Comparison of different detectors in low field NMR measurements

Low-field (LF) liquid sample NMR measurements using different detectors are performed at Larmor frequencies fL ranging from 300 Hz to 35 kHz. Five different sensors cooled with liquid nitrogen are compared: a wire-wound coil, a printed planar copper coil, both resonant with capacitance C and read out by a room-temperature amplifier, a high critical temperature Tc thin film rf (bare) SQUID and two tuned SQUID circuits, one with a wire-wound input coil and another with a high-Tc superconducting tape coil. The resonant frequency of each LC circuit is adjusted to fL. The sensitivity of any LC circuit is determined by its inductance, quality factor, and fL. The magnetic field resolution of the sensors was evaluated with a homogenous magnetic field source at different frequencies and with a small source at different positions. We discuss the signal-to-noise ratio of the LF NMR signals recorded by these detectors and find the high-Tc tape coil to be unsuitable for LF NMR measurement. The homogeneity of the magnetic measurement field is deteriorated by this bulk superconducting object. That effect leads to a dramatic reduction of the decay time of the free induction decay (FID) signal. In contrast, the bare SQUID oriented parallel to the measurement field does not influence the FID significantly.