Transcript Document 7261246

Geant4
- a simulation toolkit Makoto Asai (SLAC Computing Services)
On behalf of the Geant4 Collaboration
December 1st, 2003
ACAT03 @ KEK
Contents

General introduction and brief history

Geant4 kernel

Geometry

Physics

Highlights of the new developments

Highlights of user applications

User support processes

Summary
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General introduction
and brief history
What is Geant4?

Geant4 is an object-oriented toolkit for simulation of elementary
particles passing through and interacting with matter. It includes a
complete range of functionality including tracking, geometry, physics
models and hits.

Geant4's application area includes high energy and nuclear physics
experiments, accelerator and shielding studies, space engineering,
medical physics and several other fields.

Geant4 is the successor of GEANT3, the world-standard toolkit for HEP
detector simulation. Geant4 is one of the first successful attempt to
re-design a major package of HEP software for the next generation of
experiments using an Object-Oriented environment.
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Flexibility of Geant4

In order to meet wide variety of requirements from various application
fields, a large degree of functionality and flexibility are provided.

Geant4 has many types of geometrical descriptions to describe most
complicated and realistic geometries


CSG, BREP, Boolean

XML interface
The physics processes offered cover a comprehensive range including
electromagnetic, hadronic and optical processes for a large set of longlived particles in materials and elements, over a wide energy range. The
applicable energy range begins, in some cases, from 100 eV and
extends up to the TeV energy range.
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Physics in Geant4


It is rather unrealistic to develop a uniform physics model to cover wide
variety of particles and/or wide energy range.
Much wider coverage of physics comes from mixture of theory-driven,
parameterized, and empirical formulae. Thanks to polymorphism
mechanism, both cross-sections and models (final state generation) can
be combined in arbitrary manners into one particular process.

Standard EM processes

Low energy EM processes

Hadronic processes

Photon/lepton-hadron processes

Optical photon processes

Decay processes

Shower parameterization

Event biasing technique
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Geant4 – Its history and future

Dec ’94 - Project start

Apr ’97 - First alpha release

Jul ’98 - First beta release

Dec ’98 - Geant4 0.0 (first public) release

Jul ’99 - Geant4 0.1 release

…

Jun ’03 - Geant4 5.2 release

Dec 12th, ’03 - Geant4 6.0 release (planned)

We currently provide two to three public releases and bimonthly
beta releases in between public releases every year.
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HARP
Geant4 Collaboration
Univ. Barcelona
Lebedev
Helsinki Inst. Ph.
PPARC
Collaborators also from nonmember institutions, including
Budker Inst. of Physics
IHEP Protvino
MEPHI Moscow
Pittsburg University
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Geant4 kernel
Geant4 kernel

Geant4 consists of 17 categories.

Geant4
Independently developed and
maintained by WG(s) responsible to
Visuali
zation
each category.

Run
Persis
tency
Interfaces between categories (e.g.
top level design) are maintained by
Inter
faces
Readout
Event
Tracking
the global architecture WG.

Digits +
Hits
Geant4 Kernel

Handles run, event, track, step, hit,
trajectory.

Processes
Track
Geometry
Particle
Graphic
_reps
Material
Provides frameworks of geometrical
representation and physics processes.
Intercoms
Global
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Tracking and processes

Geant4 tracking is general.


It is independent to

the particle type

the physics processes involving to a particle
It gives the chance to all processes

To contribute to determining the step length

To contribute any possible changes in physical quantities of the
track

To generate secondary particles

To suggest changes in the state of the track

e.g. to suspend, postpone or kill it.
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Processes in Geant4

In Geant4, particle transportation is a process as well, by which a particle
interacts with geometrical volume boundaries and field of any kind.

Because of this, for example, shower parameterization process can
take over from the ordinary transportation without modifying the
transportation process.

Each particle has its own list of applicable processes. At each step, all
processes listed are invoked to get proposed physical interaction lengths.

