Effect of 14N Nucleus Capture in Nuclear Quadrupole Resonance at Small Relaxation Parameters

Abstract

Experiments on detecting induction signals of nuclear quadrupole resonance (NQR) on 14 N nuclei in hexahydro-1,3,5-trinitro-s-triazine C 3 H 6 N 6 O 6 (RDX) with preliminary changed population densities of NQR energy levels upon exposure to a saturating pulse are described. It is demonstrated that this actually causes the induction signal amplitude to change; however, complete saturation of the transition cannot be reached because of the NQR relaxation time. It is also demonstrated that the results of calculations without regard for the relaxation processes in RDX lack experimental support. Experimental data demonstrate the feasibility of application of the methods of preliminary change of the population densities (by pumping) for multifrequency experiments in NQR and explosive detection.In the method of direct NQR of 14 N with spins I = 1, simultaneously two and more transitions can be excited [1]. Interest in the application of multifrequency methods in this field has considerably increased recently [2]. This is primarily due to the possibility of detecting NQR signals on transitions unaffected by radio-frequency pulses [3]. This allows one to avoid the influence of the transition processes in the equipment and piezo-effects in various media. One of the best known applications of multifrequency NQR is attribution of spectral lines and change of the signal intensity for saturated adjacent transitions [4]. The last phenomenon is called capture in NQR. One of the methods of realizing this effect, in which a saturating pulse changing the initial energy level population densities is applied to the adjacent transition before the application of radio-frequency pulses, was suggested in [5]. In the present work, results of analogous experiments in hexahydro-1,3,5-trinitro-s-triazine C 3 H 6 N 6 O 6 (RDX) at room temperature are considered. Because RDX is a widespread explosive, results of our investigations can be used to increase the reliability of detection of this substance from two frequencies in the NQR spectrum.For spins Ι = 1 of the systems of 14 N nuclei, there is a system of three energy levels with frequencies of nuclear quadrupole resonance transitions [3] ( ) ( ) 0 , 3 , 3 2 4 4 eQq eQq eQqwhere zz eQq is the quadrupole interaction constant, η is the asymmetry parameter of the electric field gradient, and isPlanck's constant. The conditions and methods of 14 N capture in NQR must depend on the quadrupole system parameters. This is due to the fact that the spin temperature also depends on the relaxation parameters of the system. Having estimated the spin temperature T s for the corresponding transition, we can estimate the capabilities of changing the population density for the adjacent transition that can affect the NQR signal amplitude on the other transition. To this end, we take advantage of the following expression [6]:Kaliningrad State University.

Secondary signals in two-frequency nuclear quadrupole resonance on 14N nuclei with I=1

“…in the case of hexahydro-1,3,5-trinitro-s-triazine C 3 H 6 N 6 O 6 (RDX). It has been shown that for this substance the results of the model calculations cannot be applied even in the simplest case of secondary signals related with the coherence transfer between adjacent transitions [13]. Another reason is the experimental complications that appear in multifrequency NQR experiments.…”