Patent Publication Number: US-2005135533-A1

Title: Coded target for neutron source

Description:
TECHNICAL FIELD  
      The present invention concerns a coded target for a neutron source such as a neutron generating tube or a particle accelerator.  
      Neutron generating tubes are mainly used to carry out imaging or analysis, for example in the non-destructive testing of objects. The neutrons make it possible to determine the presence of practically all of the elements in the periodic table. One may thus use neutron generating tubes for analysing, controlling, dosing or searching for materials, for example drugs, explosives and radioactive waste in drums, ore or bulk materials.  
      Particle accelerators such as linear accelerators supply neutrons with wide energy ranges. The applications may also be radiography applications or analysis of materials or even applications in nuclear medicine or in physics research.  
     STATE OF THE PRIOR ART  
      In said devices, the neutrons are generated by a target when it is bombarded with a highly accelerated particle beam.  
      In methods using the fusion of hydrogen, the targets contain heavy isotopes of hydrogen: deuterium and/or tritium. A fusion nuclear reaction takes place between the nuclei of deuterium or tritium of the target and the accelerated particles from, in general, an ion source.  
      In neutron generators, the particles are hydrogen isotope ions. The ions are accelerated between two electrodes.  
      In accelerators, the particles are also hydrogen ions (deuterium or tritium) or other heavier ions. The acceleration may be achieved by means of a succession of electrodes or by coupling in quadripolar radiofrequency accelerating cavities.  
      The collision between a nucleus of deuterium and a nucleus of tritium supplies a neutron with an energy close to 14 MeV and an α particle with an energy of around 3.6 MeV. The neutrons are emitted in all directions, in other words in 4π steradians. For each neutron emitted in a direction an α particle is emitted substantially in the opposite direction. This mechanism also takes place with the reaction between two deuterium nuclei. The energy of the neutron is in this case 2.45 MeV. It is associated with a particle of helium  3  of 0.8 MeV emitted in the opposite direction, at about a low angle.  
      In order for the nuclear reaction efficiency to be high, the targets should have a high density of nuclei. One generally fixes the tritium and/or deuterium nuclei in a hydrogen fixing metal such as, for example, titanium, zirconium or erbium. The particle beam also has to be sufficiently intense and homogeneous. One therefore tends to focus the particle beam and make it bombard a small homogeneous target. However, a compromise has to be found between the lifetime of the target and the efficiency. Indeed, for a total given emission, the lifetime of a target is proportional to its surface area. Moreover, it is not easy to focus the particle beam with precision. Said focusing must be consistent with the geometry of the target. In neutron generating tubes, one is obliged to provide a series of electrodes of appropriate geometry and to raise them to voltages of a specific value. The focusing device may become very sophisticated. Finally, the smaller the target, the lower the flow of emitted neutrons and the longer the analysis time.  
      Small targets (substantially point shaped) are required for transmission radiography. They lead to a better image quality than that obtained with a more extended target, particularly when one wishes to detect small point objects (for example, defects in a structure). One may refer to  FIG. 1A  which shows, in an image plane  5 , the image  3  of a point  1  of an object  4 , given by a point target  2 . The size d of said image  3  is expressed by the following relation: d=dc.L/l, where dc is the size of the target  2 , L is the distance between the object  4  and the image plane  5  and l the distance between the object  4  and the target  2 .  
      In  FIG. 1B , the target  2 ′ is no longer point shaped but instead extended. The image  3 ′ of the point object  1  in the image plane  5  is a lot bigger than in  FIG. 1A , which leads to an overall image of the object which will be blurred. The size d′ of said image  3 ′ is expressed by the following relation: d′=dc′.L/l, where dc′ is the size of the target  2 ′, L is the distance between the object  4  and the image plane  5  and l the distance between the object  4  and the target  2 ′.  
     DESCRIPTION OF THE INVENTION  
      The aim of the present invention is, as a matter of fact, to propose a target for a neutron source that does not have the limitations and difficulties cited above.  
      One aim of the invention is to form a target that has both a longer lifetime and an acceptable efficiency.  
      Another aim of the invention is to form a target that does not need to be associated with a precisely focused particle beam.  
      A further aim of the invention is to provide a target suited to delivering a higher flow of neutrons than that of point targets.  
