Since the tragic events of September 11, 2001, anyone who has been to an airport is aware that security efforts in the U.S. have been greatly intensified. However, according to many experts, terrorist threats to homeland security are equally likely to come by way of the sea. The U.S. Customs Service reports that some 6 million cargo containers arrive through U.S. seaports every year. Ninety percent of the trade goods brought into the U.S. each year – some 2 billion metric tons worth – enter through the country’s 361 seaports. Presently, less than 2 percent of these containers are ever opened and inspected by Customs Service officials.
      These trailer-sized, steel-walled cargo containers are typically sealed in foreign ports and not opened again until delivered by trucks to points all across the United States. It’s not hard to imagine these containers being used for smuggling contraband – even a weapon of mass destruction. To physically open each container and extract and inspect the contents by hand would be too time-consuming and unrealistic. Clearly, what’s needed is some means of searching the containers quickly and thoroughly without disrupting the flow of goods. An X-ray imaging system like Varian Medical Systems’ Linatron® linear accelerator is an excellent candidate for the job. It can generate steel-piercing X rays that “see” through container walls and allow contraband nowhere to hide.
      “The challenge is to provide customs officials with a solution that lets them look inside these containers quickly and efficiently. You need to generate enough energy to penetrate up to 440 mm (17 inches) of solid steel and produce high-quality images that show even small objects in fine detail,” says Lester Boeh, vice president for Varian’s Security and Inspection Group. “The Linatron meets those specifications. It has already been incorporated into cargo screening systems all over the world, but there are comparatively few in the U.S. The impact of September 11 could change all that for the U.S. and many nations engaged in international trade.”
      Varian’s Linatron, which generates high-energy X-ray beams, has already been incorporated into cargo inspection systems in countries like Australia, Belgium, China, France, Germany, Ghana, Indonesia, Israel, Japan, Korea, Mexico, Saudi Arabia, Taiwan, Turkey, and the U.K. Japan operates multiple units at its six busiest ports; the U.K. at more than a dozen. Eurotunnel uses the technology to scan freight cars that pass between France and the U.K.

Imaging the Contents of a Cargo Container
The Linatron has been incorporated into fixed-site and mobile cargo scanning systems built by companies like ARACOR, Heimann Systems, L3, and RapiScan. These systems work like a giant airport baggage screening system. They use the high-energy X rays generated by the Linatron to send a beam of photons through a cargo container. The photons are absorbed and scattered in varying amounts by the materials in their path, depending on their densities. On the far side of the cargo container, a detector array collects and records the photons that make it through unabsorbed, generating an electronic signal that is translated into an image. The image, which shows the container’s contents, can be viewed on a monitor. A Linatron-based cargo screening system can scan a full container in less than three minutes. Fixed-site systems are built into garage-like facilities, and trucks carrying cargo containers are moved through these facilities the way cars are moved through a carwash. The truck passes between the Linatron X-ray beam and the photon detector. Electronic images are captured and transmitted to a computer monitor at an operator’s station.
      For customers who need to move a cargo inspection system from site to site, mobile systems can be mounted on trucks.
      Similarly, it is also necessary to scan air cargo. Each year, more than 30,000 tons of air freight are transported in cargo and commercial aircraft in the U.S. and virtually none of it is ever inspected. The Linatron M is an ideal solution to this problem and is already working in airports outside the U.S.

Penetration, Contrast, Resolution
Three basic physics criteria are used to measure the effectiveness of any imaging system: penetration, contrast, and resolution. All three are related to the level of energy – and hence the number of photons – sent through whatever is being scanned. A Linatron-based screening system generates higher energy X rays than competing gamma-based systems.
      Penetration is probably foremost of the three criteria for cargo screening. The inspection obviously fails if the imaging photons lack the energy to punch through a container’s thick steel walls. The key to penetration is photon energy – the more energetic, the deeper the photons penetrate into a material. Steel is the bar by which the penetration capabilities of an imaging technique are measured. Varian’s Linatron can generate X rays at energies of 9 million electron volts (MeV). That’s enough power to pass through 440 millimeters (17 inches) of solid steel and still provide enough energy to produce a high-contrast image – a critical issue for scanning big trucks and containers.
      Says Boeh, “Without full penetration of a cargo container and its contents, too much can be missed.”

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