EP3224655A1 - Pulsed backscatter imaging apparatus and method - Google Patents

Pulsed backscatter imaging apparatus and method

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Publication number
EP3224655A1
EP3224655A1 EP15800886.2A EP15800886A EP3224655A1 EP 3224655 A1 EP3224655 A1 EP 3224655A1 EP 15800886 A EP15800886 A EP 15800886A EP 3224655 A1 EP3224655 A1 EP 3224655A1
Authority
EP
European Patent Office
Prior art keywords
electrons
inspecting
imaging apparatus
backscatter imaging
backscattered
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15800886.2A
Other languages
German (de)
French (fr)
Inventor
Robert Menzies DEAS
David NEELY
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
UK Secretary of State for Defence
Original Assignee
UK Secretary of State for Defence
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by UK Secretary of State for Defence filed Critical UK Secretary of State for Defence
Publication of EP3224655A1 publication Critical patent/EP3224655A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
    • G01V5/20—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
    • G01V5/22—Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays
    • G01V5/222—Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays measuring scattered radiation

Definitions

  • the invention relates to a single-sided imaging apparatus and method, and more specifically to a pulsed backscatter imaging apparatus and method capable of producing a spatially resolved image of an object across an increased stand-off range or greater penetrative depth.
  • X-ray imaging systems are well known for producing high resolution images of hidden objects in non-invasive examination roles such as inspection, diagnosis and detection and are commonly used in industries such as medicine, security, defence and exploration.
  • X-rays transmitted " through an object, to a detector on the far side can be used to obtain good quality images of structures within the object.
  • Transmitted X-rays interact with matter through photoabsorption, inelastic scattering and elastic scattering. The strength of these interactions is dependent on the energy of the X-rays and the composition i.e. atomic number and density of the material of the structures.
  • X-rays which are scattered by objects can also be utilised to form an image.
  • An advantage of using backscattered X-rays is that the detector is located on the same side of the object as the X-ray source, allowing X-ray imaging in situations where only one side of the object is available for inspection i.e. single sided imaging.
  • X-rays are more likely to scatter in the forward direction and this likelihood increases with increasing energy. Whilst X-rays having energies in excess of lMeV can be used in transmission imaging, such high energy X-rays are not useful for backscatter imaging because the degree of backscatter diminishes with increased energy. As a consequence of the lower energy X-rays necessarily used in backscatter techniques the resultant images show a greater sensitivity to lower atomic number materials, compared to X- ray transmission imaging, and there is the potential to distinguish between materials based on a combination of their atomic number and density. These properties make X-ray backscatter imaging particularly suitable for a range of security and defence applications such as scanning cargo containers, vehicles and people for contraband and for explosive screening.
  • backscattered component reduces in intensity as forward scattering becomes more likely. Furthermore, as the X-ray energy is increased, the number of multiple scattering events increases, reducing the potential horizontal resolution of the backscattered image.
  • WO2011/069024 discloses an X-ray backscatter imaging system in which the X-ray beam is pulsed on for ⁇ - ⁇ and off for 1- ⁇ . By determining the time difference between when the X-ray source is switched on and when the backscattered signal is detected it is possible to determine the distance between the detector and the object and hence the surface profile of the object can be determined.
  • a backscatter imaging apparatus for inspecting an object located at a distance from the apparatus, comprising: a radiation source for propagating a pulsed, collimated beam of radiation directed towards the object to be inspected; an X-ray detector adapted to detect X-rays backscattered by a plurality of scattering events within the object and to generate a signal representative of the intensity of X-rays backscattered by each scattering event within the object; a signal processor adapted to determine the time difference between the emission of a pulse from the source and the return of a backscattered X-ray signal in order to determine the range of the scattering event from the apparatus; and an output device adapted to provide data representative of the spatially resolved density profile of the object; wherein the radiation source is a laser wakefield accelerator configured to emit a short pulsed, collimated beam of electrons for projection towards the object to be inspected.
  • object is intended to include any discrete object, a container containing further objects e.g. a vehicle or item of luggage, a barrier concealing further objects or a mass material concealing further embedded objects.
