Patent Publication Number: US-2013235977-A1

Title: Electromagnetic Scanning Apparatus for Generating a Scanning X-ray Beam

Description:
The present application claims the priority of U.S. Provisional Patent Application Ser. No. 61/607,232, filed Mar. 6, 2012, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates a source of scanned x-ray radiation, and, more particularly, to an apparatus for generating a scanned x-ray beam by electromagnetic scanning of a beam of charged particles with respect to a concave target surface. 
     BACKGROUND ART 
     Scanning x-ray beams generated by electromagnetically scanning a pencil beam of electrons over an anode have been envisioned from many years, though no commercial systems are yet available. All of the methods use the so-called transmission arrangement, exemplified in  FIG. 1  and described in U.S. Pat. No. 6,282,260 (to Grodzins, entitled “Unilateral Hand-Held X-ray Inspection Apparatus”), which is incorporated herein by reference. Beam  20  of electrons emitted by cathode  18  is accelerated toward target  22 , typically an anode, and referred to, hereinafter, as such. Electron beam  20  may be scanned with respect to anode  22  such that the orientation of beam  14  may be varied. In this example, the generated x-rays exit out of the thin, typically high-Z, anode into a conical enclosure, and only exit from an aperture at the apex of the cone. Other examples of scanning x-ray beams produced by scanning electron beams are listed below. In all cases the x-rays emanate in the forward hemisphere. 
     Other configurations of electromagnetically steered x-ray beams entailing a geometry based on bremsstrahlung emission in the forward direction are described, for example, in U.S. Pat. No. 6,421,420 (to Grodzins, entitled “Method and Apparatus for Generating Sequential Beams of Penetrating Radiation”) and U.S. Pat. No. 6,542,574 (to Grodzins, entitled “System for Inspecting the Contents of a Container”), both of which patents are incorporated herein by reference. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     In accordance with various embodiments of the present invention, an apparatus is provided for generating a scanned beam of penetrating electromagnetic radiation. The apparatus has a source for producing an electron beam characterized by a propagation direction and an anode for receiving the electron beam and emitting electromagnetic waves in response thereto. The apparatus also has an electromagnetic beam director for directing the propagation direction of the electron beam such that electrons impinge upon a succession of specified locations on the anode, and an exit aperture for emitting electromagnetic waves from the succession of specific locations on the anode, such that a direction of a beam of electromagnetic waves exiting from the aperture scans over a range of angles within a scan plane in response to angular scanning of the electron beam, wherein the scan plane is displaced from the propagation direction of the electron beam by at least 45 degrees. 
     In accordance with other embodiments, an apparatus for generating a scanned beam of penetrating electromagnetic radiation is provided that has a source for producing an electron beam, and an anode having a concave surface as viewed from the source, where the anode receives the electron beam and emits electromagnetic waves. An electromagnetic beam director directs the electron beam to a succession of specified locations on the anode, and electromagnetic waves are emitted via an exit aperture in direction that are scanned in response to angular scanning of the electron beam. 
     In any of the foregoing embodiments, the electromagnetic beam director may scan the electron beam within an electron beam plane. The exit aperture may lie within the electron beam plane in certain embodiments, although, in other embodiments, it may lie outside the electron beam plane. 
     In further embodiments of the invention, the apparatus may have multiple exit apertures. The electromagnetic beam director may be adapted to switch the electron beam in a lateral plane transverse to the electron beam plane. The apparatus may have a plurality of anodes, and a filter may be disposed within one or more exit aperture. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying figures, in which: 
         FIG. 1 , shows a prior art electronic beam scanner, as described U.S. Pat. No. 6,282,260. 
         FIG. 2  is a conceptual drawing of an electronic beam scanner having a “one-sided” reflection geometry in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic cross-section, as viewed from above, of a reflection-scanned x-ray beam system in accordance with an embodiment of the present invention, showing the plane of a resulting x-ray beam taking off at an angle of about 150° from the plane of the scanning electron beam. 
         FIG. 4  a schematic cross-section, as viewed from above, of a stereoscopic reflection-scanned x-ray beam system in accordance with an embodiment of the present invention, for generating two simultaneous scanning x-ray beams. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     In accordance with embodiments of the present invention, now described with reference to  FIG. 2 , a reflection geometry is employed to generate a scanning x-ray beam  217 . Electrons  201  derived from a cathode source  203  are accelerated toward an anode  205  in an electron beam  303  characterized by a propagation direction which is varied with time as described below. 
     In the embodiment of the invention depicted in  FIG. 2 , anode  205  has a concave surface as viewed from source  203 , such as a circular arc. However, it is to be understood that anode  205  may have any shape, within the scope of the present invention. X-rays  207 , generated by a bremsstrahlung process at anode  205 , are emitted in the back hemisphere  209 , exiting from an aperture  211  in that hemisphere. The present invention is described herein in terms of x-ray radiation for heuristic convenience and without limitation, although it is to be understood that any penetrating radiation derived in the bremsstrahlung process described is within the scope of the present invention. The reflection arrangement, shown in the drawings of  FIGS. 2 ,  3  and  4 , is versatile, with a number of advantages over the prior art transmission geometry represented in  FIG. 1 . 
