• calorimeter
• direction
• ISS
• photons

Gamma radiation represents the most energetic part of the electromagnetic spectrum: therefore, it provides information about the most energetic processes and phenomena in the Universe. High energy γ rays can be produced through acceleration of charged particles by galactic astrophysical objects like Supernova Remnants or Pulsars, or by extragalactic sources (Active Galactic Nuclei and Blazars). Because photons are neutral particles, they are not deflected by galactic magnetic fields and their incoming direction points back directly to the source.
The Alpha Magnetic Spectrometer (AMS-02), a general purpose experimental apparatus installed in the International Space Station on May 16th 2011, has the capability to detect high energy γ rays because of its electromagnetic calorimeter (ECAL), which is able to trigger and identify γ rays. ECAL has an excellent energy resolution (about 1.4% at 1 TeV). At present no other γ ray detector operating in Space has a better energy resolution for γ rays in the TeV region. Furthermore, from test beam studies, ECAL is expected to have a good angular resolution, a crucial feature to associate γ rays to known sources. In this thesis, the angular resolution of ECAL for high energy γ rays is studied from flight data.
Assuming the angular resolution is the same for electrons and photons (both develop similar electromagnetic showers in the calorimeter), electrons have been used because they can be traced by means of the AMS-02 silicon tracker. In fact, the electron direction was obtained by applying three different algorithms to the electromagnetic shower shape in ECAL, then compared with the direction reconstructed by the silicon tracker. The best agreement was found using an algorithm that provides an angular resolution of about 0.5 degrees at 1 TeV. Thus, this algorithm was chosen to analyze γ rays. In the analysis, photons were identified using a Boost Decision Tree, which enhanced the separation between photons and background. The angular error associated to each γ is transferred into galactic coordinates to obtain its Point Spread Function (PSF), which depends on the energy and on the impact angle of the photon. A catalog of γ sources above 10 GeV has been published in summer 2013 by the Fermi collaboration: only a fraction of these sources can be detected by AMS-02, owing
to the AMS-02 acceptance (geometric acceptance plus efficiency) and to its exposure time. Sources expected to be detectable by ECAL were taken into account in this thesis.
Given the PSF, the distance for each selected γ from the nearest source was used to associate γ rays detected by ECAL to Fermi sources. To estimate the amount of fake associations due to diffused photons, a Monte Carlo simulation was developed to generate random events distributed according to the expected diffused photon flux.
This thesis shows that, thanks to the extraordinary capability of ECAL to reconstruct the incoming direction, AMS-02 can identify sources, among the known ones, emitting γ rays from 50 GeV to TeV energies. Furthermore, it is also shown that future larger
statistics will allow to identify new γ sources with high flux at high energy.