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

TL;DR

This study used high-resolution gamma-ray spectrometry in an underground lab to search for rare double electron capture decays in cerium isotopes, setting new limits on their half-lives with no positive signals observed.

Contribution

It provides improved constraints on the half-lives of Ce-136 and Ce-138 double electron capture decays using a large radiopure crystal and Bayesian analysis.

Findings

No signals observed for 35 decay modes.

Improved half-life limits for most decay modes.

New best limit of 5.0e18 years for Ce-136 2nee decay.

Abstract

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.