6.5  The Selector McStas Component

velocity selector (helical lamella type) such as <b>V_selector</b> component

Identification

• Author: Peter Link, <a href="mailto:Andreas.Ostermann@frm2.tum.de">Andreas Ostermann</a>

• Origin: Uni. Gottingen (Germany)

• Date: MARCH 1999

Description

Velocity selector consisting of rotating Soller-like blades defining a helically twisted passage. Geometry is defined by two identical apertures at 12 o’clock position, The origin is at the ENTRANCE of the selector.

Example: Selector(xmin=-0.015, xmax=0.015, ymin=-0.025, ymax=0.025, length=0.25,

nslit=72,d=0.0004, radius=0.12, alpha=48.298, nu=500)

These are values for the D11@ILL Dornier ’Dolores’ Velocity Selector (NVS 023)

%VALIDATION Jun 2005: extensive external test, one minor problem Jan 2006: problem solved (for McStas-1.9.1) Validated by: K. Lieutenant

%BUGS for transmission calculation, each neutron is supposed to be in the guide centre

Input parameters

Parameters in boldface are required; the others are optional.

Links

• Component source code found in file Selector.comp.

• See also Additional notes March 1999 and Jan 2000.

another approach to describe a rotating velocity selector

The component Selector describes the same kind of rotating velocity selector as V_selector - compare description there - but it uses different parameters and a different algorithm:

The position of the apertures relative to the z-axis (usually the beam centre) is defined by the four parameters \(xmin, xmax, ymin, ymax\). Entry and exit apertures are always identical and situated directly before and behind the rotor. There are \(num\) blades of thickness \(width\) twisted by the angle \(\alpha \) (in degrees) on a length \(len\). The selector rotates with a speed \(feq\) (in rotation per second); its axle is in a distance \(radius\) below the z-axis.

First the neutron is propagated to the entrance window. The loss of neutrons hitting the thin side of the blades is taken into account by multiplying the neutron weight by a factor

\begin {equation} p(r) = \theta _i(r) / \theta _o \end {equation}

\begin {equation} \theta _o = 360^o / num \end {equation}

\(\theta _i\) is the opening between two blades for the distance \(r\) between the neutron position (at the entrance) and the selector axle. The difference between \(\theta _o\) and \(\theta _i\) is determined by the blade thickness. The neutron is now propagated to the exit window. If it is outside the regarded channel (between the two actual blades), it is lost; otherwise it remains in the exit plane.

WARNING - Differences between Selector and V_selector:

• Selector has a different coordinate system than V_selector; in Selector the origin lies in the entrance plane of the selector.

• The blades are twisted to the other side, i.e. to the left above the axle in Selector.

• Speed of rotation is given in rotation per second, not in rotations per minute as in V_selector.