ZnO-based scintillating bolometers: new prospects to study double beta decay of 64Zn

The first detailed study on the performance of a ZnO-based cryogenic scintillating bolometer as a detector to search for rare processes in zinc isotopes was carried out. A 7.2 g ZnO low-temperature detector, containing more than 80% of zinc in its mass, exhibits good energy resolution of baseline noise 1.0-2.7 keV FWHM at various working temperatures resulting in a low-energy threshold for the experiment, 2.0-6.0 keV. The light yield for /3/y events was measured as 1.5(3) keV/MeV, while it varies for a particles in the range of 0.2-3.0 keV/MeV. The detector demonstrates an effective identification of /3/y events from a events using time-properties of only heat signals. The radiopurity of the ZnO crystal was evaluated using the Inductively Coupled Plasma Mass Spectrometry, an ultra-low-background High Purity Ge y-spectrometer, and bolometric measurements. Only limits were set at the level of O(1-100) mBq/kg on activities of 40K, 137Cs and daughter nuclides from the U/Th natural decay chains. The total internal a-activity was measured as 22(2) mBq/kg, with a major contribution caused by 6(1) mBq/kg of 232Th and 12(2) mBq/kg of 234U. Limits on double beta decay (DBD) processes in 64Zn and 70Zn isotopes were set on the level of O (1017-1018) yr for various decay modes, profiting from 271 h of acquired background data inthe above-ground lab. This study shows a good potential for ZnO-based scintillating bolometers to search for DBD processes of Zn isotopes, especially in 64Zn, with the most prominent spectral features at similar to 10-20 keV, like the two-neutrino double electron capture. A 10 kg-scale experiment can reach the experimental sensitivity at the level of O(1024) yr.