\section{Source\_simple: A simple continuous source
with a flat energy/wavelength spectrum}
\label{source-simple}
\index{Sources!Simple continuous source}

%\component{Source\_simple}{System}{ $r_\textrm{s}$, $z_\textrm{foc}$, $w$, $h$, $E_0$, $\Delta E$, $\Psi$}{$\lambda_0$, $d\lambda$}{Validated. t=0}
\mcdoccomp{sources/Source_simple.parms}

This component is
a simple source with an energy distribution which is uniform
in the range $E_0 \pm dE$
(alternatively: a wavelength distribution in the range $\lambda_0 \pm d\lambda$).
This component is not used for detailed time-of-flight simulations,
so we put $t=0$ for all neutron rays.

The initial neutron ray position is chosen randomly from within a
circle of radius $r_\textrm{s}$ in the $z=0$ plane.
This geometry is a fair approximation
of a cylindrical cold/thermal source with the beam going out along
the cylinder axis.

The initial neutron ray direction is focused onto a rectangular target of width
$w$, height $h$, parallel to the $xy$ plane placed at $(0,0,z_\textrm{foc})$.

The initial weight of the created neutron ray, $p_0$, is set to the
energy-integrated flux, $\Psi$, times the source area, $\pi r_\textrm{s}^2$
times a solid-angle factor, which is basically the
solid angle of the focusing rectangle.
See also the section \ref{s:neutron-flux} on source flux.

This component replaces the obsolete components
Source\_flux\_lambda, Source\_flat, Source\_flat\_lambda, and Source\_flux.
