On RF-Pulse-Phase Dependence of Nuclear Quadrupole Resonance Signal Under Short-Repetition-Time Pulse Sequences

Abstract

In the most conventional pulsed nuclear quadrupole resonance (NQR) systems, radio frequency (RF) pulses for irradiation are produced by chopping a continuous RF wave with a DC pulse. Therefore, RF pulses have phase coherency but NQR signals after each RF pulse have different initial phase. In this case, a phase-sensitive detection (PSD) using the same continuous RF wave is necessary to accumulate NQR signal. Meanwhile, recent improvement of device technology enables a phase controlled RF pulse and a direct sampling of NQR signal in the laboratory frame. We developed a new pulse NQR system using phase controlled RF pulse, in which RF pulses with the same initial phase are produced and NQR signal after each pulse is directly acquired in the laboratory frame, and then the accumulated signal is processed by digital quadrature detection and fast Fourier transform. This paper provides and compares the feature of NQR signals from both types of pulse NQR system. Main findings are that NQR signal obtained by a train of phase coherent pulses and PSD shows the periodical variation of the intensity depending on the offset frequency of the irradiation RF pulse from the resonance frequency but the signal obtained by the phase controlled RF pulse train does not. This feature would be important for the application of NQR to detection system for such as explosives.