Physics List Guide

The Physics List is one of the three mandatory user classes of the Geant4 toolkit. In this class all Geant4 particles and their interaction processes should be instantiated. This class should inherit from the base class G4VUserPhysicsList and should be given to G4RunManager:

G4MTRunManager* runManager = new G4MTRunManager;
runManager->SetUserInitialization(physicsList);

Here physicsList is a pointer to the user defined class. Initially [eal03], it was proposed for users to create custom class based on variants of PhysicsList in Geant4 example applications. After first Geant4 official releases, a conception of reference Physics Lists was introduced [eal06]. In the beginning, the default Geant4 Physics List was QGSP_BERT [eal09]. Since the Geant4 release 10.0 the default Physics List becomes FTFP_BERT [eal16]. The main advantage of working with the reference Physics List is in common method of instantiation of physics objects in Geant4 tests, in Geant4 examples, and in user applications. Geant4 developers develop and validate these physics configurations and any user or group of users may reproduce the same physics in their Geant4 applications. Geant4 developers establish various tests and benchmarks, which are used for validation and verification of the Geant4 toolkit before making a new public version. Users may compare results obtained in the same conditions in different setups.

Number of reference Physics Lists is not small, because there are many alternative physics models in the toolkit applicable for simulation of very different problems. Reference Physics Lists are available in Geant4 physics_list sub-library. All these classes inherit from virtual interface G4VModularPhysicsList (which is an extension of the base class G4VUserPhysicsList. The advantages of the modular design are in maintenance of modules by different Geant4 working groups, independent development of particular modules, possibility for combining of various modules (following G4VPhysicsConstructor interface) in reference Physics Lists and in user custom Physics Lists. There are following types of modules:

  • electromagnetic physics;

  • extra physics processes for gamma and leptons;

  • decay;

  • hadron elastic;

  • hadron inelastic;

  • stopping particles capture processes;

  • ion nuclear interactions;

  • step limiters;

  • others.

The last category may include any type of physics processes, for example, optical, exotic physics, thermal neutron transport model, and others. A user may customise reference Physics List using following interfaces of the G4VModularPhysicsList class:

  • void RegisterPhysics(G4VPhysicsConstructor* );

  • void ReplacePhysics(G4VPhysicsConstructor* );

  • void RemovePhysics(G4VPhysicsConstructor* );

The following reference Physics Lists are available in the physics_lists sub-library:

  • FTFP_BERT

  • FTFP_BERT_ATL

  • FTFP_BERT_HP

  • FTFP_BERT_TRV

  • FTFP_INCLXX

  • FTFQGSP_BERT

  • FTF_BIC

  • LBE

  • NuBeam

  • QBBC

  • QBBC_ABLA

  • QGSP_BERT

  • QGSP_BERT_HP

  • QGSP_BIC

  • QGSP_BIC_AllHP

  • QGSP_BIC_HP

  • QGSP_BIC_HPT

  • QGSP_FTFP_BERT

  • QGSP_INCLXX

  • QGS_BIC

  • Shielding

  • ShieldingLEND

These Physics List classes may be included directly to the user code. It is also possible instantiate reference Physics List by name using helper class G4PhysListFactory. Variants of usage of this helper class are demonstrated in Geant4 extended examples:

  • geant4/examples/extended/hadronic/Hadr00.cc - for multi-threaded mode;

  • geant4/examples/extended/hadronic/Hadr01.cc - for sequential mode.

In the case of usage of this helper class, an additional possibility does exist to extend electromagnetic physics configuration by simply adding an extension to a physics list name, for example FTFP_BERT_EMZ means , that the default electromagnetic physics is substituted by the configuration providing the most accurate simulation of electromagnetic physics (see details in EM physics constructors). Following extensions are available:

  • EMV EM Opt1 less precise, but faster set of electromagnetic physics is used. Otherwise known as electromagnetic option 1.

  • EMX EM Opt2 less precise, but faster set of electromagnetic physics is used. Otherwise known as electromagnetic option 2.

  • EMY EM Opt3 it uses a set of EM processes with accurate simulation of gamma and charged particle transport. Only the Urban multiple scattering model is used for all charged particles and all energies. Also known as electromagnetic option 3, the detailed physics causes longer execution times than the standard package.

  • EMZ EM Opt4 the best set of electromagnetic physics models selected from the low energy and standard packages. With its concentration on the best possible physics, electromagnetic option 4 is slower than the standard EM package.

  • LIV EM Liv is made on top of electromagnetic option 3 by substitution of standard models for gamma and electrons from Livermore set of models.

  • PEN EM Pen is made on top of electromagnetic option 3 by substitution of standard models for gamma, electrons and positrons from Penelope-2008 set of models.

  • _GS EM GS is made on top of the default electromagnetic configurations by substitution of the Urban multiple scattering model for electrons and positrons by the Goudsmit-Saunderson model.

  • _LE EM LE is made on top of the default electromagnetic configurations by substitution of the Urban multiple scattering model for electrons and positrons by the LowEWentzelVI model. Also, using 5D gamma conversion model and Lindhard-Sorensent model for ion ionisation.

  • WVI EM WVI is made on top of the default electromagnetic configurations by substitution of the Urban multiple scattering model for electrons and positrons by the WentzelVI model and ATIMA ion ionisation model.

  • _SS EM SS is made on top default electromagnetic configurations by substitution of all multiple scattering models by single scattering models.

The additions on top of any reference Physics list may be implemented via the ReplacePhysics interface:

  • G4RadioactiveDecayPhysics

  • G4NeutrinoPhysics

  • G4ChargeExchangePhysics

  • G4OpticalPhysics

For those using shared object libraries, the extensible physics list factory g4alt::G4PhysListFactory is an option. This factory extends the capability of the original factory and can be substituted in by changing the include header and adding using namespace g4alt. This code is demonstated in the extended example.

  • geant4/examples/extended/physicslists/extensibleFactory

This factory is different from the one described above by allowing one’s own custom physics lists to be registered with the factory, as well as allowing for the customization of physics lists with physics constructors. The later uses “_” for ReplacePhysics() and “+” for RegisterPhysics(), such that valid physics lists might look like:

  • FTFP_BERT_HP_EMZ+G4RadioactiveDecayPhysics which starts with a standard physics list FTFP_BERT_HP, substitutes the EMZ electromagnetic configuration, and adds radioactive decay.

  • MySpecialPhysList_GS+G4OpticalPhysics+G4NeutronTrackCut which uses a pre-registered custom build physics list, substitutes _GS EM physics, and adds G4OpticalPhysics and G4NeutronTrackCut.

Bibliography

eal06

J. Allison et al. Geant4 developments and applications. IEEE Transactions on Nuclear Science, 53:270–278, feb 2006. URL: http://ieeexplore.ieee.org/document/1610988/?reload=true, doi:10.1109/TNS.2006.869826.

eal16

J. Allison et al. Recent developments in geant4. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 835:186–225, nov 2016. URL: https://doi.org/10.1016/j.nima.2016.06.125, doi:10.1016/j.nima.2016.06.125.

eal09

J. Apostolakis et al. Geometry and physics of the geant4 toolkit for high and medium energy applications. Radiation Physics and Chemistry, 78(10):859–873, oct 2009. URL: https://doi.org/10.1016/j.radphyschem.2009.04.026, doi:10.1016/j.radphyschem.2009.04.026.

eal03

S. Agostinelli et al. Geant4—a simulation toolkit. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 506(3):250–303, jul 2003. URL: https://doi.org/10.1016/S0168-9002(03)01368-8, doi:10.1016/s0168-9002(03)01368-8.