=================================================================
                     Channeling effect in Geant4
=================================================================
            Enrico Bagli - INFN and University Ferrara (Italy)
			 bagli@fe.infn.it

This example shows how channeling in bent crystal can be simulated
in Geant4

1.INTRODUCTION
The example simulates the channeling of 400 GeV/c protons in bent
Si crystal. Channeling occurs when particles enter a crystal aligned
with atomic planes or axes. In bent crystals, the particles are 
trapped between atomic planes and follow the crystal curvature 
being deflected. If the particle direction is tangent to a bent
crystal plane is reflected to the opposite direction with respect
to channeling, i.e., it suffer ‘volume reflection’. The example 
provides the physical model for planar channeling and volume
reflection in bent crystals.

2.GEOMETRY
The geometry is a bent Si crystal with three Si detectors placed at
-9.998 m, -0.320 m and 10.756 m with respect to the position of
the bent crystal itself. The Si detectors allows to measure 
incoming and outgoing angle after the interaction with the Si bent crystal.
The geometry is all under vacuum.

3.PRIMARY EVENT
The primary events are 400 GeV/c protons launched at -10.5 m from the
crystal with 13.36 microrad x 11.25 microrad divergence.

4.PHYSICS
In the example the physics of channeling and volume reflection
has been added to the standard Geant4 physics. The description 
of the used model can be found in the paper ‘A model for the 
interaction of high-energy particles in straight and bent 
crystals implemented in Geant4’ by E. Bagli et al., available
online at http://arxiv.org/abs/1403.5819

5.EXECUTION & OUTPUT 
The executable must be run from within the source directory of the example
to ensure that it can find the path for crystal data files.

Data files for Si crystal interplanar potential, nuclei and electron density
 are stored in a subdirectory named ’data’

Upon execution, the 2009_PLB680_129.mac macro will automatically run the
example with 1000 protons.

Use
/xtal/setBR XXX 0. 0. m
To change crystal bending to XXX meters

Use
/xtal/setSize 1.0 70. XXX mm
To change crystal length to XXX millimeter

Use
/xtal/potfilename data/Si220pl
To select the (110) Si crystal plane of channeling

GPS commands are used for the primary generator.

The output is the ExExhCh.root file with the TTree ExExChTree
has the leaves:
- angXin : incoming particle X angle at the crystal
- angYin : incoming particle Y angle at the crystal
- posXin : hitting X position of the particle at the crystal
- posYin : hitting Y position of the particle at the crystal
- angXout: outgoing particle X angle out of the the crystal
- angYout: outgoing particle Y angle out of the the crystal
