A Detection System and Method for Investigating a Content of an Item

Abstract

A detection system and method for investigating a content of an item to be inspected, comprising an inspection space for receiving said item and a neutron generator for generating a directional beam of energetic neutrons, directed towards said inspection space. A detector is responsive to interaction products coming from said inspection space and impinging substantially along a detection axis upon interaction of said energetic particles with nuclei of material of said item. Said neutron generator is configured to expose said inspection space to a uni-directional beam of energetic neutrons along an interrogation axis through said inspection space. Said directional beam has a smaller cross section than a corresponding cross section of said inspection space and smaller than a corresponding cross section of said item to be inspected. Said detector detects said interaction products along a detection axis upon interaction of said uni-directional beam of energetic neutrons with said item to be inspected.

Claims

1. A detection system for investigating a content of an item, the detection system comprising: a particle source comprising a neutron beam generator, configured and arranged for generating a directed beam of neutrons along an interrogation axis toward the item, and detection means comprising at least two gamma ray detectors, configured and arranged to detect gamma ray products of neutron interactions with the item along individual detection axes, wherein said gamma ray detectors are configured and arranged to detect gamma ray products uniquely from individual voxels within the item, and wherein an overlap between two individual voxels is less than 20 percent, particularly less than 10 percent and more particularly less than 5 percent, of a volume of a smallest one of said voxels.

2. A detection system according to claim 1, further comprising an inspection space for accommodating said item, wherein said neutron beam generator is configured to direct said directed beam of neutrons substantially along said interrogation axis crossing said inspection space, said directed beam of neutrons having a cross section that defines a corresponding cross section of said voxels which is smaller, particularly at least said several times smaller, than a corresponding cross section of said inspection space, and wherein said gamma ray detectors are responsive to gamma ray products along said individual detection axes crossing said interrogation axis in consecutive voxels along said interrogation axis to detect gamma ray products from said consecutive voxels.

3. Detection system according to claim 1, wherein said gamma ray detectors comprise a gamma ray detector that is displaceable over individual detection axes.

4. Detection system according to claim 1, wherein said gamma ray detectors comprise adjacent gamma ray detectors in an array of gamma ray detectors that are distributed over said individual detection axes.

5. Detection system according to claim 1, wherein said gamma ray detectors generate electronic signals in response to an exposure to said gamma ray products, wherein said gamma ray detectors are coupled to a data processor receiving said electronic signals from at least said gamma ray detectors, and wherein said data processor is configured to generate a signature out of said electronic signals and to comparing said signature with at least one of stored reference signatures.

6. Detection system according to claim 1, wherein said detection means comprise one or more gamma ray detectors that are arranged opposite said neutron beam generator to detect gamma ray products that passed through said item, and wherein a central axis of each of the voxels associated with said gamma ray detectors lie in a single plane.

7. Detection system according to claim 1, wherein said detection means comprise at least one neutron detector is configured to detect neutrons that have passed through said item.

8. Detection system according to claim 7, wherein said neutron detector is a position sensitive neutron detector.

9. Detection system according to claim 1, wherein said neutron generator is configured to generate a pulsed beam of neutrons.

10. Detection system according to claim 9, wherein said gamma ray detectors are synchronized with said neutron beam generator to detect gamma ray products during a pulse of said pulsed beam of neutrons or in between consecutive pulses of said pulsed beam of neutrons.

11. Detection system according to claim 1, wherein said beam of neutrons comprises at least primarily neutrons having an energy greater than 6 MeV.

12. Detection system according to claim 1, wherein said neutron generator comprising a collimator for creating a fan beam of neutrons around an interrogation axis, wherein at a first distance from the neutron beam generator a first dimension of the fan beam in a first direction perpendicular to the interrogation axis is at least three times larger than a second dimension of the fan beam in a second direction perpendicular to the interrogation axis, the first direction being substantially perpendicular to the second direction.

13. The detection system according to claims 12, wherein a neutron detector is positioned opposite the neutron beam generator having dimensions substantially matching the dimensions of the fan beam at the position of the neutron detector.

14. The detection system according to claim 1, wherein the detection system is configured to move the item through the neutron beam for investigating the whole item.

