KAHIBARO
14.3. Energy Deposition Actor
Table of Contents
Deposited energy
The energy deposition actor is the basic tool in GATE to record how much energy particles deposit in a given volume. It does not try to convert energy to dose or to mimic a detector’s electronics. Instead, it reports the raw physical quantity: the sum of energy deposited by all particle steps that occur inside the target volume.
When you attach an energy deposition actor to a volume, GATE monitors every particle step in that volume. For each step, it adds the step’s deposited energy, usually called $dE$ or edep, to internal counters maintained by the actor. The result is typically stored per event and per volume, although the exact content depends on the actor configuration.
Conceptually, you can think of the actor as computing, for each event:
$$
E_{\text{dep, volume}} = \sum_{\text{all steps in volume}} dE_i
$$
where $dE_i$ is the energy lost by the particle in step $i$ within the selected volume.
Important: The energy deposition actor reports energy, not dose. To obtain dose, you must divide the deposited energy by the mass of the material in which it is deposited, or use a dedicated dose actor.
Energy deposition can be recorded in different units, most often in keV, MeV, or joules, depending on your unit choices in the simulation script. It is important to remain consistent about units everywhere in your configuration, especially when comparing results with analytical calculations or experimental measurements.
You typically attach the actor to a specific volume, such as a detector crystal, a phantom, or a small region of interest. If the volume is non‑voxelized and you use a simple (non‑binned) energy deposition actor, the output will often be a list of total deposited energies per event and per attached volume. This is very useful for building energy spectra for detectors or for checking that a source deposits the expected amount of energy in a test volume.
Because energy deposition is a stochastic process, the values you obtain will fluctuate from event to event. When you run more events, the statistical uncertainty on the mean deposited energy decreases. For any analysis based on energy deposition, you should always consider both the mean and its statistical spread, especially when validating your simulation or comparing different configurations.
The energy deposition actor is particularly useful as a debugging and validation tool. For example, if you change materials or geometry and the total deposited energy in a reference region changes in a non‑physical way, that may indicate a geometry or physics configuration problem. Likewise, if almost no energy is deposited where you expect strong interactions, this is often an early sign that the source, physics list, or geometry assignment is incorrect.
Spatial distributions
Beyond simple totals per volume, the energy deposition actor can record where the energy is deposited. In this mode, the actor divides the attached volume into a 3D grid of voxels. For each step inside the volume, it accumulates the deposited energy into the corresponding voxel, producing a map of energy deposition.
Mathematically, if the volume is discretized into voxels indexed by $(i,j,k)$, the actor records
$$
E_{\text{dep}}(i,j,k) = \sum_{\text{steps in voxel }(i,j,k)} dE_s,
$$
where $dE_s$ is the deposited energy in step $s$ that occurs inside voxel $(i,j,k)$.
This results in a 3D image of energy deposition. Each voxel contains the total energy deposited in that spatial region during the simulation. The grid configuration is defined by the number of voxels in each direction and the physical size of the volume. If the actor is attached to a volume of size $L_x \times L_y \times L_z$ and you choose $N_x, N_y, N_z$ voxels along each axis, then the voxel sizes are:
$$
\Delta x = \frac{L_x}{N_x}, \quad
\Delta y = \frac{L_y}{N_y}, \quad
\Delta z = \frac{L_z}{N_z}.
$$
Important: A finer voxel grid provides higher spatial detail but increases memory use and output file size. Choose voxel dimensions that match your physics question and available computational resources.
In practice, spatial distributions of energy deposition are saved as 3D images, such as MHD, NIfTI, or other image formats supported by GATE and your configuration. You can load these images into your preferred visualization or analysis software to inspect the spatial patterns of energy deposition. Typical uses include observing beam penetration in a phantom, visualizing hot spots where many interactions occur, and checking symmetry or uniformity in a detector.
For detector development, voxelized energy deposition maps reveal how energy is shared between neighboring detector elements and how edge effects appear near crystal boundaries. For dosimetry‑oriented studies, energy deposition maps are the starting point before converting to dose distributions, either by dividing by density and voxel mass or by using a dedicated dose actor, which handles this conversion for you.
You can also analyze spatial distributions statistically. For example, by averaging energy maps over multiple runs or by examining line profiles through the 3D data, you can compare different geometries or physics models. When validating a setup, you may compare the simulated spatial pattern of energy deposition with analytical expectations, such as exponential attenuation in a simple slab, or with measured profiles in a calibration phantom.
Finally, spatial distributions recorded by the energy deposition actor are often used alongside other actors. For example, you might use an energy deposition actor to obtain a qualitative view of where interactions occur, while a dose actor provides quantitatively correct absorbed dose. Keeping these roles distinct helps you interpret the outputs correctly and build a clear analysis workflow.
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