Geant4—a simulation toolkit

TLDR: The Gelfant 4 toolkit as discussed by the authors is a toolkit for simulating the passage of particles through matter, including a complete range of functionality including tracking, geometry, physics models and hits.

Abstract: G eant 4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics.

Figures

Fig. 9. Isotope production cross-sections for neutron induced production of important isotopes as simulated using the isotopeproduction code in Geant4. Large points are simulation results, small points are evaluated data from the MENDL2 data library.

Fig. 4. Smart Voxels. A mother volume with divisions along the horizontal axis. Each one of these slices has an independent set of vertical divisions (voxels). Here, the first one from the left is shown.

Fig. 5. Multiple scattering of 6:56 MeV protons by 92:6 mm of silicon: the angular distribution of exiting protons. Comparison of Geant4, GEANT3 and data from [55]: G3 and G4 results are nearly identical, except at small angle.

Fig. 6. Radial shower profile of E0 ¼ 1 GeV pencil electron beam in a cylinder of water, length 10 radiation lengths, radius 0.7 radiation lengths. The radial profile is the relative energy deposited per radiation length of radius. The cumulative profile is the energy deposited within a cylinder of given radius (the binning is 0.1 radiation lengths). Comparisons of Geant4, GEANT3 and measurements are from [56]. G3 and G4 results are nearly identical.

Fig. 1. The Top Level Category Diagram of the Geant4 toolkit. The open circle on the joining lines represents a using relationship; the category at the circle end uses the adjoined category.

Fig. 11. Comparison of the branchings in two particle final states in proton anti-proton annihilation [100] with the predictions of CHIPS.

Full text

ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303
Gea nt4—a simulation toolkit
S. Agostinelli
ae
, J. Allison
as,
*, K. Amako
e
, J. Apostolakis
a
, H. Araujo
aj
,
P. Arce
l,m,x,a
, M. Asai
g,ai
, D. Axen
i,t
, S. Banerjee
bi,l
, G. Barrand
an
, F. Behner
l
,
L. Bellagamba
c
, J. Boudreau
bd
, L. Broglia
ar
, A. Brunengo
c
, H. Burkhardt
a
,
S. Chauvie
bj,bl
, J. Chuma
h
, R. Chytracek
a
, G. Cooperman
az
, G. Cosmo
a
,
P. Degtyarenko
d
, A. Dell’Acqua
a,i
, G. Depaola
y
, D. Dietrich
af
, R. Enami
ab
,
A. Feliciello
bj
, C. Ferguson
bh
, H. Fesefeldt
l,o
, G. Folger
a
, F. Foppiano
ac
,
A. Forti
as
, S. Garelli
ac
, S. Giani
a
, R. Giannitrapani
bo
, D. Gibin
m,bc
, J.J. G
!
omez
Cadenas
m,bp
, I. Gonz
!
alez
q
, G. Gracia Abril
n
, G. Greeniaus
p,h,ag
, W. Greiner
af
,
V. Grichine
f
, A. Grossheim
m,z
, S. Guatelli
ad
, P. Gumplinger
h
, R. Hamatsu
bk
,
K. Hashimoto
ab
, H. Hasui
ab
, A. Heikkinen
ah
, A. Howard
aj
, V. Ivanchenko
a,ba
,
A. Johnson
g
, F.W. Jones
h
, J. Kallenbach
aa
, N. Kanaya
i,h
, M. Kawabata
ab
,
Y. Kawabata
ab
, M. Kawaguti
ab
, S. Kelner
at
, P. Kent
r
, A. Kimura
ay,bb
,
T. Kodama
aw
, R. Kokoulin
at
, M. Kossov
d
, H. Kurashige
am
, E. Lamanna
w
,
T. Lamp
!
en
ah
, V. Lara
a,l,bq
, V. Lefebure
l
, F. Lei
bh,be
, M. Liendl
l,a,br
,
W. Lockman
j,bn
, F. Longo
bm
, S. Magni
k,au
, M. Maire
ao
, E. Medernach
a
,
K. Minamimoto
aw,al
, P. Mora de Freitas
ap
, Y. Morita
e
, K. Murakami
e
,
M. Nagamatu
aw
, R. Nartallo
b
, P. Nieminen
b
, T. Nishimura
ab
, K. Ohtsubo
ab
,
M. Okamura
ab
, S. O’Neale
s
, Y. Oohata
bk
, K. Paech
af
, J. Perl
g
, A. Pfeiffer
a
,
M.G. Pia
ad
, F. Ranjard
n
, A. Rybin
ak
, S. Sadilov
a,ak
, E. Di Salvo
c
, G. Santin
bm
,
T. Sasaki
e
, N. Savvas
as
, Y. Sawada
ab
, S. Scherer
af
, S. Sei
aw
, V. Sirotenko
i,al
,
D. Smith
g
, N. Starkov
f
, H. Stoecker
af
, J. Sulkimo
ah
, M. Takahata
ay
, S. Tanaka
bg
,
E. Tcherniaev
a
, E. Safai Tehrani
g
, M. Tropeano
ae
, P. Truscott
be
, H. Uno
aw
,
L. Urban
v
, P. Urban
aq
, M. Verderi
ap
, A. Walkden
as
, W. Wander
av
, H. Weber
af
,
J.P. Wellisch
a,l
, T. Wenaus
u
, D.C. Williams
j,bf
, D. Wright
g,h
, T. Yamada
aw
,
H. Yoshida
aw
, D. Zschiesche
af
a
European Organization for Nuclear Research (CERN) Switzerland
b
European Space Agency (ESA), ESTEC, The Netherlands
c
Istituto Nazionale di Fisica Nucleare (INFN), Italy
d
Jefferson Lab, USA
e
KEK, Japan
*Corresponding author. Tel.: +44-161-275-4179; fax: +44-161-273-5867.
E-mail address: john.allison@man.ac.uk (J. Allison).
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0168-9002(03)01368-8

