Gabriele Cosmo
344 lines
13 KiB
Plaintext
344 lines
13 KiB
Plaintext
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LXe Example
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------------
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Introduction
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------------
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This example demonstrates usage of optical physics.
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-----------------------------
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Geometry and primary particle
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-----------------------------
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The main volume is a box of LXe. PMTs are placed around the outside. There
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may be a reflective sphere placed inside the box, and a wavelength shifting
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slab and fibers.
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The geometry implementation is different from many of the other examples.
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See the discussion below.
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G4ParticleGun creates the primary particle. The type of particle is selectable
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by the user.
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-------
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Physics
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-------
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The physics list is FTFP_BERT, with G4EmStandard_option4 electromagnetic
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physics and G4OpticalPhysics.
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-----------
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Macro files
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-----------
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cerenkov.mac disables scintillation, so the optical photons that are produced
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are Cerenkov photons.
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wls.mac implements a scintillating slab and wavelength shifting fibers.
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---------------------------
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List of built-in histograms
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---------------------------
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1 "hits per event"
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2 "hits per event above threshold"
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3 "scintillation photons per event"
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4 "Cerenkov photons per event"
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5 "absorbed photons per event"
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6 "photons absorbed at boundary per event"
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7 "energy deposition in scintillator per event"
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-------------
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How to start?
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-------------
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- execute LXe in 'batch' mode from macro files, e.g.
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$ ./LXe cerenkov.mac
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- execute LXe in 'interactive' mode with visualization, e.g.
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$ ./LXe
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Then type commands, for instance
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Session: /run/beamOn 1
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-----------------------------------------------
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Detailed Explanation of Geometry Implementation
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-----------------------------------------------
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The way the geometry is constructed is an experiment for a new, more object
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oriented, way to construct geometry. It separates the concept of how a volume
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is built from where it is placed. Each major volume in the geometry is defined
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as a class derived from G4PVPlacement. In this example, just the main LXe
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volume, the WLS scintillator slab, and the WLS fibers were chosen. To place
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one of these volumes, simply create an instance of it with the appropriate
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rotation, translation, and mother volumes.
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LXeMainVolume(G4RotationMatrix *pRot,
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const G4ThreeVector &tlate,
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G4LogicalVolume *pMotherLogical,
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G4bool pMany,
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G4int pCopyNo,
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LXeDetectorConstruction* c);
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Also necessary are the pMany and pCopyNo variables with the same usage as in
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G4PVPlacement. Additionally, the detector construction must be passed to the
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main volume as a way to communicate the many parameters to the volume and its
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sub-volumes. The communication is done from the CopyValues() function which
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retrieves the information from the detector constructor.
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Notably, the name and logical volume parameters are no longer part of the
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constructor. This is because they are both to be decided by the volume itself.
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The volume must specify its own name and a temporary logical volume. The
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constructor will then procede to define its logical volume in the normal way.
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Once complete, the logical volume can be assigned to the physical volume using
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the SetLogicalVolume() function.
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To handle instances of the same type of volume, a new logical volume should not
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be defined for each one. Instead, the logical volume is kept as a static member
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and defined only once.
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if (!housing_log || updated) {
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//...
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//Define logical volume
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//...
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}
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SetLogicalVolume(housing_log);
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The updated variable is to signal that the volume needs to be updated and a new
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logical volume made.
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---------------------------------
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Modifying the geometry at runtime
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---------------------------------
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This example allows the user to modify the geometry definition at runtime. This
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is accomplished through LXeDetectorMessenger, a derived class of G4UImessenger.
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The commands it adds change variables stored in LXeDetectorConstructor that
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are used when constructing the geometry.
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void LXeDetectorConstruction::UpdateGeometry(){
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// clean-up previous geometry
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G4SolidStore::GetInstance()->Clean();
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G4LogicalVolumeStore::GetInstance()->Clean();
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G4PhysicalVolumeStore::GetInstance()->Clean();
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//define new one
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G4RunManager::GetRunManager()->DefineWorldVolume(ConstructDetector());
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G4RunManager::GetRunManager()->GeometryHasBeenModified();
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}
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----------------------
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PMT sensitive detector
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----------------------
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The PMT sensitive detector cannot be triggered like a normal sensitive detector
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because the sensitive volume does not allow photons to pass through it. Rather,
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it detects them in the OpBoundary process based on an efficiency set on the
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skin of the volume.