15. Detection system according claim 14, wherein said inspection space comprises a displaceable support platform for receiving said item to be inspected, wherein said support platform is coupled to drive means that are configured to force said platform into a translation and/or a rotation during an investigation that is controlled by controller means.

16. Detection system according to claim 15, wherein said support platform is suspended for axial displacement along a traverse axis that is substantially perpendicular to said interrogation axis and/or wherein said support platform is suspended for a rotation around said traverse axis, wherein said drive means are configured to force said platform into an axial displacement along said traverse axis and/or said drive means that are configured to force said platform into a rotation around said traverse axis.

17. Detection system according to claim 1, wherein at least one further inspection space is provided along said interrogation axis of said directional beam of neutrons, in line with said first inspection space, said at least one further inspection space accommodating g a further item to be inspected concurrently with said first item to be inspected.

18. Detection system according to claim 17, wherein adjacent inspection spaces are shielded from one another by means of a neutron shield that has a window at said interrogation axis.

19. Detection system according to claim 18, wherein collimator means are provided along said window that are configured to collimate said at least one beam of energetic neutrons along said interrogation axis.

20. Detection system according to claim 1, wherein a pre-inspection space is provided receiving said item to be inspected prior to said inspection space, wherein said item is subjected to a flood inspection at said pre-inspection space.

21. Detection system according to claim 20, wherein said flood inspection comprises at least one of a visual inspection, an X-ray inspection and a beam of neutrons interrogation of said item.

22. Detection system according to claim 18, wherein said pre-inspection space is in line with said inspection space and said item is exposed at said pre-inspection space to said at least one beam of energetic neutrons at a diverged cross section that exposes a corresponding cross section of said pre-inspection space, particularly a corresponding cross section of said item to be inspected.

23. Detection system according to claim 20, characterized by transportation means, particularly comprising a conveyor belt, that carry said item to be inspected through said pre-inspection space and to either said inspection space or an output depending on an inspection outcome of said flood inspection of said item at said pre-inspection space.

24. A detection system according to claim 1, wherein said gamma ray detectors are accommodated in a housing in between collimator walls that collimate said gamma ray products created from the interaction of said item with said beam of neutrons, thereby focusing the subject detector on a particular voxel.

25. A detection system according to claim 24, wherein said gamma ray detectors and said collimator walls are axially displaceable with respect to one another to thereby shifting said detector in between said walls.

26. A detection system according to claim 1, wherein said interrogation axis by said beam of neutrons is inclined with respect to a face of said item under investigation.

27. A detection system according to claim 26, wherein said gamma ray detectors and said item under investigation are movable with respect to one another in a direction parallel to said item.

28. A detection system according to claim 27 wherein said gamma detectors move synchronously with the item under investigation.

29. A detection system according to claim 1, wherein said beam of neutrons emanate from said neutron beam generator through an aperture that is reduced to produce a substantially pencil shaped beam of neutrons.

30. A detection system according to claim 1, wherein said beam of neutrons emanate from said neutron beam generator through an aperture that is increased to produce a flood illumination by said beam of neutrons.

31. A method of non-invasive investigating a content of an item, wherein said item is exposed to a beam of neutrons that interact with material of said item to generate interaction products, wherein said interaction products are detected and analysed by means of processing means, wherein said item is exposed to an at least substantially uni-directional beam of energetic neutrons along an interrogation axis through said item, particularly a fan shaped beam, wherein said at least substantially uni-directional beam is provided with a cross section that is smaller, particularly at least said several times smaller, in at least one direction than a corresponding cross section of said item to be inspected to define a cross section of a voxel of a number of adjacent voxels within said item, wherein said interaction products are detected by means of at least one detector that is focused to a particular voxel to detect said interaction products along at least one detection axis upon interaction of said at least substantially uni-directional beam of energetic neutrons with local material within said voxel of said item to be inspected, and wherein said item is scanned in consecutive stages to cover said adjacent voxels in three cardinal directions along said item.

32. Method according to claim 31, wherein said beam of neutrons is pulsed and delivered as a series of consecutive bunches of energetic neutrons during a pulse time at a repetition rate.

33. Method according to claim 32, wherein said interaction products are detected and analysed during each bunch and/or in between bunches.

