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there may be a significant difference in estimating dose rates a year
or more after detonation and in estimating doses that might occur at
these later periods.
This is because (tame) "?*? is intended to apply
to disintegrations of atoms.
However, in estimating the reduction of
gamma dose rates above a plane with time there must be considered the
changing numbers and energy spectra of gamma photons released per
disintegration, and the effects of weathering.
During the first two weeks after fallout there was no rainfall
and the winds were light.
storm occurred.
About the end of the second week a tropical
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 a much greater reduction in contamination
was produced by the first weathering events than for later ones.
The theoretical curve of Graph One would flatten out with time
due to the dominance of Cesium-137 with its 33 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.
The continued drop
in actual dose rates versus theoretical might be explained on the basis
of the effects of weathering.
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