WO2012065948A1 - X-ray fluorescence imaging device - Google Patents

Abstract

The invention relates to an X-ray fluorescence imaging device comprising at least one energization source (1) capable of emitting an interrogating beam (2) toward an object (3) such that the object emits radiation (4) as a result of the interaction between the interrogating beam (2) and the object (3), said radiation (4) emitted by the object due to the irradiation including X-ray fluorescence radiation, an image-generating device (5) provided with a detector (5.1) for detecting the radiation emitted by the object due to the irradiation, and a collimator (8) arranged upstream from the detector (5.1) for limiting the field of view of the latter. The detector (5.1) is a pixel detector, the pixels of which are to perform a function of an energy-based selection of the detected radiation, and supply an image signal relating to the radiation detected in a given energy band.

Description

 X-FLUORESCENCE IMAGING DEVICE

DESCRIPTION

TECHNICAL AREA

 The invention relates to the technical field of X-ray fluorescence, and more specifically to the detection of X-ray fluorescence radiation, under the effect of the irradiation of a material with incident radiation, such as X or gamma radiation.

 When ionizing radiation, for example a gamma ray or an X-ray, interacts in matter, there is a certain probability that electrons are torn from the electronic corpuscle to which they belong, by photoelectric effect. By rearranging itself, the electronic procession can emit an X-ray, whose energy corresponds to the difference between the energy of the starting layer and the energy of the arrival layer of the electron. This phenomenon is called X-ray fluorescence. It is well known and commonly used in analytical methods.

STATE OF THE PRIOR ART

As is commonly practiced, X-ray fluorescence consists in irradiating a material with an interrogating beam, this beam being formed by electromagnetic X or gamma rays, produced for example by an X-ray generator or a radioactive source, respectively. This interrogating beam excites the material and the latter emits radiation of ionizing photons having: a so-called fluorescence component corresponding to fluorescence X photons emitted by the zone of the material irradiated by the interrogating beam

 an inelastic scattering component, or Compton scattering, corresponding to the inelastic scattering of the interrogator beam radiation in the material.

 A monolithic X-ray detector is used which will detect radiation emitted by the material under the effect of irradiation by the interrogator beam. This radiation is an ionizing radiation whose energy is of the order or greater than a few keV. From this monolithic X-ray detector, the energy of the radiation detected by the detector will be acquired. The monolithic X-ray detector can be a germanium type detector.

 By placing the monolithic X-ray detector facing the material which is excited by the interrogating beam, the monolithic detector having a spectrometric function, an energy distribution of the radiation emitted by the material is obtained under the effect of the irradiation resulting from the source, this energy distribution being commonly called energy spectrum.

In general, the energy of the X or gamma ionizing photon of the interrogating beam is chosen to be greater than or equal to the electron binding energy of the K layers, or, in some cases, L layers on which the electrons constituting the material to be analyzed. When an X photon (or gamma) incident has an energy equal to the binding energy of an electron, it has a certain probability of ejecting this electron by photoelectric effect. The atom that included this electron is then in an excited state. The de-excitation is done by an electronic transition, with emission of an X photon or an Auger electron. The heavier the atom, the more the emission of an X photon is favored to the detriment of the emission of an Auger electron.

 In the present monolithic detector patent application is meant a device capable of carrying out a transformation of the radiation which it detects in order to extract a signal which is generally the energy spectrum of the ionizing photons of the detected radiation. This monolithic detector can not provide information on the geometric location of received ionizing photons having a given energy relative to other ionizing photons having another energy, received simultaneously.

 On the energy spectrum of the radiation detected by the detector, can be highlighted lines corresponding to fluorescence X-rays, whose energy (or energies) allows (tent) to identify the element (or elements) constituting the material. The determination of the fluorescence energy makes it possible to obtain information on the elemental composition of the sample. Such a technique is commonly used for the elemental analysis of liquid or solid samples.

These measurements can be performed using a very fine interrogator beam, crossing for example a collimator. This is called microfluorescence. In this case, the monolithic detector can receive substantially all the radiation emitted by the material corresponding to a position of the excitation source emitting the interrogating beam. An elementary mapping of a heterogeneous material can be carried out by moving the excitation source emitting the interrogating beam with respect to the monolithic detector by providing for associating with the monolithic detector means for processing a plurality of measurements made by the monolithic detector. . One measure can not lead to this mapping.

 A monolithic X-ray detector does not give spatial information as to the zone containing the detected chemical element, it only gives information on the presence of the given chemical element.

Still according to the prior art, alternatively, the interrogator beam may be an electron beam, according to the principle of a Castaing probe, or microprobe. The kinetic energy of the interrogator beam electrons can result in the ejection of an electron from the atom. In a way similar to what results from the photoelectric effect, the atom is found excited. If the ejected electron is part of the K (or L in some cases) layers, the excited atom returns to its initial state by emitting an X-ray characteristic of the electronic transitions. In the same way as X-ray fluorescence, its energy therefore depends on the element. Alternatively, the interrogator beam may consist of an ion beam instead of an electron beam. In general, this technique is directed to elements having a high fluorescence yield, typically having an atomic number Z> 20.

