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
radionuclides, with only minor
face of Bikini Island,
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,
and Eneu Islands.
respond to the gamma flux on the
Any technique
the entire surand at about
Since the response of
they were
expected
to over-
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.,
component — and,
nonlinear energy response,
portability of equipment,
therefore,
a lower
energy — than the point source.
and
The
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
They are virtually
The response of the detector
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.
available pressurized ion chamber,
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 1376g 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 exposure rates at 1] m above the ground
It exhibits a
over the gamma~ray energies of
interest in a typica:t
environmental