Thermal neutron detectors were fabricated from 10B enriched h-BN epilayers of different thicknesses. The charge carrier generation and energy loss mechanisms as well as the range of alpha daughter particles generated by the nuclear reaction between thermal neutrons and 10B atoms in hexagonal boron nitride (h-BN) thermal neutron detectors have been investigated via their responses to alpha particles from a 210Po source. The ranges of alpha particles in h-BN were found to be anisotropic, which increase with the angle (θ) between the trajectory of the alpha particles and c-axis of the h-BN epilayer following (cos θ)-1 and are 4.6 and 5.6 μm, respectively, for the alpha particles with energies of 1.47 MeV and 1.78 MeV at θ = 0. However, the energy loss of an alpha particle inside h-BN is determined by the number of layers it passes through with a constant energy loss rate of 107 eV per layer due to the layered structure of h-BN. Roughly 5 electron-hole pairs are generated when an alpha particle passes through each layer. It was also shown that the durability of h-BN thermal neutron detectors is excellent based on the calculation of boron vacancies generated (or 10B atoms consumed) by neutron absorption. The results obtained here provide useful insights into the mechanisms of energy loss and charge carrier generation inside h-BN detectors and possible approaches to further improve the overall performance of h-BN thermal neutron detectors, as well as the ultimate spatial resolution of future neutron imaging devices or cameras based on h-BN epilayers.