Simulation Driven Design of a Cryogenic Anti-coincidence Detector for X-ray Microcalorimeter Space Missions

X-ray microcalorimeter space missions require an anti-coincidence detector to mitigate background events from Galactic Cosmic Rays (GCRs). These high-energy particles mimic an X-ray signal by depositing energy in the microcalorimeter absorber, with the amount proportional to their track length. An anti-coincidence detector, positioned below the main microcalorimeter array, identifies these background events and should respond quickly to a GCR signal, have a low threshold and high dynamic range, and use a similar readout scheme as the main detector array. For the proposed Line Emission Mapper (LEM) X-ray probe concept, a W and Al-based athermal phonon sensor design, originally developed for the Cryogenic Dark Matter Search, was adapted to cover a large Si substrate ( 14 sq. cm) with 12 parallel networks of these sensors. This initial prototype for the LEM mission was developed in collaboration with SLAC and Northwestern University, and tested at NASA Goddard Space Flight Center. The prototype demonstrated a combination of low threshold, high dynamic range, and millimeter-scale position discrimination, far exceeding all mission requirements. At the same time, detector modeling efforts were undertaken to understand the rich data sets produced by the prototype. In this work, we expand upon these modeling capabilities to explore new anti-coincidence particle detection designs enabled by the high performance of the LEM prototype. Using G4CMP, an add-on framework to Geant4 for phonon and charge transport in cryogenic detectors, we model the athermal phonon signals generated by GCR events and their collection as quasiparticles in the sensor network. We consider alternative sensor network layouts to maximize GCR event position discrimination, which could reduce false-coincidence events by localizing event rejection in the main array.