Current sensing noise thermometry using a low T-c DC SQUID preamplifier

We describe here the design and performance of a current sensing noise ther mometer using a low T-c DC SQUID as the front end amplifier. The DC SQUID i s used to measure the thermal noise current in a resistor and the temperatu re is then obtained from the Nyquist formula. The thermometer is fast, abso lute and precise and is usable over a wide temperature range below 4.2 K, i n principle down to well below 1 mK. The excellent energy sensitivity of th e DC SQUID, operated at fixed temperature, enables the use of a relatively large noise resistor, in the m Omega range. This requires relatively short averaging times when measuring the spectrum of noise fluctuations. We have shown that it is possible to determine absolute temperature with a precisio n of 1% in a measuring time of 10 seconds with an amplifier noise temperatu re, T-N, of the order of 30 muK, and to an accuracy better than 0.3%. The p ercentage precision is independent of temperature for temperatures much gre ater than T-N. OUT method of heat sinking the noise resistor ensures proper cooling of the electrons, We incorporate a fixed point device for checking the gain calibration. We have cooled the thermometer successfully to below 1 mK, achieving a minimum electron temperature of 300 muK. We present the results of a preliminary comparison with a He-3 melting curve thermometer ( MCT) above 4.5 mK, and with a platinum NMR thermometer down to the lowest t emperatures.