KAHIBARO
26.1. Project Overview
Table of Contents
Simulate proton transport in water
In this example you will build and study a simple but very important Geant4 simulation: a monoenergetic proton beam entering a water volume, often called a water phantom. This setup is a standard reference in medical physics and proton therapy, because water behaves similarly to soft tissue. By simulating proton transport in water you can observe how protons lose energy, where they stop, and how the dose is distributed with depth.
The goal of the project is to reproduce the characteristic depth dose curve of a proton beam and, in particular, to see the Bragg peak. When protons travel through matter, they lose a small amount of energy at the entrance, then deposit most of their remaining energy in a sharp peak near the end of their range. In clinical proton therapy, this peak is used to deliver a high dose to a tumor while sparing healthy tissue in front of and behind it. Your simulation will let you visualize this behavior.
You will create a simple geometry where a block of water is the main scoring volume. A narrow proton beam will enter from one side and travel along a straight line into the phantom. Using a suitable Geant4 physics list for proton and electromagnetic interactions, the toolkit will handle multiple scattering, ionization energy loss, nuclear interactions, and the creation of secondary particles. You will not need to implement the physics formulas yourself, but you will configure which physics models are used and at what level of detail.
To quantify the energy deposition with depth, you will divide the water phantom into slices along the beam direction. Each slice will act as a small volume where you measure the deposited energy per incident proton. From the simulated energy deposition you can reconstruct the depth dose curve. You will then use the Geant4 analysis tools to store the results in histograms or ntuples, typically written to a ROOT file. After running the simulation for many events, you will analyze the output with ROOT and plot the depth dose distribution, clearly identifying the Bragg peak and its position.
Throughout the project you will practice several core Geant4 skills in a realistic context. You will define materials using the NIST database to obtain a correct description of water, construct a simple but meaningful geometry, configure a proton beam as the primary particle source, select and initialize an appropriate physics list, implement scoring of energy deposition, and control and analyze the run through macro commands and ROOT. By the end of this example you will have a complete workflow, from building the application and running the simulation, to extracting and plotting a physical observable that is directly relevant to real proton beam measurements.
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