Search for double beta decay of zinc and tungsten with low background ZnWO4 crystal scintillators

Double beta processes in 64Zn, 70Zn, 180W, and 186W have been searched for with the help of large volume (0.1–0.7 kg) low background ZnWO4 crystal scintillators at the Gran Sasso National Laboratories of the INFN (Italy). 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, in particular (all the limits are at 90% C.L.): T 1 / 2 0 ν 2 e ⩾ 1.1 × 10 20 yr , T 1 / 2 2 ν e β + ⩾ 7.0 × 10 20 yr , and T 1 / 2 0 ν e β + ⩾ 4.3 × 10 20 yr . In addition, new T 1 / 2 bounds were set for different modes of 2β processes in 70Zn, 180W, and 186W at the level of 1017–1020 yr.