Physics processes describe how particles interact with materials. Geant4 provides seven major categories of processes:
electromagnetic,
hadronic,
transportation,
decay,
optical,
photolepton_hadron, and
parameterisation.
All physics processes are derived from the G4VProcess base class. Its virtual methods
AtRestDoIt
,
AlongStepDoIt
, and
PostStepDoIt
and the corresponding methods
AtRestGetPhysicalInteractionLength
,
AlongStepGetPhysicalInteractionLength
, and
PostStepGetPhysicalInteractionLength
describe the behavior of a physics process when they are implemented in a derived class. The details of these methods are described in Section 5.2.
The following are specialized base classes to be used for simple processes:
AtRestDoIt
AlongStepDoIt
PostStepDoIt
Another 4 virtual classes, such as G4VContinuousDiscreteProcess, are provided for complex processes.
The G4ProcessManager class contains a list of processes that
a particle can undertake. It has information on the order of
invocation of the processes, as well as which kind of DoIt
method is valid for each process in the list. A
G4ProcessManager object corresponds to each particle and is
attached to the G4ParticleDefiniton class.
In order to validate processes, they should be registered with
the particle's G4ProcessManager. Process ordering
information is included by using the AddProcess()
and
SetProcessOrdering()
methods. For registration of simple
processes, the AddAtRestProcess()
,
AddContinuousProcess()
and AddDiscreteProcess()
methods may be used.
G4ProcessManager is able to turn some processes on or off
during a run by using the ActivateProcess()
and
InActivateProcess()
methods. These methods are valid only
after process registration is complete, so they must not be used in
the PreInit phase.
The G4VUserPhysicsList class creates and attaches
G4ProcessManager objects to all particle classes defined in
the ConstructParticle()
method.
G4VUserPhysicsList is the base class for a "mandatory user
class" (see Section 2.1), in which all physics
processes and all particles required in a simulation must be registered.
The user must create a class derived from
G4VUserPhysicsList and implement the pure virtual method
ConstructProcess()
.
For example, if just the G4Geantino particle class is
required, only the transportation process need be registered. The
ConstructProcess()
method would then be implemented as
follows:
Example 2.17. Register processes for a geantino.
void ExN01PhysicsList::ConstructProcess() { // Define transportation process AddTransportation(); }
Here, the AddTransportation()
method is provided in the
G4VUserPhysicsList class to register the
G4Transportation class with all particle classes. The
G4Transportation class (and/or related classes) describes
the particle motion in space and time. It is the mandatory process
for tracking particles.
In the ConstructProcess()
method, physics processes
should be created and registered with each particle's instance of
G4ProcessManager.
An example of process registration is given in the
G4VUserPhysicsList::AddTransportation()
method.
Registration in G4ProcessManager is a complex procedure for other processes and particles because the relations between processes are crucial for some processes. Please see Section 5.2 and the example codes.
An example of electromagnetic process registration for photons is shown below:
Example 2.18. Register processes for a gamma.
void MyPhysicsList::ConstructProcess() { // Define transportation process AddTransportation(); // electromagnetic processes ConstructEM(); } void MyPhysicsList::ConstructEM() { // Get the process manager for gamma G4ParticleDefinition* particle = G4Gamma::GammaDefinition(); G4ProcessManager* pmanager = particle->GetProcessManager(); // Construct processes for gamma G4PhotoElectricEffect * thePhotoElectricEffect = new G4PhotoElectricEffect(); G4ComptonScattering * theComptonScattering = new G4ComptonScattering(); G4GammaConversion* theGammaConversion = new G4GammaConversion(); // Register processes to gamma's process manager pmanager->AddDiscreteProcess(thePhotoElectricEffect); pmanager->AddDiscreteProcess(theComptonScattering); pmanager->AddDiscreteProcess(theGammaConversion); }