212
PERKINS, THOMAS, AND NIELSEN
centrations of "y and !“Sb during January 1964 help explain the ob-
served high fallout rates of these radionuclides during the summer of
1964 (see -Fig. 7). The relatively low concentrations of '“*Cs in the
stratosphere are in accord with the '“Cs to '3’Cs ratio measurements
on biological samples,’ which show a large drop in this ratio since
1961 and indicate that a major injection of “Cs into the atmosphere
occurred prior to the past test series.
Stratospheric samples were available which covered latitudes
from about 26 to 70°N at longitudes of 97 to 146°W. This covers an
area from southern Mexico to northern Alaska. These samples were
collected between Jan, 8, 1964, and Feb. 18, 1964. In Fig. 10 the ratios
10
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57 ,_95Nb
T
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10
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I
30 40 50 60
DEGREES NORTH
ke
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1
70
20
30 Oo 50 50
DEGREES NORTH
70
Fig. 10—Ratios of the concentrations of various airborne radionu-
clides to 9’7Cs as a function of latitude (stratospheric samples).