Bolometers equipped with thin film Anderson insulator sensors: their resolution for nuclear recoil events

Bolometers are very low temperature detectors which measure a deposited energy by the resulting thermal effects. All the excitations in its absorber finally convert to heat (phonons), so that bolometers are universal detectors capable of measuring radiation's energy, even if it has no ionizing component. The ultimate resolution of a bolometer, from the thermodynamical point of view, is a decreasing function of the operating temperature. However, in the case of nuclear recoil events in a solid absorber, trapping effects degrade the resolution. These phenomena are studied here in order to find the resolution limit in ion beam analysis and in direct searches for WIMPs (the hypothetical candidates for non baryonic Dark Matter). Bolometers of different masses and geometries, all equipped with Nb_xSi_1-x thin film sensors, were made and characterized. The properties of these films as an Anderson insulator, and their sensitivities to the non-equilibrium phonons are studied. Furthermore, the experimental resolution in the thermal and athermal regimes were compared. The best resolution obtained is 0.34% (18 keV on 5.5 MeV alpha particles). This result is above the theoretical resolution estimated in this work, so that these bolometers could achieve further progresses. (author)