Detection and Study of Galactic Cosmic Rays with the DAMPE and HERD space missions

The Cosmos and our (somehow limited) knowledge of it, can be divided -for the sake of simplicity- in two categories: the macrocosm and microcosm. The former comprises immense structures like stars, planets, galaxies, clusters, the Interstellar Medium, etc, while the latter focuses on the building blocks constituting the aforementioned structures, such as subatomic particles, radiation and heavier elements, covering a vast energy range. A prominent part of the microcosm is manifested through Cosmic Rays (CRs) in the Universe. These can be elements such as protons, helium, carbon, oxygen (all the way up to iron, nickel and beyond) nuclei, electrons/positrons, antiprotons, gamma-rays and neutrinos. Understanding their (inherently complex) origin, acceleration and propagation mechanisms in the Universe is of paramount importance to Astroparticle Physics. As a consequence to the above-stated questions, a multitude of theoretical models have been introduced, along with an impressive number of sophisticated experiments focusing on CR detection in a broad energy range. Such experiments are devoted either to direct or indirect CR detection, with the former including space or balloon-borne instruments and the latter corresponding to ground-based experiments. This work focuses on the study and direct detection of highly energetic Galactic Cosmic Rays with space-borne detectors, specifically oriented towards the Dark Matter Particle Explorer (DAMPE) and High Energy cosmic-Radiation Detector (HERD) experiments. DAMPE, is a space-borne detector designed for direct CR and gamma ray studies along with indirect searches for Dark Matter (DM). The instrument is operative since December 2015, comprising a plastic scintillator detector, a silicon tracker, a deep calorimeter and a neutron detector. Such a configuration allows for precise measurements of electrons/positrons and gamma rays in the energy range of 5 GeV to 10 TeV, as well as protons and heavier nuclei spanning from 50 GeV to hundreds of TeV. An important part of this thesis is dedicated to spectral measurements pertaining to the BCNO nucleonic group, focusing mainly on carbon and oxygen fluxes along with complimentary insights on flux ratios such as C/O, B/C and B/O. DAMPE is able to provide precise measurements of the aforementioned nuclei in the multi-TeV/n region, for the first time, thus exhibiting a novelty amongst direct measurements. The fluxes of carbon and oxygen are precisely measured in the energy range of 10 GeV/n (energy per nucleon) to approximately 20 TeV/n and 10 TeV/n respectively, with the flux ratios reaching up to a few TeV/n. Pertaining to the fluxes, the main objective will be to validate features detected previously as well as extending current spectral measurements in the multi-TeV region for the first time, with unprecedented precision. In this region, a softening feature could be evident for carbon and oxygen, as in recent measurements of DAMPE proton and helium fluxes, where the aforementioned spectral feature was detected approximately at 15 TV. Regarding the C/O, B/C and B/O flux ratios, nuclei such as carbon and oxygen are of primary origin while boron originates from the spallation of heavier primaries with Interstellar Medium particles. Extending the prevailing knowledge on CR nucleonic spectra up to multi-TeV/n energies, while understanding the nature of CR propagation in the Galaxy via secondary-to-primary ratios (such as B/C and B/O), will be an important part of this work. Benefiting from the successful and long-standing experience of DAMPE in orbit, along with the experience gained from the various challenges that accompany a space mission, an additional part of this work is dedicated to the development and realization of a novel CR detector. HERD or, the High Energy cosmic-Radiation Detection facility is a future space-borne payload proposed to be installed on-board the China Space Station (CSS). Due to its unique geometrical design and deep calorimeter (55 $\mathrm{X_{0}}$) HERD will be detecting incoming particles from both its top and four lateral sides, thus leading to an increase of more that an order of magnitude in acceptance, while maintaining manageable payload dimensions for a space mission. Main goal of this activity pertains to the optimal configuration of the Plastic Scintillator Detector (PSD), an instrument aiming to provide gamma-ray and charged particle trigger signals, together with an essential charge measurement for particle identification purposes. This work is based on the preparation, construction and validation of various configurations regarding plastic scintillator bars' readout by silicon photomultipliers (SiPMs). Each setup is thoroughly examined in order to provide high detection efficiency, large dynamic range and satisfactory energy resolution. The entirety of all aforementioned configurations were tested both in Gran Sasso Science Institute (GSSI) and National Laboratories of Gran Sasso (LNGS) as well as in CERN Super Proton Synchrotron (SPS) and Proton Synchrotron (PS) facilities, verified via a multitude of particles, ranging from: electrons, CR muons, pions and protons in a wide energy range (from a few MeV to a few hundred GeV).