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10.3. Source Position
Table of Contents
Point source
In GATE, the simplest way to define where particles start is a point source. A point source emits all particles from a single position in space. This is useful when you want an idealized or highly localized source, for example a calibration source placed at the center of a detector or a small radioactive marker.
You define a point source by specifying a single position, usually as coordinates $(x, y, z)$ in the simulation world. In OpenGATE Python, this is typically done with a position type such as "point" and a position value. The coordinates are always combined with the correct length units, for example $0 mm, 0 mm, 0 * mm$ for the center of the world.
For a point source, the entire activity is concentrated at one position. The position must be given with units and in the world coordinate system unless you explicitly attach the source to a volume.
You can move a point source by changing its position coordinates. This is often used to study detector response at different locations or to scan a phantom by repeating the simulation with the point source at different points. When interpreting results from a point source, remember that the dose or fluence you obtain is due to a highly localized emission, not a distributed activity.
Point sources are also useful for debugging geometry. Placing a point source at a suspected problematic region and visualizing particle tracks can help you see whether materials, overlaps, or boundaries behave as expected.
Box source
A box source spreads particle emission uniformly inside a rectangular region. Instead of all particles starting from one point, their initial positions are sampled randomly within the volume of a box. This is useful when you want a simple homogeneous activity distribution in a region such as a block phantom, a slab, or part of a detector.
To define a box source you need the size of the box (its half lengths in X, Y, and Z in GATE) and the center position. The source position type is typically set to "box", and you provide the box dimensions and possibly an offset relative to the world or to a parent volume.
A box source represents a uniform spatial distribution over a rectangular volume. You must specify the box size with units and ensure the box is fully inside the intended volume (phantom or detector) if you want all particles to start in that region.
Box sources are often used to represent uniform activity in a simple water phantom or a test object. By adjusting the box dimensions you can study edge effects or partial volume effects, for example by shrinking the box so it does not fill the entire phantom.
If you place a box source so that it intersects a boundary, some particles will start in one material and some in another. This can be intentional, for example to model activity crossing an organ boundary, but you should be aware of this when interpreting resulting dose maps or detection patterns.
Sphere source
A sphere source defines particle emission uniformly inside a spherical region. Positions are sampled randomly within the volume of the sphere. This is useful for modeling more realistic, compact activity distributions such as small tumors, spherical lesions, or calibration sources that have a finite size.
To create a sphere source you specify its radius and center position. The radius must have length units, for example $10 * mm$. The position type is usually "sphere", and GATE takes care of sampling random positions with uniform probability in the sphere volume.
A sphere source provides a uniform distribution inside a radius $R$. The radius and center must be given with correct units, and the sphere should be fully contained in the target volume if you want all particles to start in that region.
Compared with a box source, a sphere source has smoother boundaries and can better approximate compact sources in tissue. Because emission is three dimensional in all directions from the center, the resulting dose or fluence often has a more radially symmetric pattern around the center of the sphere.
You can also use a small sphere to approximate an almost pointlike source while avoiding numerical artifacts that can appear if all particles originate at exactly the same point, for example in very high statistics simulations.
Volume source
A volume source is defined so that particles are emitted within an existing geometry volume instead of specifying a simple analytic shape like a box or sphere. In this case, GATE uses the shape and placement of a named volume in the geometry to sample initial positions. This is particularly useful for complex geometries such as realistic phantoms, patient models, or detailed detector components.
To use a volume source, you choose a volume already defined in the geometry, and set the source position type to something that refers to this volume. GATE then ensures that generated positions lie inside that volume, regardless of whether the shape is a box, cylinder, sphere, or a more complex construct like a voxelized phantom.
A volume source uses the existing geometry volume as the spatial region. The source is tied to the volume name, so if you change or move that volume, the source region moves with it. The volume must be fully defined before you configure the source.
Volume sources are especially powerful when combined with voxelized geometries and activity maps. For example, you can define a patient phantom volume and then map local activity from an image, so that the emission probability varies with position inside the volume. Even when you use a uniform volume source, attaching it to a specific organ or component helps keep your simulation configuration clear and easier to maintain.
If the volume is translated or rotated in the world, the source automatically follows, because its position is relative to that volume. This makes volume sources convenient when modeling moving detectors or rotating systems where the activity distribution is fixed in an object but the whole object moves during the simulation.
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