Low Frequency Nuclear Quadrupole Resonance with SQUID Amplifiers*

The dc SQ U ID (Superconducting QUantum Interference Device) can be configured as an ampli­ fier of spin-echos with a noise temperature of approximately 10 mK (f/1 MHz) at an operating temperature of 1.5 K. A Fourier transform spectrometer based on a SQ U ID with a superconducting input circuit and operated in a flux-locked loop is used to obtain nuclear quadrupole resonance (NQR) spectra in a broadband mode over the bandwith 0 -1 MHz. Spin-echo spectra of 14N in N H 4C 104 reveal sharp NQ R resonances, obtained simultaneously, at 17.4, 38.8 and 56.2 kHz. At 1.5 K, the measured longitudinal and transverse relaxation times Tx and T2 for the 38.8 kHz transi­ tion are 63 ± 3 ms and 22 + 2 ms, respectively.