• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 List of Figures
 List of Tables
 Abstract
 Introduction
 Theory
 Experimental system
 NaI (T1) system
 NaI (T1) performance
 Ge (Li) system performance
 Guarded system performance
 Active sample system
 Comparison of system on environmental...
 Evaluation of detection system...
 Conclusions
 Appendices
 References
 Biographical sketch














Title: Development and comparison of Compton suppression techniques for low-level radio-nuclide analysis
CITATION PDF VIEWER THUMBNAILS PAGE IMAGE ZOOMABLE
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STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00097628/00001
 Material Information
Title: Development and comparison of Compton suppression techniques for low-level radio-nuclide analysis
Physical Description: xii, 117 leaves. : illus. ; 28 cm.
Language: English
Creator: Markwell, Frank Russell, 1948-
Publication Date: 1972
Copyright Date: 1972
 Subjects
Subject: Scintillation spectrometry   ( lcsh )
Gamma ray spectrometry   ( lcsh )
Radioactivity -- Measurement   ( lcsh )
Engineering Sciences thesis Ph. D
Dissertations, Academic -- Engineering Sciences -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 115-116.
Additional Physical Form: Also available on World Wide Web
Statement of Responsibility: by Frank R. Markwell.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097628
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000582632
oclc - 14147955
notis - ADB1009

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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
        Page v
    List of Figures
        Page vi
        Page vii
        Page viii
    List of Tables
        Page ix
    Abstract
        Page x
        Page xi
        Page xii
    Introduction
        Page 1
        Page 2
        Page 3
    Theory
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Experimental system
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
    NaI (T1) system
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
    NaI (T1) performance
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
    Ge (Li) system performance
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
    Guarded system performance
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
    Active sample system
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
    Comparison of system on environmental samples
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
    Evaluation of detection systems
        Page 94
        Page 95
        Page 96
    Conclusions
        Page 97
        Page 98
    Appendices
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
        Page 113
        Page 114
    References
        Page 115
        Page 116
    Biographical sketch
        Page 117
        Page 118
        Page 119
        Page 120
Full Text











DEVELOPMENT AND COMPARISON OF

COMPTON SUPPRESSION TECHNIQUES FOR

LOW-LEVEL RADIONUCLIDE ANALYSIS











By

Frank R. Markwell


A DISS-ETATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE 0? DOCTOR OF PHILOSOPHY IN
NUCLEAR ENGINEERING SCIENCES








UNIVERSITY OF FLORIDA


1972








ACKNOWLEDGEMENTS


The author wishes to express his appreciation to Dr. W.E.

Bolch, Dr. M.J. Ohanian, Dr. C.E. Roessler, Dr. W.H. Ellis,

and Dr. H.A. Bevis, members of his supervisory committee, for

their advice and assistance in the preparation of this disserta-

tion. He especially wishes to thank Dr. W.E. Bolch for the

suggestion of the dissertation subject and for his encouragement

and valuable advice. Special thanks are due Dr. C.E. Roessler

for his aid, helpful suggestions and constructive criticism.

Thanks are also expressed to Dr. W.H. Ellis for his helpful

suggestions and encouragement.

Special appreciation is extended to the the staff members

of the Nuclear Engineering Department and the Environmental

Engineering Department in the University of Florida, without

whom this work could not have been performed.

The author acknowledges the support by the Atomic Energy

Commission for the award of a Special Fellowship in Nuclear

Science and Engineering during his graduate studies and the

support by Florida Power Corporation.

In addition, the author wishes to thank Nuclear Physics

for the loan of the Ge(Li) spectrometer.













TABLE OF CONTENTS


Page


ACKNOWLEDGEMENTS ..........

LIST OF FIGURES ...........

LIST OF TABLES ............

ABSTRACT ..................

Chapter


... *. *..#..


* . . .


I. INTRODUCTION ...... ... ..... .....

II. THEORY ..................... ......

A. Ge(Li) Detector ........................

B. Anticoincidence Guarding ...............

C. Active-Sample Guarding .................

III. EXPERIMENTAL SYSTEM ........................

A. Primary Detector .......................

B. Guard Detector ....................

C. Sample Detector ........................


D.

E.

IV. Nal

A.

B.

V. Nal

A.

B.


Counting Room and Shielding ............

Electronics ....................... ..... ...

(Tl) SYSTEM .............................

Configuration ..........................

Electronics ............................

(Tl) PERFORMANCE ........................

Detector Background ....................

Efficiency .............................


iii









TABLE OF CONTENTS -- continued


Page

Chapter -- continued

C. Minimum Detectable Activity ............ 42

D. Peak-to-Compton Ratio .................. 44

E. Complex Spectrum ....................... 44

VI. Ge(Li) SYSTEM PERFORMANCE ................ 51

A. System Background ...................... 51

B. Efficiency ................... ...... ... 55

C. Minimum Detectable Activity ............ 55

D. Peak-to-Compton Ratio .................. 59

E. Complex Spectrum ...................... 59

VII. GUARDED SYSTEM PERFORMANCE ................. 62

A. System Background ...................... 62

B. Guard-Detector Efficiency .............. 64

C. Photopeak Reduction .................... 6g

D. Minimum Detectable Activity ............ 69

E. Peak-to-Compton Ratio .................. 69

F. Complex Spectrum ....................... 73

VIII. ACTIVE SAMPLE SYSTEM ....................... 75

A. Efficiency ............................. 75

B. Minimum Detectable Activity ............ 75

C. Peak-to-Compton Ratio .................. 76

D. Separation of Decay Modes .............. 76

IX. COMPARISON OF SYSTEMS ON ENVIRONMENTAL

SAMPLES .................................... 82








TABLE OF CONTENTS -- continued


Page


Chapter -- continued

A. Standard Samples .......................

B. Air Sample .............................

C. Water Samples ................... ..... ..

D. Solid Samples ..........................

X. EVALUATION OF DETECTION SYSTEMS ............

A. NaI(T1) System ....................... ...

B. Unguarded Ge(Li) System ................

C. Guarded Ge(Li) System ..................

D. Active-Sample System ...................

XI. CONCLUSIONS ...............................


Appendices

A. ELECTRONIC CIRCUITS ...........

B. GUARD-DETECTOR CHARACTERISTICS

C. SPECTRUM ENERGY CALIBRATION ...

D. EXAMPLE OF DIGITAL DATA .......


.............



.............
OOOOOeooooo


LIST OF REFERENCES ................ ....... ........

BIOGRAPHICAL SKETCH ..............................


