TWO-STEP METHOD FOR ANALYZING AN OBJECT USING TRANSMITTED RADIATION FOLLOWED BY A Scattered Spectrum.

Abstract

The invention relates to a method for analyzing an object which takes place in two parts: - a first part comprising the steps: irradiation of the object by incident photonic radiation; acquisition of radiation transmitted by the object by means of a detector placed in transmission; determination of at least one first characteristic of the object from the measured transmission radiation; verification of the fulfillment of at least one suspicion criterion relating to the first characteristic of the object and indicating that the object contains a material potentially suspect for a given application; - and a second part carried out only when the suspicion criterion is fulfilled, and comprising: acquisition of an energy spectrum diffracted by the object by means of a spectrometric detector placed in scattering at an angle of less than 15°; determination of a second characteristic of the object from at least the measured scattering spectrum;- comparison at least of the second characteristic of the object with characteristics of standard materials stored in a database, for the purpose of identifying a constituent material of the object.;

Description

The present invention relates to a method for analyzing an object by diffractometry, and an associated detection system. Diffractometry corresponds to the irradiation of a material or object using a source of electromagnetic radiation, of the X or gamma type, followed by the analysis of the low-angle elastic scattering radiation. The expression "low angle elastic scattering radiation" designates the radiation diffused by the material or object, in a coherent manner, according to an angle less than 15°, or even less than 10°, with respect to the direction of the incident radiation at the material 10 or object. Beyond 10°, in fact, elastic scattering, or Rayleigh scattering, becomes progressively negligible. It is known to use diffractometry to detect certain crystalline substances such as most explosives or many other dangerous or illegal structures. The invention thus finds an application in the field of security, more particularly in the field of the detection of explosive materials in luggage. It is also useful in the medical field, for example, for locating a tumor in a breast. Indeed, a UCL publication (Pani, S. et al. “Characterization of breast tissue using energy-dispersive X-ray diffraction 20 computed tomography”. Applied Radiation and Isotopes 68, No. 10 (2010): 1980- 1987) was able to demonstrate the possibility of differentiating (adipose and fibrous tissues, benign tumours, adenofibromas, carcinomas, etc.) objects of biological tissues from breast biopsies thanks to the diffusion measurement of these tissues. Analysis of radiation scattered at a low angle (note that the term "diffracted" is generally used for a crystalline material while the term "scattered" is generally used for an amorphous material, but these two terms are used here interchangeably , as well as the terms diffusion and diffraction) by a material is a method of physico-chemical analysis which provides information on the structure of the material thus allowing a better characterization of the materials. It is known that the analysis of the spectrum of radiation scattered at low angle, or diffusion spectrum, makes it possible to establish a signature of the material examined. For crystalline materials, for example, when the wavelength of the irradiating X-rays is of the same order of magnitude as the inter-reticular distances (a few angstdims), the scattered rays generate constructive or destructive interference depending on their energy and their beam angle. diffusion. The conditions under which interference is constructive are determined by Bragg's law. For a crystalline material, this law links the inter-reticular distance, the energy of the diffused radiation and the angle of diffusion, according to the following equation: E hkl n 2dhi, sin(8 / 2) With: dhk,: distance inter-reticular between the crystallographic planes of the irradiated crystal; {hk1}: Miller indices; θ: scattering angle, that is to say the angle formed between the scattered radiation analyzed and the beam incident on the irradiated crystal; h: Planck's constant; c: speed of light; n: the order of the interference. Bragg peaks can be identified by the momentum transfer defined by the following equation: sin(9 / 2) n x = 2dhki The advantage of expressing the scattering profiles (measured intensity) as a function of x is due to the fact that an intensity peak can be measured for different couples (λ,13) but for a single value of x (fixed n). By way of examples, the appended FIG. 1 shows the Bragg peaks of two crystals, namely TNT (trinitrotoluene) and salt (NaCl). In the case of non-crystalline materials, the low angle scattering spectrum is also representative of the material being examined. Like the interference determined by Bragg's law for a crystalline material, interference phenomena can also occur between the atoms and / or molecules of an amorphous material such as a liquid, this time involving a distribution continuum of distances (molecular interference function, denoted FIM). Indeed, many amorphous materials have regular arrangements over nanometric distances (we speak of short-range order). This type of order is determined by strong chemical bonds for covalent and ionic bonds. This short range order causes hc intramolecular and intermolecular interferences. The appended figure 2 illustrates examples of molecular interference functions, namely the normalized FIM of water (H20), the normalized FIM of hydrogen peroxide (H202), the normalized FIM of acetone (C2H6C0), and the standardized FIM of a material known under the trade name plexiglas® ((C502H8)n). The most common diffractometers are called angular dispersion ADXRD (acronym for “Anguier Dispersive X-ray Diffraction”). The energy is fixed by the use of monochromatic radiation and the number of diffracted photons is measured as a function of the angle. Although these devices are very precise, they require the use of a powerful monochromatic source and cannot be used for imaging due to their size. Developed more recently, the EDXRD technique (acronym for “Energy Dispersive X-Ray Diffraction”) makes it possible to overcome these difficulties. This time it consists of working at a fixed angle using a set of collimators and illuminating the object with a polychromatic beam to measure the diffracted photons with an energy-resolved spectrometric detector. The diffraction peaks then appear at certain energies in the measured spectrum. The EDXRD technique, and more generally, any technique of analysis by spectrometry, requires the implementation of a spectrometric detector sufficiently resolved in energy to allow the separation and the identification of the various characteristic peaks of the material constituting the object to be analyzed. The known detectors having the best energy resolution are of the Germanium type. But this type of detector must be cooled to very low temperatures, by complex and / or expensive processes (thermoelectric cooling or by a liquid nitrogen tank). Also, the analysis devices implementing such a detector are very bulky. The recent emergence of spectrometric detectors that can be used at room temperature, such as detectors of the CdTe or CdZnTe type, or scintillator materials, offers an interesting alternative to Germanium detectors. Indeed, these detectors are compact, uncooled and less expensive. However, their performances in terms of energy resolution are still lower than those obtained with Germanium detectors, although correct. To know if a given crystalline or amorphous substance is contained in an object, it is therefore known to: / irradiate the object using an incident beam, emitted by a source of ionizing