Study on the environmental movements and distributions of natural radioactive nuclides on the granite area (III)

The natural radionuclides as K-40, uranium decay series and thorium decay series etc. are widely distributed on environment, but are not uniformly. These have influences various forms as the sources of terrecial environmental γ radiation and of radon in to the human life environment and make wide fluctuation seasonal and spatially on the environment. The environmental radiation is higher than that of the other on the west Japan where generally consist rich of granite strata. We deeply appreciate in regard to natural radiation and would carry the studies on the movement and distribution of natural radioactive nuclides on high background radiation area consisted rich granite strata. We have selected and observed on Ikeda mineral spring district Ota-shi Shimane pref., Misasa spa district, Tohaku-gun, Tottori pref. on Chugoku area, Muro district Uda-gun, Hachibuse district, Nafa-shi Nara pref., and Arima spa district, Hyogo-pref., Kawanishi-shi, Hyogo pref. on Kinki area and Masutomi spa, Koma-gun, Yamanashi pref., for HBRA, and Higashi-osaka-shi, Osaka pref., for CA. We have carried out the study on the environmental movement and distribution of natural radioactive nuclides containing radon and decay nuclides, and reported these results on following; (1) Radon measurements have been carried using cup typed radon and thoron monitors which are easy handling in spite of need of long sampling period, pico-rad method by active charcoal sampling and Pilon scintillation-cell with 300 ml volume by grub sampling. Accumulated radon monitors have been used cellulose nitrate (LR-115 type II, Kodak Co.) as solid state track detector. Among these characteristics of radon monitors, though minimum detectable limit of cup method for 3 months sampling is higher than those by the other method, it is able to measure mean Rn-222 concentration for 3 months. Rn-222 concentration by pico-rad method is able to get briefly mean concentration for 24 hours, is small detector and many sample are measured at the same time to be available on draw of the concentration distribution map. (2) Mean Rn-222 concentrations in air on every districts at Misasa spa environs measured for five years until December 1999 were fluctuated largely ND ∼ 150 Bq/m3 outdoor and 8 ∼ 194 Bq/m3 indoor, the latter generally are higher than those outdoor and these ratio were fluctuated ranged 1.1 ∼ 2.3. On the results measured on Misasa spa environs, Rn-222 concentrations on Misasa district, Asahi district along Takeda stream located southward from Misasa spa district, and Takeda district along Tenjin stream are high, on Osika district and Mitoku district (up the Takeda stream) located east-southward from Misasa spa district are low level. On comparison of R-222 concentration on geological map, it turned out that Rn-222 concentrations on Misasa district, Asahi district and Takeda district geologically formed from granite strata are high, and those on Osika district and Mitoku district formed from volcanic rocks (Andesite and Basalt) are low level. Indoor and outdoor Rn-222 concentrations in air on granite strata layers are 2-3 times higher than those of volcanic rock. Rn-222 concentration in air was influenced by the strata of the area. (3) On the results of variations in time-course of indoor Rn-222 concentrations in air at Misasa spa district and Kawanishi district, the concentration on summer is low level and that on winter is high. As people live at open-door situation on summer and on rain season, Rn-222 concentration in air are dilute with outdoor air of low concentration. Indoor Rn-222 concentrations at Higashi-osaka district are high in summer and low level in winter. The life situation of air ventilation and conditioning are more influenced than variation on area condition. It was shown that outdoor concentrations variation in time course at three districts are low level on summer and are high on winter. Result on Higashi-osaka faced the Pacific is similar with literature. But as Kawanishi and Misasa are continental districts and have a similar tendency with the Pacific side. (4) Rn-222 concentration in Tansan spring among spa waters at Arima spa area, Hyogo prefecture, is a little high level, those of the other spring sources are comparatively low level. Outdoor Rn-222 concentrations in air are a little higher than the control area, thought samples are a few numbers. (5) At the summer 1999 investigation on Masutomi spa area, Koma-gun, Yamanashi pref., indoor Rn-222 concentrations by pico-rad method were fluctuated 17 ∼ 300 Bq/m3, 13 ∼ 136 Bq/m3 outdoor and are about ten times higher than the mean values on Higashi-osaka as control area. By the Scinti-cell method, Rn-222 concentrations in air at the near and the surface of spring source are ranged largely 67 ∼ 13000 Bq/m3 outdoor. Outdoor radiation dose rate at the near spring is 4.0 μ Gy/h, about 70 times higher than that at Higashi-osaka. As outdoor γ radiation dose rate at places with high Rn-222 concentration are tendency to be high, γ radiation dose are under the influence of Rn-222 concentration in air. (author)