  • radiation is used in a broad sense to include energy emitted in the form of waves or subatomic particles and is not limited to electromagnetic radiation.
  • High energy electrons are able to penetrate to greater depths than X-rays through an object to be imaged and their energy can be tuned to produce a peak in X-ray backscatter emission at a particular depth.
  • X-rays are produced via bremsstrahlung emission.
  • the electrons penetrate further into the object they lose more kinetic energy which is converted into more X-rays, which then backscatter and are attenuated as they travel back to the detector.
  • the interaction between these effects increasing X-ray emission caused by electron deceleration and X-ray attenuation, leads to a peak in backscatter emission at a particular depth.
  • the compromise between stand-off and efficient X-ray backscatter dictates that lower electron beam energies are preferred.
  • the electron beam energy lies in the range 0.1-1000MeV and more preferably in the range l-200MeV.
  • Each pulse of electrons has potential to have a duration of less than 100 picoseconds and preferably less than 1 picosecond. At these pulse widths it is the detector performance which is the limiting factor on spatial resolution.
  • the use of high energy electrons allows X-ray backscatter imaging across an increased standoff range through air or greater penetrative depth, depending on the application, compared to known X-ray backscatter techniques.
  • the object to be inspected may advantageously be located up to 10m through air from the laser wakefield accelerator.
  • the effective imaging depth through sand may exceed 200mm. In both situations the maximum effective imaging distance will be limited by detecting the backscattered X-rays rather than by propagating the electrons .
  • the invention is particularly applicable to ground penetration imaging and for this application it is convenient to mount the apparatus on a vehicle which can be used to traverse the ground.
  • the invention also encompasses a method of inspecting an object, the method comprising the steps of: positioning a laser wakefield accelerator and an X-ray detector at a distance from an object to be inspected; operating the laser wakefield accelerator to propagate a short pulsed, collimated beam of electrons projected towards the object; detecting X-rays backscattered by a plurality of scattering events within the object; generating a signal representative of the intensity of X-rays backscattered by each scattering event within the object; determining the time difference between the emission of a pulse of electrons from the laser wakefield accelerator and the return of a backscattered X-ray signal in order to determine the range of each scattering event from the apparatus; and providing output data representative of the spatially resolved density profile of the object.
  • Fig. 1 shows a comparison of the total backscattered X-ray energy as a function of depth in sand
  • Fig. 2 shows a schematic of a backscatter imaging apparatus in accordance with the invention
  • Fig. 3 shows a schematic plan view of an experimental arrangement of the radiation source and the detector in accordance with the invention.
  • Fig.4 shows an output image generated in accordance with the invention.
  • FIG. 1 shows the results of modelling using the Geant4 Monte Carlo toolkit to simulate the total backscattered X-ray energy per unit length from sand (Si0 2 with a density of 1.78 g/cm3) as a function of distance when probed with 350keV X-rays, lOOkeV X-rays and 140MeV electrons.
  • the simulation was conducted with 10 6 incident electrons.
  • the backscattered ' X-ray flux was measured using a detector plane on the front surface of the sand sample.
  • FIG. 1 illustrates the fact that high energy electrons are able to penetrate to greater depths through a barrier than X-rays having energies suitable for backscatter imaging.
  • the electrons decelerate X-rays are produced via bremsstrahlung emission.
  • the X-rays produced then backscatter and are attenuated as they travel back to the detector, the interaction between these effects leads to a peak in backscatter emission.
  • the peak in backscatter emission can be tuned to occur at a particular depth.
  • FIG. 2 shows a schematic of a backscatter imaging apparatus 10 according to the invention, comprising a laser wakefield accelerator 2 directing a short pulsed, collimated beam of electrons 3 towards a barrier 4 and a concealed object 5.
  • a fast detector 6 receives a backscattered X-ray signal 7, which includes a response 14 in respect of scattering events at the barrier and a response 15 in respect of scattering events at the object.
  • the detector 6 generates an electronic signal proportional to the intensity and energy of X-rays backscattered by each scattering event.
  • the detector 6 is connected to a signal processor 8 which analyses the signal intensity and time domain data to determine the range of each detected scattering event from the X-ray detector position.
