First limits on neutrinoless resonant 2ε captures in 136Ce and new limits for other 2β processes in 136Ce and 138Ce isotopes

A small CeCl3 crystal (6.9 g) was measured deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of the INFN (Italy) with a low background HP Ge detector (244 cm3) during 1280 h. The crystal is not polluted by 232Th, 40K, 60Co, 137Cs (limits were set on the activity at a level of 0.05–1 Bq/kg), but contains radioactive 138La (0.68 Bq/kg) and, probably, is polluted by 235U and 238U (0.3–0.7 Bq/kg). The half life limits for different double β processes in 136Ce and in 138Ce were set in the range of (1–6)×1015 yr. In particular, the 2ε captures in 136Ce to various excited levels of 136Ba were considered for the first time. The limits for the resonant neutrinoless transitions to the 2392.1 keV and 2399.9 keV levels of 136Ba are: T1/2>2.4×1015 yr and T1/2>4.1×1015 yr at 90% C.L., respectively.