An energy degrader (1) for attenuating the energy of a charged particle beam (2), comprising a first energy attenuation member (10) presenting a annular beam entry face (11) and a continuous helical beam exit face (12), a second energy attenuation member (20) presenting a continuous helical beam entry face (21) and an annular beam exit face (22), the first and second helical surfaces facing each other, and a drive assembly configured for rotating the first and/or the second energy attenuation members around respectively a first axis (A1) and a second axis (A2) which are parallel to each other. Such a degrader can have a small moment of inertia, allowing more accurate and faster variation of the beam energy.
