Explosives detectors are used to screen people, packages, and infrastructure, as well as conduct investigation of unknown substances. Optical detection techniques hold promise to enable stand-off detection of a range of threat materials, precursors and by-products. However, conventional Raman instruments, which rely on visible and near IR excitation sources, often exhibit fluorescence obscuration and poor sensitivity. An area of research interest is deep-UV Raman spectroscopy as a solution to both issues. Photon Systems RPL-200 is a deep-UV Raman and fluorescence microscope. This report presents preliminary experiments using this technology, for studies on the phenomenology of trace explosive detection, to inform future efforts that aspire to realize high sensitivity stand-off detection of explosives.
The capabilities of the dual-comb spectrometer for stand-off sensing of traces of explosives in scanning mode were systematically tested at a distance of 0.5 m with RDX and PETN with the mass loadings of 5-8 μg/cm2 deposited onto nonporous surfaces: glass, transparent acryl, white and black plastic. Total scanning time of 400 points area (12 x12 cm) with the beam diameter of ~ 1 cm in the target area was 14 s, limited by the processing time of the frequency comb data using our current hardware. Data analysis of the scanning area comprised baseline removal and plotting of Pearson correlation coefficients, as the heatmaps take 30-90 s to acquire depending on both the algorithms for baseline removal and the computer hardware used currently (24-core processor). Baseline removal with the asymmetric least squares (ALS) algorithm is more reliable but takes longer to process (90 s). The probability of detection on nonporous surfaces with the ALS method is 92-98% for PETN and 65-90% for RDX with the confidence level of 90%, depending on the surface material. With the false positive rate being set to 10%, the true positive rate of the system reaches 90% with the ALS method for baseline removal for RDX and PETN on tested surfaces.
The progress in development of a dual-comb spectrometer for detection of traces of explosives at stand-off distances is reported. The spectral range of the spectrometer was extended to 1205-1305 cm-1, the stand-off distance was shortened to 0.5 m to access more potential use-cases, and the speckle contrast was decreased to 0.3%. Tests of the dual-comb spectrometer on RDX and PETN deposited on glass surfaces with a surface concentration of ~10 g/cm2 deposited using a sieving method will be presented and compared with the measurements carried out using a laboratory grade FTIR instrument.
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