Description
McStas plays an important part in the development and optimization of neutron instrumentation. In the McStas package there is a specific need for a component that simulates bent single crystals. These crystals grant a high resolution and high peak reflectivity in specific focusing conditions, at the cost of lower integrated reflectivity, and a heavy dependence on the diffraction geometry. These crystals therefore give great power to the experiment, but also great responsibility to the instrument setup. These crystals are therefore often used
for engineering diffractometers and triple-axis spectrometers.
The current crystal components in McStas either implement a very simplified model of a crystal (infinitely thin), or a very detailed model of a crystal (NCrystal), neither of which supports the case of a bent single crystal of finite thickness. This work therefore implements a new component in the McStas library called Monochromator Bent. The component is based upon the crystal component in the SIMRES software package. The component simulates neutron diffraction in a crystal of finite thickness, which can be bent or mosaic, and any combination of these.
We then define showcase configurations, that highlights the capabilities of the component. In these configurations, we compare the component to its sister in the SIMRES software, and the NCrystal component in McStas. Finally we also simulate the SALSA instrument at the ILL, and compare the results to data taken from SALSA. The results from these comparisons show that the component is able to simulate the effects of a bending mosaic/non-mosaic crystal with a finite thickness. The added features to McStas from this component is then both diffraction off of bent single crystals as well as a more realistic crystal simulation than the current monochromators, whilst still being faster than the NCrystal component.
