Search for $2\beta$ decay of $^{106}$Cd with enriched $^{106}$CdWO$_4$ crystal scintillator in coincidence with four HPGe detectors

Abstract

A radiopure cadmium tungstate crystal scintillator, enriched in $^{106}$Cd to 66%, with mass of 216 g ($^{106}$CdWO$_4$), was used to search for double beta decay processes in $^{106}$Cd in coincidence with four ultra-low background high purity germanium detectors in a single cryostat. New improved limits on the double beta processes in $^{106}$Cd have been set on the level of $10^{20}- 10^{21}$ yr after 13085 h of data taking. In particular, the half-life limit on the two neutrino electron capture with positron emission, $T_{1/2}^{2\nu\varepsilon\beta^+}\geq 1.1\times 10^{21}$ yr, has reached the region of theoretical predictions. With this half-life limit the effective nuclear matrix element for the $2\nu\varepsilon\beta^+$ decay is bounded as $M^{2\nu\varepsilon\beta^+}_{eff}\le 1.1$. The resonant neutrinoless double electron captures to the 2718 keV, 2741 keV and 2748 keV excited states of $^{106}$Pd are restricted at the level of $T_{1/2} \geq (8.5\times10^{20}-1.4\times10^{21}$) yr.