The process which requires the shortest interaction length (in spacetime) limits the step.
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Cuts in Geant4


A Cut in Geant4 is a production threshold.
 Only for physics processes that have infrared divergence
 Not tracking cut, which does not exist in Geant4
Energy threshold must be determined at which discrete energy loss is
replaced by continuous loss
 Old way:
 Track primary particle until cut-off energy is reached, calculate
continuous loss and dump it at that point, stop tracking primary
 Create secondaries only above cut-off energy, or add to
continuous loss of primary for less energetic secondaries
 Geant4 way:
 Specify range (which is converted to energy for each material) at
which continuous loss begins, track primary particle one more step
to make it down to zero range
 Create secondaries only above specified range, or add to
continuous loss of primary for less energetic secondaries
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Energy cut vs. range cut



500 MeV/c proton in liq.Ar (4mm) / Pb (4mm) sampling calorimeter
Geant3 (energy cut)
 Ecut = 450 keV
Geant4 (range cut)
 Rcut = 1.5 mm
 Corresponds to
Ecut in liq.Ar = 450 keV, Ecut
in Pb = 2 MeV
liq.Ar
Pb
liq.Ar
Pb
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Geometry
Volume


Three conceptual layers

G4VSolid -- shape, size

G4LogicalVolume -- daughter physical volumes,

material, sensitivity, user limits, etc.
G4VPhysicalVolume -- position, rotation
Hierarchal three layers of geometry description allows maximum reuse
of information to minimize the use of memory space.
G4VSolid
G4Box
G4Tubs
G4LogicalVolume
G4Material
G4VisAttributes
G4VSensitiveDetector
G4VPhysicalVolume
G4PVPlacement
G4PVParameterised
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Solid

Geant4 geometry module supports variety of
representations of shapes.

CSG (Constructed Solid Geometry) solids



G4Polycone, G4Polyhedra, G4Hype, …
BREP (Boundary REPresented) solids



Analogous to simple GEANT3 CSG
solids
Specific solids (CSG like)


G4Box, G4Tubs, G4Cons, G4Trd, …
G4BREPSolidPolycone,
G4BSplineSurface, …
Any order surface
Boolean solids

G4UnionSolid, G4SubtractionSolid, …
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Physical volume





G4PVPlacement
1 Placement = One Volume
 A volume instance positioned once in its mother volume
G4PVParameterised
1 Parameterized = Many Volumes
 Parameterized by the copy number
 Shape, size, material, position and rotation can be parameterized,
by implementing a concrete class of G4VPVParameterisation.
 Reduction of memory consumption
G4PVReplica
1 Replica = Many Volumes
 Slicing a volume into smaller pieces (if it has a symmetry)
G4ReflectionFactory
1 Placement = a set of Volumes
 Generating a pair of placements of a volume and its reflected volume
 Useful typically for end-cap calorimeter
G4AssemblyVolume
1 Placement = a set of Placements
 Position a group of volumes
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Smart voxelization


In case of Geant 3.21, the user had to carefully implement his/her
geometry to maximize the performance of geometrical navigation.
While in Geant4, user’s geometry is automatically optimized to most
suitable to the navigation. - "Voxelization"

For each mother volume, one-dimensional virtual division is performed.

Subdivisions (slices) containing same volumes are gathered into one.


Additional division again using second and/or third Cartesian axes, if
needed.
"Smart voxels" are computed at initialisation time

When the detector geometry is closed

Does not require large memory or computing resources

At tracking time, searching is done in a hierarchy of virtual divisions
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Geometry checking tools




An overlapping volume is a contained volume
which actually protrudes from its mother
volume
 Volumes are also often positioned in a
same volume with the intent of not
provoking intersections between
themselves. When volumes in a common
mother actually intersect themselves are
defined as overlapping
Geant4 does not allow for malformed
geometries
The problem of detecting overlaps between
volumes is bounded by the complexity of the
solid models description
Utilities are provided for detecting wrong
positioning
 Graphical tools
 Kernel run-time commands
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Physics
Physics processes in Geant4







