WO2008142446A2

WO2008142446A2 - Energy dispersive x-ray absorption spectroscopy in scanning transmission mode involving the calculation of the intensity ratios between successive frequency bands

Info

Publication number: WO2008142446A2
Application number: PCT/GB2008/050358
Authority: WO
Inventors: Max Robinson; Ian Radley; Gary Gibson; Arnab Basu
Original assignee: Durham Scientific Crystals Ltd
Priority date: 2007-05-17
Filing date: 2008-05-19
Publication date: 2008-11-27
Also published as: WO2008142446A3

Abstract

A method and apparatus for obtaining radiation transmission data comprising information about the composition of an object are described. The principle involves providing a radiation source such as an x-ray or gamma-ray source and a radiation detector system such as an x-ray or gamma-ray detector system, the detector system being capable of detecting and collecting spectroscopically resolvable information about incident radiation; collecting a dataset of information about radiation incident at the detector; resolving the dataset spectroscopically.across a plurality of frequency bands within the spectrum of the source to produce a corresponding plurality of frequency-specific datasets; determining an intensity ratio for at least one pair of frequency specific datasets; using this to identify materials present.

Description

Method and Apparatus for the Inspection and Characterisation of

Materials

The invention relates to a method and apparatus for the inspection and characterisation of materials. The invention relates especially to a method and apparatus that operates by or in conjunction with the generation of an image, but is not limited to such imaging.

The invention in particular relates to an apparatus and method making use of high energy radiation such as x-rays or gamma-rays to scan objects where it is desirable to gain information about their internal contents and/ or composition. This principle is widely employed for example in the security industry, but might also be employed in other areas, for example, without limitation, medical imaging, imaging for quality control purposes or the purposes of determining the integrity of the structure, or the like.

X-Ray absorption has been used as the basis for screening objects to create some form of representational image of the contents or components thereof relative to each other in three-dimensional space. The thicker or more dense an object is then the more it will attenuate an x-ray beam. By use of suitable detectors and a suitable source, radiographs of an item under screening in the form of images based on the absorption of an object or set of objects can be generated.

Typically, an x-ray source generates an essentially 2-dimensional beam and detectors of transmitted x-rays are used to build up successive image slices in cross-section based on transmitted x-rays (and hence differentiating by absorption). A computer is used to generate images of cross-sections of the object so they can be looked at one at a time. The cross-sections are then put together to form an image reflecting at least some three-dimensional cues. It is for example known to employ a line- scan principle, in which three dimensional objects are caused to move through a scanning zone and imaging information collected as it moves and an image built up from successive linear slices. It is also for example known to employ a computed axial tomography (CAT or CT) principle in which an image is built up from a series of two-dimensional images taken around a single axis of rotation. The precise way that an image might be generated from transmitted radiation is not pertinent to the present invention.

These known apparatus and methods tend to give limited information about the material content. In essence, at its simplest, all that is being measured is transmissivity of the object to the source radiation. The detector merely collects amplitude information, and does not discriminate transmitted radiation spectroscopically. In most practical systems even this is measured indirectly. At its simplest, a typical linear array x-ray detector comprises in combination a scintillator material responsive to transmitted x-rays, which is then caused to emit lower frequency radiation, and for example light in or around the visible region, in combination with a semiconductor detector such as a silicon or gallium arsenide based detector which is responsive to this lower frequency radiation.

However, it is known that spectroscopic information from transmitted x- rays could be used to give additional information about the material content of the objects or components being scanned. It is known that the x-ray absorption properties of any material can vary spectroscopically, and that this effect depends in particular on atomic number. This has led to development of dual-band or dual-energy detectors which are capable of separately identifying low- and high-energy bands from the full spectrum of x-ray emissions. When exploited as part of a security or like material identification system, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category. The practical implications of this have led to the use of such detectors in the security industry, and for example in airport x-ray scanners, either to create separate images of metallic items inside luggage (to reveal hidden metal items, for example weapons, such as guns, and knives) or to identify plastic explosives.

Most explosives are dense organic materials usually high in nitrogen content. There is therefore some limited merit in the use of dual energy detectors but it is far from being a precise explosive detector since many other items in luggage, such as soaps, creams, leather goods etc, are also dense organic materials.

A dual energy system confers only limited information about composition. The organic/inorganic division is crude and approximate. Conventional detectors do not give any real spectroscopic information about the spectrum of transmitted x-rays although they detect the presence or otherwise of x-rays within two distinct bands of the spectrum. Ultimately decisions are made based on the attenuation radiograph which is based on the shape of items and their proximity to other objects.

Systems such as described above typically use combinations of multiple sources and/ or multiple detectors with filters to select from the broad band spectrum of a broad band source a narrow frequency band, and for example lower and higher frequency bands, from which data can be collected and analysed. This does not exploit the full potential of information that could be obtained from transmission levels across a greater part of the full spectrum of a broad band source. Recent development of detectors that can resolve spectroscopic information about the transmitted X-rays more effectively has led to the development of apparatus that discriminate across a larger range of bands and generate a larger plurality of images across these bands to generate multispectral images. For example US5943388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three images. This better exploits the effect of differential spectral absorption by different materials and better approximates transmissivity to composition but is still limited to the information that can be conveyed by a displayed image, and by the approximate and indicative nature of any relationship between colour on a multispectral image, especially based on relatively wide energy bands, and composition of material in the transmission path.

It is desirable to generate compositional information from a spectroscopically resolved dataset, and in particular to supplement that available by plural band imaging alone. It is desirable to do so in a manner that exploits a greater part of the full spectrum of a broad band source and reduces the system complexity inherent in systems employing multiple sources and/ or multiple detector arrays with frequency filters.

According to one aspect of the invention there is provided a method of obtaining radiation transmission data comprising information about the composition of an object comprising the steps of: providing a single broad spectrum radiation source such as an x-ray or gamma-ray source and a radiation detector system such as an x-ray or gamma-ray detector system spaced therefrom to define a scanning zone therebetween, the detector system being capable of detecting and collecting information about incident radiation resolvable spectrally across at least a part, and especially a major part, of the spectrum of the source; collecting a dataset of information about radiation incident at the detector and hence transmissivity of an object in the scanning zone at at least one and preferably a plurality of scanning positions; resolving the dataset spectroscopically across a plurality of frequencies/ frequency bands within the spectrum of the source to produce a corresponding plurality of frequency-specific datasets comprising at least data about intensity of radiation incident at the detector at a given frequency/ band; numerically analysing at least one pair of frequency-specific datasets to produce a frequency-comparative dataset, at least by applying a comparative function to at least one pair of frequency-specific intensity datasets to determine an intensity ratio for at least one pair of frequency- specific datasets.

Thus, the invention exploits the principles of multispectral resolution of transmitted radiation to gather useful information that can be related to the composition of an object in a scanning zone. The detector system is adapted to generate spectroscopic information about the transmitted radiation. That is, the detector exhibits a spectroscopically variable response across at least a substantial part of the spectrum of the radiation source allowing spectroscopic information to be retrieved. A plurality of datasets over a plurality of frequency bands are resolved, which term as used herein includes resolution at effectively single discrete frequencies/ energies. These are generated using a single radiation source capable of producing broad spectrum emission over a wide range of energies within a desired operating bandwidth. The radiation source preferably comprises a source to deliver high-energy radiation such as ionizing radiation, for example high energy electromagnetic radiation such as x-rays and/ or gamma rays, or subatomic particle radiation, and the detection system is adapted correspondingly to detect radiation in this spectrum. The radiation source for example is a broadband x-ray or gamma-ray source capable of producing broad spectrum emission over a wide range of x-ray or gamma-ray energies. Such a source will be familiar, and is widely used. However, in accordance with the invention a single source is used, preferably in conjunction with a single detector or detector array, from which spectroscopically resolved information can be obtained from the inherent broad spectrum properties of the detector, avoiding much of the complexity and loss of information inherent in using multiple sources and/ or multiple detector arrays with narrow band frequency filters.

