WO2017191563A1

WO2017191563A1 - Detector, apparatus and method for performing a non-invasive radiographic control of items

Info

Publication number: WO2017191563A1
Application number: PCT/IB2017/052550
Authority: WO
Inventors: Giuseppe BERTUCCIO; Bruno GARAVELLI; Pietro Pozzi
Original assignee: Xnext S.R.L.; Politecnico Di Milano
Priority date: 2016-05-04
Filing date: 2017-05-03
Publication date: 2017-11-09
Also published as: ITUA20163149A1

Abstract

A detector (11A, 11B) for performing a non-invasive radiographic inspection of at least one item (2) is described, such a detector (11A, 11B) comprising at least one first detection module (20) including a plurality of n elementary detectors (PA1-PAn) that are adjacent to each other and aligned so as to form a first row, the elementary detectors (PA1-PAn) being apt to convert into an electric signal the photon energy of an incident radiation emitted from at least one source (9A, 9B), the detector (11A, 11B) further comprising at least one second detection module (21) adjacent and identical to the first detection module (20) and in turn including a plurality of n elementary detectors (PB1-PBn) that are adjacent to each other and aligned so as to form a second row. Conveniently, the first detection module (20) and the second detection module (21) are associated with at least one electronic unit (22, 23) processing the electric signals generated therefrom such that each photon incident on these detection modules (20, 21) is associated with an energy band of a plurality of energy bands, the first detection module (20) having a different sensitivity with respect to the one of the second detection module (21) in at least one subset of these energy bands.

Description

Title: "Detector, apparatus and method for performing a non-invasive radiographic control of items"

DESCRIPTION

Field of application The present invention relates to a detector, an apparatus and a method for performing a non-invasive radiographic inspection of items.

More particularly, the present invention relates to a detector for performing a non-invasive radiographic inspection of items during the transfer thereof, the detector comprising at least one first detection module including a plurality of n elementary detectors adjacent to each other and aligned so as to form a first row, these elementary detectors being able to convert into an electric signal the photon energy of an incident radiation emitted from at least one source.

The present invention also relates to a corresponding method for performing a non-invasive radiographic inspection of items and the following description is made with reference to this application field with the only purpose of simplifying the exposition thereof.

Prior art

As it is well-known in this specific technical field, it is currently of great importance to be able to easily identify dangerous objects and/ or contraband materials and goods inside baggage or merchandise containers, all of this with a view to combating terrorism and preventing smuggling and any falsification activities.

It is also known the problem of identifying contaminants and foreign bodies, especially with small dimensions, present in food and foodstuffs.

More generally, it is known the problem of identifying defective products during industrial quality control procedures.

The present-day technology allows detecting and identifying solid objects inside a baggage under inspection, as well as high-density foreign bodies in food directly on the production line during quality control. However, the detection of liquid materials which could be used as compounds for explosives is still a slow process and subject to false alarms.

Moreover, the known solutions are unable to identify contaminants of biological origin and foreign bodies having a molecular density less than that of the host product and/ or a chemical composition which is similar or equivalent in terms of atomic number. The density of the objects which can be identified by means of X-ray analysis is closely related to the X-ray absorption properties of these materials. In particular, the materials having a lower density absorb at low X-ray energies, generally less than 20 keV.

Moreover, it is known that the superposition of many materials hinders inspection.

Therefore, there is the need for automatic scanning apparatus performing in an efficient manner the aforementioned functions, examining a constant flow of objects along a path intended for the transfer thereof. This is the case for example of baggage arranged on a conveyor belt of an airport or packaged products undergoing quality control in an industrial food or pharmaceutical product plant, and more generally the case of systems for quality control along a production line. Furthermore, there is the need to effectively perform the inspection of welds, as well as the separation of plastic materials so that they can be better recycled subsequently, or more generally the identification and tracking of products in single batches. The current technology still has a number of limitations due to the fact that the systems proposed by the prior art in order to fulfill the aforementioned needs aim to identify the objects based on their physical properties, such as the equivalent atomic number or the density, but not based on their chemical properties. For this reason, the systems currently proposed by the prior art are still subject to a high number of false alarms and have limited possibilities of distinguishing between substances having physical properties similar to each other, such as water and hydrogen peroxide. Furthermore, the known systems are not yet able to identify localized alterations of a same substance such as the initial formation of mould or the initial deterioration of a food product. Furthermore, these known systems are unable to identify a foreign body having a density lower than the one of the material in which this body is contained, which is of great importance in the food field.

Most of the currently used methods are based on Computerized Tomography (CT) or on X-ray scanning.

CT systems produce sectional images of objects whose volume is reconstructed based on absorption measurements acquired from different angles. For example, the tomography systems used in the medical field are well-known, these systems however being too slow to be used effectively in the industrial field or for the checking of hand baggage along a conveyor line.

On the other hand, he systems based on X-ray scanning envisage a linear scanning of the object to be inspected, typically by exposing this object, which is moving on a conveyor belt, to a collimated X-ray beam and then obtaining two-dimensional images which are then superimposed in order to determine any discrepancies. A system of this type is described for example in the European Patent No. EP 2 405 260 B l . A first improvement to this technology was achieved by means of the so- called dual energy systems, for example systems using monochromatic high-energy X-rays at two different preset energy levels with consequent acquisition of two attenuation profiles for each scanning line. By analyzing and comparing the two different profiles, it is possible to estimate with a certain degree of uncertainty the equivalent atomic number of the material under inspection. Although advantageous in many respects, with this known solution however it is not possible to detect low density objects.

A further improvement was introduced by using the so-called multiple- views method, for example by means of apparatuses where the object is linearly scanned from two or more viewpoints generally arranged at angles orthogonal to each other.

Other known methods and systems are based on X-ray diffraction. For example, the US patent n. 6, 1 18,850 discloses a system based on X-ray diffraction having an energy resolution where various volumes inside the object being examined are irradiated with a polychromatic X-ray beam and the spectra of the diffracted radiation are acquired by means of an energy dispersion detector arranged at preset angles with respect to the incident radiation.

Finally, spectroscopy systems for measuring the absorption of X-rays by an object under inspection are known.

Although the apparatuses based on the aforementioned methods are becoming increasingly widespread in response to the growing demand of the market, none of them may be properly defined as being effective and advantageous for the purpose of a correct identification of an object, in particular the composition thereof. In particular, none of the known solutions described above is able to identify objects with a molecular density less than that of the material in which they are contained. The limited precision of the determination of the chemical and physical characteristics of the objects being examined therefore leads to erroneous results of the recognition algorithms.

Moreover, the aforementioned apparatuses are generally not suitable for fast initial inspections, but instead they are generally suitable for inspections of previously selected objects which require further slow-speed scanning.

Finally, it must be mentioned that, in the case of the detectors used in the known solutions, the resolution cannot be easily increased since those detectors are composed of elementary detectors or pixels, the minimum dimensions of which cannot be easily reduced. The US patent application number US 2005/0247882 A 1- describes a detector in which. a firstjface of a crystal, which is irradiated by X-rays on a second opposite face, is provided with a plurality of electrodes corresponding to a plurality of pixels, the pixels and the corresponding electrodes being arranged in two parallel rows that may be staggered with respect to each other.

