Nuclei identification with the AMS-02 Silicon Tracker

The AMS-02 Magnetic Spectrometer is a state-of-the art particle physics detector operating as an external module on the International Space Station. The purpose of the experiment is to study with unprecedented level of statistics the fluxes of cosmic rays in an energy range from 0.5 GeV to few TeV. The Silicon Tracker system accurately determines the trajectory and absolute electric charge of cosmic rays by multiple measurements of the coordinates and energy loss. It is composed of 2284 double-sided silicon micro-strip detectors arranged in nine layers. A charged particle traversing the silicon volume looses energy through ionization. This energy loss is proportional to the square of the particle charge thus allowing the distinction between different nuclei. The analog readout and the high dynamic range of the front end electronics allow to identify nuclear species from hydrogen up to iron and above. The charge resolution is naturally degraded by a number of detector effects that need to be correctly accounted for. In-flight calibration of the Tracker is done using the statistics accumulated during two years of operation in space, allowing to perform a precise equalization and linearization of the responses of the 3072 amplifier chips. This results in an excellent overall charge discrimination power. In this contribution we describe the Tracker system and the main challenges of its operation in space. We then present the procedure that has been used to accurately calibrate the Tracker's response and optimize its performances in terms of charge resolution.