Run
Basic concept of Run
In Geant4, Run is the largest unit of simulation. A run
consists of a sequence of events. Within a run, the detector
geometry, the set up of sensitive detectors, and the physics
processes used in the simulation should be kept unchanged. A run is
represented by a G4Run class object. A run starts with
BeamOn() method of G4RunManager.
Representation of a run
G4Run represents a run. It has a run identification
number, which should be set by the user, and the number of events
simulated during the run. Please note that the run identification
number is not used by the Geant4 kernel, and thus can be
arbitrarily assigned at the user's convenience.
G4Run has pointers to the tables
G4VHitsCollection
and G4VDigiCollection. These tables are associated
in case sensitive detectors and
digitizer modules are
simulated, respectively. The usage of these tables will be
mentioned in and .
Manage the run procedures
G4RunManager manages the procedures of a run. In the
constructor of G4RunManager, all of the manager classes in
Geant4 kernel, except for some static managers, are constructed.
These managers are deleted in the destructor of
G4RunManager. G4RunManager
must be a singleton, and
the pointer to this singleton object can be obtained by the
getRunManager() static method.
As already mentioned in , all of the
user initialization classes and
user action classes
defined by the user should be assigned to G4RunManager
before starting initialization of the Geant4 kernel. The
assignments of these user classes are done by
SetUserInitialization() and SetUserAction()
methods. All user classes defined by the Geant4 kernel will be
summarized in .
G4RunManager has several public methods, which are listed
below.
Initialize()
All initializations required by the Geant4 kernel are triggered
by this method. Initializations are:
construction of the detector geometry and set up of sensitive
detectors and/or digitizer modules,
construction of particles and physics processes,
calculation of cross-section tables.
This method is thus mandatory before proceeding to the first run.
This method will be invoked automatically for the second and later
runs in case some of the initialized quantities need to be updated.
BeamOn(G4int numberOfEvent)
This method triggers the actual simulation of a run, that is,
an event loop. It takes an integer argument which represents the
number of events to be simulated.
GetRunManager()
This static method returns the pointer to the
G4RunManager singleton object.
GetCurrentEvent()
This method returns the pointer to the G4Event object
which is currently being simulated. This method is available only
when an event is being processed. At this moment, the application
state of Geant4, which is explained in the following sub-section,
is "EventProc". When Geant4 is in a state other than
"EventProc", this method returns
null.
Please note that the return value of this method is
const G4Event * and thus you cannot modify the
contents of the object.
SetNumberOfEventsToBeStored(G4int nPrevious)
When simulating the "pile up" of more than one event, it is
essential to access more than one event at the same moment. By
invoking this method, G4RunManager keeps nPrevious
G4Event objects. This method must be invoked before proceeding
to BeamOn().
GetPreviousEvent(G4int i_thPrevious)
The pointer to the i_thPrevious G4Event object can be
obtained through this method. A pointer to a const object
is returned. It is inevitable that i_thPrevious events
must have already been simulated in the same run for getting the
i_thPrevious event. Otherwise, this method returns
null.
AbortRun()
This method should be invoked whenever the processing of a run
must be stopped. It is valid for GeomClosed and
EventProc states. Run processing will be safely aborted even
in the midst of processing an event. However, the last event of the
aborted run will be incomplete and should not be used for further
analysis.
G4UserRunAction
G4UserRunAction is one of the user action
classes from which you can derive your own concrete class. This base class
has two virtual methods, as follows:
BeginOfRunAction()
This method is invoked at the beginning of the
BeamOn() method but after confirmation of the conditions
of the Geant4 kernel. Likely uses of this method include:
setting a run identification number,
booking histograms,
setting run specific conditions of the sensitive detectors
and/or digitizer modules (e.g., dead channels).
EndOfRunAction()
This method is invoked at the very end of the BeamOn()
method. Typical use cases of this method are
store/print histograms,
manipulate run summaries.
Geant4 as a state machine
Geant4 is designed as a state machine. Some methods in Geant4 are
available for only a certain state(s). G4RunManager controls
the state changes of the Geant4 application. States of Geant4 are
represented by the enumeration G4ApplicationState. It has
six states through the life cycle of a Geant4 application.
