Cosmic-Ray Physics with the AMS experiment on the International Space Station
Picture: © NASA
AMS is a detector designed for precision spectroscopy of cosmic rays that was installed on the International Space Station in May 2011 (see picture above). With dimensions of 5x4x3 m³ and a weight of 7.5 tons, AMS is the largest cosmic-ray spectrometer ever built.
Its construction began in 1995, and a successful prototype flight aboard the Space Shuttle Discovery proved the feasibility of the detector concept in 1998. Led by Nobel laureate Professor Samuel Ting from MIT, AMS has been constructed and is now operated by an international collaboration of more than 200 scientists and engineers, from Europe, America and Asia. The overall construction costs, including the flight of AMS to the Space Station aboard Space Shuttle Endeavour, have amounted to 1.5 billion US dollars.
In Germany, RWTH Aachen has been strongly involved in the AMS project since its inception. One of the main components of AMS, the transition radiation detector (TRD), has been designed and constructed by the I. Physikalisches Institut B under the direction of Professor Stefan Schael. Today, the Aachen group, comprising 20 scientists and students, plays a major role in the analysis of the data gathered by AMS and in the operation and calibration of the instrument.
Since their discovery in 1912, cosmic rays have held many surprises in stock for us, from the discovery of new elementary particles to the most violent processes taking place in the Universe and accelerating cosmic rays to enormous energies.
As a multi-purpose instrument for the precision spectroscopy of cosmic rays, AMS was conceived to answer fundamental questions about our Universe: What is the nature of Dark Matter? What happened to the antimatter that must have been produced in the Big Bang? Where are cosmic rays accelerated and how do they propagate through the Milky Way? Answers to these questions will have a profound impact on our understanding about the inner workings of our Universe and help advance fundamental science. In particular, the search for dark matter complements the endeavour to search for new elementary particles at the Large Hadron Collider (LHC) at CERN, Geneva.
AMS so far has recorded more than 80 billion individual particle crossings (so called “events“). The raw data volume collected is on the order of 40 TB per year. AMS employs three different sub-detectors for particle identification (the TRD, an electromagnetic calorimeter and a ring-imaging Cherenkov counter) and two sub-detectors for energy or momentum measurements (a silicon tracker and a time-of-flight system).Before any physics analysis of the data can be performed, the information from all these subdetectors has to be pieced together and complicated reconstruction algorithms have to be run for each of them. The resulting high-level data serves as the input for physics analyses and occupies a volume of 160 TB per year of AMS flight on disk.
Several processing runs of AMS data have already been conducted successfully on the JUROPA and JUAMS clusters at JSC as the result of the cooperation within JARA.
Two major papers have appeared in 2015. They deal with measurements of the cosmic-ray proton and helium spectra. Surprisingly, they reveal a progressive hardening of the spectral indices of both protons and helium at high energies, with high precision. Remarkably, the spectral index of the proton to helium flux ratio increases with rigidity up to 45 GV and then becomes constant. Both papers have been selected as Editor's Suggestion by Physical Review Letters.