Abstract:
Disclosed is a detector array comprising a first linear array for detecting a first ray and a second ray which penetrate through a first plurality of parts of the inspected object to acquire first values and second values for the first plurality of parts, wherein the second ray is alternately emitted with the first ray; and a second linear array arranged parallel to the first linear array for detecting the first ray and the second ray which penetrate through a second plurality of parts of the inspected object to acquire third values and fourth values for the second plurality of parts, wherein the first plurality of parts is partly identical to the second plurality of parts. With the detector array, the efficiency and material discrimination accuracy can be improved in the scanning inspection of the inspected object by use of alternate dual-energy rays.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a detector for radiograph imaging of an object, and more particularly, to a detector array and a device using the same, which can eliminate error and inaccuracy of material discrimination at the edge occurring during the process of the object inspection using alternately generated rays, and can improve the efficiency of scanning inspection by multiples. 
       BACKGROUND OF THE INVENTION 
       [0002]    As the requirement for a security inspection system such as at Customs is constantly increased, the relevant technology has been widely applied as in U.S. Pat. No. 5,044,002, in which X-rays having two different energy levels are utilized to perform non-destructive inspection on an object while the material of the object is identified. Recently, the dual-energy method is resumed to implement material identification within high-energy range (&gt;1 MeV) in the non-destructive inspection of large-sized objects, as disclosed in U.S. Pat. No. 5,524,133. 
         [0003]    The physical principle of the dual-energy method for discriminating material is that when two X-ray beams having different energy levels interact with the same object, since the photon energy levels of the two beams are different from each other, there exists a difference between their interactions with the object. Such difference as a whole can simply be represented by the difference in attenuation index. Based on such principle various methods of alternately generating X-rays having two energy levels have been proposed, such as in U.S. Pat. No. 6,069,936 and international application WO 00/43760 there is disclosed a single radiation source which modulates a high energy spectrum by means of material absorption. In addition, international application WO 2004/030162 A2 discloses a method of alternately generating X-rays having high and low energy spectra by an accelerator. However, when the alternately generated X-rays having high and low energy spectra are utilized to scan an object, a severe defection occurs as follows. Since the X-rays having two energy levels are alternately generated at certain frequency, there is certain time interval between the generation of each ray. The inspected object always move at certain speed, and thus it will move by some distance during the time interval between the generation of X-rays having high and low energy levels. Therefore, when used to scan the inspected object (e.g., luggage, container, etc.), the interactions between the two kinds of X-rays and the object are not completely identical. This will have a negative impact on the discrimination accuracy, especially at the edge of the inspected object where the rays having two energy levels may interact with different objects, thereby incurring a false discrimination result. Meanwhile, in order to suppress the error due to rays having high and low energy levels interacting with different positions, the conventional method is to slow down the movement of the inspected object. This method severely limits the efficiency of object inspection and can&#39;t solve the false discrimination occurring at the edge of the object. 
       SUMMARY OF THE INVENTION 
       [0004]    In view of the above problem with the prior art, the present invention is accomplished. It is an object of the present invention to provide a detector array and a material discrimination system using this detector array, which can suppress discrimination errors occurring at the edge of the inspected object during the process of object inspection utilizing rays alternately generated. 
         [0005]    At the first aspect of the present invention, there is provided a detector array comprising: a first linear array for detecting a first ray and a second ray which penetrate through a first plurality of parts of the inspected object to acquire first values and second values for the first plurality of parts, wherein the second ray is alternately emitted with the first ray; and a second linear array arranged parallel to the first linear array for detecting the first ray and the second ray which penetrate through a second plurality of parts of the inspected object to acquire third values and fourth values for the second plurality of parts, wherein the first plurality of parts is partly identical to the second plurality of parts. 
         [0006]    According to an embodiment of the present invention, the first linear array is arranged closely to the second linear array. 
         [0007]    According to an embodiment of the present invention, the distance between the first linear array and the second linear array is adjustable. 
         [0008]    According to an embodiment of the present invention, the distance depends on the moving speed of the inspected object and the time interval between the alternate generation of the first ray and the second ray. 
         [0009]    According to an embodiment of the present invention, each detector element of the first linear array and the second linear array comprises a scintillator or a gas detector. 
         [0010]    According to an embodiment of the present invention, the first linear array and the second linear array are both connected to the same processing apparatus. 
         [0011]    According to an embodiment of the present invention, each of the first linear array and the second linear array is connected to respective processing apparatuses. 
         [0012]    According to an embodiment of the present invention, the first ray and the second ray are generated by the same radiation source. 
         [0013]    According to an embodiment of the present invention, the scintillator is CdWO 4  or Csl. 
         [0014]    In addition, the present invention further provides a device comprising the detector array described above. 
