HIGH-SENSITIVITY CW NMR PROBE FOR LOW-TEMPERATURES AND ULTRAHIGH FREQUENCIES

A double-tuning technique for tank-circuit design is presented for con tinuous-wave NMR measurements at ultrahigh frequencies and very low te mperatures (in the millikelvin range) where long lossy transmission li nes must be used to reduce the transfer of heat to the sample region. The technique uses two separate sets of tuning capacitors, one set at low temperatures to match the tank circuit closely (but not exactly) t o a lossy (low thermal conductivity) transmission line, and a second s et to achieve an accurate match to 50 Omega at the input of a quadratu re hybrid-tee bridge. The system allows for a stable, tunable, wide fr equency coverage and an improvement in the signal-to-noise ratio by a factor of about 40 over conventional probe designs. The absorption sig nal of a monolayer coverage of hydrogen absorbed on boron nitride (app roximate to 10(17) spins) with a filling factor of 10(-5) has been obs erved at 50 mK with an applied RF field of 3 mG. (C) 1996 Academic Pre ss, Inc.