Lab scientists have developed a rugged, inexpensive neutron detector—made largely of plastic—that could be mass-produced to provide more-widespread border screening for nuclear contraband.
Government agencies are currently fielding neutron detectors at seaports, airports, rail yards, and border crossings to detect contraband plutonium from its neutron emissions. The aim is to foil terrorist attempts to smuggle a plutonium-fueled nuclear bomb or its plutonium parts into the country. Detonating a nuclear bomb in a city would be devastating.
But preventing such an attack is not easy because there are so many entry points to the United States. Each year, 7 million freight containers are unloaded at nearly 400 seaports; 800,000 commercial airline flights and 130,000 private flights land on U.S. soil; and 11 million trucks and 2 million railroad cars enter the country from Canada and Mexico. At each of the fifty or more vehicular border crossings, there are at least ten traffic lanes. To cover all these entry points would require several thousand neutron detectors, possibly tens of thousands.
The most commonly deployed neutron detector—a proportional counter—costs at least $30,000 for a model with a detection area of 1 square meter. Ten thousand of these detectors would cost at least $300 million.
Los Alamos scientist Kiril Ianakiev has developed an attractive alternative: a new breed of neutron detector. The detector's major parts include spark plugs, welding gas, and a briefcase-sized block of plastic that forms its body. The detector is rugged and inexpensive enough to be widely deployed—which is the whole idea. [figure: detector prototypes]
Ianakiev's detector is also a good neutron detector: it detects 10 percent of the neutrons emitted by plutonium-240 that strike it. (Weapons-grade plutonium typically contains about 5 percent plutonium-240.) By comparison, a proportional counter detects 15 percent of the neutrons. But a proportional counter is also nearly ten times more expensive. One of Ianakiev's detectors with a 1-square-meter detection area will cost about $4,000. Ten thousand detectors would cost only $40 million.
Leveraging the Microchip
If the electric field is high enough, however, the electrons gain enough energy to ionize more gas atoms, a process that produces more electrons. The resulting "avalanche" of electron-ion pairs—called gas multiplication—amplifies the current pulse.
In the early days of radiation detectors, it was far easier to amplify the current pulses with gas multiplication than it was to amplify them with vacuum tubes, which had just been invented in 1906. Now, however, an inexpensive microchip can amplify the current pulses without gas multiplication, allowing Ianakiev to develop a detector that overcomes the limitations of early detector designs. (The sidebar explains how gas-filled radiation detectors work.)
neutron + 3He triton + proton, and
neutron + 10B 7Li + alpha particle,
where Li is lithium, He is helium, B is boron, and the superscripts are isotopic numbers. A triton is the nucleus of a tritium atom (hydrogen-3); an alpha particle is the nucleus of a helium atom.
The reaction rates are significant only for neutrons with kinetic energies close to the thermal energy of their surroundings, about 0.025 electronvolt at room temperature. For the 1-million-electronvolt neutrons emitted by plutonium-240, the reaction rates are about one-thousandth those of thermal neutrons. To be detected, therefore, the plutonium neutrons must first lose energy in many glancing blows with a succession of nuclei, a process called moderation. Because light nuclei such as those from hydrogen atoms efficiently moderate neutrons, neutron detectors usually include a block or sheet of a hydrogenous moderator, such as paraffin or polyethylene.
Tough, Smart, and Modular
Embedded in the detector's body are electronic modules that condition and analyze the detection signal and monitor detector performance. An onboard microprocessor makes the detector easy for untrained operators to use and permits detectors to be networked.
The bottom of the detection cell looks like an oversized metal soap dish. Deposited on the cell's inner surface is a thin layer of lithium-6, which absorbs moderated neutrons and produces alpha particles and tritons. The layer is thick enough for a high reaction rate yet thin enough for about half of the tritons and alpha particles to penetrate the layer and ionize the cell's gas. The optimal thickness for the layer was calculated by Los Alamos scientist Martyn Swinhoe.
Because lithium will not bond directly to polyethylene, the lithium is deposited on a metal substrate that does bond to the plastic. The substrate also prevents the gases emitted by polyethylene from entering the detection volume.
A flat polyethylene lid with a lithium undercoat covers the top of the cell and provides a flat surface for an O-ring gas seal. The lid is bolted to the detector's body. Filling the cell with argon at atmospheric pressure simplifies adding the gas during detector manufacture and eliminates the safety problems of pressurized vessels.
With the lid in place, the detection volume is completely enclosed by metal, which improves detection sensitivity by shielding the volume from the electrical noise produced by power lines and other external sources. The metal enclosure is also the detector's negative electrode. A thin aluminum sheet on the detector's exterior electrically shields the embedded electronic modules.
The cell's positive electrode is a metal ball screwed onto the end of a modified spark plug, which extends from the lid into the detection cell. In addition to providing an insulated connection to the positive electrode, the spark plug—built to withstand the harsh, percussive environment of an internal combustion engine—will not vibrate if the detector is bumped. [figure: computer rendering of detection cells]
The shortest dimension of the detection cell—its depth—equals the longest distance a lithium-produced triton will travel in argon at atmospheric pressure before coming to rest. Because a lithium-produced alpha particle will travel an even shorter distance, both the tritons and the alpha particles ionize as much argon as possible, providing maximum detection sensitivity. [figure: detector performance]
Head-to-Head with the Proportional Counter
Because a proportional counter uses gas multiplication, its detection signal is highly sensitive to gas impurities. Thus, the gas in a proportional-counter tube must be at least 99.999 percent pure. In fact, about half the cost of a helium-3 proportional-counter tube is in its high-purity gas. In contrast, Ianakiev's detector—which does not use gas multiplication—works even with inexpensive welding-grade argon, which has a purity of 99.5 percent. Furthermore, the small amounts of oxygen, water vapor, and carbon dioxide slowly emitted from the detector's interior surfaces will be absorbed by the lithium coating, so that outgassing will not affect detector performance for twenty years or more.
Finally, because the proportional counter's wire electrode can easily be made to vibrate—and thereby to produce spurious signals—the detectors are susceptible to shock and vibration. Supported by a robust spark plug, the relatively massive spherical electrode in Ianakiev's detector resists vibration.
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