HIGH-SENSITIVITY NMR MEASUREMENTS AT LOW-TEMPERATURE AND FREQUENCY

The sensitivity of continuous-wave NMR can be increased with a high-Q resonance tank circuit by using superconducting wire and a GaAs FET preamplifier operating at liquid helium temperatures. We discuss impedance matching to obtain an optimal signal-to-noise ratio for input circuits operating at almost-equal-to 500 kHz, with Q almost-equal-to 10(3)-10(4) at an impedance level of 1 MOMEGA and a noise voltage of 1.3 nV/square-rootHz. High rf compensation of 1:10(5) is achieved with orthogonally oriented transmitter coils and an astatically wound pick-up coil. The performance is evaluated on the basis of resolution that we achieved in the measurement of the NMR line in normal liquid He-3; this is better than 1 part in 10(3) of the peak amplitude, while in the integrated intensity the standard deviation from the mean is better than 0.3%.