Published by the MIT News Office at the Massachusetts Institute of Technology, Cambridge, Mass.
Published by the MIT News Office at the Massachusetts Institute of
Technology, Cambridge, Mass.
Grodzins Explores Physics Of Finding Hidden Bombs by Eugene F. Mallove News Office With the Persian Gulf war renewing fears of terrorist attacks, Professor Lee Grodzins of the Department of Physics, an expert on technical aspects of airport security, reviewed the science of bomb detection last week at a Laboratory for Nuclear Science seminar. Highlighting the severity of the situation, Dr. Grodzins noted that today if you want to fly to Athens, Greece, you must pay a $500 security surcharge to the one airline that still flies there from the US. Moreover, no electronic devices whatsoever are permitted in luggage to that destination--not even hair dryers or electric shavers. There have been almost no bomb problems aboard aircraft flying within the US or leaving the country, he said. The problems are mostly within Europe or South America, some of them related to drug traffic. "International terrorism has really not come to our shores," he said. Even though more than 1,000 home-made bombs exploded in the US in 1988-- most thought to be set for personal grudges--few, if any, were aboard aircraft. The serious problem for airlines comes not from crude black-powder pipe bombs, but from modern high explosives that can be fashioned into objects of virtually any size or shape. There are 20 or 30 generic kinds of such explosives, Professor Grodzins said, with thousands of possible variations. Terrorists have lined suitcases with 1/8th-inch-thick sheets of the popular Semtex explosive. Such plastic compositions are essentially impossible to detonate, except by triggering with a tiny blast from another explosive--a few milligrams of which might suffice. For example, a tiny amount of explosive might be detonated in a personal calculator "timer" resting on a difficult-to-detect Semtex sheet. Hence the appeal of plastic explosives to terrorists who know what they are doing. Easily concealed explosives have one advantage for detection technology. Professor Grodzins noted that the chemical composition of all the "appealing" shock-resistant explosives has a characteristic atomic signature that is very high in nitrogen. He showed a two-dimensional plot of nitrogen and oxygen content of various explosives in which all such compositions fell within a defined rectangular zone. Since no other kinds of material even come close to being in the zone, a detector could in principle search for specific parameters of high nitrogen/oxygen composition and low carbon content. Easily concealed explosives also pose a problem for detection technology because conventional explosives detectors are chemical "sniffers" for airborne nitrogen compounds. Such devices rely on the vapors coming to equilibrium in surrounding air. However they seldom are sensitive enough to detect vapors of modern explosives which may contain as little as only one nitrogen molecule per trillion molecules of air. In use now are certain types of X-ray bomb detectors that go beyond making mere two-dimensional images of the contents of luggage. Explosives detectors using X-rays rely on analyzing the absorption of X- rays at specific energies and at specific angles to detect zones of high concentration of materials of low atomic number within a bag. There are about 15 nuclear-type bomb detection technologies that have been proposed or are now in use. So-called thermal neutron analysis detectors are the only ones commercially available today. Such detectors literally bathe luggage in a bath of penetrating low-energy (thermal) neutrons; they rely on detecting gamma rays emitted when some of the neutrons fuse with nitrogen-14 to make nitrogen-15. It turns out that the high energy (10.8 million electron volt) gamma ray from that fusion sticks out like a sore thumb amidst almost all other kinds of background gamma radiation. Other kinds of neutron detectors are in the works, such as one that pulses bursts of "fast" neutrons in a kind of tomography--akin to a CAT scan--to define suspicious localized nitrogenous regions within a piece of luggage. Requirements on explosives detectors are exceptionally stringent, because over a billion pieces of luggage must be examined annually. For operational reasons, the false-alarm rate and the probability of missing a bomb must be extremely low. Still, Professor Grodzins believes that for no more than about $5 per airline ticket, we will eventually be able to have a very strong, nearly perfect security system. Professor Grodzins has consulted in the field for four years, specializing in evaluating systems that employ nuclear particles and radiation to detect concealed explosives. He is now working full time on the problem, one that he asserts will be with us for decades. Prior to his involvement, Professor Grodzins headed the Heavy Ion Physics Group within LNS.