A survey of terrestrial gamma ray dose rates of Nara Prefecture

Terrestrial gamma ray dose rates were measured in Nara Prefecture. The portable 1''φ x 2'' Na(Tl) scintillation counter was used and spectra of gamma ray of >50 keV gave the dose rate by response matrix correction. Measurement was conducted at 207 points in the prefecture. The mean dose rate at all points was 46.8 nGy/h with standard deviation of 13.6 nGy/h and variation coefficient of 0.29. The above statistic values were close to those in all Japan areas, the variations in each drainage system were different, dose rates of granite in the northern area were rather lower than those in Japan, and the rates were lower in Minamikawa and Chichibu zones in mid-prefecture than southern Shimanto zone. (N.I.)