Double beta decay of76Ge(0+) to 0+and 2+states in76Se studied by the β-γ coincidence method

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Abstract
Double beta decays ( beta beta ) from the 76Ge ground state (0+) to the 0+ and 2+ states in 76Se have been studied by means of a 171 cm3 Ge detector surrounded by a 4 pi -geometry NaI detector for 8621 h at the Kamioka underground laboratory. All beta -ray signals from the Ge detector and gamma -ray ones from the NaI detector were recorded on magnetic tape with a list mode, and were used to select candidates of beta beta events from background events. Lower limits on the neutrinoless double beta decays (0 nu beta beta ) are obtained as T12/0 nu 0+ to 0+)>0.73*1023 year and T12/0 nu (0+ to 2+)>0.57*1023 year at the 68% confidence level (CL). The limit on T12/0 nu (0+ to 0+) gives an upper limit on the Majorana neutrino mass of (mnu )2*1020 year. This leads to an upper limit on the coupling strength with the Majoron field as mod (gB) mod <(0.7-1.8)*10-3. The lower limit on the half-life for the two-neutrino beta beta is T12/2 nu (0+ to 0+)>1*1020 year.