Nuovo Cimento 12, 549,71 (1951),

Table of Isotopes and Hollander, J. M. , Perlman, I., and

Seaborg, G. T., Rev. Mod. Phys . 25, 469 (1953) and

Way, K. , et. al., "Nuclear Data "STTeets", U. S. Government

Printing Office, Washington 25, D.C., 1963.

Parameters used in calculation:

1. Flux, 10 13 -2 -1 ^ n.cm sec

2. Significant ocunt rate at energy of interest,

1.0 count ch.

3. Time of irradiation, 15 h or saturation

whichever is less.
An additional -advantage of this type of spectrometer is

the low background associated with it. Using a count rate

of twice the background as a lower limit of detection,, a table of theoretical sensitivities (table 6) for activation

analysis of nuclides which decay by (3-y-y coincidence was

prepared.

We wish to express our appreciation to Mr. R. W. Shideler

for supplying the plug-in preamplifiers and Mr. H. Kocol for

performing the calculations for table 6.

(W. P. Reed and J. R. DeVoe)

69
H. Abstracts of Papers and Publications

The following are abstracts of manuscripts that were

presented at Texas A & M in April 1965.

1. Copper and Zinc in Lung Tissue

Keenan, R. G-. , Marcus, J. H. , and DeVoe, J. R. , "Simultaneous Determination of Copper and Zinc

in Human Lung Tissue by Neutron Activation

Analysis", Proceedings of International

Conference, "Modern Trends in Activation

Analysis" Symposium, College Station, Texas,

April 1965^ to be published.

"Certain trace elements, including copper and zinc, are

constituents of numerous enzymes and function as essential

catalysts in biosyntheses . The expanded interest in these

substances has supplied a demand for micro-analytical

techniques of increased sensitivity, accuracy, and precision

if meaningful conclusions are to be derived from the analytical data. This paper shows that good precision may be

realized in the activation analysis of complex biological

specimens with only a few, one-step chemical separations.

"The nuclear reactions Cu (n,y)Cu (12. 8h) and

68 6Qm

Zn (n,y)Zn y (13.8h) were used for the analysis. The

13 samples are irradiated in a thermal neutron flux of 10

-2 -1

n«cm sec . The copper is extracted as the acetylacetonate

into chloroform from an acetate system buffered at a pH of

4.75. The remaining aqueous phase is extracted with chloro70
form to remove most of the acetylacetone dissolved in the

aqueous phase. The zinc is extracted as the dithizonate

into chloroform, and is back-extracted into 1% HC1 for

counting. Careful internal standardization was used for

correction of results for variation in chemical yields.

"Application of the method to the simultaneous determination of copper and zinc in lung samples provided values

of the same order of magnitude as those reported by Tipton

and Cook for U.S. adults using emission spectrographic

procedures which were between 7 and 10 ppm for copper and

40 and 70 ppm for zinc. Comparison of the precision of this

method with that of emission spectroscopy indicates that

this activation technique has at least three times better

precision.

2. NBS Facilities and Program

DeVoe, J. R. 3 and Smith, G. W. s "Activation Analysis

Program and Facilities at the National Bureau of

Standards", Proceedings of International Conference,

"Modern Trends in Activation Analysis" Symposium,

College Station, Texas, April 1965 , to be published.

"The National Bureau of Standards ' Radiochemical

Analysis Section is now developing a capability in activation

analysis to complement other analytical techniques using

radioisotopes in analysis.

"Activation analysis can be likened to spectrometric

methods in which the re-emitted radiation from excited

71
states ( e.g. radioisotopes) which are often long (e.g.

milliseconds to years) are measured and related to the kind

and amount of element which interacted with the primary

radiation. The analyst learns to discriminate from unwanted

excitation processes by varying the type and energy of

incident radiation, the length of time of the excitation,

and the type of detector that measures the re-emitted or

secondary radiation.

"To be most effective for our applications of activation analysis a computer is best used in a regenerative

feedback loop which consists of selecting, in sequence, the

proper nuclear reaction, time of irradiation, complexity

of radiochemical separation, appropriate detector, stripping

of time dependent radiation spectra, and judgment of the

analyst. It should be emphasized that a selective combination of the above variables does not place too large a

burden on any one of the variables.

"Plans for the development of facilities to carry out

these concepts are discussed. Facilities which are planned

or under construction are: a 10 MWh nuclear reactor, a 40 kW-100 MeV Linac linear electron accelerator, a wide

variety of pneumatic tube facilities, hot laboratories, a clean room complex, and low background counting facility.

5. Tellurium in Cartridge Brass

"Becker, D. A., and Smith, G. W. , "Activation Analysis

Determination of Tellurium in Standard Reference

72
Cartridge Brass and White Cast Iron", Proceedings

of International Conference , "Modern Trends in

Activation Analysis", College Station, Texas,

April 1965 j to be published.

"For the first time, neutron activation was used for

elemental analysis of NBS Standard Reference Materials at

the National Bureau of Standards. The materials were

cartridge brasses and white cast irons, analyzed for

tellurium content utilizing the nuclear reaction:

Te^0 (n5Y)Te131

^

X131

1~31

The samples were irradiated and the I separated by a

destructive distillation and solvent extraction technique.

125 Chemical yields were obtained by means of I ^ tracer.

Tellurium concentrations of 2 to 800 ppm were determined,

and the results compared with those obtained on identical

samples by polarography and emission spectroscopy."

75
4. RADIATION TECHNIQUES

The major effort of this group in the past year was the

design of M<5ssbauer spectrometers which have a very high

precision and accuracy. The electromechanical M8ssbauer

drive described last -year has been updated to some extent

and a complete mechanical design of the transducer mechanism

is reported here. Engineering prints of this system may be

obtained by writing to this section (510.01).

The optical M<5ssbauer drive reported at the American

Physical Society in April 1964 has been improved by the

design of a sensitive frequency comparator which is described

in both this and the instrumentation part of this report.

Both of these machines will be used in calibration of

the first Standard Reference Material (SRM) for Mflssbauer

spectroscopy. A description of this standard and preliminary

spectra of it are included within this report. This SRM will

be available in about three months, and all orders which have

been received to date will be filled by that time.

In addition to this effort the group has been able to

clarify the structure of iron in a variety of EDTA complexes,

to indicate detectability of about 1% of iron carbides such

as austenite in an iron matrix, and to determine the species

of iron on certain organic resin cation exchangers.

A number of experiments have been made in an effort to

make M<5ssbauer sources with narrow line widths and large

fractions of recoiless emission. These efforts have been

74
rewarded through the production of a Sn-*--

J-"m source that will

increase, to a considerable extent, the effectiveness of

Mossbauer spectroscopy in studying the structure of tin

compounds

.

The group's leader, Dr. J. J. Spijkerman was one of four

representatives from the United States who took part in a panel discussion in April on the "Application of M8ssbauer

Effect" sponsored by the International Atomic Energy Agency

in Vienna, Austria. This panel discussion brought into focus

the fact that M8ssbauer spectroscopy is a very promising tool

for quantitative and structural analysis.

A. Standard Reference Material for the Differential Chemical

Shift of Iron Compounds in Mossbauer Spectroscopy

M<5ssbauer spectroscopy shows great promise in the

elucidation of chemical structure. In fact it may be stated

that this technique will take its place along with other

spectroscopies such as nuclear magnetic resonance and electron spin resonance by providing supplementary information

that assists in identifying chemical structure.

The technique has been described in detail by many

authors [9^10,11,12]. For the purposes of this presentation

it is instructive to describe the technique with regard to

57 its use for measuring chemical effects. A Fe Mossbauer

57

source is prepared by placing Co into the lattice of a crystalline .material in such a way that a significant number

(theoretical approximately 70^) of the 14.4 keV gamma rays

75
are emitted from the source with a negligible fraction of

energy removed from it through recoil of the nucleus. This

will produce a 14.4 keV gamma ray with a theoretical line

width (FWHM) of 4.7 x 10"" eV. This line width is so narrow

that it is possible to observe very small changes in the

difference of the energy levels between the l4.4 keV excited

57 state and the ground state of Fe . For example., the change

in the energy levels caused by the interaction of the extranuclear electron configuration with the nucleus can be

measured. This effect can manifest itself through a hyperfine coupling with the nucleus producing a monopole^ dipole,,

and quadrupole interaction. When no field gradient is

present at the nucleus (e.g. high symmetry in the electron

distribution) a displacement in the energv level of the

14.4 keV excited and ground states of the nucleus can result

from direct interactions between the penetrating electrons

and the nucleus through I J coupling (I total nuclear angular

momentum^ J total electron angular momentum) . This monopole

interaction is called the chemical shift. The quadrupole

interaction results from a net electric field gradient at the

nucleus which removes the degeneracy of the spin states of

the energy levels in the nucleus by producing a nuclear

electric quadrupole splitting (for 1^1). Internal magnetic

fields or an externally applied magnetic field further

removes the degeneracy and results in a nuclear Zeeman effect

(6 lines in the case of the 3/2 to 1/2 transition in Fe ).

76
Let us omit from consideration any electric quadrupole

or magnetic interaction and discuss the spectroscopy used to

measure the chemical shift. The source is emanating a 14.4

keV gamma ray of the very narrow line width that is produced

as a result of the transition from the chemically shifted

14.4 keV state to the chemically shifted ground state. An

iron bearing chemical compound is placed in the gamma-ray

beam and most often a detector is placed so as to measure the

transmitted gamma rays. If the chemical environment of the

absorber is different than the source, no Mtissbauer absorption

occurs because the energy levels of Fe J in the absorber are

displaced from that of the source. However , the M<Bssbauer

spectrometer produces a Doppler motion along the gamma-ray

vector of perpendicular incidence to the absorber which adds

or subtracts energy from the gamma ray depending upon the

direction of the motion velocity vector. By plotting the

transmitted gamma-ray intensity as a function of the relative

Doppler velocity a M<5ssbauer spectrum is obtained. We

measure a chemical shift, $ , which can be indicated as

b = ^EQas- aE where aE is the difference between the 1^.4 keV

a

state and ground state of the Fe^ nucleus in the absorber,

and aE is the difference between the same levels in the

s source

.

The M8ssbauer spectrum measures a relative chemical

shift. Therefore, one can compare the chemical shift of a series of iron chemical compound used as absorbers with

77
respect to a single source. Such measurements have been made

for a number of compounds. A serious difficulty arises, however, when one spectroscopist wishes to compare data on the

chemical shift of his compound with that of another spectroscopist. The spectroscopists must find a compound which can

be measured in both laboratories so that a normalization

factor may be calculated. The accuracy of the normalization

factor is affected by the linearity of the spectrometer, of

course; but it also is affected by the degree of chemical

identity, either between the sources or between the absorbers

in each laboratory. It is believed that some of the serious

discrepancies "chat appear in the literature are the result

of the use of improper normalization factors due to the

nonuniformity in chemical environment in supposed identical

sources or absorbers prepared by each laboratory.

Very recent work in our laboratory indicates that the

method of preparation of an iron absorber can seriously affect

the shape of the six lined spectrum obtained. This has been

found to be due to presence of iron carbides whose phase

transitions are very dependent on the type of heat treatment

that is used [13].

