EP4266273A1

EP4266273A1 - Method for identifying a valuable document under test, a device, and a sorting machine

Info

Publication number: EP4266273A1
Application number: EP22168753.6A
Authority: EP
Inventors: Andrea Brambilla; Vincent MOULIN
Original assignee: European Central Bank
Current assignee: European Central Bank
Priority date: 2022-04-19
Filing date: 2022-04-19
Publication date: 2023-10-25

Abstract

The present invention relates to a method for identifying at least one valuable document under test, a device, and a sorting machine. The method comprises or consists of the steps irradiating at least a portion of the valuable document under test with a wide band X-ray beam, detecting at least one or more electric signal pulses corresponding to photons emitted by the valuable document under test in response to the irradiation using the wide band X-ray beam, and evaluating the detected electric signal pulses in order to identify the valuable document under test as being genuine or not and/or as being of a particular sort or not.

Description

FIELD OF THE DISCLOSURE

The present invention relates to a method for identifying at least one valuable document, a device for identifying at least one valuable document, and a sorting machine.

BACKGROUND

Reliable identification methods for valuable documents are of high importance since such valuable documents are often subject to counterfeiting. Elaborate evaluation methods are usually performed at specialized institutions such as reserve bank institutes, which have the need to continuously identify/authenticate large amounts valuable documents such as banknotes. Accordingly, the evaluation methods are desired to fulfill the requirements of high reliability and large throughput.
One approach for inspecting objects relates to the use of X-ray beams utilizing multi-energy X-ray transmission (MEXRT) imaging. For example, this technique is applied in security-relevant areas, such as security checkpoints at airports or similar locations. In this regard, a beam is directed towards an object, such as baggage, and the transmitted spectrum is obtained. From the difference of the spectra, conclusions can be determined as to dangerous substances being subject to the beam, such as for example explosives.
However, valuable documents are usually extremely thin. Moreover, the amount of dopants arranged within or on a surface of a document under test interacting with the X-ray beam is low. Accordingly, the absorption efficiency is too low, such that it has been found that MEXRT is not suitable for practicability regarding identification of valuable documents.
In consequence, a need exists to provide methods for identifying valuable documents with a high certainty. Preferably, the method may at the same time be suitable for high throughput rates.

