X-ray cargo screening systems: the technology behind image quality



Dr William A. Reed, Product Manager, Varian Medical Systems Inc., Security & Inspection Products, Las Vegas, NV, USA


X-ray cargo screening for container security and contraband interdiction has emerged quickly and is fast becoming a common feature in ports throughout the world. While early systems were custom adaptations of industrial inspection equipment, today’s x-ray inspection systems are specially tailored for the inspection of sea containers, trucks, and rail cars within small confined areas.

The latest technology allows systems to rapidly produce high quality images  while minimising the disruption of commercial port activities. The question has changed from “whether to use x-ray inspection?” to “what to look for in current technology?” Clearly, image quality is the critical objective. Only with systems that can discriminate between subtle differences in density, shape, and outline, can a user effectively identify contraband materials.

Two primary components are responsible for determining the ultimate quality of the x-ray image a system produces. First, the x-ray source must have sufficient power and dose to fully penetrate the most densely loaded container, yet, not too much power, which would result in excessive cost, size, and operating area space  requirements. Second, the detector array, which is the component that captures the x-rays that penetrate the container and converts them to electrical signals, must be highly sensitive and possess a wide dynamic range to provide data that accurately reflects the object being scanned.

X-ray source technology

The first x-rays were produced with specially designed vacuum tubes, which are still used today for medical applications, industrial inspection, and airport baggage screening. While today’s tubes offer much more sophisticated technology, they still are limited in energy output to about 450 KeV (kilo-electron-volts). This energy level
limits the penetrating power to less than 100 mm of steel, making it inappropriate for container screening applications. A more effective solution is to generate the x-rays using linear accelerators (linacs), which are scaleable in energy output up to 9 MeV (mega-electronvolts), producing the ability to penetrate over 400 mm of steel. The x-ray penetration of steel and water is often used to determine how much x-ray energy is needed for an application.

The chart in Table 1 shows the maximum penetration in water and steel for x-rays at varying energies. Note that as energy changes, materials act differently in absorbing x-rays. This is reflected by a changing ratio of penetrating power. The ratios shown are the penetration through water divided by the penetration through steel for each energy range. While the optimal energy level for cargo screening is a function of the detector components and overall system design, many cargo screening system manufacturers have found that 3 MeV to 6 MeV linacs produce the best tradeoff for overall performance and cost. Below 2 MeV, x-ray penetration is not complete for heavily loaded containers. Conversely, energies significantly above 9 MeV require extensive shielding and produce neutrons as an unwanted by-product.

Linacs used for cargo screening have X-ray outputs that are pulsed rather than constant. Pulses are on the order of 4 microseconds long and have adjustable repetition rates of 50 to 500 pulses per second. While pulsed operation is a specific design technique to produce x-rays efficiently, it also allows smaller and more cost effective accelerators to be manufactured. Higher repetition rates do not affect the quality of the output and allow faster vehicle throughput. For example, at 500 pulses per second, cargo containers can be scanned as they move at speeds of nearly 10 kilometres per hour and still achieve a 5 mm horizontal resolution.

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