1. Geant4 concepts and functionalities
Part III
Geant4 concepts
and functionalities

2. The Geant4 kit Code Documentation Examples
~1M lines of code, ~2000 classes
(continuously growing)
publicly available from the web
Documentation
6 manuals
Examples
distributed with the code
navigation between documentation and examples code

3. Basic concepts Run, Event Track, Step, Trajectory Process and Physics
These lectures provide an overview of Geant4 basic concepts and functionality. Please refer to Geant4 User Documentation if you want to learn more, and contact your Geant4 Technical Board Representative, if you need further information.
Geant4 home page
Run, Event
Track, Step, Trajectory
Process and Physics
Sensitive detector and Hit

4. Run and Event
Run
Conceptually, a run is a collection of events which share the same detector condition
The Run Manager can handle multiple events
possibility to handle the pile-up
Multiple runs in the same job
with different geometries, materials etc.
Event
At the beginning of processing an event, primary particles are pushed into a stack
When the stack becomes empty, the processing of an event is done
Powerful stacking mechanism
three levels by default: can handle trigger studies, loopers etc.
It has following objects at the end of its processing:
List of primary vertices and particles
Trajectory collection (optional)
Hit collections
Digi collections (optional)

5. Interface to event generators
Through an ASCII file for the generators that support /HEPEVT/
currently used event generators are mostly in Fortran
no mixture with Fortran in Geant4
Interface to Pythia7
Abstract interface agreed between Geant4 and Pythia7 authors

6. Track, Step and Trajectory
A track is a snapshot of a particle
A step is a “” information to a track
A step has two points and “” information of a particle (energy loss on the step, time-of-flight spent by the step, etc.)
A track is deleted when
it goes out of the world volume
it disappears (e.g. decay)
it goes down below cut-off
the user decides to kill it
A track is made out of three layers of classes
G4Track
Position, volume, track length, global ToF
ID of mother track
G4DynamicParticle
Momentum, energy, local time, polarization
Decay channel
G4ParticleDefinition
Mass, lifetime, charge, other physical quantities
Decay table
A Trajectory is a record of a track. It stores information of all steps done by the track

7. Tracking
Decoupled from physics: all processes handled through the same abstract interface
Independent from particle type
It is possible to add new physics processes without affecting the tracking
Geant4 has only production thresholds,
no tracking cuts
all particles are tracked down to zero range
energy, TOF ... cuts can be defined by the user
The design choices allow to
optimise the performance
fully exploit the validity limits of physics processes
The design allows to handle full and fast simulation
Stepping algorithm based on Geant3 experience

8. Geometry Requirements to satisfy:
detailed description of the experimental set-up
efficient navigation performance
exchange with CAD
CAD exchange
interface through ISO STEP (Standard for the Exchange of Product Model Data)
Fundamental concepts
pure geometrical shape (solid)
non-positioned element (logical volume)
positioned element (physical volume)
single element (touchable, new concept)

9. Geometry representations
Multiple representations
CSG (Constructed Solid Geometries)
simple solids
STEP extensions
polyhedra
spheres
cylinders
cones
toroids
etc.
BREPS (Boundary REPresented Solids)
volumes defined by boundary surfaces
include solids defined by NURBS (Non-Uniform Rational B-Splines)

10. New geometry features New algorithm to position a volume
efficient memory usage
New navigation methods
hierarchy of volumes (Geant3)
flat geometry (CAD)
smart voxels (Geant4)
very efficient search in the volume database
Boolean operations between solids
External tool for g3tog4 geometry conversion

11. Fields miss distance Step Chord real trajectory
To propagate a particle inside a field (magnetic, electric, or mixture of them, etc.), we integrate the equation of motion of the particle in the field
Several Runge-Kutta methods are available for the integration
Once a method is chosen to calculate the track's motion in a field, the curved path is broken into linear chord segments
The chords are used to interrogate the Navigator, whether the track has crossed a volume boundary
One can set the accuracy of the volume intersection, by setting a parameter (the “miss distance”)
One step can consist of more than one chord
miss distance
Step
Chord
real trajectory

