1. Electromagnetic Physics
Maria Grazia Pia
CERN/IT and INFN Genova
S. Chauvie, V. Grichine, P. Gumplinger, V. Ivanchenko, R. Kokoulin,
S. Magni, M. Maire, P. Nieminen, M.G. Pia, A. Rybin, L. Urban
on behalf of the Geant4 Collaboration
MC2000 Conference, Lisbon, October 2000
Budker Inst. of Physics
IHEP Protvino
MEPHI Moscow
Pittsburg University

2. Highlights A wide domain of applications with
GLAST
Highlights
A wide domain of applications with
a large user community in many fields
CMS
at LHC, CERN
Gamma-ray Large Area Space Telescope
HEP, astrophysics, nuclear physics, space sciences, medical physics, radiation studies etc.
ATLAS at LHC, CERN
X-ray telescope
An extensive set of physics processes and models over a wide energy range
XMM
Courtesy of L3
Courtesy of the Italian Nat. Inst. for Cancer Research
E (MeV)
Photon attenuation
BaBar at SLAC
Low energy photons
High energy m
Borexino
at Gran Sasso Laboratory
A rigorous approach to software engineering
Budker Inst. of Physics
IHEP Protvino
MEPHI Moscow
Pittsburg University

3. Geant4 is a simulation Toolkit designed for a variety of applications
It has been developed and is maintained by an international collaboration of > 100 scientists
RD44 Collaboration
Geant4 Collaboration
The code is publicly distributed from the WWW, together with ample documentation
1st production release: end 1998
2 new releases/year since then
It provides a complete set of tools for all the typical domains of simulation
geometry and materials
tracking
detector response
run, event and track management
PDG-compliant particle management
visualisation
user interface
persistency
physics processes
It is also complemented by specific modules for space science applications

4. plays a fundamental role in Geant4
Geant4 architecture
Software Engineering
plays a fundamental role in Geant4
formally collected
systematically updated
PSS-05 standard
User Requirements
Domain decomposition has led to a hierarchical structure of sub-domains linked
by a uni-directional flow of dependencies
Software Process
spiral iterative approach
regular assessments and improvements
monitored following the ISO model
OOAD
use of CASE tools
Object Oriented methods
essential for distributed parallel development
contribute to the transparency of physics
commercial tools
code inspections
automatic checks of coding guidelines
testing procedures at unit and integration level
dedicated testing team
Quality Assurance
Use of Standards
de jure and de facto

5. Features of Geant4 Physics
OOD allows to implement or modify any physics process without changing other parts of the software
open to extension and evolution
Tracking is independent from the physics processes
The generation of the final state is independent from the access and use of cross sections
Transparent access via virtual functions to
cross sections (formulae, data sets etc.)
models underlying physics processes
An abundant set of electromagnetic and hadronic physics processes
a variety of complementary and alternative physics models for most processes
Use of public evaluated databases
No tracking cuts, only production thresholds
thresholds for producing secondaries are expressed in range, universal for all media
converted into energy for each particle and material
The transparency of the physics implementation contributes to the validation of experimental physics results

6. Electromagnetic physics
multiple scattering
Bremsstrahlung
ionisation
annihilation
photoelectric effect
Compton scattering
Rayleigh effect
g conversion
e+e- pair production
synchrotron radiation
transition radiation
Cherenkov
refraction
reflection
absorption
scintillation
fluorescence
Auger (in progress)
It handles
electrons and positrons
g, X-ray and optical photons
muons
charged hadrons
ions
energy loss
Comparable to Geant3 already in the 1st a release (1997)
High energy extensions
fundamental for LHC experiments, cosmic ray experiments etc.
Low energy extensions
fundamental for space and medical applications, neutrino experiments, antimatter spectroscopy etc.
Alternative models for the same physics process

7. OO design
Class diagram of electromagnetic physics
Alternative models, obeying the same abstract interface, are provided for the same physics interaction

8. Standard electromagnetic processes
1 keV up to O(100 TeV)
Photons
Compton scattering
g conversion
photoelectric effect
Electrons and positrons
Bremsstrahlung
ionisation
continuous energy loss from Bremsstrahlung and ionisation
d ray production
positron annihilation
synchrotron radiation
Charged hadrons
Shower profile, 1 GeV e- in water
J&H Crannel - Phys. Rev August69

9. Features of Standard e.m. processes
Multiple scattering
6.56 MeV proton , 92.6 mm Si
Multiple scattering
new model
computes mean free path length and lateral displacement
New energy loss algorithm
optimises the generation of d rays near boundaries
Variety of models for ionisation and energy loss
including the PhotoAbsorption Interaction model
Differential and Integral approach
for ionisation, Bremsstrahlung, positron annihilation, energy loss and multiple scattering
J.Vincour and P.Bem Nucl.Instr.Meth (1978) 399

10. Photo Absorption Ionisation Model
Ionisation energy loss produced by charged particles in thin layers of absorbers
3 GeV/c p in 1.5 cm Ar+CH4
5 GeV/c p in 20.5 mm Si
Ionisation energy loss distribution produced by pions, PAI model

