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There are valuable arguments to perform neutrinoless double beta (0ν2β) decay experiments with several nuclei: the uncertainty of nuclear-matrix-element calculations; the possibility to test these calculations by using the ratio of the measured lifetimes; the unpredictability of possible breakthroughs in the detection technique; the difficulty to foresee background in 0ν2β decay searches; the limited amount of isotopically enriched materials. We propose therefore approaches to estimate the Majorana neutrino mass by combining experimental data collected with different 0ν2β decay candidates. In particular, we apply our methods to a next-generation experiment based on scintillating and Cherenkov-radiation bolometers. Current results indicate that this technology can effectively study up to four different isotopes simultaneously (82Se, 100Mo, 116Cd and 130Te), embedded in detectors which share the same concepts and environment. We show that the combined information on the Majorana neutrino mass extracted from a multi-candidate bolometric experiment is competitive with that achievable with a single isotope, once that the cryogenic experimental volume is fixed. The remarkable conceptual and technical advantages of a multi-isotope investigation are discussed. This approach can be naturally applied to the proposed CUPID project, follow-up of the CUORE experiment that is currently taking data in the Gran Sasso underground laboratory.

Two-neutrino double β decay can create an irremovable background even in high energy resolution detectors searching for neutrinoless double β decay due to random coincidence of 2 ν 2 β events in the case of poor time resolution. Some possibilities for suppressing this background in cryogenic scintillating bolometers are discussed. It is shown that the present bolometric detector technologies enable one to control this form of background at the level required to explore the inverted hierarchy of the neutrino mass pattern, including the case of bolometers searching for the neutrinoless double β decay of 100 Mo, which is characterized by a relatively short two-neutrino double β decay half-life.

A search for double-beta decay of 190Pt and 198Pt with emission of γ-ray quanta was realized at the HADES underground laboratory with a 148 g platinum sample measured by two ultralow-background HPGe detectors over 8946 h. The isotopic composition of the platinum sample has been measured with high precision using inductively coupled plasma mass spectrometry. New lower limits for the half-lives of 190Pt relative to different channels and modes of the decays were set on the level of limT1/2∼1014–1016 year. A possible exact resonant 0νKN transition to the 1,2 1326.9 keV level of 190Os is limited for the first time as T1/2≥2.5×1016 year. A new lower limit on the double-beta decay of 198Pt to the first excited level of 198Hg was set as T1/2≥3.2×1019 year, one order of magnitude higher than the limit obtained in the previous experiment.

Experimental search for spontaneous fission and cluster decays of 234 U, 235 U, 236 U and 238 U was carried out with a certified 230-g uranium oxide sample enriched in 235 U to 93.2%. The sample was measured over 46.3 days by using a 70 cm 3 broad energy germanium detector. The search for the fission fragments and their daughters was performed by using γ  quanta of energy > 1.5  MeV expected in β -  decays. No such γ  peaks were observed in the experimental data, thus, upper limits on the fragments’ yields are set. The lower limits on partial half-lives are obtained at the level of 5.0 × 10 10 - 2.6 × 10 17  years for the corresponding cold fission and cluster decay channels. In particular, 5 limits for 235 U and 238 U cold fission exceed the prediction made by S.B. Duarte et al. in framework of the effective liquid drop model by a factor of 1.6–227. Therefore, the obtained results can be used to improve the theoretical frameworks to study the cold fission processes.

A long-term measurement was conducted to search for α, double-α and double-β decays with γ quanta emission in naturally occurring osmium isotopes. This study took advantage of two ultra-low background HPGe detectors and one ultra-low background BEGe detector at the Gran Sasso National Laboratory (LNGS) of the INFN. Over almost 5 years of data were taken using high-purity osmium samples of approximately 173 g. The half-life limits set for α decays of 184 Os to the first 2 + 103.6 keV excited level of 180 W ( T 1/2  ≥ 9.3 × 10 15  yr) and of 186 Os to the first 2 + 100.1 keV of 182 W ( T 1/2  ≥ 4.8 × 10 17  yr) exceed substantially the present theoretical predictions that are at level of T 1/2  ~ (0.6–3) × 10 15  yr for 184 Os and T 1/2  ~ (0.3–2) × 10 17  yr for 186 Os. New half-life limits on the 2EC and ECβ + decay of 184 Os to the ground and excited levels of 184 W were set at level of T 1/2  > 10 16 –10 17  yr; a lower limit on the 2β – decay of 192 Os to the 2 + 316.5 keV excited level of 192 Pt was estimated as T 1/2  ≥ 6.1 × 10 20  yr. The half-life limits for 2α decay of 189 Os and 192 Os were set for the first time at level of T 1/2  > 10 20  yr.

We report the measurement of the two-neutrino double-beta ( 2 ν β β ) decay of 100 Mo to the ground state of 100 Ru using lithium molybdate ( Li 2 100 MoO 4 ) scintillating bolometers. The detectors were developed for the CUPID-Mo program and operated at the EDELWEISS-III low background facility in the Modane underground laboratory (France). From a total exposure of 42.235 kg × day, the half-life of 100 Mo is determined to be T 1 / 2 2 ν = [ 7 . 12 - 0.14 + 0.18 ( stat . ) ± 0.10 ( syst . ) ] × 10 18  years. This is the most accurate determination of the 2 ν β β half-life of 100 Mo to date.

