GB2357414A - Fast detection of X-rays using detector arrays and energy discrimination - Google Patents

Abstract

This invention relates to the field of X-ray inspection systems and more particularly those that use X-ray diffraction to analyse an object under inspection. X-ray diffraction has long been used as an aid to structural analysis and information about a diffracting material is commonly derived by one of two methods: energy dispersion or angular dispersion. The present invention proposes the use of an array of semiconductor detector elements and associated electronics which are capable of extracting an essentially monochromatic diffraction pattern from scattered polychromatic or quasi-monochromatic X-rays. Electrical pulses with a parameter representative of the response magnitude are discriminated by electronics which output a digital signal. If the pulse parameter lies between first and second discriminator values a digital signal is produced and counted. Therefore, X-rays over a range of energies, as defined by the discriminator values, are detected and counted over a range of angles, determined by the position of sensors on the array.

Description

APPARATUS FOR FAST DETECTION OF X-RAYS This invention relates to the field

of X-ray inspection systems and more particularly those that use X-ray diffraction to analyse an object under inspection.

X-ray diffraction has long been used as an aid to structural analysis. The technique derives from a well-known property of crystalline materials: that they diffract incident X-rays in accordance with the Bragg equation:

nX = 2d sinO where 20 is the angle, measured relative to an axis through an X-ray source and scattering centre within the crystalline material, through which incident X-rays of wavelength; are coherently scattered; d is the crystal lattice spacing (d- spacing) and n is an integer. It is also known from, for example, Harding and Kosanetzky J. Opt.

Soc. Am. A 4(5) 933 - 944, 1987, that information can also be extracted from X-ray diffraction patterns derived from non-crystalline or poorly-ordered materials.

A crystal lattice will possess a large characteristic set of d-spacings. The range of d spacings present in a material can be extracted from measurements taken while either X or 0 is varied. The position of each spot (or peak) in a diffraction pattern, formed when the diffracted beam hits some form of X-ray detector, arises from a characteristic set of values {d, 2, 0}. The spot intensities contain information regarding the molecular content of the material. Information about a diffracting material is commonly derived from the position and intensity of peaks in the diffraction pattern by either of two methods: detecting the range of wavelengths diffracted through a constant scattering angle (energy dispersion) or looking at monochromatic X-rays scattered through a range of angles (angular dispersion).

Essential to prior art diffraction systems employing angular dispersion is the provision of a monochromatic incident X-ray beam. The terms monochromatic and polychromatic are used herein to refer to narrow W. and broad AX finite spectral ranges respectively, 3X << AX. The midpoint XO of the narrow spectral range 3X is taken to be the wavelength of monochromatic X-rays.

4 0 a 2 - In a conventional X-ray source, polychromatic X-rays are generated by bombardment of a target anode material such as copper or tungsten with high-energy electrons.

The X-ray spectrum so-produced comprises a continuous spectrum ("continuum") superimposed with peaks at characteristic (of the anode material) energies. The continuum emission in a conventional X-ray source tends to dominate the emission.

Monochromation is achieved by filtering out the spectral region around a peak.

Quasi-monochromation refers to any source that will provide a narrow spectral output in comparison with a conventional source. Whereas in a conventional source the continuum emission dominates the line emissions (peaks), in a quasi- monochromatic source the continuum will be background and the line emission will dominate. An example of a quasi-monochromatic source would be a Fluorescence X-ray source in which a high energy X-ray source is used to excite X-ray fluorescence in a material.

Although a quasi-monochromatic source produces a much narrower spectral range than a conventional polychromatic source it is not truly monochromatic and so full monochromation will still be achieved by filtering out the spectral region around a peak.

To achieve a degree of monochromation suitable for applications in which the X-ray scattering power of a substance under investigation is weak, it is necessary to avoid contamination of the diffraction pattern by diffracted X-rays from other spectral regions. Thus, while for some applications, a single filter is sufficient for the peak wavelength to dominate in the diffraction pattern, more discriminating applications require a balanced filter technique to be used.

