A chopper for a particle beam comprises at least one control element, which is divided into at least two regions A and B, wherein region B is less transparent to the particle beam than region A, and at least one drive source for moving the control element through the particle beam in such a way that the beam impinges on regions A and B in a chronologically alternating manner. The control element has a band-shaped design and is non-positively seated against the outer circumference of at least one element that can be caused to rotate by the drive source. It was found that the chopper can have a significantly more space-saving design when the control element is designed as a band-shaped element than as a wheel-shaped or ring-shaped chopper according to the known art. In particular, the drive source, which is bulky compared to the control element, can be disposed spatially separated from the beam path, while the band itself transmits the force of the drive source.