The technical problem on the basis of the present invention is that of devising a detector and a method for performing a non-invasive radiographic inspection of an object moving along a transfer path, having structural and functional characteristics such as to overcome the drawbacks mentioned with reference to the prior art solutions, in particular being able to identify objects in a simple and efficient manner, including those items which are small in size and have varying densities, with a high resolution and at high speed.

In particular, an object of the invention is to identify micro contaminants, also of biological origin, having dimensions even smaller than one millimeter, such as a hair and/ or the presence of toxins in foodstuffs.

A further object of the invention is to identify micro-defects in materials resulting from alterations of the material itself, such as a weld or a thermomechanical process altering the composition or the local structure of the material.

Yet another object of the invention is to allow the identification of objects having a molecular density less than that of the host material, with an analysis capacity performed in a time period which is essentially halved and at a cost lower than that of the known solutions.

Finally, another object of the invention is that of identifying materials before they are recycled, and therefore for example separating from each other plastics or noble metals in electronic boards in order to facilitate the recycling process.

Summary of the invention

The solution idea at the basis of the present invention is that of using a particular configuration of a detector in order to speed up the acquisition and increase the quality of the attenuation data to be processed for the identification of an item under inspection. Basically, the detector is configured so as to have two rows of elementary detectors or pixels which are adjacent to each other, each row privileging the collection of information in energy bands that are different from those of the other row, this configuration allowing the acquisition speed and energy resolution of the acquisition to be increased, such that the biological origin of many contaminants can also be identified. In this way, a first row privileges the detection of light or not very dense materials, while the other row privileges the detection of heavy or denser materials, detecting different characteristics of the inspected object.

On the basis of this solution idea, the aforementioned technical problem is solved by a detector for performing a non-invasive radiographic inspection of at least one item, such a detector comprising at least one first detection module including a plurality of n elementary detectors that are adjacent to each other and aligned so as to form a first row, these elementary detectors being apt to convert into an electric signal the photon energy of an incident radiation emitted from at least one source, the detector further comprising at least one second detection module adjacent and identical to the first detection module and in turn including a plurality of n elementary detectors that are adjacent to each other and aligned so as to form a second row, the detector being characterized in that the first detection module and the second detection module are associated with at least one electronic unit processing the electric signals generated therefrom such that each photon incident on these detection modules is associated with an energy band of a plurality of energy bands, the first detection module having a different sensitivity with respect to the one of the second detection module in at least one subset of these energy bands.

More in particular, the invention comprises the following additional characteristics, considered individually or, if necessary, in combination.

The features and advantages of the detector, the apparatus and the method according to the invention will become apparent from the following description of an embodiment thereof, given by way of non-limiting example with reference to the accompanying drawings. Brief description of the drawings

In those drawings:

- Figure 1 shows a schematic view of an apparatus for performing a noninvasive radiographic inspection of an item according to the present invention; - Figure 2 shows a two-dimensional schematic view of a detector, an object to be inspected and an X-ray source included in the apparatus of Figure 1 ;

- Figure 3 shows a two-dimensional schematic view of the detector of Figure 2 and its electrical connections with electronic units;

- Figure 4 shows a block flow diagram illustrating successive steps of the method according to the present invention; and

- Figure 5 shows a schematic sequence of a movement of an object along a transfer path, the attenuation data of that object being detected by the detector according to the present invention.

Detailed description

With reference to those figures, and in particular to the example of Figure 1 , an apparatus for performing a non-invasive radiographic inspection of at least one item 2 according to a method illustrated below is described, such an apparatus being schematically and globally indicated with the reference number 1.

It is worth noting that the figures represent schematic views and are not drawn to scale, but instead they are drawn so as to emphasize the important features of the invention. Moreover, in the figures, the different elements are depicted in a schematic manner, their shape varying depending on the application desired. Finally, it is noted that in the figures the same reference numbers refer to elements that are identical in shape or function. The apparatus 1 comprises an inspection and control station 4 including an active part of the components of this apparatus 1, among which a conveyor belt 3 for moving the at least one item 2 which is to be inspected. It is emphasized that, although Figure 1 shows only an item 2, the present invention may be applied also to a plurality of items 2 being transferred along the conveyor belt 3. Moreover, the item 2 may comprise internally elements which must be detected, including very small elements having dimensions even smaller than 1 mm, such as for example micro contaminants of biological origin. For the sake of easier description, these elements will be indicated always by the reference number 2. The item 2 is transferred inside the inspection and control station 4 by means of the conveyor belt 3 in the direction of the arrow F.

The inspection and control station 4 further comprises an inspection tunnel 5 which is substantially passed through by the conveyor belt 3 via an entry opening 6 and an exit opening 7, these openings 6 and 7 being defined in this inspection tunnel 5 above the conveyor belt 3, according to the local reference system of Figure 1. The entry and exit openings 6 and 7 are equipped with suitable movable partitioning plate 8, which are shielded and can be opened during the passage of the item 2 being transferred on the conveyor belt 3, only the plate on the side of the entry opening 6 being shown.

The inspection tunnel 5 is essentially a screened chamber wherein at least one X-ray source 9A and at least one detection chain 10A, which is intended to collect the radiation emitted from the at least one X-ray source 9A, are arranged.

It can be noted that Figure 1 shows a first X-ray source 9A and a second X-ray source 9B, as well as a first detection chain 10A and a second detection chain 10B associated with the first X-ray source 9A and the second X-ray source 9B, respectively, this configuration being considered the preferred one, but not limiting the scope of the present invention, whereby the number of X-ray sources and the number of detection chains may vary depending on the needs and/ or the circumstances. The X-ray sources 9A and 9B are positioned preferably below the conveyor belt 3, even though other arrangements are obviously possible. The support structure of the conveyor belt 3 is radio-transparent in such a way that it does not introduce disturbances in the analysis of attenuation data of the radiation incident on the item 2, namely it does not attenuate the incident radiation X.

The X-ray sources 9A and 9B are therefore associated with respective detection chains lOA and 10B, which are apt to collect the radiation attenuated by the passage of the item 2 to be inspected in front of the X- ray sources 9 A and 9B, the item 2 absorbing part of the incident radiation. These detection chains 10A and 10B are schematically shown in Figure 1 by means of respective dashed lines and comprise at least one detector 1 1A and at least one detector 1 IB, respectively.

The at least one detector 1 1A and at least one detector 1 1B are therefore arranged inside the inspection tunnel 5 so as to collect the radiation emitted by the X-ray sources 9A and 9B, this radiation being attenuated by the passage of the item 2 to be inspected.

Each one of the detection chain 10A and the detection chain 10B may comprise a plurality of detectors, which may be arranged inside the inspection tunnel 5 according to different configurations. For example, the detectors may be arranged in a L- shape forming one row on the top side and one row on the rear side of the inspection tunnel 5, the terms "top" and "rear" being understood according to the local reference system of Figure 1 , even though obviously other arrangements are possible. For the sake of a simpler illustration, only a single detector 1 1A of the detection chain 10A and only a single detector 1 1B of the detection chain 10B will be considered in the following.

