C (p, p alpha) Reactions for the PANDORA Project

The PANDORA project plans to investigate photo-nuclear reactions in the mass region below A ∼ 56. The main motivation of this project is to understand the energy loss process of nuclei consisting of ultra-high-energy cosmic rays (UHECRs) in inter-galactic propagation. The origin, acceleration mechanism, and composition of UHECRs are still unknown. However, UHECRs with energies above 10²⁰ eV are observed by large cosmic-ray air-shower observatories such as Pierre Auger and Telescope Array. Analyses of air-shower depth distributions revealed a trend to be heavier in the mass composition between protons and iron at the highest energies.

UHECR nuclei are predicted to primarily lose their energy by emitting particles after photo-nuclear excitation by absorbing a cosmic microwave background (CMB) photon. Therefore, photonuclear reaction cross-sections and decay branching ratios are crucial elements for understanding the energy and mass evolution of UHECRs during inter-galactic propagation.

In the experiment, virtual photon exchange by proton scattering will be used to excite target nuclei and determine the photo-absorption cross-sections covering the giant dipole resonance. The detection of decay particles will extract branching ratios.

Data for the ^12,13C (p, p alpha) reactions at 392 MeV were measured in a test experiment in Dec. 2022 and some improvement points were identified.

I will report the status of the experimental setup for a beam time in Oct. 2023.