ITR-15
SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
INTRALABORATORY TECHNICAL REPORT
November 6, 1967
PRELIMINARY REPORT ON THE
PERFORMANCE OF A 10 X 10 INCH PLASTIC
SCINTILLATOR IN THE WHOLE-BODY COUNTER
David Dif Skov
SUMMARY
A 10 x 10 inch plastic scintillator was installed in the whole-body
counting chamber at the Southwestern Radiological Health Laboratory
(SWRHL) for the purpose of evaluating its spectral response in com-
parison with a 4 x 9 inch and 4 x 11.5 inch Nal(Tl) crystal detector.
The investigation took place in one room (Whole-body counting chamber)
of a pair of steel chambers located 20 feet below ground level in a
large basement enclosure shielded by concrete and earth. Each
crystal assembly was coupled with a 400 channel analyzer with
integrator-resolver, magnetic and paper tape output and input, type-
out, and x-y plotter accessories.
The sensitivity of the plastic scintillator, measured in photopeak ef-
ficiency over an energy band width from 0. 66 MeV to 1.12 MeV,
matched closely the gamma efficiency'for the 4 x 11.5 inch Nal(Tl)
detector.
The resolution of the plastic scintillator for an energy band 'of the
same width was of a magnitude from five to seven times larger than
observed with either the 4x9 inch or 4 x 11.5 inch detector. Other
investigations included: total room background activity; rotation of
the plastic scintillator on an axis to detect any drift of activity; and
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fall-off rate of a point source moved on a directed line away from the
4x11 inch and plastic detector. Each of the detector systems and
the method and results of the investigation are described .in detail.
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EXPERIMENTAL
The SWRHL Whole-body counting facility consists of a large under-
ground room (20 ft. below surface level) which encloses a pair of
steel chambers identical in physical .dimensions (12'L x 8'W x 9'H).
Each chamber is constructed of clean, pre-World War II, 6 inch
thick plate steel and is lined with 1/8 inch each of lead and stainless
steel. Both chambers weigh 160 tons and each room is entered
through a single swing-type steel door. One chamber contains an
Ohio-nuclear scanner consisting of a single overhead Nal crystal and
four smaller crystals below bed level. The other room contains the
whole-body detector, a 4 x 11.5 inch Nal(Tl) crystal (Figure 1), which
is mounted on an overhead support, allowing three-dimensional move-
ment for crystal placement.
The 4 x 11.5 inch Nal(Tl) crystal is used on a regular basis at the
SWRHL facility for the whole-body counting of individuals who have
been exposed to fission-yield products connected with operations at
the Nevada Test Site.
The crystal is optically coupled to a 2 inch unactivated Nal light pipe,
canned in stainless steel, and attached to seven matched verietian-
blind dynode photomultipliers. A 4 x 9 inch Nal(Tl) crystal is also
canned in stainless steel and is optically coupled to four photo -
multipliers. A .plastic scintillator (Figure 2) is optically coupled to
an unactivated (Vycor) light pipe, canned in aluminum and attached
to a single 5 inch photomu!.tiplier.
The spectral response from a plastic scintillator when even a mono-
energetic source such as 137Cs is counted, presents special problems
not confronted in Nal crystal spectrometry. Figure 3 shows a
typical spectrum obtained by counting the same source (137Cs) in the
same geometry under the 4 x 11.5 inch Nal and 10x10 inch plastic
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crystals. The extent of peak broadening from the plastic scintillator
is greatly in excess of the Nal generated peak.
For a Nal crystal, the cross sections for Comptoh and photoelectric
interaction vary according to gamma-energy. The 137Cs peak is the
result of total energy absorption either by single photoelectric col-
lision or multiple Compton interaction. The contribution of Compton
scattered photons resulting in partial energy loss is diminished due
to the density and size of a large detector. This leads to a peak
with minimal spectral broadening (the photopeak), since the contri-
bution of the total energy loss, due mostly to the photoelectric effect,
is maximal.
The pulse-amplitude spectrum from a plastic scintillator differs in
that the total cross-section for absorption is dominated, by the Compton
interaction. Primarily due to the density of the plastic detector,
the probability is high that an initial incoming photon or Compton-
scattered gamma photon will escape the volume of the detector.
Therefore, the pulse from this type of spectrum can best be described
as a "Compton-peak" which results from Compton-scatter followed
by photon absorption or escape.*
Another contribution to spectral broadening is the multiple inter-
action in the scintillator from back-scattered photons which increases
the total energy deposited beyond the energy corresponding to the
Compton edge. This event broadens the spectrum on the high-energy
side.1 Consequently, the delineation of a photopeak by Compton sub-
traction techniques suitable for Nal spectrometry magnifies to a
problem of identifying a peak corresponding to some given pulse
height from a Compton smear. This.becom.es a nearly impossible
task for multipeak spectrums.
