SANS from a 2D S(Qx,Qy) map on a grid (beam along z).
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
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)
Parameters in boldface are required; the others are optional.
|
Name |
Unit |
Description |
Default |
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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 |
Å\(^{3}\) |
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 |
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Component source code found in file MCViNE_SANS2D_ongrid.comp.
MCViNE documentation: https://mcvine.github.io