Moreover, instead of merely using this to generate plural band multispectral images of the object, which can give only limited indicative information about composition, the dataset is analysed numerically by numerically comparing frequency-resolved information including at least intensity information for at least one pair, and preferably multiple pairs, of frequency-specific datasets to obtain a comparative numerical result that represents in quantified manner a relational aspect between them including at least an intensity ratio. From this analysis, it can be possible to obtain a result dataset of frequency-comparative intensity ratio data that is more specifically characteristic of an aspect of the composition of the object than could be achieved by a mere multispectral image.

Since the method of the invention involves using a broad spectrum detector or detector array with a single broad spectrum source to resolve information across the spectrum of source using the inherent properties of the detector rather than using multiple sources and/ or multiple detector arrays with narrow band frequency filters it offers the potential for much more sophisticated numerical analysis, and much more complete collection of and use of information across the source spectrum, than is provided by prior art systems relying for example on multiple sources with different frequency filters.

The source is capable of generating a sufficiently broad spectrum of radiation to enable the spectral resolution necessary for the performance of the invention. Preferably the source generates radiation across at least one or more parts of the range of 20 keV to 1 MeV, and more preferably across at least a part, and for example a major part, of the range of 20 keV to 160 keV. For example the source generates radiation ranging across at least one bandwidth of at least 20 keV within the given range. For example the spectrum is such that at least three 10 keV bands can be resolved within that range. Preferably, a library of results is maintained giving characteristic frequency-comparative data for a range of numerically comparative analysis methods and a range of target materials. The method preferably comprises the further step of comparing the result dataset of frequency- comparative data with a predetermined library of characteristic frequency- comparative data for a range of target materials appropriate to the numerically comparative analysis method being used, and thus to deriving from the frequency-comparative dataset a numerical indication of the composition of an object being scanned.

A frequency-specific dataset is collected that comprises at least data about intensity of radiation incident at the detector at a given frequency band, and for example comprises data representative of the average intensity of radiation incident at the detector across a given frequency band or at least a sufficiently representative part thereof. Thus, a numerical analysis of at least one pair of frequency-specific datasets to produce a frequency-comparative dataset preferably comprises applying a comparative function to at least one pair of frequency-specific intensity datasets and especially average intensity datasets. More preferably yet the numerical analysis step comprises determining an intensity ratio, and for example an average intensity ratio (that is, a ratio of average intensities across a given frequency band or at least a part thereof as previously defined), for at least one pair of frequency-specific datasets.

Intensity ratios can represent a particularly useful quantification of the dataset of transmitted radiation that can be particularly characteristic of specific material composition.

Appropriate and where applicable different numerical weighting factors may be applied to data in different frequency-specific datasets prior to or as part of the process of any numerical comparison therebetween to produce a suitably modified/ meaningful result dataset without departing from the principle of the invention. Such weightings might for example correct for intensity variations in a given source spectrum, for noise of any kind, or for any other factor that it might be desirable to account for to improve the numerical result.

It will be understood that although reference is made herein for convenience to the scanning of an object this should not be considered to limit the application of the invention to the scanning of single homogenous objects. Indeed, for many envisaged applications, an "object" is likely to consist of multiple heterogeneous materials and/or to be a container or other agglomeration of multiple articles, so that any transmitted radiation path is likely to pass through multiple different materials having varied properties. One of the particular advantages of the invention is that it can facilitate resolution of such varied materials. The method of the invention is not limited in its application to the scanning of objects moving through a scanning zone in a scanner. Information pertinent to characteristic numerical result data inherent in the transmitted dataset for a given scanning event, and hence the material composition of an object or objects in a transmission path, can be obtained by a single scanning event, for example of a stationary object being scanned by a single beam of appropriate geometry, for example a pencil beam or conical beam. In such circumstance the method merely includes placing the object in a scanning zone to obtain such a single scan and single dataset of data of information about radiation incident at the detector.

In one possible embodiment the method comprises placing a sample under test in the scanning zone and supporting it in the scanning zone on sample retention means. In particular, the method is applicable to a sample under test comprising a liquid in a container of other material, such as a bottle, flask, carton or the like, and the method comprises placing a container under test in the scanning zone and supporting it in the scanning zone on sample retention means comprising container holding means.

However, in a preferred embodiment information is collected regarding the transmissivity of an object under test in the scanning zone in a plurality of scanning positions between which the object is translated and/ or rotated. In accordance with this embodiment of the method, the method comprises the additional step of causing an object to move relative to and for example through the scanning zone as a plurality of successive datasets of information about radiation incident at the detector are collected.

The detector system is capable of being used to detect a plurality of, and preferably at least three and more preferably at least five, specific energy bands. So long as they are resolved, the bandwidth of each band is not directly pertinent to the invention and useful results can be obtained by any suitable approach to dividing the spectrum, either in whole or in part, into separate bands. For example, the entire spectrum or a substantial part thereof may simply be divided between such a plurality of bandwidths, and each frequency-specific dataset may be derived as a measure representative of intensity across the entire band, and for example an average intensity. Alternatively, a plurality of relatively wide bands, but with discrete gaps therebetween, may be envisaged and analysed on the same basis. Alternatively, "bands" may be narrow even to the point where they essentially approximate to an evaluation of intensity at a single energy. As used herein the concept of intensity at an energy "band" includes evaluation of intensity at such a discrete single energy as well as evaluation of intensity at an energy across a narrow or broad bandwidth.

Similarly the source may be a single broad spectrum source across which a plurality of bandwidths or single energies may be identified. Alternatively or additionally sources may be provided having narrow bandwidths or generating incident radiation at one or more discrete energies to provide some of the energies for comparison in accordance with the method of the invention. In this case the radiation source is a plural source comprising a combination of sources at different energies to provide the necessary total spectrum spread to allow resolution by the detector across a plurality of energies/ energy bands.

For example a plural source comprises an x-ray source having a relatively lower energy spectrum, for example operating below 60 keV and for example at 10 to 50 keV and one or more radioisotope sources generating radiation at higher energies, for example above 100 keV. At its most basic, the invention allows identification of materials from collected and spectrally resolved transmission data based on a numerical analysis that provides, for example with reference to a suitable data library of characteristic spectrally resolved transmission data based on equivalent numerical analysis for at least one and preferably a range of target materials and/ or objects likely to be encountered in a given application, an indication of material content. The data library may comprise information in any suitable form which can be related in a numerically analytical manner to the frequency-comparative data calculated from transmission collected across the resolved energy bands in accordance with the invention. The data library may include standard preset reference materials and/or user input reference materials and/or reference data may be generated from known materials in accordance with the foregoing method. That is, a library of data may be built up by the system, which can in effect "learn" material characteristics, over time. The data library may comprise electronically stored data and/or data stored on a hard medium, such as a printed resource, and may be held and accessed locally and/or remotely, manually and/or automatically, none of which is directly pertinent to the operation of the method of the invention.

Thus, at its most basic, the invention allows improved indication of identification of materials from collected transmission data based on characteristic transmission behaviour across different resolved parts of the spectrum. It is not necessary to generate an image. No particular transmission beam geometry is mandated. A simple, effectively one- dimensional beam geometry incident upon a simple, single detector may be sufficient.

However, for practical purposes it may be preferable that the invention forms part of and supplements the information offered by a scanning imaging system. Preferably, the method further comprises the generation of an image from transmitted intensity data, which is preferably a multispectral image generated from data resolved across a plurality of frequency bands. In accordance with a preferred embodiment, the dataset of information about radiation incidence collected at the detector is also used to generate an image of an object in the scanning zone. In particular, in a preferred mode of operation, a succession of images is generated. Preferably an image is resolved spectroscopically across a plurality of frequency bands within the spectrum of the source which are allocated to generate a series of energy-differentiated images.

The method of the invention conveniently further provides the additional step of displaying such generated image or images, and in the case of multiple images might involve displaying such images simultaneously or sequentially.

For clarification it should be understood that where used herein a reference to the generation of image is a reference to the creation of information dataset, for example in the form of a suitable stored and manipulatable data file, from which a visual representation of the underlying structure of the object under investigation could be produced, and references to displaying this image are references to presenting an image generated from such a dataset in a visually accessible form, for example on a suitable display means.

