Search for 2β decay of 106Cd with enriched 106CdW4 crystal scintillator in coincidence with four HPGe detectors

A radiopure cadmium tungstate crystal scintillator, enriched in Cd106 to 66%, with mass of 216 g (Cd106WO4), was used to search for double-β decay processes in Cd106 in coincidence with four ultra-low-background high-purity germanium detectors in a single cryostat. Improved limits on the double-β processes in Cd106 have been set on the level of 1020−1021 yr after 13 085 h of data taking. In particular, the half-life limit on the two-neutrino electron capture with positron emission, T2νɛβ+1/2≥1.1×1021 yr, has reached the region of theoretical predictions. With this half-life limit the effective nuclear matrix element for the 2νɛβ+ decay is bounded as M2νɛβ+eff≤1.1. The resonant neutrinoless double-electron captures to the 2718-, 2741-, and 2748-keV excited states of Pd106 are restricted at the level of T1/2≥(8.5×1020−1.4×1021) yr.