The `MCViNE_SANS2D_ongrid_process` Component

Union process: SANS from a 2D S(Qx,Qy) map on a grid (beam along z).

Identification

Author: Fahima Islam (McStas port of the MCViNE SANS2D_ongrid_Kernel (mcvine.acc); MCViNE by J. Y. Y. Lin et al.)
Origin: MCViNE, https://github.com/mcvine/acc (acc/kernels/SANS2D_ongrid.py)
Date: 2026-09-29

Description

Qx, Qy are sampled uniformly over the grid range; the neutron gets
v_perp -= Q*K2V and vz = sqrt(vi^2 - vx^2 - vy^2) (elastic). The weight is the
bilinearly interpolated S(Qx,Qy) (no solid-angle Jacobian, as in mcvine.acc;
use scattering_coefficient to set the absolute scale). File layout: first
numeric row Qx values (nx, ascending), second row Qy values (ny), then ny rows of
nx values (row index = Qy), i.e. the (y,x) image layout used by mcvine.acc.

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).
Orientation: this process is anisotropic; its frame (Q vectors, reciprocal
vectors, atom positions) follows the ROTATED placement of this component.
The kernel code is shared with the standalone component MCViNE_SANS2D_ongrid (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_SANS2D_ongrid_process(S_QxQy="MCViNE/SANS2D_example.dat", scattering_coefficient=1)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
S_QxQy	str	2D grid file (see description)	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_SANS2D_ongrid_process Component

Identification

Description

Input parameters

Links

The `MCViNE_SANS2D_ongrid_process` Component