The method of the invention makes use of a detector system enabled to generate spectroscopic information about the transmitted radiation, and for example comprising an array one or more detectors that can generate spectroscopic information about the transmitted radiation. That is, the detector exhibits a spectroscopically variable response across at least a substantial part of the radiation spectrum of the source allowing spectroscopic information to be retrieved.

This is exploited in accordance with this preferred embodiment in that spectroscopic resolution of transmitted radiation in each of a plurality of relatively broad "imaging" bands is represented in a generated image or series of images. For example, spectroscopic differentiation in the collected data is represented in the image as differentiated colour, shading or marking. A banded mapping is used in that the source spectrum is divided into a plurality of bands, for example between four and eight bands, and different colours are used to represent each such band in the displayed image. The apparatus conveniently includes suitable image processing means to effect this mapping.

An image or composite image or succession of images so generated is preferably displayed on a suitable display means.

Some possible embodiments of the invention involving suitable numerical analysis of frequency-specific datasets based on an intensity ratio analysis to obtain a frequency-comparative dataset of quantified information correlatable to composition will now be described. It will be appreciated that these are examples only and that the principle of the invention is applicable to any numerical analysis technique that will yield quantified information correlatable to composition as a means of obtaining useful data from a multispectral resolved dataset that supplements that obtained from an image, or from an image alone.

In accordance with a first possible embodiment at least one of the said plurality of frequency bands is selected to correspond to a characteristically scattered wavelength of a target material to be identified, and the numerical analysis step comprises identifying an absence of or substantial reduction in a transmitted signal intensity at the characteristic frequency band and interpreting this as the presence of the said target species.

This possible embodiment is especially effective at distinguishing objects which are superimposed in the x-ray path, and/ or in deriving information concerning the crystalline or polycrystalline nature of an object.

Polycrystalline materials scatter x-rays and a resulting x-ray image may hardly detect such polycrystalline material because a very large proportion of the X-rays which have not been absorbed by the material will have been scattered and so not received by the detector. This is unfortunate as in security x-ray screening a number of threat items are polycrystalline in nature, in particular plastic explosives such as CP4, RDX, PETN and proprietary formulations thereof, drugs and the like, and these are therefore difficult to detect by using conventional x-ray systems.

Crystalline or polycrystalline objects are capable of diffracting an x-ray beam if certain conditions are satisfied.

The situation is outlined using Bragg's Law which is:-

nλ = 2dsinθ

n is an integer (order of diffraction) λ is the wavelength of the diffracted ray d is the atomic lattice parameter θ is the angle of diffraction At the specified wavelength (energy) the effect is close to 100%.

Attempts have been made to overcome detection problems associated with characteristic Bragg reflection by searching for the diffracted beam. If the threat material is specified then the information would be available concerning the diffraction angle θ, the lattice parameter d, and the x-ray wavelength λ. In addition therefore, scanners have been proposed which make use of characteristic diffraction by including scatter detectors at appropriate scatter angles for particular target materials. Earlier patents GB2360685 and GB2329817 refer to just such an attempt. The energy of the diffracted photons is given by:-

λ

where h is Planck's constant and c is the speed of light.

The method of the invention makes use of a multi band detector system that is capable of detecting quite specific energy bands. Instead of placing detectors at the appropriate positions to detect the diffraction beam angle θ of given energy E_Ph, a detector system is used in accordance with the invention to show that the particular energy of interest is NOT there in the primary beam.

It has been noted that many target materials are crystalline or poly- crystalline in structure. Such crystalline materials exhibit characteristic scattering of high energy electromagnetic radiation typically in the x-ray region. With conventional apparatus using a high energy electromagnetic source, and for example an x-ray source, this can present a problem. The primary beam might be scattered at these characteristic frequencies, making detection more difficult. However, where prior art systems treated this as a problem to be addressed by trying to detect the scattered secondary beams, this embodiment of the present invention takes a profoundly different approach, in that it seeks instead to detect absences in the primary beam.

This approach confers a number of advantages. Secondary scattered beams can be difficult to detect, requiring very precisely placed secondary detectors. The present method dispenses altogether with the need for such secondary detectors. Instead, the primary detector system detects transmitted data in the primary transmitted beam and, by an appropriate numerical analysis technique, resolves this in such a way that characteristic scattering can be identified by the absence in the primary beam of a characteristic energy rather than by the presence of a secondary beam at a characteristic scatter angle.

Thus, in accordance with the method of the embodiment, for a given target species at least one frequency band is allocated which corresponds to a characteristic Bragg scattering condition. In particular, this might correspond to a first order Bragg scattering condition for a given target species, although additionally (for example to provide a confirmation) or alternatively a frequency band might be allocated to a characteristic lower order scattering.

The collected transmission data is resolved spectroscopically across a plurality of frequency bands in accordance with the basic principles of the invention. In the or each frequency band allocated to be characteristic of a given target species scattering occurs in accordance with Bragg's law at a characteristic energy within the frequency band, reducing the amplitude of the transmitted signal. If the frequency band is sufficiently narrow to correspond sufficiently closely to the characteristic scattering frequency a substantial and measurable reduction in transmitted amplitude will be resolved at the detector.

This reduction in amplitude over the specifically defined frequency band encompassing a characteristic scattering frequency, relative to the transmission data across the spectrum as a whole, is specifically characteristic of the target material, or at least characteristic of that class of materials having the same Bragg scattering properties as the target material. Thus, a characteristic absence or substantial reduction in a transmitted signal intensity in the primary transmitted beam at the frequency band may be interpreted as the presence of the said target species, and a result to that effect can be generated.

In practice, such a reduction in amplitude is determined numerically by comparison with intensity data representing at least one other frequency band away from the characteristic, and optionally also by comparison with a known and for example prerecorded source spectrum.

Preferably, an image is also generated, which is preferably a multispectral image. Proper resolution of spectroscopic information confers two advantages. It offers the potential by imaging across a series of relatively broad bands to create several images which to some extent can reflect the different responses of materials and thus, by distinguishing between each image across each relatively broad band, for example by representing them differently (such as in different colours) in a resultant combined image, it assists in resolution of different objects, components or parts of the image. However, by also offering the potential to collect data across relatively narrow bands characteristic of Bragg scattering conditions for one or more given target crystalline species it can effect in accordance with the invention a genuine and much more specific identification of a target material or narrow class of materials.

In accordance with a preferred mode of operation this embodiment of the invention, each collected image is resolved spectroscopically across a plurality of frequency bands within the spectrum of the source comprising a plurality of relatively narrow "characteristic" frequency bands, each corresponding to and containing within the band a characteristic scatter frequency of a given target species, and/ or a plurality of relatively broad "imaging" bands each intended to generate an image across a broader part of the overall spectrum, so that the imaging bands together allow the generation of an energy-differentiated composite image or succession of images in familiar manner. The number of characteristic frequency bands will be determined by the number of target species, and by whether a target species is mapped onto one or more than one characteristic scatter frequency. The number of imaging frequency bands is conveniently between 2 and 10, and for example between 4 and 8.

In accordance with a second possible embodiment a plurality of pairs of frequency bands are compared numerically, in particular to determine intensity ratios, and thus to obtain a numerical indicator in functional relationship with a mass attenuation coefficient.

In this embodiment the method preferably comprises collecting a dataset comprising intensity data for radiation incident at the detector and comprises the additional steps of resolving such an intensity dataset across at least three frequency bands within the spectrum of the source to produce a frequency resolved intensity data item for each band; evaluating the ratio between intensity data items for at least two pairs of such frequency bands in a given intensity dataset and for example each successive such frequency band to obtain a numerical indicator in functional relationship with a mass attenuation coefficient associated with the intensity dataset; comparing the same with a library of data indicative of characteristic mass attenuation coefficients, and in particular for example with mass attenuation coefficients characteristic of target materials such as suspect materials, in order to obtain an indication of the likely material content of material in a transmission path producing such intensity dataset.

In accordance with the embodiment, intensity data from transmitted x-rays, gamma rays or the like is thus collected as before. For each "scanning event" (that is, for a measurement of transmitted intensity via a given transmission path through a given object in a given position) an "intensity dataset" is collected representing the transmitted intensity across at least part of a source energy spectrum. However, the key to the embodiment is the resolution of intensity data for a given "scanning event" into at least three separate energy bands across the spectrum of the source. An intensity dataset thus constitutes a dataset of intensity information related to frequency/energy which is resolvable into at least three such bands to produce at least three intensity data measurements or data items relating to a given scanning event and hence a given transmission path through the object/material under test.