SUMMARY

The subject matter of the independent claims satisfies at least part of the respective need. Further embodiments are indicated within the dependent claims and the following description, each of which, individually or in a suitable (sub)combination, may represent aspects of the invention. Some specifics of the present invention are described with regard to corresponding methods or devices. However, the advantages and preferred embodiments described with regard to the indicated devices are correspondingly to be transferred to the according methods and vice versa.
A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide a brief summary of these embodiments and that these aspects are not intended to limit the scope of the present invention. The invention may encompass a variety of aspects that may not be set forth below.
Any of the aspects disclosed below may be part of the present invention based on a sole basis or based on any suitable (sub-)combination with other aspects provided that the skilled artisan considers the respective combination to further develop the present invention. In particular, all facultative aspects ("may", "can", etc.) may also be true aspects of the present invention.
According to a first aspect, a method for identifying at least one valuable document under test is provided. The method comprises or consists of the steps of irradiating at least a portion of the valuable document under test with a wide band X-ray beam. The method further comprises the step of detecting one or more electric signal pulses corresponding to photons emitted by the valuable document under test in response to the irradiation using the wide band X-ray beam, wherein the one or more electric signal pulses respectively contain a peak amplitude, which is proportional to the emitted photons by the valuable document under test. In addition, the method comprises the step of evaluating the detected one or more electric signal pulses in order to identify whether the document under test is a genuine valuable document or not in dependence of a quantitative match and/or is of a particular sort in dependence of a qualitative match. In particular, the evaluation step may include comparing one or more electric signal pulses with a reference electric signal pulse emitted by a genuine valuable document in the same portion as irradiated above. The valuable document under test may then be identified as being a genuine valuable document in case the detected one or more electric signal pulses match with a predetermined tolerance range of the reference electric signal pulse. In case the detected one or more electric signal pulses do not match with a predetermined tolerance range of the reference signal pulse, then the valuable document under test may be identified as not being genuine.
In the context of the present invention, a wide band X-ray beam is considered a beam comprising multiple electromagnetic waves having different wavelengths, also called broad band X-ray beam or a beam having a white spectrum. Since atoms of the document under test are to be excited by the beam used for irradiation, thus, all possible excitation energies are generally provided.
The so-designed inventive method utilizes an appropriate X-ray technique, such as e.g. the X-ray fluorescence (XRF) mechanism. Alternative X-ray techniques may comprise X-ray (powder) diffraction (XRD), use of energy dispersive X-ray spectrometers (XDL or XDLM), X-ray magnetic circular dichroism (XMCD), particle induced X-ray emission (PIXE), etc..
According to the present invention, a wide band X-ray beam is used to irradiate at least a portion of the document under test (DUT). Atoms / chemical elements, also called Xtags, arranged within or on the surface of the irradiated portion of the DUT may then be ionized. Within the electron shell model this may be described by an inner shell electron being emitted from the atom such that the atom is excited into a high-energy state. The ionization process takes place at a certain probability (more specifically at a certain cross section) if an incident photon with sufficient energy to eject an electron from the inner shells of the atom (i.e., the electrons with higher binding energy) is impinging the atom under irradiation. Within this context, this mechanism is based on the Photoelectric effect. Generally, for evaluating the DUT all secondary photons generated as a consequence of the interactions of the incident beam with the DUT may be used. This includes fluorescence photons, but also elastic and inelastic scattering.
The excited state of the atom is instable as to the unoccupied inner shell free valance. Accordingly, a secondary process of the excited atom includes that an outer shell electron may transition into the inner shell free vacancy. This secondary process includes the emission of a secondary photon of a characteristic energy, also at a specific probability (cross section) of the process. Notably, the photon emission process is angle-independent, i.e. the photon may be emitted at all possible angles with equal probability. The photon energy is transition-dependent (so-called Kα-, Kβ-, Lα-, Lβ-, Lγ-lines, etc.; in the following called: K-lines, L-lines, M-lines, etc.) as well as element specific. Since a DUT usually comprises atoms of different elements as Xtags within the irradiated portion, the excitation of these atoms will lead to characteristic emission of electric signal pulses, which may be transformed into an emission signal spectra showing at least one or multiple peaks, which at least partially reside within the X-ray energy range. Of course, the extrema may also be shown as dips (minima) if the sign of the spectra is changed.
In general, as the detected one or more electric signal pulses and, thus, its peak amplitude, respectively correspond to the energy emitted by the irradiated portion of the document under test (DUT) in response to the broad band X-ray beam, it is possible to verify the presence of pulse amplitudes A at respective characteristic predetermined (synonym: predefined) ranges A_min to A_max, which are due to a given Xtag present in a respectively irradiated portion (synonym: "area") of a genuine valuable document. Thus, in case the number N_measure of counts of determined, e.g. measured, pulse amplitudes A at the predetermined amplitude range A_min to A_max (synonym: "amplitude window [A_min, A_max]") is within (synonym: "does match with") a predetermined tolerance range when compared with the reference number N_ref of counts of the respective pulse amplitudes of a genuine valuable document determined in the same irradiated portion and for the same Xtag in this portion, the DUT may be considered to be genuine. In case the number N_measure of counts of determined pulse amplitudes A at the predetermined amplitude range A_min to A_max is outside of (synonym: "does not match with") the predetermined tolerance range of the reference number N_ref of counts of the respective pulse amplitudes of a genuine valuable document measured in the same specific portion and for the same Xtag in this portion, the DUT may be considered to be counterfeited or at least not genuine.
According to the present invention, the use of evaluating the detected electric pulses including pulse amplitudes is advantageous, as it allows the real time evaluation and, thus, sorting of valuable banknotes in a high throughput machine.
In view of the present invention, the predetermined tolerance range, such as the predetermined tolerance range of the reference number N_ref of counts of pulse amplitudes, may be selected of suitable tolerance ranges applicable in the art, such as ± 20 % or less, ± 15 % or less, ± 10 % or less, ± 5 % or less, ± 4 % or less, ± 3 % or less, ± 2 % or less, ± 1 % or less.
According to one embodiment of the present invention, an X-ray beam may irradiate a portion, such as an area, e.g., a square area, of the DUT as test area (e.g., AREA1).
In case AREA1 may only comprise one Xtag, the number N_measure of counts of pulse amplitudes in at least one fluorescence peak of this Xtag may be determined. The fluorescence peak may be selected from Kα-, Kβ-, Lα-, Lβ-, and/or Lγ-lines, e.g., the Kα-line. Generally, a characteristic fluorescence peak for a respective Xtag may be selected. A range of a minimum amplitude A_min and a maximum amplitude A_max for the respective fluorescence peak may be predetermined thereby forming in other words a respective amplitude window [A_min, A_max] (synonym: "energy window [A_min, A_max]") for the respective fluorescence peak. The number N_measure of counts of amplitudes pulses A for the respective fluorescence peak of the Xtag within the range A_min < A < A_max may be determined with suitable means.