34. Method according to claim 31, wherein said item is rotated during inspection around an axis of rotation.

35. Method according to claim 34, wherein said item is translated parallel to, particularly along, said axis of rotation during said inspection.

36. Method according to claim 31, wherein one or more of: elastically scattered neutrons, inelastically scattered neutrons, transmitted neutrons, emitted neutrons and transmitted photons, particularly gamma ray photons, are being detected and analysed as interaction products.

37. Method according to claim 31, wherein several items are inspected concurrently using a common at least one, at least substantially uni-directional beam of energetic neutrons along said interrogation axis.

38. Method according to claim 31, wherein a neutron beam generator is used, comprising a Radio Frequency Quadrupole (RFQ) with an ion source and an target, wherein said ion source generates deuterium ions and said target holds deuterium within a metal.

Description

[0043] Hereinafter the invention will be described in further detail with reference to a number of specific embodiments and a drawing, that will reveal further details, embodiments and variations of the detection system and method according to the invention. In the drawing:

[0044] FIG. 1 shows a schematic setup of a first embodiment of the detection system according to the invention;

[0045] FIG. 2 shows a schematic setup of a further embodiment of the detection system according to the invention;

[0046] FIG. 3 shows a schematic setup of a further embodiment of the detection system according to the invention;

[0047] FIG. 4 shows a schematic setup of an array of gamma ray products detectors along an interrogation axis of a detection system according to the invention;

[0048] FIG. 5 shows a schematic setup of an array of gamma ray products detectors traverse to an interrogation axis of a detection system according to the invention;

[0049] FIG. 6 shows a schematic setup of a further embodiment of the detection system according to the invention;

[0050] FIG. 7 shows a schematic setup of a further embodiment of the detection system according to the invention;

[0051] FIG. 8 shows a schematic setup of a further embodiment of the detection system according to the invention;

[0052] FIG. 9A-C show a schematic setup of a further embodiment of the detection system according to the invention in different stages of operation; and

[0053] FIG. 10 shows a schematic setup of a further embodiment of the detection system according to the invention.

[0054] It should be noted that the figures are drawn purely schematically and not to scale. Particularly, certain dimensions may be exaggerated to a greater or lesser extent with an aid to better understanding the invention. Similar parts of the system are generally denoted by a same reference numeral throughout the drawing.

[0055] FIG. 1 depicts in a side view the basic setup of an embodiment of a detection system according to the invention, hereinafter also briefly referred to as scanner. A parcel P is brought into an inspection space 10 of the system by means of a suitable transportation system T, where it is aligned along an axis of a narrow beam B that is generated by a neutron source N. This beam axis I provides an interrogation axis I along which the parcel P is being inspected. The inspection space 10 is surrounded by a number of detectors in specific locations to detect particular interaction products, along their respective detections axes, that are a result of interaction by the emitted neutrons with the chemical contents of the parcel that is within the beam, i.e. along the interrogation axis.

[0056] The generator N sends one or more thin neutron bunches to the parcel P. These are synced with the gating properties of the detectors DG,DN1 . . . 4. The parcel P is moved through the beam B. The detectors DG,DN1 . . . 4 take measurements along their respective detection axes D of gamma rays generated from inelastic collisions and neutron capture (DG), of neutrons that pass through the parcel (DN1) and of neutrons (back) scattered out of the parcel (DN2,DN3,DN4). The detectors DG,DN1 . . . 4 output their detection signals to a sophisticated Content Analysis System CAS that uses the information from all or some of these detectors to provide a detection response. The system CAS uses deep learning and other classification algorithms, or a combination of these, to determine the chemical composition of a volume area V,1,1 . . . V,4,4 of the parcel that is being scanned, based on reference signatures of known substances that could be suspicious. The parcel P exits at the other side of the scanner and is either cleared for onward travel or diverted to a quarantine area. Note that the expression “parcel” is used through this application to denote any kind of item to be inspected and can equally be used for luggage or standard post.

