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

Abstract

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 T _1/2(0ν)≥5.1×10²² yr with 90% C.L. The limits on T _1/2 for transitions to the excited 2 ⁺₁ and 0 ⁺₁ 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 T _1/2(0νM1)≥1.4×10²¹ yr and T _1/2 (0νM2)≥4.1×10²⁰ yr (at a 90% C.L.). The corresponding constraints on the neutrino mass and the neutrino-Majoron coupling constant are 〈m _v〉 ≤ 3.1 eV and 〈g _M〉 ≤ 1.9 × 10⁻⁴, respectively.