Rf Sub-Systems for Cargo and Vehicle Inspection D.J
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5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-WEIB02 RF SUB-SYSTEMS FOR CARGO AND VEHICLE INSPECTION D.J. Mistry, T.A. Cross, C.R. Weatherup, e2v Technologies (UK) Ltd., Chelmsford, England Abstract X-RAY SOURCES X-ray screening for security is a well-established inspection technique. Whilst in terms of fielded systems The choice of X-ray source is dependent upon the the vast majority consist of low energy X-ray sources, physics of X-ray absorption in materials and is strongly typically used for hand baggage or mail screening. There influenced by detector performance. These constraints is a smaller but high value niche market servicing the need to be adapted to match requirements for object and requirements for border security, and cargo and vehicle material discrimination, throughput, reliability, safety and inspection (CVI). This latter application requires higher size. There are currently three main technologies used for X-ray energies of up to 10 MeV using an electron linear security systems which are characterised by their accelerator (linac) source to penetrate fully loaded penetration in steel, in Fig. 1. shipping containers. Increasingly, methods are required to improve throughput and provide a higher level of material discrimination during inspection. This paper will briefly review the elements required to make an effective X-ray source, whilst outlining the RF technology required to drive a linac-based X-ray security system. Following this, potential new developments in radiofrequency (RF) sub- systems will be discussed in the context of user benefits. INTRODUCTION Over 90% of the world’s non-bulk cargo is transported by ship in standardised containers. The containerisation system was developed after World War II and its cost Figure 1: A diagram to show the approximate range of reduction has been a major driver of international trade steel penetration for each type of X-ray source [2, 3, 4, 5]. and globalisation. There are well over half a billion container shipments per annum. China tranships about Vacuum tube designs rely on the acceleration of an 25% of the total with Europe at 13% and the US at under electron beam by an electric field and its subsequent absorption by a high atomic number (Z) anode target. The 7% [1]. 2014). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. electron deceleration occurring during absorption Security screening is a broad term which encompasses © intelligent logistics, sniffer dogs and X-ray inspection at produces a ‘stopping’ radiation, which is characterised by ports, airports, borders and mail depots. X-ray inspection a Bremsstrahlung energy spectrum. Energies between 50 - systems offer rapid, non-invasive detection of contraband 450 keV are achievable with this technique, however and explosive devices, and can be combined with other above 150 keV energies become difficult to attain techniques to identify nuclear material. Although fast and reliably, due to a reduction in anode lifetime principally effective, only a small fraction of container movements caused by cooling issues. These sources are typically used are X-rayed, as screening adds considerable time and cost for baggage and mail scanning. to the industry. However, the recognition of the economic Radioactive isotopes such as Cobalt-60 and Caesium- 137 are used to produce gamma rays with energies of 1.25 value in protecting customs revenue has driven additional 1 growth in the market, since the detection and confiscation MeV and 662 keV respectively [6]. These systems are widely used for vehicle inspection and non-destructive of contraband can quickly recover the capital cost of the system. testing (NDT) [7]. This paper will concentrate on security screening Linac based systems for heavy container and vehicle scanning produce the 2 - 10 MeV X-rays required for systems which require a linac X-ray source, as these systems produce the energies required for CVI. The increased penetration [5]. Although, Cobalt-60 sources accelerators and RF sub-systems used in radiotherapy offer a similar steel penetration to low energy linacs, it is markets have been established for many years, and thus unsuitable for imaging cargo containers. Most act as a technology baseline for X-ray security systems. manufacturers typically design linacs in the range of 3 - 6 Despite this fact, cargo scanning imposes new MeV to balance the trade-off between poor penetration at requirements such as, portability, material discrimination low energies, versus unwanted neutron production and and higher throughput, leading to a shorter time between shielding requirements at higher energies. object screening. So, it is likely that some of these 1 The quoted energy is the average energy of the two gamma rays technology developments will in turn offer real benefit emitted by a Co-60 source. The individual energies are 1.17 and 1.33 back to the radiotherapy market. MeV respectively [7]. 09 Session for Industry, Technology Transfer and Industrial Relations WEIB02 Content from this work may be used under the terms of the CC BY 3.0 licence ( T30 Industrial Collaboration 1917 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-WEIB02 The pulse repetition frequency (PRF) of linac systems X-ray Backscatter can be adjusted to match the speed of the cargo or vehicle A key requirement of the X-ray source for backscatter under inspection. Higher repetition rates permit faster is that the energy range must match that required to throughput, thereby decreasing scanning time. For observe Compton scattering for low-Z materials, see Fig. example a PRF of 500 Hz, allows cargo containers to be 3. This can be achieved using a low voltage vacuum tube scanned as they move at speeds of nearly 10 km/h, whilst for luggage and personnel screening to identify hidden achieving a 5mm horizontal resolution [5]. threats under clothing, or with a linac system to scan LINAC SCREENING SYSTEMS vehicles using a large field-of-view [9]. Originally, backscatter systems were designed to address user The technology for linac screening systems is primarily requirements for single-sided detection, [10] in cases driven by user requirements for high throughput, and where access to the far side of an object was restricted. accurate material identification. These requirements These systems detect photons which have interacted determine the choice of X-ray source, RF sub-system and with the object material via Compton scattering. The fan the detector array. In the following section a range of beam produced by the X-ray source is further collimated imaging modalities is discussed in terms of their detection using a ‘chopper wheel’ - to create a pencil beam at the capabilities and the RF technology required to drive the desired position to ‘line scan’ the object [9]. This process linac and thus generate X-rays. is repeated to create an image as the scattered photons are Most linac based systems operate in transmission detected by a set of large area integrating detectors [8]. mode, see Fig. 2. In this method X-rays are imaged after Backscatter system rely upon the strong signal received passing through the object under inspection, with photons from low-Z materials, as scattering forms a significant being attenuated along their path. This technique is proportion of the total interaction cross-section for this sensitised to denser materials which absorb more X-rays, energy range. Hence, this method can be used to identify and thus appear as brighter objects on positive contrast organic contraband i.e. narcotics and explosives [9]. images. However, due to the poor penetration of denser Forward-scatter imaging is a key variant of the materials, many commercial systems now incorporate transmission imaging method, in that a pencil beam of X- both forward and backscatter imaging modalities into a rays is used to obtain spatial information for the object. single system, using two sets of detectors, and in some This modality relies upon the same attenuation cases two X-ray sources depending on the application. mechanism as the conventional transmission method [8]. These complementary detection capabilities offer increased penetration and improved discrimination across high and low-Z materials. 2014). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. © Figure 2: A schematic diagram to highlight the RF sub-system required for a typical fixed transmission linac screening system. The RF sub-system is highlighted by the components in the red box, items coloured green are currently offered by e2v and those in purple form future development work. Content fromWEIB02 this work may be used under the terms of the CC BY 3.0 licence ( 09 Session for Industry, Technology Transfer and Industrial Relations 1918 T30 Industrial Collaboration 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-WEIB02 Dual Energy Systems range of attenuation paths are created, which can vary as much as 100000:1 [5]. In this method, a linac is used to create interlaced Detector technology is driven by the demand for high pulses of high and low X-ray energies with a high PRF to energy X-ray screening systems to provide higher provide a continuous imaging mechanism [11]. There are throughput, and better object and material identification several methods available for dual energy switching at the [13]. The design considerations for high energy detectors RF sub-system level these include; dumping some of the can be split up into the requirements imposed on the X- RF power from the magnetron to create a low energy ray source and the desire for better image quality.