boy
used to supplement quantitative radiochemical
analytical procedures, and has made possible very
rapid and positive identification of gamma-emitting
radionuclides. The principles of tn vive gamma
spectrometry as applied to human beings have
been previously described.*’
The steel room constructed at BNL in 1957 for
the Marshallese surveys has been described previously.® The “portable” 21-ton steel room, 5x5
x 6-ft with 4-in.-thick steel walls, was used to
supply the required shielding (see Figure 49). This
steel room was mounted on a largetrailer along
with a 7X8 10-ft air-conditioned wooden room,
designed to house the electronic components of the
counter(see Figure 50). The trailer was mounted
on the tank deck of the Navy LST, and the subjects to be counted were brought aboard up the
ship’s ramp from the beach on each islandvisited.
Also available on the tank deck were shower
facilities where each subject washed and then
changed into an uncontaminated papersuit to ~
avoid the possible risk of measuring any external
contamination that might be present.
A 5-in. Nal (TI) crystal (Harshaw) was placed
over the subject at a distance of 19 in. above the
apex of the chair (see Figure 51). The pulses from
the photomultipher were fed into a linear ampli-
fier (Cosmic Radiation Lab. Inc., Model 101), and
thence into a 100-channe! analyzer (TMC-PA
10,000 pegee
COUNTS 75 MINUTES / 20 Kev CHANNEL
_ Zo 112 Mev)
oof
-
o
1
9
O20
040
1
:
1
060 085
100
ENERGY (Mev}
Ko
(1.46Mev)
|
4
47
100, Quartz Line Storabe Pulse-Height Anaber).
Two complete detectors and analyzers with the
necessary associated electronic circuitry were
taken along, since considerable difficulty had been
experienced during the previous year in maintaining the whole-body counting system operational under the conditions of tropical heat and
humidity. Housing of the electronic equipmentin
an air-conditioned, dehumidified room during
operation was of considerable assistance in maintaining its stability.
Since the principal photopeaks in these persons
were known from the previous study to be due to
Cs'*" and Zn**, and since the levels were sufficiently high, it was only necessary to count the
subjects for 5 or 10 min with the particular geometrical arrangement described above. The data
were printed out on a paper tape of a Victor adding machine,and the (PeORARCHIVESfor analysis.
Analysis of Gamma-Ray Spectra
The quantitative interpretation of the gammaray pulse-height data for the discrete gamma
energies involved the graphical reduction ofthe
data. A typical spectrum of a Marshallese ts shown
in Figure 52. The ordinate represents counts per
20-kev pulse-height increment, and the abscissa
represents gamma energy. In the graphical stripping method employed,**** the activity of the
ordinate value ofits total absorption peak. Then a
channel-by-channelsubtraction of the distribution
corresponding to the abundanceofthis particular
gamma-rayor radionuclide is made from the spectrum. When this subtraction is completed, the
concentration of the gamma-rayof the next
highest energy, Zn** (1.14-Mev) can be estimated
from the ordinate value of its peak. The distribution corresponding to the concentration of the
second gamma-ray is then subtracted from theremaining distribution, and the process is continued
with Cs'** (0.66- Mev) until all the three major
gamma-ray-emitting components have been determined.
t
1.20
t
highest energy gamma-rayin the mixture, K‘*°
(1.46-Mev), is determined directly from the
cs!" (0.66 Mev)
1000 +
’
140)
1.60
Figure 52. Gamma spectra of Marshallese male (subject
24, age 40, wt 70 kg ~ solid line) and normalized plastic
phantom (containing 0.64 pC Zn*, 1.22 wC Cs'*", and
140 g K - dashed line), 1959.
This stripping process is illustrated on a spectrum of the plastic phantom containing known
amounts of K*°, Cs'**, and Zn® (Figure 53). It is
to be noted that at the concentrations used here
(approximately the mean concentrations found in
the Marshallese), the Compton contribution of
53