Detecting the 14N NQR Signal Using a High-Tc SQUID

A nuclear quadrupole resonance (NQR) detection system based on a high-Tc rf SQUID was developed. To improve the sensitivity of the SQUID, a normal metal transformer was used. The transformer was composed of a pickup coil, an input coil and a capacitor. With this normal metal transformer, the magnetic field resolution of a high-Tc rf SQUID could be improved from 50 ft /radicHz to about 1 fT/radicHz at 3.31 MHz. To reduce the influence the strong transmission magnetic field, the SQUID was put in magnetic shielding and pulse-controlled switches were inserted in the transformer. With the high-Tc rf SQUID and the excitation rf pulse of 3.31 MHz, the ¹⁴N NQR of hexamethylenetetramine (HMT) at 3.3075 MHz could be successfully detected. The signal-to-noise ratio was similar to that of using normal coil with a low noise preamplifier.