During the Gordon Research Conference in Inorganic

Chemistry, August 1964, the opinion of the majority of the

attendees at the M<5ssbauer symposium was that the National

Bureau of Standards should develop an absorber which can be

used as a standard reference for chemical shift measurements.

78
The. requirements of such a standard are as follows:

The standard must be stable with time, and should

not deteriorate under standard laboratory

conditions

.

The compound must be well defined chemically

and physically.

3) The line width of the M^ssbauer spectrum must

be nearly theoretical,, with a large recoilfree fraction at room temperature.

%) The chemical shift should fall in the same

region as most other iron compounds and

should have a low temperature coefficient.

A doublet spectrum would be highly desirable

to minimize the dependence on line shape and

to provide , simultaneously, a velocity

calibration.

Several materials were considered (e.g. iron metal)

,

but as indicated above it was found that it was difficult

to produce the degree of chemical uniformity that was required

At this conference it was agreed to investigate the

properties of disodium pentacyanonitrosof errate dihydrate

Na

p

[Fe(CN) (_NO] • 2HpO (sodium nitroprusside ) . It has been

found that this material has all of the requirements for a

satisfactory Standard Reference Material for Mo'ssbauer

spectroscopy. Pertinent data on the chemical,, physical, and

crystal properties appear in table 7.

To assure that the chemical system be as highly reproducible as possible, single crystals were grown from the

reagent grade salt. Platelets 1 x 1 x 0.0775 cm were cut

and polished so that the molecular axis of symmetry is along

the plane of the platelet, (see figure 25). When the crystal

79
Table 7. Chemical, physical and crystal properties

of Na

2 [Fe(CN) N0]-2H2

0.

Molecular weight 297.95

Specific gravity 1.72 g/cm

Color Red

Solubility in HgO 40 g/100 cm5 at 16 °C

ri5i Crystallographic data J 12 Orthorhombic , space group D„, - Pnnm

Cell constants

a = 6.17 + 0,,03 A

b = 11.84 ± 0,.06 A

c = 15.43 + 0,,08 A

Habit [110] Primary Rhombic Prism

[010] Brachy-pinakoid

[Oil] Primary Brachy-domal Prism

Crystallizes in needles along the b-axis

is mounted so that the M<5ssbauer radiation passes through

perpendicular (+ 2 ) to the be plane of the platelet , the

absorption spectrum shown in figure 26 is obtained. Preliminary measurements give two peaks separated by aE = 0.1712±

.0004 cm/s, which result from the electric quadrupole

splitting of the excited state of the iron nucleus. The

point half way between these two peaks is assigned a differential chemical shift value (* ) of zero. The procedure will

be to place the standard reference absorber in the spectrometer

80
Figure 25. Single crystal of sodium nitroprusside ., showing habit and orientation of platelet.

and to calibrate the spectrometer in accordance with the

point of zero differential chemical shift. The chemical

system to be measured is then placed in the spectrometer and

the differential chemical shift is measured directly.

These single platelets will soon be available for

purchase from the NBS Office of Standard Reference Materials

(Division 302). A M<5ssbauer spectrum using a Co palladium

source with the narrowest line width obtainable will be

81
100k

-O.IO -0.05 0.05 VELOCITY (cm/sec)

Na2 Fe(CN) 5 N0-2H2 SINGLE CRYSTAL

100 plane (be)

0.1 0.15

Figure 26. M<5ssbauer spectrum of Standard Reference

Material for Fe

furnished with each platelet so that the spectroscopist will

be able to calibrate his spectrometer to that of NBS in

addition to using the absorber as the reference for measuring

differential chemical shift; such a spectrum is shown in

figure 26. This- spectrum will be calibrated with a Mfissbauer

spectrometer which uses a Doppler motion drive incorporating

on optical interf erometric technique (see part B) . This

spectrometer has., theoretically , an accuracy and a precision

of 1 part in 100,000 (relative standard deviation of a single

determination)

(J. J. Spijkerman and J. R. DeVoe)

82
B. Standard Reference Material for Iron in M<5ssbauer

Spectroscopy

While major emphasis has been placed on the use of the

sodium nitroprusside as a standard for measuring chemical

shift it is important to consider its use in other areas of

M<5ssbauer spectroscopy.

The M^ssbauer drive using optical interf erometry should

provide a very accurate means of measuring the distance (in

units of velocity) between the peaks due to the quadrupole

splitting in the single crystal. This can then be used to

calibrate the spectrometer in this velocity range for use in

any experiments in physics or chemistry. As indicated in the

above section, a precision and accuracy of one part in 100,000

is theoretically obtainable with the optical spectrometer.

However, careful measurements will be made to determine

whether small errors in crystal orientation materially affect

the precision of measurement of this splitting. It is very

likely that this Standard Reference Material will be a factor

of 10 more precise than those data which have been most often

used as a reference [14].

(J. J. Spijkerman and J. R. DeVoe)

C. Differential Chemical Shift of Iron Compounds

The use of the sodium nitroprusside single crystal

absorber should result in an improvement in the quality of

data for chemical shifts for the reasons indicated in

sectionA above. In order to make a clear distinction in

83
such reporting of data, it is suggested that the term differential chemical shift ( jj be used when a chemical compound

is measured with respect to the Standard Reference Material.

The National Bureau of Standards plans to act as a clearinghouse for data on differential chemical shifts. These data

will be published annually as a part of the Radiochemical

Analysis Sections' Progress Report as well as being incorporated into an information retrieval system using a digital

computer. Details on this will be published at a later date.

It is hoped that this effort will bring stability and

integrity to the promising new tool of Mossbauer spectroscopy

for structural analysis of chemical compounds.

(J. J. Spijkerman and J. R. DeVoe)

D. Absolute Velocity Mossbauer Spectrometer Using Optical

Fringe Counting Technique

Mossbauer spectroscopy involves the measurement of the

relative nuclear energy levels between a Mossbauer gamma-ray

source and an absorber by Doppler shifting the gamma radiation

so as to produce resonant absorption.

The Doppler velocity can be produced by mechanical or

electromechanical [15] means, and either constant velocity,

or constant acceleration is employed to obtain a Mossbauer

spectrum.

Mechanical means of producing a Doppler motion have

inherent long term stability, are reproducible from one cycle

to the next, and the average velocity can be measured with

84
great accuracy; but, due to friction and vibrations, the

instantaneous velocity is not constant. Also, the slow

response of mechanical systems makes it difficult to

incorporate feedback.

Electromechanical transducers, using velocity feedback,

are used with good success in M<5ssbauer spectroscopy, either

to generate a constant velocity motion, or constant acceleration. The instantaneous velocity can be obtained from a measurement of the emf of a velocity sensing coil [18]; but

it requires calibration, either by a direct method, or from

a known McBssbauer spectrum. A direct method for measuring

velocity, with high accuracy, using an optical interferometer

will be described.

1. Optical Interferometer

The use of a Michelson interferometer for the measurement of length is well known [16,17] and with minor modification it can be used to measure velocity.

The interferometer consists of a stationary light source

beam splitter, reflector, and a movable second reflector,

as shown in figure 27. The light intensity produced by the

interferometer varies sinusoidally with the optical path

difference (aS) between the two reflectors. The current

produced by a photomultiplier is then

I(aS) = I + I cos —: > v ' D o A

where ID

is the dark current, 1^ the signal current. If the o

85
KOSTERS

PRISM

BEAM

SPLITTER

-^Hg 198 LIGHT SOURCE

X = 5460.74 A"

REFLECTOR, MOUNTED

ON FLECTURE PLATES

STATIONARY

CORNER CUBE

REFLECTOR

P.M.

FILTER

PHOTO CURRENT I = ID +

QCOSy 27Tt

2V

FREQUENCY f==f

Figure 27. Optical syst m for measuring absolute

velocity.

second reflector is not stationary, but moving with a velocity v, the photomultiplier current will be time dependent,

I(t) Lp. + I cos 2tt D o 2vt

with a frequency f = 2v/\ . For a velocity of 1 mm/sec _, 198 ° using the Hg y green line of 5460.7^ A, this frequency

corresponds to 3662.5 hertz. The interferometer reduces

the measurement of velocity to a frequency measurement 3 and
counting techniques can be used.

The conventional Michelson type interferometer would

be somewhat inconvenient to use with a M<5ssbauer spectrometer

because of alinement requirements. These requirements can be

considerably reduced by using corner cube reflectors [18]

instead of plane mirrors and a Rasters prism [19] for a beam

splitter, as shown in figure 28. This arrangement produces

circular fringes, and the center fringe is focused upon

the photomultiplier cathode.

2 . Electronic Instrumentation

The Doppler motion is generated by a loudspeaker with a double voice coil, the second coil acts as a velocity pickup

coil and is incorporated in a feedback loop, as shown in

figure 29. The travel of the loudspeaker coil is limited by

a photocell slit system, which generates the square wave

voltage required for constant velocity. The reflector and

the loudspeaker coil are suspended by two flexure plates,

which are photo-etched from 0.005-inch beryllium copper, and

provide up to one inch travel.

The fringe count frequency measures the change in distance with high accuracy. To provide high precision and

accuracy in the velocity produced by the drive, the fringe

count frequency is compared with a known frequency derived

from the NBS Standard Frequency (WW) to produce the required

velocity. This comparator is shown schematically in figure 4,

and is described in part 2 of this report „ This comparator

87
-p

CDaoU

QJ

CD PH

cdo

•H

-PftO

bO

S^ •H

CQ

0)>

?H

0) H

ft

fto

fi

<HO

•Po

!>» H

cd

<l) PiCD C5

00

OJ

a)

hO H

88
.005>aF

f-cWWV

Figure 29. Mtissbauer drive and velocity measuring

system.

is a scaling device which has four stable states, -1, -0 , +0

,

and +1 3 depending upon whether the velocity is too low,

correct, or too high, respectively. An analog output of this

scaler provides the correction signal for the drive. Accuracy and precision of this Doppler motion producing transducer is theoretically 1 part in 100,000.

(J. J. Spijkerman)

E . Design of a Drift-Free Mflssbauer Spectrometer

1. Introduction

An electro-mechanical Mossbauer spectrometer is composed

of two parts: (1) the driving electronics and (2) a mechanical
transducer slave which produces a velocity function from the

voltage function output of the electronics. The electronics

of the N. B. S. spectrometer have been described elsewhere

(see abstract). In this report the latest mechanical design,

changes in the electronics, and an additional electronic

circuit will be described.

2 . Component Design

In the design of the spectrometer several criteria were

considered. The moving parts of the spectrometer should not

be hampered by friction. The supporting mechanism for the

moving parts should constrain them to move in a reproducible

straight line. The supporting device should have a long life,

and it should be able to handle motion with amplitudes of up

to ~5/K inch.

a. The Flexure Plates . Tne moving parts of the

spectrometer are supported by a pair of 5 mil beryllium copper

flexure plates (figure 30). The flexure plates have the

ability to support the weight of the moving parts, and yet

they do not appreciably hinder the motion. Because of their

design, they have little tendency to deform under the stresses

caused by the weight of the moving parts. They do not deform

even over long periods of time; therefore, the spectrometer

is virtually free from the misalinement which causes friction

to develop.

