Union process: Isotropic S(Q,E) scatterer from an analytic expression or grid, with optional final-energy focusing.
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
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 [Å^-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)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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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 |
Å\(^{-1}\) |
Lower Q bound |
0 |
|
Qmax |
Å\(^{-1}\) |
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 |
Å\(^{3}\) |
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). |
"" |
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Component source code found in file MCViNE_SQE_process.comp.
MCViNE documentation: https://mcvine.github.io