ED STATES WENT OF MERGE

ICATION

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Library, E-01 Admin. Bldg.

FEB 17 1971

NBS TECHNICAL NOTE 541

IrisO''"

Preliminary Study on the

Characteristics and Design

Parameters for a Mbssbauer

Resonant Detector

U.S.

4RTMENT

OF

MMERCE

National

Bureau

of

andards

•.

«. BUREAU OF STANSMKS

1 r

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

[/ f S NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

'/

» o '

>

NBS TECHNICAL NOTE 541

ISSUED JANUARY 1971

Nat. Bur. Stand. (U.S.), Tech. Note 541, 65 pages (Jan. 1971)

CODEN: NBTNA

Preliminary Study on the Characteristics

and Design Parameters for a Mossbauer

Resonant Detector

J. J. Spijkerman, J. C. Travis,

P. A. Pella, and J. R. DeVoe

Radiochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

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FOREWORD

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

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

Materials Research in the February 1, 1964, reorganization.

It consists at present of nine sections and about 100 technical personnel encompassing some 60 different analytical

competences from activation analysis and atomic absorption to

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

in turn the sections which they comprise, are charged with

research at the forefront of analysis as well as awareness of

the practical sample, be it standard reference material or

service analysis. In addition it is their responsibility to

inform others of their efforts.

Formal publication in scientific periodicals is a highly

important output of our laboratories. In addition, however,

it has been our experience that informal, annual summaries of

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

is perhaps in order about the philosophy of these yearly

j progress reports. In any research program a large amount of

: information is obtained and techniques developed which never

find their way into the literature. This includes the

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

importance also are the numerous small items which are often

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

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

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

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

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

journal article.

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

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

appears will not be treated fully in regular publications are

considered in some detail here. Other results which are

being written up for publication in the journal literature

are covered in a much more abbreviated form.

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

In 1970 we plan to issue these summaries for all of our

sections. The following is the seventh annual report on

progress of the Radiochemical Analysis Section.

1

:

W. Wayne Meinke, Chief

Analytical Chemistry Division

IV
PREFACE

This progress report serves the dual purpose of providing

the status of a very important project involving detector

design for Mossbauer spectroscopy and the progress of the

Radiochemical Analysis Section for the year. Indeed as a result of cutbacks during the past eighteen months, the total

effort of the section has centered around this work and that

of the certification of Standard Reference Materials for

Mossbauer spectroscopy. A report on the Standards will be

forthcoming

.

Significant reorientation of the activities of this

section is imminent, and the section's new direction will be

reported next year.

This study has been supported in part by the Division of

Isotope Development. U. S. Atomic Energy Commission,

Washington, D. C. under contract AT-49-2-1165

•

James R. DeVoe , Chief

Radiochemical Analysis Section

v

TABLE OP CONTENTS

PAGE

1. INTRODUCTION 1

2. DETECTION METHODS FOR MOSSBAUER SPECTROSCOPY .... 6

A. Conventional Transmission Method 6

B. Conventional Scattering 6

C. Scattering with Detection of Conversion X-Rays . 8

D. Scattering with Detection of Conversion Electrons 10

E. Transmission Using a Resonant Detector 13

P. Scattering Using a Resonant Detector 13

3. THEORY 1^

A. Internal Conversion Electrons 14

B. Cross Sections 14

C. Efficiency 18

D. Signal-to-background Ratio 28

E. Line Width 33

4. PRELIMINARY MEASUREMENTS 36

119

A. Resonant Detector for Sn Using the Gel Scintillation Technique 36

B. Resonant Detector for 57pe Using a Conversion

Electron Proportional Counter 40

5. SUMMARY 50

6. REFERENCES 51

APPENDIX A: TO COMPUTE RESONANT DETECTOR EFFICIENCY

AND SIGNAL BACKGROUND RATIO AS A FUNCTION

OF ASSORTED INPUT PARAMETERS 53

APPENDIX B: TRIPLE CONVOLUTION OF SOURCE, ABSORBER

AND DETECTOR LORENTZIAN PROFILES 55

FIGURE NO.

LIST OF FIGURES

1. Energy resolution of the various types of

processes associated with the 14.4 keV gamma

radiation of cobalt-57

VII
PAGE

2. a) Schematic of internal conversion process

for the 14.4 keV transition of 57q . b) Energy

degradation process for a source, absorber and

detector system in a conventional Mossbauer

detection system 4

3. Detection methods for Mossbauer spectroscopy,

a) Conventional transmission. b) Conventional

scattering. c) Conversion x-ray scattering,

d) Conversion electron scattering. e) Transmission, using a resonant detector.

f) Scattering, using a resonant detector .... 7

4. Attenuation coefficient for 14.4 keV gamma

radiation as a function of element [19] .... 9

5. Number of 6.3 keV internal conversion x-rays

from Fe as a function of the penetration into

the sample for backscattering geometry 10

6. X-ray detection Mossbauer scattering spectra

for iron foil on stainless steel foil 11

7. Conversion-electron detection Mossbauer backscattering spectra for vacuum-deposited iron on

stainless steel foil 12

8. Transmission of parallel electrons through

FeBe^ 20

9. a) Normal geometry for electron attenuation

measurements, b) Electron generation and

attenuation geometry for resonant detector with

gamma flux incident from the right 22

10. Relationship of effective thickness "seen" by

isotopic electron source to actual thickness as

"seen" by parallel electron source 23

11. Transmission of electrons from isotropic electron

source at a specified depth in an infinitely

thick FeBe,- foil 24

12. a) Signal-to-baseline (background). b) Detector

efficiency as a function of iron-57 abundance

for a ten foil detector with a total thickness

of two resonant half values, assuming f = fn

= .95 ! • . . • 26

vm
PAGE

13. a) Signal-to-baseline (background) ratio,

b) Detector efficiency as a function of the

number of foils for a total thickness of

half value layers (3,716 A) of iron-57

enriched FeBe^ 27

5

14. a) Signal-to-baseline (background) ratio,

b) Detector efficiency as a function of total

thickness of foil(s) for 97 percent iron-57

abundance. One resonant half value layer = 1,858 A, one photoelectric hvl = 239 resonant

hvls. n = number of foils 29

15. Detector efficiency as a function of total

thickness of foil(s) for 50 percent iron-57

abundance. One resonant half value layer

(hvl) = 3,604 A; one photoelectric hvl = 123

resonant hvls. n = number of foils 30

16. a) and b) Detector efficiency as a function of

total thickness of resonant foils, expressed

in resonant half value layers, for 3 percent

iron-57 abundance. One resonant half value

layer = 60,069 A; one photoelectric hvl = 7.h

resonant hvls. n = number of foils 31

17. a) Line profile for source and absorber using

a conventional detector. b) Line profile for

a moving absorber between a stationary source

and resonant detector 3^

18. Pulse-height spectrum of tritium loaded gel.

Endpoint is at 18.1 keV 37

119

19. Mossbauer spectrum of SnO

? dispersed in

scintillation gel and moving Pd l-'-^msn source.

Peak is barely visible at -2.2 mm/s 38

20. Pulse-height spectrum of gel scintillation

counter. Upper spectra: H.V. = 2000 volts,

upper curve with, lower curve without source.

Lower spectra: same, but H.V. = 1200 volts . . 38

21. Coincidence arrangement for gel scintillation

detector. Photomultiplier tubes are shielded

from direct gamma radiation 39

119 22. Mossbauer spectrum ofin „ SnO„ dispersed in scintillation gel and Pd~ J Sn source, using

coincidence noise suppression 40

ix
PAGE

23. Isometric view of proportional counter with

top plate removed 4l

24. Pulse-height spectra for Co-57 Mossbauer

source as a function of position on 3 inch x

3 inch window 42

25. Photograph of detector for conversion electrons 44

26. Diagram of detector for conversion electrons . . 44

55 27. Pulse-height spectra of a Fe (5.5 keV endpoint) and 57co (7.3 keV endpoint) source

placed inside the detector 45

28. Mossbauer spectrum of the resonance detector

with a 0.5 mil 50 percent enriched stainless

steel foil and moving 57co in stainless steel

source 46

29. Energy distribution of photoelectrons and

conversion electrons obtained by source-detector

off resonance (a) and in resonance (b) 47

30. Energy distribution of the conversion electrons

from a 0.5 mil 50 percent enriched stainless

steel foil 48

31. Differential signal/noise of the electrons

generated in the stainless steel resonant

detector. Optimum selection would be from 2

to 6 keV 49

LIST OF TABLES

LIST OF TABLES

1. Photoelectric cross sections (barns) 16

2. Compton cross sections (barns) 16
PRELIMINARY STUDY ON THE CHARACTERISTICS AND DESIGN

PARAMETERS FOR A MOSSBAUER RESONANT DETECTOR

J. J. Spijkerman, J. C. Travis, P. A. Pella

and J . R. DeVoe

Progress in the design and fabrication of a

resonant detector for Mossbauer Spectroscopy is

described. This report begins with a review of

all of the methods of detection for this spectroscopy and describes the expected advantages of

the resonant detector. If one uses conversion

electron detection, considerable enhancement in

signal to noise ratio and decrease In linewidth

may be realized. Efforts to produce an iron

bearing resonant material are described.

