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 anti-neutrinos. Purpose: We measured the half life of the two-neutrino double-beta decay of 150 Nd to excited final states of 150 Sm by detecting the de-excitation gamma rays of the daughter nucleus. Method: This study yields the first detection of the coincidence gamma rays from the 0 + 1 excited state of 150 Sm. These gamma rays have energies of 333.97 keV 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 150 Nd 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 +0.45 −0.25 (stat) ± 0.07(syst.)) × 10 20 years. The effective nuclear matrix element was found to be 0.0465 +0.0098 −0.0054. 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 the decay to the first excited state in 150 Sm is favored over that to the ground state.

Background: Multiple large collaborations are currently searching for neutrinoless double-β decay, with the 5 ultimate goal of differentiating the Majorana-Dirac nature of the neutrino.

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