Sizeable numbers of containers enter the country each day, whether through ports, airports, or border crossings. The risk exists that these containers will be used to smuggle hazardous materials into the country, such as nuclear, chemical, or biological weapons or materials. In addition, there are many instances where particular facilities or buildings need to be secured against the threat of attack from weapons being surreptitiously brought onto the premises, either through cargo that is delivered or in vehicles themselves. Because the level of threat to a location may change, having a portable system that can be placed temporarily or semi-permanently in a location is extremely useful. Individually inspecting containers by hand or through passive techniques in these circumstances is cumbersome, inefficient, expensive, and in many cases ineffective.
In addition to safety applications, there are other needs for inspecting containers, such as with mining operations or storage of waste (including nuclear waste). It is desirable to quickly and efficiently determine whether containers full of rock, dirt, ore, or waste contain materials of interest, such as minerals, metals, or elements. High-energy sources can penetrate the container and remotely examine its contents to determine whether it contains materials of interest.
With respect to safety inspections, passive techniques to locate nuclear materials, such as using Geiger counters and neutron emission detectors, can easily be fooled and render false positives from materials as innocuous as cat litter, which can carry trace amounts of radioactivity in the clay that comprises the litter. More troubling, they can easily be fooled into rendering a false negative by effective shielding of the nuclear material. Either false result is detrimental because it wastes the time and resources of the inspectors or entirely misses the threat being sought.
To overcome these problems, others have proposed using high-energy electrons or x-rays from LINACs and Bremsstrahlung sources to examine the containers. These high-energy sources can easily penetrate the walls of the containers, enabling inspectors to examine the contents of the containers in situ without having to manually inspect them. In addition, such sources can readily distinguish false threats from real threats, thereby reducing the number of false positive indications. More importantly, it is difficult to shield threatening materials from these high-energy sources due to their penetrating power. These sources can also detect the presence of shielding materials, which raises red flags and invites further inspection.
While these high-energy sources have many desirable traits, they are not without their own problems. First, many of these sources create x-ray or electron beams with large energy spreads, which are undesirable for a number of reasons. Typically only a narrow band of energy is needed for detection, so most of the energy in these sources is wasted. This wasted energy creates problems, such as increasing the signal noise, which makes it more difficult to detect the materials of interest, and creating a greater radiation safety hazard to inspectors and those near the system. Second, these systems can be large and difficult to move due to the necessity of using a LINAC, which only has a modest accelerating field and is therefore large and heavy. Third, these systems can be unstable and difficult to synchronize to other components, such as lasers, that may be used to create x-rays. Fourth, most LINACs are not tunable, so the system has a fixed energy and cannot be easily adjusted.