Sunday, November 29, 2015

Privacy and Backscatter X-Ray Technology

The New York City Police Department owns an unknown number of high-tech vans that allow their operators to play Superman—at least with regard to his X-ray vision abilities.  An online article in The Atlantic last month describes how NYPD Commissioner Bill Bratton indirectly admitted his organization was using X-ray vans when he refused to discuss the matter at a press conference, citing security concerns.  Superman was a fictional character whose strictly limited flaws were in service of a plot that always ended in the triumph of the unquestionably good over the irrevocably bad.  But real life isn't so simple.  And there are real concerns about the way the NYPD may be using this technology.

First, how does it work?  Offhand, it sounds highly irresponsible for somebody to just shoot X-rays at random passersby.  X-rays are a form of what is called ionizing radiation, meaning that they have enough energy to knock electrons out of atoms to make ions.  Such ions can wreak havoc in the DNA of a biological target, for instance, and lead to cancer and other problems.  That is why medical X-ray systems are highly regulated and only properly trained operators are allowed to use them.

But there is apparently a sort of escape clause for non-medical equipment that uses X-rays.  If it meets a certain technical standard that limits the amount of exposure someone would get from a typical spying operation that lasts a few seconds, then the FDA is not involved and the rules change.  According to numerous sources, the type of X-ray machinery used by the NYPD uses so-called "backscatter" X-ray technology that falls into the low-dose category. 

It's really rather clever.  Conventional X-ray machines use a transmission approach, sending X-rays through the item to be examined and recording what comes out on the other side.  Your dentist uses this type of machine, and the image of your teeth shows up because bone is denser than air or soft tissue and absorbs and scatters more X-rays.  But obviously, for transmission X-rays to work, the rays have to be strong enough to get all the way through the item being examined.

Backscatter X-rays work differently.  Instead of producing a strong beam that illuminates the whole target at once and goes through it, a scanning type of backscatter unit sends a "flying spot" of X-rays sweeping across the target, which could be a person inside his or her clothes, or even a car.  These X-rays don't have to penetrate the target.  Instead, all they have to do is cause a thing called Compton scattering, which is basically what happens when an X-ray encounters an electron and is generous enough to share some of its energy.  The electron takes off with some of the energy and a new X-ray photon appears carrying the energy that's left. 

It is these new lower-energy X-rays that are detected by the backscatter machine, which consequently does not need to use X-rays that are as energetic as those used by conventional transmission machines.  That and the fact that any one point on the target is exposed to X-rays for only a small fraction of the total time of exposure, means that the X-ray dose of a backscatter unit is much smaller than, say, what you'd get from a medical chest X-ray.  Numerous sources confirm that the dose is so small from a surveillance backscatter X-ray device, that you would get something similar by just standing on a street corner for an hour or so and exposing yourself to background radiation.  This comes from sources like cosmic rays and the potassium in construction materials, and everybody gets that every day, twenty-four hours a day.  So despite some concerns on the part of investigative journalists that there are health hazards from backscatter X-ray technology, as long as the systems are working properly and used properly there are much more important things to worry about.  

To my mind, the greater concern about these systems is privacy.  You don't think that research directors of government agencies would go in for public cheesecake photos of themselves, but the Wikipedia article on backscatter X-rays shows such an image of Susan Hallowell, director of the U. S. Transportation Security Administration's research lab.  It is, er, very clear that the image is that of a woman.  We can all be glad that she wasn't a man. 

At this point, we should take a look at the U. S. Constitution's Fourth Amendment, which is short enough to quote in full here:  "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."  Updating its 1792-era language to modern terms, it means that no government official can snoop on you unless they swear or affirm that they have a good reason to do so, and the officials can say exactly where they're going to look and what they're going to look for.

I'm not a lawyer, and since the Fourth Amendment was passed it has accumulated various qualifications and exceptions, like a ship picks up barnacles.  But the principle that the writers of that amendment had in mind is still clear.  A person's body, house, papers (which was the only way to record data back then), and "effects" (as in personal effects—what a ten-year-old would call "my stuff") are inviolable against government attempts to mess with them—taking them, looking at them, or anything along those lines.  The only cause strong enough to justify such violation is when the government has a good case to show that something is amiss and can describe what they want to look for and where.  This clearly rules out fishing expeditions, in which an official simply snoops at will and starts investigating a crime when the snooping itself provides evidence.

Right now, the NYPD is in effect saying "Trust us, we're using this X-ray van the right way."  But if citizens can't even know how many vans there are, let alone how they're being used, that is asking for a heck of a lot of trust.

Sources:  The Atlantic's article "The NYPD Is Using Mobile X-Ray Vans to Spy on Unknown Targets" appeared on Oct. 19, 2015 at  I also consulted the websites of firms that make such devices, including American Science and Engineering at and the Tek84 Engineering Group at  I thank my nephew Matt, a graduate student in criminal justice, for bringing this matter up at the Thanksgiving dinner table. 

No comments:

Post a Comment