TES-based anti-coincidence detector development for the LEM Probe Mission Concept

The Line Emission Mapper (LEM) Probe concept will utilize a transition-edge sensor (TES) microcalorimeter to perform faint, diffuse X-ray observations in the presence of significant charged particle background. Achieving the needed sensitivity requires a ~14 cm^2 anti-coincidence (anti-co) detector with > 95% rejection efficiency and threshold below 20 keV. Our anti-co design employs parallel networks of quasiparticle-trap-assisted electrothermal-feedback TESs. We have developed multiple full-scale prototype anti-co detectors for LEM – both versions with Mo/Au TESs and with W TESs. Here we present the status of anti-co development and plans for continued maturation towards TRL-6, including detailed characterization of the low-energy threshold, rejection efficiency, and spatial discrimination ability. Broad energy range measurements have been performed (4.1 keV – 5.5 MeV) and demonstrate threshold and efficiency requirements can be achieved with this design. We also discuss modeling with G4CMP, a solid-state physics Geant4 add-on package, towards understanding phonon propagation and quasiparticle production in the detector, and compare the model to experimental results.