Gabriele Cosmo
294 lines
12 KiB
Plaintext
294 lines
12 KiB
Plaintext
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=========================================================
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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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=========================================================
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Extended Example B5
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--------------------
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Example B5 implements a double-arm spectrometer with wire chambers,
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hodoscopes and calorimeters. Event simulation and collection are
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enabled, as well as event display and analysis.
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1- GEOMETRY
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The spectrometer consists of two detector arms
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(see B5DetectorConstruction). One arm provides
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position and timing information of the incident particle while the
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other collects position, timing and energy information of the particle
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after it has been deflected by a magnetic field centered at the
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spectrometer pivot point.
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- First arm: box filled with air, also containing:
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1 hodoscope (15 vertical strips of plastic scintillator)
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1 drift chamber (5 horizontal argon gas layers with a
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"virtual wire" at the center of each layer)
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- Second arm: box filled with air, also containing:
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1 hodoscope (25 vertical strips of plastic scintillator)
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1 drift chamber (5 horizontal argon gas layers with a
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"virtual wire" at the center of each layer)
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1 electromagnetic calorimeter:
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a box sub-divided along x,y and z
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axes into cells of CsI
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1 hadronic calorimeter:
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a box sub-divided along x,y, and z axes
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into cells of lead, with a layer of
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plastic scintillator placed at the center
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of each cell
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- Magnetic field region: air-filled cylinder which contains
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the field (see B5MagneticField)
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The maximum step limit in the magnetic field region is also set
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via the G4UserLimits class in a similar way as in Example B2.
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The rotation angle of the second arm and the magnetic field value
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can be set via the interactive command defined using the G4GenericMessenger
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class.
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2- PHYSICS
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This example uses the reference hadronic physics list, FTFP_BERT,
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and also adds the G4StepLimiter process.
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3- ACTION INITALIZATION
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B5ActionInitialization class
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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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B5ActionInitialization::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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B5ActionInitialization::BuildForMaster()
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which is invoked only in multi-threading mode.
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4- 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 is
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is sent in the direction of the first spectrometer arm.
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The type of the particle and its several properties can be changed
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via the G4 built-in commands of the G4ParticleGun class or
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this example command defined using the G4GenericMessenger class.
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5- EVENT
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An event consists of the generation of a single particle which is
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transported through the first spectrometer arm. Here, a scintillator
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hodoscope records the reference time of the particle before it passes
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through a drift chamber where the particle position is measured.
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Momentum analysis is performed as the particle passes through a magnetic
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field at the spectrometer pivot and then into the second spectrometer
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arm. In the second arm, the particle passes through another hodoscope
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and drift chamber before interacting in the electromagnetic calorimeter.
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Here it is likely that particles will induce electromagnetic showers.
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The shower energy is recorded in a three-dimensional array of CsI
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crystals. Secondary particles from the shower, as well as primary
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particles which do not interact in the CsI crystals, pass into the
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hadronic calorimeter. Here, the remaining energy is collected in a
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three-dimensional array of scintillator-lead sandwiches.
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Several aspects of the event may be changed interactively by the user:
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- angle of the second spectrometer arm
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- strength of magnetic field
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- initial particle type
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- initial momentum and angle
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- momentum and angle spreads
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- type of initial particle may be randomized
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The initial particle type can be changed using the G4ParticleGun command:
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/gun/particle particleName
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The UI commands specific to this example are available in /B5 command
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directory:
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/B5/detector/armAngle angle unit
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/B5/field/value field unit
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/B5/generator/momentum value unit
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/B5/generator/sigmaMomentum value unit
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/B5/generator/sigmaAngle value unit
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/B5/generator/randomizePrimary [true|false]
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They are implemented in
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B5DetectorConstruction::DefineCommands(),
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B5MagneticField::DefineCommands() and
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B5PrimaryGeneratorAction::DefineCommands() methods
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using G4GenericMessenger class.
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In first execution of BeginOfEventAction()
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the hits collections identifiers are saved in data members of the class
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and used in EndOfEventAction() for accessing
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the hists collections and filling the accounted information in defined
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histograms and ntuples and printing its summary in a log file.
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The frequency of printing can be tuned with the built-in command
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/run/printProgress frequency
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6- DETECTOR RESPONSE:
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All the information required to simulate and analyze an event is
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recorded in hits. This information is recorded in the following
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sensitive detectors:
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- hodoscope:
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particle time
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strip ID, position and rotation
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(see B5HodoscopeSD, B5HodoscopeHit classes)
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- drift chamber:
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particle time
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particle position
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layer ID
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(see B5DriftChamberSD, B5DriftChamberHit classes)
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- electromagnetic calorimeter:
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energy deposited in cell
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cell ID, position and rotation
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(see B5EMCalorimeterSD, B5EMCalorimeterHit classes)
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- hadronic calorimeter:
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energy deposited in cell
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cell column ID and row ID, position and rotation
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(see B5HadCalorimeterSD, B5HadCalorimeterHit classes)
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The hit classes include methods GetAttDefs and CreateAttValues to define
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and then fill extra "HepRep-style" Attributes that the visualization system
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can use to present extra information about the hits.
