A new peer-reviewed journal article by a pair of University of California-San Francisco researchers reveals that the TSA's new x-ray body scanners may not be as safe as the government agency wants the public to think. In fact, the scanners might not even detect pancake-sized bombs containing PETN, the explosive used in the failed "underwear bomb" last Christmas.
According to Leon Kaufman and Joseph W. Carlson in the Journal of Transportation Security, the scanners could miss PETN bombs if they were taped to a person's body in a flattened, rounded manner.
The researches write about their findings in the article "An evaluation of airport x-ray backscatter units based on image characteristics."
From the introduction:
We show that the body is exposed throughout to the incident x-rays, and that although images can be made at the exposure levels claimed (under 100 nanoGrey per view), detection of contraband can be foiled in these systems. Because front and back views are obtained, low Z materials can only be reliable detected if they are packed outside the sides of the body or with hard edges, while high Z materials are well seen when placed in front or back of the body, but not to the sides. Even if exposure were to be increased significantly, normal anatomy would make a dangerous amount of plastic explosive with tapered edges difficult if not impossible to detect. [Emphasis added]
The article goes on to explain the pancake explosive theory:
It is very likely that a large (15–20 cm in diameter), irregularly-shaped, cm-thick pancake with beveled edges, taped to the abdomen, would be invisible to this technology, ironically, because of its large volume, since it is easily confused with normal anatomy. Thus, a third of a kilo of PETN, easily picked up in a competent pat down, would be missed by backscatter “high technology”. Forty grams of PETN, a purportedly dangerous amount, would fit in a 1.25 mm-thick pancake of the dimensions simulated here and be virtually invisible. Packed in a compact mode, say, a 1 cm×4 cm×5 cm brick, it would be detected.
The article, as is common in journals, is rather technical. But the conclusion remains: the "backscatter" technology has its limits and its loopholes, and may not be the savior of airport security.
The Washington Times goes as far as to call the new scanners a "fraud" in an editorial last week.
"Homeland Security Secretary Janet Napolitano insists the public should trust her when she says the expensive airport scanners are safe and effective," the editorial says. "Until now, there has been no way to verify this claim because the TSA and the scanner manufacturers have cloaked key operational data behind a veil of purported 'national security.'"
The researchers pointed out that the manufacturers of airport scanners positioned contraband like guns, knives and drugs in unnatural ways to conceal the limitations of their device. For example, the simulated drugs are always packed into tight rectangles that show up distinctly on the machine. TSA employees would have a far more difficult time spotting less tidy terrorists. "The eye is a good signal averager at certain spatial frequencies, but it is doubtful that an operator can be trained to detect these differences unless the material is hard-edged, not too large and regular shaped," Mr. Kaufman and Mr. Carlson wrote.
The editors' conclusion calls for the scanners to be scrapped:
In the end, this false sense of security creates a blindness that real terrorists will exploit. Continuing to rely on this fundamentally flawed technological crutch makes air travel more dangerous. The plug must be pulled on these invasive and ineffective machines.