Abstract
Photoacoustic (PA) spectroscopy has come into prominence for homeland security and defense applications in detecting hazardous materials and illicit drugs or nerve agents in a crowded environment. The spectral signature of an explosive is obtained from its residue left over a surface or the vapor emanating from the concealed explosive. PA spectroscopy has been used in the laboratory for trace detection of powder samples of explosives like trinitrotoluene and RDX and illicit drugs like heroin and morphine, using CO2 laser and the PA cell fitted with a microphone. The use of tunable quantum cascade laser (QCL) sources in photothermal spectroscopy (PT) is very effective for the standoff detection and identification of trace explosive residues in the field conditions. PT spectroscopy of gasses utilizing a quartz tuning fork was first reported in 2002, and the basic principle of quartz-enhanced PA spectroscopy is to accumulate the acoustic energy not in a gas-filled cell but in a sharply resonant acoustic transducer. The scattered QCL light from the target surface with RDX residue is collected by a mirror and focused on QTF at more than 10m.
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