-14Tungsten-185, with a half-life of 74 days, was produced in a number
of detonations over silica sand.
By studying this isotope it is hoped that the origin of the middle
northern hemisphere maximum of strontium- 90 fallout can be clarified,
Three main models of world-wide transport have been proposed, each
giving a different interpretation of this observed phenomenon.
Reduced
to their simplest terms, they are:
(1)
A model, described by Libby (Ref. 8), based on uniform distribution
of radioactive debris in the stratosphere.
In this model the non-uniformity
of fallout is due to non-uniform rainfall and to the contribution from lowyield tests which deposit debris in the troposphere.
The low-yiéld tests
have been mostly conducted in the middle and high latitudes of the northern
hemisphere.
This model, then, attributes a major part of the strontium-90
fallout which had occurred in the United States by late 1957 to Nevada tests
and USSR low-yield tests.
(2)
The second model, described by Machta (Ref. 6), is based on a very slow
mixing rate, both horizontally and vertically in the lower stratosphere,
together with a slow poleward circulation in both hemispheres.
In this
model material from both equatorial Pacific tests and high-latitude USSR
tests tends to drift towards higher latitudes and to enter the troposphere
non-uniformly with a maximm in the middle latitudes of both hemi spheres in
late winter and spring.
If this model is correct, the major part of the
strontium-90 fallout in the United States up to early 1958 had come from
tests in the equatorial Pacific.
(3)
DOE ARCHIVES
A third model, recently proposed by Martell (Ref. 16) is based on a
shorter stratospheric residence time for debris injected at high latitudes
than at equatorial latitudes.
This model attributes a large part of the (le