100

105

110

113


115

117











LIST OF FIGURES


Figure Page

1. Effect of.Compton suppression ................. 7

2. Guarded Ge(Li) system configuration ........... 11

3. Ge(Li) detector side view ................... 12

4. Ge(Li) detector frontal view ................ 13

5. Ge(Li) detector with extension collar ......... 15

6. Guard-detector photomultiplier tube ........... 17

7. Guard-detector with photomultiplier mounted ... 18

8. Sample port mounted in guard detector ......... 19

9. Ge(Li) detector positioned in sample port ..... 20

10. Sample container with photomultiplier mounted 22

11. Sample port for standard technique ............ 23

12. Sample port for active-sample technique ....... 24

13. Counting room and entrance .................... 25

14. Counting room ................................. 26

15. Photomultiplier shielding ..................... 28

16. Guarded Ge(Li) system electronics ............. 29

17. Ge(Li) system analyzer ........................ 30

18. NaI(T1) detector configuration ................ 34

19. NaI(Tl) crystal in lead shield ................ 35

20. NaI(T1) system in counting room ............... 36

21. NaI(T1) system electronics .................... 38

22. Background for NaI(T1) ....................... 40


vi









LIST OF FIGURES -- continued


Figure Page

23. NaI(T1) photopeak efficiency .................. 43

24. Minimum theoretical detectable activity for

NaI(Tl) ....................................... 45

25. Peak-and-Compton spectrum for NaI(T1) ......... 47

26. Peak-to-Compton ratio for NaI(T1) ............. 48

27. Complex NaI(T1) spectrum ...................... 49

28. Counting room background ...................... 52

29. Ge(Li) background with Hg shielding ........... 53

30. Ge(Li) photopeak efficiency ................... 56

31. Minimum theoretical detectable activity for

Ge(Li) ..................................... 57

32. Peak-to-Compton ratio for Ge(Li) .............. 60

33. Unguarded complex Ge(Li) spectrum ............. 61

34. Unguarded Ge(Li) background ................... 63

35. Guarded Ge(Li) background ..................... 65

36. Guard-detector efficiency ..................... 67

37. Minimum theoretical detectable activity for
guarded Ge(Li) ................................ 70

38. Peak-to-Compton ratio for guarded Ge(Li) ...... 72

39. Guarded complex Ge(Li) spectrum ............... 74
40. Active-sample minimum theoretical detectable

activity for Ge(Li) ........................... 77

41. Active-sample peak-to-Compton ratio ........... 79

42. Separation of decay modes .................... 80

vii









LIST OF FIGURES -- continued


Figure Page

43. Zn-65 source spectrum ......................... 83

44. Complex standard-sample spectrum .............. 84

45. Air sample spectrum ........................... 86

46. Water standard spectrum ....................... 87

47. UFTR primary coolant water spectrum ........... 89

48. Soil sample spectrum .......................... 90

49. Oyster sample spectrum ........................ 92

50. Seaweed sample spectrum ....................... 93

51. Sample-detector electronics circuit ........... 101

52. Guard-detector electronics circuit ............ 102

53. Ge(Li) threshold electronics .................. 103

54. System coincidence electronics ................ 104

55. Guard-detector light collection ............... 106

56. Guard-detector background .................... 107

57. Threshold calibration ......................... 108

58. Ge(Li) calibration spectrum ................... 111

59. Ge(Li) calibration curve ...................... 112


viii











LIST OF TABLES


Table Page

1. Background Count Rate for the NaI(Tl) System .. 41

2. Theoretical Minimum Detectable Activities for

the NaI(T1) System ............ .......... 46

3. Background Count Rate for the Ge(Li) System ... 54

4. Theoretical Minimum Detectable Activities for

the Ge(Li) System .......................... .. 58

5. Background Count Rate for the Guarded Ge(Li)

System ................. ......... ......... .. .. 66

6. Theoretical Minimum Detectable Activities for

the Guarded Ge(Li) System ..................... 71

7. Theoretical Minimum Detectable Activities for

the Active-Sample Ge(Li) System ............... 78

8. Digital Data for Guarded Air Sample Spectrum .. 114












Abstract of Dissertation Presented to the Graduate Council
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy in Engineering

DEVELOPMENT AND COMPARISON OF COMPTON SUPPRESSION
TECHNIQUES FOR LOW-LEVEL RADIONUCLIDE ANALYSIS


By

Frank R. Markwell


December, 1972


Chairman: Dr. W. E. Bolch
Major Department: Nuclear Engineering Sciences


Optimum analysis of low-level radionuclides requires the

removal of Compton interference from the gamma-ray spectrum.

Significant Compton suppression can be achieved by employing

high-efficiency guard detectors to detect the scattered

gamma photons. However, high detection efficiency prohibits

significant absorption between the primary detector and the

guard detector, thus restricting the sample to ore of small

mass. When large-mass samples are used Compton suppression

is greatly degraded.

To alleviate this problem a new concept has been

developed and a system constructed, and evaluated which, in

effect, makes the sample part of the guard detector. Restricted

to optically clear samples, the system requires the addition

of a liquid scintillator to the counting sample. This










"cocktail" is viewed by a separate photomultiplier tube

and associated electronics which add its detected signal to

the guard signal and both are used in anticoincidence with

the primary detector. This scheme restores the performance

of the system to that of the "small-sample" Compton reduction

system. However, it also removes photopeaks due to simultaneous

(precedingwithin 200 ns) p or decay in the sample which

can also trigger the anticoincidence logic. Alternately, by

requiring coincidence between the sample-scintillator

detector and the primary detector, all electron capture and

metastable states are excluded, leaving only a spectrum of

photopeaks from short lived A* and f- decay daughters but

with partial Compton reduction. In short, this technique

yields two spectra for each liquid sample, a "perfect"

gamma spectrum of some excited states and a "not so perfect"

gamma spectrum of the remaining excited states.

The complete detection system employs a 6% efficient

(re 3x3 inch NaI(T1) ), 2.8 keV resolution Ge(Li) spectro-

meter as the primary detector arranged to extend 12 inches

into the center of a 31 inch diameter by 26 inch long container

of Pilot Chemicalts high efficiency mineral oil scintillator.

This scintillator, which acts as the guard detector, is

viewed by a 15 inch Fairchild K2128 photomultiplier tube

and both are shielded with 2.5 inches of mercury. The sample

container arranged axially around the Ge(Li) crystal and

viewed by a 2 inch diameter RCA 6655 photomultiplier tube,

holds 1.5 litres (607 scintillator). The electronics include








preamplifier, threshold detectors, and logic circuits to give a

timing uncertainty of less than 200 ns.

Comparison of this system with a 4x4 inch NaI(T1)

system and the unguarded Ge(Li) system shows it to have

both superior peak-to-Compton ratios and superior minimum

detectable activities for counting times as short as 40

minutes. In addition to maintaining the point source peak-

to-Compton ratio for large water samples, the active-sample

technique has the advantage of dividing the spectrum into

two spectra according to the decay mode of the radionuclides.


xii












CHAPTER I


INTRODUCTION


Gamma ray spectroscopy has been an important tool in the

analysis of radionuclides for many years. The development of

the NaI(T1) detector opened the field and since then vast

improvements in efficiency and reductions in interradio-

nuclide interference have been made. Improvements in detector

efficiency and resolution have reduced this interference but

not eliminated it. The remaining interference is mostly due

to the Compton continuum associated with photopeaks in the

spectrum.

Since Compton scatter in the detector is only a partial

energy release mechanism, a lower energy gamma photon may also

exit the detector. This characteristic has been used in
(1)(2)
suppressing the Compton continuum. The original systems

for such suppressions employed NaI(T1) as the primary detector

surrounded by a large liquid scintillator which detected the

Compton scattered photons leaving the primary detector and

used them to eliminate such counts from the spectrum. Thus,

this technique is denoted as anticoincidence guarding.

