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The impact of high counting rates on the spatial resolution of cylindrical drift tubes is investigated in detail and the results are compared with simulations. Electronics effects and space-charge effects are quantitatively analysed. A spatial resolution of σ < 80 µm can be achieved even at rates as high as 1500 Hz/cm wire length (300 kHz per wire).

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Measurements of the drift-tube response to charged particle tracks are compared with a complete simulation. The measured resolution of typically 80 µm agrees well with the simulation and allows the individual factors limiting the resolution such as diffusion, charge deposit fluctuations, gas gain fluctuations and signal processing to be studied. The results with respect to the dependence of the drift chamber resolution on gas gain, gas pressure and electronics parameters are reported. IntroductionCylindrical drift tubes will be the precision tracking devices for the muon spectrometer of ATLAS [?, ?], a general purpose experiment at the Large Hadron Collider (LHC) at CERN. About 370000 drift tubes will be assembled into 1200 chambers to measure the muon tracks along a spectrometer arm of 5-15 m. To achieve a momentum resolution of 10 % for a 1 TeV muon, the position resolution of each measurement must be better than 80 µm. Although single-wire space resolutions of better than 20 µm have been achieved in drift chambers, the goal of 80 µm is a new challenge due to the large size of the system, the high background counting rates of up to 300 kHz/wire, and economic constraints.The most important requirements for a drift chamber in this high-rate environment are, in principle, a relatively fast drift velocity to reduce the occupancy, low gas gain to avoid chamber ageing effects due to large amounts of charge deposit on the wire, and linearity of the space-drift-time relation to minimize space-charge effects. However, since a good position resolution would favour high gas gain and a slow drift gas, a detailed study was performed of all the resolution limiting parameters in order to achieve good resolution in this very 'hostile' environment even for low gain and fast gases. The drift chamber resolution was measured and compared with a full chain simulation of the detector. The good agreement allowed us to make a detailed study of the individual components that contribute to the resolution and also showed that we have a satisfactory understanding of the drift chamber processes. Techniques to improve the resolution are also presented.

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We report experimental evidence showing that the electric-field-dependent yield of carriers excited in the track of an a particle stopping in liquid argon is dominated by geminate (initial) recombination of electron-ion pairs 0 This is the first confirmation of geminate carrier recombination in a noble-gas liquid. The thermalization length of the hot electrons is determined to be about 28 nm.