11. Low energy extensions: e-, g
250 eV up to 100 GeV
Based on EPDL97, EEDL and EADL evaluated data libraries
cross sections
sampling of the final state
Photoelectric effect
Compton scattering
Rayleigh scattering
g conversion
Bremsstrahlung
Ionisation
Fluorescence
Photon transmission on 1 mm Al

12. water
water
Photon attenuation coefficient
Comparison with NIST data
Standard electromagnetic package
and Low Energy extensions Fe

13. Low energy extensions: hadrons and ions
Various models, depending on the energy range and charge
E > 2 MeV  Bethe-Bloch
1 keV < E < 2 MeV  parameterisations
Ziegler 1977, 1985
ICRU 1993
corrections due to chemical formulae of materials
nuclear stopping power
E < 1 keV  free electron gas model
Barkas effect taken into account down to 50 keV
quantum harmonic oscillator model

14. Muon processes 1 keV up to 10 PeV scale Validity range
 simulation of ultra-high energy and cosmic ray physics
High energy extensions based on theoretical models
Bremsstrahlung
Ionisation and d ray production
e+e- Pair production

15. Processes for optical photons
Optical photon  its wavelength is much greater than the typical atomic spacing
Production of optical photons in HEP detectors is mainly due to Cherenkov effect and scintillation
Processes in Geant4
in-flight absorption
Rayleigh scattering
medium-boundary interactions (reflection, refraction)
Track of a photon entering a light concentrator CTF-Borexino

16. Openness to evolution
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.”
with attention to UR
Geant4 physics keeps evolving
facilitated by the OO technology
Lower energy extensions, new models for polarisation, new models for material dependence etc.

17. User support User Support is a key feature of Geant4
Users (experiments, laboratories, institutes) are members of the collaboration itself
User Requirements are formally collected and regularly updated
Extensive documentation available from the web (5 manuals)
A Geant4 Training Programme in preparation
User Support through a web interface for code-related problem reports
User Support through human interface for consultancy, investigation of anomalous results etc.
A distributed model of User Support
a large number of experts performs the support on the domain of their competence
The close relationship with user communities and their feedback is very valuable to Geant4

18. Geant4 electromagnetic physics Working Groups
M. Maire (LAPP)
P. Nieminen (ESA)
M.G. Pia (INFN Genova)
S. Agostinelli - (IST Genova)
P. Andreo (Karolinska Inst.)
D. Belkic (Karolinska Inst.)
A. Brahme (Karolinska Inst.)
A. Carlsson (Karolinska Inst.)
G. Cabras (INFN Udine)
S. Chauvie (INFN Torino)
G. Depaola (Univ. Cordova)
R. Cirami (INFN Trieste)
E. Daly (ESA)
A. De Angelis (INFN Udine)
G. Fedel (INFN Trieste)
J.M. Fernandez Varea (Univ. Barcelona)
S. Garelli (IST Genova)
R. Giannitrapani (INFN Udine)
V. Grichine (LPI Moscow)
I. Gudowska (Karolinska Inst.)
P. Gumplinger (TRIUMF)
V. Ivanchenko (Budker Institute )
R. Kokouline (MEPhI, Moscow)
E. Lamanna (INFN Cosenza)
S. Larsson (Karolinska Inst.)
R. Lewensohn (Karolinska Inst.)
B.K. Lind (Karolinska Inst.)
J. Lof (Karolinska Inst.)
F. Longo (INFN Trieste)
B. De Lotto (INFN Udine)
F. Marchetto (INFN Torino)
E. Milotti (INFN Udine)
R. Nartallo (ESA)
G. Nicco (Univ. Torino)
B. Nilsson (Karolinska Inst.)
V. Rolando (Univ. Piemonte Orient.)
A. Rybin (IHEP Protvino)
G. Santin (INFN Trieste)
U. Skoglund (Karolinska Inst.)
A. Solano (INFN Torino)
R. Svensson (Karolinska Inst.)
N. Tilly (Karolinska Inst.)
L. Urban (KFKI Budapest)
Standard
e.m. Physics
Low Energy e.m. Physics

19. Conclusions
Geant4 is a simulation Toolkit, providing advanced tools for all the domains of detector simulation
Its areas of application span diverse fields: HEP and nuclear physics, astrophysics and space sciences, medical physics, radiation studies etc.
Geant4 is characterized by a rigorous approach to software engineering
Geant4 electromagnetic physics covers a wide energy range of interactions of electrons and positrons, photons, muons, charged hadrons and ions
An abundant set of electromagnetic physics processes is available, often with a variety of complementary and alternative physics models
Low and high energy extensions have opened new domains of applications
Thanks to the OO technology, Geant4 is open to extension and evolution

20. Related presentations at this conference
V. Grichine Fast Simulation of X-ray Transition Radiation in the Geant4 Toolkit
P. Nieminen Space applications of the Geant4 Toolkit
P. Arce et al. Multiple scattering in Geant4
S. Chauvie Medical applications of the Geant4 Toolkit