4.4  The Filter_gen McStas Component

Release: McStas 1.6

This components may either set the flux or change it (filter-like), using an external data filename.

Identification

• Author: E. Farhi

• Origin: ILL

• Date: Dec, 15th, 2002

Description

This component changes the neutron flux (weight) in order to match a reference table in a filename. Typically you may set the neutron flux (source-like), or multiply it using a transmission table (filter-like). The component may be placed after a source, in order to e.g. simulate a real source from a reference table, or used as a filter (BeO) or as a window (Al). The behaviour of the component is specified using the ’options’ parameter, or from the filename itself (see below) If the thickness for the transmission data filename D was t0, and a different thickness t1 would be required, then the resulting transmission is:

D^(t1/t0).

You may use the ’thickness’ and ’scaling’ parameter for that purpose.

File format: This filename may be of any 2-columns free format (k[Å-1],p), (omega[meV],p) and (lambda[Å],p) where p is the weight. The type of the filename may be written explicitely in the filename, as a comment, or using the ’options’ parameter. Non mumerical content in filename is treated as comment (e.g. lines starting with ’#’ character). A table rebinning and linear interpolation are performed.

EXAMPLE : in order to simulate a PG filter, using the lib/data/HOPG.trm file Filter_gen(xwidth=.1 yheight=.1, filename="HOPG.trm") A Sapphire filter, using the lib/data/Al2O3_sapphire.trm file Filter_gen(xwidth=.1 yheight=.1, filename="Al2O3_sapphire.trm") A Berylium filter, using the lib/data/Be.trm file Filter_gen(xwidth=.1 yheight=.1, filename="Be.trm") an other possibility to simulate a Be filter is to use the PowderN component: PowderN(xwidth=.1, yheight=.1, zdepth=.1, reflections="Be.laz", p_inc=1e-4)

in this filename, the comment line # wavevector multiply sets the behaviour of the component. One may as well have used options="wavevector multiply" in the component instance parameters.

Input parameters

Parameters in boldface are required; the others are optional.

Links

• Component source code found in file Filter_gen.comp.

• HOPG.trm filename as an example.

Example transmission data files

Some example transmission data files are provided with McStas in the MCSTAS/data directory as *.trm files, for use with the filename parameter above: