ABSTRACT
A planar nuclear quadrupole resonance (NQR) sensor has been developed. The sensor is resilient to environmental noise and is capable of simultaneous independent multi-frequency operation. The device was constructed as an open multimodal birdcage structure, in which the higher modes, generally not used in magnetic resonance, are utilized for NQR detection. These modes have smooth distributions of the amplitudes of the corresponding radiofrequency magnetic fields everywhere along the sensor's surface. The phases of the fields, on the other hand, are cyclically shifted across the sensor's surface. Noise signals coming from distant sources, therefore, induce equal-magnitude cyclically phase-shifted currents in different parts of the sensor. When such cyclically phase-shifted currents arrive at the mode connection point, they destructively interfere with each other and are cancelled out. NQR signals of polycrystalline or disordered substances, however, are efficiently detected by these modes because they are insensitive to the phases of the excitation/detection. No blind spots exist along the sensor's surface. The sensor can be used for simultaneous detection of one or more substances in locations with environmental noise.