The flexure plates are photo-etched in a process quite

similar to that used in the production of printed circuits.

90
First an enlarged drawing and a pair of negatives are made

to scale from the drawing by photo-reduction. The negatives

are then used to expose both sides of a piece of light sensitized beryllium-copper. The coating on the beryllium-copper

is then developed and a flexure plate is etched. This

process makes the flexure plates very reproducibly and,

therefore , interchangable with each other.

Figure 30. Flexure plate.

As a consequence of the design of the flexure plates,

a translation of the moving parts of the spectrometer will

cause a small rotational torque between the inner and outer

diameters of each flexure plate. It is necessary that the

two have identical space orientations, i.e. the contact

points must be at the same angle on both ends, or an unequal

91
rotational torque will develop between them causing one of

the plates to buckle and to interfere with the desired motion.

Much of the design effort on the spectrometer was made to

insure that the flexure plates had the proper space orientation.

b. The main frame of the spectrometer . The main

frame of the spectrometer (diagram in figure 31 and photograph in figure 32) consists of (l) a speaker magnet, (2) two

flexure plates,, (3) a linear velocity transducer coil, (4) two

push rods, (5) the lamp and photocell housing, (6) a linear

velocity transducer core (LV Syn Core), (7) a speaker coil,

(8) the slit system, (9) an alinement tube, (10) two end

plates, and (11) the transducer housing.

The two end plates and the alinement tube are bolted

together and their alinement is insured by careful machining,

and by the use of two dowel pins at each end of the tube.

The speaker magnet and flexure plates are clamped to the end

plates and alined by dowel pins. Thus the alinement of the

spectrometer is assured with the exception of the transducer

core in the transducer coil.

A flexure plate is clamped into a socket on each end

plate. The inner diameter of the front flexure plate is

clamped to a push rod and the speaker coil. The push rod

passes through the hole in the speaker magnet and connects

to the L.V. Syn Core, which in turn connects to the other

push rod which is clamped to the inner diameter of the rear

92
v/y////

10

\/ ///////////-.

4 \///&/////////////////&//^7r~

Figure ~$1. Mechanical design of the Mossbauer

spectrometer

.

95
Figure 32. Electronics

for NBS Mossbauer

spectrometer

94
flexure plate. The slit system is attached to the rear push

rod and extends behind the rear end plate. The photocell

and lamp housing mount onto a ring which is bolted onto the

outside of the rear end plate. It can be rotated on the rin^

so that the slits, lamp, and photocell line up.

The spectrometer is supported by the front and rear end

plates. These plates have been made oversized, and they

have many drilled and tapped holes in them to facilitate

mounting equipment used in experimental work.

c. The alinement procedure . Except for the L.V.

Syn Core inside of the L.V. Syn Coil the spectrometer is

alined during the assembly. To aline this remaining part of

the spectrometer, the following procedure is used:

The mechanical feedback loop is opened by replacing

the L.V. Syn Coil in the circuit by a 20 kft resistor.

A small amplitude, one sine wave, is introduced at

the input of the electronics. The output of the

monitor L.V. Syn Coil is observed on an oscilloscope,

and if the sine wave has flat perturbations around

the zero voltage crossover (stick slip operation) the

spectrometer needs to be alined. By using the eight

set screws in the transducer housing to adjust the

L.V. Syn Coil position, and by observing the monitor

coil output, the spectrometer can easily be alined so

that there will be virtually no rubbing.

d. Changes in the electronics . In the past year

95
there have been several changes in the electronics in the

N. B. S. M<5ssbauer Drive. The power amplifier has been modified to utilize newer and better transistors. The feedback

loop has been modified and a new logic circuit has been added

A photograph of the present electronics is shown in figure 33

Figure 33. The main frame of the Doppler Drive

96
(1) Power amplifier . A new complementary pair

of signal transistors have been employed as the pre-drivers.

They are to replace the 2N217 and 2N647. They come in a

single package from Motorola and are designated MD985. Because

they are silicon transistors , the two 22 kn resistors have

been reduced to 10 kfi to establish the correct bias (figure

~3k) . It was determined that the amplifier has greater stability if the two pre-drivers have a larger power rating and are

in thermal equilibrium with each other.

LOUD SPEAKER COIL

MD985

PT, 2N3I7I

PT2 STCI080

+ I0V

-10 V

Figure ^>k . Schematic of Doppler Drive

97
(2) Feedback loop . A change has been made in

the feed-back loop which produced better velocity linearity

(figure J>k) . The L.V. Coil,, which was previously in series

with the signal input and the first Philbriek P65A amplifier

positive input has been placed between ground and the negative

input of the first P65A amplifier. By removing the 2.4 henry

coil from the input, the frequency response of the system has

been improved. A one Megohm resistor has also been added

from the output of the power amplifier to the negative input

of the first P65A to D.C. stabilize the power amplifier.

(3) Detector gate control board . Another

circuit board (figure 35) has been added to the electronics

of the spectrometer. It has the following logic functions:

(1) to produce a -20 volt output to inhibit the analyzer

from storing when a spring loaded switch on the front panel

is momentarily depressed downward, (2) to light an indicator

lamp when the analyzer is not storing, (3) to produce a zero

volt output which, in turn, allows the analyzer to store

after the first address reset pulse arrives when the springloaded switch is held in the upwards position, (4) to produce

a gated address reset pulse that can be counted by a scaler

when the analyzer is storing, (5) to be able to inhibit the

analyzer from storing when a preset count is reached by the

scaler, (6) to amplify the voltage-to-frequency converter

three volt output to ten volts so that it can be stored by

the analyzer, (7) to invert the positive output of our single

98
2NI308

750ftTO ANALYZER

DETECTOR GATE

€>

2.2K —WWWSINGLE CHANNEL

INPUT

VOLTAGE TO FREQUENCY CONVERTER INPUT

TO ANALYZER

DETECTOR INPUT —

®

[INHIBIT

TO SCALER COUNT INPUT

ADDRESS RESET

INPUT

+ IOV

Figure 35 • Detector gate control

„

channel analyzer to a negative ten volts so that it can be

stored by the analyzer.

The detector gate control is a convenient feature that

was added to the electronics. It is designed so that one

can accumulate counts for an integral number of sweeps. This

enables one to test the spectrometer by using the voltage-tofrequency converter so that an integral number of sweeps can

be made.

3. Conclusion

This spectrometer design meets all of the required

criteria. It has operated trouble-free for one year, and

99
only minor modifications were required for special experiments. The modified spectrometer has linearity and reproducibility to better than O.lfo in the constant acceleration

mode with a velocity range of 0.001 mm sec" and 20 cm sec" . The major improvement to the spectrometer as a result of these

modifications lies in its greater reliability. Blueprints of

the mechanical design can be obtained upon request.

The author would like to thank Joseph Brodely at

N. B. S. Shops for his excellent machining assistance.

(F. G. Ruegg)

100
F. Cosine Effect in Mflssbauer Spectroscopy

The Doppler effect has proven to be the most satisfactorymethod to obtain a M<5ssbauer spectrum. The frequency v the

absorber experiences for a relative velocity v with respect

to a source of frequency v is

V = V o 1 +

'

2

c = V o i + - cos e

c (1)

where 6 is the angle between the direction of motion and the

Y radiation.

The corresponding energy change is

E

AE = — v cos 9. (2)

c which is directly proportional to the instantaneous velocity.

Since the radiation emitted by the source is isotropic >

a range of velocity components are present for a Doppler

velocity v , and corrections for this effect must be made.

The corrections can be calculated from the angular distribution of the gamma radiation.

It is thus desirable to find the gamma-ray intensity

which is detected as a function of the angle between the

gamma radiation and the velocity vector. In figure ^6 the

geometry is shown for a circular planar source of radius Rs

separated by the distance L from a circular planar detector

of radius R , . Considering only the gamma radiation emitted

at the angle 6 5 and reaching the detector^ a cone is obtained

as shown by the shaded area. Only a fraction of the source

101
SOURCE

DETECTOR

Figure 36. Geometrical relationship between source and

detector for gamma rays emitted at angle (3. and the detector area are being utilized for this particular

group of gamma rays. The partial count rate for an angle q

is thus

N(e) = N

oA

s (e)-A

d

(e)

= arctan V/V :?)

where A (0) is the effective area of the source and A, (0) 3 the effective detector area.

N can be obtained from the total count rate which is

o

N

o

- /

max

n(0) h e

max

arctan

R , + R

d s L

, for R , > R (*)

102
The effective sourc-e area can be calculated from the radius

of source R and the detector radius R , . Since

S Q

R . = L tan 9 - R ,

.

si d^

A

B

(e-) = tt 2 ( d

" P)

, 2 R

s +

| 0„ - 9 1 ( L tan " R

d

) (5)

where H = arctan R,/L. This area is an anulus for 6 > d

a. circle for

d d

d

, and the sum of two circular areas for

0<0

d

since for the last case the radiation from a point on

the source can reach two points on the detector.

Likewise for the detector

[

9 ( c ) (Lv tan - R ) s

' (6)

' = arctan R /L

s _ s

/

N(0) = it N • R

c +

o [ s 2„ i „2 ( S

d

+ 9 >

d

(L tan 6 - R

2 ( 8

S

" 6

)

, 2 R

d

+

l e - 6 1 ( L tan e - R

s ) (7)

The normalized counting rate N(V/V ) as a function of the

ratio of the Doppler velocity V to the spectrometer velocity

V for various apertures a is shown in figure 37. The source

radius equals the detector radius 3 and the aperture is defined

by a = Radius/L.

The smearing correction becomes important for apertures

greater than a = 0.2 (e.g. a 0.2 inch source at a distance

of 1 inch for a 0.2 inch detector). Since most spectrometers

are calibrated using a known absorber^ the error is reduced.

103
Figure 37. Normalized counting rate as a function

of the ratio of Doppler velocity V to a

spectrometer velocity V . o

104
However j this smearing correction is not linear, and should

be applied to the specific geometry used. The trend in

Mo'ssbauer spectroscopy is in high speed data acquisition,

which is obtained by using stronger sources, and poorer

geometry. In this case corrections can be made without

significant losses in accuracy.

(J. J. Spijkerman)

G. The Sn ^ -Palladium M<5ssbauer Source

119m ti Existing Sn ^ Mossbauer sources show low recoilless

fractions and large line widths at room temperature. A new

source [20] has been prepared which has a line width which

is nearly theoretical and which has a large recoilless fraction

at room temperature.

-i -i o The source, consisting of 10$ by weight Sn along with

Sn produced by the (n,y) reaction in palladium, was

prepared by melting appropriate amounts of the two metals

together in an atmosphere of argon.

)urce had a radioac

119m

119m The Sn ^ used to make the source had a radioactivity

of 2.0 mCi and a radiochemical purity of 9o%. The Sn"

sample weighed 0.0410 grams. An amount of 99.9$ pure palladium foil weighing 1.355^ g was tamped in an alundum crucible.

The pellet of tin was placed in the crucible after about

three fourths of the palladium had been tamped in place.

With the crucible in place, the apparatus was evacuated

at about 0.1 torr for 50 minutes. Argon was then admitted

and the system purged for 20 minutes at an argon flow rate

105
of about 50 cm /m.

