Thicker septa in high-energy gamma-ray collimators are used to prevent septal penetration by photons that are not traveling along the intended axis. High-energy photons have a greater ability to penetrate through the walls between holes, so if the septa are too thin, many off-axis photons can slip through and reach the detector, creating background noise and blur. By increasing septa thickness, these off-axis photons are absorbed more effectively, improving image contrast and reducing background. This comes at the cost of some sensitivity, since thicker septa also block more photons that would otherwise pass through, but for high-energy imaging the reduction in septal penetration is essential to maintain image quality. In contrast, low-energy gamma rays don’t penetrate as readily, so thinner septa can be used to keep higher sensitivity while still suppressing off-axis photons.