Many nuclear facilities are located within 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 accident. A car-borne survey which used a 3-in × 3-in NaI(Tl) scintillation spectrometer was carried out throughout the prefecture for the purposes of making a dose distribution map and estimating the annual external dose. The average absorbed dose rate in air and the annual effective dose were found to be 22 ± 5 nGy h -1 and 0.20 ± 0.08 mSv, respectively. These average values for all of Aomori Prefecture were respectively 44% and 59% of the nationwide average values. The average values with standard deviations of activity concentrations in soil of Cs to the absorbed dose rates in air were judged to be negligible.KEY WORDS: absorbed dose rate in air, annual effective dose, car-borne survey, Aomori Prefecture, activity concentration, potassium-40, uranium-238, thorium-232, dose rate distribution map.

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An easily-handling radon instrument was developed for using on the job of radiation monitoring in atomic and radiation facilities and in the environment. This instrument was named as "Radon Survey Meter" because a handy-type instrument and this was driven with battery and/or AC power. The weight of radon survey meter was 3 kg. Air is sampled by open and close of drawer of gas container within 10 seconds and radon concentration is measured by 10minutes. From a practical test in natural environment and using a radon source, it is cleared that this survey-meter was performed in radon concentration of 20~10,000Bq/m 3 .

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