The development of the Ge(Li) detector has resulted in

greatly improved resolution capabilities and reduced effects

from the Compton continuum. The usefulness of such a detector




2

for low level radionuclide analysis has been shown previously
(3)
by Cooper et al. Soon thereafter investigators applied
(4)(5)(6)(7)
anticoincidence guarding to Ge(Li) detectors.

These systems, which suppressed the Compton continuum up

to a factor of 6, employed plastic scintillators as the guard

detector. Current investigators, using NaI(Tl) guards,
137(8)(9)
report a Compton reduction for Cs of up to 9 times,

and those using plastic scintillator guards report a reduction
(10)(11)
of 6 times.

Significant Compton reduction can only be achieved by

employing high-efficiency guard detectors. However, high

detector efficiency prohibits significant absorption between

the primary detector and the guard detector, thus restricting

the sample to one of small mass. When large-mass samples are
(7)
used, Compton suppression is greatly degraded.

To alleviate this problem, a new concept is proposed which,

in effect, makes the sample part of the guard detector.

Restricted to clear water samples, the system would require

the addition of a liquid scintillator to the counting sample.

This "cocktail" would be viewed by a separate photomultiplier

tube and associated electronics which would add its detected

signal to the guard signal and both would be used in anti-

coincidence with the primary detector. This scheme should

restore the performance of the system to that of the "small-

sample" Compton reduction system. However, it would also

remove many photopeaks due to simultaneous (preceding within

200 ns) A or B- decay in the sample which could also trigger

the anticoincidence logic. Alternately, by requiring coinci-





3

dence between the sample-scintillator detector and the

primary detector, all electron capture and metastable states

could be excluded, leaving only a spectrum of photopeaks from

short-lived and 8 decay daughters but with partial Compton

reduction. In short, this technique would yield two spectra

from each liquid sample, a "perfect" gamma spectrum of some

excited states and a "not so perfect" gamma spectrum of the

remaining excited states.

Presented here are the design details of a liquid

scintillator guarded Ge(Li) system with the active-sample

option, as mentioned above, and the evaluation of this system

for environmental radionuclide analysis with respect to their

relative advantages over a 4x4 inch NaI(T1) system.













CHAPTER II


THEORY


In this chapter the theory behind the active sample

anticoincidence guarded Ge(Li) spectrometer is presented.

Included are both the theory associated with the new active

sample technique and the theory of anticoincidence guarding.

The latter is presented since anticoincidence guarding is

still a relatively new technique and not widely known.


A. Ge(Li) Detector


Interaction of a gamma photon with the primary detector

can occur in one of several ways. These are photoelectric

absorption, Compton scatter, and pair production. For

germanium and typical gamma energies (<2MeV), only the first

two are significant. Photoelectric absorption occurs when

the gamma photon releases all of its energy in one interaction

with a bound electron, and has the greatest probability for

photon energies just above the ionization potential energy

of that bound electron. Thus,this method of photon energy

release is most significant for low energies and has
-3.5
approximately a (hO) probability dependance. Photoelectric

absorption transfers all of its energy to the bound electron

which in turn releases this energy by ionization and electronic






5
excitation of atoms in the immediate vicinity of the photon

interaction.

The other interaction of significance is Compton scatter,

which consists of a partial transferral of photon energy to the

bound electron and the subsequent scatter of the photon

(reduction in energy). This reaction has a (hO)-1 probability

dependance and thus is significant for energies higher than

those significant for photoelectric absorption. Again the

liberated electron from the photon interaction produces

ionization and electronic excitation of atoms in its immediate

vincinity.

Such interactions within the intrinsic region of a

semiconductor produce electron-hole pairs having lifetimes

long enough to allow their collection by an electric field.

The result is a charge collection which is proportional

to the gamma photon energy release within the semiconductor

crystal. A count rate-energy release spectrum (charge

collection) yields a peak corresponding to photoelectric

absorption and a continuum of counts corresponding to the

energy release in Compton scatter. Due to the physics of

Compton scatter this continuum is approximated by a plateau

from zero energy (no scatter) to a maximum energy signicantly

less (typically 100-200 keV) than the photopeak energy

(corresponding to the energy release in photon backscatter).

The ratio of probabilities of the two interactions depends

upon the photon energy and the detector material. For

germanium the photoelectric interaction and Compton scatter





6

are equally probable at 150 keV but at 1 MeV Compton scatter
(7)
is 100 times more probable.


B. Anticoincidence Guarding


The useful information from a Ge(Li) spectrum lies in the

photopeak energy and amplitude since the gamma photon energy

is characteristic of the radionuclide from which it originated

and the amplitude is proportional to the quantity present.

The Compton plateau is a continuum which, because of counting

statistics, is erratic and thus obscures other photopeaks which

may lie within this energy range. Anticoincidence guarding

suppresses the counting of scatter events thereby removing

this continuum and allowing recognition of small amplitude

photopeaks within the energy range (see Figure 1 ).

To achieve this Compton suppression, those gamma photons

which deposit only part of their energy in the primary

detector must be recognized. By placing the primary detector

within a high-efficiency detector, photons which are scattered

in the primary detector can be detected again in the guard

and thus recognized as being Compton scattered. These can

then be prevented from being recorded in the spectrum. To

obtain excellent Compton suppression, the guard detector

must approach 100% detection (not energy collection)

efficiency and photon absorption between the primary detector

and the guard detector must be eliminated. Since good

primary detection requires the radioactive sample to be

placed in the center of the guard detector just adjacent to

the primary detector, the sample itself can act as an























































































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8

unwanted absorber. Thus excellent Compton suppression in

standard systems can be maintained only with small samples.

In addition, large samples inherently yield a lower peak-to-

Compton ratio due to Compton scatter in the sample before the

photon reaches the primary detector.

An inherent disadvantage of anticoincidence guarding is

the removal from the spectrum of those photopeaks associated

with cascade gamma decay (2 or more simultaneous photons)

and .+ decay whose excited daughter decays simultaneously

(yielding a simultaneous gamma decay photon and annihilation

photons).


C. Active Sample Guarding


To eliminate degradation of Compton suppression and

maintain excellent peak-to-Compton ratios, the sample must be

used as a guard detector also. This allows recognition of

sample-scattered photons which enter the primary detector

and primary detector scattered photons which are absorbed

back in the sample. The combined effects yield a peak-to-

Compton ratio for large samples which approaches that for

point source samples.

An inherent problem in this technique arises when a

radionuclide in the sample decays by emission of a P

particle to its excited daughter and then immediately

emits a gamma photon. In this case gamma photons enter the

primary detector at the same time that the "- particles (poly-

energetic) release their energy in the sample detector. Thus





9

the f particles can not be distinguished from the scattered

gamma photons by the sample detector. 3adionuclides having

excited daughter lifetimes greater than the timing uncertainty

of the detector system, and those radionuclides which decay

by electron capture, are not affected by this phenomenon.

To restore these short-lived excited daughters from f"

decay and remove the long-lived and electron-papture events,

coincidence signals can be required between the sample detector

and the primary detector, indicating #" decay with an

associated prompt gamma photon. This spectrum contains only

photopeaks from these prompt gamma decay states but maintains

only partial Compton suppression.

