Cosmic-ray -produced activities in chondrites

Cosmic-ray -produced activities in the new carbonaceous chondrite Esebi and in the freshly fallen amphoterite St. Severin have been studied by non-destructive gamma-spectroscopy. The total spectrum of the radionuclides was measured. Long-lived 40K and 26Al were the dominant radioactivities detected. The radiation of the β+ emitters 22Na and 26Al was measured by a γ (511 keV)-γ coincidence method. The 26Al specific activity of the St. Severin meteorite (date of fall: 27 June, 1966) was found to be 0.058 dpm/g. The 22Na specific activity was 0.091 dpm/g. For the carbonaceous meteorite Esebi (date of fall: July 1957) upper limits of < 0.08 and < 0.01 were derived for 26Al and 22Na, respectively. Spallogenic 21Ne and 22Ne contents of these meteorites were measured by mass spectrometry. From the 22Na activity the 26Al activity, and the rare gas content, exposure ages were derived. The method was calibrated by comparison with the 26Al to 21Ne ratio of other meteorites of well-known age. Radiation ages of 13 x 106 yr for St. Severin and 1 x 106 yr for Esebi were determined. Histograms of 21Ne - 26Al radiation ages for amphoterites and carbonaceous chondrites are discussed. (author)