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Photoacoustic Spectroscopy for Trace Vapor Detection and Standoff Detection of Explosives

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Technical Report

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US Army Research Laboratory Adelphi United States

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The Army is investigating several spectroscopic techniques e.g., infrared spectroscopy that could allow for an adaptable sensor platform. Current sensor technologies, although reasonably sized, are geared to more classical chemical threats, and the ability to expand their capabilities to a broader range of emerging threats is uncertain. Recently, photoacoustic spectroscopy PAS, employed in a sensor format, has shown enormous potential to address these ever-changing threats. PAS is one of the more flexible IR spectroscopy variants, and that flexibility allows for the construction of sensors that are designed for specific tasks.PAS is well suited for trace detection of gaseous and condensed media. Recent research has employed quantum cascade lasers QCLs incombination with MEMS-scale photoacoustic cell designs. The continuous tuning capability of QCLs over a broad wavelength range in themid-infrared spectral region greatly expands the number of compounds that can be identified. We will discuss our continuing evaluation ofQCL technology as it matures in relation to our ultimate goal of a universal compact chemical sensor platform. Finally, expanding on ourpreviously reported photoacoustic detection of condensed phase samples, we are investigating standoff photoacoustic chemical detection ofthese materials. We will discuss the evaluation of a PAS sensor that has been designed around increasing operator safety during detectionand identification of explosive materials by performing sensing operations at a standoff distance. We investigate a standoff variant of PAS based upon an interferometric sensor by examining the characteristic absorption spectra of explosive hazards collected at 1 m.

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