Abstract: A device for varying the energy of a particle beam extracted from a fixed-energy particle accelerator includes a block of energy degrading material positioned in the path of the particle beam. The block of energy degrading material is preferably in the form of a ring arranged on a wheel. The ring is of a staircase configuration, having discrete steps defining a thickness between parallel entry and exit faces. According to one aspect of the invention, the block is configured so that the particle beam energy variation reaches a maximum at the edges of each step. This upper limit is also the lower limit of the next step. Thus, continuous energy variation is possible despite the fact that the thickness of the block varies in discrete steps.