radiation, preferably collimated by a collimator primary, / capture the diffracted radiation using a detection device comprising.a detector, called a spectrometric detector, capable of establishing an energy spectrum (or energy spectrum) of the scattered radiation according to a given scattering angle, that is to say a detector comprising o a detector material capable of interacting with a scattered radiation by the object and which has, on the side of the object, a so-called detection plane, o spectrometric measurement means, able to measure an energy released by each interaction of a photon with the detector material and to establish at minus one energy spectrum. . a collimator, called a detection collimator, associated with the detector, the detector and the detection collimator being arranged so as to have a detection axis D forming, with the central axis Z of the incident beam, a scattering angle 0, / analyzing the spectra or spectra measured by comparison with the energy spectrum of the substance sought. Generally speaking, an energy spectrum illustrates the energy distribution of radiation in the form of a histogram representing the number of photon interactions in the object (on the ordinate) as a function of the energy released (abscissa). Often, the energy axis is discretized into channels of width 2 bE, a channel Ci, centered on the energy Ei corresponding to the energies between EiÔE and Ei-FbE. It is known that the identification of a material by elaboration then analysis of a diffusion spectrum can be tedious when it is desired to characterize objects of large volume. Indeed, this method is all the more precise as the volume of object observed is low. The invention aims to overcome these drawbacks, in particular by proposing a faster and more effective method for analyzing an object. To do this, the invention proposes a method for analyzing an object characterized in that it takes place in two parts, namely a first part comprising the following steps: / irradiation of the object by incident photon radiation , / measurement of radiation transmitted by the object, called measured radiation in transmission, using a detector placed in transmission, / determination of at least a first characteristic of the object from the radiation in transmission measured, V verification of the achievement of at least one suspicion criterion relating to the first characteristic of the object and reflecting the fact that the object contains a potentially suspect material for a considered application; thus, for example, in the case of an application relating to the field of security such as the search for explosives in luggage, the criterion of suspicion reflects the fact that the object contains a potentially dangerous material, i.e. i.e. a material whose first characteristic approaches that of an explosive material; in the case of a medical application such as the search for a tumor in a breast, the suspicion criterion translates the fact that the object contains a potentially malignant material, that is to say a material whose first characteristic is close to that of cancerous tissue. The method according to the invention comprises a second part which is executed only when the suspicion criterion is fulfilled. Thus, when the suspicion criterion is not met, the method of analyzing the object is terminated at the end of the first part: the object being analyzed 20 is considered harmless, is moved and optionally replaced (in the object reception zone of the detection system) by a new object to be analyzed. Conversely, when the suspicion criterion is fulfilled, the second part is executed, which second part comprises: / acquisition of an energy spectrum, called measured diffusion spectrum, diffused by the object according to a diffusion angle 0 non-zero less than 15° using a spectrometric detector placed in diffusion, / determination of a second characteristic of the object from - either the measured diffusion spectrum, - or the measured diffusion spectrum and the radiation in measured transmission, 30 V comparison with characteristics of standard materials stored in a database, either of the second characteristic of the object, or not only of the second characteristic of the object but also of the first characteristic of the object, for the purpose of identifying a constituent material of the object. In summary, the definition of the process provided above covers four cases: - a first case in which .the second characteristic (for example a scattering signature) is obtained from the measured scattering spectrum alone, the transmission radiation then serving only to eliminate non-suspect objects (in the first part of the method), . the second characteristic is used alone to identify the material, the second characteristic then being able to be qualified as the “determining signature” of the material in the sense that it alone makes it possible to identify the material reliably and quickly, such as for example a scattering signature (Bragg Peaks, Molecular Interference Function), - a second case in which . the second characteristic (for example scattering signature) is obtained from the measured scattering spectrum and the measured transmission radiation (which in this case is preferably a transmission spectrum), the measured transmission spectrum then being used twice, a first time in the first part of the method to eliminate non-suspect objects and a second time in the second part of the method for the determination of the second characteristic, for example to improve the accuracy with which this second characteristic is determined, . the second characteristic is used alone to identify the material, the second characteristic then being a decisive signature as defined above, - a third case in which. the second characteristic (for example momentum transfers) is obtained from the measured diffusion spectrum alone, the transmission radiation then serving only to eliminate non-suspect objects (in the first part of the method), . both the second characteristic (eg momentum transfers) and the first characteristic (eg effective atomic number) are used to identify the material. - A fourth case in which . the second characteristic (for example the scattering spectrum itself, or the position of certain peaks of this scattering spectrum and possibly their relative intensity) is obtained from the measured scattering spectrum and the measured transmission radiation (which in this case is preferably a transmission spectrum), the measured transmission spectrum then being used twice, once in the first part of the method to eliminate non-suspect objects and a second time in the second part of the method to determine the second characteristic, for example to improve the precision with which this second characteristic is determined, . both the second characteristic (for example the diffusion spectrum) and the first characteristic (for example an attenuation coefficient, the thickness of a material constituting the object) are used to identify the material. It should be noted that it is possible to use one and the same spectrometric detector both for the measurement of the radiation in transmission in the first part of the method, which radiation measured is then advantageously an energy spectrum in transmission, and for the acquisition of the scattering spectrum in the second part of the process. It is advisable, in this case, to move the detector during the process, between the two acquisition positions (position in transmission, on the axis of the incident radiation / position in diffusion, shifted by an angle 0 from this axis) . As a variant, a detection system is used provided with two detectors which are not intended to be moved, namely a detector placed in transmission, which can be a spectrometric detector (preferred version) or another detector such as for example a detector with integration, and a spectrometric detector placed