         [0015]    The present invention further provides a radiograph imaging method utilizing the detector array described above. 
         [0016]    The present invention further provides a material identification method utilizing the detector array described above. 
         [0017]    At another aspect of the present invention, there is provided a method of detecting rays with a detector array comprising a first linear array and a second linear array arranged parallel to each other, the method comprises the steps of: generating alternately a first ray and a second ray to penetrate through an inspected object moving at a fixed speed; detecting the first ray and the second ray which penetrate through a first plurality of parts of the inspected object with the first linear array, to acquire first values and second values for the first plurality of parts, wherein the second ray is alternately emitted with the first ray; and detecting the first ray and the second ray which penetrate through a second plurality of parts of the inspected object with the second linear array, to acquire third values and fourth values for the second plurality of parts, wherein the first plurality of parts is partly identical to the second plurality of parts. 
         [0018]    According to an embodiment of the present invention, the method further comprises adjusting the distance between the first linear array and the second linear array based on the moving speed of the inspected object and the time interval between the alternate generation of the first ray and the second ray. 
         [0019]    At another aspect of the present invention, there is provided a detector array comprising: a first linear array for detecting a first ray, a second ray and a third ray which penetrate through a first plurality of parts of the inspected object to acquire first values, second values and third values for the first plurality of parts, wherein the first ray, the second ray and the third ray are alternately emitted; a second linear array arranged parallel to the first linear array for detecting the first ray, the second ray and the third ray which penetrate through the second plurality of parts of the inspected object to acquire fourth values, fifth values and sixth values for the second plurality of parts, wherein the first plurality of parts are partly identical to the second plurality of parts; and a third linear array arranged parallel to the first linear array and the second linear array for detecting the first ray, the second ray and the third ray which penetrate through a third plurality of parts of the inspected object to acquire seventh values, eighth values and ninth values for the third plurality of parts, wherein the second plurality of parts are partly identical to the third plurality of parts. 
         [0020]    It is possible to make the ray beams having high and low energy levels interact with the object at the same position and realize accurate detection by utilizing the above detector array to detect the penetrating rays as well as by using dislocation matching. Meanwhile, since the distance between the first linear array and the second linear array is determined based on the moving speed of the inspected object and the time interval between the generation of rays having high and low energy levels by the radiation source, the rays interacting with the inspected object at the same part are ensured to be detected, thereby improving the accuracy of the dual-energy method in discriminating material and suppressing the false discrimination result at the edge of the inspected object. Because the distance between the first linear array and the second linear array is adjustable, the moving speed of the inspected object is variable. Therefore, the requirement for the moving speed of the inspected object is decreased. Moreover, since the first linear array and the second linear array simultaneously collect signals, the detective area is equivalently increased. On the other hand, the small section of the individual scintillator ensures to detect objects with high detection precision and obtain detailed detective images. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is the schematic view of the material identification system using a detector array according to an embodiment of the present invention; 
           [0022]      FIG. 2  is the schematic view showing the operating principle of the detector array when a radiation source alternately generates rays having different energy levels according to an embodiment of the present invention; 
           [0023]      FIG. 3  is the schematic structural view of the detector array according to an embodiment of the present invention; and 
           [0024]      FIG. 4  is the schematic view showing the operating principle of the detector array when a radiation source alternately generates rays having different energy levels according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Hereafter, an embodiment of the present invention is described in detail with reference to the drawings. 
         [0026]      FIG. 1  is the schematic view of the material discrimination system using a detector array according to an embodiment of the present invention. 
         [0027]    As shown in  FIG. 1 , the detector array including the first linear array  104   a  and the second linear array  104   b  is used to collect the dual-energy rays generated alternately by a radiation source. The radiation source  100  can alternately generate radiations such as X-rays. The synchronization control part  105  provides a synchronization signal  110  for the radiation source  100  and the first and second linear arrays  104   a  and  104   b  to make the radiation source  100  alternately generate high- and low-energy-level rays at the timing of the synchronization signal  110 . 
         [0028]    A fan-shaped planar radiation is obtained after the rays  102  generated by the radiation source  100  pass through the collimator  101 . As shown in  FIG. 1 , the inspected object  103  moves at a fixed speed in a fixed direction perpendicular to the radiation plane. The penetrating radiation after the interaction between the planar radiation and the inspected object  103  is detected by the first and second linear arrays  104   a  and  104   b.  Here, the first and second linear arrays  104   a  and  104   b  are arranged parallel to each other, and based on the synchronization signal from the synchronization control part  105 , adjust the parameters of the collecting circuits to perform simultaneous collecting. However, this isn&#39;t necessary. 
