Temperature effects on the performances of the ATHENA X-IFU thermal filters

The X-Ray Integral Field Unit (X-IFU) detector on-board ATHENA is an array of TES micro-calorimeters that will operate at ~50 mK. In the current investigated design, five thermal filters (TF) will be mounted on the cryostat shields to attenuate IR radiative load and avoid energy resolution degradation due to photon shot noise. Each filter consists of a thin polyimide film (~50 nm thick) coated with aluminum (~30 nm thick). Since the TF operate at different temperatures in the range 0.05-300 K, it is relevant to study how temperature affects their mechanical/optical performances (e.g. near edge absorption fine structures of the atomic elements in the filter material). Such results are crucial for the proper design of the filters as well as to establish the calibration program operating temperatures. We report the preliminary results of visual inspections performed on test filters of polyimide/Al at different pressure and temperature conditions, IR transmission measurements (1-15 µm) performed in the temperature range 10300 K, and X-ray Absorption Spectroscopy measurements (175-1650 eV) performed in the temperature range 130-300