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Double beta decay of ^{64,70}Zn and ^{180,186}W isotopes

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 ZnWO_4 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 \approx3600 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 ^{64}Zn have been set: T^{2ν2K}_{1/2}(^{64}Zn) \geq 6.2(6.3) \times 10^{18} yr, T^{0ν2ε}_{1/2} (^{64}Zn) \geq 1.1(2.8) \times 10^{20} yr, T^{2νεβ^+}_{1/2}(^{64}Zn) \geq 0.7(2.1) \times 10^{21} yr, and T^{0νεβ^+}_{1/2}(^{64}Zn) \geq 4.3(5.7) \times 10^{20} yr, all the limits are at 90% (68%) C.L. The positive indication on the εβ^+ decay of ^{64}Zn with T^{(2ν+ 0ν) εβ^+}_{1/2} (^{64}Zn) = (1.1 \pm 0.9) \times 10^{19} yr suggested in [Appl. Radiat. Isot. 46 (1995) 455-456] is fully discarded by the present experiment. The half-life limits on the 2βprocesses in ^{70}Zn, ^{180}W, and two neutrino mode of 2β^- decay in ^{186}W established in the present work on the level of 10^{17}-10^{20} yr are one order of magnitude higher than those set in previous experiments. Energy resolution, relative light output, α/βratio, decay time, pulse-shape discrimination between αparticles and γrays (βparticles), and radioactive contamination of CdWO_4, PbWO_4 (undoped, and doped by F, Eu, Mo, Gd, S), and ZnWO_4 crystal scintillators were studied. Applicability of these scintillators to search for double beta decay was proved.