Muon imaging for monitoring changes in an object

Abstract

Muon imaging for change monitoring of an object involves obtaining a two-dimensional (2D) radiographic projection of the object using a vectorless muon radiography detector, determining a discrepancy

Description

Field of Invention The present invention relates to a method of and apparatus for muon imaging for change monitoring of an object, for example, monitoring degradation of concrete structures or formation of voids within the object. The present invention especially relates to a two-stage method of muon imaging to provide efficient long-term change monitoring of an object. Background of the Invention Muon radiography is an imaging technique for inspecting properties of matter. A cosmic-ray muon passing through matter may be altered by the internal composition of the matter. By measuring the muon as it exits the matter, the nature of any changes to the internal composition can be investigated, for example, changes in composition and density. Typically, in muon tomography a Muon Scattering Tomography (MST) detector can perform tomography of the matter under inspection based on scattering of cosmic ray muons by the internal structure of the matter. A three-dimensional (3D) MST image reconstruction can be used to identify and locate specific changes that have occurred within the matter. Muon imaging is a generic term that encompasses both imaging modalities, of muon radiography, and muon tomography. Figure 1 illustrates, in schematic form, a known MST detector 100 for monitoring matter 102. The apparatus 100 comprises upper and lower muon detectors above 104 and below 110 the matter 102, respectively. In this example, the upper muon detector 104 comprises an angle-sensitive set of two planes of position-sensitive muon detectors 106 and 108 configured to measure the trajectories of incoming muons entering the matter 102. The lower muon detector 110 comprises an angle-sensitive set of two planes of position-sensitive muon detectors 112 and 114 configured to measure the trajectories of outgoing muons scattered by the matter 102 in three-dimensional (3D) space. In this example, each of the planes (106,108,112 and 114) has two orthogonal layers of arrayed scintillating fibres along the x and y axes, which can detect the coordinates of the incoming and outgoing muons in two dimensions (e.g. x and y). With a known vertical separation 120 between the planes of the upper (104) and lower (110) muon detectors, a 3D trajectory can be calculated from the muon detection coordinates 122 (x1, y1) and 124 (x2, y2). With this configuration, changes in structure and composition of the matter 102 can be determined based on the scattering angle between incoming 116 and outgoing 118 muon tracks. A high-resolution 3D MST image of the matter can be constructed using the measured trajectories. The 3D MST image has a high spatial resolution of up to approximately 1mm, which allows the internal composition of the matter to be accurately characterised. Moreover, once changes have been detected in a 3D MST image, there is a need to monitor these changes over time by deploying the MST detector to reconstruct another 3D MST image of the matter at a different time, particularly when dealing with rapidly changing radioactive materials. However, the 3D MST image reconstruction of a large-scale object is an expensive (costing approximately £1.2 million per MST detector) and time-consuming task. Furthermore, it is required that the MST detector is placed around the object, or that the object is transported to the detector. The large dimensions of the MST detector or the object will make transportation difficult and cumbersome. In addition, human personnel are required to carry out the task, further increasing the logistical requirements for monitoring. Therefore, it is impractical to deploy the MST detector to perform such tasks very frequently or for long-term purposes. Summary of invention It is desirable to have a method and associated muon radiography apparatus which allow long-term monitoring of an object, in a cost-effective manner. It is also desirable to provide a cheaper, smaller and simpler muon radiography detector that can be embedded and remain in place for extended periods of time to enable longterm change monitoring of an object. According to a first aspect of the present invention, there is provided a method of muon imaging for change monitoring of an object, the method comprising the steps: - obtaining a two-dimensional (2D) radiographic projection of the object using a vectorless muon radiography detector; - determining a discrepancy between the 2D radiographic projection of the object and a reference data representation of the object; - in response to the discrepancy, making a determination of whether to inspect the object using a muon tomography detector to obtain a three-dimensional (3D) tomographic data representation of the object. Preferably, the method comprises the step of generating the 3D tomographic data representation of the object using the muon tomography detector, based on the determination. Preferably, the 3D tomographic data representation of the object is a 3D tomographic image. Preferably, the 2D radiographic projection is produced by a convolution of signals from muons that have passed through the object from a plurality of different angles and have been scattered by the object. Preferably, the reference data representation of the object is generated using the vectorless muon radiography detector. Preferably, the vectorless muon radiography detector is a scintillator-based detector. Preferably, the scintillator-based vectorless muon radiography detector is embedded in a location proximal to the object. Preferably, when the object is contained in a waste container, the scintillator-based vectorless muon radiography