  • the short pulsed collimated beam of electrons can be directed at different spatial positions of the barrier and object by magnetically manipulating the electrons or by optically manipulating the laser beam. Altering the direction of the electron beam allows two dimensional scanning of the barrier and object to improve spatial resolution.
  • the imaging device 9 displays an output image of the spatially resolved density profile of the object under inspection, which in this case will reveal the relative location and profile of an object 5 concealed behind a barrier 4.
  • FIG.3 shows a schematic plan view of an experimental arrangement of the radiation source and the detector of the invention.
  • the experiment was performed using the Gemini dual beam Ti: Sapphire 800nm laser system at the Central Laser Facility at the Rutherford - Appleton Laboratory as the radiation source 32.
  • the Gemini laser is able to produce pulses of 55 ⁇ 5fs duration and energies of 10 ⁇ 1J on target.
  • a single p-polarised beam of Gemini 34 was focused into the centre of a supersonic gas jet 35 at normal incidence using an//20 off-axis parabolic mirror 33.
  • the focal spot Full Width Half Maximum (FWHM) was measured to be 25 ⁇ , resulting in a peak laser intensity of 10 Wcm " .
  • FWHM Full Width Half Maximum
  • Gemini is capable of producing narrow energy spread electron beams of up to GeV energies.
  • highly reproducible electron beams are required with lower electron energies typically between 10 - 200MeV. This increases the fraction of lower energy X-rays (10s to 100s of keVs) that are most likely to be backscattered.
  • a high charge beam is also 1 required to increase the backscattered signal as the X-ray backscatter signal is much lower than the forward scattered beam.
  • an//20 focusing optic 33 was used to provide a large focal spot within a supersonic gas jet ( ⁇ 25 ⁇ ) 35.
  • the electron beam 37 propagated out of the vacuum chamber 36 through a 3mm thick steel window 38 on the laser axis 1.8m from the gas jet 35.
  • Samples to be scanned were typically placed in an imaging area 39 between 0.4m - 1.5m from the end of the vacuum chamber 36 (2.2m - 3.3m from the gas jet 35).
  • a 0.3m thick lead electron beam stop 40 was placed behind the imaging area 39, 2.25m from the end of the vacuum chamber 36 to provide a strong X-ray backscatter signal 44. It is possible to shield the detectors from this signal using a lead collimator, however, it could also be used as a reference X-ray scatter point.
  • Micro Channel Plate Photomultiplier Tube (MCP - PMT) detectors 41 were placed ⁇ 2.2m away from the imaging area 39 within lead shielding 42, 43 to restrict the field of view and suppress unwanted background X-ray emission associated with electrons interacting with the vacuum chamber 36 walls.
  • An array of objects chosen to give a variety of mass densities and atomic numbers including 0.14m thick foam wall insulation 55, 38mm thick aluminium 57 and a 0.10m x 0.05m x 0.15m bag of low density organic compound (C 12 H 12 On) 56 were set up as shown in FIG. 4a.
  • the detectors were positioned to the side of the vacuum chamber 2.2m from the 38mm Al sample.
  • the electron beam was scanned horizontally in 10cm increments to form the x axis of the image shown in FIG. 4B.
  • the resolution in the y axis is dependent on the combined scintillator and detector time response.
  • Each slice is an average of the BaF 2 MCP-PMT signal over 40 shots.
  • the zero position is given as the signal from the 3mm steel window 54, with , distances (in m) given from this position, with the lead beam stop 50 shown at 2.20m.
  • the image clearly shows the positions of the different density and atomic number objects shown in FIG. 4A, with the objects positioned behind the 0.14m foam "wall' 55 being observed to have a strong response demonstrating the potential for single-sided imaging of objects behind a barrier. This is due to 140MeV electrons having a much greater penetration depth than the 10s to 100s of keV X-rays required for traditional backscatter detection. These electrons are able to pass through the foam wall and generate X-rays within the other objects which then backscatter and pass back through the foam wall to the detector. Direct X-ray irradiation would require the X-rays to pass through the foam wall twice being attenuated both times and giving a weaker signal from the objects behind the wall.
  • a 140MeV electron beam generated by laser wakefield acceleration projected into air has been used to create an X-ray backscatter image of an array of objects.
  • the use of a laser wakefield generated electron beam has the advantage of producing a high charge of high energy electrons in a short time scale from a very compact accelerator.
  • the small size of this system makes it practical for mobile and field applications.
  • This method has the potential to image at greater depths than previously demonstrated with X-ray source imaging techniques due to the greater range of the electrons.
  • improvements to the image quality can be made with an increase in electron beam charge, or an increase in laser repetition rate which would allow for an increase in signal to noise ratios in a given integration time. Further work into the development of fast detection techniques would improve depth resolution of the image and larger area detectors would improve signal to noise ratios.

