Performance study of the Jalousie detector baseline design for the ESS thermal powder diffractometer HEIMDAL through GEANT4 simulations.

HEIMDAL is a thermal powder diffractometer designed to operate at the European Spallation Source, world's most intense neutron source under construction in Lund, Sweden. The instrument is expected to enter user operation in 2024/2025. The detailed design of the instrument assumes that the neutrons scattered by the powder under investigation will be collected with hundreds of large-area Multi-Wire Proportional Counters employing a $^{10}$B$_4$C-solid converter. The gas counters will exhibit large active volumes and tapered trapezoidal shapes that allow for close packing into a cylindrical shell with high solid angle coverage. The whole detector will operate in an air environment within the shielding cave and provide signals with sensitivity for locating detection in three dimensions. This paper presents the results of a GEANT4 study of the baseline design for the HEIMDAL powder diffraction detector. The detector model was used to study key performance parameters such as detection efficiency and spatial resolution. The contribution of the detector to the resolving power of the instrument, one of the key figures-of-merit for powder diffractometers, was also investigated. Most of the simulation results reported in this work cannot be validated against a sufficiently similar physical reference until the first segment or module are constructed and tested with neutron beam. However, these results can help identify possible ways of optimising the detector design and provide the first glimpse into the expected performance of the technological approach selected by the instrument team long before the whole detector or parts of it become available.