SEASONAL-VARIATION IN THE ENVIRONMENTAL BACKGROUND LEVEL OF COSMIC-RAY-PRODUCED NA-22 AT FUKUI CITY, JAPAN

The monthly deposition rate of cosmogenic Na-22 was observed at Fukui City (36 degrees N, 136 degrees E). The deposition rate varied widely, being one order of magnitude smaller in summer than in winter. The an nual deposition rate of Na-22 was 0.42 Bqm(-2) year(-1), which was fou r orders of magnitude lower than that of Be-7. This value was smaller than those observed during the first half of the 1960s. The high level s in the 1960s resulted from artificial Na-22 produced by the frequent atmospheric nuclear weapon tests. The Be-7/Na-22 activity ratio showe d a seasonal variation that was induced by the downward transport of s tratospheric air into the troposphere with different residence times b etween seasons. (C) 1998 Elsevier Science Ltd.