* In this paper the terms photopeak and Compton-peak are used inter-
changeably.
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A worthy illustration of the problem, of applying Nal spectrometry
techniques to a plastic scintillator is the variation of Compton-peak
efficiency obtained with the latter: 0. 249%; 0. 090%; and 0. 017%. **
Three different approaches to the difficulty of resolving a peak to
obtain a "figure of merit" by which the plastic scintillator could be
compared to the Nal detector, included as a basis:
a. The use of photopeak efficiency by s.electing an energy band
of sufficient -width to correspond to either;
1. One-half peak height, applicable to only those isotopes
used which gave a peak/Compton trough ratio of two or
more;
2. Or two-thirds peak height. This grew out of the need to
furnish as a basis of comparison the relative efficiency
of the photopeak, which would eliminate many of the
errors associated with the Compton continuum, and which
would permit the application of a simple, accurate, and
reliable technique.
b. The treatment of an energy band of varying width (channel
width) as a window to obtain total efficiency.
** These values were obta.ned by integration and Compton subtraction
of the 137Cs photopeak channels; 40-95; 44-78; 40-78 respectively,
and indicates initial attempts to resolve the photopeak by separation
of channels which contribute to either a definite rise or fall of activity
to either side of the peak channel(s).
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RESULTS
From the calibrated standards available, only those were chosen
which were suitable as monoenergetic sources and provided suf-
ficient activity to reduce statistical error to a minimum. * Each
standard was counted in a geometry identical for each crystal:
. 6 meter from the crystal center. The choice of . 6 meter was neces
sitated by our use of this geometry in whole-body counting measure-
ments of people at the station. Figures 1 and 2 show the final place-
ment of the plastic and 4 x 11.5 inch Nal crystals respectively
(although not shown the 4x9 inch Nal crystal placement is similar
to Figure 2).
The spectral response of the plastic scintillator is essentially con-
stant with any change in direction of the detector over a point source.
By rotating the detector .on an axis from zero to ninety degrees, the
maximum efficiency detectable was in a position where the photo -
cathode end in contact with the crystal window, faced the source.
However, this exceeded any other position by no more than 5 percent.
The slight difference in response can be traced to a change in light
collection efficiency at the photocathode which is a function of the
position of the scintillation event.2
A comparative analysis of representative spectra from 137Cs, 54Mn,
and 65Zn, obtained in each case from the Nal 4 x 11.5 inch crystal
and plastic scintillator (Figures 3, 4 and 5), reveals that with the
latter, (a) the photopeak response on a per channel basis is much
lower, (b) secondary peaks such as backscatter, annihilation, and
x-ray, are not resolved, (c) linear response is poor, especially
above 1.0 MeV, (d) Compton continuum response is very nearly
137 r-o 54,, 65
Cs, Mn, bZn in 400 ml cottage cheese containers,
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equal in magnitude to the Nal crystal between 0. 2 MeV and the lower
energy side of the photopeak. Below 0. 2 MeV the Compton electron
(and combined x-ray) response rises sharply above the Nal detector.
From Table I and the results shown plotted in Figure 6, it can be
seen that the photopeak efficiency measured at either one-half
(for 65Zn only) or two-thirds peak height are nearly identical for the
plastic and Nal 4 x 11.5 inch crystals, either of which are higher
than the 4x9 inch Nal crystal. As explained earlier, .the peak/
Compton trough count-rate ratio from the plastic detector never ap-
proached a, value of two for l37Cs and 54Mn, so that no information
is available at one-half peak height.
An interesting comparison can be made of the foregoing results with
those of Table II, and from it, Figures 7, 8, and 9. The selected
window efficiencies were normalized to unit activity for each sepa-
rate isotope in the band width 0-1.99 MeV. For all window settings,
the relative efficiency for the plastic crystal assumes values which
range from a minimum of 2. 5% (137 Cs, 0-199) to a maximum of
25% (54Mn, 20-199) below corresponding efficiencies for the
4 x 11.5 inch Nal crystal.
It is apparent that the spread in window efficiency between the two
crystals increases both with a reduction in size of the window and
with the lowering of the gj.mma photon energy characteristic of each
isotope. These results ai e due in part to and are complicated by
the superimposition of secondary peaks upon the Nal generated spec-
trum such as the 65Zn . 51 MeV annihilation and 137Cs backscatter
peaks.
i
Referring once again to Table I, and Figures 10 and 11, the resolution
of all three detectors was determined at one-half and two-thirds peak
height. At the highest energy (1. 12 MeV), the best resolution achieved
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Table I. Gamma efficiencies and resolution at one-half and two-thirds
peak height.
