Review of NQR Explosive Detection. Many researchers have been engaged in the study of mine detection using NQR. The history of NQR dates back to the discovery of NMR at Stanford University and Harvard University in 1946 [3, 98, 99]. Pound et al [100] predicted that resonances due to electric quadrupole splitting would be detected in the absence of a static magnetic field in NMR measurements, based on the effect of the presence of an electric quadrupole moment on nuclear magnetic resonance absorption. The first observation of NQR phenomena in the absence of an external magnetic field was reported in 1950 [101]. The NQR phenomenon was observed in the signal generated from chlorine nuclei from 50 g of solid polycrystalline trans-dichloroethylene (C2H2Cl2). Resonance lines with ratios of intensity corresponding to two resonance frequen- cies of 35.40±0.01 MHz and 27.96±0.01 MHz were observed from the chlorine nucleus (I=3/2) of 35Cl and 37Cl in the homopolar C-Xx xxxx. They have determined the ratio of the nuclear quadrupole resonance moments of the chlorine isotopes. Since then, research on NQR has been conducted in laboratories around the world. Early NQR spectrometers were based onCWtechnology [ 102]. They mainly used oscillators of relatively simple design, in which the sample was placed in (or near) an inductor L, the frequency was determined by the LC resonant circuit, and the frequency was swept by controlling the capacitance C mechanically or electrically. When the frequency being swept matches the NQR resonance frequency, absorption occurs and the NQR phenomenon can be observed. In contrast to the CW method, the more versatile pulse method is currently widely used in NQR measurements. The pulse method started with NMR and has been adapted to NQR. In the pulse method, a pulsed wave of NQR resonance frequency is irradiated and the emission returned from the excited nucleus is observed. As CW saturates the NQR signal and the sensitivity is limited, the pulse method tends to have higher sensitivity. In conjunction with the pulse NQR method, research on digital signal processing and Fourier transform has been conducted. The British Army considered the use of NMR and other techniques as a method of detecting explosives such as land mines, but it was difficult to apply a uniform static magnetic field to the ground. Hence, research to detect explosives using NQR has started. In the early 1960s, Southwest Research, Inc. in San Antonio, Texas, briefly considered the use of NQR for explosives d...
