Water Tank Geant4 Simulation

A comprehensive Geant4-based simulation of a water-Cherenkov calibration system designed for large-scale neutrino detectors, such as the IceCube Neutrino Observatory. This simulation models cosmic ray interactions in a cylindrical water tank with Digital Optical Module (DOM) photodetectors.

Project Overview

This simulation recreates the geometry and physics of a water-Cherenkov detector tank used for DOM calibration studies. The system consists of:

• Cylindrical polypropylene tank (71" diameter × 36" height) filled with ultrapure water
• Glass DOM photomultiplier sphere for optical photon detection
• Cosmic ray shower simulation using the CRY (Cosmic Ray Yield) library
• Comprehensive optical physics including Cherenkov radiation, refraction, and absorption
• ROOT-based data output for analysis of particle interactions and photon yields

This work serves as a foundation for developing a muon scintillator trigger array system and forms part of an undergraduate thesis project focused on calibration systems for large-scale neutrino detectors.

Physics Features

Particle Physics

• Complete electromagnetic and hadronic physics via Geant4's QBBC physics list
• Cosmic ray simulation using CRY library for realistic muon flux
• Optical physics with detailed Cherenkov radiation modeling
• Material property modeling for water, glass, and polypropylene

Detector Response

• Photomultiplier tube simulation with realistic quantum efficiency
• Optical surface modeling between water and glass interfaces
• Time-resolved photon detection for pulse shape analysis
• Energy deposition tracking in both water and DOM volumes

Realistic Geometry

• Scale-accurate tank dimensions matching existing calibration hardware
• Wavelength-dependent optical properties (300-620 nm range)
• DOM positioning optimized for maximum photon collection

Prerequisites

Required Software

• Geant4 (version 10.7 or later) with Qt/OpenGL visualization
• ROOT (version 6.0 or later) for data analysis and plotting
• CMake (version 3.6 or later)
• CRY (Cosmic Ray Yield) library v1.7 (included in repository)
• C++17 compatible compiler (GCC 7+, Clang 5+)

System Requirements

• Linux/macOS/Windows with WSL2
• Minimum 4 GB RAM (8 GB recommended for large event samples)
• OpenGL-capable graphics card for visualization

Installation and Setup

1. Clone Repository

git clone https://github.com/ChristopherPrainito/WaterTankGeantSim.git
cd WaterTankGeantSim

2. Verify Required Software

# Check Geant4 environment
echo $G4INSTALL
geant4-config --version

# Verify ROOT installation
root-config --version
which root

3. Build CRY Library (if needed)

cd cry_v1.7
make
cd ..

4. Build Simulation

mkdir build
cd build
cmake ..
make -j4

Usage

Interactive Mode (Visualization)

Launch the simulation with Qt/OpenGL visualization for geometry inspection and event display:

cd build
./exampleWaterTank

In the interactive session:

# Initialize geometry and physics
/run/initialize

# Configure visualization
/vis/open OGL 600x600-0+0
/vis/drawVolume
/vis/viewer/set/viewpointVector -1 0 0
/vis/scene/add/trajectories
/vis/scene/endOfEventAction accumulate

# Run single muon events
/run/beamOn 10

Batch Mode Simulations

Single Muon Test

./exampleWaterTank test.mac

Cosmic Ray Simulation

./exampleWaterTank test_cry.mac

Macro Commands

Primary Generator Control

# Switch between single muon and CRY modes
/watertank/generator/useCRY true/false

# Single muon configuration
/watertank/generator/muon/energy 4 GeV
/watertank/generator/muon/direction 0 0 -1
/watertank/generator/muon/position 0 0 50 cm

Physics Settings

# Optical physics parameters
/process/optical/cerenkov/setMaxPhotons 300
/process/optical/cerenkov/setStackPhotons true

# Verbosity control
/run/verbose 1
/event/verbose 1
/tracking/verbose 1

Output Data Format

The simulation generates ROOT files (output_default.root) with comprehensive event and hit-level data stored in two main trees:

Event Tree (event)

Contains 15 branches with event-level physics data:

• EventID: Unique event identifier
• PrimaryEnergy_GeV: Initial particle energy (GeV)
• PrimaryPDG: Particle type (PDG code)
• PrimaryPosX/Y/Z_cm: Initial particle position (cm)
• PrimaryDirX/Y/Z: Initial momentum direction (unit vector)
• Edep_GeV: Total energy deposited in water (GeV)
• DOMHitCount: Number of photons detected by DOM
• PhotonYield_per_GeV: Light yield efficiency (photons/GeV)
• FirstPhotonTime_ns: Time of first photon detection (ns)
• LastPhotonTime_ns: Time of last photon detection (ns)
• AvgPhotonWavelength_nm: Average detected photon wavelength (nm)

DOM Hits Tree (domhits)

Contains 12 branches with individual photon hit data:

• EventID: Associated event identifier
• Time_ns: Photon arrival time (ns)
• Energy_eV: Photon energy (eV)
• Wavelength_nm: Photon wavelength (nm)
• PosX/Y/Z_cm: Hit position on DOM surface (cm)
• DirX/Y/Z: Photon direction at detection (unit vector)
• TrackID: Geant4 track identifier
• ParentID: Parent track identifier

Analysis Tools

ROOT Analysis Macro

The simulation includes a comprehensive ROOT analysis script (analyze_watertank.C) that automatically generates detailed physics plots and statistics.

