  The ATHENA X-ray Observatory is the second large-class mission in the ESA
Cosmic Vision 2015-2025 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 mm 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 (DM), which will
be the detector able to demonstrate the critical technologies expected in the
CryoAC development programme.