The active-sample technique, in addition to the normal

guarded system spectrum, yields two additional simplified

spectra- one spectrum of gamma photons from electron capture

and metastable states maintaining total Compton suppression,

and a second spectrum of prompt gamma photons from 83 decay

maintaining partial Compton suppression.












CHAPTER III


EXPERIMENTAL SYSTEM


The complete active-sample experimental system consists

of three detectors, the primary, the guard, and the sample

detector. Figure 2 shows their relationship. This chapter

gives both the component specifications and design parameters

for each detector and establishes this relationship. In

addition, the counting room is described and shielding

considerations explained.


A. Primary Detector


In order to obtain the greatest resolution and thus the

most discriminating and accurate spectrometer, a Ge(Li)

detector was selected as the primary detector. The Ge(Li)

detector yields by far the best photopeak resolution and the

greatest peak-to-Compton ratio of any detector presently
(12)
available. However, the efficiency of the Ge(Li)

detector is typically much less than other available

detectors. Taking both properties into account, the Ge(Li)
(6)
detector still yields a superior spectrum in most cases.

The Ge(Li) detector, produced by Ortec, Inc. has a crystal

volume of 34 cc which yields 6% efficiency (re 3x3 NaI(Tl)),2.9

keV resolution, and 15 to 1 peak-to-Compton ratio for Co.

The detector as shown in Figures 3 and 4 includes a liquid

10



















4-)


Or-4
(D
a,



Ha
Ed


0

Z: H
Q)r-
4.3 0
m 0d

























































Fig. 3 Ge(Li) detector side view.


























































Fig. 4 Ge(Li) detector frontal view.







14

nitrogen container required for cooling of the crystal and a preamp-

lifier mounted on the side.. The standard Ge(Li) detector

configuration prohibits its insertion into a large guard

detector. Therefore to facilitate use of this detector, a

10 inch long by 2.5 inch diameter plexiglass collar having 0.25

inch thick walls was used to raise the detector to a more

accessible position (see Figure 5 ). Due to cooling re-

quirements of the detector, this position could be maintained

for only 24 hours without topping-up the liquid nitrogen.

(In the standard position, the nitrogen container can cool

the detector for approximately three weeks.)


B. Guard Detector


The single requirement of the guard detector is that it

sense nearly 100% of all gamma photons entering it. This

requires that each gamma photon go through at least one

Compton scatter. Since the scatter cross section is typically

much larger than the photoelectric cross section, Compton

scatter dominates the total mass attenuation coefficient
(13)
which is approximately equal (av. 0.1 cm2/g) for all

available detectors within the energy range of interest.

Thus to obtain 95% detection the mass thickness must be
2
30 g/cm2. For NaI(T1) crystals this would require a detector

depth of about 8 cm and for liquid or plastic scintillators

about 30 cm.(Ge(Li) detector configuration limits its position-

ing into a solid guard detector to 30 cm deep.)























































Fig. 5 Ge(Li) detector with extension collar.





16
Liquid scintillator was chosen for the guard detector

because of its faster decay time over NaI(T1) and its larger

output and longer light mean free path than plastic scintilla-
(13)
tor. The former lowers the energy detection limit and

decreases dead time while the latter allows use of a larger

volume detector, and thereby higher detection efficiency.

Pilot Chemical's new high-efficiency (60% re anthracene)

mineral oil scintillator was determined to have one of the

longest mean free paths (5 meters) and the shortest decay
(14)
times (2 ns). The guard detector consists of 80 gallons

of this scintillator in a 31 inch diameter by 26 inch long

cylindrical tank with a Fairchild K2128 15 inch photomulti-

plier (see Figure 6) mounted on one face (see Figure 7) and

a 5 inch diameter by 12 inch long sample port in the oppo-

site face (see Figure 8). The inner surfaces of the tank are

painted with white epoxy paint to increase photomultiplier

light collection.

The Ge(Li) detector can then be positioned in the guard

detector port as shown in Figure 9.


C. Sample Detector

The sample detector was designed in the shape-of a

Marinelli beaker (coaxial around the primary detector) to

allow detection of large-volume low-concentration samples.

Since addition of sample depth beyond 3.5 cm has little

advantage due to scatter and absorption in the sample (50%

attenuation path of 100 keV gamma photons in water), the

sample container size was restricted to 1.5 litres giving






17




































4.



-P







0



0


4-)
C)




0



41-)
a)
Id
rd

$4
cs









lii
'14 "





19































,0


42
C)

4--

0










0


E
cs
Co

0
42


c,
H:
p1

CI




rz
























































Fig. 9 Ge(Li) detector positioned in sample port.





21

about 3.5 cm of depth around the primary detector (see

Figure 10).

The sample container itself is made of 15-mil thick

aluminum to minimize absorption (2% of incident 70 keV gamma

photons), is coated with white reflective epoxy paint and is

viewed on the end by an RCA 6655 2-inch diameter photomulti-

plier tube.

In order to act as a detector, the sample must be mixed

with a liquid scintillator. Several different commercial

scintillators allow addition of up to 40% water. These include

Aquasol by New England Nuclear and Insta Gel by Packard Instru-

ments. In addition, a mixture of p-xylene and Triton N-101
(15)
has been shown to accept 40% water. However, only Aqua-

sol was used due to its superior light transmission.

Two sample ports were constructed for use with and without

the active sample container. Figure 11 shows the sample port

for use without the active sample container and is constructed

of 50 mil thick aluminum. Figure 12 shows the sample port for

use with or without the active sample container and is con-

structed of 125 mil thick plexiglass. The former absorbs 6%

of incident 70 keV gamma photons and the latter 2%.


D. Counting Room and Shielding


The counting room, surrounded by 24 inch thick low-

activity poured concrete, measured 7.5 feet by 18 feet. The

positioning of the guarded detector system in this counting

room can be seen in Figures 13 and 14. The counting room also








23











































rd
Id

z
4-,




(Z





0
4-,
$A



0









rx-





24





































cs


to



cs
0
0)



C)
H














co
C)l







r4-
G('
H




rxl





25






























4d
C0















O
C)







IT-4



d





26













































0
0
0


U
O

r e






r**

OI





27

houses a 4x4 inch NaI(Tl) system.

Gamma exposure as measured on a Nuclear Chicago model

2650 gamma survey meter was 0.01 mR/hr in the counting room

and five times higher outside of the room.

Mercury was used to shield the guarded Ge(Li) system

since it has a higher mass number, greater density, and lower

activity than lead. Five thousand pounds of mercury in 2.5

inches thick double wall tanks (approximately equivalent to 4

inches of lead) surround the guard detector and its photo-

multiplier tube (see Figure 15). With the shield, the

guard detector measures 36 inches diameter by 31 inches long.

Due to inherent activity in construction materials, the

detectors, and the mercury, additional shielding is not

significantly advantageous.

As protective measures against mercury toxicity, a

ventilator system and seamless floor were installed in the

counting room.


