X射线安全检查技术研究新进展

Recent progress on X-ray security inspection technologies

暴恐事件的频发严重影响了国际社会的安全和秩序,给客运、货运安全带来了前所未有的挑战.有效打击恐怖活动,保障人身、财产安全,是国内乃至全球公共安全领域的重要诉求.因此,如何通过高技术手段提升安全检查能力,成为人们关注的重要问题和研究热点.随着具有穿透能力的X射线技术的飞速发展,使用X射线对藏匿于货物、行李及人体内外的爆炸物及违禁品进行检查,日益成为目前行之有效的技术手段.特别是近几十年来,利用X射线所具有的多种与物质相互作用机制,研究和发展出多种查验技术,为公共安全提供了重要保障,表现出极其广泛的应用前景.本文对基于X射线的透视成像、散射成像、X-CT成像、能谱成像、衍射XRD、相衬成像等安全检查技术做了全面介绍,针对主要技术的物理机制、应用特点及其研究进展进行了论述,并对新技术的研究及应用作了展望,以期进一步推动X射线技术在公共安全领域发挥更加重要的作用.

Emerging terrorist attacks seriously impacted the global security situation which brings unprecedented challenges to public transportation. Effectual anti-terrorist ways to protect personal safety and assets security become an important global demand. Consequently, to improve security inspection capabilities with state-of-art technology has long been a serious issue and hot research topic. Recent years, with the rapid development of X-ray detection technology, utilizing X-rays to inspect hidden explosives and contrabands in cargo, luggage, vehicle and on human body become both effective and efficient. Based on different X-ray interaction behaviors with matter, a variety of inspection techniques have been researched and developed which show great potentials for security applications. This paper provides a general review on different X-ray inspection techniques such as X-ray transmission imaging, backscatter imaging, phase-contrast imaging, spectral imaging, CT reconstruction, X-ray diffraction and so on. Basic physical mechanisms, research progresses and application features are investigated. And outlooks to new technologies and applications are given. X-ray transmission imaging detects the attenuation of X-ray flux while penetrating objects. The transmission image has a strong discrimination for high-density materials with strong X-ray absorption, such as guns, knives, grenades, detonators and other high atomic number items. Dual-energy X-ray imaging is an important complement that provides preliminary material classification capability. The technology has been widely used for sea freight cargo and passenger luggage inspection. X-ray back-scatter imaging detects X-rays that retlects trom me scanned objects te.g., venlcie, container ana numan body）. It is good for imaging low density （or low atomic number） materials. The source and detectors of the scanning device are usually designed on one side of the target object, and a rotating wheel with pencil beam collimators is the key to scan in a point-by-point method. Computed tomography （CT） inspection system generates volumetric image of scanned item. It provides high resolution 3D inner structures nondestructively and without overlap. Due to the rapid development in CT reconstruction theory, various applications for security inspection have emerged. Hot research and development topics include material classification method, dual-energy and multi-energy CT imaging, non-spiral trajectory CT scan, artifacts correction and scan speed improvement. X-ray diffraction （XRD） technique has been widely used to identify minerals and other crystalline materials. It also shows good ability for detecting drug, explosives and hazardous liquids in security examination. Photo-counting detectors are utilized to measure scatter flux on different photon energy levels to form diffraction spectrum. Currently research on XRD for security application is focused on improving signal-to-noise ratio, reducing scanning time, material recognition method and system design. Phase-contrast imaging （PCI） calculates X-ray phase-shift when traversing the item. For low atomic number materials the cross section of the phase-shift can achieve up to 103 times larger than the one of attenuation. Thus it gains much better image contrast for fruits, clothes, drugs, liquids and other low atomic number items. Grating-based imaging has been regarded as the most promising PCI method because it is a first approach that requires only conventional X-ray source rather than synchrotron or micro-focus X-ray sources. Preliminary research work can be found on the security application potentials of grating-based PCI systems and methods.