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G4OpticalSurface* photocath_opsurf=
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new G4OpticalSurface("photocath_opsurf",glisur,polished,
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dielectric_metal);
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G4double photocath_EFF[num]={1.,1.};
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G4double photocath_REFL[num]={0.,0.};
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G4MaterialPropertiesTable* photocath_mt = new G4MaterialPropertiesTable();
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photocath_mt->AddProperty("EFFICIENCY",Ephoton,photocath_EFF,num);
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photocath_mt->AddProperty("REFLECTIVITY",Ephoton,photocath_REFL,num);
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photocath_opsurf->SetMaterialPropertiesTable(photocath_mt);
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new G4LogicalSkinSurface("photocath_surf",photocath_log,photocath_opsurf);
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A normal sensitive detector would have its ProcessHits
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function called for each step by a particle inside the volume. So, to record
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these hits with a sensitive detector we watched the status of the OpBoundary
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process from the stepping manager whenever a photon hit the sensitive volume
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of the pmt. If the status was 'Detection', we retrieve the sensitive detector
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from G4SDManager and call its ProcessHits function.
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boundaryStatus=boundary->GetStatus();
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//Check to see if the particle was actually at a boundary
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//Otherwise the boundary status may not be valid
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//Prior to Geant4.6.0-p1 this would not have been enough to check
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if(thePostPoint->GetStepStatus()==fGeomBoundary){
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switch(boundaryStatus){
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//...
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case Detection: //Note, this assumes that the volume causing detection
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//is the photocathode because it is the only one with
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//non-zero efficiency
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{
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//Trigger sensitive detector manually since photon is
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//absorbed but status was Detection
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G4SDManager* SDman = G4SDManager::GetSDMpointer();
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G4String sdName="/LXeDet/pmtSD";
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LXePMTSD* pmtSD = (LXePMTSD*)SDman
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->FindSensitiveDetector(sdName);
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if(pmtSD)
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pmtSD->ProcessHits_constStep(theStep,NULL);
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break;
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}
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//...
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}
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--------------------------------------------------------
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Selectively drawing trajectories or highlighting volumes
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--------------------------------------------------------
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In a simulation such as this one, where an average of 6000 trajectories are
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generated in a small space, there is little use in drawing all of them. There
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are two ways to select which ones to draw. The first of which is to decide
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while looping through the trajectory container which ones to draw and only call
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DrawTrajectory on the important ones. However, trajectories only contain a
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small portion of the information from the track it represents. This may not
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be enough to decide if a trajectory is worth drawing.
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The alternative is to define your own trajectory class to store additional
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information to help decide if it should be drawn. To use your custom trajectory
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you must create it in the PreUserTrackingAction:
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fpTrackingManager->SetTrajectory(new LXeTrajectory(aTrack));
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Then at any point you can get access to the trajectory you can update the extra
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information within it. When it comes to drawing, you can then use this to
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decide if you want to call DrawTrajectory. Or you can call DrawTrajectory for
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all trajectories and have the logic decide how and if a trajectory should
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be drawn inside the DrawTrajectory function itself.
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Selectively highlighting volumes is useful to show which volumes were hit. To
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do this, you simply need a pointer to the physical volume. With that, you can
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modify its vis attributes and instruct the vis manager to redraw the volume
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with the new vis attributes.
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G4VisAttributes attribs(G4Colour(1.,0.,0.));
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attribs.SetForceSolid(true);
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G4RotationMatrix rot;
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if(physVol->GetRotation())//If a rotation is defined use it
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rot=*(physVol->GetRotation());
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G4Transform3D trans(rot,physVol->GetTranslation());//Create transform
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pVVisManager->Draw(*physVol,attribs,trans);//Draw it
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In this case, it is done in Draw function of a PMT hit but it can be placed
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anywhere. The logic to decide if it should be drawn or not may be similar to
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the logic used in choosing which trajectories to draw.
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See /LXe/detector/volumes/sphere in "UI commands" below for info on what
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trajectories are drawn in this simulation.
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--------------------------
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Saving random engine seeds
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--------------------------
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At times it may be necessary to review a particular event of interest. To do
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this without redoing an entire run, which may take a long time, you must store
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the random engine seed from the beginning of the event. The run manager
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has some functions that help in this task.
