The device (23) varies energy of a particle beam in two irradiation procedures in such a way that a target volume is irradiated layer-wise in layers that are spatially arranged one behind another. The device vary a sequence in which the layers of the target volume are irradiated from one irradiation procedure to another irradiation procedure in terms of a direction of incidence of the particle beam. A direction of incidence of the particle beam remains identical from the former irradiation procedure to the latter irradiation procedure in the two irradiation procedures. Independent claims are also included for the following: (1) a particle therapy unit comprising a particle source (2) a method for controlling an energy of a particle beam during an irradiation of a target volume.
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