  The X-IFU is one of the two instruments of ATHENA, the next ESA large X-ray
observatory. It is a cryogenic spectrometer based on an array of TES
microcalorimeters. To reduce the particle background, the TES array works in
combination with a Cryogenic AntiCoincidence detector (CryoAC). The CryoAC is a
4-pixel detector, based on ~1 cm2 silicon absorbers sensed by Ir/Au TES. It is
required to have a wide energy bandwidth (from 20 keV to ~1 MeV), high
efficiency (< 0.014% missed particles), low dead-time (< 1%) and good
time-tagging accuracy (10 us at 1 sigma). An end-to-end simulator of the CryoAC
detector has been developed both for design and performance assessment,
consisting of several modules. First, the in-flight flux of background
particles is evaluated by Geant4 simulations. Then, the current flow in the TES
is evaluated by solving the electro-thermal equations of microcalorimeters, and
the detector output signal is generated by simulating the SQUID FLL dynamics.
Finally, the output is analyzed by a high-efficiency trigger algorithm,
producing the simulated CryoAC telemetry. Here, we present in detail this
end-to-end simulator, and how we are using it to define the new CryoAC baseline
configuration in the new Athena context.
