Patent Number: 047679301
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS For the sake of clarity, the embodiments, which contain only parts essentially known per se, are depicted in a simplified manner. Throughout the drawings, like elements are designated with the same numerals. FIG. 1 shows from a LINAC a bending magnet 1 which sends an electron beam 2 through a window 3 along a beam axis 4. The beam has a diameter of about 1 millimeter and comprises electrons of about 10 MeV. The current intensity across the beam is highest at its center and decreases gradually towards its periphery. After leaving window 3, beam 2 passes a beam diffusing lens system 5, a passage way 27 of a shielding block 28, and a beam defining jaw system with two pairs of opposite jaws 29, 30, 31. Lens system 5 contains, as depioted in FIG. 2, a quadrupole magnet consisting of two horseshoe magnets 6, 7. The two horseshoe magnets are wrapped with coils 8 and 9, respectively, which are jointly connected--via a variable resistor 10--to a current supply 11. Both magnets 6, 7 are disposed in a X-Y plane perpendicular to the beam axis, with their poles arranged such that the north pole of one magnet is placed opposite the south pole of the other one. The magnetic field is zero at the beam axis 4, directed downward to the right of the beam axis and directed upward to the left of the beam axis so that all the electrons which are not on the Y-axis are deflected away from the beam axis. The result is a flattened beam as shown by a broken line 12. The distance between adjacent poles along the X-axis is small compared with the distance between opposite Poles along the Y-axis so that the magnetic field has actually no components along the X-axis. Therefore, the beam is neither focused nor defocused along the Y-axis. By adjusting the value of resistor 10 the current through the coils and thus the strength of the beam spreading magnetic fields may be varied. The beam emitted through window 3 is most intense at the beam axis 4; the intensity drops according to a Gaussian distribution toward the beam edge. This distribution should be changed by the lens system so that the fanned beam becomes more intensive with increasing distance from the beam axis 4. Only then can the circular area swept by rotating the fan axis around the beam axis 4, receive a uniform intensity without additional means. In order to reverse the original intensity distribution, the Y-component of the magnetic field must be attenuated with increasing distance X. The exact function is obtained by properly shaping and arranging the four magnetic poles. The lens system can be rotated around the beam axis 4, as indicated by an arrow 13. With this mechanical rotation, the fan axis revolves around the beam, so that after a half cycle, the fanned beam has covered the circular area. FIG. 3 shows another embodiment having no movable parts. Here, a lens system 14 is formed by three magnetic lenses. Each lens resembles the lens of the first embodiment, with two opposite horseshoe magnets 15, 16, 17, 18, 19, 20 and a coil 21, 22, 23, 24, 25, 26, wrapped around each magnet. All three lenses are arranged in a X-Y plane perpendicular to the beam axis, consecutive lenses being offset against each other by 120.degree. with respect to axis 4. The two coils of each lens are connected in parallel and jointly connected with one of the three terminals U, V, W of a conventional three-phase current supply, as shown in FIG. 4. In operation, the lens system creates in the beam area, a field pattern with distinct tangential components perpendicular to beam axis 4. These components diffuse the beam mainly along a fan axis, and this axis moves around the beam axis with the frequency defined by the current supply; after each third of the period the same field pattern, rotated by 120.degree. around the beam axis, is built-up. If the magnetic poles are properly formed and arranged as a function of the energy, profile and diameter of the electron beam and the frequency of the alternating current, even relatively large circular areas can be homogeneously irradiated, with built-up times less than a second. The diameter of the treatment field may be varied by adjusting the amplitude of the alternating current, and irregular fields can easily be produced by laterally introducing radiation absorbing sheets into the beam. Having thus described the invention with particular reference to preferred forms thereof, it will be obvious to those skilled in the art to which the invention pertains, after understanding the invention, that various changes and modifications may be therein without departing from the spirit and scope of the invention as defined by the claims appended hereto. For example, the flat beam may be generated by electric rather than magnetic fields or, if the lens system operates with lenses individually activated according to a specific function, activation pulses without overlap for consecutive lenses could be applied. Further, the beam might be spread such that it becomes broader rather than more intense with increasing distance from the beam axis. In some instances, it may be preferable to expand the pencil beam into one instead of two directions along the fan axis or to spread first one half of the beam in one direction and afterwards the other half of the beam into the opposite direction.