Abstract:
Disclosed is a method of discriminating materials by employing fast neutron and continuous spectral X-ray and an equipment for the same. The method comprising the steps of: (a) transmitting a fast neutron beam produced by a fast neutron source and a continuous spectral X-ray beam produced by a continuous spectral X-ray source through inspected objects; (b) directly measuring the intensity of the transmitted X-rays and the intensity of the transmitted neutrons by a X-ray detector array and a neutron detector array respectively; and (c) identifying the materials of the inspected object by Z-dependency curves formed by the attenuation differences between the neutron beam and X-ray beam transmitted through different materials of the inspected object. This direct measurement of transmitted dual-ray technique has much more efficient than secondary radiations measurement such as neutron activation analysis, has much more material discrimination sensitivity than dual-energy x-ray technique. The respective measurements of neutrons and x-rays make the usages of high detect efficiency neutron detectors and x-ray detectors possible. The using continuous spectral x-ray produced by Linac adds more advantages such as: high penetration ability, high spatial resolution, and high image quality over monoenergetic dual-ray technique.

Description:
The present application claims priority of Chinese patent application Serial No. 200510086764.8, filed Nov. 3, 2005, the content of which is hereby incorporated by reference in its entirety. 
   FIELD OF THE INVENTION  
   This invention relates to a radiographic technique for containers and other voluminous objects inspection, especially to an equipment and method for discriminating materials by directly measuring the transmitted fast neutrons and continuous spectral x-rays and identifying materials by employing the Z-dependency curves formed by the attenuations differences between neutrons and x-rays transmitted through different materials. 
   BACKGROUND OF THE INVENTION  
   The present invention is driven by the global terrorism threat. As the antiterrorism situation getting severe, the radiographic container inspection systems capable of automatically detecting explosive, drugs and other contraband become pressing desired. The existing material discrimination techniques for container and other voluminous objects inspection, such as: high and dual energy radiographic technique, PFNA and container inspection CT, show more and more important. 
   High and dual energy radiographic technique employs absorption variation between materials in the megavolt range due to the Compton Effect and Pair Production Effect to determine the effective atomic number of the irradiated objects, and accordingly to discriminate different materials. But there are some physical limitations: First, absorption variation is not big enough. Second, the high-energy spectrum partially overlapped with low energy spectrum, even with spectrum filtering only can solve part of problems. Third, measurement error degrades the discrimination effect. All this factors make the unsatisfied results, and sequentially high and dual energy system mainly used to identify “organic”, “compound” and “inorganic” material in inspected container. Isotope source can provide monoenergetic gamma-ray which can solve the spectrum overlap problem, but penetration ability is too low to be used in container and other voluminous objects inspection system for material discrimination. 
   Present available PFNA systems, some have 3D material discriminating ability. But their spatial resolution is too big, throughput is too slow, and the price is too high. So PFNA can&#39;t dominate container inspection market at present and near future. Some NAA container inspection system using Cf-252 as neutron source can&#39;t be used for on-line real-time measurement, because only after the suspicious area having been detected by other equipment, the NAA measurement for suspicious area can be performed. 
   Container inspection CT system is giant and the throughput rate is too low to dominate container inspection market. 
   WO 2004/053472 discloses a radiographic equipment which directly measure transmitted monoenergetic fast neutron and monoenergetic gamma-ray. This equipment employs mass attenuation coefficient ratio to discriminate different materials, which can be used for detecting explosives, drugs and contraband. Comparing with high and dual energy X-ray technique, the direct measuring transmitted dual-ray technique has better material identification ability. Comparing with PFNA technique which measures secondary radiations such as neutron-induced gamma rays, the direct measuring transmitted dual-ray technique has much more efficient, especially has high penetration ability than thermal neutron. Comparing with container inspection CT, the dual-ray system is compact, low price and real-time measurement. 
   Unfortunately, the monoenergetic dual-ray system only can use isotope source, such as Co-60, as its gamma-ray source. Nevertheless, for containers or other voluminous objects inspection, the big disadvantage of isotope source is low penetration ability, low spatial resolution, poor image quality, and has radiation safety administration problems. This technique only provides low spatial resolution container transmission image, so is difficult to compete with Linac container inspection systems which provide high quality image. Since monoenergetic gamma-ray has low penetration ability, which limits its material identifying thickness as well, can&#39;t be used in case of fully-loaded container or thick objects inspection. All those defects limit its applications. 
