The surficial materials of solar planets without thick atmospheric layers are exposed to cosmic rays. Nuclear reactions, mainly spallation and neutron capture reactions, are caused by the interactions of cosmic rays with the surficial planetary materials. Since isotopic compositions of some rare earth elements (REE) are possibly variable with spallation and/or neutron capture reactions induced by cosmic-ray irradiation, the REE isotopic shifts recorded in extraterrestrial materials are possibly good dosimeters of cosmic-ray. In this talk, isotopic evidence for neutron capture effect induced from the cosmic-ray irradiation will be shown from the Sm isotopic variations recorded in meteorites and lunar soils.
Sm has seven stable isotopes with mass number 144, 147, 148, 149, 150, 152, and 154. One of seven Sm isotopes, 149Sm has a very large thermal neutron capture cross section, and changes to 150Sm by a neutron capture reaction, 149Sm(n,)150Sm. Since the isotopic decrement of 149Sm correspond quantitatively to the isotopic increment of 150Sm, its isotopic shift is simply and clearly shown as an isotopic shift line in a diagram between 149Sm/152Sm and 150Sm/152Sm if neutron capture reactions of 149Sm(n,)150Sm occurred in the materials. As the results of the Sm isotopic analyses of several kinds of meteorites and lunar soils, significant variations of 149Sm-150Sm isotopic shifts corresponding to the neutron fluences up to 1017 neutrons/cm2 could be detected. Sm isotopic shifts of the solar planetary materials can be used as a natural neutron dosimeter which is also an important indicator to understand the evolution process of the planetary materials in space.
