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Long before September 11, engineers at Lawrence Berkeley National Laboratory, in collaboration with researchers at Lawrence Livermore and Los Alamos National Laboratories, have been working to outsmart terrorists attempting to smuggle radioactive material into the country.
Their solution is Cryo3, a 10-pound, battery-powered detector that promises to bring state-of-the-art radiation spectrometry anywhere radioactive materials are found.
"The innovation is coupling a germanium radiation detector with a small, low-power cryogenic cooling mechanism originally designed for the aerospace industry," says Lorenzo Fabris of Berkeley Lab's Engineering Division. "This offers extremely high-resolution radiation analysis in a portable package."
The need for a hand-held radiation detector was first born from a necessity to monitor nuclear weapon stockpiles to ensure nations adhered to treaty obligations. An even more pressing need surfaced after the dissolution of the Soviet Union, when national security experts worried the former superpower's nuclear arsenal could spark a black market in fissile materials. In the wrong hands, these isotopes could be used to build both nuclear bombs and conventional bombs laden with radioactive material-a so-called dirty bomb. And rather than being delivered via intercontinental missiles, contraband isotopes can be hidden in backpacks and car trunks, meaning airports, border checkpoints, and shipping terminals provide the last best chance to thwart smuggling.
To complicate matters, any tool used to screen for isotopes in busy terminals must detect not only the presence of radiation, but also the type. A terrorist could mask radioactive material destined for a dirty bomb in a seemingly benign package of medical isotopes, and therefore sneak past a Geiger counter.
That's where the Cryo3 comes in. At the heart of the unit is a high purity germanium crystal. Energetic photons, X, and gamma rays, interact in the germanium crystal to create a corresponding charge. When further processed, this charge depicts both the quantity and type of radioactive isotope present. Although germanium offers higher radiation resolution than other semiconductor detectors, such as silicon and cadmium telluride, it must be deeply cooled, traditionally with liquid nitrogen. And although liquid nitrogen is very common in the laboratory, it is awkward to transport, store, and handle in the field.
To sidestep this limitation, Berkeley Lab engineers coupled the germanium crystal to an off-the-shelf mechanical cooling device currently used to cool low-noise cell phone antennae. The device, which utilizes the Sterling cycle to reach low temperatures, only requires 15 watts to cool the germanium to 87 degrees Kelvin. When the cryogenic mechanical cooler is vacuum sealed to a germanium detector, the result is a lightweight, highly sensitive radiation detector that operates up to six hours on two rechargeable camcorder batteries.
The mechanical cooler requires 16 hours to cool the detector from room temperature to operating temperature, but because the batteries are hot swappable, a fresh supply guarantees unlimited operational time.
To keep the system portable and low power without sacrificing resolution, Fabris and colleagues made additional refinements. Borrowing from lessons learned in satellite-based germanium detector applications, they protected the delicate crystal in a hermetically sealed, nitrogen-filled capsule. The encapsulated germanium detector is suspended with Kevlar fibers in a close-fitting utility vacuum chamber.
Another obstacle was electronic noise, a byproduct of all electrical systems that is particularly troublesome in radiation detectors because it degrades the electronic readout's depiction of the absorbed radiation. In short, electronic noise softens the readout's sharp spikes into rounded hills, meaning valuable data is lost. Fabris turned to a specially designed small, low-power preamplifier that minimizes electronic noise without sapping battery power-a critical component, given that conventional preamplifiers are too power-hungry to be used in a battery-powered device.
So far, Fabris and colleagues have developed detectors of modest size, or so-called 25 percent efficient detectors. In the future, they hope to increase the detector size and therefore the efficiency to 50 and even 100 percent by using modified mechanical coolers that only cool to 105 degrees Kelvin, a temperature still within germanium's operating parameters. The modified mechanical coolers have almost twice the heat lift for the same input power when compared to the conventional mechanical cooler.
Ultimately, Fabris foresees a time when next-generation iterations of Cryo3 safeguard the nation with lab-quality, portable radiation detection and characterization.
"Whatever you can detect with a germanium crystal, you can detect with the portable system," says Fabris. "Ideally, we would be able to place one at any customs port."
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