Irradiation Studies and Design Optimization of the ATLAS Tile Calorimeter for the High-Luminosity LHC

Deep inside the earth beneath the border of France and Switzerland, a massive machine called the Large Hadron Collider smashes particles together at nearly the speed of light. This collision creates a shower of new particles, some of which are hadrons, a heavy family of particles that includes protons and neutrons. To understand what happens in these collisions, scientists need to measure the energy of these hadrons with extreme precision. The ATLAS experiment, one of the giant detectors surrounding the collision point, uses a specialized device called the Tile Calorimeter for this job. It works like a layered sandwich: thick steel plates act as a sponge to absorb the energy of the passing particles, while plastic tiles sandwiched between the steel glow with a flash of light when hit. This light is collected and sent to sensitive electronic eyes that count the flashes to determine how much energy was deposited.

However, the machine is about to undergo a massive upgrade to become the High-Luminosity LHC, which will run for years starting in 2030. This new phase will produce collisions at a rate ten times higher than before, creating a much more intense environment filled with radiation. This radiation is like a constant, invisible storm that can damage delicate electronics, causing them to malfunction or fail completely. The challenge for the scientists is to ensure that the Tile Calorimeter can survive this storm and continue to take accurate measurements without stopping. If the electronics fail, the detector becomes blind, and years of potential discoveries are lost. The team responsible for the Tile Calorimeter had to redesign the entire system that sits right next to the detector, replacing old components with new ones built to withstand this harsh future.

The researchers began by mapping out exactly how much radiation the different parts of the detector would face over the next decade. They used computer simulations to predict the worst-case scenarios for the most exposed areas, particularly near the gaps between the detector modules. These calculations showed that the electronics would be bombarded with a level of energy and particle hits far beyond what the current system was designed to handle. To survive, the team decided to replace the entire readout system—the electronics that collect and process the data—with a new, highly modular architecture. This new design uses a mix of custom-made chips and rugged, commercially available components that have been specially selected for their ability to resist radiation damage.

A critical part of this new system is the power supply, which acts as the heart of the detector, delivering steady electricity to the sensitive sensors. The old power units were inefficient and generated too much heat, which would have been impossible to cool in the crowded detector space. The team redesigned these units to be much more efficient, improving their performance from about fifty-eight percent to seventy-two percent. This change was not just about saving energy; it was essential to keep the detector cool enough to operate safely. They also replaced the old transistors inside these units with newer models that are better at handling the electrical stress caused by radiation. Tests showed that these new components could withstand the intense radiation environment without losing their ability to regulate power, ensuring that the detector would not suffer from electrical failures.

The team also focused on the photomultiplier tubes, which are the devices that turn the faint light from the plastic tiles into electrical signals. In the most dangerous areas of the detector, where radiation is highest, they installed over a thousand new tubes that are tougher and more stable than the old ones. These new tubes are paired with active dividers, circuits that manage the high voltage needed to run the tubes. The researchers tested these dividers under extreme conditions, exposing them to radiation doses that would have destroyed standard electronics. They found that even after absorbing a massive amount of radiation, the voltage stability of the tubes remained almost perfect, with only a tiny, acceptable drop in performance. This means the light signals will remain clear and reliable, even after years of heavy bombardment.

The most complex part of the upgrade involves the digital brains of the detector: the main boards and daughter boards that process the data and send it to the computers. These boards contain advanced processors that are naturally sensitive to radiation, which can cause them to flip bits of data or freeze. To protect them, the engineers built in multiple layers of defense. They used special software and hardware techniques that constantly check for errors and fix them instantly before they can cause a problem. During testing, they bombarded these boards with protons and neutrons to simulate the radiation of the future collider. The boards survived doses of radiation that were far higher than what they would ever see in the real machine. When errors did occur, the system caught and corrected them immediately, proving that the data would remain intact and the detector would keep working without interruption.

Every single component in this new system, from the power bricks to the digital processors, underwent a rigorous series of tests to prove it could survive the High-Luminosity LHC. The researchers exposed the parts to gamma rays, neutrons, and protons, measuring exactly how they behaved under stress. The results were clear: the new design passed every test with flying colors. The components showed no signs of the catastrophic failures that would have doomed the old system. They confirmed that the new power supplies are efficient and robust, the light sensors are stable, and the digital processors are resilient. With these findings, the ATLAS collaboration has a fully certified plan to upgrade the Tile Calorimeter. The detector is now ready to be rebuilt with these new, hardened systems, ensuring it will continue to reveal the secrets of the universe when the collider turns on in 2030.