Each process can act at any of three space-time intervals
 In time - At rest (e.g. decay at rest)
 Continuously along a step (e.g. Cherenkov radiation)
 At a point - at the end of the step (e.g. decay in flight)
“Along step actions” are applied cumulatively, while others are selectively
applied.
A process can have more than one types of action according to its nature.
 For example, Ionization process has Along and End step actions
Tracking handles each type of action in turn.
 It loops over all processes with such a type of action.
The motivation for creating these categories of actions is to keep the
tracking independent of the physics processes.
All seven combinations of actions are possible.
 The traditional Continuous, Continuous-Discrete, Discrete and AtRest
are found in these cases.
The ordering of processes is important in some cases.
 e.g. Multiple scattering affects the step length.
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Electromagnetic processes





Gammas :
 Gamma-conversion, Compton scattering, Photo-electric effect
Leptons (e, m), charged hadrons, ions :
 Energy loss (Ionisation, Bremstrahlung) or PAI model energy
loss, Multiple scattering, Transition radiation, Synchrotron
radiation,
Optical photons :
 Cerenkov, Rayleigh, Reflection, Refraction, Absorption,
Scintillation
High energy m
Alternative implementation
 Standard EM package ignores the binding energy of electron to
an atom, while Low Energy EM package takes it into account.
 ‘Standard’ for applications that do not need to go below 1 KeV
 ‘Low Energy’: down to 250eV (e+/g), O(0.1mm) for hadrons
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Multiple
scattering


Examples of
comparisons:
15.7 MeV eon gold foil
Modelling &
comparisons:
L. Urban
Angle (deg)
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Hadronic and Photolepton-hadron
processes

Each hadronic process may have one or more cross section data sets,
and final state production models associated with it. Each one has its
own energy range of applicability.

The term “data set” is meant in a broad sense to be an object
that encapsulates methods and data for calculating total cross
sections.

The term “model” is meant in a broad sense to be an object that
encapsulates methods and data for calculating final state
products.
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Parameterization and data driven models


Parameterization models on the fly

high energy inelastic (Aachen, CERN)

low energy inelastic, elastic, fission, capture (TRIUMF, UBC, CERN, SLAC)
Parameterization models for stopping particles

base line (TRIUMF, CHAOS)

mu- (TRIUMF, FIDUNA)

pi- (INFN, CERN, TRIUMF)

K- (Crystal Barrel, TRIUMF)

anti-protons (JLAB, CERN)


Electromagnetic transitions of the exotic atom prior to capture; effects of
atomic binding. (Novosibirsk, ESA)
Data driven models
 Low energy neutron transport (neutron_hp),
 Radioactive decay (DERA, ESA)
 photon evaporation (INFN)
 elastic scattering (TRIUMF, U.Alberta, CERN)
 internal conversion (ESA)
 etc..
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Theory driven models




Ultra-high energy models
 Parton transport model (U.Frankfurt, in discussion)
High energy models
 ‘Fritjof’ type string model (CERN)
 Quark gluon String model (CERN)
 Pythia(7) interface (Lund, CERN)
Intra-nuclear transport models (or replacements)
 Hadronic cascade+pre-equilibrium model (HIP, CERN)
 Binary and Bertini cascade models (HIP, CERN, Novosibirsk, SLAC)
 QMD type models (CERN, Inst.Th.Phys. Frankfurt)
 Chiral invariant phase-space decay model (JLAB, CERN, ITEP)
 Partial Mars rewrite (Kyoto, Uvic, in collaboration with FNAL)
De-excitation
 Evaporation, fission, multi-fragmentation, fermi-break-up (CMS)
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Hadronic Model Inventory
CHIPS
At rest
Absorption
m, p, K, anti-p
CHIPS (gamma)
Photo-nuclear, electro-nuclear
High precision neutron
Evaporation
Fermi breakup
PreMultifragment
compound
Photon Evap
Rad. decay
FTF String (up to 20 TeV)
Bertini cascade
QG String (up to 100 TeV)
Binary cascade
Fission
MARS
LE pp, pn
HEP ( up to 20 TeV)
LEP
1 MeV
10 MeV
100 MeV
1 GeV
10 GeV
100 GeV
1 TeV
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Verification


The verification effort of the geant4 hadronic working group is grouped
into several sections:

Inclusive cross-sections

Thin target comparisons

Verification of model components

Code comparisons (least effective)

