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This example shows how to define/use OVERLAPPING field elements in Geant4. Fields might be either magnetic, electric or both.
Credit goes to Tom Roberts and Muons Inc. since much of the code and ideas were taken at liberty from the (GNU GPL) source of G4BEAMLINE release 1.12.
http://g4beamline.muonsinc.com
See field04.cc.
The example can be run with the following optional arguments:
% field04 [-m macro ] [-p physicsList] [-r randomSeed] [-s preinit|idle]
If a macro is provided with the option "-m", the program runs in a batch mode, otherwise the program open the interactive session after executing the default initialization macro init_vis.mac. The option "-s preinit" can be used to start the program without initialization in PreInit phase.
For example: to assign the F04PhysicsList:
% field04 -p QGSP_BERT
an initial random number seed with:
% field04 field04.in -r 12345
to start with a macro file and an initial seed:
% field04 -m field04.in -r 12345
The geometry consists of two solenoidal magnets: a "CaptureMgnt" followed by a (blue-colored "TransferMgnt". By definition, the axis and center of the "CaptureMgnt" coincide with the "World". The position of the "TransferMgnt" relative to the downstream end of the "CaptureMgnt", as well as its axis angle, both may vary. A cylindrical "Target" is positioned inside the "CaptureMgnt". Its axis can vary from 0 to 180 deg, and hence also the direction of the incoming proton beam wrt the "CaptureMgnt"'s axis. A "Degrader" is located inside the "TransferMgnt", its default position being at the upstream end of the "TransferMgnt". Finally, also a "TestPlane" is located inside the "TransferMgnt", by default at its downstream end.
The "World" consists of a solid cylinder made of a given material. (It is the responsibility of the user to make the world large enough to contain the rest of the geometry!)
Three parameters define the world :
Example (default values):
/field04/SetWorldMat G4_AIR /field04/SetWorldR 5.0 m /field04/SetWorldZ 50.0 m
The "Target" is a solid cylinder made of a given material. Five parameters define the target:
Example (default values):
/field04/SetTgtMat G4_W /field04/SetTgtRad 0.4 cm /field04/SetTgtThick 16.0 cm /field04/SetTgtPos 0.0 cm /field04/SetTgtAng 170
The "Degrader" is a solid cylinder made of a given material.
Four parameters define the degrader:
Example (default values):
/field04/SetDgrMat G4_Pb /field04/SetDgrRad 30.0 cm /field04/SetDgrThick 0.1 cm #/field04/SetDgrPos -7.4 m
The "CaptureMgnt" is a solenoid (vacuum cylinder). It is either a two-sided or a one-sided magnetic bottle with the B field varying linearly from the center value B1 to the edge value B2. The one-sided F04FocusSolenoid has the open end at +z and focuses on the z < 0 side.
Four parameters define the "CaptureMgnt":
Example (default values):
/field04/SetCaptureR 0.6 m /field04/SetCaptureZ 4.0 m /field/SetCaptureB1 2.5 tesla /field/SetCaptureB2 5.0 tesla
The "TransferMgnt" is a solenoid (vacuum cylinder) with a constant B-field. When the "TransferMgnt" follows immediately the "CaptureMgnt", its relative position is at 0 cm.
Four parameters define the "TransferMgnt":
Example (default values):
/field04/SetTransferR 0.3 m /field04/SetTransferZ 15.0 m /field/SetTransferB 5.0 tesla /field04/SetTransferP 0.0 m
The default geometry is constructed in F04DetectorConstruction class, but all the parameters can be changed via the commands defined in the F04DetectorMessenger class.
Material definitions are done through the singleton class F04Materials which keeps a pointer to the G4NistManager. It has a method GetMaterial by name (G4String) which in turn invokes the G4NistManager::FindOrBuildMaterial, and/or G4Material::GetMaterial methods. It has also a method CreateMaterials which, for materials absent from the NIST data base, shows how to create them using the G4NistManager::ConstructNewMaterial method.
The primary kinematic consists of a single particle which hits the target perpendicular to its upstream face. The type of the particle and its energy are set in the F04PrimaryGeneratorAction class, and can be changed via the G4 build-in commands of the G4ParticleGun class. In addition, there is a fRndmFlag, which once set allows the beam to explore randomly the whole cross section of the target. The default beam consists of 500 MeV protons, starting at the upstream face of the target, directed along dx = dy = 0, dz = 1 wrt the target frame. The default direction should NOT be changed! The arguments of the x/y/zvertex commands are relative to the target center.
