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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• Analysing Materials By The Use Of Radiation (AREA)

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• 2014
  • 2014-11-24 GB GBGB1420834.2A patent/GB201420834D0/en not_active Ceased
• 2015
  • 2015-11-20 EP EP15800886.2A patent/EP3224655A1/de not_active Withdrawn
  • 2015-11-20 WO PCT/GB2015/000303 patent/WO2016083766A1/en active Application Filing
  • 2015-11-20 GB GB1520442.3A patent/GB2534276B/en active Active

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