Baseline design of the thermal blocking filters for the X-IFU detector on board ATHENA

ATHENA is an advanced X-ray observatory designed by a large European consortium to address the science theme "Hot and Energetic Universe" recently selected by ESA for L2 - the second Large-class mission within the Cosmic Vision science program (launch scheduled in 2028). One of the key instruments of the mission is the X-ray Integral Field Unit (X-IFU), an array of Transition Edge Sensor (TES) micro-calorimeters with high energy resolution (2.5 eV @ 6 keV) in the energy range 0.2÷12 keV, operating at the focal plane of a large effective area high angular resolution (5" HEW) grazing incidence X-ray telescope. The X-IFU operates at temperatures below 100 mK and thus requires a sophisticated cryostat. In order to allow the beam focused by the telescope to reach the X-IFU detector, windows need to be opened on the cryostat thermal and structural shields surrounding the cold stage. X-ray transparent thermal blocking filters need to be mounted on such open windows to make the radiation heat-load onto the detector array negligible with respect to conduction heat load and dissipated electrical power, and to minimize photon shot noise onto the detector. After a brief survey of the heritage from space satellite and sounding rocket experiments on thermal filters operated at cryogenic temperatures, we present the selected baseline design of the thermal filters for the ATHENA X-IFU detector, show the performances, and finally discuss possible improvements in the design to increase the X-IFU quantum efficiency at low energies.