In accordance with the embodiment, for each such scanning event, ratios of at least two pairs of such resolved intensity data item measurements, and for example successive intensity data item measurements, are obtained numerically, to provide representative information which can be correlated to the mass attenuation constant necessary to produce such an intensity pattern. As is described in greater detail below, most of the variables associated with a given scanning event are coefficient with respect to the frequency/energy of the incident radiation from the source. However, the mass attenuation coefficient varies with energy in characteristic way. By performing such a ratio analysis on intensity data across at least three different energy bands for a given scanning event to generate at least two ratios, data which is representative of the functional relationship between mass attenuation coefficient and incident radiation energy can be obtained. Thus, inferences relating to the specific mass attenuation coefficient applicable to the transmission path through material under test for a given scanning event can be drawn. A comparison is then made to a suitable database of data representative of the mass attenuation coefficient for different materials and/or target objects to give a more representative indication of what is being scanned.

The transmission of x-rays through a material can be given by the exponential attenuation law, as follows:

I / Io = exp [ - (μ/p) pt]

Where μ/p = Mass attenuation coefficient, a material constant which is characteristic of the weighted elemental composition of a material I = Final intensity Io = Initial intensity P = density of the material t = thickness of the material

The approach taken in CT methodology is to vary the path taken by the x- rays through the baggage. This effectively changes the thickness of the material, the term 't' in the equation. Thus by looking at the variation in the x-ray transmission deductions can be made about the mass attenuation coefficient and the density of the material. These two parameters are characteristic of different materials and so materials identification becomes possible.

The key to the methodology of the present embodiment of the invention is the ability, by provision of suitable detectors, to resolve the transmitted radiation with respect to energy/frequency across at least three such bands so that relative values from at least two pairs of intensity data items can be calculated. This is considered to be the minimum necessary to allow the numerical analysis required to reduce any influence of other uncertainties affecting transmitted intensity, in particular material density and thickness, both of which are essentially invariant relative to incident energy for a given scanning event.

Nevertheless, whilst three might represent a fundamental minimum, for the scanning of heterogeneous objects and/or "objects" comprising containers or agglomerations of multiple articles a larger plurality of energy-resolved intensity data items is likely to be preferred for the numerical analysis as above described, for example at least five.

Transmission data may be collected through an object at many (three or more and preferably at least five) different energy bands. If we consider the exponential attenuation equation again, it can be seen that the mass attenuation coefficient is one of the terms listed. The mass attenuation coefficient itself is however dependent on the energy of the detected x- rays. The other terms in the equation have no dependence on the x-ray energy. Thus if one measures transmission at multiple energies it is possible to relate the variation in transmission to the mass attenuation coefficient. As this term is characteristic of the materials present it is therefore possible to identify or characterise particular materials, especially target materials such as explosive materials, in a methodology similar to that used in the CT methodology but without any of the drawbacks or the uncertainties encountered when changing the ray path. This means that technology based on multiple energy scans offers the potential for higher rate of success than is enjoyed by the current technology.

One of the simplest ways to eliminate the additional terms is to take a ratio of the transmission at different energies and for example a ratio of successive readings at a plurality of successive different energies. It can be seen that a ratio will in principle eliminate the material thickness and density as constant terms. This will therefore make the mass attenuation coefficient the only remaining term that will affect the transmission ratio.

By analogy, in accordance with a further aspect of the invention there is provided an apparatus for scanning of and obtaining radiation transmission data from, and preferably also an image of, an object comprising: a single broad spectrum radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween and enabled to collect in use a dataset of information about radiation incident at the detector system and hence transmissivity of an object in the scanning zone, at least one and preferably a plurality of scanning positions; a data processing apparatus to process and resolve the dataset spectroscopically across a plurality of frequency bands within the spectrum of the source to produce a plurality of frequency-specific datasets comprising at least data about intensity of radiation incident at the detector at a given frequency band; a calculation means to numerically compare at least one pair of frequency- specific datasets at least by applying a comparative function to at least one pair of frequency-specific intensity datasets to determine an intensity ratio for at least one pair of frequency-specific datasets to produce a frequency-comparative dataset.

Thus the apparatus is capable of implementing the method and the dataset is analysed numerically by numerically comparing frequency- resolved information for at least one pair, and preferably multiple pairs, of frequency-specific datasets to obtain a comparative numerical result that represents in quantified manner a relational aspect between them. If this comparative numerical result is appropriately selected, by applying to the frequency-specific datasets an appropriate comparative function, it can be possible to obtain a result dataset of frequency-comparative data that is more specifically characteristic of an aspect of the composition of the object than could be achieved by a mere multispectral image.

Preferably a comparison is made to a library of results. Thus, preferably, the apparatus further comprises one or more of: a further data register to store such frequency-comparative data; a data library of known data for known materials; and a comparator to compare the frequency-comparative data in the data register with data in the library and derive therefrom an indication of the likely material content of material in a transmission path.

The apparatus of the invention has a calculation means that effects a comparison between at least one pair of frequency-specific datasets at least by applying a comparative function to at least one pair of frequency- specific intensity datasets to determine an intensity ratio for at least one pair of frequency-specific datasets to produce a frequency-comparative dataset. The apparatus optionally further has a comparator to compare the frequency-comparative data in the data register with data in a library. Any suitable form of calculation means and/or comparator and/or library combining suitable hardware and software and combining automatic and user-input calculation steps can be envisaged. For example a calculation means and/or comparator and/or library comprises a suitably programmed data processing apparatus such as a suitably programmed general purpose or special purpose computer.

It will be understood generally that a numerical step in the method of the invention can be implemented by a suitable set of machine readable instructions or code. These machine readable instructions may be loaded onto a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a means for implementing the numerical step specified, and in particular thereby to produce a calculation means as herein described.

These machine readable instructions may also be stored in a computer readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in a computer readable medium produce an article of manufacture including instruction means to implement some or all of the numerical steps in the method of the invention. Computer program instructions may also be loaded onto a computer or other programmable apparatus to produce a machine capable of implementing a computer executed process such that the instructions are executed on the computer or other programmable apparatus providing steps for implementing some or all of the numerical steps in the method of the invention. It will be understood that a step can be implemented by, and a means of the apparatus for performing such a step composed in, any suitable combinations of special purpose hardware and/ or computer instructions. Optionally, the apparatus is adapted to collect in use transmission intensity data with an object in a single scanning position and for example includes a means to retain an object in a scanning position such as a receptacle into which an object can be placed. Additionally or alternatively it may include a conveyor to convey an object into and out of such scanning position.

In particular, the apparatus is adapted to receive a sample under test comprising a liquid in a container of other material, such as a bottle, flask, carton or the like, and comprises a container receiving means adapted to receivingly support a container within the scanning zone. The container receiving means may include holding means to hold the container static in situ. These may adjust to hold containers of different sizes. Adjustable and for example spring loaded formations may be provided to effect this.

Optionally, the apparatus is adapted to collect in use transmission intensity data with an object in a plurality of scanning positions as the object moves through the scanning zone, and preferably to collect in use data for an image of an object in the scanning zone, and preferably a succession of images as the object moves through the scanning zone, in that it further comprises an object handler to cause an object to move relative to and through the scanning zone in use.

Preferably, the apparatus further includes an image generation apparatus to generate at least a first image from the output of the detector system, being adapted co-operably with the detector to collect in use data for at least one image of an object in the canning zone and to generate at least one image from the output of the detector system; and preferably further an image display adapted to display an image. The display means is conveniently a simple two dimensional display screen, for example a conventional video display screen (which term is intended to encompass any direct display or projection system exploiting any cathode ray tube, plasma display, liquid crystal display, liquid crystal on silicon display, light emitting diode display or like technology). It is a particular advantage that the method can be envisaged for use with, and the apparatus for the invention incorporated into, the standard display screens of comparable existing systems for example in the security or medical imaging fields.

