Neutrinoless double beta decay is considered as a powerful tool to study properties of neutrinos and weak interactions. The process is forbidden in the framework of the Standard Model since it violates the lepton number and requires the neutrinos to be massive Majorana particles. 116Cd is one of the most promising double beta isotopes taking into account the theoretical calculations, the large energy of decay, the relatively high isotopic abundance and possibility to increase it by centrifugation. The Aurora experiment at Gran Sasso investigates double beta decay of 116Cd with the help of 1.16 kg cadmium tungstate crystal scintillators enriched in 116Cd to 82% (116CdWO4). The low level of background allows investigating the two neutrino mode of the decay of 116Cd with the highest up-to-date accuracy. The sensitivity of the experiment to the neutrinoless double beta decay of 116Cd to the ground state of 116Sn is estimated as T1/2 ≥ 1.9×1023 yr at 90% CL, which corresponds to the effective Majorana neutrino mass limit ≤ (1.2 − 1.8) eV. New limits are obtained for the double beta decay transitions of 116Cd to the excited levels of 116Sn, and for the neutrinoless double beta decay with emission of majorons. The radioactive contamination by thorium and the total alpha activity in a sample cut from the 116CdWO4 crystal boule were reduced by the recrystallization method by factors of 10 and 3, respectively, which confirms the possibility of ultra-radiopure 116CdWO4 crystal scintillators production.
Oksana G. Polischuk
Institute for Nuclear Research