The performance of the transition radiation detector of the AMS-02 experiment

The alpha magnetic spectrometer (AMS-02) is an experiment which will be mounted on the international space station (ISS) in 2006 to measure primary cosmic ray spectra in space and to perform an indirect search of dark matter component of universe. A key element is a transition radiation detector (TRD) to distinguish an e⁺ or p^- signal reducing the p⁺ or e^- background by a rejection factor 10³-10² in an energy range from 10-300 GeV. This will be used in conjunction with an electromagnetic calorimeter to provide overall p⁺ rejection of 10⁶ at 90% e⁺ efficiency. The detector consists of 20 layers of 6 mm diameter straw tubes alternating with 20 mm layers of polyethylene/polypropylene fleece radiator. The tubes are filled with a 80%:20% mixture of Xe:CO₂ at 1.0 bar absolute from a recirculating gas system designed to operate >3 years in space. A TRD prototype has been calibrated and its performance measured in test beams with p⁺, e^-, μ^-, π^- in the momentum range from 3 to 250 GeV/c and compared with Monte-Carlo predictions. It achieves rejection factors from 2000-140 for protons in a momentum range of 15-250 GeV/c. The design and construction of the detector is presented and results from test beam runs are discussed.