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Figure 53 Gainma spectrum of phantom lustrating
graphical strapping ol KY, Zn", and Cs’ from total
Spectr uit,
one isotope to the photopeak of the other isotopes
of lower energy is very small.
In order to carry out this stripping method, ir is
necessary to have calibrated pulse-height distribulion spectra tor each gamma emitter encountered.
Further, these spectra must ideally be obtained
trom 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)
presence and concentration of other components
of the spectrum.
This procedure was further complicated in this
study by several factors. In the held study the subjects were measured with a 5-in. Nal (T]) crystal.
‘The calibration was originally carried out in the
held with a Presdwood phantom, but when the
Alderson plastic phantom later becameavailable
it was foundto give a better approximation ofthe
spectrumfor each isotope, and therefore most of
the calibration was repeated with it at BNL.
However, the geometry in the fieid situation was
rather difficult to duplicate exactly. Also, counting
the subjects for 5 to 10 min was suffictent to estimate accurately the levels of Cs'*’ and Zn”® but
not the K** body concentration and trace amounts
of other fission products in the presence ofthe relatively large amounts of Cs'" and Zn"*. The lack
of a statistically significant number of counts to
measure K** accurately is evident from the poorly
detined K“ photopeak of the subject as compared
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 is
compared with the spectrum obtatned with the
plastic phantom containing the same concentradons of K, Cs'", and Zn"' tn nearly identical
counting geometry in Figure 52. In this way it was
possible to sintulate the multicomponent spectra
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 smail
amounts of unknown radionuclides is not always
ubvious in the presence of large concentrations of
other radionuclides, it is possible to miss the presence of very sinall amounts ofother fission products. However, when all the major components
have been stripped vut, the presence of anyremaining photopeak should serve to dently the
tigere 34
Calibration phantom tn standard counting
podon ma BN whole-body counter,