Search for $2{\hat{I}}^2$ 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, $T_{1/2}^{2ν\varepsilonβ^+}\geq 1.1\times 10^{21}$ yr, has reached the region of theoretical predictions. With this half-life limit the effective nuclear matrix element for the $2ν\varepsilonβ^+$ decay is bounded as $M^{2ν\varepsilonβ^+}_{eff}\le 1.1$. 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 $T_{1/2}\ge (8.5\times 10^{20}-1.4\times 10^{21}$) yr.

18 pages, 11 figures, 1 table