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source energies, mainly by Campton ‘Scattering ‘in air. The dose from each of the resul.ing
enervy intervals was calculated and platted as a fraction of the total dose. This was seen to
yroup roughly into three regions, with maxima at 100, 700, and 1500 kev. An exposure to such
a source was thus the resultant effect of partial doses frorn each region, making the exposure
energy conditions quite different from those of the clinic or laboratory.
Figure 1.2 illustrates the dose spectrum of 4-day-old fallout {rom a cloud sample. In the
absence of other data to the contrary, this had to be taken as representative of the fallout on
all of the islands. At this time the proportion of low energy component was at its maximum,
During the several days before and after this time, the general shape of the spectrum apparently did not vary grossly from that illustrated here, since the observed flux decay rates
closely followed that of the observed gamma dose rate. For the period between fallout and
surveys, therefore, a knowledge of instrument response to each energy region allowed a total
correction factor to be calculated, The instruments used were calibrated just prior to the
surveys, and their readings have been corrected for the spectrum shape here illustrated.
1.4.2
Rate of Decay of the Fallout Mixture
Decay rates of fallout samples were measured in the field and in the laboratory, where a
fairly consistent pattern was observed amony various locations and samples.
In addition,
theoretical considerations based on the radiochemical composition of the fallout mixture permitted decay rates to be calculated for different intervals between the times of initial exposure
and later survey readings. These agreed well with the experimental data, and were used both
in the dose calculations during the exposure intervals and in extrapolating the later survey
readings to earlier times.
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1.4.3
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Only the time of evacuation is known accurately for all the islands. On Rongerik, however,
. the time of arrival of the radioactive cloud was determined precisely by the continuously recording dose rate monitor at the weather station, The fallout became visible at the time the
instrument first indicated the presence of a radiation field above background. The material
had the appearance of snow. The times of beginning of fallout on Rongelap and Ailinginae
were estimated from similar visual observations, combined with knowledye of the relative
distances of these atolls from Bikini and the wind velocities in the area. Fallout was not observed on Utirik, hence the estimate of arrival time there was made on the basis of the Ronge-
rik fallout time, wind, and distance factors.
Two extreme possibilities exist relative to the duration of the fallout: the first, that the
fallout occurred entirely within a short time; the second, thal il was gradual and extended over
a period of many hours, The monitoring instrument on Rongerik went off-scale at 100 mryhr,
%, hour after the dose rate bepan lo rise above backpround. If this rate of increase is extrapolated to a point for which subsequent decay would reduce the dose rate to the values found at
later times, a long fallout is implied. This was taken as one limiting case, and corresponding
doses were calculated. However, the possibility does not seem yreat that this actually occurred. Existing data are inconclusive, but several indications tending to favor the short time
hypothesis are summarized below.
First: a long fallout probably would not be uniformly heavy throughout, the first portion
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Time of Arrival of the Radioactive Cloud, Duration of the Fallout, and Time of Evacuation for EachCase
being the most intense and the balance tailing off.
the effect of a shorter fallout.
The total phenomenon thes tends toward
This is supported by monitor data from other nuclear events.
Second: the estimated durations of fallout, of about 18 hours, which result from the above
extrapolation for Ronuyerik and Rongelap, appear too long to have occurred at the distances of
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these atolls from Bikini, since the wind velovily in the area was high enough for the cloud lo
pass over the islands .na considerably shorter time.
Third: the accounts of the visibtlity of the fatlout, although conflicting, de not seem to
indicate such late cessation.
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