It is thus seen that in the state of the art, the X-ray fluorescence of a sample is produced by irradiation with an interrogating beam of incident energy particles (X or gamma photons, electrons, ions). The interrogator beam is usually very thin and collimated, it irradiates an elemental area of the sample. Elementary zone means the zone irradiated by the stationary interrogator beam at a given instant. The radiation emitted by the sample under the effect of the irradiation with the interrogating beam is detected by the monolithic detector in order to extract a signal which is generally the energy spectrum of the photons X of the detected radiation. It is then possible to detect the presence at the level of the elementary zone of the sample irradiated by the interrogating beam of a chemical element considered by the presence of fluorescence lines characterizing it, but the location of the chemical element in the irradiated zone. is not known. Moreover, with some calibrations, the intensity of these lines may be related to the concentration of this chemical element in the volume (or area) of the elementary zone by the interrogator beam. When it is desired to characterize the surface of a sample and not only an elementary zone of the latter, it is necessary to move the interrogator beam, which is generally finely collimated, and to measure the detected radiation for each position of the beam. We speak of discrete measurements, made by scanning the source on the analyzed material. From these discrete measurements, we can reconstruct a two-dimensional map of the elements present on the surface of the sample characterized by carrying out an appropriate signal processing with all the measurements collected. Obtaining this mapping therefore takes a significant time. This type of system is very powerful but it requires a displacement of the excitation source near the surface to be characterized if one wishes to obtain a precise result. When the excitation source is placed at a distance, only an average elemental composition of the irradiated surface or the volume is obtained if it is not very absorbent. The spatial resolution depends on the size of the incident interrogator beam on the analyzed material. It is understood that if one wishes to obtain a good spatial resolution, it is necessary to use a finely collimated source. The duration of analysis is high.

With a location of the excitation source emitting the interrogating beam at a distance, even with a scan of the surface to be characterized with the interrogating beam, precise spatial information can not be obtained, that is to say a precise location of this or that chemical element throughout the irradiated area when scanning the interrogator beam.

 The patent application US 2007/0108387 describes an analysis device comprising a lens closed, in its central part, by a dense screen, placed between a sample excited by an interrogator beam and an image generating device for example comprising a device CCD charge transfer. In this context, an image generating device provides in addition information on the presence of a zone containing a given chemical element, geometric information namely the shape, size and location of the zone containing the element. detected in relation to its environment in the analyzed material, this environment being also irradiated by the interrogating beam and included in the field of view of the image generating device. In this context the image generator comprises a pixel detector and a monolithic detector, that is to say formed of a plurality of independent elementary detectors, each elementary detector or pixel representing a point of the acquired image. These elementary detectors are arranged in a matrix or bar and cooperate with a read circuit. Each pixel receives a fraction of the radiation emitted by the object under the effect of the irradiation and therefore of the fluorescence X-ray.

The closed lens is positioned so that the radiation, of a given wavelength, emitted by the sample under the effect of a irradiation by an interrogator beam, is focused on the CCD type image generating device. The shutter placed in the central part of the lens blocks the radiation emitted by the object under the effect of the irradiation, not focused by the lens. According to this invention, only the radiation of a predetermined wavelength translating the presence of a zone into a given chemical element constitutes the useful signal received by the image generating device. The image acquired by the image generating device thus provides information on the presence of the chemical element given in the zone, on the geometry (size and shape) and on the location of the zone. If the closed lens is removable, a change of the lens closed by another having another focal distance allows the image generating device to receive radiation of another wavelength and to acquire an image indicating the presence another chemical element in the area. The superposition of several images delivered by the image generating device and acquired with different closed lenses makes it possible to locate on the same image the different chemical elements detected. Such a device requires knowing in advance the chemical elements to be detected and having lenses having focal lengths adapted to the wavelengths to be detected. Another disadvantage is that it is necessary to provide a device that mechanically displaces the lenses and possibly the sample and / or the image-generating device. Adjusting the position of the lens with respect to the sample on the one hand and the image-generating device on the other hand is delicate. In addition, the deflection of an X-ray beam by a lens is effective only on the low energies of a few hundred electron volts (eV), even a few kilo electron volts (keV). Beyond a few keV, X photons are not deflected by such a lens and the previously described device becomes inoperative.

 The lens therefore has a function of selection in energy of the radiation emitted by the object under the effect of the irradiation which passes through it since the wavelength and the energy are magnitudes dependent on each other and characteristics of the chemical element that produces this X fluorescence. It is not the pixel detector that realizes the energy selection of the different chemical elements present.

US Pat. No. 4,987,582 discloses a device comprising a source of X or gamma radiation producing an interrogating beam intended to irradiate a sample, for example a piece of luggage during control at the airports, and a device for detecting a radiation emitted by said sample under the effect of irradiation by the interrogator beam. The detection means can be parameterized for a given wavelength and therefore for a given chemical element, as a function of the characteristic Bragg angle of said wavelength. For this, it comprises one or more blocks of crystalline material without dislocation through which the radiation emitted by said sample under the effect of irradiation passes without practically attenuation if the radiation emitted by the sample under the effect of irradiation arrive on a block of crystalline material given with the Bragg angle. At the output of the block of crystalline material, an image-generating device is placed with a pixel detector. It is also expected that the image generator cooperates with an image processing device and a display device.

 The disadvantage of the structure described in this patent is that it is also necessary to know beforehand the chemical elements to be detected so that the crystal block (s) are oriented properly with respect to the radiation emitted by the sample under the effect of the 'irradiation. An energy selection is performed upstream of the image generating device. To have an image of the whole sample, for example of luggage type during control at the airports, it is necessary to scroll the sample in front of the excitation source / detection device pair and it is necessary to make a treatment of the different acquisitions. made by the image generator device. The device described in this document can only work with radiation emitted by the sample under the effect of low energy irradiation of the order of a few keV.

The document "Energy-and-position-sensitive pixel detector Timepix for X-ray fluorescence imaging", Jan Zemlicka et al., Nuclear Instruments and Methods in Physics Research A 607 (2009), pages 202-204, discloses the possibility of performing a picture of X-ray fluorescence from objects made of metallic materials such as nickel, copper or zinc. To do this, the object is irradiated with X-radiation emitted by a source and fluorescence X-radiation is detected by the Timepix pixel detector. The Timepix detector is described in the document "Pixel detectors for imaging with heavy charged particles", Jan Jakubek et al., Nuclear Instruments and Methods in Physics Research A 591 (2008) pages 155-158. The Timepix detector is formed of a thickness of 300 microns of silicon deposited on an electrode matrix so as to materialize the pixels. Each pixel is connected to an electronic circuit making it possible to produce an energy spectrum.

