Limits to Drift Chamber Resolution

ATLAS (A Large Toroidal LHC Apparatus) will be a general-purpose experiment at the Large Hadron Collider that will be operational at CERN in the year 2004. The ATLAS muon spectrometer aims for a momentum resolution of 10% for a transverse momentum of pT=1TeV. The precision tracking devices in the muon system will be high pressure drift tubes (MDTs) with a single wire resolution of < 80μm. Approximately 370000 MDT tubes will be assembled into > 1100 chambers covering an area of ≈ 2500m2. The high counting rates in the spectrometer as well as the aim for excellent spatial resolution and high efficiency put severe constraints on the MDT operating parameters. This work describes a detailed study of all the resolution limiting factors in the ATLAS environment. A ’full chain’ simulation of the MDT response to photons and charged particles as well as quantitative comparisons with measurements was performed. The good agreement between simulation and measurements resulted in a profound understanding of the drift chamber processes and the individual contributions to the spatial resolution. An optimization of the detector physics operating parameters and the electronics parameters was performed. The detailed study of the individual contributions to the spatial resolution showed that signal rise time fluctuations due to charge deposit fluctuations are one of the major contributions to the spatial resolution of high pressure drift ch ambers. A scheme to correct for these fluctuations was developed. Resolution limiting factors from space charge effects were studied as well. A detailed analysis of the MDT efficiency by analysing pulse shapes of inefficient events gave answer to the nature of the inefficiencies. A scheme to increase the MDT efficiency by using a second discriminator was developed. Vibrations of the MDT wires induced by the ions created in the avalanche are also a resolution limiting contribution at high count rates. This effect as well as the effect of externally induced vibrations were studied. Finally a frontend electronics scheme that allows to extract all the interesting information contained in the wire chamber signal into a single output channel was developed.