The radiation source must produce a distribution of energies across a suitable spectral range for characteristic scattering, and is typically an x- ray source. Tungsten is the most appropriate target, but others could be used.

It is necessary that the detector system is enabled to detect radiation in a manner which is spectroscopically resolvable by the data processing apparatus. Preferably, a detector system, or some or all discrete detector elements making up a multi-element system, may be inherently adapted to produce spectroscopic resolution in that it exhibits a direct spectroscopic response. In particular a system or element is fabricated from a material selected to exhibit inherently as a direct material property a direct variable electrical and for example photoelectric response to different parts of the source spectrum. For example, the detector system or element comprises a wide direct bandgap semiconductor material. For example, the detector system or element comprises a semiconductor material or materials preferably formed as a bulk crystal, and for example as a bulk single crystal (where bulk crystal in this context indicates a thickness of at least 500 μm, and preferably of at least 1 mm). The materials making up the semiconductor are preferably selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT), germanium, lanthanum bromide, thorium bromide. Group N-VI semiconductors, and especially those listed, are particularly preferred in this regard. The materials making up the semiconductor are preferably selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT) and alloys thereof, and for example comprise crystalline Cdi-(a+b)Mn_aZnbTe where a and/ or b may be zero.

Combination of these and any other such materials may be considered which give spectroscopic x-ray or other radiation detection rather than merely detecting amplitude of transmitted radiation and thus enable resolution at least of characteristic absences/ amplitude reductions in the transmitted radiation indicating presence of a characteristic target species.

An image generator may be provided to generate such an image. In particular it may be adapted to receive from the data processor a plurality of spectroscopically resolved images from a plurality of "imaging" bands and display these images successively or simultaneously to aid in object differentiation as above described. For example spectroscopic differentiation in the collected data is represented in a single combined image as differentiated colour, shading or marking.

A collimator is preferably provided to produce an emitted beam of suitable geometry from the x-ray source. The geometry of the emitted beam will determine the most useful geometry of the detector system. At its simplest, particularly if the apparatus is being used purely to collect spectrally resolved transmission data for the purposes of deriving numerically an indication of mass attenuation coefficient, a simple, effectively one dimensional beam may be provided in conjunction with a simple single transmission detector. However, in the preferred embodiment, the apparatus is further adapted for the generation of imaging information. It is intended in a possible mode of operation that the material identification provided in accordance with the numerical analysis method underlying the invention will serve in conjunction with imaging as an additional aid in the scanning of suspicious objects and in the identification of articles or materials therein, rather than being used in isolation. It is an advantage of the approach of the invention that useful compositional and imaging data can be obtained in principle for the same scan. More useful imaging data will generally be obtained by more complex beam and detector geometries.

For example, a beam may be collimated to have a spread in one or two dimensions, in particular to co-operate respectively with one or more linear detectors or with an area detector. Conveniently, linear and/or area detectors comprise linear and/or area arrays of a plurality of individual detector elements as above described.

The invention in particular relates to an apparatus and method operating on the line-scan principle, in which three dimensional objects are caused to move through a scanning zone and imaging information collected.

Imaging apparatus which employs the line-scan principle is well known. Typically, such apparatus will consist of an x-ray source, the beam of which may be collimated into a curtain, usually referred to as a "curtain beam", and is then detected by a linear detector for example comprising a linear photodiode array. Image information is obtained by having the object of interest move linearly for example at right angles with respect to the beam and storing successive scans of x-ray transmission information derived from the linear array from which a complete image frame can be compiled.

Accordingly, in this line-scan embodiment, the method comprises: providing an x-ray source and an x-ray detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one and preferably a plurality of linear detectors capable of generating spectroscopically resolvable information about incident x-rays; causing an object to move relative to and through the scanning zone; resolving the resultant transmitted data in the manner above described.

Accordingly, in this line-scan embodiment, the apparatus comprises: an x-ray source and an x-ray detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one and preferably a plurality of linear detectors capable of generating spectroscopically resolvable information about incident x-rays.

In accordance with this embodiment the radiation source is preferably collimated to produce a curtain beam and is thus a curtain beam x-ray source as will be familiar from conventional line scan apparatus.

Preferably the detector system comprises a plurality of linear detectors linearly or angularly spaced apart in generally parallel conformance in serial array. Each linear detector may comprise a linear array of detector elements.

The x-ray source may comprise a single primary source adapted to generate a beam such as a curtain beam aligned to be incident upon each linear detector in the spaced serial array at a suitable angular separation, from example by a suitable beam splitting apparatus. A single beam may be generated. Alternatively, multiple beams may be generated from a single source. Alternatively, multiple sources may be provided each generating a beam such as a curtain beam incident upon a linear detector in the serial array. The x-ray source may comprise a source combining any or all of the foregoing principles.

The provision of a plurality of linear detectors in accordance with this preferred embodiment of the invention offers an additional functionality. Data can be collected for an equivalent plurality of transmission paths as an object passes through a scanning zone. The provision of such a plurality of transmission paths between a source and differently positioned linear detectors or detector arrays gives the collected information the characteristics of the information collected by a conventional CT scanning apparatus, and allows the data to be processed additionally in a manner known from that technology.

For example, multiple transmission path data may be used to generate multiple images and thus improve the information content of the imaging aspect of operation in a familiar manner. Additionally or alternatively, multiple transmission paths through a given part of an object will lead to a varying of the effective through thickness, which can be employed to draw inferences about material content, again in a manner analogous to that known from CT scanning, and reinforce or further inform the inferences drawn by the derivation of data indicative of the mass attenuation coefficient in accordance with the basic principles of the invention.

With the latter application in mind in particular, it will generally be preferable if a single source is provided and used for the multiple ray paths created by having a multiple array of linear detectors. This guarantees that the incident spectrum for each ray path is essentially the same, and eliminates one possible uncertainty. However, the same principles could be applied to systems using multiple sources of reasonable spectral reliability.

Two possible embodiments of the invention involving suitable numerical analysis of frequency-specific datasets to obtain quantified information correlatable to composition were described hereinabove.

In accordance with the first such possible embodiment at least one of the said plurality of frequency bands is selected to correspond to a characteristically scattered wavelength of a target species to be identified, and the numerical analysis step comprises identifying an absence of or substantial reduction in a transmitted signal intensity at the characteristic frequency band and interpreting this as the presence of the said target species.

By analogy in this embodiment a suitable apparatus comprises: a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween and to collect in use a dataset of information about radiation incident at the detector and hence transmissivity of an object in the scanning zone at at least one and preferably a plurality of scanning positions, and preferably to collect in use data for an image of an object in the scanning zone, and preferably a succession of images as the object moves through the scanning zone; a data processing apparatus to process and resolve each such dataset or image spectroscopically across a plurality of frequency bands within the spectrum of the source, wherein at least one of the said plurality of frequency bands corresponds to a characteristically scattered wavelength of a target species to be identified; including a comparator to identify the absence of or substantial reduction in a transmitted signal intensity at the said frequency band and to output the same as an indication of the presence of the said target species.

In accordance with the second such possible embodiment a plurality of pairs of frequency bands are compared numerically, in particular to determine intensity ratios, and thus to obtain a numerical indicator in functional relationship with a mass attenuation coefficient.

By analogy in this embodiment a suitable apparatus comprises: a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween and to collect in use a dataset of information about radiation incident at the detector system and hence transmissivity of an object in the scanning zone, at at least one and preferably a plurality of scanning positions; a data processing apparatus to process and resolve each such dataset or image spectroscopically across at least three frequency bands within the spectrum of the source and produce an intensity data item for each band; an intensity data item register to store such resolved data items for each dataset; a calculation means to evaluate the ratio between intensity data items for at least two pairs of such frequency bands in a given intensity dataset and for example each successive such frequency band to obtain a numerical indicator in functional relationship with a mass attenuation coefficient associated with the intensity dataset; and preferably further a further data register to store such numerical indicator; a data library of data indicative of characteristic mass attenuation coefficients and in particular for example with mass attenuation coefficients characteristic of target materials such as suspect materials; a comparator to compare the numerical indicator with data in the library and derive therefrom an indication of the likely material content of material in a transmission path producing the said intensity dataset.

The invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is general schematic of a possible apparatus to implement an embodiment of the invention to detect absence of Bragg scattered signals; Figure 2 illustrates a typical radiation source spectrum, and illustrates how it is partitioned to implement an embodiment of method of the in conjunction with figure 1 ;

Figure 3 is a schematic protocol for operation of the embodiment of the invention of figure 2; Figure 4 is a side view of a representation of a scanning apparatus suitable for use in an embodiment of the invention;

Figure 5 illustrates the effect that can be created by images generated by means of the multiple ray paths provided by the embodiment of figure 4;

Figure 6 is general schematic of a possible apparatus to implement an embodiment of the invention to detect ratios of transmitted intensities;

Figure 7 illustrates a typical radiation source spectrum, and illustrates how it is partitioned to implement an alternative embodiment of method of the invention to detect ratios of transmitted intensities;

Figure 8 is a schematic protocol for operation of the embodiment of the invention of figure 7.

Referring first to the general schematic representation on Figure 1 , an x- ray source 1 and laterally spaced detector array 21 together define a scanning zone Z between them. In use, an object to be scanned is brought into and through the scanning zone in the usual manner, for example on a suitable conveyor belt (not shown).

In the illustrated example, a sample of material 9 sits in the scanning zone Z. An incident beam 11 from the x-ray source is illustrated. In the example, a diffracted beam 12 is diffracted at a characteristic angle in accordance with Bragg's law reducing significantly the intensity of the transmitted beam 13 above and beyond the reduction which would be attributable to absorption alone. This illustrates the effect exploited by the invention embodiment in its numerical analysis of the overall transmitted dataset.

The transmitted beam 13 is incident upon a detector array 21 which in a preferred embodiment comprises a plural linear array of cadmium telluride detector units.

The detector array 21 is in data communication with a processor 22. The detector array is used to generate a two dimensional "slice" in familiar manner. The inherent spectral resolution of the material in the array allows the processor 22 to resolve this image differentially across a plurality of pre-set frequency/energy bands in accordance with the principles of the invention by reference to energy band boundaries stored in the data register 23.

As is illustrated in more detail in Figures 2 and 3, some of these resolved energy bands are used to build up an energy-differentiated image for transmission to the display means 27. In this regard, the apparatus follows the same basic principles as conventional energy-differentiated imaging apparatus. It differs in the additional functionality provided by the comparator 24 which performs a numerical analysis in accordance with the general principles of the invention. In the embodiment this is done in relation to some of the identified frequency bands, each of which is associated with a characteristic Bragg scattering of a target species, to identify unusual reductions in the transmitted amplitude 13 within the characteristic frequency band which are indicative of characteristic scattering. This can be effected by comparison with a previously stored spectrum for the source 1 in the data register 25. The characteristically identified species may be identified to a user of the scanning system in any suitable way, either by inclusion in the image displayed on the display 27 or by another suitable alerting system. Any of the data processing or storage elements of the apparatus, for example including one or more of the processor 22, data register 23, comparator 24 and data register 25, may be provided by a suitably programmed data processor means such as a special purpose or general purpose computer.

The source 1 generates x-rays across a relatively broad spectrum of energy, so that this resolution maye be exploited. It may be a plural source, or a single source with the necessary spread. The source 1 is preferably tungsten source, which gives a characteristic plot of x-ray intensity (I) versus wavelength (λ) as is illustrated in Figure 2. Figure 2 illustrates how this spectrum might be divided to operate a system in accordance with the principles of the present embodiment. In Figure 2a the overall spectrum is divided into successive relatively broad bands b1 to b5. These are imaging bands used to draw up a relatively conventional energy-differentiated image. In Figure 2b the spectrum is additionally processed to target certain narrow frequency bands c1 to c3. These are "characteristic" bands and each is associated with a characteristic Bragg scattering wavelength for a given target species. A given target species may have more than one characteristic band identified. With the spectrum suitably resolved in the manner indicated in Figure 2 by means of the processor 22 identified in Figure 1 an image is generated and other information retrieved in accordance with the flow chart process represented in Figure 3.

Reading from top to bottom, the collected dataset is resolved both into the series of image bands and into the series of characteristic bands in the manner illustrated in Figure 2. Resolution of the image bands produces a series of images b1 , b2, b3, b4 and b5 which together represent intensities of transmitted x-rays across relatively broad band widths but differentiated for energy for across the spectrum. In this way a degree of differentiation between objects of different composition is possible. Objects of different composition, and in particular a different atomic number, will tend to exhibit varying responses. If the different images b1 to b5 are for example successively displayed, or, more preferably, given distinctive colourations and displayed simultaneously in a single composite image, additional resolution of objects from the scan can be provided. This process is reasonably conventional.

Where the embodiment notably differs from an imaging system alone is in the additional resolution of characteristic bands c1 to c3. These characteristic bands are relatively narrow, and each is intended to focus on and correspond to a characteristic Bragg scattering wavelength for a given target species. The resolved transmission data for these bonds in the register 25 are processed by a comparator to identify, for example with reference to a stored spectrum and/or with reference to intensity data in the vicinity of the characteristic band, any significant reduction in amplitude within the characteristic band suggestive of presence of characteristic scattering and hence of presence of the target species. The presence or absence so identified is then displayed, for example in combination with the complex image generated from the imaging band resolution or as an additional information display in association with the image or on a bespoke display.

In a preferred embodiment, the apparatus employs a line scan principle to generate an x-ray image. In airline security applications, the principle is encountered in particular in relation to hand baggage scanners. X-ray imaging might also be used in principle as a supplementary system for hold baggage (the reduced CT scan of the detection application being limited as regards imaging capability) but this is less common.

Figure 4 illustrates a suitable apparatus. An envisaged apparatus may combine the materials identification capability of the energy-resolved data collection and manipulation aspect of the invention with the information provided by generating an image in order to reinforce the scanning of an unknown object, in particular where the unknown object is a container such as a baggage item including multiple articles, for example for security applications, and for example for the detection of explosives. With this application in mind the illustrated embodiment uses a single x-ray source collimated to produce a curtain beam incident upon the three linear detectors 3a to 3c (which in the embodiment each comprise a linear array of detector elements). Thus, a plurality of ray paths 5a to 5c are generated in the scanning zone by means of a plurality of curtain beams incident upon a linearly or angularly spaced array of such linear detectors. Incident ray paths 5a to 5c are shown through the scanning zone between the x-ray source 1 and, respectively, the detectors 3a to 3c.

In the embodiment, the linear array detectors 3a to 3c comprise material capable of spectroscopic resolution of incident x-rays, and in the specific example comprise cadmium telluride although the skilled person will appreciate that other material selections may be appropriate. To exploit this spectral resolution, the x-ray source emits x-ray across a broad energy spectrum. In the example a tungsten source is used, although the skilled person would appreciate that other materials might be appropriate.

An endless belt conveyor 7 causes an object to be scanned 9 to move in a direction d so as to intercept the ray paths 5a to 5c in the scanning zone. The envisaged application of this embodiment of the invention is as a security scanner, and object 9 can be considered typically to be a container that is expected to contain a variety of distinct objects which it would be useful and desirable to characterise compositionally and to view effectively in a third dimension (for example, an item of airline hold baggage). However, the skilled person would readily appreciate that the same principles can be applied for example to the scanning of objects for internal examination purposes, to medical scanning, and to similar applications.

Datasets of transmitted intensity information are generated by building up transmitted information from each of the three detectors 3a to 3c. The processing of a dataset of information by resolving, at least to some extent, a relationship between incident energy/ wavelength and transmitted intensity for both numerical analysis in accordance with the principles of the invention and spectroscopically resolved imaging purposes is carried out (for example as illustrated in figures 2 and 3 or 6 and 7, although the principles illustrated in figures 4 and 5 can operate independently of the numerical analysis method).

Although the invention, especially in non-imaging mode of operation, requires only a single ray path, the embodiment of figure 4 presents plural ray paths through an object. Figure 5 illustrates an additional effect that can be created by images generated by means of the multiple ray paths provided by the embodiment of figure 4 which can further enhance the information provided.

As an object 9 passes through incident ray paths 5a to 5c (see figure 5a) three images are generated in which the object is oriented differently relative to the x-ray source 1. Successive display of these images will cause the object to appear to rotate as is illustrated in figure 5b.

