Spurious signals caused by the piezoelectric ringing of NaN0 2 in pulsed NMR

Abstract

Spurious signals caused by the piezoelectric ringing of single crystals of NaNO 2 in pulsed NMR have been studied at 14.5 MHz using AM detection. Their amplitude is about two orders of magnitude larger than the 23Na FID when the a axis of the crystal is along the axis of the coil, and about one order of magnitude larger than the FID when the b or c axis is along the coil axis. The buildup of the piezoelectric signals during a pulse is found to be characterized by a time constant of about 10 μec. However, the decay following the pulse is characterized by an altogether different time constant which varies with the pulse length but is of the order of 100 μsec. Measurements at different locations of the crystal relative to the rf coil show that the piezoelectric signals are generated by a wholly electrostatic interaction, primarily involving the longitudinal electric field generated by the pulse. A theoretical expression for the signal amplitude has been derived. It accounts for many properties of the signals but gives an amplitude which is two orders of magnitude larger than the experimental. Ways of suppressing the piezoelectric signals to facilitate the observation of NMR are discussed.