X-ray imaging for homeland security

Since 9/11 there has been a growing interest to improve the security both of air travel and of cargo shipping. Visual inspection is the simplest tool and is very efficient, when single objects of small size are to be checked. The complexity of inspection increases as we have large "boxes" and when several objects are packed into a closed space, such as in a luggage or even in larger volumes like shipping containers or trucks. An obvious method to check these objects is using X-rays, which can penetrate through the shipping "boxes" and let us see what is inside. It is well known that the X-ray absorption of an object depends on the absorption coefficient and the thickness of a given material. So X-ray is an excellent tool to check luggage without opening and manually checking each of them. This paper gives an overview of different x-ray luggage inspection techniques from simple transmission imaging to multi-energy and 3D X-ray computer tomography (CT). However, none of the X-ray imaging methods give a full differentiation between a plastic explosive and a bag of flour of the same shape. X-ray diffraction is a more sophisticated method to differentiate materials with and without crystalline structure and among materials of different crystalline structures. There is also a real danger that not only traditional explosives but nuclear materials can be smuggled into the US in cargo containers and trucks. Not only the explosion of a nuclear bomb but also a so called "dirty bomb" could cause death and panic in large cities. So to check the containers before they enter the country, for explosives and especially for nuclear materials is a primary goal. As it was mentioned earlier the X-ray absorption of the materials depends on the X-ray energy. Higher energy mega-electronvolt X-rays can penetrate through thick steels and other highly absorbent materials. This makes it possible to check the content of shipping containers or trucks with steel walls in the loading area. The theory and limits of high energy X-ray imaging, dual energy X-ray imaging, and high energy CT are also demonstrated. A further application using X-rays is back scatter imaging. This technique is especially useful for detection of landmines and IEDs (Improvised Explosive Devices) as it will be shown below. Finally, the use of neutron detection for identifying nuclear materials will be mentioned. Neutron detection is a viable alternative to X-ray imaging for finding nuclear materials.