NewAthena X-IFU non-x-ray background: veto optimization and laboratory validation

We report on Geant4 simulations of the non–X-ray background (NXB) for the NewAthena X-IFU, focusing on updated mass models, veto strategies, and validation. The mass model evolved with mission redefinition, including a new cryostat and focal plane assembly, L-grooves, and absorber layouts. High-statistics simulations show modest changes in background relative to the 2017 model, and confirm spatial homogeneity. Although the design is not yet finalized in all its elements, the results’ representativeness increased with respect to previous results. A sector veto strategy exploiting CryoAC segmentation can keep 75% of the TES array active, raising NXB by ~50%; extending the veto region by Δ pixels reduces the increment to 6% for Δ=5. A preliminary solar energetic particle simulation confirmed background levels exceeding requirements by three orders of magnitude. Validation against CryoAC prototype data demonstrated strong agreement, supporting the robustness of the simulation framework.