Double beta decay of 64,70Zn and 180,186W isotopes
Abstract
The results of the experimental investigations of double beta processes in Zinc and Tungsten isotopes with the help of middle volume (117 g, 168 g and 699 g) low-background ZnWO4 crystal scintillators are presented. The experiment was carried out in the low-background "DAMA/R&D" set-up at the Gran Sasso National Laboratories of the INFN (Italy) at a depth of ≈3600 m w.e. The total measurement time exceeds ten thousand hours. New improved half-life limits on double electron capture and electron capture with positron emission in 64Zn have been set: T2 2K1/2(64Zn) ≥ 6.2(6.3) × 1018 yr, T0 2ε1/2 (64Zn) ≥ 1.1(2.8) × 1020 yr, T2 ε β+1/2(64Zn) ≥ 0.7(2.1) × 1021 yr, and T0 ε β+1/2(64Zn) ≥ 4.3(5.7) × 1020 yr, all the limits are at 90% (68%) C.L. The positive indication on the ε β+ decay of 64Zn with T(2 + 0) ε β+1/2 (64Zn) = (1.1 0.9) × 1019 yr suggested in [Appl. Radiat. Isot. 46 (1995) 455-456] is fully discarded by the present experiment. The half-life limits on the 2β processes in 70Zn, 180W, and two neutrino mode of 2β- decay in 186W established in the present work on the level of 1017-1020 yr are one order of magnitude higher than those set in previous experiments. Energy resolution, relative light output, α/β ratio, decay time, pulse-shape discrimination between α particles and γ rays (β particles), and radioactive contamination of CdWO4, PbWO4 (undoped, and doped by F, Eu, Mo, Gd, S), and ZnWO4 crystal scintillators were studied. Applicability of these scintillators to search for double beta decay was proved.