Positron ratio measurement with AMS-02

The Alpha Magnetic Spectrometer (AMS-02) is a state of the art particle detector on the International Space Station (ISS). Equipped with a transition radiation detector (TRD) and an electromagnetic calorimeter (ECAL) AMS-02 is able to clearly distinguish positrons from the large background of protons in the cosmic rays. To optimize the proton rejection power of the TRD as many hits with a good estimation of the particles path length in the straw as possible are needed. A new tracking method has been used, which does not only use the particle path information provided by the tracker, but also uses the inefficiency of the single layers in the TRD. A missing signal in a layer of the detector adds additional high precision points of passage of the measured particle. These points help to correct the particle path for multiple scattering inside the TRD or small interactions in the material between TRD and Tracker. With this tracking method the path length estimation in the TRD straws is improved and therefore the proton rejection power is increased. This assures a positron spectrum with a high purity.