4.3  The Beamstop McStas Component

Rectangular/circular beam stop.

Identification

• Author: Kristian Nielsen

• Origin: Risoe

• Date: January 2000

Description

A simple rectangular or circular beam stop. Infinitely thin and infinitely absorbing. The beam stop is by default rectangular. You may either specify the radius (circular shape), or the rectangular bounds.

Example: Beamstop(xmin=-0.05, xmax=0.05, ymin=-0.05, ymax=0.05) Beamstop(radius=0.1)

Input parameters

Parameters in boldface are required; the others are optional.

Links

• Component source code found in file Beamstop.comp.

A neutron absorbing area

The component Beamstop can be seen as the reverse of the Slit component. It sets up an area at the \(z=0\) plane, and propagates the neutrons onto this plane (by the kernel call PROP_Z0). Neutrons within this area are ABSORB’ed, while all other neutrons are unaffected.

By using this component, some neutrons contributing to the background in a real experiment will be neglected. These are the ones that scatter off the side of the beamstop, or penetrates the absorbing material. Further, the holder of the beamstop is not simulated.

Beamstop can be either circular or rectangular. The input parameters of Beamstop are the four coordinates, \((x_\textrm {min}, x_\textrm {max}, y_\textrm {min}, y_\textrm {max})\) defining the opening of a rectangle, or the radius \(r\) of a circle, depending on which parameters are specified.

If the "direct beam" (e.g. after a monochromator or sample) should not be simulated, it is possible to emulate an ideal beamstop so that only the scattered beam is left; without the use of Beamstop: This method is useful for instance in the case where only neutrons scattered from a sample are of interest. The example below removes the direct beam and any background signal from other parts of the instrument

1COMPONENT MySample=V_sample(...) AT (...) 
2EXTEND 
3%{ 
4  if (!SCATTERED) ABSORB; 
5%}