geant4如何获取一个次级光子从产生到消失的全部过程每一步的能量变化，用代码演示

以下是一个简单的代码示例，展示了如何通过G4UserSteppingAction类获取一个次级光子从产生到消失的全部过程每一步的能量变化：

#include "G4UserSteppingAction.hh"
#include "G4Step.hh"
#include "G4Track.hh"
#include "G4ParticleDefinition.hh"

class MySteppingAction : public G4UserSteppingAction
{
public:
    virtual void UserSteppingAction(const G4Step* step)
    {
        G4Track* track = step->GetTrack();
        const G4ParticleDefinition* particle = track->GetDefinition();

        // 判断当前粒子是否为次级光子
        if (particle->GetParticleName() == "opticalphoton" && track->GetParentID() > 0)
        {
            G4StepPoint* prePoint = step->GetPreStepPoint();
            G4StepPoint* postPoint = step->GetPostStepPoint();

            // 获取当前步长的能量沉积
            G4double deltaE = step->GetTotalEnergyDeposit();

            // 获取当前步长的起点和终点
            G4int copyNo = prePoint->GetTouchableHandle()->GetCopyNumber();
            G4ThreeVector startPos = prePoint->GetPosition();
            G4ThreeVector endPos = postPoint->GetPosition();

            // 输出当前步长的信息
            G4cout << "Optical photon " << track->GetTrackID() << ":" << G4endl;
            G4cout << "  Delta E: " << deltaE << G4endl;
            G4cout << "  Start position: " << startPos << G4endl;
            G4cout << "  End position: " << endPos << G4endl;
        }
    }
};

在模拟运行中，只需要将MySteppingAction对象添加到G4UserActionInitialization中即可：

#include "MySteppingAction.hh"
#include "G4UserActionInitialization.hh"

class MyActionInitialization : public G4UserActionInitialization
{
public:
    virtual void Build() const
    {
        SetUserSteppingAction(new MySteppingAction());
    }
};

这样，每当一个次级光子在模拟中产生和消失时，MySteppingAction类的UserSteppingAction()方法就会被调用，输出该光子的能量变化情况。