N-14 NQR using a high-Tc rf SQUID with a normal metal transformer

Abstract

We have improved our high-Tc SQUID-based N-14 nuclear quadrupole resonance (NQR) detection system. Byusing a reed relay SIL5-1A72-71L (MEDER electronic) in the normal metaltransformer, the isolation to the excitation field was much improved. Thehigh-Tc rf SQUID could operate stably when the rf excitation field was over 20 mT.By optimizing the input coil of the transformer, better sensitivity of0.5 fT Hz−1/2 was obtained at the resonant frequency of the tuned normal metal transformer. Aspin-locking spin-echo (SLSE) multi-pulse sequence was also used in the system to detectthe weak NQR signals produced by samples with longer spin–spin relaxation timeT2, such as trinitrotoluene (TNT).