tronic equipment (Figure 52). The air conditioning and dehumidification were of considerable
value in maintaining the stability of the instru-
can be explained in terms of the thinner shielding
in the portable counter and the omission of Pb
lining for the ceiling of the counting room. To
some extent the thinner shielding was compensated
ments in the tropical climate.
The subjects were ferried out to the ship, and,
tor by the shielding provided bythe water under
before being counted, showered and donned paper
and around the ship.
Since difficulties had been experienced in identifving small photopeaks of various isotopes deposited in the Marshallese in the presenceofrelatively large amounts of Cs'” and Zn”’, the count-
coveralls and slippers in order to minimize the
possibility of counting external contamination
from the island environment on their bodies and
clothing. The subjects were seated on a folded
hospital cot and placed in a standard. fixed position under the detector, as shown in Figure 33.
Figure 54 shows a Marshallese subject leaving the
counting room through the pneumatically-driven
ing time was increased for a number of subjects
over that used in previous years. In addition. a
larger crystal detector was substituted for the 3-in.
detector formerly used. The majority of the sub-
shding door.
jects were counted for 10 min. and a large number
were counted for 30 min.
The counting geometry emploved in the port-
able counter is identical to that used in the per-
An 8x 4-in. NaI (TI) crystal (Harshaw) detec-
manent whole-bodycounter at BNL, which makes
tor was placed above the patient at a distance of
19 in. (see Figure 53). Pulses from three 3-in.
possible the cross calibration of the two units.'*'
The efficiency and precision of the portable counter for the various isotopes are presented in Table
16. The values obtained are very similar to those
photomultiplier tubes were fed into a Nuclear
Data 256-channel transistorized pulse-height
analyzer (Model 120). The analyzer fed the data
for the whole-body counter at BNL.
The background observed in the Marshall
Islands counting room in the range 100 kevto 2
Mev was 1796 cpm (counts per minute), a value
directly to an IBMtypewriter and simultaneously
toa Tally paper punch unit, Model 420. Pro-
vision 1s made in this analyzer for transferring
spectra recorded on papertapes into the memory
of the analyzer so that calibration spectra can be
compared with the incoming data when desired.
The data recorded on the punched paper tapes
were transferred to IBM cards and thenceto the
somewhat highér than the average background
level observed at BNL (1400 cpm). The increased
background count in the Marshall Islands was
chiefly in the verv lowenergy range and probably
Table 16
Properties of Portable Whole-Body Counter for Measuring Specific Radionuclides
Cs? 7
Photopeak energy ( Mev)
Co”
0.66
Energy band measured (Mev)
1.17
0.61-0.71
Background (cpm)
Calibration factor (C.F.) (cpm/pC)
Precision (P ) of counter* (muC)
73.3
6114
0.3560
Percent standard deviation of count rate**
+£0.23
Integrated background (cpm) at 0.2 to 2 Mev
1.12-1.22
Zn”*
1.12
1.07-1.17
K"
1.46
L.4f-1.51
29.6
4320 .
0.324
33.7
1733
0.866
40.2
0.86 (cpm kg)
3.8 (g)
10.0
1.73
3.38
1796
*For 70-kg phantom in standard counting geometry,
P
VIR t+ CR, ty)
where &. = Sample plus background counting rate (cpm), &, =background counting rate (cpm), ¢. = sample plus background counting timeimin), and ¢, = background counting time (min).
** Average Marshallese adult male.
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