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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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TestEm14
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How to compute cross sections from the direct evaluation of the mean
free path ( see below, item Physics).
How to plot final state of a process.
1- GEOMETRY DEFINITION
It is a single box representing a 'semi infinite' homogeneous medium.
Two parameters define the geometry :
- the material of the box,
- the (full) size of the box.
The default geometry (100 m 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 'artificial' constraints on the step size, the
multiple scattering is not instanciated, and all processes are
registered as discrete : there is no continuous energy loss.
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 interaction of the incident paticle.
The absorption length, also called mean free path, is computed as
the mean value of the track length of the incident particle.
This is why the medium must be 'infinite' : to be sure that interaction
occurs at any events.
The result is compared with the 'input' data, i.e. with the cross
sections stored in the PhysicsTables and used by Geant4.
The energy spectrum and the angular distribution of the scattered
particle (if any) and of the created secondaries are plotted (see
SteppingAction).
A set of macros defining various run conditions are provided.
The processes are actived/inactived in order to survey the processes
individually.
5- HISTOGRAMS
The test contains 6 built-in 1D histograms, which are managed by the
HistoManager class and its Messenger. The histos can be individually
activated with the command :
/analysis/h1/set id nbBins valMin valMax unit
where unit is the desired unit for the histo (MeV or keV, etc..)
(see the macros xxxx.mac).
1 "scattered primary particle: energy spectrum"
2 "scattered primary particle: costheta distribution"
3 "charged secondaries: energy spectrum"
4 "charged secondaries: costheta distribution"
5 "neutral secondaries: energy spectrum"
6 "neutral secondaries: costheta distribution"
The histograms are managed by the HistoManager class and its Messenger.
The histos can be individually activated with the command :
/analysis/h1/set id nbBins valMin valMax unit
where unit is the desired unit for the histo (MeV or keV, deg or mrad, etc..)
One can control the name of the histograms file with the command:
/analysis/setFileName name (default testem14)
It is possible to choose the format of the histogram file : root (default),
hbook, xml, csv, by using namespace in HistoManager.hh
It is also possible to print selected histograms on an ascii file:
/analysis/h1/setAscii id
All selected histos will be written on a file name.ascii (default testem14)
Using hbook format
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Need a special treatement : the Cern Library must be installed and the
environment variable CERNLIB correctly set. Then, *before* compiling,
activate G4_USE_HBOOK in GNUmakefile and g4hbook.hh in HistoManager.hh
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 ?
execute TestEm14 in 'batch' mode from macro files :
% TestEm14 compt.mac
execute TestEm14 in 'interactive mode' with visualization :
% TestEm14
Idle> control/execute vis.mac
....
Idle> type your commands
....
Idle> exit
Gabriele Cosmo