Complex Geant4 Simulation Study for the Optimisation of Multi-Grid Detector

The unique neutron yield and high performance requirements of the European Spallation Source (ESS) ERIC, and the current
³
He-shortage opened a new frontier for neutron detector development. A potent new solution for large area detectors is the Multi-Grid thermal neutron detector: an Ar/CO
₂
gas filled detector with solid
¹⁰
B
₄
C converter, developed for the chopper spectrometers at the ESS. For these instruments a high Signal-to-Background Ratio (SBR) is a key requirement, so understanding the effect and sources of the scattered neutron background, considering the intrinsic detector background as well is essential for instrument optimisation. Thanks to the recently introduced neutron simulation tools, a detailed Geant4 model of the Multi-Grid detector was developed and validated against measured data for Monte Carlo simulations. These simulations allow a novel, holistic approach to understand the intrinsic detector background and to determine the effect of different detector components as sources of neutron scattering, and their impact on the measured signal. The model became a powerful tool in the development of the detector design and was applied for the optimisation of the SBR via background reduction. The model was used to determine the impacts of different detector components e.g. the background contribution from the neutron scattering on the aluminium vessel and grids, the shielding properties of additional coating layers and their impact on the detection efficiency and the background-reducing potential of a complex, internal detector shielding etc. All these phenomena were studied in a detailed simulation of detector response. The model was also applied to dissect and understand the composition and sources of the measured signal from recent prototype tests at the SEQUOIA instrument. The obtained results contribute to the optimisation of the shielding, coating and vessel design of the Multi-Grid detector module, leading to instruments with better SBR by design.