% Emacs settings: -*-mode: latex; TeX-master: "manual.tex"; -*-

\chapter{Monochromators}

In this class of components, we are concerned with elastic Bragg
scattering from monochromators. \textbf{Monochromator\_flat}
models a flat thin mosaic crystal with a single scattering vector
perpendicular to the surface.
The component \textbf{Monochromator\_curved} is physically similar,
but models a singly or doubly bend monochromator crystal arrangement.

A much more general model of scattering from a single crystal is
found in the component \textbf{Single\_crystal},
which is presented under Samples, chapter~\ref{c:samples}.

\input{optics/Monochromator_flat.tex}

\input{optics/Monochromator_curved.tex}

\section{Single\_crystal: Thick single crystal monochromator plate with multiple scattering}
\index{Optics!Monochromator, thick}

The \textbf{Single\_crystal} component may be used to study more complex monochromators, including incoherent scattering, thickness and multiple scattering. Please refer to section \ref{s:Single_crystal}.

\section{Phase space transformer - moving monochromator}
\index{Optics!phase space transformer}

Eventhough there exist a few attempts to write dedicated phase space transformer components, there is an elegant way to put a monochromator into move, by mean of the EXTEND keyword. If you define a SPEED parameter for the instrument, the idea is to change the coordinate system before the monochromator, and restore it afterwards, as follow in the TRACE section:
\begin{lstlisting}
DEFINE INSTRUMENT PST(SPEED=200, ...)
(...)
TRACE
(...)
COMPONENT Mono_PST_on=Arm()
AT ...
EXTEND %{
  vx = vx + SPEED; // monochromator moves transversaly by SPPED m/s
%}

COMPONENT  Mono=Monochromator(...)
AT (0,0,0) RELATIVE PREVIOUS

COMPONENT Mono_PST_off=Arm
AT (0,0,0) RELATIVE PREVIOUS
EXTEND  %{
  vz = vz - SPEED; // puts back neutron in static coordinate frame
%}
\end{lstlisting}
This solution does not contain acceleration, but is far enough for most
studies, and it is very simple. In the latter example, the instance \verb+Mono_PST_on+ should itself be rotated to reflect according to a Bragg law.
