Environmental Radiation Monitoring and External Dose Estimation in Aomori Prefecture After the Fukushima Daiichi Nuclear Power Plant Accident

Jpn. J. Health Phys.，51 (1)， 41 ～ 50 (2016) DOI: 10.5453/jhps.51.41 Original Paper

Environmental Radiation Monitoring and External Dose Estimation in Aomori Prefecture after the Fukushima Daiichi Nuclear Power Plant Accident

Masahiro HOSODA,*1 Kazumasa INOUE,*2 Mitsuaki OKA,*3 Yasutaka OMORI,*4 Kazuki IWAOKA*5 and Shinji TOKONAMI*5

（Received on September 16, 2015）（Accepted on November 24, 2015）

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 40K, 238U and 232Th were 234 ± 148, 15 ± 6, 12 ± 6 Bq kg–1, respectively. The average values of contributions of 40K, 238U and 232Th to absorbed dose rates in air were 39%, 29% and 32%, respectively. The contributions of 134Cs and 137Cs 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.

the Japanese government.11) These results are available on the I INTRODUCTION website of the Nuclear Regulation Authority.12) The minimum Large amounts of arti¿cial radionuclides were released level of the distribution maps of ambient dose equivalent by the Fukushima Daiichi Nuclear Power Plant (FDNPP) rate which were reported by the government was shown as accident on 11 March 2011.1) The evaluations of contamination 0.1 ȝSv h–1. by the radionuclides,2) distribution of ambient dose rate,3, 4) A fact that is often neglected by the general public is that external dose 5, 6) and internal dose 7~10) have been carried out people were exposed to natural radiation sources such as by staff members of the national and local governments and radon, cosmic-rays and terrestrial gamma-rays before the researchers of universities and institutes. Moreover, air-borne FDNPP accident,13) and that such exposure continues on a surveys for making distribution maps of ambient dose rate daily basis. In order to estimate the external dose for Japanese, (ambient dose equivalent rate) and contaminations by 134Cs a nationwide survey of the ambient dose rate (exposure rate) and 137Cs in the whole area of Japan were also carried out by by natural radiation was carried out by researchers of the National Institute of Radiological Sciences (NIRS),14) and *1 Department of Radiological Life Sciences, Division of Medical Life the distribution map of ambient dose rate (converted to the Sciences, Hirosaki University Graduate School of Health Sciences; absorbed dose rate in air) was drawn.15) Portable type NaI(Tl) 66–1 Honcho, Hirosaki-shi, Aomori 036–8564, Japan. scintillation survey meters which were calibrated by the same *2 Department of Radiological Sciences, Graduate School of Human Health Sciences, Tokyo Metropolitan University; 7–2–10 Higashiogu, ionization survey meter were used for this nationwide survey, Arakawa-ku, Tokyo 116–8551, Japan. and almost all the measurements were made on the grounds of *3 Environmental Radioactivity Monitoring Center, Safety schools located in each prefecture.14) According to the results – Management Division, Japan Nuclear Fuel Limited; 4 108 Aza by this nationwide survey,15, 16) the arithmetic mean of absorbed Okitsuke, Oaza Obuchi, Rokkasho-mura, Aomori 039–3212, Japan. *4 Department of Radiation Physics and Chemistry, Fukushima dose rate in air by terrestrial gamma-rays was reported as Medical University; 1 Hikarigaoka, Fukushima-shi, Fukushima 960– 50 nGy h–1, and the annual effective dose was 0.33 mSv. 1295, Japan. Two nuclear power plants (NPPs) and Japan’s ¿rst 5 * Department of Radiation Physics, Hirosaki University, Institute of commercial nuclear fuel cycling facilities are located in Radiation Emergency Medicine; 66–1 Honcho, Hirosaki-shi, Aomori 036–8564, Japan. Aomori Prefecture (Fig. 1). The operation of the Higashidori E-mail: [email protected] NPP was stopped following the FDNPP accident. The 42 Masahiro HOSODA, Kazumasa INOUE, Mitsuaki OKA, Yasutaka OMORI, Kazuki IWAOKA and Shinji TOKONAMI