Key words: Conversion electrons; iron;

Mossbauer Spectroscopy;

resonant detector.

1. INTRODUCTION

Conventional type Mossbauer spectrometers use low energy

gamma-ray detection systems . Although these counters have a high counting efficiency, their resolution is very poor, typically about 1 keV, compared to the energy width of 10 eV for

the Mossbauer radiation. Since Mossbauer sources are not monochromatic, the resolution of the counter determines the amount

of unwanted background radiation. Figure 1 shows a typical

57 spectrum of a Co source near the lH.k keV Mossbauer radi-

_ o ation which consists of three major components; a) the 10

_:> eV wide recoil free radiation, b) the 2x10 eV wide recoil

broadened radiation and c) a wide band of Compton and photoelectric effect radiation (degradation products) arising from

the 122 keV precursor radiation. Most of the Compton background is generated by the counting gas and walls of the

proportional detector or by the scintillating material in a scintillation detector.
f\J

I0~ 8 eV Mossbauer

Radiation

r*j

15.4 keV

Figure 1. Energy resolution of the various types of processes

associated with the 14.4 keV gamma radiation of

cobalt-57
Much of the background radiation in Mossbauer spectroscopy

can be suppressed by using a resonant detector. The possibility

of using a resonant detector was first discussed at the Allerton

Park Mossbauer Conference and several Russian papers on this

type of detector have been published [1-9]. The principle of

its operation is based upon the detection of internal conversion

electrons emitted by resonance-excited nuclei in a detector.

Since this detector makes use of the Mossbauer effect, its

— R resolution is near 10 eV. The background in this type of

detector arises from secondary photoelectrons . The process of

decay of the 14.4 keV excited nuclear level in iron-57 is shown

schematically in Figure 2. For every 100 nuclei in the 14 .

4

keV excited state ten nuclei decay by emitting the 14 . 4 keV

gamma-ray, and the other 90 nuclei decay by the internal

conversion process (e.g. the internal conversion coefficient

is 9). This means that a K shell electron is emitted with

approximately 7-3 keV energy. An electron from the L shell

enters the K shell and emits the characteristic 6.45 keV x-ray.

Of these 90 - 6.4 keV x-rays, approximately 63 cause 5.6 keV

electrons to be emitted by the Auger effect, and the remaining

27 - 6.4 keV x-rays leave the atom. M to L cascade produces

< 1 keV x-rays. Internal conversion with the L shell occurs,

but with significantly less probability. Because of the fact

that out of 90 - 7-3 keV conversion electrons emitted there are

simultaneously 63 - 5.6 keV Auger electrons, the probability for

electron detection is enhanced.

The resonant counter can be operated in two modes; a) by

placing the material to be analyzed inside the detector, b) by

using a stationary source and resonant detector and moving an

absorber placed between the source and detector. The first

2 method has the advantage that very small samples (10 ug/cm of

iron) can be used, which in most cases is a surface measurement.

Because of the small sample, self-absorption broadening is

absent and the spectrum shows very narrow lines predicted by

the thin absorber limit. Because of the small sample, the

3
7.3 keV e-

; 5.6 keV e6.3 keV x—ray

b)

Compton

Photo-elect.

Compton

Photo-elect.

14.4 keV Y

Filter

Figure 2. a) Schematic of internal conversion process for the

14.4 keV transition of 57c . b) Energy degradation

process for a source, absorber and detector system

in a conventional Mossbauer detection system.

4
efficiency is poor, which is partially compensated by the high

signal/noise ratio, so that the data accumulation time may

approximate that obtained for conventional detectors. The

(second method has many advantages, but the detector is difficult

j to fabricate. To obtain maximum efficiency and resolution,

the source and absorber must be in resonance at zero relative

velocity, with a very narrow resonance profile. Furthermore,

the counter material must be of low Z to reduce the Compton

background. The efficiency can be increased by using enriched

57Fe and several converter plates. The resolution of the

second method is greatly improved, due to the selective absorp1 tion of the counter. For a thin absorber, the line width is

reduced by 26 percent. Since the Mossbauer cross section is

200 times the photoelectric cross section, a large reduction

in data accumulation time is expected.
2. DETECTION METHODS FOR MOSSBAUER SPECTROSCOPY

For comparison j the different methods used for detection

of the Mossbauer effect will be described.

A. Conventional Transmission Method

The conventional technique, shown in Figure 3a uses a proportional, scintillation or solid state detector. The proportional and scintillation counters have a very high counting

efficiency, and can be used at high counting rates, but have

poor energy resolution. The solid state detector has better

energy resolution, but the counting rate is much lower, hence

the data accumulation time is longer. The Compton and photoelectric effect background accounts for the large background,

B . If a very thin sample is used, most of the radiation will

pass through the sample; and the baseline B will increase, and

the effect will decrease. For a thick sample, the atomic

absorption by the sample will attenuate the Mossbauer radiation,

but not the higher energy radiation present, and the Compton

and photoelectric effect background will increase. Shimony [10]

and Protop and Nistor [11] have derived the optimum conditions

for the sample thickness with background radiation.

B

Conventional Scattering

The resonantly scattered 14.4 keV gamma ray can be detected by the conventional low energy gamma-ray detector. Because

of internal conversion, the efficiency for detecting the Mossbauer effect is low, and the background is significant due to

the Compton and photoelectric effect in the absorber produced

by the 122 keV precursor gamma ray.

The geometrical arrangement is very important for overall

efficiency. A recently developed toroidal counter greatly increases the geometrical efficiency [12]. Shielding of the

source is also essential to reduce the background, and depleted

uranium gives the best total attenuation for shielding. Pene-
a)

Y

-r- Vo

B Bb

A

Vo

C)

D

b Bx

A

Vo

d)

Bb A

Vo

e)

<-v-»

Bb

Vo

f)

Figure 3« Detection methods for Mossbauer spectroscopy,

a) Conventional transmission. b) Conventional

scattering. c) Conversion x-ray scattering,

d) Conversion electron scattering. e) Transmission, using a resonant detector,

f) Scattering, using a resonant detector.

7
tration depth into the sample is about .5 mils* [13]. The

observed line width is broadened due to self-absorption in the

sample of the reemitted Mossbauer radiation.

C . Scattering with Detection of Conversion X-Rays

The low detection efficiency of the conventional scattering method can be improved by detection of the conversion x-rays

Since the K conversion is 90 percent with about a 30 percent

fluorescent yields a 270 percent increase in efficiency is

observed. For iron, the attenuation of the 6.3 keV x-ray

radiation by the sample is the same as that of the 14 . 4 keV,

due to the K absorption edge at 7-1 keV. Mass absorption

coefficients as a function of element are given in Figure 4.

The use of x-ray detection also eliminates the possibility of

detecting reabsorbed resonantly scattered Mossbauer radiation

by the sample, and a narrower linewidth is observed. The

penetration depth in the sample has been calculated by [14],

and the amplitude of the backscattered radiation as a function

of thickness is shown in Figure 5.

To determine the sample depth that enters into an iron-57

Mossbauer scattering spectrum when 6.3 keV x-rays are being

counted, two spectra were obtained for a sample consisting of

a stainless steel foil 0.001 inch thick covered with iron foil.

For one spectrum the thickness of iron foil was 0.5 mil. The

Mossbauer scattering spectra are shown in Figure 6. Argon

with 10 percent methane was used for a flow gas, to reduce the

Compton effect in the detector. As seen in Figure 6, the

central peak, which is due to the underlying stainless steel

foil, decreases in relative intensity as the thickness of iron

covering the stainless steel is increased. The calculated

areas beneath the iron and stainless steel peaks show that 78

percent of the backscattered signal is from the first 0.2 mil

SI system of units requires conversion to meters (multiply by

2.5^ x 10~5). This conversion should be made at other similar

entries in this report

8
Figure 4. Attenuation coefficient for 14.4 keV gamma radiation

as a function of element [19].

of the sample depth and that 93 percent of the backscattered

signal originates in the first 0.5 mil of sample depth. These

figures are in general agreement with those predicted by Terrell

and Spijkerman [14]. For all practical purposes, an iron-57

Mossbauer backscattering spectrum obtained by detecting the

9
y(cm)

Figure 5. Number of 6.3 keV internal conversion x-rays from

Pe as a function of the penetration into the sample

for backscattering geometry.

