Lasers detecting explosives from 20 meters away
Oak Ridge National Laboratory (ORNL) researchers have developed a super-sensitive explosives detector which uses a laser and a device that converts reflected light into sound. Interestingly, the technique they've used is based on earlier works of Alexander Graham Bell in the late 1880s. In their experiments, the researchers used three explosives such as TNT and 'were able to detect trace residues with lasers 100 times less powerful than those of competing technologies.' Right now, they can detect explosives 20 meters away from their system, but 'they believe they can achieve detection at distances approaching 100 meters.' But read more...
You can see above one of the ORNL researchers, Charles Van Neste, using his prototype laser to check if explosives are inside a briefcase. (Credit: Jason K. Richards, Creative Media, Oak Ridge National Laboratory, via this Science Daily link)
This research work has been conducted at the Nanoscale Science & Devices Group, which is part of the Biosciences Division at Oak Ridge National Laboratory (ORNL). The team was composed of Thomas Thundat, leader of the group, Larry Senesac and Charles Van Neste.
Here are some details about the ORNL's technique. It "involves illuminating the target sample with an eye-safe pulsed light source and allowing the scattered light to be detected by a quartz crystal tuning fork. '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. 'This produces pressures that drive the tuning fork into resonance.' The amplitude of this vibration 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."
This research work has been published by several scientific journals in 2008.
The first article was included in Applied Physics Letters under the name "Standoff detection of explosive residues using photothermal microcantilevers" (Volume 92, Article 134102, April 1, 2008). Here is a link to the abstract. "Standoff detection of trace explosives is gaining attention due to its immediate relevance in countering terrorist threats based on explosive devices. However, most currently available standoff techniques rely on expensive, complex, and bulky equipment. We have demonstrated highly selective and sensitive standoff detection of explosive residues on surfaces by using photothermal spectroscopy carried out with bimaterial microcantilever sensors. The demonstrated sensitivity of the technique, 100 ng/cm2, is sufficient to detect the explosive contamination generally found on explosive devices. The sensitivity of the technique can be further improved by optimizing the bimaterial cantilever and by using higher intensity infrared sources."
The researchers also published a paper in the Journal of Applied Physics under the title "Trace explosive detection using photothermal deflection spectroscopy" (Volume 103, Issue 9, Article 094906, May 2, 2008). Here is a link to the abstract. "Satisfying the conditions of high sensitivity and high selectivity using portable sensors that are also reversible is a challenge. Miniature sensors such as microcantilevers offer high sensitivity but suffer from poor selectivity due to the lack of sufficiently selective receptors. Although many of the mass deployable spectroscopic techniques provide high selectivity, they do not have high sensitivity. Here, we show that this challenge can be overcome by combining photothermal spectroscopy on a bimaterial microcantilever with the mass induced change in the cantilever's resonance frequency. Detection using adsorption-induced resonant frequency shift together with photothermal deflection spectroscopy shows extremely high selectivity with a subnanogram limit of detection for vapor phase adsorbed explosives, such as pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), and trinitrotoluene (TNT)."
Finally, Applied Physics Letters accepted a third paper under the name "Standoff photoacoustic spectroscopy" (Volume 92, Article 234102, June 12, 2008). Here is a link to the abstract. Here, we demonstrate a variation of photoacoustic spectroscopy that can be used for obtaining spectroscopic information of surface adsorbed chemicals in a standoff fashion. Pulsed light scattered from a target excites an acoustic resonator and the variation of the resonance amplitude as a function of illumination wavelength yields a representation of the absorption spectrum of the target. We report sensitive and selective detection of surface adsorbed compounds such as tributyl phosphate and residues of explosives such as trinitrotoluene at standoff distances ranging from 0.5–20 m, with a detection limit on the order of 100 ng/cm2."
Sources: Oak Ridge National Laboratory news release, June 25, 2008; and various websites
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