The Design of the Input Stage for the Very Front-End of the CUORE Experiment

We describe the criteria for the selection of the input transistor of the very front-end for the detector, an array of 988 macro bolometers. Each of such macro bolometer is composed of a crystal of TeO₂, having a mass of 750 g, to which a thermistor is glued. Due to the very large mass of the crystals, the detector response time is very slow and limited to a few Hz. The quoted characteristic for the CUORE (Cryogenic Underground Observatory on Rare Events) detectors make it attractive a solution based on a complete room temperature operated front-end. The very important requirements of such a solution are a very small parallel and low frequency series noise. We worked to the selection of a Si JFET having a gate area and pinch-off voltage that fulfils the CUORE requirements. The selected JFET has an input gate current of less than 60 fA at 40°C (the expected operating temperature at regime). The noise at 1 Hz is about 3 nV/ at the operating point of I _DS of 0.5 mA and V _DS of 0.5 V, selected for having small power dissipation. Thanks to the quoted results the limit in the energy resolution given by the preamplifier is about 10 times better than that presently achievable by the crystals tested and candidate for CUORE. The wafer from which the semi-custom JFET has been cut was previously probed. All the production consisting of 1200 JFETs pairs is already available.