in diffusion. Advantageously and according to the invention, when the detector placed in transmission is an imager, the first characteristic may correspond to a geometric shape of the object, or to one of these dimensions, such as for example its thickness along the axis of the incident radiation .Advantageously and according to the invention, the detector placed in transmission is a spectrometric detector, which makes it possible to estimate, as the first characteristic of the object, in addition to those mentioned above, an attenuation coefficient for one or more predetermined energies, or for a given range of energies, or even the effective atomic number Zeff of a material constituting the object. It can also be an indication as to the nature of a constituent material of the object, such as for example: - a family of materials to which the constituent material of the object would belong, the second part of the process allowing then to specify within this family 5 what is the exact nature of the material and therefore to refine the hypothesis provided by the first part, - a hypothesis on the very nature of the material, the second part of the process then making it possible to confirm the hypothesis provided by the first stage as to the nature of the material, these indications being obtained from the analysis of the spectrum in transmission. The effective atomic number Zeff is one of the parameters that can characterize a material. Remember that the atomic number of a simple body is defined as the number of protons present in the nucleus of an atom of this body. On the other hand, when considering a chemical compound, one must appeal to the notion of effective atomic number. This is defined as the atomic number of a simple body which would lead to the same transmission spectrum in a given energy band. In general, the effective atomic number of a chemical compound is obtained by means of a combination of the atomic numbers of the simple constituent bodies of the compound, each atomic number being assigned a weighting coefficient depending on the mass or atomic fraction. of the simple body in the compound. Thus, in practice, the effective atomic number eff of a Zeff = (Zieff y composed of N simple bodies, checks where zieff is the atomic number = of the simple body i1,...,Net P is a constant related to the effect , photoelectric (p- 462) 25 The suspicion criterion preferably defines a range of values ​​of the first characteristic within which it is considered that the object is potentially suspicious with regard to the application considered, and to outside of which the object is considered not to be suspicious The step of verifying the fulfillment of the suspicion criterion then comprises a comparison between the first characteristic determined for the object and the range of predefined values ​​eu An advantage of determining the first characteristic from a measured transmission spectrum is that such a spectrum requires a relatively short acquisition time, so that the first part of the method which does not allow to precisely identify the material but allows to eliminate a large number of harmless objects is fast. The second part of the process, which allows better discrimination of the material but which is longer due to the acquisition of a diffusion spectrum, is only carried out on suspect objects. A considerable saving of time is thus achieved. The second characteristic of the object, determined from at least one measured scattering spectrum, is for example a function called scattering signature, which represents scattering peaks (Bragg peaks for a crystalline material or interference function molecular for an amorphous material). As a variant, it may be a physical quantity extracted from such a diffusion signature (inter-reticular distance for a crystalline material for example). The second characteristic of the object can also be a momentum transfer. As defined previously, this second characteristic is determined from at least the measured diffusion spectrum. In a preferred embodiment, it is determined from not only the scattering spectrum measured during the second part of the method but also the radiation in transmission measured during the first part method. The combined use of a scattering spectrum and a transmission spectrum to reconstruct a scattering signature of a material is described in more detail below in the context of an example embodiment of the invention.This combination makes it possible to improve the identification of the constituent material of the object. In a preferred version, the spectrometric detector placed in diffusion is configured so as to have a detection axis forming, with a central axis of the incident radiation, a diffusion angle θ of less than 10° and preferably less than 5°. In a preferred embodiment, the analysis method according to the invention comprises a first preliminary step of calibrating a response matrix of the spectrometric detector placed in diffusion (which calibration step can be omitted in particular if the response of the detector is deemed satisfactory, that is to say if the energy resolution of the detector used is particularly good), a second preliminary step of calibrating a response matrix of the detector placed in transmission when the latter is a spectrometric detector ( which calibration step can be omitted in particular if the response of the detector is judged to be satisfactory, that is to say if the energy resolution of the detector used is particularly good), and a third step of calibrating an angular response matrix of the detection system, these first, second and third prior calibration steps being able to be carried out in one order or another, before any irradiation of an object to be analyzed, that is to say "off-line". The invention extends to a detection system capable of implementing the method according to the invention, and in particular to a detection system for analyzing an object comprising: V a source of photonic radiation; V a zone for receiving an object to be analyzed; Y detection means, downstream of the object reception zone, for measuring radiation transmitted by the object, called measured transmission spectrum and for acquiring an energy spectrum, called measured spectrum. measured scattering, scattering by the object according to a non-zero scattering angle θ of less than 15°; these detection means comprising a spectrometric detector; Y computer processing means comprising 20 . means for determining at least a first characteristic of the object from the measured transmission radiation, the first characteristic possibly being a shape, a dimension of the object, an attenuation coefficient for one or more predetermined energies, or for a predetermined range of energies, or an effective atomic number 25 Zeff of a material constituting the object, or an indication as to the nature of a material constituting the object (first estimate of the nature of the material -to be confirmed using the diffusion spectrum in the second part of the process- or identification of a family or type of material), and possibly its thickness. 