We have designed and demonstrated a standoff Raman system for detecting high explosive materials at distances up to 50 meters in ambient light conditions. In the system, light is collected using an 8-in. Schmidt-Cassegrain telescope fiber-coupled to an f/1.8 spectrograph with a gated intensified charge-coupled device (ICCD) detector. A frequency-doubled Nd : YAG (532 nm) pulsed (10 Hz) laser is used as the excitation source for measuring remote spectra of samples containing up to 8% explosive materials. The explosives RDX, TNT, and PETN as well as nitrate- and chlorate-containing materials were used to evaluate the performance of the system with samples placed at distances of 27 and 50 meters. Laser power studies were performed to determine the effects of laser heating and photodegradation on the samples. Raman signal levels were found to increase linearly with increasing laser energy up to approximately 3 x 10(6) W/cm2 for all samples except TNT, which showed some evidence of photo- or thermal degradation at higher laser power densities. Detector gate width studies showed that Raman spectra could be acquired in high levels of ambient light using a 10 microsecond gate width.
A chemical kinetic model for the thermal decomposition of the solid high explosive pentaerythritol tetranitrate (PETN) is developed for prediction of times to thermal explosion using the Chemical TOPAZ heat transfer computer code. The model is based on times to thermal explosion measured in a new One Dimensional Time to Explosion (ODTX) apparatus. ODTX experiments are reported for pure PETN and for Semtex 1A. The pure PETN results are accurately modeled using a four reaction decomposition process in which an autocatalytic process produces intermediate reaction product gases, which subsequently react in a second order gas phase process to produce the final reaction products. Semtex 1A exhibits longer times to explosion than PETN at low temperatures, indicating that its endothermic binder decomposition absorbs heat produced by PETN decomposition. This binder reaction is modeled as a first order endothermic process. Three experiments on 5.08 cm diameter unconfined cylinders of PETN ramp heated to explosion at different rates are reported. The PETN model accurately predicts the thermocouple records and explosion times for these unconfined experiments in which only intermediate gaseous products can form.
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