The balanced filter technique involves taking two diffraction images of the same spectral region through two high-pass filters whose cut-off edges are either side of the desired spectral peak. One image is subtracted from the other, and the result is a filtered monochromatic (occupying the spectral region between the two filters' absorption edges) diffraction pattern. There are numerous disadvantages of this technique. In practice any filter will inevitably attenuate the beam to some extent within the spectral range of interest; this is compounded by the need to provide two filters. It is very difficult to arrange for close matching of the attenuation properties of the filters, which is necessary to ensure that a narrow energy band is sampled. Two images are required of the same scattering target (one per filter) with the result that either the object under investigation must be stationary during data acquisition to allow successive images to be taken or two detector systems are required.

Furthermore the subtraction of the two images results in data of poor statistical quality.

There is therefore a perceived need to provide an alternative technique to improve the extraction of structural information using angular dispersive X-ray diffraction from weakly diffracting or otherwise noisy materials.

It is an object of this invention to provide apparatus suitable for structural investigation by angular dispersive X-ray diffraction without the intensity loss inherent to the filter techniques of the prior art.

Accordingly this invention provides a detection system for detecting quasi monochromatic or polychromatic X-rays scattered by a target material across an angular range O - d wherein the detection system comprises an array of semiconducting detector elements subtending the angular range O d respectively connected to a corresponding array of readout channels characterised in that the detector elements are fabricated from a semiconducting material with band gap transport responsive to irradiation by X-rays whereby each detector element generates an electrical response whose magnitude is dependent on incident X-ray energy and each readout channel comprises front-end electronics arranged to transform the semiconductor electrical response to an electrical pulse with a parameter representative of response magnitude; discriminating electronics arranged to output a digital signal if the pulse parameter lies between first and second pre selected discriminator values and a counter arranged to count the number of digital signals output from the discriminating electronics.

This invention provides the advantage that information about the target material can be extracted with shorter interrogation time of the target material in comparison with the prior art. Scattering is detected across the spectrum of the quasi- monochromatic or polychromatic X-rays within all angles subtended by the detector array. Each parameter is separable. The scattering angle is determinable from the position of the detector element at which an X-ray beam is intercepted. To provide a rapid assessment of X-ray energy a relatively simple electronic circuit is used in which an analogue detector element response is converted to a digital output - the output having one of two logic states to indicate merely the presence or absence of X-ray incidence within a particular narrow energy range.

The detector system is particularly suitable for use in X-ray diffraction systems. In such systems it is desirable to know both energy (or equivalently wavelength) and angular deflection of an X-ray beam in order to deduce the lattice spacings d characteristic of the material under investigation. Detector arrays have been used in the prior art to determine scattering angle, but detector elements have generally been unsuitable for providing energy resolution, certainly within a reasonable exposure timescale. This has necessitated the use of X-ray monochromation techniques prior to diffraction pattern detection. The monochromation process itself inevitably leads to a reduction in intensity of the detected probe beam, leading to a consequent increase in target interrogation time. By way of contrast, this invention provides a detection system which is capable of measuring both scattering angle and energy of diffracted X-rays, and so reduces the need for beam monochromation prior to detection. All X rays scattered within the accepted angular range will be detected by the semiconducting detector elements. It is only after detection that the electronics provide a fast means of recording an essentially monochromatic diffraction pattern, with no significant discarding of detection events within the required energy band. In this way two of the characteristic set of values {d, E:(X), 0} linked by Bragg's equation are measurable and the characteristic d-spacings can be calculated in order to extract information about the target material.

The system may include at least two arrays of readout channels, each semiconducting detector element being connected to respective readout channels, one from each array, wherein readout channels in the same array have discriminating electronics set to substantially the same first and second discriminator values, and which in turn are different from discriminator values appropriate to readout channels in other arrays.

This embodiment of the detection system of the invention provides a means by which further information can be extracted simultaneously with the monochromatic diffraction pattern described above. Different arrays of readout channels are set to measure monochromatic diffraction patterns occupying different parts of the quasi monochromatic/polychromatic spectral range. Thus more information can be extracted in the same interrogation time, further increasing the advantage in speed that this detection system has over the prior art.

