The `MCViNE_Phonon_CoherentInelastic_SingleXtal` Component

Coherent one-phonon scattering from a single crystal, using a full phonon dispersion on a grid.

Identification

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

Description

Single-crystal coherent one-phonon scattering with dispersion and polarization
vectors from MCViNE IDF files (periodic over the reciprocal cell b1,b2,b3 given
in Qgridinfo; orient the crystal by rotating the component). For a final
direction (4pi, or a cone towards a target when focus_r>0) and a random
branch among those with Emin < 1.5 Ei, all final speeds solving
omega(Q) = |Ei-Ef| are found (Ridders' method on nsteps sub-intervals of
[0,2vi]); one is chosen and weighted by the delta-function Jacobian,
|sum_d b_d/sqrt(M_d) exp(iQ.d)(Q.e_d)|^2, Bose and Debye-Waller factors.

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_SingleXtal(idf_dir="MCViNE/fcc_toy_phonons", atoms="MCViNE/fcc_toy_atoms.dat", T=300, xwidth=0.01, yheight=0.01, zdepth=0.01)

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
target_x	m	Target position (component frame) x	0
target_y	m	Target position y	0
target_z	m	Target position z	0
target_index	1	Relative index of a component to aim at (overrides target_x/y/z when non-zero)	0
focus_r	m	Radius of the target disk; 0: scatter into 4pi (MCViNE default)	0
deltaV_Jacobi	1	Relative velocity step for the numerical Jacobian	0.001
nsteps	1	Sub-intervals for the root search in vf	100
xacc	m/s	Root accuracy in vf	10
unbiased	1	0: MCViNE sampling (retries up to 100 direction/branch draws until omega(Q)=\|Ei-Ef\| has a solution, without correcting the weight: over-estimates when many directions have no solution). 1: single attempt, 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_SingleXtal Component

Identification

Description

Input parameters

Links

The `MCViNE_Phonon_CoherentInelastic_SingleXtal` Component