have been established within the past two years.
However, sufficient
data have been collected to indicate world-wide distribution of artificially radioactive material and that a good qualitative relationship
exists between the total fission products and the strontium-90 activity
which is falling out.
The data indicate that on the average about 1 millicurie of
_strontium-90 per square mile has fallen out up to 1 September 1954 in
the United States.
About 0.25 millicurie per square mile has been
estimated for England and varies for other countries over the world
from this value to a low of 0.03 millicurie per square mile in Africa.
Stations in the Philippine Islands and other stations in that area show
higher values, probably due to easterly winds at high altitudes from
the Pacific Proving Ground conibined with the unusually high rainfall
in the South China Sea area.
Such rainfall is known to scavenge radio-
active particles quite effectively from the troposphere.
There is a
large world area in the Communist-dominated countries where no samples
have been collected.
However, on the basis of the data available, it
can be assumed that an average of 0.25 millicurie of strontium-90 per
square mile has fallen to date in Europe, 0.1 millicurie per square
mile in the Southern Hemisphere and 1.0 millicurie per square mile in
the United States.
It should be noted at this point that the total
activity from fall-out falls off quite slowly after about one year
following a detonation.
The decay factor of tie is no longer a use-
ful approximation much after that time, since the decay rate is then
controlled by only the few long-lived isotopes remaining.
For a period
of from one to six months after a detonation, the rare earth elements
constitute about 30% of the total activity.
Domination of the activity
slowly chenges so that after about one year a few specific isotopes
such as cesium-137, strontium-90 and a few of the longer lived rare
earth elements determinethe effective over-all decay rate.
The effect of the environment upon which the fall-out occurs is
to tend to fractionate certain elements, particularly strontium-90, so
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