      In order to attain these aims, the invention is a target intended to emit neutrons when it is bombarded with particles and said target comprises neutron emissive parts and neutron non-emissive parts which are juxtaposed, said emissive and non-emissive parts forming a pattern of the type of that of a coded mask.  
      The emissive parts may be formed from at least one metal hydride, the metal of the metal hydride being deposited on a support in non-hydrogen fixing material through a stencil.  
      In one embodiment, the target may comprise an extended neutron emissive zone formed from at least one metal hydride, said extended zone cooperating with a mask in neutron non-emissive material, the non-emissive material of the mask partially covering up the extended emissive zone vis-à-vis the particles and forming non-emissive parts.  
      The extended emissive zone may be supported by a support in a non-hydrogen fixing material.  
      The non-hydrogen fixing material of the support may be chosen from among copper, silver or gold, said metals being used alone or in combination.  
      The metal of the metal hydride may be chosen from among titanium, zirconium, erbium, scandium and vanadium.  
      The non-emissive material of the mask may be chosen from among molybdenum, steel, iron, copper, tungsten or tantalum, said metals being used alone or in combination.  
      The present invention also concerns a particle accelerator which comprises a target thus defined.  
      The present invention also concerns a neutron generating tube which comprises a target thus defined.  
      The present invention also concerns the application of the neutron generating tube thus defined to transmission radiography, in which the target cooperates with geometric deconvolution means to decode an untreated image given by the neutrons having crossed through an object to be radiographied in a reconstructed image of the object.  
      The present invention also concerns the application of the particle accelerator thus defined to transmission radiography, in which the target cooperates with geometric deconvolution means to decode an untreated image given by the neutrons having crossed through an object to be radiographied in a reconstructed image of the object.  
      The present invention also concerns a neutron generating tube thus defined, said tube being equipped with an α particle detector associated with the emission of neutrons.  
      The present invention also concerns a particle accelerator thus defined, said accelerator being equipped with an α particle detector associated with the emission of particles.  
      The α particle detector may comprise a plurality of pixels arranged in a matrix.  
      Advantageously, the target may be inclined in relation to the direction of the particles that are bombarding it in order to facilitate the detection of α particles without overly disrupting the interaction of the particle beam in the target.  
      In another configuration, the target may be substantially parallel to the α particle detector.  
      The present invention also concerns the application of the neutron generating tube thus defined with an α particle detector to the analysis of substances and/or the imaging of substances that may be hidden, the tube cooperating with at least one γ radiation detector and geometric deconvolution means for a gamma pseudo-image obtained by coincidence of gamma events and α particles detected by the α particle detector.  
      The present invention also concerns the application of the particle accelerator thus defined with an α particle detector to the analysis of substances and/or the imaging of substances that may be hidden, the accelerator cooperating with at least one γ radiation detector and geometric deconvolution means for a gamma pseudo-image obtained by coincidence of gamma events and α particles detected by the α particle detector.  
      The present invention also concerns the application of the neutron generating tube thus defined with an α particle detector to the imaging of substances that may be hidden, the tube cooperating with at least one neutron detector.  
      The present invention also concerns the application of the particle accelerator thus defined with an α particle detector to the imaging of substances that may be hidden, the accelerator cooperating with at least one neutron detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be more fully understood on reading the description of embodiments, given purely by way of indication and in nowise limitative, and by referring to the appended drawings in which:  
       FIGS. 1A, 1B  are diagrams illustrating the principle of transmission radiography of an object with uniform targets, respectively point or extended targets;  
       FIGS. 2A, 2B  show a front view of two examples of targets according to the invention;  
       FIG. 3A  shows a transmission radiography installation for an object that makes use of a neutron generating tube according to the invention and  FIG. 3B  shows the radiography obtained with the said installation and an object less transparent to neutrons;  
       FIGS. 4A  to  4 C show different steps of forming one embodiment of a target according to the invention;  
       FIGS. 5A, 5B  show different steps of forming another embodiment of a target according to the invention;  
       FIG. 6  shows a particle accelerator that is the subject of the invention;  
       FIG. 7A  shows a neutron generating tube having associated particles that is the subject of the invention and  FIGS. 7B, 7C  accelerators having associated particles that are the subject of the invention;  
       FIGS. 8A, 8B  and  8 C show material imaging and/or analysis installations that make use respectively of a point target neutron generating tube, an extended target neutron generating tube and a neutron generating tube or particle accelerator that are the subject of the invention.  