detector is located inside the container. Preferably, when the object is contained in a waste container, the scintillator-based vectorless muon radiography detector is embedded into a container body of the waste container. Alternatively, the vectorless muon radiography detector is a gas-based detector. Preferably, the vectorless muon radiography detector comprises a single-surface sensor for detecting muons that have passed through the object from a plurality of different angles and have been scattered by the object. Preferably, the single-surface sensor is a single planar sensor aligned with a horizontal plane of the object. Preferably, the single-surface sensor has a spatial resolution of coarser than that of the muon tomography detector. Preferably, obtaining the 2D radiographic projection comprises using a detector assembly, wherein the detector assembly comprises a plurality of vectorless muon radiography detectors. Preferably, the plurality of vectorless muon radiography detectors of the detector assembly are arranged in an array. According to a second aspect of the present invention, there is provided a computer program product comprising a computer usable medium, where the computer usable medium comprises a computer program code that, when executed by a computer apparatus, determining the discrepancy according to the method of the first aspect. According to a third aspect of the present invention, there is provided an apparatus for muon imaging for change monitoring of an object, the apparatus comprising: - a muon radiography system configured to: - obtain a 2D radiographic projection of the object using a vectorless muon radiography detector; and - a processor configured to: - determine a discrepancy between the 2D radiographic projection of the object and a reference data representation of the object; and - in response to the discrepancy, make a determination of whether to inspect the object using a muon tomography detector to obtain a 3D tomographic data representation of the object. Preferably, the apparatus comprises the muon tomography detector to obtain the 3D tomographic data representation of the object, based on the determination. Preferably, the vectorless muon radiography detector comprises a single-surface sensor for detecting muons that have passed through the object from a plurality of different angles and have been scattered by the object. Preferably, the single-surface sensor is a single planar sensor aligned with a horizontal plane of the object. Preferably, the single-surface sensor has a spatial resolution of coarser than that of the muon tomography detector. Preferably, the vectorless muon radiography detector is a scintillator-based detector or a gas-based detector. Preferably, the apparatus comprises a detector assembly comprising a plurality of vectorless muon radiography detectors. Preferably, the muon tomography detector comprises a plurality of single-surface sensors arranged on one side of the object and a plurality of single-surface sensors arranged on an opposite side of the object. Brief description of drawings Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which: Figure 1 illustrates, in schematic form, a cross-sectional view of a known Muon Scattering Tomography (MST) detector. Figure 2 illustrates, in schematic form, a cross-sectional view of a vectorless muon radiography detector, for use in accordance with an embodiment of the present invention. Figures 3a and 3b illustrate, in schematic form, 2D radiographic images of an object obtained at points T-1 and T in time, respectively, obtained using the vectorless muon radiography detector of Figure 2. Figures 4a and 4b illustrate, in schematic form, cross-sections of a waste container, wherein the vectorless muon radiography detector of Figure 2 is embedded in the container body. Figures 5a and 5b illustrate, in schematic form, cross-sections of a waste container, wherein a detector assembly comprising a plurality of the vectorless muon radiography detectors of Figure 2 is embedded in the container body. Figure 6 is a flowchart of a method of muon radiography for change monitoring of an object, in accordance with an embodiment of the present invention. Figure 7 illustrates, in schematic form, an apparatus of muon radiography for change monitoring of an object, in accordance with an embodiment of the present invention. Description of embodiments In the Figures, elements labelled with reference numerals found in the preceding Figures represent the same elements as described for the respective preceding Figure. For example, feature 202 in Figure 4a corresponds to the same feature 202 as described with reference to Figure 2. Features described in the context of an embodiment may be provided separately or in any suitable combination to provide other embodiments. Figure 2 illustrates a vectorless muon radiography detector 202 for change monitoring of an object 201. Vectorless is defined to mean that measurements from a vectorless detector are of positions, but not directions, of the muons that have passed through the detector. In this example, the vectorless muon radiography detector 202 comprises a singlesurface sensor for detecting muons that have passed through the object from a plurality of different angles and have been scattered by the object. In this example, the single-surface sensor is a single planar sensor aligned with a horizontal plane of the object, which may be any one of the planes (106, 108,112 and 114) illustrated with reference to Figure 1. In use, the vectorless muon radiography detector 202 is positioned at a location in close proximity to the object 201 to ensure an optimal sensitivity of the detector with respect to the object 201. It will be understood by the person skilled in the art how to best deploy the detector. The vectorless muon radiography detector 