Abstract: A chopper for a particle beam comprises an annular guiding element and an element for controlling the intensity of the particle beam. The control element is supported on the guiding element so that at least one point under consideration on the control element can revolve along the circumference of the guiding element. Mounting along a circumference allows for accommodation of considerably higher disturbance torque than mounting on a rotational axle, using the same bearing force. Furthermore, it is possible to dispense with the entire rotational axle, and the control element can be designed, for example, as a ring. This brings about considerable weight savings as compared to chopper wheels according to the prior art, which accordingly enables higher circumferential speeds and therefore higher modulation frequencies for the particle beam, while at the same time increasing operational safety.