Spectral measurements of cosmic Li, Be and B nuclei with the DAMPE space mission

The measurement of secondary cosmic ray nuclei such as lithium, beryllium and boron can offer insights into the mechanisms of cosmic ray propagation in our Galaxy. For example, the flux ratio of secondary over primary nuclei is a direct handle on the energy dependence of the galactic diffusion coefficient. Moreover, a comparison of the observed spectral features between secondary and primary cosmic rays makes it possible to probe the origin of these structures, disentangling between the hypotheses of a source-related and a diffusion-related effect. Currently available measurements of secondary nuclei at high energies are the ones from PAMELA, AMS-02 and CALET. This work details the measurement of cosmic lithium, beryllium and boron with the DAMPE space mission. The boron flux and the B/C ratio have been measured from 10 GeV/n to 5.6 TeV/n. The lithium, beryllium fluxes and Li/C and Be/C flux ratios have been measured from 14.6 GeV/n to 3.2 TeV/n. The fluxes and ratios have also been measured as a function of rigidity. The results of this work show good agreement with previous results from PAMELA, AMS-02 and CALET, extending to higher energies. Fits of models including a break were performed on all the measured fluxes and flux ratios in order to characterize the hardening position and the change of slope. Notably, the break is found at a consistent rigidity in the fluxes of all three nuclei. The weighted mean of the hardening position is 292 +- 88 GV. The boron flux shows strong evidence for the hardening, also observed in the B/C ratio, with significance over 7 sigmas. These results highly favour the hypothesis of such a structure being originated by a propagation-related effect. The measurements of lithium and beryllium and their ratio to carbon confirm these findings, even if with lower significance due to the higher uncertainty in the measurement. When compared to primaries, such as proton and helium nuclei measured by DAMPE, secondary nuclei harden twice as much, as expected in the case of a break in the diffusion coefficient. Given the unprecedented energy reach of the DAMPE measurements presented in this work, additional theorized effects involving cosmic ray acceleration and propagation could be tested. The presence of grammage at the source, secondary production at sources and re-acceleration of secondaries at shock fronts are some of the predicted effects that are expected, for instance, to influence the B/C ratio. The results presented in this thesis will contribute to the improvement of our understanding of cosmic ray propagation in the Galaxy.