1995 Volume 44 Issue 12 Pages 846-855
The natural background radiation was measured at 109 places in Fukuoka Prefecture by the ionization chamber and theNaI (Tl) scintillation detector. The mean values of the absorbed dose rate for the total ionizing radiation and the terrestrial γ-ray components were 76.4 and 40.7 nGy/h, respectively. The absorbed dose rate for the ionizing cosmic ray component was calculated to be 31.8 nGy/h from the regression analysis between the total ionizing radiation and the terrestrial γ-ray components.
If we assume that the outdoor occupancy factor is 1, the collective effective dose equivalent due to the natural background radiation is calculated to be 2 720 man·Sv/y in Fukuoka Prefecture by the present data.
With respect to geological classification, the natural background radiation is fairly high in the plutonic rock area and low in the volcanic rock area. In the high background radiation area, the surface soil and rock contain more abundant 40K and other nuclides from the238U-series and 232Th-series.