48
3
=
3
e
3
&
4
g
ji
“
5
=
=
~~
ww
5
zntts a!”
CONTRIBUTION
i
r
‘
.
=
Boor
SPECTRUM |
1
24
4 tt aba 4:
45
72
7
TToTa 4
‘
b+
3
Kt) 46Mer)
ocs®”’ specTRUM |t
r
x
'
100 NET ow on
r
b
7
S
io
ENERGY (Mey)
bot
144
ahiis
#«x1[s8
1
a4
Le
Figure 53. Gamma spectrum of phantom illustrating
graphical stripping of K*°, Zn*®, and Cs'’’ from total
spectrum.
one isotope to the photopeakof the other isotopes
of lower energy 1s very small.
‘In orderto carry out this stripping method,it is
necessary to have calibrated pulse-height distribution spectra for each gamma emitter encountered.
Further, these spectra must ideally be obtained
from a subject of the same size and bodybuild. To
obtain these spectral data, known amounts of
Cs'*? and Zn** were administered to subjects at
BNL,and their spectra were obtatned. Laterin
field with a Presdwood phantom, but when the
Alderson plastic phantom later becameavailable
it was found to give a better approximation of the
spectrum for each isotope, and therefore most of
the calibration was repeated with it at BNL.
However, the geometryin the field situation was
rather difficult to duplicate exactly. Also, counting
the subjects for 5 to 10 min wassufficient to estimate accurately the levels of Cs'*’ and Zn** but
not the K*° body concentration and trace amounts
of otherfission products in the presence ofthe relatively large amounts of Cs'’ and Zn*®. Thelack
of a statistically significant numberof counts to
measure K* accurately is evident from the poorly
defined K*° photopeakof the subject as compared
DOE ARCHIVES
iPnal
enalt
a
t
z 1000
ty
Ze
presence and concentrationof other components
of the spectrum.
This procedure was further complicated in this
study byseveral factors. In the field studythe subjects were measured with a 5-in. Nal (TI) crystal.
The calibration wasoriginally carried out in the
har gs.
|
ft
TOT OT
Ce” (066 Mev}
AY
TF
pon
10,000 4-74
the study, a plastic phantom (REMAB-Alderson)
was obtained and usedfor calibration (Figure 54).
Spectra were also obtained from the phantom
with known amounts of KCl, Cs'*7, and Zn®.
From these spectra, an average spectrum for each
isotope was obtained. The pulse-height distribuuon spectrum of one of the Marshallese subjects is
compared with the spectrum obtained with the
plastic phantom containing the same concentra-
tions of K, Cs'*’, and Zn® in nearly identical
counting geometry in Figure 52. In this wayit was
possible to simulate the multicomponent spectra
of the Marshallese by use of the phantom.
Since it is not possible to measure a photopeak
unul the contributions of other peaks of higher
energy and their Compton continua have been
subtracted out, and since the presence of small
amounts of unknown radionuclides is not always
obvious in the presence of large concentrations of
other radionuclides, it is possible to miss the pres-
ence of very small amounts of other fission prod-
ucts. However, when all the major components
have been stripped out, the presence of any remaining photopeak should serve to identifythe
Figure 54. Calibration phantom in standard counting
position in BNL whole-bodycounter.