 Although FIG. 4 of the latter document shows that it is possible to produce energy spectra of a radiation received at energies close to 60 keV, the inventors believe that with materials containing heavy chemical elements, it is the atomic number of which is greater than that of gold, lead, or even uranium, a Timepix detector can not produce a signal whose characteristics are satisfactory both from a point of view of the sensitivity and of the resolution in energy.

STATEMENT OF THE INVENTION

The object of the present invention is precisely to propose an X-ray fluorescence imaging device which does not have the disadvantages mentioned above, ie which does not require displacement of the excitation source with respect to the detector, which does not requires no prior knowledge of the chemical elements contained in the object to be imaged and which gives a satisfactory image.

 In order to achieve this, the present invention uses a pixel detector to detect radiation emitted by an object irradiated by an interrogating beam emitted by an excitation source, this pixel detector integrating a function for selecting energy from the detected radiation, and providing an image signal relative to the detected radiation in a given energy band. The radiation emitted by the object under the effect of the irradiation and detected by the pixel detector does not undergo any sorting in wavelengths between its emission and its acquisition by the pixel detector. Using a pixel detector avoids the relative movement between itself and the excitation source.

More specifically, the present invention relates to an X-ray fluorescence imaging device comprising at least one excitation source able to emit an interrogating beam towards an object to be imaged so that the object emits a radiation that is a consequence of the interaction between the interrogating beam and the object, the radiation emitted by the object under the effect of the irradiation including fluorescence X-ray, an image generating device provided with a detector for detecting the emitted radiation by the object under the effect of irradiation, a collimator upstream of the detector to limit its field of view. According to the invention, the detector is a pixel detector whose pixels are intended to perform a function of energy selection of the detected radiation and to provide an image signal relative to the radiation detected in a given energy band.

 The energy selection function can be energy thresholding, energy windowing or even more advantageously even energy spectrometry.

 The collimator may be a coded mask collimator, which makes it possible to improve the sensitivity and leads to a better noise ratio, or a parallel channel collimator, especially if the object is close to the pixel detector, or a pinhole collimator. , the latter having an infinite depth of field.

 The excitation source may be an X - ray tube, a radioactive source, an electron gun or an ion gun.

 The image generating device may furthermore comprise a device for displaying the image signal, this image signal possibly being processed by an image processing device.

The pixel detector may comprise a scintillator upstream of a charge transfer device, for example of the CCD type or of a device with CMOS transistors, the scintillator having to convert the radiation emitted by the object under the effect of the irradiation which reaches it in a light signal delivered to the charge transfer device or to the CMOS transistor device. The pixel detector may further comprise an image intensifier tube disposed between the scintillator and the charge transfer device or the CMOS transistor device and optionally a bundle of optical fibers connecting the image intensifier tube to the device. charge transfer or to the device with CMOS transistors.

 In another embodiment, the pixel detector may comprise independent semiconductor elementary detectors for detecting the radiation emitted by the object under the effect of the irradiation and connected to an integrated reading circuit.

 In yet another embodiment, the pixel detector may comprise a monolithic semiconductor material for detecting the radiation emitted by the object under the effect of the irradiation, and connected to a reading circuit, generally matrix, defining the pixels.

 In addition, shielding may be provided between the excitation source and the image generating device.

 The excitation source may be equipped with a collimator which defines an axis of the interrogator beam.

The collimator disposed upstream of the pixel detector defines a collimation axis of the pixel detector. This collimator defines the solid detection angle of the detector. The axis of the interrogating beam and the collimation axis being preferably at an angle of the order of 150 ° to reduce the Compton scattering component in the radiation emitted by the object under the effect of irradiation.

 The device may further comprise an auxiliary camera associated with the image-generating device, able to take a visible image of an area of the object observed by the pixel detector.

 A separation prism of the radiation emitted by the object under the effect of the irradiation of visible radiation from the object can be used, this prism being placed on the path taken by the radiation emitted by the object under the effect of irradiation to the pixel detector.

 The present invention also relates to an X-ray fluorescence imaging method, comprising:

 irradiating an object to be imaged by an interrogating beam emitted by an excitation source,

 to detect radiation emitted by the object under the effect of irradiation by a pixel detector of an image-generating device, after passing through a collimator, this pixel detector having an energy selection function detected radiation, and

 to provide, by each of several pixels of the pixel detector, an image signal relative to the detected radiation in a given energy band.

The detection of the radiation emitted by the object under the effect of the irradiation is made of preferably substantially simultaneously with the irradiation of the object with the interrogator beam.

 Energy selection can be energy thresholding, energy windowing, or energy spectrometry.

 The method may further include identifying a chemical element from the detected radiation in a given energy band.

 When the energy selection is spectrometry, the pixels provide a spectral image signal, the method also includes a peak detection step in each spectral image signal.

 To identify a chemical element of the object, the method may comprise when the energy selection is a spectrometry, a comparison of the energy of each peak with the X-ray fluorescence energy of known chemical elements.

 The method may further include displaying the intensity of the detected radiation in a given energy band.

BRIEF DESCRIPTION OF THE DRAWINGS

 The present invention will be better understood on reading the description of exemplary embodiments given, purely by way of indication and in no way limiting, with reference to the appended drawings in which:

 FIG. 1 schematically illustrates an X-ray fluorescence imaging device according to the invention;

Figures 2A, 2B show different variants for the collimator; FIG. 3 illustrates, in an imaging device according to the invention, an image generating device associated with an auxiliary camera with a separation prism of the X-ray fluorescence radiation;

 FIG. 4 illustrates an image-generating device of the imaging device according to the invention with an image pixel detector capable of acquiring both X-ray fluorescence images and visible images;

 FIGS. 5A, 5B, 5C, 5D, 5E show different variants of the pixel detector;

 FIGS. 6A, 6B, 6C respectively illustrate the function of energy thresholding, windowing energy and energy spectrometry.

 Identical, similar or equivalent parts of the different figures bear the same numerical references so as to facilitate the passage from one figure to another.

