Constraints on partial half-lives of Ce-136 and Ce-138 double electron captures

The gamma-ray emissions from a radiopure cerium-bromide crystal with a mass of 4381 g were measured for a total of 497.4 d by means of high-resolution gamma-ray spectrometry in the HADES underground laboratory at a depth of 500 m.w.e. A search for 0/2 nu epsilon epsilon and 0/2 nu epsilon beta(+) double beta decay transitions of Ce-136 and Ce-138 was performed using Bayesian analysis techniques. No signals were observed for a total of 35 investigated decay modes. 90% credibility limits were set in the order of 10(18)-10(19) yr. Existing constraints from a cerium oxide powder measurement were tested with a different cerium compound and half-life limits could be improved for most of the decay modes. The most likely accessible decay mode of the Ce-136 2 nu epsilon epsilon transition into the 0(1)(+) state of Ba-136 results in a new best 90% credibility limit of 5.0 x 10(18) yr.