Maria Grazia Pia

1

• Maria Grazia Pia
• INFN Genova
• Geant4 Collaboration
• XI Giornate sui Rivelatori
• Torino, 1-2 March 2001

2
Introduction

• Author Maria Grazia Pia

Geant4 Training Kit
3
An example of how simulation can be
mission-critical
Courtesy of NASA/CXC/SAO
4
Chandra X-ray Observatory Status
Update September 14, 1999 MSFC/CXC CHANDRA
CONTINUES TO TAKE SHARPEST IMAGES EVER TEAM
STUDIES INSTRUMENT DETECTOR CONCERN Normally
every complex space facility encounters a few
problems during its checkout period even though
Chandras has gone very smoothly, the science and
engineering team is working a concern with a
portion of one science instrument. The team is
investigating a reduction in the energy
resolution of one of two sets of X-ray detectors
in the Advanced Charge-coupled Device Imaging
Spectrometer (ACIS) science instrument. A series
of diagnostic activities to characterize the
degradation, identify possible causes, and test
potential remedial procedures is underway. The
degradation appeared in the front-side
illuminated Charge-Coupled Device (CCD) chips of
the ACIS. The instruments back-side illuminated
chips have shown no reduction in capability and
continue to perform flawlessly.
An excerpt of a press release
Courtesy of NASA/CXC/SAO
5
What can affect CCDs on X-ray astronomy missions?

• Radiation belt electrons?
• Scattered in the mirror shells?
• Effectiveness of Magnetic brooms
• Electron damage mechanism? - NIEL?
• Other particles? Protons, cosmic rays?
• Path to CCD? Wall penetration?

• Proposal set the problem up in Geant4 as a
  case-study

6
XMM
7
Courtesy of
ESA Space Environment Effects Analysis Section
RGS
EPIC
Q2
Q1
Q1
8
Courtesy of
ESA Space Environment Effects Analysis Section
CCD displacement damage front vs.
back-illuminated.
30 mm Si ? 1.5 MeV p
30 mm
2 mm
30 mm
2 mm
Active layerPassive layer
Electron deflector
Low-E (100 keV to few MeV), low-angle (0-5)
proton scatteringObscure problem not much
analysed
9
How well can Geant4 simulate low energy protons?
Courtesy of R. Gotta, Thesis
10
What happened next?
XMM was launched on 10 December 1999 from Kourou
EPIC image of the two flaring Castor components
and the brighter YY Gem
Courtesy of
11
The role of simulation

• The scope of these lectures (and of Geant4)
  encompasses the simulation of the passage of
  particles through matter
• there are other kinds of simulation components,
  such as physics event generators,
  detector/electronics response generators, etc.
• often the simulation of a complex experiment
  consists of several of these components
  interfaced to one another

Simulation plays a fundamental role in various
domains and phases of an experimental physics
project

• design of the experimental set-up
• evaluation and definition of the potential
  physics output of the project
• evaluation of potential risks to the project
• assessment of the performance of the experiment
• development, test and optimisation of
  reconstruction and physics analysis software
• contribution to the calculation and validation of
  physics results

12
Domains of application

• HEP, nuclear, astrophysics and astro-particle
  physics experiments
• the most traditional field of application
• Radiation studies
• evaluation of safety constraints and shielding
  for the experimental apparatus and human beings,
  on earth and in space
• Medical applications
• radiotherapy
• design of instruments for therapeutic use
• Biological applications
• radiation effects (in human beings, food etc.),
  at cellular and DNA level
• and more

13
What is required

• Modeling the experimental set-up
• Tracking particles through matter
• Interaction of particles with matter
• Modeling the detector response
• Run and event control
• Accessory utilities (random number generators,
  PDG particle information, physical constants,
  system of units etc.)

• User interface
• Interface to event generators
• Visualisation (of the set-up, tracks, hits etc.)
• Persistency
• Analysis

14
Fast and full simulation

• Usually in a typical HEP experiment there are two
  types of simulations
• Fast simulation
• mainly used for feasibility studies and quick
  evaluations
• coarse set-up description and physics modeling
• usually directly interfaced to event generators
• Full simulation
• used for precise physics and detector studies
• requires a detailed description of the
  experimental set-up and a complex physics
  modeling
• usually interfaced to event generators and event
  reconstruction
• Traditionally fast and full simulation are done
  by different programs and are not integrated in
  the same environment
• complexity of maintenance and evolution
• possibility of controversial results