The determined number N_measure of counts may be compared to a reference number N_ref of counts given in the same amplitude window of the same fluorescence peak in the same portion of a genuine DUT. In order to determine whether the DUT may be considered genuine, the determined number N_measure of counts of pulse amplitudes may be compared with a reference number N_ref of pulse amplitudes emitted by a genuine valuable document, which is irradiated in the same portion. In case the determined number N_measure is within a predetermined tolerance range of minimum count rate N_{ref_min} of pulse amplitudes and a maximum count rate N_{ref_max} of pulse amplitudes, the DUT may be considered genuine. In case the determined number N_measure of counts is N_measure < N_{ref_min} or N_measure > N_{ref_max}, the tested DUT may be considered counterfeited or at least not genuine.
In order to increase reliability of the inventive method, the number of fluorescence peaks to be detected per Xtag may be increased. In general, in view of the selected computational device for carrying out the inventive method, the computational load imposed by the evaluation of the respective fluorescence peaks may form a limitation.
In case the test area AREA1 comprises two or more Xtags (Xtag_i, wherein i = 2, 3, 4, 5, 6, 7, 8, 9, 10 etc.), respective two or more amplitude windows [A_{min i}, A_{max i}] with i = 2, 3, 4, 5, 6, 7, 8, 9, 10 etc. for the respective fluorescence peaks for each Xtag_i may be predetermined. In order to increase correctness of the identification method, the respective fluorescence peaks for the respective Xtags may be selected on the basis that they enable qualitative and/or quantitative identification of each of the different Xtag_i in the irradiated portion. The number N_{measure_i} of counts of pulse amplitudes in each of the different amplitude windows [A_{min i}, A_{max i}] of the different Xtag_i may be determined. The determined number of counts N_{measure_i} with i = 2, 3, 4, 5, 6, 7, 8, 9, 10 etc. may be compared to a predetermined range of a minimum reference number N_{ref_min i} of counts and a maximum reference number N_{ref_max i} of counts with i = 2, 3, 4, 5, 6, 7, 8, 9, 10 etc.. Again, in case the determined number N_{measure_i} of counts is in the range of N_{ref_min i} < N_{measure_i} < N_{ref_max i}, the DUT may be considered genuine, whereas in case N_{measure i} < N_{ref min i} or N_{measure i} > N_{ref_max i}, then the DUT may be considered counterfeited or at least not genuine.
In addition or alternatively, two or more different fluorescence peaks of the same Xtag may be used for determining whether the DUT may be considered genuine or counterfeited. As an example, the two or more fluorescence peaks may be selected from the group of Kα-, Kβ-, Lα-, Lβ-, Lγ-lines of the same Xtag. In case two fluorescence peaks are chosen, such as Kα- and Kβ-lines or Lα- and Lβ-lines, a respective amplitude window [A_min, A_max] and respective minimum and maximum reference numbers [N_{ref_min}, N_{ref_max}] of counts of pulse amplitudes for each of the selected fluorescence peaks may be predetermined in order to compare the respectively determined number N_measure of counts of pulse amplitudes for each of the respective two or more fluorescence peaks. As set out above, in case the determined number N_measure of counts of pulse amplitudes for each of the respective two or more fluorescence peaks is within the predetermined tolerance range of minimum and maximum reference numbers [N_{ref_min}, N_{ref_max}] of counts of pulse amplitudes for each of the selected fluorescence peaks, then the DUT may be considered genuine and otherwise counterfeited or at least not genuine.
Again, in order to increase reliability of the inventive method, the number of Xtags and their respective one or more fluorescence peaks may be increased. In general, in view of the selected computational device for carrying out the inventive method, the computational load imposed by the evaluation of the two or more Xtags may form a limitation.
According to another embodiment of the present invention, the Xtags may be present in two or more portions of the genuine valuable document, so that two or more test areas (AREA_n with n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 etc.) may be selected for carrying out the inventive identification method as also described above with respect to one test area AREA1. Choosing two or more portions to be irradiated again increases reliability of the inventive method, as the risk of random matches in one portion may be reduced.
For each of the two or more of the n test areas AREA_n, a set of two or more respective amplitude windows [A_{min_n_m}, A_{max_n_m}] may be predetermined, wherein m = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 etc., such as [A_{min_1_1}, A_{max 1_1}], [A_{min_2_2}, A_{max_2_2}] ... [Amin_n_m, Amax_n_m], and the corresponding reference numbers [N_{ref_min_n_m}, N_{ref_max_n_m}] of counts of pulse amplitudes may be predetermined, such as [N_{ref_min_1_1}, N_{ref_max_1_1}], [N_{ref_min_2_2}, N_{ref_max_2_2}], ... [N_{ref_min_n_m}, N_{ref max n_m}]. The number N_{measure_n_m} of counts of pulse amplitudes may be determined for each of the two or more respective amplitude windows [A_{min_n_m}, A_{max_n_m}] in each of the two or more test areas AREA_n.
If N_{ref_min_n_m} < N_{measure_n_m} < N_{ref_max_n_m} for all n and m, then the DUT may be considered genuine. In case for one or more of the determined number N_{measure_n_m} of counts of pulse amplitudes the comparison shows that N_{measure_n_m} < N_{ref_min_n_m} and/or N_{measure_n_m} > N_{ref_max_n_m}, then the DUT may be considered counterfeited or at least not genuine.
Alternatively, the detected electric pulses may be transformed as a function of their amplitude forming at least one X-ray signal spectrum. In this case, peaks of the X-ray signal spectrum are evaluated in order to identify whether the document under test is a valuable document or not. In this case, the presence of characteristic predetermined peaks of the X-ray signal spectrum at respective energies are due to Xtags present in a genuine valuable document. Thus, in case one or more determined, such as measured, characteristic peaks are within / match with a predetermined tolerance range in comparison to a reference X-ray signal spectrum, the DUT may be considered to be genuine. The absence of the characteristic peaks, peaks outside the predetermined tolerance range or the presence of peaks at different energies may be considered as proof that the DUT may be a counterfeit or that the DUT may at least not be genuine. In particular, the presence of peaks at different energies gives evidence that the DUT contains chemical elements that are not part of the nominal composition of a genuine valuable document in the respective irradiated portion. Furthermore, the intensity of the background signal may be correlated to an electron density per unit surface area (which is at first approximation proportional to the mass of the DUT per unit area) in the irradiated DUT. Too low or too high background signal levels may be considered an indication of abnormal document thicknesses, which leads to an identification that the DUT not genuine; or else the presence of other chemical compounds whose fluorescence peaks are outside the energy range of the determination. For example, this may be the case with organic compounds applied to the DUT.
In this regard, the level of Compton scattering CS_measure may additionally or alternatively be determined, such as measured, for a DUT in a respective one or more portions, preferably test areas AREA_n and may be compared to a predetermined minimum to maximum tolerance range of a reference Compton scattering [CS_{ref_min}, CS_{ref_max}] of a genuine valuable document emitted by the same irradiated portion. In case the determined level of Compton scattering CS_measure is not within the range CS_{ref min} < CS_measure < CS_{ref_max}, then the DUT may be considered to have a different thickness in the irradiated portion in comparison to a genuine valuable document. Using the Compton scattering in addition or alternatively in the inventive method, the reliability of the identification that the DUT may be considered counterfeited or at least not genuine may be increased.
