Geant4 入射粒子设置

Geant4 入射粒子设置

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在 PrimaryGeneratorAction 中设置 G4ParticleGun.

注意：关于粒子种类的定义，有两个位置可以定义，第一是在 PrimaryGeneratorAction 类的构造函数中定义，第二是在 GeneratePrimaries() 函数中定义。

• 在 PrimaryGeneratorAction 类的构造函数中定义，粒子种类可以在 .mac 文件中修改，举例代码取自 example B2：

B2PrimaryGeneratorAction::B2PrimaryGeneratorAction()
 : G4VUserPrimaryGeneratorAction()
{
  G4int nofParticles = 1;
  fParticleGun = new G4ParticleGun(nofParticles);

  // default particle kinematic

  G4ParticleDefinition* particleDefinition 
    = G4ParticleTable::GetParticleTable()->FindParticle("proton");

  fParticleGun->SetParticleDefinition(particleDefinition);
  fParticleGun->SetParticleMomentumDirection(G4ThreeVector(0.,0.,1.));
  fParticleGun->SetParticleEnergy(3.0*GeV);
}

• 在 GeneratePrimaries() 函数中定义，粒子种类可以在 .mac 文件中修改，举例代码取自 example B5：

#include "B5PrimaryGeneratorAction.hh"

#include "G4Event.hh"
#include "G4ParticleGun.hh"
#include "G4ParticleTable.hh"
#include "G4ParticleDefinition.hh"
#include "G4GenericMessenger.hh"
#include "G4SystemOfUnits.hh"
#include "Randomize.hh"

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

B5PrimaryGeneratorAction::B5PrimaryGeneratorAction()
: G4VUserPrimaryGeneratorAction(),     
  fParticleGun(nullptr), fMessenger(nullptr), 
  fPositron(nullptr), fMuon(nullptr), fPion(nullptr), 
  fKaon(nullptr), fProton(nullptr),
  fMomentum(1000.*MeV),
  fSigmaMomentum(50.*MeV),
  fSigmaAngle(2.*deg),
  fRandomizePrimary(true)
{
  G4int nofParticles = 1;
  fParticleGun  = new G4ParticleGun(nofParticles);
  
  auto particleTable = G4ParticleTable::GetParticleTable();
  fPositron = particleTable->FindParticle("e+");
  fMuon = particleTable->FindParticle("mu+");
  fPion = particleTable->FindParticle("pi+");
  fKaon = particleTable->FindParticle("kaon+");
  fProton = particleTable->FindParticle("proton");
  
  // default particle kinematics
  fParticleGun->SetParticlePosition(G4ThreeVector(0.,0.,-8.*m));
  fParticleGun->SetParticleDefinition(fPositron);
  
  // define commands for this class
  DefineCommands();
}

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

B5PrimaryGeneratorAction::~B5PrimaryGeneratorAction()
{
  delete fParticleGun;
  delete fMessenger;
}

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

void B5PrimaryGeneratorAction::GeneratePrimaries(G4Event* event)
{
  G4ParticleDefinition* particle;  
  if (fRandomizePrimary) {
    G4int i = (int)(5.*G4UniformRand());
    switch(i) {
      case 0:
          particle = fPositron;
          break;
      case 1:
          particle = fMuon;
          break;
      case 2:
          particle = fPion;
          break;
      case 3:
          particle = fKaon;
          break;
      default:
          particle = fProton;
          break;
    }
    fParticleGun->SetParticleDefinition(particle);
  }
  else {
    particle = fParticleGun->GetParticleDefinition();
  }
  
  auto pp = fMomentum + (G4UniformRand()-0.5)*fSigmaMomentum;
  auto mass = particle->GetPDGMass();
  auto ekin = std::sqrt(pp*pp+mass*mass)-mass;
  fParticleGun->SetParticleEnergy(ekin);
  
  auto angle = (G4UniformRand()-0.5)*fSigmaAngle;
  fParticleGun->SetParticleMomentumDirection(
                  G4ThreeVector(std::sin(angle),0.,std::cos(angle)));
  
  fParticleGun->GeneratePrimaryVertex(event);
}

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

void B5PrimaryGeneratorAction::DefineCommands()
{
  // Define /B5/generator command directory using generic messenger class
  fMessenger 
    = new G4GenericMessenger(this, 
                             "/B5/generator/", 
                             "Primary generator control");
            
