Measurements of Natural Radioisotopes in Granites Around Hiroshima City

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  • 広島市周辺の花崗岩中の天然放射能の測定

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Abstract

Approximately 2 years ago, the contents of potassium (K), uranium (U) and thorium (Th) in granites around Hiroshima City were estimated by determining the natural gamma rays from powdered samples. The powdered samples were stored in plastic capsules, and the K, U and Th contents were estimated again. The K, U and Th contents in the stored samples were well coincident with those estimated 2 years ago. This suggests that the powdered granite sample sufficiently retained K and elements in the U and Th series. A granite from Kurahashi Island near Hiroshima City contained about 5% biotite. Removing the biotite from the granite, the K content decreased by 13%, but neither the U content nor Th content changed. The minerals of feldspar (mainly potassium feldspar) and quartz were extracted from a Kurahashi granite. The U and Th contents were 2.3 ppm and 8.6 ppm in the feldspar, and 1.7 ppm and 9.6 ppm in the quartz, respectively, while the Kurahashi stone contained 4.7 ppm U and 25 ppm Th. This suggests that small minerals, except potassium feldspar, quartz and biotite, contain a large part of the U and Th elements in the Kurahashi stones. In this study, four quarries around Hiroshima City were surveyed. The granite stones examined up to the present covered totally eight quarries and six rock bodies on the surface of the earth. Only the granites from Kurahashi Island and Akasaka quarry (Fukuyama City, Hiroshima Pref.) contained more than 5 ppm U. Th contents higher than 25 ppm were observed only in the granites from Kurahashi Island. In Kurokami Island (Syunan City, Yamaguchi Pref.), stones were taken from a large rock body since before the Second World War. The granite samples from the rock body contained about 4.3 ppm U, which were about twice the amount of those in the granites from a rock body recently developed. The black-and-white stones from Akasaka quarry and Kitagi (Kasaoka City, Okayama Pref.) quarry were visually similar to each other. The data set of K, U and Th contents for the stones from Akasaka and Kitagi will be useful to differentiate each one.

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