Import Geant4 11.4.0 source tree
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Gabriele Cosmo
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\page ExampleB2 Example B2
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This example simulates a simplified fixed target experiment.
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## GEOMETRY DEFINITION
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The setup consists of a target followed by six chambers of increasing
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transverse size at defined instances from the target. These chambers are
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located in a region called the Tracker region.
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Their shape are cylinders, constructed as simple cylinders
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(in B2a::DetectorConstruction) and as parametrised volumes
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(in B2b::DetectorConstruction), see also B2b::ChamberParameterisation class.
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In addition, a global, uniform, and transverse magnetic field can be
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applied using G4GlobalMagFieldMessenger, instantiated in
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B2a::DetectorConstruction::ConstructSDandField with a non zero field value,
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or via interactive commands.
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For example:
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```
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/globalField/setValue 0.2 0 0 tesla
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```
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An instance of the B2::TrackerSD class is created and associated with each
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logical chamber volume (in B2a) and with the one G4LogicalVolume associated
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with G4PVParameterised (in B2b).
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One can change the materials of the target and the chambers
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interactively via the commands defined in B2a::DetectorMessenger
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(or B2b::DetectorMessenger). For example:
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```
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/B2/det/setTargetMaterial G4_WATER
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/B2/det/setChamberMaterial G4_Ar
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```
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## PHYSICS LIST
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The particle's type and the physic processes which will be available
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in this example are set in the FTFP_BERT physics list. This physics list
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requires data files for electromagnetic and hadronic processes.
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See more on installation of the datasets in
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<a href="http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/InstallationGuide/html/ch03s03.html">
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Geant4 Installation Guide, Chapter 3.3: Note On Geant4 Datasets </a>.
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The following datasets: G4LEDATA, G4LEVELGAMMADATA, G4SAIDXSDATA and
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G4ENSDFSTATEDATA are mandatory for this example.
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In addition, the build-in interactive command:
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```
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/process/(in)activate processName
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```
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allows the user to activate/inactivate the processes one by one.
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## ACTION INITALIZATION
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A newly introduced class, B2::ActionInitialization,
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instantiates and registers to Geant4 kernel all user action classes.
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While in sequential mode the action classes are instatiated just once,
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via invoking the method:
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B2::ActionInitialization::Build()
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in multi-threading mode the same method is invoked for each thread worker
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and so all user action classes are defined thread-local.
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A run action class is instantiated both thread-local
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and global that's why its instance is created also in the method
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B2::ActionInitialization::BuildForMaster()
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which is invoked only in multi-threading mode.
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## PRIMARY GENERATOR
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The primary generator action class employs the G4ParticleGun.
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The primary kinematics consists of a single particle which starts close
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to the world boundary and hits the target perpendicular to the entrance
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face. The type of the particle and its energy can be changed via the
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Geant4 built-in commands of the G4ParticleGun class.
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## RUNS and EVENTS
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A run is a set of events.
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The user has control:
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- at Begin and End of each run (class B2::RunAction)
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- at Begin and End of each event (class B2::EventAction)
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- at Begin and End of each track (class TrackingAction, not used here)
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- at End of each step (class SteppingAction, not used here)
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The event number is written to the log file every requested number
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of events in B2::EventAction::BeginOfEventAction() and
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B2::EventAction::EndOfEventAction().
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Moreover, for the first 100 events and every 100 events thereafter
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information about the number of stored trajectories in the event
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is printed as well as the number of hits stored in the G4VHitsCollection.
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The run number is printed at B2::RunAction::BeginOfRunAction(), where the
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G4RunManager is also informed how to SetRandomNumberStore for storing
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initial random number seeds per run or per event.
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## USER LIMITS
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This example also illustrates how to introduce tracking constraints
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like maximum step length, minimum kinetic energy etc. via the G4UserLimits
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class and associated G4StepLimiter and G4UserSpecialCuts processes.
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See B2a::DetectorConstruction (or B2b::DetectorConstruction).
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The maximum step limit in the tracker region can be set by the interactive
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command (see B2a::DetectorMessenger, B2b::DetectorMessenger classes).
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For example:
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```
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/B2/det/stepMax 1.0 mm
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```
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## DETECTOR RESPONSE
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A HIT is a step per step record of all the information needed to
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simulate and analyse the detector response.
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In this example the Tracker chambers are considered to be the detector.
