Gabriele Cosmo
422 lines
16 KiB
Plaintext
422 lines
16 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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field04 Example
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---------------
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This example shows how to define/use OVERLAPPING field elements
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in Geant4. Fields might be either magnetic, electric or both.
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Credit goes to Tom Roberts and Muons Inc. since much of the code
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and ideas were taken at liberty from the (GNU GPL) source of
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G4BEAMLINE release 1.12.
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http://g4beamline.muonsinc.com
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**************
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*Classes Used*
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**************
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1 - main()
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Note:
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// runManager->Initialize();
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is commented out and must be executed with: /run/initialize
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either in a macro or on the command line. Note: One can change
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the geometry (see below) only BEFORE the initialization!
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Is is possible to assign the PhysicsList via an argument to
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program submission:
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field04 -p QGSP_BERT
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and an initial random number seed with:
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field04 field04.in 12345
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It is also possible to specify more than one macro file which are
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then executed in succession:
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field04 field04.in field04.in 12345
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(Note, in this case the initial random # must also be provided)
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(1) argc holds the number of arguments on the command line. Since
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this MUST include at least the program name, argc value is >= 1!
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(2) argv[0] is always the name of the program
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argv[1] points to the first argument, and so on ...
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(3) argv[argc-1] is assumed to be the intial random # seed
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(4) the loop over macro files goes from optind = 1 to (argc-1)
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2- GEOMETRY DEFINITION
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The geometry consists of two solenoidal magnets: a "CaptureMgnt"
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followed by a (bloe-colored "TransferMgnt". By definition, the
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axis and center of the "CaptureMgnt" coincide with the "World". The
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position of the "TransferMgnt" relative to the downstream end of the
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"CaptureMgnt", as well as its axis angle, both may vary. A cylindrical
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"Target" is positioned inside the "CaptureMgnt". Its axis can vary
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from 0 to 180 deg, and hence also the direction of the incoming
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proton beam wrt the "CaptureMgnt"'s axis. A "Degrader" is located
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inside the "TransferMgnt", its default position being at the
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upstream end of the "TransferMgnt". Finally, also a "TestPlane" is
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located inside the "TransferMgnt", by default at its downstream end.
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The "World" consists of a solid cylinder made of a given material.
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(It is the responsibility of the user to make the world
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large enough to contain the rest of the geometry!)
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Three parameters define the world :
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- the material of the world,
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- the world radius,
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- the world length.
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Example (default values):
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/field04/SetWorldMat G4_AIR
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/field04/SetWorldR 5.0 m
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/field04/SetWorldZ 50.0 m
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The "Target" is a solid cylinder made of a given material.
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Five parameters define the target:
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- the material of the target,
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- the target radius,
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- the target thickness,
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- the target position inside the "CaptureMgnt",
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- the target axis angle relative to that of the "CaptureMgnt".
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Example (default values):
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/field04/SetTgtMat G4_W
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/field04/SetTgtRad 0.4 cm
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/field04/SetTgtThick 16.0 cm
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/field04/SetTgtPos 0.0 cm
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/field04/SetTgtAng 170
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The "Degrader" is a solid cylinder made of a given material.
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Four parameters define the degrader:
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- the material of the degrader,
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- the degrader radius,
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- the degrader thickness,
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- the degrader position relative to the "TransferMgnt" center.
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Example (default values):
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/field04/SetDgrMat G4_Pb
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/field04/SetDgrRad 30.0 cm
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/field04/SetDgrThick 0.1 cm
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#/field04/SetDgrPos -7.4 m
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The "CaptureMgnt" is a solenoid (vacuum cylinder). It is either
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a two-sided or a one-sided magnetic bottle with the B field
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varying linearly from the center value B1 to the edge value B2.
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The one-sided 'FocusSolenoid' has the open end at +z and focuses
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on the z < 0 side.
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Four parameters define the "CaptureMgnt":
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- the magnet radius,
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- the magnet length,
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- the weaker magnetic field at the center B1
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- the stronger magnetic field at the edge B2
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Example (default values):
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/field04/SetCaptureR 0.6 m
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/field04/SetCaptureZ 4.0 m
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/field04/SetCaptureB1 2.5 tesla
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/field04/SetCaptureB2 5.0 tesla
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The "TransferMgnt" is a solenoid (vacuum cylinder) with a
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constant B-field. When the "TransferMgnt" follows immediately
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the "CaptureMgnt", its relative position is at 0 cm.
