Nuclear quadrupole resonance

Advanced Nuclear Quadrupole Resonance (NQR) techniques can be used to detect explosives in any surroundings. The quadrupole charge distribution of the atom results in alignments of nuclear spins. A radio frequency pulse (rf-pulse) generated by a transmitter coil causes the excitation of nuclear spins to higher quantisized energy levels. When the nuclear spins return to their equilibrium position, they follow a particular precession frequency. This specifies the atoms and functional groups in the molecules. Nitrogen is a quadrupole atom that appears in every type of explosive. Because of very distinct NQR frequencies the false alarm rate due to other nitrogen containing materials is extremely low [10]. The study of this type of sensor results from a collaboration between the King's College in London, the RMA and the KUL. Fig.(9) and (10) below give an idea on the NQR process.

  

Figure 9: quadrupole moment

  

Figure 10: Detection process