maria grazia pia, infn genova 1 part iii geant4 concepts and functionalities

Maria Grazia Pia, INFN Genova 1

Part III

Geant4 concepts and functionalities


The Geant4 kit

Code ~1M lines of code, ~2000 classes (continuously growing) publicly available from the web

Documentation 6 manuals publicly available from the web

Examples distributed with the code navigation between documentation and

examples code


Basic concepts

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 http://wwwinfo.cern.ch/asd/geant4/geant4.html

Run, Event Track, Step, Trajectory Process and Physics Sensitive detector and Hit


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)


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


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


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


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)

G4Box

G4Tubs

G4VSolid G4VPhysicalVolume

G4Material

G4VSensitiveDetector

G4PVPlacement

G4PVParametrized

G4VisAttributes

G4LogicalVolume


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)


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


Fields

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

StepChord

real trajectory


DAVID

New tool for geometry debugging DAVID allows to indentify intersecting volumes


Materials and particles

Materials 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


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


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


PhysicsPhysics

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.”


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


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


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


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.


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)


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


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


Hadronic physicsParameterised 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.


Hadronic physicsTheoretical 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


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!


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


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....


ESA projects in Geant4

Low energy extensions of electromagnetic physics Source Particle Module Radioactivity Decay Module Sector Shielding Analysis Tool CAD Tool Front-End


Detector response

Hits 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


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


Visualisation and UI

Visualisation 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


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


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...


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


User actions

Mandatory actions (abstract classes) 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


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


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


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


How Geant4 runs: initialization

main Run manager user detector const ruction

user physics list

1: initialize2 : const ruct

3: material const ruct ion

4: geometry construct ion5: world volume

6 : const ruct

7 : physics process const ruction

8: set cuts


How Geant4 runs

Initialization Construction of materials and geometry Construction of particles, physics

processes and calculation of physics tables

“Beam-On” = “Run” Close geometry Optimize geometry Event Loop


How Geant4 runs: event loop

main Run Manager Geometry manager

Event generator

EventManager

1: Beam On2: close

3: generate one event

4: process one event

5: open


How Geant4 runs: event processing

Event manager

Stacking manager

Tracking manager

Stepping manager

User sensitive detector

1: pop

2: process one track3: Stepping

4: generate hits

5: secondaries

6: push


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


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


Examples: extended level

ExampleE01 CLHEP histogramming

ExampleE02 Persistency by Objectivity/DB (CERN RD45)

ExampleE02 STEP geometry interface

Advanced level examples To be prepared Expect users’ contributions


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 http://wwwinfo.cern.ch/asd/geant4/reports/gallery/

Publication and Results web page http://wwwinfo.cern.ch/asd/geant4/reports/reports.html

maria grazia pia, infn genova 1 part iii geant4 concepts and functionalities

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