6.3 keV x-rays resulting from internal conversion of the ik.k

keV Mossbauer gamma rays represents a layer of sample approximately 0.5 mil thick. A major background contribution is the

6.3 keV x-ray fluorescence in the absorber resulting from

the 122 keV precursor, as shown in Figure 3c.

D. Scattering with Detection of Conversion Electrons

The third method for the detection of the scattered Mossbauer radiation is by conversion electron detection [15] « It

can be seen from Figure 2 that this conversion process is 90

percent efficient, resulting in 7-3 keV electrons followed

70 percent of the time by 5-6 keV Auger electrons.

The range of these conversion electrons in solids is very

short which limits the sample depth that is effective in producing a Mossbauer spectrum. To determine the sample depth, two

10
>-ZLUI—z

UJ>

I—<

LU

108.0

100.0

Ay

STAINLESS STEEL

PEAK

-z.

<• (a) 0.2 MIL IRON FOIL

106.0

jL /A .B ° <B>i£ 1 / V

100.0 - <$/

(b) 0.5 MIL IRON FOIL

SOURCE VELOCITY, MM/SEC —>-

Figure 6. X-ray detection Mossbauer scattering spectra for

iron foil on stainless steel foil.

one-mil-thick stainless steel foils were covered with 600 A

and 3000 A respectively of vacuum deposited iron and placed

inside the detector. Helium-10 percent CH^. flow gas was used to

detect the conversion electrons and discriminate against x-ray

and gamma-ray detection. The two spectra are shown in Figure 7-

The calculated areas for the iron peaks and stainless steel

peak show that 65 percent of the intensity originates within

the first 600 \ and 96 percent within the first 3000 A. The

11
102.0

t

z

>

<

STAINLESS STEEL

PEAK

600 A IRON

100.0

103.2

100.0

3000 A IRON

-6.4 —3.2 0.0 3.2

SOURCE VELOCITY, MM/SEC —

6.4

Figure 7 Conversion-electron detection Mossbauer backscattering spectra for vacuum-deposited iron on

stainless steel foil.

geometrical efficiency for this detection method is nearly 50

percent which makes this method ideal for surface analysis of

iron bearing materials.

12
E. Transmission Using a Resonant Detector

The conversion electron detector described in D can be

used for Mossbauer transmission measurements [16], as shown in

Figure 3e. While in transmission geometry using a conventional

detector, the movement of the source or absorber is entirely

equivalent, this is not the case for a resonant detector. The

source and the conversion plate (absorber) must be in resonance

at zero relative velocity, and a moving absorber placed between

the stationary source and the detector is used. To increase

the detection efficiency of the counter, several iron-57

enriched converter plates should be used. The extremely high

energy resolution of the detector greatly reduces the background, and the selective absorption by the detector reduces

the observed linewidth by 27 percent compared to a nonselective

(conventional) detector [8]. The background is caused by the

photoelectric effect, and therefore, the detector's geometrical

design and selection of materials must be carefully considered

to minimize this effect. Since the background is greatly

reduced, the detector can be used with much stronger sources

before counting rate saturation occurs. Such saturation is

a serious limitation with the conventional detector.

F. S cattering Using a Resonant Detector

The same arguments considered in the previous section (E)

hold for this case. Since it is desirable to analyze bulk material nondestructively , the source and detector have to be moved

in opposite directions in order to obtain zero relative velocity

The small physical dimensions of this detector present no

difficulty with the available Mossbauer spectrometers. Sample

penetration depth is the same as discussed in Section B (0.5

mils) . With the low background of this detector, the improved

Mossbauer spectral resolution, and the possibility of using

much stronger sources, the data accumulation times can be

greatly reduced.

13
3. THEORY

AInternal Conversion Electrons

The ideal detector for Mossbauer spectroscopy would be one

which generates a pulse for every incoming recoil-free photon

from the source, but does not respond to any other incoming

radiation. In principal, the best discrimination between

recoil-free photons and other radiation is provided by an

absorber in resonance with the source such that Incoming recoilfree photons are signalled by resonance absorption events . The

principal problem encountered in incorporating this principal

into a detector, however, is the requirement that resonant

absorption events in the detector, and only such events, be

translated into an electronic signal suitable for counting.

In iron-57j every one hundred resonant absorption events

are followed in 10 ' seconds by a distribution of reemitted

radiation consisting roughly of ten 14 . 4 keV gamma photons,

ninety 7^3 keV internal conversion electrons, twenty seven

6.3 keV x-rays, sixty three 5*5 keV Auger electrons, and a cascade of low energy x-rays and Auger electrons. Electron

counting to signal the occurrance of resonant absorption events

obviously yields the highest efficiency, or fraction of incoming

signal detected, by virtue of the numbers available. In addition, a gas filled counter may be built with a high cross

section for electrons, but very low cross section for incoming

gamma and x-radiation, minimizing the background from external

sources. Discounting electronic noise, the primary source of

unwanted counts would be photoelectrons and Compton electrons

created inside the detector by incoming radiation. With appropriate detector optimization, such background may be kept to a small fraction of the signal.

B

Cross Sections

Theoretical estimates of detector discrimination and efficiency, for optimization purposes, require consideration of the

processes which produce electrons and the probability of the

14
electrons escaping from the foil to be detected by the gas.

For radiation of the energy range of interest to us, the only

significant contributions to electron production are Compton,

photoelectric, and resonant interactions. For the Compton and

2 photoelectric effects, the number of electrons produced per cm

in a layer of thickness At at depth t, say AN and AN , may be

c p

expressed as

ANn . = \i n . I. (t) At Ci K Ci 1

AN . = y . I. (t) At pi pi l

tot = y» AN ANtot = V AND . C *r-> Cl 5 p

^

Pi

(1)

{E

±

\ = {14.4, 122, 1361

2 where I.(t) is the number of photons per cm of energy E. incident on the layer of depth t, and y p

. and y . are the energy

dependent Compton and photoelectric linear attenuation coefficients. For an intermetallic foil, the linear attenuation

coefficients may be obtained from the cross sections characteristic of the various metals included,

*» = L n

i

a

i (2)

i

2 where n is the number of atoms/cm and a. is the interaction

-2 x cross section, in cm /atom, of the ith species.

Approximate expressions for the photoelectric and Compton

cross sections as functions of E , Z (absorber foil atomic

number) and E,, the K shell absorption edge, as given by

Berthelot [17], have been used to calculate representative

cross sections for iron and beryllium, as shown in Tables 1

and 2 in units of 1 barn = 10" cm

2

.

15
Table 1. Photoelectric cross sections (barns)

14. ll 122 136

2.69 .0018 0012

26 8450.00 12.2000 8.5

Table 2. Compton cross sections (barns)

"^^-^hv 14.4 122 136

4 2.52 2.18 2.22

26 16.39 14.16 14.46

:

The number of internal conversion electrons produced per

2

cm by Mossbauer interactions in a layer of thickness At at

depth t, ANTrJ is given by

ANT „ = y I (t) At ~-

IC K r r 1+a

a = 9

(3)

where a is the internal conversion coefficient, and where

2

I (t) is the number of recoilless photons per cm reaching

depth t. The resonant linear attenuation coefficient, y . is

given by

o

y r

n

Pe aD

f

D "l+/S\2

c = 2.2 x 10 cm

2 o (4)

r = T + T ~.15 mm/sec

o L) 16
3 where n,-, is the number of iron atoms per cm , a^ is the iron-57

Fe D

isotopic abundance and f is the recoilless fraction of the

detection foil(s), S is the isomer shift between the centers

of the source and detector lines, and F^ and T are the source

and detector halfwidths.

In addition to the "primary" electrons created by the

three absorption processes, the K fluorescence yield of about

.3 for iron means that about 7 Auger electrons are emitted for

every ten ejected K electrons. Thus, for a pure iron detector

foil, the total number of electrons created is given by

1-7 IE AN

C

. + £ ANpl + ANIC

For an intermetallic foil, the numbers of electrons created in

each species would need to be tallied separately to keep proper

account of the different Auger electron yields of the different

metals

.

