The cryogenic AntiCoincidence detector for ATHENA X-IFU: Advancement in the project

The Athena observatory is the second large class ESA mission to be launched on early 2030's. One of the two on board instruments is the X-IFU, which is a TES based kilo-pixels array able to perform simultaneous high grade energy spectroscopy (2.5eV@7keV) and imaging over the 5' FoV. The X-IFU sensitivity is degraded by primary particles background (bkg) of both solar and Galactic Cosmic Rays origin, and secondary electrons produced by primaries interacting with the materials surrounding the detector. The TES-array main sensor therefore needs a Cryogenic AntiCoincidence detector (CryoAC) to veto as much as possible such particles. The required residual bkg is 0.005 cts/cm/s/keV in 2-10 keV energy bandwidth. The CryoAC is at present baselined as 4 pixels detector made of Silicon suspended absorbers sensed by a network of IrAu TESes, and placed at a distance < 1 mm below the TES-array. On November 2019, Athena has successfully passed the Mission Formulation Review (MFR), thus entering in Phase B. Next close goal is the MAR (Mission Adoption Review) planned in second half of 2022 where all the critical technologies must demonstrate a Technology Readiness Level (TRL) equal to 5. Here we will provide an overview of the CryoAC program advancement involving: 1) the present particle background assessment; 2) the assembly design concept and the related trade-off studies between the present baseline (4 pixels) against a monolithic solution (1 pixel); 2) the technology status (i.e., some results from the integrated chipset test; warm electronics). We will conclude with programmatic aspects.