      The different parts shown in the figures are not necessarily represented on the same scale, in order to make the figures more legible.  
      Identical, similar or equivalent parts in the different figures described below have the same numerical references in order to facilitate going from one figure to another. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS  
      We will now refer to  FIG. 2  which shows a front view of an example of a target  10  according to the invention. It comprises juxtaposed emissive parts  11  and non-emissive parts  12 , said emissive parts  11  and non-emissive parts  12  forming a pattern of the type of that of a coded mask.  
      It will be recalled that a coded mask is a mask that is pierced with a plurality of holes placed in a more or less random manner. The surface area of said holes may attain 50% or more of the total surface area of the mask. Coded masks are used in the medical field or in astronomy in telescopes. In these applications, the coded masks are placed between the object to be observed (planet, star) and the observer (or the device capturing the image).  
      The emissive parts  11  of the target  10  according to the invention correspond to the holes that the coded mask would have and the non-emissive parts  12  correspond to the solid parts of the coded mask. Obviously, the reverse is possible.  
      Each emissive part  11  may also be considered as a pinhole. All of the emissive parts  11  together constitute a multi pinhole.  
      The emissive parts  11  may be assimilated to point shaped targets and the whole target  10  to a quasi-point shaped target. The pattern represented in  FIG. 2A  is relatively complex and other coded mask patterns may be used, such as that illustrated in  FIG. 2B , which only comprises three emissive parts  11 , one substantially circular, a second substantially square and a third substantially rectangular.  
      Consequently, when such a target  10 , included in a source of electrons  23  schematically shown in  FIG. 3A , is bombarded for example by ions of deuterium or accelerated deuterium and tritium  20 , only the emissive parts  11  are going to emit neutrons. Said neutron source  23  may be a neutron generating tube or a particle accelerator. It comprises a jacket  21 , the target  10  and an ion source  22 , for example a Penning source type, in which deuterium or a mixture of deuterium and tritium is ionised. The ions are accelerated between the ion source  22  and the target  10  by acceleration means, which are not represented for reasons of clarity.  
      The pattern of the target  10  is simpler than in  FIG. 2A . The target  10  is going to emit a heterogeneous flow of neutrons  13 . In this example, the flow of neutrons  13  is divided into a plurality of secondary flows  13 . 1 ,  13 . 2  and said secondary flows are only initiated in order not to overcrowd the figure.  
      The flow of neutrons  13  emitted will “convey” with it the pattern of the target  10  through an object  4  to be examined or analysed.  
      One observes that in the invention, the target  10 , which plays the role of a coded mask, is not inserted between the object and an observer as in normal applications of coded masks but it is placed upstream of the object.  
      The flow of neutrons  13  is a flow marked or coded by the pattern of the target  10 . In the manner of  FIG. 1A , an image  30  of a point  1  of an object  4 , given by the target  10  according to the invention, is projected on an image plane  5 . But now, the image obtained  30  in the image plane  5  is an untreated image of the object  4  and said untreated image must be treated in order to obtain a reconstructed image  31 . In a way, the untreated image comprises as many superimposed images of the object as there are emissive parts in the target, each of said emissive parts acting like a point target. The untreated image  30  may be acquired, for example, by a neutron detector  33  configured as a matrix, formed from materials sensitive to rapid neutrons such as recoil proton scintillating materials.  
      The treatment is called geometric deconvolution and is performed in a computerised manner by means of an appropriate algorithm. Means of treating the untreated image  30  are shown schematically with the reference  32 .  
      In fact, the untreated image  30  obtained conveys two items of information, one relating to the pattern of the target  10  and the other relating to the transparency of the object  4 . Deconvolution consists in separating the two items of information. It is a well known method and widely used in optics or in techniques using coded masks. This treatment is possible because the pattern of the target  10  is known by experimentation, modelling or calculation.  