f
Lebedev Institute, Russia
g
Stanford Linear Accelerator Center (SLAC), USA
h
TRIUMF, Canada
i
ATLAS Collaboration, CERN, Switzerland
j
BaBar Collaboration, USA
k
Borexino Collaboration, Italy
l
CMS Collaboration, CERN, Switzerland
m
HARP Collaboration, CERN, Switzerland
n
LHCb Collaboration, CERN, Switzerland
o
RWTH, Aachen, Germany
p
University of Alberta, Canada
q
ALICE Collaboration, CERN, Switzerland
r
University of Bath, UK
s
University of Birmingham, UK
t
University of British Columbia, Canada
u
Brookhaven National Laboratory, USA
v
Kfki, Budapest, Hungary
w
Universit
"
a della Calabria and INFN, Italy
x
CIEMAT, Italy
y
University of Cordoba, Spain
z
University of Dortmund, Germany
aa
FNAL, USA
ab
Fukui University, Japan
ac
IST Natl. Inst. for Cancer Research of Genova, Italy
ad
INFN Genova, Italy
ae
Universit
"
a di Genova, Italy
af
Inst. f
.
ur Theoretische Physik, Johann Wolfgang Goethe Universit
.
at, Frankfurt, Germany
ag
HERMES Collaboration, DESY, Germany
ah
Helsinki Institute of Physics (HIP), Finland
ai
Hiroshima Institute of Technology, Japan
aj
Imperial College of Science, Technology and Medicine, London, UK
ak
IHEP Protvino, Russia
al
North Illinois University, USA
am
Kobe University, Japan
an
IN2P3/LAL, Orsay, France
ao
IN2P3/LAPP, Annecy, France
ap
IN2P3/LLR, Palaiseau, France
aq
EPFL, Lausanne, Switzerland
ar
Lyon University, France
as
Department of Physics and Astronomy, The University of Manchester, UK
at
MEPhI, Moscow, Russia
au
INFN, Milan, Italy
av
MIT, USA
aw
Naruto University of Education, Japan
ay
Niigata University, Japan
az
Northeastern University, USA
ba
Budker Institute for Nuclear Physics, Novosibirsk, Russia
bb
Osaka Institute of Technology, Japan
bc
Universit
"
a di Padova, Italy
bd
University of Pittsburg, USA
be
QinetiQ, UK
bf
SCIPP/UCSC, Santa Cruz, USA
bg
Ritsumeikan University, Japan
bh
University of Southampton, UK
bi
TIFR, Mumbai, India
bj
INFN, Torino, Italy
bk
Tokyo Metropolitan University, Japan
ARTICLE IN PRESS
S. Agostinelli et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303 251

