ABSORPTION IMAGING APPARATUS AND DETECTOR ARRAY THEREFOR

Abstract

An array (1) for detecting electromagnetic radiation is provided for a radiographic inspection system (20). The array has a plurality of detector elements (2) arranged consecutively along a scan line which extends in a first direction (Y). Each of the detector elements has a detection surface (3) for receiving electromagnetic radiation and converting the received electromagnetic radiation into a corresponding detection signal. Each detection surface (3) has a surface normal (4, N) that extends in a common plane (S) and converges into a common focus (5). The common plane (S) extends in the first direction (Y). The distances between the common focus and the detection surfaces along the respective surface normal (4, N) are different for at least two detector elements.

Claims

1. An array for detecting electromagnetic radiation, comprising: a plurality of detector elements arranged consecutively along a scan line which extends in a first direction for receiving electromagnetic radiation and converting the received electromagnetic radiation into a detection signal; wherein each detector element receives the electromagnetic radiation on a detection surface which has a plurality of surface normals, such that at least one surface normal of each detection surface extends in a common plane in the first direction and converges into a common focus; wherein at least two of the detector elements are arranged such that a distance from the common focus to a first of the at least two detection surfaces is different than a distance from the common focus to the second of the at least two detection elements, when each distance is measured along the surface normal of the respective detection surface.

2. The array of claim 1, wherein each of at least one pair of detector elements has an end surface, such that the end surfaces, in facing relationship, are aligned along a straight line that passes through the common focus.

3. The array of claim 2, wherein, for each of the detector elements, a neighboring detector element is positioned such that a distance from the common focus to the detection surface is different than a distance from the common focus to the detection surface of the neighboring detection element, when each measurement is made along the surface normal of the respective detection surfaces.

4. The array of claim 3, wherein, for each of the detection surfaces, the surface normal thereof that converges in the common focus passes through a centroid of the detection surface.

5. The array of claim 4, wherein the centroids of each of the detection surfaces are aligned along a straight line.

6. The detector array of claim 4, further comprising: an inspection plane for arranging an article to be inspected, the inspection plane located between the common focus and at least one pair of the detector elements, such that: a first of the detector elements has a surface normal through the centroid of the detector surface to the common focus that intersects the inspection place at a point A; a second of the detector elements has a surface normal through the centroid of the detector surface to the common focus that intersects the inspection plane at a point B; along the respective surface normals, point A is at a distance a from the common focus and at a distance a′ from the first detector, and point B is at a distance b from the common focus and at a distance b′ from the second detector, wherein a ratio a/a′ is equal to a ratio b/b′.

7. The array of claim 1, wherein at least one of the detection surfaces comprises a flat surface.

8. The array of claim 1, wherein at least one of the detection surfaces has a rectangular circumference.

9. The array of claim 1, wherein the detector elements are adapted to detect X-ray radiation.

10. The array of claim 1, wherein each of the detector elements comprises a stacked-type dual-energy electromagnetic radiation detection element that comprises a first detector member, adapted to detect radiation at a first energy level, and a second detector member, adapted to detect radiation at a second energy level that is higher than the first energy level, wherein the first detector member is positioned atop the second detector member to be closer to the common focus.

11. The array of claim 9, wherein each of the detector elements comprises a scintillator.

12. The array of claim 1, further comprising: a support, on which each detector element is detachably mounted.

13. The array of claim 1, wherein, for each of the detector elements, a neighboring detector element is positioned such that a distance from the common focus to the detection surface is different than a distance from the common focus to the detection surface of the neighboring detection element, when each measurement is made along the surface normal of the respective detection surfaces.

14. The array of claim 1, wherein, for each of the detection surfaces, the surface normal thereof that converges in the common focus passes through a centroid of the detection surface.

15. The detector array of claim 14, further comprising: an inspection plane for arranging an article to be inspected, the inspection plane located between the common focus and at least one pair of the detector elements, such that: a first of the detector elements has a surface normal through the centroid of the detector surface to the common focus that intersects the inspection place at a point A; a second of the detector elements has a surface normal through the centroid of the detector surface to the common focus that intersects the inspection plane at a point B; along the respective surface normals, point A is at a distance a from the common focus and at a distance a′ from the first detector, and point B is at a distance b from the common focus and at a distance b′ from the second detector, wherein a ratio a/a′ is equal to a ratio b/b′.

16. A system for radiographic inspection of an article to be inspected, comprising: a radiation source for producing a radiation beam that comprises a bundle of rays spanning a radiation plane, the radiation beam comprising a focal spot; a detector array according to claim 1; a transport means for transporting the article to be inspected along a transport path transverse to the radiation plane, the transport path being interposed between the radiation source and the detector array, wherein the detector array is arranged such that the common plane of the detector array coincides with the radiation plane, and the common focus coincides with said focal spot.