The arrangement of the X-ray sources 9A and 9B inside the inspection tunnel 5 is suitably chosen so that the item 2 is struck by an X-ray beam having an angular coverage that is sufficient to irradiate the whole surface of the item 2 facing these X-ray sources 9A and 9B. Obviously, it is possible to increase the number of X-ray sources or to angle them relative to each other so as to cover the entire surface of the item 2 by exposing it to X-rays from several angles. However, these aspects are not of decisive importance for the purposes of the present invention and a person skilled in the art may understand that a greater or lesser exposure of the item 2 to the X-ray beams may be appropriate depending on the examination needs and/ or depending on the need to optimize / reduce the costs both of the apparatus and of the single inspection operation. The arrangement and the relative orientation of the detectors 1 1A and 1 IB with respect to the X-ray beam is suitably chosen so as to maximize the collection of the photons attenuated by the item 2 and their conversion into electric signals.

The apparatus 1 also comprises conventional power supply units 12, which are able to supply power both to power circuits and to command and control circuits of the whole apparatus 1 , these circuits not being shown in the figures since they are conventional.

Furthermore, the apparatus 1 includes a monitoring and control computerized unit 13, which manages all the functions of the apparatus 1. The computerized unit 13 may be on the outside of the inspection and control station 4 and may be easily accessed by the user. Obviously, the computerized unit 13 may be electrically connected to the inspection and control station 4 by means of electric cables intended both for the supplying of electric power and for signal connection. Alternatively, the signal connection may be performed by means of a wireless interconnection, the inspection and control station 4 and the computerized unit 13 comprising wireless data reception/ transmission means TX1 and TX2, respectively. The computerized unit 13 may control the movement of the conveyor belt 3 according to an inspection program started by the operator, or alternatively it may control the movement of the conveyor belt 3 also in a completely automated manner upon receiving input signals for example from a proximity detector 14, e.g. a photodetector, arranged near the entry opening 6 of the inspection tunnel 5.

The computerized unit 13 also controls the emission of the X-ray sources 9A and 9B, this emission being synchronized with the passage of the item 2 inside the inspection tunnel 5.

The computerized unit 13 is also responsible for performing the calibration, the acquisition and possibly the processing of the signals of the detection chains 10A and 10B associated with the respective X-ray sources 9 A and 9B.

Conveniently, the structure and arrangement of the detectors 1 1A and 1 IB ensure an improved acquisition of the incident X-rays attenuated by the moving item 2 to be inspected, in particular an improved analysis capacity of the attenuated radiation in different energy bands, as will be explained in detail in the following.

In particular, at least one of the detection chains 10A or 10B comprises at least one detector 1 1A or 1 1B structured with a double row of adjacent elementary detectors able to convert the photon energy of an incident radiation into an electric signal. Preferably, both the detection chains 10A and 10B comprise at least one detector 1 1A and 1 IB structured with such a double row of adjacent elementary detectors. With reference to Figure 2, the structure of the detector 1 1A, which collects the radiation from the X-ray source 9A, is now described, said radiation being attenuated by the item 2 being transferred on the conveyor belt 3. In particular, the detector 1 1A is structured so as to include at least one first detection module 20, comprising a plurality of n elementary detectors PAl-PAn adjacent to each other and aligned so as to form a first row, and at least one second detection module 21 adjacent to the first detection module 20 and in turn including a plurality of n elementary detectors PB l-PBn adjacent to each other and aligned so as to form a second row, the elementary detectors PB l-PBn of the second detection module 21 being preferably longitudinally offset with respect to the elementary detectors PAl-PAn of the first detection module 20, namely they are offset along a longitudinal axis H-H of the detector 1 1 A, such a longitudinal axis H-H indicating a direction along which the detector 1 1A has its greater dimension.

In the example of Figure 2, the first detection module 20 and the second detection module 21 are arranged above one another, the first detection module 20 being arranged below the second detection module 21 according to the local reference system of Figure 2, the elementary detectors PAl-PAn of the first detection module 20 being adjacent to the corresponding elementary detectors PB l-PBn of the second detection module 21.

It is also emphasized that, although Figure 2 shows only the detector 1 1 A, the detector 1 1B or any other detector included in the apparatus 1 is also structured in the same way, the figures being provided only by way of example and not limiting in any way the scope of the present invention.

The detector 1 1A therefore comprises the plurality of elementary detectors PAl-PAn and the plurality of elementary detectors PB l-PBn, organized on the first and on the second detection modules 20 and 21, respectively, these elementary detectors being the so-called pixels, the number n of elementary detectors PAl-PAn of the first detection module 20 corresponding to the number n of elementary detectors PB l-PBn of the second detection module 21.

The number n of elementary detectors PAl-PAn and PB l-PBn may vary depending on the needs and/ or circumstances, for example it is possible to have 16, 32, 64, 128 or even 1024 pixels.

In a preferred embodiment of the present invention, the elementary detectors PAl-PAn have the same physical structure of the elementary detectors PB l-PBn, namely they are made of the same material and have the same shape and dimensions.

In particular, the elementary detectors PAl-PAn and PB l-PBn all have the same dimensions at least along the longitudinal axis H-H of the detector 1 1A. Even more particularly, the first detection module 20 and the second detection module 21 are adjacent to each other, but with the respective elementary detectors PAl-PAn and PB l-PBn longitudinally offset with respect to each other by a distance S/2 equal to half the longitudinal dimension S of a single elementary detector of the plurality of elementary detectors PAl-PAn and PB l-PBn, where "longitudinal dimension" means the dimension of the elementary detector measured along the longitudinal axis H-H of the detector 1 1A.

In other words, in a preferred embodiment, the elementary detectors PAl- PAn and PB l-PBn have the same dimensions and spatial measurements, at least along the axis H-H, and their offset arrangement results in the formation of the first detection module 20 and the second detection module 21 arranged adjacent to each other, but offset relative to each other by half an elementary detector, i.e. by a distance equal to S/2.

Moreover, Figure 2 shows the limits of incidence of the X-ray beam, indicated as LI and L2. Preferably, the projection of the X-ray beam strikes perpendicularly the detector 1 1A; however, it is possible for the projection of the X-ray beam to strike the detector 1 1A also at different angles.

In the embodiment shown in Figure 2, each elementary detector has the shape of a square with a side equal to S. Therefore, each detection module, which comprises n elementary detectors, has a height equal to S and a longitudinal extension equal to n*S, this figure however not limiting the scope of the present invention, since the elementary detectors of the two detection modules may also have other shapes. In this case, the aforementioned offset arrangement of the first detection module 20 and the second detection module 21 is therefore equal to half the side of each square, i.e. S/2. The first detection module 20 therefore comprises the plurality of elementary detectors PAl-PAn and it is shown with a first element PA1 , i.e. a first elementary detector or pixel, having a squared shape with side S, the term "first" referring to a starting point on the left-hand end of the detector 1 1A according to the local reference system of Figure 2, The first element PA1 is followed in sequence by n-2 i-th elements PAi having a squared shape with side S and by a final element PAn, also having a squared shape with side S.

The second detection module 21 instead comprises the plurality of elementary detectors PB l-PBn and it is shown with a first element PB 1 , namely a first elementary detector or pixel, having a squared shape with side S, the term "first" always referring to a starting on the left-hand end of the detector 1 1A according to the local reference system of Figure 2, the first element PAI being followed in sequence by n-2 i-th elements PBi having a squared shape with side S and by a final element PBn, also having a squared shape with side S.