bl
Universit
"
a di Torino, Italy
bm
Universit
"
a di Trieste and INFN Trieste, Italy
bn
UCSC, Santa Cruz, USA
bo
Universit
"
a di Udine and INFN Udine, Italy
bp
University of Valencia, Spain
bq
IFIC Instituto de Fisica Corpuscular de Valencia, Spain
br
Vienna University of Technology, Austria
Geant4 Collaboration
Received 9 August 2002; received in revised form 11 March 2003; accepted 14 March 2003
Abstract
Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of
functionality including tracking, geometry, physics models and hits. The physics processes offered cover a
comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived
particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in
others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle
complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit
is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting
software engineering and object-oriented technology and implemented in the C++ programming language.
It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical
physics.
r 2003 Elsevier Science B.V. All rights reserved.
PACS: 07.05.Tp; 13; 23
Keywords: Simulation; Particle interactions; Geometrical modelling; Software engineering; Object-oriented technology; Distributed
software development
1. Introduction
Modern particle and nuclear physics experi-
ments pose enormous challenges in the creation of
complex yet robust software frameworks and
applications. Of particular importance is the
ever-increasing demand for large-scale, accurate
and comprehensive simulations of the particle
detectors used in these experiments. The demand
is driven by the escalating size, complexity, and
sensitivity of the detectors and fueled by the
availability of moderate-cost, high-capacity com-
puter systems on which larger and more complex
simulations become possible. Similar considera-
tions arise in other disciplines, such as: radiation
physics, space science, nuclear medicine and, in
fact, any area where particle interactions in matter
play a role.
In response to this, a new object-oriented
simulation toolkit, Gea nt4, has been developed.
The toolkit provides a diverse, wide-ranging, yet
cohesive set of software components which can be
employed in a variety of settings. These range from
simple one-off studies of basic phenomena and
geometries to full-scale detector simulations for
experiments at the Large Hadron Collider and
other facilities.
In defining and implementing the software
components, all aspects of the simulation process
have been included: the geometry of the system,
the materials involved, the fundamental particles
of interest, the generation of primary particles of
events, the tracking of particles through materials
and external electromagnetic fields, the physics
processes governing particle interactions, the
response of sensitive detector components, the
ARTICLE IN PRESS
S. Agostinelli et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303252