The metals were outgassed by heating the crucible and

contents to 800 C for one minute. The temperature was then

increased at a rate of 200 G/min to 1600 C. At this temperature the Pd just began to melt (1550 C) and the alloy was

formed. The temperature was maintained at 1600 C for about

50 seconds before being lowered to 1000 G. From 1000 C

the temperature was decreased 100 C every thirty seconds to

a level of 500 C where the temperature was maintained for

10 minutes while the alloy annealed.

The alloy obtained had a shiny silver color. A negli119m gible amount of Sn distilled out of the crucible and

deposited on the chamber walls. Virtually 100^ of the alloy

was recovered.

The source has a line width of 1.14 ± 0.02 mm/s using

a natural (white) 0.005" tin foil. A 0.002" Pd foil filter

was placed over the source to eliminate the Sn X-rays. Preliminary measurements show a recoiless fraction (f factor)

greater than 70$ at 20 °G.

The Sn-Pd source offer-s two important advantages over

existing Sn y ^sources such as Sn y

2 [21] and Mg?Sn

[22] ^ in that it has a very high f factor and a line width

approaching the theoretical value of 0.64 mm/s at room

temperature

106
Effect

m

1 Sn11 ^

12

1 MggSn11^

4

| Sn119m-Pd 17

Line width mm/sec

(corrected for

absorber thickness)

1.5

0.9

0.74 + 0.02'

Standard deviation for a single determination.

The Pd-Sn alloy system will be further investigated,

since this system shows great promise for a single line,

119m Sn y ' source, with almost natural line width and a large

recoil-free fraction at room temperature.

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

H. Abstracts of Papers and Publications

The following are abstracts of manuscripts accepted for

publication during FY '65.

1. Ruegg, F. C.j Spijkerman, J. J., and DeVoe, J. R.

,

"Drift-Free M<5ssbauer Spectrometer ", Rev. Sci.

Inst. Vol. 36, 556-359. (1965)

"in order to eliminate drifts encountered in electromechanical M<5ssbauer spectrometers, a drive has been developed which is drift free with regard to velocity as well as

position. The spectrometer can be driven in two modes:

constant acceleration and constant velocity. The constant

acceleration input signal is the analog output of the address

scalers of a multichannel analyzer running as a multiscaler

107
(time mode). The constant velocity is obtained from a photocell slit system which drives the input of the spectrometer

with a rectangular wave. In both modes, the photocell system

is used to eliminate all position drifts by starting each

new cycle from the same position. To analyze the velocity

of the spectrometer a voltage-to-frequency converter and

multichannel analyzer are used."

2. Spijkerman, J. J., Ruegg, F. C, and DeVoe, J. R.

,

"A Standard Reference Material for MtBssbauer

Spectrometry of Iron and Its Compounds ", Panel

on the Application of the M<5ssbauer Effect in

Chemistry and Solid State Physics, Vienna,

April 1965j> to be published.

"The rapid development of the M<5ssbauer effect has

resulted in a new spectrometric method for chemical structure

analysis. The various sources used in M<5ssbauer spectroscopy

require a reference material to provide M<5ssbauer data on a uniform basis. A series of standard reference materials for

M<5ssbauer spectroscopy will be made available from the

National Bureau of Standards, U.S.A. The first in this series

is a single crystal of disodium pentacyanonitrosylf errate

dihydrate, Na^ [Fe (CN) [-NO ]

• 2HpO. The crystals are supplied

by a commercial manufacturer, and calibrated by NBS on a M&ssbauer spectrometer, using an optical fringe counting

technique

"The Mfissbauer spectrum of a single crystal absorber,

108
cut along the be plane, is a well resolved, symmetric doublet,

The center of this doublet is defined as the zero reference

point for the differential chemical shift , with the containing

absorber at 25.0 C. The absorber crystals supplied will be

2

1 cm x 1 cm, with 25.0 mg/cm of natural iron."

3. Herber, R. H. , and Spijkerman, J. J., "Mossbauer

Line Broadening in SnOp Due to Twinning Effects",

J. Phys. Chem. , to be published.

"Stannic Oxide (cassiterite, SnOp) in the tetragonal

system, class 4/m 2/m 2/m. The space group is P4p/mm, with

two SnOp molecules per unit cell. The most detailed investigation of the structure of a single crystal SnOp by Marley

o and MacAvoy* showed a single Sn-0 bond distance of 2.055 A,

with six oxygen atoms occupying the corners of an octahedron.

This should lead to a vanishing field gradient tensor at the

tin atom lattice point, and zero quadrupole splitting, if all

of the O-Sn-0 bond angles are either it or tt/2 . Mttssbauer

spectra of SnOp with MgpSn119"1 source (23.8 keV, 3/2-+ 1/2

transition) showed definite, though not completely resolved,

quadrupole splitting. This splitting is attributed to

twinning, which was observed even in the best single crystals

grown by the vapor transport method. This twinning occurs

primarily in the (051) plane, so that the oxygen octahedron

around the tin atom in the twin plane is slightly deformed

*J. A. Marley and T. C. MacAvoy, AFCRL-62-771 "investigation

of the Mechanism of Single Crystal Growth in High Temperature Systems; J. App. Phys. _3£j 250^-i- (196l)

109
with a resulting non-vanishing electric field gradient

tensor .

4. Spijkerman, J. J., Ruegg, F. C, and DeVoe, J. R. , "Standardization of the Differential Chemical

57 Shift for Fe , Proceedings of the Symposium

of Mossbauer Effect Methodology, New York

January (1964)

57 "To provide M<5ssbauer data for Fe on a uniform basis,

to eliminate recalculation of data from various laboratories,

and to provide tables of M<5ssbauer spectra, the National

Bureau of Standards has included disodium pentacyanonitrosylferrate dihydrate, Na

2 [Fe (CN) [-NO] • 2H

2 0, in the Standard

Reference Materials Program. Single crystals of sodium

nitroprusside will be available by April 5, 1965 for this

standardization. The crystals are supplied by a commercial

manufacturer, and calibrated by NBS on a Mossbauer spectrometer, using an optical fringe counting technique.

"The Mossbauer spectrum of a single crystal absorber,

cut along the be plane, is a well resolved," symmetric doublet

The center of this doublet is defined as the zero reference

point for the differential chemical shift, with the containing absorber at 25.0 C. The absorber crystals supplied

will be 1 cm x 1 cm, with 25. mg/cm of natural iron."

5. Ruegg, F. C, "A Flip-Flop with adjustable Bipolar

Output", Electronics - Designers Casebook, to be

published. (No abstract is available.)

110
5. RADIOISOTOPE TECHNIQUES

This group uses the principle of radioisotope dilution

and radioactive indicators (radiometric techniques) to

develop trace analytical methods. The standard radioisotope

dilution method measures a change in specific activity of a radioisotope. Since this is often difficult to do directly

for |ig quantities of elements, Ruzicka [25,24] has developed

a method called substoichiometric radioisotope dilution.

Considerable difficulty in this method has been found with

respect to maintaining good precision at very low concentrations. It is apparent that contamination can seriously

affect the precision as well as the accuracy at the levels

of ng/ml of many elements.

It appears that it is possible to apply the general

principle of radioisotope dilution using radioisotopes to

many chemical systems without the use of the principle of

substoichiometry . In each of these procedures it is necessary

to be able to determine a change in specific activity of a radioisotope before and after adding an unknown amount of

stable element. Using a substoichiometric amount of reagent

allows measurement of the ratio of "true" specific activities.

In some of the other methods such a "true" ratio is not

measured, but a ratio which is dependent upon the concentration of the desired element can be measured.

Radiometric techniques have been used in a number of

analyses and this group is attempting to apply the technique

111
to the u.g level and below.

The use of radioisotopes for trace analysis using the

above principles has some inherent advantages. The techniques

have very high sensitivity if the specific activity of the

radioisotope is high. The cost is less than many other trace

analytical methods and the procedure s, although they are not

automated at present, are amenable to it. The main disadvantage from which most other trace analytical methods suffer

(except activation analysis) is that the reagent blank can be

appreciable. However, presuming that most laboratories which

are producing or analyzing very pure materials have been able

to resolve their contamination problems (i.e. they do produce

a pure substance), these techniques should allow trace analysis at a reasonable cost in time and equipment.

A. A Method of Trace Element Analysis With Radioisotopic

Tracers in a System Using Heterogeneous Equilibrium

1. Introduction

This method is based upon the fact that it is possible

to alter the distribution of an elemental species be'tween

two phases by altering the initial concentration of the first

phase. By combining this process with the change in specific

activity of a radioisotope present initially in the first

phase that results by changing the initial concentration,

one obtains a relationship between the initial concentration

in the first phase and the distribution of radioactivity

between the two phases for a constant amount of radioisotope.

112
It is necessary to separate the two phases, of course,

and to measure the radioactivity in each one. One system

that has been investigated is the separation of cobalt on

manganese dioxide impregnated in chromatographic paper.

2. Experimental

To evaluate the method, model solutions of cobalt nitrate

and solutions containing NBS Standard Reference Material (671,

672, and 673) of nickel oxide were analyzed. The nickel

oxide samples contained traces of Fe, Cu, Mn, Mg, Al, and Ti.

a. Preparation of paper . The manganese dioxide

paper is prepared by impregnating strips of chromatography

paper (4 cm x 45 cm) in a bell jar, using an ascending flow,

with 0.5 M manganous chloride. After drying, the manganese

dioxide paper is dipped into a solution mixture containing

15 ml of 30% hydrogen peroxide diluted to a liter with 1 N

sodium hydroxide to precipitate the manganese dioxide on the

paper. The oxidation step takes 4 minutes in order to obtain

a uniform manganese dioxide precipitate throughout the strip

of paper. The manganese dioxide paper is then washed in

distilled water and air dried.

b. Analytical Procedure . The nickel oxide Standards were dissolved in 9 M hydrochloric acid and diluted

to volume so that 100 ml of solution contained 10 g of sample.

To 50 ml beakers are added 1.00 ml of Co of known specific

activity, known amounts of stable cobalt, 7.00 ml of ammoniaammonium hydrochloride buffer (pH 10.50)_, 0.100 ml of the

113
Standard and enough distilled water to make a constant

aqueous volume of 11.1 ml in all sample beakers. Then to

each sample solution is added the pre-weighed piece of

manganese dioxide paper (approximately 15 mg) and a magnetic

stirring bar. -Each beaker was covered with parafilm to

prevent evaporation. The samples are stirred for forty

minutes with a magnetic stirrer and allowed to stand for

five hours with occasional stirring. After five hours the

radioactivity of 1.00 ml aliquot s of sample solution are

measured and the manganese dioxide paper is removed, washed

with distilled water, dried and counted. Likewise, a blank

sample solution containing all the constituents mentioned

above except the stable cobalt is carried through the same

procedure

c. Method of Calculation of Results . After

obtaining the number of counts on the manganese dioxide

paper, the counts per minute per gram of paper is calculated

and a plot of the amount of cobalt adsorbed on the paper

versus the total concentration of cobalt added can be obtained. A least squares fit of the data is made to a straight

line function.

