Materials can produce scattering effects on incident photons and ultrasound waves, by collecting scattered signal, the objects can be imaged. Backscattering imaging is a unique technique whereby the photons or ultrasound from an object in the generally backward direction are detected. Backscatter technology has been widely used in astronomy1, 2, medicine3, 4, security inspection5-9 and other fields. Especially in the field of security, X-ray transmission is also the main means of inspecting dangerous goods, such as gun, knife, explosive, drug and so on. This technique is effective for detecting metal objects but difficult for organics such as drug and plastic explosive, because the attenuation coefficient of most organic matter to X-ray radiation is very low, the material appears to be transparent. The main elements of drug and plastic explosive are carbon, hydrogen, oxygen and nitrogen, which can produce strong scattering for incident X-ray. And, by collecting backscattered X-ray, bright images of organics such as drug and plastic explosive can be obtained. Relative to X-ray transmission, the backscattering imaging has three advantages: First, the backscatter images highlight organic materials, which appear significantly brighter than most metallic or inorganic materials. As a result, organic threats or contraband materials such as explosives and drugs can be more easily detected in the backscatter images than in the corresponding transmission images. Second, by changing the energy of X-ray, backscattering imaging can be utilized for detecting dangerous matters hidden inside or on the surface of objects, such as container and human body. Third, X-ray detectors can be placed on the same side of the objects being inspected as the X-ray source, which allow inspection of moving car or container vehicles by mounting the detectors and X-ray source on a mobile platform that drives past the objects of interest10. In addition, the equipment based on this technology can be miniaturized, which enable to extend the devise into a narrow space (car, container, etc.) and scan internally. So, X-ray backscattering imaging has become a significant method for detecting dangerous organics in the field of security inspection. At present, several inspection technologies based on X-ray backscattering imaging have been developed, for example, “Flying Spot” scanner 5 , coded aperture imaging9 and lobster-eye imaging6-8, etc.
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