Direct evidence for two-neutrino double-beta decay inSe82

The two-neutrino mode of double-beta decay in ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ has been observed in a time-projection chamber at a half-life of ${(1.1\mathrm{}}_{\mathrm{-}0.3}^{+0.8\mathrm{}}$)×${10}^{20}$ yr (68% confidence level). This result from direct counting confirms the earlier geochemical measurements and helps provide a standard by which to test the double-beta-decay matrix elements of nuclear theory. It is the rarest natural decay process ever observed directly in the laboratory.