Compact Linac with Dynamic Controls for Image Driven Optimisation of X-ray screening

Linac based X-ray sources are commonly used by customs officers and homeland security worldwide to scan the contents of shipping containers and trucks for illicit materials and goods, as well as dangerous materials. The linac consists of a small particle accelerator and a target. Modern scanning systems require more flexible X-ray sources to provide better scanning of objects where the X-ray output can be closely matched for each object to be scanned, known as image driven optimisation. However such flexible X-ray sources are not currently available.

Lancaster University and STFC have recently developed a compact controllable X-ray source as part of the PNPAS scheme. We propose to improve the engineering design of this linac to increase its technology readiness level as well as upgrading this linac to a higher beam energy, 3 MeV, and implement a sophisticated control system developed for high energy physics accelerators to allow the development of a flexible X-ray source suitable for image driven optimisation.

The linac is also significantly smaller than conventional linacs, allowing a reduction in weight that improves the
manoeuverability of the linac.

Major international events in recent years has made secure screening of cargo and baggage at ports and airports a priority for many nations. This is typically performed using X-rays produced by firing electron beams into tungsten targets. The electron beam is accelerated to the required energy using an RF linac. The global markety for these devices is around $600 Million per year. However future screening technology requires a new generation of RF linacs, which allow the X-ray source dose rate and energy to be matched to the object to be scanned. A flexible, mobile scanning system with energy between 1-3 MeV with a small exclusion zone could open up new sectors in scanning at airports, public events and mobile screening of loaded ULDs. The expected market for these applications is around 100 linacs per year that could be met with the two linac products to be developed under CLASP.

Current linac technology is dominated by S-band linac technology as the larger RF wavelength relaxes cavity requirements in terms of complexity of design and of manufacturing tolerances. Currently 90% of all security scanners utilise S-band linac technology. However operating with a higher RF frequency, such as X-band, allows a transversely smaller linac as well as increased gradient capability meaning shorter linacs as well. Reducing the size of a linac, and hence the weight of the shielding required, is a key requirement for mobile security screening as it allows more flexible gantry systems and reduces the weight on the rear axel of truck mounted scanners. Such a compact linac system has been developed at the Cockcroft Institute as part of the PNPAS project, however this linac has a fixed energy of 1 MeV as it is focussed on the air cargo inspection application. The CLASP project would intitially focus on further development of the 1 MeV system to allow this market to be addressed in a shorter timescale.

There are no linacs currently available that allow image driven optimisation of the X-ray source. However the experience and skills developed at the Cockcroft Institute and the Engineering Technology Centre will allow such a concept to be developed. In this project we propose to also upgrade the PNPAS 1 MeV linac to 3 MeV while developing flexible controls to allow fast optimisation of the systems energy and dose rate for each object to be scanned. This will allow a more advanced product to be produced on a slightly longer timescale.