Maria Grazia Pia, INFN Genova 1 Part I The motivations for Geant4

Maria Grazia Pia, INFN Genova 2 Part I: outline 4The role of simulation: an example 4The role of simulation 4The market of simulation packages 4Geant: what is and how it evolved 4Geant4: the motivations behind it

Maria Grazia Pia, INFN Genova 3 Once upon a time there was a X-ray telescope...

Maria Grazia Pia, INFN Genova 4 Chandra scheme

Maria Grazia Pia, INFN Genova 5 Chandra CCDs

Maria Grazia Pia, INFN Genova 6 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 Chandra’s 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 instrument’s back- side illuminated chips have shown no reduction in capability and continue to perform flawlessly. An excerpt of a press release

Maria Grazia Pia, INFN Genova 7 Chandra in Geant4

Maria Grazia Pia, INFN Genova 8 XMM

Maria Grazia Pia, INFN Genova 9 Geant4 simulations of Chandra and XMM ]Simulations to study the response of the instruments to the radiation environment on orbit and the lifetime of detectors  Hadron ionisation with  ray production, hadron multiple scattering, electron ionisation, electron Bremsstrahlung, e + e - annihilation,  ionisation,  Bremsstrahlung,  pair production, photoelectric effect, Compton scattering,  conversion ]Protons of energies from ~hundreds keV to a few MeV can scatter at low angles through the mirror shells of X-ray astronomy missions, producing a high non-ionising dose in unshielded CCDs ]Experimental measurements of proton reflectivity of XMM grating and mirror samples are in good agreement with Geant4 simulation XMM was launched on 10 December 1999 from Kourou

Maria Grazia Pia, INFN Genova 10 XMM

Maria Grazia Pia, INFN Genova 11 CCDs 30  m 2  m 30  m 2  m Active layer Passive layer CCD displacement damage: front vs. back-illuminated. 30  m Si  ~1.5 MeV p+

Maria Grazia Pia, INFN Genova 12 The role of simulation ]Simulation plays a fundamental role in various domains and phases of an experimental physics project l design of the experimental set-up l evaluation and definition of the potential physics output of the project l evaluation of potential risks to the project l assessment of the performance of the experiment l development, test and optimisation of reconstruction and physics analysis software l contribution to the calculation and validation of physics results ]The scope of these lectures (and of Geant4) encompasses the simulation of the passage of particles through matter l there are other kinds of simulation components, such as physics event generators, electronics response generators, etc. l often the simulation of a complex experiment consists of several of these components interfaced to one another

Maria Grazia Pia, INFN Genova 13 Domains of application ]HEP and nuclear physics experiments l the most “traditional” field of application l used by nearly all experiments l applications in astrophysics experiments too ]Radiation background studies l evaluation of safety constraints and shielding for the experimental apparatus and human beings ]Medical applications l radiotherapy l design of instruments for therapeutic use ]Biological applications l radiation damage (in human beings, food etc.) ]Space applications l they encompass all the aspects of the other domains above + In some of these areas simulation is mission critical

Maria Grazia Pia, INFN Genova 14 Requirements for physics validation The validation of the overall physics results of an experiment impose some requirements on simulation ]Transparency l the user has access to the code l and he/she can understand its content and how it is used l and he/she has control on what he/she uses in his/her physics application l the data and their use are kept distinct ]Public distribution of the code l the code is the same for all users and applications l no hand-made “specially tuned” versions of the code l the validation is done by independent users, not only by the authors of the code ]Use of evaluated databases and of published data l no “hard coded” numbers or parameters of unknown source l but use of the commonly accepted body of knowledge Geant4 implements all these guidelines

Maria Grazia Pia, INFN Genova 15 Components ] Modeling of the experimental set-up ] Tracking of particles through matter ] Interaction of particles with matter ] Modeling of the detector response ] Run and event control ] Visualisation of the set-up, tracks and hits ] User interface ] Accessory utilities (random number generators, PDG particle information etc.) ] Interface to event generators ] Persistency

Maria Grazia Pia, INFN Genova 16 The world of simulation packages ]Simulation of particle interaction with matter has been an active field for many years ]Many specialised and general purpose packages available on the market n GEANT3 n EGS n ITS n HETC n MCNP n MORSE n MICAP n CALOR n VENUS n LHI n CEPX-ONELD n TRIM, SRIM n TART... n etc.

Maria Grazia Pia, INFN Genova 17 Integrated suites vs specialised codes ]Specialised packages cover a specific simulation domain Pro: –the specific issue is treated in great detail –often 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 ]Integrated packages cover all/many simulation domains 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 approach: take all or nothing –limited or no options for alternative models –usually complex to install and use –difficult maintenance and evolution

Maria Grazia Pia, INFN Genova 18 Fast and full simulation ]Usually there are two types of simulations in a typical experiment Fast simulation l mainly used for feasibility studies and quick evaluations l coarse set-up description and physics modeling l usually directly interfaced to event generators Full simulation l used for precise physics and detector studies l requires a detailed description of the experimental set-up and a complex physics modeling l 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 ècomplexity of maintenance and evolution èpossibility of controversial results

Maria Grazia Pia, INFN Genova 19 The Toolkit approach...that is, how to get the best of all worlds ]A toolkit is a set of compatible components l each component is specialised for a specific functionality l each component can be refined independently to a great detail l components can be integrated at any degree of complexity l components can work together to handle inter- connected domains l it is easy to provide (and use) alternative components l the simulation application can be customised by the user according to his/her needs l maintenance and evolution - both of the components and of the user application - is greatly facilitated ]...but what is the price to pay? l the user is invested of a greater responsibility l he/she must critically evaluate and decide what he/she needs and wants to use