17. The system of claim 16, wherein the transport means comprises a conveyor belt.

18. The system of claim 16, wherein the radiation source comprises an X-ray source.

19. The system of claim 17, wherein the radiation source comprises an X-ray source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In the following description, the invention will be specified in greater detail by way of example with reference to the drawings. In the drawings,

[0037] FIG. 1 is a side view of a detector array according to the present invention, wherein a common focus of the detector array is arranged in the focal spot of a radiation source,

[0038] FIG. 2 is a similar view to the one shown in FIG. 1, which illustrates geometrical relationships of the detector array,

[0039] FIG. 3 is an enlarged view of region D in FIG. 2,

[0040] FIG. 4 is a perspective view of a radiographic inspection system according to the present invention,

[0041] FIG. 5 is a side view of a detector array and a radiation source according to the state of the art,

[0042] FIG. 6 is a perspective view of a radiographic inspection system according to the state of the art.

DETAILED DESCRIPTION

[0043] FIG. 1 is a side view of a detector array 1 according to the present invention. The detector array 1 comprises a plurality of detector elements 2 arranged consecutively along a scan line which extends in a first direction Y.

[0044] FIG. 1 further shows a radiation source 30 emitting electromagnetic radiation comprising a bundle of rays. In FIG. 1, the beam is in the form of a fan-shaped radiation beam 31, but the invention is not limited to this. The rays of the radiation beam span a radiation plane RP. In FIG. 1, the radiation plane RP coincides with the paper plane.

[0045] An article to be inspected may be transported through the radiation plane RP. To this end, a belt conveyor may be used. The belt conveyor may transport the article in an inspection plane 8 transverse, in particular perpendicular, to said radiation plane RP.

[0046] Each of said detector elements 2 has a detection surface 3 for receiving electromagnetic radiation and operative to convert the received electromagnetic radiation into a corresponding detection signal. Surface normals 4 of each of the detection surfaces 3 extend in a common plane S, said common plane S extending in said first direction Y. The surface normals 4 converge into a common focus 5. I. e., the surface normals 4 all lie within the common plane S. Each surface normal 4 defines a normal direction N. Unlike in an arc-shaped arrangement of the detection elements as it is known in the art, there is for each detector element 2 a neighboring detector element 2 such that the distances between the common focus 5 and detection surfaces 3 along the normal direction N are different for the two neighboring detector elements 2. I. e., if one defines as d1 the distance between a detection surface 3 of one of the two detector elements 2 and the common focus 5 along the surface normal N of the one detector element 2 converging in the common focus 5, and if one defines as d2 the distance between the detection surface 3 of the other one of the two neighboring detector elements 2 and the common focus 5 along the surface normal N of the other detector element 2 converging in the common focus, then d1 is different from d2. As it is immediately clear from FIG. 1, the height of the detector array 1, i. e. the extension of the detector array 1 in the common plane S, is limited by the length l of the detection surfaces 3 shown in FIG. 1. In this way a very compact arrangement of the detector elements, and thus a very compact detector array 1, can be realized. Furthermore, the detector array 1 allows for an easy mounting and/or replacement of individual detector elements 2.

[0047] Furthermore, as can be seen best in FIG. 3, which is an enlarged view of the region D of the embodiment shown in FIGS. 1 and 2, facing end surfaces 6 of neighboring detector elements 2 are aligned along a straight line L through the common focus 5. In this way, there is no gap between neighboring detector elements 2 in the first direction Y where radiation of the fan-shaped beam 31 is not detectable. This improves the detection efficiency of the detector array 1 compared to arrangements where there is a gap between the neighboring detector elements such that radiation is not received from either of the two neighboring detector elements 2.

[0048] Furthermore, in the embodiment shown in FIG. 1, there exists for each of said detection surfaces 3 a surface normal 4 at a centroid 7 of the respective detection surfaces 3, wherein said surface normal 4 converges in the common focus 5 (see the enlarged view of FIG. 3). The detection surfaces 3 may have a rectangular circumference and may be flat. Then, the centroid 7 corresponds to the center of the flat, rectangular detection surface. I. e., in the case shown in FIG. 1 where a focal spot 32 of the radiation source 30 coincides with the common focus 5 of the detector array 1, the radiation emitted in the fan-shaped radiation beam 31 is incident on the detection surfaces 3 of the detector elements 2 in the centroid 7 of the detection surfaces 3 at an angle of 90° (see FIG. 3).

