  Our team is developing the Cryogenic Anticoincidence Detector (CryoAC) of the
ATHENA X-ray Integral Field Unit (X-IFU). It is a 4-pixels TES-based detector,
which will be placed less than 1 mm below the main TES array detector. We are
now producing the CryoAC Demonstration Model (DM): a single pixel prototype
able to probe the detector critical technologies, i.e. the operation at 50 mK
thermal bath, the threshold energy at 20 keV and the reproducibility of the
thermal conductance between the suspended absorber and the thermal bath. This
detector will be integrated and tested in our cryogenic setup at INAF/IAPS, and
then delivered to SRON for the integration in the X-IFU Focal Plane Assemby
(FPA) DM.
  In this paper we report the status of the CryoAC DM development, showing the
main result obtained with the last developed prototype, namely AC-S9. This is a
DM-like sample, which we have preliminary integrated and tested before
performing the final etching process to suspend the silicon absorber. The
results are promising for the DM, since despite the limitations due to the
absence of the final etching (high thermal capacity, high thermal conductance,
partial TES surface coverage), we have been able to operate the detector with
TB = 50 mK and to detect 6 keV photons, thus having a low energy threshold
fully compatible with our requirement (20 keV).
