Geant4：全能粒子物理模擬器

一、Geant4官網

Geant4是歐洲核子研究組織(CERN)開發的全能粒子物理模擬器，可用於各種應用場合，包括核醫學、高能物理、輻射防護等。

在Geant4官網上，你可以找到Geant4的最新版本和文檔，以及廣泛的應用示例和教程。Geant4的設計理念基於現代軟體開發的最佳實踐，並採用模塊化結構，允許使用者根據需要選擇特定模塊，以便更好地適應其應用場合。

下面是一個簡單的例子，展示如何使用Geant4模擬一個簡單的電子束：

#include "G4RunManager.hh"
#include "G4UImanager.hh"
#include "G4VisExecutive.hh"

#include "DetectorConstruction.hh"
#include "PhysicsList.hh"
#include "PrimaryGeneratorAction.hh"

int main(int argc, char** argv)
{
    // Create the detector construction
    DetectorConstruction* detector = new DetectorConstruction();

    // Create the physics list
    PhysicsList* physics = new PhysicsList();

     // Create the primary generator action
    PrimaryGeneratorAction* primary = new PrimaryGeneratorAction();

    // Create the run manager and configure it
    G4RunManager* runManager = new G4RunManager();
    runManager->SetUserInitialization(detector);
    runManager->SetUserInitialization(physics);
    runManager->SetUserAction(primary);

    // Initialize the Geant4 UI manager
    G4UImanager* uiManager = G4UImanager::GetUIpointer();

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

    // Run in interactive mode
    uiManager->ApplyCommand("/control/execute vis.mac");

    // Clean up and exit
    delete visManager;
    delete runManager;
    delete primary;
    delete physics;
    delete detector;

    return 0;
}

二、Geant4學習資源

除了Geant4官網上的資源，還有其他很多學習Geant4的資源。其中，grant是一個基於網路的遠程桌面服務，提供一個訪問Geant4的實驗環境。

另外，Geant4的官網上也有一些教程，包括入門教程和高級教程。這些教程都提供源代碼和示例數據，方便用戶學習。此外，還有一些第三方機構和個人提供的課程材料和教學視頻。

以下是Geant4官網上提供的一個簡單的教程例子，展示如何使用Geant4建立一個基本的探測器和模擬一個質子束：

#include "G4RunManager.hh"
#include "G4UImanager.hh"
#include "G4UIterminal.hh"
#include "G4UItcsh.hh"
#include "G4VisExecutive.hh"
#include "G4UIExecutive.hh"

#include "DetectorConstruction.hh"
#include "PhysicsList.hh"
#include "PrimaryGeneratorAction.hh"

int main(int argc, char** argv)
{
    // Create the run manager and configure it
    G4RunManager* runManager = new G4RunManager();
    runManager->SetUserInitialization(new DetectorConstruction());
    runManager->SetUserInitialization(new PhysicsList());
    runManager->SetUserAction(new PrimaryGeneratorAction());

    // Initialize the Geant4 UI manager
    G4UImanager* uiManager = G4UImanager::GetUIpointer();

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

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

    // Clean up and exit
    delete ui;
    delete visManager;
    delete runManager;

    return 0;
}

三、Geant4設置通用粒子源

Geant4支持多種粒子源類型，包括單個粒子源、多個粒子源、泊松分布粒子源等。可以通過設置相應的源參數，控制粒子源的屬性和生成數目。

以下是一個簡單的例子，展示如何使用Geant4建立一個基本的探測器和模擬一個通用粒子源：

#include "G4RunManager.hh"
#include "G4UImanager.hh"
#include "G4VisExecutive.hh"

#include "DetectorConstruction.hh"
#include "PhysicsList.hh"
#include "PrimaryGeneratorAction.hh"
#include "GeneralParticleSource.hh"

int main(int argc, char** argv)
{
    // Create the detector construction
    DetectorConstruction* detector = new DetectorConstruction();

    // Create the physics list
    PhysicsList* physics = new PhysicsList();

    // Create the primary generator action
    PrimaryGeneratorAction* primary = new PrimaryGeneratorAction();

