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

TL;DR

This study used low-background ZnWO_4 crystal scintillators at Gran Sasso to investigate double beta decay in Zinc and Tungsten isotopes, setting new half-life limits and analyzing scintillator properties for decay detection.

Contribution

It provides improved half-life limits for double beta decay in several isotopes and demonstrates the applicability of ZnWO_4 scintillators for such rare decay searches.

Findings

New half-life limits for ^{64}Zn double beta processes.

Disproved previous positive indication of ^{64}Zn decay.

Validated scintillator properties for double beta decay experiments.

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 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.