Two-neutrino double-beta decay of ¹⁵⁰Nd to excited final states in ¹⁵⁰Sm

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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. We measured the half life of the two-neutrino double-beta decay of ¹⁵⁰Nd to excited final states of ¹⁵⁰Sm by detecting the de-excitation gamma rays of the daughter nucleus. This study yields the first detection of the coincidence gamma rays from the 0+₁ excited state of ¹⁵⁰Sm. 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. The enriched Nd₂O₃ sample consisted of 40.13 g ¹⁵⁰Nd 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 T₁/₂=(1.07⁺⁰.⁴⁵₋₀.₂₅(stat)±0.07(syst.))× 10²⁰ 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. 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 ¹⁵⁰Sm is favored over that to the ground state.