Particle accelerators

Intense Positron Sources

Intense Positron Source

High-energy particle colliders are essential tools for exploring fundamental physics. These colliders, especially electron-positron (e+e−) colliders, offer a cleaner collision environment compared to hadron colliders, making them ideal for precise measurements of the Standard Model (SM) processes and searches for new physics beyond the Standard Model (BSM). A key component in these colliders is a reliable source of positrons, which has traditionally been a significant challenge.

Conventional Positron Sources

Linear and circular colliders are essential tools for particle physics research. Electron-positron (e+e−) colliders enable high-precision measurements due to their cleaner collision environment compared to hadron colliders, although the latter achieve higher energies. Generating intense, low-emittance electron and positron beams is critical for luminosity, but positron production remains a major technical challenge.

Conventional positron sources use a high-Z target (e.g., tungsten) struck by a high-energy electron beam, producing Bremsstrahlung photons that generate e+e− pairs. However, energy deposition in the target causes significant thermo-mechanical stress, evaluated using Peak Energy Deposition Density (PEDD) to ensure structural integrity.

Crystal-Based Positron Sources

An alternative approach leverages lattice coherent effects to enhance photon production. Studies on Ge and Si crystals demonstrated high photon yields, later confirmed experimentally. Experiments at CERN (WA103) and KEK validated the use of thick tungsten crystals for positron production, but heat load remained a challenge.

To mitigate thermal stress, hybrid targets are preferred: a thin crystalline radiator (1-2 mm) efficiently generates photons, while an amorphous converter produces positrons. Studies indicate that granular converters help reduce thermal shock. Recent experiments with 5.6 GeV and 6 GeV electrons on a tungsten crystal measured radiation losses and pair production in a copper converter, validating the performances of the oriented crystal radiator and allowing the use of an improved simulation toolkit and comparing hybrid and conventional positron sources for future high-energy colliders.

Future Prospects

The development of crystal-based positron sources opens new avenues for the design of future lepton colliders. This technology can significantly enhance the luminosity and efficiency of these colliders, contributing to more precise experiments and discoveries in particle physics. Ongoing research and further experiments will continue to refine these methods, paving the way for their implementation in next-generation collider projects.

Indeed, in collaboration with IJCLab, leader of the FCC-ee Injector Studies WP3 'Positron Source: Target and Capture System' group, the e+BOOST team has finalized the design of a crystal-based positron source in a single-crystal configurationfor both the 6 GeV/c and 2.86 GeV/c electrons, which aligns with the new FCC-ee Injector baseline for the positron source. Moreover, the results of this test beam will be essential for advancing the CHART PSI Positron Production (P3) Project for the FCC-ee Injector, where the crystal-based positron source will be tested with the full capture system. The e+BOOST team is leading the efforts to optimize the crystal for this future upgrade at PSI.