Speaker
Description
Current constraints on the Majorana neutrino mass are strongly affected by uncertainties in the nuclear matrix element calculations underlying neutrinoless double-beta decay. A key step toward improving nuclear calculations within a data-driven framework is validating their predictions through challenging nuclear processes, such as highly forbidden $\beta$-decays. The ACCESS (Array of Cryogenic Calorimeters for Rare Event Search) project aims to establish a novel experimental approach using cryogenic calorimeters for high-precision studies of forbidden $\beta$-decays. By operating a pilot array of natural and doped crystals that embed $\beta$-emitting radionuclides, ACCESS enables a unified and complementary investigation of both naturally occurring isotopes (such as $^{113}Cd$ and $^{115}In$) and synthetic isotopes (such as $^{99}Tc$ and $^{36}Cl$). This technique allows the simultaneous measurement of decay half-lives and spectral shapes with excellent energy resolution and efficiency. In this contribution, after a brief overview of recent advances in the field, we present a new low-threshold measurement of the fourth-forbidden $\beta$-decay of $^{115}In$, obtained from the combined analysis of two cryogenic calorimeters with indium iodide and indium oxide absorbers. We further discuss two complementary experimental approaches for the study of $^{99}Tc$ and potentially several other isotopes, based respectively on neutron-activated crystals and on tellurium dioxide slabs with a deposited $^{99}Tc$ source.