About this Article
Written by: Farzana Ansari
Written on: July 11th, 2005
Tags: electrical engineering, security & defense
Thumbnail by: Adam Ciesielski/SXC
About the Author
In the summer of 2005, Farzana Ansari was a sophomore majoring in Biomedical Engineering at the University of Southern California. Her love of traveling compelled her to take a deeper look into the airport security systems she has encountered.
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Volume IX Issue II > Security Versus Privacy: The Engineering of X-Ray Vision

X-rays and the Photoelectric Effect

An explanation of backscatter technology starts with two fundamental physical phenomena: X-rays and the photoelectrical effect. X-rays are a form of electromagnetic radiation, or light. They are composed of tiny energized particles known as photons that travel in a wavelike pattern [7]. In relation to visible light, X-rays have a shorter wavelength and higher frequency. X-ray frequencies vary from 1016 to 1021Hertz. As the frequency increases, the magnitude of energy of a wave's photons increases. This direct relationship corresponds to how far an electromagnetic wave can penetrate an opaque surface. X-rays are located at the higher end of the spectrum, and thus they can pass through solid objects, such as clothes and skin, that visible light cannot.
How does a wave's energy determine whether it will pass through or deflect off a surface? Upon interaction with an atom, photons can transfer their energy to one or more electrons, thereby elevating the latter to a higher energy state [8]. A photon's ability to energize an electron depends on the frequency of the wave it travels in. This dependency is highly specific; transfers occur only when the energy of a certain frequency equals the energy between specific electron energy levels [9]. In other words, an X-ray will not energize an electron to a particular state (and thus will not be absorbed) if its frequency is too small or too large. This relationship is known as the photoelectric effect.
The absorption capability also depends on atomic structure. Smaller atoms (with lower atomic numbers) tend to hold on to their electrons more tightly, deflecting high frequency X-rays. Larger atoms (higher atomic numbers) absorb these high frequency X-rays since they have more electrons that are farther from the nucleus [8]. Thus, varying the frequency could allow X-rays to deflect off one surface and not another. In the case of airport security systems, this allows the penetration of clothing but not of skin and foreign objects.
Photons that are deflected off electrons follow a route known as the Compton Scattering Effect (see Fig. 2). This effect occurs when a photon does not completely transfer its energy to an electron with which it interacts. Instead, the electron absorbs a fraction of the particle's energy and deflects an X-ray wave with a lower frequency. These lower-energy waves are known as "backscatter" X-rays and are often emitted by organic materials, or pure elements with low atomic numbers (e.g. hydrogen, oxygen, and carbon). Since explosives and threatening chemicals are composed of organic matter, backscatter waves are ideal for individually identifying these potential threats. Engineers have taken advantage of the Compton Scattering Effect by developing systems that recognize backscatter X-rays that correspond with specific organic materials.
Hyperphysics/Georgia​ State University
Figure 2: Compton Scattering describes the effect of electrons absorbing a fraction of a particle's energy and deflects an X-ray of lower frequency.

SmartCheck​'s Virtual Strip Search

The TSA's desire for a security device that minimizes human involvement inspired several companies to take up the challenge of creating a virtual strip search. In 2002, American Science and Engineering, Inc., (AS&E) presented the TSA with SmartCheck (see Fig. 3), a personnel inspection system designed to detect "both metallic and non-metallic objects, such as guns and knives, plastic and liquid explosives, composite weapons, drugs and other hidden threats and contraband" [10]. This $100,000 system uses AS&E's Z Backscatter technology to provide an "easy-to-read" display of hidden weapons or contraband (see Fig. 3).
American Science and Engineering, Inc./SmartCheck
Figur​e 3: The SmartCheck Personnel Screening System is a personnel X-ray screening system designed to detect all objects that a person may be carrying on their body.