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  • Physics & Mathematics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Geophysics (AREA)
  • Analysing Materials By The Use Of Radiation (AREA)

Abstract

A backscatter imaging apparatus (10), comprises a laser plasma (wakefield) accelerator (2) directing a short pulsed, collimated beam of electrons (3) towards a barrier (4) and a concealed object (5). A fast detector (6) receives a backscattered X-ray signal (7) and generates an electronic signal representative of the intensity of X-rays backscattered by each scattering event. Signal processor (8) analyses the signal intensity and time domain data to determine the range of the scattering event from the apparatus. The imaging device (9) displays an output image of the spatially resolved density profile of the object (5) concealed behind a barrier (4). The use of a laser wakefield generated electron beam allows X-ray backscatter imaging across an increased stand-off range or greater penetrative depth.

Description

PULSED BACKSCATTER IMAGING APPARATUS AND METHOD
Technical Field of the Invention
The invention relates to a single-sided imaging apparatus and method, and more specifically to a pulsed backscatter imaging apparatus and method capable of producing a spatially resolved image of an object across an increased stand-off range or greater penetrative depth.
Background to the Invention
X-ray imaging systems are well known for producing high resolution images of hidden objects in non-invasive examination roles such as inspection, diagnosis and detection and are commonly used in industries such as medicine, security, defence and exploration. X-rays transmitted" through an object, to a detector on the far side, can be used to obtain good quality images of structures within the object. Transmitted X-rays interact with matter through photoabsorption, inelastic scattering and elastic scattering. The strength of these interactions is dependent on the energy of the X-rays and the composition i.e. atomic number and density of the material of the structures. In order to achieve high transmission it is necessary to utilise high energy X-rays, however as the energy of X-rays increases they undergo less interactions with delicate structures and materials so there is generally a compromise between depth of inspection and contrast. Furthermore, high energy X-rays become increasingly difficult to detect because they do not interact with detector materials at higher energies.
X-rays which are scattered by objects, particularly backscattered X-rays, can also be utilised to form an image. An advantage of using backscattered X-rays is that the detector is located on the same side of the object as the X-ray source, allowing X-ray imaging in situations where only one side of the object is available for inspection i.e. single sided imaging.
However, X-rays are more likely to scatter in the forward direction and this likelihood increases with increasing energy. Whilst X-rays having energies in excess of lMeV can be used in transmission imaging, such high energy X-rays are not useful for backscatter imaging because the degree of backscatter diminishes with increased energy. As a consequence of the lower energy X-rays necessarily used in backscatter techniques the resultant images show a greater sensitivity to lower atomic number materials, compared to X- ray transmission imaging, and there is the potential to distinguish between materials based on a combination of their atomic number and density. These properties make X-ray backscatter imaging particularly suitable for a range of security and defence applications such as scanning cargo containers, vehicles and people for contraband and for explosive screening.
Unfortunately, lower energy X-rays are significantly attenuated over distance especially through dense and high atomic number materials. This imposes a stand-off limitation on the use of X-ray backscatter imaging, particularly in applications such as ground penetrating imaging. Attempts to apply X-ray backscatter imaging technology to the imaging of underground objects, such as archaeological artefacts or landmines, have encountered difficulties due to the fact that X-rays having energy in the range 10s - 100s of keV, which are required for a strong backscattered signal, are highly attenuated by the soil. In practice, X- ray backscatter imaging through soil is limited to a depth of approximately 50mm. At more than 50mm depth the attenuation and scattering of the X-rays becomes too large to allow good imaging. The use of higher energy X-rays allows greater penetration but the
backscattered component reduces in intensity as forward scattering becomes more likely. Furthermore, as the X-ray energy is increased, the number of multiple scattering events increases, reducing the potential horizontal resolution of the backscattered image.
It is also known to use a pulsed X-ray source. WO2011/069024 discloses an X-ray backscatter imaging system in which the X-ray beam is pulsed on for Ι-ΙΟΟμβ and off for 1- ΙΟΟμβ. By determining the time difference between when the X-ray source is switched on and when the backscattered signal is detected it is possible to determine the distance between the detector and the object and hence the surface profile of the object can be determined.
