-17Using a more indirect procedure, Libby suggested a value
of 2.4 megacuries of Sr-90 in the fall of 1956.
This estimate is
based not on direct measurements in the stratosphere, but rather
The
on what might be describedas material balance studies.
amount of Sr-90 produced in detonations, to date, can be
estimated with some certainty.
Estimates of the amounts of
Sr-90 that are deposited in the intense fallout in the vicinity
of a detonation are availeble from extensive investigations
conducted during the test progrems in Nevada and in the Pacific.
The total strontium produced in a detonation, less the Sr-90
which falls out in the immediate vicinity of the detonation,
cives the total inventory of 5r-90 that is available for subsequent deposition at places renote from the site of detonation.
It is apparent that the rate of precipitation of the Sr-90
must be considered in any estimation of future hazard.
time of descent was infinitely
If the
great, the Sr-90 would decay
before it reached the earth's surface end it would not constitute
a potentiel hazard.
Actually, we heve learned that the time of
descent is relatively short in relation to the half-life of
Sr-90.
Libby has estimated that the everage residence time is
approximately 10 years.
In mathenaticeal terms this would be
equivalent to a half-life of 7 years.
It is possible this
estimate is too long and the average life is as little as
6-7 years (half-life about 43 years).
Our discussion of the foreseeable levels of Sr-90 will thus
be simplified by the assimption that the material now contsined in
Dor