Isotopic anomalies in the early solar system

An early irradiation model for the explanation of the isotopic anomalies is described in this report. It is considered that interstellar matters are repeatedly exposed not only to the products of nucleosynthesis, but also to the cosmic-ray irradiation from nearly supernova explosion. An astronomical model proposed by Kuroda seems to be quite consistent with the observed relative abundances of the extinct nuclides I-129 and Pu-244. The difference in the isotopic compositions of xenon in the atmosphere and carbonaceous chondrites can be attributed to the combined effect of mass fractionation, neutron capture and cosmic-ray irradiation processes which took place in the early solar system. The duration of cosmic-ray irradiation for producing Xe-126 can be calculated. Intense irradiation from supernova explosion is considered. The total estimated neutron fluence exceeds the limit set by Reeves et al. This means that the isotopic ratios of some of rare earth elements should have been altered appreciably. The data on neodymium isotopes recently obtained seem to support this view. The barium isotope data can also be explained in a similar manner. The case of calcium isotope data is not quite so clear-cut. Small enrichment of Ca-42 may be due to the contribution from the neutron capture of K-41 and the successive beta-decay. Amounts of magnesium isotopes, oxygen isotopes and uranium isotopes are also discussed. (Kato, T.)