Isotopic study of neutron capture effects on Sm and Gd in chondrites

Isotopic compositions of Sm and Gd in eight chondrites (Mezb-Madaras, Parna llee, Otis, Suizhou, Zhaodong, Allende, Bruderheim and Y-74191) were measur ed to study neutron capture effects due to the interaction of cosmic rays w ith planetary materials in space. The anti-correlation of Sm-149/Sm-152 wit h Sm-150/Sm-152 and of Gd-157/Gd-160 With Gd-158/Gd-160 in the meteorites p rovides evidence for neutron capture effects in these chondrites. Isotopic deviations observed in the eight chondrites are epsilon(149Sm) = -0.35 to - 1.93, epsilon(150Sm) = +0.87 to +4.35, epsilon(157Gd) = -2.82 to -1.21 and epsilon(158Gd) = +0.33 to +1.66, which correspond to thermal neutron fluenc es of (1.03-5.17) x 10(15) x 10(15) n cm(-2). The combination of isotopic d ata from Srn-Gd in this study and Ne-Kr from the literature provides inform ation on the secondary produced thermal (<0.1 eV), epi-thermal (30-300 eV) and fast ( > 5 MeV) neutron fluences for chondrites, and it is used here to estimate the neutron energy distribution for six L- and LL-chondrites. Fro m the difference of distribution of thermal, epi-thermal and fast neutron f luences in individual chondrites, the depth of the samples inside the meteo roid is discussed. In Allende, the isotopic anomalies observed may be due n ot only to neutron capture reactions but also to additions of r-process nuc lides of Sm and Gd. (C) 2000 Elsevier Science B.V. All rights reserved.