      We have shown that the intensity of each point of the untreated image is the integral on the object of the product of the transparency of each point  1  of the object  4  multiplied by a term depending on the target  10 . In the Fourier space or frequency space, the Fourier transform of the untreated image is the product of the Fourier transforms of the object and an image of the target  10  obtained through an infinitely small pinhole. It will be recalled that a pinhole is, in photography, a small hole placed in a screen arranged in place of the objective of a photographic device. In order to obtain the reconstructed image of the object, it is therefore necessary to take the inverse Fourier transform of the ratio of Fourier transforms of the untreated image and of the target. The choice of the pattern of the target makes it possible to carry out this division correctly. With a traditional extended target, which is emissive in a uniform manner, the differences between neighbouring points of the object do not stand out. Other calculation methods exist; they enable the same result to be obtained.  
      The reconstructed image  31  of the object  4  (or the point  1  of the object) obtained after geometric deconvolution is substantially that which one would have obtained if one had used a point target.  
      Consequently, according to the invention, one obtains a reconstructed image  31  of quality comparable to that which would have been obtained with a point target, in other words a sharp image with a good resolution. Since the target according to the invention has dimensions greater that those of the point target which would have given the same image, it has a longer lifetime compared to the point target. The gain in lifetime may be very important because the ratio of surface areas between the target according to the invention and the point target which would have given the same image may attain, depending on the case, factors of one hundred-fold or one thousand-fold. The flow of neutrons is thus increased compared to that of the point target. With equal emission surface area and equal emission intensity, a target according to the invention makes it possible to obtain much better reconstructed image quality than a conventional homogeneous target.  
      The pattern of the target  10  appears on the image plane  5  in a more or less intense manner depending on whether the object  4  allows the neutron flow to pass to a greater or lesser extent. The difference between the untreated image  30  obtained with the transmission radiography installation in  FIG. 3A  and that in  FIG. 3B  is only at the level of its intensity. The untreated image  30  in  FIG. 3B  is less intense than that in  FIG. 3A . This means that the object that has given the untreated image  30  in  FIG. 3B  is less transparent to neutrons than that shown in  FIG. 3A .  
      We will now describe a first embodiment of a target according to the invention, while referring to  FIGS. 4A, 4B  and  4 C. The target comprises a non-emissive support  14 , in non-hydrogen fixing material. Said material may be chosen from among copper, silver, gold or other, said metals being used alone or in combination. On the support  14 , usually circular without this being obligatory, is deposited at least one hydrogen fixing material  15  through a stencil  16 . Said stencil  16  has a pattern that corresponds to that which the target should have, in other words a pattern of the type of that of coded masks. Said stencil  16  may be formed from metal such as iron or aluminium for example. As hydrogen fixing metal, one may employ for example titanium, zirconium, erbium, scandium or vanadium. The hydrogen fixing material  15  forms blocks  17  on the support  14 . Said blocks  17  are going to contribute to forming the emissive parts  11  of the target. The parts of the support  14  located between the blocks  17  are going to form the non-emissive parts  12  of the target. The hydrogen fixing material may be deposited, for example, by cathode sputtering, vacuum evaporation or other method. The thickness of the blocks  17  is typically several micrometers. The support  14  equipped with blocks  17  is then brought into the presence of hydrogen so that the block  17  material traps the hydrogen and fixes it. The hydrogen fixed blocks  17  are represented in  FIG. 4B . This hydrogen fixing step takes place while the target is placed in a vacuum jacket (which is that of the tube or the accelerator that uses the target) containing a gaseous mixture of tritium and/or deuterium under low pressure. Once hydrogen fixed, the blocks  17  are going to make it possible to emit neutrons when they are bombarded with a particle beam.  FIG. 4C  shows a front view of a finished target, in the form of a disc.  
       FIGS. 5A and 5B  show another embodiment of a target according to the invention. As in the previous example, the support  14  in non-hydrogen fixing material is shown. Said support  14  is covered in a uniform manner with a metallic hydrogen fixing material  18 . As metallic hydrogen fixing material one may use, for example, titanium, zirconium, erbium, scandium or vanadium, said metals being used alone or in combination. Said deposition may be carried out by sputtering or as in the previous example.  