15
The zoo
NMTC HERMES FLUKA EA-MC DPM SCALE GEM MF3D
EGS4, EGS5, EGSnrc MCNP, MCNPX, A3MCNP, MCNP-DSP,
MCNP4B Penelope Geant3, Geant4 Tripoli-3,
Tripoli-3 A, Tripoli-4 Peregrine MVP,
MVP-BURN MARS
MCU MORSE TRAX MONK MCBEND VMC LAHET RTST-2000
...and I probably forgot some more
Many codes not publicly distributed A lot of
business around MC
Monte Carlo codes presented at the MC200
Conference, Lisbon, October 2000
16
Integrated suites vs specialised codes
Specialised packages cover a specific simulation
domain
Integrated packages cover all/many simulation
domains

• Pro
• the specific issue is treated in great detail
• sometimes the package is based on a wealth of
  specific experimental data
• simple code, usually relatively easy to install
  and use
• Contra
• a typical experiment covers many domains, not
  just one
• domains are often inter-connected

• Pro
• the same environment provides all the
  functionality
• Contra
• it is more difficult to ensure detailed coverage
  of all the components at the same high quality
  level
• monolithic take all or nothing
• limited or no options for alternative models
• usually complex to install and use
• difficult maintenance and evolution

17
The Toolkit approach

• A toolkit is a set of compatible components
• each component is specialised for a specific
  functionality
• each component can be refined independently to a
  great detail
• components can be integrated at any degree of
  complexity
• components can work together to handle
  inter-connected domains
• it is easy to provide (and use) alternative
  components
• the simulation application can be customised by
  the user according to his/her needs
• maintenance and evolution - both of the
  components and of the user application - is
  greatly facilitated
• ...but what is the price to pay?
• the user is invested of a greater responsibility
• he/she must critically evaluate and decide what
  he/she needs and wants to use

18
The role of Geant

• Geant has been a simulation tool, that provides a
  general infrastructure for
• the description of geometry and materials
• particle transport and interaction with matter
• the description of detector response
• visualisation of geometries, tracks and hits
• The user develops the specific code for
• the primary event generator
• the geometrical description of the set-up
• the description of the detector response

19
The past Geant3

• Geant 3
• has been used by most major HEP experiments
• frozen since March 1994 (Geant3.21)
• 200K lines of code
• equivalent of 50 man-years, along 15 years
• used also in nuclear physics experiments, medical
  physics, radiation background studies, space
  applications etc.
• The result is a complex system
• because its problem domain is complex
• because it requires flexibility for a variety of
  applications
• because its management and maintenance are
  complex
• It is not self-sufficient
• hadronic physics is not native, it is handled
  through the interface to external packages

20
New simulation requirements

• Physics extensions to high energies
• LHC, cosmic ray experiments...
• Physics extensions to low energies
• space applications, medical physics, X-ray
  analysis, astrophysics, nuclear and atomic
  physics...
• Reliable hadronic physics
• not only for calorimetry, but also for PID
  applications (CP violation experiments)
• ...etc.

• High statistics to be simulated
• robustness and reliability for large scale
  production
• Exchange of CAD detector models
• especially relevant for large scale experiments
• Transparent physics
• for validation of physics results

User requirements formally collected and coded
according to PSS-05 standard Geant4 User
Requirements Document
21
What is Geant4?

• OO toolkit for the simulation of next generation
  HEP detectors
• ...of the current generation too
• ...not only of HEP detectors
• already used also in nuclear physics,
  astrophysics, medical physics, space
  applications, radiation background studies etc.
• It is also a successful experiment of distributed
  software production and management, as a large
  international collaboration with the
  participation of various experiments, labs and
  institutes
• It is also a successful experiment of application
  of rigorous software engineering and Object
  Oriented technologies to the HEP environment

22
RD44
Approved as RD end 1994 gt 100 physicists and
software engineers 40 institutes, international
collaboration responded to DRCC/LCB

• Milestones end 1995
• OO methodology, problem domain analysis, full
  OOAD
• tracking prototype, performance evaluation
• Milestones spring 1997
• release with the same functionality as Geant 3.21
• persistency (hits), ODBMS
• transparency of physics models
• Milestone July 1998
• public release
• Milestone end 1998
• production release Geant4.0, end of the RD
  phase

• All milestones have been met by RD44
• Reconfiguration at the end of the RD phase
• International Geant4 Collaboration since 1/1/1999
• Management of the production phase
• Continuing RD also in the production phase

23
Geant4 Collaboration

• MoU based
• Distribution, development and User Support

• Atlas, BaBar, CMS, HARP, LHCB
• CERN, JNL,KEK, SLAC, TRIUMF
• ESA, Frankfurt Univ., IGD, IN2P3, Karolinska
  Inst., Lebedev, TERA
• COMMON (Serpukov, Novosibirsk, Pittsburg etc.)
• other memberships currently being discussed

• Collaboration Board
• manages resources and responsibilities
• Technical Steering Board
• manages scientific and technical matters
• Working Groups
• maintenance, development, QA, etc.

