488
LOCKHART, PATTERSON, SAUNDERS, AND BLACK
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1961
1962
Fig. 7—Tropos pheric burdens of IS y activity.
after April 1962 resulted from the U. S. Christmas Island tests. The
small magnitude of the equatorial crossover of tropospheric debris
would not significantly change the conclusions.
The small effect of the U. S, Christmas Island tests on the radioactivity levels in the northern hemisphere is shown in Fig. 8, where
89Sr/Mgr ratios in 80th meridian air samples and in precipitation at
Westwood, N. J.”8 and Rijswijk, The Netherlands,”® are plotted against
time. The solid line extrapolating back to a shot date of Oct. 15, 1961,
during the U.S.S.R. 1961 series, characterizes very well the decay of
the "*sr/"sr activity ratio during the spring of 1962. The relatively
constant composition at all the northern-hemisphere sites is evidence
of the well-mixed condition of the troposphere. A slow, progressive
change in the source material is indicated by the gradual increase in
the relative amount of the shorter lived component, ®*Sr. No apprecia-
ble change in this pattern occurred until the July —August collection
period. Though some “°Ba was detected in rainwater at Westwood,
N. J., in May and June,which must have come from the Operation
Dominic I tests, an upper limit of 10 to 15% can be set on the contribu-