 The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Referring to Figure 1 which shows an example of X-ray fluorescence imaging device object of the invention. It comprises an excitation source 1 able to emit an interrogating beam 2 towards an object 3 to be imaged and characterized. The excitation source 1 is equipped with a collimator (not referenced) which defines an axis 2.1 of the beam interrogator 2, the interrogating beam propagates along this axis 2.1. The term "object" has been used in a general sense, namely everything that, whether animate or inanimate, affects the senses, mainly sight.

 The interrogating beam 2 is capable of interacting with the material of the object 3 so as to excite it so that it emits ionizing radiation 4, said radiation emitted by the object under the effect of an irradiation by the interrogator beam 2. This radiation 4 emitted by the object under the effect of irradiation has a fluorescence component and an inelastic scattering component as already mentioned above. This radiation 4 emitted by the object under the effect of the irradiation can be emitted only by a region z of the object 3, only this region z having been irradiated by the interrogating beam 2. The interrogating beam 2 can be an X-ray or gamma ray beam, an electron beam or an ion beam. It can be poly energetic, that is to say convey several wavelengths and not one, and this will be the case when the excitation source 1 is an X-ray generator.

The excitation source 1 may be an X-ray generator as has just been asserted, but other variants exist. It can be for example a radioactive source for example at ⁵⁷ Co, an electron gun or an ion gun.

The radiation 4 emitted by the object 3 under the effect of the irradiation is directed towards an image generating device 5 capable of detecting X-rays. The image-generating device 5 comprises a detector at 5.1 pixels, which in this example, has a sensitive portion 5.10 radiation 4 emitted by the object under the effect of irradiation. This sensitive portion 5.10 may be formed of a plurality of independent elementary detectors 5.13 sensitive to radiation 4 emitted by the object under the effect of irradiation, these elementary detectors 5.13 form the pixels. They cooperate with an electronic reading circuit 5.11. The image generating device 5 may further comprise an image processing circuit 5.2 connected at the output of the pixel detector 5.1, that is to say connected to the electronic reading circuit 5.11 and a display device 5.3. connected to the image processing circuit 5.2.

 The 5.1 pixel detector can take several configurations, as will be seen later.

The image generating device 5 is intended to provide for each of its pixels, an image signal corresponding to a radiation that it detects in a given energy band. The detected radiation comes from the radiation 4 emitted by the object under the effect of the irradiation reaching the detector with pixels 5.1. The radiation emitted by the object under the effect of the irradiation is free of wavelength filtering between its emission by the object 3 and its detection by the pixel detector 5.1 of the image generator device 5 The image-generating device 5 makes it possible to obtain, for a plurality of pixels 5.13, an image signal corresponding to the intensity of the radiation detected in a given energy band. In other words the 5.1 pixel detector makes it possible to obtain an image of the radiation detected in a given energy band, hence the use of the expression image generator. By image is meant a matrix of pixels, each pixel 5.13 delivering a spatially resolved information, relating to the detected radiation from an elementary zone of the observed object.

 According to the invention, the 5.1 pixel detector therefore integrates a power selection function. We then combine information spatially resolved, but also spectrally resolved, that is to say resolved in energy. It is no longer an additional mechanical device that performs this function as in the prior art. Thus, it is no longer necessary to know in advance the different chemical elements to be detected. This energy selection function applies to each of the pixels but only certain pixels can use it at a given moment. This energy selection function means that the pixel detector is capable of delivering an image signal relative to the energy of the radiation that it detects in at least one given energy band.

Thus one can simultaneously acquire the radiation 4 emitted by the object under the effect of irradiation by the interrogator beam through a plurality of pixels 5.13 of the 5.1 pixel detector. Advantageously, the pixel detector 5.1 comprises several tens, or even several hundreds, or even several thousand pixels 5.13. Each of these pixels 5.13 is associated with an elementary surface of the irradiated object 3, so as to deliver information spatially resolved. Thus one can simultaneously obtain the intensity of the radiation emitted by the object 3 under the effect of the irradiation in a given energy range. The spatial resolution of the image generating device 5 is determined by the elementary surface of the object 3 seen by a single pixel 5.13. It is not necessary to move the excitation source 1, nor the 5.1 pixel detector, nor the object 3 to obtain spatially resolved information of the radiation 4 emitted by the object under the effect of the irradiation .

 It is possible to arrange for the pixels 5.13 of the 5.1 pixel detector to simultaneously deliver an image signal relating to the intensity of the radiation 4 emitted by the object under the effect of the irradiation in the same energy band. One can speak of image of the radiation 4 emitted by the object associated with said energy band. Thus, if the object 3 comprises an element whose X-ray fluorescence energy is located in this energy band, the pixels 5.13 of the pixel detector 5.1 corresponding to the position of this element in the object 3 detect a signal more intense than the other pixels. When implementing the image generator 5, it is possible to realize at its pixel detector 5.1 a scanning energy, by varying the energy band associated with the radiation image 4 emitted by the object under the effect of irradiation. We then obtain as many images as bands of energy.

The 5.1 pixel detector has a line of sight 6 which is substantially perpendicular to a plane, called the input plane, through which the radiation 4 emitted by the object under the effect of the irradiation reaches the sensitive part 5.10 of the pixel detector 5.1.

 The 5.1 pixel detector and the excitation source 1 as well as possibly the object 3 can be static with respect to each other, which simplifies the assembly of the different components of the imaging device with respect to each other.

 A collimator 8 is placed upstream of the pixel detector 5.1 and therefore of the sensitive part 5.10 of said pixel detector. The concept of upstream and downstream is based on the direction of propagation of radiation 4 emitted by the object under the effect of irradiation. The radiation 4 emitted by the object under the effect of the irradiation reaching the sensitive part 5.10 of the pixel detector 5.1 has thus passed through the collimator 8. The collimator 8 defines a collimation axis of the pixel detector which is substantially merged with the line of sight 6 of the pixel detector. The collimator 8 delimits the field of view of the pixel detector 5.1, this field of view extends around the line of sight 6. The collimator 8 delimits a solid angle of the pixel detector 5.1, this solid angle s' extending around the collimation axis 6. This solid angle is named Θ in FIG. 2B.

