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     Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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                            Example B4
                            -----------

 This example simulates a simple Sampling Calorimeter setup.
 To demonstrate several possible ways of data scoring, the example
 is provided in four variants: B4a, B4b, B4c, B4d.
 (See also examples/extended/electromagnetic/TestEm3)

 1- GEOMETRY DEFINITION

   The geometry is constructed in B4[c,d]::DetectorConstruction class.
   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.

   Four parameters define the geometry of the calorimeter :
	- the thickness of an absorber plate,
 	- the thickness of a  gap,
 	- the number of layers, and
 	- the transverse size of the calorimeter (the entrance face is a square).

   In addition, a global, uniform, and transverse magnetic field can be
   applied using G4GlobalMagFieldMessenger, instantiated in
     B4[c,d]::DetectorConstruction::ConstructSDandField
   with a non zero field value, or via interactive commands.
   For example:

   /globalField/setValue 0.2 0 0 tesla


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

   A more general version of this geometry can be found in:
   examples/extended/electromagnetic/TestEm3
   where all the geometry parameters, the absorber and gap materials
   can be modified interactively via the commands defined in the DetectorMessenger
   class.

 2- PHYSICS LIST

   The particle's type and the physic processes which will be available
   in this example are set in the FTFP_BERT physics list. This physics list
   requires data files for electromagnetic and hadronic processes.
   See more on installation of the datasets in Geant4 Installation Guide,
   Chapter 3.3: Note On Geant4 Datasets:
   http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/InstallationGuide/html/ch03s03.html
   The following datasets: G4LEDATA, G4LEVELGAMMADATA, G4SAIDXSDATA and
   G4ENSDFSTATEDATA are mandatory for this example.

   In addition the build-in interactive command:
               /process/(in)activate processName
   allows to activate/inactivate the processes one by one.

 3- ACTION INITALIZATION

   A newly introduced class, B4[a,b,c,d]::ActionInitialization,
   instantiates and registers to Geant4 kernel all user action classes.

   While in sequential mode the action classes are instatiated just once,
   via invoking the method:
      B4[a,b,c,d]::ActionInitialization::Build()
   in multi-threading mode the same method is invoked for each thread worker
   and so all user action classes are defined thread-local.

   A run action class is instantiated both thread-local
   and global that's why its instance is created also in the method
      B4[a,b,c,d]::ActionInitialization::BuildForMaster()
   which is invoked only in multi-threading mode.

 4- PRIMARY GENERATOR

   The primary beam 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 B4::PrimaryGeneratorAction class, and can
   be changed via the G4 built-in commands of the G4ParticleGun class (see
   the macros provided with this example).

 5- RUNS and EVENTS

   A run is a set of events.
   The user can choose the frequency of printing via the Geant4 interactive
   command, for example:

     /run/printProgress 100

 6- DETECTOR RESPONSE

   The energy deposit and track lengths of the charged particles are recorded on
   an event by event basis in the Absober and Gap layers.

   In order to demonstrate several possible ways of data scoring,
   the example is provided in four variants:

   Variant a: User Actions

     These 4 quantities are data members of the B4a::EventAction class.
     They are collected step by step in
     B4a::SteppingAction::UserSteppingAction(), and passed to the event action
     via two methods: B4a::EventAction::AddAbs() and B4a::EventAction::AddGap().

     In B4a::EventAction::EndOfEventAction(), these quantities are printed and
     filled in H1D histograms and ntuple to accumulate statistic and compute
     dispersion.

   Variant b: User data object

     In order to avoid dependencies between action classes, a user object
     B4b::RunData, derived from G4Run, is defined with data members needed
     for the accounted information.
     In order to reduce the number of data members a 2-dimensions array
     is introduced for each quantity.
     Then the quantities are collected step by step in user action classes:
     B4b::SteppingAction::UserSteppingAction() and
     B4b::EventAction::EndOfEventAction() in a similar way as in variant a.

   Variant c: Hits and Sensitive detectors

     In this option, the physics quantities are accounted using the hits
     and sensitive detectors framework defined in the Geant4 kernel.
     The physics quantities are stored in B4c::CalorHit via two B4c::CalorimeterSD
     objects, one associated with the Absorber volume and another one with Gap
     in B4c::DetectorConstruction::ConstructSDandField().

     In contrary to the B2 example (Tracker) where a new hit is created
     with each track passing the sensitive volume (in the calorimeter), only one
     hit is created for each calorimeter layer and one more hit to account for
     the total quantities in all layers. In addition to the variants a and b,
     the quantities per each layer are also available in addition to the total
     quantities.

