Pulse sequences for the detection of RDX at 5.192 MHz: steady state free precession (SSFP) versus free induction decay

Nuclear quadrupole resonance is a promising technique for the detection of illicit substances. It relies on the magnetic properties of some specific nuclei, such as nitrogen and chlorine, widely spread among explosives, narcotics or counterfeit medicines. In the basic NQR experiment, the signal (Free Induction Decay (FID)) is generated by a single radio frequency pulse. Because of its small amplitude, the signal is enhanced by averaging several measurements. However, the excitation cannot be repeated until the spin system relaxes back towards equilibrium and this recovery depends on the spin-lattice relaxation time (T 1 ). This can be sorted out by using multi-pulse sequences. One type of multi-pulse sequence, Steady State Free Precession (SSFP), could be used when the spin-spin relaxation time (T 2 ) of the compound is of the same order as T 1 . It has been claimed that SSFP is a more efficient acquisition sequence than the accumulation of ordinary FIDs. The present study will show, by using simulations and experimental data, that SSFP is a useful sequence for RDX measurements at 5.192 MHz, but is not more effective than a series of well-separated FIDs with a repetition rate lower than 1/T 1 .