Precise gain measurement of the LHCb muon chambers

The muon detector of the LHCb experiment, which will operate at the Large Hadron Collider (LHC) at CERN, consists of five muon tracking stations placed along the beam axis and equipped mainly with multi-wire proportional chambers (MWPC). In the present paper we report the results of a precise measurement of the gain of the MWPC's as a function of the anode voltage. A precise evaluation of the primary ionization current (of about 5 pA) was performed. The absolute gain of the chambers was deduced as a function of the anode voltage and compared with the prediction of the Diethorn formula.