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Four parameters define the "TransferMgnt":
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- the magnet radius,
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- the magnet length,
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- the magnet field,
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- the magnet relative position
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(its upstream face wrt the downstream face of the "CaptureMgnt".)
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Example (default values):
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/field04/SetTransferR 0.3 m
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/field04/SetTransferZ 15.0 m
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/field04/SetTransferB 5.0 tesla
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/field04/SetTransferP 0.0 m
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The default geometry is constructed in DetectorConstruction class,
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but all the parameters can be changed via the commands defined in
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the DetectorMessenger class.
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3- MATERIAL DEFINITION
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Material definitions are done through the singleton class Materials
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which keeps a pointer to the G4NistManager. It has a method
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GetMaterial by name (G4String) which in turn invokes the
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G4NistManager::FindOrBuildMaterial, and/or G4Material::GetMaterial
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methods. It has also a method CreateMaterials which, for materials
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absent from the NIST data base, shows how to create them using the
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G4NistManager::ConstructNewMaterial method.
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4- AN EVENT: THE PRIMARY GENERATOR
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The primary kinematic consists of a single particle which hits the
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target perpendicular to its upstream face. The type of the particle
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and its energy are set in the PrimaryGeneratorAction class, and can
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be changed via the G4 build-in commands of the ParticleGun class.
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In addition, there is a rndmFlag, which once set allows the beam to
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explore randomly the whole cross section of the target. The default
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beam consists of 500 MeV protons, starting at the upstream face of
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the target, directed along dx = dy = 0, dz = 1 wrt the target frame.
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The default direction should NOT be changed! The arguments of the
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x/y/zvertex commands are relative to the target center.
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Example:
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/gun/random on
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#/gun/xvertex 0 mm
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#/gun/yvertex 0 mm
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#/gun/zvertex -100 mm
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5- DETECTOR RESPONSE
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Information is extracted from the program via SteppingAction
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at the TestPlane.
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6- PHYSICS
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The PhysicsList is adopted from examples/extended/hadronic/Hadr01.
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It uses components which are distributed with Geant4 in
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/geant4/physics_lists subdirectory. So, before compiling field04
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it is necessary to compile physics_lists.
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There are two std::vector<G4VPhysicsConstructor*> PhysicsListVectors;
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one for EM- and one for HadronPHysics and a user defined process
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"StepMax".
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The choice of the physics is provided by the UI command:
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/exp/phys/addPhysics emstandard_opt1
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/exp/phys/addPhysics QGSP_BERT
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To see the list of available configurations one can use:
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/exp/phys/list
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The command:
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/exp/phys/addPhysics PHYSLIST
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allows to download a physics configuration defined by an
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environment variable PHYSLIST.
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The cuts for electromagnetic phsyics can be established via:
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/exp/phys/allCuts 1 mm
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/exp/phys/gammaCut 0.1 mm
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/exp/phys/electronCut 0.2 mm
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/exp/phys/positronCut 0.3 mm
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The cut for the StepMax process via:
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/exp/phys/stepMax
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The two PhysicsListVectors can be cleared via:
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/exp/phys/clearEMPhysics
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/exp/phys/clearHadronPhysics
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and individual processes can be removed from the vectors via,
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for example:
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/exp/phys/removeEMPhysics msc
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/exp/phys/removeHadronPhysics gamma_nuc
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Furthermore, the following commands are also available, but
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may only be used AFTER /run/initialize
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/process/inactivate msc
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/process/activate msc
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The decay of pions can be assigned via (pi -> e nu, pi -> mu nu):
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/decay/pienu
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/decay/pimunu
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The pienu assignment includes a small fraction of radiative decay:
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e nu gamma (G4PionRadiativeDecayChannel).
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The standard/default muon decay chain is modified to be 98.6%
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G4MuonDecayChannelWithSpin and 1.4% G4MuonRadiativeDecayChannelWithSpin
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in ConstructParticle().
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The pion decay process G4PolDecay inherits from G4Decay and implements
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the virtual method - empty in the base class - DaughterPolarization
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The muon decay process is G4DecayWithSpin
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7- Overlapping Fields
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The GlobalField (a singleton) is instantiated in DetectorConstruction()
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and assigned to the global field manager in updateField():
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fFieldManager = GetGlobalFieldManager();
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fFieldManager->SetDetectorField(this);
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The GlobalField has a std::vector<ElementField*> FieldList
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The field from each individual beamline element is given by a
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ElementField object. Any number of overlapping ElementField
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objects can be added to the global field. Any element that
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represents an element with an EM field must add the appropriate
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ElementField to the global GlobalField object.