In view of the present invention, the predetermined minimum to maximum tolerance range of the reference Compton scattering may be selected of suitable tolerance ranges applicable in the art, such as ± 20 % or less, ± 15 % or less, ± 10 % or less, ± 5 % or less, ± 4 % or less, ± 3 % or less, ± 2 % or less, ± 1 % or less. In addition or alternatively, a predetermined minimum to maximum tolerance range of a reference thickness of a respectively irradiated portion of a genuine valuable document may be used, wherein a suitable tolerance range for the thickness of the valuable document applicable in the art may be selected, such as ± 20 % or less, ± 15 % or less, ± 10 % or less, ± 5 % or less, ± 4 % or less, ± 3 % or less, ± 2 % or less, ± 1 % or less.
Since the present method includes detecting one or more electric signal pulses emitted by the document under test in response to the broad band X-ray beam, wherein the one or more electric signal pulses respectively contain a peak amplitude, which is proportional to the emitted photons by the valuable document under test, and optionally transforming the detected electric signal pulses into a X-ray signal spectrum corresponding to the photons emitted by the DUT in response to irradiating at least a portion of the DUT, the electric signal pulses and optionally the X-ray signal spectrum can be evaluated in order to not only identify the genuineness (synonym: "authenticity"), but also to identify the sort of DUT. In this context, identifying the sort of DUT may include determining at least one of the species, type, and denomination of the DUT, such as a banknote. As an example, the number of the pulse amplitudes in a given tolerance range A_min to A_max and/or the general shape of the X-ray signal spectrum may be evaluated in order to retrieve information on elements included within the DUT. Based on these determined elements in comparison to reference elements of different sorts of genuine valuable document, such as different species, types or denominations, an evaluation unit may be configured to draw the conclusion / determine that the DUT has to be grouped into a specific sort of genuine valuable document, e.g. a specific denomination, such as a 20 € banknote, since other genuine valuable documents, such as banknotes include different combinations of Xtags / chemical elements. The identification of a respective sort of DUT may be conducted prior to, at the same time or after identifying the genuineness / authenticity, wherein the number of counts of pulse amplitudes within a given tolerance range A_min to A_max and/or the spectrum may be compared to expected / predetermined values such as provided through reference data of a respective genuine valuable document. In other words, the identification of the sort of the DUT may be considered as a qualitative evaluation of the detected electric signal pulses or the transformed X-ray signal spectrum, while a quantitative evaluation of the detected electric signal pulses or the transformed X-ray signal spectrum may be conducted when identifying the genuineness / authenticity of the DUT. Consequently, the present invention provides a method, which is advantageously applicable for reliably testing and identifying the sort and / or genuineness of DUTs.
Within the context of the present invention, a genuine valuable document may be considered a security-related document, in particular one of a banknote, a check, a bill, a ticket, a passport, or a flight ticket.
Within the context of the present invention, a wide band X-ray beam may be considered a beam having a so-called white spectrum. In other words, the beam may comprise a variety of photons at different energy levels forming a substantially continuous energy spectrum. The wavelength of the beam may be 10 nm or below.
The X-ray response (synonym: "emitted X-ray signal spectrum") of the DUT may be caused by so-called Xtags. These may comprise or consist of dopants. The atomic number of the dopants is decisive with regard to the features included within the detected electric signal pulses, and optionally the transformed X-ray signal spectra. Due to the available wavelength of the incident beam chemical elements having too low atomic numbers, such as lower than 20, may generally rather be not suitable for acquiring respective spectra. Xtags may be included in the documents as part of security features or elements contained in or arranged on a surface of the DUT. For example, the DUT may comprise a substrate including such Xtag dopants and/or security features. Moreover, the DUT may comprise a layer, printing, or different structure, such as a foil, thread, planchette, fibre comprising respective Xtag dopants and/or Xtag security features arranged on a surface of the DUT. As an example, an ink, coating or varnish may include the Xtags as dopant and/or as security feature, which may be considered compounds or molecules including atoms of corresponding types such that these may be excited and emit respective photons in response. Preferably, Xtags may comprise atoms of at least one of chemical elements having an atomic number in the range of 20 or higher, preferably 20 to 85, in particular including the following elements: Ti, Fe, Cu, Zn, Y, Sn and Ba.
At least one step of the method, in particular the evaluation of the detected electric signal pulses and optionally the transformed X-ray signal spectrum, may be carried out computer-implemented. In other words, a data processing unit may be utilized to evaluate the detected electric signal pulses and/or the transformed X-ray signal X-ray signal spectrum.
In an alternative or cumulatively the method may further comprise the step of evaluating a spectral distribution of the at least one transformed X-ray signal spectrum in order to determine an elemental composition of the at least one portion of the valuable DUT which emitted the X-ray signal spectrum. The transformed X-ray signal spectrum comprises characteristics such as one or multiple peaks. These peaks occur at individual photon energies (wavelengths) corresponding to specific atomic electron transitions. Since the transitions are element-dependent, the spectrum may be evaluated to determine the element species included within the transformed X-ray signal spectrum. In this regard, one difficulty arises since the X-ray signal spectrum may generally be considered a superposition of emission spectra caused by atoms of different elements. However, the X-ray signal spectrum may still at least be evaluated to determine which element types are generally included in the X-ray spectrum. In other words, it may be determined based on which element types the X-ray spectrum was emitted by the DUT. Accordingly, these elements can be assumed to be contained within the DUT or arranged on a surface of the DUT.
In an alternative or cumulatively the method may further comprise evaluating an energy-dependent intensity distribution of the X-ray signal spectrum in order to determine at least a relative concentration-dependent elemental composition of the at least one portion of the valuable DUT which emitted the X-ray signal spectrum. Typically, an energy-dependent (wavelength-dependent) count number is detected when detecting the electric signal pulses. In other words, a detector counts the number of detected photons at different energies. Accordingly, the specific characteristics of the transformed X-ray signal spectrum show intensity (amplitude) variations. These intensity variations generally correspond to the relative element-dependent atom concentrations within the irradiated portion of the DUT. Therefore, evaluating the energy-dependent intensity distribution within the detected electric signal pulses, and optionally the transformed X-ray signal spectrum provides the possibility to draw conclusions on the element concentrations within the irradiated portion of the DUT.
