2000 年 35 巻 2 号 p. 187-192
We analyzed data on gamma-ray dose rates measured with an NaI(Tl) scintillator, and made 3 groups: dry days, rainy days, and rain. The hourly gamma-ray dose rate from April 1991 to December 1995 in Dazaifu City was 14.4cps on average. The average on dry days was 14.2cps. The daily variation of dose rates was inversely correlated to daily variation of temperature, showing cyclical variations with high values in the morning and low values in the afternoon. The difference between days was 0.8cps on average, which was relatively small. Monthly variations on dry days were generally high during the period from October to January, and low during the period from June to September. The difference between months was 1.9cps on average. Annual variations did not appear to be significant. The average on rainy days was 14.6cps and the range of change was high, 11.1cps on average. The average in the rain was 15.9cps and the highest was 24.1cps. Monthly variations in the rain were high in January and low in August. The gamma-ray dose rate increased rapidly with rain and was changed by the intensity, time and interval of the rainfall. After it stopped raining, it decreased gradually. Measurements that were higher than the value obtained by adding three times the standard deviation of the monthly gamma-ray dose rate to the monthly average gamma-ray dose rate were 2.2% a year on average. Most of them were observed when it rained. They were also observed when it rained after yellow sand was blown in from China.