[0057] When neutrons interact with materials the event can be classed as either scattering or absorption. Scattering is further broken down into elastic and inelastic and absorption can be broken down into electromagnetic (production of a gamma ray), charged (production of a charged particle), neutral (production of one or more neutrons), and fission (atom splits into two or more smaller, lighter nuclei). The depicted system of FIG. 1 comprises a detector DG for the direct measurement of gamma-rays, produced by inelastic scattering or neutron absorption, and one or more detectors DN1 . . . DN4 for the detection of (back) scattered (DN2 . . . DN4) or transmitted (DN1) neutrons to provide information on the content of the investigated object.

[0058] The information of the interaction mechanisms described above provide specific information about the atomic composition of the substance under investigation. Although most elements can be identified in this way, the elements under consideration include, but are not limited to, C, H, O, N, S, Na, Cl, B, Br, Li, F. Furthermore, the imaging of the transmitted neutrons provides additional information about the location of the substances present in the parcel.

[0059] FIG. 1 shows the main configuration of the system. The neutron generator N emits a narrow beam of neutrons along an interrogation axis I towards a parcel P that is in the inspection space. Gamma-rays that are being produced within the parcel are detected by one or more gamma-ray detectors DG. Fast neutrons that pass through the parcel are detected by a fast neutron imaging device DN1. Neutrons that have lost part of their energy through scattering inside the parcel are detected by one or more neutron detectors DN2 . . . DN4.

[0060] The beam that is produced by the neutron source is several times narrower than a corresponding cross-section of the parcel P such that only a portion, or certain portions, of the parcel is being scanned. This will provide localized information of the parcel P relating to a particular, local volume portion, referred to as voxel, of that parcel P. FIG. 1 schematically shows a matrix of sixteen of such volume portions V,1,1 . . . V,4,4 that are in a same plane V of the drawing and that are selectively scanned by the system by moving the parcel P stepwise or continuously through the beam B in all Cartesian directions. Every detector DG,DN1 . . . 4 has its own line of sight, referred to as detection axis D, which is directed towards a particular volume area within the parcel. To be able to discriminate between adjacent voxels the detectors are focused such that an overlap between adjacent voxels is kept below 20% of their volume. In case of different volumes the smallest volume is being taken. Preferably both the lateral and vertical overlap between adjacent voxels is maintained below 10% and more particularly below 5% or avoided anyway.

[0061] The system is self-contained within a surrounding shielding 20 that provides an entrance IN and exit OUT for the parcels P, as shown in top view in FIG. 2. The parcel is being carried and transported by a conveyor belt 30. At the entrance IN and exit OUT, the parcel and a conveyor belt 30 pass around a maze-like extension 25 of the shield 20 that prohibits radiation from escaping from the enclosure. Once past the entrance maze, the parcel is conveyed to the scanning and inspection space 10. Any necessary parcel rearrangement may be carried out between the entrance IN and the scanning area 10. This rearrangement may include repositioning of the parcel P on the belt 30 or rotating it. To achieve optimum positioning of the parcel the system may use information from external sources. This could include a visual image of the parcel or other intelligence.

[0062] Behind the inspection space 10 is a beam stop 40. One of the advantages of using a directed beam of neutrons is that neutron shielding will be easier. The majority of all neutrons that are generated will move in the forward direction towards the parcels after which the beam stop 40 is placed. This beam stop 40 is responsible for slowing down the neutrons as well as absorbing them and the associated secondary radiation. This means that shielding requirements for the overall system can be less stringent than for typical neutron sources that generate neutrons omni-directionally. The beam stop is for instance made of several layers of neutron modelling and neutron absorbing materials.

[0063] As neutrons are scattered and captured they will generate gamma rays. This can occur from any atom in the beam but also from atoms outside the beam that are subsequently hit. Those not from the area of interest may add to the gamma background that is seen by the gamma-ray detector DG and need to be screened out. The conveyor belt 30 is designed to produce a minimal gamma background in the inspection space 10 from its interaction with the beam of neutrons. To reduce the amount of background signal from the conveyor belt, the use of materials with components equal to the ones that are mostly sought after (C, N, O, H) should be avoided. Also, materials that produce secondary radiation with energies close to the ones of the commonly investigated substances should be avoided. This has led to the use of stainless steel and aluminium as preferred materials for the conveyor belt in the scanner area.