It is an aim of the invention to provide an improved pulsed backscatter imaging apparatus and method capable of producing a higher resolution image of an object across an increased . stand-off range. Summary of the Invention
According to an embodiment of the invention, there is provided a backscatter imaging apparatus for inspecting an object located at a distance from the apparatus, comprising: a radiation source for propagating a pulsed, collimated beam of radiation directed towards the object to be inspected; an X-ray detector adapted to detect X-rays backscattered by a plurality of scattering events within the object and to generate a signal representative of the intensity of X-rays backscattered by each scattering event within the object; a signal processor adapted to determine the time difference between the emission of a pulse from the source and the return of a backscattered X-ray signal in order to determine the range of the scattering event from the apparatus; and an output device adapted to provide data representative of the spatially resolved density profile of the object; wherein the radiation source is a laser wakefield accelerator configured to emit a short pulsed, collimated beam of electrons for projection towards the object to be inspected.
The term "object" is intended to include any discrete object, a container containing further objects e.g. a vehicle or item of luggage, a barrier concealing further objects or a mass material concealing further embedded objects.
The term "radiation" is used in a broad sense to include energy emitted in the form of waves or subatomic particles and is not limited to electromagnetic radiation.
High energy electrons are able to penetrate to greater depths than X-rays through an object to be imaged and their energy can be tuned to produce a peak in X-ray backscatter emission at a particular depth. As the electrons are decelerated by the material of the object, X-rays are produced via bremsstrahlung emission. As the electrons penetrate further into the object they lose more kinetic energy which is converted into more X-rays, which then backscatter and are attenuated as they travel back to the detector. The interaction between these effects, increasing X-ray emission caused by electron deceleration and X-ray attenuation, leads to a peak in backscatter emission at a particular depth. This depth can be tuned dependent on the initial electron energy and the material properties of the object. In contrast, in known X-ray backscatter techniques the intensity of backscattered X-rays reduces with increasing incident X-ray energy whilst low energy X-rays suffer greater attenuation during both the outward and the return journeys. The combination of these effects imposes fundamental limitations on the depth at which successful backscatter imaging can be achieved.
It is known to utilise a laser to induce an X-ray beam, which can be used for transmission and backscatter imaging. However, laser induced X-rays produced in this manner are not sufficiently collimated to significantly improve the resolution of backscatter imaging. In order to narrow the laser induced X-ray beam significantly it would be necessary to induce synchrotron motion of electrons. Synchrotron electrons can be induced using an undulator (a periodic arrangement of magnets) which is bulky or betatron motion of electrons can be induced within the laser plasma acceleration process itself. However, both methods are presently unable to produce X-rays of sufficient energy to be useful in penetrative imaging.
It is also known to use a laser plasma (or wakefield) accelerator to produce an extremely high energy, collimated beam of electrons. However, typically this is used in the study of plasma or particle physics where the interest is in maximising electron energies, generally in excess of lGeV. For the purpose of imaging, in accordance with the invention, the compromise between stand-off and efficient X-ray backscatter dictates that lower electron beam energies are preferred. Preferably the electron beam energy lies in the range 0.1-1000MeV and more preferably in the range l-200MeV. These lower energies are achieved by altering the density and type of gas target and making adjustments to the focussing conditions of the laser into the gas.
Using laser wakefield acceleration not only enables production of a collimated electron beam but also reduces the time period that the electrons are produced to such a degree that time of flight imaging performance is limited by the speed of X-ray detection rather than by the pulse width of the source. Each pulse of electrons has potential to have a duration of less than 100 picoseconds and preferably less than 1 picosecond. At these pulse widths it is the detector performance which is the limiting factor on spatial resolution.
