Fabio Cappella

Phonon-mediated cryogenic calorimeters find application in rare event searches due to their intrinsically low energy threshold. Achieving optimal sensitivity for these kinds of detectors is crucial for signal identification, leading to various optimization techniques. In this work, we present two complementary methods to increase the sensitivity of cryogenic detectors read out by transition-edge sensors, developed and tested in the context of the NUCLEUS experiment. The first procedure maps the signal-to-noise ratio of the device across a wide range of operating points, to identify the configuration with maximal sensitivity to be used during data taking. The second method exploits the double readout of the detector, combining the information from different channels with a two-dimensional optimum filter analysis that effectively lowers the energy threshold. By combining both techniques at the same time, we obtain a baseline resolution of 2.94 ± 0.05 _stat ±0.74 _syst eV using a CaWO ₄ based detector, achieving a promising result ahead of the first run of NUCLEUS at the experimental site.

A small series of four caesium hafnium chloride (Cs 2 HfCl 6 , CHC) crystal scintillators has been measured in a low-background setup at the Gran Sasso National Laboratory of the INFN, Italy. After characterization of the detectors, new measurements concerning the rare alpha activity of natural Hf isotopes have been done. Based on cumulative data collected over 85.23 d, a new half-life value for the 174 Hf α decay has been obtained: ( 2.95 − 0.14 + 0.15 ( stat ) ± 0.09 ( sys ) ) × 10 16 year. This result is consistent with that reported earlier, which was obtained using one of the CHC crystals employed in the present work, albeit within a different experimental setup. The combined value is T 1 / 2 = (2.95 ± 0.16) × 10 16 year. These measurements have resolved the long-standing discrepancy between the previously available experimental values and theoretical predictions. The experimental limits on the half-lives for α decay of other natural Hf isotopes have been derived in the range of 10 19 − 10 20 year (90% C.L.). The limit for the α decay of 180 Hf was set for the first time.

Double-electron capture in ¹⁵² Gd was searched for over 1336 h with a 286-g GAGG:Ce crystal scintillator at the Gran Sasso underground laboratory of the INFN (Italy). New, improved by four orders of magnitude, limit on the near-resonant 0ν2EC capture in ¹⁵² Gd was set as lim T _1/2 = 8.1 x 10 ¹⁶ yr at 90% C.L. Moreover, for the first time, a laboratory experiment has established a lower limit on the half-life of the allowed 2νKL process in the nuclide, with lim T _1/2 = 9.2 x 10 ¹⁶ yr at 90% C.L.

The NUCLEUS experiment aims to detect coherent elastic neutrino-nucleus scattering of reactor antineutrinos on CaWO 4 targets in the fully coherent regime, using gram-scale cryogenic calorimeters. The experimental apparatus will be installed at the Chooz nuclear power plant in France, in the vicinity of two 4.25 GW th reactor cores. This work presents results from the commissioning of an essential version of the experiment at the shallow Underground Laboratory of the Technical University of Munich. For the first time, two cryogenic target detectors were tested alongside active and passive shielding systems. Over a period of two months, all detector subsystems were operated with stable performance. Background measurements were conducted, providing important benchmarks for the modeling of background sources at the reactor site. Finally, we present ongoing efforts to upgrade the detector systems in preparation for a technical run at Chooz in 2026, and highlight the remaining challenges to achieving neutrino detection.