Instrumentation and

Field Measurements

The instruments used in a typical survey included portable
scintillation counters,

the NaI(Tl)

spectrometric system,

a

high pressure ionization chamber, and an altimeter (Fig. 1).
The portable scintillation detectors were used to survey the

area to assure that the dose rate was relatively uniform through-

out.

The large NaI(Tl1) detector was placed near the center of

the area, one meter above the ground, and connected by a 50 foot
coaxial cable to a multichannel analyzer system in the HASL Corvan
vehicle,
Power was supplied by the car battery through a 12
volt DC~AC converter,
The gain of the system was adjusted to
obtain a spectrum extending from zero to about 3.5 Mev.
A
twenty minute live time was found to provide adequate statistics.

Two typical spectra are shown in Figure 2.

The high pressure ionization chamber filled with argon was
used to measure the total dose rate and was placed a few
feet from the crystal.
The altimeter provided the elevation

data necessary to infer the cosmic ray contribution to the

ionization current measured by the pressure chamber.
In addition to the instrument measurements, notes were made of any

unusual conditions prevalent such as recent rainfall, very wet

soil, unusual instrumental fluctuations, and ground depressions.
A complete set of measurements usually required about 35 minutes from time of arrival to time of departure.
DOSE RATE DETERMINATIONS
Total Terrestrial

The field reading of the ionization chamber (current) can
be converted directly into terrestrial and cosmic dose rate

values since the chamber has been calibrated in the laboratory

for gamma rays*® and its cosmic ray response and the variation
of the cosmic radiation dose rate as a function of altitude
have been determined fairly accurately.*
The total terrestrial
dose rate can also be calculated from the gamma ray spectrum obtained from the NaI(Tl) detector.
The total “energy" (counts
per channel multiplied by the gamma ray energy represented by
that channel) in the spectrum from 0.15 to 3.4 MeV has been
found to correlate very well with the ionization chamber terrestrial dose rate values, even for locations whose spectra

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