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     =========================================================
     Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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                        purgin_magnet
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                     s. Larsson, May 2004

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Acknowledgments to GEANT4 people, in particular to J. Apostolakis,
J Generowicz, G. Folger, Vladimir Ivanchenko,  M.G.Pia and 
S. Guatelli. 
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0. Introduction
----------------

This example simulates electrons traveling through a 3D magnetic field. 

The Purging Magnet example is an application of Geant4 in a medical
environment. It simulates a strong purging magnet in a treatment head. 
The geometry is very simplified. The major idea of this example is to 
implement an external magnetic field grid and test if the deviation of 
electrons are as expected in this particular field. The data (position, 
energy and momentum) are collected in a measurement volume.The data is 
stored in a HBOOK file if the user has set up the AIDA 3.0 environment 
and Anaphe is available.
 
  
1. Setting up the environment variables
---------------------------------------
-example enviroment with the compiler 3.2.3
 -a standard Geant4 example GNUmakefile is provided                     

setup with:                                                             
compiler = gcc-3.2.3
G4SYSTEM = linux-g++                                                    

The following environment variables need to be set:                     

G4LEDATA: points to low energy data base - G4EMLOW2.3                   

setup for analysis: AIDA 3.2.1, PI 1.2.1                                

Users can download the analysis tools from:
                                                                        
http://aida.freehep.org/
http://www.cern.ch/PI

The required data-file for the field map can be downloaded from:
   http://cern.ch/geant4/source/source/PurgMag3D.TABLE.gz


2. How to run the example
-------------------------

- Run the "PurgMag" executable.
- File PurgMag.TABLE needs to be available in the current directory in order to run correctly.
- For visualisation use vis.mac. 
  Default visualization is with OpenGL
- Interactive or batch modes possible.
  Default: Interactive mode.

To run a certain number of events in interactive mode, 
type the following at the "idle>" prompt:

idle> run/beamOn  NumberOfEvents
idle> exit

- Simulation histogram output is stored in purgmag.hbk

1)Ntuple with position, energy and momentum for electrons
2)Ntuple with position, energy and momentum for photons 
  (not needed in this example, will be used in further development)
3)Ntuple with position, energy and momentum for positrons
  (not needed in this example, will be used in further development)

A default vizualisation macro (vis.mac) is pre-loaded before interactive runs.
Executing it
    osmachine.3% $G4WORKDIR/bin/Linux-g++/PurgMag 
runs vis.mac before giving you an interactive prompt.

	
3. Detector description
-----------------------

Simply a measurement volume. All particles entering the volume are scored.


4. Physics processes
--------------------

This example uses the standard Electromagnetic processes.


5. Particle Generator
----------------------

The PurgMagPrimaryGeneratorAction sets the initial state of tracks to

-electrons 50MeV
-Start position (0, 0, 15cm)
-Momentum direction (0, 0, -1)

 
6. Geometry and materials
--------------------------

The world consists of Vacuum to minimize interactions of the electrons
with the medium. The purging magnet is implemented as a 3D field grid 
of field values and geometerically as a pole gap made of iron. The 
measurement volume also contains vacuum. 

The field is interpolated using a simple linear interpolation in two 
dimensions (z and rho).

7. Comparison
--------------

The design of the magnetic field was made with the OPERA 3D package 
which is an electromagnetic finite element and finite difference 
analysis software. The deviation in the y-direction (ey in Ntuple 1)
has also been calculated in the OPERA 3D module TOSCA for comparison. 

TOSCA:  deviation y-direction: 35.112 cm
GEANT4: deviation y-direction: 35.170 cm (updated after PurgMag.pdf)

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* More information about the setup and geometry in PurgMag.pdf *
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