A systematic study of the inclusive ($0\nu +2\nu$) double beta ($\beta \beta$) decay of ${}^{100}\mathrm{Mo}$ to various excited final states of ${}^{100}\mathrm{Ru}$ was performed. Utilizing two large HPGe detectors operated in coincidence, a search for the subsequent deexcitation $\gamma \gamma$ cascades was conducted. A 1.05-kg sample of isotopically enriched $(98.4\%)$${}^{100}\mathrm{Mo}$ was investigated for 455 days, yielding an unambiguous observation of the $\beta \beta$ decay of ${}^{100}\mathrm{Mo}$ to the $0_1^{+}$ state (1130.3 keV) of ${}^{100}\mathrm{Ru}$. This excited final state decays via the $0_1^{+}\to 2_1^{+}\to 0_{\mathrm{gs}}^{+}$ sequence (with $E_{\gamma 1}=590.8$ keV and $E_{\gamma 2}=539.5$ keV), and 22 such coincidence events were detected, with a continuous background estimated to be 2.5 events. This counting rate corresponds to a decay half-life for the $\beta \beta (0^{+}\to 0_1^{+})$ transition of $T_{1/2}^{(0\nu +2\nu )}=[6.0_{-1.1}^{+1.9}(\mathrm{stat})\pm 0.6(\mathrm{syst})]\times {10}^{20}$ years. Lower limits on decay half-lives were achieved for higher excited final states.

• Received 19 December 2005

DOI:https://doi.org/10.1103/PhysRevC.74.044314