  The CG-2 beamline at the High Flux Isotope Reactor (HFIR) exhibits a notable
discrepancy between observed count rates and the count rates we would expect
based on a Monte-Carlo neutron ray-trace simulation. These simulations
consistently predict count rates approximately five times greater than those
observed in four separate experimental runs involving different instrument
configurations. This discrepancy suggests that certain factors are causing
losses in measurements that are not adequately accounted for in the simulation,
in particular guide reflectivity or misalignment. To investigate these
discrepancies, a high-dimensional simulation parameter approach is applied in
order to understand the losses. Region of Interest (ROI) groups along the
instrument are assigned to different surfaces of the guide components within
the simulation. This allows the parameters of those guide components to be
varied as a group to minimize the complexity of the search space. The result is
an optimization of simulation parameters using an iterative scheme that aims to
minimize the difference between experimentally measured count rates and
simulated count rates across all tested collimator combinations. This proposed
methodology holds the potential to reveal previously unrecognized sources of
intensity loss in the CG-2 beamline at HFIR and improve the accuracy of
simulations, leading to enhanced understanding and performance of the beamline
for various scientific applications.
