Nuclear Detective

A Logical Approach to Computer Security

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Ionel Dragos Hau research interests also include medical applications for nuclear energy.

When the dentist is preparing to x-ray your mouth, he or she usually drapes your torso in a lead blanket to shield the rest of your body from the radiation. The metal is an excellent radiation shield. Some fear that terrorists might take advantage of that same phenomenon by using lead containers to smuggle radioactive materials for weapons. However, lead is no match for the new radiation detector built by UC Berkeley nuclear engineering graduate student Ionel Dragos Hau and his colleagues. Along with nonproliferation applications, their technology will likely have broader uses in nuclear science, astrophysics, and materials science.

"We're trying to design an instrument that identifies hidden materials that emit radiation, but also characterizes known materials for scientific study," says Hau, who works in Lawrence Livermore National Laboratory's (LLNL) Advanced Detector Group as part of the University of California's Student Employee Graduate Research Fellowship Program (SEGRF). His faculty adviser is Jasmina L. Vujic, chair of Berkeley's Department of Nuclear Engineering, while the detector project's lead scientist at LLNL is Stephan Friedrich.

A photo of the ultrahigh-resolution neutron spectrometer with the various key components labeled. (courtesy LLNL)

Radiation detectors are at the core of many well-known technologies and scientific fields, from medical imaging machines to surveys of deep space. For example, gamma-ray detectors on satellites have enabled UC Berkeley physicists to study huge explosions on the other side of our galaxy. Meanwhile, physicians use similar technology for early diagnosis and improved treatment of cancer. In recent years, researchers at LLNL and elsewhere have developed gamma-ray imaging spectrometers as part of airport and ocean port security systems to detect the transport of nuclear materials.

"The problem is that lead around a gamma ray source shields much of the radiation," Hau says. "Neutrons aren't absorbed by lead as easily though. So if we can detect neutrons with a very high energy resolution, then we can determine the nature of that neutron source."

In fact, the researchers' instrument, called an ultrahigh-resolution neutron spectrometer, is sensitive enough to detect a single neutron. And not only can it detect whether a sample is radioactive but it determine the composition of the nuclear material.

The Berkeley instrument doesn't look directly for neutrons, but rather measures how the neutrons heat up another material within the machine. The detector on the instrument is installed at the end of a "cold finger" containing a lithium fluoride crystal. Pointed at a radioactive source, the lithium fluoride absorbs the neutrons. That induces a nuclear reaction that produces excess energy released as heat. The temperature increase can be below one millikelvin (less than 1/500 of a degree Fahrenheit).

Detecting such a minute change demands an incredibly sensitive thermometer that operates at temperatures close to absolute zero. The LLNL researchers fabricated such a thermometer from molybdenum and copper that, thanks to an effect called superconductivity, changes its resistance very rapidly with a tiny shift in temperature. As a result, the temperature pulses are converted into electrical pulses that can then be digitized for computer analysis.

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According to Hau, the instrument is likely to be ten times more precise than current detectors that use semiconductors or pressurized gas. They're also more compact than neutron spectrometers that conduct measurements based on the time it takes for the particles to travel over a fixed distance.

With improvements in speed and other factors, Hau believes the system could be ready for the field in a few years.

"This unprecedented resolution could lead to a new generation of instrumentation," he says. "However, its field applications will depend on how small and portable the instrument can be made without sacrificing too much energy resolution.

Advanced Detector Group (LLNL)

High Resolution Neutron Spectrometers

Jasmina L. Vujic's home page

"Radiation Detection on the Front Lines" by Gabriele Rennie (Science & Technology Review, September 2004)

"Protecting Our Ports" by David Pescovitz (Lab Notes, November 2003)

Student Employee Graduate Research Fellowship

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