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Radiation-based techniques for use in the border protection context
Radiation Physics and Chemistry ] (]]]]) ]]]–]]]
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Radiation-based techniques for use in the border protection context Dudley Creagh n Faculty of Applied Science, University of Canberra, Canberra ACT 2601, Australia
H I G H L I G H T S c c c c c
Radiation-based techniques used by border protection agencies are reviewed. Passenger portals use THz and mm-wave radiation, CCTV is used for crowds. For the examination of luggage, air-cargo, and shipping containers, X-rays are used. The need for excellent materials discrimination software is demonstrated. Materials discrimination using a combined neutron and X-ray system is described.
a r t i c l e i n f o
abstract
Article history: Received 8 October 2012 Accepted 3 December 2012
Most airline travelers will be familiar with the current overt passenger examination procedures: metal detectors and small tunnel X-ray examination systems. The mix of overt and covert systems used to prevent dangerous goods and contraband from passing through the portal is constantly changing, dictated by policy decisions made by governments. The United States of America and the European Union are the largest regulatory bodies, and their procedures are adopted by smaller countries: Australia, for example. This paper discusses a wide variety of techniques used by Border Protection Agencies. Most of these examination systems involve the use of the emission, absorption, and scattering of electromagnetic radiation and descriptions of these systems will comprise the bulk of this paper. However, a brief discussion of the use of neutron scattering will be given to demonstrate how systems for the examination of large objects may develop in the future. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Border protection THz radiation mm-wave radiation CCTV X-ray scattering Neutron scattering
1. Introduction When passengers enter the security portal of an airport they are requested to remove from their person many of their personal possessions: hats, coats, jackets, mobile phones, keys, wallets,y. The list seems endless. These are placed on plastic trays which are then examined with the passenger’s hand luggage using a small tunnel X-ray system. But that is not the end of the pre-scan procedures. The passenger must remove any computers, bottles above a certain size, aerosol cans, and the like from the handluggage, and place them on trays to be X-rayed. All this tests the patience of the passengers and adds significantly to the through-put time for the passengers, and hence the loading time for an aircraft. But it is necessary, because all governments are concerned about the possibility of bomb, hand guns, knives, and other dangerous goods being taken onto aircraft. They have regulations, for example, the European Parliament
n
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(2002), about what is not allowed: and what equipment and personnel will be deployed for the examination process. But what the passenger sees does not necessarily reflect the totality of the examination to which he is subjected. A range of unseen systems may have scanned the passenger. He may have been examined by covert systems using passive or active THz radiation or mm-radiation. He will certainly been observed by Closed Circuit Television Systems (CCTV) and his image compared with lists of banned passengers. Recent changes regulations may force the passenger to be scanned in an active mm-wave cabinet, or subjected to examination in a low energy low intensity X-ray body scanner. His carryon baggage and other effects will have been examined by an X-ray system. As well, his hold baggage will have been examined by a more sophisticated X-ray system that that in the passenger hall. In all, the passenger and his effects would have been examined by electromagnetic radiation spanning the photon energy range from o1 meV to 0.5 MeV. I shall not discuss here the covert non-radiation-based examinations made by dogs trained to detect explosives and drugs, and systems using ion beam mass spectroscopy. Nor shall I address
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the complexities of facial recognition software for identifying suspicious people. In discussing X-ray systems I shall concentrate on systems used in the passenger hall: small tunnel systems ( o1000 mm 1000 mm tunnel aperture). I shall not discuss the large pallet and shipping container X-ray systems used for examining air cargo and shipping containers. I shall, however, discuss the principles of operation of a new X-ray/neutron system for the examination of shipping containers.
2. Examination systems A distinction has to be made, first, between ionizing and nonionizing radiation. Ionizing radiation has sufficient energy to break chemical bonds and alter the genetic structure in cells. The boundary between the non-ionizing and the non-ionizing regimes could be thought to occur in the ultra-violet region of the optical spectrum ( 350 nm), due to which skin cancers can be induced. For wavelengths longer than this the radiation is deemed to pose no health threat to humans. THz and mm-wave radiation fall into the non-ionizing category. A distinction must also be made between passive and active systems. Passive systems are those in which the subject generates its own radiation. Active systems irradiate the subject with radiation. 2.1. Passive passenger portals Passive passenger systems rely on measuring the black body radiation from the incoming passenger and the variations in spectral emissivity of the item carried from that of the passenger. The emission obeys the Stephan Boltzmann Law: energy flux density¼J¼ esT4. Here e ¼emissivity (0o e o1); s ¼ 5.7604 10 8 W m 2 K 4; T¼temperature (K). Currently available systems are sensitive to radiation from the person in the mm wave ( 100 GHz) or the THz ( 250 GHz) regions. The design and construction of mm-wave and THz detectors is a rapidly growing field (Knoll, 2000;Rieke, 2003; Lee, 2009). A wide range of multipixel semiconductor devices have been developed to operate at room temperature. These new detectors will be incorporated in systems using existing digital TV technology. Commercially available passenger portals tend to use only a line of detectors, and the image of the subject is scanned across this line onto the detector array using a tilting mirror (Fig. 1). Several images are available to the operator: the Closed Circuit Television (CCTV) image; the raw image presented by the detector, and enhanced images (edge-enhancement, for example) (Fig. 2).
Fig. 1. Exploded schematic view of a mm-wave passenger scanner.
Fig. 2. CCTV and enhanced images of a passenger with a wallet in his hip pocket.
These systems examine only one side of the passenger. Either two systems have to be deployed so that the front and back of the passenger can be viewed simultaneously, or the passenger has to turn around so that a second image can be taken. Screening authorities, such as those in airports, are very concerned with ‘‘throughput’’, the number of passengers per hour passing the portal. As well, they are concerned with false alarm rates, both positive (there is a threat), and negative (there is no threat). Both have a deleterious effect on throughput. Advantages of these systems are that: measurements can be made at a distance (stand-off capability); observations can be made whilst the subject is approaching the portal; objects at temperatures different from the body or having different emissivities will provide an image; the system can see what is hidden under a moderately thick layer of clothing; examination is rapid. Disadvantages include: lack of materials discrimination; poor resolution; inability to detect items in cavities and crevices; inability to detect objects in the foot and ankle region; poor detection of objects not in contact with the body. 2.2. Active mm-wave passenger portal technology Active passenger portals consist of mm-wave or THz generator and antenna system and a corresponding detector system. Beams of radiations are scanned over the subject, and an image corresponding to the point-to-point reflectivity of the subject is formed. In a sense this is a RADAR map of the subject. For portals which are large enough to encompass a human body it is necessary for the system to be enclosed in a cabinet because of radiation licensing regulations. In all countries the electromagnetic spectrum is divided into bands which are allocated to corporations and other entities by the relevant Broadcast Authority. Enclosing the system minimizes the problem of encroaching on another user’s band. By their nature these systems do not have a ‘‘stand-off’’ capability. Fig. 3 shows a photograph of a cabinet passenger mm-wave scanning system. As well, an image of a passenger is shown. To meet the requirements of privacy legislation the outline of the image is a computer generated anthropomorphic shape. Some details of his clothing are evident. A circular shape has been placed around the genital area to ensure privacy. The operator can, however, turn off this privacy filter: but he has no visual contact with the subject. The passenger is rotated through 180o to ensure that both front and rear images are produced. Images of three square objects of
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Fig. 3. Cabinet mm-wave passenger scanner (left), and an image of a subject taken with the system (right) are shown. (Taken with an L3 ProVision).