202 may be placed above the ground, in-ground or under the ground in close proximity to the object 201. In an example as illustrated in Figure 2, the vectorless muon radiography detector 202 is positioned below the object 201. In another example, the vectorless muon radiography detector 202 is positioned at a side of the object 201. In embodiments, for example, as shown in Figures 1 to 4, the vectorless muon radiography detector 202 is a scintillator-based detector. In another example, the vectorless muon radiography detector 202 is a gas-based detector. Advantageously, the vectorless muon radiography detector may be placed inside or on a bridge to monitor the formation of voids or degradation of steel reinforcement bars, forming a “smart bridge”. Preferably, the vectorless muon radiography detector can be embedded in structures permanently or for an extended period of time to provide continuous monitoring. According to an embodiment of the present invention, the scintillator-based vectorless muon radiography detector is at least partially embedded in a container body of a waste container. According to embodiments of the present invention, the vectorless muon radiography detector 202 can be of any length, width or depth dimensions, which may be indicated by applications the detector is used for or upon request by end users. Preferably, the vectorless muon radiography detector 202 has a typical weight approximately 25 percent of the mass of a typical MST detector (e.g. the detector 100 of Figure 1) for detecting a similar active area. A two-dimensional (2D) radiographic projection of the object 201 can be obtained using the vectorless muon radiography detector 202. The 2D radiographic projection is a convolution of signals from muons that have passed through the object 201 from a plurality of different angles, for example 204, 206 and 208, and have been scattered by the object 201. The 2D radiographic projection can be in a variety of forms. In an example, the 2D radiographic projection corresponds to raw muon detection data obtained directly from the vectorless muon radiography detector 202. In another example, the 2D radiographic projection refers to a 2D radiographic image produced from the muon detection data. Figures 3a and 3b illustrate, in schematic form, 2D radiographic images of an object 302 captured at two different points of time T-1 and T, respectively. These 2D radiographic images have a comparatively coarse spatial resolution relative to 3D MST images. Preferably, the scintillator-based vectorless muon radiography detector has a spatial resolution of between 1cm and 10cm, which is coarser than that of the known MST detector 100 (e.g of approximately up to 1 mm). Alternatively, the gasbased vectorless muon radiography detector can have a spatial resolution as fine as 50pm. Advantageously, 2D radiography images captured by the vectorless muon radiography detector have significantly more statistics compared to 3D MST images. This is due to the fact that any muon that hits the vectorless muon radiography detector contributes to the 2D radiographic images, whereas for 3D MST images only those muons known to pass through all detectors (e.g. 106, 108, 112 and 114) will be taken into account. Advantageously, the vectorless muon radiography detector 202 is significantly cheaper than the MST detector of Figure 1 and takes a much shorter time to form a 2D radiographic image than a 3D MST image reconstruction of the object. The vectorless muon radiography detector 202 provides rapid detection of changes in the object based on differences between two subsequent 2D radiographic projections of the object. Factors able to indicate that a change has occurred in a region of the object comprise changes in outline of the region and material density of the region or across the entire object. An example of a change in outline could be an increase or decrease in area of the region or changes in physical shape (e.g. removal of a part of the object). An example of a change in material density refers to either a bulk density change across the entire object, or a density change in a localised region of the object. These changes could indicate oxidation (volume expansion and density reduction) of waste inside the container, deformation in the container walls, pitting or porosity forming in the concrete shielding, etc. Referring to Figures 3a and 3b, it can be seen that changes in both area and shape have occurred in a lower region (304 at point T-1 in time and 306 at point T in time) of the object 302. Although the 2D radiographic images cannot be used to exactly localise the change in 3D space, or to determine the exact nature of the change, useful information can still be extracted. For example, when inspecting contents of a nuclear waste container, qualitative trends in the rate of change of area can be determined based on the differences between sequential coarse-resolution 2D radiographic images. This allows a determination of how rapidly the contents are degrading prior to further detailed inspection using the high-resolution MST detector. Advantageously, the coarse-resolution 2D radiographic images can be used to guide the use of the high-resolution MST detector on the relevant region of interest containing the changes. Figure 4a illustrates, in schematic form, a cross section of a waste container, wherein a container body of the waste container comprises side walls 402 and a bottom wall 406 defining an internal volume 410 for receiving hazardous contents 404. The waste container of Figure 4 is a “smart” waste container, wherein the vectorless muon radiography detector 202 is fully embedded in the bottom wall 406, forming an integral part of the waste container. This makes it possible to achieve long-term monitoring of the hazardous contents 404 contained within the waste container. In