ENERGY DEGRADER

Disclosed embodiments include an energy degrader for attenuating the energy of a charged particle beam and comprising two energy attenuation members having different masses. The degrader further comprises a drive unit configured to move simultaneously the two energy attenuation members at respectively a first and a second speed across the particle beam during a first movement and to move the lightest of the two energy attenuation members at a third speed across the particle beam during a second movement, the third speed being higher than the first speed. More accurate and faster variation of the energy of the charged particle beam can hence be achieved.