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WHOLE BODY COUNTING RESULTS FROM1974 TO 1979
measurements were made in 1974 (Con75)
and in 1977 (Coh77). In August 1977, the
responsibility for providing body burden
measurements was transferred from the
Medical Department to the Safety and
Environmental Protection Division at Brookhaven National Laboratory. The 1978 and
1979 body burden measurements of the Bikini
population were conducted by the latter
organization.
In this report. the results of four whole
body counting measurements on the Bikini
population that were conducted in 1974 and
1977-79 are presented. Because the body
burden measurements were performed by
two different organizations, the current
experimental design included a cross check
mechanism to ensure that previous and current results are directly comparable. The approach to the problem was muitidirectional.
First, key detection components were duplicated. Second. the systems were calibrated in
the same manner (Coh63). Third, the operational procedures and counting geometries
were basically similar. and an intercomparison study was conducted using Marshallese and Brookhaven personnel to ensure
system comparability
EXPERIMENTAL DESIGN
Instrumentation
The detector chosen for field use by both
Brookhaven organizations is a
28-cmdiameter, 10-cm-thick, sodium iodide thallium
activated scintillation crystal Nal(TI). It is
optically coupled to seven, 7.6-cm-diameter
low background magnetically shielded, photomultiplier tubes. In the current system the
signal output from each photomultiplier tube
is connected in parallel through a summing
box with the combined output routed to a
preamplifier-amplifier
and
then
to
a
microprocessor-based computer/pulseheight
no angeate: * a
analyzer (PHA). The PHA datais stored on a
magnetic diskette, and the results may be
analyzed either in the field or at BNL using a
matrix reduction, minimization of the sum of
squares technique (TP76).
Calibration
on
Analysis of Nal(Tl) spectra by the matrix
reduction technique requires that the computer library contain
a standard
for
each radionuclide that is expected in the field
Measurement and that the fleld measurements and standards have the same geometry.
To accomplish this. a review of the previous whole body counting data (Con75§; Coh77)
indicated the need to calibrate for “K. Co
and '"Cs. The current svstem was calibrated
ustng an Anderson REMCAL phantom
(Coh63). Each radionuclide was introduced
into the phantom’s organs in an amount
equivalent
to
the
fraction
in
organ
of
reference of that in total body as defined by
the ICRP in Publication 2 (ICRP59). To
verify the activity in the phantom prior to use
as a standard, an aliquot of the phantom
solution was counted on a lithium drifted
germanium detector which was calibrated
with NBS standard sources.
The phantom was then counted in a
shadow whole body counter (WBC) (Paé5).
The whole body counting system consists of
a stationary crystal and stationary bed. The
counter detects radioactive material located
principally in the thorax, so positioning of the
phantom and the in vivo counting subjects
must be as similar as possible. To facilitate
reproducible counting geometries, each subject and the standard phantom was positioned
such that the central axis of the crystal intersected the central axis ‘of the body about
25 cm below the sternal notch. The distance
between the surface of the bed and the bottom of the detector ts 32.4cm. The total
system efficiencies for “K, “Co and '’Cs are
listed in Table | as are typical minimum
detection limits for these nuclides.
Quality control
The quality control (QC) program consisted
of a cross comparison of the radionuclide
quantities estimated to be in the phantom
volume vs NBS calibration standards.
Agreement between these two activity concentrations is within +5% for all radionucl-
ides.
Other
quality
control
mechanisms
employed were repetitive counting of secon-
dary point source standards, multiple counts
of Brookhaven personnel, repetative counting
of the Marshallese (blind replicates) and an
intercomparison study.
(