Geant4 KISTI Tutorial Marc Verderi LLR – Ecole polytechnique October 2012 Physics II: Overview, Processes, Production Threshold

30/10/12 ›Gean4 provides a wide variety of physics components, coded as processes ›Processes are organized into three main categories: “Electromagnetic“, “Hadronic“ and “Decay & Parameterized“ ›Each process provides methods to determine... –...at what point a particle interacts –...what happens to the particle when it interacts ›In most cases a user will not worry about the structure of the process class and merely choose which processes to apply Introduction MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 2

30/10/12 I.Physics Overview –Overview of Geant4 physics capability II.Processes –How physics processes are modeled in Geant4 III.Production Thresholds (aka cuts) Outline MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 3

I.Physics Overview

30/10/12 ›Standard : Complete set of processes covering charged particles and gammas. –Energy range 1 keV - ~PeV › Low energy : More precise description at low energy for e +, e , , charged hadrons incident particle. –More atomic shell structure detail –Some processes valid down to hundreds of eV –Some processes not valid above 1 GeV › Optical photon : Long wavelength  (X-ray, UV, visible) –Reflection, refraction, absorption, wavelength shifts, Rayleigh scattering Electromagnetic Processes MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 5

30/10/12 ›Pure Hadronic Processes (0 - ~TeV) –elastic –inelastic –capture –fission ›Strong Radioactive Decay –at rest –In flight ›Photo-Nuclear (~10 MeV - ~TeV) –Gamma-nuclear reactions ›Lepto-Nuclear (~10 MeV - ~TeV) –e+, e- nuclear reactions –muon nuclear reactions Hadronic Processes MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 6

30/10/12 Variety of Hadronic Physics Models Capture at rest Fission / evaporation String Models: Quark Gluon String, Fritiof  evaporation Intra-Nuclear Cascade Models: Binary, Bertini Intermediate Energy Nucleon dominating behavior Low Energy Nucleus dominating behavior High Energy Quark/gluon dominating behavior Main Models Precompound Radioactive decay MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 7

30/10/12 ›Decay processes –Decay of particles having a width narrow enough ›i.e. : exclude hadronic resonances –weak decay (leptonic decays, semi-leptonic decays, radioactive decay of nuclei) –electromagnetic decay (e.g. π 0, Σ 0 ) ›Parameterized –Fast Simulation functionnality –Provide hook to the user to shortcut the detailed tracking ›and develop for example a parameterized EM shower Decay and Parameterized MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 8

II.Processes

–PostStep actions: For describing point-like (inter)actions, like decay in flight, hard radiation… G4VProcess : 3 kind of actions (1/2) ›Abstract class defining the common interface of all processes in Geant4: –Used by all « physics » processes –but is also used by the transportation, etc… – Defined in source/processes/management ›Define three kinds of actions: –AlongStep actions: To describe continuous (inter)actions, occuring along the path of the particle, like ionisation; –AtRest actions: Decay, e + annihilation … AlongStep PostStep MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL10 30/10/12

G4VProcess : 3 kind of actions (2/2) › A process can implement any combination of the three AtRest, AlongStep and PostStep actions: – eg: decay = AtRest + PostStep ›Each action defines two methods: – GetPhysicalInteractionLength() : ›Used to limit the step: – either because the process « triggers » an interaction, a decay – or any other reasons, like fraction of energy loss, geometry boundary, user’s limit … – DoIt() : ›Implements the actual action to be applied on the track; ›And the related production of secondaries. MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL11 30/10/12

Process Handling by the Stepping 1.At the beginning of the step, determine the step length: – Consider all processes attached to the current G4Track ; –Define the step length as the smallest of the lengths among: › All AlongStepGetPhysicalInteractionLenght() › All PostStepGetPhysicalInteractionLength() MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL12 30/10/12

Process Handling by the Stepping 1.At the beginning of the step, determine the step length: – Consider all processes attached to the current G4Track ; –Define the step length as the smallest of the lengths among: › All AlongStepGetPhysicalInteractionLenght() › All PostStepGetPhysicalInteractionLength() 2.Apply all AlongStepDoIt() actions, « at once »: –Changes computed from particle state at the beginning of the step; – Accumulated in the G4Step ; – Then applied to the G4Track, from the G4Step. 3.Apply PostStepDoIt() action(s) « sequentially », as long as the particle is alive: – Apply PostStepDoIt() of process which limited the step (if any); –And apply any other « forced » processes (not discussed here) MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL13 30/10/12

III.Production Thresholds (aka « cuts »)

