KAHIBARO
38.3. Phase-Space Files
Table of Contents
Reusing particle distributions
Phase space files are a compact way to store the state of many particles at a given scoring surface or volume boundary. In GATE you usually create them with a phase space actor, then you can later reuse them as an input source for another simulation. This two step workflow is very common in medical physics, for example when an accelerator head is simulated once, and the resulting beam is then used many times with different patients, phantoms, or geometries.
A phase space file typically contains, for each recorded particle, information such as position, direction, kinetic energy, particle type, weight, and time. The exact content depends on how you configured the phase space actor, but the key idea is that the file represents a statistical sample of particles that cross a given plane or region. When reused as a source, GATE samples from this stored distribution instead of generating particles from an analytical or simple geometric model.
When you read a phase space file as a source, you effectively "replay" those particles at the scoring surface. The downstream simulation, for example transport through a patient phantom or a detector, can then be modified and rerun without any change to the upstream accelerator or beamline.
A phase space file is only valid to reuse if the geometry and physics upstream of the scoring surface stay unchanged. If you modify anything before the scoring plane, you must regenerate the phase space data.
You should also be aware that phase space sources are inherently limited by the number of particles stored in the file. If you run a new simulation that requests far more primary histories than are available in the phase space, GATE will reuse particles by looping over the file, typically with randomization of the order. This is acceptable for many applications, but it introduces correlations that must be considered when you estimate statistical uncertainties.
Another important aspect is normalization. When a phase space file is created, the phase space actor usually records the number of primary particles or the total simulated time that produced the file. When reusing the file, you must decide what effective fluence or intensity it represents in the new simulation. For instance, you might use one pass through the file to represent a given number of monitor units in external beam radiotherapy, or a certain number of particles per second in a beamline study. You then scale your dose or fluence results accordingly.
Using phase space files is particularly useful when the upstream part of the simulation is expensive. Accelerator heads, collimator systems, or complex source models can be simulated once with many primary particles, and the resulting phase space is stored. Later, you can attach different patient phantoms, alter scoring actors, or change detector designs, all driven by that same phase space source. This greatly reduces the total computing time required for parametric studies or optimization.
Another advantage is reproducibility. A phase space file is a complete snapshot of the particle distribution at the scoring surface. As long as you keep the file, anyone can repeat the downstream simulation exactly, even with a different GATE version or on a different machine, provided the physics configuration is compatible. For published work, keeping your phase space files and documenting how they were created is part of good scientific practice.
Finally, phase space files can be used hierarchically. You may record phase space once at an upstream plane, then reuse that to generate a second phase space at a downstream location with a different geometry, such as a specific collimator or range shifter configuration. This chained approach can help separate complex systems into modules, each with its own validated phase space, while still allowing flexible combinations in later simulations.
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