E. Electronics


The complete system electronics is block diagramed in

Figure 16. Each of the detectors has its own preamplifier,

linear amplifier, and threshold detectors which drive coincidence

and anticoincidence gating. In addition, the Ge(Li) detector

electronics include an ORTEC model 120-2B preamplifier (supplied

with the detector), a model 485 linear amplifier, and a model 444

biased amplifier. Except for these, the ORTEC high voltage

supply, and a Packard Model 115 400 channel multichannel

analyzer (MCA) (shown in Figure 17) the electronics were











*. .., F-No


It






29













Cd
0



4-)
2. 2






C)

0)













4-)

I) H



Cd)

Ei)





02
cts
PC1





























$-4
:E 0 c


0 cs 0
-r-4 4-



0 cs 0
4-) > 4) Cd4.
a) ) 41












i l






31
custom designed and built in the laboratory.

The signal from the Ge(Li) crystal after amplification

in the 120-2B preamplifier is split, being amplified by the 485

linear amplifier and 444 biased amplifier before entering the MCA,

and being amplified, sensed by a threshold detector, and then

given control of the gate leading to the anticoincidence in-

put of the MCA. Similarly, the guard detector and sample

detector have their own identical preamplifiers, linear amplifiers,

and threshold detectors. In the sample-guard mode, these two

threshold signals are combined by an OR gate which feeds the

Ge(Li) AND gate controlling input to the MCA anticoincidence

input. In the coincidence sample mode, only the guard thres-

hold feeds the Ge(Li) AND gate while the sample threshold con-

trols the coincidence input of the biased amplifier.

The effect of this circuitry for the sample-guard mode

is to send an anticoincidence control pulse to the anticoin-

cidence input of the MCA only when the Ge(Li) detector yields

a signal above its noise level and the guard detector or the

sample detector yields a signal above its respective noise

levels. The effect for the coincidence sample mode is to

allow amplification of the Ge(Li) signal only when the sample

detector yields a signal above its noise level and to send an

anticoincident pulse to the anticoincidence input of the MCA

only when the Ge(Li) detector and guard detector yield signals

above their respective noise levels.

The circuit diagrams for the electrons of the three

detectors, the coincidence units, control pulse generators,




32
and cable drivers are given in Appendix A.












CHAPTER IV


NaI(T1) SYSTEM

In order to better show the advantages of a guarded

Ge(Li) system, comparison is made with a NaI(T1) system. In

this chapter both the NaI(Tl) configuration and its associated

electronics are presented.


A. Configuration


The NaI(Tl) system used as a standard for comparison

consists of a Harshaw 4x4 inch NaI(T1) crystal viewed by a

photomultiplier tube and positioned in the center of a 2 foot

cubic, 2 inch thick lead shield lined with Cu and Cd (gee

Figure 18). The top side of the shield is supported on

rollers to allow access to the detector crystal. (see Figure

19 ). The shield and detector system occupies the rear corner

of the low level counting room as shown in Figure 20.

The NaI(Tl) detector is customarily used to count samples

in both 1.0 litre and 3.5 litre Marinelli beakers. In addition,

samples in 1.0 litre "ice cream" containers and flat samples are

frequently counted on this crystal.


B. Electronics

The electronics consists of a Power Designs Pacific

model HV-1565 high voltage power supply, Nuclear Data models

33

























































Fig. 18.NaI(T1) detector configuration.



















wc~E





37

NTD-180F analog to digital converter, ND-180N 512 channel

memory unit, and ND-180R readout control unit, a Tally

Corporation model 1506 paper punch, a Tektronix model RM503

oscilloscope, a Nuclear Data model ND-316 autofinger mounted

on an IBM electric typewriter, and a Dohrmann Instruments

model 299 chart recorder. The system block diagram is shown

in Figure 21.

The signal after collection by the photomultiplier tube

is amplified, digitized and recorded in the appropriate

memory channel. Upon command, the memory can either be

displayed on the oscilloscope, plotted, or paper punched.

The latter allows computer analysis of the data.




38















o
0
0 0





CO
EO 4) 4*)

















0 r.O
CO0
H" 0 N4

4- H

d -) 0 1p4

0














H Ho
4--

















-4
CO
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002
< 0 rz
(_) CH
I ----- I -- I --N












CHAPTER V


NaI(T1) PERFORMANCE


In this chapter the optimum performance of the 4x4 inch

NaI(T1) system in the low-level counting room as described

in Chapter IV is stated. Included are system background,

efficiency, minimum detectable activity, peak-to-Compton

ratios, and ability to analyze complex spectrum.


A. Detector Background


The 4x4 inch NaI(T1) crystal background count-rate was

determined in the absence of a sample and with the shield in

place. Since standard counting procedure in this laboratory uses

40-minute counts, a 40-minute background was accumulated and

is plotted in Figure 22. Only traces of the 0.35 MeV RaB

and the 1.46 MeV 4K photopeaks are visible in the spectrum.

Table 1 lists the background count-rate grouped in various

energy ranges and shows a cumulative background count-rate of

just over 500 cpm for the 70 to 2500 keV range.


B. Efficiency


The absolute efficiency of the 4x4 inch NaI(T1) crystal

was measured for both point sources (1.0 cm from the crystal)

and 3.5 litre sources with calibrated sources obtained from

39











































x0t 91"'T














gu 3c"


TaNNV1H3/SlNnOD


OO
o0
-4






41










4-)
cs
r \- P4-) 104 -.t V 0

0





a,
4'
CO
C




00 0 0

j N N N
-4 O 0 0 0 0
> I I I I
1 0 0 0 0
S 0 0 0





0






0 0
4.1








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00
0 0
Oc O










O O 0 0 0
0 a 0 co 0o\ 0










p4 0 0 O O O 0 0 0
-.g C- 0 0 0 0 0 0 0
N 00 co 0 OV 0
t0> 1 1 1 1 1





42

Baird Atomic and the Public Health Service. The plots of

photopeak (full width at tenth maximum) efficiency vs. energy

are shown in Figure 23 and include data from the Public Health
(16) (17)
Service and previous data taken on this system. The

data yields essentially straight lines on log-log paper, showing

increasing efficiency with decreasing energy down to a point

where detector window absorption becomes significant. These

data also show that the 3.5 litre sample efficiency is

typically 1/3 of that for the point source, the difference

being attributed to differences in the average source-to-

detector distance.


C. Minimum Detectable Activity


The theoretical minimum detectable activity depends upon

the background count and photopeak efficiency. It was

determined for the 4x4 inch NaI(T1) detector by requiring a 95%

detection certainty. This means that the theoretical minimum

detectable activity equals two times the standard deviation

in net count (gross minus background) under the photopeak

energy spread, corrected for detector efficiency. For a 40-

minute count, the minimum detectable count rate in cpm equals


2 4/ (gross count + background count)
40

which yields a minimum detectable activity in dpm of


Using the measured photopeak efficiency and background


21/2 x background count (under Dhotopeak)
40 (efficiency)










I I I I I


100
90
80
70
60


I I


I I


t~i*


**


4-


21-


SPoint source
* 7 ml sample (16) (17)
S3.5 1. configuration


L. * C I


.2 .3 .4 .5 .6.7 .8 1.0
Energy (MeV)


photopeak efficiency.


0.O


50

40


10
10 -
S9
" 8
U 7
I-

5


2.0


3.0


.I .


I I I I I I I I I I a j I i I


! i


I | I


Fig.23. NaI(T1)




44

data, the theoretical minimum detectable activity was

calculated for nine radionuclides and photopeak energies.

