Multi-pulse spin-locking

The application of various multi-pulse methods in NQR requires a clear theoretical understanding of physical processes first of all behind the simplest "basic" multi-pulse sequences which include such a popular sequence as multi-pulse spin locking—MW-4, and a sequence of identically spaced coherent radio frequency (RF) pulses with intervals between them less than the time constant of free induction decay (FID) —strong off-resonance comb— SQRC. ... 

The independence of Hgff on the time interval of the sequence under the condition that this interval lies between T2 and Tie can be used for determining proper Heff. This approach was first used in multi-pulse nuclear magnetic resonance in paper, and later, in NQR, for the analysis of quasi-stationary state in multi-pulse spin locking in the nuclear system with the spin of 5/tF at exact resonance. We extended this approach to the arbitrary offset of the pulse carrier frequency in relation to the resonance transition. 

It follows from (27) that for the multi-pulse spin-locking sequence ri2 = 0, so we shall obtain... 

The scheme of the experiment on polarization enhancement of NQR signals for PETN detection is shown in Figure 7. A PETN sample was placed for a fixed period of time between two identical ring magnets after which it was manually transferred inside the solenoidal coil of the detector of an NQR spectrometer where the signal from N nuclei was detected using a multi-pulse spin-locking sequence MW-4 (py — (t — - t) . 

Figure 8 Dependence of enhancement factor F on the time of protons polarization in magnetic field 6q 80 mT, measured for NQR signals received before and after polarization. NQR signal detection was performed with multi-pulse spin locking sequence (t — — t) with the...

The chapfer consists of four sections, which are devoted to theoretical and practical issues of the detection of nitrogen-containing substances. Section 2 deals with theoretical aspects of the two most popular multi-pulse sequences multipulse spin-locking—MW-4 and "strong off-resonant comb"—SORC. In spite of the fact that the development of these sequences has enabled a dramatic increase in the sensitivity of NQR methods, until recently a number of theoretical and experimental peculiarities of these sequences have not been studied adequately. The primary issue concerns the sequence SORC, and also behaviour of both sequences in case of close times of spin-lattice and dipolar relaxations, which is especially important for the detection of such a popular explosive as hexogen (RDX). There are also demonstrated some experimental techniques for the detection of some other explosives (PETN and trinitrotoluene (TNT)). 

Podkorytov and Skrynnikov derived analytical equations for the relaxation rate in case of the application of alternating-phase spin-lock field and demonstrated that the exchange parameters ean be determined from the dependenee of the relaxation rate on the spin-loek pulse duration. Such measurements can be used in combination with rotating-frame methods to analyse multi-site exchange. 

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