The system may also include display means arranged to display for each readout array an X-ray scattering pattern derived from position of each detector element in the array plotted against number of counts registered in respectively connected readout channels. This provides the advantage of simplicity, the pattern is displayed in a format commonly used for powder diffraction patterns. This facilitates a quick comparison with known diffraction patterns to assist in identification of an unknown substance.

The semiconducting elements are preferably fabricated from cadmium zinc telluride, gallium arsenide, lead iodide or mercury iodide. These high atomic number semiconducting materials are currently fabricated into commercially available arrays and meet the preferred criteria of detector element materials for use in this aspect of the invention. These materials can be used to provide detectors sensitive to the 60keV region of enhanced emission from a tungsten source. Such materials, with their high attenuation coefficients and high photoelectric absorption to Compton scatter ratios, enable the detecting elements to record practically all incident photons.

Additionally, they can be used to construct detector elements with sufficient energy resolution (less than 10% at full-width-half-maximum) in the 60keV energy region to be able to record the true energy of all relevant incident X-ray photons.

Preferably, the angular range O to d is 20 to 80. This corresponds to the angular range of interest in measuring diffraction patterns from powdered materials using the 60keV tungsten enhanced emission band. Moreover the materials referred to in the previous paragraph provide sufficient spatial resolution over this angular range.

In an alternative aspect, this invention provides an X-ray inspection system comprising an X-ray source, a target material to be investigated and a detection system arranged such that a quasi-monochromatic or polychromatic X-ray beam generated by the source is scattered from the target material to the detection system characterised in that the detection system comprises the detection system detailed in the preceding paragraphs. This aspect of the invention provides a system which is capable of rapid data collection due to the use of a detection system in which monochromation is effectively carried out after diffracted X-rays within the angular range of interest have been detected.

The quasi-monochromatic/polychromatic X-ray beam is preferably collimated into a fan beam in order to illuminate a coplanar two-dimensional array of voxels within the target material and the system includes focusing collimation means arranged to pass only X-rays scattered from a single voxel at one depth and height within the illuminated array to the detection system. The system may also include an array of focusing collimation means and a respective array of detection systems, the collimation array members being stacked so as to pass simultaneously to respective detection systems X-rays scattered from respective voxels at different heights within the illuminated voxel array. Further, the array of collimation means is preferably moveable relative to the target material in the direction of unscattered X-rays in order to enable detection of X-rays scattered from voxels at different depths within the target material.

These features lend themselves to implementation in a scanning X-ray inspection system which can be used to inspect the entire volume of a bulk target in realistic time periods. This reduces the total time taken to complete a thorough inspection in comparison with prior art inspection systems. These embodiments are therefore particularly applicable to scanning airport baggage in which a high throughput is desired within a timescale which falls within passenger tolerance and yet with a very high probability of detection of any explosives, drugs or other contraband material.

The inspection system may include multiple detection systems symmetrically oriented to intercept a conical distribution of diffracted X-rays at symmetrically equivalent regions. This provides the advantage of improved accuracy. The diffraction patterns detected by different detection systems can be averaged over the same monochromation range to provide more accurate counting statistics and hence increase the certainty with which a target material may be identified.

In order that the invention may be more fully understood embodiments thereof will now be described with reference to the accompanying drawings in which Figure I illustrates schematically an embodiment of the X-ray detection system of the invention.

Figures 2a and 2b are schematic illustrations of the readout electronics of the detection system of Figure 1.

Figure 3 is a schematic illustration of the pulse processing performed by the readout electronics of Figure 2.

Figure 4 is an illustration of a multi-channel readout circuit for use with the detection system of Figure 1.

Figure 5 is an illustration of X-ray diffraction in a prior art system for baggage scanning.

