The ${}^{138}\mathrm{La}\hspace{0.5em}{\beta }^{-}$ decay constant was determined geophysically. The isotopic growths of ${}^{138}{\mathrm{C}\mathrm{e}/}^{142}\mathrm{Ce}$ and ${}^{143}{\mathrm{N}\mathrm{d}/}^{144}\mathrm{Nd}$ were examined from ancient igneous crystalline rocks. For the Ce isotope measurement, a newly developed dynamic multicollector technique was applied. The crystallization age of the rocks was calculated to be $2037\pm 13$ Myr from the growth of ${}^{143}\mathrm{Nd}.$ From the age and the growth of ${}^{138}\mathrm{Ce},$ ${\lambda }_{\beta }$ of ${}^{138}\mathrm{La}$ was estimated to be $(2.37\mathrm{}\pm 0.10)\times {10}^{-12}\hspace{0.5em}{\mathrm{yr}}^{-1}.$ This value is consistent with those of the latest two $\gamma$-ray counting data of lanthanum compounds within the analytical error.

• Received 17 June 1999

DOI:https://doi.org/10.1103/PhysRevC.62.017601