Fi g. 1 The route map in Aomori Prefecture and measurement points of gamma- ray pulse height distribution using a NaI(Tl) scintillation spectrometer. construction of the Oma NPP was also suspended at that time draw a detailed dose rate distribution map using the survey and re-started from October 2012. In the nationwide survey results. Additionally, an external dose estimation was carried by NIRS, measurements of ambient dose rate were carried out out using the obtained data. at only 23 points in Aomori Prefecture 14) and these were not II MATERIALS AND METHODS enough to draw a detailed dose rate distribution map of the prefecture. IYOGI et al.17) measured absorbed dose rate in air 1. Survey route at 109 points in residential areas of Aomori Prefecture using A car-borne survey which used a 3-in × 3-in NaI(Tl) radio-photoluminescence glass dosimeters (RPLGDs), and scintillation spectrometer (EMF-211, EMF Japan Co., Japan) they provided the ¿ rst reported dose rate distribution map of was carried out from the 12th to the 18th in August 2013. the prefecture. Their survey included suf¿ cient data to estimate The survey route covered all 40 cities of Aomori Prefecture the external dose for residents. However, in order to draw a and it is shown in Fig. 1. This route map was drawn using more detailed dose rate distribution map of Aomori Prefecture the Generic Mapping Tools (GMT) created by WESSEL and which has many nuclear industries, it is necessary to make SMITH.19) Measurements of the counts inside the car were measurements at many more locations, including mountain carried out every 30 s along the route. Simultaneously with the regions. gamma-ray count measurements, the latitude and longitude According to air-borne survey results made available by at each measurement point were measured with a global the Nuclear Regulation Authority 12) for the northern Tohoku positioning system (GPS). Car speed was kept at 40–60 km h–1 region, consisting of Aomori, Iwate, and Akita Prefectures, for ordinary roads and 20–40 km h–1 for mountain regions. the observed ambient dose rates were below 0.1 ȝSv h–1 for The estimated distances between measurement points ranged almost all areas. Environment radiation monitoring at ¿ xed from 170 m to 500 m. The weather, except from 16:30–17:10 points around the nuclear fuel cycling facilities at Rokkasho on the 16th of August, was sunny or cloudy throughout the Village (Nos. 21 and 22 in Fig. 1) is carried out by the local entire measurement period. Count rates which were inÀ uenced government, and the monitoring results have been used to by rain on the 16th were re-measured on the 18th. Thus, the discuss the dose contribution from the arti¿ cial radionuclides estimated absorbed dose rates in air were not affected by by the FDNPP accident.18) However, no dose rate distribution rainfall. The total distance of the car-borne survey was map for all of Aomori Prefecture since the FDNPP accident 2,501 km. has been obtained. In this study, a car-borne survey for all of Aomori 2. Evaluation of the conversion factor to the aEsorEed Prefecture, including its mountain regions, was carried out to dose rate in air Environmental Radiation Monitoring and External Dose Estimation in Aomori Prefecture after the Fukushima Daiichi Nuclear Power Plant Accident 43

