NOISE IN REFRIGERATING TUNNEL JUNCTIONS AND IN MICOROBOLOMETERS

摘要

We analyzed the possibility to use in space-born experiments designed for astronomical observations, normal metal-insulator-superconductor (NIS) tunnel junctions as refrigerators for microcalorimeters and microbolometers. This was done by presenting very briefly the progress made in the construction of such refrigerators ―as much as the author knows it―and by studying the noise sources in the detectors. Although the subsystem directly involved in the radiation detection is the electron system of the thermal sensing element (TSE), the noise equivalent power (NEP) can be significantly reduced if the ionic lattice of the TSE is also cooled down to about 100 mK. It was shown explicitly that the NEP is not determined just by the fluctuations of the power fluxes or of the temperature of the electron system in the TSE, but depends strongly on the measurement process. The correlations between fluctuations of the quantity physically measured by the thermometer and the fluctuations of the electronic temperature or electron number in the TSE may lead to a decrease of the NEP (Eqs. 11 and 12). Finite size effects and the impedance of the external circuit that connects the NIS junctions to the current and voltage sources are important in the evaluation of the NEP and should be taken into account in future calculations.