16.4. Hit Attributes

Table of Contents

Energy

Each hit represents a single interaction of a particle in a sensitive volume, and one of its core attributes is the deposited energy. In GATE, the hit energy is usually stored as the energy deposited during one simulation step inside that volume, often called $E_\text{dep}$.

The deposited energy is typically given in units of keV or MeV, depending on how the digitizer and output are configured. The choice of units does not change the physics but is essential when you read and analyze the data, since you must consistently apply the same unit system in your analysis scripts.

Energy in hits is used later by the digitizer to approximate the detector’s measured signal. In scintillation detectors, the deposited energy is proportional to the light yield, and then, through the detector response model, to the recorded energy in singles. In semiconductor detectors, it is related to the number of charge carriers produced. Multiple hits in the same crystal or readout cell may be combined, summed, or otherwise processed to form one detector event.

In many analyses, you will use hit energy to:

Build energy spectra at the hit level, which can help debug physics lists or geometry.
Check that the maximum deposited energy is consistent with the incident particle energy.
Evaluate where energy is deposited inside a detector or phantom.

Always check and remember the energy units of your hit data. Mixing keV and MeV or forgetting to apply unit conversions is a common and serious source of analysis errors.

When you compare hit energies to theoretical expectations, it is important to keep in mind that individual hits often represent partial energy deposits, especially for charged particles that undergo many steps or for photons that undergo multiple interactions before leaving the volume.

Position

The position attribute of a hit records where in space the interaction took place. This is given by three coordinates, usually $x$, $y$, and $z$, in the global simulation coordinate system. The units are typically millimeters, but you should verify the units used in the output and remain consistent in all visualizations and analyses.

Hit positions are crucial for reconstructing spatial patterns of energy deposition. In detector simulations, they allow you to see where interactions occur inside crystals, modules, or entire scanner geometries. In dosimetry and phantom studies, the spatial distribution of hit positions helps you understand where particles deposit energy inside a patient or experimental setup.

The recorded hit position is usually the position at the middle or end of the simulation step where the interaction occurs. For charged particles that take many small steps, a dense cluster of nearby positions can represent a track through the material. For photons, isolated positions correspond to discrete interactions such as Compton scattering or the photoelectric effect.

When you later group hits to form detector signals, you often map hit positions to higher level detector elements. For example, a crystal index may be assigned based on which volume or region contains the hit position. For more advanced analysis, you can use hit positions to generate 2D or 3D maps of interaction density, to check alignment of geometry, or to debug suspicious detector regions that appear inactive or overly active.

Hit positions are recorded in the simulation coordinate system, not in image index coordinates. Always apply the correct coordinate transforms when comparing hit locations to images or physical drawings.

Time

The time attribute of a hit records when the interaction occurred within the simulation’s time frame. It is usually expressed as the global time since the beginning of the event or acquisition, often in nanoseconds. Time is essential whenever you are modeling detectors that have time resolution or when you analyze time-dependent processes.

For each hit, the time tells you when the particle reached that volume and deposited energy. In detector simulations, these times form the basis for constructing detector timestamps after digitization. For time-of-flight PET or fast timing detectors, the precise time of each interaction is critical for estimating arrival time differences between detectors.

The recorded time also reflects the particle’s history. For instance, secondary particles produced later in an event will naturally have larger times than the primaries, and scattered photons arriving after multiple interactions will be delayed compared to unscattered ones. This makes hit times very useful in studying scatter, delayed events, or slow decays in complex simulations.

In postprocessing, you can use hit times to:

Build time distributions that show how interactions occur as a function of time.
Identify late or out-of-time hits that might contribute to random or delayed events.
Validate timing models and transport parameters.

Be careful to distinguish between global time, event time, and any local time definitions used in your simulation. Misinterpreting the time reference can lead to incorrect conclusions about delays or coincidence windows.

Particle type

Each hit is associated with the particle that produced it, and the particle type attribute captures this information. Internally, this is often stored as a string or an identifier that corresponds to a specific particle definition, such as gamma, electron, positron, proton, or more complex ions.

Knowing the particle type is vital when you want to separate contributions from different species in a detector or phantom. For example, you might want to plot energy deposition from gammas separately from that from electrons, or you might be interested only in hits from secondary electrons created by photon interactions. In PET simulations, distinguishing between primary 511 keV photons and secondary scattered photons can be especially important.

The particle type also helps you verify that your physics list and transport settings work as expected. If you see unexpected particle types contributing hits, such as heavy ions where you expected only photons and electrons, it can signal that your physics configuration produces interactions you did not account for.

When you filter hits during analysis, you can restrict your data to one or several particle types. This is often done before constructing energy spectra, spatial maps, or timing distributions, to focus on the processes relevant to your specific scientific question.

Always check the particle type when interpreting hit data. A spectrum or spatial map that mixes multiple particle species can be misleading if you assume it represents only one type of particle.

Event ID

The event ID is an attribute that links each hit to the primary event that generated it. In Monte Carlo simulations, an event usually corresponds to one primary particle, or one decay, along with all secondary particles created by that primary. All hits that share the same event ID belong to the same physical history.

This attribute is essential whenever you need to group or correlate hits. For example, if you want to reconstruct the full interaction chain of a single photon entering a detector, you can gather all hits with the same event ID and track their positions, times, and energies. In PET, you may later relate detector-level data back to the event ID of the original positron annihilation.

Event IDs are also important for calculating per-event quantities, such as total energy deposited by one primary, or the number of hits generated by each event in a given volume. When you study statistical properties like energy resolution or efficiency, it is often necessary to work at the event level rather than at the individual hit level.

In more advanced simulations, event IDs can also be combined with other identifiers such as track IDs to provide a complete view of particle histories. This allows you to distinguish between hits from different tracks within the same event, which can be useful when studying complex cascades or coincidences.

Never mix hits from different event IDs when you intend to analyze the behavior of a single primary particle or decay. Always group by event ID first, then perform per-event calculations.

Volume ID

The volume ID attribute connects each hit to the specific geometric volume in which the interaction occurred. This can be stored as an integer index, a string name, or a structured identifier that encodes the position within a detector hierarchy, such as ring, module, and crystal indices.

Volume IDs are key to interpreting where in the detector or phantom the energy deposition took place. In complex medical imaging detectors, many crystals and modules share the same material and dimensions, so geometry alone is not enough to identify which element recorded the hit. The volume ID bridges the gap between coordinates and the logical or physical detector element.

You can use volume IDs to:

Build energy or count maps for each detector element.
Identify hot or cold elements that may represent misconfigured geometry or analysis errors.
Aggregate hits per crystal, module, or region before digitization or postprocessing.

In practice, volume IDs often relate directly to the readout scheme of the detector. A given volume ID may correspond to a single readout channel in the electronics, or it might be used together with other indices to reconstruct a more complex mapping. This connection is crucial when converting hit data into signals that resemble what a real detector would output.

When you analyze simulation results, volume IDs also help isolate specific regions of interest, such as a particular organ in a phantom or a particular ring in a PET scanner. You can filter hits based on volume IDs to focus your analysis on only the volumes that contribute to your measurement.

Volume IDs depend on your geometry definition and naming strategy. Changing volume names, hierarchy, or replication patterns can change these IDs, so always verify your mapping between volume IDs and detector elements after any geometry modification.