Neutrons due to Natural Radioactivities in Granite Rock Samples in Hiroshima City and the Surrounding Areas

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Abstract

In order to prepare data for estimating the precise amounts of the naturally produced long life nuclide of ^<36>Cl (3 x 10^5 y of half life) in granites distantly exposed to the Hiroshima atomic bomb, contents of potassium (K), uranium (U) and thorium (Th) were determined by measurements of gamma rays from natural radioactivities. The contents of K, Th and U in granites not exposed to the atomic bomb were also determined for examining how the contents varied amongst different lithospheres around Hiroshima. This yielded interesting facts, such as that the K contents in Iyo stones from Iyo-Ohshima Island, Ehime Prefecture were much lower than those in Giin stones from Kurahashi Island near Hiroshima City. Seventeen samples were taken out at various depths in a large granite body near the top of the mountain for the Giin quarry. The results showed that the U and Th contents were very high in a small part of the body. The maximum values were 6.5 ppm U and 55 ppm Th. This suggests that the U and Th contents vary widely according to location in the Giin quarry.

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