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Identification of Cosmic Nuclei with the CALET Electron Telescope on the International Space Station

By Francesco Stolzi


CALET (CALorimetric Electron Telescope) is a high energy astroparticle physics experiment designed for long-term observations of high-energy Cosmic Rays (CRs) on the International Space Station (ISS). The mission is funded by the Japanese Space Agency (JAXA), in collaboration with the Italian Space Agency (ASI) and NASA. CALET reached the ISS on August 24, 2015 and, at the time of writing, the instrument is operating in science data mode: a two year period of observations has started with a target of 5 years. The purpose of the experiment is to perform precise measurements of high energy cosmic rays, with an extensive physics program that includes the detection of possible nearby sources of high energy electrons; searches for signatures of dark matter in the spectra of electrons and γ rays; monitoring gamma-ray transients and solar modulation; long exposure observations of cosmic nuclei from proton to iron and trans-iron elements; measurements of the CR relative abundances and secondary-to-primary ratios. The telescope is an all-calorimetric instrument, with a total thickness of 30 radiation length (X 0 ) and 1.3 proton interaction length (λ I ), preceded by a particle identification system. The instrument is composed of three main subsystem: at the top a Charge Detector (CHD) identifies the individual chemical elements in the cosmic-ray flux, then a fine granulated pre-shower IMaging Calorimeter (IMC) is followed by a Total Absorption Calorimeter (TASC) measuring the energy released. The CHD is designed to provide incident particle charge measurement over a wide dynamic range, from Z = 1 to Z = 40 with sufficient charge resolution to resolve individual elements. The IMC can image the early shower profile and reconstruct the incident direction of cosmic rays with good angular resolution. The TASC measures the total energy of the incident particle and discriminates electrons and gamma-rays from hadrons. The main subject of the present thesis is the study of the performance of CALET in the identification of cosmic nuclei, by combining both IMC and CHD detectors. A clear identification of the incoming nuclei and the measurement of their energy is crucial both to measure absolute fluxes and the ratio of the fluxes of secondary-to-primary elements. From measurements of the energy dependence of the flux ratio it is possible to discriminate among different models of CR propagation in the galaxy. The first chapter of the thesis provides an introduction to cosmic-ray physics, cosmic-ray propagation and acceleration. In chapter 2 the CALET instrument is described in detail and the anticipated telescope performances and the experimental program are reported. Chapters 3 and 4 include the original work of this thesis. The third chapter is focused on two aspects of the energy calibration process: the first one related to the energy position dependence correction in CHD, the second one to the correction for the quenching effect in the CHD and IMC. The fourth chapter is focused on the identification of cosmic nuclei: charge tagging using both the IMC and the CHD and the performance in terms of the charge resolution of both detectors are described in view of an accurate determination of light element fluxes with CALET. Finally in chapter 5 an overview on the current status of the CALET mission is given

Topics: CALET, CHD
Publisher: Università di Siena
Year: 2017
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