14 N Nuclear Quadrupole Resonance Signals in Paranitrotoluene and Trinitrotoluene. Spin-Lock Spin-Echo Off-Resonance Effects

A simple, yet effective technique to enhance the 14 N NQR tri- nitrotoluene notoriously low sensitivity is the use of multipulse sequences. Here we investigate the off-resonance effects of the Spin-Lock Spin-Echo multipulse sequence, a predecessor of many advanced pulse sequences used for the same enhancement. Two samples have been used: paranitrotoluene, with a single 14 N site as a model compound for trinitrotoluene, and tri- nitrotoluene itself, with six 14 N sites. Our main focus has been the irradiation frequency dependence of the NQR signal, which is important when 14 N NQR is used for remote detection of explosives. The two related principal issues are: the target temperature uncertainty and the existence of multiplets with several closely spaced resonance frequencies. The first applies to any explosive, since in remote detection the temperature is only approximately known, whereas the second applies mainly to trinitrotoluene, with 12 reson- ance frequencies between 837 and 871 kHz. Our frequency dependent investigation shows that the signal intensity as well as the effective spin- spin relaxation time varies substantially with irradiation frequency in both samples. We provide a theoretical explanation of this variation which describes very well the observations and can be useful for increasing the reliability of remote detection signal processing.

[1]  M D Ginsberg,et al.  Optimization of offset frequency in the SORC pulse sequence using feedback. , 1999, Journal of magnetic resonance.

[2]  C. Slichter.  Principles of magnetic resonance , 1963 .

[3]  Robert Matthews,et al.  Remote sensing by nuclear quadrupole resonance , 2001, IEEE Trans. Geosci. Remote. Sens..

[4]  R. Blinc,et al.  Polarization enhanced “single shot” N14 nuclear quadrupole resonance detection of trinitrotoluene at room temperature , 2006 .

[5]  S. S. Kim,et al.  Experimental Investigations of the Strong Off-Resonant Comb (SORC) Pulse Sequence in 14N NQR , 1992 .

[6]  Andreas Jakobsson,et al.  Frequency-selective detection of nuclear quadrupole resonance signals , 2005, IEEE Transactions on Geoscience and Remote Sensing.

[7]  S. M. Klainer,et al.  Multiple spin echoes in pure quadrupole resonance , 1977 .

[8]  D.Ya. Osokin,et al.  Coherent multipulse sequences in nitrogen-14 NQR , 1982 .

[9]  Jürgen Altmann,et al.  1H-14N cross-relaxation in trinitrotoluene - a step toward improved landmine detection , 2002 .

[10]  T. N. Rudakov,et al.  On the NQR detection of nitrogenated substances by multi‐pulse sequences , 2004 .

[11]  John S. Waugh,et al.  Pulsed spin locking in pure nuclear quadrupole resonance , 1980 .

[12]  Y. Hiyama,et al.  Determination of the 14N electric field gradient tensor in the nitro group: A single crystal nuclear double resonance study of p‐nitrotoluene , 1981 .

[13]  R. F. Connors,et al.  Orthorhombic and monoclinic TNT: a nitrogen -14 NQR study , 1983 .

[14]  Alan Gregorovic,et al.  Relaxation during spin-lock spin-echo pulse sequence in (14)N nuclear quadrupole resonance. , 2008, The Journal of chemical physics.

[15]  K. Sauer,et al.  Spin dynamics in the pulsed spin locking of nuclear quadrupole resonance , 2006 .

[16]  L. M. Pant,et al.  The structure of p‐nitrotoluene , 1971 .

[17]  Maricq.  Quasistationary state and its decay to equilibrium in the pulsed spin locking of a nuclear quadrupole resonance. , 1986, Physical review. B, Condensed matter.

[18]  T. N. Rudakov,et al.  Multiple NQR spin echoes in phase cycled pulse experiments , 2003 .