Ty
cs37 (0.66 Mev)
OTT
TTD
TOT TTT]
1
Zn*5(1.12 Mev)
L
<7
riaiiyl
1000
(1. 46Mev)
4
40
'
‘
‘
‘
24
48
2
pacaitl
Kn"
\
‘
\
4
\
\
36
120
ENERGY (Mev}
a
+
a
Cc
=
SPECTRUM
m
1
wa
7v
mT TTT
T
zn? cg!27
CONTRIBUTION
NET IN vivo “7
Cs"
—T
COUNTS / MINUTE / 2Okey CHANNEL
E
144
168
4
10,000 -—+—t—
pop pd
48
|72
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 is very small.
In order to carry out this stripping method,itis
presence and concentration of other components
of the spectrum.
This procedure was further complicatedin this
study by severalfactors. In the field study the subjects were measuredwith a 5-in. Nal (T1) crystal.
Thecalibration was originally carried out in the
field with a Presdwood phantom, but when the
Alderson plastic phantom later became available
it was found to give a better approximation ofthe
spectrum for each isotope, and therefore most of
the calibration was repeated with it at BNL.
However, the geometry in the field situation was
rather difficult to duplicate exactly. Also, counting
the subjects for 5 to 10 min wassufficienttoestimate 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°*. The lack
of a statistically significant numberof countsto
measure K*" accurately is evident from the poorly
defined K*° photopeak of the subject as compared
necessary to have calibrated pulse-height distribu-
tion spectra for each gamma emitter encountered.
Further, these spectra must ideally be obtained
from a subject of the same size and body build. To
obtain these spectral data, known amounts of
Cs’*" and Zn®* were administered to subjects at
BNL, and their spectra were obtained. Later in
the study, a plastic phantom (REMAB-Alderson)
was obtained and used for calibration (Figure 54).
Spectra were also obtained from the phantom
with known amounts of KCl, Cs’**, and Zn".
From these spectra, an average spectrum for each
isotope was obtained. The pulse-height distribu-
tion spectrum of one of the Marshallese subjects is
compared with the spectrum obtained with the
plastic phantom containing the same concentrations of K, Cs?*’, and Zn® in nearly identical
counting geometry in Figure 52. In this way it was
possible to simulate the multicomponentspectra
of the Marshallese by use of the phantom.
Since it is not possible to measure a photopeak
until the contributions of other peaks of higher
energy and their Compton continua have been
subtracted out, and since the presence of small
amounts of unknownradionuclidesis not always
obvious in the presence of large concentrations of
other radionuclides, it is possible to miss the presence of very small amounts of other fission prod-
i)
‘
mo
or
ucts. However, when all the major components
have been stripped out, the presence of anyremaining photopeak should serve to identify the
Figure 54, Calibration phantom in standard counting
position in BNL whole-body counter.