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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.

AMoRE-II aims to search for neutrinoless double beta decay ( 0 ν β β ) with an array of 423 Li 2 100 MoO 4 crystals operating in the cryogenic system as the main phase of the Advanced Molybdenum-based Rare process Experiment (AMoRE). AMoRE has been planned to operate in three phases: AMoRE-pilot, AMoRE-I, and AMoRE-II. AMoRE-II is currently being installed at the Yemi Underground Laboratory, located approximately 1000 m deep in Jeongseon, Korea. The goal of the experiment is to reach an exclusion half-life sensitivity to the 0 ν β β of 100 Mo on the level of T 1 / 2 0 ν β β > 6 × 10 26 year that covers completely the inverted Majorana neutrino mass hierarchy region of (15–46) meV. To achieve this, the background level of the experimental configurations and possible background sources of gamma and beta events should be well understood. We have intensively performed Monte Carlo simulations using the GEANT4 toolkit in all the experimental configurations with potential sources. We report the estimated background level that meets the 10 - 4 counts/(keV · kg · year) requirement for AMoRE-II in the Region Of Interest (ROI) and show the projected half-life sensitivity based on the simulation study.

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.

The AMoRE collaboration searches for neutrinoless double beta decay of 100 Mo using molybdate scintillating crystals via low temperature thermal calorimetric detection. The early phases of the experiment, AMoRE-pilot and AMoRE-I, have demonstrated competitive discovery potential. Presently, the AMoRE-II experiment, featuring a large detector array with about 90 kg of 100 Mo isotope, is under construction. This paper discusses the baseline design and characterization of the lithium molybdate cryogenic calorimeters to be used in the AMoRE-II detector modules. The results from prototype setups that incorporate new housing structures and two different crystal masses (316 g and 517–521 g), operated at 10 mK temperature, show energy resolutions (FWHM) of 7.55–8.82 keV at the 2.615 MeV 208 Tl γ line and effective light detection of 0.79–0.96 keV/MeV. The simultaneous heat and light detection enables clear separation of alpha particles with a discrimination power of 12.37–19.50 at the energy region around 6 Li ( n , α ) 3 H with Q-value = 4.785 MeV. Promising detector performances were demonstrated at temperatures as high as 30 mK, which relaxes the temperature constraints for operating the large AMoRE-II array.

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.

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.

Low-background anisotropic scintillators represents an innovative approach to study the presence, in the galactic halo, of those Dark Matter (DM) candidate particles able to induce just nuclear recoils, by exploiting the directionality approach. ZnWO4 crystal scintillators are particularly well-suited for such investigations, since the light output and scintillation pulse shape vary depending on the angle of incidence of heavy particles (e.g., α particles and nuclear recoils) relative to the crystal axes. Due to this anisotropic behavior, a signal induced by those DM candidates can be investigated in two independent modes: studying the directionality variation both of the signal rate and of the pulse shape discrimination from the γ/β radiation (that does not give rise to any anisotropic effects). Additionally, the detector’s sensitivity spans a wide range of DM masses, attributed to the differing atomic masses of its target nuclei (Zn, W, and O). Building on these characteristics, the ADAMO project carried out new studies to examine the anisotropic response of ZnWO4 scintillators to α particles and nuclear recoils induced by neutron scattering. A summary of these investigations are presented in this paper.

Studies on double beta decay processes in 106Cd were performed by using a cadmium tungstate scintillator enriched in 106Cd at 66% (106CdWO4) with two CdWO4 scintillation counters (with natural Cd composition). No effect was observed in the data that accumulated over 26,033 h. New improved half-life limits were set on the different channels and modes of the 106Cd double beta decay at level of limT1/2∼1020−1022 yr. The limit for the two neutrino electron capture with positron emission in 106Cd to the ground state of 106Pd, T1/22νECβ+≥2.1×1021 yr, was set by the analysis of the 106CdWO4 data in coincidence with the energy release 511 keV in both CdWO4 counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range T1/2∼1021−1022 yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of 106Pd is restricted at the level of T1/20ν2K≥2.9×1021 yr.

Cadmium-116 is one of the favorable candidates for neutrinoless double-beta decay ( 0 ν β β ) searches from both theoretical and experimental points of view, in particular thanks to the high energy of the decay (2813.49 keV), the possibility of the industrial enrichment in 116 Cd and its use in the well-established production of cadmium tungstate crystal scintillators. In this work, we present low-temperature tests of two 0.6 kg 116 CdWO 4 crystals enriched in 116 Cd to 82 % as scintillating bolometers. These detectors were operated underground, with one at the Laboratoire Souterrain de Modane (LSM) in France and the second at the Laboratorio Subterraneo de Canfranc (LSC) in Spain. The two crystals are coupled to bolometric Ge light detectors in order to register the scintillation light. The double readout of heat and scintillation enables reduction in the background in the region of interest by discriminating between different populations of particles. The main goal of these tests is the study of the crystals’ radiopurity and the detectors’ performance. The achieved results are extremely promising, in particular, the detectors demonstrate a high energy resolution (11–16 keV FWHM at 2615 keV) and a high-efficiency discrimination of the alpha background ( ∼ 20 σ ). These results, achieved for the first time with large mass enriched 116 CdWO 4 crystals, demonstrate prospects of the bolometric technology for high-sensitivity searches of 116 Cd 0 ν β β decay.