different sizes can be seen. Because the passenger is supposed to have divested himself of everything except their clothing these images would be regarded as suspicious. Note that the extremities of the passenger are not imaged. The advantages of this system are: mm-waves have no deleterious interactions with people; the spatial resolution is 2 mm; the throughput is acceptable ( 600 subjects/h); the system maps the surface of the passenger, illicit items being identified by their shape and contrast; the passenger’s clothing has only a small effect on the image. Disadvantages include: lack of materials discrimination; inability to detect items in cavities and crevices, and in the lower extremities; sensitivity to metallic items; poor detection of objects not in contact with the body. If no portal is available the passenger may be manually scanned using a handheld device (referred to as a ‘‘wand’’). Because of their low power and the fact that they can be positioned in close proximity to areas of the person the problem of encroachment on other bands in the electromagnetic spectrum is considered by the licensing authority to be of little significance. A wand contains a THz antenna and a THz detector. The highly collimated THz photon pulse excites ground state electrons in the object under investigation to higher excited states. In relaxing to the ground state photons are emitted corresponding to the energy difference between the levels: Eupper–Eground ¼hc/l. The spectral lines are characteristic of the material examined. Comparison of the spectrum with entries in a spectral database enables materials to be identified. The ability to identify materials is important: the ability to produce an image does not, itself, give the operator sufficient information about the object under scrutiny to be certain of his identification. 2.3. Active devices: metal detectors In its simplest form a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. When this coil is placed
close to conductive material eddy currents are induced in the material and an alternating magnetic field is created. Another coil can be used to detect the absorption or re-transmission of the radiation due to the presence of the conducting material. The frequency of the oscillator can be tuned to indicate the presence of particular metals, the most common being gun metal and steel. Arrays of oscillator/detector coils can be manufactured, and crude imaging is possible. Metal detectors are used commonly in passenger portals (where systems can contain a number of oscillators tuned to different frequencies), hand held devices (tuned to a fixed frequency range), or tunnel devices, used (for example in the examination of parcel post for firearms). Fig. 4 shows the image of the barrel of a Glock pistol concealed in a parcel with other metallic items, and beside it, an X-ray image of it, with a threat ellipse generated by the metal detector. This image was taken using a system using the phenomenon of Nuclear Quadrupole Resonance (NQR). See Miller and Barrall, 2005 for a description of how this technique is used for the detection of explosives. The principal advantage of metal detectors is that they provide a relatively inexpensive and rapid assessment of whether the passenger is carrying threat items. The disadvantages are that: the systems are relatively insensitive to the type of metal carried, with the result that false positives occur regularly, especially if the passenger has metal prostheses and implants; they can have an adverse effect on heart pacemakers. 2.4. Active devices: X-ray passenger portals A discussion of the variety X-ray systems used in the border protection context is given by Creagh (2012). X-ray systems may cause harm to passengers if not used correctly because they cause ionizing radiation. Their use is therefore closely regulated by Government instrumentalities. Used in an X-ray passenger portal context the X-ray energies lie in the range 50 to 160 keV. The interactions of radiation with matter are summarized in Fig. 5.
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Fig. 4. Metal detector image (left) and X-ray image (right) of a Glock pistol concealed in a postal parcel. Only the barrel of the pistol is metal. (Using the QRSciences QRX1000 system).
Fig. 5. Scattering of photons by free atoms: carbon.
This shows the cross sections for different scattering processes by atoms, in this case, carbon. In practice the systems are either transmission systems (the X-ray passes through the subject to the detector) or backscattered (the X-ray is scattered back towards the detector). The dominant scattering processes are photoelectric (spe) and backscattered (Compton) (sinel). Because the photon energy changes as a function of scattering angle in Compton scattering the backscattered photons have energies of 0.83 of the incident photons in the 50 to 160 keV energy range. Fig. 6 shows a schematic diagram of a transmission X-ray system currently in use by border protection agencies and prison authorities. Full body scans may be required to enable the contents of the vaginal, rectal, and abdominal cavities to be made. The X-ray source has a maximum energy of 160 keV. The X-rays are collimated into a fan shaped beam and detected using a linear array of scintillation detectors comprising 1000 elements. The absorption follows the Beer–Lambert Law: I ¼I0 exp (mlt); where I0 is the incident energy, t is the path length in the material; ml ¼linear absorption coefficient. The operator makes judgments based on differences in contrast and brightness in the images caused by the variation of absorption coefficients and thicknesses for the materials present in the object. He then relates the image he has observed to images he has seen in the past. The training and experience of the
examining officer are important ingredients in the examination process. Fig. 6 includes a typical image. The disadvantages of the system are that: it has low throughput (10 passengers/h); relatively high doses are delivered to vital organs (3 mSv/scan); the skeletal image can obscure areas of interest; materials discrimination is not possible, although the contrast increases as the atomic number of the target material increases. Systems using backscattered radiation operate on a somewhat different system. A collimation system is used to create to a fan beam and a rotating slit system is used to form a pencil beam which moves horizontally across the subject. The X-ray tube and rotating slit system is then translated downwards so that the flying spot completely traverses the subject. The X-rays scattered back from the subject are collected on large sheets of scintillation material (often gadolinium oxysulphide), one on each side of the plane of rotation of the collimator system. Only two scintillation detectors are required. The data is assembled for display in much the same way as TV images. Fig. 7 shows a schematic diagram of such a system, and an image of a subject with concealments attached to his person. The passenger has to be scanned twice so that both its front and back can be examined. If two systems were to be used the throughput could be doubled. The operator uses contrast and shape variations to identify anomalies. As with all security applications detection depends on the alertness and training of the operator. Advantages of the backscatter type of portal compared to the transmission type are that they: operate at lower voltage (100 keV); do not penetrate deeply into the subject; are not affected by skeletal artefacts; deliver a lower dose per scan (0.1 mSv); have a greater throughput (60–100 passengers/h); is sensitive to low atomic weight materials, such as are in liquids and gels. Disadvantages include: the inability to inspect cavities and crevices; poor materials discrimination; the contrast observed depends on both the material of the object and the material behind it. 2.5. X-ray baggage, pallet, and container systems The simplest X-ray systems used in baggage, pallet (and air cargo), and container examination consist of the following components: an X-ray source, a slit system which forms a fan beam (see Fig. 6), a detection tunnel which incorporates a means for translating the object under examination through the fan beam, a detector array which measures the changes in intensity of the X-ray beam as it passes through the X-ray fan beam, and a system for accumulating, storing and manipulating the data into an image which can be assessed by an operator. X-ray systems fall into three categories: those for baggage (tunnel size o1200 mm 1200 mm), pallet ((1200 mm)2 oH Wo (2000 mm)2), shipping container (H W4(2000 mm2)) applications.
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Fig. 6. Schematic diagram of a transmission X-ray portal (left), and a typical image of a passenger (right).
Fig. 7. Schematic diagram of a backscatter X-ray portal (left), and an image (right) of a subject carrying concealments. Note that images of the subjects shin bones can be seen. (Taken with an AS&E Smart Check).
For tunnel sizes less that (2000 mm)2 sealed X-ray tubes are the preferred source. For shipping containers the preferred source is the linear accelerator (LINAC). Both sources are sealed, highly evacuated systems in which electrons from a heated filament are accelerated to a high voltage and collide with a cooled target (usually tungsten). The resultant change in energy causes the radiation of electromagnetic radiation (bremsstrahlung). For passenger baggage E0 is usually 160 kV; for pallet, 450 kV, and for containers, 3 to 9 MV Whilst sealed tube X-ray systems are operated as constant potential devices, LINACs are pulsed devices, with the pulse rate determined by the speed of transport of the object past the X-ray fan beam. The target for the electrons is typically a water-cooled 1 mm thick gold plated tungsten disc. Fig. 8 shows intensitywavelength plots for bremsstrahlung from a sealed X-ray tube for different values of accelerator potential E0, together with intensity versus energy curves for a LINAC operated at E0 ¼2.5 MeV. The method of transport of the object past the object varies from application to application.
