Seasnal variation in the environmental background level of cosmic-ray-produced 22Na at Fukui City, Japan

The monthly deposition rate of cosmogenic 22Na was observed at Fukui City (36 °N, 136 °E). The deposition rate varied widely, being one order of magnitude smaller in summer than in winter. The annual deposition rate of 22Na was 0.42 Bq m−2 year−1, which was four orders of magnitude lower than that of 7Be. This value was smaller than those observed during the first half of the 1960s. The high levels in the 1960s resulted from artificial 22Na produced by the frequent atmospheric nuclear weapon tests. The 7Be/22Na activity ratio showed a seasonal variation that was induced by the downward transport of stratospheric air into the troposphere with different residence times between seasons.