It is desirable to achieve a co-incident investigative kV source for a therapeutic MV source - a so-called "beams-eye-view" source. It has been suggested that bremsstrahlung radiation from an electron window be employed we propose a practical structure for achieving this which can switch easily between a therapeutic beam and a beam-eye-view diagnostic beam capable of offering good image resolution. Such a radiation source comprises an electron gun, a pair of targets locateable in the path of a beam produced by the electron gun, one target of the pair being of a material with a lower atomic number than the other, and an electron absorber insertable into and withdrawable from the path of the beam. In a preferred form, the electron gun is within a vacuum chamber, and the pair of targets are located at a boundary of the vacuum chamber. The lower atomic number target can be Nickel and the higher atomic number target Copper and/or Tungsten. The electron absorber can be Carbon, and can be located within the primary collimator, or within one of a plurality of primary collimators interchangeably locateable in the path of the beam. Such a radiation source can be included within a radiotherapy apparatus, to which the present invention further relates. A flat panel imaging device for this source can be optimised for low energy x-rays rather than high energy Caesium Iodide-based panels are therefore suitable.Il est souhaitable de parvenir à une source kV dinvestigation pour une source MT thérapeutique – une source dite « beams eye view » (fonction « vue de la source »). Il a été suggéré quun rayonnement de freinage provenant dune fenêtre délectron soit utilisé la présente invention propose une structure pratique pour parvenir à cela qui peut commuter facilement entre un faisceau thérapeutique et un faisceau de diagnostic beams eye view capable doffrir une bonne définition dimage. Une telle source de rayonnement comprend un canon à électrons, une paire de cibles pouvant être sit
