GEANT4 The physics simulation toolkit by Mike Perricone
You go to a home supplies store and purchase the oak one of the biggest projects ever mounted in science: the board for those shelves you need to add to your base- development of the proposed International Linear Collider. ment. You haul it home and discover integrated within the Serving as combination instruction manual/toolkit/support wood are instructions for building not just the bookcase network is geant4, a freely-available software package but your entire basement redesign, along with all the tools that simulates the passage of particles through scientific you’ll need–and the expertise to use them, plus a support instruments, based on the laws of particles interacting with group standing by to offer help and suggestions while you matter and forces, across a wide energy range. Besides cut and drill, hammer and fit. its critical use in high-energy physics (its initial purpose) This do-it-yourself fantasy is not far removed from the geant4 has also been applied to nuclear experiments, way physicists are now working on facilities that include and to accelerator, space, and medical physics studies.
20 21 CMS detector, which weighs in at nearly 14,000 Illustration: ESA-Ducros sitive X-raytelescopeeverbuilt. in 1999.XMM-Newtoncarriesthemostsen to launchbytheEuropeanSpaceAgency spacecraft weremodeledwith on theinstrumentsofXMM-Newton environment radiation the of effects The Image: CMScollaboration the a datavolumeequivalentto10,000copiesof tons. DuringonesecondofCMSrunning, Left Photo:A in the full Compact Muon Solenoid detector.in thefullCompact Muon Solenoid Image: CMScollaboration decaying totwo Right Image: transverse energy), with a Higgs particle Higgs a with energy), transverse event(showingleptonsandmissing symmetry Encyclopaedia Britannica geant geant Z particlesandtofourmuons 4 4 simulationofasuper- visualizationofthe isrecorded. geant 4 prior
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symmetry | volume 02 | issue 09 | november 05
“The benefit that has struck me most strongly But geant4 is an ongoing worldwide collabo- is the way geant4 can be used to help in treat- ration as much as it is a software package with ing tumors,” says Fermilab’s Daniel Elvira. The worldwide use. The software from its beginnings European Particle Physics Laboratory (CERN) was an independent project, developed through and Stanford Linear Accelerator Center (SLAC) the efforts of more than 100 scientists from more are the focal points of geant4 resources; Elvira than 40 institutes and experiments in Europe, heads up a small but growing geant4 team at Russia, Japan, Canada, and the United States, Fermilab, providing support for individual users all signatories to a Memorandum of Under- and experiment collaborations at the lab. He standing that is renewed every two years. Gov- continues: “This is a very nice example how a ernance is through a Collaboration Board (CB), physics-based tool, developed in a physics envi- a Technical Steering Board, and several work- ronment, has filtered to other communities in groups. The Technical Steering Board, chaired such effective ways. Simulations are now used by CERN’s John Apostolakis, is judge and jury in all places in society. Simulations enable the for software and physics issues, for user requests, optimization of designs for better performance and for suggested software revisions. Respon- with lower costs, in the same way that the sibilities of the Collaboration Board, chaired by Internet saves money through the sharing of SLAC’s Richard Mount, include monitoring the information.” overall allocation of resources. “The CB consists, in the main, of people with Building with objects the power to allocate resources within their geant4 is “object-oriented” in design and function. institution,” Mount explains. “When resource- A segment of data (an “object”) is encapsulated related issues are brought to the CB, it allows with the routines or methods that operate on institutional decisions to be made on the basis of that data. In other words, the information knows a full knowledge of what geant4 needs, and how to process itself when a user retrieves it. what is likely to be available from other parts of Almost by definition, geant4 can handle virtually the collaboration.” any simulation task within reasonable bounds; maybe even within unreasonable bounds. GEANT4 and ILC “Practically everything is an object–a detector, geant4 is being used to design three International a particle, a particle track, a vertex, a hit in the Linear Collider detector concepts by three detectors,” says Fermilab’s Maya Stavrianakou, interdependent global collaborations. “The soft- working with geant4 at CERN in the completion ware used in this design process must provide stages of the Compact Muon Solenoid detector full simulation capabilities for the whole for the Large Hadron Collider. International Linear Collider physics program, The complex CMS detector has about 100 including physics simulations, detector designs, million events used for physics preparation stud- and machine backgrounds,” says Norman Graf ies. The attributes and interactions of a dynamic of SLAC, leader of the North American Detector particle in the detector can be described and simulation group. “The goal is to have a common retrieved in geant4–with all requisite tools and simulation environment used in all ILC studies operations attached. which allows sharing of detectors, algorithms, and code. The system should be flexible, powerful, GEANT4 heritage yet simple to install and maintain.” As with any tool so valuable and easy to use, geant4 (from “geometry and tracking”) was years in the making–decades, in fact. It also embodies the transition of scientific computing code from the programming language fortran (“formula translation”), developed by IBM in the 1950s, to C++, the object-oriented language developed in the 1980s. C++ grew from the C language devel- oped in the 1970s at Bell Labs for the UNIX operating system. Ancestors of geant4 software written in fortran date back to CERN experi- ments in the 1980s at the Large Electron-Positron collider, but the specific genesis of geant4–the current indispensable package–springs from a 1994 proposal to the CERN Detector Research and Development Committee, for rewriting the geant4 is being used to design highly efficient detectors to software code into C++. The proposal was explore the physics from events produced at the ILC, such as - approved as RD44, reporting to the CERN Large this simulated response to e + e- p Z (p μ+ μ-) + higgs (p bb) Hadron Collider Committee. Image: Norman Graf