The detailed method of the car-borne survey was described analyzer was divided into unequal intervals as energy bins in in previous reports by the authors 20~22); only an outline is the gamma-ray energy range from 0 to 3.2 MeV. The gamma- described here. The NaI(Tl) scintillation counting system used ray energies over 3.2 MeV were not included for evaluation, can provide the absorbed dose rate in air using the response since the maximum value of the gamma-ray energy from matrix method while the car is being driven.23, 24) However, natural radionuclides is 2.615 MeV which is emitted from photon peaks obtained with the NaI(Tl) scintillation detector 208Tl. The energy ranges were set up so that the gamma-rays are affected by temperature.25) Furthermore, it is dif¿cult to of 1.464 MeV from 40K, 1.765 MeV and 2.205 MeV from 214Bi obtain the photon peak for each gamma-ray energy in a 30-s (238U-series) and 2.615 MeV from 208Tl (232Th-series) could be measurement in the natural environment. Thus, the accuracy stored in each bin, individually. A 22 × 22 matrix for the 3-in × of the energy calibration will be low for these reasons. 3-in NaI(Tl) scintillator for an isotropic ¿eld was calculated Accordingly, the relationship between the total counts per using the Monte Carlo code, SPHERIX.27) The gamma-ray minute (cpm) of a gamma-ray pulse height distribution and Àux density and dose rate per unit solid angle are considered an absorbed dose rate in air (nGy h–1) was examined for the almost isotropic in the natural environment.28) The contribution estimation of dose rate conversion factor (nGy h–1 cpm–1). of cosmic-rays to the gamma-ray pulse height distribution Measurements of gamma-ray pulse height distributions were was subtracted using the energy bin number 22 which was carried out 1 m above the ground surface at 35 points along the stored as the gamma-ray energy range from 3.0 to 3.2 MeV. survey route. Counting time at each measurement point was Moreover, the contribution of 40K contamination due to photo- set as 900 s. The gamma-ray pulse height distributions were multiplier tube was also subtracted from the gamma-ray pulse unfolded using a 22 × 22 response matrix for the estimation of height distribution. Clear peaks from 40K (bin number 14, absorbed dose rate in air.23, 24) energy range: 1.39–1.54 MeV), 214Bi (bin numbers 16 and 18; energy range, 1.69–1.84 MeV and 2.10–2.31 MeV) and 3. Evaluation of the shielding factor Ey car Eody 208Tl (bin number 20; energy range, 2.51–2.72 MeV) were Since count rate is measured inside the car, it is necessary to observed in the unfolded spectrum of the gamma-ray pulse estimate a shielding factor of the car body towards terrestrial height distribution. The activity concentrations of the 238U, gamma-rays in order to represent the unshielded external 232Th and 40K could be estimated by comparing them with dose rate.26) The shielding factor was estimated by making the theoretically evaluated gamma-ray Àux density spectrum measurements inside and outside the car at 73 points and due to these nuclides. In order to evaluate each activity correcting them with the inside count rates. Measurements of concentration of the natural radionuclides from an energy the count were recorded over consecutive 30 s intervals during bin spectrum, it is necessary to calculate the gamma-ray Àux a total recording period of 2 min. densities per unit activity concentration of the 238U, 232Th and 40K. The primary and scattered gamma-ray Àux densities 4. Evaluation of the effect of pavement cover on per unit activity concentration could be calculated using measurement the one-dimensional Monte Carlo gamma transport code, In order to evaluate the effect of pavement cover toward the MONARIZA/G2.29) A total of one million histories were traced absorbed dose rate in air, measurements of count rates on the for each natural radionuclide. The nuclear data of gamma- soil surface and the pavement were carried out at 20 points. ray energies and disintegration rates used the values reported Measurements on the soil surface and the pavement were also by BECK 30) and BECK et al.31) for this Monte Carlo simulation. individually set for the recording period of 2 min. The activity concentration of each natural radionuclide was evaluated by a successive approximation which used a 3 × 3 5. Analysis of the activity concentrations in soil of 40K, matrix determined by MINATO 24) to the values of energy bins 238U and 232Th for 40K, 238U and 232Th. Incidentally, the energy resolutions Measurements of the gamma-ray pulse height distributions of the photon peak from each natural radionuclide have were carried out 1 m above the ground surface at the 25 points wide variations for commercially available scintillators. in Fig. 1. These measurement points were included in the So a diagonal element ¿tting (DEF) technique which was 35 points used for evaluation of the conversion factor to the developed by MINATO 24) was used in this calculation. A new absorbed dose rate in air. In order to evaluate the activity matrix was reconstructed easily and quickly from the standard concentration in soil, ten measurements taken above the response matrix using this DEF technique. This calculation pavement were not used for this analysis. The gamma-ray assumed that a semi-in¿nite volume source was formed in the pulse height distributions were unfolded using a 22 × 22 ground.23, 24) Counting time at each measurement point was response matrix for the analysis of activity concentrations set as 900 s. The relative standard deviations for absorbed in soil of 40K, 238U and 232Th and the contributions of their dose rate in air and activity concentrations in soil for 40K, 238U nuclides to absorbed dose rate in air.23, 24) By following the and 232Th obtained using this software depend on the integral method presented in references by MINATO,23, 24) a gamma- absorbed dose (nGy) at each measurement point,32) and these ray pulse height distribution obtained by measurements was were evaluated in this study as 2–3%, 2–4%, 6–13% and converted to the energy bin spectrum of incident gamma- 4–9%, respectively. Since the standard uncertainties of these rays which is a distribution of gamma-ray Àux density to each values were not reported in the literature,32) these relative energy bin. In this method, each channel of a multi-channel standard deviations were used in the present analysis. 44 Masahiro HOSODA, Kazumasa INOUE, Mitsuaki OKA, Yasutaka OMORI, Kazuki IWAOKA and Shinji TOKONAMI

