Institute for Nuclear Research (NASU)

An experimental search for alpha and 2 alpha decays of naturally occurring europium isotopes, Eu-151 and Eu-153, was carried out at the Gran Sasso underground laboratory using a 313.8 g highly radiopure sample of europium oxide and a 118 cm(3) broad-energy HPGe detector. In the spectrum measured over 4211 h, no gamma peaks expected from the processes of interest were observed. Improved partial half-life limits were set for the alpha decays of Eu-151 to the 91.1-keV daughter level and Eu-153 to all energetically allowed levels: > 4.0 & times; 10(18) y and > 1.6 & times; 10(18) y, respectively, at 90% C.L. The first experimental limits were obtained for 2 alpha decay of Eu-151 ( > 5.0 & times; 10(11) y) and Eu-153 ( > 2.6 & times; 10(17) y).

Probing inelastic dark matter at large mass splittings requires heavy target nuclei, an extended recoil-energy range, and the high-velocity tail of the dark-matter distribution. We exploit these features with the RES-NOVA prototype detector, featuring a PbWO4 cryogenic calorimeter, produced from archaeological Pb and operated at the deep-underground laboratory of Gran Sasso of INFN (Italy), analyzing a 32.4 g day exposure over 2.5 keV - 1 MeV under both the Standard Halo Model (SHM) and a Large Magellanic Cloud (LMC)-motivated velocity distribution. We extend direct-detection constraints beyond the 330 keV reach of established technologies (e.g. Xe-based TPCs), probing splittings up to 510 (780) keV in the SHM (LMC) benchmark, while future exposures will probe new regions of the parameter space.

As a step towards the realization of cryogenic-detector experiments to search for neutrinoless double-beta decay (such as CROSS, BINGO, and CUPID), we investigated a batch of 10 Ge light detectors (LDs) assisted by Neganov-Trofimov-Luke (NTL) signal amplification. Each LD was assembled with a large cubic light-emitting crystal (45 mm side) using the recently developed CROSS mechanical structure. The detector array was operated at milli-Kelvin temperatures in a pulse-tube cryostat at the Canfranc underground laboratory in Spain. We achieved good performance with scintillating bolometers from CROSS, made of (Li2MoO4)-Mo-100 crystals and used as reference detectors of the setup, and with all LDs tested (except for a single device that encountered an electronics issue). No leakage current was observed for 8 LDs with an electrode bias up to 100 V. Operating the LDs at an 80 V electrode bias applied in parallel, we obtained a gain of around 9 in the signal-to-noise ratio of these devices, allowing us to achieve a baseline noise RMS of O(10 eV). Thanks to the strong current polarization of the temperature sensors, the time response of the devices was reduced to around half a millisecond in rise time. The achieved performance of the LDs was extrapolated via simulations of pile-up rejection capability for several configurations of the CUPID detector structure. Despite the sub-optimal noise conditions of the LDs (particularly at high frequencies), we demonstrated that the NTL technology provides a viable solution for background reduction in CUPID.

RES-NOVA is a cryogenic experiment based on PbWO4 scintillating bolometers produced from archaeological lead, originally designed to detect coherent elastic neutrino-nucleus scattering (CEvNS) from galactic core-collapse supernovae. We show that the same high-density, high-Z absorbers and sub-keV energy resolution make RES-NOVA an appealing probe of solar axions. We compute the expected signal from the Primakoff, ABC (atomic recombination and de-excitation, Bremsstrahlung, Compton), longitudinal-plasmon (LP), and 57Fe nuclear-line components of the solar axion flux, folded through the inverse-Primakoff and axioelectric detection channels in PbWO4, which probe the axion couplings to photons (g_a-gamma), electrons (g_ae), and nucleons (g_aN), respectively. We derive projected sensitivities for a 1 ton*y exposure of the RES-NOVA demonstrator in the (g_ae,g_a-gamma), (g_ae,g_aN), and (g_a-gamma,g_aN) planes. The projected reach in g_ae approaches that of XENONnT to within a factor of 2, despite a background roughly four orders of magnitude higher, thanks to the large target mass, the high-Z enhancement of the axioelectric and inverse-Primakoff cross sections, and the sub-keV energy resolution. Because the inverse-Primakoff constraint on g_a-gamma relies on absorption rather than coherent conversion, it is independent of the axion mass, unlike bounds from magnetic helioscopes. We complement these projections with the first solar-axion exclusion limit obtained on real data with a 13 g PbWO4 prototype grown from archaeological lead, using a background-model-independent optimum-interval analysis. These results establish RES-NOVA as a promising multi-coupling probe of solar axions, with simultaneous sensitivity to g_ae, g_a-gamma and g_aN that is complementary to existing helioscope and direct-detection searches.

The influence of metallic environments and cryogenic temperatures on nuclear decay rates remains an open question in nuclear physics, with potential implications for both fundamental science and radioactive waste management. Following a review of experimental studies on environmental effects in β decays, electron capture, and α decays, a phenomenological parametrization of electron screening effects is developed, taking into account competing theoretical predictions. The A-DREAM (Accelerated Depletion of RadioactivE wAste in Metals) project is then presented, aiming to investigate whether the α -decay rate of ^226 Ra can be enhanced at cryogenic temperatures when embedded in a metallic matrix. A wet-chemistry protocol was developed to produce low-activity (a few tens of Bq) sources of metallic ^226 Ra homogeneously incorporated into solid Ga-Hg alloy matrices, ensuring incorporation of Radium atoms into the metallic lattice in a structurally coherent manner, as well as long-term sample stability.