Equations (1) and (3) account for the number of primary

electrons produced at depth t as long as the total photon flux

at that depth, I., and the resonant portion of that flux,

I (t), are known. Therefore, we must account for photons

removed from the beam over each increment, At, as well as electrons created. Neglecting secondary Compton photons, we may

say that one photon is removed for each primary Compton or

photoelectron produced, and ten resonant photons are removed

for each nine internal conversion electrons created, or

il.(t) = -(^AN„. + AN . + ^-

AN T ~6. ) 1 \ Ci pi a IC 10/

6. *0ifi^0; 6. = 1 if i = 10 ' 10

(5)

where the negative sign denotes a decrease over the interval,

At. The reduction of the resonant photon beam is somewhat more

difficult to describe:

17
1+a

J r

(t)

/ A I (t) = - ±^- AN T ~ + T i

, c /AN„ + AN * (6) r a IC I (t) ^ Co poy ^ u '

The second term on the right implies that the fraction of

non-resonantly removed photons which is taken from the resonant

photon supply is equal to the fraction of resonant photons in

the incident beam. It may be seen from (5) and (6) that,

althoughto at depth J^ zero I ^(0)/I ' (0) - f , the source recoilless

o s fraction, the fraction of resonant photons in the E beam,

I (t)/I (t), changes with increasing t. This is a significant

point in optimization of efficiency and selectivity.

For i ? in (5), i.e. for gamma-ray energies other than

the Mossbauer line, such as the 122 and 136 keV lines for

cobalt-57, (5) may be written

AI. = -(ANC1 + ANpl ) = -I.(t) (p cl

+ U

p

.jAt (7)

dl. = -I.(t) (y cl + u pl )dt

which may be integrated to yield the familiar exponential

expression,

I

± (t) = I.(o) :(w ci + v)* (8)

For the Mossbauer line, however, the peculiar interplay of

equations (5) and (6) would make any analytical expressions

for I (t) and I (t) exceedingly cumbersome. Thus, for computational purposes, these values are kept track of numerically

by applying the appropriate decrements from (5) and (6) at each

layer

C. Efficiency

The detector effiency, the fraction of incoming resonant

photons detected, is limited by at least two essentially invariant factors, and numerous variable factors. For iron-57, the

internal conversion coefficient of 9 places a limit of 90 per18
cent on the maximum efficiency of a resonant detector relying

on conversion electron counting. A second "invariant" factor

is the need for a substrate to support the thin foils required.

Thus, at least half of the electrons created in even the thinnest of foils would be "lost" into the substrate, for a 50

percent geometrical efficiency limit. The combined limit due

to these invariants is thus about 45 percent. The primary

efficiency limiting factor which is subject to optimization

is the attenuation of the internal conversion and accompanying

Auger electrons in exciting the foil. Obviously, only an

infinite number of infinitesimally thin films in the counter

could reach the 45 percent limit.

In order to set practical limits on the design of the

resonant films, it is advantageous to simulate via computer

the efficiency behavior of a given resonant material as a function of number and thickness of films, Pe-57 abundance,

source-detector shift, and so forth. The calculations on the

following pages relate to the alloy FeBe^, with a density of

3 3.272 g/cm and a molecular weight of 101. The calculations

may be greatly simplified by noting from Tables 1 and 2 that

the iron photoelectric cross section for 14.4 keV radiation

is orders of magnitude larger than any other cross section.

We thus include only this one nonresonant interaction in the

computations as a first approximation.

The two interactions being considered, the resonant one

and the 14.4 keV iron photoelectric one, give rise to four

species of electrons. The primary internal conversion electron

has an initial energy given by the difference between 14.4 keV

and the 7«1 keV K edge for a value of 7.3 keV. The accompanying

x-ray energy is given by the K edge less the .8 keV L edge for

a total of 6.3 keV, such that the Auger electron energy is 6.3

keV less the L edge for a total of 5-5 keV.

The iron K shell photoelectrons from the 14.4 keV radiation

also carry 7*3 keV of energy, and the accompanying Auger electrons carry 5.5 keV.

19
Attenuation curves for 5j 10 , 15, and 20 keV electrons in

copper and gold have been given by Cosslett and Thomas [18].

Expressed in thickness units of g/cm , the copper and gold results are surprisingly similar. Furthermore, these workers

state that the results for aluminum are practically identical

with those for copper so that the attenuation curves are

essentially Z independent. For this reason, we have employed

the copper results to give the approximate attentuation curves

for FeBe^., shown in Figure 8. The curves as shown have been

"specialized" to FeBe^ by expressing the thickness in angstroms,

using the FeBe,- density, but it may be generalized to any alloy

by applying the density ratio to the thickness scale.

o

o

^.'oo li.oo 36.00 45,00 eSToo

THICKNESS (ANGSTROMS)

7$. 00

hict

so.ft. 00

Figure 8. Transmission of parallel electrons through FeBe,-.

5

20
The curves of Figure 8 were generated by fitting Lorentzian curves to several data points obtained from the 5, 10,

and 15 keV data of Cosslett and Thomas [18] by parabolic interpolation. The original data were obtained by impinging parallel

beams of electrons of the various energies on a series of foils

and recording the total fraction of electrons emerging from

the far side of the foil regardless of direction. The distinction between this geometry and that of our experimental arrangement is illustrated in Figure 9, with their geometry shown in

9(a). For our geometry, (9b), we assume as a first approximation that the electrons created at a depth, t, in a foil have

no angular preference (Photoelectrons have a certain bias in

favor of the incident photon direction, but this is not very

significant for lower energies). Measuring the depth, t, from

the "active" face of the foil (side next to the counting gas

as opposed to the substrate) implies that those electrons

emitted in a small increment of solid angle at = (where

is measured with respect to the normal to the face of the

foil) must travel a distance, t, to exit the foil, whereas

those emitted at an angle 0^0 must travel a distance t cos 0.

(Again, this is an approximation since electrons do not travel

straight paths through matter) . Considering the electrons to

have a maximum range R, those electrons traveling in a direction for which > cos" (t/R) will not leave the foil. In

other words, 2tt(1 - t/R) steradians of solid angle will contain

electrons with a possibility of escaping, compared to the 4tt steradian emission geometry, so the fraction (1 - t/R)/2 of

the electrons have a finite probability of escaping.

To determine a realistic attenuation factor for the fraction of electrons which may escape, we assign an effective

depth, t ff , in terms of the true depth, t, and electron range,

R s by finding the expectation value of tcos over the solid

angle discussed above:

21
a) H—t —

b)

\\\V\\\\\\\\

Foir

Substrate

±_

electron

photon

-HN-At

Figure 9- a) Normal geometry for electron attenuation measurements, b) Electron generation and attenuation

geometry for resonant detector with gamma flux incident from the right

22
eff

/

cos

1 (t/R)

t cosG sin0 d0

/

cos

1 (t/R)

sine d6

t ln(R/t)

(1 - t/R)

(9)

where sin 6, the weighting factor, is proportional to the

number of electrons emitted at angle 6, for isotopic emission.

Figure 10 shows the relationship of t ff to t, in units of the

Figure 10

0T2O 0.U0 0.80

THICKNESS/RANGE

0.80 l.oo

Relationship of effective thickness "seen" by

isotopic electron source to actual thickness as

"seen" by parallel electron source.

23
electron range. As might be expected, the two may be seen to

be equal for the end points, t = and t = R. If L, (t) and

Lp(t) are the Lorentzian descriptions of parallel electron

attenuation, as shown in Figure 8 for the geometry of Figure

9(a), then electron attenuation curves for the geometry of 9(b)

may be expressed approximately by

L^(t) = .5(1 - |) L

±

(t eff ) = .5(1 - |) L.(t ln(|)/(l-|) (10)

The curves L! and L' are shown in Figure 11 for FeBe,-

i 2

to 5

35.00

DEPTH

ui.oo

(ANGSTROMS)

90.00

Figure 11 Transmission of electrons from isotropic electron

source at a specified depth in an infinitely thick

FeBe c foil.

5

24
The various equations presented above for photon attenuation, electron production, and electron attenuation, have been

incorporated into the computer program listed in Appendix .A. This program computes the efficiency and signal-to-baseline

(or background) ratio as a function of foil thickness, number

of foils, degree of source-resonant material overlay, source

and detector recoilless fractions, and detector iron-57 isotopic abundance.