   SUMMARY OF THE INVENTION  
   The present invention has solved above problems and provides material discrimination methods and equipments by directly measuring transmitted fast neutron and continuous spectral X-ray. Since the mass attenuation coefficient ratio of fast neutron and continuous spectral X-ray can&#39;t be simply used to determine Z (Line-of-Sight Effective Atomic Number of inspected objects), the present invention employs Z-dependence n-x curves to do material identification. Take advantage of high penetration ability of Linac X-ray and fast neutrons, it can do material identification well even in the cases of fully-load container or thick objects. This invention not only has all the advantages of monoenegetic dual-ray system such as: high material discriminating sensitivity, compact configuration, high throughput, low price, real-time measurement, but also has the added advantages such as: high penetration ability, high detect efficiency, high spatial resolution, high image quality, high material identification precision and veracity. 
   According to one aspect of the invention, a method of discriminating materials by employing fast neutron and continuous spectral X-ray comprising the steps of: (a) transmitting a fast neutron beam produced by a fast neutron source and a continuous spectral X-ray beam produced by a continuous spectral X-ray source through inspected object; (b) directly measuring the intensity of the transmitted X-rays and the intensity of the transmitted neutrons by an X-ray detector array and a neutron detector array respectively; and (c) identifying the materials of the inspected object by Z-dependency curves formed by the attenuation differences between the neutron beam and the X-ray beam transmitted through different materials of the inspected object. 
   In a preferred embodiment of the invention, the method further comprising a step of: (d) forming 2-dimensional X-ray transmission image and neutron transmission image at same scan. 
   In a preferred embodiment of the invention, the fast neutron source is one of a neutron generator, an isotope neutron source and a photoneutron source; and the continuous spectral X-ray source is one of an electron linear accelerator and an X-ray machine. 
   In a preferred embodiment of the invention, the photoneutron source is an accelerator for producing an X-ray beam part of which impinges on photoneutron converter and is converted into photoneutrons. 
   In a preferred embodiment of the invention, a distribution collimator divides the X-ray beam produced by the accelerator into two beams, one beam is collimated by X-ray beam-limited collimator so as to form an X-ray beam, the other beam impinges on photoneutron converter and is converted into photoneutrons to form a photoneutron beam by beam-limited collimator. 
   In a preferred embodiment of the invention, the fast neutron beam and the continuous spectral X-ray beam are measured by an X-ray detector array having high X-ray detecting efficiency and a neutron detector array having high neutron detecting efficiency respectively. 
   In a preferred embodiment of the invention, along the scanning tunnel a neutron scanner frame comprising the fast neutron source and the neutron detector array is located in parallel with an X-ray scanner frame comprising the X-ray source and the X-ray detector array; and along the direction of scanning, the X-ray scanner frame is preceding and the neutron scanner frame is behind so that the inspected object is scanned by X-ray scanner frame first, and then scanned by neutron scanner frame. 
   In a preferred embodiment of the invention, the neutron source and the X-ray source are pulsed in synchronism, and the emitting time of pulse neutron source is delayed a period of time, for example several milliseconds, from the emitting time of the pulse X-ray source. 
   In a preferred embodiment of the invention, wherein the identifying of the materials comprising: measuring the intensity T n , of neutron transmitted through the inspected object by every neutron detector of the neutron detector array; measuring the intensity T x  of X-ray transmitted through the inspected object by every X-ray detector in the X-ray detector array; composing Z-dependence curves by the pairs of (c 1 ,c 2 ), wherein c 1 =f 1 (T x ) is used as x-coordinator and c 2 =f 2 (T n ,T x ) is used as y-coordinator, wherein f 1 (T x ) denotes a function of the attenuation of X-ray, and f 2 (T n ,T x ) denotes a function of the attenuation difference of neutron and X-ray; identifying the different materials of the inspected object by using the Z-dependence curves; and displaying the identified different materials by different colors in an material discrimination image. 
   In a preferred embodiment of the invention, one pixel value of the neutron transmission image is paired with the mean of one or several pixels value of the X-ray transmission image to compose a (c 1 ,c 2 ) pair on one of the Z-dependence curves. 
   In a preferred embodiment of the invention, there are two scanning models for forming the X-ray transmission image and the neutron transmission image, one is a neutron scanner frame and an X-ray scanner frame moving, while the inspected object holds still; the other is the inspected object moving along the scanning tunnel, while the neutron scanner frame and the X-ray scanner frame are stationary. 