Optionally, the evaluation may also be based on the absence or presence of an amplitude in the detected electric signal pulses or - in relation with the transformed X-ray signal spectrum - the presence or absence of a peak within the spectra at respective energies. Spoken differently, the identification of sort and/or authentication mechanism may be based on the count number per energy window. Thereby, correlating the count number to the concentration of the corresponding element within the DUT causing the respective spectral feature may be omitted. Accordingly, the evaluation process can be made more simple.
In an alternative or cumulatively, the evaluation of the spectra may also be based on the general shape of the spectra. In other words, the shape of the transformed X-ray signal spectrum may be compared to the shape of a reference X-ray signal spectrum. The shape comparison may lead to a match value based on which the DUT may be identified and/or authenticated.
Any of the techniques for evaluating the spectra may include scaling the respective spectra based on a known spectral feature. For example, the scaling could be performed based on the background. Alternatively, the scaling could be performed based on a rather sharp spectral feature known to be caused by the X-ray source itself.
The inventive method may additionally or alternatively also comprise the step of comparing at least one portion of the X-ray signal spectrum to at least one reference X-ray signal spectrum. For example, a reference X-ray signal spectrum may be provided in advance based on a document including no or only single or defined combinations of dopants. Preferably, the reference X-ray signal spectrum may also be provided based on genuine documents, such as genuine banknotes. Optionally, the Xtags may have pre-defined element-dependent concentrations. The reference X-ray signal spectra may for example be stored within a data storage medium. A data processing unit used for evaluating the X-ray signal spectrum may then be coupled to this data storage medium. Accordingly, at least a portion of the X-ray signal spectrum may be compared to the reference X-ray signal spectrum. For example, the peaks may be compared to reference X-ray signal spectra in order to quantitatively determine dopant element concentrations of the DUT.
The comparison may also be utilized to compensate secondary effects, such as spectral characteristics arising from different scattering mechanisms, for example Compton scattering processes. Compton scattering arises from inelastic photon scattering at charged particles, such as electrons. The photon energy is partially transferred to the charged particle. Consequently, the photon energy changes while the photon is scattered. Compton scattering is angle-dependent. In other words, the probability (cross section) of the process and the energy distribution of the scattered photons are angle-dependent. Therefore the spectral characteristics within the X-ray signal spectrum generally depends on the relative orientations of the detector and the DUT. Furthermore, the spectral characteristics depend on the electron concentrations (densities) per unit of surface area of the DUT. Accordingly, reference spectra may be detected using arrangements which are also used for inspecting the DUT. In particular, for each measurement setup reference X-ray signal spectra may independently be detected such that deviations emerging from the difference between multiple measurement setups may be avoided. Afterwards, the Compton background features may be compared to the transformed X-ray signal spectrum of the DUT in order to gain insight into Compton scattering relating effects.
Hence, in an alternative or cumulatively, the inventive method may also comprise the step of determining an electron density per unit of irradiated surface area of the device under test. As soon as reference X-ray signal spectra are compared with the transformed X-ray signal spectra under known and calibrated measurement angles, any differences between the spectra may be considered to arise from variations regarding the electron density per unit of surface area. The electron density may for example change if the DUT comprises a different substrate, a substrate having a different thickness, or different amounts of ingredients, such as different amounts of glue. Variations may occur if the document is based or is subject to counterfeiting actions. Accordingly, conclusions can be drawn in view of the document thickness as well as other substrate parameters. Ultimately, evaluating the electron density may also be used to conclude whether the DUT is genuine or not. Thus, the identification process is further improved.
In an alternative or cumulatively, the inventive method may further comprise the step of subtracting at least a portion of a reference background signal spectrum from at least a portion of the detected electric signal pulses, and optionally the transformed X-ray signal spectrum. For example, the features relating to the Compton scattering mechanism may be subtracted such that they may be compensated within the transformed X-ray signal spectrum. Then, an X-ray signal spectrum may be achieved substantially comprising XRF related features only. Hence, the evaluation of the XRF features may be performed at improved reliability and accuracy.
In an alternative or cumulatively, the inventive method may also comprise the steps of determining at least one value based on at least one portion of the transformed X-ray signal spectrum, comparing the determined value to at least one reference value of a genuine valuable document, and considering the valuable document under test to be genuine if a difference between the determined value and the reference value is within at least one tolerance range. The tolerance range may extend from zero to a first tolerance limit. The tolerance range may also extend from a first tolerance limit to a second tolerance limit. The value may for example relate to an intensity of a spectral feature at a defined energy window. Moreover, energy windows may be defined according to specific wavelength ranges. For these energy windows values depending on the spectral features may be determined which may then be compared to reference values detected from genuine documents. Thus, the accuracy of the identification process may be further improved.
The wide band X-ray beam may be collimated such that the at least one irradiated portion of the valuable document under test is delimited. Collimators are widely used to restrict a beam of electromagnetic radiation to a certain cross-sectional shape. Accordingly, the collimator may be arranged and configured such that the portion of the DUT under irradiation fits the desired shape. For example, a circular surface area (spot) having a pre-defined diameter may be irradiated. Also, different shapes, such as ellipsoidal surface areas may be chosen. The irradiated portion of the DUT may be at least one of a single spot, a one-dimensional line-like section, and an extended or elongated two-dimensional portion. The irradiated surface area may also at least in part depend on the relative orientations of a signal source providing the wide band X-ray beam and the DUT. In particular, the collimator may be used to delimit the irradiated surface area to portions of the DUT comprising Xtag features suitable for emitting a respective X-ray signal spectrum as a response.
In an alternative or cumulatively, the valuable DUT may be arranged at a reception. The reception may then be configured to be moved relative to the wide band X-ray beam during irradiation. Accordingly, elongated portions such as for example stripe-like portions of the DUT may be irradiated, optionally extending from a first edge of the DUT towards a second opposite edge of the DUT. Thus, the DUT may be inspected according to its entire width or length.
Furthermore, the spectral distribution of the detected electric signal pulses and in particular of the transformed X-ray signal spectrum may be evaluated so as to be spatially resolved. The transformed X-ray signal spectrum may be temporally correlated to the irradiated portion of the DUT. Thus, it may be determined based on which portion of the DUT the X-ray signal spectrum is emitted. Then, the conclusions regarding the transformed X-ray signal spectrum can be drawn spatially resolved. This way, for example spatially resolved element-specific concentrations may be determined such that genuineness of the DUT can be achieved at improved accuracy.
