The `MCViNE_Phonon_CoherentInelastic_PolyXtal` Component

Coherent one-phonon scattering from a powder, using a full phonon dispersion (energies + polarizations) on a grid.

Identification

Author: Fahima Islam (McStas port of the MCViNE phonon CoherentInelastic_PolyXtal kernel; MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/mcvine (mccomponents/lib/kernels/sample/phonon/CoherentInelastic_PolyXtal.cc)
Date: 2026-09-29

Description

Powder average of the coherent one-phonon cross section, computed from phonon
energies and polarization vectors tabulated on a grid over one reciprocal cell
(MCViNE IDF format, e.g. from phonopy via MCViNE tools). A random branch and a
random Q vector in a cube are drawn until the event is kinematically allowed;
phonon creation/annihilation, Bose factor and Debye-Waller factor included.
Scattering coefficient: total coherent cross section / Vc.

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_CoherentInelastic_PolyXtal(idf_dir="MCViNE/fcc_toy_phonons", atoms="MCViNE/fcc_toy_atoms.dat", T=300, max_omega=45, radius=0.01, yheight=0.05)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
idf_dir	str	Directory with MCViNE IDF phonon files: Qgridinfo, Omega2, Polarizations[, DOS]	0
atoms	str	Atoms file: one row per atom 'x y z mass b_coh sigma_inc sigma_abs' ([AA] cartesian, [amu], [fm], [barn], [barn]), same order as in the IDF files	0
T	K	Temperature	300
dw_core	AA^2	Debye-Waller core; <0: computed from the DOS (idf_dir/DOS or dos)	-1
dos	str	Optional DOS file for the Debye-Waller factor (default: idf_dir/DOS)	0
Vc	AA^3	Unit cell volume; 0: (2pi)^3/\|b1.(b2 x b3)\| from Qgridinfo	0
max_omega	meV	Maximum phonon energy	50
min_omega	meV	Phonons below this energy are skipped	0.01
unbiased	1	0: MCViNE sampling (rejection + empirical accessible reciprocal volume, a few % high in tests). 1: single Q sample in the cube with its exact volume, unbiased	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

AT (	,	,		) RELATIVE
ROTATED (	,	,		) RELATIVE

The MCViNE_Phonon_CoherentInelastic_PolyXtal Component

Identification

Description

Input parameters

Links

The `MCViNE_Phonon_CoherentInelastic_PolyXtal` Component