   In another aspect of the invention, an equipment for performing the method for discriminating materials by employing fast neutron and continuous spectral X-ray, comprising: a fast neutron source for producing neutrons; a continuous spectral X-ray source for producing X-rays; a neutron detector array for detecting the neutrons; and an X-ray detector array for detecting the X-rays; wherein the fast neutron source and the continuous spectral X-ray source are located at one side of a scanning tunnel, and the neutron detector array and the X-ray detector array are located at opposite side of the scanning tunnel. 
   In a preferred embodiment of the invention, the X-rays produced by the X-ray source are collimated into an X-ray beam which aims at the X-ray detector array, the X-ray beam transmits through the inspected object, and is received by the X-ray detector array; the neutrons produced by the fast neutron source are collimated into a neutron beam which aims at the neutron detector array, the neutron beam transmits through the inspected object, and is received by the neutron detector array. 
   In a preferred embodiment of the invention, the fast neutron source is one of a neutron generator, an isotope neutron source and a photoneutron source; and the continuous spectral X-ray source is one of an electron linear accelerator and an X-ray machine. 
   In a preferred embodiment of the invention, along the scanning tunnel, a neutron scanner frame comprising the fast neutron source and the neutron detector array is located in parallel with an X-ray scanner frame comprising the X-ray source and the X-ray detector array. 
   In a preferred embodiment of the invention, along the direction of scanning, the X-ray scanner frame is preceding, the neutron scanner frame is behind so that the inspected object is scanned by the X-ray scanner frame first, and then scanned by the neutron scanner frame. 
   In still another aspect of the invention, an equipment for performing the method for discriminating materials, comprising: an accelerator producing continuous spectral X-rays and photoneutrons; a neutron detector array for detecting photoneutrons; and an X-ray detector array for detecting X-rays; wherein the accelerator is located at one side of a scanning tunnel, the neutron detector array and the X-ray detector array are located at the other side of the scanning tunnel. 
   In a preferred embodiment of the invention, the equipment further comprising: an X-ray distribution collimator, which is installed at an X-ray beam emitting window of the accelerator, and divides the X-ray beam into two beams, one beam is collimated by the X-ray beam-limited collimator so as to form an X-ray beam, the other beam is collimated and leaded into a photoneutron enhancement chamber. 
   In a preferred embodiment of the invention, the equipment further comprising: a photoneutron converter, which is placed in the photoneutron enhancement chamber, and is interposed in the path of the X-ray beam, the X-ray beam impinges on the photoneutron converter and is converted into photoneutrons to form a photoneutron beam by the photoneutron enhancement chamber and a beam-limited channel connected with the photoneutron enhancement chamber. 
   In a preferred embodiment of the invention, the X-ray beam transmits through the inspected object, and is received by the X-ray detector array, the photoneutron beam transmits through the inspected object, and is received by the neutron detector array. 
   In a preferred embodiment of the invention, along the direction of scanning, an X-ray scanner frame comprising the X-ray beam and the X-ray detector array is preceding, a neutron scanner frame comprising photoneutron beams and the neutron detector array is behind, that is, the inspected object is scanned by X-ray scanner frame first, and then scanned by the neutron scanner frame. 
   In a preferred embodiment of the invention, the photoneutron converter comprises beryllium or other material, and has a shape of spherical dome, cylinder, cone, L-shaped plate or other shape. 
   In a preferred embodiment of the invention, between the photoneutrons emitting window of the photoneutron enhancement chamber and the beam-limited channel, a bismuth filter is interposed on the way of the photoneutron beam. 
   In a preferred embodiment of the invention, the photoneutron enhancement chamber comprises lead and graphite layers or other material. 
   Due to above technologies employed in this invention, the n-x curves are Z-dependence only and not related to the thickness of inspected objects. The invention has following advantages: compact equipment, high detecting efficiency. By this method that container is scanned by the X-ray scanner frame first and by the neutron scanner frame later, the affect on the X-ray transmission image by gamma ray induced by neutron activation is eliminated. By employing time-dividing technology, that is the delay of neutron beam emitting time relative to the emitting time of the Linac X-ray beam, the interfere of the neutrons with X-ray transmission image and the X-rays with neutron transmission image can be reduced, and the images quality can be improved. As the fan-shaped X-ray beam and the fan-shaped neutron bean are narrow-beam, the scattering interference can be reduced, easy to radiation protection and has high spatial resolution. In the case of fully-loaded container or thick objects inspection, the material discrimination can perform well. So it can be used to detect explosive, drug, contraband, special nuclear material, radiation material and other material in container, container truck, railcar or other voluminous objects. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic illustration showing the configuration of one equipment according to an embodiment of the invention; 
       FIG. 2  is a schematic illustration showing the configuration of another equipment according to another embodiment of the invention; 
       FIG. 3  is a schematic illustration showing the structure of the X-ray beam distributing collimator and photoneutron converting and enhancing facility. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
   Hereinafter, the embodiments of the invention will be described with the accompanying drawings. For convenience&#39;s sake, such components in  FIGS. 1-3  will be indicated with the same or similar reference numerals. 
