[ Identification | Description | Input parameters | Links ]

The MCViNE_SANS2D_ongrid Component

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.

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_SANS2D_ongrid(S_QxQy="MCViNE/SANS2D_example.dat", scattering_coefficient=1, xwidth=0.01, yheight=0.01, zdepth=0.001)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
S_QxQy	str	2D grid file (see description)	0
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	AA^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

AT (	,	,		) RELATIVE		Generate to clipboard
ROTATED (	,	,		) RELATIVE

Links

• Source code for MCViNE_SANS2D_ongrid.comp.
• MCViNE documentation: https://mcvine.github.io

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

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