Kosmogener Überschuß von 40K und Fluß schneller Neutronen in Meteoriten / Cosmogenic Excess of 40K and the Flux of Fast Neutrons in Meteorites

Abstract

Abstract Results are reported of a mass spectrometric investigation of the content and isotopic com-position of potassium from the mesosiderite Emery (silicate phase) and the chondrite Elenovka (bulk). Normalized to the Nier value of 39 K/ 41 K=13.47 the 40 K in Emery (K-content 220 + 25 ppm) was found to be enriched by (4.03 ± .30)%, the potassium from Elenovka (760 ± 50 ppm) to be indistinguishable from terrestrial potassium. - Evidence is presented that the excess 40 K in the silicates from Emery (Ca-content 6.06 weight %) has been produced essentially by secondary cosmic ray neutrons via the 40 Ca (n,p) -reaction. The total excess of (2.57 ± .39) x 1014 40 K-atoms/g Ca together with the excitation function of the 40 Ca (n,p) -reaction and the neutron flux spectrum of Arnold, Honda and Lal yields a dose of fast neutrons (2 MeV ≦ En ≦ 100 MeV) Φ · T = (7.2 ± 1.1) x 1016 neutrons /cm2 and an average flux during the cosmic ray exposure age T = (134 ± 12) Myrs of Φ = (17.4 ± 3.1) neutrons /cm2 sec. - A comparison of the neutron induced production rate of 40 K from Ca with that of 21 Ne from Mg and 38 Ar from Ca indicates the Arnold, Honda, Lal flux spectrum to be adequate for En ≲ 20 MeV but that it underestimates the number of neutrons with higher energies.