In this contribution, we describe two innovations of the structure of large mass bolometers, proposed by the cryogenic group of the Insubria University (Como) and developed in collaboration with the Firenze group. First, up to now, low temperature calorimeters do not have any sort of spatial resolution. This means that it is not possible to reject events coming from the material that faces the detectors (holder, refrigerators shields, ...). In order to cope this problem, we developed a new kind of composite bolometers able to discriminate, by means of active ultra-pure semiconductor shields, external surface events from those coming from the absorber bulk.
A second innovation that we discuss here concerns the temperature sensors. Presently, neutron transmutation doped Ge thermistors are the most common kind of phonon sensors. Unfortunately, this kind of readout dissipates power on the detector because of the thermistor biasing and also introduces a Johnson noise term. To improve energy resolution we studied and test the application of capacitive sensors that in principle could allow us to achieve a better signal-to-noise ratio. Modeling, simulations and first encouraging measurements on surface sensitive bolometers will be discussed along with preliminary results on capacitive sensors.
We have designed and built a very simple and efficient instrument that allows performing very accurate noise measurements of transistors at any biasing conditions, from room temperature down to cryogenic temperatures. This way a study has been possible of the noise behavior of Silicon JFETs for both the low frequency and the high frequency white noise. We explored a wide range of biasing conditions, starting from a power dissipation of only 2 μW up to 1 μW. Concerning white noise, evidence was found for the hot electron effect: it was negligible at small power dissipation and evident at large power. An experimental study was made of the low frequency noise. Its interpretation was developed based on the Generation Recombination theory. Many JFET samples were investigated, made with different technologies and having different gate area.
High-Z low-temperature calorimeters are developed by an Italian collaboration (Milano-Como-Gran Sasso Underground Laboratories) in order to search for rare nuclear events and Dark Matter massive candidates. They exhibit an excellent energy resolution, close to that of Ge-diodes, but a much higher efficiency. Different high-Z materials were initially employed . A many-years optimisation work on tellurium oxide (TeO2) lead to impressive results: devices with total masses around 750 g present FWHM energy resolutions on gamma-ray peaks ranging from 1 KeV (close to the 5 KeV energy threshold) to 2.6 KeV at 2615 KeV (208Tl gamma line). A 3.2 KeV FWHM energy resolution was obtained at 5.4 MeV (210Po alpha line), which is by far the best one ever achieved with any alpha detector. These devices, operated at about 10 mK, consist of a TeO2 single crystal thermally coupled to a 50 mg Neutron Transmutation Doped (NTD) Ge crystal working as a temperature sensor. Special care was devoted to methods for response linearization and temporal stabilisation. Devices based on the same principle and specifically optimised could find applications in several fields like gamma-ray astrophysics, nuclear physics searches, environmental monitoring and radiation metrology.
We are presenting our recent developments to measure the electron antineutrino mass by studying the 187Re (beta) - spectrum end-point with high resolution thermal detectors. We will discuss the preliminary results of an array of 8 bolometers made up of AgReO4 absorbers (2.309 mg of total mass corresponding to a total 187Re active mass of about 0.905 mg of with an expected (beta) total rate of about 1.3 Hz). Their risetime of 0.7 - 1.2 ms together with their energy resolution, ranging between 21 eV and 26 eV at 1.5 KeV, should allow to set a limit of about 10 - 12 eV after one year of real time measurements.
Angelo Alessandrello, Chiara Brofferio, David Camin, C. Cattadori, Oliviero Cremonesi, Ettore Fiorini, Andrea Giuliani, A. Maglione, Benno Margesin, Angelo Nucciotti, Maura Pavan, Gianluigi Pessina, Giorgio Pignatel, Ezio Previtali, Luigi Zanotti
We are developing Si-implanted thermistors to realize high resolution microcalorimeters. We plan to use these devices in an experiment for the determination of the neutrino mass. The measure implies the evaluation of the correct end-point energy of a beta spectrum with a calorimetric approach. Our study is devoted to outline the optimum fabrication process concerning performances and reproducibility. For such reasons we have realized Si thermistors with different concentration of dopant impurities and with different implant geometries. Tests are performed between 4.2 and 1.2 K using a pumped helium cryostat, and selected samples are characterized at very low temperatures in a dilution refrigerator. Good reproducibility of the devices is necessary for producing an array of detectors. At the same time suitable electronics are developed to optimize the detectors preamplifiers link: minimization of the parasitic capacitance is necessary to reduce the integration of signal and to maximize the speed response of the detector.
A massive thermal detector consisting of a 73 g TeO2 crystal, to be used to search for double beta decay of 130Te and to detect high energy gamma-rays, is operating at aprox. 15 mK in the Gran Sasso Underground Laboratory. The FWHM resolution achieved with this detector is 5-8 KeV, slightly dependent on energy from 100 KeV to 3 MeV, showing the competitive performances of this detector in gamma-ray spectroscopy comparable with those of conventional Ge solid state detectors. Moreover this is the first massive high-resolution gamma-ray detector of atomic number larger than 32, which implies a peak to Compton ratio comparable to that of a Ge diode of a mass larger by an order of magnitude.
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