-
‘
-
1—-NET TOTAL ~
:
po
24
|
30DY
| {| SPECTRUM 2
4
18
=
:+
‘
\ /
36
12
ENERGY (Mew)
i
\
146.
Vo
6BSC«d 2
Figure 53. Gamma spectrum of phantom illustrating
graphical stripping of K‘’, Zn"*, and Cs'*’ from total
However, the geometry in the field situation was
rather difficult to duplicate exactly. Also, counting
the subjects for 5 to 10 min was sufficient to estimate accuratelythe levels of Cs'?* and Zn** but
not the K*° body concentration and trace amounts
spectrum.
of other fission products in the presenceoftherelatively large amounts of Cs'** and Zn®*. Thelack
one isotope to the photopeakof the other isotopes
of a statistically significant nu:mber of counts to
of lower energy is very small.
In order to carry out this stripping method,it is
necessary to have calibrated pulse-height distribution spectra for each gamma emitter encountered.
measure K*° accurately is evident from the poorly
defined K*® photopeak of the subject as compared
Further, these spectra must ideally be obtained
from a subject of the samesize 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 KCI, Cs'**, and Zn®.
From these spectra, an average spectrum for each
isotope was obtained. The pulse-height distribution spectrum of one of the Marshallese subjects ts
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 way it was
possible to simulate the multicomponent spectra
of the Marshallese by use of the phantom.
Since it is not possible to measure a photopeak
i
i
until the contributions of other peaks of higher
energy and their Compton continua have, been
subtracted out, and since the presence of smalt
amounts of unknown radionuclidesis 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 products. However, when all the major components
have been stripped out, the presence of any remaining photopeak should serve to identify the
Figure 54. Calibration phantom in standard counting
position in BNL whole-body counter.
7
CONTRIBUTION
10
tt séMew!
COL NET UN Vg 7
=“ ¢s'3? SPECTRUM |1
“
1
1
1
1
wo.
1
1
study by several factors. In the field study the sub-
jects were measured with a 5-in. Nal (TI) crystal.
The calibration wasoriginally carried out in the
held with a Presdwood phantom, but when the
Alderson plastic phantom later became available
it was found to give a better approximationofthe
spectrum for each isotope, and therefore most of
the calibration was repeated wit at BNL.
Ae
\
This procedure was further complicated in this
=
.
Ny eg
a dhaide
COUNTS / MINUTE / 20kev CHANNEL
Zn 8301 12 Mew}
pres. ice and concentration of other components
of the spectrum.