30 . means for verifying at least one criterion of suspicion relating to the first characteristic previously determined for the object and reflecting the fact that the object contains a potentially suspect material for a considered application, for example by comparing the first characteristic with a predefined range of values ​​with respect to the application concerned; . means for determining a second characteristic of the object, from either the measured scattering spectrum, or both from the measured scattering spectrum and from the measured transmission radiation, and means for comparing with characteristics of standard materials stored in a database, either of the second characteristics, or of the first and second characteristics, for the purposes of identifying a constituent material of the object. According to a first possible embodiment, the detection means comprise, in addition to the previously defined spectrometric detector, positioning means for the successive positioning of said spectrometric detector in a transmission position for measuring the measured transmission radiation, and in a position scattering for the acquisition of the measured scattering spectrum. According to a second possible embodiment, the detection means comprise, in addition to the spectrometric detector previously defined, which is then placed in diffusion, another detector, placed in transmission, which detector can be a spectrometric detector or another detector such as a detector to integration.The detection system according to the invention also preferably has one or more of the following characteristics: the first characteristic is chosen from among a shape of the object, a dimension of the object, an attenuation coefficient at one or more predetermined energies of a material constituting the object, an effective atomic number Zeff of a material constituting the object, an indication as to the nature or possibly a thickness of a material constituting the object, / the second characteristic is chosen from among a scattering signature representing Bragg peaks or a molecular interference function, momentum transfers. / The computer processing means are capable of implementing a method based on an approach of the inverse problem type and of estimating an overall response matrix of the detection system. / The computer processing means are capable of estimating an incident spectrum attenuated by the object using the measured transmission spectrum. / The computer processing means are capable of constructing an overall response matrix of the detection system from an estimated attenuated incident spectrum, a calibrated angular resolution matrix of the detection system as well as, possibly, a matrix calibrated response of the spectrometric detector placed in diffusion, a calibrated response matrix of the detector placed in transmission. / The radiation source is polychromatic. / The radiation source is an X-ray source. / The spectrometric detector placed in diffusion is configured in such a way as to present a detection axis forming, with a central axis of the incident radiation, a diffusion angle 0 of less than 10° and preferably less than 5°. / The detection system comprises a first collimator, called a source collimator, placed between the source of photonic radiation and the object reception zone, so as to collimate the radiation emitted by the source into an incident beam with a central axis Z , 15 V The detection system comprises a second collimator, called the diffusion collimator, between the object reception zone and the spectrometric detector placed in diffusion, the diffusion collimator having a central axis of collimation forming a diffusion angle 0 with the central incident axis Z, which scattering angle θ is preferably less than 5°. 20 v The spectrometric detectors used contain direct conversion sensors, i.e. incident X photons on the sensor are absorbed by a semiconductor (CdTe for example) and create a cloud of electronic charges (typically 10,000 electrons for an X photon of 60 keV). These charges are then collected by electrodes and form a transient electrical signal called a pulse. If collection is complete, the integral of the measured pulse for each detected particle is proportional to its energy deposited in the semiconductor. An electronic circuit makes it possible to measure this integral. After digitization, the various measurements of photons of the same energy are summed, which makes it possible to reconstruct the spectrum of X-rays having interacted with the irradiated object. The invention extends to a detection system and to a method for analyzing an object characterized in combination by all or some of the characteristics mentioned above and below. Other details and advantages of the present invention will appear on reading the following description, which refers to the appended schematic drawings and relates to preferred embodiments, provided by way of non-limiting examples. In these drawings: / Figure 1 is a graph representing the Bragg peaks of TNT (trinitrotoluene) and salt (NaCl), which peaks illustrate the normalized intensity (ordinate axis), i.e. the number relative number of photons detected during an acquisition operation, as a function of the momentum transfer x (axis of the abscissas) in nm-1 of the photons detected. / Figure 2 is a graph representing the normalized molecular interference function of water (H20), the normalized molecular interference function of hydrogen peroxide (H202), the normalized molecular interference function of acetone (C2H6C0), and the normalized molecular interference function of Plexiglas® ((C502H8)n), with, on the ordinate, the number of photons detected and, on the abscissa, the momentum transfer x in nm-1. v Figure 3 is a schematic view of a detection system according to the invention. / Figure 4 is a graph representing transmission spectra for cylindrical samples 40 mm in diameter of water (H20) and acetone (C2H6C0), as measured by the spectrometric detector placed in transmission 20 of the detection system of figure 3. / Figure 5 is a graph representing diffusion spectra for cylindrical samples of 40mm diameter of water (H20) and acetone (C2H6C0), as measured by the spectrometric detector placed in diffusion of the system Figure 6 is a graph representing the diffusion signature (estimated FIM) of a cylindrical sample of water 40 mm in diameter and that of a cylindrical sample of acetone 40 mm in diameter in diameter, as reconstructed from a method and a detection system according to the invention, / FIG. 7 is a graph representing on the one hand the diffusion signature (“reconstructed FIM”) of a cylindrical sample of water from 40 mm in diameter and that of a cylindrical sample of acetone 40 mm in diameter, as reconstructed from a method and a detection system according to the invention, and on the other hand the diffusion signatures (“theoretical FIM”) of these samples, / Figure 8 is a graph representing a response matrix of the spectrometric detector placed in transmission of the detection system of Figure 3. / Figure 9 is a graph representing a response matrix of the spectrometric detector placed in diffusion of the detection system of FIG. 3. / FIG. 10 is a graph representing the angular distribution of the spectrometric detector placed in diffusion of the detection system of FIG. 3. / FIG. 11 is a graph representing an angular response matrix of the detection system of Figure 3. 10 V Figure 12 is a graph depicting estimated attenuated incident spectra for cylindrical specimens of 40 mm in diameter of acetone and water, as estimated according to the invention from the measured transmission spectra of FIG. 4. / FIG. 13 is a diagram illustrating an operation for constructing a response matrix of the detection system of FIG. 3. / FIG. 14 is a graph representing the overall response matrix A of the detection system of FIG. 3 for a cylindrical sample of water 40 mm in diameter. / Figure 15 is a graph representing the overall response matrix 20 A of the detection system of Figure 3 for a cylindrical sample of acetone 40 mm in diameter. The detection system according to the invention illustrated in FIG. 3 comprises: . a polychromatic source 1 of ionizing radiation, such as an X-ray tube, 25 . a source collimator 2, which makes it possible to channel the radiation from the source 1 into an incident beam with a central axis Z, - a zone 4 for receiving an object to be analyzed, . a diffusion collimator 5 having a collimation axis D,.a spectrometric detector 6 placed in diffusion, associated with the diffusion collimator 5 so that the detector 6 detects radiation diffused according to a diffusion angle θ (angle between the incident axis Z and the axis