The use of high energy electrons allows X-ray backscatter imaging across an increased standoff range through air or greater penetrative depth, depending on the application, compared to known X-ray backscatter techniques. The object to be inspected may advantageously be located up to 10m through air from the laser wakefield accelerator. Alternatively, the effective imaging depth through sand may exceed 200mm. In both situations the maximum effective imaging distance will be limited by detecting the backscattered X-rays rather than by propagating the electrons .
The skilled person will understand that by introducing relative movement between the beam of electrons and the object to be inspected two dimensional scanning can be achieved.
In view of the increased range through soil/sand the invention is particularly applicable to ground penetration imaging and for this application it is convenient to mount the apparatus on a vehicle which can be used to traverse the ground.
The invention also encompasses a method of inspecting an object, the method comprising the steps of: positioning a laser wakefield accelerator and an X-ray detector at a distance from an object to be inspected; operating the laser wakefield accelerator to propagate a short pulsed, collimated beam of electrons projected towards the object; detecting X-rays backscattered by a plurality of scattering events within the object; generating a signal representative of the intensity of X-rays backscattered by each scattering event within the object; determining the time difference between the emission of a pulse of electrons from the laser wakefield accelerator and the return of a backscattered X-ray signal in order to determine the range of each scattering event from the apparatus; and providing output data representative of the spatially resolved density profile of the object.
Brief Description of the Drawings
The invention will now be described by way of example only and with reference to the accompanying drawings, in which:
Fig. 1 shows a comparison of the total backscattered X-ray energy as a function of depth in sand;
Fig. 2 shows a schematic of a backscatter imaging apparatus in accordance with the invention;
Fig. 3 shows a schematic plan view of an experimental arrangement of the radiation source and the detector in accordance with the invention; and
Fig.4 shows an output image generated in accordance with the invention.
The drawings are for illustrative purposes only and are not to scale. Detailed Description
FIG. 1 shows the results of modelling using the Geant4 Monte Carlo toolkit to simulate the total backscattered X-ray energy per unit length from sand (Si02 with a density of 1.78 g/cm3) as a function of distance when probed with 350keV X-rays, lOOkeV X-rays and 140MeV electrons. The simulation was conducted with 106 incident electrons. The backscattered' X-ray flux was measured using a detector plane on the front surface of the sand sample.
FIG. 1 illustrates the fact that high energy electrons are able to penetrate to greater depths through a barrier than X-rays having energies suitable for backscatter imaging. As the electrons decelerate X-rays are produced via bremsstrahlung emission. The X-rays produced then backscatter and are attenuated as they travel back to the detector, the interaction between these effects leads to a peak in backscatter emission. By adjusting the energy of the electron beam the peak in backscatter emission can be tuned to occur at a particular depth. In contrast, X-ray attenuation occurs during both the outward and the return journeys in standard X-ray backscatter techniques so that the backscattered X-ray flux, and hence image resolution, diminishes rapidly as distance through sand increases. In the example of FIG.1 use of an incident electron beam provides improved X-ray backscatter, compared to known X-ray backscatter techniques, beyond a depth of approximately 50mm in sand.
FIG. 2 shows a schematic of a backscatter imaging apparatus 10 according to the invention, comprising a laser wakefield accelerator 2 directing a short pulsed, collimated beam of electrons 3 towards a barrier 4 and a concealed object 5. A fast detector 6 receives a backscattered X-ray signal 7, which includes a response 14 in respect of scattering events at the barrier and a response 15 in respect of scattering events at the object. The detector 6 generates an electronic signal proportional to the intensity and energy of X-rays backscattered by each scattering event. The detector 6 is connected to a signal processor 8 which analyses the signal intensity and time domain data to determine the range of each detected scattering event from the X-ray detector position. The short pulsed collimated beam of electrons can be directed at different spatial positions of the barrier and object by magnetically manipulating the electrons or by optically manipulating the laser beam. Altering the direction of the electron beam allows two dimensional scanning of the barrier and object to improve spatial resolution. The imaging device 9 displays an output image of the spatially resolved density profile of the object under inspection, which in this case will reveal the relative location and profile of an object 5 concealed behind a barrier 4.
