Double-beta decay of ${}^{150}$Nd to excited levels of ${}^{150}$Sm

Abstract

The $2\nu 2{\hat{I}}^2$ decay of ${}^{150}$Nd to the first excited 740.5 keV $0_1^{+}$ level of ${}^{150}$Sm was measured over 5.845 yr with the help of a four-crystal low-background HPGe ${\hat{I}}^3$ spectrometry system in the underground low-background laboratory STELLA of LNGS-INFN. A 2.381 kg highly purified Nd-containing sample was employed as the decay source. The expected de-excitation gamma-quanta of the $0_1^{+}$ level with energies 334.0 keV and 406.5 keV were observed both in one-dimensional spectrum and in coincidence data resulting in the half-life $T_{1/2}=[0.83_{-0.13}^{+0.18}{(\mathrm{stat})}_{-0.19}^{+0.16}(\mathrm{syst})]\times 10^{20}$ yr. Interpreting an excess of the 334.0-keV peak area as an indication of the $2{\hat{I}}^2$ decay of ${}^{150}$Nd to the 334.0 keV $2_1^{+}$ excited level of ${}^{150}$Sm with a half-life of $T_{1/2}=[1.5_{-0.6}^{+2.3}(\mathrm{stat})\pm 0.4(\mathrm{syst})]\times 10^{20}$ yr, the $2\nu 2{\hat{I}}^2$ half-life of ${}^{150}$Nd for the transition to the 0${}_1^{+}$ level is $T_{1/2}=[1.03_{-0.22}^{+0.35}{(\mathrm{stat})}_{-0.19}^{+0.16}(\mathrm{syst})]\times 10^{20}$ yr, in agreement with the previous experiments. Both half-life values reasonably agree with the theoretical calculations in the framework of proton-neutron QRPA with isospin restoration combined with like nucleon QRPA for description of excited states in the final nuclei. For $2\nu 2{\hat{I}}^2$ and $0\nu 2{\hat{I}}^2$ transitions of ${}^{150}$Nd and ${}^{148}$Nd to several excited levels of ${}^{150}$Sm and ${}^{148}$Sm, limits were set at level of $T_{1/2}>10^{20}-10^{21}$ yr.

61 pages, 19 figures, 14 tables