ground surface at each measurement point can be estimated using Eq. (1). III RESULTS AND DISCUSSION D =2N ×1.50×(1.9×10–3) (1) 1. Correction factors for count rates out in –1 A correlation between absorbed dose rate in air (nGy h ) In this study, the counts (Nin) inside the car were obtained which was calculated by software using the 22 × 22 response by the measurements for 30 s. Since the dose rate conversion matrix method and total count rate (cpm) outside the car is factor was given as dose rate (nGy h–1) for counts per minute shown in Fig. 2. The dose rate conversion factor and the (cpm) it is necessary to double Nin in order to convert it into standard uncertainty were evaluated as 1.9 × 10–3 and 2.2 × the counts per minute.21) 10–5 (nGy h–1 cpm–1), respectively. The difference in the The standard uncertainty of one time measurement can abundance ratio of 238U, 232Th and 40K at the measurement be given as a standard deviation of the measured value. The points may inÀ uence variation of the dose rate conversion values of Nin were distributed from 1,789 to 8,970 (counts factor. However, the relative standard deviation of the ratios per 30 s). The standard uncertainties depend on the counts, of total count rates and the absorbed dose rates in air at each and they were given as 85 to 189 (counts per 30 s). These measurement point in this study was 8.0%. Thus, this result standard uncertainties were converted to the relative standard suggests that the difference among measurement points for the uncertainties to estimate the combined relative standard dose rate conversion factor is likely negligible. uncertainty, and they ranged from 2.1% to 4.8%. Relative According to many previous reports,3, 20, 21, 33~39) the shielding standard uncertainties for dose rate conversion factor and factors due to the body of passenger cars depend on the car shielding factor were 1.2% and 0.67%, respectively. Thus, the type, and these values range from 1.3 to 1.9. A correlation maximum value of the combined relative standard uncertainty between count rates outside and inside the car is shown in to the estimated absorbed dose rate in air by this method was Fig. 3, and the shielding factor and the standard uncertainty evaluated as 4.9%. were evaluated as 1.50 and 0.01, respectively. Thus, this estimated value was similar to the previously reported value. 2. DistriEution of the aEsorEed dose rate in air From comparison of the measured count rates on pavement The distribution map of absorbed dose rate in air of and soil (unsealed) surfaces, the average value of count rates Aomori Prefecture is shown in Fig. 4. This map was drawn on the soil surface was approximately 4% higher than that on using 7,425 data which excluded the data that could have the pavement surface. Thus, the effect of cover on soil toward been inÀ uenced by rain. This map was also drawn using the the absorbed dose rate in air was negligible. Similar results GMT. A minimum curvature algorithm was used for the data were reported for measurements made in other prefectures.33, 34) interpolation by GMT. This is the method for interpolating Moreover, this result suggests that the pavements were data by presuming a smooth curved surface from the data of constructed using rocks which came from nearby locations. each point. According to the explanation accompanying the 40) Thus, the absorbed dose rate in air (Dout) 1 m above the geological map for Aomori Prefecture, loam which was

