S. H. COHN, R. A. CONARD, E, A. GUSMANOand J. 5. ROBERTSON
17
Table 1. Properties ofportable whole-body counterfor measuring specific radionuclides
Cs137
Photopeak energy (meV)
Energy band measured (meV)
Background (cpm)
Co%
0.66
0.61-0.71
75.5
Calibration factor (C.F.) cpm/yc
6114
Precision (P) of counter* (nc)
0.360
Standard deviation of count ratet (°)
Integrated background (cpm)
0.1-2 meV
1.17
1.12-1.22
29.6
4320
788
1.12
1.07-1.17
33.7
1733
0.324
+0.23
K40
0.866
+10.0
+1.73
1.46
1.41-1.51
40.2
0.86
(cpm/kg)
3.8 (g)
+3.58
1796
* For 70 kg phantom in standard counting geometry, 30 min counting time.
E _ VRIte + ale) |
CLF.
where
P = precision ( os
C.F.
in ne
&, = combined counting rate (cpm)
R, = background counting rate (cpm)
t, = combined counting time (min)
t, = background counting time (min)
C.F. = calibration factor (cpm/nc)
+ Average Marshallese adult male.
the computer with a Fortran program. In this
operation the spectrum of each individual
isotope is removed from the total spectrum
- obtained for the subject, which represents the
combination of the contributions from all the
isotopes deposited in that subject.
Spectra for each of the individual isotopes
quantitated in this study were obtained with
the use of an Alderson phantom (REMCAL).
Solutions of known concentration of each radio-
nuclide were placed in the phantom to approximate the effects of tissue absorption and scatter.
.The spectrum of the phantom for each of the
isotopes was obtained under conditions of
counting geometry identical with that used in
counting the subjects. By this technique it was
possible to simulate quite closely with the
phantom the multi-component spectra of the
Marshallese. A representative Marshallese
spectrum obtained by adding K, Cs!87 and Zn®
at average levels (as determined in the medical
study of 1959) to the phantom to simulate the
multi-component spectrum of the Marshallese is
shown in Fig. 5. The K, Cs!87 and Zn® were
distributed homogeneously throughoutthe phantom, while Co® was placed in the liver only.
2
Analyses of the complex spectra were performed by subtracting the calibrated pulseheight spectrum for each y-emitter to be
quantified. Although these spectra are obtained
ideally from a subject of identical build, an
approximation is obtained with the use of the
plastic phantom. Computation was carried out
by an IBM-704 computer.
Starting with the
highest energy photopeak, that of K*°, (after
correction for background and normalization of
the K4° photopeak to that of the subject), the
computer performs a channel-by-channel sub-
traction of the normalized K* spectrum. In
similar manner, the normalized spectra for
Zn®, Co® and Cs!3? were subtracted from the
total spectrum (see Fig. 5).
RESULTS
A spectrum for an average Marshallese adult
male, obtained in the 1961 study, is shown in
Fig. 6. In the same figure, the spectrum of a
member of the U.S. medical team of the same
body weight and age is also illustrated for
comparison. Almost identical amounts of the
radionuclide K‘° are noted along with large
differences in the Cs!87 and Zn®levels between