Contrast sensitivity, the second important criterion for cargo screening, is extremely important for distinguishing between items inside a container. Imaging experts say that the higher the contrast sensitivity, the greater the chances for detecting contraband. Linatron-based systems have proven to be ten times more contrast sensitive than other systems.

      “The objective of nonintrusive screening is to image the contents of a cargo container with enough clarity to make a decision about the contents,” says Jim Johnson, general manager for Varian Industrial Products.
      The third criterion, resolution, is a measurement of the ability to see spatial details in an image. If you are looking for hundreds of pounds of drugs, just knowing there’s something large and unexpected inside the container is enough. However, if you’re looking for nuclear weapons components, which can be surprisingly small, you want the best resolution you can get. While resolution depends to a large degree on the quality of the detector that is collecting the imaging photons, the more photons that penetrate the cargo’s interior, the better your chances are of obtaining high resolution. Again, the advantage falls squarely to linac-generated X rays.

A Distinct Technological Advance
In addition to the three main criteria, several other factors involved in imaging give linac-generated high-energy X rays a distinct technological advantage for use in cargo screening. Varian’s Linatrons can also provide dual views by sending in two perpendicular beams to help overcome the problem of a lighter material being shadowed behind a denser material. And thanks to the high energy and photon output of the Linatron, images can be obtained very quickly – an important consideration for a busy port.
      “Before September 11, the U.S. Customs Service was mostly interested in screening cargo containers to find illegal drugs. Now their primary concern is finding weapons of mass destruction,” Boeh explains. “For this task, there does not seem to be any competitive technology on the horizon better than high-energy X rays.”
      Numerous customs services, both in Europe and Asia, have installed cargo-screening systems that use Varian Linatrons to generate X rays. They are successfully finding illegal drugs, weapons, and other contraband. According to Boeh, many of these governments have found that the ability to verify manifests, find deliberate falsifications, and levy taxes on the undeclared contents generates enough revenue to pay for the inspection systems.

Non-Destructive Testing
Varian’s Linatron technology is also useful in other forms of nondestructive testing. Highway engineers use a portable version called the Linatron MP to test the structural integrity of large steel and concrete structures like bridges and overpasses. A major manufacturer of jet engines is using the Linatron M with a Varian flat-panel image detector to inspect turbine blades.
      “They bought the Linatron to replace a kilovoltage (kV) imaging system,” says Boeh. “They needed a system that could penetrate the larger cross-section of the new blades. Using the previous kV system was taking them 30 minutes to scan a turbine blade for structural flaws. With the Linatron, they have reduced this to about 30 seconds. In addition, we were able to design a compact shielding package so that the system fit into their existing facility.”
      Varian’s Linatron has also been used to inspect large castings, rocket motors, and pressure vessels – large metal containers that carry pressurized contents. The technology enables engineers to find tiny cracks and flaws. “These are not things that you want to see fail,” says Johnson.

Sterilization
The Linatron technology has additional applications in sterilization. It is being used in medical settings to irradiate and sterilize medical products. A system in Hawaii is used to treat papayas, which are subject to a federal fruit-fly quarantine and cannot be distributed on the U.S. mainland without treatment. Unlike other solutions that Hawaiian growers had tried, including the use of chemicals and heat, the Linatron solution does not adversely impact the appearance or nutritional value of the fruit, or damage the environment, according to the grower, Hawaii Pride LLC. Food irradiation can be used to instantly eliminate the threat of harmful food-borne pathogens such as E. coli, listeria, and salmonella in meat and poultry, as well as fruits and vegetables, without changing their texture or taste.
      “The Linatron enables us to harness and focus energy, and put it to work in a number of different ways,” says Boeh. “High energy X rays are very useful for inspection and for sterilization. There are a lot of as-yet-untapped potential applications for this technology.”


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