   [Configuration] 
     FIG. 1  is a schematic illustration showing the configuration of one equipment according to an embodiment of the invention. 
   Referring to  FIG. 1 , the equipment  10  according to the first embodiment of the invention comprises a container convey track  32 , at least one inspected containers or other voluminous objects  34  that can be placed on the container convey track  32 , a fast neutron source  12  for producing neutrons, a continuous spectral X-ray source  22  for producing X-rays, a neutron detector array  18  having a high detecting efficiency with respect to neutron, an X-ray detector array  28  having a high detecting efficiency with regard to X-ray, a fan-shaped neutron beam  16  and a fan-shaped X-ray beam  26 . 
   The fast neutron source  12  is one of a neutron generator and isotope neutron source. The continuous spectral X-ray source  22  is one of an electron linear accelerator (Linac) and an X-ray machine. The fast neutron source  12  and the continuous spectral X-ray source  22  are located at one side of the container convey track  32 . The neutron detector array  18  and the X-ray detector array  28  are located at opposite side of the container convey track  32 . 
   The neutrons emitted from the fast neutron source  12  are collimated into a fan-shaped neutron beam  16  which transmits through container  34  and then is received by the neutron detector array  18 . The X-rays emitted from continuous spectral X-ray source  22  is collimated into a fan-shaped X-ray beam  26  which transmits through container  34  and then is received by the X-ray detector array  28 . 
   A neutron scanner frame formed by the fast neutron source  12  and the neutron detector array  18  is located in parallel with an X-ray scanner frame formed by the continuous spectral X-ray source  22  and the X-ray detector array  28  and they move along the container convey track  32 . The direction of scanning  36  is opposite to the moving direction  38  of the inspected container  34 . Along the direction of scanning, the X-ray scanner frame is preceding, and the neutron scanner frame is behind. That is, the inspected container  34  is scanned by the X-ray scanner frame first, and then scanned by the neutron scanner frame. 
     FIG. 2  is a schematic illustration showing the configuration of another equipment according to another embodiment of the invention, and  FIG. 3  is a schematic illustration showing the structure of the X-ray beam distributing collimator and photoneutron converting and enhancing facility. 
   Referring to  FIG. 2  and  FIG. 3 , another equipment  11  according to the second embodiment of the invention comprises a container convey track  32 , at least one inspected container or other voluminous object  34  that can be placed on the container convey track  32 , an accelerator  42  which can produce continuous spectral X-ray beam and part of which is converted into photoneutrons, a neutron detector array  18  and an X-ray detector array  28 . 
   Accelerator  42  is located at one side of the container convey track  32 . The neutron detector array  18  and the X-ray detector array  28  are located at another side of the container convey track  32 . A special designed X-ray distribution collimator  52  is installed at the X-ray beam emitting window of the accelerator  42 . And the X-ray distribution collimator  52  divides the X-ray beam produced by the accelerator into two beams: one beam is collimated by the X-ray beam-limited collimator  24  so as to form a fan-shaped continuous spectral X-ray beam  26 ; the other beam  58  is collimated and leaded into a photoneutron enhancement chamber  50  which is made of lead, graphite layers or other materials. 
   A photoneutron converter  56  comprising beryllium or other material, and having a shape of spherical dome, cylinder, cone, L-shaped plate or other shape is placed in the photoneutron enhancement chamber  50 , and is interposed in the path of the X-ray beam  58 . The X-ray beam  58  impinges on the photoneutron converter  56  and is converted into photoneutrons to form a fan-shaped photoneutron beam  16  by the photoneutron enhancement chamber  50  and a neutron beam-limited channel  51  connected with the photoneutron enhancement chamber  50 . Between the photoneutrons emitting window of the photoneutron enhancement chamber  50  and the neutron beam-limited channel  51 , a bismuth cylinder filter  60  is interposed on the way of the photoneutron beam. 