Figure 24 shows a plot of minimum detectable activity in

gamma photopeak dpm vs. energy. In addition, the minimum

detectable activity in pCi and the minimum detectable con-

centration in pCi/1 for several radionuclides of interest

are listed in Table 2.


D. Peak-to-Comoton Ratio


The peak-to-Compton ratio of a detector is important since

it is an indication of the amount of interference between

radionuclides in the complex spectrum. Figure 25 shows the

peak and Compton relationship in a typical point-source

spectrum. Statistical variation in large Compton plateaus

greatly increases the minimum detectable activity in that

energy range. This uncertainty increases with large-volume

samples due to Compton scatter within the sample before

detection. Also in complex spectra addition of Compton

plateaus at the lower energies becomes very significant.

The peak-to-Compton ratio for the 4x4 inch NaI(Tl) de-

tector was measured and plotted in Figure 26 for both point

sources and 3.5-litre sources. The degradation in peak-to-

Compton ratio for the large sample can be readily seen to in-

crease at lower energies.


E. Complex Spectrum


Figure 27 shows a complex spectrum for the 4x4 inch

NaI(T1). The complex spectrum of 226Ra was used in order to
















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48











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50

standardize comparison. It can be seen that both the poor

resolution and the addition of Compton continuum raise the

minimum detectable activity in the low-energy range of the

spectrum to approximately the activity of the high-energy

photopeak radionuclides in the spectrum.













CHAPTER VI


Ge(Li) SYSTEM PERFORMANCE


In this chapter the performance of the standard Ge(Li)

system is presented. System background, efficiency, minimum

detectable activity, peak-to-Compton ratios, and complex

spectrum resolution are tabulated.


A. System Background


In order to show both shielding benefits and shielding

problems, a 24-hour unshielded background in the low-level

counting room was taken with the Ge(Li) spectrometer. The

spectrum given in Figure 28 shows a complex collection of

photopeaks from both radon and thoron daughters. Background

with 2.5 inches of mercury shielding, given in Figure 29,

shows a considerable reduction across the spectrum with

fewer photopeaks being discernible. The background still

contains small radon daughter photopeaks mainly due to their

presence in the large air space between the shielding and

the detector. Background with this air space filled with

liquid is discussed in Chapter VIII. Integrated background,

both shielded and unshielded, is tabulated in Table 3 for

several energy ranges.





















0
a
















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C

































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I










a)


C) C) N C'- n V Nq n' H? V' N 0 \O
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0 0 0 0 0 0 0 0 0 0 0 0
00 0 0 0 0 0 0 0
--- 0 0 0 0 0 0 0 c' 0 0 0
N 0 C\ o 0 0 N -- 0
i ~ ~ o~~ ,-I -I ,-





55

B. Efficiency


The efficiency of Ge(Li) detectors is much less than

that of NaI(T1) detectors, ranging up to only 20% as

efficient as 3x3 NaI(Tl) for 6Co. The Ge(Li) detector

used for this system was rated only 6% efficient (re 3x3 inch

NaI(T) 6Co source). To allow activity calibration, an

absolute efficiency for the Ge(Li) detector was determined

for both calibrated point sources and for 1.5-litre sources

(configuration of the active-sample container). Both are

plotted in Figure 30, and essentially are straight lines on

log-log paper with efficiency increasing at decreasing energies.

The 1.5-litre configuration is again about 1/3 as efficient

as that for the point source (1 cm distance) due to the

difference in geometry factors for the two configurations.


C. Minimum Detectable Activity


The theoretical minimum detectable activity was determined

as in Chapter V. For a 40-minute count, it equals in dpm


24 2 x Background count (under nho opeak)
40 x efficiency (of photopeak)


Figure 31 shows the theoretical minimum detectable photo-

peak activity for the 34 cc Ge(Li) detector with 2.5 inches of

mercury shielding and shows a slight increasing sensitivity at

lower energies. In addition, the theoretical minimum detectable

activity in pCi and theoretical minimum detectable concentration

in pCi/1 are tabulated in Table 4 for several radioisotopes of

interest.











I I I I I I I I


6_


1.5 litre
Point source


[I I I I I I


.2 .3 .4 .5 .6.7 .8 1.0
ENERGY (MeV)


2.0 3.0


Fig.30. Ge(Li) photopeak efficiency.


4


3-


.1
.1


I I I I I I I I


I I


I


I I


I I


2k


I


I



















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O-4
0
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w6a )V dOLOHd


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.-






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c
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4 E O N 4 N 0 H 0 4
0 N N \ H H r H C' H

S0


p p crT 0 H
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) HH

0 0
0*

o aH (\ "0 H N N H c' \0 N
i 0 H HH




H 0
-PP

-i o o H






,0
< 0 .4



H
S ( 0 i 0 n


0 P z H
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E oS 0 + Nj c^- ( o

>0 C) 0 r \ H c N i c N N l
4 o H un H o f
o5i4-3 H i -Qt




OP4
H CC).r-\ 0 C0 V40


40 4




o 4 0 O 4 ) 0
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SN o H r
ce o













0 \0 N C- \ H 4
04 N N 0a 0 COr 4 \0
N N H HI H H
4> C` aD *








*^~c CI +s C (





59

D. Peak-to-Compton Ratio


Due to the high resolution of Ge(Li) detectors, their

peak-to-Compton ratios are typically several times larger

than those for NaI(Tl). The peak-to-Compton ratio for

several radionuclide photopeaks was determined for the 34 cc

Ge(Li) detector and is plotted in Figure 32. Both point-

source data and 1.5-litre data are shown. Again it can be

seen that the peak-to-Compton ratios for the large-volume

sources are much less than for the point sources, being

particularly worse at low energies.


E. Complex Spectra


To demonstrate the usefulness of the Ge(Li) detector

for analysis of complex spectra, the complex spectrum for
226
Ra is shown in Figure 33. It can be seen that the

Compton continuum contributes most to the increase in the

practical minimum detectable activity and not the photopeaks

which are now only 3 keV wide. Thus the minimum detectable

activity at low energies is comparable to the Compton

continuum and can be predicted by the cumulative activity of

all radionuclide photopeaks of higher energies divided by

the average peak-to-Compton ratio.


















0
C.,

H 0



.0


0 *t






*L


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0




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0














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m 4-)



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*




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as 4













CHAPTER VII


GUARDED SYSTEM PERFORMANCE


In this chapter the performance of the anticoincidence

guarded Ge(Li) spectrometer is presented. As in Chapter VI,

the system background, peak-to-Compton ratio, and minimum

detectable activity are specified. In addition, factors

determining guard-detector efficiency and reduction in

photopeak efficiency are explained.


A. System Background


In the guarded mode, gamma-ray spectrum background is

reduced due to the additional passive shielding of the liquid

scintillator, the removal of Compton scattered events in the

guard which are detected in the Ge(Li) crystal, and the

removal of Compton scattered events in the Ge(Li) crystal

which are detected in the guard.

Figure 34 shows a 24-hour background spectrum with the

guard off. Comparison of this with Figure 29 (mercury

shielded background) shows a decrease in the background at

high energies but an increase in the background at very low

energies (200 keV) when the liquid scintillator is present.

