An electron accelerator comprising: (a) a resonant cavity consisting of a hollow closed conductor comprising: • an outer wall comprising an outer cylindrical portion having a central axis, Zc, and having an inner surface forming an outer conductor section, and, • an inner wall enclosed within the outer wall and comprising an inner cylindrical portion having the central axis, Zc, and having an outer surface forming an inner conductor section, the resonant cavity being symmetrical with respect to a mid-plane, Pm, normal to the central axis, Zc, and intersecting the outer cylindrical portion and inner cylindrical portion, (b) an electron source adapted for radially injecting a beam of electrons into the resonant cavity, from an introduction inlet opening on the outer conductor section to the central axis, Zc, along the mid-plane, Pm, (c) an RF system coupled to the resonant cavity and adapted for generating an electric field, E, between the outer conductor section and the inner conductor section, oscillating at a frequency (f RF), to accelerate the electrons of the electron beam along radial trajectories in the mid-plane, Pm, extending from the outer conductor section towards the inner conductor section and from the inner conductor section towards the outer conductor section, (d) at least one magnet unit comprising a deflecting magnet composed of first and second magnets positioned on either side of the mid-plane, Pm and adapted for generating a magnetic field in a deflecting chamber in fluid communication with the resonant cavity by at least one deflecting window, the magnetic field being adapted for deflecting an electron beam emerging out of the resonant cavity through the at least one deflecting window along a first radial trajectory in the mid-plane, Pm, and to redirect the electron beam into the resonant cavity through the at least one deflecting window or through a second deflecting window towards the central axis along a second radial trajectory in the mid-plane, Pm, said second radial trajectory being different from the first radial trajectory, wherein the resonant cavity is formed by: • a first half shell, having a cylindrical outer wall of inner radius, R, and of central axis, Zc, • α second half shell, having a cylindrical outer wall of inner radius, R, and of central axis, Zc, and • a central ring element of inner radius, R, sandwiched at the level of the mid-plane, Pm, between the first and second half shells, wherein the surface forming the outer conductor section is formed by an inner surface of the cylindrical outer wall of the first and second half shells, and by an inner edge of the central ring element, and characterised in that a portion of the central ring element extends radially beyond an outer surface of the outer wall of both first and second half shells, and wherein the at least one magnet unit is fitted onto said portion of the central ring element. (a) a resonant cavity consisting of a hollow closed conductor comprising: • an outer wall comprising an outer cylindrical portion having a central axis, Zc, and having an inner surface forming an outer conductor section, and, • an inner wall enclosed within the outer wall and comprising an inner cylindrical portion having the central axis, Zc, and having an outer surface forming an inner conductor section, the resonant cavity being symmetrical with respect to a mid-plane, Pm, normal to the central axis, Zc, and intersecting the outer cylindrical portion and inner cylindrical portion, • an outer wall comprising an outer cylindrical portion having a central axis, Zc, and having an inner surface forming an outer conductor section, and, • an inner wall enclosed within the outer wall and comprising an inner cylindrical portion having the central axis, Zc, and having an outer surface forming an inner conductor section, (b) an electron source adapted for radially injecting a beam of electrons into the resonant cavity, from an introduction inlet opening on the outer conductor section to the central axis, Zc, along the mid-plane, Pm, (c) an RF system coupled to the resonant cavity and adapted for generating an electric field, E, between the outer conductor section and the inner conductor section, oscillating at a frequency (f RF), to accelerate the electrons of the electron beam along radial trajectories in the mid-plane, Pm, extending from the outer conductor section towards the inner conductor section and from the inner conductor section towards the outer conductor section, (d) at least one magnet unit comprising a deflecting magnet composed of first and second magnets positioned on either side of the mid-plane, Pm and adapted for generating a magnetic field in a deflecting chamber in fluid communication with the resonant cavity by at least one deflecting window, the magnetic field being adapted for deflecting an electron beam emerging out of the resonant cavity through the at least one deflecting window along a first radial trajectory in the mid-plane, Pm, and to redirect the electron beam into the resonant cavity through the at least one deflecting window or through a second deflecting window towards the central axis along a second radial trajectory in the mid-plane, Pm, said second radial trajectory being different from the first radial trajectory, • a first half shell, having a cylindrical outer wall of inner radius, R, and of central axis, Zc, • α second half shell, having a cylindrical outer wall of inner radius, R, and of central axis, Zc, and • a central ring element of inner radius, R, sandwiched at the level of the mid-plane, Pm, between the first and second half shells,