Two-neutrino double-beta decay of $^{150}$Nd to excited final states in $^{150}$Sm

Background: Double-beta decay is a rare nuclear process in which two neutrons in the nucleus are converted to two protons with the emission of two electrons and two electron antineutrinos. Purpose: We measured the half-life of the two-neutrino double-beta decay of Nd-150 to excited final states of Sm-150 by detecting the deexcitation. rays of the daughter nucleus. Method: This study yields the first detection of the coincidence gamma rays from the 0(1)(+) excited state of Sm-150. These. rays have energies of 333.97 and 406.52 keV and are emitted in coincidence through a 0(1)(+) -> 2(1)(+) -> 0(gs)(+) transition. Results: The enriched Nd2O3 sample consisted of 40.13 g Nd-150 and was observed for 642.8 days at the Kimballton Underground Research Facility, producing 21.6 net events in the region of interest. This count rate gives a half-life of T-1/2 = [1.07(-0.25)(+0.45)(stat) +/- 0.07(syst)] x 10(20) yr. The effective nuclear matrix elementwas found to be 0.0465(-0.0054)(+0.0098). Finally, lower limits were obtained for decays to higher excited final states. Conclusions: Our half-life measurement agrees within uncertainties with another recent measurement in which no coincidence was employed. Our nuclear matrix element calculation may have an impact on a recent neutrinoless double-beta decay nuclear matrix element calculation which implies that the decay to the first excited state in Sm-150 is favored over that to the ground state.