The collimator 8 may be a collimator with parallel channels as shown in FIG. 1. In this case, it is preferably located close to the object 3 and the pixel detector 5.1. The distance between the pixel detector 5.1 and the collimator 8 must be in this configuration, less than approximately 1 or 2 centimeters, the distance between the collimator 8 and the object 3 being also limited to a few centimeters, if it is desired to obtain a spatially resolved information, which may appear as a constraint.

 Alternatively, a masked mask collimator 8 as illustrated in FIG. 2A could be used, which makes it possible to improve the sensitivity and leads to a better noise ratio at the expense of a more complex deconvolution algorithm.

 Yet another variant would be to use a pinhole collimator 8 as shown in FIG. 2B. This offers the advantage of a very large depth of field, a simplicity of design as well as a solid angle Θ allowing the observation of important surface objects. In general, a pinhole collimator is in the form of a small diameter hole, generally between a few hundred micrometers and a few millimeters, in a dense material. Such collimators are usually used in gamma cameras intended for the observation of irradiating sources, for example that described in the patent application EP 0 425 333.

The image generating device 5 will provide an image 7 of the irradiated region z in the field of view of the pixel detector 5.1. This image makes it possible to obtain information on the chemical elements present in the intersection between the solid angle of the pixel detector and the irradiated part of the object. It should be noted that these are The depth depends on the energy of the interrogator beam 2. The image makes it possible to highlight the arrangement of fluorescent zones, in a given energy, of the irradiated region z of the object 3 included in the field of view of the pixel detector 5.1.

 It is advantageous to provide a shield 9 placed close to the collimator 8 of sufficient thickness so as to limit the influence of the interrogator beam 2 on the pixel detector 5.1 or to limit any diffusion effects. It may be made for example of lead and / or tungsten or other X-ray arresting materials.

 It can be provided to associate the image generating device 5 with an auxiliary camera 10 sensitive to visible light capable of acquiring a visible image of the object 3 to be imaged or at least a visible image of the area observed by the detector to pixels 5.1. With the reference 10.1 in FIG. 3, visible radiation is shown which reaches the auxiliary camera 10.

This auxiliary camera 10 may be a color or black and white camera. It may be secured to the image generating device 5. It will preferably have an optical axis 11 which is substantially parallel to the line of sight or collimation 6 of the pixel detector 5.1. In the image processing circuit 5.2, a computer parallax correction can be provided, after a calibration step, between the pixel detector 5.1 and the As a variant, it is possible to provide a separation prism 12 to perform this parallax correction as shown in FIG. 3. The separation prism 12 is placed on the path of the radiation 4 emitted by the object 3 under the effect of the irradiation, upstream of the detector with pixels 5.1 and the auxiliary camera 10. It is transparent vis-à-vis the radiation 4 emitted by the object under the effect of the irradiation , which then is not deflected and can reach the image generator device 5. On the other hand, it deflects the visible radiation 10.1 which will be detected by the auxiliary camera 10.

Alternatively as illustrated in Figure 4, it is conceivable that it is directly the pixel detector 5.1 of the image generator device 5 is able to acquire visible images, including black and white of the object. In this case, the sensitive part 5.10 is also sensitive to visible light. A retractable mechanical shutter 13 is provided, placed in a non-retracted position upstream of the sensitive portion 5.10 to prevent the visible light reflected by the object from reaching the sensitive portion 5.10. This mechanical shutter 13 passes the radiation 4 emitted by the object under the effect of irradiation. This mechanical shutter 13 is preferably located downstream of the collimator 8. When it is in the retracted position, the image acquired by the pixel detector 5.1 is a visible image and when it is in the non-retracted position, the acquired image by the 5.1 pixel detector is an image of the radiation detected by the pixel detector. The sensitive part 5.10 of the pixel detector 5.1 can be made by a charge transfer device, for example CCD, or CMOS transistor 14 possibly covered with a scintillator 15, for example cesium iodide, as shown in FIG. 5A. In this case, a read circuit is integrated with each pixel of the pixel device. This read circuit delivers image signals to the display device. The read circuit is not referenced and the pixels are not differentiated.

 The CCD or CMOS transistor device is preferably matrix. In this context a matrix device comprises several sensitive sites, the pixels, arranged in matrix or bar.

The scintillator 15 converts the radiation 4 emitted by the object under the effect of the irradiation into a light signal and the CCD or CMOS transistor matrix device formed of a plurality of sensitive sites, in this case photosites, generally photodiodes, converts the light signal into electrical image signals to be displayed by the display device 5.3 after possible processing in the processing device 5.2, if the display device and the processing device are provided. It is these electrical image signals that carry the energy selection, the radiation 4 emitted by the object under the effect of the irradiation when it reaches the detector at 5.1 pixels did not undergo energy selection, which was not the case in the prior art.

 Such a 5.1 pixel detector is particularly simple, but suffers however from a disadvantage because of the CCD matrix device or CMOS transistors. It does not allow to provide a spectrometric function, that is to say that it does not allow to know, for a given photosite, the number of ionizing photons received at a given energy. The image obtained provides spatially information on the number of ionizing photons received, that is to say on the intensity of the radiation detected from the radiation emitted by the object under the effect of the irradiation and this is the integrated reading circuit which will perform the energy selection function which can then be thresholding or windowing type as will be explained later.

 In order to reduce the diffusion component of the radiation emitted by the object under the effect of the irradiation, it is possible to adjust the angle between the axis 2.1 of the interrogating beam 2 and the collimation axis 6 of the pixel detector 5.1. This angle may be for example about 150 °.

