New limits on 2EC, ECbeta+, and 2beta+ decay of 136Ce and 138Ce with deeply purified cerium sample

A search for double electron capture (2Ec), electron capture with positron emission (ECbeta+), and double positron emission (2beta+) in 136Ce and 138Ce was realized with a 465 cm3 ultra-low background HP Ge spectrometer over 2299 h at the Gran Sasso underground laboratory. A 627 g sample of cerium oxide deeply purified by liquid-liquid extraction method was used as a source of quanta expected in double decay of the cerium isotopes. New improved half-life limits were set on different modes and channels of double decay of 136Ce and 138Ce at the level of yr.