         [0029]    Then, the processing module of the dual-column detector array matches the newly collected signals of the two ray beams and outputs the detection values obtained after the high- and low-energy rays interact with the inspected object  103 . The detection values are sent to the image processing and material discrimination part  106  via the network. The image processing and material discrimination part  106  finally identify the material property of the inspected object such as inorganic matter, organic matter, heavy metal, etc. by use of the dual-energy algorithm and the relevant image processing algorithm. 
         [0030]      FIG. 2  is the schematic view showing the operating principle of the detector array when the radiation source  100  alternately generates rays having different energy levels according to the embodiment of the present invention. 
         [0031]    As shown in  FIG. 2 , on the basis of the timing  203 , the radiation source  100  alternately generates rays  102 H and  102 L having high and low energy levels, which are alternately emitted at a fixed frequency with the time intervals t between the emission of two ray beams are equal. The object  103  moves at a fixed speed along certain direction. It is assumed that the radiation source  100  emits a high-energy ray  102 H, which is collimated and then interacts with the parts  1  and  2  of the inspected object  103 . The penetrating ray is collected and buffered by the first and second linear arrays  104   a  and  104   b,  respectively, and the detection values are referred as  102 H- 1 A and  102 H- 2 B. 
         [0032]    Then, the radiation source  100  emits a low-energy ray  102 L when the time t has elapsed. At this time, the inspected object  103  has moved forward by a distance of one pixel, i.e., V*t. The low-energy ray  102 L penetrate through the parts  2  and  3  of the inspected object  103 , and is subsequently collected and buffered by the first and second linear arrays  104   a  and  104   b,  respectively, with the detection values being referred as  102 L- 2 A and  102 L- 3 B. The processing module of the detector array pairs the previously buffered detection value  102 H- 2 B, which is collected after the high-energy ray  102 H interacts with the part  2  of the inspected object  103 , and the newly buffered detection value  102 L- 2 A, which is collected after the low-energy ray  102 L interacts with the part  2  of the inspected object  103 , and outputs the pair to the image processing and material identification part  106 . 
         [0033]    Next, on the basis of the timing  203 , the radiation source  100  generate a high-energy ray  102 H again, while the inspected object  103  moves further by a distance of one pixel V*t. Therefore, the high-energy ray  102 H interacts with the parts  3  and  4  of the inspected object  103 . After such interaction, the detection values are collected respectively by the first and second linear arrays  104   a  and  104   b,  and referred as  102 H- 3 A and  102 H- 4 B. Subsequently, the processing module of the detector array pairs the previously buffered detection value  102 L- 3 B, which is collected after the low-energy ray  102 L interacts with the part  3  of the inspected object  103 , and the newly collected detection value  102 H- 3 A, which is collected after the high-energy ray  102 H interacts with the part  3  of the inspected object  103 , and outputs the pair to the image processing and material discrimination part  106 . In this way, as the inspected object  103  moves, the signal detection is performed after the high- and low-energy rays interact with the same part of the inspected object  103 . 
         [0034]    Since the paralleled first and second linear arrays  104   a  and  104   b  are utilized, the first ray, which is an approximate narrow beam of high energy and first generated by the radiation source  100 , can be collected by the first and second linear arrays  104   a  and  104   b  after the interaction with the parts  1  and  2  of the inspected object  103 . The first linear array  104   a  detects the first ray penetrating through the part  1  of the inspected object  103  and outputs the first detection value for the part  1 , and the second linear array  104   b  detects the first ray penetrating through the part  2  of the inspected object  103  and outputs the first detection value for the part  2 . Immediately following is that the radiation source  100  emits the second ray of a low energy level. Since the inspected object  103  has move forward by a distance of one pixel, the second ray will interact with the part  2  and  3  of the inspected object  103 . The first and second linear arrays  104   a  and  104   b  detect the signals for the parts  2  and  3  penetrated through by the second ray, and output the second detection value for the part  2  and the first detection value for the part  3 , respectively. Accordingly, the first and second detection values for the part  2  are the values outputted after the first and second rays penetrate through the part  2  of the inspected object  103 , respectively. Thus, the effective atomic number in the part  2  of the inspected object  103  can be determined based on the first and second detection values for this part, thereby determining the material property of the part  2 . 
