  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 0/2nee and 0/2neb+ 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 1e18-1e19 a. 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 2nee transition into the 0+1 state of
Ba-136 results in a new best 90% credibility limit of 5.0e18 a.
