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ATHENA+ is a space mission proposal for the next ESA L2-L3 slot. One of the focal plane instruments is the X-ray integral field unit (X-IFU) working in the energy range 0.3–10 keV. It is a multi-array based on TES detectors aimed at characterizing faint or diffuse sources (e.g. WHIM or galaxy outskirt). The X-IFU will be able to achieve the required sensitivity if a low background is guaranteed. The studies performed by GEANT4 simulations depict a scenario where the use of an active anticoincidence (AC) is mandatory to reduce the background expected in L2 orbit down to the goal level of 0.005 cts cm $$^{-2}$$ s $$^{-1}$$ keV $$^{-1}$$ . This is possible using a cryogenic anticoincidence (CryoAC) detector placed within a proper optimized environment surrounding the X-IFU. We propose a 2 $$\times $$ 2 array of microcalorimeter detectors made by silicon absorber (each of about 1 cm $$^{2}$$ and 300 $$\upmu $$ m thick) and sensed by an Ir TES. In order to better understand the involved physics and optimize the performance, we have produced several samples featured by different absorber areas, TES size, and QPs Al collectors. Here we will discuss, as a review, the obtained results and the related impact to the final detector design.

“The Hot and Energetic Universe” is the scientific theme approved by the ESA SPC for a Large mission to be flown in the next ESA slot (2028th) timeframe. ATHENA is a space mission proposal tailored on this scientific theme. It will be the first X-ray mission able to perform the so-called “Integral field spectroscopy”, by coupling a high-resolution spectrometer, the X-ray Integral Field Unit (X-IFU), to a high performance optics so providing detailed images of its field of view (5’ in diameter) with an angular resolution of 5” and fine energy-spectra (2.5eV@E