Fi g. 2 Correlation between absorbed dose rate in air which was calculated by software using the response matrix Fi g. 3 Correlation between count rates outside and inside method and total count rate outside the car. The slope of this the car. The slope of this regression line was used as the regression line was used as the dose rate conversion factor. shielding factor of the car body. Environmental Radiation Monitoring and External Dose Estimation in Aomori Prefecture after the Fukushima Daiichi Nuclear Power Plant Accident 45

Fi g. 4 The distribution map of absorbed dose rate in air in Aomori Prefecture. Mt. Iwaki shown with the white triangle is one of the landmarks of Aomori Prefecture and, as the highest mountain in the prefecture, has an elevation of 1,624 m. The areas of T1, T2, T3, N1 and N2 were classi¿ ed roughly according to the geological map.40)

deposited from the Pliocene to Pleistocene epochs is widely Thus, the relatively high level dose rates in the T1 area might distributed in the N1 area. Moreover, alluvium (forming the be inÀ uenced by gamma-rays emitted from exposed rocks Tsugaru plain) is distributed in the T2 area. It was reported of the cliffs. Absorbed dose rates in air at the T3 area were that the absorbed dose rates in air for loam and alluvium similarly observed as approximately 50 nGy h–1. Middle-upper of N1 and T2 areas were lower than those of other types of Miocene andesitic lava and pyroclastic rocks and mudstone weathered rocks.34, 41) Thus, the absorbed dose rates in air in which are reported to have a low level dose rate are distributed N1 and T2 were observed as lower values. In the N2 area, in this area.40) It might be considered that this area was also andesitic lava and pyroclastic rocks and rhyolitic (dacitic) inÀ uenced by gamma-rays emitted from the exposed rock of lava and pyroclastic rocks (middle-upper Miocene epoch) are roadside cliffs. distributed. Absorbed dose rates in air for these materials were The frequency distribution of absorbed dose rates in air reported as being at a relatively low level.42, 43) Cretaceous obtained by the car-borne survey is shown in Fig. 5. Normality granodiorite is distributed in part of the T1 area, and absorbed of the distribution of the obtained values was evaluated using dose rates in air around this bedrock area were found to be the Kolmogorov-Smirnov test by EZR,44) and the signi¿ cance 25–40 nGy h–1. According to MINATO,41) the absorbed dose level was set as p < 0.05. Although these statistic results were rate in air in a granodiorite area is relatively lower than that in not observed as a normal distribution, the standard deviations areas of other granite groups. Moreover, absorbed dose rates in to the results obtained in this study were evaluated for air exceeding 50 nGy h–1 were observed around Mt. Shirakami comparison with the results by other researchers. Absorbed in the T1 area. Cretaceous granodiorite and lower to upper dose rates in air for the Tsugaru and Nanbu areas, and the Miocene sedimentary rocks (including andesitic lava and whole prefectural area ranged from 14 to 51, 10 to 45, and 10 pyroclastic rocks) which have relatively low level dose rates to 51 nGy h–1, respectively. The medians and geometric means are distributed around T1.40) However, since T1 is primarily (averages and standard deviations) of absorbed dose rates mountainous, cliffs are found along most of the roadsides. in air for the Tsugaru and Nanbu areas (indicated in Fig. 1), 46 Masahiro HOSODA, Kazumasa INOUE, Mitsuaki OKA, Yasutaka OMORI, Kazuki IWAOKA and Shinji TOKONAMI

