Nuclear and atomic data determination with metallic magnetic calorimeters

Abstract We are developing metallic magnetic calorimeters (MMC) to improve accuracy on nuclear and atomic data determination such as electron capture probabilities and beta spectrum shape factors. We describe the concept and realization of MMC enclosing radioactive sources ( 55 Fe, 36 Cl) in a 4 π geometry allowing for unprecedented detection efficiency for both electrons and X-rays throughout an energy range of more than two-orders of magnitude. First results of detector characterization are presented.