The AMS-02 transition radiation detector (TRD) gas control system

The Alpha Magnetic Spectrometer (AMS-02) is a large experiment in the International Space Station (ISS) intended to measure primary cosmic ray spectra in space. It sits outside of the station, looking out into space. High-energy particles pass through it, interacting with different detectors on the way. They all travel at about the speed of light, so the TOF and the RICH cannot distinguish them. The TRD, thought, is sensitive to =E/m, the energy divided by the mass. This quantity is very different for electrons and protons so the TRD can tell them apart. In particular this is important for dark-matter search, for which we need a good signature for positrons. To reach its physics goals a TRD was designed to identify positrons with a rejection factor of 103 against protons from 1.5 to 300 GeV. The gas control system of the TRD is an essential item for its performance. To obtain the required discriminating power, a stringent control of gas parameters is necessary.