      The hydrogen fixing material  18  is covered with a mask  19  that comprises a pattern of the type of that of a coded or multi-pinhole mask. Said mask  19  is formed of a material that does not emit neutrons, which does not sputter or which sputters as little as possible when it is bombarded by particles and which is compatible with vacuum techniques. Said material may be, for example, chosen from among molybdenum or copper, iron, tungsten, tantalum or steel, said metals being used alone or in combination. The mask  19  is substantially parallel with the surface of the layer  18 . A gap of around one millimetre or more can separate the mask  19  and the layer  18 . Obviously, the mask may be in contact with the hydrogen fixing material instead of being slightly separated.  
      The assembly formed of the support  14 , the layer  18  and the mask  19  is then brought into the presence of hydrogen so that the layer  18  traps the hydrogen and fixes it. This hydrogen fixing step takes place, as previously, while the target is placed in a vacuum jacket (which is that of the tube or accelerator that uses the target) containing a gaseous mixture of tritium and/or deuterium under low pressure. The layer  18  then forms an extended zone capable of emitting neutrons but it is partially masked by the mask  19 . The hydrogen fixed material of the layer  18  located at the openings of the mask  19  forms the emissive parts  11  of the target whereas the solid parts of the mask  19  form the non-emissive parts  12  of the target. The parts of layer  18  placed behind the solid parts of the mask  19  are inactive even though impregnated with hydrogen. Obviously, it is possible for the hydrogen fixing of the layer  18  to take place before the application of the mask  19 .  
      With a target such as that represented in  FIGS. 5 , the particle beam which is going to bombard the hydrogen fixed layer  18  in order to make it emit neutrons is coded because it is going to be partially stopped by the mask  19  and only reach the emissive parts  11 . The neutron beam is also coded since only the emissive parts  11  are going to emit neutrons.  
      Such targets may be integrated into a neutron generating tube, particularly to carry out rapid neutron transmission radiography. The energy of the neutrons may be 14 MeV or 2.45 MeV. The reconstructed image then has a resolution comparable to that obtained with a point target and an intensity substantially equal to n times that obtained with a point target, where n is the number of emissive parts  11  of the target.  
      The target may be included in a particle accelerator such as that represented in  FIG. 6 . Said accelerator comprises a tubular body  60  cooperating with pumping means  61  for extracting the non-ionised gas and cooling means  62 . At one of the ends of the accelerator is placed means of injecting  63  an ion beam  64  (protons or deuterons). At the other end is placed the target  65  according to the invention that is to be bombarded. In the body  60  of the ion accelerator the ions are firstly focused then accelerated before they reach the target  65 . The focusing means referenced  66  may be formed by electrodes. The acceleration means referenced  67  may be formed for example by various high voltage devices. This type of particle accelerator finds application in neutron radiography or the analysis of matter.  
      It is also advantageous to integrate a target according to the invention in a neutron generating tube having associated particles, in other words a neutron generating tube equipped with an α particles detector. An example of this type of tube is represented in  FIG. 7A . A particularly interesting application of this type of associated particles tube is the imaging and/or the analysis of substances such as the detection of mines in the ground, the detection of illicit materials or explosives in various containers.  
      A neutron generating tube equipped with an α particle detector is known for example from the patent application FR-A1-2 738 669 in the applicant&#39;s name. Said neutron generating tube is equipped with an α particle detector which cooperates with the means of focusing the α particles emitted by a uniform emission target. The means of focusing may be a coded mask type and be placed in the path of the α particles. In the present invention, it is either the ions projected on the target that are coded in the case of the mask, or the target itself which is coded.  
      The disadvantages of this type of tube compared to that of the present invention are that it is more bulky, since the α particle detector is relatively distant from the target, that it is more complicated to produce and more difficult to use. The distance of the coded mask from the target introduces a fuzziness on the origin of the neutrons which makes the use of the deconvolution algorithm less easy to implement.  
      We will now refer to  FIG. 7A . The tube comprises a vacuum jacket  41  intended to contain a gaseous mixture of tritium and/or deuterium under low pressure (of around several tenths of Pascals). The getter referenced  42  enables the supply of a gaseous mixture and acts as a pressure regulator. The jacket  41  is sealed by an electrically insulating part  43  which is crossed by high voltage connections  44  for the energy supply for an ion source  45  placed in said jacket  41 . Said ion source  45  ionises the gaseous mixture present in said jacket  41 . Said ion source  45  is intended to produce ions that form an ion beam  46  directed towards the patterned target  47  of the invention when they are accelerated by an accelerating electrode  48  placed between the ion source  45  and the patterned target  47 . Said ion source  45  may be a Penning type one and comprises an anode  71 , a cathode  72  and a permanent magnet  73  which creates a magnetic field normal to the electric field established between the cathode  72  and the anode  71 . A ferromagnetic casing  74  surrounds the cathode  72  and the anode  71  and guides the magnetic field generated by the magnet  73  so that it re-closes at the level of an air gap crossed by the ion beam  46 .  
