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CHAPTERI
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External Gamma Radiation
GAMMA DOSE RATES WERE taken periodically on
severalislands in the Pacific over a time ranging
from about two days to more than two years.
PACIFIC MARSHALL ISLANDS
The attached mapis an estimate of the gamma
dose rates at three feet above the ground at
D-+1 (one day after the detonation on March
1, 1954), A very rough approximation of the
degree of contamination may be made by dividing these readings by four to arrive at units of
gamma megacuries per square mile. (The beta
to gammaratio varies with time but at one day
sunCEE
ATOLL
we
AILINGINAE ED
ATOLL
‘Statute Miles
ATOLL
may be near unity, so these values may also be
thought of as beta activities.) However, the
gamma dose rates do indicate the relative degrees of contamination on the islands, therefore are useful in this respect: when evaluating
the data in subsequent sections of this report.
Graph 1 shows the decay with time of gamma
dose rates on the Island of Rongelap. Similar
decay curves were found on otherislands in the
Atoll and in nearby Atolls (Ailinginae and
Rongerik).
The decay of activity of mixed
fission products is assumed to follow (time)~*?
principle. This is intended to apply to disintegrations of atoms. However, in estimating
the reduction of gamma dose rates above 6
plane with time there must be considered the
changing numbers and energy spectra of gamma
photons released per disintegration, and the
effects of weathering. When computing the infinity radiation doses from fallout that occurs
within a few hours after detonation, integration
of the (time)~'? curve gives a fair approxima-
tion since mostof this total dose is accumulated
during the early periods when this curve lics
near the theoretical gamma decay curve. However, in extrapolating by (time)-'? there may
be a significant difference in estimating dose
rates a year or more after detonation and in
estimating doses that might occur st these later
periods.
During thefirst two weeks after fallout there
was no rainfall and the winds were light. About
the end of the second week a tropical storm
occurred. For these reasons, a straight line
was drawn for the first two weeks followed by
a break in the curve. The readings are not to
be considered precise, due to the nature of such
measurements, but the curves suggest that
whatever was the reduction of gamma dose
rates by weathering, it occurred principally
with the first heavy rainfalls. Except for the
last data point on Graph 1 for the Island of
Rongelap, which may be somewhat high, the
actual and theoretical decay curves correspond
fairly well.
Thetheoretical curve of Graph 1 would flatten
out with time due to the dominance of Cesium~
137 with its 27 year half-life. The last survey
of Rongelap Island in late July 1956 indicates
a range of gamma dose rates at three feet above
the ground of 0.2—0.5 milliroentgens per hour
with an average of 0.4 mr/hr. These values are
higher than suggested by Graph 1 and are due
to the small additional fallout resulting from
Opcration Redwing (Spring and Summer 1956).
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