\chapter{Coherent elastic scattering}

\section{Nucleon-Nucleon elastic Scattering}
The classes G4LEpp and G4LEnp provide data-driven models for
proton-proton (or neutron-neutron) and neutron-proton elastic
scattering over the range 10-1200 MeV.  Final states (primary
and recoil particle) are derived by sampling from tables of
the cumulative distribution function of the centre-of-mass scattering
angle, tabulated for a discrete set of lab kinetic energies from
10 MeV to 1200 MeV.  The CDF's are tabulated at 1 degree intervals and
sampling is done using bi-linear interpolation in energy and
CDF values.  The data are derived from differential cross sections
obtained from the SAID database, R. Arndt, 1998.

In class G4LEpp there are two data sets: one including Coulomb effects
(for p-p scattering) and one with no Coulomb effects (for n-n
scattering or p-p scattering with Coulomb effects suppressed).
The method G4LEpp::SetCoulombEffects can be used to select the
desired data set:
\begin{itemize}
\item       SetCoulombEffects(0):    No Coulomb effects (the default)
\item        SetCoulombEffects(1):    Include Coulomb effects
\end{itemize}
The recoil particle will be generated as a new secondary particle.
In class G4LEnp, the possiblity of a charge-exchange reaction is
included, in which case the incident track will be stopped and both
the primary and recoil particles will be generated as secondaries.

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