The behavior of the detector efficiency as a function of

the various design parameters may best be illustrated by an

arbitrary selection of sample calculations shown in the following figures. The principal obstacle to high efficiency is the

limited electron range. For 97 percent iron-57 enrichment,

the resonant half-value-layer (half-thickness) in FeBe,- is

o 5

1,858 A, such that fully half of the "Mossbauer" electrons

created in a single foil detector are created at depths greater

than this amount. From Figure 11 it is obvious that electrons

created at such depths are severely attenuated. We would like

to bring the electrons created in the foil as close to the

counting surface as possible. This may be done by (1) enriching the foil in iron-57 and thus decreasing the resonant half

value layer, or (2) segmenting the foil into several thinner

ones so that the total foil thickness traversed by the photon

flux is no longer the depth from which the electrons must

escape. Since, as we have seen, even 97 percent enrichment is

inadequate, a combination of the two methods is necessary.

Figure 12(b) shows the improvement as a two half value layer

97 percent enriched foil of two half value layer thickness is

subdivided into an increasing number of foils. Since the

efficiency is an ever-increasing number of foils, the limitation is a technological one because of the difficulty in

fabricating extremely thin layers. In addition, the signal-tobackground ratio described below must also be considered.

25
(a)

'B.QO 0.2Q Q.40 0.80

FE-57 ABUNDANCE

0.80 1.00

(b)

'B.OO

Figure 12 .

FE-57 ABUNDANCE

a) Signal-to-baseline (background) . b) Detector

efficiency as a function of iron-57 abundance for

ten foil detector with a total thickness of two

resonant half values, assuming f = f~ ° s D

26

= .95.
(a)

c*

n

CD .

tnQ. o

+ + + + + + + + + + + + + «- 18.00 u.'oa aToo 12.00 T. 18.00 r 20.00

o

«no ..*

>- +

a +

ty +

(b)

EFFICIE

<g.oa

o.io

a.20

+

+

+

+

00 Ui00 fl.'QO t£.0Q 16.00 20

Figure 13.

NO. OF FOILS

a) Signal-to-baseline (background) ratio.

b) Detector efficiency as a function of the number

of foils for a total thickness of half value layers

(3,716 A) of iron-57 enriched FeBe,-.

27
Figures 1Mb), 15, and 16 show studies of the efficiency

of 1, 3, 5, and 7 foil detectors for 97 percent, 50 percent,

and 3 percent enrichment as a function of total foil thickness.

An interesting feature shared by all of these figures is that

the efficiency monotonically increases with increasing thickness for a single foil detector, but reaches a maximum at a finite thickness for detectors having more than one foil. For

a single foil, the monotonic increase is explained by the fact

that more recoilless photons are converted to electrons as the

foil thickness is increased; the rate of increase, however,

decreases because the additional electrons emitted due to increased thickness are less likely to escape from the foil and

be counted. For multi-foil counters, the efficiency initially

rises faster with thickness than for the single foil counter

because electrons created in the second through the last foils

are closer to the surface than they would have been for the

same traversed thickness in a single foil. As the thickness

increases, however, more and more photons are stopped in the

first foil, reducing the opportunity of the succeeding foils

to contribute to electron production. Thus, in the limit of

infinite thickness, all of the curves must converge to the same

limit as the single foil curve, since at infinite thickness,

all of the photons are absorbed in the first foil.

D. Signal-to-background Ratio

Although the attainment of a high efficiency is important

for the most economical source utilization, in principal, the

real selling point for the resonant detector, is its ability

to respond selectively to the desired radiation. This selectivity can be measured by a signal-to-noise ratio, which we

prefer to call a signal-to-background or signal-to-baseline

ratio to avoid confusion with the Mossbauer spectroscopist 's

normal utilization of "noise" as descriptive of the statistical

scatter associated with random radiative processes . The ratio

of the signal (desired counts) to the background (undesired
a

o

-o.oo I. 00 2.00 3.00 4.00

TOTAL THICKNESS CHVL3

5.00

^.00

Figure 14.

una iToo 3^00 y.ao

TOTAL THICKNESS (HVL)

5. OS

a) Signal-to-baseline (background) ratio.

b) Detector efficiency as a function of total thickness of foil(s) for 97 percent iron-57 abundance.

One resonant half value layer = 1,858 A, one photoelectric hvl = 239 resonant hvls . n = number of foils

29
'B.OO 1.00 2.00 3.00 q.00

TOTAL THICKNESS (HVL)

5.00

Figure 15. Detector efficiency as a function of total thickness of foil(s) for 50 percent iron-57 abundance.

One resonant half value layer (hvl) = 3,604 A; one

photoelectric hvl = 123 resonant hvls . n = number

of foils.

counts) may be observed by moving the source relative to the

detector, thereby taking a Mossbauer spectrum of the detector,

such as that shown in Figure 28. The height of the baseline

of the spectrum thus generated represents the background, while

the signal is the height of the resonant peak above the baseline, hence the "signal-to-baseline" terminology. Although

such spectra o_f the detector are useful for optimization, they

should not be confused with spectra taken with the detector,

for which the source and detector remain in resonance (at zero

relative velocity).

Figure 12(a) shows the expected signal-to-baseline improvement with increasing iron-57 enrichment. This occurs because

the total thickness required to produce a given number of the

desired Mossbauer electrons decreases with increasing enrichment, hence the thickness available for producing unwanted

30
eno

(a)

CM

UJ

j—

i

o

o

D

n = 7

n = 5

n = 3

n = 1

=9.00 0.20 0.40 0.60 O.flO

TOTAL THICKNESS [HVL3

l.c

CO

a

(b)

o

CM \______ n = 7

C-)«" —

UJ n = 5

j—i U

U_o n = 3

fc=

n = 1

o

o

1 1 i i 5.00

Figure 16

1.00 2.00 3.00 q.oo 5.00

TOTAL THICKNESS MVL1

a) and b) Detector efficiency as a function of

total thickness of resonant foils, expressed in

resonant half value layers, for 3 percent iron-57

abundance. One resonant half value layer = 60, 069

A; one photoelectric hvl = 7-4 resonant hvls . n = number of foils.

31
photoelectrons may be decreased. When the number of foils is

increased, however, the signal-to-baseline ratio decreases with

increasing partition of the resonant material, in contrast to

the behavior of the efficiency. This behavior relates to the

fact mentioned earlier that the proportioning between resonant

and nonresonant photons in the beam changes as a function of

the thickness traversed. Specifically, resonant photons are

removed from the beam at a higher rate than are nonresonant

photons so that, as more material is traversed, a higher proportion of unwanted photoelectrons are produced per thickness

increment. In other words the "incremental signal-to-background" ratio decreases with increasing photon penetration.

The overall ratio is a weighted average of the incremental

ratios with the weighting factor being proportional to the

detectability of the electrons from a given depth. Thus, for

a single foil, the overall ratio is weighted in favor of the

shallow depth increments with higher incremental ratios . As

the foil is partitioned, electrons produced further along the

photon penetration path become more detectable, due to the

additional counting surfaces introduced, thereby lowering the

overall weighted average.

The signal-to-baseline ratio as a function of total thickness for 97 percent iron-57 enrichment is shown in Figure 14(a)

The convergence of the curves for different numbers of foils in

the thin limit is expected because the change in resonant

proportion discussed above requires a finite thickness to be

effective. Because of the linearity of signal-to-baseline with

enrichment, as shown in Figure 12(a), the relationship shown

in Figure 14(a) may be reproduced for other enrichment factors

by applying the enrichment ratio to the signal-to-baseline

axis of 14(a). For instance, for 50 percent abundance, the

numbers on the signal-to-baseline axis of 14(a) should be

multiplied by 50/97.

The signal-to-baseline and efficiency values resulting

from these computations are far greater than any observed

32
experimentally to date. This is due partly to the fact that

enrichment and thin film production is less expensive and more

easily done theoretically than experimentally. However,

there are undoubtedly additional errors resulting from the

simplifying assumptions regarding the production of "noise"

electrons in the foils), the neglect of electron "straggling"

effects, lack of consideration of noise production in the

counting gas and counter walls, and others. The final optimization process must certainly be experimental, but these

computations, and future ones, should provide useful guidelines

despite these deficiencies.