G4State_PreInit state
A Geant4 application starts with this state. The application
needs to be initialized when it is in this state. The application
occasionally comes back to this state if geometry, physics
processes, and/or cut-off have been changed after processing a
run.
G4State_Init state
The application is in this state while the
Initialize() method of G4RunManager
is being invoked. Methods defined in any
user initialization classes
are invoked during this state.
G4State_Idle state
The application is ready for starting a run.
G4State_GeomClosed state
When BeamOn() is invoked, the application proceeds to
this state to process a run. Geometry, physics processes, and
cut-off cannot be changed during run processing.
G4State_EventProc state
A Geant4 application is in this state when a particular event
is being processed. GetCurrentEvent() and
GetPreviousEvent() methods of
G4RunManager are
available only at this state.
G4State_Quit state
When the destructor of G4RunManager is invoked, the
application comes to this "dead end" state. Managers of the Geant4
kernel are being deleted and thus the application cannot come back
to any other state.
G4State_Abort state
When a G4Exception occurs, the application comes to this
"dead end" state and causes a core dump. The user still has a hook
to do some "safe" opperations, e.g. storing histograms, by
implementing a user concrete class of G4VStateDependent. The
user also has a choice to suppress the occurence of
G4Exception by a UI command
/control/suppressAbortion. When abortion is suppressed, you
will still get error messages issued by G4Exception, and there is
NO guarantee of a correct result after the G4Exception error
message.
G4StateManager belongs to the intercoms
category.
User's hook for state change
In case the user wants to do something at the moment of state
change of Geant4, the user can create a concrete class of the
G4VStateDependent base class. For example, the user can
store histograms when G4Exception occurs and Geant4 comes to the
Abort state, but before the actual core dump.
The following is an example user code which stores histograms
when Geant4 becomes to the Abort state. This class object
should be mabe in, for example main(), by the user code.
This object will be automatically registered to
G4StateManager at its construction.
Header file of UserHookForAbortState
#ifndef UserHookForAbortState_H
#define UserHookForAbortState_H 1
#include "G4VStateDependent.hh"
class UserHookForAbortState : public G4VStateDependent
{
public:
UserHookForAbortState(); // constructor
~UserHookForAbortState(); // destructor
virtual G4bool Notify(G4ApplicationState requiredState);
};
Source file of UserHookForAbortState
#include "UserHookForAbortState.hh"
UserHookForAbortState::UserHookForAbortState() {;}
UserHookForAbortState::~UserHookForAbortState() {;}
G4bool UserHookForAbortState::Notify(G4ApplicationState requiredState)
{
if(requiredState!=Abort) return true;
// Do book keeping here
return true;
}
Customizing the Run Manager
Virtual Methods in the Run Manager
G4RunManager is a concrete class with a complete set of
functionalities for managing the Geant4 kernel. It is the only
manager class in the Geant4 kernel which must be constructed in the
main() method of the user's application. Thus, instead of
constructing the G4RunManager provided by Geant4, you are
free to construct your own RunManager. It is recommended,
however, that your RunManager inherit
G4RunManager. For this purpose, G4RunManager has
various virtual methods which provide all the functionalities
required to handle the Geant4 kernel. Hence, your customized run
manager need only override the methods particular to your needs;
the remaining methods in G4RunManager base class can still
be used. A summary of the available methods is presented here:
public: virtual void Initialize();
main entry point of Geant4 kernel initialization
protected: virtual void InitializeGeometry();
geometry construction
protected: virtual void InitializePhysics();
physics processes construction
public: virtual void BeamOn(G4int n_event);
main entry point of the event loop
protected: virtual G4bool ConfirmBeamOnCondition();
check the kernel conditions for the event loop
protected: virtual void RunInitialization();
prepare a run
protected: virtual void DoEventLoop(G4int n_events);
manage an event loop
protected: virtual G4Event* GenerateEvent(G4int i_event);
generation of G4Event object
protected: virtual void AnalyzeEvent(G4Event* anEvent);
storage/analysis of an event
protected: virtual void RunTermination();
terminate a run
public: virtual void DefineWorldVolume(G4VPhysicalVolume * worldVol);
set the world volume to G4Navigator
public: virtual void AbortRun();
abort the run
Customizing the Event Loop
In G4RunManager the event loop is handled by the
virtual method DoEventLoop(). This method is implemented
by a for loop consisting of the following steps:
construct a G4Event object and assign to it primary
vertex(es) and primary particles. This is done by the virtual
GeneratePrimaryEvent() method.
