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    Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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                       Extended Example for G4GeneralParticleSource (GPS)
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  exGPS is created to demonstrate the usage of G4GeneralParticleSource for generating 
  primary incident particle according to user defined distributions. These range from simple 
  monocromatic point source to complicated mutiple sources with various biasing schemes. 
  
  More detailed information on the usage of GPS are available on its home site: 
	http://reat.space.qinetiq.com/gps

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  1. GEOMETRY

     Simple geometry consists of a "Vacuum" world and, in it with two other components:	     
 
       - An alunimium box : 20 x 20 x 20 cm in size, cerntered at the origin.
		  
       - A SiO2 sphere: 5 cm in radius and is placed in the centre of the aluminium box. 

  2. PHYSICS

	Tranportation process only for all particles.

  3. EVENT:

     The event generator is the G4GeneralParticleSource (GPS). The instantiation of G4GeneralParticleSource
     is same as that for G4ParticleGun. See the exGPSPrimaryGeneratorAction.cc file for details .  
  
  5. VISUALIZATION:
 
     Visualisation of the geometry and the tracks is possible with many of the G4 visualisation packages. An
     example of displaying the geometry and tracks using VRML is given in the macro file display_vrml.mac.  

  6. ANALYSIS:

     This example implements an AIDA-compliant analysis manager which creates histograms and ntuples. 
     If user has an AIDA-compliant tool such as AIDAJNI, ANAPHE, or PI installed on his/her system, 
     the analysis part of this example can be activated by: 

	setenv G4ANALYSIS_USE 1

     This needs to be done before building the executable.  

     At the end of an excution, an xml file "exgps.aida" is created by default which contains 
     histograms and ntuples. User can change the name and type of this output file with the commands
	
	/analysis/filetype new-type
	/analysis/fileName new-filename

	e.g.

	/analysis/filetype hbook
	/analysis/filename exgps.hbook

     these change the output file type to "hbook" and name to "exgps.hbbok"

     The output file conatins 6 histograms and one ntuple:

	histogram 1: The energy spectrum of the primary particles.
	histogram 2: 2d histogram of primary particle incident position distribution in the X-Y plane.
	histogram 3: 2d histogram of primary particle incident position distribution in the X-Z plane.
	histogram 4: 2d histogram of primary particle incident position distribution in the Y-Z plane.
	histogram 5: 2d histogram of primary particle incident angle distribution in terms of Phi/Cos(Theta).
	histogram 6: 2d histogram of primary particle incident angle distribution in terms of Phi/Theta.

     The binnings of the histograms can be changed with the UI commands avialable under the 
     /analysis directory.
 
     In the ntuple the following data are recorded for each incident particle:  

	Particle Name
	Incident Position (x,y,z);
	Incident Angle (theta,phi);
	Particle weight;


  7. GETTING STARTED:

     i) If you have an AIDA-compliant analysis package installed, then you should switch on the analysis 
     part of the example by: 

	setenv G4ANALYSIS_USE 1

     otherwise make sure the G4ANALYSIS_USE is not defined:
 
	unsetenv G4ANALYSIS_USE
 
     ii) Build the exGPS executable:

         cd to exgps
         gmake clean
         gmake

     gmake will create tmp and bin directories in your $G4TMP and $G4BIN directories. 
     The executable, named exGPS, will be created in the $G4BIN/$G4SYSTEM/ directory.

     iii) Run the executable: 

     while in the exgps directory do 

         $G4BIN/$G4SYSTEM/exGPS exrgps.in

     If G4ANALYSIS_USE is defined, one should see a display of the six histograms. If JAIDA is used, one has 
     to close the plotter window in order to terminate the execution. After the termination one will find 
     the "exgps.aida", as well as a vrml file "g4_00.wrl" in the directory.

 8. FURTHER EXAMPLES of MACRO FILES:

    There are a number of g4mac files in the ./macros subdirectory, to show the various features of GPS. 
    Most of them will lead to the creation of an aida file in the same name as the macro file. 
    These aida files can be examed and analysed with an analysis tool such as JAS3.

	Please see the README file there for further information.