The signal-to-noise ratio (SNR) of the NQR and NMR experiment is of critical importance for sensors striving to achieve low detection limits in short times on the order of seconds, for example those with applications for real-time bulk ore sorting [1]. Presenting a significant challenge to this is the fact that solids typically suffer from short nuclear spin-spin relaxation times (T2), leaving detection limits heavily dependent on minimisation of probe ring down times; however, T2 relaxation time enhancement has been observed in the presence of a weak static magnetic field B0 both historically in 35Cl, 37Cl quadrupole resonances in solid NaClO3 crystals [2], and recently in 75As quadrupole resonances in FeAs2 (lollingite) powders [3]. We report here a similar effect in the 63Cu quadrupole resonance in the mineral CuS (covellite).
The room temperature 14.28 MHz 63Cu NQR transition was excited in CuS powder using spin-echo sequences in the presence of weak B0 in the range 0 – 4.04 mT aligned perpendicular to the radiofrequency (RF) excitation field direction. The T2 relaxation time was observed to increase by a factor of 2.48 from T2 =123 us in zero-field to T2 =305 us in B0 = 4.04 mT; however, an optimum SNR increase by a factor of 3.16 occurred at a lower static field strength of B0 = 2.02 mT. This is likely due to simultaneous line broadening at higher B0 as was observed by Lehmann-Horn et al in lollingite.
The result has implications for the design of high-sensitivity CuS NQR sensors in which the optimisation of SNR is critical for reaching desirable detection limits in the short times required for effective deployment for bulk ore sorting in the minerals industry.