As a variant illustrated in FIG. 5B, the pixel detector 5.1 may comprise a cascade with, in this order, from the collimator 8, a scintillator 15, an image intensifier tube 16 and a CCD or CMOS transistor device 14 The image intensifier tube 16 serves to amplify the light signal delivered by the scintillator 15. A bundle of optical fibers 17 (typing in English) can connect the image intensifier tube 16 to the CCD or CMOS transistor type device 14. This configuration makes it possible to obtain an image signal relating to the intensity of the radiation emitted by the object under the effect of irradiation in a bounded energy band whose maximum and minimum energies can be established.

 Preferably, to examine the fluorescence radiation and in particular the radiation resulting from transitions to the K layers of heavy chemical elements, the pixel detector 5.1 is made of a semiconductor material for the detection of ionizing photons whose energy exceeds a few keV, even a few dozen keV. In particular for lead, energies higher than about 70 keV are obtained. In spite of a lower energy resolution than that of a germanium detector, it will be preferred to use a detector material that can be used at room temperature.

This semiconductor material is, for example, CdTe cadmium telluride, CdZnTe zinc doped cadmium telluride, Hgl ₂ mercury iodide. These materials are known for their spectrometric properties at room temperature. As an alternative to detectors made of semiconductor material such as cadmium telluride CdTe, zinc-doped cadmium telluride CdZnTe, mercury iodide HgΣ ₂ , detectors made of scintillating material such as lanthanum bromide LaBr ₃ can advantageously be used. .

The detectors made of these materials do not therefore have the disadvantages of detectors silicon. The spectra obtained are sufficiently defined and the use of deconvolution techniques is avoided to isolate the different fluorescence lines, the reliability of these deconvolution techniques being random.

 A pixel detector is illustrated in Figure 5C. The pixel detector 5.1 comprises independent elementary detectors 18 preferably arranged in a matrix of semiconductor material sensitive to the radiation emitted by the object under the effect of the irradiation. The thickness of the detector is typically from a few hundred micrometers to a few millimeters, even a few centimeters. Preferably, the front faces of the elementary detectors 18, that is to say the faces exposed to the radiation emitted by the object under the effect of the irradiation, are coplanar. The elementary detectors 18 define a detection plane. The elementary detectors 18 are connected to a read integrated circuit 19, the latter being connected to the processing circuit 5.2. This read integrated circuit 19 processes the signal resulting from the interaction of an ionizing photon with the elementary detector concerned and able to measure the energy thereof. The integrated reading circuit delivers the image signals.

The elementary detectors 18 form the pixels. Specifically, the 5.1 pixel detector comprises one or more blocks, each block comprising a matrix of a plurality of elementary detectors 18, for example 4x4 up to 16 × 16, and one or more integrated circuits 19 forming an electronics module. proximity. When there are several blocks, they are contiguous. In FIG. 5C, only one column of four elementary detectors 18 is represented, but of course the pixel detector may comprise several columns and each column may have many more elementary detectors. The integrated circuit 19 for reading can be realized with ASICs. The image generating device could thus include a gamma camera such as that developed for the INTEGRAL program (INTErnational Gamma-Ray Astrophysics

Laboratory) of the European Space Agency.

 As a variant illustrated in FIG. 5D, the pixel detector 5.1 of the image-generating device could be made of monolithic semiconductor material 20 cooperating with a reading circuit 21. It is the reading circuit 21 that defines the pixels and which also delivers the image signals. The read circuit is connected to the not shown processing device. Such a pixel detector may for example be a MEDIPIX chip.

 In general, as illustrated in FIG. 5E, it is advantageous to have a pixel detector 5.1 made of semiconductor material 20. In the example, the semiconductor material 20 is of parallelepipedal shape.

A first electrode 50 polarized at a first potential is contiguous with a first face of the semiconductor material 20. A second electrode 51 polarized at a second potential is contiguous with a second face of the semiconductor material 20, the second face being opposed to the first face. The second potential is greater than the first potential; thus, the first electrode 50 is a cathode, while the second electrode is an anode. The second electrode 51 is composed of a plurality of elementary electrodes, or elementary anodes, 51i ... _51n arranged on the second face of the detector 20 in a matrix material. Each elementary electrode 51i ... _51n is connected to a read circuit 22, capable of collecting and processing electrical pulses produced as a result of interactions of the radiation detected by the detector 5.1 from the radiation emitted by the object under the effect of irradiation. Thus, each elementary electrode, in cooperation with the read circuit, defines a pixel. The first electrode 50 and the second electrode 51 are polarized by polarization means 53, represented here in the same block as the read circuit 21. The semiconductor material 20 may be for example cadmium telluride CdTe, telluride telluride. CdZnTe zinc-doped cadmium, mercury iodide Hgl ₂

 We will now return to the operation of such an imaging device.

The present invention also relates to an X-ray fluorescence imaging method. In general, the method consists in irradiating an object to be imaged by an interrogating beam emitted by an excitation source so that the object emits radiation emitted by the object under the effect of the irradiation by the interrogator beam, to detect the radiation emitted by the object under the effect of the irradiation after crossing a collimator, by a pixel detector, to make a selection in energy in the radiation detected by several pixels of the pixel detector, to deliver an image signal corresponding to the detected radiation in a given energy band. The image signal produced by each of the pixels can be processed by processing means and displayed by display means. These image signals reflect the intensity of the radiation emitted by the object under the effect of the irradiation in the given energy band. The detection of the radiation emitted by the object under the effect of the irradiation, including the fluorescence X-ray radiation, is substantially simultaneously with the irradiation of the object with the interrogating beam.