2.5.1. X-ray baggage, pallet, and container systems: gray scale images For baggage and pallet applications the source and detector systems remain stationary and the object moves through the fan beam on a conveyor belt or a driven roller system. For container
systems the container is either towed through the fan beam or the container remains stationary and the source and detector are scanned along the container. Typical scan rates are 200 mm/s. Modern detector systems usually comprise arrays of scintillation detectors. Scintillation materials used include cesium iodide (CsI), cadmium tungstate (CdWO4) and mercury cadmium telluride (HgCdTe). The scintillations caused by the incident photons are detected by solid state detectors such as avalanche photodiodes (APD). The arrays usually comprise more that 1000 separate elements which are grouped in units of 32 elements. Each element presents an active area of 6 mm 6 mm or less to the beam. The output of each of the detectors is interrogated using multiplexing techniques, and that output is ratioed with the output of elements which were not impeded by the object (giving the ratio I/I0, from which the average attenuation coefficient can be computed using the Beer–Lambert Law.). Each time slice is the output of the linear array, and the overall image is the outcome of linking these time slices, and displaying the result on a computer monitor. The resulting black and white (Gray Scale) image can then be manipulated by the operator so that he can better understand the components of the image. For simple objects it is easy to relate the X-ray image to the object examined. In practice objects are rarely simple, and a number of objects may overlay one another. The intensity of the image depends on the materials through which the X-ray beam passes, and
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Fig. 8. Intensity–wavelength plots for bremsstrahlung from a sealed tube X-ray source operated at a number of accelerator potentials (left) and intensity plotted against energy for a LINAC operated at 2.5 MeV (right).
ln (I/I0)¼ (m1t1 þ m2t2 þ m3t3 ¼)yy., where m1t1 is the absorption thickness: the product of the linear attenuation coefficient of the material and the thickness of that material. Note that ml can itself be either that of an atom or a chemical compound. Each manufacturer has his own set of algorithms for enhancing the image. Techniques may include contrast and brightness adjustment, edge enhancement, histogramming, and so on. These algorithms are similar to those available in photographic programs in personal computers.
2.5.2. X-ray baggage, pallet, and container systems: materials discrimination Some degree of materials discrimination is possible if measurements along the same path length can be made at different photon energies. As can be seen in Fig. 8 the intensity-energy spectra are continuous spectra. To a good approximation it is possible to establish a mean value for each of the spectra, and use these in calculations. Fig. 5 shows that the photo-electric scattering factor and the Compton scattering factor are very different in the energy range used in baggage and pallet systems. If the mean energy is changed, therefore, the value of (I/I0) will be changed, and a new value of mlt will result. From the simultaneous equation it is possible to determine ml and t for a two component system, independent of the material thickness. The ratio M ¼ln(I1/I2)¼ m2/m has a fixed value for a given material. This ratio can be determined empirically for a chosen X-ray system. The variation of mean energy can be effected by: using two separate X-ray source energies (switching from 6 MeV to 3 MeV kV at a rate of, say, 200 Hz); by having two separate identical detector arrays with an absorber placed in front of one of the detector arrays to absorb the low energy part of the spectrum; and by having two detectors of the same type in line with one another, the first detector absorbing the low energy part of the spectrum, and the second detector absorbing the high energy part of the spectrum. Systems of these types are referred to as Dual Energy Systems. The first option is better because there is no spatial difference in the images, and the energy difference between the spectra is better characterized. The separation of the detector arrays in the second case creates spatial resolution problem, which leads to blurring of the image. The effect of spatial separation on image resolution is demonstrated in Fig. 9. Here a composite single energy bank of 32 detectors (CsI in front of CDWO4) is placed into a conventional dual array system. The resolution of the inline system is demonstrably superior to the side-by-side array, and the signal-to-noise ratio of the inline system is better than the conventional dual system by about an order of magnitude. The dynamic range is 212 or 12 Gy scales.
Fig. 9. Images of two urns taken using dual and inline X-ray detectors. (Taken with an L3 CX450DV system).
If more than one X-ray beam (referred to as ‘‘views’’) are used to scan the sample the number of simultaneous equations for which solutions can be found increases, and M can be found for increasingly more complicated packing in the scanned object. Dual view, triple view, and quadruple view systems are marketed at present. Computed tomography (CT) systems can give as many views as the manufacturer may wish. The image processing time for CT systems determines the throughput, and throughput is a major determinant in the organization of the operation of airports, in both the passenger and air cargo contexts. A compromise has to be made between certainty of detection and throughput. High throughput causes increased false positive and false negative rates. The additional data available in multi-view examination systems makes it possible to calculate the effective atomic number, Zeff (Manohara et al., 2008) and the density r for any voxel (volume element) in the object. Zeff ¼ SifiAi(mm)i/Sjfj(Aj/Zj)(mm)j; f is the molar fraction, and A and Z are the atomic weight and the atomic number of the atomic species; mm is the mass absorption coefficient ( ¼ ml/r). Zeff and r can be used as a discriminant between materials. For metals: Zeff 419. For inorganic materials:19 4Zeff 4 11. For organic materials: 11 4Zeff. Most explosive materials have parameters in the range: 8.34Zeff 46.5 and 0.9 o r o1.8 g/ml. If an object has a measurement of Zeff which lies in the explosives range there is reason to proceed to further examination of the baggage by other means.
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All US manufactured X-ray examination systems are accredited by the United States Transport Security Administration (http://www.tsa.gov) as explosives detection systems (EDS). These purport to make the assessment as to whether the object contains an explosive threat without human intervention. In practice, however, confirmation by an operator on the basis of shape and contrast is required. The performance of X-ray examination systems is usually described in terms of a Receiver Operator Characteristic (ROC) diagram (Fig. 10) (Green and Smets, 1966). For multi-view systems materials discrimination using automatic computation of Zeff and r can lead to reasonably high throughputs, but the systems are prone to false positives being recorded because innocuous materials may well lie in the threat band (for example water, chocolate, and other innocuous substances have Zeff and r in the threat region). To overcome this deficiency EDS systems can be used as a primary screening device and questionable objects diverted into a system which has a much smaller throughput, but which can give a spectroscopic analysis of the questionable area of the object. Such a system uses energy dispersive X-ray diffraction (EDXRD) (Knoll, 2000). 2.5.3. X-ray baggage, pallet, and container systems: materials discrimination: diffraction systems In Energy Dispersive X-ray Diffraction (EDXRD) systems the tube source (which is a bremsstrahlung source (see Fig. 8)), its
Fig. 10. Idealized Receiver Operator Characteristic (ROC) curves. The diagonal line represents the results if the operator simply guesses.
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collimator system, and an energy dispersive X-ray detector are mounted so that that the beam emitted from the source can pass through the suspect region of the object and be scattered into the detector which is offset by an angle of about 51. The elastically scattered photons from the suspect region interact with the detector giving output pulses proportional to these photon energies (Laine and Lateenmaki, 2008). Most systems are crystalline and elastic scattering occurs according to Bragg’s Law: 2dhkl sin y ¼ l ¼hc/E. Because y is fixed the diffraction peaks occur whenever photon energy corresponds to a particular interplanar spacing dhkl which satisfies the Bragg equation. Fig. 11 (Harding et al., 2002; Harding, 2009) shows the diffraction patterns of an explosive, Semtex (C3H8N4O12 (76%) þC3H6N6O6 (4.6%)), and chocolate taken with a laboratory X-ray system. Chocolate is often confused with explosives by operators. They have clearly different diffraction patterns which can be rapidly compared with entries in the JCPDS database. The International Commission on Diffraction Data (http://www.icdd.com) maintains the JCPDS files which include some 500,000 entries. Recent developments in detector design and improvements in system sensitivity have made it possible to identify some amorphous materials and liquids as well as crystalline materials.
