Detailed study of the gain of the MWPCs for the LHCb muon system

The gain of a multiwire proportional chamber (MWPC) of the LHCb muon detector was measured precisely. The chamber, filled with a CO2/Ar/CF4 gas mixture, 55/40/5% in volume, was irradiated with a 1.3 GBq 137Cs radioactive source and the current drawn by the chamber was measured. By precisely determining the primary ionization current it was possible to evaluate the absolute gain of the chamber. The dependence of the gain on the anode voltage and the gas density was measured and the need for a gain control system during the LHCb data taking is considered. Our experimental results are compared with those predicted for the chamber gain by Diethorn formula.