Table 3—- ESTIMATED BURST DATES OF FALLOUT IN
RAIN WATER
Sampling
Burst month (calculated
Burst month (calculated
month
from average Sr®?/sr*® ratios)
March
September 1955
August 1955
November 1954
May
June
September 1955
October 1955
November 1951
February 1954
July
February 1956
October 1954
April
from average % Sr*)
March 1954
to be realized. This is possibly due to enrichment of the Sr*® in fallout that would occur ina
mixture of material from different test series.
Therefore, the Sr**/Sr® ratio is the more sensitive indicator of the age of fallout and can
be expected to yield a more valid estimation of relative freshness than the percent Sr. It is
apparent, then, that July rainout is of more recent origin than any of the preceeding months.
It has been hypothesized that fallout material that is entering the troposphere from the
stratosphere has a selective entrance zone near the mid-latitude region. If the fallout material is old and originated from a high-yield device, it is probable that the debris is strato-
spheric, and upon entrance into the troposphere it can be brought down in rainfall. It follows
that there would be moreactivity in rain occurring near the mid-latitudes.
Scatter diagramsof Sr®activity vs inches of rainfall for all months are plotted in Fig. 2.
All values, regardless of location, are plotted, but stations below 40°N (an arbitrary limit)
appear as black dots on the plots. It is visually apparent from the scatter of the points that
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RAINFALL, IN.
Fig. 2a— Regression of Sr™ in rain water on the amountof rainfall in March 1956.
there is little correlation on an over-all basis. Considering a separation of the data at 40°N,
there is better correlation found among southern stations. The specific activity (millicuries
per square mile per inch) of rain water for these stations is generally lower than for northern
sites for the duration of the sampling, and the specific activity at each site does not vary conSiderably from month to month.
Since it has been possible to obtain better correlation by using the 40°N separation, this
idea is expanded by considering even smaller areas. For this purpose four areas in different
347