16
.
USE OF A PORTABLE WHOLE-BODY COUNTER
y-spectra on 227 people. Details of the exposure
and findings of these medical studies have been
reported.(4—7)
In February 1961, another survey was conducted. At that time the portable shield was
used in conjunction with an improved detection
and data-recording apparatus. The y-ray
spectra of 110 Marshallese people were obtained.
Half of the people measured had been exposed
to the fallout in 1954;
the other half of the
group were people (most of whom wererelated
by blood) who had been living under the
identical conditions of the exposed groupfor the
past four years, but who had notbeen involved
in the original accident.
This report describes our experience during
the 1961 survey of the Marshall Islanders using
the portable whole-body counter, with details on
the measurements made, the methods used, and
the automatic data-handling techniques devel-
oped for recording and analyzing the large
amountof data associated with such a project.
METHOD
Shielding was provided by a 21 ton room with
walls constructed of laminated } in. thick steel
panels bolted together, which can be disassembled. The inside dimensions of the room are
5 ft x 5ft x 6ft high. In 1959 the room was
mounted on a 25 ton trailer and positioned with
a tractor on the tank deck of a U.S. Navy LST
(Fig. 1). In 1961 the room was set up in the
cargo hold of a civilian cargo ship (Fig. 2).
Along with the steel room, an air-conditioned
prefabricated wooden room 7 ft x 8 ft x 10 ft
was set up to house the pulse-height analyzer
and other electronic equipment (Fig. 1). The
air-conditioning and dehumidification were of
considerable value in maintaining thestability
of the instrumentin the tropical climate.
The subjects were ferried out to the ship for
the counting. Prior to being counted, the
subjects showered and donned paper coveralls
and slippers. This procedure was necessary 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 (see Fig. 3). A Marshallese subject
is seen leaving the counting room through
the pneumatically-driven sliding door (Fig. 4).
The counting geometry employed in the
portable counter is identical to that used in the
permanentwhole-body counter at BNL, making
possible the cross calibration of the two units.‘
The efficiency and precision of the portable
counter for the various isotopes are presented in
Table 1. The values obtained are very similar
to those of the whole-body counter at BNL.
The background observed in the Marshal
Islands counting room in the range 100 keV to
2 meV was 1796 cpm, a value somewhathigher
than the average background level observed
at BNL (1400 cpm). The increased background
count in the Marshall Islands waschiefly in the
very low energy range and probably 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 com-
pensated for by the shielding provided by the
water under and aroundtheship.
Since, in previous years, difficulties were
experienced in identifying small photopeaks of
various isotopes deposited in the Marshallese in
the presence of relatively large amounts of
Cs!8? and Zn®, the counting time was increased
for a number of 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 subjects was
counted for 10min; a large number was
counted for 30 min.
An 8in. x 4in. NaI (T1) crystal (Harshaw)
detector was used. The detector was placed
above the patient at a distance of 19 in. (see
Fig. 3). Pulses from three 3 in. photomultiplier
tubes were fed into a Nuclear Data 256channel transistorized pulse-height analyzer
(Model 100). The analyzer fed the data
directly to an IBM typewriter and simultaneously to a Tally paper punch unit, Model 420.
Provision is made in this analyzer for transferring spectra recorded on papertapesinto the
memory of the analyzer so that calibration
spectra can be compared with the incoming
data whenitis so desired.
The data recorded on the punched paper
tapes were transferred to IBM cards and thence
to the magnetic tape of a 704 computer. The
task of “spectral stripping” was carried out on