[0049] Each of the detector elements 2 of the embodiment shown in FIG. 1 may comprise a stacked-type dual-energy X-ray detector element. This will be explained in more detail with reference to FIG. 3 below.

[0050] As mentioned above, the detector elements 2 may comprise detection surfaces 3 having a flat surface. Furthermore, said detection surfaces 3 may have a rectangular circumference. In this way, it may be particularly easy to arrange the detector elements 2 in the detector array 1.

[0051] In absorption imaging, a region of a size s of an article to be inspected is represented as an image on the detector array, wherein the size s′ of the image of the region is larger than the size s of the region of the article. This effect is called magnification, and s′/s is called the magnification factor. A problem of state of the art detector arrays is a non-constant magnification factor. I.e., two regions of the same size of the article to be inspected are represented as images of different size at the detector. This problem may be overcome by the detector array of the present invention. To this end, it is referred to FIG. 2 which is a similar view to the one shown in FIG. 1, which illustrates geometrical relationships of the detector array 1. Let A and B denote two points, each of which being located at an intersection of a surface normal 4 at a centroid 7 of a detection surface 3 of first and second detector elements 2 and the inspection plane 8. The distance between the common focus 5 and the point A will be denoted as a, the distance between the common focus 5 and the point B will be denoted as b, the distance between the point A and the first detector element of the detector array 1 along the surface normal 4 will be denoted as a′, and the distance between the point B and the second detector element of the detector array 1 along the surface normal 4 will be denoted as b′. Then, as all centroids 7 of the detector elements 2 are arranged on a single straight line G, it follows from the intercept theorem that the distance a/a′=b/b′ is constant. Due to this relationship, a more constant magnification factor can be guaranteed. I. e., a region of size s of an article in the vicinity of point A and a region of size s of an article in the vicinity of point B on a conveyor belt in the inspection plane 8 will be enlarged to an image of (approximately) the same size at the respective detector elements 2 of the detector array 1. Preferably, the distance ratio defined above is the same for each pair of detector elements 2.

[0052] FIG. 3 is an enlarged view of the region D in FIG. 2. FIG. 2 depicts a detector element 2 comprising a stacked-type dual-energy X-ray detector element. Each of the detector elements 2 comprises a first detector member 2a and a second detector member 2b stacked on top of each other. The first detector member 2a faces the radiation source 30. The first detector member 2a may be adapted to detect radiation at an energy E1, and the second detector member 2b may be adapted to detect radiation at an energy E2, wherein E2 is bigger than E1. I. e., the first detector member 2a may be considered as a low-energy detector, and the second detector member 2b may be considered as a high-energy detector. As the first and second detector members 2a, 2b are stacked on top of each other, a ray 33 of the radiation beam 31 is incident at a centroid 7 of the detection surfaces 3 of the first and second detector members 2a, 2b at an angle of 90°. There may be a distance between the first and second detector members 2a, 2b. Thus, the problem of the prior art where a ray incident on an upper detector member of a detector element is not incident on a lower detector member of the same detector element but on a neighboring detector element is prevented with a detection array according to the present invention. In this way, the resolution of the detector array 1 is improved compared to arrangements in the state of the art where the detection surfaces are arranged on a straight line.

[0053] FIG. 4 is a perspective view of a radiographic inspection system 20 according to the present invention. The radiographic inspection system 20 comprises a radiation source 30. The radiation source 30 may be an X-ray source. The radiation source produces a fan-shaped radiation beam 31, wherein the rays of said radiation beam span a radiation plane RP. The fan-shaped radiation beam 31 comprises a focal spot 32.

[0054] The radiographic inspection system 20 further comprises a detector array 1 according to the present invention. The detector array 1 may be the detector array 1 shown in FIGS. 1 and 2.

[0055] Furthermore, the radiographic inspection system 20 comprises transport means 40 for transporting an article 50 to be inspected along a transport direction T transversely, in particular perpendicularly, to said radiation plane RP. The transport means 40 comprises a conveyor belt 41 to transport the article 50 along the transport direction T. The article is transported in the inspection plane 8. The transport path T is interposed between the radiation source 30 and the detector array 1. The detector array 1 is arranged such that the common plane S coincides with the radiation plane RP. Furthermore, the common focus 5 of the detector array 1 mentioned above coincides with the focal spot 32 of the radiation source 30. The radiographic inspection system 20 shown in FIG. 4 is very compact, as the detector array 1 is very compact. Furthermore, the detector array 1 according to the present invention allows for high precision imaging of the article 50 to be inspected. The radiographic inspection system 20 may be used for X-ray absorption imaging.