Events at Physics
program designed to search for the neutrino-less double beta
decay of Xe-136. The current phase of the experiment, EXO-
200, uses 200 kg of liquid xenon with 80% enrichment in Xe-
136, and also serves as a prototype for a future 1-10 ton scale
EXO experiment. The detector has been taking low background
physics data with enriched xenon at Waste Isolation Pilot Plant
(WIPP) in New Mexico since early May 2011. After
performing the first charge only analysis of the data, we report
the observation of two-neutrino double-beta decay in Xe-136
with T(1/2) = 2.11 +/- 0.04 (stat.) +/- 0.21 (sys.) x 10^21 yr.
This second order process, predicted by the Standard Model, has
been observed for several nuclei but not for Xe-136. The
observed decay rate provides new input to matrix element
calculations and to the search for the more interesting neutrinoless double-beta decay mode.