This ability in effect to get a view of the object which is in effect rotatable in a third dimension can be seen in some respects as analogous to CT scanning. In a conventional CT scanner, relative rotational movement between scanner and scanned object (usually, by orbital movement of the scanner) allows a rotatable image to be collected. The multiple image generated in this example offers some of these features as a result of the multiple ray paths provided by the apparatus, but with a less complex geometry, and for example on a simple linear conveyor such as is typically used in security scanning systems. This offers an additional image functionality.

In this way, in accordance with the invention, an apparatus and method is described which can offer specific material characterisation based on resolved energy detection and data processing to identify materials by the absence or reduction of characteristically scattered band. All this information is obtained from the primary transmitted beam by spectroscopic resolution and processing of the primary collected dataset and without the need for specific detection of characteristically scattered signals. Figures 6 to 8 illustrate an alternative example protocol for numerical analysis of transmitted intensity data. Datasets of transmitted intensity information are generated as above. The processing of a dataset of information by resolving, at least to some extent, a relationship between incident energy/ wavelength and transmitted intensity for both numerical analysis in accordance with the principles of the invention and spectroscopically resolved imaging purposes is illustrated in the figures.

In the general schematic representation on Figure 6, a single ray path only is shown for simplicity. An x-ray source 101 and laterally spaced detector apparatus assembly 121 together define a scanning zone Z between them. In use, an object to be scanned is brought into and through the scanning zone in the usual manner, for example on a suitable conveyor belt as above.

In the illustrated example, an object 109 sits in the scanning zone Z. An incident beam 111 from the x-ray source is illustrated. In this simple schematic, the incident beam is represented by the line 111. The transmitted beam 113 is incident upon a detector array 121.

The detector array 121 is in data communication with a processor 122. The detector array is used to generate a two dimensional "slice" in familiar manner. The inherent spectral resolution of the material in the array allows the processor 122 to resolve this image differentially across a plurality of pre-set frequency/energy bands in accordance with the principles of the invention by reference to energy band boundaries stored in the data register 123. In the example embodiment a tungsten x-ray source, is used. A typical spectrum such as might be generated by tungsten of initial intensity against wavelength is illustrated in Figure 7.

The main purpose of Figure 7 is to illustrate two possible ways in which the spectrum may be resolved in accordance with the principles of the invention. In each case, the spectrum is resolved across five frequency bands. Although in mathematical principle some useful information can be derived from just three bands, it is suggested that five is a more practical minimum for complex heterogeneous objects if a reasonable inference about the functional variation of transmitted intensity with incident energy/frequency, and therefore about the mass attenuation coefficient, is to be derived.

The schematic illustrates two ways in which the spectrum may be resolved. In Figure 7, the bulk of the generated spectrum is divided between five relatively broad energy bands b1 to b5. In Figure 7b, five relatively narrow bands, which may approximate even to individual energies, are defined c1 to c5. Neither alternative is in contradiction with the principles of the invention, and any combination may be used to generate useful results either for the numerical analysis of the invention or, in a preferred embodiment, for spectroscopically resolved imaging to give further information about an object under investigation.

In the preferred embodiment, the data is also used to generate an image, and most preferably a spectrally resolved image which is spectrally resolved itself across a plurality of frequency bands to give further information to the image. In such an embodiment, some of the resolved energy bands in figure 7, for example those illustrated in Figure 7a, could be used to build up an energy-differentiated image for transmission to the display means 129. In this regard, the apparatus follows the same basic principles as conventional energy-differentiated imaging apparatus.

It differs in the functionality provided by the processor 122 which further acts in relation to a series of identified frequency bands, for example those in Figure 7b, but in this function uses the data to generate a representative quantification of, and for example an average of, transmitted intensity in each band, which is then passed to the intensity data item register 124 for storage.

A calculation means 125 evaluates the ratio between successive intensity data items (for example, where data items are collected 11 to I5 relating to energy bands c1 to c5, the calculation means evaluates the quotient 11/12, 12/13, 13/14, 14/15). This calculation of such a quotient is capable in principle of removing from consideration variables, such as density and thickness, which do not vary with incident radiation energy, and therefore of providing a numerical indicator which is functionally related to energy, and consequently indicative of the primary energy-dependent variable, the mass attenuation coefficient.

A comparator 126 compares the data thereby produced with a library of data 127. The library of data may include pre-stored data of similar or at least numerically comparable nature which is related to or depends upon the mass attenuation coefficient for a range of materials, and in particular specified target materials. This may be a manually or automatically address library. Data may be preloaded or referenced, or may be generated or added to over time by operation of the apparatus with known materials. Any of the data processing or storage elements of the apparatus, for example including one or more of the processor 122, data register 124, calculation means 125, comparator 126 and data library 127, may be provided by a suitably programmed data processor means such as a special purpose or general purpose computer.

By virtue of this comparison, inferences may be drawn about the likely material content in the transmission path. This may be displayed on the display means 130, for example in association with the image display 129. In addition to its value in isolation, this may be used in conjunction with the image displayed on the display means 129 the better to characterise the contents or composition of an object under investigation.

The data collection and manipulation process is illustrated by the flow chart of Figure 8, again for a preferred embodiment in which spectral resolution of transmitted intensity is used both for the numerical identification process of the invention and for an additional imaging purpose. Reading from top to bottom, the collected dataset is resolved both into the series of image bands and into the series of bands for numerical analysis in the manner illustrated in Figure 7.

Resolution of a transmitted intensity dataset into image bands produces a series of images b1 , b2, b3, b4 and b5 which together represent intensities of transmitted x-rays across relatively broad band widths but differentiated for energy for across the spectrum. In this way a degree of differentiation between objects of different composition is possible. Objects of different composition, and in particular a different atomic number, will tend to exhibit varying responses. If the different images b1 to b5 are for example successively displayed, or, more preferably, given distinctive colourations and displayed simultaneously in a single composite image, additional resolution of objects from the scan can be provided. This process is reasonably conventional.

Where the embodiment notably differs is in the additional resolution of the transmitted intensity dataset into bands c1 to c5 and in the resultant numerical analysis. In the embodiment these bands are relatively narrow, but this is illustrative only. There is no reason in principle why the same bands could not be used for both purposes. The resolved transmission data for these bands in the register 125 are processed as above to generate intensity ratios and thus a numerical representation of the variation of intensity with energy and then a comparator references equivalent stored data to allow inferences to be drawn about material content. This may be displayed for example in combination with the complex image generated from the imaging band resolution or as an additional information display in association with the image or on a bespoke display.

In an example security or like use the apparatus is employed for the detection of contraband materials, for example explosives or other dangerous or prohibited materials. Data suitable to alert for the presence of these materials may be stored in the library. The apparatus may include visible and/ or audio alarm means, e.g. as part of the display 130, actuated when the comparator produces an indication that a target contraband material is likely to be present.

In this way, in accordance with the invention, an apparatus and method is described which can offer specific material characterisation based on resolved energy detection and data processing and also offer the option of generating an image and in particular an image which has some general energy differentiation to facilitate in distinguishing between different objects of different composition. In this combined mode the invention offers in a single apparatus a materials (e.g. explosive) detection capability analogous to that of prior art CT scanners commonly used for hold baggage scanning (and which typically have limited or no imaging application) in combination with an imaging capability with the advantages of a line scan such as is commonly used for hand baggage scanning. All this information is obtained from the primary transmitted beam by the provision of specific detectors having a functionality to effect spectroscopic resolution of transmitted intensity and by suitable numerical analysis of the transmitted intensity dataset.

Claims

1. A method of obtaining radiation transmission data comprising information about the composition of an object comprising the steps of: providing a single broad spectrum radiation source such as an x-ray or gamma-ray source and a radiation detector system such as an x- ray or gamma-ray detector system spaced therefrom to define a scanning zone therebetween, the detector system being capable of detecting and collecting information about incident radiation resolvable spectrally across at least a part of the spectrum of the source; collecting a dataset of information about radiation incident at the detector and hence transmissivity of an object in the scanning zone at at least one and preferably a plurality of scanning positions; resolving the dataset spectroscopically across a plurality of frequency bands within the spectrum of the source to produce a corresponding plurality of frequency-specific datasets comprising data about intensity of radiation incident at the detector at a given frequency band; numerically analysing at least one pair of frequency-specific datasets to produce a frequency-comparative dataset, at least by applying a comparative function to at least one pair of frequency- specific intensity datasets to determine an intensity ratio for at least one pair of frequency-specific datasets.

