[en] The invention concerns an electron accelerator with a target exposed to the electron beam, with an electron absorber topped downstream the target in direction of the beams, and with a collimator for localization of the X-ray bekam cone. With such electron accelerators the radiation exposure of the patient is, in addition to the therapeutically desired X-ray quanta, also increased by neutrons in a most undesired way. For reduction of the neutron level the invention provides for the rim zone of the collimatior turned towards the target being made of a material with a small effective cross-section for (ß, n)-processes. For dimensioning of this rim zone made of a material with a small effective cross-section for (ß-n)-processes the invention provides that this rime zone arranged radially to the target has a dimension which approximately corresponds to the half-value depth of the X-ray radiation in that material. (orig.)