Optionally, an additional collimator may be applied. The secondary collimator may be configured and arranged such that backscattered X-ray photons emerging from a surrounding of the valuable DUT are avoided contributing to the transformed X-ray signal spectrum. The incident X-ray beam may at least partially irradiate also components within a surrounding of the DUT. In addition, since the DUTs may be moved relative to the incident wide band X-ray beam, there may occur configurations where at least for certain time periods no DUTs are present within the X-ray beam. For example, if multiple DUTs are arranged on a conveyor belt under the incident X-ray beam, these DUTs may comprise certain distances between them. Also, the X-ray beam may at least partially transmit through the DUT and interact with components on a back side of the DUT. All these undesired interaction processes may eventually cause backscattering photons to emerge from structural non-DUT components which in principle can contribute when a detector is used to detect the one or more electric signal pulses corresponding to photons emitted by the DUT and optionally to be transformed to the X-ray signal spectrum. However, such contributions would negatively affect the detected electric signal pulses and in particular the transformed X-ray signal spectrum since they are not caused by the DUT and its dopants. Since the interaction with such components take place at different locations in space, a secondary collimator may be arranged such that backscattered photons caused by such processes are hindered from reaching the detector. The secondary collimator can for example comprise a radiopaque material such that the photons are absorbed. Accordingly, unwanted signal contributions may advantageously be avoided within the detected electric signal pulses and in particular the transformed X-ray signal spectrum.
As an alternative or cumulatively, at least two different portions of the valuable DUT may be irradiated. A radiopaque component may be arranged such that two portions of the DUT may be irradiated simultaneously. The component may for example comprise several blades such that different portions may be irradiated. Thus, the options to adapt the inspected area may be tailored to the respective needs.
The inventive method may also comprise transforming the detected electric signal pulses into multiple X-ray signal spectra which may then be averaged. The average may then be evaluated to identify the DUT. Averaged spectra are well suited to compensate quantum noise. Accordingly, the identification method may be performed with improved accuracy.
All features and embodiments disclosed with respect to the first aspect of the present invention are combinable alone or in a suitable (sub-)combination with any one of the other aspects of the present invention including each of the preferred embodiments thereof, provided the resulting combination of features is reasonable to a person skilled in the art.
According to a second aspect of the present invention, a device for identifying at least one valuable DUT is provided. The device comprises at least one X-ray signal source, at least one X-ray detector, and at least one data processing device coupled to the X-ray detector. The device is configured for performing the above-disclosed method. The X-ray source is configured and arranged for irradiating at least a portion of the valuable DUT with a wide band X-ray beam. The X-ray detector is configured and arranged for detecting one or more electric signal pulses corresponding to photons emitted by the valuable DUT in response to the irradiation using the wide band X-ray beam, wherein the one or more electric signal pulses respectively contain a peak amplitude, which is proportional to the emitted photons by the valuable document under test. The data processing unit is configured for quantitatively and/or qualitatively evaluating the detected electric signal pulses and / or the transformed X-ray signal spectrum in order to identify the valuable DUT as being genuine or not and/or to identify the valuable DUT to be of a particular sort. Thus, a device is provided capable of reliably inspecting DUTs so as to determine their genuineness and/or sort. In particular, the data processing unit may be configured and arranged to store reference data, such as of amplitude windows and tolerance ranges as set out with respect to the first inventive aspect.
The device may further comprise at least one conveyor belt for moving at least one valuable DUT. In an alternative, a different transporting device may be used to move the DUTs relative to the incident beam. In particular, the DUT may be moved relative to the X-ray source providing the wide band X-ray beam while being irradiated. Accordingly, elongated two-dimensional surface areas which may be considered stripe-like portions may be tested. Also, the conveyor belt provides the possibility to test DUTs at high throughput rates. The speed at which the DUT is moved may for example be 8 to 10 m/s and/or >10 to 15 m/s. In particular when using higher speeds, the evaluation of the electric signal pulses including the pulse amplitudes is preferred in order to allow real time evaluation. Furthermore, when using higher speeds, the number of tested DUTs may be increased. In addition, the measurement may alternatively also be performed in a static arrangement instead. Accordingly, when using longer irradiation time periods a better signal-to-noise ratio may be achieved, which in turn may increase the accuracy of the inventive method.
In an alternative or cumulative embodiment, the inventive device may additionally comprise at least one radiopaque portion or component arranged such that at least two different portions of the valuable DUT are irradiated. Specifically, the at least two portions of the DUT may be irradiated simultaneously. Also, more than two portions may be irradiated using only a single beam emitted by the X-ray source. Therefore, specific portions of the DUT, which for example could comprise pre-determined Xtags including different elements may be tested at once. Thus, the identification routine may be even more elaborate such that counterfeit actions are made even more difficult.
The inventive device may in an additional or alternative embodiment also comprise at least one collimator such that the wide band X-ray beam is collimated. The collimator may also include the radiopaque portion mentioned before. Collimation may be used to delimit portions of the DUT that contains Xtags, and excluding other portions without Xtags that only contribute to the background signal. Thus, the signal-to-noise ratio is improved. Consequently, the accuracy of the evaluation process may be enhanced.
All features and embodiments disclosed with respect to the second aspect of the present invention are combinable alone or in a suitable (sub-)combination with any one of the other aspects of the present invention including each of the preferred embodiments thereof, provided the resulting combination of features is reasonable to a person skilled in the art.
According to a third aspect, a sorting machine for valuable DUTs is provided. The sorting machine comprises an inventive device according to the above-mentioned first and second aspects and, thus, is configured to identify the genuineness and/or the sort of the respective valuable DUT. Sorting machines are commonly used in reserve bank institutes. Such institutes handle large amounts of banknotes and, thus, reliable high-throughput identification methods as provided by the inventive method according to the first aspect and the inventive device according to the second aspect are preferred.
All features and embodiments disclosed with respect to the third aspect of the present invention are combinable alone or in a suitable (sub-)combination with any one of the previous aspects of the present invention including each of the preferred embodiments thereof, provided the resulting combination of features is reasonable to a person skilled in the art.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects, characteristics and advantages of the invention will ensue from the following description of the embodiments with reference to the accompanying drawings, wherein