send the G4Event object to G4EventManager
for the detector simulation. Hits and
trajectories will
be associated with the G4Event object as a
consequence.
perform bookkeeping for the current G4Event object.
This is done by the virtual AnalyzeEvent() method.
DoEventLoop() performs the entire simulation of an
event. However, it is often useful to split the above three steps
into isolated application programs. If, for example, you wish to
examine the effects of changing discriminator thresholds, ADC gate
widths and/or trigger conditions on simulated events, much time can
be saved by performing steps 1 and 2 in one program and step 3 in
another. The first program need only generate the hit/trajectory
information once and store it, perhaps in a database. The second
program could then retrieve the stored G4Event objects and
perform the digitization (analysis) using the above threshold, gate
and trigger settings. These settings could then be changed and the
digitization program re-run without re-generating the
G4Events.
Changing the Detector Geometry
The detector geometry defined in your
G4VUserDetectorConstruction concrete class can be changed
during a run break (between two runs). Two different cases are
considered.
The first is the case in which you want to delete the entire
structure of your old geometry and build up a completely new set of
volumes. For this case, you need to set the new world physical
volume pointer to the RunManager. Thus, you should proceed
in the following way.
G4RunManager* runManager = G4RunManager::GetRunManager();
runManager->DefineWorldVolume( newWorldPhys );
Presumably this case is rather rare. The second case is more
frequent for the user.
The second case is the following. Suppose you want to move
and/or rotate a particular piece of your detector component. This
case can easily happen for a beam test of your detector. It is
obvious for this case that you need not change the world volume.
Rather, it should be said that your world volume (experimental hall
for your beam test) should be big enough for moving/rotating your
test detector. For this case, you can still use all of your
detector geometries, and just use a Set method of a
particular physical volume to update the transformation vector as
you want. Thus, you don't need to re-set your world volume pointer
to RunManager.
If you want to change your geometry for every run, you can
implement it in the BeginOfRunAction() method of
G4UserRunAction class, which will be invoked at the
beginning of each run, or, derive the RunInitialization()
method. Please note that, for both of the above mentioned cases,
you need to let RunManager know "the geometry needs to be
closed again". Thus, you need to invoke
runManager->GeometryHasBeenModified();
before proceeding to the next run. An example of changing geometry
is given in a Geant4 tutorial in Geant4 Training kit #2.
Switch physics processes
In the InitializePhysics() method,
G4VUserPhysicsList::Construct is invoked in order to
define particles and physics processes in your application.
Basically, you can not add nor remove any particles during
execution, because particles are static objects in Geant4 (see
and
for details).
In addition, it is very difficult to add and/or remove physics
processes during execution, because registration procedures are
very complex, except for experts (see
and ).
This is why the initializePhysics() method is assumed
to be invoked at once in Geant4 kernel initialization.
However, you can switch on/off physics processes defined in your
G4VUserPhysicsList concrete class and also change parameters
in physics processes during the run break.
You can use ActivateProcess() and
InActivateProcess() methods of G4ProcessManager
anywhere outside the event loop to switch on/off some process. You
should be very careful to switch on/off processes inside the event
loop, though it is not prohibited to use these methods even in the
EventProc state.
It is a likely case to change cut-off values in a run. You can
change defaultCutValue in
G4VUserPhysicsList
during the Idle state. In this case, all cross section
tables need to be recalculated before the event loop. You should use the
CutOffHasBeenModified() method when you change cut-off
values so that the SetCuts method of your
PhysicsList concrete class will be invoked.