-3rapidly but during fewer years.
but over more years.
Other soils give them up more slowly
We need vastly more research information on rates of uptake and rates
of leaching of different isotopes for different soils, for different
crops, and for different seasons.
The accumulated observations from many sources show that the rate of
clearance of material from the stratosphere is much more complex than
Recent data (including stratospheric sampling)
originally thought.
indicate that the simple exponential expression for the rate at which
debris is deposited from the stratosphere does not adequately describe
the situation.
Since January 1958 the fallout rates of the longer lived products
could be better described by quasi-symmetrical peaks characterized by
maxima appearing from 1.5 to about 11 months post-detonation and by
widths at half maximum varying from 3 to 8 months.
Although these
time intervals seem to be inversely correlated witn the latitude at
which detonations have occurred, the influence of the height reached
by the cloud and of the season of year at which tests occur remains to
be assessed,
In any event, all indications suggest that most of the
fission debris injected into the atmosphere during 1958 has already
been deposited on the ground except for that fraction pertaining to
weapons detonated at high altitude in August 1958. This debris has not
yet reached its maximum deposition rate as of March 1, 1960 and seems
to account for a sizable fraction of the present radioactivity in the
air.*
It seems likely, therefore, that some modification of past appraisals
and future extrapolations in the matter of radiation dose will be
necessary.
Most obvious from present information is the expectation
that the increase in the skeletal deposition rate of Sr?9 will be
considerably less than that postulated in the past, since the present
rate of deposition in soil is already comparable to the rate of radloactive decay of the sr70 therein.
An attempt to corroborate this prediction from the latest values of sr20
content of milk is beset with the difficulty of estimating the relative
contribution of foliar contamination of pastures.
Although this effect
is believed to be important, evaluation of its ultimate disappearance
must await return of the herds to spring pasture. Complicating factors
such as leaching and drainage of surface waters in meadows will require
*The lively debated question of a seasonal rise in fallout rate in the
spring remains undecided at the moment because of this coincidental
contribution of the Johnston Island tests.