Complete application tests

Robustness.
A few examples of each are given in the following slides.
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Proton reaction
total
cross-section:
J.P.Wellisch
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Pion production examples, QGS:
Rapidity distributions and invariant
cross-section predictions in quark gluon string model
100 GeV pi+ on Gold
400GeV protons on Lithium
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Forward peaks in proton
induced neutron production
Beryllium
Lead
256 MeV data
Neutrons at 7.5deg.
Iron
Aluminum
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p Production from 730 MeV p
(Bertini Model)
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Low energy neutron capture:
gammas from 14 MeV capture on Uranium
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10-9 pC/incident proton
Nuclear interactions with Geant4 versus
experiment
Channel
Phantom and experimental results from H.Paganetti, B.Gottschalk, Medical physics
Vol. 30, No.7, 2003
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Atlas HEC (e/p ratio)
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CMS ECAL + HCAL testbeam
GEANT3 - GEANT4 comparison
100 GeV pi+
ECAL+HCAL
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"Educated guess" physics lists




In Geant4, the physics lists serve the same purpose as the "packages"
(GHEISHA, FLUKA, GCALOR) in geant3.
 Conceptually, the two are identical.
 Both ways provide the physics and its modeling to an application.
 Each "package" is built of a complete and consistent set of models
In Geant4, the number of "packages" is quite large. Each option comes
with trade-offs in descriptive power and performance.
It simply became clear that writing a good physics list is not trivial, in
particular when hadronic physics is involved.
 It is nice to be able to exploit the full power in the flexibility and
variety of hadronic physics modeling in geant4, but being forced to do
so is not what we want.
 It is also nice to have the physics transparently in front of the user and
to exploit it in the best possible way, but being forced to understand
everything is (very understandably) not what people want, either.
We have systematically accumulated experience with various combinations
of cross-section and models over the past years. Today we provide a set
of physics lists institutionalizing this knowledge.
 "Educated guess" physics lists
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Use-case driven packages










LCG simulation project.
HEP calorimetry.
HEP trackers.
'Average' collider detector
Low energy dosimetric
applications with neutrons
low energy nucleon penetration
shielding
linear collider neutron fluxes
high energy penetration
shielding
medical and other life-saving
neutron applications
low energy dosimetric
applications





high energy production targets
e.g. 400GeV protons on C or Be
medium energy production
targets
e.g. 15-50 GeV p on light
targets
LHC neutron fluxes
low background experiments
Air shower applications (still
working on this)
Each package has several
physics lists suitable to the
use-case
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Decay

A decay table is associated to the definition of each particle type.

A track can have a decay channel. If it has, it exactly decays through
this channel without randomizing by the decay ratio.

This allows the user to import decay chains generated by physics
generators such as Pythia, and rely on Geant4 tracking for such
unstable particles.
Primary particle list
B0
G4Track
B0
K0L
…
K0L
…
“pre-defined”
decay channel
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Optical processes


Geant4 has a particle named “Optical Photon”, which is distinguished
from gamma. It interacts by optical processes.
Geant4 is an ideal framework for modeling the optics of scintillation
and Cerenkov detectors and their associated light guides. This is
founded in the toolkit's unique capability of commencing the simulation
with the propagation of a charged particle and completing it with the
detection of the ensuing optical photons on photo sensitive areas, all
within the same event loop.

This functionality is now employed world-wide in experimental
simulations as diverse as ALICE, ANTARES, AMANDA, Borexino,
Icarus, LHCb, HARP, KOPIO, the Pierre Auger Observatory, and the
GATE (Imaging in Nuclear Medicine) Collaboration.
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Optical processes



Optical photons are generated if one or more of following processes
are activated.

Cerenkov radiation,

Transition radiation,

Scintillation
Optical processes built in Geant4

Absorption,

Rayleigh scattering

Boundary Processes (reflection, refraction)
Optical properties, e.g. dielectric coefficient and surface smoothness,
can be set to a volume.
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Shower parameterization framework

Geant4 includes a built-in framework for shower parameterization
scheme. Currently, the user has to concrete his/her own
parameterization assigned to a logical volume, which is then called as
an “envelop”.

Regardless of the existence of granular daughter geometry, a
particle comes into the envelop can be fully treated by the shower
parameterization process.


