Geant4 User's Guide for Application Developers - Geant4 - CERN

Tracking and Physics5.2.6.8. Gflash ParameterizationThis section describes how to use the Gflash library. Gflash is a concrete parameterization which is based onthe equations and parameters of the original Gflash package from H1(hep-ex/0001020, Grindhammer & Peters,see physics manual) and uses the "fast simulation" facilities of GEANT4 described above. Briefly, whenever ae-/e+ particle enters the calorimeter, it is parameterized if it has a minimum energy and the shower is expectedto be contained in the calorimeter (or " parameterization envelope"). If this is fulfilled the particle is killed, aswell as all secondaries, and the energy is deposited according to the Gflash equations. An example, provided inexamples/extended/parametrisation/gflash/, shows how to interface Gflash to your application. The simulationtime is measured, so the user can immediately see the speed increase resulting from the use of Gflash.5.2.6.9. Using the Gflash ParameterisationTo use Gflash "out of the box" the following steps are necessary:• The user must add the fast simulation process to his process manager:void MyPhysicsList::addParameterisation(){G4FastSimulationManagerProcess*theFastSimulationManagerProcess = new G4FastSimulationManagerProcess();theParticleIterator->reset();while( (*theParticleIterator)() ){G4ParticleDefinition* particle = theParticleIterator->value();G4ProcessManager* pmanager = particle->GetProcessManager();pmanager->AddProcess(theFastSimulationManagerProcess, -1, 0, 0);}}• The envelope in which the parameterization should be performed must be specified (below: G4Regionm_calo_region) and the GFlashShowerModel must be assigned to this region. Furthermore, the classesGFlashParticleBounds (which provides thresholds for the parameterization like minimal energy etc.),GflashHitMaker(a helper class to generate hits in the sensitive detector) and GFlashHomoShowerParamterisation(which does the computations) must be constructed (by the user at the moment) and assigned to theGFlashShowerModel. Please note that at the moment only homogeneous calorimeters are supported.m_theFastShowerModel = new GFlashShowerModel("fastShowerModel",m_calo_region);m_theParametrisation = new GFlashHomoShowerParamterisation(matManager->getMaterial(mat));m_theParticleBounds = new GFlashParticleBounds();m_theHMaker= new GFlashHitMaker();m_theFastShowerModel->SetParametrisation(*m_theParametrisation);m_theFastShowerModel->SetParticleBounds(*m_theParticleBounds) ;m_theFastShowerModel->SetHitMaker(*m_theHMaker);The user must also set the material of the calorimeter, since the computation depends on the material.• It is mandatory to use G4VGFlashSensitiveDetector as (additional) base class for the sensitive detector.class ExGflashSensitiveDetector: public G4VSensitiveDetector ,public G4VGFlashSensitiveDetectorHere it is necessary to implement a separate interface, where the GFlash spots are processed.(ProcessHits(G4GFlashSpot*aSpot ,G4TouchableHistory* ROhist))A separate interface is used, because the Gflash spots naturally contain less information than the full simulation.Since the parameters in the Gflash package are taken from fits to full simulations with Geant3, some retuningmight be necessary for good agreement with Geant4 showers. For experiment-specific geometries some retuningmight be necessary anyway. The tuning is quite complicated since there are many parameters (some correlated)178