The advantages due to this offset spatial arrangement of the elementary detectors will be explained in the description below.

Figure 3 schematically shows a two-dimensional scheme of the detector 1 1 A, as well as a scheme of the electrical connections thereof, the first and second detection modules 20 and 21 being physically assembled in electronic boards.

The computerized unit 13 drives a first electronic unit 22 associated with the first detection module 20 of the detector 1 1A and a second electronic unit 23 associated with the second detection module 21 of the detector 1 1 A.

In particular, each elementary detector PAi of the first detection module 20 is associated with the first electronic unit 22 and each elementary detector PBi of the second detection module 21 is associated with the second electronic unit 23, these first and second electronic units 22 and 23 performing the polarization, the control and the analog/ digital conversion of the signal emitted from each elementary detector PAi and PBi. More specifically, the elementary detectors PAl-PAn of the first detection module 20 generate by means of the first electronic unit 22 corresponding first attenuation data DATAl-DATAn, while the elementary detectors PB 1- PBn of the second detection module 21 generate by means of the second electronic unit 23 corresponding second attenuation data DATB l-DATBn. The first and the second electronic units 22 and 23 cyclically scan each elementary detector in order to acquire and count the X-ray photons separated into energy bands and collect and pack the first and second attenuation data DATAl-DATAn and DATB l-DATBn detected and converted for the subsequent processing thereof. In other words, the attenuation data DATAl-DATAn and DATB l-DATBn, following the processing in the electronic units 22 and 23 of the electric signals generated by the detection modules 20 and 21 , are available in the form of a photon count, each photon being associated with an energy band of a plurality of energy bands, which can be modified dynamically during the processing procedure.

Each elementary detector PAi of the first detection module 20 generates therefore a corresponding attenuation data DATAi, as well as each elementary detector PBi of the second detection module 21 generates a corresponding attenuation data DATBi. The first and second attenuation data DATA l-DATAn and DATB l -DATBn may be then transmitted to the computerized unit 13, as described above, which can comprise a processing unit (not shown in the figures) for processing data. Alternatively, the processing unit may be included in the inspection and control station 4. Moreover, the first electronic unit 22 and the second electronic unit 23 are adapted to perform a first self-calibration of the detector 1 1 A, calculating the offset and the gain thereof before starting the inspection of the item 2.

Conveniently, due to the structure of the detector according to the invention, the first electronic unit 22 and the second electronic unit 23 are independent and physically separated from each other, the first detection module 20 being connected to the first electronic unit 22 and the second detection module 21 being connected to the second electronic unit 23.

Furthermore, the structure of the detector according to the present invention allows performing a simple connection between the first detection module 20 and the first electronic unit 22 as well as between the second detection module 21 and the second electronic unit 23, adopting a symmetrical geometry in which the first and second electronic units 22 and 23 are configured as two coplanar modules connected to the detector by means of conductor wires (shown as dashed lines in Figure 3) or bump bonding. The simplicity of the connections between detector and electronic units results in manufacturing costs which are much lower than those for a connection between pixel matrices and a reading electronics also having a matrix structure. Alternatively, in an embodiment not shown in the figures, other configurations are possible, all of them falling within the scope of the present invention. For example, it is possible to provide a detector comprising three detection modules, i.e. comprising a third detection module adjacent to one of the first detection module 20 and the second detection module 21, each detection module of the detector comprising a respective plurality of n elementary detectors aligned with each other, but offset longitudinally with respect to the corresponding elementary detectors of the adjacent detection module by a distance equal to one third of the spatial dimension of a single elementary detector. This concept may be extended so as to provide a detector comprising n' detection modules, each detection module comprising a respective plurality of n elementary detectors which are aligned with each other but are longitudinally offset with respect to the corresponding elementary detectors of the adjacent detection module by a distance equal to 1 /n' of the spatial dimension of a single elementary detector.

Each elementary detector is a solid-state component capable of directly converting the X-rays into an electric signal. The base material, which is adapted to absorb and detect the incident radiation and which the elementary detectors are made of, is preferably selected from cadmium telluride (CdTe), cadmium zinc telluride (CZT) or silicon (Si), even though any suitable semiconductor may be used.

These detectors are particularly efficient for energy conversion, have a low noise level and allow a high-frequency acquisition, also above 100 MHz (and therefore at rates of more than one hundred million counts per second). With these detectors, it is also possible to perform a count of the photons collected according to their energy.

As mentioned above, the elementary detectors of the first detection module 20 are preferably made of the same material of the elementary detectors of the second detection module 21 , but the use of different materials is also possible. For example, the elementary detectors of the first detection module 20 may be made of cadmium telluride, while the elementary detectors of the second detection module 21 may be made of silicon.

The aforementioned structure with offset rows of the detector 1 1 A is achieved by means of deposition of metal contacts onto the base material of this detector 1 1 A, which base material therefore maintains its original shape and structure.

Owing to the particular structure of the detector according to the present invention, it is possible to achieve a spatial resolution twice that of the known detectors, since the minimum detectable dimensions of an object correspond to half the longitudinal spatial dimension of the pixels of the detector, therefore resulting in significant advantages.

Furthermore, the electronic unit 22 of the first detection module 20 is separate from the electronic unit 23 of the second detection module 21 (namely the production and the processing of the first attenuation data DATAl-DATAn occurs separately from and at the same time as the production and processing of the second attenuation data DATB l-DATBn) and therefore, when a photon strikes one elementary detector of only one of the first detection module 20 and the second detection module 21 , it is not detected by the elementary detectors of the other detection module and it is therefore possible to establish in an unambiguous manner to which detection module the elementary detectors struck by the photons belong. It is therefore clear that, owing to the offset arrangement of the detection modules, if an object being transferred is detected (namely it causes a variation of the grey value) by one i-th elementary detector of only one of the two detection modules 20 or 21 (and not by the corresponding i-th elementary detector of the other detection module), this object has minimum dimensions at least equal to half the longitudinal spatial dimension of the elementary detector considered.

Advantageously according to the present invention, owing to the configuration adopted, each detection module is easily provided with a different energy specialization, each detection module being adapted to detect photons having different wavelengths, namely photons belonging to a given portion of the energy spectrum. For example, a detection module of the detector may be sensitive only to high energy X-ray photons, while the other detection module may be sensitive only to X-ray photons with a lower energy. Consequently, the present invention allows the first detection module 20 and the second detection module 21 to be specialized for low energy and high energy, or vice versa. In the present description, the term "sensitivity" means the capacity of a detection module to privilege the detection of photons (and therefore the collection of information) in a particular energy range or energy band.

Owing to this energy specialization, it is possible to have one detection module, for example the first detection module 20, and the respective electronics, specialized to work in the low energy range starting from 0 keV, where preference is given to the low noise characteristics rather than the response speed, and another detection module, for example the second detection module 21 , which is specialized to work in higher energy range. As a result of the present invention, it is therefore possible to have one detection module working at a low noise level, leaving the other detection module the task of working with a higher response speed.