[0064] The parcel is moved backwards and forward and up and down as required within the inspection space 10 to provide a complete scan over several individual voxels within the parcel P.

[0065] Alternatively the parcel is moved up and down while being rotated 360 degrees around a vertical axis to provide a complete image.

[0066] Neutrons are generated within the neutron source N by accelerating ions towards a target where, at impact, mainly forward directed neutrons are created to form the beam B. The choice of ion, acceleration energy and target material determine the emitted neutron spatial and energy distribution. The neutron generator is pulsed and produces relatively short, thin, intense bunches of neutrons at a high bunch repetition rate. The accelerator N that is used in this embodiment is based on the use of a Radio Frequency Quadrupole (RFQ), which provides ion bunches in a compact space. To further enhance the quality of the beam of neutrons, a neutron collimator C may be used. This has the additional advantage that shielding of fast neutrons that are emitted within the source N but that are not directed towards the parcel, and hence will not contribute to the parcel scanning process, is done close to the source. This contributes to lower shielding requirements at the peripheral shielding 20 of the system.

[0067] One or more gamma-ray detectors DG are placed above the inspection space 10 accommodating the parcel P. The detector DG measures the energy of gamma rays that impinge on the sensitive detector area. To get depth information about the location of certain materials within the parcel P, either a single detector can move along the z-direction or multiple detectors may be placed in a line or in a pattern. This is indicated in FIG. 3. These detectors DG may be contained within a shield (collimator) 50 so as to limit the cross talk between them and to limit the detection of any background radiation. FIG. 4 (side view) and FIG. 5 (front view) show a possible configuration of a group of detectors DG in a shielded enclosure 50 where all detectors are pointing to a voxel V along the interrogation path I of the neutrons through the Z plane.

[0068] A position-sensitive neutron detector DN1 may be placed in the beam of neutrons B behind the parcel, see FIG. 1. This provides a visual representation of the location of items within the parcel P, similar as to x-ray images. Imaging with neutrons have some distinct advantages compared to x-ray images as neutrons have much better penetration capabilities through dense materials.

[0069] The neutron detector DN1 can be used for multi-energy imaging if the neutron source N facilitates this option. This provides the option to use neutron resonance imaging to determine the fractions of C, N, O, H and indicate the presence and location of explosives and/or drugs in the investigated object P.

[0070] In addition to the transmission imaging, information about the content of the parcel P can also be determined by measuring or imaging neutrons with lower (or even thermal) energies. A likely location for these detectors DN2,DN3 is in line with the end of the generator N, see FIG. 1. These detectors DN2,DN3 measure neutrons that are scattered back in the Z direction. One or more of such neutron detectors DN4 could also be placed for example below or behind the parcel P, see FIG. 1. A gating of the detectors is synchronized to the neutron generator's pulses. The inelastic scattering gamma-ray detection DG and fast neutron imaging DN1 is done during the neutron pulse; the capture gamma-rays and lower energy neutron detection DN2,DN3 are performed off-pulse.

[0071] Time coded information for some or all of the detectors DG, DN1 . . . DN4 is used to provide an analysis of the parcel contents. The classification of the content is done using one or more algorithms, for example classification algorithms such as boosted trees, or by machine learning algorithms, for example based on deep learning, or a combination of multiple algorithms to obtain a higher certainty.

[0072] Initially the algorithm will be trained to look for suspected substances and indicate whether for example a drug or explosive is inside the parcel. This will create reference signatures that may be stored such that later realtime detector information may be compared against these reference signatures. The algorithm will be able to determine with a high certainty which substance and what amount is likely to be present in the investigated object.

[0073] In addition to the above analysis, images can also be created from the gamma detector(s) DG and fast neutron detectors DN1 to highlight the area V that is suspected to contain contraband material. This result in detailed location information about the suspected substance required for faster manual inspection.

[0074] Furthermore, also information from external sources may be used by the analysis algorithm(s). This could include x-ray or visual images of the parcel or other intelligence, which may include shipping information. In addition, the physical properties of the parcel may be used. These may include size, weight, weight distribution and external packaging. The algorithm may be suited (trained) to filter standard, known packaging materials from the output signals.

