6
EPFECTS OF [UONIZING RADIATION
diation ax it emanated from the material itself
made preible an approximate calculation of
the weresrticcs ob ienmd dee clelivered in exch
of weveral energy regions. Such a calculation.
mung spectrometric data on the source material
of mixed fireson products ailtaking inte ac:
count thin energy degradation by Compton wattering along the path in air, (1) led to the
dose-eneryy histogram shown in Figure Lz.
Roughly there were three regions, with max-
Imnat 100, 70 and La REV.
A geometry correction factor
was also calculated. The total correction resultIng from this procedure was found to be about
twenty percent.
Tie
tween the time of initial etpesure and later sur-
vey readings (4). These agree well with the
experimental data, and were used both in the
daw calculations during the exposure intervals
and in extrapolating the later survey readings
to earlier times.
The total ex-
posure was thus the resultant effect of partial
(loses from each energy region, making the exposure energy condition significantly different
from those of radiation therapy or experimental
radiobiology.
The data in Figare (2 are based on the spectrum of 4 diay old tission products from a fallout sample. dn the absence of other data, this
was taken as representative of the fallout on
all of the islands to which the individuals were
exposed. An energy correction factor for the
radiation mensuring instrument was calculated
by weighting the dose from each energy interval by ano avernge meter response factor for
that energy (2).
cations and sampies. In addition, theoretical
considerations haamb an the radsocheoniicat «ames
pemition of the fallout mizture permitted | ceay
rates to be calculated for different snterva.s be-
Using this correction, the dose rates on the
ishinds at the time of survey were determined.
Since radioactive decay of the fission products
had occurred between the start of the exposure
whi this time, it Was necessary to obtain a value
for this decay rate during the exposure period
in order to calculate a total dose in each case.
A large number of radioisotopes are present in
Varying proportions tn the fission product mix:
ture, and the total rate of change of radiation
intensity resulting from them may differ somewhat with place and time. The best data avail
wble in this case came from fallout: samples
tuken soon after the detonation at points some
distance from the contaminated atolls. Decay
rates Of these saneples were measured in the
field wncoin the luboratery, and a fairly consistent pattem: Was observed amiene various lo-
1.22
Duration of the Exposures
The time of evacuation is known accurately
for all the islands: however, the time of arrival
of the radioactive clond was determined precisely only for Rongerik by means of a continu.
ousty recording dese rate monitor located at the
wenther station on that atoll, As the radiation
Intensity rose above the background, a material
with aomisty appearance began to fall. The
trines of beginning of fallout for Rongelap and
Ailinginae atolls were estimated from similar
visual observations. These estimates were consistent with the relative distances from the site
of detonation and the known wind velocities.
Fallout was not observed on Utirik. hence the
extimate of arrival time was made on the basis
of wind velocity and distance.
Two extreme possibilities exist relative to the
duration of the fallouts: the first. that the fallout occurred entirely withing a short tame; the
second, that it was gradual and extended over a
longer period, “The monitoring instrument on
Rongenk went off scale at looome hr, one-half
hour after the dose rate began to rise above
background. Df this rate of increase ts taken as
constant, and is extrapolated te a point for
Whieh sobsequent decay would reduce the dose
rate to the values found at later times. the assumption of along fallout of about 16 hours is
found to be necessary, This slow rate of fall
and late maxim time of dose rate was one
Tamtinge conse: however this stcuation was net
considered Likely. Fixistinge data are meonelusve, bat several indications favor oa) shorter
“effective fallout tine hypothesis and are sum
tarized below.