geant4当同时有几个粒子打在闪烁体时，如何确定同一个粒子的次级光子，并获取它的每一步变化信息

在 Geant4 中，通过设置敏感探测器 (Sensitive Detector) 来获取粒子与材料相互作用产生的次级粒子信息。当多个粒子打在闪烁体时，每个粒子的次级粒子信息都会被记录下来，并且可以通过敏感探测器来区分不同粒子的次级粒子。下面是一个示例：

首先定义一个敏感探测器类 `MySD`，继承自 `G4VSensitiveDetector`。在 `ProcessHits()` 函数中，可以获取粒子与材料相互作用产生的次级粒子信息。在这个函数中，我们可以根据粒子的信息来确定同一个粒子的次级光子，并获取它的每一步变化信息。

class MySD : public G4VSensitiveDetector {
public:
    MySD(const G4String &name) : G4VSensitiveDetector(name) {}
    virtual ~MySD() {}

    virtual void Initialize(G4HCofThisEvent *) override {}
    virtual G4bool ProcessHits(G4Step *step, G4TouchableHistory *) override {
        G4Track *track = step->GetTrack();
        G4String particleName = track->GetDefinition()->GetParticleName();
        G4double energy = track->GetKineticEnergy();
        G4double stepLength = step->GetStepLength();
        G4double deltaEnergy = step->GetDeltaEnergy();
        G4ThreeVector position = track->GetPosition();
        G4double time = track->GetGlobalTime();
        G4int parentId = track->GetParentID();
        G4int trackId = track->GetTrackID();

        if (particleName == "opticalphoton" && parentId != 0) {
            G4cout << "Optical photon from track " << parentId
                   << ", Energy: " << energy / eV << " eV"
                   << ", Step length: " << stepLength / mm << " mm"
                   << ", Delta energy: " << deltaEnergy / eV << " eV"
                   << ", Position: (" << position.x() / mm << ", " << position.y() / mm << ", " << position.z() / mm << ") mm"
                   << ", Time: " << time / ns << " ns"
                   << G4endl;
        }

        return true;
    }
};

然后在主函数中，需要将敏感探测器添加到相应的逻辑体中。在下面的示例中，我们将敏感探测器添加到了一个叫做 `scintillator` 的逻辑体中。

int main(int argc, char **argv) {
    G4RunManager *runManager = new G4RunManager();

    G4UImanager *uiManager = G4UImanager::GetUIpointer();

    G4VisManager *visManager = new G4VisExecutive();
    visManager->Initialize();

    G4UIExecutive *ui = new G4UIExecutive(argc, argv);
    uiManager->ApplyCommand("/control/execute vis.mac");

    G4LogicalVolume *scintillator = new G4LogicalVolume(new G4Box("scintillator", 10 * cm, 10 * cm, 10 * cm), G4NistManager::Instance()->FindOrBuildMaterial("G4_PLASTIC_SC_VINYLTOLUENE"), "scintillator");
    G4SDManager *sdManager = G4SDManager::GetSDMpointer();
    MySD *mySD = new MySD("MySD");
    sdManager->AddNewDetector(mySD);
    scintillator->SetSensitiveDetector(mySD);

    G4VPhysicalVolume *world = new G4PVPlacement(0, G4ThreeVector(), scintillator, "world", 0, false, 0);
    runManager->SetUserInitialization(new G4OpticalPhysics());
    runManager->SetUserAction(new RunAction());
    runManager->SetUserAction(new EventAction());
    runManager->SetUserAction(new TrackingAction());
    runManager->SetUserAction(new SteppingAction());

    ui->SessionStart();
    delete ui;

    delete visManager;
    delete runManager;

    return 0;
}

在上述代码中，我们将敏感探测器 `MySD` 添加到了逻辑体 `scintillator` 中，并将 `scintillator` 设置为敏感探测器。这样，当粒子与 `scintillator` 相互作用时，产生的次级光子信息就会被记录下来，并且可以根据粒子的信息来确定同一个粒子的次级光子，并获取它的每一步变化信息。