X-Ray Energy
(MeV)
0.66
0.84
1. 12
0.66
0.84
1. 12 .
Gamma Efficiency
at 1/2 peak -height
(%)
Plastic
_(b)
-
0. 13
Nal
. 4x11. 5"
-
-
0. 14
.Nal
4x9"
-
-
- .
Gamma Efficiency
at 2/3 peak-height
(%)
0.089
0.092
0.074
0. 088
0. 089
0.074
0. 038
0.032
-
Resolution at 1/2
peak-height
. W1/2/Ax 100 (%)(*)
Plastic
> 89.0(C>
> 49. 5
39.6
Nal
4x11. 5"
9.90
9. 32
8. 20
Nal
4x9"
10.4
9- 04
-
Resolution at 2/3
peak-height
W2/3/A x 100 (%) (a)
54
39
.26
7. 55
7.45
6.54
8. 30
7. 35
-
(a) Wl/2 is width of peak at half-height.
A is peak position.
^2/3 is width of peak at two-thirds height.
(b) (-) indicates no information available.
(c) > indicates greater than or equal to.
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Table- II. Window efficiencies # from plastic and Nal(Tl) detector units.
Energy Band Width 0. 66 MeV (137 Cs) 0. 84 MeV (54Mn) ' 1 . 'l 2 MeV (65 Zn)
in Channels (a) (b) Percent (a) (b) Percent (a) (b) Percent
(10 KeV/Channel) NaI4xll.5in. Plastic Below (a) Nal 4x1 1 . 5 in. Plastic Below (a) NaI4xll.5in, Plastic Below (a)
0
10
20
40
-199
-199
-199
-199
1.
0.
0.
0.
000
958
841
633
0.975
0. 833
0. 675
0.475
2. 50
13. 1
19.9
25.0
1.000
0.883
0. 810
0. 547
.810
.722
.606
.467
19.0
18. 2
25. 3
14.7
1.000
0.933
0.859
0.677
0.950
0.884
0, 760
0. 620
5. 00
5.40
11. 6
8. 60
^Efficiencies have been normalized to unit activity in the energy band. 0-1. 99 MeV.
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from the plastic detector was 39. 6% compared to 8. 20% from the Nal
4 x 11.5 inch detector. As the gamma energy decreases below about
1. 0 MeV, the resolution at either one-half or two-thirds pulse-height
becomes astronomical. Part of the explanation for the very poor
resolution obtainable with the plastic detector lies in the use of a
single photomultiplier tube. Better resolution can be achieved with
the addition of phototubes of the same or larger size (which among
other benefits, would increase the light collection efficiency). *
With the information from Tables I and II in mind, an interesting
comparison can be made with the background observation shown in
Figure 12. The background obtained first from the Nal 4 x 11.5 in
crystal reveals the presence of 226Ra decay peaks (which originate
by radon emanation from the concrete walls below ground level) and
the usual 40K photopeak. The background -from the plastic scintillator
indicates (a) the absence of any photopeaks, (b) that the Compton con-
tinuum in an energy band from about 0. 2 MeV to 2. 0 MeV is nearly
identical to Nal crystal, (c) that the rise in activity below 0. 2 MeV
accounts for by far the greatest response in total activity. These
facts are borne out more clearly in Table III which presents data that
has been single-channel analysed from Figure 12.
Of immediate concern is the four-fold increase of total activity
(channel 0-199) from the plastic over the Nal crystal. As seen
earlier, the plastic scintillator, using the same energy band width,
gave a window efficiency slightly lower than the Nal crystal.
*The best results for improving one-half resolution (for a plastic
detector 6 1/2 x 10 x 20 in. ) have come from the use of a smaller
number of large diameter photomultipliers (two - 7 in tubes) rather
than the use of a large number of smaller size photomultipliers
(four - 5 in tubes). 3 .
10
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Table III. Response of a plastic and Nal(Tl) 4 x 11.5 inch
detector units to background (CPM/CHANNEL).
Detector Channel Limits (lOKeV/Channel)
Unit 0-199 0-10 11-20 21-40 41-80 81-130
Plastic 8041 4944 1759 477 435 224
Scintillator
NaI(Tl)4xll.5 1978 654 248 494 506 237
Inch
131-199
164
157
Only a minimum of information was compiled from the whole-body
counting of individuals in the arc chair under the plastic crystal.