[0075] By moving the parcel in the X- and Y-direction, either continuously or stepwise, through the narrow beam B consecutive volume areas V,1,1 . . . V4,4 (voxels) may be scanned individually in the above described manner to be searched for illicit materials. Instead of scanning each parcel thoroughly, an alternative approach would be to initially flood illuminate every parcel and to determine the resulting gamma-ray spectrum. In case there are no indications for illicit goods, the parcel can move directly to the exit. Only if the flood illumination highlights materials of interest the parcel is scanned more closely with a narrow beam as described hereinbefore. This principle is depicted in FIG. 6.

[0076] Advantageously such flood exposure is given by means of fast neutrons that passed through the inspection space 10. To that end this embodiment provides an further inspection space 11 that is inline with the first inspection space 10 to be exposed to these transmitted neutrons. The second inspection space 11 is also equipped with one or more gamma ray detectors DG and neutron detectors (not shown) to provide information on the general contents of the entire parcel P. A conveyor belt T carries the parcel(s) P first through the second inspection space. If no suspicious contents is detected the parcel may continue directly to the exit. In the other case it will be shifted to a further transportation mechanism T1 that will carry and/or manipulate the parcel in the first inspection space 10 to obtain a detailed scan by the narrow beam B over consecutive partial volume areas (voxels).

[0077] The main advantage of this approach is that, depending on the number of parcels that need to go through the detailed screening, the system can operate at much higher speed than when every parcel needs to be fully scanned. A buffer area T2 to hold parcels waiting for the more detailed scan may also be provided. A further addition could be to add a conventional X-ray machine to the setup to do a pre-scan of the parcel and preselect items of interest.

[0078] Another way in which the screening speed can be increased is to simultaneously scan multiple parcels with one and the same beam of neutrons B. A significant proportion of the beam of neutrons B will not interact with a parcel P that is placed in the inspection space 10. Instead this portion of the beam will continue its path along the interrogation axis I and may be used to scan one or more parcels P in consecutive inspection spaces 11,12,13 that are aligned along said axis as shown in FIG. 7.

[0079] A single neutron generator N may be used in this manner to scan several parcels simultaneously or within quick succession. The parcels P are carried by separate conveyor belts T1,T2,T3,T4 and can be moved through the beam sequentially, all at the same time or with a random pattern. Parcel sizes may vary significantly and likewise also a total scan time to search the entire parcel.

[0080] To decrease cross talk in some implementations, a layer of shielding S may be placed between consecutive inspection spaces 10 . . . 14 with consecutive conveyor belts T1 . . . T4. This shielding S comprises a small aperture or slit for the beam B to pass through. This aperture may also act as a collimator.

[0081] Each parcel P may be scanned by translating the parcel in two Cartesian directions through the beam B; for instance left-right and up-down. An alternative to such scanning left-right of a parcel would be to rotate the parcel through the beam on a platform that is moved up or down during the rotation. The total parcel may be scanned in this manner in a single continuous movement, thus avoiding many start-stop actions that may be associated with left-right scanning. This provides similar information on the content of the parcel as with a left-right scanning technique.

[0082] FIG. 8 shows a further embodiment of the detection system according to the invention. The detection system comprises a neutron generator N delivering a directional beam B of energized neutrons along an interrogation axis I. This beam crosses a parcel P to be investigated. To that end the system comprises an array of adjacent gamma ray detectors DG1-DG4 at opposite sides of the parcel P. The detectors DG1-DG4 are accommodated within individual enclosures in between collimating walls 80. These walls 80 limit the detection aperture, defined by the lines of sight L1-L4 of the individual detectors DG1-DG4. As a result the detectors DG1-DG4 are focused to receive gamma ray products from particular voxels V1-V3 only, that are hence uniquely associated with a specific detector DG1-DG4. As shown in the figure the detectors DG1-DG4 and their lines of sight L1-L4are laid out such that the individual voxels V1 show hardly no overlap OV with one another in order to provide spatial resolution to the information obtained from the output signals of the individual detectors DG1-DG4. In accordance with the present invention said overlap OV is maintained below 20% of the volume of the smallest voxel V1-V4 being inspected concurrently.

