The Cryogenic AntiCoincidence Detector for ATHENA X-IFU: Assessing the Role of the Athermal Phonons Collectors in the AC-S8 Prototype

The ATHENA X-ray observatory is the second large-class mission in the ESA Cosmic Vision 2015\u20132025 science programme. One of the two on-board instruments is the X-IFU, an imaging spectrometer based on a large array of TES microcalorimeters. To reduce the particle-induced background, the spectrometer works in combination with a cryogenic anticoincidence detector (CryoAC), placed less than 1\ua0mm below the TES array. The last CryoAC single-pixel prototypes, namely AC-S7 and AC-S8, are based on large-area (1 cm2) silicon absorbers sensed by 65 parallel-connected iridium TES. This design has been adopted to improve the response generated by the athermal phonons, which will be used as fast anticoincidence flag. The latter sample is featured also with a network of aluminum fingers directly connected to the TES, designed to further improve the athermals collection efficiency. In this paper, we will report the main results obtained with AC-S8, showing that the additional fingers network is able to increase the energy collected from the athermal part of the pulses (from the 6% of AC-S7 up to the 26 % with AC-S8). Furthermore, the finger design is able to prevent the quasiparticle recombination in the aluminum, assuring a fast pulse rising front (L/R limited). In our road map, the AC-S8 prototype is the last step before the development of the CryoAC demonstration model, which will be the detector able to demonstrate the critical technologies expected in the CryoAC development programme