PANDORA Project: Study of Photonuclear Reactions below A = 60

Photo-nuclear reactions in the mass region below a mass of 60 will be systematically investigated in the project PANDORA (Photo-Absorption of Nuclei and Decay Observation for Reactions in Astrophysics). Virtual photon exchange by proton scattering and a high-intensity real-photon beam by laser Compton scattering will be used to excite target nuclei to determine the photo-absorption cross-sections covering the giant dipole resonance. Decay particles and gammas will be measured in coincidence. The primary motivation of the project is to determine the energy loss process of the nuclei consisting the Ultra-high-energy cosmic rays (UHECRs) in the inter-galactic propagation. UHECRs are observed on earth up to above 10²⁰ eV by observatories such as Pierre Auger and Telescope Array. The origin and the acceleration mechanism remain a mystery. A recent analysis of the air-shower depth distribution showed a trend to be larger in mass between a proton and an iron at the highest energy. UHECR nuclei are predicted to lose their energy primarily by emitting particles after photo-nuclear excitation by absorbing a cosmic microwave background (CMB) photon. Thus, photo-absorption cross-sections and decay branching ratios are the key ingredients for understanding the energy and mass evolution of UHECRs during the inter-galactic propagation.

I will introduce the experimental plan and the recent status at the Research Center for Nuclear Physics, Osaka University for the PANDORA project.