E. Line Width

The line profile L(v,T) observed in Mossbauer spectroscopy

is the convolution of the source L~(Vo,r„) and the absorber

LA(vA

,T

A

) profiles, or

(5b «*« v„,r) = J

L

s

(v s,r s

)

• L

A

(v

s

- v

A

, r A )dv

A (ii)

where r,r„,r

A are the half-widths at half-maximum intensity for

the observed, source, and absorber line profiles. The source

and absorber profiles can be expressed by the Lorentzian

function,

L ('n." ,„° „ (12) o

1 + (W

In the case of a thick absorber, the linewidth is increased

due to absorption broadening. Successive layers of material in

the absorber will preferentially absorb the radiation in the

center of the profile with respect to the wings. The line

broadening can be calculated as a function of absorber thickness from the approximate relation,

r obs

= V2 ' 00 + 0.27t

A

) for t

A

<5 (13)

33
with the absorber thickness t

A

= f^nko t, where f is the fraction of the recoil-free radiation, knt is the number of Mossbauer nuclei per unit area, and a the resonance cross section.

For the resonant detector, the line profile is now a function of the source, absorber, and detector line profiles,

which can be calculated from the convolution integral

l

r

[(v

s

, v

D )-v

A

, rR

] = (14)

/

l q (v q , r Q )

• l a (v„-v a

, r A

)

• Ln

(vn , r n )dv

s

v s A

v S A A 'Dx D' D A

where T D

is the detector half-width. The selectivity of the detector reduces the observed half-width, as shown in Figure 17,

for the case where r s

> r A'

r D

= 1 and v

s

= V

D

* The llne P roflle

was obtained by numerical integration, using a computer. A

l=a 2.50

Figure 17-

-1.50 -0.50 0.50 1.50

VEL. [NORM. HflLFWIDTHSJ

2.50

a) Line profile for source and absorber using a con- ventional detector. b) Line profile for a moving

absorber placed between a stationary source and

resonant detector.

34
computer program for this convolution written in BASIC is listed

in Appendix B.

For the case of the resonant detector, the thickness broadening can be calculated from the line profile obtained by numerical integration, as was performed by Mitrofanov et al. [8].

They derived an analytical expression for the thickness broadening, given by

T R

= T Q (1.47 + 0.27t

A

) for t

A

<5 (15)

For a thin absorber a 26 percent reduction in linewidth

is obtained, which increases the resolution of the spectrometer. Conversely, the resonant detector makes it possible to

use much thicker samples without loss of resolution beyond that

of the conventional detector, thereby increasing the effect.

This will result in a considerable decrease in data accumulation time.

35
4. PRELIMINARY MEASUREMENTS

Several preliminary measurements were made to obtain data

on the efficiency and signal/noise ratio for the different detection methods and types of detectors available. Although the

internal conversion process provides the possibility of using

either conversion x-ray or conversion electron detection, the

x-ray fluorescence and low yield of conversion x-rays due to

secondary processes makes the conversion electron detection

more efficient. There are several types of detectors available

for low energy electron detection, and their inherent noise

and sensitivity to x- or gamma-ray radiation will be a decisive

factor in the selection of the proper counter. Since the inherent thickness of the conversion material will degrade the

energy of the initial conversion electron, the energy resolution of the detector will not be critical. Fast response time

for the required high counting rates will be essential. These

requirements favor the proportional, electron multiplier, and

scintillation detectors, but place the solid state detectors

at a disadvantage.

119

A . Resonant Detector for Sn Using the Gel Scintillation

Technique

The internal conversion factor of 5.5 and the 19-6 keV

119

energy of the conversion electron made Sn a logical choice.

Furthermore the gel scintillation technique is well developed

and simple to use. The resonant material in finely powdered

form is mixed with a scintillation gel and placed on a photomultiplier tube. The gel was prepared from 10 g of PPO (2,5

Diphenyloxazole) and 0.6 g of Dimethyl POPOP ( 1, 4-bis-2-( 4-Methyl-5-Phenyloxazolyl) -benzene) , which was mixed with enough

toluene to yield 2 liters of solution. Calibration of the

pulse-height spectrum was made by using a tritium loaded gel.

Tritiated glucose, 10.7 mg was mixed with 1.0 ml of H„0 and

added to 10 ml of scintillator solution. Various amounts of

Cabosil (trade name for silica gel) were then added to this

36
Figure 18. Pulse-height spectrum of tritium loaded gel.

Endpoint is at 18.1 keV.

solution to obtain the proper gel viscosity. Counting efficiency was the highest for the rigid gel. The pulse height

spectrum for the tritium loaded gel is shown in Figure 18.

The endpoint energy of the beta-decay is 18.1 keV, which provides

a calibration point. For the resonant detector experiments,

119 the gel was made by dispersing finely powdered SnOp in

Cabosil and scintillator solution. The Mossbauer spectrum of

a moving Pd^ Sn source and the enriched SnO„ absorber-gel

is shown in Figure 19. The results were very unsatisfactory

due to the large baseline count and the low effect. Most of

the baseline counts observed were due to photomultiplier noise,

since the pulse height spectrum shown in Figure 20 did not

change appreciably when the source was present or not. The

early measurements were made with an EMI 6097S tube. Replacing

this tube with a RCA C3100 photomultiplier with much less dark

current gave very little improvement. It was observed that the

photomultiplier tube itself is very sensitive to gamma-radiation, due to photoelectrons being produced in the envelope and

photocathode . In order to eliminate the direct gamma radiation from the photomultiplier and also to reduce the tube noise,

37
+ 5 mm/

119, Figure 19. Mossbauer spectrum of "J~"l''/ SnOp dispersed

tion gel and moving Pd 119mSn source

visible at -2.2 mm/s

'3 scintillaPeak is barely

HUH " zzz: ::::

-.zzzzzz

S

I

Figure 20 . Pulse-height spectrum of gel scintillation counter.

Upper spectra: H.V. = 2000 volts, upper curve with,

lower curve without source. Lower spectra: same,

but H.V. = 1200 volts

38
GEL

figure 21. Coincidence arrangement for gel scintillation

detector. Photomultiplier tubes are shielded from

direct gamma radiation.

'two Amperex AVP 53 tubes were placed in coincidence, and

connected by a light pipe to the gel sample. See Figure 21.

The AVP 53 tubes were selected because of their fast rise time,

which is required for high counting rates . The Mossbauer

spectra obtained with this arrangement is shown in Figure 22.

The results are poor, and confirmed the measurements made by [5]

The energy of the conversion electron from Sn is very

close to the detection limit of present scintillation systems

.

Furthermore, the energy degradation in a solid such as SnO p is

large, which will lower the energy of the conversion electron

escaping the SnOp particle. The use of a tin compound soluble

in the scintillator solution would help the finite particle

problem, but all Mossbauer data on organo-tin compounds show

39
B

i

I

-5 T

Figure 22. Mossbauer spectrum of

lation gel and Pd^ 1 '" noise suppression

119

m. SnO

+5 mm/s

2 dispersed in scintil- • Sn source, using coincidence

a very small effect (low f-factor) , whereas a resonant detector

requires a very high f-factor.

The scintillation process is very inefficient. Electrons

o interact with the scintillator, producing 3000 A wavelength

photons, which interact with the photocathode to produce a very low energy electron, which is then used in the multiplication process

57 B . Resonant Detector for Fe Using a Conversion Electron

Proportional Counter . The background noise in the gel scintillation technique and

57 the lower energy (7-3 keV) for the conversion electrons in Fe

made this technique very unattractive for iron Mossbauer spectroscopy. However, He filled proportional counters have a low

background, are rather insensitive to x-ray radiation and are

very efficient in detecting low energy electrons. To optimize

the geometrical efficiency a flat proportional counter was

designed.

HO

i
H.V. CONNECTOR

GAS INLET

Figure 23 • Isometric view of proportional counter with top

plate removed.

As shown in Figure 23, the proportional counter is rectangular in appearance with removable top and bottom plates to

facilitate rearrangement of the wire anodes as desired. The

anode configuration used in this work consisted of two one-mildiameter stainless steel wires spaced three inches apart.

During operation of the counter, the anodes were kept at approximately 1^00 V with respect to the grounded aluminum shell. The

41
counter has two windows . One is 1/2 inch in diameter and the

other is 3 inch x 3 inch square. The choice of window covering

and flow gas varied somewhat, as will be noted later. After

an initial purge to remove air from the counter, the gas flow

rate was maintained at approximately 1 cm /s

5 MIL ALUMINUM FOIL

3" x 3" WINDOW

?W?J??J>J/)»j;/A/7

WIRE ANODE

5 MIL ALUMINUM FOIL W WINDOW

7

Co-57 SOURCE

V\6" HOLE

zzzzzzzzz?-*

F 3ST

Al SHIELD

(a) EXPERIMENTAL ARRANGEMENT

t

<cm

O

u

QLUM

<

o

1.0- •

1/i6

,/ HOLE LOCATED

OFF £ OF PROPORTIONAL

COUNTER

!/i6" HOLE LOCATED ON <L OF PROPORTIONAL COUNTER

6.3 14.4

RADIATION ENERGY, keV —»-

(b) PULSE-HEIGHT SPECTRA

Figure 2k. Pulse-height spectra for Co-57 Mossbauer source as

a function of position on 3 inch x 3 inch window.

