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$Id: README,v 1.20 2010-12-01 05:55:04 allison Exp$
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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
=========================================================

ExampleN03
----------

This example simulates a simple Sampling Calorimeter setup.

1- GEOMETRY DEFINITION

The calorimeter is a box made of a given number of layers. A layer
consists of an absorber plate and of a detection gap. The layer is
replicated.

Six parameters define the calorimeter :
- the material of the absorber,
- the thickness of an absorber plate,
- the material of the detection gap,
- the thickness of a  gap,
- the number of layers,
- the transverse size of the calorimeter (the input face is a square).

In addition a transverse uniform magnetic field can be applied.

The default geometry is constructed in DetectorConstruction class,
but all of the above parameters can be modified interactively via
the commands defined in the DetectorMessenger class.

|<----layer 0---------->|<----layer 1---------->|<----layer 2---------->|
|			|			|			|
==========================================================================
||		|	||		|	||		|	||
||		|	||		|	||		|	||
||   absorber 	|  gap	||   absorber 	|  gap	||   absorber 	|  gap	||
||		|	||		|	||		|	||
||		|	||		|	||		|	||
beam	||		|	||		|	||		|	||
======>	||		|	||		|	||		|	||
||		|	||		|	||		|	||
||		|	||		|	||		|	||
||		|	||		|	||		|	||
||		|	||		|	||		|	||
||    		| 	||    		| 	||    		| 	||
==========================================================================

NB. The thickness of the absorber or of the gap can be set to zero
(but not together), and the number of layers to 1. In this case we
have a unique homogeneous block of matter, which looks like
a bubble chamber rather than a calorimeter ...
(see the macro of commands: newgeom.mac)

A more general version of this geometry can be found in :
examples/extended/electromagnetic/TestEm3

2- AN EVENT : THE PRIMARY GENERATOR

The primary kinematic consists of a single particle which hits the
calorimeter perpendicular to the input face. The type of the particle
and its energy are set in the PrimaryGeneratorAction class, and can
be changed via the G4 build-in commands of ParticleGun class (see
the macros provided with this example).

In addition one can choose randomly the impact point of the incident
particle. The corresponding interactive command is built in
PrimaryGeneratorMessenger class (see run2.mac).

A RUN is a set of events.

3- DETECTOR RESPONSE

Per event, one wants record total energy deposit in Absober and Gap
(total = sum of all layers), and also total track length of charged particles
in Absober and Gap.

Therefore, these 4 quantities are data members of class EventAction.
They are collected step by step in
SteppingAction::UserSteppingAction(), and passed to EventAction via methods

In EndOfEventAction(), these quantities are printed and passed to RunAction
to accumulate statistic and compute dispersion.

It should be noticed that informations are collected and treated via 'direct'
methods. One do not need to use (SensitiveDetector + HitsCollection) tool,
though this can be done (see N02).

4- VISUALIZATION

The Visualization Manager is set in the main().
The initialisation of the drawing is done via a set of /vis/ commands
in the macro vis.mac. This macro is automatically read from
the main in case of interactive running mode.

By default, vis.mac opens an OpenGL viewer.
You can switch to other graphics systems by commenting out this line
and instead uncommenting one of the other /vis/open statements, such as
HepRepFile or DAWNFILE (which produce files that can be viewed with
HepRApp and DAWN respectively).

The DAWNFILE, HepRepFile drivers are always available
(since they require no external libraries), but the OGL driver requires:
1- the visualisation & interfaces categories have been compiled
with the environment variable G4VIS_BUILD_OPENGLX_DRIVER.
2- exampleN03.cc has been compiled with G4VIS_USE_OPENGLX.
(This is best done through Configure or CMake.)

install and run DAWN, OpenGL and HepRApp, see the visualization tutorials,
for example,
http://geant4.slac.stanford.edu/Presentations/vis/G4OpenGLTutorial/G4OpenGLTutorial.html

The detector has a default view which is a longitudinal view of the
calorimeter.

The tracks are drawn at the end of event, and accumulated to end of run.
vis.mac shows how to draw and filter trajectories.

Additional visualization tutorial macros are available in the visTutor
subdirectory.  They can be tried as:
% \$G4BINDIR/exampleN03
idle > /control/execute visTutor/exN03VisX.mac
For details, see comment lines  described in the macro files.

5- PHYSICS DEMO

The particle's type and the physic processes which will be available
in this example are set in PhysicsList class.

In addition the build-in interactive command:
/process/(in)activate processName
allows to activate/inactivate the processes one by one.
Then one can well visualize the processes one by one, especially
in the bubble chamber setup with a transverse magnetic field.
(see run2.mac and newgeom.mac)

As a homework try to visualize a gamma conversion alone,
or the effect of the multiple scattering.

6- RANDOM NUMBERS HANDLING

CLHEP provides several random number engines. In this example the Ranecu
engine is choosen at beginning of the main (exampleN03.cc).

By default, G4RunManager does not save the rndm seed.
To do so the user must set in BeginOfRunAction:
G4RunManager::GetRunManager()->SetRandomNumberStore(true);

Then the rndm seed is systematically saved at beginning of run
(currentRun.rndm) and beginning of event (currentEvent.rndm)
Therefore, in case of abnormal end, the seed of the last event processed
is available in currentEvent.rndm

Even in case of normal run processing, the user may wish to preserve the
rndm seed of selected events. At any time in the event, put the
following statement:
if (condition) G4RunManager::GetRunManager()->rndmSaveThisEvent();
currentEvent.rndm will be copied to runXXevntYY.rndm
(see ExN03SteppingAction::UserSteppingAction() )

To restart a run from a given rndm seed, use the UI command :
/random/resetEngineFrom  fileName

The macro rndmSeed.mac shows how to save and reset the random number
seed between runs, from UI commands.

7- USER INTERFACES

The default command interface, called G4UIterminal, is done via
standart cin/G4cout.
On Linux and Sun-cc on can use a smarter command interface G4UItcsh.
It is enough to set the environment variable G4UI_USE_TCSH

On can use a Motif driven command interface (called G4UIXm) if:
1- interfaces category has been compiled with G4UI_BUILD_XM_SESSION
2- exampleN03.cc has been compiled with G4UI_USE_XM.

8- HOW TO START ?

- compile and link to generate an executable
% cd N03
% gmake

- execute N03 in 'batch' mode from macro files
% exampleN03   run1.mac

- execute N03 in 'interactive mode' with visualization
% exampleN03
....
Idle>      ---> type your commands. For instance:
Idle> /run/beamOn
....
Idle> /run/beamOn 10
....
Idle> /control/execute newgeom.mac
....
Idle> exit

9- HISTOGRAMS

There is no histograms in novice examples. For N03 + histograms, see
examples/extended/analysis/AnaEx01