The irradiation with energetic proton beams impinging normal to the axis of a multiwalled carbon nanotube (MWCNT) is studied both experimentally and theoretically, in the 2-10 keV energy range. The MWCNTs are dispersed on top of a very thin film of holey amorphous carbon (a-C) substrate. Measurements of the proton energy loss distribution are performed after MWCNT irradiation with energetic proton beams by the transmission technique. The resulting energy loss spectra in the forward direction show two well differentiated peaks, whose origin is elucidated by using a semi-classical simulation of the proton trajectory through the nanotube. We found that the experimental lower-energy loss peak mostly comes from protons travelling between the outer walls of the MWCNT, whereas the high-energy loss peak is mainly due to protons that interact with the MWCNTs just on top of the supporting a-C substrate. (C) 2012 Elsevier Ltd. All rights reserved.