KAHIBARO
11.4. Gamma Emitters
Table of Contents
Gamma energies
In medical physics, a gamma emitter is a radionuclide that releases photons from its nucleus during de-excitation. In GATE, you do not manually define every individual photon, but it is essential to understand what gamma energies your radionuclide produces so that you can interpret spectra, choose appropriate energy windows, and validate your simulation.
Each gamma transition has a discrete energy, usually given in keV or MeV. For example, Tc‑99m has a dominant gamma at about 140 keV, while I‑131 has several lines, including a strong photon near 364 keV. When you specify a radionuclide in a radioactive source in GATE, the underlying Geant4 radioactive decay database provides this list of discrete gamma energies.
From a simulation point of view, gamma energy influences what you should expect to see in your detector:
At low energies, such as around 30 to 100 keV, photoelectric absorption is very probable in high‑Z detector materials. You will often see a strong photopeak and relatively short photon ranges in matter.
At intermediate energies used in SPECT, typically 100 to 300 keV, both photoelectric effect and Compton scattering contribute. Detectors are still efficient, but scatter in the patient and collimator becomes important.
At higher energies, such as the several hundred keV to MeV photons from therapeutic radionuclides, Compton scattering dominates in tissue, photons penetrate deeper, and detector efficiency usually decreases.
An energy spectrum recorded in a detector will not show perfect delta peaks at the gamma energies. Detector response, including Compton continua, escape peaks, and energy resolution, spreads the ideal lines into more complex shapes. When you configure gamma physics and digitizers later, you will see how these discrete nuclear energies translate into realistic detector spectra.
In practice, you typically look up the main gamma energies for a radionuclide in a nuclear data table, then check that your simulated spectrum contains peaks at approximately the right positions. Small differences can appear because of detector calibration, energy blurring, and binning, but the correspondence in peak location is one of the simplest ways to validate that your radioactive source and physics configuration are correct.
For a given radionuclide, the set of discrete gamma energies is fixed by nuclear structure. These energies appear as characteristic peaks in ideal spectra and are the basis for choosing detector energy windows in imaging.
Branching ratios
A radionuclide does not always emit the same gamma in every decay. Instead, there is a probability that a particular transition and its associated photon will occur. This probability is called the branching ratio, often expressed as a percentage per decay. For instance, if a line is listed with a branching ratio of 0.9, it means that, on average, 90 percent of all decays produce that gamma.
In nuclear data tables, you typically see, for each gamma line, an energy and an intensity or branching ratio. Some photons are very probable, others are rare. For example, Tc‑99m emits its 140 keV photon with a high branching ratio, which makes it efficient for imaging, while many weaker lines are often ignored in basic discussions but are still present in precise data.
In GATE, when you choose a radionuclide for a radioactive source, the decay engine uses these branching ratios to stochastically decide which photons are produced in each simulated decay. You do not explicitly specify the probabilities. Instead, the simulation samples decay modes and gamma emissions according to the nuclear database, so that, over many events, the fraction of emitted photons at each energy reproduces the branching ratios.
This has several practical consequences for beginners:
The relative heights of peaks in a simulated energy spectrum are influenced by both branching ratios and detector efficiency. A gamma with a high branching ratio but low detection efficiency can produce a peak comparable in height to a lower branching ratio gamma that is detected more efficiently.
When you design energy windows, you may choose to include or exclude photons with low branching ratios. Including only the dominant line can reduce scatter and improve image quality, but it may discard some useful counts.
For therapeutic radionuclides, branching ratios of higher energy photons affect dosimetry and shielding considerations, because even relatively low probability emissions can contribute to dose outside the target or staff exposure.
In time dependent simulations, branching ratios remain constant for a given radionuclide. The overall count rates change with activity and decay, but the relative frequencies of the different gamma lines stay the same, as long as the same decay scheme applies.
When validating your simulation against measured spectra, differences between expected and simulated peak intensities can come from incorrect assumptions about branching ratios, detector response, or acquisition settings. By comparing measured peak areas to those predicted from nuclear data and simulated spectra, you can check whether your source definition is consistent with reality.
For each gamma line of a radionuclide, the branching ratio is the probability that this photon is emitted in a decay. In GATE, radioactive decay sampling uses these branching ratios automatically, so that, over many decays, the relative number of photons at each energy matches the nuclear data.
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