   Variant d: Scorer

     In this option, the Geant4 scorers which are defined on the top of hits
     and sensitive detectors Geant4 framework are used.
     In practice this means that the user does not need to define hits and sensitive
     detector classes but rather uses the classes already defined
     in Geant4. In this example, the G4MultiFunctionalDetector with
     G4PSEnergyDeposit and G4PSTrackLength primitive scores are used (see
     B4d::DetectorConstruction::ConstructSDandField()).

     The scorers hits are saved in form of ntuples in a Root file using Geant4
     analysis tools. This feature is activated in the main () function with instantiating
     G4TScoreNtupleWriter.

     Also with this approach, the quantities per each layer are available
     in addition to the total quantities.

  7- HISTOGRAMS

   The analysis tools are used to accumulate statistics and compute the dispersion
   of the energy deposit and track lengths of the charged particles.
   H1D histograms are created in B4[b]::RunAction::RunAction() for the
   following quantities:
   - Energy deposit in absorber
   - Energy deposit in gap
   - Track length in absorber
   - Track length in gap
   The same values are also saved in an ntuple.

   The histograms and the ntuple are saved in the output file in a format
   according to a specified file extension, the default in this example
   is ROOT.

   The accumulated statistic and computed dispersion is printed at the end of
   run, in B4::RunAction::EndOfRunAction().
   When running in multi-threading mode, the histograms and the ntuple accumulated
   on threads are merged in a single output file. While merging of histograms is
   performed by default, merging of ntuples is explicitly activated in the B4::RunAction
   constructor.

   The ROOT histograms and ntuple can be plotted with ROOT using the plotHisto.C
   and plotNtuple.C macros.

 8- HOW TO RUN

    This example handles the program arguments in a new way.
    It can be run with the following optional arguments:
    % exampleB4a [-m macro ] [-u UIsession] [-t nThreads] [-vDefault]

    The -vDefault option will activate using the default Geant4 stepping verbose
    class (G4SteppingVerbose) instead of the enhanced stepping verbose with best
    units (G4SteppingVerboseWithUnits) used in the example by default.

    The -t option is available only in multi-threading mode
    and it allows the user to override the Geant4 default number of
    threads. The number of threads can be also set via G4FORCENUMBEROFTHREADS
    environment variable which has the top priority.

    - Execute exampleB4a in the 'interactive mode' with visualization
        % exampleB4a
      and type in the commands from run1.mac line by line:
        Idle> /tracking/verbose 1
        Idle> /run/beamOn 1
        Idle> ...
        Idle> exit
      or
        Idle> /control/execute run1.mac
        ....
        Idle> exit

    - Execute exampleB4a in the 'batch' mode from macro files
      (without visualization)
        % exampleB4a -m run2.mac
        % exampleB4a -m exampleB4.in > exampleB4.out

    - Execute exampleB4a in the 'interactive mode' with a selected UI session,
      e.g. tcsh
        % exampleB4a -u tcsh

 The following paragraphs are common to all basic examples

 A- VISUALIZATION

   The visualization manager is set via the G4VisExecutive class
   in the main() function in exampleB4a.cc.
   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 function when the example is used in interactive running mode.

   By default, vis.mac opens an OpenGL viewer (/vis/open OGL).
   The user can change the initial viewer 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 the
   HepRApp and DAWN viewers, respectively).  Note that one can always
   open new viewers at any time from the command line.  For example, if
   you already have a view in, say, an OpenGL window with a name
   "viewer-0", then
      /vis/open DAWNFILE
   then to get the same view
      /vis/viewer/copyView viewer-0
   or to get the same view *plus* scene-modifications
      /vis/viewer/set/all viewer-0
   then to see the result
      /vis/viewer/flush

   The DAWNFILE, HepRepFile drivers are always available
   (since they require no external libraries), but the OGL driver requires
   that the Geant4 libraries have been built with the OpenGL option.

   For more information on visualization, including information on how to
   install and run DAWN, OpenGL and HepRApp, see the visualization tutorials,
   for example,
   http://geant4.slac.stanford.edu/Presentations/vis/G4[VIS]Tutorial/G4[VIS]Tutorial.html
   (where [VIS] can be replaced by DAWN, OpenGL and HepRApp)

   The tracks are automatically drawn at the end of each event, accumulated
   for all events and erased at the beginning of the next run.

 B- USER INTERFACES

   The user command interface is set via the G4UIExecutive class
   in the main() function in exampleB4a.cc
   The selection of the user command interface is then done automatically
   according to the Geant4 configuration or it can be done explicitly via
   the third argument of the G4UIExecutive constructor (see exampleB4a.cc).
