Suggestive evidence for the two-neutrino double-β decay ofGe76

A dramatic reduction in background was achieved in the latest Pacific Northwest Laboratory–University of South Carolina germanium detectors. Two 1.05-kg natural-isotopic-abundance detectors were operated for 1.92 kg yr. The residual spectrum, after straightforward corrections, has a significant region resembling the theoretical spectrum of the two-neutrino ββ decay of ${}_{}{}^{76}{}_{}{}^{}\mathrm{Ge}$. A fit to the data yields ${\mathit{T}}_{1/2}^{2\nu }$ ${(}^{76}$Ge)=(1.$1_{\mathrm{-}0.3}^{+0.6\mathrm{}}$)×${10}^{21}$ yr at the 95% C.L., which agrees with shell-model predictions.