Patent Number: 
Section: description

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 illustrates an exemplary x-ray inspection apparatus 10 constructed in accordance with the present invention. The apparatus 10 comprises several components disposed sequentially along a central beam axis, denoted A (which is parallel to the x-axis of the overall apparatus), including a high energy X-ray source 12, a pre-target collimator 14, a target support structure 16 which supports a target 18, and a detector assembly 20. The source 12, pre-target collimator 14, and detector assembly 20 are suspended from a support structure such as a gantry 13 illustrated in FIG. 1 including a horizontal member 15 which may be raised or lowered in a known fashion to move the X-ray components in the vertical or z-direction relative to the target 18. All three of these components may also be moved individually along the x-axis of the apparatus 10 by known means, as indicated by the arrows in FIG. 1. The X-ray source 12 may be any known X-ray source which is capable of producing X-rays having the energy level required for the particular application. The X-ray source 12 includes a first collimator 22, for example a lead slit-type collimator having an aperture 24 (see FIG. 6) which limits the vertical dispersion of the beam. In the illustrated example the aperture 24 has a vertical dimension (height) of about 3 mm (0.12 in.), and a horizontal dimension (width) of about 100 mm (3.9 in.) One suitable X-ray source is a Linatron M6 linear accelerator of 6 MeV output, available from Varian Industrial Products, 3100 Hansen Way, Palo Alto, Calif., 84104 USA. In an exemplary embodiment, the focal spot of the source 12 is positioned about 2.4 m (96 in.) away from the detector (described below). Referring to FIGS. 2, 3, and 4, a pre-target collimator is shown in detail. The pre-target collimator 14 comprises a plate-like body 28 having an opening 30 formed therethrough. A pair of end pieces 32 (see FIG. 3) are attached to each end of the body 28, for example with fasteners 34. The end pieces include a means for attaching the pre-target collimator 26 to a support structure 36, for example with studs 38. A pair of collimator jaws 40 is mounted to the front surface 42 of the body 28. The collimator jaws 40 include a pair of parallel spaced apart bars 44 which each carry a jaw piece 46. The generally rectangular space between the jaw pieces 46 defines the aperture 48 of the pre-target collimator 14. The jaw pieces 46 are constructed of a radio-opaque material, such as tungsten, and have a length sufficient to stop the beam in the x-direction, for example about 7.62 cm (3 in.). The bars 44 are held in parallel, movable relationship to each other by a pair of pivoting links 50, which are attached to the collimator body 28 and the ends of the bars 44 by pivot pins 52. One of the links 50 has an extended arm 54 which is connected to a controllable motor 56 with a pivot pin 50 and a threaded rod 58. This arrangement allows the vertical dimension (height) of the aperture 48 to be adjusted to suit a particular application by operating the motor 56, which causes pivoting of the links 50 which in turn causes the jaw pieces 46 to move towards or away from each other. The operating mechanism of the pre-target collimator 14 includes suitable known means for providing feedback to the means (not shown) used to control the motor 56. For example, a position sensor 57, such as an LVDT, may be mounted on the upper bar 44, with its moveable probe or rod 59 projecting through a hole in the upper jaw piece 46 and contacting the lower jaw piece 46. The output of the position sensor provides a direct measurement of the gap between the jaw pieces 46, and allows control of the aperture 48 independent of any excess motion in the moving parts between the motor 56 and the jaw pieces 46. In the illustrated example the aperture 48 may be adjusted from approximately 0 mm (0 in.), that is, completely closed, to approximately 6 mm (0.24 in.). The jaws 46 have a width extending in the y-direction (perpendicular to both the x-axis and the z-axis) a distance sufficient to encompass the beam spread at the location of the pre-target collimator 14. In the illustrated example the jaws 36 are approximately 66 cm (26 in.) wide. The target support structure 16 provides means for supporting and manipulating the target 18. The exemplary support structure 16 illustrated in FIG. 1 comprises a turntable 60 which is powered so as to be able to rotate the target 18. The target 18 is mounted to the turntable with suitable tooling 64, such as a pedestal as shown in the illustrated example. The tooling 64 incorporates known means for securing the target 18, such as clamps or fasteners (not shown). If required, the support structure 16 may also include known means for manipulating the target 18 in other ways, for example rotating the target 18 about other axes than that of the turntable 60, or by moving the target 18 in the x-, y-, or z-axes. The detector assembly 20 includes an X-ray detector 19, for example a linear array detector 19, and a post-target collimator 21. The post-target collimator is of a known type generally comprising an array of radio-opaque plates arranged to collimate the beam in-plane (i.e. horizontally, or perpendicular to the direction of the first collimator 22 and pre-target collimator 14). FIGS. 5 and 6 illustrate the operation of the X-ray inspection apparatus 10 in comparison to that of a prior art system. Referring to FIG. 5, a prior art X-ray inspection system 210 includes a source 212 having a vertical collimator 214. In operation the source 212 produces a fan shaped X-ray beam 300 which diverges as it travels toward a target 18, the centerline of which is denoted B in FIG. 5. This divergence causes the target 18 to be illuminated by X-rays over a zone having a dimension in the z-direction, denoted H1 in FIG. 5. Unfortunately, the z-dimension (height) of the desired inspection zone, denoted H2, is much smaller, for example as small as about 0.5 mm (0.02 in.) The stray radiation outside of the desired inspection zone causes scatter of the X-rays, which degrades both contrast resolution and measurement accuracy of the inspection system. FIG. 6 illustrates the operation of the X-ray inspection apparatus 10 of the present invention. A x-ray beam 100 is generated in an X-ray source 12. The X-ray beam 100 propagates from a focal point 110 and passes through the horizontal slit aperture 24 of the first collimator 22. The beam 100 then diverges as is travels towards the target 18 along the central beam axis A. The beam 100 subsequently passes through the aperture 48 of the pre-target collimator 14, which is placed as close as physically possible to the target 18. Because the distance from the pre-target collimator 14 to the target 18 is minimized, the portion of the target 18 exposed to the X-ray beam measured in the z-direction, denoted H3 in FIG. 6, is substantially equal to the height H2 of the desired inspection zone, thus eliminating stray radiation and scattering. In the illustrated example, the pre-target collimator 14 is positioned about 2.5 cm (1 in.) away from the target 18. In comparison, in a similar prior art application not having the pre-target collimator 14, the target 18 is located about 125 cm (49 in.) away from the source collimator. These dimensions are of course merely representative and can be varied to suit a particular application. The important consideration is to locate the pre-target collimator 714 so that it is as close as possible to the target 18 without physically interfering with manipulation of the target 18. The arrangement of components of the present invention minimizes the collimation requirements at the x-ray source 12 and completely eliminates the need for vertical collimation between the target 18 and the detector assembly 20. In addition, the improved imaging performance of the x-ray inspection apparatus 10 of the present invention enables it to be effectively applied for such applications as composite material and part inspection, reverse engineering of complex parts and assemblies, high resolution x-ray metrology, and first article inspection and validation. An apparatus constructed in accordance with the present invention has shown improvements of as much as 70% in contrast resolution and as much as 40% in measurement accuracy over prior a prior art system, without reducing inspection speed. The foregoing has described an X-ray inspection system having an X-ray source and first and second collimators, wherein the first and second collimators are arranged in relation to the source and the target such that the portion of the target actually illuminated by The X-ray beam is substantially equal to the size of a selected inspection zone. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.