Final results of experiment to search for 2 beta processes in zinc and tungsten with the help of radiopure ZnWO4 crystal scintillators

A search for the double beta decay of zinc and tungsten isotopes has been performed with the help of radiopure ZnWO4 crystal scintillators (0.1–0.7 kg) at the Gran Sasso National Laboratories of the INFN. The total exposure of the low background measurements is 0.529 kg × yr. New improved half-life limits on the double beta decay modes of 64Zn, 70Zn, 180W and 186W have been established at the level of 10^18–10^21 yr. In particular, limits on double electron capture and electron capture with positron emission in 64Zn have been set: T1/2_2ν2K ≥ 1.1 × 10^19 yr, T1/2_0ν2ε ≥ 3.2 × 10^20 yr, T1/2_2νεβ+ ≥ 9.4 × 10^20 yr and T1/2_0νεβ+ ≥ 8.5 × 10^20 yr all at 90% CL. Resonant neutrinoless double electron capture in 180W has been restricted on the level of T1/2_0ν2ε ≥ 1.3 × 10^18 yr. A new half-life limit on α transition of 183W to the metastable excited level 1/2^− 375 keV of 179Hf has been established: T1/2 ≥ 6.7 × 10^20 yr.