Dose rate, dy, is calculated from Equations 4.1 and 4.2.
For any point at which the gamma
energy emission rate is R:
d = 4,R
(4-3)
Since R cannot be calculated directly from the experimental data,
follows:
d was obtained as
4.4
(44)
{—
—
=
d= (3 ) al 4
Values for R/I were obtained from values of R and I calculated for an arbitrary number
of gamma photons. The experimentally determined gamma spectra for Shots 5 and 6
and known response of the ionization detector to various gamma photon energies were
used. I/A was determined experimentally with actual water samples? . A was caleulated' from the measured activity of water samples.
Values of d thus obtained were plotted for the geographical coordinates at which
fallout was received and dose rate contours were drawn.® Further details of the calculations are given in the following sections and illustrative calculations are provided.
4.1
SAMPLE ANALYSIS
The gammaactivity of all sea water samples received was determined in general
by counting 15-ml aliquots in a gamma scintillation counter (UDR—9) through approximately 1600 mg Al em~?. The UDR-—9 counter was equipped with a 1% inch by Y, inch
Nal crystal detector. The overall efficiency of the instrument was estimated to be 5 to
8 percent for the sample geometry used. In some cases samples of low activity from
Shot 6 were counted in a Nal crystal well counter. By counting samples in both instru-
ments the ratio of counts in the crystal well to those from the UDR—9 was found to be
~ 12. All counting data were converted to UDR—9 counts and expressed as counts per
minute at H + 218 hours for Shot 5 and H + 171 hours for Shot 6. The results are shown
in Tables 4.1 and 4.2.
In the case of Shot 5 samples, decay corrections from time of counting to H + 218
hours were made by use of an experimentally determined decay curve. Shot 6 samples
were received and analyzed in two separate shipments. Unfortunately from the time of
analysis of the first group of samples to the time of analysis of the second group 12 or
13 days later, no decay data were recorded.
It was necessary, therefore, to use a cal-
culated decay curve based upon disintegrations per minute from a mixtureof fission
product and induced activities shown in Figure 4.1. The relative amountsof fission
product and induced activities were consistent with the capture to fission ratio deter-
2 Use was made of the gamma ionization detector since its response per gamma
photon was better known as a function of photon energy than was the response of the
gamma counters used. In principle, a similar calibration of the gamma counter would
have permitted its use and obviated use of the gammaionization instrument.
3 If the fallout had been received by an actual land surface the dose rates would be
decreased by the “ roughness factor”, and probably slightly increased by scattering
from beneath the source.
Footnote?.
Neither correction has been applied.
55
See also Section 4.7,