  HEIMDAL is a thermal powder diffractometer designed to operate at the
European Spallation Source, world's most intense neutron source. The detailed
design of the instrument, which is expected to enter user operation in
2024/2025, 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 consists of 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 this technological
approach.