of collimation and detection D) for example equal to 2.5° (the scale not being respected in FIG. 3); the spectrometric detector 6 placed in diffusion is capable of establishing a measured diffusion (energy) spectrum, that is to say an energy spectrum of the radiation diffused by the object in the direction D; the spectrometric detector 6 is, in the example, a semiconductor material detector, such as a CdTe or CdZnTe detector; - a spectrometric detector 7 placed in transmission, able to establish a spectrum (energy) in measured transmission, that is to say an energy spectrum of the radiation transmitted by the object in the direction Z; the spectrometric detector 7 placed in transmission is, in the example, a semiconductor material detector, such as a CdTe or CdZnTe detector; - computer means 8 for processing the measured spectra supplied by the spectrometric detectors 6 and 7. The terms "transmitted radiation" designate the radiation consisting of photons which have not undergone any interaction in the object examined. By “transmission spectrum”, is meant the spectrum of the radiation transmitted along the axis of the incident beam to the object, consisting of the photons which have not undergone any interaction in the object. The expression “placed in transmission” denotes a detector capable of detecting the radiation transmitted by the material. Thus, a detector placed in transmission is located on the axis of the radiation incident to the object, the object being placed between the detector and the radiation source. “Spectrometric detector” means a detector capable of generating an energy spectrum of the radiation detected. The method according to the invention aims to determine whether an object to be analyzed is dangerous (explosive, tumour, etc.) or not. To do this, the process takes place in two parts. The objective of the first part of the process is not the precise identification of the constituent material of the object; this first part aims only to determine whether the object is suspicious or not, that is to say to determine whether the object deserves further analysis. This first part thus makes it possible to shorten the analysis for a large number of objects which turn out to be manifestly harmless. The method described here was implemented by the inventors to study two cylindrical samples 40 mm in diameter consisting respectively of acetone (C2H6C0) and water (H20). For each sample inspected, the first part of the method uses a measured transmission spectrum as supplied by the spectrometric detector 7, and whose direct model is the following: h=RE, - (S,' x An) With - h: the measured transmission spectrum vector of size (NbEit x 1) - REt: the response matrix of the spectrometric detector placed in transmission, of size (NbEit x NbE,). In the case of a perfect detector, this matrix is ​​a diagonal matrix. Each term REt (j,i) of the matrix represents the probability of detection of an energy value equal to j when the incident photon at the detector has an energy i. -: the vector of the incident spectrum of the X-ray tube 1 of size (1 x NbEi) - Att: the attenuation vector of size (1 x NbE,) which takes into account the attenuation effects in the object. - The vector (Sinc x Att) represents the spectrum of the radiation source attenuated by the object. In the example described here, and which provides very precise signatures, this vector is taken into account in the construction of an overall response matrix A of the system, as will be understood later. - NbEit and NbE, correspond respectively to the number of channels of the spectrum in transmission measured (i.e. to the number of channels of the spectrum of the energy detected by the detector placed in transmission) and to the number of channels of the spectrum of the incident energy. - The symbol x corresponds to a product term by term (S,n, and Att are multiplied term by term, we then obtain a vector which is the same size). - The symbol . corresponds to the classical matrix product. The transmission spectra measured with the spectrometric detector 7 for the aforementioned acetone (C2H6C0) and water (H20) samples can be seen in Figure 4.Several characteristics can be extracted from such a transmission spectrum: attenuation coefficient, effective atomic number Zeff of a constituent material of the object, an estimate of the nature, or even the nature and thickness, of a constituent material of the object. 30 The effective atomic number Zeff of the material can for example be obtained by a statistical method which compares the transmission spectrum measured for the object with spectra of calibration samples (standard materials) measured under the same experimental conditions, and which uses a physical model of evolution of the spectrum according to the effective atomic number and the thickness of the material. This technique is described in French patent application No. 1363174 by Alexia Gorecki in the name of the CEA, incorporated here by reference. . The transmission spectrum can also be used to estimate the nature of the material examined. The estimation of the nature of a material, as well as possibly its thickness, from a transmission spectrum is described in applications WO2012000993 or W02011069748, in the name of the CEA, incorporated here by reference. Once this or these characteristics of the object have been estimated, a decision is made. If the inspected object seems suspicious, i.e. if one or more suspicion criteria are met (doubtful form, subject to using an imager as a detector placed in transmission, or atomic number belonging to a range critical values ​​of potentially dangerous materials defined beforehand according to the application concerned), then a diffusion spectrum of the material will be acquired and studied to allow better discrimination of the material. This second part is not done systematically on all materials because, although very discriminating, it requires a relatively long exposure time to X-rays. If there is no doubt about the harmlessness of the object, then the first part of the process is carried out on a new object. The effective atomic number of the two samples inspected (water and acetone) was evaluated from the spectra in FIG. 4. The following results were obtained: V for the water sample: estimated Zeff=7.21. In theory, such a sample 25 has an effective atomic number: theoretical Zeff=7.22. It can therefore be seen that the result obtained is very close to the theoretical value of the sample. V For the acetone sample: estimated Zeff = 6.05, for a theoretical atomic number theoretical Zeff = 6.03. A critical zone has been defined for this application (detection of explosives); it corresponds to materials whose Zeff is between 6 and 8.5. In other words, the step of verifying the fulfillment of the suspicion criterion consists in comparing the effective atomic number determined, estimated from the measured transmission spectrum, with the range of values ​​[6;8,5]. Note that several discontinuous critical zones (several ranges of values) can be defined for the same first characteristic (atomic number for example). It is also possible to define several suspicion criteria relating to distinct characteristics; in this case, it can be considered either that the object is suspect (and therefore eligible for the second part of the process) when at least one of these suspicion criteria is met, or that the object is not suspect only if all of these criteria for suspicion are met. The two samples inspected here (water and acetone) have Zeffs that are in the critical zone; they are therefore considered suspect and the second part of the method according to the invention, namely a study of these two samples in small angle scattering, must therefore be carried out. The second part of the process aims to precisely identify the constituent material of the object when the latter has been considered suspect at the