File:Bachelorthesis Xu 2016.pdf

An accurate measurement of absolute temperature is essential in low temperature physics. In this thesis, we will present a current sensing noise thermometer (CSNT) based on SQUIDs1 amplifier for temperatures from 7 K down to micro-kelvin regime. The current noise for determining the temperatures is from a piece of high purity bulk silver, which is monitored by two current sensing dc-SQUIDs. Operating both SQUIDs in voltage biased mode in a 2-stage SQUID amplifier allows to reduce both power dissipation and the noise of the SQUIDs to a very low level. In comparison with a Magnetic Field Fluctuation Thermometer, after subtracting an appropriate background noise from the measured noise spectrum, the temperatures given by two thermometers agree on the level of 0:9% from 9mK to 6K, with acquisition time of 500s. This indicate that the relatively error of the our CSNT should be lower than 0:9%.