    // Create the general particle source and configure it
    GeneralParticleSource* gps = new GeneralParticleSource();
    gps->SetParticleEnergy(1.0*GeV);
    gps->SetParticleMomentumDirection(G4ThreeVector(0.,0.,1.));
    gps->SetParticlePosition(G4ThreeVector(0.,0.,-10.));

    // Set the GPS to be the primary generator action
    primary->SetParticleSource(gps);

    // Create the run manager and configure it
    G4RunManager* runManager = new G4RunManager();
    runManager->SetUserInitialization(detector);
    runManager->SetUserInitialization(physics);
    runManager->SetUserAction(primary);

    // Initialize the Geant4 UI manager
    G4UImanager* uiManager = G4UImanager::GetUIpointer();

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

    // Run in interactive mode
    uiManager->ApplyCommand("/run/beamOn 100");

    // Clean up and exit
    delete visManager;
    delete runManager;
    delete primary;
    delete physics;
    delete detector;

    return 0;
}

四、Geant4論壇

Geant4論壇是一個開放的社區，任何人都可以在這裡提出問題、分享經驗和知識。論壇上有很多關於Geant4的討論和技術支持，對於新手和高級用戶都很有用。

以下是論壇上的一個問題，關於如何使用Geant4模擬一個粒子在材料中的運動軌跡：

// Use the Geant4 tracking manager to initialize the track
G4Track* track = new G4Track(new G4DynamicParticle(G4Proton::Proton(), G4ThreeVector(0.,0.,500.*MeV), 0.), 0., 0.);
G4TrackingManager* trackingManager = G4EventManager::GetEventManager()->GetTrackingManager();
trackingManager->SetStoreTrajectory(true);
trackingManager->SetTrajectory(new G4Trajectory(track));
trackingManager->ProcessOneTrack(track);

// Get the trajectory from the track and store it in a file
G4Trajectory* trajectory = (G4Trajectory*) track->GetUserInformation();
std::ofstream outFile("trajectory.txt");
for(int i=0; iGetPointEntries(); i++) {
    G4TrajectoryPoint* point = (G4TrajectoryPoint*) trajectory->GetPoint(i);
    outFile <GetPosition().x()/mm << " " <GetPosition().y()/mm << " " <GetPosition().z()/mm << std::endl;
}
outFile.close();

五、Geant4有時間概念嗎？

Geant4支持時間模擬，可以記錄粒子在模擬中經過的時間和到達某個特定地點的時間。這對於某些應用場合非常重要，例如核醫學和粒子物理學中的時間解析度。

以下是一個簡單的例子，展示如何使用Geant4模擬一個粒子在時間上的運動軌跡：

#include "G4RunManager.hh"
#include "G4UImanager.hh"
#include "G4VisExecutive.hh"

#include "DetectorConstruction.hh"
#include "PhysicsList.hh"
#include "PrimaryGeneratorAction.hh"

int main(int argc, char** argv)
{
    // Create the detector construction
    DetectorConstruction* detector = new DetectorConstruction();

    // Create the physics list
    PhysicsList* physics = new PhysicsList();

    // Create the primary generator action
    PrimaryGeneratorAction* primary = new PrimaryGeneratorAction();

    // Create the run manager and configure it
    G4RunManager* runManager = new G4RunManager();
    runManager->SetUserInitialization(detector);
    runManager->SetUserInitialization(physics);
    runManager->SetUserAction(primary);

    // Initialize the Geant4 UI manager
    G4UImanager* uiManager = G4UImanager::GetUIpointer();

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

    // Run in interactive mode with time tracking
    uiManager->ApplyCommand("/tracking/storeTrajectory 1");
    uiManager->ApplyCommand("/run/beamOn 100");

    // Clean up and exit
    delete visManager;
    delete runManager;
    delete primary;
    delete physics;
    delete detector;

    return 0;
}

結束語

Geant4是一個強大的粒子物理模擬器，且用於各種應用場合。Geant4的學習曲線較陡峭，但是在掌握相關技術後，可以幫助研究人員進行複雜的物理模擬，並為相關領域的研究提供支持。