Throughput performance factors in X-ray cargo screening systems



William A. Reed, Ph.D. & Edward Haines, Ph.D., Varian Medical Systems Security & Inspection Products, Las Vegas, NV, USA



As X-ray cargo screening becomes more prominent at ports and border crossings, its impact on container traffic is frequently discussed. This is essentially a question of system throughput, which varies by the type of X-ray system chosen and how it’s operated within a port facility.

Screening objectives

X-ray cargo screening has been adopted at ports and border crossings throughout the world because this technology has solved a number of important problems. It first gained prominence for manifest verification, allowing countries to better enforce import tariffs. Authorities also found that the image quality achieved with X-ray screening allowed them to interdict contraband, including drugs, cash, weapons, and other illicit materials.

During the last few years, attention has shifted to homeland security concerns, where X-ray screening has become a major tool in prohibiting the smuggling of weapons of mass destruction (WMDs) into countries. These different objectives often determine which systems are chosen and how they are operated at ports and border crossings. In addition, X-ray capability is often implemented as part of a ‘layered’ security strategy (Kutz, G. D. & Cooney, J. W. , 2007), frequently supplemented by other technologies and methods, which allows authorities some discretion in how to inspect incoming vehicles.

For example, some X-ray screening systems allow scanning at different speeds so that enhanced images can be obtained from a particularly suspicious vehicle or container. Consequently, system throughput performance must be judged in terms of meeting specified objectives, rather than simply comparing average inspection times per vehicle.

X-ray cargo system types

X-ray cargo screening systems are typically available in four basic types. All four types of X-ray inspection systems can provide good quality images. They differ in how their designers have addressed certain system objectives related to factors such as: intended use, installed location, life-cycle costs, and overall performance.
From an operational perspective, these four types can be further divided into stationary and mobile systems. Mobile systems are fully contained in vehicles that can be moved to a location, set up, and ready to scan within about 30 minutes. Advantages of mobile systems include their ability to be operated at temporary locations
and rapidly moved from place to place, creating an inspection capability across a large area. They are also smaller and may be less expensive than stationary systems, which typically require a shielded and enclosed operating area. Stationary systems are available in various configurations. Some have re-locatable mechanisms that can be disassembled and moved to other locations, while others are designed into a permanent structure. Another difference among stationary systems is their shielding configuration. Some systems are heavily shielded near the X-ray source, which minimises the requirement for structureshielding. Others  have less source shielding, but require more substantial structure shielding.

Stationary X-ray inspection

Figure 1 illustrates the complexity in determining actual throughput performance of an X-ray cargo system. This diagram represents a simplified timeline for scanning a trailer or cargo container using a stationary screening system. It assumes that the truck is driven into a scanning bay and the driver leaves the truck and moves to a secure area while the X-rays are active. Then the driver moves the truck from the scanning bay, completing the cycle. A somewhat different
timing diagram for a mobile system will be described later.

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