Running the Analysis

# From the build directory
root -l -b -q "../analyze_watertank.C(\"output_default.root\")"

# Or interactively in ROOT
root
.x analyze_watertank.C

Generated Output

The analysis creates three detailed plot sets:

1. Event Analysis (water_tank_event_analysis.png) - 6-panel event-level physics
2. Photon Analysis (water_tank_photon_analysis.png) - 6-panel detector response
3. Physics Analysis (water_tank_physics_analysis.png) - 2-panel efficiency/yield plots

Plot Descriptions

Event Analysis Plots

1. Primary Particle Energy - Distribution of incident muon energies showing beam characteristics
2. Energy Deposition in Water - Energy lost by muons through ionization and electromagnetic processes
3. DOM Hit Multiplicity - Number of Cherenkov photons detected per event, indicating light collection efficiency
4. Photon Yield vs Primary Energy - 2D correlation showing relationship between muon energy and light production
5. First Photon Arrival Time - Timing distribution of earliest detected photons, crucial for trigger studies
6. Average Photon Wavelength - Spectral characteristics of detected Cherenkov light (should peak ~400-450nm)

Photon Analysis Plots

1. Photon Energy Spectrum - Energy distribution of individual detected photons (1.5-4.5 eV range)
2. Photon Wavelength Spectrum - Detailed wavelength distribution showing Cherenkov 1/λ² spectrum
3. Photon Arrival Times - Temporal spread of photon hits (log scale), reveals scattering and detector response
4. DOM Hit Positions (X-Y View) - Spatial distribution of photon hits on DOM hemisphere
5. DOM Hit Positions (Z-R View) - Cylindrical view showing hit pattern vs depth and radius
6. Photon Direction Distribution - Angular distribution of detected photons in azimuth/polar coordinates

Physics Analysis Plots

1. Cherenkov Light Yield - Light production efficiency vs muon energy, fundamental physics validation
2. Detection Efficiency - Fraction of events producing detectable light, critical for trigger design

Analysis Statistics

The macro automatically calculates and displays:

• Mean primary energy and energy deposition
• Average photon detection rates
• Overall detection efficiency
• Light yield in photons per GeV
• Event processing statistics

Complete Analysis Workflow

1. Run Simulation

cd build
./exampleWaterTank test_cry.mac        # Cosmic ray simulation
# or
./exampleWaterTank test.mac            # Single muon test

2. Analyze Results

# Generate comprehensive analysis plots
root -l -b -q "../analyze_watertank.C(\"output_default.root\")"

3. View Output

The analysis generates three PNG files with detailed physics plots:

• water_tank_event_analysis.png: Event-level distributions and correlations
• water_tank_photon_analysis.png: Individual photon characteristics and detector response
• water_tank_physics_analysis.png: Light yield efficiency and detection performance

4. Interpret Results

Expected Physics Signatures

• Cherenkov Light Yield: ~200-300 photons/GeV for high-energy muons
• First Photon Time: ~9-10 ns (speed of light propagation + detector response)
• Wavelength Spectrum: Peak at 400-450 nm with 1/λ² falloff toward UV
• Detection Efficiency: >90% for muons crossing full detector volume
• Energy Deposition: ~100-200 MeV for 4 GeV muons (minimum ionizing)

Quality Checks

• Energy conservation: Primary energy >> Energy deposition
• Timing consistency: First photon < Last photon times
• Spectral range: Detected wavelengths within 300-700 nm window
• Spatial distribution: Hits concentrated on DOM hemisphere facing muon track

Configuration Files

CRY Setup (cry_setup.file)

Controls cosmic ray generation:

returnMuons 1          # Generate muons
returnElectrons 0      # Disable electrons
returnGammas 0         # Disable gammas
latitude 42.36         # Latitude (degrees)
altitude 0             # Sea level (meters)
subboxLength 3         # Generation box size (meters)
date 1-1-2024          # Cosmic ray flux date

Visualization (vis.mac)

/vis/open OGL 800x600-0+0
/vis/drawVolume worlds
/vis/scene/add/trajectories smooth
/vis/modeling/trajectories/create/drawByCharge
/vis/viewer/set/autoRefresh false

References and Related Work

IceCube Collaboration

• IceCube Neutrino Observatory
• IceCube Upgrade Technical Design Report

Geant4 Documentation

• Geant4 User Guide
• Geant4 Optical Physics Reference

CRY Cosmic Ray Library

• CRY: Cosmic Ray Shower Library
• CRY Physics Documentation

Contact and Support

Author: Christopher Prainito Institution: Harvard College

For questions or bug reports, please open an issue on GitHub.