High-Sensitivity CW NMR Probe for Low Temperatures and Ultrahigh Frequencies

✍ Scribed by Kiho Kim; J.R. Bodart; N.S. Sullivan

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 112 KB
• Volume: 118
• Category: Article
• ISSN: 1064-1858
• DOI: 10.1006/jmra.1996.0005

✦ Synopsis

A double-tuning technique for tank-circuit design is presented dilution refrigerator. A Kel-F NMR sample cell of improved for continuous-wave NMR measurements at ultrahigh frequencies design that is superfluid 4 He leak tight has been developed and very low temperatures (in the millikelvin range) where long for very low temperatures. This sample-cell design is suitlossy transmission lines must be used to reduce the transfer of able for studies of two-dimensional systems with very low heat to the sample region. The technique uses two separate sets spin densities and will be reported elsewhere.

of tuning capacitors, one set at low temperatures to match the

In previous tank-circuit designs, large signal losses occur tank circuit closely (but not exactly) to a lossy (low thermal conmainly due to impedance mismatching at the junction of the ductivity) transmission line, and a second set to achieve an accucold end of the coaxial transmission line and the so-called rate match to 50 V at the input of a quadrature hybrid-tee bridge.

conventional series tank circuit consisting of a low-tempera-

The system allows for a stable, tunable, wide frequency coverage and an improvement in the signal-to-noise ratio by a factor of ture NMR radiofrequency coil connected via a 50 V coaxial about 40 over conventional probe designs. The absorption signal transmission line to variable series-tuning and parallel-matchof a monolayer coverage of hydrogen absorbed on boron nitride ing capacitors located at room temperature. This conventional ( É10 17 spins) with a filling factor of 10 05 has been observed at tank-circuit/transmission-line design has a serious mismatch 50 mK with an applied RF field of 3 mG.