37
tronic equipment(Figure 52). The air conditioning and dehumidification were of considerable
value in maintaining the stability of the instruments in the tropical climate.
The subjects were ferried out to the ship, and,
before being counted, showered and donned paper
coveralls and slippers in order to minimize the
possibility of counting external contamination
from the island environmenton their bodies and
clothing. The subjects were seated on a folded
hospital cot and placed in a standard, fixed position underthe detector, as shown in Figure 53.
Figure 54 shows a Marshallese subject leaving the
counting room through the pneumatically-driven
sliding door.
The counting geometry employed in the portable counteris identical to that used in the permanent whole-body counter at BNL, which makes
possible the cross calibration of the two units.*”
The efficiency andprecision of the portable counter for the various isotopes are presented in Table
16. The vaiues obtained are very similar to those
for the whole-body counter at BNL.
The background observed in the Marshall
Islands counting room in the range 100 kev to 2
Mev was 1796 cpm (counts per minute), a value
somewhat higher than the average background
level observed at BNL (1400 cpm). The increased
background count in the Marshal] Islands was
chiefly in the very low energy range and probably
can be explained in termsof the thinnershielding
in the portable counter and the omission of Pb
lining for the ceiling of the counting room. To
some extent the thinnershielding was compensated
for by the shielding provided by the water under
and aroundtheship.
Since difficulties had been experienced in identifying small photopeaks of various isotopes deposited in the Marshallese in the presenceofrelatively large amounts of Cs’** and Zn°®°, the counting time wasincreased for a numberof subjects
over that used in previous years. In addition, a
larger crystal detector was substituted for the 5-in.
detector formerly used. The majority of the sub-
jects were counted for 10 min, and a large number
were countedfor 30 min,
An 8x4-in. NaI (T1) crystal (Harshaw) detec-
tor was placed above the patient at a distance of
19 in. (see Figure 33). Pulses from three 3-in.
photomultiplier tubes were fed into a Nuclear
Data 256-channel transistorized pulse-height
analyzer (Model 120). The analyzer fed the data
directly to an IBM typewriter and simultaneously
toa Tally paper punch unit, Model 420. Provision is made in this analyzer for transferring
spectra recorded on paper tapes into the memory
of the analyzerso that calibration spectra can be
compared with the incoming data when desired.
The data recorded on the punched papertapes
were transferred to IBMcards and thenceto the
Table 16
Properties of Portable Whole-Body Counter for Measuring Specific Radionuclides
Cs!37
Photopeak energy (Mev)
Energy band measured (Mev}
Background (cpm)
Calibration factor (C.F.) (cpm/pC)
Precision (P ) of counter* (mpC)
Percent standard deviation of count rate**
Co**
0.66
0.61-0.71
1.17
1.12-1.22
73.5
29.6
6114
0.360
* +£0.23
Integrated background (cpm)at 0.2 to 2 Mev
4320
0.324
10.0
Zn"
1.12
1.07-1.17
33.7
1733
0.866
#1.73
RK’
1.46
1.41-1.51
40.2
0.86 (cpm/kg)
3.8 (g)
23.58
1796
*For 70-kg phantém in standard counting geometry,
pVRISE)
at
where R, = Sample plus background counting rate (cpm), &, =background counting rate (cpm), ¢, = sample plus back-
r
4
*
ground counting time(min), and ¢, = background counting time(min).
“* Average Marshallese adult male.