2.5.4. X-ray, pallet, and container systems: materials discrimination: X-ray & neutron system A system has been developed for the X-ray & neutron examination of air cargo ships and shipping containers (Eberhardt et al., 2005). A commercial realization of this system (Nuctech AC series(FINDEX): http://nuctech.com) uses a switched 6/3 MV LINAC X-ray source with two collimators which produce two vertical fan beams at an angle of 121 to one another. The system has two array detector systems, one for each fan beam. The objects are towed through the fan beams at a constant 200 mm/s and conventional images are taken at 3 and 6 MeV energies in each detector system. The resulting images can be combined to produce a binocular view through the object, and the switched voltage source can give a crude indication of materials discrimination as indicated in Section 2.5.3. Incorporated in the system is a conventional sealed tube neutron generator which produces 1010 fast neutrons per second of energy 14 MeV. The neutrons as formed as a result of the 2 H1 þ 3H1-4He2 þ 1n0 reaction Neutrons are emitted isotropically from the target, but, similar to the X-ray case, they are collimated to produce a vertical fan beam using a slit. After the object has traversed the beam the attenuated beam intensities are detected by an array of BF3 proportional detectors. The attenuation of the X-ray beams and the neutron beams are measured, and the output of pixels from the two arrays are mapped to form the ratio, R(¼ln(I0/I)neutron/ln(I0/I)X-ray). Whilst the X-ray attenuation cross
Fig. 11. Diffraction patterns for semtex (76% PETN þ5% RDX), (left), chocolate (right).
Please cite this article as: Creagh, D., Radiation-based techniques for use in the border protection context. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2012.12.012i
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Fig. 12. TNT concealment a clock/radio: conventional:rectangular shape in bottom left of Left Image; backscatter: white rectangle at the left hand corner of the Central Image; CT: rectangular shape at the bottom right of the right image (and the corresponding thumbnail images).
Fig. 13. Image of two containers taken in one scan of the gantry (360 s).
Table 1 R-values for a number of common materials. Material
Lead
Iron
Aluminum
Glass
Teflon
Graphite
TNT
Paper
Heroin
Water
HDPE
R-value
0.3
5.2
6.8
8.8
1.08
1.15
1.25
1.41
1.44
1.56
1.76
sections are regular functions of the atomic number the neutron cross sections are not. The ratio R can be determined for a given element and is a characteristic of that element. For nitrogenous explosive materials R lies in the range 1.18 oRo1.28. However, as in the case of Zeff, some non-threat materials have R-values in this range. The system ultimately relies on the skill of the operator to interpret the high resolution image seen.
3. Examples
3.2. X-ray shipping container examination systems X-ray shipping container examination systems are usually gantry systems and use single view LINAC sources. Dual view systems exist, but most large systems are single view systems. Tunnel dimensions are such that shipping containers are able to be examined (typically 4 m 4 m). The gantry, which contains the source on one side and the detector array on the other side, can be stationary (the container is towed through the gantry), Fig. 13 shows a typical image taken with a moving gantry system. Identification of anomalies is effected by shape and contrast resolution.
3.1. The performance of X-ray baggage systems The performance of two extreme configurations of X-ray baggage scanners will be discussed. The first contains a conventional single view dual energy system as well as a separate system which provides a backscattered image. The second is CT system. The object is a clock radio in which there is a concealment of TNT (Fig. 12). Note that these images would usually be in color, and the threat material would have been colored orange. The left hand image is from a conventional system. The center image is a backscattered image. The right hand image was taken with a CT system. The thumbnail images on the right show the shape of the TNT block and lines link these with the position of the threat. As well, the value of Zeff is displayed on the monitor screen (in this case, 7.42, which corresponds to the value for TNT). The CT system produces a clearer image, and provides the operator with the value of Zeff at any location at which he places his mouse-cursor. This increase in precision is gained at the expense of throughput.
3.3. X-ray and neutron examination system The absorption coefficients for any particular material for X-rays and neutrons are completely different. Whereas the Xray scattering is caused by interactions of the photons from the electrons within an atom (Fig. 5), and is a strong function of Z. The neutrons are scattered by the nucleus, and the neutron absorption cross-section generally remains fairly constant as the atomic weight varies. The ratio R ( ¼(ln(I0/I)neutron/ln(I0/I)X-ray) for a number of common materials is shown in Table 1. Comparing the measured R-values with a tabulation of R-values enables the operator to identify the materials within the container under examination. Materials like aluminum, water, glass, and many of the explosive materials are difficult to indentify using solely X-ray systems. They are easily identified using the R-value method.
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4. Protocols: passenger, air cargo/pallet, shipping containers
Acknowledgments
It is imperative that equipment is properly designed and maintained, and that operators are properly screened and trained. Protocols for the proper maintenance and testing of all the systems exist (ASTM: http://www.astm.org; ANSI:http://www. ansi.org.). For passenger baggage systems the standard is ASTM F792e01. For large tunnel systems the standard is ANSI 42.45. Research is continuing on the creation of appropriate inspection standards. Recently standards have been developed for pallet and shipping container X-ray examination systems (Blagejovic and Creagh, 2011; Creagh and Blagejovic, 2008).
This chapter is a summary of my work in the field of Border Technology and Border Protection during the past fifteen years. Many have helped me. To them I am grateful for their friendship and advice. And I give my heartfelt thanks to them for their generous support.
5. Conclusions Over the past decade there have been many advances in the technology associated with border protection. The principles of operation of the systems outlined in this chapter will not change much in the next decade, but the detailed construction of systems and their operator interfaces will undergo constant change. One area in which advances may be made in the near future is in the routine development of neutron systems for the imaging of baggage and air cargo. To cope with increasing passenger and freight traffic improvements have been made to simplify the presentation of information to the operators at checkpoints. It must be stressed that, with the increasing complexity of examination systems comes the risk of system failure due to incompetent maintenance strategies. Risks are involved associated with inappropriate management practices such as the acquisition of systems which are not fit-for-purpose and the failure to employ operators with appropriate skills. The human dimension must not be neglected: operators must be properly security screened, have the innate skills necessary for image interpretation, be properly trained, and be motivated perform their tasks consistently at the highest level.
References American Society for the Testing of Materials. /http://www.astm.orgS. West Conshohoken, PA, USA. American National Standards Institute. /http://www.ansi.orgS. Washington, DC, USA. Blagejovic, N., Creagh, D.C., 2011. 0266427. Imaging test piece for medium and large security X-ray scanners. US Patent and Tradmarks Office. Washington, DC, USA. /http://www.upto.govS. Creagh, D.C., 2012. Technology for Border Security. In: Grupen, C., Buvat, I., McGregor, D. (Eds.), Handbook of Particle Detection and Imaging. Springer Verlag, Heidelberg, Germany, pp. 633–652. Creagh, D.C., Blagejovic, N., 2008. 2008903226. A new system for the evaluation of the performance of medium- and large-tunnel X-ray examination systems. Australian Patents Office, Canberra, Australia. /http://www.ipaustralia.gov. auS. Eberhardt, J.E., Rainey, S., Stevens, R.J., Sowerby, B.D., Tickner, J.R., 2005. Fast neutron radiography scanner for the detection of contraband in air cargo. Appl. Radiat. Isotopes 63, 179–188. European Parliament: Regulation (EC) No 300/2002: common rules in the field of air safety security. Green, D.M., Smets, J.M., 1966. Signal detection Theory and Psycophysics. Wiley, New York. Harding, A., Schlomka, J.-P., Harding, G., 2002. Simulations and experimental feasibility study of fan beam coherent scatter CT. Proc. SPIE 4786, 202–209. Harding, G., 2009. Systems and methods for using a crystallinity of a substance to identify the substance, US Patent 751954. Knoll, G.F., 2000. Radiation Detection Measurement, 3rd Ed. Wiley, New York. Laine, E., Lateenmaki, J., 2008. The energy dispersive X-ray diffraction method. J. Mater. Sci. 15 (2), 269–277. Lee, Y.S., 2009. Principles of THz Science and Technology. Springer, New York. Manohara, S.R., Hanagodimath, S.M., Thind, K.S., Gerward, L., 2008. On the effective atomic number and electron density: for all types of materials and energies above 1 keV. Nucl. Instrum. Methods B266, 3906–3912. Miller, J., Barrall, G., 2005. Explosives detection with nuclead quadrupole resonance. American Scientist 93, 1–8. Rieke, G., 2003. Detection of light: from the ultraviolet to the sub-millimeter, 2nd Ed. Cambridge University Press, Cambridge.
