Variation of tritium concentration in rain at Fukuoka

Tritium concentrations have been measured for rain collected at Fukuoka for these 8 years. The annual averages of tritium concentration have showed a gradual decline with an apparent half-life of 8.1 y until 1988. However, the decline on tritium concentration was not clear after 1988, suggesting no significant contribution of bomb produced tritium to rain. Variation of tritium concentration, relatively high in winter and low in summer season, indicates that origin of air masses is important on tritium concentration in rain. (author)