Modeling the energy thermalization of X-ray photons in a microcalorimeter with superconducting absorber

We present a modeling of the response of a microcalorimeter to the absorption of X-ray photons, based on the main microscopical processes responsible for the energy thermalization. In particular, we have modeled a microcalorimeter with superconducting tin absorber (350 μm × 350 μm × 7 μm) and neutron transmutation doped (NTD) germanium thermistor (75 μm × 50 μm × 150 μm). Such a detector, operated at 60 mK, is expected to achieve a spectral resolution as good as 1 eV FWHM in the soft X-ray energy range, based on the known sources of thermal and electronic noise. Nevertheless, the best spectral resolution measured in laboratory experimental tests is of about 5 eV FWHM (at 5.89 keV). We have investigated how the microscopic processes of energy thermalization, involving both quasiparticles and phonons, and the position of absorption of the photons may affect the spectral resolution of the detector.