The specific activities of several samples of methyl iodide containing ${\mathrm{I}}^{129}$ were measured with proportional counters, and the ratios of ${\mathrm{I}}^{129}$ to ${\mathrm{I}}^{127}$ were determined with a 60° sector type mass spectrometer. The half-life of ${\mathrm{I}}^{129}$ was found to be (1.72±0.09)×${10}^7$ years. The time interval between the formation of the elements and the formation of the earth calculated from this value is 2.7×${10}^8$ years. It was assumed that most of the ${\mathrm{Xe}}^{129}$ present on the earth at present originated from decay of ${\mathrm{I}}^{129}$ after the formation of the earth and that the original cosmic abundance of ${\mathrm{I}}^{129}$ was about equal to that of the stable ${\mathrm{I}}^{127}$.

• Received 15 February 1951

DOI:https://doi.org/10.1103/PhysRev.82.688