The Transition Radiation Detector (TRD) is a specialized subdetector on AMS. Its goal is to identify particles through the detection of the X-rays emitted by light particles.
Why do we need the TRD?
To many detectors, extremely-high-energy particles look all similar. Because of special relativity the speed of light cannot be exceeded. It follows that if particles have a kinetic energy of 200 billion electron volts, it’s hard to make the niggling distinction between their masses: protons with about 940 million electron volts (MeV) of mass, electrons with 0.5 MeV, pions and muons with about 100 MeV, all will look the same. They will have about the same momentum, so the Tracker can’t distinguish them. They all travel at about the speed of light, so the ToF and the RICH cannot distinguish them. The TRD, though, is sensitive to the quantity γ = E/m, i.e., 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 the dark-matter search, for which we need to look at anti-electrons (positrons). If we routinely mistake protons for positrons, we would not be in a position to perform this search. This aspect is so important that in order to have the highest rejection between protons and positrons at high energy, in addition to the TRD another specialized detector has been included in the AMS-02 spectrometer: an electromagnetic calorimeter, the ECAL.
How does the TRD work?
At high energy, the TRD is able to tell the difference between an electron and a proton: at high energy, indeed, an electron will emit X-rays while crossing the TRD detector while a proton will not. These X-rays are produced when the particle crosses several (100’s) interfaces characterized by an abrupt change in index of refraction: in our case, the interface between a layer of plastic or felt and vacuum.
X-rays pass through the fabric but are converted into a heavy, Xenon-CO₂, gas mixture. The ionized gas starts an ionization avalanche in the proximity of a thin wire at high voltage. This abrupt current change induces a fast electric signal that can be read out at the end of the wire. The X-ray contribution to the ionization signal adds up to the normal ionization of a charged particle traversing the gas. Only electrons and positrons are characterized by this double contribution to ionization, heavier particles like protons do not.
How is the TRD built?
The AMS-02 TRD, placed on top of the magnet vacuum case, is made up of 328 modules., each comprising 16 straw tubes. The modules are arranged in 20 layers supported by a conical octagon made of aluminum-honeycomb walls with carbon-fiber skins and bulkheads. Each modules contains:
1) 20 mm of radiator made of polypropylene/polyethylene fiber fleece corresponding to 0.06 g/cm³. The large number of interfaces increases the probability of production of X-rays.
2) 16 tube straws filled with a Xe:CO₂ (80%:20%) gas mixture operating at 1,600 V (full avalanche regime). The Xenon-rich gas mixture has an high efficiency for X-rays conversion.
The tubes are very difficult to manufacture, especially since they have to be gas-tight in vacuum; there are something like 10,000 separate pieces to be sealed together with glue, which must not leak or crack for many years in space!
TRD need for the special Xe:CO₂ mixture. The mixture must be carefully regulated (80% xenon, accurate to better than 1%), free of contamination (less than 1 part per million of fluorocarbons or oxygen, for example). The detector must be leak proof, and the pressure must be monitored continuously.
To keep the detector filled with clean gas, AMS has a gas recirculation system; an extra 100 lbs of Xe and 5 of CO₂, stored in lightweight bottles, will accompany the TRD into space. A network of valves and pressure sensors can whip up fresh Xe:CO₂ 80:20, at a rate of 7 liters per day, in to the 300 liter detector volume. Within the TRD, the gas is flushed in a closed circuit and there are pumps, valves, and CO₂ analyzers to monitor its properties.