KAHIBARO
46.1. Project Overview
Table of Contents
Simulate gamma attenuation
In this project you will build a small, self‑contained GATE simulation that models how gamma rays are attenuated when they pass through shielding materials. The goal is to connect basic Monte Carlo concepts to the familiar exponential attenuation law from radiation physics, and to learn how to design and analyze a simple yet realistic shielding experiment.
You will simulate a monoenergetic gamma source that emits photons toward a slab of shielding material. Behind the shield you will place a scoring region that records the transmitted photons. By varying the material type and thickness you will be able to measure how many photons get through, compute transmission and attenuation coefficients, and compare the Monte Carlo results with analytical predictions.
The structure of the project follows the later subsections of this practical example chapter. You will first define a gamma source with a fixed energy and a collimated direction that approximates a narrow beam. Next you will create a shield volume between the source and the scoring region. You will then select among common shielding materials such as lead, aluminum, or concrete, and adjust the shield thickness to see its effect on transmission.
Behind the shield you will configure an actor that records the photons that pass through, for example by counting them or storing their energies and directions. From these data you will calculate the transmitted fraction of photons for each configuration. This allows you to obtain the transmission $T$ and the corresponding attenuation, and to compare them with the theoretical exponential attenuation law for narrow beams.
The heart of the project is the comparison between simulation and the analytical model. In a narrow‑beam geometry, the unscattered transmitted intensity through a slab of thickness $x$ is predicted by the exponential law.
Exponential attenuation law for a narrow beam:
$$
I(x) = I_0 e^{-\mu x}
$$
where
$ I_0 $ is the incident intensity,
$ I(x) $ is the transmitted intensity after thickness $ x $,
$ \mu $ is the linear attenuation coefficient of the material.
The transmission is
$$
T = \frac{I(x)}{I_0} = e^{-\mu x}.
$$
By plotting the simulated transmission as a function of thickness and comparing it with $e^{-\mu x}$, you will check how well the Monte Carlo results reproduce the analytical expectation. You will also see how scattered photons, if included in the tally, can cause deviations from the ideal narrow‑beam law, which is an important point in real shielding design.
At the end of this project you should understand how to set up a simple linear geometry in GATE, how to create and position a shield, how to record transmitted photons, and how to translate raw Monte Carlo counts into physically meaningful quantities such as transmission and attenuation. This example prepares you for more complex shielding and dosimetry simulations where scattered radiation and realistic geometries play a major role.
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