Results of a search for 2$\beta$-decay of $^{136}$Xe with high-pressure   copper proportional counters in Baksan Neutrino Observatory
  The experiment for the 2$\beta$-decay of $^{136}$Xe search with two high-pressure copper proportional counters has been held in Baksan neutrino observatory. The search for the process is based on comparison of spectra measured with natural and enriched xenon. No evidence has been found for 2$\beta$(2$\nu$)- and 2$\beta$(0$\nu$)-decay. The decay half lifetime limit based on data measured during 8000 h is T$_{1/2}$$\geq8.5\cdot10^{21}$yr for 2$\nu$-mode and T$_{1/2}$$\geq3.1\cdot10^{23}$yr for 0$\nu$-mode (90%C.L.). 
Ju.M. Gavriljuk, A.M. Gangapshev, V.V. Kuzminov, S.I. Panasenko, S.S.   Ratkevich