Members of National Institutes, Laboratories and
Experiments participating in Geant4 Collaboration
acquire the right to the Production Service and
User Support For others free code and user
support on best effort basis
Budker Inst. of Physics IHEP Protvino MEPHI
Moscow Pittsburg University
24
Software engineering

• Geant4
• rigorous approach to software

25
Outline

• Motivations for software engineering in HEP
• The software process
• Components of the software life-cycle
• Object Oriented technologies
• Brief digression on basic OO concepts
• OOAD in Geant4
• Quality Assurance
• Standards

26
The benefits of software engineering
The goal producing better software at lower
cost, within predictable resource allocations and
time estimates, and happier users of the software

• the people involved
• the organization of the development process
• the technology used

Three key components

• The way to progress is to study and improve the
  way software is produced
• better technology only helps once the
  organizational framework is set
• there is evidence that going for new technology
  instead of improving the process can make things
  worst
• The practices of SPI are well established, and
  have been applied in a large number of
  organizations for several years
• the results prove that the economical benefits
  are largely worth the investment
• early defect detection, time to market, and
  quality also improve, to the point that the
  return on investment for SPI is about 500

27
Software life-cycle

• Various phases
• User Requirements definition
• Software Requirements definition
• Architectural Design
• Detailed Design and construction
• Delivery to the user
• Operations
• Frequently the tasks of different life cycle
  phases are performed somewhat in parallel
• to consider them disjoint in time is a
  simplification
• It is however important
• to distinguish them logically
• to identify documents that are the outcome of the
  various phases

28
The software process
It is the set of actions, tasks and procedures
involved in producing a software system, through
its life-cycle

• Complex domain, evolving, with many types of
  models available
• Examples of software process models
• The Waterfall model
• analysis ? design ? coding
• each phase starts following the completion of the
  previous one
• The Iterative Incremental Development model
• cycles of analysis ? design ? coding, with
  incremental refinement

29
Software process standards

• Development or Engineering processes system and
  software requirements analysis, software design,
  software construction, software integration and
  unit testing, software maintenance
• Documentation
• Configuration and Change Management
• Problem Resolution
• Quality Assurance and Measurement
• System Testing, Acceptance and Releasing
• Verification and Validation
• Reviews, Audits and Joint Reviews
• Project tasks Management
• Improvement Process
• Process Establishment
• Human resource Management
• Infrastructure
• User Support, Distribution

• Capability Maturity Model
• Software Engineering Institute
• SPICE, ISO 15504
• the path to an international standard
• PSS-05, ECSS
• ESA

Process categories
Primary life-cycle of software development Support
ing life-cycle Management process Organizational
life-cycle User-supplier processes
etc.
30
Why software engineering in HEP?

• Software engineering is somewhat new to the HEP
  environment
• other engineering branches more consolidated in
  this environment (mechanics, electronics,
  accelerators etc.)
• Benefits derive from a rigorous approach to
  software
• the lesson can be learned from the world of
  software professionals!
• even the most talented professionals need an
  organized environment to do cooperative work
• advanced technology cannot be fully effective
  without an organizational framework

Software Engineering plays a fundamental role in
Geant4
Software process SPI
User requirements OOAD Quality Assurance
31
The software process in Geant4

• a large international collaboration
• complex software
• mature categories in production and maintenance
  mode as well as categories in full development
• sensitive and mission-critical user applications
• product with a long life-time

A challenge

• Spiral-type life-cycle model adopted in most
  domains
• both iterative and incremental

• Software Process Improvement
• understand, determine and propose procedures to
  software development and maintenance
• gradual process, life-cycle driven
• regular assessment, according to the ISO 15504
  model

32
Requirements

• Requirements are the quantifiable and verifiable
• behaviours that a system must possess
• constraints that a system must work within
• User requirements
• this phase defines the scope of the system
• Software requirements
• this is the analysis phase of a software project
• builds a model describing what the software has
  to do (not how to do it)
• Requirements are subject to evolution in the
  lifetime of a software project!
• ability to cope with the evolution of the
  requirements

33
Geant4 requirements

• Geant4 has adopted a rigorous approach to
  requirements
• user requirements collected from the user
  communities in the initial phase
• coded according the PSS-05 software engineering
  standard
• continuously updated
• Geant4 User Requirements Document

34
Object Oriented technology

• OO technology is built upon a sound engineering
  foundation, whose elements are called the object
  model
• The object model encompasses the principles of
• abstraction
• encapsulation
• modularity
• hierarchy
• typing
• concurrency
• persistence
• brought together in a synergistic way
• Geant4 is based on Object Oriented technology

35
What is an object?

• G. Booch (in OOAD with Applications)
• An object has state, behavior and identity the
  structure and behavior of similar objects are
  defined in their common class.

