This example simulates a simplified fixed target experiment.

GEOMETRY DEFINITION

The setup consists of a target followed by six chambers of increasing transverse size at defined instances from the target. These chambers are located in a region called the Tracker region. Their shape are cylinders, constructed as simple cylinders (in B2a::DetectorConstruction) and as parametrised volumes (in B2b::DetectorConstruction), see also B2b::ChamberParameterisation class.

In addition, a global, uniform, and transverse magnetic field can be applied using G4GlobalMagFieldMessenger, instantiated in B2a::DetectorConstruction::ConstructSDandField with a non zero field value, or via interactive commands. For example:

/globalField/setValue 0.2 0 0 tesla

An instance of the B2::TrackerSD class is created and associated with each logical chamber volume (in B2a) and with the one G4LogicalVolume associated with G4PVParameterised (in B2b).

One can change the materials of the target and the chambers interactively via the commands defined in B2a::DetectorMessenger (or B2b::DetectorMessenger). For example:

/B2/det/setTargetMaterial G4_WATER
/B2/det/setChamberMaterial G4_Ar

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: Environment Variables for Datasets . 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 the user to activate/inactivate the processes one by one.

ACTION INITALIZATION

A newly introduced class, B2::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: B2::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 B2::ActionInitialization::BuildForMaster() which is invoked only in multi-threading mode.

PRIMARY GENERATOR

The primary generator action class employs the G4ParticleGun. The primary kinematics consists of a single particle which starts at the world boundary and hits the target perpendicular to the entrance face. The type of the particle and its energy can be changed via the G4 built-in commands of the G4ParticleGun class. Note that this particular case of starting a primary particle on the world boundary requires shooting in a direction towards inside the world.

RUNS and EVENTS

A run is a set of events.

The user has control:

at Begin and End of each run (class B2::RunAction)
at Begin and End of each event (class B2::EventAction)
at Begin and End of each track (class TrackingAction, not used here)
at End of each step (class SteppingAction, not used here)

The event number is written to the log file every requested number of events in B2::EventAction::BeginOfEventAction() and B2::EventAction::EndOfEventAction(). Moreover, for the first 100 events and every 100 events thereafter information about the number of stored trajectories in the event is printed as well as the number of hits stored in the G4VHitsCollection.

The run number is printed at B2::RunAction::BeginOfRunAction(), where the G4RunManager is also informed how to SetRandomNumberStore for storing initial random number seeds per run or per event.

USER LIMITS

This example also illustrates how to introduce tracking constraints like maximum step length, minimum kinetic energy etc. via the G4UserLimits class and associated G4StepLimiter and G4UserSpecialCuts processes. See B2a::DetectorConstruction (or B2b::DetectorConstruction).

The maximum step limit in the tracker region can be set by the interactive command (see B2a::DetectorMessenger, B2b::DetectorMessenger classes). For example:

/B2/det/stepMax 1.0 mm

DETECTOR RESPONSE

A HIT is a step per step record of all the information needed to simulate and analyse the detector response.

In this example the Tracker chambers are considered to be the detector. Therefore, the chambers are declared 'sensitive detectors' (SD) in the B2a::DetectorConstruction (or B2b::DetectorConstruction) class. They are associated with an instance of the B2::TrackerSD class.

Then, a Hit is defined as a set of 4 informations per step, inside the chambers, namely:

the track identifier (an integer),
the chamber number,
the total energy deposit in this step, and
the position of the energy deposit.

A given hit is an instance of the class B2::TrackerHit which is created during the tracking of a particle, step by step, in the method B2::TrackerSD::ProcessHits(). This hit is inserted in a HitsCollection.

The HitsCollection is printed at the end of each event (via the method B2::TrackerSD::EndOfEvent()), under the control of the command:

/hits/verbose 2

The following paragraphs are common to all basic examples

VISUALISATION

The visualization manager is set via the G4VisExecutive class in the main () function in exampleB2a.cc (or exampleB2b.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,

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

USER INTERFACES

The user command interface is set via the G4UIExecutive class in the main () function in exampleB2a.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 exampleB2a.cc).

HOW TO RUN

Execute exampleB2a in the 'interactive mode' with visualization

% exampleB2a
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 exampleB2a in the 'batch' mode from macro files (without visualization)
```
% exampleB2a run2.mac
% exampleB2a exampleB2.in > exampleB2.out
```