generation of event data, the storage of events and
tracks, the visualisation of the detector and
particle trajectories, and the capture for subse-
quent analysis of simulation data at different levels
of detail and refinement.
Early in the design phase of the project, it was
recognised that while many users would incorpo-
rate the Geant4 tools within their own computa-
tional framework, others would want the
capability of easily constructing stand-alone ap-
plications which carry them from the initial
problem definition right through to the production
of results and graphics for publication. To this
end, the toolkit includes built-in steering routines
and command interpreters which operate at the
problem setup, run, event, particle transportation,
visualisation, and analysis levels, allowing all parts
of the toolkit to work in concert.
At the heart of this software system is an
abundant set of physics models to handle the
interactions of particles with matter across a very
wide energy range. Data and expertise have been
drawn from many sources around the world and in
this respect Geant4 acts as a repository that
incorporates a large part of all that is known about
particle interactions; moreover it continues to be
refined, expanded and developed. A serious
limitation of many previous simulation systems
was the difficulty of adding new or variant physics
models; development became difficult due to the
increasing size, complexity and interdependency of
the procedure-based code. In contrast, object-
oriented methods have allowed us effectively to
manage complexity and limit dependencies by
defining a uniform interface and common organi-
sational principles for all physics models. Within
this framework, the functionality of models can be
more easily seen and understood, and the creation
and addition of new models is a well-defined
procedure that entails little or no modification to
the existing code.
Geant4 was designed and developed by an
international collaboration, formed by individuals
from a number of cooperating institutes, HEP
experiments, and universities. It builds on the
accumulated experience of many contributors to
the field of Monte Carlo simulation of physics
detectors and physical processes. While geogra-
phically distributed software development and
large-scale object-oriented systems are no longer
a novelty, we consider that the Geant4 Collabora-
tion, in terms of the size and scope of the code and
the number of contributors, represents one of the
largest and most ambitious projects of this kind. It
has demonstrated that rigorous software engineer-
ing practices and object-oriented methods can be
profitably applied to the production of a coherent
and maintainable software product, even with the
fast-changing and open-ended requirements pre-
sented by physics research.
In the following sections we present a detailed
overview of Geant4 and its features and capabil-
ities, including the design and implementation of
the various categories of physics models. Many
new physics models have been developed, and
others have been refined or extended. They have
been created to support a growing range of
applications for the software, including particle,
nuclear, medical, accelerator and space physics.
The code and documentation, as well as tutorials
and examples, are available from our Web site [1].
1.1. History of Geant4
The origin of Geant4 development can be
traced back to two studies done independently at
CERN and KEK in 1993 [2]. Both groups sought
to investigate how modern computing techniques
could be applied to improve what was offered by
the existing GEANT3 program [3], which was a
benchmark and source of ideas and valuable
experience. These two activities merged and a
proposal was submitted to the CERN Detector
Research and Development Committee (DRDC)
[4] to construct a simulation program based on
object-oriented technology. The resulting project
was RD44, a worldwide collaboration that grew to
include the efforts of 100 scientists and engineers,
drawn from more than 10 experiments in Europe,
Russia, Japan, Canada and the United States.
The design choices faced by RD44 and the
decisions arrived at are described in later sections,
but key to its success was a careful design adapting
object-oriented methodology and an early decision
to use the practical C++ language.
ARTICLE IN PRESS
S. Agostinelli et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303 253

The R&D phase was completed in December
1998 [1] with the delivery of the first production
release. Subsequently the Geant4 Collaboration
was established in January 1999 to continue the
development and refinement of the toolkit, and to
provide maintenance and user support.
1.2. Organisation of the collaboration
A Memorandum of Understanding (MoU) [5]
signed by all participating parties governs the
formal collaboration. It is subject to tacit renewal
every 2 years and sets out a collaboration structure
composed of a Collaboration Board (CB), a
Technical Steering Board (TSB) and several work-
ing groups. The MoU also defines the way in
which collaboration resources—money, man-
power, expertise, and key roles and activities (such
as program librarian and documentation man-
ager)—are measured in Contribution Units (CU),
and it further delineates how the boards are
constituted depending on the CU count for each
signatory. Participating groups include experimen-
tal teams and collaborations, laboratories and
national institutes.
It is the CB’s mandate to manage these
resources and to monitor the agreed responsibil-
ities among the affiliates. This body is also charged
with the evolution of the MoU. The TSB, on the
other hand, is the forum where technical matters,
like software engineering details and physics model
implementation issues, are discussed and decided
and where priorities are given to user requests. Its
primary tasks are the supervision of the produc-
tion service and the user support and the over-
seeing of ongoing further development of the
program. The TSB is chaired by the spokesperson
of the Collaboration, who is appointed by and
reports to the CB. The spokesperson is (re)elected
every two years.
Every domain of the Geant4 software that
corresponds to a releasable component (library) is
individually managed by a working group of
experts. In addition, there is a working group for
each of the activities of testing and quality
assurance, software management and documenta-
tion management. A coordinator who is selected
by the TSB heads each group. There is also an
overall release coordinator. This clean overall
problem decomposition makes the distributed
software design and development possible in a
worldwide collaboration. Every group can work in
parallel, allowing an optimal use of manpower and
expertise.
1.3. User support, documentation and source code
The Collaboration provides documentation and
user support for the toolkit. The support model is
described in more detail in Section 3.6.
Documentation [6] includes installation, user
and reference guides, and a range of training kits
(see also Section 1.4). It is intended to cover the
need of the beginner through to the expert user
who wishes to expand the capabilities of Gea nt4.
User support covers help with problems relating
to the code, consultation on using the toolkit and
responding to enhancement requests. A user may
also expect assistance in investigating anomalous
results.
A Web-based reporting system and a list of
frequently asked questions (FAQs) are available
on the Geant4. Web site [1]. The Collaboration
also runs a Web-based user forum [7], with sub-
forums according to areas of different interest.
Regular releases of the source code and doc-
umentation are freely available on the Web.
1.4. Examples and training kits
The toolkit includes examples at three levels:
*
Novice: for understanding basic functionalities;
*
Extended: focused on specific domains of
application (they may also need additional
third party libraries);
*
Advanced: full programs created to utilise
Geant4 in HEP experiments, and for space
and medical applications.
They are intended to develop the user’s under-
standing in many areas. Initial emphasis is on the
classes describing the user’s setup, which are
required by the toolkit. These classes are explained
in Section 2.4.
Geant4 also provides a training kit. It consists
of a modular set of units, each covering a specific
ARTICLE IN PRESS
S. Agostinelli et al. / Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303254

