DEVELOPMENT AND COMPARISON OF
COMPTON SUPPRESSION TECHNIQUES FOR
LOW-LEVEL RADIONUCLIDE ANALYSIS
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
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
LIST OF FIGURES ...........
LIST OF TABLES ............
... *. *..#..
* . . .
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 ........................
Counting Room and Shielding ............
Electronics ....................... ..... ...
(Tl) SYSTEM .............................
(Tl) PERFORMANCE ........................
Detector Background ....................
TABLE OF CONTENTS -- continued
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
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 ...............................
A. ELECTRONIC CIRCUITS ...........
B. GUARD-DETECTOR CHARACTERISTICS
C. SPECTRUM ENERGY CALIBRATION ...
D. EXAMPLE OF DIGITAL DATA .......
LIST OF REFERENCES ................ ....... ........
BIOGRAPHICAL SKETCH ..............................
LIST OF FIGURES
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
LIST OF FIGURES -- continued
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
LIST OF FIGURES -- continued
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
LIST OF TABLES
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
Frank R. Markwell
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.
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
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
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
for low level radionuclide analysis has been shown previously
by Cooper et al. Soon thereafter investigators applied
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,
report a Compton reduction for Cs of up to 9 times,
and those using plastic scintillator guards report a reduction
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
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-
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.
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
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
excitation of atoms in the immediate vicinity of the photon
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
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
are equally probable at 150 keV but at 1 MeV Compton scatter
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
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
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
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.
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
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
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)
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
Fig. 3 Ge(Li) detector side view.
Fig. 4 Ge(Li) detector frontal view.
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
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
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.
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-
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
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
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
Fig. 9 Ge(Li) detector positioned in sample port.
about 3.5 cm of depth around the primary detector (see
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-
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
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
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
As protective measures against mercury toxicity, a
ventilator system and seamless floor were installed in the
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
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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,
and cable drivers are given in Appendix A.
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.
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.
The electronics consists of a Power Designs Pacific
model HV-1565 high voltage power supply, Nuclear Data models
Fig. 18.NaI(T1) detector configuration.
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.
EO 4) 4*)
H" 0 N4
d -) 0 1p4
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I ----- I -- I --N
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.
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
P4-) 104 -.t V 0
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
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
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
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-
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)
which yields a minimum detectable activity in dpm of
Using the measured photopeak efficiency and background
21/2 x background count (under Dhotopeak)
I I I I I
* 7 ml sample (16) (17)
S3.5 1. configuration
L. * C I
.2 .3 .4 .5 .6.7 .8 1.0
I I I I I I I I I I a j I i I
I | I
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
Uh 0 H
0 0 O
0 F. 0
p o o - r- as
) ,.-4-) ;-4-
0 o' -Ot
P 4 0 0 4O P0 > 0 \O 00 C.- \.0 rO<
M 0 0 H H H
d >p <
-0 0 C) 0
S"o op o
S H4-) H
0 rf 4-.) C )
S0 r-0 *
41) 1 0 1 r-H .
0 4 P 'd 0 H
" 0 crH N N C- H \ N x
4'> C- H H
N .I 0 .4-)
4 0 0 -r4 0 0
H oi ( O Pr \
P 0 ? 0 bD
C ) 1 0 o wr
O 4 O LO N JO H ) Ft
4, 0 r 0
ri 4- PA F C
) 0a N H I 0 n H c
PO CC- H >~ H
2 0 w
4- -Qi tO0
e us 0 N 0
. C N H 4 Pt' 4
4 N 0 0 O
ENN H H
\0l m N CO r-4 V -
cd -V m P, z
-- -- T
, I I
O H ZTIT
DVE 19 0
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.
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.
