A high-flux electron detection system to measure non-linear Compton scattering at LUXE

In the vast, silent theater of the quantum world, particles do not always behave as simple, predictable billiard balls. When charged particles like electrons encounter electromagnetic fields of extraordinary intensity, the rules of physics shift. Under normal circumstances, light and matter interact in straightforward ways, but when the electric field becomes strong enough—approaching a theoretical limit known as the Schwinger limit—the interaction becomes non-linear. In this extreme regime, a single electron can absorb multiple photons at once, or a photon can spontaneously transform into a pair of particles. These phenomena, predicted by the theory of quantum electrodynamics, are difficult to observe because creating such intense fields in a laboratory requires immense power. Nature provides these conditions in the hearts of neutron stars and black holes, but to study them on Earth, scientists must build machines capable of colliding high-energy electron beams with the most powerful lasers available.

The Laser Und XFEL Experiment, or LUXE, is a project designed to bring these extreme conditions into a controlled setting at the DESY research center in Germany. The experiment aims to collide electrons from a massive accelerator with a high-intensity laser pulse. When these two beams meet, the electrons scatter, losing energy and emitting new particles. By measuring the energy of the scattered electrons with extreme precision, physicists hope to test the limits of our current understanding of the universe. However, this measurement presents a formidable engineering challenge. The collision produces a flood of electrons—more than a billion in a single burst—spread across a wide range of energies. To understand the physics, scientists must capture the energy spectrum of this chaotic spray, distinguishing between the original beam and the scattered particles, all while surviving a harsh environment of radiation and electromagnetic noise.

To solve this problem, a team of researchers has developed and tested a new high-flux electron detection system. This device is designed to act as a sophisticated eye, capable of seeing the energy distribution of the electron spray. The system combines two complementary technologies to ensure accuracy. The first part is a scintillating screen, a special material that glows when struck by electrons. This screen is observed by a set of high-resolution cameras that capture the pattern of light, revealing where the electrons land and how much energy they carry. The second part is a segmented detector based on the Cherenkov effect. When electrons travel through a medium faster than light can travel through that same medium, they emit a faint blue glow. The detector uses a series of hollow tubes and solid rods to capture this light, converting it into electrical signals that can be counted. By using both the glowing screen and the light-emitting tubes, the system can cross-check its own measurements, reducing errors and providing a clear picture of the electron spectrum.

The researchers built prototype versions of this system to test how well they would perform under real-world conditions. They took these prototypes to two different accelerator facilities: ARES at DESY and FACET-II at the SLAC National Accelerator Laboratory in California. At ARES, they focused on testing the individual components, such as different types of glowing screens and the sensors that detect light. They discovered that the screens responded linearly to the number of electrons hitting them, meaning the brightness of the glow increased steadily as the beam got stronger, without sudden jumps or drops. They also found that the position of the beam on the screen did not change the brightness, ensuring that the location of the electrons could be measured accurately. The team tested various screen materials and found that while some were brighter than others, all of them performed well within the expected range for the final experiment.

At the FACET-II facility, the tests became more rigorous, simulating the actual conditions the system will face at LUXE. Here, the team used a high-energy electron beam that had been dispersed by a magnetic field, creating a spread of energies similar to what will be seen after the laser collision. They placed the scintillating screen and the Cherenkov detector in the path of this beam to see how they worked together. The cameras successfully captured the spread of the electron beam, and the tube detectors registered the light produced by the electrons passing through them. The results showed that the system could distinguish between different parts of the energy spectrum. However, the tests also revealed some challenges. The researchers noticed that the beam appeared slightly wider when measured by the tube detectors compared to the screen, likely due to the way the beam scattered as it passed through the first layer of tubes. They also observed that the sensors lost some sensitivity over time when exposed to the intense radiation of the accelerator, a factor that will need to be managed in the final design.

Despite these hurdles, the tests confirmed that the core concept is viable. The system demonstrated the ability to handle the high flux of electrons and to measure their energy distribution with the precision required for the experiment. The team identified specific sources of background noise, such as stray particles hitting the sensors directly or electronic interference, and found ways to distinguish these unwanted signals from the true data. They also learned that by adjusting the angle of the tubes and the type of material used, they could optimize the sensitivity for different parts of the energy spectrum. The glass rods used in the low-energy part of the detector showed promise for capturing faint signals, though their performance needs further refinement to match the theoretical predictions.

The journey from a prototype to the final instrument is not yet complete, but the path is now clear. The results from these tests provide the necessary data to optimize the design of the detection system for the LUXE experiment. The researchers know exactly which components work best, where the system needs improvement, and how to calibrate the instruments to account for radiation damage and background noise. With this high-flux electron detection system, the LUXE experiment will be able to peer into the strong-field regime of quantum electrodynamics with unprecedented clarity. By measuring the energy of the scattered electrons, the experiment will test the fundamental laws of physics in a regime that has never been explored before, potentially revealing new insights into how matter and light interact at the most extreme scales. The successful development of this detector marks a significant step toward unlocking the secrets of the quantum universe.