Nuclear structure of Kr-82 and Se-82 relevant for neutrinoless double-beta decay

Nuclear Resonance Fluorescence (NRF) experiments on the nuclei Kr-82 and Se-82 were performed, that are a candidates for a mother-daughter pair for the hypothetical neutrinoless double-beta (0v beta beta) decay. The experiment aimed at providing high-precision data to benchmark theoretical calculations of the nuclear matrix elements involved in this exotic decay mode. We have investigated the excitation energy range from 2.3 MeV to 4.2 MeV, where the nuclear scissors mode is expected to be located in nuclei of this mass region. Our experiment was able to constrain decay branches of the scissors mode down to a level of a few percents, comparable to previous experiments on heavy deformed 0v beta beta decay candidates. Reduced transition strengths of the magnetic dipole-excited states have been determined by a method that exploits the non-resonant background in the NRF spectra. They are compared to a calculation within the nuclear shell model for Se-82, which reveals their mixed orbital and spin character, hinting at a more complex microscopic structure of low-lying 1(+) states.