42
The arrangement shown in Figure 24a was employed to obtain

pulse-height spectra of the radiation from a cobalt-57 Mossbauer source as different locations on the 3 inch x 3 inch

window were irradiated. Both proportional counter windows were

covered with 5-mil-thick aluminum foil. An argon-10 percent

methane flow-gas was used. A lead-and-aluminum shield with a 1/16 inch-diameter hole restricted the radiation to the desired

area of the window. The pulse-height spectra obtained at all

points on the 3 inch x 3 inch window were virtually identical

in terms of resolution, counting efficiency, and gas multiplication, except for those pulse-height spectra taken at points

along the counter centerline midway between the two wire anodes

.

The efficiency of the counter at these points was decreased by

approximately a factor of two for the 6.3 keV x-ray; furthermore, the 14.4 keV gamma ray was practically undetected. A

comparison of the two types of pulse-height spectra are shown

in Figure 24b. As little as 0.2 in deviation from the centerline of the counter resulted in the full pulse-height spectrum

illustrated by the upper curve in Figure 24b. This characteristic of low detection efficiency at the center of the

counter results in an improved signal-to-noise ratio for backscatter measurements when the collimated incident radiation

is directed through the center of the counter where it is less

likely to contribute to the background. Typical Mossbauer

spectra taken with this counter are shown in Figure 7- The

area of the described counter is too large for a resonant detector. A smaller counter, with a 0.5 inch diameter aperture and

0.25 inch thick was designed, as shown in Figures 25 and 26, and

the two 1 mil wires were spaced for optimum efficiency and resolution. This counter operates at 1200 V, using a He-8 percent

CH^ flow gas. The energy calibration of the detector was determined by placing a Fe source in the counter, which gave the

55 typical spectrum with a 5-5 keV endpoint for Fe and a 7-3 keV

57 endpoint for a Co source, shown in Figure 27. To evaluate

this counter for a resonant detector, a commercial 0.5 mil 50

43
Figure 25- Photograph of detector for conversion electrons.

IN

jn. _ax 1

Figure 26. Diagram of detector for conversion electrons

44
5.5 7.3 keV

Figure 27. Pulse-height spectra of a

J

'~Fe (5-5 keV endpoint)

and -^'Co (7-3 keV endpoint) source placed inside

the detector.

percent enriched stainless steel foil was placed inside the

57 counter over the exit window. The spectrum of a moving Co

in stainless steel source with this detector is shown in Figure

"28. The signal/background is 325 percent. The energy dependjence of the background electrons. Figure 29a, was obtained by

taking a pulse height spectrum with the source scanning at high

velocity. Figure 29b shows the electron spectrum with the

source at rest, which corresponds to the sum of the background

and resonantly generated electrons . Both spectra show a large

energy dependence, and for optimal signal/background, pulseheight selection will be required. The energy dependence of

j'the conversion electrons, obtained by subtracting the spectra

of 29a and b is shown in Figure 30. The differential signal/

noise shown in Figure 31 can be used for optimum energy selection by proper setting of the single channel analyzer. The

performance of this detector agrees well with the theoretical

( analyses made in Section 2, for a resonant detector made with

a single thick foil.

4 5
o

o SPECTRUM NO- JS1298

•^.BO -z.to 1. 40 -0.7D -0.00 0.70

VELOCITY (MM/S)

lTyo 2. 10 2. BO

Figure 28. Mossbauer spectrum of the resonance detector with

a 0.5 mil 50 percent enriched stainless steel foil

and moving 57co in stainless steel source.

The use of a stainless steel converter material is far

from optimum. The 18 percent Co and 8 percent Ni content in

this alloy increases the photoelectron contribution and attenuates the Mossbauer gamma radiation. Thin foils of this

material are also difficult to fabricate. The theoretical

analyses showed that a low Z element for alloying to collapse

the magnetic field in iron is very important, and berylium

would be the best material available. Pe-Be alloys near the

composition Be,-Fe were found to be nonmagnetic and exhibited

a narrow Mossbauer spectrum, with a large resonant effect. The

alloy composition, thickness of the foils and number of foils

46
U.00 6.00

ENERGY (KEV)

.00

Figure 29. Energy distribution of photoelectrons and conversion electrons obtained by source-detector off res^

onance (a) and in resonance (b).

required and the final construction and fabrication of the

counter will be the basis of the second report (FY1970) on

this investigation.

47
RH.OO 1.80 3. 60 5.U0

ENERGY CKEV)

7.20 9.00

Figure 30. Energy distribution of the conversion electrons

from a 0.5 mil 50 percent enriched stainless steel

foil.

48
&.00 3.75 5.00 6.25

ENERGY CKEV)

10.00

Figure 31. Differential signal/noise of the electrons generated

in the stainless steel resonant detector. Optimum

selection would be from 2 to 6 keV.

49
5 . SUMMARY

It is apparent that there are many advantages in using the

resonant detector for Mossbauer spectroscopy. Because the

detector is so selective for the Mossbauer radiation it can be

used to great advantage where the background (non-resonant)

radiation is high. This is the case with samples exhibiting

low Mossbauer effect or from a close scattering geometry or

when a very small amount of material is available to be measured such as particulate matter on a substrate of filter paper.

In addition, this detector will prove to be very valuable in

the Mossbauer spectroscopy of complex materials because of

its high resolution.

The same detector system can be used to measure the struco ture of surface films on bulk material that is less than 200 A

in thickness

Because the resonant detector is so selective the combination of increased source strength with the optimal number and

thickness of resonant material will significantly decrease the

collection time for a spectrum of a given quality. Evaluation

of the various detector systems described in this report will

be made by quantitative experimentation.

50
6 . REFERENCES

[1] Mitrofanov, K. P., Illarinova, N. V. and Shpinel, V. S.

Proceedings of the Dubna Conference on the Mossbauer

Effect, Consultants Bureau Enterprises, Inc., New York,

(1963). Translation from Russian.

Mitrofanov, K. P. and Shpinel, V. S., Zhur. Eksp . i

Teoret. Fix., 40 983 (1961).

[3] Mitrofanov, K. P., Illarinova, N. V., Shpinel, V. S.,

Prit. Tekh. Eksper. N3, 49 (1963).

[4] Aleksandrov, A. Yu. , Kerasev, A. N., Mitrofanov, K. P.,

Sorokin, A. A. and Polak, L. S., Coll: Problems of

Nuclear Geophysics (in Russian) Izd. "NEDRA" (1964).

[5] Levy, L., Mitrani, L., Ormandjiev, S., Nucl. Instr.

Methods 31, 233-6 (1964).

[6] Bonchev, ZW, Jordanov, A., and Minkova, A., Nucl. Instr.

Methods 7_0, 36-40 (1969).

[7] Mitrofanov, K. P., Instruments and Experimental Techniques, (PTE), No. 3 60 (1965).

[8] Mitrofanov, K. P., Plotnikova, M. V., and Rokhlov,

Instruments and Experimental Techniques, (PTE), No. 4,

790 (1965)

•

[9] Mirtofanov, K. P., and Rikhlov, N. I., Instruments and

Experimental Techniques, (PTE) No. 5, 1256 (1965).

Shimony, U., Nucl. Instr. Methods, 37, 350 (1965).

Protop, C. and Nistor, C., Rev. Roum. Phys . , 12, 658

(1967).

Chow, H. K., Weise, R. F., Flinn, P., "Mossbauer Effect

Spectrometry for Analysis of Iron Compounds", AEC Rept

.

NSEC-4023-1 TID-4500, Contract No. At-( 30-1) -4023

(1969).

Ord, R. N., Appl. Phys. Letters, 1_5, 279 (1969).

Terrell, J. H. and Spijkerman, J. J., Appl. Phys. Letters

13, 11 (1968)

.

51
[15] Swanson, K. R. and Spijkerman, J. J., June 1970.

DeVoe , J. R. , Editor, NBS Technical Note 501, Superintendent of Documents, U. S. Government Printing Office'

Washington, D. C, 20402 (1970).

