Example B5 implements a double-arm spectrometer with wire chambers, hodoscopes and calorimeters. Event simulation and collection are enabled, as well as event display and analysis.

GEOMETRY

The spectrometer consists of two detector arms (see B5::DetectorConstruction). One arm provides position and timing information of the incident particle while the other collects position, timing and energy information of the particle after it has been deflected by a magnetic field centered at the spectrometer pivot point.

First arm: box filled with air, also containing:
- 1 hodoscope (15 vertical strips of plastic scintillator)
- 1 drift chamber (5 horizontal argon gas layers with a "virtual wire" at the center of each layer)
Second arm: box filled with air, also containing:
- 1 hodoscope (25 vertical strips of plastic scintillator)
- 1 drift chamber (5 horizontal argon gas layers with a "virtual wire" at the center of each layer)
- 1 electromagnetic calorimeter: a box sub-divided along x,y and z axes into cells of CsI
- 1 hadronic calorimeter: a box sub-divided along x,y, and z axes into cells of lead, with a layer of plastic scintillator placed at the center of each cell
Magnetic field region: air-filled cylinder which contains the field (see B5::MagneticField)

The maximum step limit in the magnetic field region is also set via the G4UserLimits class in a similar way as in Example B2 (see USER LIMITS in B2 example documentation).

The rotation angle of the second arm and the magnetic field value can be set via the interactive commands defined using the G4GenericMessenger class.

PHYSICS

This example uses the reference hadronic physics list, FTFP_BERT, and also adds the G4StepLimiter process.

ACTION INITALIZATION

B5::ActionInitialization class 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: B5::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 B5::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 is is sent in the direction of the first spectrometer arm.

The type of the particle and its several properties can be changed via the G4 built-in commands of the G4ParticleGun class or this example command defined using the G4GenericMessenger class.

EVENT

An event consists of the generation of a single particle which is transported through the first spectrometer arm. Here, a scintillator hodoscope records the reference time of the particle before it passes through a drift chamber where the particle position is measured. Momentum analysis is performed as the particle passes through a magnetic field at the spectrometer pivot and then into the second spectrometer arm. In the second arm, the particle passes through another hodoscope and drift chamber before interacting in the electromagnetic calorimeter. Here it is likely that particles will induce electromagnetic showers. The shower energy is recorded in a three-dimensional array of CsI crystals. Secondary particles from the shower, as well as primary particles which do not interact in the CsI crystals, pass into the hadronic calorimeter. Here, the remaining energy is collected in a three-dimensional array of scintillator-lead sandwiches.

Several aspects of the event may be changed interactively by the user:

angle of the second spectrometer arm
strength of magnetic field
initial particle type
initial momentum and angle
momentum and angle spreads
type of initial particle may be randomized

The initial particle type can be changed using the G4ParticleGun command:

/gun/particle particleName

The UI commands specific to this example are available in /B5 command directory:

/B5/detector/armAngle angle unit
/B5/field/value field unit
/B5/generator/momentum  value unit
/B5/generator/sigmaMomentum value unit
/B5/generator/sigmaAngle value unit
/B5/generator/randomizePrimary [true|false]

They are implemented in

B5::DetectorConstruction::DefineCommands(),
B5::MagneticField::DefineCommands() and
B5::PrimaryGeneratorAction::DefineCommands() methods using G4GenericMessenger class.

In first execution of B5::EventAction::BeginOfEventAction() the hits collections identifiers are saved in data members of the class and used in B5::EventAction::EndOfEventAction() for accessing the hists collections and filling the accounted information in defined histograms and ntuples and printing its summary in a log file. The frequency of printing can be tuned with the built-in command

