Polarization Enhanced Nqr Detection at Low Frequencies

Abstract

In this contribution we present our current research on polarization enhanced nuclear quadrupole resonance (NQR) detection at low frequencies with the emphasis on 14 N NQR trinitrotoluene (TNT) detection at room temperature. Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes 14 N NQR in TNT fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for 14 N NQR lines in TNT were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. 14 N NQR signals in a 15 g trinitrotoluene sample of predominantly monoclinic modification were measured in 15 s in different polarization magnetic fields. The conditions for optimal polarization enhancement were determined. Introduction Nuclear Quadrupole Resonance (NQR), with its ability to identify specific molecules, is potentially a powerful method in solid state physics, chemistry and pharmacy. In the last 10 to 15 years, several attempts have been made to improve the detection of military explosives, improvised explosive devices (IED) and other illicit materials – mainly narcotics by 14 N NQR [1-9]. Unfortunately, many of these substances have 14 N NQR frequencies in the low frequency domain between 100 and 1000 kHz, hence a rather low signal to noise (s/n) ratio. Therefore, the measuring times for the required signal averaging can be hours and they are thus too long for practical applications. With a combination of proton polarization transfer to nitrogen nuclei and multi-pulse spin-locking sequences the measuring time can be significantly reduced to an acceptable level of the order of 10 s, provided the proton and the nitrogen spin-lattice relaxation times (T1) are suitable. There are two ways to increase the s/n ratio by proton-nitrogen level crossing polarization transfer: a) proton-nitrogen nuclear double resonance techniques [10-13] using changes in the proton NMR signal as an indirect indication of the 14 N NQR transitions; and b) direct 14 N NQR detection where the signal is enhanced by proton polarization transfer via proton-nitrogen level crossing in a time variable magnetic field [14-19]. The first technique requires a homogeneous applied external magnetic field and is therefore not convenient for work in the