Argon 40 in Potassium Minerals

An investigation has been made of the isotopic composition of the argon from four potassium minerals. In each case a high ${\mathrm{A}}^{40}$/${\mathrm{A}}^{36}$ ratio compared to that of atmospheric argon is observed showing directly that ${\mathrm{K}}^{40}$ decays to both ${\mathrm{Ca}}^{40}$ and ${\mathrm{A}}^{40}$. From the absolute amounts of radiogenic ${\mathrm{A}}^{40}$ and ${\mathrm{K}}^{40}$ in the minerals a lower limit on the branching ratio $\frac{{\lambda }_K}{{\lambda }_{\beta }}$ can be made. If the half-life, 7×${10}^8$ yrs., is assumed for ${\mathrm{K}}^{40}$→${\mathrm{Ca}}^{40}$, $\frac{{\lambda }_K}{{\lambda }_{\beta }}$ must be at least 0.02. The possibility of using this method for measuring geological age is suggested.