36
Some fundamental concepts in OOD -1

• The Open Closed Principle
• Open for extension, Closed for modification
• A software module that is designed to be
  reusable, maintainable and robust must be
  extensible without requiring modification
• new features are added by adding new code, rather
  than by changing old, already working, code
• The primary mechanisms behind are abstraction and
  polymorphism
• The Liskov Substitution Principle
• Functions that use pointers or references to base
  classes must be able to use objects of derived
  classes without knowing it
• Derived types must be substitutable for their
  base types
• It is an important feature for conforming to the
  OCP

37
Some fundamental concepts in OOD -2

• The Dependency Inversion Principle
• Modules that implement high level policy should
  not depend on the modules that implement low
  level details
• Both high level policy and low level details
  should depend on abstractions
• This ensures reusability and maintainability
• The interdependence makes a design rigid, fragile
  and immobile a single change triggers a cascade
  of changes in dependent modules
• The Interface Segregation Principle
• Clients should not be forced to depend on
  interfaces that they do not use
• Polluted interfaces generate unnecessary
  couplings
• We want to separate interfaces whenever possible
  to avoid the disadvantages of couplings

38
Analysis

• Webster definitions
• separation or breaking up of a whole into its
  fundamental elements or component parts
• a detailed examination of anything complex
• the practice of proving a mathematical
  proposition by assuming the result and reasoning
  back to the data or already established
  principles
• In the software world
• it is the decomposition of a problem into its
  constituent parts
• it is accomplished by beginning with a set of
  stated requirements, and reasoning back from
  those requirements to a set of established
  software components and structures
• OOA is the act of determining the abstractions
  that underlie the requirements
• In OOA the components are objects and their
  collaborations

39
Design

• Design embodies the set of decisions that
  determine how the components will look like
• In OOD typically class inheritance and
  composition hierarchies are among the decisions
• OOA and OOD cooperate synergically
• they are best done concurrently
• The output of OOAD is a set of class and object
  diagrams, showing
• the static structure
• the collaborations

40
UML Unified Modeling Language
UML is the industry-standard language for
specifying, visualising, constructing and
documenting the design of software systems

• UML represents a unification of the concepts and
  notations previously in use (Booch, OMT, Jcobson)
• UML has a standard data representation (the
  Meta-Model)
• the Meta-Model is a description of UML in UML
• it describes the objects, attributes and
  relationships necessary to represents the
  concepts of UML within a software application
• UML notation is comprised of two major
  subdivisions
• a notation for modeling the static elements of a
  design (classes, attributes, relationships...)
• a notation for modeling the dynamic elements of a
  design (objects, messages, finite state
  machines...)

41
C

• OO technology and C are not equivalent!
• OO methodologies can be implemented in a variety
  of languages, not only in C
• One can write procedural code in C, that is not
  object oriented
• C provides many features that make it suitable
  for OO implementations of large scale software
  projects
• An overview of C language features and OO
  technology is beyond the scope of these lectures
• Many textbooks, courses and online material are
  available as learning aids, eg.
• I. Pohl, OO programming using C
• S. B. Lippman, J. Lajoie, C primer
• B. Stroustrup, The C programming language
• G. Booch, OO analysis and design
• R. Martin, Designing OO C applications using
  the Booch method

42
OO technology in Geant4

• OO design fundamental for distributed parallel
  approach
• Every part can be developed, refined, maintained
  independently
• Problem domain decomposition and OOAD result
  into a unidirectional dependency of class
  categories

• Booch methodology adopted for OOAD
• choice resulting from a thorough study of
  various models

• Flexibility
• alternative models and implementations
• Interface to external software, without
  dependencies
• databases for persistency
• visualisation libraries
• tools for UI
• etc.

• Openness to evolution
• Extensibility, implementation of new models and
  algorithms without interfering with existing
  software
• The user can extend the toolkit with his/her
  model and data
• Transparency
• decoupling from implementation

43
Geant4 architecture
exploits advanced Software Engineering techniques
and Object Oriented technology to achieve
transparency of physics implementation.
44
OO design an example of top level design
45
OO design an example of a detailed design
Class diagram of Low Energy e.m. processes
hadrons
46
Quality Assurance
Extensive use of QA systems in Geant4
fundamental for a toolkit of wide public use

• Testing
• Unit testing
• in most cases down to class level granularity
• Integration testing
• sets of logically connected classes
• Test-bench for each category
• eg. test-suite of 375 tests for hadronic physics
  parameterised models
• System testing
• exercising all Geant4 functionalities in
  realistic set-ups
• Physics testing
• comparisons with experimental data

• Commercial tInsure, CodeWizard, Workshop etc.
• C coding guidelines
• scripts to verify their applications
  automatically
• Code inspections
• within working groups and across groups
• Walk-throughs with specialized tools for
  monitoring against violations of coding rules
• Checks on run-time memory management
• Checks for violations of the dependency structure
  of categories
• Performance benchmarks and monitoring

47
Standards
Geant4 adopts standards, ISO and de facto

• OpenGL e VRML for graphics
• CVS for code management
• C as programming language

• STEP
• engineering and CAD systems
• ODMG
• RD45

Have you heard of the incident with NASAs Mars
Climate Orbiter (125 million)?