Figure 1 illustrates an X-ray detection system indicated generally by 10. The system comprises a collimator (not shown) arranged to pass X-rays 12 scattered from a volume element (voxel) 14 of a substance under investigation towards an arrangement of four linear detector arrays 16, 18, 20, 22. Each array, for example 16, comprises a series of cadmium zinc telluride (CZT) detector elements 16a, 16b, 16c, ........ Each detector element 16a, 16b, 16c. is connected to a respective readout circuit, respective arrays 24, 26 of readout circuits for two detector arrays 16, being shown in the Figure. Each array of readout circuits 24, 26 processes signals received from one of the detector arrays 16, 18, 20, 22 and outputs a diffraction pattern 28, 30 representative of the intensity of X-ray radiation 12 incident at the positions of each detector element 16a, 16b, 16c. in the detector array 16, 18, 20,22.

Figure 2a illustrates a single detection channel 40 of the X-ray system illustrated in Figure 1. Each such channel comprises one CZT detector element 16a connected to an associated readout circuit 24a. The detector element 16a develops an electrical response when struck by an incident X-ray photon 42a. The magnitude of this response is proportional to the energy of the striking photon(s) and it is this which is processed by the readout circuit 24a. The circuit 24a comprises a preamplifier 44, a 0 a 00 a shaping amplifier 46, a lower-level discriminator 48, an upper-level discriminator 50 and a counter / buffer 52.

Figure 2b illustrates an array 60 of the detection channels 40 illustrated in Figure 2a.

The array 60 comprises an array 24 of the readout circuits respectively connected to an array 16 of CZT detector elements.

Figure 3 illustrates the principle of operation of the readout circuit in producing a pulse signal from the detector response. The Figure shows an array 16 of CZT detector elements 16a-h. Each detector element 16a-h develops an electrical response to an incident X-ray photon 42a-h, the magnitude of this response being proportional to the energy of the striking photon(s). This response is amplified and shaped by readout array 24 pre- 44 and shaping- 46 amplifiers to produce an electrical pulse 72a-h. The discriminators 48, 50 within the readout array 24 are arranged to produce a digital output response (or count) 74a, c, e, g, h if the magnitude of the electrical pulse 72a-h fails between lower Fs, and higher 82 discriminator levels. The counter 1 buffers 52 are arranged to measure the number of counts in each channel 40 over a given period of time.

With reference to Figure 1, the detection of an X-ray diffraction pattern in accordance with this invention will now be explained. The target voxel 14 is an elemental volume that is to be investigated by X-ray diffraction. For the moment, it is assumed that the voxel 14 and its associated diffraction pattern 12 are isolated from neighbouring voxels and their associated diffraction patterns i.e. either the voxel 14 comprises the entire object under investigation or some arrangement is used whereby such isolation can be effectively achieved. An arrangement of apparatus known to be able to effectively isolate a voxel in this way is described in a PCT patent application, publication number WO 96124863. However, this aspect is not central to the invention, although the two systems can be very advantageously combined.

Accordingly, a description of how isolation is achieved will be given later. For the present however, X-rays diffracted from the voxel 14 are uncontaminated by neighbouring voxel diffraction patterns.

A collimated beam (not shown) of polychromatic X-rays is incident on the voxel 14.

(Note: the skilled man will appreciate that a quasi-monochromatic beam can also be used). The voxel 14 comprises a number of crystallites with a range of d- spacings which diffract X-rays in accordance with the Bragg equation. In a powdered material, the crystallites are randomly oriented and each d-spacing is present in a number of orientations. For a given d-spacing and incident wavelength X, the diffracted beams therefore lie along the surface of a cone with semi-angle 0. With a polychromatic incident beam, the diffracted X-rays 12 form a continuous series of cones with a range of semi-angles 0 to Omax. However, for each value of 0, there is a range of combinations of d and X which satisfy the Bragg equation and hence possible wavelengths of X-rays forming the diffracted beams 12. The set 12 of diffracted beams can be envisaged as forming a spatial superposition of a number of monochromatic (covering wavelength region Sk) angular-dispersive diffraction patterns.

