Long-term performance of non-ALD MCP-PMTs in the high-radiation environment of ALICE

In the heart of Europe, beneath the border of France and Switzerland, a massive machine known as the Large Hadron Collider smashes particles together at speeds approaching that of light. When these collisions occur, they create a shower of new particles that fly outward in all directions. To understand the fundamental building blocks of the universe, scientists must catch these fleeting fragments and measure them with extreme precision. One of the most critical tasks is to determine exactly when a collision happened. If the timing is off by even a tiny fraction of a second, the data becomes a blur, and the story of the collision is lost. To solve this, the ALICE experiment, one of the major detectors at the collider, uses a specialized system designed to act as a high-speed stopwatch. This system relies on a phenomenon called the Cherenkov effect, which occurs when a charged particle moves through a transparent material faster than light can travel through that same material. This creates a faint flash of blue light, similar to a sonic boom but for light. By catching this flash with sensitive cameras, the detector can timestamp the collision with incredible accuracy, down to a few trillionths of a second.

However, keeping these cameras working in such a violent environment is a formidable challenge. The area where the collisions happen is flooded with radiation and a relentless stream of particles. The sensors must be sensitive enough to detect a single flash of light, yet robust enough to survive being bombarded by billions of particles every second without wearing out. The ALICE team recently reported on the long-term performance of the specific sensors they chose for this job, known as microchannel plate photomultiplier tubes. These are vacuum tubes filled with millions of tiny glass channels that amplify a single photon of light into a measurable electrical signal. The researchers found that while these sensors do degrade over time due to the heavy use, they can be kept working effectively by adjusting their operating voltage. More surprisingly, they discovered that the sensors have a hidden ability to heal themselves when the machine is turned off for maintenance, regaining some of their lost sensitivity without any human intervention.

The ALICE detector is equipped with a subsystem called the Fast Interaction Trigger, which sits very close to the collision point to monitor the very first moments of a crash. This system uses fifty-two of these specialized light sensors arranged in two arrays. The innermost sensors face the most intense traffic, dealing with a photon load equivalent to two hundred million photoelectrons per square centimeter every second. This is an enormous amount of electrical current for such a delicate device. To handle this, the team selected a specific version of the sensor that does not use a special atomic coating intended to extend its life, because that coating would have made the sensor too weak to handle the high current. Instead, they used the standard version, accepting that it would age faster, but betting that they could manage the wear and tear through careful operation.

Over several years of operation, the researchers tracked how these sensors performed under the relentless bombardment. They monitored the total electrical charge that passed through each sensor, a measure of how much work the device had done. By the time of their report, the most heavily used sensors had accumulated a charge of two coulombs per square centimeter. As expected, the sensors began to lose sensitivity; the most worn-out units produced only about forty percent of the signal they once did. However, the team found a simple way to compensate for this loss. By gradually increasing the voltage supplied to the sensors, they were able to boost the signal back to its original strength. Crucially, this adjustment did not ruin the timing precision. Even with the sensors aged and operating at higher voltages, the system maintained its ability to distinguish between collisions with a time resolution of roughly seventeen picoseconds for proton collisions and even better for heavier lead-ion collisions. This proved that the sensors could survive the harsh environment of the forward region of the detector, provided the team was willing to adjust the power settings as the devices wore in.

The study also revealed that not all sensors age at the same rate. While most of the equally illuminated sensors degraded at a nearly identical speed, two specific units wore out much faster than the rest. The researchers traced this back to the very beginning of the experiment, finding that these two fast-aging units were already the noisiest sensors before they were ever turned on. This suggests that the initial quality of the sensor, specifically its background noise level, is a strong predictor of how long it will last under heavy use. This finding helps scientists understand that while the environment is harsh, the inherent characteristics of the individual components play a major role in their survival.

Perhaps the most intriguing discovery came during the long periods when the collider was shut down for technical maintenance. The researchers noticed that when the sensors were left in the dark for months, they did not just sit idle; they actually improved. During these extended breaks, the aged sensors spontaneously recovered about eight percent of their lost sensitivity. This self-recovery happened without any changes to the equipment or the environment, which remained at a steady temperature. The sensors that had not aged significantly showed no such recovery, indicating that the healing process is specifically linked to the wear and tear the sensors had endured. The team observed this pattern consistently during two separate shutdowns, each lasting about 160 days. While the exact physical mechanism behind this recovery remains a mystery, the effect is real and measurable. It suggests that the damage caused by the high current is not entirely permanent and that the material inside the sensor can partially repair itself when given a rest.

This work marks the first large-scale use of these specific sensors in a high-energy physics experiment. The results show that by choosing the right type of sensor and managing the operating conditions, it is possible to run a high-speed timing detector in one of the most radiation-heavy environments in the world. The team demonstrated that even with significant aging, the detector can maintain the precise timing needed to study the universe's smallest particles. The discovery of the self-recovery effect adds a new layer of hope for the longevity of these devices, suggesting that the wear and tear of the collider might be less permanent than previously thought. As the experiment continues, the researchers plan to extract the most aged sensors at the end of the current run to study them in detail, hoping to finally uncover the secret behind their ability to bounce back. Until then, the detectors continue to tick away, capturing the fleeting moments of cosmic collisions with remarkable resilience.