2. A method in accordance with claim 1 wherein a frequency-specific dataset is collected comprising data representative of average intensity of radiation incident at the detector across a given frequency band or representative part thereof.

3. A method in accordance with claim 2 wherein the numerical analysis step comprises determining an average intensity ratio for at least one pair of frequency-specific datasets.

4. A method in accordance with any preceding claim wherein numerical weighting factors are applied to data in different frequency-specific datasets as part of numerical comparison therebetween to produce a suitably modified result dataset.

5. A method in accordance with any preceding claim wherein the dataset of information about radiation incident at the detector is additionally used to generate an image of an object in the scanning zone.

6. A method in accordance with any preceding claim comprising the additional step of causing an object to move relative to and through the scanning zone and thereby collecting a plurality of successive datasets of of information about radiation incident at the detector.

7. A method in accordance with any preceding claim wherein the dataset of information about radiation incident at the detector is additionally used to generate an image of an object in the scanning zone.

8. A method in accordance with claim 7 wherein an object is caused to move relative to and through the scanning zone to collect a plurality of successive datasets and generate a corresponding plurality of successive images.

9. A method in accordance with one of claims 7 to 8 wherein each image is resolved spectroscopically across a plurality of frequency bands within the spectrum of the source which are allocated to generate a series of energy-differentiated images.

10. A method in accordance with one of claims 7 to 9 comprising the additional step of displaying a generated image or images.

11. A method in accordance with any preceding claim operating on the line-scan principle and comprising: providing a single broad spectrum x-ray source and an x-ray detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one and preferably a plurality of linear detectors capable of generating spectroscopically resolvable information about incident x-rays; causing an object to move relative to and through the scanning zone; resolving the resultant transmitted data in accordance with any preceding claim.

12. A method in accordance with claim 11 wherein the radiation source is used to generate a curtain beam.

13. A method in accordance with claim 1 1 or 12 wherein the detector system comprises a plurality of linear detectors in a laterally spaced serial array at a suitable angular separation, and wherein data is collected from the resultant multiple ray paths between source and detectors.

14. A method in accordance with any preceding claim wherein at least one of the said plurality of frequency bands is selected to correspond to a characteristically scattered wavelength of a target material to be identified, and the numerical analysis step comprises identifying an absence of or substantial reduction in a transmitted signal intensity at the characteristic frequency band and interpreting this as the presence of the said target species.

15. A method in accordance with claim 14 wherein a reduction in amplitude is determined numerically by comparison if intensity data at a characteristic frequency band with intensity data representing at least one other frequency band away from the characteristic, and optionally also by comparison with a known and for example prerecorded source spectrum.

16. A method in accordance with any preceding claim wherein a dataset is collected comprising data about intensity of radiation incident at the detector and comprising the additional steps of: resolving such an intensity dataset across at least three frequency bands within the spectrum of the source to produce a frequency resolved intensity data item for each band in the manner of any preceding claim; evaluating the ratio between intensity data items for at least two pairs of such frequency bands in a given intensity dataset and for example each successive such frequency band to obtain a numerical indicator in functional relationship with a mass attenuation coefficient associated with the intensity dataset; comparing the same with a library of data indicative of characteristic mass attenuation coefficients, and in particular for example with mass attenuation coefficients characteristic of target materials such as suspect materials, in order to obtain an indication of the likely material content of material in a transmission path producing such intensity dataset.

17. An apparatus for scanning of and obtaining radiation transmission data from an object comprising: a single broad spectrum radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween and enabled to collect in use a dataset of information about radiation incident at the detector system and hence transmissivity of an object in the scanning zone, at least one and preferably a plurality of scanning positions; a data processing apparatus to process and resolve the dataset spectroscopically across a plurality of frequency bands within the spectrum of the source to produce a plurality of frequency-specific datasets comprising data about intensity of radiation incident at the detector at a given frequency band; a calculation means to numerically compare at least one pair of frequency-specific datasets, at least by applying a comparative function to at least one pair of frequency-specific intensity datasets to determine an intensity ratio for at least one pair of frequency- specific datasets to produce a frequency-comparative dataset.

18. An apparatus in accordance with claim 17 further comprising: a data register to store such frequency-comparative data; a data library of known data for known materials; and a comparator to compare the frequency-comparative data in the data register with data in the library and derive therefrom an indication of the likely material content of material in a transmission path.

19. An apparatus in accordance with claim 17 or 18 further comprising of an object handler to cause an object to move relative to and through the scanning zone in use.

20. An apparatus in accordance with one of claims 17 to 19 further including an image generation apparatus adapted co-operably with the detector to collect in use data for at least one image of an object in the canning zone and to generate at least one image from the output of the detector system.

21. An apparatus in accordance with claim 20 further comprising an image display adapted to display an image.

22. An apparatus in accordance with one of claims 17 to 21 wherein a detector is adapted to produce spectroscopic resolution in that it is fabricated from a material selected to exhibit inherently as a direct material property a direct variable electrical response to different parts of the spectrum.

23. An apparatus in accordance with claim 22 wherein the detector comprises a semiconductor material selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT), germanium, lanthanum bromide, thorium bromide.

24. An apparatus in accordance with claim 22 or 23 wherein the detector comprises a semiconductor material or materials formed as bulk crystal including a Group N-Vl semiconductor material.

25. An apparatus in accordance with claim 24 wherein the detector comprises a semiconductor material selected from cadmium telluride, cadmium zinc telluride (CZT), cadmium manganese telluride (CMT).

26. An apparatus in accordance with one of claims 17 to 25 operating on the line-scan principle, comprising: an x-ray source and an x-ray detector system spaced therefrom to define a scanning zone therebetween, the detector system comprising at least one linear detector capable of generating spectroscopically resolvable information about incident x-rays.

27. An apparatus in accordance with claim 26 wherein the radiation source is a collimated to produce a curtain beam.

28. An apparatus in accordance with claim 26 or 27 wherein the detector system comprises a plurality of linear detectors in a laterally spaced serial array at a suitable angular separation such that intensity data may be collected in use from the resultant multiple ray paths between source and array of linear detectors.

29. An apparatus in accordance with one of claims 19 to 30 comprising: a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween and to collect in use a dataset of information about radiation incident at the detector and hence transmissivity of an object in the scanning zone at at least one and preferably a plurality of scanning positions, and preferably to collect in use data for an image of an object in the scanning zone, and preferably a succession of images as the object moves through the scanning zone; a data processing apparatus to process and resolve each such dataset or image spectroscopically across a plurality of frequency bands within the spectrum of the source, wherein at least one of the said plurality of frequency bands corresponds to a characteristically scattered wavelength of a target species to be identified; including a comparator to identify the absence of or substantial reduction in a transmitted signal intensity at the said frequency band and to output the same as an indication of the presence of the said target species.

30. An apparatus in accordance with one of claims 19 to 31 comprising: a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween and to collect in use a dataset of information about radiation incident at the detector system and hence transmissivity of an object in the scanning zone, at at least one and preferably a plurality of scanning positions; a data processing apparatus to process and resolve each such dataset or image spectroscopically across at least three frequency bands within the spectrum of the source and produce an intensity data item for each band; an intensity data item register to store such resolved data items for each dataset; a calculation means to evaluate the ratio between intensity data items for at least two pairs of such frequency bands in a given intensity dataset and for example each successive such frequency band to obtain a numerical indicator in functional relationship with a mass attenuation coefficient associated with the intensity dataset; and preferably further a further data register to store such numerical indicator; a data library of data indicative of characteristic mass attenuation coefficients and in particular for example with mass attenuation coefficients characteristic of target materials such as suspect materials; a comparator to compare the numerical indicator with data in the library and derive therefrom an indication of the likely material content of material in a transmission path producing the said intensity dataset.