Speaker
Description
The study of photo-nuclear reactions is crucial for understanding nuclear structure and astrophysical processes. The PANDORA (Photo-Absorption of Nuclei and Decay Observation for Reactions in Astrophysics) project [1] aims to systematically investigate these reactions in stable nuclei with mass numbers below 60, both experimentally and theoretically. We use virtual photon exchange through proton scattering and high-intensity real-photon beams from laser Compton scattering to excite target nuclei. The subsequent decay particles and gamma-rays are detected to measure the photo-absorption cross-section and the decay branching ratio for each decay channel, covering the giant dipole resonance.
Several nuclear models, including anti-symmetrized molecular dynamics, mean-field type models, large-scale shell model, and ab initio models, will be employed to predict the systematic behavior of photo-nuclear reactions. The primary objective of the PANDORA project is to elucidate the energy loss mechanisms of ultra-high-energy cosmic ray (UHECR) nuclei during intergalactic propagation.
UHECRs, observed on Earth up to energies above 1020 eV by large cosmic-ray air-shower observatories such as Pierre Auger and Telescope Array, remain a mystery in terms of origin, acceleration mechanisms, and composition. Recent analyses suggest a heavier mass composition for UHECRs at the highest energies. UHECR nuclei are predicted to lose energy primarily by emitting particles following photo-nuclear excitation by cosmic microwave background photons. Thus, understanding photonuclear reaction cross-sections and decay branching ratios is essential for interpreting the energy and mass evolution of UHECRs.
I will introduce the experimental method [2] for studying the electric dipole excitation of nuclei and photo-nuclear reactions by proton scattering at the Research Center for Nuclear Physics, Osaka University, and report on the experiments conducted in 2023 and 2025.
References
[1] A. Tamii et al., PANDORA White Paper, Euro. Phys. J. A. 59, 208 (2024).
[2] P. von Neumann-Cosel and A. Tamii, Euro. Phys. J. A 55, 110, (2019)