45.4. Creating a Gamma Source

Table of Contents

Overview

In this part of the gamma‑ray detector example you define the radiation source that illuminates your scintillation detector. The goal is not to cover every possible source configuration in GATE, but to set up a simple, controlled gamma source that is suitable for building and interpreting an energy spectrum.

You will typically create a monoenergetic or nearly monoenergetic gamma source, position it with respect to the detector, and choose a realistic activity or number of events for a fast yet informative simulation.

Choosing a Simple Gamma Source Model

For an introductory detector simulation, the focus is the detector response, not a complex source. The most straightforward starting point is:

A single gamma energy, for example 511 keV or 662 keV.
A point‑like source, placed in front of the detector crystal.
A simple emission pattern, most often isotropic or directed toward the detector.

This configuration produces a clear peak in the energy spectrum and allows you to understand how energy blurring and thresholds modify that peak later in the example.

If you are interested in specific radionuclides, such as Cs‑137 with a 662 keV gamma or Na‑22 with a 511 keV gamma, you can mimic them by selecting the corresponding monoenergetic gamma energy, without modeling the full decay scheme.

Creating a Gamma Source in OpenGATE

In OpenGATE, you add particle sources to the simulation object. You should already have created the simulation and world in earlier steps of the practical example.

A typical pattern is to create a source, set its particle type to gamma, choose the energy, define position and direction, and then specify the way you control the number of emitted particles.

A minimal conceptual sequence is:

Add a new source object to the simulation.
Set particle to gamma.
Define energy as monoenergetic, for example $E_\gamma = 662\ \text{keV}$.
Define the source position relative to the detector.
Choose the emission pattern (directed or isotropic).
Specify how long the source emits or how many events you simulate.

The exact Python commands are not the focus here, but you should recognize that these attributes map directly to the ideas you learned in the dedicated chapter on particle sources: type, position, direction, energy and intensity.

Selecting the Gamma Energy

For a basic scintillation detector example, you usually select one gamma energy that produces a clear line in the spectrum. Two common choices are:

Radionuclide (approximation)	Gamma energy	Typical use
Cs‑137	662 keV	General calibration, teaching examples
Na‑22	511 keV	PET‑related examples and detector studies

In the full radionuclide chapters you will learn about complete decay schemes. Here, you simply use a single gamma energy, which makes the interpretation of the resulting energy spectrum straightforward.

A key rule for this example is to use a single, fixed gamma energy.
This creates a clear photopeak in the energy spectrum and simplifies learning about detector resolution and thresholds.

You may choose any convenient energy, but it should be high enough that the detector crystal absorbs a significant fraction of the photons. For common scintillators like NaI(Tl) or LYSO, energies between about 100 keV and 1 MeV work well.

Defining Source Position and Geometry

The source position must be selected relative to the detector you created in the previous section of the example. For absolute beginners, a common choice is to place the point source on the detector axis, at a small distance from the crystal surface.

You should:

Place the source outside the detector volume, slightly in front of the entrance face, so photons travel through air before reaching the crystal.
Align the source position with the center of the crystal face, for example at the same $x$ and $y$ coordinates as the crystal center, and a smaller $z$ coordinate if the detector is placed along the $z$ axis.

Make sure the source is not located exactly on a boundary shared by multiple volumes and not inside any shielding volumes, unless you specifically want to study attenuation as in a later shielding example.

Because this is a practical detector example, you do not need to create extended source shapes such as boxes or spheres. A single point is usually sufficient and eliminates additional geometric complications.

Choosing Source Direction and Emission Pattern

For a point gamma source, you have two main options:

Isotropic emission, where photons are emitted uniformly in all directions.
A directed beam, where photons are emitted predominantly toward the detector.

For this example, isotropic emission is often the simplest conceptually, because it corresponds directly to a small physical source placed near a detector. The fraction of emitted photons that reach the crystal will be determined by the solid angle covered by the detector, which is a realistic situation.

If you prefer to reduce simulation time and increase the count rate in the crystal for a given number of events, you can restrict emission to a cone that points toward the detector. In that case, you define a mean direction and possibly an angular spread around that direction.

Regardless of the choice, keep the angular model as simple as possible, so that any structure observed in the energy spectrum can be attributed mainly to the detector and not to complex angular effects.

Controlling Intensity: Activity, Time, and Events

In the detector example, you will usually be more concerned with obtaining a sufficient number of detected counts than with modeling a physically realistic activity. You have two equivalent viewpoints:

You can define an activity in units like Bq or MBq and a total acquisition time.
You can define a fixed number of primary events and ignore the physical time.

From the point of view of building an energy spectrum, both approaches simply determine the total number of photons simulated. In an introductory example, you can:

Select a moderate number of primaries, enough to produce a smooth energy spectrum but still run quickly.
Consider that statistical uncertainty scales roughly as $1/\sqrt{N}$, where $N$ is the number of detected events.

For this example, the important quantity is the number of simulated events, not the absolute activity.
You should choose event counts that are high enough for a smooth spectrum but low enough to keep run times short.

Later, when you study activity and time dependent simulations, you will learn how to interpret dose rates and realistic acquisition durations.

Preparing for Later Steps in the Example

As you configure the gamma source, it helps to think ahead to the following steps in this practical example:

You will record energy deposition in the detector crystal to form an energy spectrum.
You will apply energy resolution and possibly an energy window.

Therefore, keep the source configuration simple and well controlled. In particular:

Use a single gamma energy to produce a well defined photopeak.
Avoid complex spatial or angular distributions that are not required by the learning goal.
Place the source in a reproducible position, such as on the detector axis.

With a clean source definition, the structures observed in your final energy spectrum can be attributed mainly to physical processes in the detector and the digitization chain, which is precisely the purpose of this gamma‑ray detector example.