Results of the Analysis of the August 1998 Material Runs with Silicon Telescope

The muons produced at LHC will pass through a large amount of matter before they enter the muon chambers. On their way through the \mbox{ATLAS} inner detector, calorimeter and support structures, they can produce electromagnetic showers. In order to study the influence of the shower production on the chamber performance, three runs were taken in August 1998; one run with no absorber in front of the MDT chamber BOS; one with an iron absorber of 5~cm thickness 27~cm in front of BOS; and one with the iron absorber 47~cm distance from the chamber. A silicon telescope in front of BOS was used to determine a reference muon track. The presence of the iron absorber reduces the tracking efficiency by 1\% at a muon energy of 180~GeV. The analysis of the data verifies the reliability of a GEANT Monte-Carlo simulation at the muon energy of 180~GeV. The comparison of Monte-Carlo and data is difficult due to the lack of precise knowledge of the impurities of the H8 muon beam. The Monte-Carlo simulation, however, provides reliable upper limits for the decrease of the tracking efficiency at higher energies: At 1~TeV muon energy, the tracking efficiency in a chamber is reduced by 15\% in the worst case, i.e. for iron absorbers of more than 20~cm thickness.