The user still have a dynamic choice to take his/her
parameterization or to follow the ordinary tracking in the
granular geometry.
The shower parameterization process can directly contact to a
sensitive detector associating to the volume to produce more than
one distributed hits.
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Highlights of
new developments
Event biasing in Geant4



Event biasing (variance reduction) technique is one of the most
important requirements, which Geant4 collaboration is aware of.
This feature could be utilized by many application fields such as

Radiation shielding

Dosimetry
Since Geant4 is a toolkit and also all source code is open, the user can
do whatever he/she wants.


CMS, ESA, Alice, and some other experiments have already had their
own implementations of event biasing options.
It’s much better and convenient for the user if Geant4 itself provides
most commonly used event biasing techniques.
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Current features in Geant4






Partial MARS migration
 n, p, pi, K (< 5 GeV)
 Since Geant4 0.0
General particle source module
 Primary particle biasing
 Since Geant4 3.0
Radioactive decay module
 Physics process biasing in terms of decay products and
momentum distribution
 Since Geant4 3.0
Cross-section biasing (partial) for hadronic physics
 Since Geant4 3.0
Leading particle biasing
 Since Geant4 4.0
Geometry based biasing
 Weight associating with real volume or artificial volume
 Since Geant4 5.0
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Geometrical importance biasing
I = 1.0
W=1.0
I = 2.0
W=0.5
W=0.5
P = 0.5




Define importance for each
geometrical region
Duplicate a track with half (or
relative) weight if it goes toward
more important region.
Russian-roulette in another
direction.
Scoring particle flux with weights
 At the surface of volumes
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Cuts per Region




Geant4 has had a unique and uniform production threshold (‘cut’)
expressed in length (range of secondary).
 For all volumes
 One cut in range for each particle
 By default is the same cut for all particles.
 Consistency of the physics simulated
 A volume with dense material will not dominate the simulation
time at the expense of sensitive volumes with light material.
Yet appropriate length scales can vary greatly between different areas of
a large detector
 E.g. a vertex detector (5 mm) and a muon detector (2.5 cm).
 Having a unique (low) cut can create a performance penalty.
Requests from ATLAS, BABAR, CMS, LHCb, …, to allow several cuts
 Enabling the tuning of production thresholds at the level of a subdetector, i.e. region.
 Cuts are applied only for gamma, electron and positron.
‘Full release’ in Geant4 5.1 (end April, 2003)
 Comparable run-time performance compared to global cuts.
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Region

Introducing the concept of region.



Or any group of volumes;
Default
Region
Region B
A cut in range is associated to a
region;


Set of geometry volumes,
typically of a sub-system;
a different range cut for each
particle is allowed in a region.
Typical Uses


barrel + end-caps of the
calorimeter can be a region;
Region
B
Region A
Region
B
C
C
Region B
“Deep” areas of support
structures can be a region.
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Highlights of
Users Applications
Geant4 in HEP

ATLAS (CERN-LHC)

22 x 22 x 44 m3

15,000 ton

4 million channels

40 MHz readout
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CMS
Sliced view of CMS barrel detectors
View of CMS muon system
View of 180 Higgs event simulated
in CMS Tracker
detector
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LHCb
A Typical event in the Testbeam
Red lines:
Charged particle
Green lines :
Optical Photons.
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Geant4 for beam transportation
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INTEGRAL
Gamma ray astronomy from 15 keV to 10 MeV
 Launched 17 October 2002
 Length 5 m, diameter 3.7 m, mass 4 tons
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INTEGRAL Geant4 model by
University of
Southampton
INTEGRAL in the ESA/ESTEC test center
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Space Environments
and Effects Section
International Space Station (ISS)
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Space Radiation: Solar Events of
October-November 2003!
Images by the ESA/NASA SOHO spacecraft
The effects of space radiation on spacecraft
and on astronauts can be simulated with Geant4
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ESA Space Environment &
Effects Analysis Section
DESIRE (Dose Estimation by Simulation of the ISS





Radiation Environment)
KTH Stockholm, ESTEC, EAC, NASA Johnson
Prediction of the ambient energetic particle
environment (SPENVIS & additional models)
Construction of COLUMBUS geometry in Geant4
Radiation transport, including secondary particle
production, through the geometry
Calculation of astronaut radiation doses
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User Support
User Support

Geant4 Collaboration offers extensive user supports.

