Geant4 入射粒子設置

在 PrimaryGeneratorAction 中設置 G4ParticleGun.

注意：關於粒子種類的定義，有兩個位置可以定義，第一是在 PrimaryGeneratorAction 類的構造函數中定義，第二是在 GeneratePrimaries() 函數中定義。

在 PrimaryGeneratorAction 類的構造函數中定義，粒子種類可以在 .mac 文件中修改，舉例代碼取自 example B2：

 1 B2PrimaryGeneratorAction::B2PrimaryGeneratorAction()
 2  : G4VUserPrimaryGeneratorAction()
 3 {
 4   G4int nofParticles = 1;
 5   fParticleGun = new G4ParticleGun(nofParticles);
 6 
 7   // default particle kinematic
 8 
 9   G4ParticleDefinition* particleDefinition 
10     = G4ParticleTable::GetParticleTable()->FindParticle("proton");
11 
12   fParticleGun->SetParticleDefinition(particleDefinition);
13   fParticleGun->SetParticleMomentumDirection(G4ThreeVector(0.,0.,1.));
14   fParticleGun->SetParticleEnergy(3.0*GeV);
15 }

在 GeneratePrimaries() 函數中定義，粒子種類可以在 .mac 文件中修改，舉例代碼取自 example B5：

  1 #include "B5PrimaryGeneratorAction.hh"
  2 
  3 #include "G4Event.hh"
  4 #include "G4ParticleGun.hh"
  5 #include "G4ParticleTable.hh"
  6 #include "G4ParticleDefinition.hh"
  7 #include "G4GenericMessenger.hh"
  8 #include "G4SystemOfUnits.hh"
  9 #include "Randomize.hh"
 10 
 11 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
 12 
 13 B5PrimaryGeneratorAction::B5PrimaryGeneratorAction()
 14 : G4VUserPrimaryGeneratorAction(),     
 15   fParticleGun(nullptr), fMessenger(nullptr), 
 16   fPositron(nullptr), fMuon(nullptr), fPion(nullptr), 
 17   fKaon(nullptr), fProton(nullptr),
 18   fMomentum(1000.*MeV),
 19   fSigmaMomentum(50.*MeV),
 20   fSigmaAngle(2.*deg),
 21   fRandomizePrimary(true)
 22 {
 23   G4int nofParticles = 1;
 24   fParticleGun  = new G4ParticleGun(nofParticles);
 25   
 26   auto particleTable = G4ParticleTable::GetParticleTable();
 27   fPositron = particleTable->FindParticle("e+");
 28   fMuon = particleTable->FindParticle("mu+");
 29   fPion = particleTable->FindParticle("pi+");
 30   fKaon = particleTable->FindParticle("kaon+");
 31   fProton = particleTable->FindParticle("proton");
 32   
 33   // default particle kinematics
 34   fParticleGun->SetParticlePosition(G4ThreeVector(0.,0.,-8.*m));
 35   fParticleGun->SetParticleDefinition(fPositron);
 36   
 37   // define commands for this class
 38   DefineCommands();
 39 }
 40 
 41 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
 42 
 43 B5PrimaryGeneratorAction::~B5PrimaryGeneratorAction()
 44 {
 45   delete fParticleGun;
 46   delete fMessenger;
 47 }
 48 
 49 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
 50 
 51 void B5PrimaryGeneratorAction::GeneratePrimaries(G4Event* event)
 52 {
 53   G4ParticleDefinition* particle;  
 54   if (fRandomizePrimary) {
 55     G4int i = (int)(5.*G4UniformRand());
 56     switch(i) {
 57       case 0:
 58           particle = fPositron;
 59           break;
 60       case 1:
 61           particle = fMuon;
 62           break;
 63       case 2:
 64           particle = fPion;
 65           break;
 66       case 3:
 67           particle = fKaon;
 68           break;
 69       default:
 70           particle = fProton;
 71           break;
 72     }
 73     fParticleGun->SetParticleDefinition(particle);
 74   }
 75   else {
 76     particle = fParticleGun->GetParticleDefinition();
 77   }
 78   
 79   auto pp = fMomentum + (G4UniformRand()-0.5)*fSigmaMomentum;
 80   auto mass = particle->GetPDGMass();
 81   auto ekin = std::sqrt(pp*pp+mass*mass)-mass;
 82   fParticleGun->SetParticleEnergy(ekin);
 83   
 84   auto angle = (G4UniformRand()-0.5)*fSigmaAngle;
 85   fParticleGun->SetParticleMomentumDirection(
 86                   G4ThreeVector(std::sin(angle),0.,std::cos(angle)));
 87   
 88   fParticleGun->GeneratePrimaryVertex(event);
 89 }
 90 
 91 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
 92 
 93 void B5PrimaryGeneratorAction::DefineCommands()
 94 {
 95   // Define /B5/generator command directory using generic messenger class
 96   fMessenger 
 97     = new G4GenericMessenger(this, 
 98                              "/B5/generator/", 
 99                              "Primary generator control");
100             
101   // momentum command
102   auto& momentumCmd
103     = fMessenger->DeclarePropertyWithUnit("momentum", "GeV", fMomentum, 
104         "Mean momentum of primaries.");
105   momentumCmd.SetParameterName("p", true);
106   momentumCmd.SetRange("p>=0.");                                
107   momentumCmd.SetDefaultValue("1.");
108   // ok
109   //momentumCmd.SetParameterName("p", true);
110   //momentumCmd.SetRange("p>=0.");                                
111   
112   // sigmaMomentum command
113   auto& sigmaMomentumCmd
114     = fMessenger->DeclarePropertyWithUnit("sigmaMomentum",
115         "MeV", fSigmaMomentum, "Sigma momentum of primaries.");
116   sigmaMomentumCmd.SetParameterName("sp", true);
117   sigmaMomentumCmd.SetRange("sp>=0.");                                
118   sigmaMomentumCmd.SetDefaultValue("50.");
119 
120   // sigmaAngle command
121   auto& sigmaAngleCmd
122     = fMessenger->DeclarePropertyWithUnit("sigmaAngle", "deg", fSigmaAngle, 
123         "Sigma angle divergence of primaries.");
124   sigmaAngleCmd.SetParameterName("t", true);
125   sigmaAngleCmd.SetRange("t>=0.");                                
126   sigmaAngleCmd.SetDefaultValue("2.");
127 
128   // randomizePrimary command
129   auto& randomCmd
130     = fMessenger->DeclareProperty("randomizePrimary", fRandomizePrimary);
131   G4String guidance
132     = "Boolean flag for randomizing primary particle types.\n";   
133   guidance
134     += "In case this flag is false, you can select the primary particle\n";
135   guidance += "  with /gun/particle command.";                               
136   randomCmd.SetGuidance(guidance);
137   randomCmd.SetParameterName("flg", true);
138   randomCmd.SetDefaultValue("true");
139 }
140 
141 //..oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

