Double Beta Decay of ${}^{100}$Mo to Excited Final States

Abstract

A systematic study of the inclusive ($0\nu +2\nu$) double beta ($\beta \beta$) decay of ${}^{100}$Mo to various excited final states of ${}^{100}$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%) ${}^{100}$Mo was investigated for 455 days, yielding an unambiguous observation of the double beta decay of ${}^{100}$Mo to the $0_1^{+}$ state (1130.3 keV) of ${}^{100}$Ru. This excited final state decays via the $0_1^{+}\to 2_1^{+}\to 0_{gs}^{+}$ 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 ($0^+ rightarrow 0_1^+)$transitionof$T_{1/2}^{(0ν+2ν)} = [ 6.0^{+1.9}_{-1.1} (\rm{stat}) \pm 0.6(\rm{syst})] \times 10^{20}$ years. Lower limits on decay half-lives were achieved for higher excited final states.

10 pages, 8 figures, submitted to Physical Review C