IMPROVED 14 N NQR DETECTION IN TNT AT ROOM TEMPERATURE BY PROTON POLARIZATION
J. Luznik, J. Pirnat, V. Jazbinsek, Z. Trontelj, T. Apih and A. Gregorovic
Institute of Mathematics, Physics and Mechanics, Faculty of Mathematics and Physics and Institute J. Stefan, University of Ljubljana, Slovenia
Introduction
Nuclear Quadrupole Resonance (NQR) with its ability of identification specific molecules in measured sample is potentially powerful method in solid state physics, chemistry and pharmacy. In the last 10 or 15 years, also the efforts to detect several illicit materials and dangerous situations with explosives by the 14N NQR are more and more intense1,2.. Unfortunately, many of those substances have the 14N NQR frequencies in the low frequency domain (from about 100 kHz to few MHz). Hence, their detection is difficult because of the low signal-to-noise (S/N) ratio. The required signal averaging and measuring times are therefore of the order of hours and are thus too long for practical applications. There have been several suggestions in the past how to increase the S/N ratio in the detection of low frequency 14N NQR spectra.
Most of these were based on the use of proton-nitrogen nuclear double resonance techniques3-5 using the changes in the proton signal as an indirect indication of the 14N NQR transitions. These techniques require a homogeneous applied external magnetic field and are therefore not very well suited for daily applications or ''far field'' detection of mines and explosives.
The second group uses direct 14N NQR detection6-9 and here is no requirement for the homogeneity of the external magnetic field. The 14N NQR signal intensity is enhanced by proton polarization transfer via proton-nitrogen level crossing in adiabatically decaying external magnetic field (Fig.2).
Here we show that with a combination of proton polarization transfer and a multi-pulse spin-locking sequence the measuring time can be significantly shortened so that the 14N NQR lines of a 15 gram sample of TNT can be measured at room temperature in a ''single shot'' experiment in 20 seconds.
Experimental
TNT (C7H5O6N3) appears in two crystal modifications, orthorhombic and monoclinic, with six inequivalent nitrogen sites giving six different sets of +, - and 0 14N NQR lines for each crystal modification10, 11. The monoclinic phase is a stable phase. In the three different cylindrical samples of military grade TNT we investigated (diameter 16mm, length 50mm, with a mass of about 15 grams), the monoclinic phase was the more abundant one. This is probably due to the remelting procedure which was undertaken to simulate the real situation in an anti-personnel mine.
Protons in TNT were polarized by a NdFeB permanent magnet (70 mm X 65 mm X 65 mm). The polarizing device consists of a movable vehicle (Fig.1), carrying the magnet towards and away from the sample, and a propulsion system with triggers and delays. The pulsed spin-locking sequence (PSL) was applied 0.5 seconds after removing the polarizing magnetic field. The multipulse PSL sequence – ( – –-n (Fig.2) was used. Here n refers to the number of pulse-train repetitions and the pulse width is chosen to optimize the signal. Several echoes in a single shot were observable and thus the signal to noise ratio effectively enhanced. One level crossing cycle and one PSL sequence together took about 20 seconds. For the procedure to be effective the nitrogen T1 must be long enough to complete the level crossing cycle. For the + line of TNT the theoretical improvement in the signal to noise ratio due to proton polarization transfer is about a factor of 20 for a polarizing field of 0.3 T. The signal averaging of 16 echoes in the ‘’single shot’’ PSL sequence yields another factor of 4. Altogether the measuring time should be thus reduced in the ideal case by more than three orders of magnitude.
Figure 3.: When averaging 9 PSL sequences and using proton polarization transfer six 14N NQR lines at 837 kHz, 842 kHz, 844 kHz, 848 kHz, 859 kHz and 870 kHz could be resolved.
Figure 4.: The ‘’single shot’’ 14N signals at 842 kHz, 844 kHz and 848 kHz of the central part of the NQR spectrum of 15 g of TNT at room temperature are shown.
The obtained signal is good enough for rough quantitative measurements.
Figure 5.: The dependence of the averaged signal enhancement of the 14 N NQR line at 842 kHz, 844 kHz and 848 kHz on the time of polarization and magnetic field at the center of the sample
Figure 1.:The movable vehicle with the permanent magnet
Figure 2.: The experimental procedure:
adiabatic demagnetization and the PSL sequence
References
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Conclusions
We have demonstrated that the proton polarization with higher magnetic field considerably improves the S/N ratio in the detection of 14N NQR signal in TNT at room temperature. The successful “single shot” detection of 14N NQR signal in TNT at room temperature was possible because the conditions for the level crossing were fulfilled and the PSL sequence provided 15 good echoes.