      The jacket  41  also contains an α particle detector  49  formed, for example, of a plurality of junction diodes or a scintillator or a series of scintillating optic fibres sensitive to α particles and insensitive to neutrons and photons. The scintillator may be associated with a photomultiplier tube or a microchannel wafer device (not represented).  
      When an ion hits an emissive part of the target  47 , there is a simultaneous emission of a neutron n and an α particle in opposite directions. In the target  47 , the emissive parts and their support constitute an impassable obstacle to the α particles that sink into it. The useful α particles reach the α particle detector  49 . An α particle provides information on a neutron n emitted in the opposite direction.  
      The patterned target  47  may be inclined in relation to the ion beam  46 , for example by around 45°, in order to optimise the detection of α particles without disrupting the ion beam  46 —target  47  interaction. The α particle detector  49  is inclined in relation to the target  47 ; it is directed substantially like the particle beam  46 . It could be substantially parallel to the target as shown in  FIG. 8C .  
      In the same way, a particle accelerator may include an α particle detector as illustrated in  FIGS. 7B and 7C . The α particle detector is referenced  69 . It may be the same type as that described in  FIG. 7A . In  FIG. 7B , the target  65  is inclined in relation to the ion beam  64  whereas in  FIG. 7C , it is normal to the ion beam  64 .  
       FIGS. 8A and 8B  schematically illustrate installations for imaging and/or analysis materials, which may be hidden, using a conventional neutron generating tube  53  having associated particles. In  FIG. 8A , the neutron generating tube  53  is a point target  53  and in  FIG. 8B  it is an extended target  56 ′. In said neutron generating tubes  53 , the ion source is referenced  54  and the ion beam that it produces  55 . The ion source  54  and the target  56  or  56 ′ are contained within a vacuum jacket  57 . The jacket also contains an α particle detector  58 . The reference  51  illustrates an object in which one wishes to analyse the material and/or which one wishes to view in one or two dimensions.  
      One makes use of the fact that certain neutrons stemming from the target  56  penetrate into the object  51  and interact with the nuclei of atoms in such a way that each nucleus emits in return one or several gamma rays (γ rays) or γ event with an energy characteristic of the energy of the chemical element from which its stems. At least one γ radiation detector  50  is placed near to the object  51 . The analysis of the γ spectrum obtained provides information on the composition, chemical element by chemical element, of the materials encountered by the neutron flow emitted by the tube in a given volume of material bombarded with neutrons. But in fact this type of spectrum is difficult to use on its own, because it is drowned in the noise that results from the detection of numerous γ rays generated outside the volume of the object  51 , for example by γ radiation generated by the interaction of neutrons with the γ radiation detector  50 .  
      One aims to determine the constitution and/or to obtain a pseudo-image of an object placed at a distance h from the target  56 ,  56 ′. Said pseudo-image in two or three dimensions is called a pseudo-image because it is an indirect image of the object. It is not given by the neutrons that cross through the object but by the γ radiation gathered near to the object. It is representative of its chemical constitution. The distance h corresponds to a time of flight t for the neutrons emitted by the target  56 ,  56 ′. Said time of flight t is known from the speed of a neutron (5.2 cm/s for neutrons of 14 MeV energy), from its moment of departure known through the detection of the associated α particle which has, for its part, a speed of 1.3 cm/s.  
      One is interested in γ events detected by said γ radiation detector  50 , associated with this characteristic time of flight t. Said γ events are all linked to the detection of α particles by the detector  58  composed of a plurality of pixels  76  arranged in a matrix. Said α particle detector and the γ radiation detector are linked to treatment means  59  which determine the γ events in coincidence with a pixel p x,y  for a given time of flight t.  