This difference is due to additional energy removal in

Compton scatter by the low-mass-number liquid,thus reducing the




63










0
C,

O













o










z
60





0 00- o d










-00
I












00 0
ggi Z~'O -












0o0 0-

qsNNVMDA lsNno3





64

energy of the background gamma photons. Figure 35 shows the

effect of anticoincidence guarding on the background spectrum.

A comparison of background is tabulated in Table 5 and in

general shows a guarded background reduction of approximately

6 times over the unguarded for all energies. The photopeaks

remaining in the spectrum are due to the unshielded 3% solid

angle geometry of the sample port and the activity within the
40
detector system. The 4K photopeak in the spectrum corresponds
226
to a point-source activity of 45 pCi and the Ha daughter

photopeaks correspond to an activity of 10 pCi. Thus, this

sets the lower limit on the detectable activity for these

two radionuclides.


B. Guard Detector Efficiency


The main purpose of the guard is to detect Ge(Li)

crystal scattered gamma photons. Interaction in the guard

detector may deposit only part of the photon's energy. Thus

for the highest detection efficiency, the threshold of detection

for the guard should be as low as practical. This was deter-

mined by allowing a photopeak reduction of 10% due to unre-

lated chance pulse counts in the guard detector system and

corresponds to a background count rate of 2,400,000 cpm.

Appendix B gives the background spectrum of the guard system

and shows the threshold corresponding to 2,400,000 cpm to be

deep in the noise region of the photomultiplier tube.

With the guard threshold set at this point (approximately

15 keV) its efficiency was determined and plotted in Figure 36.




