[ Identification | Description | Input parameters | Links ]

The NCrystal_filter Component

Filter or window (box or cylinder) of a material from NCrystal, which only attenuates the beam. Simple, fast and GPU friendly.

Identification

• Author: Thomas Kittelmann
• Origin: ESS
• Date: 2026

Description

A filter or window of an isotropic material, which attenuates the beam
according to the total cross section (scattering plus absorption) of the
material, as provided by the NCrystal library. Neutrons are not scattered:
the weight of a neutron travelling a distance L through the material is
multiplied by exp(-sigma*L), where sigma is the macroscopic cross section
at its wavelength. This is suitable for e.g. Be filters or Al windows, where
the scattered neutrons are not of interest. For the full NCrystal physics,
use NCrystal_sample or the Union components instead.

The cross sections are tabulated during initialisation, so the component can
also be used on GPUs. The table is provided by NCrystal itself, unless the C
API function ncrystal_filtertable is not available, in which case the
component will create the table itself as a fallback workaround.

The material is given by an NCrystal cfg-string, e.g.
"stdlib::Be_sg194.ncmat;temp=80K" for a cooled Be filter, or "mycrystal.ncmat"
for a local file. Oriented materials such as single crystals are not
supported, but a fast sapphire filter can be emulated with an assumption that
no reflections satisfy the Bragg condition:
"stdlib::Al2O3_sg167_Corundum.ncmat;bragg=0;temp=200K". Multiphase materials
are also supported:
"phases<0.98*stdlib::Al_sg225.ncmat&0.01*stdlib::Fe_sg229_Iron-alpha.ncmat&0.01*stdlib::Si_sg227.ncmat>;temp=180K".

The geometry is a box (xwidth, yheight, zdepth) or a cylinder (radius,
yheight) along the y axis.

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
cfg	str	NCrystal material configuration string (details on this page).	"void"
xwidth	m	x-dimension (width) of the filter, for a box	0
yheight	m	y-dimension (height) of the filter, for a box or cylinder	0
zdepth	m	z-dimension (depth) of the filter, for a box	0
radius	m	radius of the filter, for a cylinder	0

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

Links

• Source code for NCrystal_filter.comp.
• The NCrystal wiki at https://github.com/mctools/ncrystal/wiki.

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

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Last Modified: Tuesday, 06-Oct-2026 21:19:11 CEST