The indication for the α decay of 180 W with a half-life T

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A new phase of a 116 Cd double β decay experiment is in progress in the Solotvina Underground Laboratory. Four enriched 116 CdWO 4 scintillators with total mass of 339 g are used in a set up, whose active shield is made of 15 natural CdWO 4 crystals (20.6 kg). The background rate in the energy interval 2.5-3.2 MeV is 0.03 counts/yr•kg•keV. The half-life for 2ν2β decay of 116 Cd is measured as T 1/2 (2ν) = 2.6 ± 0.1(stat) +0.7 −0.4 (syst)×10 19 yr. The T 1/2 limits for neutrinoless 2β decay of 116 Cd are set at T 1/2 ≥ 0.7(2.5)×10 23 yr at 90%(68%) C.L. for transition to ground state of 116 Sn, while for decays to the first 2 + 1 and second 0 + 1 excited levels of 116 Sn at T 1/2 ≥ 1.3(4.8)×10 22 yr and ≥ 0.7(2.4)×10 22yr with 90%(68%) C.L., respectively. For 0ν2β decay with emission of one or two Majorons, the limits are T 1/2 (0νM1) ≥ 3.7(5.8)×10 21 yr and T 1/2 (0νM2) ≥ 5.9(9.4)×10 20 yr at 90%(68%) C.L. Restrictions on the value of the neutrino mass, right-handed admixtures in the weak interaction, and the neutrino-Majoron coupling constant are derived as: m ν ≤ 2.6(1.4) eV, η ≤ 3.9×10 −8 , λ ≤ 3.4×10 −6 , and g M ≤ 12(9.5)×10 −5 at 90%(68%) C.L., respectively.

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The light output, α/β ratio, and pulse shape have been investigated at −25 • C with PbWO 4 crystal scintillators undoped, and doped by F, Eu, Mo, Gd and S. The fast 0.01 − 0.06 µs and middle 0.1 − 0.5 µs components of scintillation decay were observed for all the samples. Slow components of scintillation signal with the decay times 1 − 3 µs and 13 − 28 µs with the total intensity up to ≈ 50% have been recognized for several samples doped by Molybdenum. We found some indications of a pulse-shape discrimination between α particles and γ quanta with PbWO 4 (Mo doped) crystal scintillators.