Fig. 1 shows a simplified schematic drawing of a device for identifying at least one valuable DUT according to an embodiment;
Fig. 2 shows a simplified schematic drawing of a DUT;
Fig. 3 shows a simplified exemplary transformed X-ray signal spectrum in response to irradiating at least a portion of the DUT; and
Figs. 4a) and b) show a simplified schematic drawings of a method for identifying at least one valuable DUT according to an embodiment a) evaluating the detected electric signal pulses or b) evaluating a transformed X-ray signal spectrum.

DETAILLED DESCRIPTION OF EMBODIMENTS

The present invention is described in the following based on exemplary embodiments, which merely serve as examples and which shall not limit the scope of the present protective right.
Fig. 1 shows a simplified schematic drawing of a device 10 for identifying at least one valuable document under test (DUT) 14 according to an embodiment of the present invention. The device 10 comprises an X-ray source 12 and an X-ray detector 16.
The source 12 is configured to provide a wide band (wide range) X-ray beam 18 (so-called white spectrum) to irradiate at least a portion of the DUT 14. Atoms arranged within or on a surface of the DUT 14 may then be ionized at a certain cross section. The instable high-energy state of the ionized excited atoms leads to de-excitation by outer shell electrons occupying the free inner shell valence. This process includes the emission of photons having energies according to the difference of the binding energies of the participating valence states which, additionally, are element-dependent. The emission of the photons leading to an emitted beam 20 of photons emerging from the irradiated DUT 14 has equal probability according to all orientations and, thus, is omnidirectional. Therefore, the detection of the emitted beam 20 can generally be achieved in all relative orientations between the detector 16 and the X-ray source 12. However, locating the detector 16 according to a transmission arrangement is not preferred to avoid unwanted contributions emerging from the primary incident X-ray beam. Hence, the detector 16 is arranged within the backward scattering direction relative to the X-ray beam source 12 in view of the DUT 14.
A collimator 22 is used to focus the incident beam 18 onto the DUT 14. The present collimator 22 comprises two portions such that a single defined spot of the DUT 14 is irradiated. However, the collimator 22 may also comprise different shapes. For example, the incident X-ray beam 18 may be divided such that multiple spots of the DUT 14 may be irradiated simultaneously.
The DUT 14 is arranged at a reception (23) on a conveyor belt 24. The conveyor belt 24 may be configured such that the DUT 14 is moved relative to the source 12. Accordingly, not only a single but multiple and/or different (elongated) portions of the DUT 14 may be irradiated in order to test the DUT 14. The conveyor belt 24 may fit or may be considered a conveyor belt of a commercial sorting machine moving at typical speeds of 8 to 15 m/s, for example 8 to 10 m/s and/or 10 to 15 m/s. Accordingly, the device 10 may be suitable to evaluate DUTs 14 at high throughput rates.
The X-ray beam 18 may at least partially pass through the DUT 14 and interact with components 26 arranged in a surrounding of the DUT 14 at a distance 25. These interactions may include backscattering processes. Then, backscattered photons may cause a beam 28 which may generally reach the detector 16 and contribute to the detected electric signal pulses and, thus, optionally the transformed X-ray signal spectrum. In order to avoid such unwanted influences the device 10 may optionally comprise a second collimator 30 arranged such that that backscattered X-ray signal portions emerging from a surrounding of the valuable DUT 14 are avoided within the detected electric signal pulses and, thus, the transformed X-ray signal spectrum.
Fig. 2 shows a simplified schematic drawing of a DUT 14. According to this exemplary configuration, the DUT 14 is a banknote having a denomination 40 of "20 EURO". The DUT 14 may be moved relative to the incident wide band X-ray beam 18 provided by the source 12 along its length indicated by the arrow 42. The wide range X-ray beam 18 may generally irradiate a spot shaped portion 44 of the DUT 14. As the DUT 14 is moved relative to the source 12 while being irradiated, an elongated substantially two-dimensional portion 46 of the DUT 14 is tested.
The DUT 14 may comprise Xtags 47 including dopants which may generally interact with the incident beam to provide XRF features. The Xtags 47 may be contained within a substrate portion of the DUT 14 or may be arranged on a surface of the DUT 14. For example, they may be contained in an ink. Genuine documents may comprise regions 48a, 48b having different concentrations of Xtags 47 to make counterfeiting of the documents more difficult. Since the DUT 14 is moved relative to the X-ray source 12 while being irradiated, the regions 48a, 48b may be inspected at different points in time. Therefore, the transformed X-ray signal spectra measured by means of the detector 16 may be correlated to the irradiation of the different regions 48a, 48b at different time stamps. Then, conclusions can be independently made in view of the regions 48a, 48b and the detected Xtag 47 concentrations with regard to the genuineness of a DUT 14.
In this regard, documents of different types, such as different denominations 40 may for example comprise different spatial distributions of Xtags 47 which may be used for identification of the document sort as well as its genuineness as set out with respect to the first inventive aspect in the general description.
Fig. 3 shows a simplified exemplary transformed X-ray signal spectrum 50 in response to irradiating at least a portion of the DUT 14.
On the x-axis 52 of the transformed X-rays signal spectrum 50 the energy is shown, usually in electron volts or corresponding quantities. On the y-axis 54 the number of counts is resolved. Accordingly, the spectrum 50 describes the number of counts at different energy levels detected within pre-determined sampling periods. Commonly, during the detection of the entire spectrum 50 the detector 16 (not shown in Fig. 3) is modified as to its intrinsic properties so as to be sensitive for specific energy ranges (small energy intervals) for pre-determined periods of time. The number of counts measured within an individual period of time is then summed up (or integrated) and results in the y-coordinate. In other words, the energy range on the x-axis 52 is driven through by the detector 16 and it is measured how many events are detected at a given energy interval during a sampling period.
The transformed X-ray signal spectrum 50 comprises several X-ray related features. A first rather broad feature 56 emerges from the background signal including Compton scattering and elastic scattering processes as described hereinbefore. The intensity of the background signal may be correlated to an electron density per unit surface area (which is at first approximation proportional to the mass of the DUT per unit area) in the irradiated DUT. Too low or too high background signal levels may be considered an indication of abnormal document so that it can be identified whether the DUT is a valuable document or not. The background signal usually does not show pronounced peaks since the detector 16 is usually arranged such that it does not fit a backscattering angle having high cross section. Although background signal may in principle be used for determining certain aspects of the DUT, such as the electron density per unit of surface area, the focus is rather directed to the pronounced peak-shaped XRF features 58, which may facilitate authenticating a DUT. These occur at different energies and represent different de-excitation processes corresponding to the participating valence states. The XRF features 58 are element specific. Accordingly, different pronounced peaks usually refer to different so-called fluorescence lines (K-lines, L-lines, M-lines), such as Kα lines 60, Kβ lines 62. As can be seen, the XRF features are extremely sharp having good signal-to-noise ratios such that reliable conclusions can be made on the element-dependent atoms of the DUT, which have been excited by the incident beam 18 (not shown in Fig. 3). Therefore, conclusions can be made on the elemental composition of the DUT 14 (not shown in Fig. 3) relative to the irradiated spot 44 (not shown in Fig. 3). Since the DUT 14 may be moved while being irradiated, additional conclusions can be drawn on the spatial distribution of the detected elements across the DUT 14. Based on these findings, the DUT 14 may be identified belonging to a specific species. Also, genuineness of the DUT 14 may be reliably evaluated.
Figs. 4a) and b) show a simplified schematic drawings of a method 70 for identifying at least one valuable DUT according to the present invention. Optional steps are shown in dashed boxes.
In step S72 a wide band X-ray beam is provided for irradiating at least a portion of a DUT 14. Such beams may be provided using white light X-ray sources.
In optional step S74 the wide band X-ray beam may be collimated to delimit the cross-sectional shape of the beam. The cross-sectional shape may usually be circular. However, other such as quadratic or rectangular shapes are possible as well.
In optional step S76 a collimator is provided in order to avoid backscattered photons originating from scattering processes at a surrounding of the DUT 14. Such photons would influence the detected electric signal pulses corresponding to emitted photons of the DUT 14 and, thus, the transformed X-ray signal spectrum such that the evaluation would be more complex. Therefore, such influences can be prevented.
In step S78 electric signal pulses corresponding to emitted photons of the DUT 14 in response to irradiating at least a portion of the DUT 14 are detected. According to optional step S79 the detected signal pulses are transformed into an X-ray signal spectrum. The spectrum primarily includes XRF features caused by emitted photons by radiative de-excitation when an atom is left in an excited state after ionization by a photon of the primary beam. Also, the spectrum may at least partially comprise broad features caused by inelastic Compton backscattering processes.
In step S80 the detected electric signal pulses are quantitatively and/or qualitatively evaluated as such to identify the genuineness and/or the sort of the DUT 14 by comparing the detected one or more electric signal pulses with a reference electric signal pulse emitted by a genuine valuable document in the same portion as irradiated in step S78. In addition or alternatively, the detected electric signal pulses are quantitatively and/or qualitatively evaluated after transforming the electric signal pulses into the X-ray signal spectrum in order to identify the DUT 14. In particular, step S80 may be carried out computer-based.
Optionally, during the quantitative and/or qualitative evaluation a reference signal spectrum may be subtracted in step S82 from the transformed X-ray signal spectrum. For example, sample documents having no dopants may be used to determine the Compton features. These features may then be compensated within the transformed X-ray signal spectra to provide an improved basis for analysis of the XRF features. Preferably, the reference spectrum may be provided based on a genuine document. Ideally, the reference spectrum is provided using conditions identical to the measurement conditions for the DUT. If the DUT is a genuine document as well, the subtraction of the spectrum from the reference spectrum may be expected to result in a constant value for all energies. In particular, a noise spectrum close to zero may be expected to be achieved.
In an alternative or cumulatively, in step S84 the transformed X-ray signal spectrum may be compared to a reference spectrum. This may be helpful in order to evaluated general parameters such as the relative orientation of source 12, DUT 14, and detector 16 in order to rule out systematic errors.
In optional step S86 the XRF features may be determined. Accordingly, conclusions can be drawn on the elemental composition or on the amount of specific dopant concentrations being present at the irradiated DUT. Also these information may be correlated to spatial information of the portions of the DUT 14 being irradiated.
Based on the evaluated transformed X-ray signal spectrum, the genuineness and/or sort of the DUT 14 may be analyzed in optional step S88. In this regard, for certain features of the transformed X-ray spectrum values may be determined and compared to predetermined tolerance ranges. The DUT 14 may then be considered genuine if the values are quantitatively within the tolerance range. In this regard, identification of the DUT 14 may include authenticating the DUT 14, optionally considering pre-determined reference spectra. In addition or alternatively, the identification of the DUT 14 may include identification of a particular sort of DUT, such as a denomination of a banknote, in case the comparison of the values show a qualitative match.
Hence, the present method 70 allows DUTs 14 to be reliably analyzed (identification of authenticity and/or sort) at high throughput rates, even fitting typical handling periods of commercial sorting machines.
Although the invention has been described hereinabove with reference to specific embodiments, it is not limited to these embodiments and no doubt further alternatives will occur to the skilled person that lie within the scope of the invention as claimed.