FIG.3 shows a schematic plan view of an experimental arrangement of the radiation source and the detector of the invention. The experiment was performed using the Gemini dual beam Ti: Sapphire 800nm laser system at the Central Laser Facility at the Rutherford - Appleton Laboratory as the radiation source 32. The Gemini laser is able to produce pulses of 55±5fs duration and energies of 10±1J on target. A single p-polarised beam of Gemini 34 was focused into the centre of a supersonic gas jet 35 at normal incidence using an//20 off-axis parabolic mirror 33. The focal spot Full Width Half Maximum (FWHM) was measured to be 25μπι, resulting in a peak laser intensity of 10 Wcm" .
Gemini is capable of producing narrow energy spread electron beams of up to GeV energies. To provide optimal conditions for the production of a high flux of detectable backscattered X-rays from a sample, highly reproducible electron beams are required with lower electron energies typically between 10 - 200MeV. This increases the fraction of lower energy X-rays (10s to 100s of keVs) that are most likely to be backscattered. A high charge beam is also 1 required to increase the backscattered signal as the X-ray backscatter signal is much lower than the forward scattered beam. To achieve this, an//20 focusing optic 33 was used to provide a large focal spot within a supersonic gas jet (~25μπι) 35. This enables the creation of a larger non-linear plasma wave bubble within the gas, increasing the limit on the number of electrons that can be. injected. A 5mm gas jet nozzle was used with the laser focused 1mm above the tip of the nozzle. A helium and 5% nitrogen gas mix was used allowing for ionisation injection to occur to increase the total charge in the electron beam 37.
To scan an array of objects, the electron beam 37 propagated out of the vacuum chamber 36 through a 3mm thick steel window 38 on the laser axis 1.8m from the gas jet 35. Samples to be scanned were typically placed in an imaging area 39 between 0.4m - 1.5m from the end of the vacuum chamber 36 (2.2m - 3.3m from the gas jet 35). A 0.3m thick lead electron beam stop 40 was placed behind the imaging area 39, 2.25m from the end of the vacuum chamber 36 to provide a strong X-ray backscatter signal 44. It is possible to shield the detectors from this signal using a lead collimator, however, it could also be used as a reference X-ray scatter point. Micro Channel Plate Photomultiplier Tube (MCP - PMT) detectors 41 were placed ~2.2m away from the imaging area 39 within lead shielding 42, 43 to restrict the field of view and suppress unwanted background X-ray emission associated with electrons interacting with the vacuum chamber 36 walls.
An array of objects chosen to give a variety of mass densities and atomic numbers including 0.14m thick foam wall insulation 55, 38mm thick aluminium 57 and a 0.10m x 0.05m x 0.15m bag of low density organic compound (C12H12On) 56 were set up as shown in FIG. 4a. The detectors were positioned to the side of the vacuum chamber 2.2m from the 38mm Al sample. The electron beam was scanned horizontally in 10cm increments to form the x axis of the image shown in FIG. 4B. The resolution in the y axis is dependent on the combined scintillator and detector time response. Each slice is an average of the BaF2 MCP-PMT signal over 40 shots. The zero position is given as the signal from the 3mm steel window 54, with , distances (in m) given from this position, with the lead beam stop 50 shown at 2.20m. The image clearly shows the positions of the different density and atomic number objects shown in FIG. 4A, with the objects positioned behind the 0.14m foam "wall' 55 being observed to have a strong response demonstrating the potential for single-sided imaging of objects behind a barrier. This is due to 140MeV electrons having a much greater penetration depth than the 10s to 100s of keV X-rays required for traditional backscatter detection. These electrons are able to pass through the foam wall and generate X-rays within the other objects which then backscatter and pass back through the foam wall to the detector. Direct X-ray irradiation would require the X-rays to pass through the foam wall twice being attenuated both times and giving a weaker signal from the objects behind the wall.
A 140MeV electron beam generated by laser wakefield acceleration projected into air has been used to create an X-ray backscatter image of an array of objects. The use of a laser wakefield generated electron beam has the advantage of producing a high charge of high energy electrons in a short time scale from a very compact accelerator. The small size of this system makes it practical for mobile and field applications. This method has the potential to image at greater depths than previously demonstrated with X-ray source imaging techniques due to the greater range of the electrons. The skilled person will appreciate that
improvements to the image quality can be made with an increase in electron beam charge, or an increase in laser repetition rate which would allow for an increase in signal to noise ratios in a given integration time. Further work into the development of fast detection techniques would improve depth resolution of the image and larger area detectors would improve signal to noise ratios.

