An energy degrader (1) for attenuating the energy of a charged particle beam (2) and comprising : a first energy attenuation member (10) presenting a beam entry face (11) having the shape of a part of a first helical surface, a second energy attenuation member (20) presenting a beam exit face (22) having the shape of a part of a second helical surface, said beam exit face being positioned downstream of said beam entry face with respect to the beam direction, and a drive assembly (M1, M2) for rotating the first and/or the second energy attenuation members about respectively a first and/or a second rotation axis (A1, A2) while crossed by the particle beam. The first and second helical surfaces are continuous surfaces and have the same handedness, thereby allowing to build a more compact degrader with a smaller moment of inertia. More accurate and faster variation of the energy of the beam can hence be achieved.
