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

The 2ν2β decay of 150Nd to the first excited 740.5 keV 0+1 level of 150Sm was measured over 5.845 years with the help of a four-crystal low-background HPGe γ 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 ecay 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 T1/2 = [0.83+0.18−0.13(stat) +0.16−0.19(syst)] ×1020 year. Interpreting an excess of the 334.0-keV peak area as an indication of the 2β decay of 150Nd to the 334.0 keV 2+1 excited level of 150Sm with a half-life of T1/2 = [1.5+2.3−0.6(stat) ± 0.4(syst)] × 1020 year, the 2ν2β half-life of 150Nd for the transition to the 0+1 level is T1/2 =[1.03+0.35−0.22(stat)+0.16−0.19(syst)] × 1020 year, 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 2ν2β and 0ν2β transitions of 150Nd and 148Nd to several excited levels of 150Sm and 148Sm, limits were set at level of T1/2 > 1020 − 1021 year.