Answer to Question #11371 Submitted to "Ask the Experts"

The use of AS&E's Z Backscatter Vans continues to be in the news. The Health Physics Society (HPS) position statement PS017-1 "Use of Ionizing Radiation for Security Screening Individuals" discusses the very low doses reported to persons subject to scanning by these devices but makes it clear that individuals should be informed of the scans—presumably before they are administered. I have visited the website of the manufacturer (AS&E) to get more information on the technology, where I have seen posted an image created by such a scanner purportedly of a trailer in which people can be seen hiding.

My questions are:

1. If the image is created by Compton scattering off of the individuals' bodies (and off of other organic material inside the trailer), how is it that the scatter off neither the aluminum side of the trailer nor the van itself interferes with the scatter from inside the trailer?
2. Don't the x rays scatter more strongly from the aluminum sides of the vehicles than off the skin of the people hiding inside?
3. Also, why are the x rays able to penetrate the sides of the vehicles (in both directions) but not penetrate the skin of the people being scanned?
4. Finally, were all of the people shown in the images on the company's website informed of their likely radiation doses before they were scanned?

These are good questions. Backscatter imaging is indeed an effective method of security screening and, yes, anyone imaged by the x-ray beam will receive a radiation dose. Let me explain in detail.

Backscatter imaging is imaging of x rays Compton scattered off of material. Those scattered x rays are captured by an imaging system (detector), and the signal from that system is processed into an understandable image.

The nature of x rays is that they are produced by electrons impinging on a high-density target, often tungsten, and the resultant x rays are emitted in a spectrum of energies. The x-ray beam can be "hardened" by passing it through a filter that will remove the lower-energy photons but the energy of the beam is still a distribution.

When such a beam passes through an absorber (for example, the aluminum siding of a vehicle), photons directly penetrate, are scattered forward, are scattered back toward the source and detector, and are absorbed in the material. The fraction of backscattered signal would be relatively low compared to the beam penetrating because the vehicle siding is not very thick in terms of the penetrability of the x ray. Once the x rays are reflected by the cargo, contraband, stowaway individuals, etc., photons directly backscattered toward the detector will provide the majority of the signal.

The x rays scatter at different angles and intensities, depending on the type of material they interact with. This variation produces the contrast for the two-dimensional image and can be used to distinguish organic from inorganic materials.

A fortunate consideration of the makeup of cargo vehicles is that the walls of the vehicle are relatively thin. This means that an individual who is several centimeters thick will scatter more of the signal than a single centimeter of aluminum.

X rays are probably more preferentially absorbed and scattered by the aluminum. This serves to reduce the dose delivered to anyone in the car or container. Nonetheless, x rays certainly can penetrate the skin, and individuals in a scanned area (e.g., cargo trailer) will receive a radiation dose. The AS&E website itself states that exposed individuals will receive less than 0.1 microsievert (μSv), a nominal dose when compared to background radiation levels.
 
The current HPS position statement on security screening of individuals does not apply to a situation where a vehicle is being screened with individuals hidden inside.

Charles "Gus" Potter, CHP, PhD