  // momentum command
  auto& momentumCmd
    = fMessenger->DeclarePropertyWithUnit("momentum", "GeV", fMomentum, 
        "Mean momentum of primaries.");
  momentumCmd.SetParameterName("p", true);
  momentumCmd.SetRange("p>=0.");                                
  momentumCmd.SetDefaultValue("1.");
  // ok
  //momentumCmd.SetParameterName("p", true);
  //momentumCmd.SetRange("p>=0.");                                
  
  // sigmaMomentum command
  auto& sigmaMomentumCmd
    = fMessenger->DeclarePropertyWithUnit("sigmaMomentum",
        "MeV", fSigmaMomentum, "Sigma momentum of primaries.");
  sigmaMomentumCmd.SetParameterName("sp", true);
  sigmaMomentumCmd.SetRange("sp>=0.");                                
  sigmaMomentumCmd.SetDefaultValue("50.");

  // sigmaAngle command
  auto& sigmaAngleCmd
    = fMessenger->DeclarePropertyWithUnit("sigmaAngle", "deg", fSigmaAngle, 
        "Sigma angle divergence of primaries.");
  sigmaAngleCmd.SetParameterName("t", true);
  sigmaAngleCmd.SetRange("t>=0.");                                
  sigmaAngleCmd.SetDefaultValue("2.");

  // randomizePrimary command
  auto& randomCmd
    = fMessenger->DeclareProperty("randomizePrimary", fRandomizePrimary);
  G4String guidance
    = "Boolean flag for randomizing primary particle types.\n";   
  guidance
    += "In case this flag is false, you can select the primary particle\n";
  guidance += "  with /gun/particle command.";                               
  randomCmd.SetGuidance(guidance);
  randomCmd.SetParameterName("flg", true);
  randomCmd.SetDefaultValue("true");
}

//..oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

• 第 54 - 72 行，粒子源随机出射一种粒子。
• DefineCommands() 函数中定义了 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 手册原话。

代码举例，取自 HPGe_60Co

PrimaryGeneratorAction.hh 文件

#ifndef PrimaryGeneratorAction_h
#define PrimaryGeneratorAction_h 1

#include "G4VUserPrimaryGeneratorAction.hh"
#include "globals.hh"

class G4GeneralParticleSource;
class G4Event;

class PrimaryGeneratorAction : public G4VUserPrimaryGeneratorAction
{
public:
  PrimaryGeneratorAction();    
  virtual ~PrimaryGeneratorAction();

  virtual void GeneratePrimaries(G4Event* );

  const G4GeneralParticleSource* GetParticleGun() const {return fParticleGun;}

  // Set methods
  void SetRandomFlag(G4bool );

private:
  G4GeneralParticleSource*          fParticleGun; // G4 particle gun
};

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

#endif

#include "PrimaryGeneratorAction.hh"

#include "G4LogicalVolumeStore.hh"
#include "G4LogicalVolume.hh"
#include "G4Box.hh"
#include "G4Event.hh"
#include "G4ParticleGun.hh"
#include "G4GeneralParticleSource.hh"
#include "G4ParticleTable.hh"
#include "G4ParticleDefinition.hh"
#include "G4SystemOfUnits.hh"
#include "G4RandomDirection.hh"
#include "G4IonTable.hh"
#include "G4Geantino.hh"

#include "Randomize.hh"

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

PrimaryGeneratorAction::PrimaryGeneratorAction()
 : G4VUserPrimaryGeneratorAction()
{
  fParticleGun = new G4GeneralParticleSource();
}

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

PrimaryGeneratorAction::~PrimaryGeneratorAction()
{
  delete fParticleGun;
}

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

void PrimaryGeneratorAction::GeneratePrimaries(G4Event* anEvent)
{
  fParticleGun->GeneratePrimaryVertex(anEvent);
}

//....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

• 使用 G4GeneralParticleSource 的 PrimaryGeneratorAction 很简单。

/run/initialize
/tracking/verbose 0


/gps/particle ion
/gps/ion 27 60 0 0
/gps/pos/centre 0 0 0 mm

/run/beamOn 10000

• 有关 gps的 macro 文件

得到 60Co 的相关能谱，这个能谱比较好理解。

 

得到 241Am 能谱，这个就不好理解，241Am alpha 能量约为 5.486 MeV，但得到的能谱却很奇怪。

 

我认为这是我对使用 gps 产生 alpha 源不熟悉导致的，在 G4 的论坛中有类似的问题，回答中提供的图也有类似的问题，我不理解。https://geant4-forum.web.cern.ch/top/monthly