MWPC gain monitoring through time measurements

A method to measure and monitor relative gain variations in muItiwire proportional chambers (MWPC) is presented based on the measurement of the average time response of the chambers. Contrary to the classical gain measurements based on radioactive source scans or on the determination of the chamber efficiencies as a function of the high voltage, the proposed method can be applied directly online during data taking without any waste of useful physics data. The method is studied on the large muon detector of the LHCb experiment at LHC, where the excellent time resolution of the MWPC allows the measurement of relative gain variations down to 5%. Given the very good precision achievable on the relative gain measurements, this method is an excellent tool to monitor the aging of the MWPCs installed on large detector systems and is currently applied to this end to the LHCb muon detector.