Scatterer with a fixed momentum-transfer vector Q, weighted by a Gaussian in E.
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
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)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
|
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|
Qx |
Å\(^{-1}\) |
Q vector x |
0 |
|
Qy |
Å\(^{-1}\) |
Q vector y |
0 |
|
Qz |
Å\(^{-1}\) |
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 |
Å\(^{3}\) |
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 |
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Component source code found in file MCViNE_ConstantvQE.comp.
MCViNE documentation: https://mcvine.github.io