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G4RunManager::SetRandomNumberStore(G4bool)
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When set to true, this causes the run manager to write the seed for the
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beginning of the current run to CurrentRun.rndm and the current event to
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CurrentEvent.rndm. However, at the beginning of each event this file will be
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overwritten with the new event. To keep a copy for a particular event there is
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a function to copy this file to run###evt###.rndm.
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G4RunManager::rndmSaveThisEvent()
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This can be done for every event so you can review any event you like but this
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may be awkward for runs with very large numbers of events. Instead, implement
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some form of logic in EndOfEventAction to decide if the event is worth saving.
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If it is, then call rndmSaveThisEvent(). By default, these files are stored in
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the current working directory. There is a function to change this as well.
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Typically you would call that at the same time SetRandomNumberStore. The
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directory to save in must exist first. GEANT4 will not create it for you.
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G4RunManager::SetRandomNumberStoreDir(G4String)
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-----------
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UI commands
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Directories:
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/LXe/ - All custom commands belong below this directory
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/LXe/detector/ - Geometry related commands
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/LXe/detector/volumes/ - Commands to enable/disable volumes in the geometry
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Commands:
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/LXe/saveThreshold <int, default = 4500>
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-Specifies a threshold for saving the random seed for an event. If the number
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of photons generated in an event is below this number then the random seed is
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saved to ./random/run###evt###.rndm. See "Saving random engine seeds".
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/LXe/eventVerbose <int, default = 1>
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-Enables end of event verbose data to be printed. This includes information
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counted and calculated by the user action classes.
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/LXe/pmtThreshold <int, default = 1>
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-Sets the PMT threshold in # of photons being detected by the PMT. PMTs below
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with fewer hits than the threshold will not count as being hit and will also
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not be highlighted at the end of the event.
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/LXe/oneStepPrimaries <bool>
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-This causes primary particles to be killed after going only one step inside
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the scintillator volume. This is useful to view the photons generated during
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the initial conversion of the primary particle.
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/LXe/forceDrawPhotons <bool>
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-Forces all optical photon trajectories to be drawn at the end of the event
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regardless of the scheme mentioned in /LXe/detector/volumes/sphere below.
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/LXe/forceDrawNoPhotons <bool>
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-Forces all optical photon trajectories to NOT be drawn at the end of the
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event regardless of the scheme mentioned in /LXe/detector/volumes/sphere below.
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-If /LXe/forceDrawPhotons is set to true, this has no effect.
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/LXe/detector/dimensions <double x y z> <unit, default = cm>
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-Sets the dimensions of the main scintillator volume.
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/LXe/detector/housingThickness <double>
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-Sets the thickness of the housing surrounding the main detector volume.
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/LXe/detector/pmtRadius <double> <unit, default = cm>
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-Sets the radius of the PMTs
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/LXe/detector/nx
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/LXe/detector/ny
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/LXe/detector/nz
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-Sets the number of PMTs placed in a row along each axis.
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/LXe/detector/reflectivity <double>
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-Sets the reflectivity of the inside of the aluminum housing. The geometry
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uses a default value of 1.00 for a fully reflective surface.
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/LXe/detector/nfibers <int>
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-Sets the number of WLS fibers placed in the WLS scintillator slab. The
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geometry uses a default value of 15 fibers.
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/LXe/detector/scintYieldFactor <double>
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-Sets the yield factor for the scintillation process. This is cumulative with
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the yield factor set on individual materials. Set to 0 to produce no
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scintillation photons.
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/LXe/detector/defaults
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-Resets all detector values customizable with commands above to their defaults.
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/LXe/detector/volumes/sphere <bool>
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-Enables/disables the sphere placed inside the main scintillator volume. When
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the sphere is enabled, only photons that hit the sphere and hit a PMT are
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drawn. If it is disabled, then all photons that hit PMTs are drawn.
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/LXe/detector/volumes/wls <bool>
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-Enables/disables the WLS scintillator slab containing WLS fibers. By default
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this is not part of the geometry. Enabling it will place it behind the LXe
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scintillator volume.
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/LXe/detector/volumes/lxe <bool>
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-Enables/disables the main LXe scintillator volume. By default this is part of
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the geometry.
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