5. Results and Discussion

The experimental method of preparation of manganese

dioxide paper was investigated using Whatman Nos. 1, 3 js 4,

45, and 54 paper. Whatman Nos. 1 and 54 gave the most satisfactory results because they are not decomposed by the

114
oxidizing agent. The manganous ion can be oxidized to

manganese dioxide by alkaline peroxide , hypochlorite, and

potassium permanganate. The use of the alkaline peroxide

gave the most reproducible results.

The contact time was optimized to allow equilibration

with manganese dioxide paper. The conditions which gave the

best results were stirring the solution for forty minutes

and allowing the solutions to stand for five hours with

occasional stirring. If the solutions are stirred continually for five hours the paper will decompose slightly.

The sensitivity of the method using synthetic test

solutions appears to be 1 x 10" (ig/ml for cobalt and the

relative standard deviation of a single determination is

Vfo at the 0.1 p,g/ml level, 20$ at the 0.001 (ig/ml level and

50$ at the 0.0001 u.g/ml level. Preliminary results indicated

a maximum sensitivity of 0.1 u.g for silver and cadmium,

0.01 u.g for iron (III), 0.001 u.g for zinc and thallium, and

0.0001 u.g for nickel.

A standard curve for the analysis of NBS Standard

Reference Materials No. 673 is shown in Figure 38. The

following straight line function was determined by least

squares analysis:

Y = -2.54 x 109X + 2.69 x 105

where Y = radioactivity of Co on the paper, C, min" g~

X = concentration of stable cobalt added mole/1

115
280

40X10

16 28 36 44 52 60 68 76

Mole/liter Cobalt Added XIO* (final concentration)

84 92 100

Figure 38. Least square fit to straight line

Standard Reference Material No. 673.

The values of X are calculated for the Standard Reference

Materials from this equation and are shown in table 8. The

method appears to be fairly accurate since agreement with

the reported value is within about ~yfo. Although the levels

of concentration are high^ the concentration of the final

solution that was analyzed is 3 |i.g/ml. The precision

obtained is comparable with that of the test solutions. Further study of the technique will be made at lower levels of

116
Table 8. Determination of cobalt in NBS Standard

Reference Material Nos . 671, 672 and 673

of nickel oxide.

Standard Reference

Material No. Reported value Value found

W (*)

671 0.31 0.30 + .0025

0.298 + .0019

672 0.55 0.5^-6 ± .0021

673 0.016 0.0164 ± .0026

aAverage of five determinations. Error is standard

deviation of a single determination.

(A. R. Landgrebe and L. T. McClendon)

B. Differential Controlled-Potential Coulometry Utilizing

Radioisotopic Tracers

1. Introduction

This electroanalytical method which employs radioisotopes

is based upon simultaneous electrolysis at controlledpotential of a sample and a standard in two identical cells

connected in series. Under the conditions of the analysis,

the same number of equivalents are deposited in the standard

cell and in the sample cell. The radioisotope dilution

principle is then applied to obtain the concentration of an

element in the unknown material.

A differential procedure is used which involves the

addition of equal, known amounts of the element to be

analyzed and a radioisotope of that element to each cell.

117
The unknown concentration of that element is then added to

the sample cell. Since the same number of equivalents are

deposited in each cell, a mass balance relationship permits

the 'calculation of the unknown amount of element.

In this method, two electrolytic cells are connected

in series and the potential control is maintained between

the mercury pool cathode and S. C. E. in the first cell

referred to as the standard cell. (The second cell is

referred to as the sample cell). Prior to the start of the

electrolysis, atmospheric oxygen must be removed from the

solutions to be electrolyzed. The elimination of oxygen is

necessary because it is reduced at the mercury cathode in

two steps:

1. 2H+

+

2

+ 2~ = H

2 2

E = -0.1 volt vs. S. C. E.

2. 2H+

+ H

2 2

+ 2

e

~ = 2H

2

E = -0.9 volt vs. S. C. E.

Let us consider what happens in the standard cell when

+n

an amount of electroactive species, M is introduced and

electrolyzed. The potential control is maintained at a value

where the only reaction that takes place at the mercury pool

+n — is M + ne :~ M(Hg) . The current arising from the reduction

of M will follow the equation i, = i e~ where i , i, is ^ to o t

current at time o and t, respectively, K = constant. Next,

let us consider what happens at the cathode of the sample

cell. The potential of the cathode in this cell will be

determined essentially by the current passing through it

and is a function of the concentration in the standard cell.

118
Consider the case when the sample cell contains only inert

supporting electrolyte. In this case the only reduction

reaction which can occur is the reduction of hydrogen ion

and the potential of the cathode will shift in the negative

direction to a value where that reaction occurs. For the

+n

cases where equal amounts of M ' are added to each cell, the

potential of both cathodes will be identical if the cell

constants, K for each cell are identical. When an excess of

+n M ' is added to the sample cell, a measurable shift of electrode potential occurs in the positive direction. The potential of this electrode can be measured throughout the duration

of the electrolysis and must be known in order to be assured

that only the desired electrode reaction takes place. The

number of coulombs passing through the sample cell will

deposit an amount of M equal to that deposited in the standard cell for the case where both cells contain equal concen-

+h tration of M and when the sample cell contains an excess of

M+n

. The requirements for quantitative measurement are:

(1) the potential of the cathode is controlled to exclude

undesired electrode reactions, (2) the sample cell contains

a larger concentration of material to be determined than the

standard cell and (3) equal volumes of solutions are present

in both cells. The difference in concentration between cells

can be determined. The analysis for this concentration

difference can be accomplished by measurement of the current

119
via an integration procedure. Another approach was devised

which uses radioisotopic tracers in the analytical procedure

as a type of radioisotope dilution (see part D)

A mass balance relationship was derived using the radioisotopic dilution principle and can be written in the form

M 1 j ( M + M*

J = M-_2j (M + M* +AM j (3)

where a-, and a

p

new refer to counts per minute of the radioisotope, M*, left in the standard and sample cells, respectively, after a finite electrolysis time; a. is the total

radioactivity in counts per minute added to each cell, AM

is the unknown weight of the analyzed element added to the

sample cell, and M is the known weight of element added to

each cell. When M* « M , and the volumes in each cell

are identical, the above equation can be written in the form

A M = M ( — )

L t ~2 w

3. Experimental

a. Apparatus . A diagram of the apparatus employed

is shown in Figure 39.

The electrodes consist of platinum helical anodes isolated from the solution by glass tubes with fritted discs

2 of fine porosity and mercury pool cathodes about 3 cm in

area. The reference electrode is a saturated calomel electrode with a fiber tip. Stirring of the mercury pools is

120
accomplished by means of magnetic stirring bars. Prior to

the start of the electrolysis, prepurified tank nitrogen is

passed through each cell for 30 minutes in order to remove

atmospheric oxygen. A nitrogen atmosphere is maintained

above the solution during the electrolysis. Removal of

oxygen is also accomplished by pre-electrolysis of the

solutions at -0.500 volt vs. S. C. E.

8 mm. Did. Tube

Rubber Stopper

Pt Helical Anode

Fine Fritted Disc

Magnetic Stirring

Bar

Hg Pool Cathode

STANDARD CELL SAMPLE CELL

Figure 39. Apparatus for differential controlledpotential coulometry.

An automatic sample changer and scintillation counter

with a well-type Nal(Tl) crystal, a controlled-potential

coulometer, and a stopwatch are also used in these experiments,

121
A potentiostat was built according to the design of

Connally and Scott as described in their report [27], Details

on the operation of this instrument are given in their report.

The quantity of electricity is related to the bulk concentration of analyzed material which is oxidized or reduced at

a controlled potential,, and is usually measured by electronic

integration to a reported precision of +0.05/?> (standard

deviation of a single determination) [27], To obtain precise

and accurate results in the analysis i the potential control

must be maintained within relatively narrow limits (±10 millivolts). Several modifications are being made to improve this

system and will be reported at a later date.

b. Reagents . The stock solution of cadmium (II)

was made by dissolving 1.25025 gms of spectrograde cadmium

oxide in a minimum amount of perchloric acid and by diluting

with demineralized water to 500.0 ml in a volumetric flask.

This working solution contained 2.1885 mg/ml of cadmium (II).

The demineralized water was prepared by passing it through

a mixed bed ion exchanger and distilled twice in quartz. The

supporting electrolyte was prepared by dissolving ACS grade

potassium nitrate in demineralized water.

c. Procedure. To each of the cells is added 10.0 ml

of 0.1 N potassium nitrate followed by 1.00 ml of cadmium (II)

109 solution containing Cd J

. After air is expelled with nitrogen

for 50 minutes j 1.00 ml of solution is withdrawn from each

cell and counted. The potential is controlled at -0.664 volts

122
vs. S. C. E. and electrolysis is carried, out for either 10.0

or 20.0 minutes. Syringe pipettes are inserted into both

cells and 3 ml of solution are withdrawn simultaneously from

each cell. Then 1.00 ml of each of the 3 ml solutions is

accurately transferred to a test tube in the automatic well

counter and counted for ten minutes.

4. Results and Discussion

The preliminary studies carried out to date include

observing the behavior of the system when equal concentrations of cadmium are placed in both cells and when the sample

cell contains a greater concentration than the standard cell.

These studies are tabulated in table 9.

Table 9. Differential controlled-potential coulometry

of cadmium ideal solutions in 0.1 KN03

.

Difference between sample Wo. of

standards Standard Sample cell an 3- DetermiCell Cell

1.1366

b

Calculated

1.13b6bo.oooo

b

2.29b 4.58b b 2.29 u

1.00° 1.00° o.oooo

c

0.10° 0.10° 0.0000°

23.

6

C 46.36° 22.7°

250° 500° 250° 2

0.0l682b

+ .0l95

b

4

2.308b

+ . 0452b

7

0.10° + .0764° 10

0.00886° + .00558° 10

21.7° +8.6 4° 10

68.0° +11.0 3° 10

Errors are standard deviation of single determination

Weight in milligrams

.

Weight in micrograms.

123
5. Conclusion

The precision of the difference measured between the

amount of the element in the sample and in the standard cell

is about 5^ (standard deviation) of the total amount in each

cell. In addition to the poor precision,, some difficulty

exists with the accuracy of the method assuming that the

standard cadmium solution has the proper titer. These problems are now under study and several variables have been

found to be responsible for the lack of precision.

Although we have not as yet applied the system successfully to the |_ig levels 3 the high specific activity of many

radioisotopes should provide high sensitivity with this

method. Future work will consist of applying the radioisotopic principle at trace concentrations to characterize

the mass transport properties of the system and to investigate the factors governing the sensitivity and precision of

the method.

(P. A. Pella and A. R. Landgrebe)

C . Some Results on the Substoichiometric Radioisotope

Dilution Trace Element Analysis of Silver and Iron .

1. Introduction

The part of this paper on silver analysis was presented

at the ACS fall meeting in 1964 and the iron analysis at

the American Nuclear Society meeting November 1964.

