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).
Transport (same as MCViNE HomogeneousNeutronScatterer): on the first passage the neutron is forced to scatter at a uniformly chosen depth, weighted by path length, attenuation exp(-(mu+sigma)x) and the scattering coefficient; on exit it is attenuated by exp(-(mu+sigma)L_out). With order>1 further scatterings are sampled analogically (truncated exponential). With p_transmit>0 that fraction of events is kept as the attenuated direct beam. mu(v) = absorption_coefficient*2200/v. Events for which the kernel cannot scatter (kinematically forbidden) are absorbed.
Geometry: box (xwidth,yheight,zdepth), cylinder (radius,yheight), hollow cylinder (+thickness) or sphere (radius only). All vectors (Q, targets, reciprocal vectors, atom positions) are in the component’s local frame.
Uses the shared runtime share/mcvine-lib.h/.c.
Example: MCViNE_Phonon_IncoherentInelastic(dos="MCViNE/Debye_dos.dat", T=300, average_mass=50.94, sigma_inc=10.1, sigma_abs=10.1, Vc=27.6, radius=0.01, yheight=0.05)
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 |
|
sigma_abs |
barn |
Absorption cross section per unit cell at 2200 m/s |
0 |
|
Vc |
Å\(^{3}\) |
Unit cell volume |
0 |
|
radius |
m |
Outer radius of a cylinder (with yheight) or of a sphere (yheight=0) |
0 |
|
xwidth |
m |
Width of a box sample |
0 |
|
yheight |
m |
Height of a box or cylinder sample |
0 |
|
zdepth |
m |
Depth of a box sample |
0 |
|
thickness |
m |
Wall thickness of a hollow cylinder (0: filled) |
0 |
|
pack |
1 |
Packing factor (scales absorption and scattering coefficients) |
1 |
|
p_transmit |
1 |
Monte Carlo fraction of events kept as unscattered transmitted beam (0: always scatter) |
0 |
|
order |
1 |
Maximum number of scattering events per neutron (1: single scattering) |
1 |
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Component source code found in file MCViNE_Phonon_IncoherentInelastic.comp.
MCViNE documentation: https://mcvine.github.io