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Radiation-based techniques for use in the border protection context Dudley Creagh n Faculty of Applied Science, University of Canberra, Canberra ACT 2601, Australia
H I G H L I G H T S c c c c c
Radiation-based techniques used by border protection agencies are reviewed. Passenger portals use THz and mm-wave radiation, CCTV is used for crowds. For the examination of luggage, air-cargo, and shipping containers, X-rays are used. The need for excellent materials discrimination software is demonstrated. Materials discrimination using a combined neutron and X-ray system is described.
a r t i c l e i n f o
abstract
Article history: Received 8 October 2012 Accepted 3 December 2012
Most airline travelers will be familiar with the current overt passenger examination procedures: metal detectors and small tunnel X-ray examination systems. The mix of overt and covert systems used to prevent dangerous goods and contraband from passing through the portal is constantly changing, dictated by policy decisions made by governments. The United States of America and the European Union are the largest regulatory bodies, and their procedures are adopted by smaller countries: Australia, for example. This paper discusses a wide variety of techniques used by Border Protection Agencies. Most of these examination systems involve the use of the emission, absorption, and scattering of electromagnetic radiation and descriptions of these systems will comprise the bulk of this paper. However, a brief discussion of the use of neutron scattering will be given to demonstrate how systems for the examination of large objects may develop in the future. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Border protection THz radiation mm-wave radiation CCTV X-ray scattering Neutron scattering
1. Introduction When passengers enter the security portal of an airport they are requested to remove from their person many of their personal possessions: hats, coats, jackets, mobile phones, keys, wallets,y. The list seems endless. These are placed on plastic trays which are then examined with the passenger’s hand luggage using a small tunnel X-ray system. But that is not the end of the pre-scan procedures. The passenger must remove any computers, bottles above a certain size, aerosol cans, and the like from the handluggage, and place them on trays to be X-rayed. All this tests the patience of the passengers and adds significantly to the through-put time for the passengers, and hence the loading time for an aircraft. But it is necessary, because all governments are concerned about the possibility of bomb, hand guns, knives, and other dangerous goods being taken onto aircraft. They have regulations, for example, the European Parliament
n
Tel.: þ61 2 62953353. E-mail address: [email protected]
(2002), about what is not allowed: and what equipment and personnel will be deployed for the examination process. But what the passenger sees does not necessarily reflect the totality of the examination to which he is subjected. A range of unseen systems may have scanned the passenger. He may have been examined by covert systems using passive or active THz radiation or mm-radiation. He will certainly been observed by Closed Circuit Television Systems (CCTV) and his image compared with lists of banned passengers. Recent changes regulations may force the passenger to be scanned in an active mm-wave cabinet, or subjected to examination in a low energy low intensity X-ray body scanner. His carryon baggage and other effects will have been examined by an X-ray system. As well, his hold baggage will have been examined by a more sophisticated X-ray system that that in the passenger hall. In all, the passenger and his effects would have been examined by electromagnetic radiation spanning the photon energy range from o1 meV to 0.5 MeV. I shall not discuss here the covert non-radiation-based examinations made by dogs trained to detect explosives and drugs, and systems using ion beam mass spectroscopy. Nor shall I address
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the complexities of facial recognition software for identifying suspicious people. In discussing X-ray systems I shall concentrate on systems used in the passenger hall: small tunnel systems ( o1000 mm 1000 mm tunnel aperture). I shall not discuss the large pallet and shipping container X-ray systems used for examining air cargo and shipping containers. I shall, however, discuss the principles of operation of a new X-ray/neutron system for the examination of shipping containers.
2. Examination systems A distinction has to be made, first, between ionizing and nonionizing radiation. Ionizing radiation has sufficient energy to break chemical bonds and alter the genetic structure in cells. The boundary between the non-ionizing and the non-ionizing regimes could be thought to occur in the ultra-violet region of the optical spectrum ( 350 nm), due to which skin cancers can be induced. For wavelengths longer than this the radiation is deemed to pose no health threat to humans. THz and mm-wave radiation fall into the non-ionizing category. A distinction must also be made between passive and active systems. Passive systems are those in which the subject generates its own radiation. Active systems irradiate the subject with radiation. 2.1. Passive passenger portals Passive passenger systems rely on measuring the black body radiation from the incoming passenger and the variations in spectral emissivity of the item carried from that of the passenger. The emission obeys the Stephan Boltzmann Law: energy flux density¼J¼ esT4. Here e ¼emissivity (0o e o1); s ¼ 5.7604 10 8 W m 2 K 4; T¼temperature (K). Currently available systems are sensitive to radiation from the person in the mm wave ( 100 GHz) or the THz ( 250 GHz) regions. The design and construction of mm-wave and THz detectors is a rapidly growing field (Knoll, 2000;Rieke, 2003; Lee, 2009). A wide range of multipixel semiconductor devices have been developed to operate at room temperature. These new detectors will be incorporated in systems using existing digital TV technology. Commercially available passenger portals tend to use only a line of detectors, and the image of the subject is scanned across this line onto the detector array using a tilting mirror (Fig. 1). Several images are available to the operator: the Closed Circuit Television (CCTV) image; the raw image presented by the detector, and enhanced images (edge-enhancement, for example) (Fig. 2).
Fig. 1. Exploded schematic view of a mm-wave passenger scanner.
Fig. 2. CCTV and enhanced images of a passenger with a wallet in his hip pocket.
These systems examine only one side of the passenger. Either two systems have to be deployed so that the front and back of the passenger can be viewed simultaneously, or the passenger has to turn around so that a second image can be taken. Screening authorities, such as those in airports, are very concerned with ‘‘throughput’’, the number of passengers per hour passing the portal. As well, they are concerned with false alarm rates, both positive (there is a threat), and negative (there is no threat). Both have a deleterious effect on throughput. Advantages of these systems are that: measurements can be made at a distance (stand-off capability); observations can be made whilst the subject is approaching the portal; objects at temperatures different from the body or having different emissivities will provide an image; the system can see what is hidden under a moderately thick layer of clothing; examination is rapid. Disadvantages include: lack of materials discrimination; poor resolution; inability to detect items in cavities and crevices; inability to detect objects in the foot and ankle region; poor detection of objects not in contact with the body. 2.2. Active mm-wave passenger portal technology Active passenger portals consist of mm-wave or THz generator and antenna system and a corresponding detector system. Beams of radiations are scanned over the subject, and an image corresponding to the point-to-point reflectivity of the subject is formed. In a sense this is a RADAR map of the subject. For portals which are large enough to encompass a human body it is necessary for the system to be enclosed in a cabinet because of radiation licensing regulations. In all countries the electromagnetic spectrum is divided into bands which are allocated to corporations and other entities by the relevant Broadcast Authority. Enclosing the system minimizes the problem of encroaching on another user’s band. By their nature these systems do not have a ‘‘stand-off’’ capability. Fig. 3 shows a photograph of a cabinet passenger mm-wave scanning system. As well, an image of a passenger is shown. To meet the requirements of privacy legislation the outline of the image is a computer generated anthropomorphic shape. Some details of his clothing are evident. A circular shape has been placed around the genital area to ensure privacy. The operator can, however, turn off this privacy filter: but he has no visual contact with the subject. The passenger is rotated through 180o to ensure that both front and rear images are produced. Images of three square objects of
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Fig. 3. Cabinet mm-wave passenger scanner (left), and an image of a subject taken with the system (right) are shown. (Taken with an L3 ProVision).
