We investigated the scintillation characteristics, including the fluorescence decay time, energy resolution, and pulse height spectrum, of a Bi4Ge3O12 (BSO) crystal by using a 661-keV ??- ray source. The light response of the BSO crystal was measured for proton beams with energies in the energy range from 16 MeV to 36 MeV by using the MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS) with different thicknesses aluminum degraders. We determined the proton beam energy incident on the crystal after it had passed through the degraders and 200 cm of air by using a Monte Carlo simulation based on the Geant4 code. We show that the light response of the BSO crystal to the kinetic energy of the proton beam in the above energy range is approximately linear. We compared these results with our previous results obtained by using a Bi4Ge3O12 (BGO) crystal.