Union process: Elastic scatterer with a vector structure factor S(Qx,Qy,Qz) (expression or 3D grid).
Author: Fahima Islam (McStas port of the MCViNE SvQkernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/SvQkernel.cc (+ GridSvQ))
Date: 2026-09-29
Elastic scattering into a uniformly random direction, weighted by S(Q) with Q = ki - kf in the sample frame. S is either an expression in Qx,Qy,Qz or a 3D grid file: three rows ’min max n’ for Qx, Qy, Qz followed by nx*ny*nz values (Qx slowest, Qz fastest), trilinearly interpolated (0 outside).
Functions are given as strings evaluated at run time (MCViNE uses fparser): + - * / ^ (or **), % , comparisons, && ||, if(c,a,b), sin cos tan asin acos atan sinh cosh tanh exp log log10 log2 sqrt abs floor ceil int sign cbrt, pow atan2 min max hypot fmod, constants pi and e.
Union process. Part of the Union components: define this process, collect it into a material with Union_make_material (absorption is set there with my_absorption, the absorption inverse penetration depth at 2200 m/s), assign the material to Union_box/Union_cylinder/Union_sphere/Union_mesh geometries, and add a Union_master after them. Geometry, attenuation and multiple scattering are handled by Union_master; this component provides the MCViNE kernel: the scattering coefficient and the final-state sampling (weight). Orientation: this process is anisotropic; its frame (Q vectors, reciprocal vectors, atom positions) follows the ROTATED placement of this component. The kernel code is shared with the standalone component MCViNE_SvQ (mcvine-lib.c). Uses share/mcvine-lib.h/.c and share/mcvine-union-lib.h/.c; the process type MCViNE is declared in share/union-lib.h and registered with Union_master in share/mcvine-union-lib.h.
Example: MCViNE_SvQ_process(SvQ="exp(-((Qx-1)^2+Qy^2+Qz^2)/0.01)", scattering_coefficient=10)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
|
|||
|
SvQ |
str |
S(Qx,Qy,Qz) expression (ignored if SvQ_file given) |
"1" |
|
SvQ_file |
str |
3D grid file (see description) |
0 |
|
scattering_coefficient |
m\(^{-1}\) |
Scattering coefficient (inverse penetration depth for scattering) |
0 |
|
sigma_scat |
barn |
Alternative: scattering cross section per unit cell (used when Vc>0) |
0 |
|
Vc |
Å\(^{3}\) |
Unit cell volume; when >0 the coefficient is sigma_scat/Vc |
0 |
|
packing_factor |
1 |
Packing factor (scales the scattering coefficient) |
1 |
|
interact_fraction |
1 |
Union: fraction of interactions forced to this process (-1: by cross section) |
-1 |
|
init |
string |
Deprecated and unused, accepted like on the other Union components (see Union_init). |
"" |
|
|
|||
|
|
|||
|
|
|||
Component source code found in file MCViNE_SvQ_process.comp.
MCViNE documentation: https://mcvine.github.io