Detection of plastic explosives in luggage with N-14 nuclear quadrupole resonance spectroscopy

Nuclear quadrupole resonance (NQR) of N-14 nuclei has many advantages as a method for detecting nitrogen-containing explosives. thr most important are very high chemical specificity, true noninvasive operation and detection o f bulk explosive in situ only (no vapor or particular capture needed). One of the most high explosives is hexogen (RDX) often used by terrorists in pl asticized Forms, The ring nitrogen nuclei in an RDX molecule generate three sets of NQR frequencies corresponding to three physically nonequivalent po sitions of the molecule in the crystal lattice. The prototype device ws hav e constructed is intended for inspection of suitcases for the presence of p lastic explosives containing RDX or octogen by N-14 quadrupole resonance. I t is essentially a fully automated PC-controlled pulsed FT NQR spectrometer equipped with a large volume (70 1) radio-frequency (RF) sample coil to ac commodate a typical suitcase. The device consists of a measure chamber with an RF coil, tuning and matching box, an RF pulse transmitter and a control PC with dedicated cards like digital receiver, frequency synthesizer, puls e programmer and probe-tuning controller. The control software finds the NQ R lines and measures their frequencies. An alarm is produced if any of thes e frequencies matches the characteristic NOR frequency of the explosive and the signal-to-noise ratio exceeds the preset threshold. Multipulse sequenc es of the type SORC (strong off-resonance comb) or SLSE (spin-locked spin e cho) were used in order to increase the allowed data acquisition rate. We c ould detect 230 g of PMW-8, a plastic explosive (containing 81% of RDX) in 10 s or 100 g in 30 s. Detection probability was not less than 90%.