Isotopic compositions of terrestrial thorium: A search in terrestrial thorium for the unidentified excess 4.6 meV alpha-activity of lunar thorium

Since thorium in lunar crystalline rocks was reported to be excessive in 4.6 MeV alpha-activity up to 80%, which was attributed to a long-lived isomer of²³²Th with an origin different from normal thorium, we have measured alpha-spectra of various terrestrial thorium in order to see if terrestrial thorium also contains some excess 4.6 MeV alpha-activity. The results showed that the content of the excess 4.6 MeV alpha-activity in terrestrial thorium, if any, is much less than 3%. An approach to set an upper limit for the excess alpha-activity in terrestrial thorium is proposed. The origin of the excess alpha-activity is discussed.