12. DAVID New tool for geometry debugging
DAVID allows to indentify intersecting volumes

13. Materials and particles
Geant4 provides tools to describe any
elements
isotopes
compounds
chemical formulae
Particles
Particles descriptions are compliant with PDG
The whole set of PDG data is contained in Geant4
and more, for specific Geant4 use, like ions

14. Processes
Processes describe how particles interact with material or with a volume itself
Three basic types
At rest process
(e.g. decay at rest)
Continuous process
(e.g. ionization)
Discrete process
(e.g. decay in flight)
Transportation is a process
interacting with volume boundary
A process which requires the shortest interaction length limits the step

15. Cuts In Geant4 the user defines cut-offs by length
(instead of energy, as it is usually done)
A cut-off represents the accuracy of the stopping position
It makes poor sense to use the energy cut-off:
Range of 10 keV  in Si ~ a few cm
Range of 10 keV electron in Si ~ a few m

16. Physics
From the Minutes of LCB (LHCC Computing Board) meeting on 21 October, 1997:
“It was noted that experiments have requirements for independent, alternative physics models. In Geant4 these models, differently from the concept of packages, allow the user to understand how the results are produced, and hence improve the physics validation. Geant4 is developed with a modular architecture and is the ideal framework where existing components are integrated and new models continue to be developed.”

17. The approach to physics
Ample variety of independent, alternative physics models available in Geant4
No more black boxes of packages
The users are directly exposed to the physics they use in their simulation
This approach is fundamental for the validation of the experiments’ physics results

18. Physics: general features
Abstract interface to physics processes
tracking independent from processes
Distinction between processes and models
often multiple models for the same process
Data encapsulation and polymorfism
Transparent access to cross sections, from files, interpolation from tables, analytical formulae etc.
Distinction between the calculation of cross sections and their use
Calculation of the final state independent from tracking
Uniform treatment of electromagnetic and hadronic physics
Open system
Users can easily create and use their own models

19. Data libraries
Systematic collection and evaluation of experimental data from many sources worldwide
Databases
ENDF/B, JENDL, FENDL, CENDL, ENSDF,JEF, BROND, EFF, MENDL, IRDF, SAID, EPDL, EEDL, EADL, SANDIA, ICRU etc.
Collaborating distribution centres
NEA, LLNL, BNL, KEK, IAEA, IHEP, TRIUMF, FNAL, Helsinki, Durham, Japan etc.
The use of evaluated data is important for the validation of physics results of the experiments

20. Electromagnetic physics
Comparable to Geant3 and EGS already in the -release
Substantial further extensions
Multiple alternatives for various processes
High energy extensions
models for  up to PeV
fundamental for LHC experiments, cosmic ray experiments etc.
Low energy extensions
e, down to 250 eV
(EGS, ITS etc. to 1 keV, Geant3 to 10 keV))
low energy hadrons and ions models based on Ziegler and ICRU data and parameterisations
models for antiprotons
fundamental for space and medical applications, neutrino experiments, antimatter spectroscopy etc.

21. E.M. processes in Geant4 multiple scattering energy loss
Bremsstrahlung
ionisation
annihilation
photoelectric effect
Compton scattering
pair production
synchrotron radiation
transition radiation
Cherenkov
Rayleigh effect
rifraction
reflection
absorption
scintillation
fluorescence
Auger (in progress)

22. Hadronic physics Completely different approach w.r.t. the past
transparent
native
no longer interface to external packages
clear separation between data and their use in algorithms
Cross section data sets
transparent and interchangeable
Final state calculation
models by particle, energy, material

23. Completeness of Geant4 hadronic physics
Ample variety of models
the most complete hadronic simulation kit on the market
alternative and complementary models
it is possible to mix-and-match, with fine granularity
data-driven, parameterised and theoretical models
Consequences for the users
no more confined to the black box of one package
the user has control on the physics used in the simulation, which contributes to the validation of physics results

24. Hadronic physics Parameterised and data-driven models
Based on experimental data
Some models originally from GHEISHA
completely reengineered into OO design
refined physics parameterisations
New parameterisations
pp, elastic differential cross section
nN, total cross section
pN, total cross section
np, elastic differential cross section
N, total cross section
N, coherent elastic scattering
Other models are completely new, such as
stopping particles (- , K- )
neutron transport
isotope production
All databases existing worldwide used in neutron transport
Brond, CENDL, EFF, ENDFB, JEF, JENDL, MENDL etc.

