KAHIBARO
13.2. Range Cuts
Table of Contents
Gamma cuts
In GATE, a range cut tells Geant4 which secondary particles are worth creating explicitly. Instead of asking for a minimum secondary energy, you specify a minimum range in material, for example $0.1\ \text{mm}$. Geant4 then converts this range into an equivalent energy threshold for each particle and material.
For gammas, a range cut does not mean a physical range like for charged particles, because photons do not lose energy continuously. Instead, the cut is interpreted in terms of the range of the secondary charged particles that would be produced by the gamma interaction.
When a gamma interacts, it can produce secondary electrons or positrons. If those secondaries would have a range shorter than the gamma range cut in the current material, Geant4 does not create them as separate tracks. Their energy is deposited locally instead. If their range is longer than the cut, they are created and tracked.
Key rule for gamma cuts
A gamma range cut effectively controls production of secondary electrons and positrons from gamma interactions.
If the estimated range of these secondaries is shorter than the gamma cut value, they are not created as tracks and their energy is deposited locally.
In medical-physics simulations, gamma cuts are important in regions where photon interactions dominate, such as detector crystals and patient or phantom volumes. A smaller gamma cut leads to more low-range secondary electrons being created and tracked, which increases accuracy for dose or detailed energy deposition, but also increases computation time.
Typical practice is to use a relatively small gamma cut in sensitive regions where energy deposition details matter, and possibly a larger cut in regions that are less important for the final result. In GATE you usually set gamma cuts by defining a global default and optionally region-specific values. The numerical values should be interpreted as lengths, for example $1\ \text{mm}$ or $0.1\ \text{mm}$, not energies.
Electron cuts
For electrons, the concept of a range cut is more direct. Electrons lose energy continuously as they travel through matter. Geant4 uses the electron range cut to decide whether to create secondary electrons (such as $\delta$-rays) and to define the minimum range for transported electrons.
If an electron that would be produced as a secondary has a range shorter than the electron cut in the current material, Geant4 does not create it as a separate track; instead, the energy is deposited locally. Similarly, when an existing electron’s remaining range falls below the cut, the remaining energy is deposited and the track is stopped.
Key rule for electron cuts
The electron range cut is a minimum transport range.
Electrons (and electron secondaries) with a shorter range than the cut are not transported as individual tracks; their energy is deposited locally instead.
Smaller electron cuts lead to a more detailed description of the electron transport, with more steps and more short-range secondaries. This is often necessary for accurate dose calculations or for simulations sensitive to fine spatial patterns of energy deposition. However, smaller cuts significantly increase simulation time and memory use.
In practice, you choose electron cuts according to the spatial resolution and accuracy you need. For example, if you are scoring dose on $2\ \text{mm}$ voxels, using an electron range cut much smaller than $0.1\ \text{mm}$ may not noticeably improve the scored voxel dose, but will slow down the simulation. For detector simulations where internal fine details of electron tracks are not needed, a moderately larger electron cut can provide sufficient accuracy with better performance.
Positron cuts
Positrons behave similarly to electrons with respect to transport, but with the additional step of annihilation when they slow down in matter. The positron range cut controls how finely their transport is described and which secondary positrons are created.
As with electrons, if a secondary positron would have a range shorter than the positron cut in the current material, it is not created as a separate track. Its energy is added locally instead. An existing positron is transported until its remaining range falls below the cut, then its remaining kinetic energy is deposited locally, and annihilation is handled according to the physics list.
Key rule for positron cuts
The positron range cut controls the minimum range for transported positrons and for any secondary positrons.
Positrons with a range below the cut are not tracked, and their energy is deposited locally before annihilation is treated.
In PET simulations, positron transport is important because the finite positron range affects the spatial blurring of the annihilation point. A smaller positron cut allows a more accurate description of this range. If the positron cut is too large, the effective positron range may be underestimated, which can slightly reduce the apparent spread between the emission point and the annihilation location.
For general dose calculations involving positron emitters, positron cuts are usually set close to electron cuts, since both types of particles have similar ranges at comparable energies. As always, smaller cuts improve physical detail while larger cuts improve computation speed, so the choice depends on whether accurate modeling of positron transport is central to your simulation goal.
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