• Units
• Geant4 is independent from the system of units
• all numerical quantities expressed with their
  units explicitly
• user not constrained to use any specific system
  of units

48
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

49
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.
50
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

51
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

• Abstract interface to physics processes
• tracking independent from the type of process
• Distinction between processes and models
• often multiple models for the same process
• The generation of the final state is independent
  from the access and use of cross sections and
  from tracking

• Transparent access to
• cross sections (formulae, data sets etc.)
• models underlying physics processes
• An abundant set of electromagnetic and hadronic
  physics processes and models, both complementary
  and alternative
• Use of public evaluated databases

The transparency of the physics implementation
contributes to the validation of experimental
physics results
52
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

53
What is needed to run Geant4

• Geant4 source code and libraries are freely
  available at
• http//wwwinfo.cern.ch/asd/geant4/source/source.ht
  ml

• Graphics
• OpenGL, X11, OpenInventor, DAWN, VRML...
• OPACS, GAG, MOMO...
• Persistence
• it is possible to run in transient mode
• in persistent mode use a HepDB interface, ODMG
  standard

• Platforms
• DEC, HP, SUN native compilers, g
• Linux g
• Windows-NT Visual C
• Commercial software
• ObjectStore STL (optional)
• Free software
• CVS
• gmake, g
• CLHEP

54
Documentation
http//wwwinfo.cern.ch/asd/geant4/geant4.html

• 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

• User Documentation
• Introduction to Geant4
• Installation Guide
• Geant4 Users Guide - For
  Application Developers
• for those wishing to use Geant4
• Geant4 Users 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

55
User support

• The Geant4 User Support covers the
• provision of help and analysis of code-related
  problems
• the consultancy
• the requests for enhancement or new developments
• the investigation of anomalous results
• The User Support is provided by the Geant4
  Collaboration
• Major advantages for the users of this
  distributed approach are
• a large number of people performs the support,
  and always on the domain of their competence or
  even on the code they developed themselves
• a large number of contact/reference points for
  the users are available, avoiding the channeling
  of all problems through the same support people
  and thus improving efficiency
• Geant4 User Support is extensively described at
• http//wwwinfo.cern.ch/asd/geant4/G4UsersDocuments
  /Welcome/IntroductionToGeant4/html/introductionToG
  eant4.html7

56
Kernel

• Author Makoto Asai

Geant4 Training Kit
57
Run

• As an analogy of the real experiment, a run of
  Geant4 starts with Beam On.
• Within a run, the user cannot change
• detector geometry
• settings of physics processes
• ---gt detector is inaccessible during a run
• Conceptually, a run is a collection of events
  which share the same detector conditions.

58
Event

• At beginning of processing, an event contains
  primary particles. These primaries are pushed
  into a stack.
• When the stack becomes empty, processing of an
  event is over.
• G4Event class represents an event. It has
  following objects at the end of its processing.
• List of primary vertexes and particles
• Trajectory collection (optional)
• Hits collections
• Digits collections (optional)

59
Track

• Track is a snapshot of a particle
• Step is a delta information to a track
• A track is made out of three layers of class
  objects.
• G4Track
• Position, volume, track length, global ToF
• ID of itself and mother track
• G4DynamicParticle
• Momentum, energy, local time, polarization
• Pre-fixed decay channel
• G4ParticleDefinition
• Shared by all G4DynamicParticle of same type
• Mass, lifetime, charge, other physical quantities
• Decay table

60
Step

• Step has two points and also delta information
  of a particle (energy loss on the step,
  time-of-flight spent by the step, etc.).

End of step point
Step
Begin of step point
Boundary
Trajectory

• Trajectory is a record of a track history. It
  stores some information of all steps done by the
  track as objects of G4TrajectoryPoint class.