25. Hadronic physics Theoretical models
They fall into different parts
the evaporation phase
the low energy range, pre-equilibrium, O(100 MeV),
the intermediate energy range, O(100 MeV) to O(5 GeV), intra-nuclear transport
the high energy range, hadronic generator régime
Geant4 provides complementary theoretical models to cover all the various parts
Geant4 provides alternative models within the same part
All this is made possible by the powerful Object Oriented design of Geant4 hadronic physics
Easy evolution: new models can be easily added, existing models can be extended

26. Theoretical models Two String Models
Traditional Cascade Model, Kinetic Model, Quantum Molecular Dynamics Model
some are still in progress
Precompound Model
Evaporation, Fission, Fermi break-up, Multifragmentation, Photon Evaporation
Possibility to interface to Pythia7 for hard scattering
Lepton-hadron interactions
nucleus interactions, photo-fission, -meson conversion
...and more: area under active development!

27. Event biasing Geant4 provides facilities for event biasing
The effect consists in producing a small number of secondaries, which are artificially recognized as a huge number of particles by their statistical weights
Event biasing can be used, for instance, for the transportation of slow neutrons or in the radioactive decay simulation

28. Components for space applications
ESA participates in the Geant4 Collaboration
Various simulation packages previously used by ESA (and its contractors), including Geant3
Comparative evaluation of the main existing simulation packages: Geant3, ESABASE, Shieldose, HETC, LHI, MORSE, MCNP, ITS, MICAP, CEPX-ONELD, EGS
Specific requirements for space applications
environment
cosmic rays
Van Allen belts
fluxes of energetic particles (solar flares)
simulation studies (mission critical!) for
degradation of electronic components
single event logic-flips in electronic equipment
backgrounds
risks for astronauts etc....

29. ESA projects in Geant4
Low energy extensions of electromagnetic physics
Source Particle Module
Radioactivity Decay Module
Sector Shielding Analysis Tool
CAD Tool Front-End

30. Detector response Hits Digis Hits are user-defined Examples of hits:
position and time of the step
momentum and energy of the track
energy deposition of the step
Each geometrical volume can be associated to a sensitive detector
A sensitive detector creates hits using the information given in the Step object
Hit objects are collected in an Event object at the end of that event
Digis
They describe detector response (e.g. ADC/TDC count, trigger signal)
While Hits are generated at tracking time automatically, the digitization function must be explicitly invoked by the user’s code

31. Read-out geometry
Read-out geometry is a virtual and artificial geometry, which can be defined in parallel to the real detector geometry
A read-out geometry is associated to a sensitive detector

32. Visualisation and UI Visualisation User Interfaces Various drivers
OpenGL, OpenInventor, X11, Postscript, DAWN, OPACS, VRML...
Functionality for
detector, trajectories, steps, hits etc.
selections for, cut-views etc.
User Interfaces
Command-line, Tcl/Tk, Tcl/Java, batch+macros, OPACS, GAG, MOMO
The Graphic Geometry Editor (GGE)
automatic code generation for geometry and material description
The Graphic Physics Editor
same as above for physics processes

33. Communication
The intercoms category is used by almost all other Geant4 categories for exchanging information without having pointers
e.g. the user can apply the “abort event” command from the user stepping action without knowing the pointer to G4EventManager
(G)UI also accepts commands dynamically
G4UImanager receives the application of a command and passes it to a messenger
The messenger brings the command to the target destination class object

34. Persistency
The big HEP experiments require a large number of complex events to be stored
Possibility to run either in transient or in persistent mode
Persistency is handled through abstract interfaces to ODBMS
Geant4 does not depend on any specific persistency model
Open to any persistency model and to future evolutions
The user can implement the concrete persistency model that he/she likes
Persistency of Events, Geometry, Hits Trajectories...

