New limit on two neutrino electron capture with positron emission in ¹⁰⁶Cd

An experiment to search for double beta processes in ¹⁰⁶Cd is in progress with the help of ¹⁰⁶CdWO₄ crystal scintillator (enriched in ¹⁰⁶Cd to 66%) in coincidence with two large volume CdWO₄ crystal scintillators in close geometry at the Gran Sasso Underground Laboratory. The time resolution of the set-up was analyzed, and the methods of pulse-shape discrimination of α and γ(β) events was applied to estimate the radioactive contamination of the crystal and to reduce the background counting rate. The possible double beta decay processes and the background components have been simulated by a Monte Carlo method with GEANT4. The experimental sensitivity is approaching the theoretical predictions for the two-neutrino electron capture with positron emission in ¹⁰⁶Cd: lim T_1/2≥4×10²¹ yr at 90% C.L. (the best previous limit was established in the previous stage of the experiment as T_1/2≥1.1× 10²¹ yr, while the theoretical predictions are in the region of T_1/2∼10²¹−10²² yr).