14
In order to be able to predict the future levels of radioactivity in the plants and animals of the Eniwetok Test Site area,
the rate of decline has been determined for many organisms.
The
rate of decline is defined as the rate at which the radioactivity
is decreasing in a given tissue,
environment
(Held,
physical decay.
1957).
organ,
or organism in its native
The only constant factor is that of
Prediction of rate of decline would be extremely
difficult by other than empirical means.
The decline rates and
decay rates for many organisms in various areas in the vicinity of
the Eniwetok Test Site are reported by Bonham (1958).
In general the decay curves are steeper than the decay curve
for mixed fission products which is generally expressed as being
t7l-2
-
The mean of about 100 decay curves was about t
“1.5.
The
greatest deviation from the mean was for muscle samples from birds
and rats,
for which the decay rate was much slower.
The decline rates were equal to or greater than the decay
rate of tole?
The difference between the decay rate for mixed
fission products and the decay rate and decline rate of the biological samples means that the radioactivity is declining faster
in plants and animals than it would from fallout that included the
entire spectra of fission products.
Reasons for this are that
there is biological selectivity of the radioisotopes and other
factors such as weathering.
The decline in radioactivity
of
representative samples from just after the Nectar shot at Eniwetok