GAGG:Ce Scintillator Crystals in Space Research

The HERMES Pathfinder mission focuses on creating a group of small satellites to monitor and study fast astronomical events, particularly gamma-ray bursts. These satellites will operate in low Earth orbit and will use a special type of material known as a Scintillator to detect various types of radiation. The specific scintillator being used is called GAGG:Ce, which is a crystal that produces light when exposed to radiation. This study evaluates how this crystal behaves after being bombarded with high-energy particles and light.

#The Importance of GAGG:Ce Scintillator Crystals

GAGG:Ce is a cerium-doped crystal that glows when hit by radiation. This Afterglow can last for a long time, which means the crystal continues to emit light even after the radiation source is removed. The study is essential because understanding this afterglow is crucial for ensuring that the detectors in the satellites will work correctly when they are launched into space.

#Initial Observations

In earlier studies, researchers noticed that the afterglow of the GAGG:Ce crystal could last for days or even weeks after exposure to high-energy Protons and ultraviolet light. However, there were issues in the initial experiments, such as temperature changes and timing inaccuracies, that could affect the results. To improve the measurements, a new set of tests was planned.

#Experimental Setup

The experiment had two phases. First, the crystal was exposed to light from an LED, and later, it was irradiated with protons. The goal was to measure the light emitted during and after each exposure. Two types of devices measured the light: a silicon drift detector (SDD), which was active during radiation exposure, and a photomultiplier tube (PMT), which measured the afterglow.

To get the best results, the experiment was carried out in a dark room where temperature could be carefully controlled. This was important because both the crystal and the detectors are sensitive to light and temperature changes.

#Measurement Process

For the LED phase, the crystal was illuminated for various amounts of time, and the afterglow was measured afterward. Measurements were taken at two different temperatures to see how temperature influenced the afterglow. The data collected during these tests were then processed to make them easier to analyze.

During the proton phase, the crystal was exposed to high-energy protons. The goal was to see how the afterglow from proton exposure compared to the LED results. Data from this phase were collected over several days as the crystal was irradiated at different times.

#Understanding Afterglow Through Modeling

The long-lasting glow of the GAGG:Ce crystal is attributed to the formation of traps within the crystal structure, where energy from the radiation can become stored. These traps release energy over time, resulting in light. The study created a model to explain how this process works, taking into consideration the types of traps present in the crystal and how long they take to release energy.

The model generated parameters that helped to predict how much afterglow could be expected under various conditions. It differentiated between different types of traps based on their lifetimes, allowing for a clearer understanding of the afterglow behavior.

#Results from the LED Tests

The tests using the LED produced data that helped characterize the afterglow. The analysis showed how the afterglow varied based on how long the crystal was exposed to light and the temperature. The model fit the data from these tests quite well, but there were challenges in accurately capturing the initial rise of the afterglow. This issue highlighted limitations in both the model and measurement techniques.

#Comparing LED and Proton Irradiation

Next, the crystal underwent proton irradiation. This phase was crucial because it more closely mirrored conditions the crystal would face in space. The results from the proton exposure showed differences in afterglow compared to LED exposure.

During proton exposure, the crystal experienced damage that could affect its light output. Some initial data from this phase also showed challenges in isolating the afterglow signal because of the activated nuclei's contribution, which was not present in LED tests.

#Implications for Satellite Performance

The findings from both the LED and proton tests were key in assessing how the GAGG:Ce crystals would perform in the HERMES Pathfinder satellites. The afterglow currents produced by the crystal were low enough that they would not interfere with satellite electronics. This is critical since the electronics are designed to function within certain limits.

The study also reviewed how the satellites would be subjected to high-energy radiation while in orbit, particularly in regions known for increased radiation, like the South Atlantic Anomaly. The results indicated that the afterglow current, which behaves like leakage current, would remain below the operational limits for the satellites.

#In-Orbit Considerations

Once the satellites are in orbit, they will encounter energetic particles trapped in Earth's magnetic field. This can affect the materials used in the satellites, including the scintillator crystals. The design aim is to ensure that these crystals can handle radiation exposure without degrading their performance.

The research modeled potential scenarios the satellites might face in space, particularly during periods of increased radiation exposure. The findings indicated that the afterglow current would most likely remain low, ensuring the electronic systems could operate effectively.

#Conclusion

The study of the residual luminescence from GAGG:Ce scintillator crystals proved vital for the HERMES Pathfinder mission. The successful development of a model to describe the afterglow was achieved through two phases of testing. The results indicated that the afterglow, while significant, would not hinder satellite operation. The project, funded by European Union research initiatives and partnerships, represents significant progress in understanding materials used in space applications.

In summary, the research not only illustrates the characteristics of GAGG:Ce scintillator crystals but also lays the groundwork for their use in future space missions. With further experiments planned, this study has advanced the understanding of how scintillator technology can contribute to detecting cosmic events. The insights gained from this research will be crucial for ensuring the reliability and effectiveness of the HERMES Pathfinder satellites as they monitor the universe.