Detection of Nitrogen Substances by Nuclear Quadrupole Resonance in Large Volumes

In this work, we describe the design of a sensing system for the NQR detection of nitrogen substances in a large inspection volume. The system consists of a Tecmag Scout NQR console, Tomco linear amplifier (4kW), a large distance/volume RF probe (either planar gradiometer or toroidal coil), a duplexer and a high-power Q-factor spoiler. We studied the conditions for optimal unilateral detection at distances up to 20 cm as well as for the detection inside a large volume. It has been shown that depending on RF probe configuration and ambient RF noise level), a
$14_\mathrm{N}$
NQR signal of a small amount (100 g) of hexamethylenetetramine (HMT) as the reference nitrogen substance can be detected using both types of RF detectors. Furthermore, a possibility of NQR detection for a partially shielded HMT sample has been also shown. The influence of conducting bodies near an NQR sample for detection performance has been studied as well. We have also demonstrated that this system can be applied to the detection of ammonium nitrate.