Enrichment of the ${}^{100}\mathrm{Ru}$ isotope, resulting from the double beta $\left(\beta \beta \right)$ decay of ${}^{100}\mathrm{Mo}$, has been found in old molybdenites from Australia. Using $\mathrm{Re}–\mathrm{Os}$ ages determined here together with the amount of excess ${}^{100}\mathrm{Ru}$ determined via isotope dilution mass spectrometry, consistent half-lives were obtained from two different molybdenite samples of varying ages (2.90 billion and 1.05 billion years old) with an average half-life of $(2.1\pm 0.3)\times {10}^{18}$ years. This half-life is highly consistent with the theoretical model for a two-neutrino $\beta \beta$ decay.

• Received 20 February 2004

DOI:https://doi.org/10.1103/PhysRevC.70.025501