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

Tracking and PhysicsIn cases where the scintillation yield of a scintillator depends on the particle type, different scintillation processesmay be defined for them. How this yield scales to the one specified for the material is expressed with the ScintillationYieldFactorin the user's PhysicsList as shown in Example 5.6. In those cases where thefast to slow excitation ratio changes with particle type, the method SetScintillationExcitationRatiocan be called for each scintillation process (see the advanced underground_physics example). This overwrites theYieldRatio obtained from the G4MaterialPropertiesTable.Example 5.6. Implementation of the scintillation process in PhysicsList.G4Scintillation* theMuonScintProcess = new G4Scintillation("Scintillation");theMuonScintProcess->SetTrackSecondariesFirst(true);theMuonScintProcess->SetScintillationYieldFactor(0.8);theParticleIterator->reset();while( (*theParticleIterator)() ){G4ParticleDefinition* particle = theParticleIterator->value();G4ProcessManager* pmanager = particle->GetProcessManager();G4String particleName = particle->GetParticleName();if (theMuonScintProcess->IsApplicable(*particle)) {if (particleName == "mu+") {pmanager->AddProcess(theMuonScintProcess);pmanager->SetProcessOrderingToLast(theMuonScintProcess, idxAtRest);pmanager->SetProcessOrderingToLast(theMuonScintProcess, idxPostStep);}}}A Gaussian-distributed number of photons is generated according to the energy lost during thestep. A resolution scale of 1.0 produces a statistical fluctuation around the average yield set withAddConstProperty("SCINTILLATIONYIELD"), while values > 1 broaden the fluctuation. A value ofzero produces no fluctuation. Each photon's frequency is sampled from the empirical spectrum. The photons originateevenly along the track segment and are emitted uniformly into 4# with a random linear polarization and attimes characteristic for the scintillation component.5.2.5.3. Generation of Photons in processes/optical - WavelengthShiftingWavelength Shifting (WLS) fibers are used in many high-energy particle physics experiments. They absorb lightat one wavelength and re-emit light at a different wavelength and are used for several reasons. For one, they tendto decrease the self-absorption of the detector so that as much light reaches the PMTs as possible. WLS fibers arealso used to match the emission spectrum of the detector with the input spectrum of the PMT.A WLS material is characterized by its photon absorption and photon emission spectrum and by a possible timedelay between the absorption and re-emission of the photon. Wavelength Shifting may be simulated by specifyingthese empirical parameters for each WLS material in the simulation. It is sufficient to specify in the user'sDetectorConstruction class a relative spectral distribution as a function of photon energy for the WLSmaterial. WLSABSLENGTH is the absorption length of the material as a function of the photon's momentum.WLSCOMPONENT is the relative emission spectrum of the material as a function of the photon's momentum,and WLSTIMECONSTANT accounts for any time delay which may occur between absorption and re-emissionof the photon. An example is shown in Example 5.7.168