Opening the low-background and high-spectral-resolution domain with the ATHENA large X-ray observatory: Development of the Cryogenic AntiCoincidence Detector for the X-ray Integral Field Unit

ATHENA is a large-class ESA mission selected for launch in 2031. It is designed to address the science theme "The Hot and Energetic Universe", performing X-ray observations (0.2-12 keV) at the L2 Sun-Earth Lagrangian point. The X-IFU is one of the two instruments of the payload. It is a cryogenic spectrometer providing spatially resolved high-resolution spectroscopy. The core of the instrument is a large array of TES microcalorimeters, operated at a 50 mK thermal bath. The X-IFU performances would be strongly degraded by the particle background expected in the L2 environment, thus advanced reduction techniques have been adopted to reduce this contribution by a factor $\sim$50. This is needed to enable many core science objectives of the mission. Most of the background reduction ($\sim 80\%$) is achieved thanks to the Cryogenic AntiCoincidence detector (CryoAC), a 4 pixels TES microcalorimeter which will be placed less than 1 mm below the TES array. The CryoAC is a sort of instrument-inside-the-instrument, with independent cold and warm electronics and a dedicated data processing chain. To reach the required particle rejection efficiency ($\sim 98 \%$) it will have a wide energy band (from 20 keV to $\sim$ 1 MeV) and a low deadtime ($< \sim 2\%$), while respecting several constraints to ensure mechanical, thermal and electromagnetic compatibility with the TES array. Here I will report my PhD research activity, which has been focused on the development of the CryoAC Demonstration Model (DM), a single pixel detector requested by ESA before the mission adoption. The thesis is divided in two main parts. In the first one I will mainly present the astrophysical framework of my research, and in the second one I will focus on the experimental activities carried out towards the CryoAC DM development.