Development of transition edge sensors with rf-SQUID based multiplexing system for the HOLMES experiment

Measuring the neutrino mass is one the most compelling issue in particle physics. HOLMES is an experiment funded by the European Research Council for a direct measurement of neutrino mass. HOLMES will perform a precise measurement of the end point of the Electron Capture decay spectrum of Ho-163 in order to extract information on neutrino mass with a sensitivity as low as 1 eV. HOLMES, in its final configuration will deploy a 1000 pixel array of low temperature microcalorimeters: each calorimeter consists of an absorber, where the Ho atoms will be implanted, coupled to a Transition Edge Sensor thermometer. The detectors will be kept at the working temperature of similar to 70 mK using a dilution refrigerator. In order to gather the required 3 x 10(13) events in a three year long data taking with a pile up fraction as low as 10(-4), detectors must fulfill rather high speed and resolution requirements, i.e. 10 mu s rise time and 4 eV resolution. To ensure such performances with an efficient read out technique for very large detectors array kept at low temperature inside a cryostat is no trivial matter: at the moment, the most appealing read out technique applicable to large arrays of Transition Edge Sensors is rf-SQUID multiplexing. It is based on the use of rf-SQUIDs as input devices with flux ramp modulation for linearisation purposes; the rf-SQUID is then coupled to a super-conductive lambda/4-wave resonator in the GHz range, and the modulated signal is finally read out using the homodyne technique.