   The fan-shaped photoneutron beam  16  aims at the neutron detector array  18  located on the other side of the container convey track  32 , and the phothneutron beam  16  and the neutron detector array  18  form a neutron scanner frame. The fan-shaped X-ray beam  26  aims at the X-ray detector array  28  located on the other side of the container convey track  32 , and the X-ray beam  26  and X-ray detector array  28  form an X-ray scanner frame. 
   Along the direction of scanning  36 , the X-ray scanner frame is preceding, and the neutron scanner is behind. That is, the container  34  is scanned by the X-ray scanner frame first, and then scanned by the neutron scanner frame. 
   [Operation] 
   (a) A neutron scanner frame composed of the neutron source  12  and the neutron detector array  18  is located in parallel with an X-ray scanner frame composed of the X-ray source  22  and the X-ray detector array  28  and they move along the convey track. The inspected container  34  passes through X-ray scanner frame first, and then passes through the neutron scanner frame. The fan-shaped X-ray beam  26  transmits through the inspected container  34 . The transmitted beam is received by the X-ray detector array  28 , and then forms a 2-dimensional X-ray transmission image. At the same scan, the fan-shaped neutron beam  16  transmits through the inspected container  34 . The transmitted beam is received by the neutron detector array  18 , and then forms a 2-dimensional neutron transmission image. If a pulse neutron source is used as the neutron source  12 , the neutron source  12  and Linac X-ray source  22  are pulsed in synchronism, and the emitting time of pulse neutron source is delayed a period of time from the emitting time of the Linac pulse continuous spectral X-ray source. 
   (b) The materials discrimination method is implemented by employing Z-dependence n-x curves. The count T n  of every neutron detector is the neutron intensity of the neutron transmitted through the container  34 . The count T x  of every X-ray detector is the X-ray intensity of the X-ray transmitted through the container  34 . Using c 1 =f 1 (T x ) as x-coordinator and C 2 =f 2  (T n ,T x ) as y-coordinator, the pairs of (c 1 ,c 2 ) compose the Z-dependence curves, which are employed to identify different material. Here, f 1 (T x ) denotes a function of the attenuation of X-ray, and f 2 (T n ,T x ) denotes a function of the attenuation difference of neutron and X-ray. One pixel value of neutron transmission image can be paired with the mean of one or several pixels value of the X-ray transmission image, and compose a (c 1 ,c 2 ) pair on the Z-dependence curves, which are employed for material discrimination. Different materials are displayed by different colors in a material discrimination image. 
   The physics principle of the Z-dependence curve will be described as below. 
   The attenuation of narrow-beam monoenergetic neutrons transmitted through an irradiated object with thickness x(cm) can be calculated using the equation (1):
 
 I   n   =I   n0 exp(−μ n ( Z, E )· X )   (1)
 
where I n  and I n0  denote the measured intensities with and without attenuation respectively; μ n (Z,E) denotes the linear attenuation coefficient (cm −1 ) of irradiated object&#39;s material for neutrons, which is the function of effective atomic number Z of the object under inspection and the energy of incident neutrons E(MeV).
 
   In case of narrow-beam continuous spectral neutrons, the attenuation of narrow-beam continuous spectral neutrons transmitted through an irradiated object with thickness x(cm)can be calculated using the equation (2): 
                   I   n     =       ∫   0     E   nb       ⁢         I     n   ⁢           ⁢   0       ⁡     (   E   )       ⁢     exp   ⁡     (       -       μ   n     ⁡     (     Z   ,   E     )         ·   x     )       ⁢     ⅆ   E                 (   2   )               
where I n  denotes the measured intensity of transmitted neutrons; I n0 (E) denotes the measured incident intensity of continuous spectral neutrons with boundary energy E nb (MeV); μ n (Z,E) denotes the sum of linear attenuation coefficients (cm −1 ) of irradiated object&#39;s material for neutrons, which is the function of effective atomic number Z of the object under inspection and the energy of incident neutrons E(MeV).
