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^{2\nu 2K}_{1/2}(^{64}Zn) \geq 6.2(6.3) \times 10^{18} yr, T^{0\nu 2\epsilon}_{1/2} (^{64}Zn) \geq 1.1(2.8) \times 10^{20} yr, T^{2\nu \epsilon \beta^+}_{1/2}(^{64}Zn) \geq 0.7(2.1) \times 10^{21} yr, and T^{0\nu \epsilon \beta^+}_{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 \epsilon \beta^+ decay of ^{64}Zn with T^{(2\nu + 0\nu) \epsilon \beta^+}_{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 2\beta processes in ^{70}Zn, ^{180}W, and two neutrino mode of 2\beta^- 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, \alpha/\beta ratio, decay time, pulse-shape discrimination between \alpha particles and \gamma rays (\beta particles), 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.