Simulating neutron diffraction from deformed mosaic crystals in McStas

Abstract

Monochromator and analyzer systems that rely on bent single crystals are in use throughout the neutron scattering community. We here introduce a new component to the neutron simulation software package McStas, that simulates these bent single crystals. We then compare the performance of this component to like software in SIMRES, and Ncrystal. These simulations show excellent agreement across the different software programs. Finally we compare simulations of the new McStas component with analytical calculations of secondary extinction, based on the Darwin transfer equations. Here we also find excellent agreement, further validating this new component.

Figures (11)

• Figure S1: (Left) An unbent crystal. The reflecting lattice planes are cut at an angle $\chi$ to the surface. (Right) The deformed crystal with $\chi=0$. The reciprocal lattice vectors are shown as $\boldsymbol{\tau}_0$ and $\boldsymbol{\tau}(\boldsymbol{r})$ the unbent, and bent reciprocal lattice vector, respectively.
• Figure S2: A neutron ray (purple) tracing a possible path through the crystal. $t_n$ is the n'th $t_{max}$.Notice that the algorithm uses $t_{max}$ to sample a $t$ value at every reflection. The figure is heavily inspired by NIMA.
• Figure S3: The setup used for simulating the bent mosaic crystal in both SIMRES and McStas. $x,y$ and $z$ is the coordinate system of the monochromator, and $\omega$ is the rotation angle around the $y$ axis, used for the rocking curve.
• Figure S4: Visualization of the effect of attenuation throughout the unbent mosaic crystal. The purple dots in the crystal illustrate potential paths of the neutrons.
• Figure S5: SIMRES and McStas simulations of the spatial distribution for Bragg scattering from a perfect crystal, bent at different radii. Overlain with the analytically calculated reflectivity.