Nuclear spin thermometry below 1 °K

A thermometry technique is described which consists of measuring the magnetization of a system of nuclear spins by means of pulsed nuclear magnetic resonance. Such a thermometer is useful at temperatures below that of pumped liquid helium, which is used as a calibration point. This technique has been employed to detect nuclear spin temperatures as low as 10 -5 °K in a specimen of sodium metal; these temperatures were produced by the well known method of nuclear cooling originated in the Clarendon Laboratory. The results of our experiments confirm the conclusions drawn by the original authors, namely that, after isentropic demagnetization, the nuclear spins reach temperatures of the order of microdegrees, while the metallic lattice and the conduction electrons remain at the temperature of the cooled paramagnetic salt reservoir. Spin-lattice relaxation times Ƭ 1 have been measured for copper metal at temperatures ranging from 0·02 to 0·35 °K and for platinum metal at temperatures ranging from 0·05 to 2 °K. The copper measurements give T 1 T = 1·12 s °K ±10% , in good agreement with the results obtained by Anderson & Redfield (1959) in the temperature range 1 to 4·2 °K. An earlier report of an increase in T 1 T for platinum by a factor of 2 at temperatures below 1 °K is corrected by the present measurements, which show this quantity to be constant at 30 ms °K at temperatures down to 0·07 °K. It is further shown that the erroneous earlier measurements were caused by eddy current heating in the metallic specimens. The problem of eddy current heating, which was significant for all of the experiments reported here, is analyzed in some detail. A complete description is given of the electronic circuitry developed for the present low-frequency pulsed nuclear resonance experiments.