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The ATHENA observatory is the second large-class ESA mission, in the context of the Cosmic Vision 2015 - 2025, scheduled to be launched on 2028 at L2 orbit. One of the two on-board instruments is the X-IFU (X-ray Integral Field Unit): it is a TES-based kilo-pixels order array able to perform simultaneous high-grade energy spectroscopy (2.5 eV at 6 keV) and imaging over the 5 arcmin FoV. The X-IFU sensitivity is degraded by the particles background which is induced by primary protons of both solar and Cosmic Rays origin, and secondary electrons. The studies performed by Geant4 simulations depict a scenario where it is mandatory the use of reduction techniques that combine an active anticoincidence detector and a passive electron shielding to reduce the background expected in L2 orbit down to the goal level of 0.005 cts/cm2/s/keV, so enabling the characterization of faint or diffuse sources (e.g. WHIM or Galaxy cluster outskirts). From the detector point of view this is possible by adopting a Cryogenic AntiCoincidence (CryoAC) placed within a proper optimized environment surrounding the X-IFU TES array. It is a 4-pixels detector made of wide area Silicon absorbers sensed by Ir TESes, and put at a distance < 1 mm below the TES-array. On October 2015 the X-IFU Phase A program has been kicked-off, and about the CryoAC is at present foreseen on early 2017 the delivery of the DM1 (Demonstration Model 1) to the FPA development team for integration, which is made of 1 pixel “bridgessuspended” that will address the final design of the CryoAC. Both the background studies and the detector development work is on-going to provide confident results about the expected residual background at the TES-array level, and the single pixel design to produce a detector for testing activity on 2016/2017. Here we will provide an overview of the CryoAC program, discussing some details about the background assessment having impact on the CryoAC design, the last single pixel characterization, the structural issues, followed by some programmatic aspects.
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