In Geant4, physics processes use many tables of cross sections,
energy losses and other physics values. Before the execution of an
BuildPhysicsTable() methods of
G4VProcess are invoked for all processes and
as a part of initialisation procedure cross section tables are prepared.
Energy loss processes calculate cross section and/or energy loss values for
each pair of material and production cut value used in geometry for a give run.
A change in production cut values therefore require these
cross sections to be re-calculated. Cross sections for hadronic
processes and gamma processes do not depend on the production cut but sampling
of final state may depend on cuts, so full re-initilisation is performed.
G4PhysicsTable class is used to handle cross section
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. By default the linear interpolation is used, alternatively
spline may be used if the flag
of spline is activated by SetSpline method of the
Users can assign different production cuts to different regions (see Section 5.5). This means that if the same material is used in regions with different cut values, the processes need to prepare several different cross sections for that material.
objects, each of which consists of a material paired with a cut
G4MaterialCutsCouples are numbered with an
index which is the same as the index of a
for the corresponding
G4PhysicsTable. The list of
in the current geometry setup is updated before starting the event
loop in each run.
Calculated physics tables for electromagnetic processes can be stored in files. The user may thus eliminate the time required for the calculation of physics tables by retrieving them from the files.
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.
after the list of MaterialCutsCouples is updated, the
G4VUserPhysicsList::BuildPhysicsTable() method is invoked
to build physics tables for all processes.
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
G4VProcess::RetrievePhysicsTable() method is
invoked if the
flag is asserted. After checking materials and cuts in files,
physics vectors corresponding to the MaterialCutsCouples
used in the current setup are restored.
G4VProcess::BuildPhysicsTable() method is
invoked and only physics vectors which need to be re-calculated are
At the end of program
G4PhysicsTable should be deleted.
Before deletion of a table it should be cleaned up using the method
should be called in a middle of the run if an old table is removed and
a new one is created.