different sizes can be seen. Because the passenger is supposed to have divested himself of everything except their clothing these images would be regarded as suspicious. Note that the extremities of the passenger are not imaged. The advantages of this system are: mm-waves have no deleterious interactions with people; the spatial resolution is 2 mm; the throughput is acceptable ( 600 subjects/h); the system maps the surface of the passenger, illicit items being identified by their shape and contrast; the passenger’s clothing has only a small effect on the image. Disadvantages include: lack of materials discrimination; inability to detect items in cavities and crevices, and in the lower extremities; sensitivity to metallic items; poor detection of objects not in contact with the body. If no portal is available the passenger may be manually scanned using a handheld device (referred to as a ‘‘wand’’). Because of their low power and the fact that they can be positioned in close proximity to areas of the person the problem of encroachment on other bands in the electromagnetic spectrum is considered by the licensing authority to be of little significance. A wand contains a THz antenna and a THz detector. The highly collimated THz photon pulse excites ground state electrons in the object under investigation to higher excited states. In relaxing to the ground state photons are emitted corresponding to the energy difference between the levels: Eupper–Eground ¼hc/l. The spectral lines are characteristic of the material examined. Comparison of the spectrum with entries in a spectral database enables materials to be identified. The ability to identify materials is important: the ability to produce an image does not, itself, give the operator sufficient information about the object under scrutiny to be certain of his identification. 2.3. Active devices: metal detectors In its simplest form a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. When this coil is placed
close to conductive material eddy currents are induced in the material and an alternating magnetic field is created. Another coil can be used to detect the absorption or re-transmission of the radiation due to the presence of the conducting material. The frequency of the oscillator can be tuned to indicate the presence of particular metals, the most common being gun metal and steel. Arrays of oscillator/detector coils can be manufactured, and crude imaging is possible. Metal detectors are used commonly in passenger portals (where systems can contain a number of oscillators tuned to different frequencies), hand held devices (tuned to a fixed frequency range), or tunnel devices, used (for example in the examination of parcel post for firearms). Fig. 4 shows the image of the barrel of a Glock pistol concealed in a parcel with other metallic items, and beside it, an X-ray image of it, with a threat ellipse generated by the metal detector. This image was taken using a system using the phenomenon of Nuclear Quadrupole Resonance (NQR). See Miller and Barrall, 2005 for a description of how this technique is used for the detection of explosives. The principal advantage of metal detectors is that they provide a relatively inexpensive and rapid assessment of whether the passenger is carrying threat items. The disadvantages are that: the systems are relatively insensitive to the type of metal carried, with the result that false positives occur regularly, especially if the passenger has metal prostheses and implants; they can have an adverse effect on heart pacemakers. 2.4. Active devices: X-ray passenger portals A discussion of the variety X-ray systems used in the border protection context is given by Creagh (2012). X-ray systems may cause harm to passengers if not used correctly because they cause ionizing radiation. Their use is therefore closely regulated by Government instrumentalities. Used in an X-ray passenger portal context the X-ray energies lie in the range 50 to 160 keV. The interactions of radiation with matter are summarized in Fig. 5.
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Fig. 4. Metal detector image (left) and X-ray image (right) of a Glock pistol concealed in a postal parcel. Only the barrel of the pistol is metal. (Using the QRSciences QRX1000 system).
Fig. 5. Scattering of photons by free atoms: carbon.
This shows the cross sections for different scattering processes by atoms, in this case, carbon. In practice the systems are either transmission systems (the X-ray passes through the subject to the detector) or backscattered (the X-ray is scattered back towards the detector). The dominant scattering processes are photoelectric (spe) and backscattered (Compton) (sinel). Because the photon energy changes as a function of scattering angle in Compton scattering the backscattered photons have energies of 0.83 of the incident photons in the 50 to 160 keV energy range. Fig. 6 shows a schematic diagram of a transmission X-ray system currently in use by border protection agencies and prison authorities. Full body scans may be required to enable the contents of the vaginal, rectal, and abdominal cavities to be made. The X-ray source has a maximum energy of 160 keV. The X-rays are collimated into a fan shaped beam and detected using a linear array of scintillation detectors comprising 1000 elements. The absorption follows the Beer–Lambert Law: I ¼I0 exp (mlt); where I0 is the incident energy, t is the path length in the material; ml ¼linear absorption coefficient. The operator makes judgments based on differences in contrast and brightness in the images caused by the variation of absorption coefficients and thicknesses for the materials present in the object. He then relates the image he has observed to images he has seen in the past. The training and experience of the
examining officer are important ingredients in the examination process. Fig. 6 includes a typical image. The disadvantages of the system are that: it has low throughput (10 passengers/h); relatively high doses are delivered to vital organs (3 mSv/scan); the skeletal image can obscure areas of interest; materials discrimination is not possible, although the contrast increases as the atomic number of the target material increases. Systems using backscattered radiation operate on a somewhat different system. A collimation system is used to create to a fan beam and a rotating slit system is used to form a pencil beam which moves horizontally across the subject. The X-ray tube and rotating slit system is then translated downwards so that the flying spot completely traverses the subject. The X-rays scattered back from the subject are collected on large sheets of scintillation material (often gadolinium oxysulphide), one on each side of the plane of rotation of the collimator system. Only two scintillation detectors are required. The data is assembled for display in much the same way as TV images. Fig. 7 shows a schematic diagram of such a system, and an image of a subject with concealments attached to his person. The passenger has to be scanned twice so that both its front and back can be examined. If two systems were to be used the throughput could be doubled. The operator uses contrast and shape variations to identify anomalies. As with all security applications detection depends on the alertness and training of the operator. Advantages of the backscatter type of portal compared to the transmission type are that they: operate at lower voltage (100 keV); do not penetrate deeply into the subject; are not affected by skeletal artefacts; deliver a lower dose per scan (0.1 mSv); have a greater throughput (60–100 passengers/h); is sensitive to low atomic weight materials, such as are in liquids and gels. Disadvantages include: the inability to inspect cavities and crevices; poor materials discrimination; the contrast observed depends on both the material of the object and the material behind it. 2.5. X-ray baggage, pallet, and container systems The simplest X-ray systems used in baggage, pallet (and air cargo), and container examination consist of the following components: an X-ray source, a slit system which forms a fan beam (see Fig. 6), a detection tunnel which incorporates a means for translating the object under examination through the fan beam, a detector array which measures the changes in intensity of the X-ray beam as it passes through the X-ray fan beam, and a system for accumulating, storing and manipulating the data into an image which can be assessed by an operator. X-ray systems fall into three categories: those for baggage (tunnel size o1200 mm 1200 mm), pallet ((1200 mm)2 oH Wo (2000 mm)2), shipping container (H W4(2000 mm2)) applications.
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Fig. 6. Schematic diagram of a transmission X-ray portal (left), and a typical image of a passenger (right).
Fig. 7. Schematic diagram of a backscatter X-ray portal (left), and an image (right) of a subject carrying concealments. Note that images of the subjects shin bones can be seen. (Taken with an AS&E Smart Check).
For tunnel sizes less that (2000 mm)2 sealed X-ray tubes are the preferred source. For shipping containers the preferred source is the linear accelerator (LINAC). Both sources are sealed, highly evacuated systems in which electrons from a heated filament are accelerated to a high voltage and collide with a cooled target (usually tungsten). The resultant change in energy causes the radiation of electromagnetic radiation (bremsstrahlung). For passenger baggage E0 is usually 160 kV; for pallet, 450 kV, and for containers, 3 to 9 MV Whilst sealed tube X-ray systems are operated as constant potential devices, LINACs are pulsed devices, with the pulse rate determined by the speed of transport of the object past the X-ray fan beam. The target for the electrons is typically a water-cooled 1 mm thick gold plated tungsten disc. Fig. 8 shows intensitywavelength plots for bremsstrahlung from a sealed X-ray tube for different values of accelerator potential E0, together with intensity versus energy curves for a LINAC operated at E0 ¼2.5 MeV. The method of transport of the object past the object varies from application to application.
