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 cm(3)) during 1280 h. The crystal is not polluted by Th-232, K-40, Co-60, Cs-137 (limits were set on the activity at a level of 0.05-1 Bq/kg), but contains radioactive La-138 (0.68 Bq/kg) and, probably, is polluted by U-235 and U-238 (0.3-0.7 Bq/kg). The half life limits for different double beta processes in Ce-136 and in Ce-138 were set in the range of (1-6) x 10(15) yr. In particular, the 2 epsilon captures in Ce-136 to various excited levels of Ba-136 were considered for the first time. The limits for the resonant neutrinoless transitions to the 2392.1 keV and 2399.9 keV levels of Ba-136 are: T-1/2 > 2.4 x 10(15) yr and T-1/2 > 4.1 x 10(15) yr at 90% C. L., respectively. (C) 2009 Elsevier B. V. All rights reserved.