2 March 2010—In the wake of the attempted bombing of an airplane flying into Detroit last Christmas Day, more U.S. airline passengers can expect to encounter an alternative to the traditional metal detector at security checkpoints. The U.S. Congress has appropriated funds to install 450 full-body scanners that rely on X-ray backscatter to detect objects hidden under clothing. The first units are set to be installed this month.
The technology is not particularly new, says Joe Reiss, vice president of marketing at American Science and Engineering (AS&E), in Billerica, Mass., which manufactures SmartCheck X-ray backscatter scanners. The company started marketing X-ray scanners in the 1980s to screen baggage. More recently, its personnel scanners have been used at high-security sites such as military installations.
The devices use a mechanical collimator that moves a pencil-thin beam of X-rays over a person . Elements with low atomic numbers, such as carbon, oxygen, and other materials in the human body, scatter X-rays more, while higher-number elements scatter them less. The scattered photons are picked up by a detector next to the X-ray source that contains a scintillating material, which converts the X-rays to visible light. Photomultiplier tubes measure the intensity of the light, showing how strongly any given spot on the subject scattered the X-rays. The system notes sharp changes in the scattering and uses that to draw outlines of objects the person is carrying.
The person operating the scanner sees an outline of the passenger’s body, with outlines around any objects on that person, and can react accordingly. The operator gets no more information than the shape of the object. ”You might see a rectangle, for example, and you don’t know if it’s a wallet or a cellphone or a packet of C-4 [explosives],” Reiss says.
A scan exposes a passenger to a tiny amount of ionizing radiation. AS&E says one scan from its machine exposes a person to less than 0.1 microsievert of radiation, the equivalent of less than 10 microrem. By comparison, a passenger will get 3 µSv of radiation from one hour in the air. Normal background radiation is about 8 µSv per day, and a chest X-ray delivers about 100 µSv. ”When you fly in an airplane, you’ll get the same radiation dose in two minutes on that flight as you will from one scan in our SmartCheck scanner device,” Reiss says.
The National Council on Radiation Protection & Measurements (NCRP) says the extra health risk of anything less than 10 µSv annually, which it labels a ”negligible individual dose,” is so low that it’s not worth taking extra steps to avoid. In a report on security X-ray scanners, the NCRP says it would take 2500 scans a year to meet its lower threshold for concern and that special precautions were not needed.
Despite the low levels of radiation, an internal report of the Inter-Agency Committee on Radiation Safety, a consortium of several international agencies, suggested that children and pregnant women should not be subject to scanning, according to Bloomberg News, which saw the report. Bloomberg says regulators from the European Union are studying health and privacy issues raised by the scanning and expect to complete their assessment in April.
Transportation Security Administration (TSA) spokesperson Sarah Horowitz says the scans are safe, but passengers will have the option of being patted down instead. Reiss says the public seems more worried that a screener will see their bodies through their clothing than about radiation. That’s why the company opted for the outline, replacing the grayscale image it had used previously.
Since 2007, the TSA has installed 40 X-ray scanners in 23 airports as a test. Last fall it ordered 150 more from manufacturer Rapiscan Systems, of Hawthorne, Calif. The first of those were installed in March: three at Boston’s Logan Airport and one at Chicago’s O’Hare. Horowitz says that all 450 new units, in addition to the original 40, will be installed this year, though the TSA has not announced the locations. In the United Kingdom, meanwhile, full-body X-ray scanners were installed in February at Heathrow Airport, in London, and at Manchester Airport. All of the units have so far come from Rapiscan, though Horowitz says the TSA could decide to buy them from other manufacturers as well.
The TSA has also installed at least 40 millimeter-wave detectors, which do not produce ionizing radiation and can identify the chemical composition of materials by their spectroscopic signature. But because their wavelengths are orders of magnitude longer, they naturally produce much lower resolution than X-rays do.
About the Author
Neil Savage writes about optoelectronics and other technologies from Lowell, Mass. In the March 2010 issue of IEEE Spectrum, he reported on how engineers are working to lower the dose of CT scans while maintaining their diagnostic capability.