The different energy specialization of the two detection modules 20 and 21 leads to several advantages, the main one being the possibility of increasing the efficiency of the identification of materials having different chemical and physical characteristics, in particular different density. In fact, it should be noted that different densities cannot be detected nowadays, even at low energy, using the real-time commercial spectroscopy systems of the prior art (the typical low energy values of the known system are around 20 keV). Advantageously according to the present invention, it is now possible to detect the characteristics of materials absorbing X rays even with an energy of less than 10 keV, such as thin plastics (cellophane, latex, etc.), by means of either one of the two detection modules, i.e. the module privileging the information comprised in low-energy bands, the other detection module privileging instead the information comprised in high-energy bands, therefore taking an important step forwards towards the identification of foreign bodies for example in food products. It is emphasized, however, that the detection modules are structured to collect information also relating to the other energies of the spectrum, while privileging information comprised in specific bands, as mentioned above.

In other words, one of the first detection module 20 and the second detection module 21 is specialized to privilege information comprised in low-energy bands ranging from 0 keV to 20/30 keV, while the other detection module operates at higher energies.

It is emphasized that the elementary detectors PAl-PAn of the first detection module 20 are preferably identical to the elementary detectors PB l-PBn of the second detection module 21 and consequently the capacity of the first detection module 20 to work at energy bands that are different from those of the second detection module 21 (and therefore the capability to be sensitive to photons having a different energy) is obtained by means of a suitable processing the signals generated by the detection modules. This energy specialization therefore is not derived from the structure of the single detection modules 20 and 21 , which is substantially identical, but from the different processing of the signals produced by these detection modules 20 and 21.

In particular, both the detection modules 20 and 21 are able to divide the incident radiation into different energy bands, and the electric signals from the first detection module 20 are processed differently from the electric signals from the second detection module 21, such that the number of photons detected (counted) by the first detection module 20 in a given energy band is different from the number of photons detected (counted) by the second detection module 21 in the same energy band. In other words, the elementary detectors of the first detection module 20 generate, by means of the first electronic unit 22, the first attenuation data DATAl-DATAn which comprise information relating to the absorption of photons in at least one energy band, and the elementary detectors PB1- PBn of the second detection module 21 generate, by means of the second electronic unit 23, the second attenuation data DATB l-DATBn which comprise information relating to the absorption of photons in one or more different energy bands.

In this way, the first detection module 20 has a sensitivity different from that of the second detection module 21 in a given subset of the plurality of energy bands, this subset comprising at least one energy band. In this way, the first detection module 20 has a greater sensitivity (and therefore privileges the detection of photons) in a first subset (e.g. at low energy) and the second detection module 21 has a greater sensitivity in a second subset (e.g. at high energy) of the energy bands, which second subset is different from the first subset. In a preferred embodiment of the present invention, the different processing of the signals from the two detection modules 20 and 21 is obtained by ensuring that the first electronic unit 22 processes the signals from the first detection module 20 in an integration time different from the integration time with which the signals from the second detection module 21 are processed.

By way of example, it is assumed that the source 9A emits a constant flow of about 100,000 photons per second and that both the detector rows are able to divide the incident radiation into five different energy bands. Conveniently, the first detector row may be set to collect the photons passing through the object under inspection in a time interval of 1 ms and therefore it detects ninety photons over the whole energy spectrum. If the material to be analyzed has a very high attenuation coefficient of the incident radiation in the low energy portion of the spectrum, there will be a reduced number of photons collected in the lower energy bands. On the other hand, the second detector row may be set to collect the photons passing through the object under inspection in a time interval of 10 ms and therefore it detects 900 photons over the whole energy spectrum. Since the material analyzed is the same as that analyzed by the first row, the second row collects a greater number of photons and therefore a greater number of photons is collected also in the energy bands which before did not collect anything.

Owing to the different quantity of photons collected by the two rows, it is possible to apply analysis algorithms (namely the attenuation data DATAl-DATAn and DATB l-DATBn may be processed in the computerized unit 13) so as to allow a very precise estimation of the physical characteristics of the object under inspection, since the interesting characteristics of the object are assessed simultaneously in two different incident flow conditions.

In order to understand better this aspect of the invention, an analogy may be drawn with the operation of a camera having two lenses mounted and operating simultaneously, each one having a different focus, where for example one lens may operate in the infrared range and the other lens may operate in the visible light spectrum. With such a camera, employing the optical properties of the different wavelengths of the infrared radiation and visible radiation, it is possible to observe particular details of the objects and to obtain a greater amount of different information. In the same way, advantageously according to the present invention, one of the first detection module 20 and the second detection module 21 acts like a lens specialized for identifying small-thickness and low-density objects (such as for example a lens which is sensitive to lower energies or sensitive to higher wavelengths), while the other detection module of the detector is specialized to identify objects with a higher density and thickness (such as for example a lens sensitive to higher energies or sensitive to lower wavelengths) . Owing to the use of the detection chains 10A and 10B comprising the detectors 1 1A and 1 1B according to the invention, it is also possible to execute a novel method for performing a non-invasive radiographic inspection of at least one item 2 able to innovate the online spectroscopy technology performed by means of the absorption of X rays and/ or gamma rays.

In particular, the method comprises at least the steps of:

- providing at least one X-ray source 9A or 9B emitting a polychromatic X- ray beam incident on the item 2 under inspection; and - providing at least one detector 1 1A or 1 1B along a transfer path of the item 2;

In particular, the at least one detector 1 1A or 1 1B is structured with at least one detection module 20 comprising a plurality of n elementary detectors PAl-PAn adjacent to each other and aligned so as to form a first row, and at least one second detection module 21 adjacent to the first detection module 20 and in turn comprising a plurality of n elementary detectors PB l-PBn adjacent to each other and aligned so as to form a second row, the elementary detectors PB l-PBn of the second detection module 21 being longitudinally offset, namely along a longitudinal axis H- H of the at least one detector 1 1A or 1 1B, with respect to the elementary detectors PAl-PAn of the first detection module 20, these elementary detectors PAl-PAn and PB l-PBn all having substantially the same dimensions, at least along this longitudinal axis H-H. The method according to the invention then comprises the steps of:

- collecting first attenuation data DATA 1 -DAT An of the X ray beam generated by the elementary detectors PAl -PAn of the first detection module 20, and second attenuation data DATB l-DATBn generated by the elementary detectors PB l-PBn of the second detection module 21 , wherein, during this collection step, the electric signals generated by the detection modules 20 and 21 are processed in such a way that the first detection module 20 detects photons having a different energy compared to the second detection module 21 , namely it has a sensitivity different from that of the second detection module 21 in a subset of energy bands, privileging in this way the contents of the spectral information comprised in this subset of energy bands; and

- processing the first attenuation data DATAl-DATAn and the second attenuation data DATB l-DATBn by means of a suitable data processing algorithm so as to form radiographic images of the item 2 and obtain information about the volume and the composition thereof.

The aforementioned steps of collecting and processing the first and second attenuation data DATAl-DATAn and DATB l-DATBn are performed by means of the data processing algorithm, executed for example inside a processing unit controlled by a computerized unit 13, and are performed for each elementary detector PAi and PBi of the detection modules 20 and 21 , respectively, where PAi and PBi indicate the i-th elementary detector of the first detection module 20 and the second detection module, respectively, thus allowing the complete reconstruction of the radiographic image of the item 2.

