Patent Number: 060382856
Section: claims

1. A method for producing a monochromatic beam, the method comprising: generating a plurality of beams from a polyenergetic source;  transmitting the beams through a bent crystal having non-cylindrical shape;  positioning the bent crystal with respect to the polyenergetic source to emit a plurality of divergent monochromatic beams from the bent crystal wherein the bent crystal is differentially bent in a logarithmic spiral shape;  wherein the logarithmic spiral shape is achieved by mounting and differentially bending the crystal in a four-bar bender;  wherein the two outer bars of the four-bar bender each is moved a same distance with respect to a corresponding one of two inner bars of the four-bar bender to bend the crystal into a cylindrical shape and then the outer bars are differentially displaced with respect to each other;  wherein the outer bars are differentially displaced with respect to each other by displacing one of the outer bars by a differential .DELTA.z to at least partially unbend the bent crystal and further displacing another of the outer bars by the differential distance .DELTA.z to further bend the bent crystal. 2. A method according to claim 1 wherein the beams generated have a wavelength in a X-ray bandwidth. 3. A method according to claim 1 wherein the beams are first transmitted through a concave surface of the bent crystal and then through a convex surface of the bent crystal. 4. A method according to claim 1 wherein the different distance .DELTA.z bending is governed by: ##EQU1## where 2L.sub.c is a first distance between the two inner bars and L.sub.s is a second distance between one of the outer bars and a corresponding one of the inner bars. 5. A method according to claim 1 wherein the bent crystal is rocked in a plane of diffraction until the divergent monochromatic beams are emitted from the bent crystal. 6. A method according to claim 1 wherein a plurality of white beams are transmitted through the bent crystal and the divergent monochromatic beams are separated from the white beams by a fixed angle of diffraction. 7. A method according to claim 1 wherein the divergent monochromatic beams have a two-dimensional solid angle of at least about 5 degrees by at least about 5 degrees. 8. A method according to claim 1 wherein the divergent monochromatic beams are two-dimensional. 9. A method according to claim 1 wherein an energy bandwidth (.DELTA.E/E) of the divergent monochromatic beams is about 2%. 10. A method according to claim 1 wherein the divergent monochromatic beams are tuned to above and below a K-edge of a radiographic contrast element. 11. A method according to claim 10 wherein the tuned divergent monochromatic beams are used for dual energy digital subtraction radiography. 12. A method according to claim 1 wherein the divergent monochromatic beams are tuned to above a K-edge of a radiographic contrast element. 13. A method according to claim 12 wherein the tuned divergent monochromatic beams are used to enhance an image contrast. 14. A method according to claim 1 wherein the bent crystal is a bent Laue crystal. 15. A method according to claim 1 wherein the divergent monochromatic beams are transmitted through the bent crystal with asymmetric transmission geometry. 16. A method according to claim 1 wherein the polyenergetic source is an X-ray rotating anode source.