Fi g. 5 Histogram of absorbed dose rates in air obtained by car-borne survey in Aomori Prefecture. and the whole prefectural area were evaluated as 21 and 22 height distributions which were obtained at all 35 survey (22 ± 5), 20 and 20 (21 ± 5), and 21 and 21 (22 ± 5) nGy h–1, measurement points. These results suggested that the respectively. The average value for all of Japan was reported contributions to the photon peaks from 134Cs and 137Cs at the as 50 nGy h–1 by FURUKAWA and SHINGAKI.15) Thus, the average survey measurement points were much lower than those from value of Aomori Prefecture was found to be 44% of the the natural radionuclides. It is well known that the energy average value for all of Japan. resolution of NaI(Tl) scintillation detectors is lower than that of high purity germanium detectors and that some key 3. Activity concentrations in soil and the effect of the radiocesium photon peaks cannot be resolved from those FDNPP accident of natural radionuclides, e.g. those due to 214Bi and 134Cs at The results of activity concentrations in soil and the approximately 609 and 605 keV, respectively. However, other contributions of 40K, 238U and 232Th to absorbed dose rate in air radiocesium photon peaks that are not interfered with by obtained by spot measurements are summarized in TaEle 1. peaks of natural radionuclides, such as the peak at 795 keV, The average values with standard deviations (range and were not detected. Thus, the results obtained suggest that the median) of activity concentrations in soil of 40K, 238U and 232Th contributions of 134Cs and 137Cs to the dose rates are likely were 234 ± 148 (95–776, 192), 15 ± 6 (9–34, 15), 12 ± 6 (5–33, negligible. 11)Bqkg–1, respectively. According to the UNSCEAR,13) the current worldwide average values of these nuclides are 412, 4. External dose estimation 32 and 45 Bq kg–1, respectively. The obtained average activity Annual external doses (E) for the entire prefectural area, concentrations were compared with these worldwide average and the Tsugaru and Nanbu areas were estimated using Eq. (2). values, and they represented 57% of the worldwide average E = D × DCF × T ×(Q × R + Q ) (2) for 40K, 48% for 238U, and 27% for 232Th. According to the in out explanation included with the geological map for Aomori Here, D is the absorbed dose rate in air (nGyh–1), DCF is Prefecture,40) loam and other material deposits from the the dose conversion factor from the absorbed dose in air to the Holocene epoch that include low level natural radionuclides external effective dose for adults, T is 8,760h (24h×365d), were distributed widely at almost all in-situ measurement Qin and Qout are indoor and outdoor occupancy factors, and R points of the present survey. is the ratio of indoor dose rate to outdoor dose rate in a 24-h In the present study, the average values (range and median) period.17) of contributions of 40K, 238U and 232Th to absorbed dose rates The DCF value of 0.748 ± 0.007 Sv Gy–1 was obtained in air were 39% (28–54%, 40%), 29% (19–46%, 29%) and experimentally for natural gamma-rays in various 32% (25–45%, 32%), respectively. The contributions of these environments by MORIUCHI et al..45) It should be noted that nuclides to the dose rates were similar in value. this DCF value was obtained as the conversion factor from According to the recent report by KUDO et al.,18) 134Cs absorbed dose rate in air to effective dose equivalent. On the and 137Cs were observed in pine needles and pasture grass other hand, the value of 0.7 Sv Gy–1 which was reported by collected in Aomori Prefecture from April 2011 to December UNSCEAR is generally used for external dose estimation.13) 2013. However, in the present study, no photon peaks due However, it has been reported that the external effective dose to 134Cs and 137Cs were observed in the gamma-ray pulse using this value was 6% lower than that using 0.748 Sv Gy–1.45) Environmental Radiation Monitoring and External Dose Estimation in Aomori Prefecture after the Fukushima Daiichi Nuclear Power Plant Accident 47