Documents

Examples

Users workshops

Tutorial courses

HyperNews and mailing list

Bug reporting system

Requirements tracking system

Daily “private” communications

New implementation - Technical Forum
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Documents for users




One introduction and Five user manuals
http://wwwasd.web.cern.ch/wwwasd/geant4/G4UsersDocuments/
Overview/html/index.html
 Installation guide
 User's guide for application developers
 For a user who develops a simulation application using Geant4
 User's guide for toolkit developers
 For a user who develops a module which alternates or enhances
some of geant4 functionalities
 Physics reference manual
 Detailed description of each physics process with information of
references
 Software reference manual
LXR source code browser maintained by TRIUMF and KEK.
Materials of past tutorials / presentations, HyperNews and Web pages
maintained by developers also available via Geant4 official Web page.
"Geant4 general paper" - NIM A 506.
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Examples

Along the code releases, Geant4 provides examples which help user's
understanding of functionalities of Geant4 and are reusable as
"skeletons" of user's application.

Three levels of examples

Novice examples :



Demonstrate most basic features
Extended examples :

Highlight some functionalities / use-cases in detail

Some examples require external package(s)
Advanced examples :

Most realistic applications

User's contributions
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Geant4 users workshop

Users workshops were held or are going to be held hosted by several
institutes for various user communities.

KEK - Dec.2000, Jul.2001, Mar.2002, Jul.2002, Mar.2003, Jul.2003

SLAC - Feb.2002

Spain (supported by INFN) - Jul.2002

CERN - Nov.2002

ESA/NASA - Jan.2003, May.2004

dedicated to space-related users

Helsinki - Oct.2003

Local workshops of one or two days were held or are planned at
several places.
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Geant4 tutorials / lectures


In addition to the users workshops, many tutorial courses and lectures
with some discussion time slots were held for various user communities.
 CERN School of Computing
 Italian National School for HEP/Nuclear Physicists
 MC2000, MCNEG workshop, IEEE NSS/MIC
 KEK, SLAC, DESY, FNAL, INFN, Frascati, Karolinska, GranSasso, etc.
 ATLAS, CMS, LHCb
 Tutorials/lectures at universities
 U.K. - Imperial
 Italy - Genoa, Bologna, Udine, Roma, Trieste
Near future tutorial courses
 KEK (Dec. 8th-11th, 2003)
 Vanderbilt Univ. TN. USA (Jan. 11th-13th, 2004)
 SLAC (Mar. 8th-10th, 2004)
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HyperNews

HyperNews system was set up in April 2001
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HyperNews


19 categories
Not only “user-developer”,
but also “user-user”
information exchanges are
quite intensive.
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HyperNews is quite active
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Technical Forum




In the Technical Forum, the Geant4 Collaboration, its user community
and resource providers discuss:
 major user and developer requirements, user and developer
priorities, software implementation issues, prioritized plans, physics
validation issues, user support issues
The Technical Forum is open to all interested parties
 To be held at least 4 times per year (in at least two locales)
The purpose of the forum is to:
 Achieve, as much as possible, a mutual understanding of the needs
and plans of users and developers.
 Provide the Geant4 Collaboration with the clearest possible
understanding of the needs of its users.
 Promote the exchange of information about physics validation
performed by Geant4 Collaborators and Geant4 users.
 Promote the exchange of information about user support provided
by Geant4 Collaborators and Geant4 user communities.
Next Technical Forum meeting @ CERN on February 5th, 2004.
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Summary






Geant4 is a worldwide collaboration providing a tool for simulation of
particles interacting with matter.
Geant4’s object-oriented modular structure allows a large degree of
functionality and flexibility.
Geant4 can handle most complicated and realistic geometries.
Geant4 provides sets of alternative physics models so that users can
choose appropriate models.
Geant4 is being used by not only high energy and nuclear physics but
also accelerator physics, astrophysics, space science and medical and
other applications.
Geant4 Collaboration offers extensive user supports.
http://cern.ch/geant4/
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