FISSION PRODUCTS IN THE ATMOSPHERE AND RAIN
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This excess was then determined by titrating with a standard KMnO,
solution. A blank experiment was also performed. The chemicalyield
of manganese was usually between 25 and 50%.
RESULTS AND DISCUSSIONS
Isotopic Ratios in Rain
Figure 1 shows the variation of the concentration of *°Sr in individual rains as well as in the bimonthly average concentrations. The
data show a marked seasonal variation.
Figure 2 shows the variation of the ‘°Ba/sr and ‘Ba/®sr
ratios in rain since November 1961. Owing to the short half-life of
40Ba, these ratios are sensitive to the injection of fresh nuclear debris.
During a test moratorium period, the isotope ratios decay linearly on
a semilog scale, as can be seen in the figure, The response of the
40Ba/%Sr ratio to the late December nuclear explosions appears to
have been delayed almost two months, and the “°Ba/*Sr ratios in rain
reached the straight line ab in Fig. 2 only after March 1964, A very
similar trend is seen in the °Ba/®%Sr ratio data. The straight line cd
has a slope corresponding to the difference between the decay of '*°Ba
and ®%sr,
The observed data may be explained as being due to the fact that
some of the nuclear explosions that occurred in the late fall of 1962
were high-altitude shots, and hence the debris did not reach the lower
atmosphere immediately,
Figure 3
shows the ®Sr/%Sr ratios in rain. Data obtained at
Lemont, Ill.,° and at Milford Haven, England,® are also plotted, Accu-
rate measurement of the ®%Sr/Sr in nuclear debris is tedious and
difficult. The radiochemical techniques involved in the determination
of the "sr/*sr ratio in nuclear debris have recently been thoroughly
reinvestigated by Hodges,’ and, as a result of improvementin technique, the recent data are probably more accurate than old data, A
rather large scatter of the experimental points seen in the data prior
to 1962 might be at least partly due to experimental errors.
The straight line ab in Fig. 3 has a slope corresponding to the
50.4-day half-life of **Sr. The **Sr/Sr ratios in rain after the Decem-
ber 1962 nuclear explosions approached the straight line ab asymptotically and joined it at the end of April. By a comparison of this trend
with those observed in the cases of the “°Ba/*Sr and the “°Ba/**Sr
ratio data, shown in Fig, 2, an impression is obtained that the response
of the ®*Sr/%Sr ratios to the late December 1962 nuclear explosions
were somewhat slower than those of the “°Ba/Sr and the “°Ba/®%Sr
ratios.