TaEle 1 The measured activity concentrations and the contributions of 40K, 238U and 232Th to absorbed dose rate in air. Absorbed Contribution to the dose rate Activity concentration Latitude Longitude No.a Area Measurement site dose rate in (%) (Bq kg–1) (°) (°) air (nGy h–1) 40K 238U 232Th 40K 238U 232Th 1 Tsugaru Hirosaki City 40.6044 140.4632 24 ± 1 39 27 34 246 ± 6 16 ± 2 14 ± 1 2 Tsugaru Hirosaki City 40.6038 140.4931 20 ± 1 40 31 29 200 ± 6 15 ± 2 9 ± 1 3 Tsugaru Hirosaki City 40.5888 140.4715 27 ± 1 34 36 30 228 ± 5 23 ± 2 13 ± 1 4 Tsugaru Owani Town 40.5070 140.5770 24 ± 1 36 30 34 208 ± 5 17 ± 2 13 ± 1 5 Tsugaru Hirakawa City 40.5627 140.5369 24 ± 1 43 29 28 252 ± 6 17 ± 2 11 ± 1 6 Tsugaru Kuroishi City 40.6348 140.6102 14 ± 0 (< 1) 43 31 27 148 ± 5 10 ± 1 6 ± 1 7 Tsugaru Inakadate Village 40.6388 140.5669 16 ± 1 42 30 28 181 ± 5 12 ± 1 8 ± 1 8 Tsugaru Fujisaki Town 40.6707 140.5470 18 ± 1 40 28 32 185 ± 5 12 ± 1 10 ± 1 9 Tsugaru Itayanagi Town 40.6993 140.4540 23 ± 1 43 32 25 247 ± 7 18 ± 2 9 ± 1 10 Tsugaru Aomori City 40.7845 140.7812 27 ± 1 29 31 40 192 ± 5 19 ± 2 17 ± 1 11 Tsugaru Nakadomari Town 40.9320 140.4159 23 ± 1 43 29 28 237 ± 6 15 ± 2 10 ± 1 12 Tsugaru Fukaura Town 40.5761 140.0107 60 ± 1 52 19 29 776 ± 14 28 ± 2 28 ± 1 13 Tsugaru Fukaura Town 40.4552 139.9425 53 ± 1 39 25 36 553 ± 11 34 ± 3 33 ± 1 14 Tsugaru Nishimeya Village 40.5794 140.2999 22 ± 1 33 29 38 180 ± 5 15 ± 1 13 ± 1 15 Tsugaru Nishimeya Village 40.4833 140.2764 32 ± 1 54 20 26 413 ± 9 14 ± 1 13 ± 1 16 Nanbu Sai Village 41.3935 140.9162 25 ± 1 48 24 29 285 ± 7 13 ± 1 11 ± 1 17 Nanbu Mutsu City 41.3390 141.0542 15 ± 0 (< 1) 29 46 25 95 ± 3 15 ± 2 5 ± 0 (< 1) 18 Nanbu Mutsu City 41.1684 140.8057 27 ± 1 35 26 38 209 ± 5 15 ± 1 14 ± 1 19 Nanbu Higashidori Village 41.2799 141.3345 14 ± 0 (< 1) 29 26 45 104 ± 4 9 ± 1 10 ± 1 20 Nanbu Yokohama Town 41.0842 141.2568 21 ± 1 33 32 35 157 ± 4 15 ± 1 11 ± 1 21 Nanbu Rokkasho Village 41.0625 141.3640 15 ± 1 41 28 31 156 ± 5 10 ± 1 7 ± 1 22 Nanbu Rokkasho Village 40.9792 141.3712 16 ± 1 40 26 34 150 ± 4 9 ± 1 8 ± 1 23 Nanbu Towada City 40.6140 141.2144 14 ± 0 (< 1) 36 29 34 122 ± 4 9 ± 1 7 ± 1 24 Nanbu Shichinohe Town 40.5865 141.1159 21 ± 1 28 31 41 161 ± 4 17 ± 2 15 ± 1 25 Nanbu Shingo Village 40.4375 141.1236 17 ± 1 42 24 34 169 ± 5 9 ± 1 9 ± 1 a The numbers refer to the designations in Fig. 1

