Double-beta decay of molybdenum-100 and neodymium-150 to excited final states

A systematic study of the inclusive (0ν + 2ν) double beta (ββ) decay to various excited final states was performed at the Triangle Universities Nuclear Laboratory (TUNL). While the study of neutrinoless ββ decay is a powerful tool for investigating fundamental properties of the neutrino, the proper interpretation of heretofore unsuccessful 0νββ-decay searches is virtually impossible without valuable information afforded by the study of the two-neutrino mode of decay. In particular, 2νββ transitions to excited final states provide important checks on theoretical models used to predict 0νββ-decay half-lives. The processes that were investigated in the present experiment were the ββ decays of 100Mo and 150Nd to excited states of 100Ru and 150Sm, respectively.; The measurements were performed in the Low Background Counting Facility, which is located in the basement of the physics department of Duke University. A novel technique, in which two HPGe detectors were operated in coincidence, enabled the study of these ββ transitions via the detection of the subsequent de-excitation γ-γ cascades from the excited daughter nucleus, thus bypassing the difficult measurement of the emitted short-range electrons. The profound advantage of this coincidence approach was a considerable suppression of the background near the energy regions of interest, as compared to experiments that searched for the single photons.; The most important finding was the confirmation of the ββ decay of 100Mo to the $0+1$ state (1130.3 keV) of 100Ru. The measurement was accomplished via the observation, in coincidence, of the two γ rays (E_γ1 = 590.8 keV and E_γ2 = 539.5 keV) emitted from the de-excitation sequence $0+1\to 2+1\to \; 0+gs$. A 1.05-kg disk of isotopically enriched (98.4%) 100Mo was studied for 455 days, producing 22 such γ-γ coincidence events. After precisely determining the total γ-γ detection efficiency of the apparatus, this counting rate yields a decay half-life for the ββ($0+\to 0+1$) transition of $T0n+2n1/2=5.0+1.4-0.9stat\pm 0.5syst\times 1020$ years. This result agrees very favorably with previous measurements, albeit with a vastly improved signal-to-background ratio. Lower limits on the ββ-decay half-lives of 100Mo and 150Nd were achieved for transitions to other excited states.