this example, the detector 202 is configured with particular dimensions, wherein preferably its dimension along the x-axis 408 is at least equal to the width of the hazardous contents 404, and arranged in such a way that it can detect muons that have been scattered due to passing through the hazardous contents 404 from a sufficiently wide range of angles. In another example (not shown), the vectorless muon radiography detector 202 is partially embedded in the container body. In yet another example, the vectorless muon radiography detector 202 is located inside the internal volume 410 and arranged under or at a side of the hazardous contents 404. With reference to the cross section of Figure 4b, the vectorless muon radiography detector 202 is embedded in the side wall 402 of the container, according to an embodiment of the present invention. From this position, the detector 202 detects muons at a smaller range of angles compared to the arrangement of Figure 4a, because of the blocking effect of the earth below the horizon. Accordingly, the detector 202 is configured with dimensions, often longer along the z-axis, and arranged in such a way that it can compensate for the reduction in the range of angles accepted due to the position. According to another embodiment of the present invention, a plurality of vectorless muon radiography detectors are arranged in a detector assembly so as to perform change monitoring of a large-scale object where the use of a single MST detector would be infeasible due to reasons of cost and scale. Measurements collected from individual detectors forming the detector assembly can then be combined to generate a 2D radiographic image of the entire large-scale object. Figure 5a illustrates, in schematic form, a vertical cross section of a large-scale waste container containing a larger object 504, wherein a detector assembly 502 is fully embedded in the bottom wall 406 of the container. The detector assembly 502 comprises three vectorless muon radiography detectors 506, 508 and 510 in accordance with embodiments of the present invention. In this example, the detectors 506, 508 and 510 are arranged in a linear array and are interconnected with one another. Figure 5b illustrates, in schematic form, a vertical cross section of the waste container containing a larger object 504, wherein a detector assembly 512 is fully embedded in the side wall 402 of the container. The detector assembly 512 comprises two vectorless muon radiography detectors 514 and 516 in accordance with embodiments of the present invention. In this example, the detectors 514 and 516 are arranged in a vertical array along the side wall 402 and are interconnected with one another. In this arrangement, the detector assembly 512 can further compensate, relative to a single detector as shown in Figure 4b, for the reduction in the range of angles accepted due to the detector position. Consequently the resolution of the resulting 2D radiographic image of the object is increased. In another example (not shown), the detector assembly 502, 512 is partially embedded in the bottom or side wall of the container. In yet another example, the detector assembly 502, 512 is located inside the internal volume 410, or outside the container, in close proximity to the object 504. In use, individual detectors of a detector assembly (e.g. 502 and 512) could simultaneously or sequentially collect muon detection data from a range of angles. In this example, the detectors 506, 508, 510, 514 and 516 are scintillator-based detectors. In another example, the detectors 506, 508, 510, 514 and 516 are gasbased detectors. In yet another example, the detector assembly 502, 512 comprises a combination of scintillator-based detectors and gas-based detectors. Figure 6 illustrates a flowchart of a method of muon radiography for change monitoring of an object, in accordance with an embodiment of the present invention. The method has the following steps. At step 602, at a particular point in time T, a 2D radiographic projection of the object is obtained using the vectorless muon radiography detector, as described with reference to Figure 2 to Figure 5. At this step, the vectorless muon radiography detector is configured to measure positions, but not directions, of the muons that have passed through the object from a plurality of different angles and have been scattered by the object. At step 604, a discrepancy between the 2D radiographic projection of the object and a reference data representation of the object is determined. The reference data representation of the object may be a 2D radiographic image of the object produced by the vectorless muon radiography detector, or a 3D tomographic data representation of the object produced by a high-resolution MST detector, at a previous point in time T-1. At this step, the reference data representation of the object may also refer to a naturallanguage text data, for example, “it is a square-shaped object”. This type of data format is possible if there is prior knowledge with respect to the structure or composition of the object, which can be obtained from, for example, design drawings or blueprints of the object. In this case, comparison of the external appearance (outline) of the object detected in the 2D radiographic image to the shape defined in the natural-language text data makes it possible to determine if changes have occurred. In any case, the 2D radiographic projection obtained at time T will become the reference data representation for the purpose of comparing against a future 2D radiographic image obtained at a subsequent point in time T+1. In an example, steps 602 and 604 are performed according to an automatic timebased schedule, for example, at a predefined number of points in time. In another embodiment, steps 602 and 604 are performed according to a demand-based