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Geant4 - an Object-Oriented Toolkit for Simulation in HEP
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field04 Example
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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
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*Classes Used*
**************
1 - main()
Note:
// runManager->Initialize();
is commented out and must be executed with: /run/initialize
either in a macro or on the command line. Note: One can change
the geometry (see below) only BEFORE the initialization!
Is is possible to assign the PhysicsList via an argument to
program submission:
field04 -p QGSP_BERT
and an initial random number seed with:
field04 field04.in 12345
It is also possible to specify more than one macro file which are
then executed in succession:
field04 field04.in field04.in 12345
(Note, in this case the initial random # must also be provided)
(1) argc holds the number of arguments on the command line. Since
this MUST include at least the program name, argc value is >= 1!
(2) argv[0] is always the name of the program
argv[1] points to the first argument, and so on ...
(3) argv[argc-1] is assumed to be the intial random # seed
(4) the loop over macro files goes from optind = 1 to (argc-1)
2- GEOMETRY DEFINITION
The geometry consists of two solenoidal magnets: a "CaptureMgnt"
followed by a (bloe-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 :
- the material of the world,
- the world radius,
- the world length.
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:
- the material of the target,
- the target radius,
- the target thickness,
- the target position inside the "CaptureMgnt",
- the target axis angle relative to that of the "CaptureMgnt".
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:
- the material of the degrader,
- the degrader radius,
- the degrader thickness,
- the degrader position relative to the "TransferMgnt" center.
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 'FocusSolenoid' has the open end at +z and focuses
on the z < 0 side.
Four parameters define the "CaptureMgnt":
- the magnet radius,
- the magnet length,
- the weaker magnetic field at the center B1
- the stronger magnetic field at the edge B2
Example (default values):
/field04/SetCaptureR 0.6 m
/field04/SetCaptureZ 4.0 m
/field04/SetCaptureB1 2.5 tesla
/field04/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":
- the magnet radius,
- the magnet length,
- the magnet field,
- the magnet relative position
(its upstream face wrt the downstream face of the "CaptureMgnt".)
Example (default values):
/field04/SetTransferR 0.3 m
/field04/SetTransferZ 15.0 m
/field04/SetTransferB 5.0 tesla
/field04/SetTransferP 0.0 m
The default geometry is constructed in DetectorConstruction class,
but all the parameters can be changed via the commands defined in
the DetectorMessenger class.
3- MATERIAL DEFINITION
Material definitions are done through the singleton class Materials
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.
4- AN EVENT: THE PRIMARY GENERATOR
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 PrimaryGeneratorAction class, and can
be changed via the G4 build-in commands of the ParticleGun class.
In addition, there is a rndmFlag, 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
5- DETECTOR RESPONSE
Information is extracted from the program via SteppingAction
at the TestPlane.
6- PHYSICS
The PhysicsList is adopted from examples/extended/hadronic/Hadr01.
It uses components which are distributed with Geant4 in
/geant4/physics_lists subdirectory. So, before compiling field04
it is necessary to compile physics_lists.
There are two std::vector<G4VPhysicsConstructor*> PhysicsListVectors;
one for EM- and one for HadronPHysics and a user defined process
"StepMax".
The choice of the physics is provided by the UI command:
/exp/phys/addPhysics emstandard_opt1
/exp/phys/addPhysics QGSP_BERT
To see the list of available configurations one can use:
/exp/phys/list
The command:
/exp/phys/addPhysics PHYSLIST
allows to download a physics configuration defined by an
environment variable PHYSLIST.
The cuts for electromagnetic phsyics can be established via:
/exp/phys/allCuts 1 mm
/exp/phys/gammaCut 0.1 mm
/exp/phys/electronCut 0.2 mm
/exp/phys/positronCut 0.3 mm
The cut for the StepMax process via:
/exp/phys/stepMax
The two PhysicsListVectors can be cleared via:
/exp/phys/clearEMPhysics
/exp/phys/clearHadronPhysics
and individual processes can be removed from the vectors via,
for example:
/exp/phys/removeEMPhysics msc
/exp/phys/removeHadronPhysics gamma_nuc
Furthermore, the following commands are also available, but
may only be used AFTER /run/initialize
/process/inactivate msc
/process/activate msc
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
7- Overlapping Fields
The GlobalField (a singleton) is instantiated in DetectorConstruction()
and assigned to the global field manager in updateField():
fFieldManager = GetGlobalFieldManager();
fFieldManager->SetDetectorField(this);
The GlobalField has a std::vector<ElementField*> FieldList
The field from each individual beamline element is given by a
ElementField object. Any number of overlapping ElementField
objects can be added to the global field. Any element that
represents an element with an EM field must add the appropriate
ElementField to the global GlobalField object.
Of course, the GlobalField 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 ElementField (isWithin and isOutside).
setGlobalPoint is called 8 times for the corners of the local
bounding box, after a local->global coordinate transform.
The ElementField is the interface class used by GlobalField to
compute the field value at a given point[].
A beamline element, for example the SimpleSolenoid, will derive
from ElementField and implement the computation for the element.
simpleSolenoid = new SimpleSolenoid(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 ElementField has a G4AffineTransform "global2local" 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 global2local and the bounding box. This can be done
from the RunAction::BeginOfRunAction method, for only then are we
certain that the geometry has been completely built:
FieldList* fields = GlobalField::getObject()->getFields();
if (fields) {
if (fields->size()>0) {
FieldList::iterator i;
for (i=fields->begin(); i!=fields->end(); ++i)(*i)->construct();
}
}
The ElementField constructor will also add the derived object into
GlobalField. Finally, its addFieldValue() will add the field value
for this element to field[].
8- User Action Classes
RunActionMessenger:
/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
RunAction:
BeginOfRunAction: Deal with random number storage,
initialization etc. Call the construct() method of
ElementFields in the FieldList of GlobalField object.
EndOfRunAction: random number storage/status printing.
EventActionMessenger:
/event/setverbose
/event/drawTracks none/charged/all
/event/printModulo
EventAction(RunAction* RA):
Customized BeginOfEvent printing (frequency: printModulo)
EndofEvent: condition depending drawing of trajectories,
saveEngingStatus and showEngineStatus according to flag
in RunAction
TrackingAction:
PreUserTrackingAction: Instantiate UserTrackInformation
and set the application TrackStatus.
PostUserTrackingAction: Retreive UserTrackInformation
and decide to save random number status accordingly.
SteppingActionMessenger:
SteppingAction:
UserSteppingAction: Kill primary if/when outside Target
volume. Diagnostic/histogram filling for particles at a
TestPlane. Find decay position and when particle
FIRST reverses z-momentum component via using a
UserTrackInformation object.
StackingAction:
Track only primaries, pi+ or mu+
UserTrackInformation:
Keep a application TrackStatus for the track:
undefined, left, right, reverse
SteppingVerbose:
Only print track header and step information for
pi+ and mu+
Trajectory, TrajectoryPoint:
Example of application specific implementations
9- HOW TO START ?
- compile and link to generate an executable
% cd $G4INSTALL/example/extended/field/field04
% gmake
- execute field04 in 'batch' mode from macro files e.g.
% $(G4INSTALL)/bin/$(G4SYSTEM)/field04 field04.in
- execute field04 in 'interactive' mode with visualization e.g.
% $(G4INSTALL)/bin/$(G4SYSTEM)/field04
....
Idle> type your commands, for example:
Idle> control/execute field04.in
....
Gabriele Cosmo