ENVIRONMENTAL BACKGROUND LEVEL OF COSMIC-RAY PRODUCED NA-22

The annual deposition rate of cosmogenic Na-22 has been studied al Fuk ui City, Japan, to obtain the current local environmental background l evel. The deposition rate was found to range from 0.16 to 0.22 Bq m-2 y-1 with a mean value of 0.20 Bq m-2 y-1 for the years 1986-1990. The authors' results agree fairly well with those observed for the years 1 975-1977 in Tokyo. The Na-22 concentration in rain water was also esti mated. The mean value for this period was found to be 0.09 mBq litre-1 which is about one-third of that detected during the latter half of t he 1950s. The high levels of environmental Na-22 in the 1950s are attr ibuted to the many nuclear weapon tests conducted al that time. At pre sent, however, the Na-22 from these weapons tests has practically deca yed and the concentration in rain water has decreased to the natural b ackground level.