The ^ { 138 } La ( T _ { 1 / 2 } =102 Gyr ) - ^ { 138 } Ce - ^ { 136 } 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 } Ce/ ^ { 138 } 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 } Ce/ ^ { 138 } Ce , and ^ { 138 } La/ ^ { 138 } 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 } La/ ^ { 138 } Ce \geq 20 when the ratios of ^ { 136 } Ce/ ^ { 138 } Ce and ^ { 138 } La/ ^ { 138 } 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 \nu process origin of ^ { 138 } La .