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Abstract: After the high-luminosity upgrade of the Large Hadron Collider LHC at CERN,the ATLAS muon spectrometer is expected to work at 10 times increasedbackground rates of gammas and neutrons. This is challenging as the momentumresolution of the spectrometer is expected to be 10 %. This requires a singletube resolution of the muon drift tubes of 80 mum. At background rates around1000 Hz-cm2 space charge effects will lead in the slow and non-linear AR:CO2 =93:7 gas mixture to a degradation of the drift-tube spatial resolution. Thiswas studied before experimentally for gammas and low energetic neutrons. Almostno information exists for fast neutrons. Therefore, we organized our studiesunder the following aspects: - We investigated the influence of 11 MeV neutronson the position resolution of ATLAS MDT chambers. At flux densities between 4and 16 kHz-cm2, almost no influence on the position resolution was found, itdegrades by only 10 mum at a detection efficiency of only 4*10-4. - Weinvestigated inert gas mixtures on fastness and linearity of theirposition-drifttime r-t relation. At a reduction of the maximum drift time bya factor of 2, the use of the present hardware and electronics might bepossible. For our experimental studies we used our Munich cosmic ray facility.Two gas mixtures show almost identical position resolution as the standard gas.- For spectrometer regions of highest background rates we contributed to theinvestigation of newly developed 15 mm drift tubes. Position resolutions havebeen measured as a function of gamma background rates between 0 and 1400Hz-cm2. - Garfield simulations have been performed to simulate space chargeeffects due to gamma irradiation. Results will be presented for the standardgeometry as well as for the new 15 mm drift tubes.

Autor: Albert Engl, Otmar Biebel, Ralf Hertenberger, Alexander Mlynek, Thomas A. Mueller, Felix Rauscher


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