Study on the trigger logic for the X-IFU Athena anticoincidence system

The X-IFU is one of the two instruments onboard the Athena X-ray observatory, a large ESA mission to be launched in the 2030 decade. The instrument is based on a large TES-array able to perform simultaneous high-grade energy spectroscopy and imaging. The challenging scientific goals of the instrument also require the reduction of the particle background level. This is obtained by combining the large array with a TES-based Cryogenic Anticoincidence detector (CryoAC), which allows to veto the unwanted events. One of the key elements of the CryoAC system is the trigger logic, an algorithm able to identify the events measured by the CryoAC detector. The requirements for the CryoAC lead to the development of a sophisticated algorithm capable of achieving the required trade-off between conflicting parameters, such as trigger efficiency and dead-time, with precise time-tagging and pile-up identification. The trigger logic is based on a finite state machine and it is able to identify, in addition to the pulse-shaped events, particular conditions such as the saturation of the CryoAC detector. Due to the peculiar CryoAC pulses features and timing, the developed algorithm is based on derivatives of the signal evaluated by moving averages computed in six contiguous sliding windows, updated at every sample. The algorithm has been tested with a sequence of simulated data stream, showing the compliance with the requirements.