Example:
/gun/random on #/gun/xvertex 0 mm #/gun/yvertex 0 mm #/gun/zvertex -100 mm
Information is extracted from the program via F04SteppingAction at the TestPlane.
The F04PhysicsList extends a selected Geant4 physics list. The base physics list name is provided by its name in the F04PhysicsList constructor.
In addition to processes defined in the base Geant4 physics list, there is added the F04StepMax process and the decay of pions can be assigned via dedicated commands in F04PhysicsListMessenger.
The command to define maximum step:
/exp/phys/stepMax value unit
The decay of pions can be assigned via (pi -> e nu, pi -> mu nu):
/decay/pienu /decay/pimunu
The pienu assignment includes a small fraction of radiative decay: e nu gamma (G4PionRadiativeDecayChannel).
The standard/default muon decay chain is modified to be 98.6% G4MuonDecayChannelWithSpin and 1.4% G4MuonRadiativeDecayChannelWithSpin in ConstructParticle().
The pion decay process G4PolDecay inherits from G4Decay and implements the virtual method - empty in the base class - DaughterPolarization
The muon decay process is G4DecayWithSpin
Furthermore, the following commands are also available, but may only be used AFTER /run/initialize
/process/inactivate msc /process/activate msc
The F04GlobalField (a singleton) is instantiated in F04DetectorConstruction() and assigned to the global field manager in UpdateField():
fFieldManager = GetGlobalFieldManager(); fFieldManager->SetDetectorField(this);
The F04GlobalField has a std::vector<ElementField*> FieldList
The field from each individual beamline element is given by a F04ElementField object. Any number of overlapping F04ElementField objects can be added to the F04GlobalField. Any element that represents an element with an EM field must add the appropriate F04ElementField to the global F04GlobalField object.
Of course, the F04GlobalField has the method GetFieldValue implemented.
Before /run/initialize in the macro file or command, the update field command must have been issued if any of the other following field commands was employed:
/field/update
Other options are:
/field/setStepperType 4 /field/setMinStep 10 mm /field/setDeltaChord 3.0 mm /field/setDeltaOneStep 0.01 mm /field/setDeltaIntersection 0.1 mm /field/setEpsMin 2.5e-7 mm /field/setEpsMax 0.05 mm
Each field element has a rectilinear bounding box in global coordinate space which is checked before a point is verified to actually be inside the F04ElementField (IsWithin and IsOutside). SetGlobalPoint is called 8 times for the corners of the local bounding box, after a local->global coordinate transform.
The F04ElementField is the interface class used by F04GlobalField to compute the field value at a given point[].
A beamline element, for example the F04SimpleSolenoid, will derive from F04ElementField and implement the computation for the element.
simpleSolenoid = new F04SimpleSolenoid(B, l, logicTransferMgnt,TransferMgntCenter);
Besides the magnetic field and the length of the simple solenoid, the constructor needs the knowledge of the G4LogicalVolume for the beamline element and where its center is located in the 'World'.
The F04ElementField has a G4AffineTransform "fGlobal2local" which allows the quick computation of coordinate transformations. It can only be determined by knowing the element's coordinate origin in the global frame and after all of the geometry has been defined. For this reason, the object is prepared in two stages, through the constructor providing it with the coordinate center and a pointer to the G4LogicalVolume. Later the Construct() method is called to calculate the fGlobal2local and the bounding box. This can be done from the F04RunAction::BeginOfRunAction method, for only then are we certain that the geometry has been completely built:
FieldList* fields = F04GlobalField::GetObject()->GetFields();
if (fields) {
if (fields->size()>0) {
FieldList::iterator i;
for (i=fields->begin(); i!=fields->end(); ++i)(*i)->Construct();
}
}
The F04ElementField constructor will also add the derived object into F04GlobalField. Finally, its AddFieldValue() will add the field value for this element to field[].
/rndm/save freq - to save rndm status in external files
0 not saved
>0 saved on: beginOfRun.rndm
1 saved on: endOfRun.rndm
2 saved on: endOfEvent.rndm
/rndm/read random/run0evt8268.rndm
/event/setverbose
% field04 -m field04.in
% field04 .... Idle> type your commands ....
% field04 -s preinit .... Idle> type your commands, then Idle> /run/initialize Idle> /control/execute vis.mac ....
1.9.8