65









0
0.0



T
r o


c








0-0
oo










(. 09'
,co
xrd











~~~('h~ ne d~d 0) I g'fiM

















03 0OT p
cs
0
-^ o













o o o
M -c
S (
_










'0o
o o o
OS 60o -^ ^


(o *A-nb -40 -[do[ ^ ScuO





















C.- _. C -.
C- 0 H H 0 co

C N O 0 0 0 0 0













- I* V '
00 c0 0 0 O r 0


















0 N C- c~ c) H c O 0 O
r-4 N 'V -+ N
H


Q)

00 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0
SN 0 -H N N C\N N N NOJ
A M I I I I I I I I I I I I
be.- 0 0 0 0 0 0 0 0 0 0 0 0
C- 0 0 0 0 0 0 0 D'.- 0 0 0
N 0 O 0 0 N 0
H H N H


a
C)








0
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\O
0-0



v\>>
o

*9Z


0 0 0 0 0
ON co O \

(%) XON3I3IddS





68

Because of the high background count, the efficiency was

calculated by first measuring the photopeak reduction for

one photopeak of a cascaded decay (2 photons) radionuclide
24 60 133 134
(e.g. Na, Co, Ba, and Cs). After deadtime correc-

tion the ratio of the guarded to unguarded count for one

photopeak gives the undetected fraction of the other gamma

photon (133Ba and 134Cs having complex cascade decay required

some approximations).

The plot of efficiency shows a loss of efficiency with

decreasing energy below 500 keV and a loss of efficiency

with increasing energy above 700 keV. Losses in efficiency

can be explained with a 3% energy independent geometry loss

by the sample port, low energy loss due to a 15 keV detection

threshold (loss 2 E(keV)/15, see Appendix C) and the decrease

in attenuation coefficient for high-energy gamma photons.

The reduction in Compton continuum is directly dependent

upon the guard detector efficiency except at low energies where

the Ge(Li) detector mounts and casing absorb some of the crystal

scattered gamma photons.


C. Photopeak Reduction


Since for highest Compton suppression the guard detector

threshold must be set as low as possible (deep in the noise

region), an acceptable photopeak reduction was arbitrarily

chosen as 10%. A 10% reduction while not substantially

affecting counting statistics or efficiency does greatly

increase the allowable guard detector background and thus

allows significant lowering of the threshold setting.




69

The guard detector background count rate which gives 10%

chance coincidence is 40,000 counts per second.


D. Minimum Detectable Activity


The minimum theoretical detectable activity was

determined as in Chapter VI. However, due to reduction in

background count for the guarded spectrum, 40-minute counts

gave backgrounds below 1 count per channel. When this

occurred, Poisson statistics were used in determination of the

minimum detectable activity (3 is the minimum photopeak count
(18)
for 95% confidence in the extreme of zero bkg.) ) Figure

37 gives the calculated minimum theoretical detectable

activity in terms of gamma photopeak decay for both point

sources and 1.5-litre sources as a function of energy. In

addition Table 6 gives the theoretical minimum detectable

activity, and minimum detectable concentration for several

radionuclides for both 40 minute and 24 hour counts.


E. Peak-to-ComDtcn Ratio


The peak-to-Compton ratios for the anticoincidence

guarded 34 cc. Ge(Li) was determined as in Chapter VI for

the unguarded Ge(Li). The guarded peak-to-Compton edge
65
ratio was 9 times higher for Zn and 8 times higher for

137Cs. Figure 38 shows the guarded peak-to-Compton ratios

as a function of energy for both point sources and 1.5-litre

water sources. The peak-to-Compton ratio can be seen to be

quite constant above 200 keV. The low-energy fall-off is


















C U
o4 wa



?ILI
0*





0




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co


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a



ro o
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S*r














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cs .c


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4f \O N C\ 0 \O C?










4, \o o \,o \0 co c"
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40 O Y0 O i O Oc











0 0 ( o

n CN c \ 0 N

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73

due to both loss of efficiency in the guard system and

absorption in the Ge(Li) crystal casing.


F. Complex Spectrum


The usefulness of the anticoincidence guarded Ge(Li)

detector for a complex spectrum is shown in Figure 39.
-226
Radium was used to show both the separation of the many

photopeaks and the small amount of Compton continuum present.

It can be seen that the minimum detectable activity doesn't

significantly increase for complex spectra.


















-2-
x






DUE 822ZT








etU 9t T





DOE 02T*T







DOU 69'*0



DUH 6090 -


WIV 25c'o


0 0
0
0
rI3NNVHD/wNnoo


o


CIO




(!













CHAPTER VIII


ACTIVE-SAMPLE SYSTEM


In this chapter the performance of the anticoincidence

guarded system with active-sample guarding is presented.

Since the active-sample technique uses the same geometry as

the 1.5 litre samples reported in Chapter VII, only differences

in performance are presented. These include efficiency, peak-

to-Compton ratios, minimum detectable activity, and separation

of decay modes.


A. Efficiency


The efficiency of the active sample geometry is the

same as that reported in Figure 30 for the 1.5-litre samples.

Again,as with the standard'anticoincidence guarding, random

coincidence reduces the photopeak count. The guard again is

set for a 40,000 count per second background while the sample

detector is set for a 30,000 count per second background. The

combined guarded background yields a 15% photopeak reduction as

compared to a 10% reduction with guard shielding only.


B. Minimum Detectable Activity


The single radionuclide minimum detectable activity for

the active sample technique is improved only at energies below





76

500 keV and then by only about 15% due to a 30% improvement

in background suppression. The minimum theoretical detectable

activities, determined as in Chapter V, are plotted in Figure

40 as a function of photopeak energy. In addition, the

minimum theoretical detectable activities and concentrations

for several radionuclides are tabulated in Table 7 for

both 40-minute counts and 24-hour counts. The increase in

the minimum detectable concentration is due to the required

dilution of the sample with liquid scintillator.


C. Peak-to-Compton Ratio


The peak-to-Compton ratio for the active-sample technique

is shown in Figure 41 as a function of energy. The peak-to-

Compton ratios are restored to 95% of their point-source

ratios at high energies but only restored to 50% at energies

below 100 keV. However improvement in peak-to-Compton ratio

is still approximately 2-fold across the spectrum.


D. Separation of Decay Modes


In addition to improving peak-to-Compton ratio, the

active-sample technique removes all short-lived f + and ft

decay excited-daughter radiations from the spectrum. By

requiring coincidence between the sample-detector and the

Ge(Li) detector, these short-lived states can be left in the

spectrum while removing all electron capture and metastable

states. Figure 42 shows this division of states; however

to show the effect with available radionuclides. (positron























































































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81

and cascade decay) the guard was disconnected for the

coincidence mode.













CHAPTER IX


COMPARISON OF SYSTEMS ON ENVIRONMENTAL SAMPLES


In this chapter the spectra produced with the various

systems described previously are compared for their ability

to identify radionuclides. Included are prepared samples

and actual environmental samples in various forms.


A. Standard Samples


To show the advantages of the active-sample technique

for water samples, two standards were prepared. The first,
65
containing only Zn, was used to produce the spectra in Figure

43. The upper two spectra are from a guarded and unguarded

point source; the guarded spectrum shows a Compton edge

suppression of a factor of 8. The remaining three spectra

are for a 1.5 litre source. The normal guarded spectrum

shows a Compton tail suppression of a factor of 7 while the

active-sample guarded spectrum shows a suppression of a

factor of 23. This additional Compton tail suppression

would permit recognition of a much smaller photopeak within

this energy region.

The other standard contained 6 radionuclides having

different decay modes. The spectra are shown in Figure 44.

In addition to the suppression of annihilation radiation,




83








Point Source
Unguarded-Guarded o
o
reduction
(x6)
(x6 (x7) (x8\


0


1.5 Litre Source


Unguarded




00
4.








o
Reduction
z
Gu(x7) (x.5) (x
0






Active Sample

O
reduction
(x7) (x84) (x5.3)

o0





000

CHANNEL NUMBER


Fig. 43. Zn-65 source spectrum.




84







9 34ce Ge(Li)
\_ 5Sample -1.5 litre

0 0


e ci EC be

i ,,' o..

I'' 1










Reduction E.C.& Metastable
ACTIVE SAMPLEANTICOINCENCE400
x33




Effect With Available
Standard Sources
.Short Lived
Reduction & M tsal












0 ACTIVE SAMPLE COINCIDENCE 400



CHANNEL NUMBER

Fig. 44. Complex standard-sample spectrum.
Fig. 44. Complex standard-sample spectrum.




85

cascade events, and the Compton tail in the normal guarded

mode, the anticoincidence sample-guarded mode suppressed

the Compton tail another factor of 3. Also the coincidence

sample mode suppressed electron capture and metastable states

while restoring the rest.


B. Air Sample


To demonstrate the usefulness of the normal guarded mode,

an air sample was taken of 45,000 cubic feet of atmospheric

air and allowed to decay for 3 days. Comparison of 40 minute

count spectra for NaI(T1), unguarded Ge(Li), and guarded

Ge(Li) shows no identification of any radionuclide in either

the NaI(T1) or unguarded Ge(Li). However, data analysis on

the guarded Ge(Li) spectrum indicates the presence of 100 pCi

of 7Be, 8 pCi of 103Ru, and 12 pCi of 95Nb (all activities

determined from digital data see Appendix D).


C. Water Samples


To demonstrate the advantages and disadvantages of the

active-sample technique, two environmental water samples were

counted on the various systems. The first sample was an

environmental standard sample prepared by the Public Health

Service and its spectra are shown in Figure 46. The NaI(Tl),

unguarded Ge(Li) and guarded Ge(Li) spectra all show the

presence of 200 pCi of 106Ru and 200 pCi of 144Ce although

their presence in the guarded Ge(Li) is easiest recognized.

The active-sample mode does not show their presence due to

the dilution of the sample with scintillator.

















Nal (Tl)

with background substraction


200


I V ,I


Unguarded Ge(Li)


45,000 cubic feet
3 day decay
40 min. count


~PiJV.AIL&JIL. J. n..


400

u u
00O
o Guarded Ge(Li)
0
I I I
c~- ON
-v\t
0-0- r(N
0'-C 0\OrI1


, LL. ..-. I


00 O
...A L.. L ,


4b0
CHANNEL NUMBER


Fig. 45. Air sample spectrum.


'I JllPlSii ^nnl7~LIEIhnlnyiW


0 \


800


* -d A -in--- warp


800


IT- -J -4 L11 A .. ....... . . ..L


-LlhLYLCLII _L*I_~I-L--~-IL-- ----L


a













NaI (Tl)
with background substractio


3.5 litre


300







0





20


0


1.5 litre


Guarded Ge(Li)


O -I



ScU

0


4o0


1.5 litre


400


400


Active Sample


Water standard
from PHS
40 min. count


0.7 litre


30 reduction


400


400
CHANNEL NUMBER


Fig. 46. Water standard spectrum.


Unguarded Ge(Li)


Z

20

E 0

0 0


U U
o f
.-o-0


,I I II


I


r C1CrrC~p6AF~J ) IrknlL.LhUC


- ----


_ ___-


zWc~ud





88

The other environmental water sample was taken from the

University of Florida Training Reactor coolant water. The

spectra shown in Figure 47 indicates the presence of 20,000
24
pCi of activated Na. The NaI(Tl) spectrum is completely
24
swamped by the Na, however the unguarded Ge(Li) clearly

identifies the presence of 1,400 pCi of 99mTc, The guarded

Ge(Li) spectrum also shows the presence of 700 pCi of 115In

and 115Cd. The active sample mode spectrum, because of the

removal of 24Na ( a short-lived, ) and its Compton continuum,

shows the presence of 13 pCi of 133Ba and 90 pCi of 65Zn

(both contamination in the sample container). It should be
24
noted that the 2Na Compton continuum is less than the back-

ground in the active-sample spectrum and thus does not raise

the minimum detectable activity in that region.


D. Solid Samples


To further show the usefulness of the guarded Ge(Li)

system three environmental solid samples were analyzed. The

first, whose spectra are shown in Figure 48, consists of an

800-gram soil sample taken from undisturbed top soil. All

three spectra for this sample; the NaI(Tl), the unguarded Ge(Li),

and the guarded Ge(Li) show the presence of 1200 pCi of 226Ra and

its daughter products, 500 pCi of 137Cs, and 400 pCi of 40K.

The NaI(T1) spectrum shows considerable interference between

these radionuclides, especially between the 662 keV 137Cs peak

and the 609 keV RaC peak. The guarded Ge(Li) greatly reduces

peak-to-peak interference and Compton-to-peak interference




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