Study of cosmic-ray light nuclei on the ISS: identification of the interaction point in the CALorimetric Electron Telescope (CALET)

At present, Astro-particle Physics is one of the most interesting and alive fields in experimental physics. Direct measurements of cosmic rays will help answering many open questions about the sources and the processes of acceleration and propagation in the interstellar medium of high-energy particles (from GeV to PeV energy scale). For example, the measurement of both light and heavy nuclei spectra and their relative abundances for energies of tens of TeV/nucleon is one of the main topic to understand acceleration and propagation mechanisms in our Galaxy. The CALorimetric Electron Telescope (CALET) is a Japanese-led international space mission by JAXA (Japanese Aerospace Agency) in collaboration with the Italian Space Agency (ASI) and NASA, designed to perform precise measurements of high energy cosmic rays. CALET reached the ISS on August 24th , 2015 and started a campaign of scientific observations on October 13th the same year. After the first two-year period of the mission, an extension has been approved for additional three years. The broad scientific program of this space-based experiment includes many topics: 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 cosmic-ray relative abundances and secondary-to-primary ratios. The detector is composed by a Total Absorption Shower Calorimeter (TASC), an homogeneous calorimeter with a thickness of 27 radiation length (corresponding to 1.2 interaction length), a sampling IMaging Calorimeter (IMC) that adds 3 radiation length to the total thickness of the instrument and a CHarge Detector (CHD) for the identification of nuclear species in a wide dynamic range up to Z=40. The TASC detector has the function of measuring all the energy deposited by crossing particles. The IMC with its fine granularity allows for the reconstruction of the incident direction of particles with high angular resolution. This thesis illustrates the study on light nuclear components (such as protons and helium nuclei) with the aim to reconstruct the interaction vertex in the CALET detector. In addition to a previously developed algorithm for the reconstruction of the interaction point inside the IMC, a completely new one has been developed and tested for the TASC detector. Knowing with high precision the starting point of a shower inside the IMC detector is of fundamental importance for the CALET experiment because it allows for a redundant measurement of the charge of incoming particles with the IMC detector, in addition to the independent one provided by the CHD. Studying instead the interaction taking place in the depth of the TASC can help to understand whether those events have to be taken into account for flux measurements. The first chapter of this thesis is dedicated to a review of cosmic-ray physics with particular attention to acceleration and propagation mechanisms. Chapter 2 describes the CALET detector in detail, summarizes the architecture of the trigger system, provides the acceptance definition, and underlines the expected performance of the CHD detector, the electron/proton separation and the tracking reconstruction. Chapter 3 introduces the main scientific objectives of the CALET mission anticipating the expected measurements after five years of data taking. The original work of this thesis is described in chapters 4 and 5. The former contains the description of the interaction point reconstruction inside CALET while the latter shows how the potentiality of the algorithm can be exploited to observe energy deposits inside the TASC detector. Last, chapter 6 summarizes the results obtained by CALET after two years of data taking.