[16] Muray, J. J., Worster, B. W,, IEEE Transaction on

Nuclear Science, Vol. NS-14, No. 4, 11-16 (1967).

[17] Berthelot, Andre, Radiations and Matter , Leonard Hill

[Books] Limited, London (1958).

[18] Cosslet, V. E. and Thomas, R. N., Brit. J. Appl. Phys .

,

15, 883-907 (1964).

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

DeVoe, J. R. , NBS Misc. Publ. 260-13, Superintendent

of Documents, U. S. Government Printing Office,

Washington, D. C, 20402 (1967).

52
APPENDIX A: TO COMPUTE RESONANT DETECTOR EFFICIENCY AND

SIGNAL BACKGROUND RATIO AS A FUNCTION OF

ASSORTED INPUT PARAMETERS .

1 REM PROGRAM /SELEC/ T0 COMPUTE RESONANT DETECTOR EFFICIENCY AND

2 REM SIGNAL/BACKGR0UND RATI0 AS A FUNCTI0N 0F ASS0RTAD INPUT

PARAMETERS.

9 0PEN 4, OUTPUT, /ATTN2/

10 0PEN 3* OUTPUT* /ATTEN/

11 : #.#####! ! ! ! #.#####! ! ! ! 12:#####.

25 DIM U(50),V(50,3),T(2),K<2),EC2>

30 REM A=IC C0EFJ 14,12*16=14.4,122,136 KEV TRANSITION PR0BS.

35 REM CONVERTED TO INCIDENT INTENSITIES

41 V9 = 42 V8 = 45 READ A, 14, 12, 16

49 I4=I4/(1+A)

50 FOR J=l T0 3

51 FOR 1=1 TO 50

52 V(I,J)=0.

53 NEXT I 54 NEXT J

55 REM F1 = S0URCE FRACT ION, F2 = DETECT0R FRACTI 0N, A 1 = DETECT0R

60 REM ABUNDANCE, D5=S0URCE-DETECT0R OFFSET, G1=S0URCE HW,G2=DET.

HW

65 REM SO=RES0NANT X-SECTI0N, M0=RES . LINAAR ATTEN . C0EF.

70 REM Sl= 14.4 FE PH0T0 X-SECTION, Ml ^CORRESPONDING LIN. ATTEN.

COEF.

75 READ F1,F2,A1,D5,G1,G2

80 S0=2.2E-18*Al*F2/( 1 .+CD5/(G1+G2) )t2)

85 REM Rl=FE-BE-5 DENSITY, N 1 =# IR0N/CC, N2=#BE /CC, T2=HALF VALUE

LAYER

90 READ R1,N1,N2,S1

95 M0=N1*S0*10. t-8

96 Ml=Nl*Sl*10.t-8

97 T(2)=.693/M1

98 TCI )=.693/M0

99 PRINT "RESONANT HALF VALUE LAYER=", T( 1 ), "ANGSTROMS"

100 PRINT "NON-RESONANT HALF VALUE LAYER=", T< 2 >, "ANGSTROMS"

103 PRINT "CONVERSION ELECTK0N RANGE=8735 ANGSTROMS"

104 PRINT "AUGER ELECTRON KANGE=5807 ANGSTROMS"

105 PRINT "FOIL THICKNESS, ANGSTK0MS:"i

110 INPUT Tl

115 N3=T<1 )/Tl

116 D8=T(1)/10.

120 PRINT N3, "FOILS PER RESONANT HALF VALUE LAYER

125 PRINT "NUMBER 0F FOILS:"*

130 INPUT N4

135 F3=N4*T1/T( 1

)

-dcm

140 PRINT "TOTAL THI CKNESS=", F3; " HALF-VALUE LAYERS

145 REM E=ELECTR0N ENERGY, R=RANGE, E 1 =ELECTR0N EXIT ENERGY

53
150 READ EC 1 ),EC2),RC 1 >,RC2>

151 DEF FNACX)=RC 1 )*X*L0GCRC 1 )/X)/CR( 1 )-X)

152 DEF FNBCX)=RC2)*X*L0GCRC2)/X)/CRC2>-X)

153 DEF FNCCX)=.991/C 1 .+C C3.205-X)/36.76) t2)

154 DEF FNDCX> = 1 .01/C 1 . + C C-6.7-A>/66.86)»2)

155 REM Q1=RES0NANT PH0T0NS REACHING GIVEN DEPTH

160 REM Q2=T0TAL PHOTONS REACHING GIVEN DEPTH

165 REM P1=N0. M0SSBAUER IC ELECTR0NS CREATED AT GIVEN DEPTH

170 REM .7*P1=N0. M0SSBAUER AUGER ELECTRONS CETC)

175 REM P2=N0. PHOTOELECTRIC K ELECTRONS CREATED

176 REM .7*P2=N0. PH0T0ELEC TK I C AUGER ELECTRONS

180 Q1=F1*I4

185 02=14

194 D=RC 15/10

195 IF RC1)<T1 THEN 198

196 D=T1/10

198 FOR K=l T0 N4

200 Dl=D/2

205 REM D=DEPTH INCREMENT, D1=DEPTH

210 P1=Q1*M0*D

215 P2=Q2*M1*D

220 Q1=Q1-P1-P2*CQ1/Q2)

225 Q2=Q2-P1-P2

226 IF D1>RC1 ) THEN 340

227 D2=D1*R1/100.

228 D3=FNACD1 )*R1/100.

229 D4=FNBCD1 )*R1/100.

231 P1=P1*A/CA+1 ) 232 V1=.5*C 1-D1/RC 1 ) )*FNDCD3)*P1

235 V2=.5*C 1 -D1/RC2) )*FNCCD4)*P1*.7

236 IF D1<RC2) THEN 239

237 V2=0

239 V9=V9+C Vl+V2)/1 .7

240 V3=.5*C 1-D1/RC2) )*FNCCD4)*P2*.7

241 IF D1<RC2) THEN 245

242 V3=0.

245 V4=.5*C 1-D1/RC 1 ) )*FNDCD3)*P2

246 V8=V8+CV3+V4)/1 .7

330 G0 TO 360

340 D1=D1+D8

350 GO TO 365

360 D1=D1+D

365 IF D1>T1 THEN 375

370 G0 T0 210

375 NEXT K

495 E9=V9/I4

500 S9=V9/V8

505 PRINT ,, EFFICIENCY =

,, ;E9; M SIGNAL/BSLN = "> S9

900 DATA 9,89,89, 1

905 DATA .95, .95, .50,0., .08, .08

910 DATA 3.272, 1 . 84E+22, 9 . 22E+22, 8 . 466E-2

915 DATA 7. 3, 5. 5, 8735. ,5807.

9999 END

5k
APPENDIX B: TRIPLE CONVOLUTION OF SOURCE, ABSORBER AND

DETECTOR LORENTZIAN PROFILES

REM CONRES PROGRAM FOR TRIPLE CONVOLUTION OF SOURCE* ABSORBER

REM AMD DET ECTOR LORENTZIAN PROFILES.

) OPEN 3* OUT PUT*/ORESDT/

I DIM NC200)

. READ AC1)*PC1),GC1)

> READ AC2)*PC2)*GC2)

J READ AC3)*PC3)*GC3)

. DEF FNSCX)=AC1 )/Cl .+CCPC 1)-X>/GC 1 )

)

t2)

> DEF FNACX)=AC2)/C1 .+ C ( PC 2 ) -X) /G C 2 ) ) t 2-)

5 DEF FNDCX )=AC 3 ) / C 1 . + C C P C 3

) -X) /G C 3 ) ) t 2

>

) PRINT "N ORMAL HW"* "RESONANT HW"

.

)0 FOR Y=l TO 100

.0 LET N(Y )=0.

>0 FOR X= - 100 TO 100

50 LET N(Y)=N CY> + FNSC X) *FNACX+ Y)*FNDCX)

>0 NEXT X

50 NEXT Y

55 M=0

>0 FOR 1 = 1 TO

>2 IF NCI )<M THEN

>5 M=NCI

)

)0 NEXT I )5 FOR 1 = 1 TO

.0 LET NC I )=N

.5 WRITE #3*1*

20 NEXT I 50 1=0

10 1=1 + 1 SO IF NCIX.5 THEN 240

SO I1=I-CNCI) - .5>/CNC'i )-NCI-l ))

JO H=PC2) - II

iO PRINT 2. *GC2)*H

)01 DATA 1 . *0. * 10.

)02 DATA 1 . *50.*10.

)03 DATA 1 .*0.*10.

100

THEN

100

C I ) /M

N CI )

55

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