DOE's super-sensitive explosives detector

The Energy Department's Oak Ridge National Laboratory can now detect explosives at distances exceeding 20 yards using new photoacoustic spectroscopy methods that use a laser and a device that converts reflected light into sound.

The technique involves illuminating a target sample with an eye-safe, pulsed light source and allowing the scattered light to be detected by a quartz crystal tuning fork. The method enabled researchers to detect trace explosive residue using lasers 100 times less powerful than those of competing technologies. By using larger collection mirrors and stronger illumination sources, researchers believe they can achieve detection at distances approaching 109 yards.

"We match the pulse frequency of the illuminating light with the mechanical resonant frequency of the quartz crystal tuning fork, generating acoustic waves at the tuning fork's air-surface interface," said Charles Van Neste of Oak Ridge's Biosciences Division. "This produces pressures that drive the tuning fork into resonance."

The vibration's amplitude is proportional to the intensity of the scattered light beam falling on the tuning fork which, because of the nature of quartz, creates a piezoelectric voltage. Piezoelectricity is the ability of some materials, such as crystals and some ceramics, to generate voltage across the material in response to applied mechanical stress.

The method is a variation of photoacoustic spectroscopy, in which materials exposed to rapidly interrupted light transform the energy into sound. Plotting the sound against the light wavelength creates a photoacoustic spectrum of a sample that can be used to identify the absorbing components of the sample.

The new method overcomes a number of problems associated with the technique. Most notably, agency researchers are able to probe and identify materials in open air instead of having to introduce a pressurized chamber, which renders photoacoustic spectroscopy virtually useless for security and military applications.

Researchers believe the new technology can be an inexpensive and more exact technique for detecting explosives in the field than is currently available.

Researchers tested the technology using tributyl phosphate and three explosives'cyclotrimethylenetrinitromine, trinitrotoluene (better known as TNT) and pentaerythritol tetranitrate.

About the Author

Kathleen Hickey is a freelance writer for GCN.

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