2016 年 51 巻 1 号 p. 27-40
Many nuclear facilities are located in Aomori Prefecture, Japan. However, no detailed dose rate distribution map of Aomori Prefecture, including its mountain regions has been reported since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Thus, a car-borne survey which used a 3-in × 3-in NaI(Tl) scintillation spectrometer was done throughout the prefecture for the purposes of making a dose distribution map and estimating the annual external dose. These results have been published in the same issue of this journal. On the other hand, many researchers have performed car-borne surveys in Fukushima Prefecture after the FDNPP accident. In this paper, the methods of car-borne survey of the present author are explained in detail. 1) The relationship between the total counts per minute of a gamma-ray pulse height distribution and an absorbed dose rate in air is examined for the estimation of dose rate conversion factor. 2) The relationship between count rates outside and inside the car is examined for the estimation of the shielding factor by car body. 3) The uncertainties to the counts inside the car, the dose rate conversion factor and the shielding factor are evaluated according to the Guide to the Expression of Uncertainty in Measurement published by the Joint Committee for Guides in Metrology. 4) The gamma-ray pulse height distributions are unfolded using a 22 × 22 response matrix for the analysis of activity concentrations in soil of 40K, 238U and 232Th and the contributions of their nuclides to absorbed dose rate in air. 5) The distribution map of absorbed dose rate in air of Aomori Prefecture is drawn using the Generic Mapping Tool which was developed by Hawaii University.