end of the first part. This second part consists for example in supplying a diffusion signature of said material from a measured diffusion spectrum (provided by the spectrometric detector 6), the terms “diffusion signature” designating a function representative of the material and showing peaks diffusion (Bragg peaks or Molecular Interference Function depending on the nature -crystalline or amorphous- of the material). The diffusion spectra measured by the detector 6 for the two samples can be observed in FIG. 5. In the example described here, it was chosen to exploit, in an original way, the diffusion spectra measured in combination with the transmission spectra measured during the first part of the process. As a variant, it is possible to exploit the measured diffusion spectra of FIG. 5 according to another technique such as a known technique used in EDXRD. In the example chosen here, the second part of the method uses the following model, describing the relationship between the scattering signature f of the material and the measured scattering spectrum g: g = (RF(] x S11. X Au). f=i4.f With - g: the measured (coherent) diffusion spectrum vector, of size (NbEid×1) - REd: response matrix of the spectrometric detector placed in diffusion, of size (Nbqd×NbE,).In the case of a perfect detector, this matrix is ​​a diagonal matrix. Each term REd (j,i) of the matrix represents the probability of detection of an energy value equal to j knowing that the incident radiation at the detector has an energy equal to i. In general, the response matrix of a spectrometric detector establishes a probabilistic relationship between an energy detected by the detector and the energy of the incident radiation at this detector. Each column i of REd (j,i), with j varying from 1 to NbEjd, corresponds to the probability density of energy detected by the detector when the latter is subjected to incident radiation of energy i. : vector of the incident spectrum of the X-ray tube of size (1 x NbE,) Att: attenuation vector of size (1 x NbE,) which takes into account the attenuation effects in the object. Due to the low value of the angle 0, less than 15° and preferably less than 10° or even 5°, we will make the approximation that the attenuation by the object along the path towards the two detectors (in broadcast and in transmission) is the same. R0: angular response matrix of the detection system, of size (N bE, x Nbx). Each term Ro(j,k) of the matrix Ro corresponds to a probability that the energy of a photon detected at energy j corresponds to a momentum transfer equal to k. In other words, Ro (j, k) corresponds to the probability that a transfer of momentum k gives rise to the detection of a photon of energy j. Each column k Ro(j,k), with j varying from 1 to NbEjd, corresponds to the probability density of energy detected by the detector during a momentum transfer equal to k. More generally, the angular response matrix Ro makes it possible to establish a probabilistic relationship between the energy detected by the detector placed in diffusion and an elastic diffusion parameter of a material constituting the object, in particular the transfer of momentum. f: signature in diffusion, of size (1 x Nbx), specific to the material constituting the object, and which makes it possible to describe either the theoretical Bragg peaks of the material in the case of a crystalline material, or the interference function molecular in the case of an amorphous material, - A: global response matrix of the system in diffusion, of size (NbEid x Nbx). Each term A(j,k) of A corresponds to a probability that the energy of a photon detected, by the scattering detector, at energy j corresponds to a momentum transfer equal to k . In other words, A(j,k) corresponds to the probability that a transfer of momentum k gives rise to the detection of a photon at energy j. - The symbol x corresponds to a product term by term (S1 and Att are multiplied term by term, we then obtain a vector which is the same size). - The symbol . corresponds to the classical matrix product. - NbEid, NbE, and Nb x correspond respectively to the number of channels of the spectrum in diffusion measured (i.e. to the number of channels of the spectrum of the energy detected by the detector placed in diffusion), to the number of channels of the incident energy spectrum and the number of vector channels describing the momentum transfer. . It should be noted that the number of photons detected in each channel of the vector g follows a Poisson law with parameter the average number of photons in this channel. The method according to the invention then comprises an operation of constructing an overall response matrix A of the detection system, using the above model. To do this, it is first necessary to determine the terms Ro and (Sinc x 20 Att), and possibly REd. Each of these steps is individually described below. The preliminary step of calibrating the response matrix REd of the spectrometric detector placed in diffusion is not necessary but advantageous, because it takes into account the degradation of the spectra due to the response of the detector. However, this step is optional or even useless, in particular for detectors with sufficient energy resolution and when the response of a detector is judged to be satisfactory. The same is true for the response matrix REt of the spectrometric detector placed in transmission, used later. Once the global response matrix A has been constructed using the aforementioned model, the method according to the invention reconstructs the signature f (molecular interference function for the amorphous, distribution of the dhk, for the polycrystalline) from the model g=A f (where A and g are then known) by implementing a method based on an inverse problem type approach. The Maximum Likelihood - Expectation Maximization algorithm (denoted MLEM) proposes to estimate the spectrum to be calculated by iterative maximization of the log-likelihood function. This type of calculation is very frequent when it is a question of estimating a maximum likelihood, and uses a more general algorithm, called Expectation-Maximization (EM). This method has the advantage of taking into account the Poisson nature of the measured data. The coefficients of the global response matrix A of the system are denoted aii. We want to maximize the probability that the estimate f of dimension Nbx generates the 10 measures g. It is also known that the measured data obey a Poisson statistic, by their physical nature. We can therefore write the likelihood function of the estimate f: Its log-likelihood is then expressed: cp(f) Io 15 We then seek to maximize this function, by canceling its derivative açb(f) f k a f k The iterative resolution of this problem is then written: 20 By initializing the vector I'°~k with positive values, we are guaranteed to have non-negative results. Thus, from an estimate of A and the measurement of g, we can reconstruct f by iterating the MLEM algorithm. The results of this reconstruction (denoted “FIM reconstructed” in FIG. 6) for the samples of water and acetone 25 inspected, by implementing an MLEM algorithm with 100 iterations, are illustrated in FIG. 6. In d In other words, from measurements taken in transmission and in diffusion on an unknown object, it is possible to reconstruct a function relating to the structure Nbx ici=1"j,la j j,k of a material constituting the object. values ​​of this function are represented in the matrix A. Since this material is unknown, the objective is then to identify it. To do this, a set of standard materials is used (of the explosive and non-explosive type in the case of a baggage analysis application for example; of the healthy biological tissue and tumor tissue type in the case of a medical analysis application) whose effective atomic numbers or other first characteristic and whose diffusion signatures (interference function