schedule, for example an administrator of the object manually initiates the steps. In another example, the vectorless muon radiography detector is configured to operate continuously, so that any discrepancy between subsequent 2D radiographic images can be detected. At step 606, in response to the discrepancy determined at step 604, a decision is made with respect to whether or not there is a need to further inspect the object using a high-resolution MST detector to obtain a three-dimensional (3D) tomographic data representation of the object. This 3D tomographic data representation of the object may be compared with a previous 3D tomographic data representation of the object that was obtained at an earlier time. This step 606 comprises first making a decision as to whether the discrepancy determined at step 604 is an actionable discrepancy. In an example, a set of predefined criteria is used to guide the administrator of the object to make the decision, wherein the predefined criteria refer to the appearance, degree and coarse location of the discrepancy. If there is an actionable discrepancy, a confirmation notification will be issued associated with reliability metrics to ensure that the administrator is aware of the confidence level of the change that has occurred. An actionable discrepancy would refer to the discrepancy caused by an actual change of the object rather than by an irrelevant circumstance. An example of an actionable discrepancy would be the formation of a new void within the object. On the other hand, an example of a non-actionable discrepancy would be, for example, where the vectorless muon radiography detector is embedded inside a bridge to monitor a support beam, changes in subsequent 2D radiographic images of the support beam are attributed to changes due to a heavy lorry sitting above the detector for a period of time due to breakdown. Therefore, relevant knowledge of the administrator will be needed in interpreting the detected 2D radiographic images of the object so as to determine whether the discrepancy is an actionable discrepancy. At this step 606, an alert may be generated to inform the administrator that the discrepancy determined at step 604 is an actionable discrepancy. In response to the actionable discrepancy, an appropriate course of action will be taken by the administrator to investigate the change underlying the discrepancy. The administrator may decide to deploy the high-resolution MST detector immediately to comprehensively inspect the object in order to understand the nature of the change underlying the actionable discrepancy, or alternatively, delay the deployment of the high-resolution MST detector to a later date. This step 606 further comprises using other inspection tools (e.g. X-ray, ultrasound, visual inspection) to study the nature of the change before deciding whether inspection using the high-resolution MST detector is needed. Additionally, Al or machine learning techniques can also be used to facilitate the interpretation of the 2D radiographic images (e.g. Figures 3a and 3b) obtained from the vectorless muon radiographic detector, which can further improve knowledge as to the nature of the change prior to the deployment of the high-resolution MST detector. If there is a need to use a high-resolution MST detector to comprehensively inspect the object, at step 608, the known MST detector of Figure 1 may be used to obtain a 3D tomographic data representation of the object. In another example, a different high-resolution MST detector may be used, wherein each of the upper 104 and lower 110 muon detectors comprises an angle-sensitive set of more than two planes of positionsensitive muon detectors for measurement of positions and angles of incoming and outgoing muons, respectively. In yet another example, a high-resolution MST detector used for obtaining the three-dimensional (3D) tomographic data representation of the object comprises a plurality of single-surface sensors arranged on one side of the object and a plurality of singlesurface sensors arranged on an opposite side of the object. At step 608, in an example, the 3D tomographic data representation of the object is a 3D tomographic image. The method 600 according to embodiments of the present invention uses detectors of different spatial resolution at different stages of a two-stage process to achieve longterm monitoring of an object. The relatively low-resolution vectorless muon radiography detector ensures safety of the deployment of the object by providing an early-warning system, whilst being more cost-effective than using only a high-resolution MST detector for all stages of monitoring. Such two-stage process has particular application to long-term monitoring of hazardous contents of a waste container (e.g. a nuclear waste container). Figure 7 illustrates an apparatus of muon radiography for change monitoring of an object, wherein the apparatus comprises a muon radiography system 702 configured to obtain a 2D radiographic projection of the object using a vectorless muon 5 tomography detector; and a processor 704 configured to a) determine a discrepancy between the 2D radiographic projection of the object and a reference data representation of the object; and b) in response to the discrepancy, determining whether to inspect the object using a MST detector to obtain a 3D muon tomographic data representation of the object. The apparatus may be used in accordance with the 10 method described with reference to Figure 6. A computer program product 706 is illustrated in Figure 7. It comprises a computer usable medium, in this example a memory card, where the computer usable medium comprises a computer program code that, when executed by a computer apparatus 15 (such as processor 704), performs change monitoring according to the method described with reference to Figure 6.

