Precise muon drift tube detectors for high background rate conditions

The muon spectrometer of the ATLAS-experiment at the Large H adron Collider consists of drift tube chambers, which provide the precise m easurement of trajec- tories of traversing muons. In order to determine the moment um of the muons with high precision, the measurement of the position of the m uon in a single tube has to be more accurate than σ ≤ 100 m. The large cross section of proton-proton-collisions and th e high luminosity of the accelerator cause relevant background of neutrons and γ s in the muon spectrome- ter. During the next decade a luminosity upgrade [1] to 5 10 34 cm − 2 s − 1 is planned, which will increase the background counting rates consider ably. In this context this work deals with the further development of the existing drift chamber tech- nology to provide the required accuracy of the position meas urement under high background conditions. Two approaches of improving the dri ft tube chambers are described: • In regions of moderate background rates a faster and more lin ear drift gas can provide precise position measurement without changing the existing hardware. • At very high background rates drift tube chambers consistin g of tubes with a diameter of 15 mm are a valuable candidate to substitute the CSC muon chambers. The single tube resolution of the gas mixture Ar : CO 2 : N 2 in the ratio of 96 : 3 : 1 Vol %, which is more linear and faster as the currently used dr ift gas Ar : CO 2 in the ratio of 97 : 3 Vol %, was determined at the Cosmic Ray Measu rement Facility at Garching and at high γ -background counting rates at the Gamma Irradiation Facility at CERN. The alternative gas mixture shows similar resolution without background. At high background counting rates it shows bett er resolution as the standard gas. To analyse the data the various parts of the set up have to be aligned precisely to each other. The change to an alternative gas mix ture allows the use of the existing hardware. The second approach are drift tubes with halved radius, whic h also provide better high rate capability. The single tube resolution of these 15 mm drift tubes was determined to be 95 m at the H8 test facility at CERN using a muon beam with an energy of 140 GeV.