Speaker
Description
During LHC Run1 and Run2 the LHCb muon system consisted of over a
thousand gas detectors spread across five distinct stations, mostly
MWPCs but also triple-GEM detectors in the central part of the first
station.
The muon detector provided the input to the first level muon trigger and
was also used to identify muons in the reconstruction process.
The first station of the muon detector, located upstream of the
calorimetric system, consisted of 274 separate chambers. The 12 chambers
around the beam pipe were double triple-GEM detectors with pad readout.
These triple-GEM detectors, with an active area of 200x240 mm2, were
operated at rates as high as 300 kHz/cm2. They used a gas mixture of
Ar/CO2/CF4 in a 45/15/40 ratio and achieved the required efficiency (96%
in a 20 ns time window for the logical OR of the two sensitive gaps)
while operating at a gain of about 5000.
For over nine years, from 2010 to 2018, these triple-GEM detectors
operated in the challenging LHCb conditions without major signs of
performance degradation. However, we often observed increased leakage
currents in the GEM foils and in some cases we experienced GEM foils
shorts following repeated discharge phenomena.
In this presentation, we will report on the performance of these 24
triple-GEM detectors during Run1 and Run2, the main problems we
encountered and the solutions we implemented to keep the muon detector
efficiency close to 100%.
