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PON
r
and
had
presence and concentration of other components
of the spectrum.
This procedure was further complicatedin this
study by several factors. In the field study the subjects were measured with a 5-in. NaI (Tl) crystal.
The calibration was originally carried out in the
field with a Presdwood phantom, but whenthe
Alderson plastic phantom later became available
it was found to give a better approximationofthe
spectrum for each isotope, and therefore most of
rr
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K(1 Mev)
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Figure 53. Gamma spectrum of phantom illustrating
graphical stripping of K*°, Zn**, and Cs**’ from total
spectrum.
one isotope to the photopeak of the other isotopes
of lower energy is very small.
In orderto carry out this stripping method,it is
necessary to have calibrated pulse-height distribution spectra for each gammaemitter encountered.
Further, these spectra must ideally be obtained
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 wassufficient to estimate accurately the levels of Cs'*’ and Zn® but
not the K*° body concentration and trace amounts
of otherfission products in the presenceoftherelatively large amounts of Cs?*" and Zn**. Thelack
of a statistically significant number of counts to
measure K*° accurately is evident from the poorly
defined K*° photopeakof the subject as compared
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 usedfor 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 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 multicomponent spectra
of the Marshallese by use of the phantom.
Sinceit 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 unknownradionuclides is not always
obvious in the presence of large concentrations of
other radionuclides, it is possible to miss the presence of very small amounts ofotherfission products. However, when all the major components
have been stripped out, the presenceof anyremaining photopeak should serve to identify the
Figure 54. Calibration phantom in standard counting
position in BNL whole-body counter.