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

TL;DR

This paper reports the first detection of gamma rays from two-neutrino double-beta decay of $^{150}$Nd to an excited state in $^{150}$Sm, measuring its half-life and nuclear matrix element with implications for neutrinoless decay models.

Contribution

It provides the first detection of gamma-ray coincidence signals from this decay and measures its half-life, advancing understanding of nuclear matrix elements in double-beta decay.

Findings

Half-life of (1.07^{+0.45}_{-0.25} (stat) ± 0.07 (syst)))×10^{20} years.

First detection of gamma-ray coincidences from the 0+_1 excited state.

Effective nuclear matrix element of 0.0465^{+0.0098}_{-0.0054}.

Abstract

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 $^{150}$Nd to excited final states of $^{150}$Sm 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 $^{150}$Sm. These gamma rays have energies of 333.97 keV and 406.52 keV, and are emitted in coincidence through a 0$^+_1\rightarrow$2$^+_1\rightarrow$0$^+_{gs}$ transition. The enriched Nd$_2$O$_3$ 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 $T_{1/2}=(1.07^{+0.45}_{-0.25}(stat)\pm0.07(syst.))\times 10^{20}$ 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. 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.