Long-term and efficient operation of the MWPC muon detector at LHCb

Abstract

The Multi-Wire-Proportional-Chamber (MWPC) muon detector of LHCb is one of the largest instrument of this kind worldwide, and one of the most irradiated. All of the MWPCs are supplied with a 40% Ar + 55% CO 2 + 5% CF 4 gas mixture. For most of the LHC operation so far we took data at an instantaneous luminosity of 4 10 32 cm-2 s-1. The most irradiated MWPCs integrated over the past nine years ~0.6 C/cm of charge per unit length of wire. In nine years of MWPCs operation in a high radiation environment, the chambers didn't show a gain reduction or any other apparent symptom of ageing. However, many gas gaps were affected by the sudden appearance of high currents. This effect, originating from localized areas in the individual gaps of the MWPCs, results in an increased noise rate and a trip of the high-voltage (HV) supply system due to a current exceeding the set threshold. The observed phenomenon of currents triggered by high radiation, but self-sustained if beam goes off, suggests that most of the trips are due to Malter-like effects (ME). This phenomenon can be due to thin insulator deposits on the cathode coming from either the construction process, or from polymerization reactions taking place near the anode wires in the gas discharge plasma. During operation of the LHC, about 100 gas gaps were affected every year by HV trips. Most of the problematic chambers could be recovered successfully in situ during data taking under nominal beam conditions by means of a long HV training performed on the affected MWPC gaps. This method has proven to be very effective, allowing recovery of normal operation conditions for most of the MWPC gaps affected by HV trips, so that the muon detection efficiency could be kept close 100%. This is a remarkable result, since the recovery of gas discharge detectors without disassembling is topical for many modern experiments.