35. Fast simulation
Geant4 allows to perform full simulation and fast simulation in the same environment
Geant4 parameterisation produces a direct detector response, from the knowledge of particle and volume properties
hits, digis, reconstructed-like objects (tracks, clusters etc.)
Great flexibility
activate fast /full simulation by detector
example: full simulation for inner detectors, fast simulation for calorimeters
activate fast /full simulation by geometry region
example: fast simulation in central areas and full simulation near cracks
activate fast /full simulation by particle type
example: in electromagnetic calorimeter e/ parameterisation and full simulation of hadrons
parallel geometries in fast/full simulation
example: inner and outer tracking detectors distinct in full simulation, but handled together in fast simulation

36. User actions Mandatory actions (abstract classes) Optional actions
detector construction
event generation
Optional actions
run, event, track, step, stacking actions
process-list, particle-list actions
Main
Geant4 does not provide the main()
In his/her main() the user must
construct G4RunManager (or its derived class)
perform mandatory actions
G4VUserDetectorConstruction
G4VUserPhysicsList
G4VUserPrimaryGeneratorAction
One can define VisManager, (G)UI session, optional user action classes and/or a persistency manager

37. The recipe: Describe your detector
Derive your own concrete class from G4VUserDetectorConstruction.
In the virtual method Construct()
Construct all necessary materials
Construct your detector geometry
Construct your sensitive detector classes and set them to the detector volumes
Optionally you can define visualization attributes of your detector elements

38. The recipe: Select physics processes
Geant4 does not have any default particles or processes
even for the particle transportation, you have to define it explicitly
Derive your own concrete class from G4VUserPhysicsList and
Define all necessary particles
Define all necessary processes and assign them to proper particles
Define cut-off range

39. The recipe: Generate primary event
Derive your concrete class from G4VUserPrimaryGeneratorAction
Pass a G4Event object to one or more primary generator concrete class objects
Geant4 provides two generators
G4ParticleGun
G4HEPEvtInterface
PYTHIA interface will be available when the C++ version of PYTHIA is ready

40. How Geant4 runs: initialization

41. How Geant4 runs Initialization “Beam-On” = “Run”
Construction of materials and geometry
Construction of particles, physics processes and calculation of physics tables
“Beam-On” = “Run”
Close geometry  Optimize geometry
Event Loop

42. How Geant4 runs: event loop

43. How Geant4 runs: event processing

44. Required hardware and software
Platforms
AIX, HP, DEC, Sun, (SGI): native compilers, , g++
Linux: g++
Windows-NT: Visual C++
Commercial software
ObjectStore STL (on platforms not yet supporting the Standard)
Free software
native STL (not supported yet by all platforms)
CVS
gmake, g++
CLHEP
Graphics
OpenGL, X11, OpenInventor, DAWN, VRML...
OPACS, GAG, MOMO...
Persistency
it is possible to run in transient mode
in persistent mode use a HepDB interface, ODMG standard

45. Examples: novice level
ExampleN01
Demonstrates how Geant4 kernel works
ExampleN02
Simplified tracker geometry with magnetic field
Electromagnetic processes
ExampleN03
Simplified calorimeter geometry
Various materials
ExampleN04
Simplified collider detector with readout geometry
EM + Hadronic processes
Pythia interface
Event filtering by stack
ExampleN05
Simplified BaBar-like calorimeter
Shower parameterization
ExampleN06
Optical photon processes

46. Examples: extended level
ExampleE01
CLHEP histogramming
ExampleE02
Persistency by Objectivity/DB (CERN RD45)
STEP geometry interface
Advanced level examples
To be prepared
Expect users’ contributions

47. Documentation http://wwwinfo.cern.ch/asd/geant4/geant4.html
User Documentation
Introduction to Geant4
Installation Guide
Geant4 User’s Guide - For Application Developers
for those wishing to use Geant4
Geant4 User’s Guide - For Toolkit Developers
for those wishing to extend Geant4 functionality
Software Reference Manual
documentation of the public interface of all Geant4 classes
Physics Reference Manual
extended documentation on Geant4 physics
Examples
a set of Novice, Extended and Advanced examples illustrating the main functionalities of Geant4 in realistic set-ups
The Gallery
a web collection of performance and physics evaluations
Publication and Results web page