47
used to supplement quantitative radiochemical
analytical procedures, and has madepossible very
rapid andpositive identification of gamma-emitting
radionuclides. The principles of in 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 pre-
viously.* 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 large trailer along
with a 7*810-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 oneachislandvisited.
Also available on the tank deck were shower
facilities where each subject washed and then
changed into an uncontaminated paper suit 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 photomultiplier were fed into a linear amplifier (Cosmic Radiation Lab. Inc., Model 101), and
thence into a 100-channel analyzer (TMC-PA
100, Quartz Line Storage Pulse-Height Analyzer).
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 main-
tainingits stability.
Since the principal photopeaksin these persons
were knownfrom the previous study to be dueto
Cs'37 and Zn®*, and since the levels were sufh-
ciently high, it was only necessary to countthe
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 tapes were returned to BNL
for 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 is shown
in Figure 52. The ordinate represents counts per
20-kev pulse-height increment, and the abscissa
represents gammaenergy. In the graphicalstripping method employed,**** the activity of the
10,000
fr
+
r
i
cs'37 (0,66 Mev)
ordinate valueofits total absorption peak. ‘Then a
al
Ww
=
4
2M Mev) |
Zn 65 (1.12
x 1000 +
3
>
ov
x
Qo
oO
~
an
Ful
e
>
=
=
©
p
40
K
4
(1.46Mev)
100+
z=
D2
oO
Oo
channel-by-channel subtraction of the distribution
corresponding to the abundanceofthis particular
gamma-ray or radionuclide is made from the spectrum. When this subtraction is completed, the
concentration of the gamma-ray of the next
highest energy, Zn®* (1.14-Mev) can be estimated
from the ordinate valueof its peak. The distribution corresponding to the concentration of the
second gamma-ray is then subtracted from the remaining distribution, and the process is continued
with Cs*37 (0.66- Mev) until all the three major
gamma-ray-emitting components have been deter-
1
1
oO
1
O
1
L
O20 040
1
O60 080
J
1
100
1.20
LL
140
1.60
ENERGY (Mev)
Figure 52. Gammaspectra of Marshallese male (subject
#4, age 40, wt 70 kg — solid line) and normalized plastic
phantom (conggining 0.64 pC Zn*, 1.22 wC Cs*8’, and
co
ca
a3
|
on
ca
140 gK - dashing) 1959.
cn
highest energy gamma-ray in the mixture, K*°
(1.46-Mev), is determined directly from the
mined.
This stripping processis 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 usedhere
(approximately the mean concentrations found in
the Marshallese), the Compton contribution of