The ratios of fission product pairs with similar half-lives in the atmosphere

The ratios of pairs of fission products with similar half-lives (58.5-day 91Y to 50.8-day 89Sr and 13.6-day 143Pr to 12.8-day 140Ba) were measured radiochemically in a series of rain samples collected at Fayetteville (36°N, 94°W), Arkansas, after the Chinese explosions of March 18, 1972 (the fourteenth test), and of June 27, 1973 (the fifteenth test). The activity ratios in rain were found to show a marked variation: the 91Y/89Sr ratio in rain varied between the extreme values of 0.29 (March 24, 1972) and 10 (April 3, 1972), and the 143Pr/140Ba ratio varied from 0.21 (March 24, 1972) to 6.3 (April 13 and 14, 1972). The observed large variations of the activity ratios appear to be attributable to the difference in size distributions of particulate matter in rain. The 91Y and 143Pr become highly enriched in particles larger than a few microns in size, in relation to 89Sr and 140Ba, respectively, whereas the reverse appears to be the case in smaller submicron size particles. A similar trend was found in the data obtained after the seventh Chinese test of December 24, 1967. The results indicate that these isotopic pairs may turn out to be extremely useful as atmospheric tracers in the studies of meteorological phenomena.