 
   In the case of narrow-beam continuous spectral X-ray, the attenuation of the narrow-beam continuous spectral X-ray transmitted through an irradiated object with thickness x(cm)can be calculated using the equation (3): 
                   I   x     =       ∫   0     E   xb       ⁢         I     x   ⁢           ⁢   0       ⁡     (   E   )       ⁢     exp   ⁡     (       -       μ   x     ⁡     (     Z   ,   E     )         ·   x     )       ⁢     ⅆ   E                 (   3   )               
where I x  denotes the measured intensity of transmitted X-ray; I x0 (E) denotes the measured intensity of incident continuous spectral X-ray with boundary energy E xb (MeV); μ x (Z,E) denotes the sum of linear attenuation coefficients (cm −1 ) of irradiated object&#39;s material for X-rays, which is the function of effective atomic number Z of the object under inspection and the energy of incident X-rays E(MeV).
 
   In the case of both the neutron beam and the X-ray beam are continuous spectral distributions, the following nonlinear integration equation set (4) is employed to do material discrimination: 
                 {               T   n     ⁡     (     E   ,   x   ,   Z     )       =         ∫   0     E   nb       ⁢         I     n   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅇ       -       μ   n     ⁡     (     E   ,   Z     )         ⁢   x       ⁢     ⅆ   E             ∫   0     E   nb       ⁢         I     n   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅆ   E                           T   x     ⁡     (     E   ,   x   ,   Z     )       =         ∫   0     E   xb       ⁢         I     x   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅇ       -       μ   x     ⁡     (     E   ,   Z     )         ⁢   x       ⁢     ⅆ   E             ∫   0     E   xb       ⁢         I     x   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅆ   E                           (   4   )               
where T n (E,x,Z) denotes the transparency of the irradiated object with effective atomic number Z and thickness x(cm) for flux of neutrons with boundary energy E nb  (MeV); I n0 (E) denotes the intensity of incident neutrons with energy E(MeV); μ n (Z,E) denotes the sum of linear attenuation coefficients (cm −1 ) of irradiated object&#39;s material for neutrons, which is the function of effective atomic number Z of the irradiated object and the energy of incident neutrons E(MeV); T x (E,x,Z) denotes the transparency of the irradiated object with effective atomic number Z and thickness x(cm) for flux of X-rays with boundary energy E xb (MeV); I x0 (E) denotes the intensity of incident X-ray with energy E(MeV); μ x (Z,E) denotes the sum of linear attenuation coefficients (cm −1 ) of irradiated material for X-rays, which is the function of effective atomic number Z of the object under inspection and the energy of incident X-rays with energy E(MeV).
 
   In case of the neutron beam  16  is monoenergetic and the X-ray beam  26  is continuous spectral distribution, the following nonlinear integration equation set (5) is employed to do material discrimination: 
                 {               T   n     ⁡     (       E   n     ,   x   ,   Z     )       =         I     n   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅇ       -       μ   n     ⁡     (       E   n     ,   Z     )         ⁢   x                         T   x     ⁡     (     E   ,   x   ,   Z     )       =         ∫   0     E   xb       ⁢         I     x   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅇ       -       μ   x     ⁡     (     E   ,   Z     )         ⁢   x       ⁢     ⅆ   E             ∫   0     E   xb       ⁢         I     x   ⁢           ⁢   0       ⁡     (   E   )       ⁢     ⅆ   E                           (   5   )               
where T n (E,x,Z) denotes the transparency of the irradiated object with effective atomic number Z and thickness x(cm) for flux of neutrons with energy E(MeV); I n0 (E) denotes the intensity of incident neutron with energy E(Me V); μ n (Z,E) denotes the linear attenuation coefficient (cm −1 ) of irradiated object&#39;s material for neutrons, which is the function of effective atomic number Z of the object under inspection and the energy of incident neutrons E(MeV); T x (E,x,Z) denotes the transparency of the irradiated object with effective atomic number Z and thickness x(cm) for flux of X-rays with boundary energy E xb (MeV); I x0 (E) denotes the intensity of incident X-ray with energy E(MeV); μ x (Z,E) denotes the sum of linear attenuation coefficients (cm −1 ) of irradiated material for X-rays, which is the function of effective atomic number Z of the object under inspection and the energy of incident X-rays with energy E(MeV).
 
   The solutions of equation set (4) or (5) are not related to the thickness of the irradiated object, but only are Z-dependency. So it can be used for material discrimination. 
   In present invention, there are two scanning models: One is neutron scanner frame and X-ray scanner frame moving, while the inspected object  34  holds still. The other is the inspected object  34  moving along the convey track  32 , while the neutron scanner frame and X-ray scanner frame are stationary. 
   It is to be understood that the present invention may be carried out in any other manner than specifically described above as embodiments, and many modifications and variations are possible within the scope of the invention.