Ageing studies on small Thin Gap Chambers for the ATLAS New Small Wheel

The instantaneous luminosity of the Large Hadron Collider at CERN will be increased up to a factor of five with respect to the design value by undergoing an extensive upgrade program over the coming decade. The largest upgrade project for the ATLAS Muon System is the replacement of the present first station in the forward regions with the so-called New Small Wheels (NSWs), to be installed during the LHC long shutdown in 2018/19. Small-Strip Thin Gap Chambers (sTGC) detectors are one chosen technology to provide fast trigger and high precision muon tracking under the high luminosity LHC conditions. We study ageing effects of sTGC detectors with a gas mixture of 55% of CO2 and 45% of n-pentane. A sTGC detector was irradiated with beta-rays from a Sr-90 source. Three different gas flow rates were tested. We observed no deterioration on pulse height of the sTGC up to an accumulated charge of 6.7 C/cm.