Patent Number: 059404698
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the FIGURE, an x-ray source is shown having an inwardly hollow anode ring 1 that is provided with a hollow, inwardly projecting wedge ring 4 in a middle plane 3 perpendicular to the ring axis 2. In the wedge ring 4, a further wedge in turn projects as a liquid guide surface 5 in order to conduct coolant, which enters the anode ring via an inlet 6a and leaves it at an outlet 6b, past the inside surface of the wedge ring 4 where most of the heat due to the incident decelerated electrons is to be removed. Respective cathodes 7 and 8, each in the form of an annular helix, are arranged symmetrically relative to the rotational axis 2 above and below the middle plane 3. Focusing coils 9 and 10, and 11 and 12, respectively focus the electrons emanating from the cathodes 7 and 8 onto the ring regions of the ring surfaces 13 and 14 of the wedge ring. Dependent on whether the upper or lower electrode 7 or 8 is activated, only the upper or lower ring surface 13 or 14 is irradiated with electrons, so that x-ray bremsstrahlung proceeds only from that surface. This x-ray bremsstrahlung strikes a needle-shaped target 15 that is secured to the carrier of the upper cathode 7 symmetrically relative to the rotational axis 2. The target is divided in the middle plane 3, and the lower target half 15a is composed of a different material from the upper target half 15b. With the change of the cathodes 7 and 8, thus, either the target half 15b or the target half 15a is irradiated in alternation, so that the material-specific fluorescence radiation is respectively emitted and proceeds toward the exterior via the x-ray exit window 16. This x-ray exit window 16 is seated on a carrying tube 17 for the lower cathode 8 and the focusing coils 1 1 and 12 thereof, and projects into base opening 18 in a bottom plate 19 of the anode ring 1. In the illustrated exemplary embodiment, the wedge angle .alpha. of the wedge ring 4 is selected such that it is less than, or at most equal to, twice the heel angle of the wedge ring (see E. E. Christensen et al., "An Introduction to the Physics of Diagnostic Radiology", Lea & Febiger, Philadelphia, 1972, pages 13 and 14). By so doing, radiation is reliably prevented from proceeding from the upper ring surface onto the lower target part 15a or from the lower ring surface 14 onto the upper target part 15b. Alternatively, or in addition thereto, this risk could be precluded by bringing the tip 20 of the wedge ring extremely close to the target 15. In practice, however, this is generally prevented because the target is at cathode potential in order to prevent a thermal load due to back-scatter electrons, so that the wedge ring 4 and anode potential, of course, cannot be brought too close. The invention is not limited to the illustrated exemplary embodiment. As was already described in detail above, thus, the target, instead of being transversely divided in the plane 3, can be longitudinally divided in a plane proceeding through the symmetry axis 2. A division of the cathode into two parts lying at both sides of this separating plane must likewise then ensue in order to optionally irradiate the left or right half of the anode ring with electrons, and thus to trigger x-ray bremsstrahlung only at the irradiated half. The electrodes irradiate only one of the target halves at a time for generating a fluorescence radiation. It would also be possible to combine the two divisions with one another and to thus produce an x-radiator with four frequencies. Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.