0.135 in.
stainless steel and effectively discriminates
against all beta radiation.
The ionization current is read
out on a Victoreen Model #475A Dynamic Capacitor Electrometer.
This chamber has been shown to have a flat energy response
over all y-ray energies of importance in environmental
radiation studies and to allow determination of total y-ray
exposure rates from about 1 ur/hr to 200 Lr/hr with a
precision usually better than 2% and an accuracy of better
than 5%°.
A careful calibration was done in the laboratcry
with standard NBS calibrated *®*°®Ra and °°co sources both
before and after the survey and periodic checks for
consistency were made in the field with a small **°Ra check
source.
A correction for the contribution to the ionizaticn
current from cosmic rays was determined by measuring the
cosmic ray component alone on top of the bridge deck of the
ship in the middle of the Lagoon.
The value obtained {3.4
ur/hr) was consistent with the value of the cosiric ray
exposure rate for this latitude inferred from our previcus
extensive cosmic ray measurements®.
Total exposure rates were measured with the ionization
chamber at almost all the field spectrometer locations. as
well as at several other sites.
These total exposure rates
were later compared with the sum of the individaal compcnent
exposure rates inferred from the fieid spectrometer data.
Portable Scintillation Detector - The HAS portable scintii-
lation detector consists of ai5in. x 1 ain, NaTt{Tl} crystal
and 1 in. photomultiplier tube attached to a very stable
count rate circuit.
This instrument is relatively insensitive
to beta and cosmic radiation.
It is calibrated for particular
field conditions by comparing readings with y-ray exposure
rates determined from the ionization chamber at a number of
locations.
The "field calibration" was dcne poth on Bikini
and Nam with identical results.
This type of calibration allowed us to use the instrument
to extend the results of a few very accurate and precise
ionization chamber and spectrometer measurements over a wide
area.
We thus quickly obtained a large amount of data which,