Cosmic-Ray Muon Tomography Setup: Long-Term Life of Drift Tube Chambers

For investigation of the cosmic muon scattering tomography (MST) concept at NRC ''Kurchatov Institute''—IHEP the large-scale setup has been constructed. The setup includes 8 planes ($3\times 3$ m${}^2$ each) of 3-layer drift tube chambers, the total quantity of drift tubes is 2304. The drift tubes were filled with gas mixture Ar $+$ 7$\%$ CO${}_2$ at 0.5 bar overpressure on April 2011 and the gas system was closed. Up to now the gas mixture was not refreshed but the chambers are still working. We are presenting long term observation of the drift chambers' behavior and some performances of the setup for this long period.