The contribution from the decay of each radioiodine nuclide to thy-
roid absorbed dose was dependent upon the time post-detonation and upon the
fractionation of the isobaric chains giving rise to the radioiodines. Both factors influenced our estimate of thyroid absorbed dose. James assumed theoreti-
cal fission yields and that one third the ingestion intake occurred at ten hours
and two thirds at 30 hours post-detonation.
We assumed Bikini ash fractionation
and that one third the ingestion intake occurred at 5.5 hours (lunch) and two
thirds at 12 hours (dinner) post-detonation.
and !35r in the thyroid-dose estimate.
James considered only
31y, 133,,
We considered all likely iodine and
tellurium nuclides. Additionally, James adjusted the thyroid dose downward to
10.5 gray (1050 rad) for a 3.5~year-old to account for the possibility that part
of the intake was due to inhalation. We considered inhalation intake to be insignificant relative to ingestion intake and compared the James ingestion estimate to our ingestion estimate.
c. Maximum Thyroid Absorbed Dose. We used several methods to estimat>
a range of fallout material ingested. One was to ingest known quantities of phmaceutical-grade CaCO3 with meals and subjectively arrive at descriptions of
taste similar to those given by the Rongelap people at the time of evacuation ir.
March 1954. A group of five adult white males at BNL reported that 200 mg of
CaCO3 could not be sensed by taste when mixed with food. Another method was to
assume that the range of weights associated with the contents of the stomach in .
cases of sudden death corresponded to the range of activity intake (Ev66). This
range ~ 0 to 380 g, mean 82 g ~- implies a maximum intake of about 5 times the
mean value. Another method was to examine the range of 137¢g daily activity in-
take estimated from 1957 to 1983 for Rongelap and Utirik people (Le84).
The in-
take rate was estimated from whole-body counting results. The range of
Cs intake rate was about 5 times the mean value. Another method was to examine the
range of 137¢g body burden exhibited by the population inhabiting Bikini Island
from 1974 to 1978 (Mi83). The range was about 3.2 times the mean value. From
the above range values, we assumed a value of 4 times the intake and thus 4
times the mean thyroid absorbed dose for estimates made here (see Table 23).
d. External Sources of Thyroid Dose. External thyroid absorbed dose
estimates were based on integrated photon exposure given previously and on an adjustment for living pattern in a variable exposure-rate environment. Further de-
tail about the adjustment can be found in (Na80) and (Le84).
Some questions about the external beta radiation penetrating to
the depth of the thyroid were expressed by Cronkite (Cr81).
The thickness of
tissue overlying the thyroid ranges from 0.4 to 2.0 cm, average 0.82 cm, and
does not correlate well with age or body weight (ICRP74). A minimum beta energy
of 1.8 MeV was estimated by us for penetration of 0.82 cm of tissue. At
Rongelap Island, about 70% of the population had skin lesions on some part of
the neck.
The lesions appeared initially about 21 days post-exposure (Cr56).
This ->.uld imply a skin surface dose of tens of gray (several thousand rad).
Only a small percent of the beta flux was above 1.8 MeV in kinetic energy. Of
this higher energy flux, only a small fraction would penetrate 0.82 cm of tissue
and deposit energy in the thyroid. Thus, we considered thyroid dose from this
pathway to be insignificant.
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