61
How Geant4 runs

• Initialization
• Construction of material and geometry
• Construction of particles, physics processes and
  calculation of cross-section tables
• Beam-On Run
• Close geometry --gt Optimize geometry
• Event Loop
• ---gt More than one runs with different
• geometrical configurations

62
Initialization
63
Beam on
loop
64
Event processing
65
Detector Description

• Authors John Apostolakis and Gabriele Cosmo

Geant4 Training Kit
66
Concepts for Detector Description

• The following concepts will be described
• Material
• Detector Geometry
• Sensitive Volumes
• Hits

67
Definition of Materials

• Different kinds of materials can be defined
• isotopes ltgt G4Isotope
• elements ltgt G4Element
• molecules ltgt G4Material
• compounds and mixtures ltgt G4Material
• Attributes associated
• temperature, pressure, state, density

68
Creating a Detector Volume

• Start with its Shape Size
• Box 3x5x7 cm, sphere R8m
• Add properties
• material, B/E field,
• make it sensitive
• Place it in another volume
• in one place
• repeatedly using a function

Solid
Logical Volume
Physical volume
69
Define detector geometry

• Three conceptual layers
• G4VSolid -- shape, size
• G4LogicalVolume -- daughter phys. volumes,
• material, sensitivity, user limits,
  etc.
• G4VPhysicalVolume -- position, rotation

70
Detector geometry Solids

• Many Solids exist in G4 (G4VSolid)
• CSG solids
• G4Box, G4Tubs, G4Cons, G4Trd, etc.
• Analogous to simple GEANT3 solids
• BREP solids
• G4BREPSolidPolycone, G4BSplineSurface, etc.
• Boolean solids
• G4UnionSolid, G4SubtractionSolid, etc.
• STEP interface
• to import BREP solid models from CAD systems

71
Solids

• STEP compliant solid modeller
• Constructed Solids (CSGs)
• Boxes, Cylinders, Spherical shells
• Boundary Represented (BREPs)
• any order surface, NURBS
• Could be
• User defined 7 functions
• inside, distance in/out (x2), extent

72
What is a BREP?

• BREPBoundary Represented Solid
• Listing all its surfaces specifies a solid
• e.g. 6 squares for a cube
• Surfaces can be
• planar, 2nd or higher order
• Splines, B-Splines, NURBS
• NURBSNon-Uniform B-Splines

73
How we use CAD geometries

• Our BREP library contains all code
• needed for ISO STEP AP203
• We import the solid descriptions of detector
  models from CAD systems
• for example from Euclid Pro/Engineer
• using STEP AP203 files
• So we support tracking in boundary represented
  solids created in CAD

74
Physical Volumes

• Placement it is one positioned volume
• Repeated a volume placed many times
• can represent any number of volumes
• reduces use of memory.
• Replica simple repetition, like G3 divisions
• Parameterised
• A mother volume can contain either
• many placement volumes OR
• one repeated volume

placement
repeated
75
Magnetic field

• In order to propagate a particle inside a field
  (e.g. magnetic, electric or both), we integrate
  the equation of motion of the particle in the
  field.
• In general this is best done using a Runge-Kutta
  method for the integration of ordinary
  differential equations. Several Runge-Kutta
  methods are available.
• In specific cases other solvers can also be used
  
• In a uniform field, as the analytical solution is
  known.
• In a nearly uniform field, where we perturb it.

76
Things one can do with Geant4 geometry
One can do operations with solids
These figures were visualised with Geant4 Ray
Tracing tool
...and one can describe complex geometries, like
Atlas silicon detectors
77
A selection of geometry applications
BaBar at SLAC
XMM-Newton (ESA)
ATLAS at LHC, CERN
GLAST
CMS at LHC, CERN
Borexino at Gran Sasso Lab.
78
Magnetic field

• Once a method is chosen that allows G4 to
  calculate the track's motion in a field, we break
  up this curved path into linear chord segments.
• We determine the chord segments so that they
  closely approximate the curved path.
• We use the chords to interrogate the Navigator,
  to see whether the track has crossed a volume
  boundary.

79
Magnetic field

• You can set the accuracy of the volume
  intersection,
• by setting a parameter called the miss distance
• it is a measure of the error in whether the
  approximate track intersects a volume.
• Default miss distance is 3 mm.
• One step can consist of more than one chords.
• In some cases, one step consists of several turns.

miss distance
Step
Chords
real trajectory
80
Readout geometry

• Readout geometry is a virtual and artificial
  geometry which can be defined in parallel to the
  real detector geometry.
• A readout geometry is optional.
• Each one is associated to a sensitive detector.

81
Sensitive detector and Hit

• Hit is a snapshot of the physical interaction of
  a track or an accumulation of interactions of
  tracks in the sensitive region of your detector.
• A sensitive detector creates hit(s) using the
  information given in G4Step object. The user has
  to provide his/her own implementation of the
  detector response.
• Hit objects are collected in a G4Event object at
  the end of an event.

82
Hits
Digitisation

• You can store various types information by
  implementing your own concrete Hit class.
• For example
• Position and time of the step
• Momentum and energy of the track
• Energy deposition of the step
• Geometrical information
• or any combination of above

• Digit represents a detector output (e.g. ADC/TDC
  count, trigger signal).
• Digit is created with one or more hits and/or
  other digits by a concrete implementation derived
  from G4VDigitizerModule.

