New phase of the 116Cd 2β-decay experiment with 116CdWO4 scintillators

A new step of the 116Cd 2β-decay experiment is in progress at the Solotvina Underground Laboratory. The upgraded setup consists of four enriched 116CdWO4 crystal scintillators of total mass 339 g. As an active shield, 15 CdWO4 crystals (20.6 kg) are used. The background rate (in the energy interval 2.7–2.9 MeV) is 0.06 count/(yr kg keV), one order of magnitude lower than in the previous apparatus. Combined with results of preceding measurements, the half-life limit for the neutrinoless 2β decay of 116Cd to the ground state of 116Sn is determined as T1/2(0ν)≥5.1×1022 yr with 90% C.L. The limits on T1/2 for transitions to the excited 21+ and 01+ levels of 116Sn are 8.6×1021 and 4.1×1021 yr, respectively (at a 90% C.L.). For 0ν2β decay accompanied by the emission of one and two Majorons, the constraints on the half-life are T1/2(0νM1)≥1.4×1021 yr and T1/2 (0νM2)≥4.1×1020 yr (at a 90% C.L.). The corresponding constraints on the neutrino mass and the neutrino-Majoron coupling constant are 〈mv〉 ≤ 3.1 eV and 〈gM〉 ≤ 1.9 × 10−4, respectively.