substantial fall-out from, @fAVO_ indicated activities ranging from 455 to 2740 counts/min
per cubic foot of air.
In addition to the portable air samplers, a cascade impactor was installed in the radiac
repair shop on board the USS Bairoko near the ventilation outlet. Additional impactors were
also located on Parry and Eniwetok islands to be utilized if fall-out should occur on these islands. The purpose of the impactor was to determine the percentage of the total activity associated with particles 5 p in diameter or less in order to evaluate the inhalation hazard to
personnel subjected to the fall-out. The only cascade-impactor data obtained were collected
during the fall-out that occurred on the USS Bairoko. An average of 65 per centof the activity
collected by the cascade impactor war's found to be associated with particles less than 5 p in
diameter.
3. Decay and energy measurements: As mentioned previously, the mission of the Radia-
tion Analysis Section included the determination of the characteristic radioactive properties
encountered in the various samples collected throughout Operation Castle. Beta- and gammaenergy measurements, however, were difficult to obtain by absorption methods becauseof the
very low energy of the gammapresent in the samples collected. Low-energy gamma photons
being absorbed on aluminum filters gave rise to misleading beta-absorption curves and yet
could not be readily detected using lead absorbers because of the combination of low-counting
efficiency for gamma with a Geiger-Mueller tube and the high attenuation in the smallest lead
filter. Beta-energy measurements using absorption methods actually should more properly be
referred to as the corhbination of beta plus low-energy gamma.
In order to obtain a gamma-absorption curve of significance, it was necessary to use a
sample having a very high specific activity in order to compensate for the low counting efficiency. Such a-curve is illustrated in Fig. 5.4. It is noteworthy that the low-energy component
is less than 50 kev and the high-energy component is 0.6 Mev. Other samples for which the
gamma energy was determined fell within this range of values.
Beta energies (beta plus low-energy gamma) varied from 0.2 to 2.2 Mev, depending on the
sample being studied,
Log-log plots of counts per minute vs time after detonation were utilized to obtain decay-
rate data, and many different types of samples were studied throughout Castle. Rather than to
present all the data, the most interesting samples for each detonation have been selected, and
the decay curves representing these samples are included in Figs. 5.3, 5.5, and 5.6. A tabulation of the slopes of the other decay curves is included in Table 5.1. It is interesting to note that
the drinking-water samples from Rongelap indicated an average slope of —1.37 from|@Wv0]J+
4.2 to 10 days and --1.78 until last counted, This deviation from the 1.2 law, especiallyduring
the early period, obviously has important implications with respect to the total amountof radiation received by the Rongelap natives, assuming that this law held from the beginning of
their exposure.
_.
As shownin Table 5.1, the lagoon samples studied after KOmEGjndicated a slope of —1.35.
This same slope was obtained irrespective of whether the sample was taken on the surface,
below the surface, or even four days after the detonation.
‘ “7 Miscellaneous samples andprobléms:the course of the operation the Section
was called upon to study samples of food, soil, and water brought back from the resurvey of
Rongelap and Uterik islands. The food samples consisted of coconuts, papaya, pandanus,
arrowroot, and breadfruit, all of which are important constituents of the native diet, in addition
to sea food. All the material from Rongelap, the more heavily contaminated island, showed
considerable surface activity; however, no internal activity was detected. This finding was
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