Techniques Used to Measure Gamma-Ray Exposure Rates
Because the external dose is
at about 2500 locations on a 30-m
primarily due to gamma-emitting
rectangular grid over the entire sur-
radionuclides, with only minor
face of Bikini Island, and at about
contributions from alpha and beta
200 locations on a 120-m grid on
emitters, we had to obtain the best
Eneu Island.
possible description of the geo-
the detectors was energy-dependent
graphical variability of the
and they were calibrated with a point
gamma-ray exposure rates on Bikini
source, they were expected to over-
and Eneu Islands.
respond to the gamma flux on the
Any technique
Since the response of
for measuring gamma exposure rates
atoll because the flux is depth dis-
has its own set of limitations
tributed and has a higher scatter
(e.g., nonlinear energy response,
component — and, therefore, a lower
portability of equipment, and
energy — than the point source.
extent of geographical coverage).
detectors could be carried easily,
We therefore used four different
which allowed us to make measurements
techniques to obtain the detailed
at many locations on a uniform grid
geographical coverage and accuracy
of the islands.
we desired: portable, hand-held Nal
insensitive to cosmic radiation.
scintillation detectors, a commercially
The
They are virtually
The response of the detector
available pressurized ion chamber, and
was compared with that of the pres-
two types of thermoluminescent dosime-
surized ion chamber over the entire
ters (TLD's).
range of observed exposure rates.
The portable scintillation
The ion chamber consists of a
detectors consisted of a 2.5-cm-diam
stainless steel sphere filled with
x 3.8-cm-long Nal crystal with rate
high-pressure ultra-pure argon.
meter readout.
The current produced by the radiation-
The detectors were
calibrated in microroentgens per
induced ionization within the
hour (uR/h) against a 1376, point
chamber is measured by a sensitive
source on the primary calibration
electrometer with digital readout.
range of the National Environmental
The detector was calibrated by the
Research Center, Las Vegas, Nevada.
manufacturer and verified by several
Calibration was repeated on selected
ERDA laboratories.
instruments following the survey.
relatively flat energy response
The detectors measured the ex-
posure rates at 1 m above the ground
It exhibits a
over the gamma-ray energies of
interest in a typical
environmental