A search for two neutrino and neutrinoless double beta decay of 136Xe in the Gran Sasso underground laboratory

Abstract We have searched for two neutrino and neutrinoless double beta decay of 136 Xe with a multicell proportional counter in the Gran Sasso underground laboratory. The search for the two neutrino channel has been based on the comparison of spectra obtained in runs with natural and enriched xenon and with xenon which had been specifically cleaned with an ultracentrifuge. Special dedicated runs have been carried out with the various gases to evaluate the background from the rate of α-particles. No evidence has been found for two neutrino or neutrinoless double beta decay. The corresponding lower limits on the lifetime are evaluated with a new statistical procedure.