/run/printProgress  frequency

DETECTOR RESPONSE

All the information required to simulate and analyze an event is recorded in hits. This information is recorded in the following sensitive detectors:

hodoscope:
- particle time
- strip ID, position and rotation
  - see B5::HodoscopeSD, B5::HodoscopeHit
drift chamber:
- particle time
- particle position
- layer ID
  - see B5::DriftChamberSD, B5::DriftChamberHit classes
electromagnetic calorimeter:
- energy deposited in cell
- cell ID, position and rotation
  - see B5::EmCalorimeterSD, B5::EmCalorimeterHit classes
hadronic calorimeter:
- energy deposited in cell
- cell column ID and row ID, position and rotation
  - see B5::HadCalorimeterSD, B5::HadCalorimeterHit classes
The hit classes include methods GetAttDefs and CreateAttValues to define and then fill extra "HepRep-style" Attributes that the visualization system can use to present extra information about the hits. For example, if you pick a B5::HadCalorimeterHit in OpenGL or a HepRep viewer, you will be shown the hit's "Hit Type", "Column ID", "Row ID", "Energy Deposited" and "Position". These attributes are essentially arbitrary extra pieces of information (integers, doubles or strings) that are carried through the visualization. Each attribute is defined once in G4AttDef object and then is filled for each hit in a G4AttValue object. These attributes can also be used by commands to filter which hits are drawn: /vis/filtering/hits/drawByAttribute Detector Geometry and trajectories also carry HepRep-style attributes, but these are filled automatically in the base classes. HepRep is further described at the HepRep Home Page

ANALYSIS

The analysis tools are used to accumulate statistics. Histograms and an ntuple are created in B5::RunAction::RunAction() constructor for the following quantities:

1D histograms:
- Number of hits in Chamber 1
- Number of hits in Chamber 2
2D histograms:
- Drift Chamber 1 X vs Y positions
- Drift Chamber 2 X vs Y positions
Ntuple:
- Number of hits in Chamber 1
- Number of hits in Chamber 2
- Total energy deposit in EM calorimeter
- Total energy deposit in Hadronic calorimeter
- Time of flight in Hodoscope 1
- Time of flight in Hodoscope 2
- Vector of energy deposits in EM calorimeter cells
- Vector of energy deposits in Hadronic calorimeter cells

The histograms and ntuple are saved in two output files in a default (Root) file format.

Another file format (for example xml) can be selected either by changing the generic analysis manager default file type:

analysisManager->SetDefaultFileType("xml");

or by providing the file names with the extension:

analysisManager->SetFileName("B5.xml");
analysisManager->SetNtupleFileName(0, "B5ntuple.xml");

When running in multi-threading mode, the histograms and ntuple accumulated on threads are automatically merged in a single output file.

PLOTTING:

This example comes with a commented plotter.mac that shows how to use the plotting coming with some of the visualization drivers (for example the ToolsSG ones) to see the histograms. In it you will see how to activate the vis driver (create a "scene handler"), create a viewer, create a scene containing a plotter model object, create plotting "regions" (here 2x2 regions) and attach the histograms to each region. When done, each run beamOn should display at end the content of the histograms.

In the second part of plotter.mac, is shown various ways to customize the regions, for example changing the bins color, the axis labels fonts, etc... This could be done by using default embedded styles, defining styles with commands, or setting up directly parameters of the various parts of a plot by using a dedicated command.

By default the fonts used are the Hershey vectorial ones that do not need an extra package, but you can use some freetype fonts if building with the cmake flag -DGEANT4_USE_FREETYPE=ON. Two embedded ttf fonts comes with the ToolsSG plotting: roboto_bold (some open source kind of the Microsoft arialbd) and lato_regular (close to an helvetica). You can use your own .ttf files by using the TOOLS_FONT_PATH environment variable to specify the directory where they could be found.

The following paragraphs are common to all basic examples

VISUALISATION

The visualization manager is set via the G4VisExecutive class in the main () function in exampleB5.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.

vis.mac has additional commands that demonstrate additional functionality of the vis system, such as displaying text, axes, scales, date, logo and shows how to change viewpoint and style. To see even more commands use help or ls or browse the available UI commands in the Application Developers Guide.

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 exampleB5.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).

HOW TO RUN

Execute exampleB5 in the 'interactive mode' with visualization

% exampleB5
and type in the commands from run1.mac line by line:
Idle> /control/verbose 2
Idle> /tracking/verbose 1
Idle> /run/beamOn 10
Idle> ...
Idle> exit

or

Idle> /control/execute run1.mac
....
Idle> exit

Execute exampleB5 in the 'batch' mode from macro files (without visualization)
```
% exampleB5 run2.mac
% exampleB5 exampleB5.in > exampleB5.out
```