Union process: One-phonon incoherent inelastic scattering from a phonon DOS (incoherent approximation), optional Ef focusing.
Author: Fahima Islam (McStas port of the MCViNE phonon IncoherentInelastic / IncoherentInelastic_EnergyFocusing kernels; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/phonon/IncoherentInelastic.cc, IncoherentInelastic_EnergyFocusing.cc)
Date: 2026-09-29
S_inc(Q,E) = exp(-2W) hbar^2Q^2/(2M) g(|E|)/|E| [n(E)+1 or n(E)] with the DOS g(E) normalised to 1, Debye-Waller factor computed from the DOS (MCViNE DWFromDOS), and the DOS preprocessed like MCViNE (resampled to >=500 points, parabolic low-energy fit, normalised). The final direction is uniform over 4pi; Ef is uniform in [Ei-Emax_dos, Ei+Emax_dos]. With dEf>0, Ef is restricted to Ef +/- dEf/2 (MCViNE IncoherentInelastic_EnergyFocusing). DOS file: 2 columns E [meV] g(E) (a ’#...THz’ comment switches the unit to THz) or an MCViNE IDF binary ’DOS’ file. Multiphonon terms are not included (see Union IncoherentPhonon_process or NCrystal for those).
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_Phonon_IncoherentInelastic (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_Phonon_IncoherentInelastic_process(dos="MCViNE/Debye_dos.dat", T=300, average_mass=50.94, sigma_inc=10.1, Vc=27.6)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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dos |
str |
Phonon DOS file |
0 |
|
T |
K |
Temperature |
300 |
|
average_mass |
amu |
Average atomic mass |
0 |
|
Ef |
meV |
Final energy for energy focusing (used when dEf>0) |
0 |
|
dEf |
meV |
Full width of the final-energy window; 0 disables focusing |
0 |
|
sigma_inc |
barn |
Incoherent scattering cross section per unit cell |
0 |
|
Vc |
Å\(^{3}\) |
Unit cell volume |
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_Phonon_IncoherentInelastic_process.comp.
MCViNE documentation: https://mcvine.github.io