Radioisotopic dilution analysis described first by

Hevesy and Robbie [28] is based on the determination of the

124
Radioisotopic dilution analysis described first by

Hevesy and Hobbie [28] is based on the determination of the

change of specific activity caused by mixing a radioisotope

with the naturally occurring element to be analyzed. The

component to be analyzed must be isolated in a pure state

and the change in the specific activity of this part is

determined. Prior to the last few years, the method of

radioisotopic dilution analysis has not been used to analyze

trace amounts of inorganic substances because the measurement of specific activity of small amounts of the separated

component is difficult. These difficulties were manifest by

the problem in obtaining very pure separations of these small

quantities. The method of substoichiometric radioisotope

dilution analysis [23] reduces the severity of these difficulties . Substoichiometric radioisotopic dilution analysis

requires that the experimenter separates the same number of

atoms from the bulk of the system before and after radioisotopic dilution. This can be accomplished in two ways:

(1) Always remove the same number of atoms from solution.

Solvent extraction, using a substoichiometric amount 'of

complexing reagent in the organic phase, is an example of

this. (2) Always have the same number of atoms left in

solution. The formation of a complex combined with the

separation of the noncomplexed ions by some means, such as

ion exchange, is an example of this method.

125
Single radioisotopic dilution analysis is described by

the equation:

X = Y

A

1

A,

- 1

2

(1)

where X is the weight of the unknown and Y is a known total

weight of the element in the radioisotope, A-, is the specific

activity of the radioisotope, and A? is the resultant specific

activity of the radioisotope after the addition of the element to be analyzed. Specific activity can be defined as:

(2)

a a

An = —- and A„ = —

2 m m 2

1

where a and a

? are the activities in counts per minute (C/min)

and iru and iru are the isolated weights of the element before

and after radioisotopic dilution, respectively. When the

same amount of the element is isolated with a substoichiometric amount of reagent, nu = m„.

Then in relation (l) 3 the specific activities can be

replaced directly by relative activities measured under the

same chemical conditions and counting geometry. Since very

X = Y

1

a, - 1

2

(5)

small amounts can be isolated with good precision, this

method is very useful for trace analysis. [24,25^,26,29,30,31]

A method of internal standardization "of the system is

double radioisotopic dilution. The analysis is carried out

as follows. Different but known weights, W-, and W

?

, of

126
naturally-occurring element to be analyzed are added to equal

aliquot s of the sample containing an unknown weight, w\ } of

the element along with a weight W, of radioisotopic tracer.

An equal weight of the element is separated from each aliquot

through the use of a substoichiometric amount of reagent and

their relative activities are determined. If the weight -of

radioactive tracer is negligible with respect to W-, , W?

, and

¥ , the following formula can be used:

a W - a W

W =

(ara

2

)

a., and a

? are the radioactivities (C/min) of the aliquot

s

containing equal amounts of (W + W-, ) and (W + W ), respectively, after successive radioisotopic dilution.

An extraction procedure for silver using dithizone as

the substoichiometric reagent with EDTA as a complexant to

reduce the effect of interfering elements was devised.

A method was developed for iron using a substoichiometric amount of the sequestering reagent, EDTA. The noncomplexed ions were removed with a cationic exchanger (Dowex

50-X8) leaving a constant amount of iron EDTA complex in

solution.

2 . Experimental

NBS certified Standard Reference Material Sample No.

124d, which contained Gu 85.6^, Pb 5.2$, Zn 5.0$, Sn 4.6$,

Ni 1.0$, Fe 0.18$, Sb 0.17$, and Ag 0.02$ was analyzed for

its silver content as were cartridge brass standards Nos.

127
C1100, 1101, C1101, C1102, and 1102 which contained Gu (67-

73)#j Zn (52-27.l)^3 plus trace amounts of Pb, Fe, Sn, Ni,

Al, Sb, As, Be, Bi, Cd, Mn, P, Si, Ag, and Te. NBS Standard

Samples (671 5 672, and 673) of nickel oxide were analyzed for

iron. These samples contained trace amounts of Co, Cu, Fe,

Mg, Mn, Si, Ti, Al, and Cr.

a. Equipment . An automatic sample changer with a sodium iodide scintillation well counter ( 1 -r- x 2 inch

diameter with a 7- inch diameter x 1 — inch well) was used.

4 2

A pH meter capable of measurement to 0.01 pH units, a wristaction mechanical shaker, 60 ml glass stoppered separatory

funnel, and counting tubes were also used. An ion exchange

column of borosilicate glass, 300 x 10 mm diameter was used.

b. Reagents and Purification . Precautions must

be taken to keep the reagent blank small, so careful purification is necessary in order to eliminate the element to

be analyzed from all chemicals in contact with the system.

All of the water was triple distilled in quartz and

stored in polyethylene containers. Disodium ethylenedinitrilotetraacetate (EDTA), Lot No. 26355 from J. T. Baker Company, and diphenylthiocarbazone (dithizone), Lot No. 712255

from Fisher Scientific Company, were found not to require

further purification. All other salts and acids were of

reagent quality and were purified by distillation or crystallization before use. Carbon tetrachloride, ACS reagent grade,

was distilled twice. The acetate buffer, pH 4.5^ was

128
prepared by dissolving 137 g of sodium acetate trihydrate and

60 ml of glacial acetic acid in the special distilled water

and diluting to one liter. Radioactive silver , As. 110m

>

t-, = 270 days, was obtained from Oak Ridge National Laboratory,

"2 The Ag with specific activity of 3130 mCi/g was diluted

to the desired specific activity with stable silver nitrate

solution. The Fe-59 (t-, = ^5d) with a specific activity of

31,800 mCi/g was diluted to the desired specific activity

with stable ferric nitrate solution.

Ion exchange resin, Dowex 50 X-8, 20-50 mesh cannot be

used without purification to remove trace amounts of iron

and cobalt. The resin was washed with 30 bed volumes of

demineralized water followed by elution with 6 N HC1 to

remove iron. After washing with 30 bed volumes of demineralized water, the resin was converted to the sodium form

with a 5^ solution of sodium hydroxide. The resin was

treated with 60 bed volumes of a ±% solution of EDTA, followed

by 30 bed volumes of demineralized water.

c . A procedure using single substoichiometric

radioisotope dilution .

(l) Silver . A known amount of radioactive

silver of specific activity Y was added to 10 ml aliquots of

the aqueous solutions of the sample to be analyzed. The

aqueous solutions were extracted with 2 ml of carbon tetrachloride containing dithizone. After shaking the solutions

for thirty minutes, the organic phase was separated and the

129
radioactivity of a 1.00 ml aliquot of the organic layer

(a ?

) was measured. At the same time, a solution containing

the same amount of radioactive silver of specific activity

Y was extracted.

The radioactivity of a 1.00 ml aliquot of the carbon

tetrachloride layer from the solution containing only the

radioisotope was counted (a.,). The amount of silver was

calculated by use of equation 5.

(2) Iron . The column containing 10 ml of ion

exchanger was equilibrated with 60 ml of the same EDTAbuffer solution that is used in the influent. The influent

60 contained 5 nil of EDTA solution, 5 ml of Co radioisotope

solution, and varying amounts of stable cobalt as a test

solution or in an unknown sample when conducting the analysis.

The solution was diluted to 50 ml in a volumetric flask and

the influent was passed through the column at a constant flow

rate of 1 ml/min. The column was then washed with 50 ml

of demineralized water. The combined effluent and wash were

diluted to 100 ml, and a 1.00 ml aliquot was placed in a

test tube for counting. For the single substoichiometric

radioisotope dilution method the same procedure above is

conducted twice; once on the radioisotope solution to give

a-, , and once after dilution with the standard cobalt or the

unknown sample to give a

?

.

130
d. Procedure for sub stoichiometric radioisotopic

dilution analysis of Standard Reference Materials .

(1) Single dilution procedure for silver . The NBS Standard Materials of ounce metal, Standard Material

124d, and the standards of cartridge brass, Nos . C1100, 1101,

CHOI, C1102, and 1102 were dissolved in nitric acid and

evaporated to near dryness. Acetate buffer and a twofold

excess of EDTA were added. This solution was heated at

60 C for 30 minutes, then cooled to room temperature. The

solution was diluted with acetate buffer so that 100 ml of

solution contained 1.00 g of sample.

(2) Double radioisotopic dilution procedure for silver . When the samples were analyzed by the method

of double radioisotopic dilution, they were dissolved in

nitric acid and evaporated to near dryness. Acetate buffer

and a twofold excess of EDTA were added. The naturallyoccurring silver ¥-, and Wp containing the same amount of

radioisotope were added to two equal 'weights of the same

sample. In order to insure complete masking of the copper

by EDTA, these solutions were heated to 60 C for 30 minutes,

then cooled to room temperature. The samples were diluted

with acetate buffer so that 100 ml of solution contained 1.00

g of sample. Ten ml aliquots were extracted with 2.00 ml of

dithizone in carbon tetrachloride in all experiments as before.

One (1.00) ml aliquots were counted from each extract to give

a-, and an which are used in equation k to. calculate W .

131
(3) Single dilution procedure for iron . Ten

grams of the nickel oxide NBS Standard Reference Materials

(671;, 672, and 673) were dissolved in 9 M HC1 and passed

through an anion exchange column which was prepared by

washing it in 9 M HG1. The effluent from the column containing nickel was discarded. The columns were eluted with

water and the solution was evaporated and then diluted to

volume in a 50 ml volumetric flask. A 5 ml aliquot of this

solution was then added to another 50 ml volumetric flask

60 along with 5 ml of the Co , 20 ml of diethylamine buffer

and 5 ml of 1 x 10" 5 M EDTA. The solution (pH 11) was

diluted to volume and passed through the ion exchange column.

The effluent was diluted to 100 ml and a 1 ml aliquot was

counted^, using procedures indicated above.

2. Results and Discussion

The results of this analytical procedure for silver in

a variety of brass standard reference materials (SRM) are

shown in table 10 along with the emission spectroscopic

results from which the SRM was certified.

Although several numbers on the precision of the spectroscopic method are' missing., it appears that the radioisotope

dilution methods, both single and double, have precision

comparable to that of the spectroscopic method.

The amounts of silver in Standard Reference Materials

C1100, 1102, and C1102 show reasonable agreement for the

precision indicated between all of the methods. The poor

132
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133
precision and accuracy of the double method for 1101 or CHOI

indicate a serious problem which has yet to be resolved. The

single method for these samples shows good agreement for the

precisions indicated (e.g. the differences of the averages

could be due to chance variation)

The standard reference materials which were analyzed

for silver contained so much copper that it was necessary

to perform the analysis of these samples at pH 4.5 in an

acetate buffer in order to increase the solubility of EDTA.

The concentration of copper (II) ion in the presence of a

i R

two-fold excess of EDTA is insignificant (10~ M) and the

copper is not extracted under the conditions employed. Using

EDTA as a masking agent permitted the determination of silver

in high copper concentrations.

The results of the single dilution method using the

cation exchange separation for iron in SRM nickel oxide are

shown in table 11. The agreement is considered to be

excellent for the precision obtained. This method using

EDTA as a complexing agent is quite specific because few

elements form as strong an EDTA complex as Fe III does.

We have had considerable difficulty in obtaining the

precision which has been reported for iron using the same

method as that of Ruzicka [26]. Our precision (standard

deviation,, single determination) varies from about \% at the

U-g/ml concentration level to about 5$ at the 10 to 100 u.g/m.1

level. Other data at more dilute concentrations indicate a

154
precision as poor as 15$ at the ng/ml level (see table 12).

