KAHIBARO
Phase Space Actor
Table of Contents
Particle information
A phase space actor in GATE records detailed information about particles as they cross a chosen surface or volume. Instead of scoring a derived quantity like dose or fluence, it saves the microscopic state of many individual particles. This state is often called the particle phase space and includes at least the particle type, position, direction, and energy, plus optional extra attributes such as time or event identifiers.
When you attach a phase space actor to a volume, GATE writes an entry every time a particle fulfills the actor’s recording condition. Typical conditions are entering a surface, exiting a surface, or simply being inside a scoring plane. Each recorded entry corresponds to one particle crossing and is similar to a “snapshot” of that particle at that location.
Because the phase space actor can easily produce very large files, it is important to think carefully about which particles to record and which attributes are required for your later analysis. For instance, if you only need photons for a detector study, you should filter out all other species in order to reduce file size and speed up the simulation.
A typical phase space record contains at least these items:
- Particle type, for example gamma, electron, positron, proton.
- Spatial coordinates, usually three components $(x, y, z)$ in the simulation reference frame.
- Direction cosines, usually three components $(u_x, u_y, u_z)$ that describe the direction of motion.
- Kinetic energy $E$ of the particle.
- Time $t$ at which the particle reaches the plane or volume.
- Event or track identifiers so that you can relate multiple phase space entries to the same primary event.
A phase space actor records individual particle states, not integrated quantities like dose. You must post-process phase space data to obtain spectra, fluence maps, or other high-level results.
Many workflows use phase space data in two stages. In the first stage, you simulate a complex geometry such as a linac head or a CT tube and record particles on a plane downstream. In the second stage, you use that phase space file as a source in another simulation, for example one that includes only the patient or detector. This reuse significantly reduces computation time, because the expensive upstream transport is performed once and then recycled.
Position
The position stored in a phase space record describes where the particle crosses the scoring surface or volume. In GATE, positions are expressed in the global simulation coordinate system, using the units that you have defined, typically millimeters or centimeters.
For each recorded particle, the phase space actor saves three coordinates $x$, $y$, and $z$. If you use a planar scoring surface, one coordinate will usually be approximately constant for all particles, for example $z = z_0$ for a plane perpendicular to the $z$ axis. The other two coordinates then provide the spatial distribution of the beam over that plane. This is essential when you want to reconstruct fluence profiles or spatial distributions at that location.
In many medical physics applications, the phase space plane is chosen to coincide with a physical interface. For example, in external beam radiotherapy it is common to place a phase space actor just below the treatment head, above the patient or a water phantom. The recorded positions then describe the incident particle field on the patient surface. In PET or SPECT detector studies, a phase space surface can be placed just in front of the crystal to characterize the incoming gamma field.
To interpret these positions correctly, you must be aware of the origin and orientation of your simulation geometry. The world volume has its own coordinate system, and each child volume can be placed with a translation and rotation. The phase space actor output is always in the global frame, so if you want to express results in a local detector coordinate system, you may need to apply coordinate transformations during analysis.
Always check the coordinate system and units of phase space positions. Misinterpreting axes or units, for example mixing up mm and cm, can lead to incorrect beam profiles or misaligned dose calculations.
When analyzing phase space positions, common tasks include:
- Building 2D fluence maps in $(x, y)$ at a fixed $z$ plane.
- Extracting 1D beam profiles along one direction, for example $x$ or $y$.
- Computing beam spot size or field size using statistical descriptors such as full width at half maximum.
Energy
The energy stored in a phase space record is the kinetic energy of the particle at the moment it crosses the scoring surface or volume. In GATE and Geant4, energies are typically expressed in units such as keV or MeV, chosen via the units system.
The phase space actor writes one energy value $E$ per recorded particle. This quantity allows you to reconstruct energy spectra, evaluate beam quality, and verify that your transport and physics settings produce the expected energy distributions at a given location.
For an x‑ray or gamma beam, the phase space energy distribution corresponds to the photon spectrum at that plane. For a proton or electron beam, it reflects both the initial spectrum and the energy losses that have occurred upstream, for example in collimators, filters, or air. If you are using a phase space file as an input source for a second simulation, the energies in the file directly define the incident energy distribution on that downstream geometry.
From phase space data, you can form histograms of $E$ to obtain energy spectra. You can also compute mean energy, energy spread, or energy windows. This is particularly useful when validating that your simulation reproduces known spectra from literature or from measurements.
When recording and later reusing phase space data, you must use consistent energy units throughout your simulations and analysis. Mixing keV and MeV in configuration or analysis will produce incorrect spectra and dose estimates.
It is also common to apply energy-based filters directly in the phase space actor. For example, you may choose to record only photons within a specific energy range, such as a PET photopeak window or a megavoltage therapeutic beam range. This reduces file size and ensures that the stored data is tailored to your planned analysis.
Direction
The direction of motion stored by the phase space actor specifies where the particle is heading when it crosses the scoring surface or volume. In GATE, this is expressed as a normalized direction vector with three components, commonly written as $(u_x, u_y, u_z)$, where
$$
\sqrt{u_x^2 + u_y^2 + u_z^2} = 1.
$$
These components are the direction cosines relative to the global coordinate axes. They allow you to reconstruct angular distributions, beam divergence, and scattering patterns. For example, a perfectly collimated beam along the positive $z$ axis would have $u_x \approx 0$, $u_y \approx 0$, and $u_z \approx 1$ for all particles, while a scattered field would exhibit a spread of directions.
Direction information is essential when converting phase space data into angular fluence distributions or when modeling detector response that depends on incident angle. It is also required when phase space files are turned into sources for a second simulation, because the direction vector tells the new simulation how to propagate each particle from the phase space plane into the downstream geometry.
In many analyses, directions are converted to polar and azimuthal angles, for example a polar angle $\theta$ relative to a given axis and an azimuthal angle $\phi$ in the transverse plane. You can derive these from the direction cosines and then build angular histograms. This provides insight into how much of the beam is primary and how much has been scattered by upstream materials.
Always treat direction vectors as normalized quantities. If your analysis inadvertently assumes unnormalized components, any derived angles or angular distributions will be incorrect.
Combining position and direction from the phase space actor lets you fully define lines of flight for each particle at the scoring surface. This combined information is the basis for many advanced applications, such as generating realistic entrance conditions for patient dose calculations, studying detector acceptance as a function of angle, or building detailed phase space sources for complex imaging and therapy simulations.
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