KAHIBARO
6.1. The World Volume
Table of Contents
World dimensions
In every GATE simulation there is a special volume called the world. This is the root of the entire geometry tree. All other volumes, including detectors, phantoms, and air gaps, live inside the world as children or descendants. Geant4 will ignore anything that is not contained in the world, so defining this volume correctly is the first essential geometry step.
Conceptually, the world must be large enough that particles cannot realistically leave it during the simulation. If a particle reaches the boundary of the world, it is killed and its history stops. For medical-physics simulations, this usually means the world should fully contain the scanner or phantom, plus a safety margin where secondaries can still be tracked.
A common practical strategy is to estimate the largest linear size of your geometry, then multiply by a factor to obtain the world size. For example, if a PET scanner has an outer diameter of 90 cm and an axial length of 20 cm, a cubic world of side 2 m is usually sufficient. For a patient CT phantom, you would consider the head‑to‑toe extent, lateral width, and add generous margins.
In OpenGATE the world is usually configured as a box volume attached directly to the simulation. A typical pattern in Python looks like:
import opengate as gate
import opengate_core as gate_core
sim = gate.Simulation()
world = sim.world # created automatically
world.size = [2 * gate.m, 2 * gate.m, 2 * gate.m]
Here the world is a box whose half lengths in $x$, $y$, and $z$ are specified. GATE uses Geant4 conventions where a box size is given as full lengths or half lengths depending on the API; in OpenGATE, world.size is a 3‑component list of full lengths. You should always double‑check the documentation of the version you use, then inspect the geometry in a visualization to confirm that all objects fit comfortably inside the world.
Choosing world dimensions is always a compromise between realism and computational efficiency. A larger world contains more material, which can slightly increase the number of interactions and tracking steps, especially for photons and electrons that travel long distances. On the other hand, a world that is too small can cut off particle tracks, remove scattered radiation that would otherwise reach the detector, and bias dose distributions at the boundaries.
For many medical applications a margin of at least several tens of centimeters around the outermost component is reasonable. In proton therapy simulations, where protons have finite range, you must also ensure the world is long enough in the beam direction to contain the full Bragg peak and all nuclear secondaries. If you are unsure, start with a larger world, run a small test, and then reduce the dimensions if necessary.
Important rule: The entire simulated system, including detectors, phantoms, and all possible particle tracks of interest, must be fully contained inside the world volume. Particles that reach the world boundary are terminated, which can distort imaging or dose results if the world is too small.
World material
In addition to its size, the world volume must have a material. This material fills every part of the world that is not occupied by other volumes. Whenever you do not explicitly place a solid object somewhere, the particle will move through the world material in that region.
For most medical‑physics simulations the natural choice is air, which approximates the room or surrounding environment of a scanner or a patient. Air has low density, so it introduces minimal attenuation and scattering while still being realistic. In GATE you can directly use standard Geant4 materials, for example:
world.material = "G4_AIR"This name refers to a predefined material from the Geant4 NIST database. You usually do not need to define it yourself. If you are simulating a laboratory setup in vacuum or trying to isolate a detector from environmental scattering for a test, you might instead set:
world.material = "G4_Galactic"which is a very low‑density vacuum material that minimizes interactions.
The choice of world material can influence results in subtle ways. Scatter in air around a gamma camera can contribute to detector counts outside the primary beam. In PET, photons may scatter in air between the patient and detectors. In dose simulations, wall materials such as concrete or lead may matter if you are interested in stray radiation beyond the patient or phantom. When these effects are not of interest, a simple air world is usually sufficient.
The world material also serves as a default whenever you forget to specify a material for a child volume. In many frameworks, an undefined material will cause an error, but if a child is incorrectly placed outside its intended parent, it can end up directly in the world and inherit the world material. This is a common source of geometry bugs. Visual inspection and material printouts help to detect such mistakes.
In OpenGATE you will usually rely on existing Geant4 materials for the world, but you can also define a custom air‑like material if needed. For example, to simulate a gas‑filled room with slightly different composition, you would first define that material at the simulation level, then assign it to the world. The detailed procedure for defining materials is described elsewhere, but the important point is that the world material must always be a valid Geant4 material.
Important rule: Always set the world material explicitly, usually to "G4_AIR" or "G4_Galactic". The world material fills all empty regions not occupied by other volumes, so an incorrect world material can change attenuation, scattering, and dose outside your main objects.
Once the world dimensions and world material are defined, you have a stable environment in which you can safely add all other geometry components such as detectors, phantoms, and shielding.
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