"Real World" Particulate Explosives Test Coupons for Optical Detection Applications

Trace or residue explosives detection typically involves examining explosives found as solid particles on a solid substrate. Different optical spectroscopy techniques are being developed to detect these explosives in situ by probing how light interacts with the surface bound particles of explosives. In order to evaluate these technologies it is important to have available suitable test coupons coated with particles of explosives. When fabricating test coupons to evaluate detection performance or help train a detection algorithm, it is important to use realistic test coupons and consider how the physicochemical properties of the explosives particles, related chemicals, and substrate may affect the spectra produced or signal intensities observed. Specific features of interest include surface fill factor, particle sizes, areal density, degree of particle contact with a substrate and any other chemicals in addition to the explosives and substrate. This level of complexity highlights the need to fabricate test coupons which mimic "real world" particle coated surfaces. With respect to metrics derived from fingerprints, we compare the properties of test coupons fabricated by sieving and inkjetting for ammonium nitrate, TNT, RDX, and sucrose on stainless steel, automotive painted steel, glass and polyethylene substrates. Sieving provides a random distribution of particles, allows fractionation of relevant particle sizes and allows relevant surface fill factors to be achieved. Inkjetting provides precise control of aerial density but because of complications related to solvent-substrate interactions, relevant fill factors and particle sizes are difficult to achieve. In addition, we introduce a custom image analysis technique, NRL ParticleMath, developed to characterize and quantify particle loadings on test coupons.