Furthermore, the first attenuation data DATAl-DATAn are collected and processed simultaneously with the second attenuation data DATB l- DATBn during the collection and processing steps, namely the elementary detectors or pixels PAl-PAn and PB l-PBn of the first and second detection modules 20 and 21 are read and processed simultaneously, with significant advantages in terms of scanning speed. In other words, the processing flows of the two detection modules 20 and 21 take place in parallel.

It is emphasized that the step of collecting the first and second attenuation data DATAl-DATAn and DATB l-DATBn of the X-ray beam involves the reading of the grey value generated by each elementary detector, these attenuation data therefore including the information generated by each elementary detector for the reconstruction of the image of the item 2. For easier description, in the following only the detector 1 1A will be considered, the method according to the invention being applied in an identical manner also to the detector 1 IB.

Further details regarding the method of the invention are provided in Figure 4, which shows a block flow diagram of the operating steps performed for the implementation of the method.

As mentioned above, the first attenuation data DATAl-DATAn and the second attenuation data DATB l-DATBn are collected simultaneously, the reading of the elementary detectors PAl-PAn of the first detection module 20 occurring at the same time as the reading of the elementary detectors PB l-PBn of the second detection module 21 , the number n of elementary detectors in each detection module being preset in the data processing algorithm.

The step of collecting the first attenuation data DATAl-DATAn and the second attenuation data DATB l-DATBn generated by each elementary detector further comprises a step of storing in a visualization matrix these first attenuation data DATAl-DATAn and these second attenuation data DATB l-DATBn. In particular, for each i-th elementary detector, the data processing algorithm reads the corresponding grey value and stores it in the visualization matrix.

Once the step of collecting the first and second attenuation data DATAl- DATAn and DATB l-DATBn has been completed, the aforementioned step of processing these first and second attenuation data DATAl-DATAn and DATB l-DATBn (which are memorized by means of the data processing algorithm) starts, this step comprising initially a first step of verifying the presence of the item 2 in the elementary detectors PAl-PAn and in the elementary detectors PB l-PBn separately. In particular, by analyzing the grey value generated by the elementary detectors and memorized in the visualization matrix, it is possible to verify whether the elementary detectors have detected or not the presence of the item 2, this grey value relating to the absorption of the X-ray photons by the object 2.

As mentioned above, owing to the structure of the detector 1 1A, the item 2, which may be for example a contaminant of biological origin, may be detected only by the i-th pixel of only one of the two detection modules 20 and 21.

Consequently, the processing step comprises, after the aforementioned first verification step, a second step of verifying if the item 2 is simultaneously detected by an i-th elementary detector PAi of the first detection module 20 and by a corresponding i-th elementary detector PBi of the second detection module 21 , the second step of verifying being carried out for all the n elementary detectors of the first detection module 20 and the second detection module 21.

Essentially, in the second verification step, the first attenuation data DATAl-DATAn of the first detection module 20 are duly compared with the second attenuation data DATB l-DATBn of the second detection module 21. If the item 2 is detected only by the i-th elementary detector of either one of the first detection module 20 and the second detection module 21 , it is possible to establish that the dimensions of the object 2 are equal to half the longitudinal spatial dimension of a single elementary detector. In general, the item 2, during its transfer, which may occur for example in the direction of the arrow F of Figures 1 and 5, covers both elementary detectors of the first detection module 20 and elementary detectors of the second detection module 21. When the presence of the item 2 is detected in a same i-th elementary detector (or in several i-th elementary detectors) of both the detection modules 20 and 21 , the item 2 has at least a dimension equal to S*i' + S/2, where i' is the number of i-th elementary detectors in which the item 2 was detected by both the detection modules 20 and 21.

During this second verification step it is therefore possible to recognize in a certain manner an item having a minimum dimension of at least S/2.

It is therefore evident that the detectors according to the present invention have a structure such that their spatial resolution may be doubled, since the minimum detectable dimension is no longer equal to the spatial dimension of the single elementary detector, but to half thereof.

During the transfer of the item 2, the collection and processing steps are continuously repeated in sequence until the item 2 has completely passed through the inspection tunnel 5 so as to obtain a complete reconstruction of its image.

Furthermore, for each elementary detector, the attenuation data DATA1- DATAn and DATB l-DATBn are provided as a photon count, each being associated with an energy band, these energy bands being possibly modified dynamically during the processing step. As mentioned above, the aforementioned processing step comprises a step of providing the first- detection module 20 with an energy resolution (i.e. specialization) different from that of the second detection module 21. During this step, the electric signals generated by the first detection module 20 are processed separately from and simultaneously with the electric signals generated by the second detection module 21. In this way, the first attenuation data DATA 1 -DAT An comprise information relating to the absorption of photons in a subset of the energy bands, and the second attenuation data DATBl-DATBn comprise information relating to the absorption of photons in a different subset, the electric signals from the first detection module 20 being acquired, during the step of collecting, in an integration time different from that of the electric signals from the second detection module 21 , resulting in different attenuation data such that those detection modules have a different energy specialization, as illustrated above.

The processing of the signals of the first detection module 20 is therefore separate from the processing of the signals of the second detection module _.21 , this different specialization of the detection modules being therefore obtained during the acquisition in the collecting step. As mentioned above, this different energy specialization is useful for recognizing objects having a density different from that of a host material, whereby these objects may also be very small owing to the high spatial resolution which can be achieved with the detector and the method according to the invention.

In other words, the first attenuation data DATA 1 -DAT An comprise information relating to the absorption of photons in given energy bands, and the second attenuation data DATB l-DATBn comprise information relating to the absorption of photons in different energy bands, the electric signals from the first detection module 20 being acquired in an integration time different from that of the electric signals from the second detection module 21.

With reference now to Figure 5, the method according to the present invention is illustrated in greater detail below by means of an example of application thereof. In particular, Figure 5 shows a schematic sequence of a movement of the item 2, which may be for example a contaminant of biological origin inside a foodstuff, along a transfer path in five successive time instants, indicated in Figure 5 as time instants TO, Tl , T2, T3 and T4, the presence of the item 2 being detected for example by the detector 1 1A. For simpler illustration, Figure 5 shows only five pixels PA1-PA5 of the first detection module 20 and five pixels PB 1-PB5 of the second detection module 21 of the detector 1 1 A.

According to the local reference system of Figure 5, the detector HA is arranged in a lower plane with respect to that of the item 2, while the X- ray source is arranged above the item 2, this item 2 moving according to the direction of the arrow F and attenuating the radiation emitted from the source towards the detector 1 1A.

First of all, it should be remembered that the measurable dimensions of an item are related to the dimensions of the elementary detector or pixel of the detector and, for the same signal to noise ratio, the spatial resolution of a detector increases with a reduction in the size of the pixels.

The item 2 is moving at a certain speed and at the instant TO is outside of the detector 1 1A, which therefore in this instant TO is unable to detect the presence of the item 2 in any of its pixels.

At the time instant Tl the item 2 covers partially the pixel PA3 of the first detection module 20 and the pixel PB2 of the second detection module 21 of the detector 1 1A. Since the item 2 covers partially the two elementary detectors, they detect the same grey level, because the object which covers first the elementary detector PA3 and then the elementary detector PB2 is the same and therefore corresponds to a same absorption of photons of the X-ray beam. In this instant Tl therefore there is not yet sufficient information to be able to reach conclusions about the dimensions of the item 2.

