In Panter (2013) I wrote about backscatter imaging as a pre-flight security clearance tactic. Backscatter imaging operates much differently than x-ray technologies, the later is more frequently used and has been in operation in airports for a longer amount of time. Yet, security professionals often do not appreciate how x-ray technologies work or understand the science behind x-ray imaging. This is often due to the academic separation between practitioners and scientists (this will be discussed in another post). Therefore, this post aims to present a better scientific understanding of x-ray technologies as a security measure to those who do not have a degree in physics or a specialization in gamma radiation.
X-ray technology operates via 3 modes of photon/ electron interaction. These are the photoelectric effect, Compton scattering, and pair production. It should be noted that many x-ray devices used in security screening are based upon the photoelectric effect, hence why we will only are discussing this. If you are curious about the Compton scattering effect, which is how backscatter imaging works, please see my previous posts.
X-ray technologies work by using gamma radiation. Gamma radiation is a type of electromagnetic radiation that has an extremely high frequency and consists of high-energy photons. These photons vary in their reaction when they enter an atom based on the type of atom they enter. The photoelectric effect occurs when an incoming photon of gamma radiation is absorbed by an electron. This results in the ejection of the electron into its orbit.
In the photo of a suitcase below notice how different materials show up on an x-ray as being darker in colour. In theory, the more dense an object is (like the revolver) the less likely it is to absorb the same amount of gamma radiation produced by an x-ray screening machine. This is why certain materials appear on a screening monitor as areas of light and dark shading. This is noticeable when looking at the can of tennis balls located to the right versus the revolver. The material of a tennis ball (rubber/ air) is not as dense as the material of the revolver (metal), therefore the gamma radiation from x-rays pass more easily through the tennis balls which creates an observed visual difference between material density. In simplest terms, x-ray photoelectric technology is based upon the fact that x-ray absorption in atomic matter varies with the energy of the beam and the density of the absorbing material.
