Union process: Scatterer with a fixed momentum-transfer vector Q, weighted by a Gaussian in E.
Author: Fahima Islam (McStas port of the MCViNE ConstantvQEKernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/ConstantvQEKernel.cc)
Date: 2026-09-29
kf = ki - Q (Q vector fixed in the sample frame); the event weight is multiplied by exp(-(E-E0)^2/(2 dE^2)) where E = Ei-Ef. Used for single-crystal resolution studies.
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_ConstantvQE (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_ConstantvQE_process(Qx=1, Qy=0, Qz=0.5, E=10, dE=1, scattering_coefficient=10)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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Qx |
Å\(^{-1}\) |
Q vector x |
0 |
|
Qy |
Å\(^{-1}\) |
Q vector y |
0 |
|
Qz |
Å\(^{-1}\) |
Q vector z |
0 |
|
E |
meV |
Nominal energy transfer |
0 |
|
dE |
meV |
Gaussian sigma of the energy weight |
1 |
|
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). |
"" |
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Component source code found in file MCViNE_ConstantvQE_process.comp.
MCViNE documentation: https://mcvine.github.io