At the time instant T2, the item 2 has moved further and a further area of the pixel PA3 is covered, this pixel generating therefore a darker grey level, while the pixel PB2 is covered exactly as at the instant Tl and therefore it has the same grey level as at the instant Tl . At the instant T2 it is therefore possible to establish that the item 2 has a dimension equal to the common side of the pixels PA3 and PB2, namely half of the longitudinal dimension of the pixels. By iterating the process of comparing the grey levels and the displacement of the object 2, namely by repeating in sequence the steps of the aforementioned method for each time instant (each time instant corresponding to a different transfer condition of the object 2 and therefore to a different coverage of the detector 1 1A), it is thus possible to identify a minimum dimension equal to half the spatial dimension of a single pixel.

In conclusion, the present invention provides a detector and a method for performing a non-invasive radiographic inspection of items, wherein the detector comprises two adjacent detection modules or rows of elementary detectors that are longitudinally offset with respect to each other, this offset value being in particular equal to half the longitudinal spatial dimension of a single elementary detector. Advantageously according to the present invention, the spatial resolution of the detector is doubled, as well as the acquisition speed of a radiographic image is doubled.

For the same resolution, the detector according to the present invention uses half the number of elementary detectors or pixels used by the detectors of the prior art, with a significant reduction in the manufacturing costs.

Moreover, the structure of the detectors according to the invention allows the number of photons collected to be doubled for the same intensity of the X-ray beam, thus allowing an improvement in the statistics for the reconstruction of the image of the item under inspection. As a consequence, for the same number of photons to be acquired for a given image quality, it is possible to reduce the intensity of the source by a factor of two, thus reducing the photon rate (photons/ second) on each pixel. The reduction in the photon rate allows increasing the amount of time dedicated to the signal processing, which allows a greater collection of the charge generated in the detector, thus increasing the signal intensity.

It should also be noted that the structure of the detector according to the invention is such that the first detection module and the second detection module can be provided with control electronic units independent of each other and connected thereto in a simple way and in symmetrical arrangements, the simplicity of this design resulting in particularly low manufacturing costs. Furthermore, owing to the separation of the electronics of the first and second detection module, it is possible to process separately and simultaneously the data generated by the first and second detection modules, thereby allowing an increase in the calculation capacity of the detector and the implementation of the method described above.

The possibility of processing in parallel the data of the first detection module and of the second detection module also results in an extremely high data processing speed, in particular a speed twice that of the known solutions, which would not be possible with a single row of elementary detectors due to the intrinsic limitations of the control electronics, such a processing speed being combined with the doubled spatial resolution of the detector.

In addition to the aforementioned advantages, in particular there is the fact that the adopted configuration, besides doubling the spatial resolution, conveniently allows providing each detection module with a different energy specialization, thus doubling the quantity of information obtained in a single scan. For example, one detection module of the detector can be more sensitive to high energy X-ray photons, while the other detection module can be more sensitive to X-ray photons with a lower energy. The different high-energy and low-energy specialization of the two rows of elementary detectors leads to several advantages, for example the possibility of detecting simultaneously items with different densities, and therefore it increases the probability of identifying elements with smaller dimensions and characteristics which, in the current inspection methods, cannot be distinguished from other characteristics of the material. Moreover, it should be noted that in this way a reduction in the number of false alarms may be achieved for the same scanning speed.

In particular, advantageously according to the present invention, the possibility of specializing the two detection modules in such a way that they operate in different energy ranges increases the probability of identifying items with smaller dimensions which cannot be detected with systems according to the known solutions. In fact, owing to the different amount of photons collected simultaneously by the two rows, it is possible to apply analysis algorithms to the generated attenuation data in such a way that it is possible to estimate in a very precise manner the physical characteristics of the item under inspection, since the interesting characteristics of the object are analyzed simultaneously in two different incident photon flow conditions. Compared to the known solutions, the present invention thus allows increasing the probability of detecting small items, as well as significantly increasing the signal to noise ratio (SNR) of the measurement. In fact, differently from the known solutions, there are no longer two different detectors (for example made of different materials) whose absorption band has a profile centered around a particular wavelength and generally has a limited bandwidth; instead, now both the detection modules of the detector have the same X-ray absorption properties, wherein the spectrum is divided into a plurality of energy bands and then the count of the photons in each band is performed, the signals from each detection module being processed differently so that these detection modules are sensitive to different portions of the spectrum (i.e. one module being specialized at low energy and the other module being specialized at high energy), thereby significantly increasing the SNR of the measurement. In other words, compared to the known solutions, there is a greater probability of detecting photons, after they have passed through the object under inspection, in a portion of the spectrum which is wider than that of the known solutions, the parameters of interest being identified by means of the ratio between photons emitted by the source and photons which have passed through the object under inspection.

It can be clearly understood that there is the great advantage of obtaining a reduction in the costs of the detector for the same resolution, since half the number of detectors is used to obtain the same resolution, or vice versa. Moreover, the energy levels used to decode the detector signals may be increased up to 1024 levels, with a significant increase in resolution compared to the solutions of the prior art.

Finally, it should be noted that the aforementioned advantages are obtained without increasing the overall dimensions of the detector. In fact, given a single row of pixels, if the base material is for example a cadmium telluride crystal, it is very difficult to reduce the longitudinal dimension of the single pixels below 0.8 mm, while the transverse dimensions of the base crystal are generally equal to 3 mm in order to ensure a good resistance to mechanical stress. By using two offset rows of pixels, offset relative to each other by about half the longitudinal spatial dimension of the single pixels, it is therefore possible to double the resolution while maintaining the same transverse dimensions of the crystal on which the pixels are formed, thereby guaranteeing the resistance to mechanical stress and at the same time ensuring a simple electronic configuration. The invention therefore solves the technical problem and achieves numerous advantages compared to the known solutions.

In particular, the detector, the apparatus and the method according to the present invention allows identifying micro-contaminants which are also of biological origin and have dimensions even smaller than 1 mm, for example a hair or toxins in foodstuffs, which is not possible with the current on line spectroscopy and X-ray methods.

Moreover, it is possible to identify micro-defects in the materials caused by various factors, for example following welding or thermomechanical processes which alter the local composition/ structure of the materials, as well as it is possible to separate plastics so that they can be better recycled subsequently, or generally to identify and trace products in single batches.

Objects with a molecular density less than that of the host material may also be inspected.

The studies and experiments carried out by the Applicant show also that the analysis capacity is doubled and the analysis time is halved.

Obviously, a person skilled in the art, in order to meet particular needs and specifications, can carry out several changes and modifications to the detector, apparatus and method described above, all included in the protection scope of the invention as defined by the following claims.

Claims

1. A detector ( 1 1A, 1 1B) for performing a non-invasive radiographic inspection of at least one item (2), said detector (1 1A, 1 1B) comprising at least one first detection module (20) including a plurality of n elementary detectors (PAl-PAn) that are adjacent to each other and aligned so as to form a first row, said elementary detectors (PAl-PAn) being apt to convert into an electric signal the photon energy of an incident radiation emitted from at least one source (9A, 9B), said detector (1 1 A, 1 1B) further comprising at least one second detection module (21) adjacent and identical to said first detection module (20) and in turn including a plurality of n elementary detectors (PB l-PBn) that are adjacent to each other and aligned so as to form a second row, said detector (1 1A, 1 1B) being characterized in that said first detection module (20) and said second detection module (21) are associated with at least one electronic unit (22, 23) processing the electric signals generated therefrom such that each photon incident on said detection modules (20, 21) is associated with an energy band of a plurality of energy bands, said first detection module (20) having a different sensitivity with respect to the one of said second detection module (21) in at least one subset of said energy bands.

