Radiation damage and recovery of SiPMs exposed to low-momentum muon beams for innovative scintillation-based muon beam monitors
This study demonstrates that eight SiPM models from various manufacturers maintain operational stability and recover optimal performance after thermal annealing following exposure to high-intensity 29 MeV/c muon beams, validating their suitability for innovative scintillation-based low-momentum muon beam monitors.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the world of particle physics, scientists often need to track invisible streams of subatomic particles to understand how the universe is built. One such particle is the muon, a heavy cousin of the electron that rains down on us from space or is created in powerful machines. To catch these fleeting visitors, researchers use detectors that glow when a muon passes through them, a process called scintillation. The light from this glow must be captured and counted, and for decades, the standard tool for this job has been a device called a Silicon PhotoMultiplier, or SiPM. Think of it as a highly sensitive electronic eye that can see single flashes of light. However, these devices are delicate. When placed directly in the path of a particle beam to do their job, they can be damaged by the very particles they are trying to measure. This damage creates a kind of electronic noise, making the device less reliable. The challenge for scientists is to design detectors that can survive this harsh environment, or at least recover from it, so they can continue to provide clear data for experiments in materials science and fundamental physics.
At the ISIS Neutron and Muon Source in the United Kingdom, a team of researchers set out to test how well these electronic eyes hold up when exposed to a specific, intense type of muon beam. They were particularly interested in low-momentum muons, which move relatively slowly and deposit a large amount of energy as they stop. These slow muons are valuable for studying materials, but their high energy deposit poses a significant risk to the sensors placed in their path. The team gathered eight different models of SiPMs from various manufacturers and subjected them to a beam of positive muons moving at a momentum of 29 MeV/c. The exposure was intense, lasting for 13 hours with a steady stream of about one million muons every second. The goal was to see if the sensors would be permanently ruined or if they could be repaired.
To monitor the damage in real time, the researchers watched the "dark current" of the sensors, which is the electrical signal that flows even when no light is present. As the muons bombarded the devices, this background noise increased, a sign that the internal structure of the silicon was being altered by the radiation. The muons were creating tiny defects in the crystal lattice of the silicon, essentially tripping up the electrons and causing them to generate false signals. However, the team suspected that this damage might not be permanent. Just as a broken bone can heal, the researchers hypothesized that heating the silicon could allow the atomic structure to repair itself, a process known as thermal annealing.
After the intense bombardment, the team took the damaged sensors and heated them to 125 degrees Celsius for 24 hours. This was not a random guess but a carefully chosen temperature designed to encourage the silicon atoms to rearrange and fix the defects without melting or destroying the device. The results were encouraging. Once the sensors cooled down, they were tested again, and the electrical noise had dropped significantly. While the devices did not return to their exact original state, they recovered enough to be considered stable and operational. The breakdown voltage, which is the specific electrical pressure needed to make the sensor work, remained largely unchanged, indicating that the fundamental ability of the sensor to detect light was intact. The researchers also noted a small, systematic shift in this voltage after the experiment, which they suspect might be due to a tiny amount of stray neutrons in the beam, but the primary conclusion was clear: the heat treatment successfully reversed most of the radiation damage.
This work is part of a larger effort to build new types of beam monitors for low-momentum muons. Current detectors often use thick layers of material or complex arrangements that can block the beam or interfere with the experiment. The team is developing a much simpler design: a single, thin sheet of scintillating material read out by a flat grid of these SiPMs. This design offers high precision but requires the sensors to sit directly in the beam. The study confirms that this direct exposure is feasible, provided the sensors are given the chance to heal through thermal annealing. By proving that these devices can withstand the rigors of a muon beam and recover their performance, the researchers have paved the way for more compact and efficient detectors. These new tools will allow scientists to peer into the atomic structure of materials with greater clarity, ensuring that the quest to understand the building blocks of matter continues without the sensors themselves becoming the bottleneck.
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