Results of a search for the two neutrino double β decay of136Xewith proportional counters

Results of a search for ${}^{136}\mathrm{Xe}$ double β decay with high-pressure multiwire wall-less proportional counters at the Baksan Neutrino Observatory are presented. The experimental method and the characteristics of the detectors are described. The detector background in the energy range 0.5–3.5 MeV has been reduced due to event position discrimination and pulse shape discrimination. Results of the analysis of background components for different event types and source positions are described. A new lower limit of $T_{1/2}>~0.81\times {10}^{21}\mathrm{yr}$ (90% C.L.) is found for the ${}^{136}\mathrm{Xe}$ 2νββ decay mode.