The `MCViNE_ConstantvQE` Component

Scatterer with a fixed momentum-transfer vector Q, weighted by a Gaussian in E.

Identification

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

Description

kf = ki - Q (Q vector fixed in the sample frame); the event weight is multiplied
by exp(-(E-E0)^2/(2 dE^2)) where E = Ei-Ef. Used for single-crystal resolution
studies.

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_ConstantvQE(Qx=1, Qy=0, Qz=0.5, E=10, dE=1, scattering_coefficient=10, xwidth=0.01, yheight=0.01, zdepth=0.01)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
Qx	AA^-1	Q vector x	0
Qy	AA^-1	Q vector y	0
Qz	AA^-1	Q vector z	0
E	meV	Nominal energy transfer	0
dE	meV	Gaussian sigma of the energy weight	1
absorption_coefficient	m^-1	Absorption coefficient at 2200 m/s (MCViNE convention; scales as 1/v)	0
scattering_coefficient	m^-1	Scattering coefficient	0
sigma_abs	barn	Alternative: absorption cross section per unit cell at 2200 m/s (used when Vc>0)	0
sigma_scat	barn	Alternative: scattering cross section per unit cell (used when Vc>0)	0
Vc	AA^3	Unit cell volume; when >0, coefficients are computed from sigma_abs, sigma_scat	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_ConstantvQE Component

Identification

Description

Input parameters

Links

The `MCViNE_ConstantvQE` Component