V H 98l'
IaOe 9C Z
Ot(-X 0917 T
gTiLL -1 'O
(t OX) SiNnfo
ovg 806"0 0
lV CO H- H N H
C) C) N C'- n V Nq n' H? V' N 0 \O
S0 0 0 C) 0 0 0 0
> I I I I I I I I I I I
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 ,-
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
I I I I I I I I
[I I I I I I
.2 .3 .4 .5 .6.7 .8 1.0
Fig.30. Ge(Li) photopeak efficiency.
I I I I I I I I
0 0 O o 0 0
w6a )V dOLOHd
4 E O N 4 N 0 H 0 4
0 N N \ H H r H C' H
p p crT 0 H
O S C VI A N r r H 0
4-) 4 0, 0 0 -- r
o aH (\ "0 H N N H c' \0 N
i 0 H HH
-i o o H
< 0 .4
S ( 0 i 0 n
0 P z H
Ei- ) 0 *
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
H CC).r-\ 0 C0 V40
o 4 0 O 4 ) 0
SN o H r
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 (
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
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.
D 3H W1OZ"
D-6H 79Z" *T -
DlH E2 *T
D6uH 6" oo
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
0 00- o d
ggi Z~'O -
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
detector system. The 4K photopeak in the spectrum corresponds
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
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.
~~~('h~ ne d~d 0) I g'fiM
03 0OT p
o o o
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
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
0 0 0 0 0
ON co O \
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
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.
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
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
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
I I I I
0 0 o o
0 o0 \1 0\
O O NO\ hi
S-- o r4i
N \ 0 N
to N r
4- N 0 H
N 4 H
^f S 3
0 \0 (\1
ut Ps P
o H H O H 4-0
Sr-4 O H O CN 0
H (N N H 4 N
H\N \O N\ O N H
4f \O N C\ 0 \O C?
4, \o o \,o \0 co c"
40 O Y0 O i O Oc
0 0 ( o
n CN c \ 0 N
H' o\ o co' \o vn
rH r rH H l
I I I - 1
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.
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.
etU 9t T
DUH 6090 -
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.
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
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
NdG XV MdOIOHd
^o a Z
) Ud 0 V O 0 CN 0 N 0 -.
, 0 C co C' O O (- 0 V-) N
3 N ?
4) 4 -I
,v n *i N
) H 4)
0 4 H 4 N \ N NO N H
-P 0 -^ -' -- -
) 0 \0 N H 4
4 N n ( ON O 4 O \0 v
N H N H rH H
ft +-> Uoo~o~
0dm kQ =F
0) ^ *N
+1r o- }.- c^> V^ ^} Cs O ^ C
g ^ + C H
21 %1 ^%69
aTqlqs'es m _-g %96
' %001 i7y I- SO
CC T --e
elT-~a 80* 0
and cascade decay) the guard was disconnected for the
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,
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,
(x6 (x7) (x8\
1.5 Litre Source
Gu(x7) (x.5) (x
(x7) (x84) (x5.3)
Fig. 43. Zn-65 source spectrum.
9 34ce Ge(Li)
\_ 5Sample -1.5 litre
e ci EC be
i ,,' o..
Reduction E.C.& Metastable
Effect With Available
Reduction & M tsal
0 ACTIVE SAMPLE COINCIDENCE 400
Fig. 44. Complex standard-sample spectrum.
Fig. 44. Complex standard-sample spectrum.
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.
with background substraction
I V ,I
45,000 cubic feet
3 day decay
40 min. count
~PiJV.AIL&JIL. J. n..
o Guarded Ge(Li)
I I I
, LL. ..-. I
...A L.. L ,
Fig. 45. Air sample spectrum.
'I JllPlSii ^nnl7~LIEIhnlnyiW
* -d A -in--- warp
IT- -J -4 L11 A .. ....... . . ..L
-LlhLYLCLII _L*I_~I-L--~-IL-- ----L
with background substractio
40 min. count
Fig. 46. Water standard spectrum.
,I I II
r C1CrrC~p6AF~J ) IrknlL.LhUC
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
pCi of activated Na. The NaI(Tl) spectrum is completely
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
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