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Homeland Security Gets High-Tech Help
    Long before the tragic events of September 11th, researchers at Berkeley Lab were working to outsmart terrorists attempting to smuggle dangerous materials into the country.

One solution is a 10-pound, battery-powered detector that promises to bring state-of-the-art radiation spectrometry anywhere radioactive materials might be found. The device, called Cryo3, was developed in collaboration with researchers at Lawrence Livermore National Laboratory. At the heart of the unit is a high purity germanium crystal. The crystal absorbs energetic photons emanating from isotopes and creates a corresponding charge. When further processed, this charge depicts both the quantity and type of radioactive material present.

  Lorenzo Fabris, of Berkeley Lab's Engineering Division, holds the Cryo3, a highly sensitive, portable radiation detector.

The need for such devices is underscored by the sobering fact that isotopes can be used to build conventional bombs laden with radioactive material -- so-called dirty bombs. Furthermore, contraband isotopes can be hidden in backpacks and car trunks, which means that 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.

Fortunately, germanium-based detectors characterize radiation type. They also offer higher radiation resolution than other semiconductors such as silicon and cadmium telluride. But there's one problem. The element must be deeply cooled with liquid nitrogen. And although liquid nitrogen is very common in the laboratory, it is awkward to transport 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 requires only 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 innovation is coupling a germanium radiation detector with a small, low-power cryogenic cooling mechanism," says Lorenzo Fabris of the Engineering Division. "This offers extremely high-resolution radiation analysis in a portable package."

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."

Seeing the Unseen

Another Berkeley Lab team has developed a portable device that uses neutrons to peer inside luggage and shipping containers to determine if explosive and fissile materials lurk inside.

  Ka-Ngo Leung (right) and Jani Reijonenl, both from the Accelerator and Fusion Research Division, stand beside the compact neutron generator.

The device, developed by Ka-Ngo Leung and his colleagues in the Accelerator and Fusion Research Division, represents a marked improvement over commercial compact neutron generators, which are typically expensive and short-lived. The tiny, tube-shaped generator costs roughly $80,000 -- that's $20,000 cheaper than generators of comparable capacity. It has also shown promise as a portable screening tool.

"We use different energies of neutrons to penetrate different materials such as steel and aluminum," says Ka-Ngo Leung. "We are currently testing how much luggage can be interrogated at a time, and how long it takes to examine a container."

In general, neutron generators fire an ionized gas composed of hydrogen isotopes, either deuterium or tritium ions, at a metal target that also contains deuterium or tritium. The ions fuse with their counterparts in the target plate in a process that emits neutrons. These neutrons are then directed toward a structure that researchers want to examine--anything from brain tissue to crystals to luggage. The neutrons and gamma rays that bounce back are used to elucidate the internal makeup of the structure.

Unfortunately, today's compact generators have several drawbacks. Once the deuterium or tritium in the target plate is depleted, the generator no longer works. In addition, most neutron generators use an ion beam that is largely composed of two or three-atom molecules, which are less likely to produce neutrons in fusion reactions than single atoms. Perhaps most troublesome, today's portable neutron generators rely on deuterium-on-tritium reactions, which produce more neutrons than deuterium-on-deuterium reactions. However, any process that uses the unstable element tritium is burdened with layers of transport and safety concerns --not an ideal characteristic for a generator that may eventually be placed in airports and at customs checkpoints.

Berkeley Lab's portable neutron generator tackles these problems head-on. First, the target plate no longer contains deuterium or tritium ions. Instead, a thin sheet of titanium and copper pitted with water-cooling channels is used. The deuterium or tritium beam hits the target and continually adds new ions to the plate. This means the target cannot be depleted.

Second, the team increased the number of single atoms in the ion beam. Ninety percent of the ion beam is composed of single atoms, compared to 20 percent in beams produced by commercial generators.

"The beam is composed of single atoms instead of molecules. This emits more neutrons at the same energy," explains Jani Reijonenl, also with the Accelerator and Fusion Research Division. "This means we can use it in the field at powers only offered by much larger, lab-based generators."

Finally, the Berkeley neutron generator is engineered to rely on deuterium-on-deuterium reactions without decreasing the number of neutrons produced. The team accomplished this by using a cylindrical target instead of a two-dimensional plate. The rod-shaped ion source, which nests inside the cylinder, emits ions along its entire length. These ions strike the target that envelops it, a process that produces tens of trillions of neutrons per second.

"We can use deuterium reactions, which are much easier, cheaper, more field-ready than reactions involving tritium," says Ka-Ngo Leung. "With these developments, we are striving to make the generator as efficient as possible."

-- Dan Krotz

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