Measurement of In-115 𝛽-decay with ACCESS

Current bounds on the neutrino Majorana mass are affected by significant uncertainties in the nuclear calculations for neutrino-less double-𝛽 decay. A key issue for a data-driven improvement of the nuclear theory is the actual value of the axial coupling constant 𝑔$_A$, which can be investigated through non-unique forbidden 𝛽-decays. The ACCESS (Array of Cryogenic Calorimeters to Evaluate Spectral Shapes) project aims to establish a novel technique to perform precision measurements of forbidden 𝛽-decays, which can serve as an important benchmark for nuclear physics calculations and represent a significant background in astroparticle physics experiments. ACCESS will operate a pilot array of cryogenic calorimeters based on natural and doped crystals containing 𝛽-emitting radionuclides. In this way, natural (e.g. Cd-113 and In-115) and synthetic isotopes (e.g. Tc-99) will be simultaneously measured with a common experimental technique. Here we present a summary of the the first measurement of the fourth-forbidden 𝛽-decay of In-115 with a cryogenic calorimeter based on indium iodide. Exploiting the enhanced spectrum-shape method for the first time to this isotope, our study accurately determines simultaneously spectral shape, 𝑔$_A$, and half-life