In a first mode of operation, an image generation device is used, the pixel detector of which has an energy thresholding function, that is to say, a thresholding means 60. The thresholding means 60 may be included in the read circuit or in processing means arranged downstream of the read circuit. In FIG. 1, the thresholding means 60 are schematized in the reading circuit 5.11. Referring to Figure 6A. Each pixel of the detector is capable of delivering an image signal relative to the quantity of ionizing photons detected whose energy is greater or less than a threshold σ in energy. This thresholding can be performed with all the pixels of the pixel detector or only on a few. It is thus possible to reject interactions between the interrogator beam and the object whose energy is below or above the threshold σ. In other words, thresholding helps to define a band of energy. This means that for a given threshold, for example equal to 50 keV, the image signals delivered by the image-generating device will reveal the location of one or more zones corresponding to chemical elements of the object irradiated by the beam. interrogator emitting radiation whose energy is at least or at most the given threshold. By performing several detections with different energy thresholds, the processing device can deliver a resulting image showing one or more zones corresponding to a given energy band. By way of nonlimiting example, it is assumed that two different thresholds SI and S2 are used successively, SI corresponds to the energy E1 and S2 corresponds to the energy E2. It is assumed that the energy El is greater than the energy E2. Two images PI, P2 of the object are acquired, the threshold S1 having been applied during the acquisition of the image PI and the threshold S2 having been applied during the acquisition of the image P2. The interrogating beam retains the same energy during the acquisition of the two images. The treatment device, by subtracting the two images P1-P2, will deliver a resultant image P to locate, one or more fluorescence zones corresponding to chemical elements of the object irradiated by the interrogating beam emitting radiation whose energy is between El and E2. This location is possible of course only if such areas exist. In a second and more interesting mode of operation, an image generating device is used, the 5.1 pixel detector of which has a windowing function in terms of energy, that is to say windowing means 61. The windowing means 61 may be included in the read circuit or in the processing means arranged downstream of the read circuit. In FIG. 1, the windowing means 61 are schematised in the reading circuit 5.11. The pixels of the pixel detector, concerned with this energy windowing, produce image signals relating to the quantity of X photons detected in at least one energy band ΔΕ given. Referring to Figure 6B. It is thus possible to obtain images corresponding to different spectral bands of energy. This windowing can be performed with all pixels of the pixel detector or only on a few. Interactions between the interrogator beam and the object whose resulting fluorescence energy is external to the band can thus be rejected. The terminals of the band can be adjustable or fixed. The image generating device then directly delivers image signals highlighting one or more zones corresponding to chemical elements of the object irradiated by the interrogating beam emitting radiation including fluorescence X-radiation containing a concentration of elements. whose fluorescence energy is included in the energy band. One can thus want to highlight a chemical element or a group of chemical elements of which the fluorescence energy is in the energy band. The energy band is then fixed. One can also want to achieve the most exhaustive possible mapping of the object by making several acquisitions of images with different energy bands, each of these bands of energy being correlated with one or more chemical elements. These different images can be overlaid to provide only one resulting image.

 These two functions can be done with the five embodiments of the pixel detector described in FIGS. 5A to 5E.

In a still more advantageous third mode of operation illustrated in FIG. 6C, the semiconductor pixel detector is provided with a spectrometry function, that is to say with spectrometry means 63. The spectrometry means 63 can be included in the reading circuit of each pixel or in the processing means of the read circuit of each pixel. In FIG. 5D, the spectrometric means 63 are schematized in the reading circuit 21 of each pixel. Each pixel of the pixel detector concerned by the spectrometry delivers an energy spectrum of the radiation detected from the radiation emitted by the object under the effect of the irradiation. Also the selection in energy corresponds to a discretization in energy of the detected radiation in a multitude of elementary bands of energy, whose width is typically of a few tenths of keV to a few keV. Referring to Figure 6C. In fact, we measure a "number of shots", that is to say a number of pulses generated by the pixel, the ratio between the number of photons penetrating the pixel and the number of shots generated depends on the performance of the detector. This spectrometry can be performed with all the pixels of the pixel detector or only on a few. The qualitative analysis, ie the knowledge of the chemical elements present in the object consists in detecting, in the different spectra obtained, the fluorescence peaks whose energy ε gives an indication of the nature of the material. Quantitative analysis, ie determining the concentration of one or more chemical elements requires knowing the height and / or the surface of a peak of the spectrum.

 Such a detector makes it possible to obtain spatially resolved spectral information. In other words, it makes it possible to obtain the energy spectrum of the radiation emitted by the object under the effect of irradiation by the source, and this in each of the pixels. This gives a spectral information of the radiation emitted by the different elementary zones of the object included in the intersection of the solid detection angle and undergoing irradiation of the source.

The object can be irradiated during a certain irradiation period T, varying between a few seconds and a few minutes, or even tens of minutes, and then, over several pixels, the energy spectrum of the radiation emitted by the object under the effect of irradiation.

 When the irradiation is completed, the acquisition of the spectrum also ends. We can then proceed to the counting of the measurements. This stripping can consist in visually representing a histogram in three dimensions: two dimensions correspond to the coordinates of the pixels in the detection matrix, and the third dimension corresponds to the energy.

 The information can also be represented by projecting the previously described histogram according to at least one dimension. For example, it is possible to represent, on each pixel, the intensity of the image signal in a given energy band ΔΕ. An image of the emission of the object is then obtained as a function of the energy band ΔΕ. It is then possible to vary this energy band and to obtain as many images as bands of energy ΔΕ. When an elementary zone of the object, corresponding to a pixel of the detector, comprises an element whose X fluorescence energy corresponds to this energy band, the intensity of the image signal detected at this energy has a maximum local, corresponding to the peak of fluorescence X. We can then conclude to the presence of the chemical element in said elementary zone.

The means for processing the signals of the pixels can also comprise peak extraction algorithms, commonly used in gamma spectrometry. These algorithms make it possible to obtain, in each of the pixels, the energies corresponding to detected peaks. It is then easy to conclude to the presence of particular chemical elements in the elementary zone corresponding to each pixel, by comparing the energies of the peaks thus detected with the X-ray fluorescence energies of said chemical elements, the latter being known.

