The ATLAS electromagnetic barrel calorimeter is a lead-liquid argon sampling calorimeter that measures the energy of electrons, positrons and photons produced as a result of proton-proton collisions at CERN's Large Hadron Collider. During early operations, electromagnetic objects from well-known processes, including electrons from the decay W → ev, will be used to calibrate this detector. It is important to identify the location and amount of material in front of the calorimeter, as this will influence its energy calibration. A method has been developed to determine the calorimeter's sensitivity to extra material using simulated electrons from W→ ev decays. With 160 pb -1, obtainable within two years at 10 TeV, the calorimeter is sensitive to a material increase of approximately 5% of a radiation length, for electron energies of up to 100 GeV. The validity of using the location of bremsstrahlung in the inner detector to pinpoint the location of extra material is also investigated.
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