Sensing System for 14N NQR Remote Detection of Explosives

In this report, we describe the design of the sensing system for the remote detection of nitrogen-based explosive compounds, which is based on the method of nuclear quadrupole resonance (NQR). The NQR technique is applicable for the detection of the solid explosives hidden in the baggage of passengers, underclothes, or buried under the ground. The sensing device includes a subsystem for the active mitigation of the external radiofrequency interference (RFI) and a subsystem for the spoiling of the quality factor of the RF probe. The system for the attenuation of RFI has three main components: the main NQR detection channel, two additional channels to mitigate the effect of RFI and digital post-processing. The first additional channel is used to decrease the effect of RFI before the NQR console and it includes the amplifier with the autoregulation of gain, the phase shifter to tune the phase and a final subtraction stage. Another additional channel consists of the set of the additional amplifier and the digital receiver to detect the RFI before it is processed digitally. Another system is used to spoil the unwanted oscillations after the application of powerful RF excitation pulses appearing in the high quality (Q) RF sensors. For remote NQR detection, the design of RF sensors is usually a trade-off between a desire to increase the sensitivity by use of high-Q sensors and a need to suppress the parasitic oscillations which duration is proportional to the Q-factor value. In this work, two versions of a Q-spoiling system have been designed. The first version is based on electromechanical switches, while the second one is made with the use of solid-state relays. We compare and discuss these two alternative approaches to shorten the dead time of the RF sensor. The designed sensing system has been tested on a model nitrogen-based substance and the prospects of its implementation to the detection of some of the real explosives have been discussed as well.