Search for double beta decay of $^{106}$Cd with an enriched $^{106}$CdWO$_4$ crystal scintillator in coincidence with CdWO$_4$ scintillation counters

Studies on double beta decay processes in Cd-106 were performed by using a cadmium tungstate scintillator enriched in Cd-106 at 66% ((CdWO4)-Cd-106) with two CdWO4 scintillation counters (with natural Cd composition). No effect was observed in the data that accumulated over 26,033 h. New improved half-life limits were set on the different channels and modes of the Cd-106 double beta decay at level of limT(1/2) similar to 10(20) - 10(22) yr. The limit for the two neutrino electron capture with positron emission in Cd-106 to the ground state of Pd-106, T-1/2(2 nu EC beta+) >= 2.1 x 10(21) yr, was set by the analysis of the (CdWO4)-Cd-106 data in coincidence with the energy release 511 keV in both CdWO4 counters. The sensitivity approaches the theoretical predictions for the decay half-life that are in the range T-1/2 similar to 10(21)-10(22) yr. The resonant neutrinoless double-electron capture to the 2718 keV excited state of Pd-106 is restricted at the level of T-1/2(0 nu 2K) >= 2.9x10(21) yr.