The primary problem involved is one of determining where the
limits of a photopeak are located energy-wise since the subject (with
normal body burdens of 137Cs and 40K) and room background spectra
are almost identical in shape. If an arbitrary band width of channels
41-80 is chosen, corresponding to the approximate area under a
137 Cs photopeak, the counting statistics standard error for a 10 minute
count under a . 6 meter arc is +7.2 percent. A similar count using
a Nal 4 x 11.5 inch crystal gives a standard error of +10. 5%. The
error may, of course, be reduced if either crystal is moved closer
to the body. Thus, at a counting distance of 10 cm. from'the plastic
crystal center to the body navel, the error reduces to +3. 7 percent.
A final study was made of the effects of moving a point source
(Mn54) a line distance away from the plastic and Nal 4 x 11.5 inch
detectors. The results are shown in Figure 13, with all measure-
ments made from the centsrs of both crystals arid with channels
11-199 integrated to give total activity. As expected, the relative
efficiency of the Nal detector is highest at all source distances and
both detectors follow the inverse square law except for a distance
greater than about 160 inches for which no measurement was made.
11
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CONCLUSION
From the information presented, it is obvious that several errors
operate in conjunction to give a poor reliability index in any series
of whole-body counting measurements. Before using the plastic
detector in this regard, it would be mandatory to assess and pos-
sibly reduce the contribution of all errors associated -with either the
gamma spectral analysis or with the detector system itself.
12
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REFERENCES
*. Butch, P. R. J. , Hughes, D. , linuma, T. A., Overton, T. R. ,
and Appleby, D. B.
Proceeding of the Symposium on Whole-Body Counting, International
Atomic Energy Agency, Vienna, Austria, (June 12-16, 1961), 65.
2 . Ibid., p. 66.
3. Ibid. , p..64.
13
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P'igure
4 x 11. S inch Nal(Tl) crystal detector supported by a
Picker X-Riiy mount in the whole-body chamber.
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Figure 2.
10x10 inch plastic scintillator detector supported by
a Picker X-Ray mount in the whole-body chamber.
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10,0*1
Sample Description---
Nal (Th;
11.5 in. Crysta1 and
_PJLa_sJ:ic Sc
Live Tim«: 10 TUP
Energy (KaV/Chonnai)
: .6m f ro
Cs 157 in a *tQO ml Cottage Cheese
Container
to ze -so- - tar so eo
IZO 1JO MO ISO 16C 170 180 TO ZOO
CHANNEL
-------
SomplS Oaariptim. Hfl 5** S p6C t TUTI f TOTI
NaI (Th; 4 x J1.5 in. Crystal and
Plastir Scinfr I 1lator
Remarks:
Mn 54 in a 400 ml Cottaae Cheese
Conta iner
iio iso . 170 "Tab
CHANNEL
-------
Sample nssEripfion--. Lf\ 65 SpeCtTUTI f COTI 3
Nal(ThJ k x 11.5 in. Crystal and
Plastic Scintillator i
Energy(KoV/Chpnnel)L-
10
r,,v -6 Ti from Crysta
ctr.
Zn 65^in a 400 ml Cottage Cheese
Container
Spectrum No
Live Time; 10 Tl f D
System;
^SE^J^a^aLi^S
CHANNEL
-------
Fig~6 GAMMA EFFICIENCY OF PLAST !C_SC INT-l LLATOR AMD NA (Th)
-V; x 11;.5", AND V- x 9! CRYST-ALSJ-A-T-T^O^-THfRDS PEAK--HE IGHT
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Gairna Energy In Mev.
-------
Fig. 7 WINDOW EFFICIENCY OF CS-137 (0.66 Mev.)
. ... . ...... . . ...
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-r~- -f r-t-
; NsI k In. x H in. Crystal
Plastic Crystai
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Window { to channel 199)
60
-------
Fig. 8 WIND6w_EFFICJ-EN£Y;._'Of_MN-5^.'.(0..8^_Mev. )
11'-'5 in. 'Crystal
"
Plastic Crystal
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10-
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-------
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-------
Fig.10 One-half Resolution of Plastic ScintiHater and
_Na(l_Jt!_nx__LL.51!_Crys.taJ
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-------
rig. 11 Two-thirds Resolution of P'iastic Scintls later and
Nat 4.' x J.1..51- Crystal
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-------
IO.OCO
Description _B& Cj
-------
Fig. 13
Response of a Mn 5^ Point Source at an increasing Distance
from a Na! h- x 11-5 in. and Plastic Detector
1000
Distance in Inches from Source to Crystal Center
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