$Id: README,v 1.4 2006/05/15 14:42:42 maire Exp $
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     Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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                            TestEm13
                            --------

	How to compute cross sections from the tramsmition coefficient
	( see below, item Physics).
	
 1- GEOMETRY DEFINITION
 
 	It is a single box representing a layer of finite thickness of
	homogeneous material.
 	Two parameters define the geometry :
 	- the material of the box,
	- the (full) size of the box.
 	
 	The default geometry (1 cm of water) is constructed in 
	DetectorConstruction, but the above parameters can be changed 
	interactively via the commands defined in DetectorMessenger.
 	
 2- PHYSICS LIST
 
 	The physics list contains the standard electromagnetic processes.
	In order not to introduce 'articicial' constraints on the step size, the
	multiple scattering is not instanciated, and there is no limitation from
	the maximum energy lost per step.  
 	 
 3- AN EVENT : THE PRIMARY GENERATOR
 
 	The primary kinematic consists of a single particle starting at the edge
	of the box. The type of the particle and its energy are set in 
	PrimaryGeneratorAction (1 MeV gamma), and can be changed via the G4 
 	build-in commands of ParticleGun class (see the macros provided with 
 	this example).
 	
 4- PHYSICS
 
	An event is killed at the first step of the incident paticle.
	Either the particle has interacted or is transmitted through the layer.
	The cross section, also called absorption coefficient, is computed from 
	the rate of unaltered transmitted incident particles.
	
	The result is compared with the 'input' data, i.e. with the cross
	sections stored in the PhysicsTables and used by Geant4.
 	
 	A set of macros defining various run conditions are provided.
 	The processes are actived/inactived in order to survey the processes 
	individually.

 	 				
 6- VISUALIZATION
 
 	The Visualization Manager is set in the main().
 	The initialisation of the drawing is done via the commands
 	/vis/... in the macro vis.mac. To get visualisation:
 	> /control/execute vis.mac
 	
 	The detector has a default view which is a longitudinal view of the 
 	box.
 	
 	The tracks are drawn at the end of event, and erased at the end of run.
	
 7- HOW TO START ?
 
	compile and link to generate an executable
 		% cd geant4/examples/extended/electromagnetic/TestEm13
 		% gmake
 		
	execute TestEm13 in 'batch' mode from macro files :
 		% TestEm13   compt.mac
 		
	execute TestEm13 in 'interactive mode' with visualization :
 		% TestEm13
		Idle> control/execute vis.mac
 		....
 		Idle> type your commands
 		....
 		Idle> exit
 