Maria Grazia Pia, INFN Genova 20 Geant: a historical overview GEANT comes from GEometry ANd Tracking ]Geant2: an attempt to build a first prototype in the late ‘70s l the ideas behind: memory management and integrated geometry/tracking/physics ]Geant3: the simulation tool of the ‘80s and ‘90s l several versions (last: Geant3.21 in 1994) ]New physics and software requirements for the LHC era triggered a R&D project for Geant4 ]With Geant4 there has been a technology transition l before: procedural software, FORTRAN l Geant4: Object Oriented technology, C++ ]Geant3 was a CERN product l developed, distributed, maintained and supported by the CERN DD/CN/IT Division l a few external individuals contributed to its development ]Geant4 is the product of an international collaboration

Maria Grazia Pia, INFN Genova 21 The role of Geant ]Geant is a simulation tool, that provides a general infrastructure for l the description of the geometry and materials of an experimental set-up l particle transport and interaction with matter l the description of detector response l visualisation of geometries, tracks and hits ]The experiment develops the specific code for l the primary event generator l the description of the experimental set-up l the digitisation of the detector response ]It plays a fundamental role in various phases of the life-cycle of an experiment l detector design l development of reconstruction and analysis software l physics studies ]Other roles in non-HEP fields (eg. treatment planning in radiotherapy)

Maria Grazia Pia, INFN Genova 22 The past: Geant3 ]Geant 3 l Has been used by most major HEP experiments l Frozen since March 1994 (Geant3.21) l ~200K lines of code l equivalent of ~50 man-years, along 15 years l used also in nuclear physics experiments, medical physics, radiation background studies, space applications etc. ]The result is a complex system l because its problem domain is complex l because it requires flexibility for a variety of applications l because its management and maintenance are complex ]It is not self-sufficient l hadronic physics is not native, it is handled through the interface to external packages

Maria Grazia Pia, INFN Genova 23 New simulation requirements New simulation requirements derive from ]the specific features of the new generation of HEP experiments l LHC, astroparticle physics etc. ]application of simulation tools to new domains l space, medical, biological etc. New simulation requirements address ]the physics capabilities of the simulation tool ]the software/computing characteristics Geant4 was born from the user communites ]User requirements formally collected from the user communities and continuously updated + Geant4 User Requirements Document

Maria Grazia Pia, INFN Genova 24 New simulation requirements: physics ]Transparent physics l for the validation of physics results ]Physics extensions to high energies l LHC experiments l cosmic ray experiments l etc.... ]Physics extensions to low energies l space applications l medical physics l X-ray analysis l astrophysics experiments l nuclear and atomic physics l etc.... ]Reliable hadronic physics l not only for calorimetry, but also for PID applications (CP violation experiments) ]...etc.

Maria Grazia Pia, INFN Genova 25 New simulation requirements: computing ]The very high statistics to be simulated requires l robustness and reliability for large scale production ]The long lifetime of the new generation of experiments requires l easy extension of the functionalities (new physics models, new data, new technologies etc.) l easy maintenance and evolution ]Independence from external software products and specific technologies requires l coupling to be managed through interfaces ]The connection between the physics design and the engineering design of the experiments requires l exchange of CAD detector descriptions ]The wide range of expertise necessary for a new complex simulation tool requires l software technologies suitable for distributed parallel development ]etc.

Maria Grazia Pia, INFN Genova 26 What is Geant4? ]Geant4 is an 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, medical physics, space applications, radiation background studies etc. ]It is also an experiment of distributed software production and management, as a large international collaboration with the participation of various experiments, labs and institutes ]It is also an experiment of application of rigorous software engineering and Object Oriented technologies to the HEP environment

Maria Grazia Pia, INFN Genova 27 Motivations for a redesign of Geant ]It had become too complicated l to maintain the program l to extend its functionality l to improve the physics transparency and content ]Geant3 was not technically adequate to the new generation experiments l the data structures are not adequate l memory handling is not adequate ]Geant3 was not physically adequate to the new generation experiments l either because of insufficient accuracy and reliability l or because of incomplete coverage of the energy scale ]Data exchange and interface with other tools was too difficult or impossible ]A fundamental component (hadronic physics) was external to Geant3 ]...etc.

Maria Grazia Pia, INFN Genova 28 Geant4 history: the R&D phase ]Approved as R&D end 1994 (RD44) l >100 physicists and software engineers l ~40 institutes, international collaboration l responded to DRCC/LCB ]Milestones: end 1995 l OO methodology, problem domain analysis, full OOAD l tracking prototype, performance evaluation ]Milestones: spring 1997  -release with the same functionality as Geant3.21 l persistency (hits), ODBMS l transparency of physics models ]Milestone: July 1998 public  -release ]Milestone: end 1998 l production release: Geant4.0, end of the R&D phase ]All milestones have been met by RD44

Maria Grazia Pia, INFN Genova 29 Geant4 history: the production phase ]Reconfiguration at the end of the R&D phase l International Geant4 Collaboration sincel 1/1/1999 l CERN, JNL, KEK, SLAC, TRIUMF l Atlas, BaBar, CMS, LHCB, TERA(IGD) l ESA, Frankfurt Univ., IN2P3, INFN(IDG), Lebedev l new membership applications being discussed ]Management of the production phase l production service l user support l continuing development ]Production releases l Geant4 0.0, December 1998 l Geant4 0.1, July 1999 l Geant4 1.0, December 1999 l...more to come l regular “reference tags” released for collaborating experiments