Status of the Design of an Annihilation Detector to Observe Neutron-Antineutron Conversions at the European Spallation Source

The goal of the HIBEAM/NNBAR program is to search for baryon number violation via conversion or oscillation neutrons into sterile and/or antineutrons at European Spallation Source. A key experimental component construction an annihilation detector directly observe production antineutron following oscillation. Design studies are presented. predicted response models studied using Geant4 simulations made with Monte Carlo signal topology and cosmic ray backgrounds. Particle identification sensitive discriminating observables, such as invariant mass sphericity, shown.