An energy degrading device for attenuating energy of a particle beam with reduced emittance growth. An energy degrader comprises an emittance control material that can preferentially scatter the beam particles that is incident on a surface with a shallow angle. In one approach, the energy degrader may include alternating layers of a low-Z and a high-Z material, wherein the low Z material serves to attenuate energy of the beam particles by virtue of scattering and the high Z material serves to suppress the emittance increase by scattering back the beam particles toward the beam axis. In another approach, the energy degrader may be composed of carbon nanotubes or a material with oriented crystalline structure that is substantially orientated in the incident direction of the particle beam. The carbon nanotubes may serve to preferentially scatter beam particles towards the central beam axis as well as attenuate energy thereof.