molecular or Br peaks agg) or other second characteristic are tabulated and stored in a database, and the analysis method according to the invention then consists in comparing the values ​​obtained for the object to be analyzed with those of the database, in order to identify the unknown object. As a variant, certain parameters making it possible to go back to the structural parameters of the material are extracted from the reconstructed signature for the object; for example, in the case of a crystalline material, the extraction of the position of the peaks present in the signature obtained makes it possible to trace the inter-reticular distances of the crystal.By comparing, on the one hand, the estimated Zeff values ​​with a base of theoretical Zeff values ​​for a set of materials and, on the other hand, the shape of the 20 reconstructed molecular interference functions with a base of the shape of molecular interference functions for a set of materials, it is possible to identify the nature of the two samples analyzed (water and acetone). FIG. 7 makes it possible to compare the reconstructed molecular interference functions of the two samples analyzed (water and acetone) and their theoretical molecular interference functions. The various steps of the construction operation of the global response matrix A of the system are now described. Prior to any analysis of an object, that is to say "off-line", calibration operations are carried out to determine certain specifications of the detection system, which depend in particular on the detectors used and the geometry of the system. , and which, unlike the attenuation vectors, do not depend on the object to be analyzed. These specifications are REt, REd, Ro. They are then stored in the computer processing means 8 . The response matrix REt of the spectrometric detector 7 placed in transmission can be obtained from the Monte-Carlo Tasmania simulation software, which makes it possible to simulate the entire detection chain of a semiconductor detector (photon interactions, transit of charge carriers. ..). Preferably, this simulation is also confronted with experimental data acquired for example with gamma sources. This makes it possible to adjust the energy resolution obtained in simulation. FIG. 8 shows the response matrix REt calibrated for the spectrometric detector 7 placed in transmission. This matrix defines the probability of detecting a photon at energy Ej while the incident energy of the photon is Ei. This probability is indicated in FIG. 8 by a shade of gray (whose scale is shown on the right of the graph), the abscissa axis of the graph representing the incident energy Ei expressed in keV, the ordinate axis corresponding to the detected energy Ej expressed in keV. In the case of a perfect detector, the matrix is ​​diagonal (if it is square). Similarly, a preliminary operation of calibrating a response matrix REd of the spectrometric detector 6 placed in diffusion is carried out “offline”, by simulation using the Monte-Carlo simulation software and / or by experimentation. The calibrated response matrix REd obtained is illustrated in FIG. 9. Again, the abscissa axis represents the incident energy Ei in keV, the ordinate axis corresponds to the detected energy Ej in keV, the probability of the pair (Ei, Ej) being translated by a shade of gray. A prior operation of calibrating an angular response matrix Ro of the detection system is also performed “off-line”. This angular response depends on two factors. The first corresponds to the geometry of the acquisition system 25 and more precisely to the opening of the source collimator 2 and to the opening of the diffusion collimator 5, knowing that it is assumed that the object fills the intersection of two cones, namely an irradiation cone and an observation cone. The irradiation cone delimits the solid angle under which the object sees the source, while the observation cone corresponds to the solid angle under which the detector sees the object. We first evaluate a 1D angular distribution of the system, either from simulations or from calibrations. Using the relationship linking x (transfer of momentum), E (energy incident to the detector placed in diffusion) and 0 (angle of diffusion), we deduce the angular response matrix function of Ei (incident energy) and of x from of the 1D angular distribution as a function of 0. . Figure 10 shows an example of 1D angular distribution of the scattering system when the collimation axis D of the scattering collimator defines a scattering angle θ equal to 2.5°, the angular distribution expressing the relative quantity of incident photons at the detector placed in diffusion (ordinate axis) as a function of the detection angle in degrees (abscissa axis). This is an example because using collimators with different configurations (especially width and length of the collimator aperture) would result in another graph being obtained. An angular response matrix Ro of the detection system can be seen in Figure 11, where the abscissa axis represents momentum transfer x in nm-1, while the ordinate axis represents energy E incident to the detector placed in diffusion in keV, shades of gray reflecting the relative quantity of incident photons. As previously mentioned, this matrix defines a probabilistic relationship between the number of incident photons at the detector placed in diffusion, at a given energy, and the momentum transfer. 15 The construction of the global response matrix A of the detection system using the model A = (REd x S x An). Ro still requires a step of estimating an incident spectrum attenuated by the object (Sin, x Att). Advantageously, this step of estimating the attenuated incident spectrum again uses the spectrum in transmission measured during the first part of the method by the spectrometric detector 7 placed in transmission. This transmission spectrum h can be written: h = RE, - (s111 x Att) In other words, it is considered in this version of the invention that the term (S,' x Att) in the expression of the matrix A is equal to the term (S,' x Att) 25 in the expression of the transmission spectrum h. The inventors have demonstrated that this approximation is quite acceptable for scattering at small angles (less than 15°) and that it makes it possible to obtain signatures f with excellent resolution and precision for angles of diffusion less than 5°. To estimate (Sin, x Att) from the measured transmission spectrum h and from the calibrated response matrix of the detector REt, the system according to the invention advantageously again uses a technique of the MLEM type. Figure 12 shows the different spectra (Sin, x Att) obtained after MLEM inversion of the experimental data recorded on the spectrometric detector 7 placed in transmission for the two samples inspected (water and acetone), which data (measured transmission spectra) are illustrated in FIG. 4. All the terms of the global response matrix A of the system have been calibrated, the method according to the invention taken as an example then consists in combining them according to the formula A=(REd x S x Att)-Ro . This combination is summarized in Figure 13 where reference numeral 10 designates line-to-line multiplication and where reference numeral 11 designates matrix multiplication. The global response matrices A obtained can be seen in Figure 14 for the water sample 10 and in Figure 15 for the acetone sample. The invention may be the subject of many variants vis-à-vis the preferred embodiment described above. Thus, for example, the second part of the method can consist of an analysis by conventional diffractometry using the EDXRD technique for example. Furthermore, the detector placed in transmission may be a simple integrating detector, in which case the first characteristic of the object evaluated for the purposes of verifying a criterion of suspicion is not the effective atomic number of the material.

