$Id: README,v 1.2 2007/12/04 10:34:17 schaelic Exp $
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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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TestPolarization
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How to compute and plot the QED processes including
polarization.
Two possible scenarios are available:
- Polarization transfer of an incoming beam to final state particles
- Material dependent transmission of a polarized beams.
The method is explained below : see item Physics.
1- GEOMETRY DEFINITION
The geometry consists of a single block of a homogeneous material,
placed in a world.
Three parameters define the geometry :
- the material of the box,
- the thickness of the box (sizeZ),
- the transverse dimension of the box (sizeXY).
The default geometry (5mm of Iron, G4_Fe) is constructed in
DetectorConstruction, but the above parameters can be changed
interactively via the commands defined in DetectorMessenger.
Its polarization can be accessed via the PolarizationMessenger
(see example macro file), and is given in the global coordinate
system.
2- PHYSICS LIST
The Physics List contains QED particle definitions (electrons,
positrons and photons) and a general transportation process.
In addition the user can add one of the two process modules:
1. "standard" - standard (unpolarized) EM physics
2. "polarized" - polarized EM physics
These physics list contain the standard electromagnetic processes.
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 (10 MeV electron).
By default the ParticleGun polarization is zero.
All parameter can be changed via the G4 build-in commands of
ParticleGun class (see the macros provided with this example).
4- PHYSICS
This example uses the following physics processes:
- electromagnetic:
photo-electric effect
Compton scattering
pair production
bremsstrahlung
ionization
multiple scattering
annihilation
or
- polarized electromagnetic:
(incl. simulation of polarization transfer, and
asymmetries for longitudinally polarized leptons, and
circularly polarized photons)
polarized photo-electric effect
polarized Compton scattering
polarized pair production
polarized bremsstrahlung
polarized ionization
multiple scattering
polarized annihilation
and
- transportation
and defines the following particles:
electron, positron, photon
5- HISTOGRAMS
The test contains 12 built-in 1D histograms, which are managed by the
HistoManager class and its Messenger. The histos can be individually
activated with the command :
/testem/histo/setHisto id nbBins valMin valMax unit
where unit is the desired unit for the histo (MeV or keV, etc..)
(see the macro histos.mac).
1 gamma energy
2 gamma cos(theta)
3 gamma phi
4 gamma polarization
5 electron energy
6 electron cos(theta)
7 electron phi
8 electron polarization
9 positron energy
10 positron cos(theta)
11 positron phi
12 positron polarization
If AIDA is available, i.e. if G4ANALYSIS_USE is set, these
histograms are stored in a compressed XML file
(pol01.aida). These results can be displayed using a small
program (plotResults.java) based on JAIDA by simply calling
% gmake plots
6- VISUALIZATION
Simulated events can be displayed on top of a representation of
the geometry, see vis.mac for an example.
7- HOW TO START ?
compile and link to generate an executable
% gmake
execute pol01 in 'batch' mode from the default macro file :
% pol01 pol01.in
or execute pol01 in 'batch' mode including aida output :
% pol01 histos.mac
if available use JAIDA to display the results via
% gmake plots
a visualisation example is available by calling
% pol01
[...]
PreInit> /control/execute vis.mac
Gabriele Cosmo