According to the results of the Monte Carlo simulation by respectively.17) SAITO et al.,46) effective dose was less than 10% lower than IYOGI et al.17) did not ¿nd a seasonal variation for the the effective dose equivalent. These results were obtained effective dose. Thus, annual effective doses to the residents using the parameters given by the International Commission were considered without seasonal variation in the present on Radiological Protection (ICRP) Publication 60.47) The study. Annual effective doses for the Tsugaru and Nanbu tissue weighting factor was revised in ICRP Publication areas, and the whole prefectural area ranged from 0.12 to 0.45, 103,48) so it will be necessary to revise the DCF for effective 0.10 to 0.43, and 0.10 to 0.45 mSv, respectively. The medians dose. However, no revised DCF for effective dose has been and geometric means (averages and standard deviations) of published yet. Thus, the value obtained by MORIUCHI et al. was annual effective doses for the Tsugaru area, Nanbu area, and used as DCF for a conservative dose estimation in this study. the entire prefectural area were estimated to be 0.18 and 0.19 Detailed data on the average time spent on various activities (0.20 ± 0.06), 0.19 and 0.19 (0.20 ± 0.07), and 0.18 and 0.19 for men and women above 10 years of age in Aomori (0.20 ± 0.08) mSv, respectively. In the previous study,17) the Prefecture were summarized by the Ministry of Internal annual effective doses in the three areas were 0.26 ± 0.04, Affairs and Communications, Japan (MIC).49) Furthermore, 0.22 ± 0.04, and 0.24 ± 0.04 mSv, respectively. IYOGI et al.17) data for the working population above 15 years of age in measured absorbed dose rates mainly in residential areas, Aomori Prefecture were also summarized by MIC.50) These and their measurement technique was quite different from the values are listed together with the values reported by IYOGI et present one. Furthermore, 17 years have passed since their al.17) in TaEle 2. The indoor and outdoor occupancy factors survey was completed in 1996, and environment conditions are were decided as 0.92 and 0.08, respectively, according to the likely to have changed, even though only slightly. The external above data. The values evaluated in this study were similar dose estimation method in this study was different from that to the values reported by IYOGI et al.,17) and were used for the used by IYOGI et al. Thus, although it is dif¿cult to compare external dose estimation. these external doses directly, the annual effective doses The ratios of the indoor dose rate to the outdoor dose rate seemed to have generally good agreement. Additionally, the in Aomori Prefecture were also reported by IYOGI et al. In nationwide average of the annual effective dose by terrestrial the Tsugaru area, Nanbu area, and the entire prefectural area, gamma-rays 16) is 0.33 mSv, and the average value for all of these ratios were 1.42 ± 0.26, 1.39 ± 0.25, and 1.48 ± 0.35, Aomori Prefecture was 59% of the nationwide average value. 48 Masahiro HOSODA, Kazumasa INOUE, Mitsuaki OKA, Yasutaka OMORI, Kazuki IWAOKA and Shinji TOKONAMI

TaE le 2 The average time per day spent on various activities for men and women above 10 years of age living in Aomori Prefecture. Time (h) Activity Indoors Outdoors 49) 17) 49) 17) MIC IYOGI et al. MIC IYOGI et al. Sleeping 8.02 8.05 Personal care 1.30 1.13 Eating meals 1.63 1.65 Commuting 0.37 0.40 Working a 2.49 2.64 0.98 1.55 School work 0.63 0.75 House keeping 1.42 1.28 Nursing 0.05 0.05 Child care 0.18 0.18 Shopping 0.38 0.32 Moving 0.42 0.33 Reading, Watching TV 2.72 2.68 Resting 1.62 1.28 Study and research 0.15 0.15 Hobbies and amusements 0.67 0.53 Sports 0.18 0.17 Community and volunteer activities 0.03 0.03 Activities with friends 0.23 0.37 Medical examination or treatment 0.15 0.12 Other activities 0.37 0.30 Sum 22.04 21.52 1.95 2.45 Occupancy factor 0.92 0.90 0.08 0.10 a Divided according to the working population in Aomori Prefecture.50)

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