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

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               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

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                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

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         Fig~6   GAMMA EFFICIENCY  OF PLAST !C_SC INT-l LLATOR AMD  NA (Th)


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Gairna  Energy  In  Mev.

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               Fig. 7  WINDOW EFFICIENCY OF CS-137 (0.66  Mev.)
                      .   ... .      ...... .  „ . ...
                                -4-
                                I
                           -I	
                               •-r~- -f	r-t-
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10-
20   ""   3^0- "  '     ^0    	50-

   Window {   to channel  199)
60

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      Fig.  8  •WIND6w_EFFICJ-EN£Y;._'Of_MN-5^.'.(0..8^_Mev. )
                                        11'-'5 in. 'Crystal
                                            	     "
            Plastic  Crystal
'0-
10-
20-"  " """30"   7 ""  4"0- ~  	  50


   Wmdow  (-  to channel 199)
60-

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            Fig.10  One-half Resolution of Plastic ScintiHater and

                           _Na(l_Jt!_nx__LL.51!_Crys.taJ	
   c
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                                   Enerav  in  MPV

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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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     .6.

Enerqy  In Mev
""i'/d

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IO.OCO
                                                                                                  Description— _B& Cj
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                        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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