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Of course, the GlobalField has the method GetFieldValue implemented.
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Before /run/initialize in the macro file or command, the update
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field command must have been issued if any of the other following
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field commands was employed:
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/field/update
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Other options are:
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/field/setStepperType 4
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/field/setMinStep 10 mm
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/field/setDeltaChord 3.0 mm
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/field/setDeltaOneStep 0.01 mm
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/field/setDeltaIntersection 0.1 mm
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/field/setEpsMin 2.5e-7 mm
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/field/setEpsMax 0.05 mm
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Each field element has a rectilinear bounding box in global
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coordinate space which is checked before a point is verified to
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actually be inside the ElementField (isWithin and isOutside).
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setGlobalPoint is called 8 times for the corners of the local
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bounding box, after a local->global coordinate transform.
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The ElementField is the interface class used by GlobalField to
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compute the field value at a given point[].
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A beamline element, for example the SimpleSolenoid, will derive
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from ElementField and implement the computation for the element.
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simpleSolenoid = new SimpleSolenoid(B, l,
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logicTransferMgnt,TransferMgntCenter);
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Besides the magnetic field and the length of the simple solenoid,
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the constructor needs the knowledge of the G4LogicalVolume for
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the beamline element and where its center is located in the
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'World'.
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The ElementField has a G4AffineTransform "global2local" which
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allows the quick computation of coordinate transformations. It can
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only be determined by knowing the element's coordinate origin in
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the global frame and after all of the geometry has been defined.
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For this reason, the object is prepared in two stages, through the
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constructor providing it with the coordinate center and a pointer
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to the G4LogicalVolume. Later the construct() method is called to
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calculate the global2local and the bounding box. This can be done
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from the RunAction::BeginOfRunAction method, for only then are we
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certain that the geometry has been completely built:
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FieldList* fields = GlobalField::getObject()->getFields();
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if (fields) {
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if (fields->size()>0) {
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FieldList::iterator i;
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for (i=fields->begin(); i!=fields->end(); ++i)(*i)->construct();
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}
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}
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The ElementField constructor will also add the derived object into
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GlobalField. Finally, its addFieldValue() will add the field value
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for this element to field[].
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8- User Action Classes
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RunActionMessenger:
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/rndm/save freq - to save rndm status in external files
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0 not saved
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>0 saved on: beginOfRun.rndm
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1 saved on: endOfRun.rndm
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2 saved on: endOfEvent.rndm
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/rndm/read random/run0evt8268.rndm
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RunAction:
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BeginOfRunAction: Deal with random number storage,
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initialization etc. Call the construct() method of
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ElementFields in the FieldList of GlobalField object.
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EndOfRunAction: random number storage/status printing.
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EventActionMessenger:
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/event/setverbose
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/event/drawTracks none/charged/all
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/event/printModulo
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EventAction(RunAction* RA):
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Customized BeginOfEvent printing (frequency: printModulo)
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EndofEvent: condition depending drawing of trajectories,
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saveEngingStatus and showEngineStatus according to flag
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in RunAction
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TrackingAction:
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PreUserTrackingAction: Instantiate UserTrackInformation
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and set the application TrackStatus.
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PostUserTrackingAction: Retreive UserTrackInformation
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and decide to save random number status accordingly.
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SteppingActionMessenger:
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SteppingAction:
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UserSteppingAction: Kill primary if/when outside Target
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volume. Diagnostic/histogram filling for particles at a
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TestPlane. Find decay position and when particle
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FIRST reverses z-momentum component via using a
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UserTrackInformation object.
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StackingAction:
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Track only primaries, pi+ or mu+
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UserTrackInformation:
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Keep a application TrackStatus for the track:
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undefined, left, right, reverse
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SteppingVerbose:
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Only print track header and step information for
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pi+ and mu+
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Trajectory, TrajectoryPoint:
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Example of application specific implementations
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9- HOW TO START ?
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- compile and link to generate an executable
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% cd $G4INSTALL/example/extended/field/field04
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% gmake
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- execute field04 in 'batch' mode from macro files e.g.
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% $(G4INSTALL)/bin/$(G4SYSTEM)/field04 field04.in
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- execute field04 in 'interactive' mode with visualization e.g.
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% $(G4INSTALL)/bin/$(G4SYSTEM)/field04
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....
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Idle> type your commands, for example:
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Idle> control/execute field04.in
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....
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