[0083] FIG. 9A-9C show a further embodiment of a detection system according to the invention. To ensure an overlap of less than 20% between adjacent voxels V-V4 is subjacent layers of the parcel P and to maintain a fixed voxel size (volume), the gamma ray detectors DG are suspended axially displaceable within their respective collimating enclosures 80. The embodiment of FIGS. 9A-9C comprises an array of four adjacent gamma ray detectors DG that may shift in between the collimating walls as indicated by the arrow in the figure.

[0084] In a first stage of operation the detectors are situated below in their enclosure to confine their lines of sight L to a relatively small detection aperture. In this stage shown in FIG. 9A, the top level of the parcel P is being scanned over a number of adjacent voxels V1-V4. The array of detectors DG is shown in only one direction along the interrogation axis of the neutron generator N. However, like in the preceding embodiment the array may be extended perpendicular to the plane of the paper to provide a matrix that covers similar voxels in the same XY plane.

[0085] To scan voxels in subjacent layers the neutron beam generator N and the parcel are moved with respect to one another in the Z-direction as shown in FIG. 9B. Concurrently the detectors

[0086] DG are shifted upwards in their collimating housing to increase their detection aperture in order to maintain a same voxel size in this lower level of the parcel P.

[0087] Finally the detectors may be moved entirely upwards as shown in FIG. 9C to scan the lowest voxel layer in the parcel P, again maintaining substantially a same voxel sight and avoiding substantial mutual overlap between adjacent voxels.

[0088] FIG. 10 shows a further embodiment of a detection system according to the invention. In thus case the neutron beam B is not directed perpendicularly to a face of the parcel B under investigation, but instead under an inclined angle a. This allows the beam B to travel diagonally through the parcel as it is being investigated, thereby crossing several subjacent layers of voxels with the parcel, The parcel P is movable in the axial direction indicated by the arrow in the figure to cover all voxels in the XY-plane. As shown in the figure the neutron beam generator N is provided with a neutron collimator C with a reduced output aperture to produces a pencil or fan shaped directional beam of neutrons.

[0089] The neutron source N may be configured to generate beams of neutrons at multiple energies. These neutrons may be used for fast neutron resonance imaging. By carrying out the imaging at multiple energies different elements may be highlighted in scanned locations. This may be added to the detection algorithm. Neutrons may be generated within a neutron source N by accelerating ions towards a target where, at impact, mainly forward directed neutrons are created to form a beam of neutrons B. The choice of ion, acceleration energy and target material determine the emitted neutrons' spatial and energy distribution.

[0090] The neutron source of the preceding embodiments is based on a deuterium-deuterium reaction in a target that holds deuterium in a metal. When the neutrons are produced, some will react with said metal to generate x-rays. By addition of one or more (secondary) detector panels in the line of the beam behind the inspection space, these x-rays may be used for x-ray imaging. The remaining neutrons leave the target. A mainly forward directed beam may be sculptured and collimated to have a relatively narrow footprint to produce a substantially uni-directional, relatively narrow beam to be employed according to the present invention.

[0091] The neutron source uses a Radio Frequency Quadrupole (RFQ), which provides ion bunches in a compact space. The RFQ neutron source comprises a ion source of deuterium which is emitted in pulses. If necessary the ions are fed through low energy beam elements so that the bunch can be accepted by an accelerator. The accelerator accelerates the ions in a vacuum and, because it is an RFQ, makes the bunches smaller. At the end of the accelerator, or at a short distance from it but still under vacuum, the beam collides with a target. This causes a fusion reaction within the target that produces and releases neutrons. By increasing the ion beam energy, these neutrons will be produced at a higher yield and/or at a higher energy. An alternative to is to use different target materials. By rotating the different target materials, one can quickly move from one energy to the next. Another alternative would be to dynamically moderate the beam of neutrons. By inducing certain amounts of material in the beam, the neutron energy of the emitted neutrons will decrease to lower values.

[0092] Although the invention has been described hereinbefore with reference to merely a few particular embodiments, it will be appreciated that the invention is by no means limited to these embodiment. On the contrary, to a person of ordinary skill many more embodiments and variations of the present invention are feasible within the framework of the invention without requiring any inventive skill.