Array of cryogenic calorimeters to evaluate the spectral shape of forbidden $$\beta $$-decays: the ACCESS project

The ACCESS (Array of Cryogenic Calorimeters to Evaluate Spectral Shapes) project aims to establish a novel technique to perform precision measurements of forbidden -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 -emitting radionuclides. In this way, natural (e.g. Cd and In) and synthetic isotopes (e.g. Tc) will be simultaneously measured with a common experimental technique. The array will also include further crystals optimised to disentangle the different background sources, thus reducing the systematic uncertainty. In this paper, we give an overview of the ACCESS research program, discussing a detector design study and promising results of In.