Claims

1. A method of muon imaging for change monitoring of an object, the method comprising the steps:- obtaining a two-dimensional (2D) radiographic projection of the object using a vectorless muon radiography detector;- determining a discrepancy between the 2D radiographic projection of the object and a reference data representation of the object;- in response to the discrepancy, making a determination of whether to inspect the object using a muon tomography detector to obtain a three-dimensional (3D) tomographic data representation of the object.

2. The method of claim 1, further comprising the step of generating the 3D tomographic data representation of the object using the muon tomography detector, based on the determination.

3. The method of claim 1 or claim 2, wherein the 3D tomographic data representation of the object is a 3D tomographic image.

4. The method of any preceding claim, wherein the 2D radiographic projection is produced by a convolution of signals from muons that have passed through the object from a plurality of different angles and have been scattered by the object.

5. The method of any preceding claim, wherein the reference data representation of the object is generated using the vectorless muon radiography detector.

6. The method of any preceding claim, wherein the vectorless muon radiography detector is a scintillator-based detector.

7. The method of claim 6, wherein the scintillator-based vectorless muon radiography detector is embedded in a location proximal to the object.

8. The method of claim 7, wherein, when the object is contained in a waste container, the scintillator-based vectorless muon radiography detector is located inside the container.

9. The method of claim 7, wherein, when the object is contained in a waste container, the scintillator-based vectorless muon radiography detector is embedded into a container body of the waste container.

10. The method of any of claims 1 to 5, wherein the vectorless muon radiography detector is a gas-based detector.

11. The method of any preceding claim, wherein the vectorless muon radiography detector comprises a single-surface sensor for detecting muons that have passed through the object from a plurality of different angles and have been scattered by the object.

12. The method of claim 11, wherein the single-surface sensor is a single planar sensor aligned with a horizontal plane of the object.

13. The method of claim 11 or claim 12, wherein the single-surface sensor has a spatial resolution of coarser than that of the muon tomography detector.

14. The method of any preceding claim, wherein the step of obtaining the 2D radiographic projection comprises using a detector assembly, wherein the detector assembly comprises a plurality of vectorless muon radiography detectors.

15. The method of claim 14, wherein the plurality of vectorless muon radiography detectors of the detector assembly are arranged in an array.

16. A computer program product comprising a computer usable medium, where the computer usable medium comprises a computer program code that, when executed by a computer apparatus, determining the discrepancy according to the method of any of claims 1 to 15.

17. An apparatus for muon imaging for change monitoring of an object, the apparatus comprising:- a muon radiography system configured to:- obtain a 2D radiographic projection of the object using a vectorless muon radiography detector; and- a processor configured to:- determine a discrepancy between the 2D radiographic projection of the object and a reference data representation of the object; and - in response to the discrepancy, make a determination of whether to inspect the object using a muon tomography detector to obtain a 3D tomographic data representation of the object.

18. The apparatus of claim 17, further comprising the muon tomography detector to obtain the 3D tomographic data representation of the object, based on the determination.

19. The apparatus of claim 17 or claim 18, wherein the vectorless muon radiography detector comprises a single-surface sensor for detecting muons that have passed through the object from a plurality of different angles and have been scattered by the object.

20. The apparatus of claim 19, wherein the single-surface sensor is a single planar sensor aligned with a horizontal plane of the object.

21. The apparatus of claim 19 or claim 20, wherein the single-surface sensor has a spatial resolution of coarser than that of the muon tomography detector.

22. The apparatus of any of claims 17 to 21, wherein the vectorless muon radiography detector is a scintillator-based detector or a gas-based detector.

23. The apparatus of any of claims 17 to 22, further comprising a detector assembly comprising a plurality of vectorless muon radiography detectors.

24. The apparatus of any of claims 17 to 23, wherein the muon tomography detector comprises a plurality of single-surface sensors arranged on one side of the object and a plurality of single-surface sensors arranged on an opposite side of the object.

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