Double beta decay ofSe82

Pairs of negative beta particles have been observed originating from a ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ source during a cloud-chamber search for double beta decay. Backgrounds recognized in previous experiments were suppressed to well below the observed event rate, and no other significant backgrounds are apparent. Within the limited statistics of the small data sample, the observed single-electron energy spectrum, the two-electron sum energy spectrum, and the opening angle distribution are consistent with expectation for neutrino-accompanied double beta decay of ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$. The tentative assignment of the observed events to double beta decay, results in a ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ half-life of (1.0±0.4)×${10}^{19}$ years, in good agreement with some very recent theoretical predictions. However, the result is in serious disagreement with the much longer half-lives measured in geochemical experiments. A planned follow-up experiment is described.