The Enriched Xenon Observatory (EXO) collaboration has observed the two-neutrino
double beta decay of 136Xe with EXO-200, a prototype to the full EXO detector in
development. This second-order process, predicted by the Standard Model, has been
observed for several nuclei but not for 136Xe. The observed decay rate provides
new input to matrix element calculations and to the search for the more interesting neutrino-less
double-beta decay, the most sensitive probe for the existence of Majorana particles and the
measurement of the neutrino mass scale.
The motivation to search for neutrino-less double-beta decay will be discussed. An overview of
experimental efforts, and the status of calculations of nuclear matrix elements will be given.
The EXO-200 detector and underground site at WIPP, New Mexico, will be presented, and the
observation of the two-neutrino decay discussed. The presentation will then focus on the development
of EXO Full, a multi-tonne detector with the ability to identify the daughter ion as a powerful background
reduction tool, with SNOLAB a possible site.