Japanese Journal of Health Physics

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 10 minutes. 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/m³.

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Recently many Japanese wear jewelry as personal ornaments, particularly bracelets. The jewelry is made of gems, jewels, noble metals and so on. Some kinds of jewelry include much amount of radioactive elements more than that of natural range in ores which are in our living-environment. The radioactive concentrations of 25 kinds of jewels and gems, which were easily purchased on a market or through the Internet, were analyzed by HPGe. The bracelet which was made of ceramics showed the highest radioactive concentration and it was estimated as approx. 800 Bq g^-1 of ²³²Th and 140 Bq g^-1 of ²³⁸U by assuming radioactive equilibrium among the nuclides in the decay series. Moreover, there was a high radioactive concentration gem including 60 Bq g^-1 of ²³²Th and 300 Bq g^-1 of ²³⁸U. The radioactive materials added in the ceramics should be monazite analyzed by a prompt gamma-ray activation analysis method. Though the ceramics became the object of the NORM management by the concentration, the radioactivity of one bracelet does not exceed 8,000 Bq. So, all these jewelry samples do not have to manage as NORM as consumer products according to the guideline by Japanese government. However, the jewelry's activity by deliberate addition of radioactive materials is deemed to be unjustified exposure, that the ICRP led the consideration. It is considered that the NORM guideline should take in the consideration of ICRP and prohibit the addition of radioactive materials to jewelry.

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The radon exhalation from soil surface varies with various meteorological and soil parameters. As the observed radon exhalation rates are simultaneously affected by various parameters, it is difficult to estimate an effect of each parameter on radon exhalation. The effect of each parameter is studied based on the time-dependent one-dimensional radon transport model we developed. The model simulates the radon exhalation rate by using various meteorological and soil parameters under the realistic environmental conditions. The effect of the precipitation on the radon exhalation is explained by change of water porosity in the soil. Under the loamy soil condition in Kanto area, the radon exhalation was found to stop during the precipitation more than 5 mm. It is well known that the radon exhalation closely depend on the change rate of atmospheric pressure. From our simulation results, we can know the mechanism of pressure effect on radon exhalation. The change of atmospheric pressure is propagated to the deep soil layer from the surface. The time lag causes the pressure difference between the surface and the deep soil layer and produces the change of radon exhalation. In addition, it is found that atmospheric pressure dependence of radon exhalation is not affected by the variation of permeability in the range of three orders. The reason of remarkable calculation results is explained in this paper. The radon exhalation increases with increasing wind speed and may rise double by the wind speed exceeding 10 m s^-1. The wind effect has a threshold value, about 5 m s^-1 in this study and is ignored in the wind speed below the value. Therefore, we can conclude that contribution of the wind speed to the radon exhalation is relatively small.

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²²²Rn fluxes from the ground surface and ²²⁶Ra contents in soil were measured on Hachijo-jima Island, which is a solitary island in the Pacific Ocean located about 200 km to the south of the main island of Japan, to evaluate fractional contributions of the locally exhaled ²²²Rn and the long-range transported one to the surface air concentration measured on this island. Averages of ²²²Rn flux and ²²⁶Ra content in dry soil were evaluated to be 0.9 ± 0.4 mBq m^-2 s^-1 and 6.8 ± 0.2 Bq kg^-1, respectively. These are considerably smaller than the respective values of 9.7 ± 0.8 mBq m^-2 s^-1 and 23.2 ± 0.4 Bq kg^-1 measured at Nagoya as a reference. The lower value of the ²²⁶Ra content and the even lower ²²²Rn flux on this island can be attributed to the basaltic geology and the soil's coarse texture and high moisture, respectively. A simple model calculation assuming a typical nocturnal condition showed that the measured ²²²Rn flux would cause only a small increase in the surface air concentration by 0.035 to 0.072 Bq m^-3 (relative contribution of 1 to 12%) in addition to the long-range transported ²²²Rn under a typical nocturnal condition. The contribution of the local flux would be smaller than that under nocturnal condition. This local ²²²Rn component is negligible as compared with the concentration of the long-range transported ²²²Rn (0.6 to 3.6 Bq m^-3). It is, therefore, concluded that Hachijo-jima Island is suitable for measuring the long-range transported atmospheric ²²²Rn in East Asia region.

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