On the Characteristics of 14N NQR in Explosives Containing Random Electronic Paramagnetic Impurities

The effect of random electronic paramagnetic impurities on the N-14 nuclear quadrupole resonance (NQR) signal intensity and spin-lattice relaxation has been measured in the samples of commercial grade explosives cyclotrimethylene trinitramine (RDX) and trinitrotoluene (TNT). It was found that in some samples the number of impurities measured by the electron paramagnetic resonance method could reach almost 1 % of the number of the host molecules. As a result, a considerable part of the N-14 nuclei is removed from the resonance leading to corresponding attenuation of the NQR signal. The temperature dependence of the nuclear spin relaxation times T (1) and T (2e) (T (2e) is the echo-signal decay time in the multiple pulse spin-locking sequence) was measured in the range from -40 to 40 A degrees C at some frequencies used for detecting this explosives. In the high temperature side of this range, the relaxation times for both the explosives decrease exponentially with increasing temperature but do not achieve a minimum. In this part of the temperature range, T-1 and T-2e are practically equal in RDX, while T (2e) is much shorter than T (1) in TNT. The exponential part of these temperature dependencies is characterized by the same activation energies of molecular motion (reorientation of the NO2 groups) equal to 72 and 60 kJ/mol in the cases of RDX and TNT, respectively. In the low temperature side, the slope of the T-1 and T (2e) temperature dependences decreases (except for T (2e) in TNT). The obtained results for RDX are compared with those reported by other authors for a similar explosive of other manufacturer and discussed in detail.