This is about four or five times less precise than much of

the work of Ruzicka. We are now attempting to determine the

cause of the relatively poor precision that we obtain for

the parts per million levels and below. Variable reagent

blanks are a possibility. Procedures which could introduce

varying amounts of contamination such as surface contamination of containers or the entrance of dust particles into

the system may also be the cause for lack of precision. It

should be pointed out, however, that precisions obtained in

this work compare very well with that obtained in most other

trace analytical methods. Further results on this precision

problem will be published at a later date.

Table 11. Single substoichiometric radioisotopic

dilution analysis of nickel oxide for

trace amounts of iron.

Standard

reference Reported

material No. value

671 0.39

672 0.079

673 0.029

b

a b Average iron found 3 m

0.394 + 0.004

.082 + 0.003

0.029 + 0.001

Average of five determinations

Error is standard deviation of a single determination

135
Table 12. Determination of trace-iron by single

substoichiometric radioisotopic dilution

analysis on ideal solutions.

Iron taken Average iron

(X) founda ; b

(lig/ml) (iig/ml)

2.24 2.28+0.09

0.224 0.24 + 0.012

0.0224 0.016 + 0.002

0.00224 0.0017 ± 0.0002

aMinimum of 6 determinations.

bError is standard deviation of a single determination.

The sensitivity of the method is limited by three

factors. (a) There is a limit to which the organic reagent

can be diluted and still retain the effect of substoichiometry. (b) The specific activity of the radioisotope must

be high enough to result in significant dilution, (c) At

low concentrations the element may be adsorbed by the glass , e.g. ^ at 10 g/ml. The last factor is an error inherent

in all trace analytical methods for silver. No attention

has been paid to the value of a preconcentration step to

further extend the sensitivity. Even though great care

must be paid to ascertain that no contamination occurs

during preconcentration some applications of this method

156
may require it. A very great advantage in the substoichiometric technique is its simplicity, and the fact that no

specialized capability of the analyst beyond that which he

uses routinely is necessary (e.g., complexometry in solution

chemistry). The equipment required is available from a variety of sources and less than microcurie amounts of

radioactivity are used. It must be remembered that chemical

exchange between the radioisotope and its stable isotopes

is a requirement for radioisotope dilution. It is possible

(usually through a series of repeated oxidation and reduction

steps) to obtain good chemical equivalence between the isotopes. The advantages which apply to regular isotope dilution

apply here also (e.g., quantitative yields are not necessary).

For this and the other reasons given above, this technique

affords considerable saving in time and equipment when

compared to most other trace analytical methods.

We wish to thank Robert Alvarez for performing the

spectroscopic analysis of silver.

(A. R. Landgrebe, L. T. McClendon, and J. R. DeVoe)

D. The Effect of Fluoride on the Distribution Coefficient

of Ferric Thiocyanate in Isobutanol-Water . Iron (III) can be extracted from an acid solution as a

f errimonothiocyanate complex into isobutanol. The formation

of the extractable species depends on the combined action of

the anion of the metal and the organic solvent. This

extraction system is referred to as an ion-association

137
type [32]. Addition of fluoride to an aqueous solution of

Fe (SCN) produces a fluoro complex

Fe(SCN) ++

+ nF"— FeF

^"n ) + (SCN)" n = 2 to 6

where the decolorizing action of fluoride on the ferric

thiocyanate complex can be observed. The instability con-

("3-n") stants of FeF. v ' where calculated by measuring this

decolorizing action spectrophotometrically [33]. It was

shown that at low concentrations of fluoride (corresponding

to [F ] = 10 to 10 M) the predominant species was the

f errimonof luoride (FeF ) complex. The step-wise instability

constants were reported by Babko and Kleiner to be as

follows

xr ^ [Fe+3

] [F~] c nn-6

Kt-, t-,++ = — = 5 X 10 FeP [FeF]++

W = [FeF+

] [F-] = 0>35 x 1Q

-4

[FeF

2

]

KFeF = [FeF^] [F"] = 0.6 x 10~ 5

[FeF^]

[FeF^] [F] = 1 x 10" 2

^FeF^" = ±

[FeF4

]"

K^-2^ [FeF4

-] [F-L

^

[FeF

5

JThe ferric fluoride complexes formed are not extracted

into isobutyl alcohol. Thus a solvent extraction procedure

was devised in the present study to investigate the distri138
bution coefficient of ferric thiocyanate as a function of

fluoride ion concentration with the prospect of using the

system as an analytical method for fluoride ion in the

-6

10 g/ml range.

1. Experimental

Apparatus ; An automatic sample changer and scintillation

counter with a well-type Nal(Tl) crystal were used. A pH

meter, capable of measurement to 0.01 pH units, and a mechanical shaker plus glass stoppered separatory funnels and

15 x 125 mm borosilicate glass counting tubes were also used.

Reagents: All the solutions were prepared from ACS

reagent grade chemicals and doubly distilled water. The

distilled water was prepared by passing it through a mixed

bed ion exchanger and distilled twice in quartz. The sodium

fluoride solutions were prepared by dissolving accurately

known quantities of sodium fluoride in distilled water stored

in polyethylene bottles.

Radioisotope : Fe y

(t-, = 45 days) of specific activity

51,800 mCi/g was obtained from the Isotopes Division of the

Oak Ridge National Laboratory. A stock Fe ^ solution was

diluted from 10.0 ml to 100.0 ml with distilled water.

RQ Measurement of the radioactivity of this stock Fe y solution

m 4 the scintillation counter yielded a value of 2.4 x 10

counts per minute per milliliter.

2. Procedure

Aliquot s (10-ml) of iron (III) nitrate tetrahydrate

159
solution in either perchloric or hydrochloric acid were

added to a series of separatory funnels. To these funnels

were added 2.00 ml of sodium fluoride solution 5 followed

59 by 2.00 ml of potassium thiocyanate and 1.00 ml of Fe ^

radio-reagent. The total volume was 15.0 ml. All concentrations referred to are the final concentrations present in

the separatory funnels. These solutions were shaken for

three minutes in a mechanical shaker with 10.0 ml of isobutanol. The phases were separated and 1.00 ml of each phase

was pipetted into counting tubes and the radioactivity measured with the scintillation counter. The distribution

coefficient was calculated from the relation:

[CPM]org [Volume] aq.

K = —. —— x — — [CPM] aq. [Volume] org.

The pH of all aqueous phases was measured.

5. Results

In Figure kO, K, is plotted versus-log [NaF], These

extractions were made from perchloric acid. In all cases

studied, a retention of Fe y was observed in the aqueous

phase as the fluoride concentration was increased. Conditions were chosen such that the fluoride ion was most effective in lowering the distribution coefficient of ferric

thiocyanate. Curves A and B in figure kO illustrate the

increase in sensitivity of the Fe(SCN) ' complex toward

fluoride as the thiocyanate concentration is decreased from

0.20 M to 0.10 M. The distribution coefficient decreased

i4o
Figure 40.

A - (o)

B - (o)

C - (A)

D - ( 6 )

Distribution coefficient versus negative

log of the concentration of sodium fluoride

expressed in mole liter"!.

Legend for Curves

Fe(lll) = 1.0 x 10" 2 M in 0.25 M HClOi.

SCN" = 0.20 M

~~ -4

pH = 1.7

Fe(lll) = 1.0 x 10" 2 M

SON- = 0.10 M

"~ pH = 1.8

"~

Fe(lll) = 1.0 x 10"^ M in 0.10 M HC10,,

SCN = 0.20 M

"~ ~~ 4

pH = 2.0 ~

Fe(lII) = 1.0 x 10"-5 M in 0.01 M HC10,,

SCN" = 0.10 M

"" "* 4

pH = 2.0 ~

141
from 0.20 M to 0.10 -M. The distribution coefficient decreased

from 7.80 to 6.80 respectively. Curves C and D show the

further shift in the curve toward smaller fluoride concentrations as the iron (II-I) concentration is reduced from

-2 -3

1 x 10 M to 1 x 10 v M. As the iron (III) concentration

-4

is reduced to 1 x 10 M distribution coefficient decreased

markedly as shown in figure 4l. (No further shift of the

curve toward smaller fluoride concentration was noted.

)

In figure 42 is shown an analogous set of curves

obtained when the extractions were made from hydrochloric

acid. Curves B' and D

1 again illustrate the enhanced sensitivity of the Fe(SCN) complex toward fluoride through a ten-fold decrease in the iron (III) concentration. A further

reduction in the iron (III) or thiocyanate concentration

produced no increase in sensitivity ^ but considerably

decreased the distribution coefficient. Comparing these

HC1 curves with those obtained with HCIOl (i.e. B and D)

,

the decrease in the distribution coefficient upon the

addition of fluoride is more pronounced when the extractions

are made from HCIOl. From this series of curves it was

concluded that conditions employed in figure 40^ for curve D., exhibited the greatest sensitivity toward fluoride. The

range of fluoride concentration most sensitive to this system

-4 ^ -3 ^Q

was 2 x 10 M to 6 x 10 ^ M. A plot of percent ¥e^ J retained

in the aqueous phase versus milligrams of sodium fluoride is

shown in figure 43. A linear response was observed for the

142
Kd VS-LOG [NaF]

3.0 3.1 3.2 -LOG [NaF] (Moles/Liter)

Figure 4l. Distribution coefficient versus negative

log of the concentration of sodium fluoride

expressed in mole liter-1.

Legend for curves

E - (o) - Fe(lII) = 1.0 x 10"4 M

SON- = 0.20 M

pH = 3.2

F ~ (A) - Fe(lII) = 1.0 x 10"4 M

SON- = 0.10 M

pH = 3.3

143
8r

10 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 -Log [NaF] (Moles/liter)

3.4 3.6 3.8 4.0 4.2 4.4

Figure k2

B< - (o) -

D« - (A)

Distribution coefficient versus negative

log of the concentration of sodium fluoride

expressed in mole liter~l.

Legend for curves

Fe(lII) = 1.3 x 10~ 2 M in 0.07 M HC1

SCN" = 0.133 M

pH = 1.2

Fe(III) = 1.3 x 10~5 M in 0.007 M HC1

SON- = 0.133 M

pH = 2.2

lkk
Q

UJZ

<

UJ

cr

l-z

UJo

crUJ

0.

65 - % Fe

59 RETAINED VS. [Na F] (mg) ^/o

60

S-"

55

50 -

45

40

35 ^s^

30

25

20 -

15

10

5

\ 1 1 1 1

1 1 1 .200 .400 .600 .800 1.000

[NaF](mg)

1.200 1.400 1.600

Figure 43. Distribution coefficient versus negative

log of the concentration of sodium fluoride

expressed in mole liter"!.

Fe(lll) = 1.0 x 10"^ M in 0.01 M HC10^_

SON" = 0.10 M

pH = 2.0

145
concentration range 0.100 to 1.500 milligrams of sodium

fluoride. The analytical application of this curve was

investigated as a possible method for the determination of

fluoride. It was found that the response of 2.00$ Fe^

retained in the aqueous phase per 0.100 milligram of sodium

fluoride does not merit consideration as an analytical method

for accurate and precise work.