The diffracted beams 12 are incident on four linear arrays 16, 18, 20, 22 of CZT detector elements. Since the diffraction pattern 12 is conically symmetric, a single linear array suffices for its detection. However four detector arrays 16, 18, 20, 22, symmetrically oriented, provide better counting statistics if detector results are averaged. The operation of one such array 16 will be explained, it being understood that the remaining three 18, 20, 22 operate in the same way. This array 16 comprises detector elements 16a, 16b, 16c, arranged to extend over an angular range 0 to d. The range d is selected to encompass a higher intensity region of the range of the diffraction beam 0 to Omax. For powdered compositions of interest at airports, this range will correspond to small angle scattering. Each detector 16a, 16b, 16c,.... in the array therefore intercepts one angular region 60 of the diffraction pattern.

Referring now to Figures 2a and 3, the function of the readout channels 24 in providing energy discrimination (equivalently resolving the superimposed monochromatic diffraction patterns) will be described. The CZT detector element 16a develops a charge pulse in response to X-ray 42a illumination. The magnitude of this response is proportional to the energy of X-rays incident on the element 16a during a response time interval. The preamplifier 44 and shaping amplifier 46 transfer the CZT detector response to an electrical pulse 72a whose height is representative of the energy of the incident X-ray 42a. If this pulse height then falls between a lower threshold E, set by the lower-level discriminator 48 and an upper threshold 62 set by the upper-level discriminator 50 a signal 74a is sent to the counter 52 which is - '. -, f 1.1: 10 01 0. 6 0,0 ?.3,., I registered as a "hit". If the peak pulse height is outside the range 5s ": 62 - El set by the discriminators 48, 50 then no hit is registered. The counter 50 then buffers the number of hits registered over an observation period and these are displayed as an intensity reading at the position of the detector element 16a in the diffraction pattern 28.

The diffraction patterns 28, 30 displayed by the apparatus 10 and shown in Figure I therefore comprise plots of scattered intensity (number of counts) against scattering angle (detector element position) of monochromatic X-rays (energy, or equivalently wavelength, within the range El - 62). These patterns are equivalent to those detected in prior art angular dispersion diffraction systems, but without the disadvantages inherent to the use of filters in obtaining monochromaticity. Any apparatus based on the prior art technique of using filters to achieve monochromation inherently introduces a loss in intensity from either the incident or diffracted beam (depending on filter positioning), with the consequent reduction in signal to noise ratio. In contrast, although in this embodiment the present invention also discards potential information carrying X-rays from outside the monochromation range, it does not consequently reduce the intensity of X-rays in the detected monochromatic spectral region.

A further advantage of this invention over the prior art is achieved if a multi-channel readout circuit, as illustrated in Figure 4, is connected to each detector element 16a.

In this Figure, the single detector element 16a is shown connected to first 24a, second 80a and third 82a readout circuits. The multi-channel circuit comprises common pre- 44' and shaping- 46' amplifiers and multiple discriminators 48, 50 and counter / buffers 52. Each readout circuit 24a, 80a, 82a is identical to that 24a described previously except that the discriminators 48, 50 in each are set to provide different threshold levels cl, 62. Thus while the first readout circuit 24a extracts information relating to the monochromation range E, - 62, the second 80a extracts information from a different monochromation range F3 - 64 and the third 82a extracts information from a third range 65 - 66- Counts from the first 24a, second 80a and third 82a readout circuits are then combined with counts from readout circuits set to the same respective thresholds from different detector elements to derive three diffraction patterns of the form of 28, each plotting intensity against scattering angle for a different part of the X-ray spectrum. Clearly this set up is not limited to only three readout circuits. A series of circuits can be provided which readout information from a series of different monochromation ranges. In this way information from other parts of the diffraction spectrum can be extracted in segments from the detector elements during a single period of voxel irradiation.

The parallel processing capability of the readout circuits enables a number of monochromatic diffraction patterns to be simultaneously extracted from a general polychromatic diffraction pattern. This reduces the information content to a number of readily interpretable conventional monochromatic presentations, which can ultimately be used to increase the accuracy of extracted structural detail.

This invention is particularly applicable to situations in which data collection needs to be completed as quickly as possible. For example baggage scanners at airports have a high throughput of passenger baggage and it is necessary to have a trusted and reliable system with which to detect possibly small amounts of illegal substances such as drugs or explosives concealed in larger containers. Similarly this invention can be used to scan rapidly foodstuffs such as meat on a conveyor belt in order to detect bone, cartilage or other inedible contaminant.