2.5.1. X-ray baggage, pallet, and container systems: gray scale images For baggage and pallet applications the source and detector systems remain stationary and the object moves through the fan beam on a conveyor belt or a driven roller system. For container
systems the container is either towed through the fan beam or the container remains stationary and the source and detector are scanned along the container. Typical scan rates are 200 mm/s. Modern detector systems usually comprise arrays of scintillation detectors. Scintillation materials used include cesium iodide (CsI), cadmium tungstate (CdWO4) and mercury cadmium telluride (HgCdTe). The scintillations caused by the incident photons are detected by solid state detectors such as avalanche photodiodes (APD). The arrays usually comprise more that 1000 separate elements which are grouped in units of 32 elements. Each element presents an active area of 6 mm 6 mm or less to the beam. The output of each of the detectors is interrogated using multiplexing techniques, and that output is ratioed with the output of elements which were not impeded by the object (giving the ratio I/I0, from which the average attenuation coefficient can be computed using the Beer–Lambert Law.). Each time slice is the output of the linear array, and the overall image is the outcome of linking these time slices, and displaying the result on a computer monitor. The resulting black and white (Gray Scale) image can then be manipulated by the operator so that he can better understand the components of the image. For simple objects it is easy to relate the X-ray image to the object examined. In practice objects are rarely simple, and a number of objects may overlay one another. The intensity of the image depends on the materials through which the X-ray beam passes, and
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Fig. 8. Intensity–wavelength plots for bremsstrahlung from a sealed tube X-ray source operated at a number of accelerator potentials (left) and intensity plotted against energy for a LINAC operated at 2.5 MeV (right).
ln (I/I0)¼ (m1t1 þ m2t2 þ m3t3 ¼)yy., where m1t1 is the absorption thickness: the product of the linear attenuation coefficient of the material and the thickness of that material. Note that ml can itself be either that of an atom or a chemical compound. Each manufacturer has his own set of algorithms for enhancing the image. Techniques may include contrast and brightness adjustment, edge enhancement, histogramming, and so on. These algorithms are similar to those available in photographic programs in personal computers.
2.5.2. X-ray baggage, pallet, and container systems: materials discrimination Some degree of materials discrimination is possible if measurements along the same path length can be made at different photon energies. As can be seen in Fig. 8 the intensity-energy spectra are continuous spectra. To a good approximation it is possible to establish a mean value for each of the spectra, and use these in calculations. Fig. 5 shows that the photo-electric scattering factor and the Compton scattering factor are very different in the energy range used in baggage and pallet systems. If the mean energy is changed, therefore, the value of (I/I0) will be changed, and a new value of mlt will result. From the simultaneous equation it is possible to determine ml and t for a two component system, independent of the material thickness. The ratio M ¼ln(I1/I2)¼ m2/m has a fixed value for a given material. This ratio can be determined empirically for a chosen X-ray system. The variation of mean energy can be effected by: using two separate X-ray source energies (switching from 6 MeV to 3 MeV kV at a rate of, say, 200 Hz); by having two separate identical detector arrays with an absorber placed in front of one of the detector arrays to absorb the low energy part of the spectrum; and by having two detectors of the same type in line with one another, the first detector absorbing the low energy part of the spectrum, and the second detector absorbing the high energy part of the spectrum. Systems of these types are referred to as Dual Energy Systems. The first option is better because there is no spatial difference in the images, and the energy difference between the spectra is better characterized. The separation of the detector arrays in the second case creates spatial resolution problem, which leads to blurring of the image. The effect of spatial separation on image resolution is demonstrated in Fig. 9. Here a composite single energy bank of 32 detectors (CsI in front of CDWO4) is placed into a conventional dual array system. The resolution of the inline system is demonstrably superior to the side-by-side array, and the signal-to-noise ratio of the inline system is better than the conventional dual system by about an order of magnitude. The dynamic range is 212 or 12 Gy scales.
Fig. 9. Images of two urns taken using dual and inline X-ray detectors. (Taken with an L3 CX450DV system).
If more than one X-ray beam (referred to as ‘‘views’’) are used to scan the sample the number of simultaneous equations for which solutions can be found increases, and M can be found for increasingly more complicated packing in the scanned object. Dual view, triple view, and quadruple view systems are marketed at present. Computed tomography (CT) systems can give as many views as the manufacturer may wish. The image processing time for CT systems determines the throughput, and throughput is a major determinant in the organization of the operation of airports, in both the passenger and air cargo contexts. A compromise has to be made between certainty of detection and throughput. High throughput causes increased false positive and false negative rates. The additional data available in multi-view examination systems makes it possible to calculate the effective atomic number, Zeff (Manohara et al., 2008) and the density r for any voxel (volume element) in the object. Zeff ¼ SifiAi(mm)i/Sjfj(Aj/Zj)(mm)j; f is the molar fraction, and A and Z are the atomic weight and the atomic number of the atomic species; mm is the mass absorption coefficient ( ¼ ml/r). Zeff and r can be used as a discriminant between materials. For metals: Zeff 419. For inorganic materials:19 4Zeff 4 11. For organic materials: 11 4Zeff. Most explosive materials have parameters in the range: 8.34Zeff 46.5 and 0.9 o r o1.8 g/ml. If an object has a measurement of Zeff which lies in the explosives range there is reason to proceed to further examination of the baggage by other means.
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All US manufactured X-ray examination systems are accredited by the United States Transport Security Administration (http://www.tsa.gov) as explosives detection systems (EDS). These purport to make the assessment as to whether the object contains an explosive threat without human intervention. In practice, however, confirmation by an operator on the basis of shape and contrast is required. The performance of X-ray examination systems is usually described in terms of a Receiver Operator Characteristic (ROC) diagram (Fig. 10) (Green and Smets, 1966). For multi-view systems materials discrimination using automatic computation of Zeff and r can lead to reasonably high throughputs, but the systems are prone to false positives being recorded because innocuous materials may well lie in the threat band (for example water, chocolate, and other innocuous substances have Zeff and r in the threat region). To overcome this deficiency EDS systems can be used as a primary screening device and questionable objects diverted into a system which has a much smaller throughput, but which can give a spectroscopic analysis of the questionable area of the object. Such a system uses energy dispersive X-ray diffraction (EDXRD) (Knoll, 2000). 2.5.3. X-ray baggage, pallet, and container systems: materials discrimination: diffraction systems In Energy Dispersive X-ray Diffraction (EDXRD) systems the tube source (which is a bremsstrahlung source (see Fig. 8)), its
Fig. 10. Idealized Receiver Operator Characteristic (ROC) curves. The diagonal line represents the results if the operator simply guesses.
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collimator system, and an energy dispersive X-ray detector are mounted so that that the beam emitted from the source can pass through the suspect region of the object and be scattered into the detector which is offset by an angle of about 51. The elastically scattered photons from the suspect region interact with the detector giving output pulses proportional to these photon energies (Laine and Lateenmaki, 2008). Most systems are crystalline and elastic scattering occurs according to Bragg’s Law: 2dhkl sin y ¼ l ¼hc/E. Because y is fixed the diffraction peaks occur whenever photon energy corresponds to a particular interplanar spacing dhkl which satisfies the Bragg equation. Fig. 11 (Harding et al., 2002; Harding, 2009) shows the diffraction patterns of an explosive, Semtex (C3H8N4O12 (76%) þC3H6N6O6 (4.6%)), and chocolate taken with a laboratory X-ray system. Chocolate is often confused with explosives by operators. They have clearly different diffraction patterns which can be rapidly compared with entries in the JCPDS database. The International Commission on Diffraction Data (http://www.icdd.com) maintains the JCPDS files which include some 500,000 entries. Recent developments in detector design and improvements in system sensitivity have made it possible to identify some amorphous materials and liquids as well as crystalline materials.
