Low-field approach to double resonance in nuclear quadrupole resonance of spin-1 nuclei

Abstract

Using double-resonance conditions, in which the Larmor frequency of a spin-1/2 nucleus is matched to one of the nuclear quadrupole resonance frequencies of a spin-1 nucleus, the authors demonstrate increased cross relaxation between the two nuclear spin species. They calculate the cross-relaxation rate using the motionally averaged heterogeneous dipole Hamiltonian as a perturbation to the combined quadrupole and Zeeman Hamiltonians. Using this cross-relaxation rate, in addition to hydrogen and nitrogen autorelaxation rates, expressions governing spin-1/2 and spin-1 spin-lattice relaxation are determined. With ammonium nitrate, containing nitrogen (spin-1) and hydrogen (spin-1/2), increased nitrogen signal and spin-lattice relaxation are demonstrated, using fields less than 120 G. The cross-relaxation rate is also measured and an overall signal/noise improvement by a factor of 2.3+/-0.1 is attained.

Measurement of the 14N nuclear quadrupole resonance frequencies by the solid effect

“…This transition probability per unit time is lower than the transition probability per unit time for the flip-flop transition in case of resonant coupling at m L = m 0 [12], but it is high enough to perform a double resonance experiment.…”

TNT detection with 14N NQR: Multipulse sequences and matched filter

“…Only the magnetic field average value determines the signal enhancement with typical values of 50 for the laboratory conditions, while practical values for remote detection are around 10. Other techniques which modestly improve the S=N include multifrequency NQR [9], circularly polarized RF NQR [10] and others [11]. In addition to signal intensity increase, the detection probability can be improved by advanced signal processing.…”

Aplicability of TNT “super- detection” to multipulse sequences

“…detection of explosives hidden in luggage. Nevertheless, the necessity for a reliable detection has in recent years seen the development of several techniques with enhanced sensitivity for NQR detection of explosives [10][11][12][13][14][15][16][17][18][19][20].…”