The advanced molybdenum-based rare process experiment (AMoRE) aims to search for neutrinoless double beta decay (\\[0\\nu \\beta \\beta \\]) of \\[^{100}\\]Mo with \\[\\sim 100\\,\\hbox {kg}\\] of \\[^{100}\\]Mo-enriched molybdenum embedded in cryogenic detectors with a dual heat and light readout. At the current, pilot stage of the AMoRE project we employ six calcium molybdate crystals with a total mass of 1.9 kg, produced from \\[^{48}\\]Ca-depleted calcium and \\[^{100}\\]Mo-enriched molybdenum (\\[^{48{{\\text {depl}}}\\hbox {Ca}^{100}\\hbox {MoO}_{4}\\]). The simultaneous detection of heat (phonon) and scintillation (photon) signals is realized with high resolution metallic magnetic calorimeter sensors that operate at milli-Kelvin temperatures. This stage of the project is carried out in the Yangyang underground laboratory at a depth of 700 m. We report first results from the AMoRE-Pilot \\[0\\nu \\beta \\beta \\] search with a 111 kg day live exposure of \\[^{48{{\\text {depl}}}\\hbox {Ca}^{100}\\hbox {MoO}_{4}\\] crystals. No evidence for \\[0\\nu \\beta \\beta \\] decay of \\[^{100}\\]Mo is found, and a upper limit is set for the half-life of \\[0\\nu \\beta \\beta \\] of \\[^{100}\\]Mo of \\[T^{0\\nu }_{1/2} > 9.5\\times 10^{22}~\\hbox {years}\\] at 90% C.L. This limit corresponds to an effective Majorana neutrino mass limit in the range \\[\\langle m_{\\beta \\beta }\\rangle \\le (1.2-2.1)\\,\\hbox {eV}\\].

From 7 naturally occurring Nd isotopes, 5 are unstable in relation to α decay. If an excited level of the daughter nucleus is populated, or the daughter nucleus is unstable, γ quanta can be emitted. We used an ultra-low background spectrometry system with 4 high purity germanium (HPGe) detectors (about 225 cm 3 volume each) to search for such decays using a highly purified Nd-containing sample with mass of 2.381 kg. Measurements were performed at the INFN Gran Sasso underground laboratory (with an overburden of about 3600 m w.e.) during 51,237 h. Half-life limits for α decays of 143 Nd and 145 Nd were determined to be T 1/2 ( 143 Nd) > 1.1 × 10 20  year and T 1/2 ( 145 Nd) > 2.7 × 10 19  year at 90% C.L. This is an increase of three and two orders of magnitude, respectively, compared with the most restrictive values currently given in literature. A limit for α decay of 144 Nd to the excited level of 140 Ce with E exc  = 1596.2 keV was determined for the first time as T 1/2 ( 144 Nd →  140 Ce * ) > 9.3 × 10 20  year. Restriction for the α decay of 146 Nd to the excited level of 142 Ce with E exc  = 641.3 keV was increased by 3 orders of magnitude to T 1/2 ( 146 Nd →  142 Ce * ) > 1.4 × 10 21  year. For α and 2 α decays of 148 Nd, first T 1/2 limits were set as 4.2 × 10 18  year and 2.1 × 10 20  year, respectively.

Highly forbidden β decays provide a sensitive test to nuclear models in a regime in which the decay goes through high spin-multipole states, similar to the neutrinoless double- β decay process. There are only 3 nuclei ( 50 V, 113 Cd, 115 In) which undergo a 4 th forbidden non-unique β decay. In this work, we compare the experimental 113 Cd spectrum to theoretical spectral shapes in the framework of the spectrum-shape method. We measured with high precision, with the lowest energy threshold and the best energy resolution ever, the β spectrum of 113 Cd embedded in a 0.43 kg CdWO 4 crystal, operated over 26 days as a bolometer at low temperature in the Canfranc underground laboratory (Spain). We performed a Bayesian fit of the experimental data to three nuclear models (IBFM-2, MQPM and NSM) allowing the reconstruction of the spectral shape as well as the half-life. The fit has two free parameters, one of which is the effective weak axial-vector coupling constant, g A eff , which resulted in g A eff between 1.0 and 1.2, compatible with a possible quenching. Based on the fit, we measured the half-life of the 113 Cd β decay including systematic uncertainties as 7 . 73 - 0.57 + 0.60 × 10 15 yr, in agreement with the previous experiments. These results represent a significant step towards a better understanding of low-energy nuclear processes.

In this article, we present current results of the experiment searching for double beta decay of 106Cd with the help of an enriched 106CdWO4 crystal scintillator in coincidence with two CdWO4 scintillation detectors. The experiment is carried out at the Gran Sasso underground laboratory of the National Institute for Nuclear Physics (LNGS INFN, Italy). After 1075 days of data-taking, no double-beta effects were observed. New half-life limits have been set for the different modes and channels of double beta processes in 106Cd at the level of limT1/2=1020−1022 years.