source enereies, maimby by Compton scattering ‘in air, The dose trot each of (ie cesta uae
energy intervals was calculated and plotted as a fraction of the total dose. ‘This was seen to
croup roughly inte three regions, with miaaxinwa at 100, 700. and 1500 key.
An exposure to such
a osouree was thus the resultant effect of partial doses from each repron, make fhe expasure
enerrey conditions quite different fram: thase of (he clinte av laboratory.
Fipure 1.2 itbusterates the dose spectrum of 4-day -old fatbout from a cloud samipbe. da the
absence of offer data to (he contrary, this had to be taken as representative of the tallout on
allof the ishimds. At this time the proportion of low energy component wis at its naaxtmuin,
During the several thays before and after this time, the general shape of the spectrum: appar
ently did not vary grossty from that illustrated here, since the observed flux decay rates
closely followed that of the observed gummi dose rate, For the period between fatlout and
surveys, therefore, a knowledge of instrument response to each energy region aiiowed 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 fillout mixture per mitted 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.
1.4.3
Time of Arrival of {he Radioactive Cloud, Duration of the Fallout, and Time of Evacuation for Fach Case
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 v cather 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 knowledge 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 onthe basis of the Rongerik 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, that it was gradual and extended over
a period of many hours. The monitoring instrument on Rongerik went off-scale at 100 mr hr,
', hour after the douse rate began to rise above background. If this rate of increase is extra-
polated to a point for which subsequent decay would reduce the dose rate to the values found at
later times, a bong fallout is implied. This was taken as one limiting case, und corresponding
doses were calculated. However, the possibility does not seem preat that this actually oc curred. Existing: dati 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
being the most intense and the balance tailing off. The total phenomenon thus tends toward
the effect of a shorter fallout. This is supported by monitor data from other nuclear events.
Second: the estiniated durations of fallout, of about 18 hours, which result from the above
extrapolation for Rongerik and Rongelap, appear too long to have occurred at the distances of
these atolls from Bikini, since the wind velocity in the area was high enough for the cloud to
pass over the islands in a considerably shorter time.
Third: the accounts of the visibility of the fallout, although conflicting, do not seem to
indicate such late cessation.