Detecting Concealed Explosives with Gamma Rays
By Joseph J. Sredniawski
In the wake of the destruction of Pan Am Flight 103 over Lockerbie, Scotland, U.S. airport security suddenly looked dismayingly vulnerable. After all, airport security systems were designed to detect hijackers carrying concealed guns or knives. Flight 103, however, was probably destroyed by the detonation of less than a pound of plastic explosive.
In response to the Aviation Security Act of 1990, drafted after the Lockerbie tragedy, the Federal Aviation Administration (FAA) has funded at least 85 projects to develop new explosives-detection technologies. Of these 40 are currently active, and 19 of the 40 are dedicated to the development of prototype detection systems.
So far, the only system that has met the FAA's classified certification standard for explosives detection is the InVision CTX 5000, an X-ray machine that uses computerized axial tomography to create images of the contents of baggage. InVision Technologies (Foster City, CA) was recently awarded a $52.2 million FAA contract to build and install 54 of these machines in 1997.
According to Aviation Week & Space Technology, "The FAA has been looking for a single machine that can automatically inspect bags with a high degree of detection and a low false-alarm rate," a so-called silver bullet. The agency is said to be under increasing pressure, however, to abandon the standard and to adopt a va1iety of technologies to meet all the security threats that airports face.
Among the most technologically demanding of these threats are explosives concealed in cargo containers. With the exception of passengers and carry-on baggage, everything that flies goes into the cargo hold. Currently, cargo containers are not screened for explosives before being loaded on an aircraft, in part because there is no practical means of doing so. The root of the difficulty is the sheer bulk of the container; whereas a typical carryon suitcase has a volume of 2.27 cubic feet, an LD-3, a medium-sized cargo container, has a volume between 145 and 158 cubic feet.
For this reason the many technologies under development for screening loose baggage, such as dual-energy X-rays, X-rays with backscatter, thermal-neutron analysis, and quadrupole resonance, are not candidates for cargo screening. Experimental cargo-screening systems rely instead either on the ability to detect minute amounts of chemicals in air samples or on energetic particles, such as fast neutrons or gamma rays.
To solve the special problems posed by cargo inspection, the FAA is considering
a technology called gamma resonance absorption (GRA), which relies on the
preferential absorption of particular gamma rays to detect explosives. Northrop
Grumman's Advanced Systems & Technology Group of Military Aircraft Systems
Division (Bethpage, NY) is developing a proof-of-principle GRA system, with
assistance from TRIUMF (Vancouver, BC) and Scientific Innovations (East Hampton,
NY) and with funding from the Advanced Research Projects Agency.
Although this system was originally intended primarily to detect drugs, analysis has shown that it might function more effectively as a bomb detector. The proof-of-principle system is expected to undergo testing early this year. In addition to providing the penetrating power needed to inspect cargo, the GRA system has several features that give it a probability of detection superior to most explosives-detection systems now under development. It can disc1iminate between materials of equal density but different composition; it has the resolution needed to detect thin sheets of explosive; and it can create three-dimensional images of a container that would reveal explosives hidden behind other objects. Although GRA's greatest asset is its ability to inspect loaded cargo containers, it could also be used to examine smaller containers or loose baggage placed in a carousel for batch processing.