83
Electromagnetic Physics

• Authors M. Maire, P. Nieminen, M.G. Pia, L.
  Urban

Geant4 Training Kit
84
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

85
Electromagnetic physics
multiple scattering Cherenkov transition
radiation ionisation Bremsstrahlung annihilation p
hotoelectric effect Compton scattering Rayleigh
effect g conversion ee- pair production refractio
n reflection absorption scintillation synchrotron
radiation fluorescence Auger effect (in progress)

• It handles
• electrons and positrons
• g, X-ray and optical photons
• muons
• charged hadrons
• ions
• 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

energy loss
86
OO design
Top level class diagram of electromagnetic physics
Alternative models, obeying the same abstract
interface, are provided for the same physics
interaction
87
Production thresholds

• 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
• It makes better sense to use the range cut-off
• Range of 10 keV gamma in Si a few cm
• Range of 10 keV electron in Si a few micron

88
Effect of production thresholds
DCUTE 455 keV
In Geant3
500 MeV incident proton
one must set the cut for delta-rays (DCUTE)
either to the Liquid Argon value, thus producing
many small unnecessary d-rays in Pb,
Threshold in range 1.5 mm
or to the Pb value, thus killing the d-rays
production everywhere
455 keV electron energy in liquid Ar 2 MeV
electron energy in Pb
DCUTE 2 MeV
89
Standard electromagnetic processes
Shower profile, 1 GeV e- in water

• 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

JH Crannel - Phys. Rev. 184-2 August69
90
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
• Ionisation features
• optimise 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. 148. (1978)
399
91
Photo Absorption Ionisation Model
Ionisation energy loss produced by charged
particles in thin layers of absorbers
3 GeV/c p in 1.5 cm ArCH4
5 GeV/c p in 20.5 mm Si

• Ionisation energy loss distribution produced by
  pions, PAI model

92
Low energy e.m. extensions
Fundamental for space and medical applications,
neutrino experiments, antimatter spectroscopy etc.
Low energy hadrons and ions models based on
Ziegler and ICRU data and parameterisations
e,? down to 250 eV Geant3 down to 10
keV (positrons in progress)
Barkas effect models for antiprotons
Photon transmission on 1 mm Al
93
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
• Bremsstrahlung
• Ionisation
• Fluorescence

10 keV limit
250 eV limit
94
Example of application of Geant4 Low Energy e.m.
processes
water
Fe
Photon attenuation coefficient
Comparison of Geant4 electromagnetic processes
with NIST data
95
Low energy extensions hadrons and ions
Various models, depending on the energy range and
the charge

• E gt 2 MeV ? Bethe-Bloch
• 1 keV lt E lt 2 MeV ? parameterisations
• Ziegler 1977, 1985
• ICRU 1993
• corrections due to chemical formulae of materials
• nuclear stopping power
• E lt 1 keV ? free electron gas model
• Barkas effect taken into account
• quantum harmonic oscillator model

96
Muon processes
Validity range 1 keV up to 1000 PeV scale

• simulation of ultra-high energy and cosmic ray
  physics

• High energy extensions based on theoretical
  models
• Bremsstrahlung
• Ionisation and d ray production
• ee- Pair production

97
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
• Optical properties, e.g. dielectric coefficient,
  surface smoothness, can be set to a
  G4LogicalVolume
• Processes in Geant4
• in-flight absorption
• Rayleigh scattering
• reflection and refraction on medium boundaries

Track of a photon entering a light concentrator
CTF-Borexino
98
Examples of application of Geant4 e.m. physics
The plot is the visible energy in silicon as a
function of the energy of the incident
electron The experimental results are from
Sicapo Collaboration, NIM A332 (85-90) 1993
Sampling calorimeter
99
Hadronic Physics

• Authors M.G. Pia

100
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
• 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

The user has control on the physics used in the
simulation, which contributes to the validation
of physics results
101
Hadronic physicsParameterised and data-driven
models (1)

• 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

p elastic scattering on Hydrogen
102
Hadronic physicsParameterised and data-driven
models (2)

• Other models are completely new, such as
• stopping particles (?- , K- )
• neutron transport
• isotope production

• All existing databases worldwide used in neutron
  transport
• Brond, CENDL, EFF, ENDFB, JEF, JENDL, MENDL etc.

103
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

104
A sample from theory-driven models
105
An example of user application
CMS HCAL Test-Beam Setup
Courtesy of CMS Collaboration
106
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

107
Fast Simulation
A shortcut to the tracking

• Author Marc Verderi

Geant4 Training Kit
108
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 per 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 e.m. 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

109
Generalities

• Fast Simulation, also called parameterisation, is
  a shortcut to the tracking.
• Fast Simulation allows you to take over the
  tracking to implement your own fast physics and
  detector response.
• The classical use case of fast simulation is the
  shower parameterisation where the typical several
  thousand steps per GeV computed by the tracking
  are replaced by a few ten of deposits per GeV.
• Parameterisations are generally experiment
  dependent.