Claims

1. A backscatter imaging apparatus for inspecting an object located at a distance from the apparatus, comprising: a radiation source for propagating a pulsed, collimated beam of radiation directed towards the object to be inspected; an X-ray detector adapted to detect X-rays backscattered by a plurality of scattering events within the object and to generate a signal representative of the intensity of X-rays backscattered by each scattering event within the object; a signal processor adapted to determine the time difference between the emission of a pulse from the source and the return of a backscattered X-ray signal in order to determine the range of the scattering event from the apparatus; and an output device adapted to provide data representative of the spatially resolved density profile of the object; wherein the radiation source is a laser wakefield accelerator configured to emit a short pulsed, collimated beam of electrons for projection towards the object to be inspected.
2. A backscatter imaging apparatus as claimed in claim 1 wherein the energy of the beam of electrons lies in the range 0.1-lOOOMeV.
3. A backscatter imaging apparatus as claimed in claim 2 wherein the energy of the beam of electrons lies in the range l-200MeV.
4. A backscatter imaging apparatus as claimed in any preceding claim wherein each pulse of electrons has a duration of less than 100 picoseconds.
5. A backscatter imaging apparatus as claimed in any preceding claim wherein each pulse of electrons has a duration of less than 1 picosecond.
6. A backscatter imaging apparatus as claimed in any preceding claim wherein the object to be inspected is located up to 10m from the laser wakefield accelerator.
7. A backscatter imaging apparatus as claimed in any preceding claim wherein the effective imaging depth through sand exceeds 200mm.
8. A backscatter imaging apparatus as claimed in any preceding claim wherein the apparatus is mounted on a vehicle.
9. A backscatter imaging apparatus as claimed in any preceding claim wherein the beam of electrons and the object to be inspected are moved relative to one another.
10. A method of inspecting an object, the method comprising the steps of:
a. positioning a laser wakefield accelerator and an X-ray detector at a distance from an object to be inspected;
b. operating the laser wakefield accelerator to emit a short pulsed, collimated beam of electrons projected towards the object;
c. detecting X-rays backscattered by a plurality of scattering events within the object;
d. generating a signal representative of the intensity of X-rays backscattered by each scattering event within the object;
e. determining the time difference between the emission of a pulse of electrons from the laser wakefield accelerator and the return of a backscattered X-ray signal in order to determine the range of each scattering event from the apparatus; and
f. providing output data representative of the spatially resolved density profile of the object.
11. A method of inspecting an object as claimed in claim 10 wherein the energy of the beam of electrons lies in the range 0.1-lOOOMeV.
12. A method of inspecting an object as claimed in claim 11 wherein the energy of the beam of electrons lies in the range l-200MeV.
13. A method of inspecting an object as claimed in any of claims 10 to claim 12
wherein each pulse of electrons has a duration of less than 100 picoseconds.
14. A method of inspecting an object as claimed in any of claims 10 to claim 13 wherein each pulse of electrons has a duration of less than 1 picosecond.
15. A method of inspecting an object as claimed in any of claims 10 to claim 14
wherein the object to be inspected is located up to 10m from the laser wakefield accelerator.
16. A method of inspecting an object as claimed in any of claims 10 to claim 15
wherein the effective imaging depth through sand exceeds 200mm.
17. A method of inspecting an object as claimed in any of claims 10 to claim 16
wherein the apparatus is mounted on a vehicle.
18. A method of inspecting an object as claimed in any of claims 10 to claim 17
wherein the beam of electrons and the object to be inspected are moved relative to one another.
19. A backscatter imaging apparatus substantially as described herein with reference to the accompanying drawings.
20. A method of inspecting an object substantially as described herein with reference to the accompanying drawings.
EP15800886.2A 2014-11-24 2015-11-20 Pulsed backscatter imaging apparatus and method Withdrawn EP3224655A1 (en)

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GBGB1420834.2A GB201420834D0 (en) 2014-11-24 2014-11-24 Pulsed Backscatter imaging apparatus and method
PCT/GB2015/000303 WO2016083766A1 (en) 2014-11-24 2015-11-20 Pulsed backscatter imaging apparatus and method

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7809104B2 (en) * 2005-11-11 2010-10-05 L-3 Communications Security and Detection Systems Inc. Imaging system with long-standoff capability
US8705692B2 (en) * 2009-05-12 2014-04-22 Nutech Ventures, Inc. Laser-based accelerator for interrogation of remote containers
GB2516794B (en) * 2009-12-03 2015-04-01 Rapiscan Systems Inc Time of flight backscatter imaging system

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GB2534276A (en) 2016-07-20
GB201420834D0 (en) 2015-01-07
GB2534276B (en) 2017-08-23
WO2016083766A1 (en) 2016-06-02

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