Patent Number: 045340528
Section: claims

1. A block for partially limiting a radiation beam emitted by a radiation source in a given reference direction, for limiting a first part of the beam corresponding to a maximum half angle of opening, said block being formed by a limiting block having moving means for allowing said limiting block to move in a parallel direction to a first axis transverse to said reference direction, said limiting block comprising a cylindrical active surface which defines limits for the beam, said limits being a function of the position which said cylindrical active surface occupies along said first axis, said moving means allowing movement of said limiting block such that the orientation of the active surface is not modified and said limits are tangential to the cylindrical active surface for all positions of said limiting block. 2. The limiting block as claimed in claim 1, wherein said cylindrical active surface comprises an edge in the form of an arc of a circle forming a directrix, whose projection on a straight reference line corresponding to said reference direction represents a height of said active surface. 3. The limiting block according to claim 2, wherein said arc of a circle comprises a radius determined by the following relationship: EQU R=H/sin .alpha. R being the radius of the arc of a circle,  H being the height of the cylindrical active surface,  .alpha. being the angle representing the maximum half angle of opening of the beam.  R being the radius of the arc of a circle,  H being the height of the cylindrical active surface,  .alpha. being the angle representing the maximum half angle of opening of the beam. 4. The limiting block as claimed in claim 2, wherein said arc of a circle comprises a center O situated on an axis perpendicular to said reference direction and passing through the end of said arc of a circle the closest to said source. 5. The limiting block according to claim 4, wherein said arc of a circle comprises a radius determined by the following relationship: EQU R=H/sin .alpha. 6. In a collimator for delimiting a useful beam from a radiation beam, comprising a first limiting assembly having a first longitudinal axis transversal to the reference direction of the radiation beam, a second limiting assembly having a second longitudinal axis perpendicular to the first one and also transveral to the reference direction, these two assemblies being superimposed and centered on said direction, the first and the second limiting assemblies each comprise a first and a second limiting block such as claimed in one of the preceding claims, cooperating with paths of rectilinear movement, for allowing said limiting blocks to move along the longitudinal axis of the assembly to which they belong. 7. The collimator as claimed in claim 6, wherein said paths of movement cooperate with rolling means formed by rollers which said limiting blocks comprise so as to allow rectilinear movement of these blocks. 8. The collimator as claimed in claim 7, further comprising drive means cooperating with studs for securing said limiting blocks so as to ensure the motorized movements of said limiting blocks. 9. The collimator as claimed in claim 6, further comprising drive means cooperating with studs for securing said limiting blocks so as to ensure the motorized movements of said limiting blocks.