Frequently asked questions

Q1. What are the contributions in "Geant4—a simulation toolkit" ?

Geant4 is a toolkit for simulating the passage of particles through matter. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. 

Q2. What are the future works mentioned in the paper "Geant4—a simulation toolkit" ?

The implementation of the toolkit in an object-oriented design allows it to be easily extended, where appropriate, to meet the requirements of the user, through class inheritance. 

Q3. What is the importance of monitoring code quality from the start?

Although unit and system testing are critical to ensure integrity and correctness of the Geant4 code, it is also important to monitor code quality from the start. 

Q4. What is the key mechanism in Geant4 for achieving customisable and state-dependent?

The intercoms category implements an expandable command interpreter which is the key mechanism in Geant4 for realising customisable and state-dependent user interactions with all categories without being perturbed by the dependencies among classes. 

Q5. What are the key domains of the simulation of the passage of particles through matter?

The key domains of the simulation of the passage of particles through matter are:* geometry and materials; * particle interaction in matter; * tracking management; * digitisation and hit management; * event and track management; * visualisation and visualisation framework; * user interface. 

Q6. What is the key to the development of a tool-kit for hadronic physics?

The use of state of the art software technology is the key that allows for distributed development of the physics base of a tool-kit for simulation of hadronic physics in the Geant4 context. 

Q7. What is the need to suspend the primary charged particle track?

The need to suspend the primary charged particle track arises in the production of Cherenkov photons because the number of such photons generated during the length of a typical step, as defined by energy loss or multiplescattering, is often very large. 

Q8. How many orders of magnitude do the basic requirements on the physics modelling of hadronic interactions?

The basic requirements on the physics modelling of hadronic interactions in a simulation toolkit span more than 15 orders of magnitude in energy. 

Q9. What is the function that takes care of the transporting of a particle?

It takes cares of all message passing between objects in the different categories which are relevant to transporting a particle (for example, geometry, interactions in matter, etc.). 

Q10. What is the reason for the demand for large scale particle and nuclear physics simulations?

The demand is driven by the escalating size, complexity, and sensitivity of the detectors and fueled by the availability of moderate-cost, high-capacity computer systems on which larger and more complex simulations become possible. 

Q11. How much error was added to the simulation results?

A systematic error of 15% was added to the simulation results to take the error in the extrapolation of the total cross-sections into account. 

Q12. What is the range for other charged hadrons?

The range for other charged hadrons is computed from the proton table by using the scaled kinetic energy Ts ¼ Tmp=m; where T is the particle kinetic energy, m is the particle mass and mp is the proton mass, which is the energy of a proton with the same velocity as the tracked particle. 

Q13. What is the approach to low energy neutron transport for radiation studies?

Data driven modeling is known to provide the best, if not only, approach to low energy neutron transport for radiation studies in large detectors. 

Q14. What tools are used to detect unsafe, nonstandard or errorprone coding practices in the source?

To achieve this end the authors employ various Quality Assurance tools such as CodeWizard [43], to detect unsafe, nonstandard or errorprone coding practices in the source code, Insure++ [43] and Valgrind [44], to detect data integrity and memory management problems in a running application. 

Q15. What is the physics of photons, electrons, muons, hadrons?

The Geant4 toolkit contains a large variety of complementary and sometimes alternative physics models covering the physics of photons, electrons, muons, hadrons and ions from 250 eV up to several PeV.