第 54 - 72 行，粒子源隨機出射一種粒子。
DefineCommands() 函數中定義了 macro 文件中使用的一些命令，有待研究。

可以使用 G4GeneralParticleSource 替代 G4ParticleGun

“For many applications G4ParticleGun is a suitable particle generator. However if you want to generate primary particles in more sophisticated manner, you can utilize G4GeneralParticleSource, the GEANT4 General Particle Source module (GPS), discussed in the next section ( General Particle Source).” Geant4 手冊原話。

代碼舉例，取自 HPGe_60Co

PrimaryGeneratorAction.hh 文件

 1 #ifndef PrimaryGeneratorAction_h
 2 #define PrimaryGeneratorAction_h 1
 3 
 4 #include "G4VUserPrimaryGeneratorAction.hh"
 5 #include "globals.hh"
 6 
 7 class G4GeneralParticleSource;
 8 class G4Event;
 9 
10 class PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
11 {
12 public:
13   PrimaryGeneratorAction();    
14   virtual ~PrimaryGeneratorAction();
15 
16   virtual void GeneratePrimaries(G4Event* );
17 
18   const G4GeneralParticleSource* GetParticleGun() const {return fParticleGun;}
19 
20   // Set methods
21   void SetRandomFlag(G4bool );
22 
23 private:
24   G4GeneralParticleSource*          fParticleGun; // G4 particle gun
25 };
26 
27 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
28 
29 #endif

 1 #include "PrimaryGeneratorAction.hh"
 2 
 3 #include "G4LogicalVolumeStore.hh"
 4 #include "G4LogicalVolume.hh"
 5 #include "G4Box.hh"
 6 #include "G4Event.hh"
 7 #include "G4ParticleGun.hh"
 8 #include "G4GeneralParticleSource.hh"
 9 #include "G4ParticleTable.hh"
10 #include "G4ParticleDefinition.hh"
11 #include "G4SystemOfUnits.hh"
12 #include "G4RandomDirection.hh"
13 #include "G4IonTable.hh"
14 #include "G4Geantino.hh"
15 
16 #include "Randomize.hh"
17 
18 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
19 
20 PrimaryGeneratorAction::PrimaryGeneratorAction()
21  : G4VUserPrimaryGeneratorAction()
22 {
23   fParticleGun = new G4GeneralParticleSource();
24 }
25 
26 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
27 
28 PrimaryGeneratorAction::~PrimaryGeneratorAction()
29 {
30   delete fParticleGun;
31 }
32 
33 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......
34 
35 void PrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
36 {
37   fParticleGun->GeneratePrimaryVertex(anEvent);
38 }
39 
40 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

使用 G4GeneralParticleSource 的 PrimaryGeneratorAction 很簡單。

1 /run/initialize
2 /tracking/verbose 0
3 
4 
5 /gps/particle ion
6 /gps/ion 27 60 0 0
7 /gps/pos/centre 0 0 0 mm
8 
9 /run/beamOn 10000

有關 gps的 macro 文件

得到 60Co 的相關能譜，這個能譜比較好理解。

得到 241Am 能譜，這個就不好理解，241Am alpha 能量約為 5.486 MeV，但得到的能譜卻很奇怪。

我認為這是我對使用 gps 產生 alpha 源不熟悉導致的，在 G4 的論壇中有類似的問題，回答中提供的圖也有類似的問題，我不理解。https://geant4-forum.web.cern.ch/top/monthly

免責聲明！

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