      Calculation means  70  (for example, a personal computer type) connected to the output of the treatment means  59  make it possible to carry out a spectral analysis of the object  51 . Spectral deconvolution means  75  connected to the calculation means  70  deliver the pseudo-image in two dimensions or in three dimensions of the object  51 , chemical element by chemical element.  
      The resolution of the pseudo-image depends on the size of the target. We will now refer to  FIG. 8A  which illustrates the case of a point target  56 . The smaller the target the better the resolution. Indeed, one p x,y  of the pixels  76  of the α particle detector  58  receives the α particles which come from the neutrons n emitted by the point target  56  and said α particles translate the interactions of neutrons n circulating in a straight line d which passes through the pixel p x,y  and the point that constitutes the point target  56 . The location of the pixel p x,y  of the α particle detector  58  makes it possible to know the trajectory of the neutrons n emitted simultaneously with the α particles in question since it is the straight line d. The γ radiation spectrum obtained for the given time of flight t and for a γ-α coincidence of the time width Δt with the pixel p x,y  is the characteristic spectrum of a volume of material, obtained by a projection in volume of the pixel p x,y  through the point target  56 . The thickness of the material of the volume is defined by the product of the speed of the neutrons multiplied by Δt. However, a neutron generating tube  53  having a point target  56  has a short lifetime. One therefore tends to increase the surface area of the target.  
       FIG. 8B  differs from  FIG. 8A  by the fact that the target  56 ′ in not a point target but a extended target. This is an extreme case. The location of an α particle on the α particle detector  58  by a pixel p x,y  does not allow the path of the corresponding neutron n to be known. It can only be localised within the interior of a solid angle A, the summit of which is the pixel p x,y , which leans on the contour of the target  56 ′ and which passes through the object  51 . This configuration excludes the notion of sharp image.  
       FIG. 8C  illustrates an installation for imaging and/or analysing materials, which may be hidden, using with reference  82  a neutron source, either a neutron generating tube of the same type as that in  FIG. 7A  or a particle accelerator of the same type as that in  FIGS. 7B  or  7 C. Said tube or accelerator contains the patterned target  80  which is the subject of the invention. The remainder of the installation is similar to that represented in  FIGS. 8A and 8B  with the exception of the fact that the geometric deconvolution means  81  are inserted between the calculation means  70  and the spectral deconvolution means  75 . The operating principle of such an installation is similar to that of the installation in  FIG. 8A  with a multitude of distinct point targets. By analogy with the use of the neutron generating tube in  FIG. 3A , in the absence of geometric deconvolution means, one would obtain at each point of the image of the object  51 , the mixed image of the object itself and of the patterned target. The geometric deconvolution means  81 , for example taking the form of an inverse algorithm, enable the two mixed images to be separated.  
      An elementary fragment (not represented) of material of the object  51  contributes to L pixels  76  of the α particle detector  58  and thus to L γ spectra. Everything takes place as if the detection of γ radiation from the object  51  bombarded with the neutrons n was carried out by the α particle detector  58  itself, through a coded mask with the pattern of the target  80 .  
      If the target  80  is composed of M point targets, the spectrum associated with each of the L pixels  76  of the α particle detector is a combination of M spectra corresponding to M elementary fragments of the object  51  through the intermediary of the target. Mathematically, one obtains a system close to a system of L equations and M unknowns. By inverting this system one may access the spectra of the elementary fragments of the object. In order to be solvable, this system requires that M (number of point targets) is less than L (number of α pixels).  
      The γ radiation detector  50  could be replaced by a neutron detector such as that bearing the reference  33  in  FIG. 3A . The principle of using the installation would be the same to carry out the imaging of substances that may be hidden. For this reason, it has not been added to the new figure.  
      Thus, with neutron generating tubes and particle accelerators according to the invention it is possible to carry out direct imaging such as neutron radiography or indirect imaging such as analysis by γ spectrometry and an associated particle tube. In both cases, the coding of the target allows the coding of the image or the pseudo-image and obtaining a “pure” image or pseudo-image of the object, cleared of the aberrations due to the non-point nature of the targets formed in practice. Consequently, the invention makes it possible to produce systems that have both high geometric resolution and a long lifetime, which were not compatible previously.  
      Although several embodiments of the present invention have been represented and described in a detailed manner, it will be understood that various changes and modifications may be made without going beyond the scope of the invention.