Patent Number: 050383701
Section: summary

BACKGROUND OF THE INVENTION The invention relates to an arrangement for generating an X-ray or gamma beam with small cross-section and variable direction, having an X-ray or gamma emitter, from the focus of which a bundle of rays emerges, and a diaphragm arrangement, which cuts out a beam from the bundle of rays and comprises a rotatable hollow-cylindrical first diaphragm body having two mutually offset helical slits on the circumference. Of interest is commonly owned copending application entitled "Device for Forming an X-ray or Gamma Beam of Small Cross-Section and Variable Direction" Ser. No. 400,188 filed Aug. 29,1989 in the name of G. Harding. Arrangements of this type are essentially known from European laid-open patent application 74,021 for medical applications and from German Offenlegungsschrift 3,443,095 corresponding to U.S. Pat. No. 4,750,196 for industrial applications. The diaphragm body of a radiation-absorbing material has in this case the form of a hollow cylinder which is provided on its circumference with two mutually offset helically encircling slits. If a bundle of parallel rays falls onto such a diaphragm body perpendicularly to its cylinder axis, there is always a point at which an X-ray beam passes through the two slits. If the diaphragm body is turned, this point shifts along the axis, so that a periodically moved X-ray beam emerges behind the diaphragm body. This periodically moved X-ray beam can be used for medical or industrial examinations. An X-ray beam with trapezoidal cross-section is defined by the two slits in the diaphragm body. What is desired, however, is a square or a circular cross-section, producing a directionally independent spatial resolution. With the same width of the two slits, the approximation to a square cross-sectional shape is all the better the larger the angle by which the two slits intersect each other. A larger angle of intersection could be achieved by using a diaphragm body with large diameter and small axial length. For many applications, however, a relatively large angle of deflection of the X-ray beam is necessary, which necessitates a corresponding axial length of the diaphragm body; a large diameter is undesirable in many applications due to the associated unit volume. SUMMARY OF THE INVENTION The object of the present invention is to design an arrangement of the type mentioned at the beginning in such a way that a favorable beam cross-section is achieved even in the case of a diaphragm body with small diameter and relatively large axial length. This object is achieved according to the invention by the fact that the slits wind around the diaphragm body in at least one turn each and are shaped in such a way that at least one straight line runs through the slits towards the focus, the position of which line can be varied by turning the diaphragm body. Thus, while in the prior art the two slits extend over an angle at circumference of 180.degree. or have only half a turn, the slits in the invention extend over an angle at circumference of at least 360.degree. or they have at least one turn (one turn corresponds to an angle at circumference of 360.degree..) The projection of the slits onto the axis of rotation or symmetry of the hollow-cylindrical diaphragm bodies therefore forms a considerably larger angle with the axis concerned, so that the X-ray beam cut out with a given slit width has considerably smaller dimensions in the direction of the said axis. With the arrangement according to the invention, as many X-ray beams are generated as there are straight lines which pass through the slits and impinge on the focus. In many applications, however, for example those in which the scattered radiation produced by the X-ray beam is to be measured, one wishes to work just with a single X-ray beam. In a development of the invention it is therefore envisaged that a second diaphragm body which only ever allows through a primary beam is arranged in the bundle of rays, and that the second diaphragm body is arranged and designed in such a way that the primary beam always coincides with one of the straight lines. In a preferred development, it is envisaged that the second diaphragm body has the form of a hollow cylinder, the axis of which lies in the plane containing the axis of symmetry and the focus and the cross-section of which is circular or semicircular and that the second diaphragm body is provided with one slit if of semicircular cross-section or with two helical slits mutually offset by 180.degree. on the circumference if of circular cross-section. If in this case the first diaphragm body is driven faster by a factor of 2n (n is an integer) than the second, an X-ray beam which moves periodically can be cut out. If the diaphragm arrangement is to form a spatially compact unit together with the X-ray or gamma emitter, the diameter of the diaphragm body is no longer negligible in comparison with its distance from the focus, so that an X-ray beam with larger axial distance emerges from the center of the diaphragm body than the beam which enters it. In order to satisfy these geometrical conditions, a further development of the invention envisages that the slits of the first diaphragm body have pitches differing from each other. In that case, the X-ray beams can only ever enter through one slit and emerge through the other slit. In a further development it is envisaged in this case that, of the slits in the first diaphragm body, the one with the greater pitch is narrower than the other one and that on the side of the first diaphragm body facing away from the focus a slit diaphragm is provided, the slit-shaped aperture of which lies in the plane formed by the focus and the axis of symmetry of the first diaphragm body. In this configuration, the dimension of the X-ray beam in the direction of the axis of symmetry is determined by the narrower of the two slits and its direction perpendicular thereto is determined by the aperture in the slit diaphragm.