Geant4 Liquid Argon Muon Voxel Study

This project builds a minimal Geant4 application for single-muon energy deposition in liquid argon and writes sparse voxelized output to a ROOT file.

Geometry

• Active volume: liquid argon sphere with radius 20 m
• World volume: vacuum box large enough to contain the active volume
• The whole liquid-argon sphere is treated as active

Primary Particle

• Particle: mu-
• One particle per event
• Kinetic energy sampled uniformly in [0.1, 10] GeV
• Initial position: origin
• Initial direction: isotropic

Physics

• Hadronic physics list: QGSP_BERT
• Electromagnetic physics: G4EmStandardPhysics

Energy Deposition

Energy deposition is taken from Geant4 step energy loss:

• G4Step::GetTotalEnergyDeposit()
• Only steps inside the active liquid-argon volume are counted
• Deposited energy is accumulated into 5 mm cubes
• Cubes are stored sparsely, so only cubes with nonzero deposited energy are written

The voxel coordinate written to output is the cube center in mm.

Derived LAr Quanta Model

In addition to edep, the code derives scintillation photons and surviving ionization electrons from deposited energy using a simple parameterized liquid-argon model.

Constants used:

• W = 23.6 eV
• Exciton-to-ion ratio alpha = 0.21
• Recombination fraction r = 0.5

Per voxel:

• Nq = Edep / W
• Ni = Nq / (1 + alpha)
• Nex = Nq - Ni
• Nphot = Nex + r * Ni
• Nelec = (1 - r) * Ni

This is a simple yield model only. It is not a field-dependent recombination model and does not simulate optical photon transport or electron drift.

Output

Default output file:

• lar_muon_voxels.root

ROOT tree:

• events

Branches:

• event
• px_MeV
• py_MeV
• pz_MeV
• cube_x_mm
• cube_y_mm
• cube_z_mm
• edep_MeV
• n_scint_photons
• n_ionization_electrons

Each tree entry corresponds to one event. The vector branches are aligned element-by-element per nonzero voxel.

Build

Requirements:

• Geant4
• ROOT
• CMake
• C++17 compiler

Build commands:

cmake -S . -B build
cmake --build build -j 4

Run

Run the default macro:

./build/lar_muon_voxels run.mac

If no macro is provided, the executable runs run.mac by default.

Files

• CMakeLists.txt
• run.mac
• src/main.cc
• src/DetectorConstruction.cc
• src/PrimaryGeneratorAction.cc
• src/EventAction.cc
• src/SteppingAction.cc
• src/RunAction.cc