KAHIBARO
31.3. Shielding
Table of Contents
Lead shielding
In a gamma camera, shielding is essential to control which photons reach the crystal. The goal is to detect only photons that pass through the collimator holes, and to block photons that come from outside the useful field of view or that scatter inside the detector housing. In a GATE simulation, you represent this shielding explicitly as one or more volumes of a high‑$Z$ material that surround the sensitive detector elements.
Lead is the standard shielding material for gamma cameras because its high atomic number and density give it a very short attenuation length for diagnostic photon energies, for example around 140 keV for Tc‑99m. In practice, a gamma camera head is enclosed in a thick lead housing. Only the front face is “open” where the collimator is mounted. In a simplified simulation, you typically model the collimator and crystal, then add a lead box or shell that surrounds them on the sides and back, with no lead directly in front of the collimator holes.
The basic workflow is to define lead as a material, create appropriate shielding volumes, place them around the camera head, and verify that they do not overlap the collimator or crystal. You then check in the simulation output that photons coming from outside the field of view are mostly absorbed by the shielding instead of contributing to detector hits.
To define lead in GATE you normally use a standard Geant4 material from the NIST database. In OpenGATE you usually just refer to "G4_Pb" as the material name when you create your shield volume. For example, a rectangular lead shell around a rectangular NaI crystal can be implemented as several box volumes with material "G4_Pb" whose sizes and positions cover the lateral sides and the back of the camera head. The front side is left open or is occupied only by the collimator.
The key geometric idea is that the crystal and collimator are fully contained inside the world volume, and the lead shielding volumes are defined as siblings or parents that enclose these detectors without touching the collimator holes. Side and back shields should extend beyond the crystal’s dimensions in all directions, so that photons entering from those directions must traverse lead before reaching the crystal. Thickness is chosen based on the photon energy. For typical Tc‑99m SPECT simulations, several centimeters of lead are often used.
In GATE, you might start with something like a lead “back plate,” a box of lead attached behind the crystal, and then add side plates as separate volumes. This keeps the geometry simple and easy to debug. You control each shield’s position and size through the usual geometry parameters, and you can adjust thicknesses to study shielding performance.
It is important to remember that shielding affects both physics and performance. A thicker lead shield increases attenuation, which reduces the number of photons reaching the crystal from undesired directions, but it also increases the number of scatter events and secondary electrons produced in the shield. These secondaries are mostly absorbed locally because of lead’s high density, but they still need to be simulated. Production cuts and region settings, which you handle elsewhere in the course, can be used to avoid unnecessary tracking deep inside the shield.
When you simulate a gamma camera with and without lead shielding, you should see a clear reduction in counts from photons arriving outside the useful field of view. The energy spectra may also change, with fewer low‑energy scattered photons reaching the detector. This is one of the simplest ways to check that your shielding setup is working as intended.
Lead in GATE is usually defined using the built‑in material "G4_Pb", and effective shielding requires that all sides and the back of the gamma camera head, except the collimated front face, are covered by sufficiently thick lead volumes that do not overlap the sensitive crystal or collimator holes.
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