Geant4 User's Guide
For Application Developers
Tracking and Physics
5.6.1 General Concepts
In Geant4, physics processes use many tables of cross sections, energy losses
and other physics values. Before the execution of an event loop, the
BuildPhysicsTable() method of G4VProcess is invoked for all
processes and cross section tables are prepared. "Standard" electromagnetic
processes calculate cross section and/or energy loss values for each material
and for each production cut value assigned to each material. A change in
production cut values therefore require these cross sections to be
Cross sections for hadronic processes do not depend on the production cut.
The G4PhysicsTable class is used to handle cross section tables. G4PhysicsTable is a collection of instances of G4PhysicsVector (and derived classes), each of which has cross section values for a particle within a given energy range traveling in a material.
The G4ProductionCutsTable has G4MaterialCutsCouple objects, each of which consists of a material paired with a cut value. These G4MaterialCutsCouples are numbered with an index which is the same as the index of a G4PhysicsVector for the corresponding G4MaterialCutsCouplein the G4PhysicsTable. The list of MaterialCutsCouples used in the current geometry setup is updated before starting the event loop in each run.
Using the built-in user command "storePhysicsTable" (see Section 7.1 ), stores physics tables in files. Information on materials and cuts defined in the current geometry setup are stored together with physics tables because calculated values in the physics tables depend on MaterialCutsCouple. Note that physics tables are calculated before the event loop, not in the initialization phase. So, at least one event must be executed before using the "storePhysicsTable" command.
Calculated physics tables can be retrieved from files by using the "retrievePhysicsTable" command. Materials and cuts from files are compared with those defined in the current geometry setup, and only physics vectors corresponding to the MaterialCutsCouples used in the current setup are restored. Note that nothing happens just after the "retrievePhysicsTable" command is issued. Restoration of physics tables will be executed in parallel with the calculation of physics tables.
Initially, the G4VProcess::PreparePhysicsTable() method is invoked. Each process creates G4PhysicsTable objects as necessary. It then checks whether the MaterialCutsCouples have been modified after a run to determine if the corresponding physics vectors can be used in the next run or need to be re-calculated.
Next, the G4VProcess::RetrievePhysicsTable() method is invoked if the G4VUserPhysicsList::fRetrievePhysicsTable flag is asserted. After checking materials and cuts in files, physics vectors corresponding to the MaterialCutsCouples used in the current setup are restored.
Finally, the G4VProcess::BuildPhysicsTable() method is invoked and only physics vectors which need to be re-calculated are built.