The `MCViNE_ConstantQE_process` Component

Union process: Scatterer with fixed momentum transfer |Q| and energy transfer E.

Identification

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

Description

Scattered neutrons have energy transfer E and |Q| fixed; the azimuth around ki
is random. S(Q,E) = delta(|Q|-Q0) delta(E-E0) (unnormalised, as in MCViNE).

Union process. Part of the Union components: define this process, collect it
into a material with Union_make_material (absorption is set there with
my_absorption, the absorption inverse penetration depth at 2200 m/s), assign
the material to Union_box/Union_cylinder/Union_sphere/Union_mesh geometries,
and add a Union_master after them. Geometry, attenuation and multiple
scattering are handled by Union_master; this component provides the MCViNE
kernel: the scattering coefficient and the final-state sampling (weight).
Isotropic process (powder/liquid-like); rotation is irrelevant.
The kernel code is shared with the standalone component MCViNE_ConstantQE (mcvine-lib.c).
Uses share/mcvine-lib.h/.c and share/mcvine-union-lib.h/.c; the process type
MCViNE is declared in share/union-lib.h and registered with Union_master in
share/mcvine-union-lib.h.

Example: MCViNE_ConstantQE_process(Q=3, E=20, scattering_coefficient=10)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
Q	AA^-1	Momentum transfer \|Q\|	1
E	meV	Energy transfer	0
scattering_coefficient	m^-1	Scattering coefficient (inverse penetration depth for scattering)	0
sigma_scat	barn	Alternative: scattering cross section per unit cell (used when Vc>0)	0
Vc	AA^3	Unit cell volume; when >0 the coefficient is sigma_scat/Vc	0
packing_factor	1	Packing factor (scales the scattering coefficient)	1
interact_fraction	1	Union: fraction of interactions forced to this process (-1: by cross section)	-1
init	string	Deprecated and unused, accepted like on the other Union components (see Union_init).	""

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

The MCViNE_ConstantQE_process Component

Identification

Description

Input parameters

Links

The `MCViNE_ConstantQE_process` Component