         [0035]    Here, as shown in  FIG. 3(A) , the two linear arrays  104   a  and  104   b  each comprising a plurality of detector elements and they can be formed of two closely-arranged scintillators, such as CdWO 4  and Csl. The first and second linear arrays  104   a  and  104   b  can be combined into a whole. The two scintillators of each row are fixed and connected to the processing module  305 . After detecting signals, the two crystals simultaneously output the signals  302 A and  302 B, which are buffered and process in the processing module  305 . When the detectors have collected the signals for the high- and low-energy rays upon two adjacent pulses, the processing module  305  matches the signals for the high- and low-energy rays and outputs the high- and low-energy detection values corresponding to the same part of the inspected object to the image processing and material discrimination part  106 . As an alternative aspect, the two linear arrays  104   a  and  104   b  can independently output the signals  301 A and  301 B to their own processing modules (not shown), respectively. Every time the detector array collects the signals after the high- or low-energy ray penetrates through the inspected object, the signals are outputted to the processing module  305  so as to pair the detection values for the high- and low-energy rays, thereby obtaining the high- and low-energy detection values for each part of the inspected object  103 . As an alternative aspect, each detector element of the two linear arrays can be formed of a gas detector. 
         [0036]    Furthermore, the distance d between the first and second linear arrays  104   a  and  104   b  is adjustable as shown  FIG. 3(B) . Here, the distance d is determined by the moving speed V of the inspected object  103  and the time interval t between the generation of high- and low-energy rays by the radiation source, i.e.,d=V*t. That is, the distance between the first and second linear arrays is adjusted based on the moving speed of the inspected object and the time interval between the generation of high- and low-energy rays by the radiation source, thereby meeting the need for the adjacent high- and low-energy rays to penetrate through the same part of the inspected object. 
         [0037]    It should be noted the two linear arrays could be extended as four or six linear arrays to increase scan speed. 
         [0038]    Although the invention has been described in the case of dual energy, the present invention could be applied to multi-energy applications. 
         [0039]      FIG. 4  is the schematic view showing the operating principle of the detector array when a radiation source alternately generates rays having different energy levels according to another embodiment of the present invention. 
         [0040]    As shown in  FIG. 4 , the present embodiment differs from the previous embodiment in that the detector array comprises three linear arrays  104   a,    104   b  and  104   c  corresponding to three rays  102 H,  102 M and  102 L. 
         [0041]    As shown in  FIG. 4 , on the basis of the timing  203 , the radiation source  100  alternately generates rays  102 H,  102 M and  102 L having high, medium and low energy levels, which are alternately emitted at a fixed frequency with the time intervals t between the emission of two ray beams are equal. The object  103  moves at a fixed speed along certain direction. It is assumed that the radiation source  100  emits a high-energy ray  102 H, which is collimated and then interacts with the parts  1 ,  2  and  3  of the inspected object  103 . The penetrating ray is collected and buffered by the first, second, and third linear arrays  104   a,    104   b  and  104   c,  respectively, and the detection values are referred as  102 H- 1 A,  102 H- 2 B and  102 H- 3 C. 
         [0042]    Then, the radiation source  100  emits a medium-energy ray  102 M when the time t has elapsed. At this time, the inspected object  103  has moved forward by a distance of one pixel, i.e., V*t. The medium-energy ray  102 M penetrate through the part  2 ,  3  and  4  of the inspected object  103 , and is subsequently collected and buffered by the first, second and third linear arrays  104   a,    104   b  and  104   c,  respectively, with the detection values being referred as  102 M- 2 A,  102 M- 3 B and  102 M- 4 C. 
         [0043]    Then, the radiation source  100  emits a low-energy ray  102 L when the time t has elapsed. At this time, the inspected object  103  has moved forward by a distance of one pixel, i.e., V*t. The low-energy ray  102 L penetrate through the parts  3 ,  4  and  5  of the inspected object  103 , and is subsequently collected and buffered by the first, second and third linear arrays  104   a,    104   b  and  104   c,  respectively, with the detection values being referred as  102 L- 3 A,  102 L 4 B and  102 L- 5 C. Thus, the transmission values of part  3  under three energy levels can be obtained, which are referred as  102 H- 3 C,  102 M- 3 B and  102 L- 3 A. 
         [0044]    Next, on the basis of the timing  203 , the radiation source  100  generate a high-energy ray  102 H again, while the inspected object  103  moves further by a distance of one pixel V*t. Therefore, the high-energy ray  102 H interacts with the parts  4 ,  5  and  6  of the inspected object  103 . After such interaction, the detection values are collected respectively by the first, second and third linear arrays  104   a,    104   b  and  104   c,  and referred as  102 H- 4 A,  102 H- 5 B and  102 H- 6 C. Subsequently, the transmission values of part  4  under three energy levels can be obtained, which are referred as  102 H 4 A,  102 M- 4 C and  102 L- 4 B. 
         [0045]    It should be noted the two linear arrays could be extended as six or nine linear arrays to increase scan speed. 
         [0046]    The above-mentioned is only the specific embodiments of the present invention, while the scope of the present invention is not limited to it. Any modification or substitution, which is obvious to the skilled in the art within the technical range disclosed in the present invention, should be included in the scope of the present invention, which is thus defined by the claims.