The AMS-02 TRD for the International Space Station

The Alpha Magnetic Spectrometer (AMS-02) is an experiment which will be mounted on the international space station (ISS) to measure primary cosmic ray spectra in space. 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 10² - 10³ in an energy range from 10 to 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 23 mm layers of polyethylene/polypropylene fleece radiator. The tubes are filled with an 80%:20% mixture of Xe : CO₂ at 1.2 bar absolute from a recirculating gas system designed to operate >3 years in space. The layers are mounted to 0.2 mm precision in a stable carbon fiber composite/aluminum honeycomb octagonal mechanical support. There are in total 5248 straw tubes which are read out by a custom-made DAQ system in less than 70 μs. The electronics must be low in power consumption and sustain the stringent requirements of operation in space. The construction of the detector and its electronics is presented in this paper.