Novel systems detect explosives

Gary Boas, gary.boas@photonics.com

The need for technologies that can detect explosives with high sensitivity and from far away is as great as ever. A number of groups are working to develop such technologies.

At the University of Málaga, researchers have demonstrated a novel hybrid sensor system that uses Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) simultaneously for instant and remote standoff analysis of explosives. Combining the atomic sensing of the former with the vibrational spectroscopy of the latter could facilitate the standoff detection of explosives residue in trace quantities – left, for instance, by human fingerprints on car door handles – at distances of up to 50 m.

Researchers have described a sensor system that combines Raman spectroscopy and laser-induced breakdown spectroscopy for standoff detection and analysis of explosives. They demonstrated the system using an experimental setup with a 532-nm Nd:YAG laser, a beam expander, a telescope, laser power sources, pulse and delay generators, spectrographs, a bifurcated optical fiber coupled into a collimating lens, a holographic SuperNotch filter and a personal computer. Reprinted with permission of Analytical Chemistry.

The system integrates a pair of Andor Shamrock SR303i spectrometers, each fitted with an iStar intensified CCD, a Cassegrain telescope and a Quantel Brilliant Twins Q-switched 532-nm Nd:YAG laser. In the Feb. 15, 2010, issue of Analytical Chemistry, the researchers reported simultaneous acquisitions of Raman and LIBS spectra for a variety of explosives, including C4 and H15 (both plastic explosives) and Goma2-ECO (Spanish-denominated dynamite class high explosive).

The hybrid sensor offers an extremely fast response and can operate at relatively long distances, said researcher José Javier Laserna, who led the team that developed it. It is a stand-alone technology and can be controlled remotely using wireless communications. As with other similar devices, a line-of-vision is needed between the sensor and the object in question to ensure effective operation. Research at the university has shown, however, that LIBS can detect explosives through car glass as well as through warehouse windows. “This capability is extremely important for a number of missions involving wide-area surveillance.”

The researchers have been in discussions with a multinational company about commercialization of the sensor. At the same time, they are working to develop data fusion strategies to fit the system with decision capabilities concerning the presence of an explosive in a particular location.

Other novel sensors can be used to detect explosives. At the US Department of Energy’s Oak Ridge National Laboratory in Tennessee, researchers are developing a device that takes advantage of the nonlinearity associated with nanoscale mechanical oscillators. Investigators typically avoid this nonlinearity, said Nickolay Lavrik, a member of the lab’s Center for Nanophase Materials Sciences Div. and a developer of the device. The Oak Ridge researchers believe that, by embracing the nonlinearity, they will be able to detect considerably smaller amounts of explosives than with currently available chemical sensors.

Researchers at Oak Ridge National Laboratory have demonstrated a sensor that takes advantage of the nonlinearity associated with nanoscale mechanical oscillators to detect trace amounts of explosives and other materials. Shown are, from left, Nickolay Lavrik and Panos Datskos, developers of the sensor.

The device uses microscale resonators much like the microcantilevers used in atomic force microscopy, which also has been explored for mass- and force-sensing applications. Panos Datskos, who heads the team developing the device, uses the example of a diving board to illustrate how the device works. A diving board will vibrate up and down with its own resonance frequency, he said. But if a swimmer climbs onto the board and stands there, the resonance frequency will shift because of the additional weight. By measuring these changes, researchers can determine the mass of the swimmer – or of trace amounts of substances, including explosives, biological agents and narcotics, that hit the microcantilevers.

The underlying premise is relatively straightforward. The challenge comes with trying to measure and analyze oscillation amplitudes as small as a hydrogen atom. Conventional approaches seek to achieve this using a sophisticated system of low-noise electronic components, which adds both cost and complexity to a device. The Oak Ridge team instead pumped energy into the system, producing considerably larger amplitudes.

With this approach, the relationship between frequency and amplitude changes is no longer described by a simple bell-shaped resonance curve. “But we used that to our advantage,” Datskos said. “Now we can see the amplitude changing more abruptly.” Lavrik added that the primary challenge in this regime was coming up with the best algorithm for the device, which would allow users to measure trace amounts of explosives in real time.

The next step in developing the chemical and biological sensor, the researchers said, is gauging interest in the commercial sector in developing a prototype device. Datskos noted that explosives detection is just one of the potential applications, and that the same platform could be used as a natural gas sensor, an emissions sensor for automobiles and more.