2.5.4. X-ray, pallet, and container systems: materials discrimination: X-ray & neutron system A system has been developed for the X-ray & neutron examination of air cargo ships and shipping containers (Eberhardt et al., 2005). A commercial realization of this system (Nuctech AC series(FINDEX): http://nuctech.com) uses a switched 6/3 MV LINAC X-ray source with two collimators which produce two vertical fan beams at an angle of 121 to one another. The system has two array detector systems, one for each fan beam. The objects are towed through the fan beams at a constant 200 mm/s and conventional images are taken at 3 and 6 MeV energies in each detector system. The resulting images can be combined to produce a binocular view through the object, and the switched voltage source can give a crude indication of materials discrimination as indicated in Section 2.5.3. Incorporated in the system is a conventional sealed tube neutron generator which produces 1010 fast neutrons per second of energy 14 MeV. The neutrons as formed as a result of the 2 H1 þ 3H1-4He2 þ 1n0 reaction Neutrons are emitted isotropically from the target, but, similar to the X-ray case, they are collimated to produce a vertical fan beam using a slit. After the object has traversed the beam the attenuated beam intensities are detected by an array of BF3 proportional detectors. The attenuation of the X-ray beams and the neutron beams are measured, and the output of pixels from the two arrays are mapped to form the ratio, R(¼ln(I0/I)neutron/ln(I0/I)X-ray). Whilst the X-ray attenuation cross
Fig. 11. Diffraction patterns for semtex (76% PETN þ5% RDX), (left), chocolate (right).
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Fig. 12. TNT concealment a clock/radio: conventional:rectangular shape in bottom left of Left Image; backscatter: white rectangle at the left hand corner of the Central Image; CT: rectangular shape at the bottom right of the right image (and the corresponding thumbnail images).
Fig. 13. Image of two containers taken in one scan of the gantry (360 s).
Table 1 R-values for a number of common materials. Material
Lead
Iron
Aluminum
Glass
Teflon
Graphite
TNT
Paper
Heroin
Water
HDPE
R-value
0.3
5.2
6.8
8.8
1.08
1.15
1.25
1.41
1.44
1.56
1.76
sections are regular functions of the atomic number the neutron cross sections are not. The ratio R can be determined for a given element and is a characteristic of that element. For nitrogenous explosive materials R lies in the range 1.18 oRo1.28. However, as in the case of Zeff, some non-threat materials have R-values in this range. The system ultimately relies on the skill of the operator to interpret the high resolution image seen.
3. Examples
3.2. X-ray shipping container examination systems X-ray shipping container examination systems are usually gantry systems and use single view LINAC sources. Dual view systems exist, but most large systems are single view systems. Tunnel dimensions are such that shipping containers are able to be examined (typically 4 m 4 m). The gantry, which contains the source on one side and the detector array on the other side, can be stationary (the container is towed through the gantry), Fig. 13 shows a typical image taken with a moving gantry system. Identification of anomalies is effected by shape and contrast resolution.
3.1. The performance of X-ray baggage systems The performance of two extreme configurations of X-ray baggage scanners will be discussed. The first contains a conventional single view dual energy system as well as a separate system which provides a backscattered image. The second is CT system. The object is a clock radio in which there is a concealment of TNT (Fig. 12). Note that these images would usually be in color, and the threat material would have been colored orange. The left hand image is from a conventional system. The center image is a backscattered image. The right hand image was taken with a CT system. The thumbnail images on the right show the shape of the TNT block and lines link these with the position of the threat. As well, the value of Zeff is displayed on the monitor screen (in this case, 7.42, which corresponds to the value for TNT). The CT system produces a clearer image, and provides the operator with the value of Zeff at any location at which he places his mouse-cursor. This increase in precision is gained at the expense of throughput.
3.3. X-ray and neutron examination system The absorption coefficients for any particular material for X-rays and neutrons are completely different. Whereas the Xray scattering is caused by interactions of the photons from the electrons within an atom (Fig. 5), and is a strong function of Z. The neutrons are scattered by the nucleus, and the neutron absorption cross-section generally remains fairly constant as the atomic weight varies. The ratio R ( ¼(ln(I0/I)neutron/ln(I0/I)X-ray) for a number of common materials is shown in Table 1. Comparing the measured R-values with a tabulation of R-values enables the operator to identify the materials within the container under examination. Materials like aluminum, water, glass, and many of the explosive materials are difficult to indentify using solely X-ray systems. They are easily identified using the R-value method.
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4. Protocols: passenger, air cargo/pallet, shipping containers
Acknowledgments
It is imperative that equipment is properly designed and maintained, and that operators are properly screened and trained. Protocols for the proper maintenance and testing of all the systems exist (ASTM: http://www.astm.org; ANSI:http://www. ansi.org.). For passenger baggage systems the standard is ASTM F792e01. For large tunnel systems the standard is ANSI 42.45. Research is continuing on the creation of appropriate inspection standards. Recently standards have been developed for pallet and shipping container X-ray examination systems (Blagejovic and Creagh, 2011; Creagh and Blagejovic, 2008).
This chapter is a summary of my work in the field of Border Technology and Border Protection during the past fifteen years. Many have helped me. To them I am grateful for their friendship and advice. And I give my heartfelt thanks to them for their generous support.
5. Conclusions Over the past decade there have been many advances in the technology associated with border protection. The principles of operation of the systems outlined in this chapter will not change much in the next decade, but the detailed construction of systems and their operator interfaces will undergo constant change. One area in which advances may be made in the near future is in the routine development of neutron systems for the imaging of baggage and air cargo. To cope with increasing passenger and freight traffic improvements have been made to simplify the presentation of information to the operators at checkpoints. It must be stressed that, with the increasing complexity of examination systems comes the risk of system failure due to incompetent maintenance strategies. Risks are involved associated with inappropriate management practices such as the acquisition of systems which are not fit-for-purpose and the failure to employ operators with appropriate skills. The human dimension must not be neglected: operators must be properly security screened, have the innate skills necessary for image interpretation, be properly trained, and be motivated perform their tasks consistently at the highest level.
References American Society for the Testing of Materials. /http://www.astm.orgS. West Conshohoken, PA, USA. American National Standards Institute. /http://www.ansi.orgS. Washington, DC, USA. Blagejovic, N., Creagh, D.C., 2011. 0266427. Imaging test piece for medium and large security X-ray scanners. US Patent and Tradmarks Office. Washington, DC, USA. /http://www.upto.govS. Creagh, D.C., 2012. Technology for Border Security. In: Grupen, C., Buvat, I., McGregor, D. (Eds.), Handbook of Particle Detection and Imaging. Springer Verlag, Heidelberg, Germany, pp. 633–652. Creagh, D.C., Blagejovic, N., 2008. 2008903226. A new system for the evaluation of the performance of medium- and large-tunnel X-ray examination systems. Australian Patents Office, Canberra, Australia. /http://www.ipaustralia.gov. auS. Eberhardt, J.E., Rainey, S., Stevens, R.J., Sowerby, B.D., Tickner, J.R., 2005. Fast neutron radiography scanner for the detection of contraband in air cargo. Appl. Radiat. Isotopes 63, 179–188. European Parliament: Regulation (EC) No 300/2002: common rules in the field of air safety security. Green, D.M., Smets, J.M., 1966. Signal detection Theory and Psycophysics. Wiley, New York. Harding, A., Schlomka, J.-P., Harding, G., 2002. Simulations and experimental feasibility study of fan beam coherent scatter CT. Proc. SPIE 4786, 202–209. Harding, G., 2009. Systems and methods for using a crystallinity of a substance to identify the substance, US Patent 751954. Knoll, G.F., 2000. Radiation Detection Measurement, 3rd Ed. Wiley, New York. Laine, E., Lateenmaki, J., 2008. The energy dispersive X-ray diffraction method. J. Mater. Sci. 15 (2), 269–277. Lee, Y.S., 2009. Principles of THz Science and Technology. Springer, New York. Manohara, S.R., Hanagodimath, S.M., Thind, K.S., Gerward, L., 2008. On the effective atomic number and electron density: for all types of materials and energies above 1 keV. Nucl. Instrum. Methods B266, 3906–3912. Miller, J., Barrall, G., 2005. Explosives detection with nuclead quadrupole resonance. American Scientist 93, 1–8. Rieke, G., 2003. Detection of light: from the ultraviolet to the sub-millimeter, 2nd Ed. Cambridge University Press, Cambridge.
Please cite this article as: Creagh, D., Radiation-based techniques for use in the border protection context. Radiat. Phys. Chem. (2013), http://dx.doi.org/10.1016/j.radphyschem.2012.12.012i