The ${}^{138}\mathrm{La}$ ($T_{1/2}=102$ Gyr)-${}^{138}\mathrm{Ce}$-${}^{136}\mathrm{Ce}$ 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 ${}^{136}\mathrm{Ce}$/${}^{138}\mathrm{Ce}$ 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 ${}^{136}\mathrm{Ce}$/${}^{138}\mathrm{Ce}$ and ${}^{138}\mathrm{La}$/${}^{138}\mathrm{Ce}$ 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 ${}^{138}\mathrm{La}$/${}^{138}\mathrm{Ce}$ ≥ 20 when the ratios of ${}^{136}\mathrm{Ce}$/${}^{138}\mathrm{Ce}$ and ${}^{138}\mathrm{La}$/${}^{138}\mathrm{Ce}$ 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 ν process origin of ${}^{138}\mathrm{La}$.

• Received 7 January 2008

DOI:https://doi.org/10.1103/PhysRevC.77.065802