Fig. 4-19 Thermoluminescence Dosimeters (TLDs) used for this study A TLD releases luminescence proportional to the irradiated radiation dose by heating. The dose accumulated during the deployed period can be determined by measuring the light intensity of the luminescence. The thermoluminescent powder is encapsulated in a glass capsule, which is enclosed in a plastic case equipped with a suitable metal gamma-ray filter to improve the energy response.

Fig. 4-20 Frequency distribution of indoor dose rates in the Tokyo Metropolitan area classified by dwelling types From these figures, it is known that concrete type houses have a relatively high and varied distribution of dose rates. The dotted red line in the figures indicates the normal radiation dose rate level on Kanto (Tokyo area) loam. The dose rate levels indoors are enhanced in every dwelling type due to urbanization of the environment.

We are continuously exposed to various kinds of radiation in daily life. JAERI continues to study natural radiation and radioactivity present in our environment. Under this research project, an experimental study of air absorbed dose rates in various types of Japanese dwellings has been performed using small integral radiation dosimeters, Thermoluminescence Dosimeters (TLDs); see Fig. 4-19. Collected in parallel with these measurements was basic information on dwelling type, structure, etc., which was necessary for the analysis of the measured data. The frequency distribution of absorbed dose rates indoors in air for concrete, wood frame, and lightweight steel frame dwellings are shown in Fig. 4-20. The radiation level was found to be higher in concrete houses because of the relatively high concentrations of natural radioactivity in the building materials. The dose rates indoors of both wood frame and lightweight steel frame dwellings are affected strongly by the radiation level outdoors because of the inherently low radioactivity in the building materials. The absorbed dose rate in air from natural radiation around Tokyo averages about 20 nGy/h due to geological features, but the measured radiation dose rates indoors for all types of dwellings are higher than the inherent level. This study determined that the radiation level in the urban environment is artificially enhanced due to natural gamma rays from the structural materials of roads and buildings, which consist of stone and concrete.

Reference K. Saito et al., Measurements of Gamma Dose Rates in Dwellings in the Tokyo Metropolitan Area, Radiat. Prot. Dosim. 69, 61 (1997).

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