detectors were mounted externally on an Army UH-1H helicopter rather than inside the helicopter as
in the 1972 survey. The data acquisition system employed was an improved second generation
version of the system used in the 1972 survey. Position information was obtained using a microwave
ranging system rather than the inertial navigation system used in the first survey. Flight lines were
flown at an altitude of 30 meters, with 45-meter line spacing. The radiation data were processed to
provide the average 241 Am concentration (in pCi/g) within the top 3 em of soil. Minimum Z4lam
detectability for the aerial survey was 7
pCi/g over islands containing low to moderate
contamination from other isotopes (mainly 13%Cg and 69Co), The actual minimum detectability
varied as a function of the background radiation present. The worst case was over Belle where the
minimum “41am detectability was 35 pCi/g. Although the results of the 1977 survey were never
formally published, they were used quite extensively during the early stages of the cleanup project as
an aid in the determination of island priority for the ground-based in situ measurements.
3.2
3.2.1
FIELD MEASUREMENTS
Introduction
Under contract to the United States Department of Energy, EG&G operated an in situ gamma ray
spectrometer system at Enewetak Atoll from July 1977 to December 1979 in support of the
Enewetak cleanup project. This system was used to determine surface {0-3 cm) concentration values
of 24l1Am as one step in the effort to characterize total transuranic surface contamination at
Enewetak arising from the nuclear testing program.
A high purity germanium (HPGe) planar detector, suspended 7.4 m_ above the ground, was used to
measure the 60 keV gamma ray from 24am (a daughter of 241 py).
Conversion factors were
established to relate the measured photopeak count rate data to average 241 Am concentration in the
soil. Using the ratio of total transuranies (TRU) to #4!Am established from soil sample data (see
Section 4.2.1), a statistical interpolation routine was then used to convert the individual 241 am
measurements into area-averaged transuranic surface concentration values (see Section 5.2). These
results formed the data base used in deciding whether removal of contaminated soil was required.
Final measurements made after soi] removal had been completed were used to document remaining
transuranic surface contamination.
Guidelines for the removal of contaminated soil existed for both surface and subsurface
contamination (see Section 2.2.4). Since the attenuation mean free path for 60 keV gamma rays in
Enewetak soil is approximately 2.0 em, the sensitivity of the in situ system to subsurface
Am
eontamination decreases rapidly with depth. For a distribution uniform with depth, approximately 95
pereent of the unseattered 60 keV gamma rays reaching the detector would originate within the top
6 cm of soil and approximately 99 percent would originate within the top 9 em. For this reason, the
in situ measurements were used to obtain only “surface” concentration values (defined for the
Enewetak cleanup as the average concentration in the top 3 cm). Subsurface soil samples were used
to evaluate and quantify subsurface contamination.
3.2.2 Instrumentation
The in situ gamma ray spectrometer utilized an HPGe planar detector having a surface area of 19
em? and a thickness of 1.6 em. The detector was mounted inside a canister suspended at the end of
a 9 m retractable pneumatic boom. This boom was mounted at the rear of a small, lightweight,
tracked vehicle (the IMP*, Figure 3-1) specifically selected for its ability to operate in soft sand.
The IMP was modified and equipped as a fully self-contained mobile data acquisition and reduction
system.
Power was supplied by a 4 kW Onan generator mounted on the front of the IMP.
A
roof-mounted air conditioner provided the necessary humidity and temperature environment for the
electronic equipment mounted in the rear section of the vehicle.
Signals from the preamplifier
(mounted on the detector) were fed inside the IMP to a microprocessor-based 4096 channel pulse
height analyzer. At the completion of a measurement, data were transferred from the analyzer to a
* The word IMP and its variations as used in this report were derived from a trademark of the
DeLorean Manufacturing Company.
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