Claims

CLAIMS 1. Method of analyzing an object characterized in that it takes place in two parts, namely a first part comprising the following steps: V irradiation of the object by incident photonic radiation, J measurement of radiation transmitted by the object, called measured transmission radiation, using a spectrometric detector placed in transmission, determination of at least one first characteristic of the object from the measured transmission radiation, ✓ verification of the achievement of at least one suspicion criterion relating to the first characteristic of the object and reflecting the fact that the object contains a potentially suspect material for a considered application; and a second part executed only when the suspicion criterion is met, which second part includes: ✓ acquisition of an energy spectrum, called a measured diffusion spectrum, diffused by the object according to a non-zero diffusion angle (9) less than 15°, using a spectrometric detector placed in diffusion, J determination of a second characteristic of the object from - either the measured diffusion spectrum, - either both the measured diffusion spectrum and the measured transmission radiation, comparison - either the second characteristic of the object, - either both first and second characteristics of the object, with characteristics of standard materials stored in a database, for the purpose of identifying a material constituting the object.

2. Analysis method according to claim 1, characterized in that the first characteristic is chosen from a shape, a dimension, an attenuation coefficient for one or more predetermined energies or for a predetermined range of energies, an indication as to the nature of a material constituting the object, an effective atomic number.

3. Analysis method according to one of the preceding claims, characterized in that the step of verifying the achievement of the suspicion criterion includes the comparison of the first characteristic determined for the object with a predefined range of values ​​with regard to the application considered.

4. Analysis method according to one of claims 1 to 3, characterized in that the second characteristic is chosen from a transfer of momentum and a function (f), called diffusion signature, representative of the object and showing diffusion peaks, reconstructed from both the measured diffusion spectrum and the measured transmission spectrum.

5. Analysis method according to claim 4, characterized in that the reconstruction of the diffusion signature of the object implements a method based on an inverse problem type approach and comprises an operation of constructing a global response matrix (A) of the detection system, which global response matrix (A) establishes a relationship between an energy detected by the detector placed in diffusion and a transfer of quantity of movement in the object.

6. Analysis method according to claim 5, characterized in that the operation of constructing the global response matrix (A) of the detection system is carried out from an estimated attenuated incident spectrum (Sine x Att) and a calibrated angular response matrix (Re) of the detection system.

7. Analysis method according to claim 6, characterized in that the operation of constructing the overall response matrix (A) of the detection system is carried out from, in addition, a response matrix (REd) of the spectrometric detector placed in diffusion (6) and a response matrix (REt) of the spectrometric detector placed in transmission (7).

8. Detection system for analyzing an object comprising: a source (1) of photonic radiation, ✓ an area (4) for receiving an object to be analyzed, S detection means (6, 7), downstream of the object reception zone (4), for the acquisition on the one hand of an energy spectrum, called measured transmission spectrum, of radiation transmitted by the object and on the other hand of a spectrum energy, called measured diffusion spectrum, diffused by the object according to a non-zero diffusion angle 0 less than 15°; these detection means comprising a first spectrometric detector (6),, computer processing means (8) including ♦ means for determining at least one first characteristic of the object from the measured transmission spectrum, ♦ means of verifying the achievement of at least one suspicion criterion relating to the first characteristic of the object and reflecting the fact that the object contains a potentially suspect material for a considered application, ♦ means for determining a second characteristic of the object from either the measured diffusion spectrum, or both the measured diffusion spectrum and the measured transmission spectrum, ♦ and means for comparing with characteristics of standard materials stored in a database, either the second characteristic of the object, or the first and second characteristics of the object, for the purposes of identifying a material constituting the object.

9. Detection system according to claim 8, characterized in that the detection means further comprise a second spectrometric detector (7), placed in a transmission position for the acquisition of the measured transmission spectrum, and in that the first spectrometric detector (6) is placed in a diffusion position for the acquisition of the measured diffusion spectrum.

10. Detection system according to claim 8, characterized in that the detection means further comprise positioning means for the successive positioning of the first spectrometric detector (6) in a transmission position for the acquisition of the measured transmission spectrum, and in a diffusion position for the acquisition of the measured diffusion spectrum.

11. Detection system according to one of claims 8 to 10, characterized in that the means for verifying the achievement of the suspicion criterion are capable of comparing the first characteristic determined for the object with a predefined range of values ​​with regard to the application concerned.

12. Detection system according to one of claims 8 to 11, characterized in that: ✓ the first characteristic is chosen from a shape, a dimension, an attenuation coefficient for one or more energies or for a predetermined range of energies, an indication as to the nature of a material constituting the object, an effective atomic number, and ✓ the second characteristic is chosen from a momentum transfer and a function (f), called the diffusion signature, representative of the object and showing diffusion peaks, reconstructed from both the measured diffusion spectrum and the measured transmission spectrum in the case where the detector placed in transmission is a spectrometric detector.

13. Detection system according to one of claims 8 to 12, characterized in that the computer processing means (8) are capable of implementing a method based on an inverse problem type approach and of constructing a global response matrix (A) of the detection system, which global response matrix (A) establishes a relationship between an energy detected by the detector placed in diffusion and a transfer of quantity of movement in the object, the global response matrix (A) being constructed from an attenuated incident spectrum (Sine x Att) estimated using the measured transmission spectrum, and from a calibrated angular response matrix (Re) of the detection system.

14. Detection system according to one of claims 8 to 13, characterized in that the first spectrometric detector (6) placed in diffusion is configured so as to have a detection axis (D) forming, with a central axis (Z) of the incident radiation, a diffusion angle 9 less than 5°.

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