Characterization tests thermal filters for the ATHENA mission X-IFU low temperature detector

The X-Ray Integral Field Unit (X-IFU) is one of the two detectors of the ATHENA astrophysics space mission approved by ESA in the Cosmic Vision Science Program (launch scheduled in 2028). The X-IFU consists of a large array of TES micro-calorimeters that will operate at ~50 mK inside a sophisticated cryostat. A set of thin filters, highly transparent to the X-rays focused by the telescope, will be mounted on the dewar and the focal plane assembly thermal/EMI/mechanical shields. These are designed to attenuate the IR radiative load avoiding energy resolution degradation due to photon shot noise, to attenuate EM interference onto the detector and the read-out electronics, and to protect the detector from contamination. The adoption of ATHENA by ESA is expected at the beginning of 2020; by that time the thermal filter design has to be consolidated, and the Technology Readiness Level 5 must be demonstrated for the selected technology. Here, we present the current filter design based on aluminum/polyimide thin bilayer membranes supported by stainless steel meshes. We describe the filter samples developed/procured so far, and present preliminary results from the ongoing characterization tests.