Test results of a full size prototype of the muon chambers for region M2/R4 of the LHCb muon system

A full size prototype M2R4-01 of the Wire Pad Chambers for region M2/R4 of the LHCb Muon System hs been constructed at PNPI and tested in the T11 beam at CERN. The prototype contained two double-gap chambers with the sensitive area of 25x120 cm2. Each double-gap chamber contained 24 wire pads of 5x25 cm2 size. The prototype was fully instrumented with the FE electronics based on SONY chips with active adapters at the input allowing to optimize the parameters of the read out channels. The chamber was tested with four different concentrations of the gas components in the Ar/CO2/CF4 gas mixture. The beam tests showed quite satisfactory performance of the M2R4-01 prototype : a wide efficiency plateau uniform over the whole sensitive area of the chamber, low noise rates and cross-talks. It was demonstrated that the gas mixtures Ar(40%)+CO2(40%,30%)+CF4(20%,30%) provide higher stability against the HV-trips thus extending essentially the operational plateau of the chamber.