Andreas Obermeier (Rheinisch-Westfaelische Tech. Hoch. (DE))
Since May 2011 the AMS-02 experiment is installed on the ISS and is observing cosmic radiation. It consists of several state-of-the-art sub-detectors, which redundantly measure charge and energy of traversing particles. Due to the long exposure time of AMS-02 of many years the measurement of cosmic-ray energy spectra is mainly limited not by statistics, but by detector response. The measurement of momentum for protons and ions, for example, is limited by the spatial resolution and magnetic field strength of the silicon tracker. The maximum detectable rigidity (MDR, rigidity is momentum per charge) for protons is about 2 TV, for Helium below 4 TV (E<2.1 TeV/amu). In this contribution we investigate the possibility to extend the range of the energy measurement for heavy nuclei (Z>=2) with the transition radiation detector (TRD). The main purpose of the TRD of AMS-02 is the discrimination between light particles (electrons and positrons) and heavy particles (protons), and was thus designed as a threshold detector. The response function of the TRD, however, shows a steep increase in signal from the level of ionization at a Lorentz factor γ of about 500 to γ ≈ 5000, where the transition radiation signal saturates. The increase of the signal over this energy range may be used to measure the Lorentz factor for very high energy cosmic-ray ions, e.g. for helium nuclei between about 500 GeV/amu and 5 TeV/amu, well beyond the limits of the silicon tracker. From the response curve and the signal fluctuations in the TRD we derive the energy resolution of the TRD and compare it to the resolution of the silicon tracker. Furthermore, the geometric acceptance available to a TRD-based measurement can be greater by an order of magnitude compared to a standard tracker-based analysis.
|Registration number following "ICRC2015-I/"||1032|
Michael Mueller (Rheinisch-Westfaelische Tech. Hoch. (DE))
Andreas Obermeier (Rheinisch-Westfaelische Tech. Hoch. (DE)) Michael Korsmeier (RWTH Aachen University)