[ Identification | Description | Input parameters | Links ]

The MCViNE_SQE_process Component

Union process: Isotropic S(Q,E) scatterer from an analytic expression or grid, with optional final-energy focusing.

Identification

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

Description

Isotropic (powder / liquid) S(|Q|,E). E is sampled uniformly in [Emin, min(Ei,Emax)]
and |Q| uniformly in the kinematically allowed part of [Qmin,Qmax]. S(Q,E) is an
expression in Q [AA^-1] and E [meV], or a grid file in the Isotropic_Sqw format
(first numeric row: q values; second: energy values; then nq rows of nw values),
bilinearly interpolated. For grid data McStas' Isotropic_Sqw already exists; the
new parts are the analytic S(Q,E) and the energy focusing: when dEf>0, E is
restricted so that Ef lies in [Ef-dEf, Ef+dEf] (MCViNE SQE_EnergyFocusing_Kernel).

Functions are given as strings evaluated at run time (MCViNE uses fparser):
+ - * / ^ (or **), % , comparisons, && ||, if(c,a,b), sin cos tan asin acos
atan sinh cosh tanh exp log log10 log2 sqrt abs floor ceil int sign cbrt,
pow atan2 min max hypot fmod, constants pi and e.

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_SQE (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_SQE_process(SQE="exp(-(E-10)^2/2)*Q^2*exp(-Q^2/4)", Qmin=0, Qmax=10, Emin=-5, Emax=30, sigma_scat=5, Vc=50)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
SQE	str	S(Q,E) expression (ignored if SQE_file given)	"1"
SQE_file	str	S(q,w) grid file (Isotropic_Sqw layout)	0
Qmin	AA^-1	Lower Q bound	0
Qmax	AA^-1	Upper Q bound	10
Emin	meV	Lower energy transfer bound	-10
Emax	meV	Upper energy transfer bound	10
Ef	meV	Final-energy focusing centre (used when dEf>0)	0
dEf	meV	Final-energy focusing half range; 0 disables focusing	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		Generate to clipboard
ROTATED (	,	,		) RELATIVE

Links

• Source code for MCViNE_SQE_process.comp.
• MCViNE documentation: https://mcvine.github.io

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[ Identification | Description | Examples | Input parameters | Links ]

Generated on mcstas 3.9.2

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Last Modified: Tuesday, 06-Oct-2026 21:19:12 CEST