Defeating x-ray backscatter detectors

There has been a lot of chatter on the “interwebs” in the past week about a scientist, engineer and activist named Jonathan Corbett who has demonstrated a trivial way of defeating the x-ray backscatter machines I wrote about a while back.

The popular response to Mr Corbett’s demonstration has received a stunningly unprofessional, incompetent response from the TSA, which thanks to the Streisand Effect has made the news that x-ray backscatter scanners can be easily defeated much better known than it would have been otherwise.

But it is worth pointing out that the problem was known in the literature sometime ago. You may need an academic account to get access to the linked paper, but the gist is that a couple of people have used GEANT4–one of the standard radiation transport codes, which is known to work well at low energy–to do some simulations of the x-ray backscatter systems, and concluded that they can be easily defeated by a variety of simple techniques, and this fact depends only on the physics of radiation transport, not on any implementation details of the scanners.

So if you are hot to suggested modified technique for using the scanners, you are barking up the wrong tree. You need to talk to God and get him to change the laws of physics to make these things work. Although that still won’t make them safe.

The problems with these systems are two-fold, and depend on the most fundamental aspect of how ionizing radiation interacts with matter. Light elements (hydrogen, oxygen, carbon, nitrogen… the things that humans and plastic explosives are made of) tend to scatter low-energy x-rays rather than absorb them.

The amount of scattering is not hugely sensitive to density and composition of the low-Z material, which means–as the above-linked paper from the Journal of Transportation Security points out–that it is really hard to detect plastic explosives that have been “pancaked” on to a person’s torso. The only reason they stand out in the TSA faked demonstration images is that they are conveniently brick-shaped, which produces edge effects that are easily detectable. Remove the sharp edges by smoothing the stuff out and you lose the ability to detect it, not as a matter of how the instrument is deployed but as a fundamental consequence of the Compton scattering cross-section as a function of Z. The only way around this is to increase the dose and/or increase the beam energy (which increases the dose.)

Secondly, and more germane to Mr Corbett’s successful demonstration of how to defeat this crude technology, is that it provides no way to tell why no x-rays are scattered back from a particular region. There are two possibilities: a) there is no scattering material there (air around the body, for example) and b) there is a strongly absorbing material there (the metal case in Mr Corbett’s successful circumvention demonstration).

All the detector knows is that it isn’t getting any x-rays back from a particular region, and it has no way to distinguish between air and metal: neither return a significant backscatter signal. This is why the background of the backscatter images is black, and why the suggestion of “just display the image on a chequerboard background” misses the point entirely: the black background is not an arbitrary choice, but determined by the basic physics of the apparatus. Any region that returns no scattered x-rays to speak of will appear as black, because the brightness of the image just encodes the flux of scattered x-rays.

So there are at least two well-known, well-documented ways of defeating these unsafe scanners, and yet no one has blown up any American aircraft recently, possibly because the United States has far fewer enemies than the Organs of the State would have one believe.

Finally, there is a point to be made regarding the TSA response to Mr Corbett’s demonstration: pointing out all the cases where a technology does work is completely irrelevant to the very important cases where it does not work. Mr Corbett has pointed out that x-ray backscatter scanners fail in certain very important and easily realized cases. Blogger Bog has “responded” by pointing out–amongst other things–that in a variety of completely unrelated cases the technology does work.

This is not useful. It is disingenuous and insulting to the intelligence of the average traveller. It is like saying that a fist-sized hole in the hull of a boat doesn’t matter because the hull is perfectly sound everywhere else. True, but hardly relevant to the massive amount of water pouring in.

Anyone who wants to can walk almost anything by the ineffective and unsafe x-ray backscatter scanners by avoiding the cases where they do work and exploiting the well-known and well-documented cases where they do not. Maybe it’s time to start talking about the things that will really make Americans safer–like genuinely universal health care, a much smaller military, and less involvement in what “some guy” referred to as “foreign entanglements”.

About TJ

Scientist, engineer, inventor, writer, poet, sailor, hiker, canoeist, father.

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