Fourth: doses calculated on the long fallout hypothesis are lower than those due to a short
fallout, since a short fallout quickly deposits a large amount of activity. On Rongerik, «a set of
film bidge readings covered the range listed below, Several badges worn both outdoors and
maide builidings on the rsland read 50) 65 r, and one badge which remained outdoors over the
28.5 hr period read 98 r. Affother proup kept indoors inside a refrigerator read 38 ce. These
dose values representa variety of condiffons, but considering the shielding and attenuttion
factors, are consistent with the assumption that the dose reached the calculated upper limit
outside, agate favoring the shorter fallout bypothesis. ‘The upper Limit of 08 ro will result if it
isaisstined that the fallout bested one hour during which the infensities rose from zero to the
maxtinunm dose rate which Chen decayed to viadues observed lator, A long fatiout will not pro
duce sucha high dose of radiation,
Fifth: on Utirik, only a short fallout time ts consistent with the later dase rates observed,
provided the fallout began as late as was estimated from wind and distance factors. A one
hour duration*® of fallout appears likely. On the other islands the actual fallout time is known
to have exceeded one hour; however, since the approximate dose discussed above was seen to
fit the film data on Rongerik, it was used for the other islands as listed in the calculations in
Table 1.1, The hour limit is thus “an effective value.”
If the long fallout case is also considered, a lower limit for the dose may alsu be estimated, though the upper limit is taken as most probable. The ranges are then as follows:
Rongerik 50 r 104 r; Rongelap 102 r -175 r; Ailinginae 53 r-69 r; and Utirik -14 r.
The dose value for Rongerik given in Table 1.1! is 75 per cent of the short fallout case
value, averaged for 28.5 and 34 hour exposures. This best expresses the average air dose received by personnel who spent roughly half their time inside structures where the dose rate
was later found to be roughly half that outdoors, On the other islands no such shielding was
present.
.
Figure 1.3, for the Rongelap atoll, illustrates the cumulative dose as a function of time
after the detonation. It can be seen that the rate of delivery of the dose varied continuously,
the major portion being received at the higher dose rate prevailing in the early portion of the
exposure period, By the time that 90 per cent of the dose had been received, for example, the
dose rate had fallen to ess than 30 per cent of its initial value. Thus the dose’ rate of exposure
differed markedly from that usually encountered using x-ray units.
1.4.4
Geometry of the Exposures
A third difference between the type of exposure encountered here and other external exposures lay in the geometry of the source. These doses were delivered from a plane source,
so that the radiation field did not follow the narrow beam geometry usually employed experimentally. In such a diffuse 360° field, the decrease of dose with depth in tissue is less pronounced than that resulting from a unilateral or bilateral exposure to an X-ray beam, so that
for a given energy, the dose at the center of the abdomen is approximately 50 per cent higher
than a given air dose would imply for the narrow beam case. Figure 1.4 illustrates an experimental simulation of the field geometry using a spherically oriented group of Co™ sources
with «va phantom placed at their center, compared with a conventional depth dose curve obtained
with a single source, It would appear under the circumstances that the midline dose, rather
(hain dose measured iffair, would be the better parameter in terms predicting bialogical ef fects. On {fis basis, the air dose values stated in Table 1.1 should be multiplied by approxtmately 1.5 an order to compare therr effects to those of an exposure using uv narrow beam
geometry, Hf this is done, assuming a fast fallout of one hour, the following doses in terms of
anair dose under Liboratory conditions result: Rongelap 260 r; Ailinginae 100 r; Ronperik
120 cr; and Utirik 21 r.
«While it is obvious that the fallout lasted longer than one hour, calculations of dose are
based on an assumed one hour fallout as explained in the text.
eevee
teoree
wees
8
e
seonse
.
.
20