22 allows manydetectors tobequickly simulated Graf says,“wehaveadoptedanapproach which in previousgeneralpurposecolliderdetectors,” of theILC detectors hastobemuch higherthan the crucialparameters. MonteCarlosimulations are acceptance angular and resolution, angular just the barest sketch of a detector. Momentum, usually, itbeginswiththephysicsmotivationand of thedetector R&Dprocess.This allowsa subdetector prototypeswhich arebuiltaspart has been determined. We can also simulate designsonceanoveralloptimum detailed very Weto level. proceed reasonable only a would at analysis of what will occur inside the detector. the inside occur will what of analysis of particleinteractionsgeneratethephysics for cancer treatment–a procedure first suggested (such asprotons) andions(such ascarbon) particles therapy usesexternal beamsofheavy therapy forthetreatmentoftumors. Hadron medical physicists working on for collaborator US major a become has SLAC GEANT4 inmedicine of GeV.” hundreds to up MeV few a from simulation Richard DuboisofSLAC.“WerequiregoodEM the bottomandtrytolooklikephotons,”says sneak into the detector through cracks or through photon signals and to explore how hadrons might simulations, weuse the radiationenvironment. simulations havebeenonthejobinexploring and space, in detector particle physics high-energy- a is GLAST purposes, practical high-energy cosmic-raybackground.Forall and ananticoincidencedetectortodiscountthe the cosmicrayparticles;adataacquisitionsys cesium-iodide barstomeasuretheenergyof using calorimeter, a detectors; silicon-strip on main components:aprecisiontracker,based ray bursts.TheprojectedlaunchisAugust2007. supermassive blackholes,pulsars,andgamma- from thecentralregionsofexoticobjectslike in France,Italy,Japan,andSweden.GLAST the US Department of Energy and with agencies Space Telescope(GLAST),ajointprojectwith spacecraft andNASA’sGammaRayLargeArea Newton ESA’s in notably design, spacecraft have benefitedfrom Agency Space European the and NASA Both GEANT4 inspace the simulationprocess.” seamless integrationoftestbeamresultsinto tions, particularly in hadron therapy and brachy and therapy hadron in particularly tions, will detect gamma rays–high-energy photons– rays–high-energy gamma detect will tem, with 32-bit radiation-hard processors; processors; radiation-hard 32-bit with tem, “Since thedesiredprecisionandperformance A detector designmustbeginsomewhere; The GLASTLargeAreaTelescopehasfour “Because oftheveryreliableelectromagnetic geant geant 4 4 tosimulateour applicationsin geant 4 applica geant
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23 through simulations.We cannot replacetheex- particles undiscovered or new of properties the Butwe cannotlearnabout of theHiggsboson. properties the example, for theorists; by dicted to better serve thoseexperts.”to better serve develop regional collaboration, and communicate advice, and issues share to America, North fast protons,” publishedin inhispaper,Wilson in 1946 “Radiologicaluseof by Fermilab foundingdirectorRobertRathbun hadronics andelectromagnetics tosoftware a cancer, using tubes or catheters. catheters. or tubes cancer, a using but theycannotteach usphysics thatwedo Not asfarmakingadiscovery, Elvirasays. launched in May of 2005. 2005. of May in launched American MedicalUsersOrganization( physics dosedetectors. Perl hasalso played medical- easy-to-reuse simulationsofstandard as such researchers, medical aid to tools tional with working and Center; Medical University applications of illustrate themosteffective useofthebeam depth-dose distributioninagivenmaterial,and Itcanalsocalculatethe distribution curves. dose proton/ion the calculate and line beam simulate atypicalhadron-therapytreatment therapy implantsaradiationsourcecloseto periments. We will always need the experiments.” problems, visualizingcomplex environ Solving Take ittothe limits expertise but rather to refine the isnottoreplicatethat Our intentionfromSLAC work insmallergroupsthroughouttheworld. applications lies with the medical physicists who Buttherealexpertise inmedical mentation. docu and visualization, interface, user kernel, in theworldof annual meeting oftheAmericanAssociation the at place took 2005, July in meeting, major as agroupwith together the of parts all in experts “comprising how far could ments, savingtime,effort, savingmoney– for particles that have certain propertiespre for particlesthathavecertain not know,” hesays.“We canlearnhowtosearch of PhysicistsinMedicineSeattle. University of California, Santa Cruz; and Stanford such asUniversityofCalifornia,SanFrancisco; tise to medical researchers at nearby institutions in hadrontherapy. colleagues at KEK in Japan to develop addi develop to Japan in KEK at colleagues a a leading role in organizing the “Simulations areanessentialpartofthework, hasoneofthelargestconcentrations “SLAC Joseph Perl headsuptheeffort onmedical geant geant geant geant geant 4 4 medicalusercommunityin take all these advantages? 4 4 4 at SLAC, offering atSLAC, exper expertise,” saysPerl, developers.The first Radiobiology. Brachy- g 4 namu geant geant geant brings brings geant 4 North North 4 g 4 toolkit 4 from from 4 namu can can -
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symmetry | volume 02 | issue 09 | november 05