Half-Life ofPb210

A new determination of the half-life of ${\mathrm{Pb}}^{210}$ has been made by the geological method. Pb${\mathrm{Cl}}_2$ extracted from uranium minerals in secular equilibrium was used to calibrate a thick-source scintillation counter for ${\mathrm{Po}}^{210}$ alpha particles. Using this calibration the absolute activity of ${\mathrm{Po}}^{210}$ in partial equilibrium with a known number of ${\mathrm{Pb}}^{210}$ atoms prepared from the decay of a measured quantity of radon was determined. From these data a half-life of 21.4±0.5 years was obtained for ${\mathrm{Pb}}^{210}$.