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samples was taken so that the variation of isotopic concentrations with depth might later be investigated.
A,
Instrument ation
Spectrometer System The HASL spectrometer system consists
of a 4 in. x 4 in. NaI detector with preamplifier and high
voltage battery pack. a 400 channel multichannel analyzer and
parallel printer. a DC~AC converter and a 12 volt storage
battery.
The calibration of the detector is described in
detail in previous HASL reports”’’*.
In brief, the exposure
rate due to a particular isotope in the ground is inferred
from the number of counts under a spectrum total absorption
peak characteristic of that isotope using calibration factors
determined from a combination of laboratory experiments and
theoretical caicuiations.
The prominent tctal absorption peaks observed in all
the field spectra (see Figure 2 for a typical field spectrum)
were at 1.17 and 1.33 Mev {°°co), .10 Mev (** Eu), .44 MeV
(*"®sb), and .662 Mev (“*°’cs}.
These isotopes appeared to
be responsible for almost all of the total expcsure rate at
the spectrometer locations.
Individual exposure rates for
*$7cas, °°co and -*"Sb were inferred for each field location.
The “°° Bu exposure rate was assumed to be of minor importance
due to its very low source energy.
For those lccations
where the ionization chamber was nct used or the other total
exposure data were inconsistent, the spectrometric data were
analyzed to obtain total exposure directiy’.
Except for the two Eneu locations, all spectra were
taken in ten minute runs.
During this time there was no
appreciable gain shift due to temperature changes.
In almost
all cases we were able to obtain excellent field spectra with
only minor equipment maifunctions even though the equipment
had to be carried by hand through heavy brush, loaded and
unloaded into small boats in fairly rough surf, and operated
and stored in ambient temperatures of approximately 85 95°F and relative humidities of 70 - 80%.
Ionization Chamber - Our high pressure ionization chamber has
a 5.6 liter sensitive volume and is filled to a pressure of
about 700 psi with pure argon gas*.,
The chamber wall is
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