2. The detector (1 1A, 1 1B) according to claim 1 , characterized in that said elementary detectors (PAl-PAn) of said first detection module (20) generate first attenuation data (DATAl-DATAn) comprising information relating to the absorption of photons in said subset of said energy bands, and said elementary detectors (PBl-PBn) of said second detection module (21) generate second attenuation data (DATB l-DATBn) comprising information relating to the absorption of photons in a different subset of said energy bands, such that said detection modules (20, 21) are specialized to work at different energies.

3. The detector ( 1 1A, 1 1B) according to claim 2, characterized in that said elementary detectors (PAl-PAn) of said first detection module (20) and said elementary detectors (PB l-PBn) of said second detection module (21) are associated with a first electronic unit (22) and a second electronic unit (23), respectively, said first electronic unit (22) and said second electronic unit (23) being separated from each other and being apt to control said first detection module (20) and said second detection module (21), respectively, in such a way that said electric signals from said first detection module (20) are acquired simultaneously with and in a different way from said electric signals from said second detection module (21), generating said different attenuation data ( DATA 1 - D ATAn , DATB 1- DATBn).

4. The detector (1 1A, 1 1B) according to claim 3, characterized in that said first electronic unit (22) is apt to acquire said electric signals from said first detection module (20) in an integration time different from that of said second electronic unit (23), resulting in said different and simultaneously processed attenuation data (DATA 1 -DAT An, DATB 1 -DATBn) .

5. The detector (1 1A, 1 1B) according to any one of the preceding claims, characterized in that one of said first detection module (20) and said second detection module (21) is specialized so as to privilege the collection of information in a low-energy subset of said energy bands, in a range from 0 keV to 30 keV.

6. The detector (1 1A, 1 1B) according to any one of the preceding claims, characterized in that said elementary detectors (PB l-PBn) of said second detection module (21) are offset along a longitudinal axis (H-H) of said detector (1 1A, 1 1B) with respect to said elementary detectors (PAl-PAn) of said first detection module (20), said elementary detectors (PAl-PAn, PB l- PBn) all having substantially the same dimensions at least along said longitudinal axis (H-H).

7. The detector ( 1 1A, 1 1B) according to claim 6, characterized in that said elementary detectors (PBl-PBn) of said second detection module (21) are longitudinally offset with respect to said elementary detectors (PA l-PAn) of said first detection module (20) by a distance (S/2) equal to half a longitudinal dimension (S) of a single elementary detector of said pluralities of elementary detectors (PAl-PAn, PB l-PBn), in such a way that said first detection module (20) is longitudinally offset with respect to said second detection module (21) by said distance (S/2).

8. The detector (1 1A, 1 1B) according to claim 6 or 7, characterized in that it comprises n' detection modules, each detection module comprising a respective plurality of n elementary detectors which are aligned with eacn otner ana are longituamaiiy onset witn respect to tne corresponamg elementary detectors of the adjacent detection module by a distance equal 1 /n' of a spatial dimension of a single elementary detector.

9. The detector (1 1A, 1 IB) according to any one of the preceding claims, characterized in that the base material, which said elementary detectors

(PAl-PAn, PB l-PBn) are made of, is selected from cadmium telluride (CdTe), cadmium zinc telluride (CZT) or silicon (Si).

10. An apparatus (1) for performing a non-invasive radiographic inspection of at least one item (2), said apparatus (1) comprising an inspection and control station (4), which in turn includes a conveyor belt (3) apt to move said at least one item (2), an inspection tunnel (5), in which at least one X-ray source (9A, 9B) is arranged, and at least one detection chain (10A, 10B) apt to collect the radiation emitted from said at least one X-ray source (9 A, 9B), said apparatus (1) being characterized in that said at least one detection chain (10A, 10B) comprises at least one detector (1 1A, 1 1B) which is made according to any one of the preceding claims.

1 1. A method for performing a non-invasive radiographic inspection of at least one item (2), said method comprising at least the steps of: - providing at least one X-ray source (9A, 9B) emitting a polychromatic X- ray beam incident on said at least one item (2) to be inspected;

- providing, along a transfer path of said at least one item (2), at least one detector (1 1A, 1 IB) apt to convert into an electric signal the photon energy of an incident radiation emitted from said at least one source (9A, 9B), each photon being associated with an energy band of a plurality of energy bands;

- collecting first attenuation data (DATA 1 -DATAn) and second attenuation data (DATB 1 -DATBn) of the beam; and

- processing said first attenuation data (DATA 1 -DATAn) and said second attenuation data (DATB 1 -DATBn) to form radiographic images of said at least one item (2) and obtain information about the volume and composition thereof, wherein said at least one detector (1 1A, 1 1B) is structured with at least one first detection module (20) comprising a plurality of n elementary detectors (PAl-PAn) which are adjacent to each other and aligned so as to form a first row, and with at least one second detection module (21) adjacent and identical to said first detection module (20) and in turn comprising a plurality of n elementary detectors (PBl-PBn) which are adjacent to each other and aligned so as to form a second row, said first attenuation data (DATAl-DATAn) being generated by said elementary detectors (PAl-PAn) of said first detection module (20) and said second attenuation data (DATB l-DATBn) being generated by said elementary detectors (PB l-PBn) of said second detection module (21), and wherein the electric signals generated by said detection modules (20, 21) are processed differently during said step of collecting, in such a way that said first detection module (20) has a different sensitivity with respect to the one of said second detection module (21) in at least one subset of said energy bands.

12. The method according to claim 1 1 , wherein said first attenuation data (DATAl-DATAn) are collected and processed simultaneously with said second attenuation data (DATBl-DATBn) during said steps of collecting and processing.

13. The method according to claim 1 1 or 12, wherein said first attenuation data (DATAl-DATAn) comprise information relating to the absorption of photons in said subset of said energy bands, and said second attenuation data (DATB l-DATBn) comprise information relating to the absorption of photons in a different subset, said electric signals from said first detection module (20) being acquired during said step of collecting in an integration time different from that of said electric signals from said second detection module (21), resulting in different attenuation data (DATAl-DATAn, DATB l-DATBn) in such a way that said detection modules (20, 21) have a different energy specialization.

14. The method according to any one of the claims from 1 1 to 13, wherein said step of processing comprises a.

15. The method according to claim 14, wherein said first step of verifying is followed by a second step of verifying if said at least one item (2) is simultaneously detected by an i-th elementary detector (PAi) of said first detection module (20) and by a corresponding i-th elementary detector (PBi) of said second detection module (21), said second step of verifying being performed in sequence to said first step of verifying for all the elementary detectors of said detector (1 1 A, 1 IB).

16. The method according to any one of the claims from 1 1 to 15, wherein said steps of collecting and processing are sequentially repeated during a transfer of said at least one item (2) in an inspection tunnel (5), until the complete reconstruction of the image of said at least one item (2) is achieved.