Union process: Scatterer with fixed momentum transfer |Q| and energy transfer E.
Author: Fahima Islam (McStas port of the MCViNE ConstantQEKernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/ConstantQEKernel.cc)
Date: 2026-09-29
Scattered neutrons have energy transfer E and |Q| fixed; the azimuth around ki is random. S(Q,E) = delta(|Q|-Q0) delta(E-E0) (unnormalised, as in MCViNE).
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). Isotropic process (powder/liquid-like); rotation is irrelevant. The kernel code is shared with the standalone component MCViNE_ConstantQE (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_ConstantQE_process(Q=3, E=20, scattering_coefficient=10)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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Q |
Å\(^{-1}\) |
Momentum transfer |Q| |
1 |
|
E |
meV |
Energy transfer |
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). |
"" |
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Component source code found in file MCViNE_ConstantQE_process.comp.
MCViNE documentation: https://mcvine.github.io