 It will be understood that this embodiment makes it possible to obtain, using a single acquisition, the energy spectrum of the radiation emitted by the object, for several pixels. This embodiment is therefore particularly advantageous because it gives access, with the same acquisition, to the intensity of the radiation emitted by the object in different energy bands.

 Due to the evolution of the X-ray fluorescence yield with the atomic number of the atom concerned, the imaging device according to the invention will have an increased sensitivity for heavy atoms vis-à-vis that obtained with light atoms . By heavy, we mean heavier than iron, or even pumph.

 Detectors capable of being used in the invention are described in the following publication: "New trends in gamma ray imaging with CdZnTe / CdTe at CEA Leti", Verger et al., And Nuclear Instruments and Methods in Physics Research section A, flight. 571, Issues 1-2, February 2007, pages 33-43.

 The different variants described must be understood as not being exclusive of each other.

Although several embodiments of the present invention have been shown and described in detail, it will be understood that various changes and modifications can be made without departing from the scope of the invention.

Claims

An X-ray fluorescence imaging device comprising at least one excitation source (1) able to emit an interrogating beam (2) towards an object to be imaged, the object being made of a material comprising at least one chemical element of which the atomic number is greater than that of the lead, so that the object emits radiation (4) as a consequence of the interaction between the interrogator beam (2) and the object (3), this radiation (4) emitted by the object under the effect of the irradiation including fluorescence X-ray, an image generating device (5) provided with a detector (5.1) for detecting the radiation emitted by the object under the effect irradiation, a collimator (8) upstream of the detector (5.1) to limit its field of view, characterized in that the detector (5.1) is a pixel detector made of a semiconductor material chosen from cadmium telluride CdTe, zinc-doped cadmium telluride CdZnTe, mercuric iodide re Hgl ₂ , the pixels being intended to perform a function of energy selection of the detected radiation and to provide an image signal relative to the detected radiation in a given energy band. An imaging device according to claim 1, wherein the energy selection function is energy thresholding, energy windowing or energy spectrometry. Imaging device according to one of claims 1 or 2, wherein the collimator (8) is a pinhole collimator, a parallel channel collimator or a masked mask collimator. 4. Imaging device according to one of the preceding claims, wherein the excitation source (1) is an X-ray tube, a radioactive source, an electron gun or an ion gun. An imaging device according to one of the preceding claims, wherein the image generating device (5) further comprises a display device (5.3) of the image signal, which image signal is optionally processed by an image processing device (5.2). Imaging device according to one of the preceding claims, wherein the pixel detector (5.1) comprises a scintillator (15) upstream of a charge transfer device or a CMOS transistor device (14). , the scintillator (15) to convert the radiation emitted by the object under the effect of the irradiation that reaches it into a light signal delivered to the charge transfer device or the CMOS transistor device (14). An imaging device according to claim 6, wherein the pixel detector (5.1) further comprises an intensifier tube of images (16) disposed between the scintillator (15) and the charge transfer device (14) or the CMOS transistor device and optionally an optical fiber bundle (17) connecting the image intensifier tube (16) to the charge transfer device (14) or the CMOS transistor device. 8. Imaging device according to one of claims 1 to 5, wherein the pixel detector (5.1) comprises elementary detectors sem ^¬ independent conductors (18) for detecting the radiation emitted by the object under the effect of the irradiation and connected to an integrated circuit (19) reading. Imaging device according to one of claims 1 to 5, wherein the pixel detector (5.1) comprises a monolithic semiconductor material (20) for detecting radiation emitted by the object under the effect of irradiation, and connected to an electronic reading circuit (21) defining the pixels. An imaging device according to one of the preceding claims, further comprising a shield (9) between the excitation source (1) and the image generating device (5). Imaging device according to one of the preceding claims, in which the excitation source (1) is collimated and the collimator defines an axis (2.1) of the interrogating beam (2), the collimator (8) arranged upstream of the pixel detector defines a collimation axis (6) of the pixel detector (5.1), the axis (2.1) of the interrogator beam (2) and the collimation axis (6) are separated by an angle of the order of 150 °. An imaging device according to one of the preceding claims, further comprising an auxiliary camera (10) associated with the image generating device (5), adapted to take a visible image of a zone (z) of the object observed by the pixel detector (5.1). 13. An imaging device according to claim 12, further comprising a separating prism (12) of the radiation (4) emitted by the object (3) under the effect of the irradiation of visible radiation from of the object, this prism being placed on the path taken by the radiation (4) emitted by the object (3) under the effect of irradiation to the pixel detector (5.1). 14. X-ray fluorescence imaging method, consisting of: irradiating an object (3) to be imaged made of a material comprising at least one chemical element whose atomic number is greater than that of the lead by an interrogating beam (2) emitted by an excitation source (1), detecting a radiation (4) emitted by the object under the effect of irradiation by a pixel detector (5.1) of an image generating device (5) after passing through a collimator (8), this pixel detector being made of a semiconductor material chosen from cadmium telluride CdTe, zinc-doped cadmium telluride CdZnTe, mercury iodide HgΣ2 and having a function of energy selection of the detected radiation, and  to provide, by each of several pixels of the pixel detector (5.1), an image signal relative to the detected radiation in a given energy band. The method according to claim 14, wherein the detection of the radiation (4) emitted by the object under the effect of the irradiation is substantially simultaneously with the irradiation of the object (3) with the interrogating beam ( 2). 16. Method according to one of claims 14 or 15, wherein the energy selection is an energy thresholding, a windowing energy or energy spectrometry. The method of one of claims 14 to 16, further comprising identifying a chemical element from the detected radiation in a given energy band. 18. The method according to one of claims 14 to 16, wherein when the energy selection is spectrometry, the pixels providing a spectral image signal, it also comprises a peak detection step in each spectral image signal. . The method of claim 18, further comprising comparing the energy of each peak with the X-ray fluorescence energy of known chemical elements. 20. Method according to one of claims 14 to 16, further comprising displaying the intensity of the radiation detected in a given energy band.