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For example, if you pick a B5HadCalorimeterHit in OpenGL or a HepRep viewer,
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you will be shown the hit's "Hit Type", "Column ID", "Row ID",
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"Energy Deposited" and "Position".
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These attributes are essentially arbitrary extra pieces of information
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(integers, doubles or strings) that are carried through the visualization.
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Each attribute is defined once in G4AttDef object and then is filled for
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each hit in a G4AttValue object.
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These attributes can also be used by commands to filter which hits are drawn:
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/vis/filtering/hits/drawByAttribute
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Detector Geometry and trajectories also carry HepRep-style attributes,
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but these are filled automatically in the base classes.
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HepRep is further described at: http://www.slac.stanford.edu/~perl/heprep/
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7- ANALYSIS:
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The analysis tools are used to accumulate statistics.
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Histograms and an ntuple are created in B5RunAction::B5RunAction()
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constructor for the following quantities:
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1D histograms:
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- Number of hits in Chamber 1
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- Number of hits in Chamber 2
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2D histograms:
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- Drift Chamber 1 X vs Y positions
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- Drift Chamber 2 X vs Y positions
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Ntuple:
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- Number of hits in Chamber 1
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- Number of hits in Chamber 2
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- Total energy deposit in EM calorimeter
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- Total energy deposit in Hadronic calorimeter
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- Time of flight in Hodoscope 1
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- Time of flight in Hodoscope 2
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- Vector of energy deposits in EM calorimeter cells
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- Vector of energy deposits in Hadronic calorimeter cells
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The histograms and ntuple are saved in the output file in a format
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according to a technology selected in B5Analysis.hh.
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When running in multi-threading mode, the histograms and ntuple accumulated
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on threads are automatically merged in a single output file.
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8- BUILD OPTIONS:
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This example can be built with excluding visualization and/or
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Geant4 user interface via G4VIS_USE and G4UI_USE compiler options
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(see exampleB5.cc).
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These options are defined by default with Geant4 configuration;
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they can be switched off at compilation time via the CMake options
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G4VIS_NONE or G4UI_NONE or via the environment variables of the
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same name if using GNUmake build.
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The following paragraphs are common to all basic examples
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A- VISUALISATION
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The visualization manager is set via the G4VisExecutive class
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in the main() function in exampleB5.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 an OpenGL viewer (/vis/open OGL).
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The user can change the initial viewer by commenting out this line
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and instead uncommenting one of the other /vis/open statements, such as
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HepRepFile or DAWNFILE (which produce files that can be viewed with the
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HepRApp and DAWN viewers, respectively). Note that one can always
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open new viewers at any time from the command line. For example, if
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you already have a view in, say, an OpenGL window with a name
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"viewer-0", then
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/vis/open DAWNFILE
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then to get the same view
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/vis/viewer/copyView viewer-0
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or to get the same view *plus* scene-modifications
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/vis/viewer/set/all viewer-0
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then to see the result
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/vis/viewer/flush
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The DAWNFILE, HepRepFile 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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vis.mac has additional commands that demonstrate additional functionality
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of the vis system, such as displaying text, axes, scales, date, logo and
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shows how to change viewpoint and style.
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To see even more commands use help or ls or browse the available UI commands
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in the Application Developers Guide.
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For more information on visualization, including information on how to
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install and run DAWN, OpenGL and HepRApp, see the visualization tutorials,
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for example,
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http://geant4.slac.stanford.edu/Presentations/vis/G4[VIS]Tutorial/G4[VIS]Tutorial.html
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(where [VIS] can be replaced by DAWN, OpenGL and HepRApp)
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The tracks are automatically drawn at the end of each event, accumulated
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for all events and erased at the beginning of the next run.
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B- 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 exampleB5.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 exampleB4a.cc).
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C- HOW TO RUN
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- Execute exampleB5 in the 'interactive mode' with visualization:
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% ./exampleB5
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and type in the commands from run1.mac line by line:
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Idle> /control/verbose 2
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Idle> /tracking/verbose 1
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Idle> /run/beamOn 10
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Idle> ...
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Idle> exit
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or
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Idle> /control/execute run1.mac
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....
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Idle> exit
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- Execute exampleB5 in the 'batch' mode from macro files
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(without visualization)
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% ./exampleB5 run2.mac
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% ./exampleB5 exampleB5.in > exampleB5.out
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