In order to facilitate this rapid scanning of bulk objects it is necessary to be able to avoid interference between diffraction patterns generated from neighbouring scattering centres. Thus each voxel must be independently addressable. As mentioned above a method of achieving this is described WO 96124863, which explicitly cites its applicability to airport baggage scanning. The advantages of combining the present invention with that one are thus readily seen.

WO 96124863 describes how illumination of a large object by an incident fan beam in combination with a particular form of collimation of the diffracted beam enables enhanced discrimination between scattering from the target voxel and that from neighbouring material. The advantage provided by that invention, namely a more rapid three-dimensional scanning of bulk objects, is further achieved by application of the present invention.

Consider a three-dimensional array of voxels illuminated by an X-ray fan beam. In a depth dimension, defined by stacking in the direction of propagation of the incident X ray beam, conic diffraction patterns, similar to 12, will be generated from successive 12 voxels. A focusing collimator, which reflects the conic symmetry of the diffraction pattern, can be used to focus in on one particular depth.

Figure 5 illustrates X-ray diffraction from a plane of voxel elements at one particular depth in a bulk three-dimensional object. This illustration is of a plane parallel to the voxel plane, displaced in the direction of propagation of the incident X- ray beam.

Diffracted beams from neighbouring voxels intersect this plane in circular profiles 90a, 90b, 90c, centred on projected voxel positions in this plane. The incident X-ray fan beam is collimated to intersect one fine only of voxels at each depth, and so defines a linear intersection 92 with the plane of Figure 5. A section 94 of the focusing collimator also intersects this plane. The section 94 comprises horizontal 94a and vertical 94b collimation sheets which respectively provide vertical and depth specificity.

At any one time, interference arising from scattering from neighbouring horizontal, i.e.

perpendicular to the plane of the fan beam, voxels 90a, 90b, 90c is thus readily avoided by collimating the beam such that only one "line" of voxels 90e, 90a, 90d is illuminated by what is effectively an X-ray line 92 formed by the fan beam. The voxels 90a, 90b, 90c can then be moved relative to the fan beam such that diffraction patterns from neighbouring voxels are separated in time. In order to avoid interference from voxels stacked in the vertical, i.e. within the plane of the fan beam, direction, only a section of each diffraction pattern, of finite height, is accepted by the focusing collimator. This is facilitated by the horizontal collimation sheets 94a.

The invention of WO 96/24863 can be very advantageously combined with the present invention to provide a fast X-ray scanner for detection of explosives and 1 or drugs carried in airport baggage. Each elementalportion of a piece of baggage acts as a scattering centre (voxel) when irradiated by X-rays. The object of an airport scanner is to scan rapidly the volume of the baggage in order first to detect whether or not any prohibited substance is present and secondly, if something is detected, to identify what it is and where within the baggage it is stowed. Each elemental diffraction pattern is therefore analysed for evidence of certain diffraction peaks, characteristic of any anticipated prohibited substance. Identification can be most rapidly achieved by comparison with a look-up table of diffraction patterns of known prohibited materials.

4 0 - 13 As described in WO 96124863, the focusing collimators can be stacked in the vertical direction of Figure 5, to provide simultaneous collimation of diffraction patterns from all voxels at a particular depth illuminated by the fan beam. One 16 of the linear detection arrays of Figure 1 and its associated array of readout circuits 24 is used in this combined system, also stacked in the same direction. Thus each detector system (comprising detector array 16 and readout circuit 24) can be used to detect, at rapid speed, one, or a number of, the spectral series of diffraction patterns generated by each voxel element isolated by the combination of fan-beam illumination 92 and each collimation system 94 of WO 96/24863.

