138La-138Ce-136Ce nuclear cosmochronometer of supernova neutrino process

The 138La (T1/2=102 Gyr) - 138Ce - 136Ce system is proposed to be used as a nuclear cosmochronometer for measuring the time elapsed from a supernova neutrino process. This chronometer is applied to examine a sample affected by a single nucleosynthesis episode as presolar grains in primitive meteorites. A feature of this chronometer is to evaluate the initial abundance ratio of 136Ce/138Ce using an empirical scaling law, which was found in the solar abundances. We calculate the age of the sample as a function of isotopic ratios, 136Ce/138Ce, and 138La/138Ce, and evaluate the age uncertainty due to theoretical and observational errors. It is concluded that this chronometer can work well for a sample with the abundance ratio of 138La/138Ce >= 20 when the ratios of 136Ce/138Ce and 138La/138Ce are measured within the uncertainty of 20%. The availability of such samples becomes clear in recent studies of the presolar grains. We also discuss the effect of the nuclear structure to the neutrino process origin of 138La.