110
Parameterisation features

• Parameterisations take place in an envelope. This
  is typically the mother volume of a sub-system or
  of a large module of such a sub-system.
• Parameterisations are often particle type
  dependent and/or may apply only to some.
• They are often not applied in complicated regions.

111
Summary Picture of Fast Simulation Mechanism

• The Fast Simulation components are indicated in
  blue.

 envelope  (G4LogicalVolume)
G4FastSimulationManager
ModelForElectrons
Placements
ModelForPions
G4Track

• When the G4Track travels inside the volume of the
  envelope, the G4FSMP looks for a
  G4FastSimulationManager.
• If one exists, at the beginnig of each step in
  the envelope, the models are messaged to check
  for a trigger.
• In case a trigger is issued, the model is applied
  at the point the G4track is.
• Otherwise, the tracking proceeds with a normal
  step.

G4ProcessManager
Process xxx
Multiple Scattering
G4FastSimulationManagerProcess
G4Transportation
112
Example of integrated Fast/Full Simulation
application

• BaBar Object-oriented Geant4-based Unified
  Simulation (BOGUS)
• Integrated framework for Fast and Full simulation
• Fast simulation available for public use since
  February 1999
• Integrated in BaBar environment
• primary generators, reconstruction, OODB
  persistency
• parameters for materials and geometry shared with
  reconstruction applications

Courtesy of G. Cosmo
113
Visualisation and (G)UI

• Authors Hajime Yoshida and Satoshi Tanaka

Geant4 Training Kit
114
Introduction

• Geant4 Visualisation must respond to varieties of
  user requirements. For example,
• Quick response to survey successive events
• Impressive special effects for demonstration
• High-quality output to prepare journal papers
• Flexible camera control for debugging geometry
• Highlighting overlapping of physical volumes
• Interactive picking of visualised objects
• Etc.

115
Visualisable Objects (1)

• You can visualise simulation data such as
• Detector components
• A hierarchical structure of physical volumes
• A piece of physical volume, logical volume, and
  solid
• Particle trajectories and tracking steps
• Hits of particles in detector components
• Visualisation is performed either with commands
  or by writing C source codes of user-action
  classes

116
Visualisable Objects (2)

• You can also visualise other user defined objects
  such as
• A polyline, that is, a set of successive line
  segments for, e.g., coordinate axes
• A marker which marks an arbitrary 3D
  position,for, e.g., eye guides
• Texts, i.e., character strings for description,
  comments, or titles

117
Visualisation Drivers

• Visualisation drivers are interfaces to 3D
  graphics software
• You can select your favourite one(s) depending on
  your purposes such as
• Demo
• Preparing precise figures for journal papers
• Publication of results on Web
• Debugging geometry
• Etc

118
Available Graphics Software

• By default, Geant4 provides visualisation
  drivers, i.e. interfaces, for
• DAWN Technical High-quality PostScript output
• OPACS Interactivity, unified GUI
• OpenGL Quick and flexible visualisation
• OpenInventor Interactivity, virtual reality,
  etc.
• RayTracer Photo-realistic rendering
• VRML Interactivity, 3D graphics on Web

119
Sample Visualisation (1)
120
Sample Visualisation (2)
121
Sample Visualisation (3)
122
Select (G)UI

• Geant4 provides the following interfaces for
  various (G)UI
• G4UIterminal C-shell like character terminal
• G4UItcsh tcsh-like character terminal with
  command completion, history, etc.
• G4UIGAG Java based GUI
• G4UIOPACS OPACS-based GUI, command completion,
  etc.
• G4UIBatch Batch job with macro file
• G4UIXm Motif-based GUI, command completion, etc.

123
Useful GUI Tools Released by Geant4 Developers

• GGE Geometry editor based on Java GUI
• http//erpc1.naruto-u.ac.jp/geant4
• GPE Physics editor based on Java GUI
• http//erpc1.naruto-u.ac.jp/geant4
• OpenScientist, OPACS Flexible
  analysis environments
• http//www.lal.in2p3.fr/OpenScientist
• http//www.lal.in2p3.fr/OPACS

124
Persistency

• Author Youhei Morita

Geant4 Training Kit
125
Category Requirements

• Geant4 Persistency makes run, event, hits, digits
  and geometry information be persistent, to be
  read back later by user programs
• Geant4 shall make use of industrial standard ODMG
  C binding and HepODBMS as persistency interface
• Kernel part of Geant4 should not be affected by
  the choice of persistency mechanism (Geant4
  should be able to run with or without persistency
  mechanism)

126
What is object persistency ?

• Persistent object lives beyond an application
  process, may be a