To scan the entire baggage volume, the fan beam is arranged to illuminate one dimension (say height). The baggage is commonly moved by conveyor belt to provide for illumination by the fan beam along its length. The collimation 94 and detection 16, 24 systems are moved relative to the baggage to scan its depth. At each scan point, sufficient information must be collected to identify (in so far as whether or not it is contained in the prohibited substances look-up table) the composition of the scattering voxel. The position of this voxel is identified from the mechanics of the conveyor belt 1 depth scanning system and which detection system registers the pattern. The ability to perform this is well known in the prior art. In order to be able to reduce the scan time of each voxel to a sufficiently short value that the entire baggage throughput of an airport can be scanned with acceptable passenger delay, it is essential to have detectors which are capable of rapidly collecting enough information for composition identification. This is achieved in the present invention by the use of energy-sensitive detectors which obviate the need for monochromation filters and their inherent reduction in X-ray intensity present in the diffraction pattern.

The time needed to collect data from each diffracting element is consequently reduced.

Claims (10)

1. A detection system 60 for detecting quasi-monochromatic or polychromatic X- rays 12 scattered by a target material 14 across an angular range O - Od wherein the detection system 60 comprises an array 16 of semiconducting detector elements 16a, b,... subtending the angular range O - d respectively connected to a corresPonding array 24 of readout channels characterised in that the detector elements 16a are fabricated from a semiconducting material with band gap transport responsive to irradiation by X-rays whereby each detector element 16a generates an electrical response whose magnitude is dependent on incident X-ray energy and each readout channel 24a comprises front-end electronics 44, 46 arranged to transform the semiconductor electrical response to an electrical pulse 72a-h with a parameter representative of response magnitude; discriminating electronics 48, 50 arranged to output a digital signal 74a, c, e, g, h if the pulse parameter lies between first F, and second 62 pre-selected discriminator values and a counter 52 arranged to count the number of digital signals output from the discriminating electronics 48, 50.

2. A detector system 60 according to Claim 1 characterised in that the system 60 includes at least two arrays of readout channels, each semiconducting detector element 16a, b,... being connected to respective readout channels, one from each array, wherein readout channels 24a in the same array have discriminating electronics 48, 50 set to substantially the same first F, and second E2 discriminator values, and which in turn are different from discriminator values &3, E4appropriate to readout channels 80a, 82a in other arrays.

3. A detector system 60 according to Claim 1 or 2 characterised in that it also includes display means arranged to display for each readout array 24 an X- ray scattering pattern 28, 30 derived from position of each detector element 16a, b, c,.... in the array 16 plotted against number of counts 74a registered in respectively connected readout channels 24a.

4. A detection system 60 according to Claim 1, 2 or 3 characterised in that the semiconducting elements 16a, b, c are fabricated from cadmium zinc telluride, gallium arsenide, lead iodide or mercury iodide.

5. A detection system according to any preceding claim characterised in that the angular range O to dis 21 to 80.

6. An X-ray inspection system 10 comprising an X-ray source, a target material to be investigated 14 and a detection system arranged such that a quasi monochromatic or polychromatic X-ray beam generated by the source is scattered from the target material to the detection system characterised in that the detection system comprises the detection system 60 of any one of Claims 1 to 5.

7. An X-ray inspection system 10 according to Claim 6 characterised in that the quasi-monochromatic or polychromatic X-ray beam is collimated into a fan beam in order to illuminate a coplanar two-dimensional array of voxels within the target material and the system 10 includes focusing collimation means arranged to pass only X-rays scattered from a single voxel 14 at one depth and height within the illuminated array to the detection system 60.

8. An X-ray inspection system 10 according to Claim 7 characterised in that the system 10 includes an array of focusing collimation means and a respective array of detection systems, the collimation array members being stacked so as to pass simultaneously to respective detection systems 60 X-rays scattered from respective voxels 14 at different heights within the illuminated voxel array.

9. An X-ray inspection system 10 according to Claim 8 characterised in that the array of collimation means is moveable relative to the target material in the direction of unscattered X-rays in order to enable detection of X-rays scattered from voxels 14 at different depths within the target material.

10. An X-ray inspection system 10 according to Claim 6 characterised in that it includes multiple 16, 18, 20, 22 detection systems 60 symmetrically oriented to 1 1 0 -. '. -.. & 0 1 1 5 & intercept a conical distribution of diffracted X-rays at symmetrically equivalent regions.