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

TL;DR

The study measures the $2\nu2\beta$ decay of $^{150}$Nd to excited levels of $^{150}$Sm using a highly purified Nd-containing source and a four-HPGe detector array in the STELLA laboratory, achieving a robust $T_{1/2}$ determination for the $0^+_1$ state and an indication for the $2^+_1$ state. A complementary maximum-likelihood analysis incorporating potential multi-channel decays yields $T_{1/2}(^{150}\mathrm{Nd}\rightarrow ^{150}\mathrm{Sm}(0^+_1))=[1.03^{+0.35}_{-0.22}(stat)^{+0.16}_{-0.19}(syst)]\times10^{20}$ yr, with a possible $2^+_1$ channel around $1.5^{+2.3}_{-0.6}\times10^{20}$ yr. The work also presents state-of-the-art theoretical calculations using pn-QRPA with isospin restoration, including exchange and radiative corrections in the phase-space factors, and provides NMEs and PSFs for multiple transitions to excited states. Overall, the results offer stringent benchmarks for nuclear matrix elements relevant to $2\nu2\beta$ processes and inform predictions for neutrinoless modes in related systems.

Abstract

The $2\nu2β$ 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 $γ$ 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 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 $T_{1/2}=[0.83^{+0.18}_{-0.13}\mathrm{(stat)}^{+0.16}_{-0.19}\mathrm{(syst)}]\times 10^{20}$ yr. Interpreting an excess of the 334.0-keV peak area as an indication of the $2β$ decay of $^{150}$Nd to the 334.0 keV $2^+_1$ excited level of $^{150}$Sm with a half-life of $T_{1/2}=[1.5^{+2.3}_{-0.6}\mathrm{(stat)}\pm 0.4\mathrm{(syst)}]\times10^{20}$ yr, the $2\nu2β$ half-life of $^{150}$Nd for the transition to the 0$^{+}_{1}$ level is $T_{1/2}=[1.03^{+0.35}_{-0.22}\mathrm{(stat)}^{+0.16}_{-0.19}\mathrm{(syst)}]\times 10^{20}$ 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 $2\nu2β$ and $0\nu2β$ 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.

Figures (19)

• Figure 1: A simplified decay scheme of $^{150}$Nd NDS150. The energies of the excited levels and of the emitted $\gamma$ quanta are in keV (the relative intensities of the $\gamma$ quanta are given in parentheses). The $Q_{2\beta}$-value for double-$\beta$ decay energy of $^{150}$Nd is taken from Wang:2021, the energies of the excited levels and of the $\gamma$ transitions are taken from the National Nuclear Data Center NuDat database NuDat3.0 accessed December 2024.
• Figure 2: A simplified decay scheme of $^{148}$Nd NDS148. The energies of the excited levels and of the emitted $\gamma$ quanta are given in keV (the relative intensities of the $\gamma$ quanta are shown in parentheses). The $Q_{2\beta}$-value for double-$\beta$ decay energy of $^{148}$Nd is taken from Wang:2021, the energies of the excited levels and of the $\gamma$ transitions are taken from the National Nuclear Data Center NuDat database NuDat3.0 accessed December 2024.
• Figure 3: Left: schematic view of the set-up with Nd-containing samples (1) installed in the HPGe-detector system: coaxial HPGe detectors (2), aluminium part of the detector system endcap (3), copper part of the endcap (4), position of radioactive $\gamma$-ray sources (5) during the calibration campaign. Right: photograph of the set-up with the Nd-containing sample installed (the shield is open).
• Figure 4: Energy spectra in the energy intervals (250–850) keV (a) and (850–2700) keV (b) measured with the Nd-containing sample over 5.845 yr (solid histogram) and without sample for 0.8969 yr (normalized to 5.845 yr, dots) by the low-background HPGe-detector system. The energy of the $\gamma$ peaks is in keV.
• Figure 5: Accuracy of the energy scale in the spectrum measured with the Nd-containing sample over 5.845 yr in the energy interval 250--630 keV. The ratio $E_{\mathrm{exp}}/E_{\mathrm{Tab}}$ was built for several intense $\gamma$ peaks present in the spectrum. The fit of the data by linear function is shown by a solid line. The positions of $\gamma$ peaks expected after $2\beta$ decay of $^{150}$Nd to the $0_1^+$ ($2_1^+$) excited levels of $^{150}$Sm are shown by blue arrows. Energy of the $\gamma$ transitions NuDat3.0 are in keV.