30/10/12 ›Every simulation developer must answer the question: how low can I go? –at what energy do I stop tracking particles? ›This is a balancing act: –need to go low enough to get the physics you’re interested in –can’t go too low because some processes have infrared divergence causing CPU to skyrocket Threshold for Secondary Production MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 15

30/10/12 ›Every simulation developer must answer the question: how low can I go? –at what energy do I stop tracking particles? ›This is a balancing act: –need to go low enough to get the physics you’re interested in –can’t go too low because some processes have infrared divergence causing CPU to skyrocket Threshold for Secondary Production MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 16 Bremstralhung: actual divergence in forward production of ultra-soft gammas.  Ionisation : large production of ionisation electrons, from loosely bound atomic ones. Soft 

30/10/12 ›Every simulation developer must answer the question: how low can I go? –at what energy do I stop tracking particles? ›This is a balancing act: –need to go low enough to get the physics you’re interested in –can’t go too low because some processes have infrared divergence causing CPU to skyrocket ›The traditional Monte Carlo solution is to impose an absolute cutoff in energy –particles are stopped when this energy is reached –remaining energy is dumped at that point Threshold for Secondary Production MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 17

30/10/12 ›But, such a cut may cause imprecise stopping location and deposition of energy ›There is also a particle dependence – range of a 10 keV  in Si is a few cm –range of a 10 keV e- in Si is a few microns ›And a material dependence –suppose you have a detector made of alternating sheets of Pb and plastic scintillator –if the cutoff is OK for Pb it will likely be wrong for the scintillator which does the actual energy measurement Threshold for Secondary Production MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 18

30/10/12 ›Geant4 solution: impose a production threshold –this threshold is a distance, not an energy –default = 1 mm –the primary particle loses energy by producing secondary electrons or gammas –if primary no longer has enough energy to produce secondaries which travel at least 0.7 mm, two things happen: ›discrete energy loss ceases (no more secondaries produced) ›the primary is tracked down to zero energy using continuous energy loss ›Stopping location is therefore correct ›Only one value of production threshold distance is needed for all materials because it corresponds to different energies depending on material Threshold for Secondary Production MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 19

30/10/12 Production Threshold vs. Energy Cut Example: 500 MeV p in LAr-Pb Sampling Calorimeter Energy Threshold Geant4 Production Range Threshold MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 20

30/10/12 MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL GeV e  in liquid Argon, with cuts of 1 km, 1 m, 1 mm and 1  m 4 meters 1 km 1 m 1 mm 1  m #track=1; #step=22; time~0s #track=8k; #step=20k; time=60ms #track=18k; #step=39k; time=90ms#track=724k; #step=1.5M; time=4.6s

30/10/12 MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL GeV e  in liquid Argon, with cuts of 1 km, 1 m, 1 mm and 1  m 1 km 1 m 1 mm 1  m #track=1; #step=22; time~0s #track=8k; #step=20k; time=60ms #track=18k; #step=39k; time=90ms#track=724k; #step=1.5M; time=4.6s

30/10/12 MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL GeV e  in liquid Argon, with cuts of 1 km, 1 m, 1 mm and 1  m 1 km 1 m 1 mm 1  m #track=1; #step=22; time~0s #track=8k; #step=20k; time=60ms #track=18k; #step=39k; time=90ms#track=724k; #step=1.5M; time=4.6s

30/10/12 MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL GeV e  in liquid Argon, with cuts of 1 km, 1 m, 1 mm and 1  m 1 km 1 m 1 mm 1  m #track=1; #step=22; time~0s #track=8k; #step=20k; time=60ms #track=18k; #step=39k; time=90ms#track=724k; #step=1.5M; time=4.6s

30/10/12 ›Geant4 proposes the default value of 0.7 mm –user needs to decide the best value –this will depend on the size and sensitive elements within the simulated detector, and on available CPU ›Be aware : not well controlled cuts may lead to inappropriate results ;) ›This value is set in the SetCuts() method of your physics list ›Instead of “secondary production threshold distance” it is more convenient to simply say “cuts” –but please remember that this does not mean that any particle is actually stopped before it runs out of energy Threshold for Secondary Production MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 25

30/10/12 ›Geant4 supplies many physics processes which cover electromagnetic, hadronic, decay physics and parameterized. ›A unique interface, G4VProcess, allows processes to specify their nature: AtRest, Along (continuous), PostStep (discrete) –A process may mix several of these ›The precision of particle stopping and the production of secondary particles are determined by a secondary production threshold –More on configuring cuts (per region) later in this tutorial Summary MARC VERDERI - LLR - GEANT4 KISTI TUTORIAL 26