Claims

A method (70) for identifying at least one valuable document under test (14), the method comprising or consisting of the steps:
a) S72: Irradiating at least a portion of a valuable document under test (14) with a wide band X-ray beam (18);

b) S78: Detecting one or more electric signal pulses corresponding to photons emitted by the valuable document under test (14) in response to the irradiation using the wide band X-ray beam (18) in step a), wherein the one or more electric signal pulses respectively contain a pulse amplitude, which is proportional to the photons emitted by the valuable document under test (14); and

c) S80: Evaluating the detected one or more electric signal pulses quantitatively and/or qualitatively in comparison to a reference electric signal pulse emitted by a genuine valuable document in the same portion as irradiated in step a) and identifying that the valuable document under test (14) is a genuine valuable document or not in dependence of a quantitative match and/or identifying that the valuable document under test (14) is of a particular sort in dependence of a qualitative match.
The method (70) of claim 1, wherein step c) further comprises determining a number N_measure of counts of pulse amplitudes A within a predetermined range of minimum pulse amplitudes A_min and maximum pulse amplitudes A_max of a fluorescence peak of a respective Xtag present in the irradiated portion of the genuine valuable document and comparing the determined number N_measure of counts of pulse amplitudes A with a reference number N_ref of counts of pulse amplitudes within the same predetermined range of minimum pulse amplitudes A_min and maximum pulse amplitudes for the respective fluorescence peak of a genuine valuable document in the same irradiated portion and identifying that the valuable document under test (14) is a genuine valuable document in case the number N_measure of counts of pulse amplitudes A match with a predetermined tolerance range of the reference number N_ref of counts of pulse amplitudes or identifying that the valuable document under test (14) is not genuine in case the detected one or more electric signal pulses do not match with a predetermined tolerance range of the reference number N_ref of counts of pulse amplitudes.
The method (70) of claim 1 or 2, wherein the wide band X-ray beam (18) is collimated such that the at least one irradiated portion of the valuable document under test (14) is delimited (S74).
The method (70) of any of the preceding claims, wherein the valuable document under test (14) is arranged at a reception (23), and wherein the reception (23) is moved relative to the wide band X-ray beam (18) during irradiation in step a).
The method (70) of any of the preceding claims, wherein at least two different portions of the valuable document under test (14) are irradiated in step a) in method step b), one or more electric signal pulses corresponding to photons emitted by the respective two or more different portions of the valuable document under test (14) are detected, and in method step c) the detected one or more electric signal pulses in the two or more irradiated areas are evaluated, wherein a valuable document under test (14) is regarded genuine in case all the electric signal pulses in the two or more portions match with the predetermined tolerance range of the respective reference electric signal pulse or wherein a valuable document under test (14) is regarded not genuine in case at least one of the electric signal pulses in one of the portions does not match with the predetermined tolerance range of the respective reference electric signal pulse.
The method (70) of any one of the preceding claims, wherein photons backscattered by a surrounding of the document under test (14) are collimated (S76).
The method (70) of any one of the preceding claims, wherein the method further comprises
b1) S79: transforming the one or more electric signal pulses detected in step b) as a function of their amplitude forming at least one X-ray signal spectrum (50) and wherein step c) further comprises

c1) Evaluating peaks of the X-ray signal spectrum (50) in order to identify whether the document under test (14) is a genuine valuable document or not.
The method (70) of claim 7, wherein the one or more electric signal pulses are transformed into multiple X-ray signal spectra (50), and wherein an average of the multiple signal spectra is determined and evaluated in step c).
The method (70) of claims 7 or 8, wherein the spectral distribution of the transformed X-ray signal spectrum (50) is evaluated so as to be spatially resolved in step c).
The method (70) of any one of the claims 7 to 9, wherein step c) further comprises:
c2) S86: Evaluating a spectral distribution of the at least one X-ray signal spectrum (50) in order to determine an elemental composition of the at least one portion of the valuable document under test (14) which emitted the energy corresponding to the detected one or more electric signal pulses.
The method (70) of claim 2, wherein step c) further comprises:
c3) S86: Evaluating an energy-dependent intensity distribution of the transformed X-ray signal spectrum (50) in order to determine at least a relative concentration-dependent elemental composition of the at least one portion of the valuable document under test (14) which emitted the energy corresponding to the detected one or more electric signal pulses.
The method (70) of any of the preceding claims, wherein step c) further comprises:
c4) S84: Comparing at least one portion of the transformed X-ray signal spectrum (50) to at least one reference signal spectrum
and/or

c5) S82: Subtracting a reference background signal spectrum from the transformed X-ray signal spectrum (50)
and/or

c6) S88: Determining at least one value based on at least one portion of the transformed X-ray signal spectrum (50), comparing the determined value to at least one reference value of a genuine valuable document, and considering the valuable document under test (14) to be genuine if a difference between the determined value and the reference value is within at least a tolerance range
and/or

c7) S89: Determining an electron density per unit of irradiated surface area of the device under test (14).
A device (10) for identifying at least one valuable document under test (14), wherein the device comprises at least one X-ray signal source (12), at least one X-ray detector (16), and at least one data processing device coupled to the X-ray detector, wherein the device is configured for performing the method of any of the preceding claims, wherein the X-ray source is configured and arranged for irradiating at least a portion of the valuable document under test with a wide band X-ray beam (18) in step a), wherein the X-ray detector is configured and arranged for detecting one or more electric signal pulses corresponding to photons emitted by the valuable document under test (14) in response to the irradiation using the wide band X-ray beam (18), wherein the one or more electric signal pulses respectively contain a peak amplitude, which is proportional to the emitted photons by the valuable document under test (14) in step b), and wherein the data processing unit is configured for performing step c).
The device (10) of claim 13, wherein the device further comprises at least one radiopaque portion (22) arranged such that at least two different portions of the valuable document under test (14) are irradiated in step a).
A sorting machine for valuable documents comprising a device (10) according to claim 13 or 14.