(P. A. Pella and A. R. Landgrebe)

E. Abstract of Publication

The following is an abstract of a manuscript presented

at the IAEA conference on Radiochemical Methods of Analysis.

1. Trace Determination of Cobalt by Substoichiometric

and Radiometric Methods

Landgrebe., A. R. 5 McClendon, L. T., and DeVoe^ J. R. s "Substoichiometric Radiometric Analysis: Determination

of Trace Amounts of Cobalt

3 International Conference

of Radiochemical Methods of Analysis,, Oct. I965 , to be published.

"This analytical method is part of a general program at

the National Bureau of Standards which utilizes radioisotopic

tracers for the analysis of impurities in highly purified

materials. Substoichiometric radiometric analysis is a modification of the isotopic dilution method of Ruzicka and

Stary. In substoichiometric isotopic dilution analysis , equal number of atoms must be isolated from the system^ while

in radiometric analysis the amount isolated need have only a

146
known relation to the initial concentration. In order to

isolate the same amount ^ or an amount proportional to the

initial concentration^ many side reactions such as hydrolytic

reactions of the complexing agent and of the metal ion ; formation of the acid or basic chelate compounds other than

the desired one_, competing side reactions of the complexing

agent with other ions present _, and the reaction of metal ions

with buffering and masking substances must be controlled.

"Two procedures of analysis (column and batch) were

developed using substoichiometric amounts of EDTA. The

excess cobalt ions were removed from solution with a cationic

ion exchanger in the sodium form.

"Experiments were performed to determined the effect of

contact time j pH 5 ionic strength and buffers on the sensitivity of the method. Buffering solutions investigated were

sodium acetate^ ammonia- ammonium chloride^ trisodium phosphate and diethylamine . The method gave reproducible results

with an acetate buffer for the concentration range of 10 u,g/ml

to 0.1 u.g/ml of cobalt. With diethylamine buffer preliminary

results appear to be meaningful at the ng/ml level. The

concentration range investigated using diethylamine buffer

-3 -10

was from 5 x 10 M to 5 x 10 M cobalt using batch experiments in which one gram of Dowex 50-X8., 20-50 mesh was

equilibrated with 10 ml of aqueous solution containing a substoichiometric amount of EDTA but an amount greater than

the total cobalt concentration.

147
"Another application of this method includes the

measurement of specific activities of radioisotopes. Measurement of C0-6O obtained from ORNL has been done. It will be

shown that substoichiometric isotopic dilution analysis is

the limiting case of a more general method which is called,,

for want of a better name, substoichiometric radiometric

analysis .

(A. R. Landgrebe,, L. T. McClendon and J. R. DeVoe)

148
6. MISCELLANEOUS

A. Sealed Source Fabrication

It is often desirable to have radioactive sources of

certain isotopes to obtain spectroscopic data on photopeak

efficiencies, resolution, peak locations, and other performance data on various radiation measuring devices. The

following paragraphs describe a fabrication technique for

such sources. The sources should be mounted on a material

of low atomic number which is durable. They must be thin

yet rigid enough to maintain a reproducible geometry each

time the source is positioned in its holder for counting.

To filfill this requirement a piece of 1/^2" plexiglass,

cut to the desired size, is taken as a base material. An

aluminum disk, l" in diameter and .005" thick, is used as

a backing for the deposited source material. After drying

under an infra-red heat lamp the radioactive area is sprayed

with a clear vinyl lacquer. A piece of adhesive plastic

film, commercially available for the purpose of laminating

identification cards and photographs, is cut to the same

dimensions as the base. The disk is then positioned at the

desired location on the adhesive material. At this time a radiation label or any other identifying label can be added,

taking care to allow a 1/8" border to insure a positive seal.

The adhesive film is then alined with the base and firmly

pressed into place (figure 44). The point of adhesion should

be traced with a smooth, hard object such as the beaded end

149
of a glass stirring rod to insure a tight seal. The edges

can be trimmed as needed to complete the source.

The attenuation of 88 keV X- or gamma-rays by the 1/32"

Plexiglas base is less than 2%. No significant difference

in the backscatter peak has been observed,, due to the 0.005

inch thick aluminum planchet.

TOP VIEW

Adhesive Film

Al. Planchet

Radiation Label

Figure 44. Exploded view of source assembly,

150
Although the finished source , by design, is fairly

rigid, a degree of flexibility remains so that under normal

handling conditions it will not be damaged. The labels are

located within the sandwich and will remain legible for the

life of the source. It is expected that radiation damage

form beta and gamma sources normally used in spectrometer

calibration will be negligible over long periods of time.

The shelf life of these sources is not known, but

137 several microcurie sources of Cs have been used in our

laboratory for six months with no apparent degradation.

The fabrication time of the source is approximately

five minutes, exclusive of the evaporation time of the

solution, and the material cost is negligible.

(F. A. Lundgren and G. W. Smith)

151
7. PERSONNEL

Radiochemical Analysis 3 J. R. DeVoe, Section

Activation Analysis

D. A. Becker

B. S. Carpenter

F. A. Lundgren

W. P. Reed

G. L. Rhinehart

G. W. Smith

G . J . Lut z J. M. English - Terminated - Summer student

R. G. Keenan - Terminated - Guest worker from U.S

Public Health Service

Cincinnati, Ohio

J. H. Marcus - Terminated

K. H. Miller - Terminated - Summer student

Instrumentation

W. N. Bettum

R. W. Shideler

A. J. Gerber - Terminated

M<3ssbauer Effect

W. L. O'Neal

F. C. Ruegg

D. K. Snediker

J. J. Spijkerman

L. A. Lynch - Terminated - Summer student

Radioisotope Tracer Techniques

T. E. Gills

A. R. Landgrebe

L. T. McClendon

P. A. Pella

152
8 . ACKNOWLEDGMENTS

The Radiochemical Analysis Section wishes to thank

Dr. J. Elliot,, Mr. E. Nowstrop and Mr. L. Harris of the

Naval Research Laboratory Reactor Facility for making the

irradiation of samples possible. We also wish to acknowledge the assistance of the NBS Health Physics staff who

have helped us with the safe handling of these samples.

We also wish to express our thanks to Dr. CO. Muehlhause,

Dr. R. S. Carter , and Dr. H. H. Landon 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, Dr. S. Penner and Mr. 0. Spokas of the

Radiation Physics Division for arrangements to use the LINAC

facilities is acknowledged. We also wish to acknowledge the

assistance of Mr. Goetz, Chief _, as well as Mr. H. J. Rouse

and Mr. T. B. McKneely, Jr.,. of the Plant Division, NBS.

The section acknowledges Mrs. R. S. Maddock for her assistance in preparing this report.

Special appreciation is expressed to Mrs. M. Fenstermaker

who typed the entire report and whose tireless efforts as

section secretary has made our entire operation more

efficient and effective.

153
9 . REFERENCES

[I] Cumming, J. B., Applications of Computers to Nuclear and

Radiochemistry, ^'CLSQ, The Brookhaven Decay Curve

Analysis Program", National Academy of Sciences

NAS-NS 3107, October 1962.

[2] Gardner, D. G. , Gardner, J. C., Laush, G. , and

Meinke, W. W. , "Analysis of Multicomponent Decay

Curves by Use of Fourier Transforms", J. Chem.

Phys. 31, 978 (1959).

[3] Watt, B. E., Reactor Handbook, Physics, USAEC, McGrawHill Co., New York, 1955- p. 621.

[4] Isenhour, T. L. and Morrison, G. H. , Anal. Chem. 36, IO89 (1964).

—'

[5] Symposium on Applications of Computers to Nuclear and

Radiochemistry, National Academy of Sciences

NAS-NS 3107, March I963.

[6] Nicholson, W. L. , Schlosser, J. E., and Brauer, F. P.,

Nuclear Instruments and Methods 25, 45 (1963).

[7] Connally, R. E., Rev. Sci. Inst. 24, 458 (1953).

[8] Perkin, R. W. , "Selective and Sensitive Analysis of

Activation Products by Multidimensional Gamma Ray

Spectrometry", Proceedings of International Conference

"Modern Trends in Activation Analysis" Symposium,

College Station, Texas, April I965, to be published.

[9] Goldanski, V. I., The Mossbauer Effect and its

Applications in Chemistry , Consultants Bureau,

New York, 19b4. ~

[10] Wertheim, G. K. , MtBssbauer Effect - Principles and

Applications , Academic Press, New York, 1964. "

[II] Danon, J., M8ssbauer Effect and Chemical Bonding in

Transition Metal Complexes , Proceedings of the Panel

on the Use of the Mtissbauer Effect in Chemistry and

Solid State Physics, International Atomic Energy

Agency, Vienna, April 1965

[12] Herber, R. H., J. Chem. Ed. 42, 180 (I965).

[13] Christ, B. ¥. , Spijkerman, J. J., Giles, P. M. , and

Ruegg, F. C, ^'Retained Austenite in 10105 Steel

Detected by Mossbauer Spectroscopy", Letter to

editor, Acta Metallurgica, to be published.

154
[14] Spirf.kerman, J. J., Ruegg, F. C., and DeVoe, J. R.,

"Standardization of the Differential Chemical Shift

for Pe57"j Mossbauer Effect Methodology, Vol. 1,

Plenum Press, New York, to be published.

[15] Ruegg, F. C.., Spijkerman, J. J., DeVoe, J. R. , Rev.

Sci. Inst. 36, 356 (1965).

[16] Morokuma, T., Oyo Buturi ]>1, 192 (1962).

[17] Hara, K. , Smith, D. S., Rev. Sci. Inst. 30, 8 (1959)

[18] Peck, E. R., Obetz, W. S., J. Opt. Soc . Am. 43, 6 (1953).

[19] Saunders, J. B., J. Res. N. B. S. 58, 21 (1957).

[20] Herber, R. H. , Private Communication.

[21] Herber, R. H. , and Spijkerman, J. J., "Mttssbauer Line

Broadening in Sn02

, J. Chem. Phys . 42, 4312, (1965).

[22] Flinn, P. A. and Ruby, S. L. , Rev. Mod. Phys. 36, 352

(1964).

[23] Ruzicka, J. and Stary, J., Talanta, 8, 228 (1961).

[24] Ruzicka, J. and Stary, J., Talanta, 8, 296 (1961).

[25] Ruzicka, J. and Stary, J., Talanta, 8, 535 (I96I).

[26] Ruzicka, J. and Stary, J., Talanta, 8, 775 (I96I).

[27] Connally, R. E., and Scott, F. A., "Hanford Controlled

Potential Coulometer", AEC Report HW-65919 (i960).

[28] Hevesy, G. and Hobbie, R.

,

Z. Analyt. Chem. 88, 1 (1932)

[29] Ruzicka, J. and Stary, J., Talanta 9, 6 17 (1962).

[30] Ruzicka, J. and Stary, J». , Talanta 9, 905 (1963).

[31] Ruzicka, J. and Stary, J., Talanta 9, 697 (1964)

[32] Morrison, G. H., Freiser, H. , "Solvent Extraction in

Analytical Chemistry", John Wiley & Sons, Inc.,

New York, 1957 . pp. 135-137.

[33] Babko, A. K. , Kleiner, K. E., J. Gen. Chem. (U.S.S.R.)

17, 1259 (19^-7).

155




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