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Therefore, the chambers are declared 'sensitive detectors' (SD) in
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the B2a::DetectorConstruction (or B2b::DetectorConstruction) class.
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They are associated with an instance of the B2::TrackerSD class.
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Then, a Hit is defined as a set of 4 informations per step, inside
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the chambers, namely:
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- the track identifier (an integer),
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- the chamber number,
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- the total energy deposit in this step, and
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- the position of the energy deposit.
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A given hit is an instance of the class B2::TrackerHit which is created
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during the tracking of a particle, step by step, in the method
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B2::TrackerSD::ProcessHits(). This hit is inserted in a HitsCollection.
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The HitsCollection is printed at the end of each event (via the method
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B2::TrackerSD::EndOfEvent()), under the control of the command:
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```
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/hits/verbose 2
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```
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<hr>
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The following paragraphs are common to all basic examples
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## VISUALISATION
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The visualization manager is set via the G4VisExecutive class
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in the main() function in exampleB2a.cc (or exampleB2b.cc).
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The initialisation of the drawing is done via a set of /vis/ commands
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in the macro vis.mac. This macro is automatically read from
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the main function when the example is used in interactive running mode.
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By default, vis.mac opens the default viewer (/vis/open).
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This chooses a graphics system (in order of priority):
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- by argument in G4VisExecutive construction.
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- by environment variable, G4VIS_DEFAULT_DRIVER.
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- by information in ~/.g4session.
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- by mode (batch/interactive) and if interactive, by your build flags.
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The user can change the initial viewer
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- with environment variable G4VIS_DEFAULT_DRIVER. The format is
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```
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<graphics-system> [<window-size-hint>]
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```
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Set this, e.g:
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- (bash) export G4VIS_DEFAULT_DRIVER=TSG
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- (tcsh) setenv G4VIS_DEFAULT_DRIVER OI
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- The window-size-hint can optionally be added, e.g:
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- (bash) export G4VIS_DEFAULT_DRIVER="RayTracerQt 1000x1000-0+0"
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- on the command line, precede the app invocation, e.g:
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- ```
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G4VIS_DEFAULT_DRIVER=Vtk ./<application-name>
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```
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- with ~/.g4session.
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For other suggestions for G4VIS_DEFAULT_DRIVER (see list of registered
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graphics systems printed at the start):
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- DAWNFILE: to create a .prim file suitable for viewing in DAWN.
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- VRML2FILE: to create a .wrl file suitable for viewing in a VRML viewer.
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- "TSG_OFFSCREEN 1200x1200": to create an image file with TSG.
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- See the tsg_offscreen.mac in examples/basic/B5 for more commands
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to change the file format, file name, picture size, etc.
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See "Choosing a graphics viewer" in the Application Guide for details.
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Of course you can change the viewer by editing the /vis/open line in vis.mac.
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Also, after the initial viewer opens, you may open a different viewer by typing
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on the command line, e.g:
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```
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/vis/open DAWNFILE
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```
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or
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```
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/vis/open RayTraceQt
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```
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(if you are using the Qt GUI).
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The view parameters of the existing viewer are copied.
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The DAWNFILE and similar drivers are always available
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(since they require no external libraries), but the OGL driver requires
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that the Geant4 libraries have been built with the OpenGL option.
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## USER INTERFACES
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The user command interface is set via the G4UIExecutive class
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in the main() function in exampleB2a.cc
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The selection of the user command interface is then done automatically
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according to the Geant4 configuration or it can be done explicitly via
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the third argument of the G4UIExecutive constructor (see exampleB2a.cc).
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The gui.mac macros are provided in examples B2, B4 and B5. This macro
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is automatically executed if Geant4 is built with any GUI session.
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It is also possible to customise the icons menu bar which is
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demonstrated in the icons.mac macro in example B5.
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## HOW TO RUN
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- Execute exampleB2a in the 'interactive mode' with visualization
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```
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% exampleB2a
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and type in the commands from run1.mac line by line:
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Idle> /tracking/verbose 1
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Idle> /run/beamOn 1
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Idle> ...
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Idle> exit
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```
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or
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```
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Idle> /control/execute run1.mac
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....
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Idle> exit
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```
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- Execute exampleB2a in the 'batch' mode from macro files
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(without visualization)
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```
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% exampleB2a run2.mac
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% exampleB2a exampleB2.in > exampleB2.out
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```
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