     An embodiment of the present invention having a spherical surface of radial distance, R, (shown in  FIG. 2 ) between an electromagnetic beam director  213 , such as the scanning magnet shown (which may be referred to herein as scanning magnet  213 , or otherwise as a “sweeping magnet”) and anode  205 , eliminates complications otherwise encountered, in the case of a planar anode, in making a uniform focal spot  215  of electrons at all points as would be called for in the case of a planar anode. However, in other applications, anodes of flat, or other, shape may be preferred. 
     Focal spot  215 , where electrons of electron beam  303  impinges upon anode  205 , is the origin focal point of the sweeping x-ray beam  217 , and the dimensions of that focal point  215  are also independent of the sweep angle θ el . Sweeping x-ray beam  217  may be referred to herein as a “reflection-scanned x-ray beam.” In certain embodiments of the invention, electromagnetic beam director  213  sweeps electron beam  303  is a plane (in  FIG. 2 , the plane of the page), which may be referred to as the “electron beam plane.” 
     The nearly constant distance D from all points of the arc of anode  205  to the exit aperture  211 , produces a uniform sweeping x-ray beam  217  across a target (not shown). 
     Scanning electron beam  220  and scanning x-ray beam  217  occupy comparable volumes so that the size of the overall system can be smaller, and the shielding can be lighter, than in the traditional geometry. 
     The “plane” of the scanning electron beam  220  and the plane of the scanning x-ray beam  217  may be made no more than a few mm thick. (As used herein, the term “plane” may be used to represent the time-integral of the path of a swept beam. Insofar as the beam is not one-dimensional, but has a finite cross-section, the term “plane” has a finite thickness, although the thickness may be ignored for most descriptive purposes.) The plane in which x-ray beam  217  sweeps is referred to herein as the “scan plane.” 
     The sweeping magnet  213  may be disposed outside a vacuum space  230  within vacuum housing  235  enclosing the electron source  203  and anode  205 . There is considerable latitude for positioning the exit aperture  211 .  FIG. 3  shows one example where an exit aperture  301  is offset from a plane containing the sweeping electron beam  303 . Angle  300  refers to the angle between electron beam  303  and the direction at which x-ray beam  307  is taken off. Within the scope of the present invention, angle  300  includes angles that are greater than 45°. 
     In accordance with embodiments of the present invention, the electron focus and the magnetic sweep are under control of a processor  305  such that a desired sweep pattern can be preprogramed or changed under operator command. For example, the angular sweep of the x-ray beam  307  can be easily changed by changing the angular sweep of the electron beam  303 . 
     A true-focus system, in which the total x-ray flux on target remains constant as scan angle is changed, can be implemented by changing the distance D from anode  205  to exit aperture  301  while changing the size of the aperture appropriately. 
     Referring, now, to the cross-sectional view of  FIG. 4 , as seen from above, two (or more) similar scanning x-ray beams  401  and  403 , one on each side of the electron scanning plane, can be simultaneously used for stereoscopic imaging of transmitted or scattered x-rays. In accordance with other embodiments of the invention, electron beam  303  may, additionally, be switched in a lateral plane (in the plane of the cross-section shown in  FIGS. 2-4 ). By switching beam  303  laterally, and by disposing x-ray-opaque element  410  in the path of x-rays emitted by anode  205 , x-ray emission may be alternated temporally between beams  401  and  403 . 
     Additionally, in accordance with yet further embodiments of the invention, multiple anodes may be provided, thereby providing distinct spectral characteristics during periods which electron beam  303  dwells on respective anodes. Apertures  421  and  423  may contain filters (or, alternatively, filters may be provided within other portions of the respective x-ray beams) such that the energy spectra of respective beams  401  and  403 . (or portions thereof) may be tailored. 
     The x-ray-defining aperture  301  (shown, for example, in  FIG. 3 ), together with changeable filters and an x-ray shutter, may be inside or, in a preferred embodiment, placed outside the vacuum  230 . 
     For heuristic convenience, the invention is described herein, without limitation, in terms of the scanning a pencil beam of x-rays in a plane. The invention can also be applied, for example, to a two-dimensional scan, in a raster fashion, or otherwise. In a preferred two-dimensional embodiment, anode  205  is a segment of a hollow sphere. 
     The one-sided scanning system, designated generally by numeral  200  in  FIG. 2 , can be applied to a wide range of applications, from large systems that scan trucks with x-rays extending to hundreds of keV, to hand-held systems that scan with beams of less than 100 keV. For electron energies below a few hundred keV, the bremsstrahlung angular distribution is essentially isotropic from a target thick compared to the electron range. Model calculations show that, in the energy range of interest, the x-ray intensity in the 180° (back) direction is, in fact, greater than the x-ray intensity at 90°. 
     The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims. 
     Where examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objective of x-ray detection. Additionally, single device features may fulfill the requirements of separately recited elements of a claim. The embodiments of the invention described herein are intended to be merely exemplary; variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.