Electromagnetic excitation of nuclei and neutron evaporation in ultrarelativistic ultraperipheral heavy ion collisions

We present a new approach for calculating the electromagnetic excitation of nuclei as well as probabilities of emission and different distributions of neutrons from decays of excited nuclear systems for ultrarelativistic, ultraperipheral heavy ion collisions. Excitation functions for γ + Pb → Pb* and γ + Au → Au* are parametrized using physics-motivated components: the excitation of giant resonances, quasideuteron absorption mechanism, excitation of nucleon resonances, as well as high-energy dissociation of protons and neutrons. Neutron emission (up to ten neutrons) from low-energy excitations of ²⁰⁸Pb and ¹⁹⁷Au is modeled in terms of the Hauser-Feshbach formalism. The probabilities of a given number of neutrons are calculated as a function of excitation energy in a Monte Carlo code gemini++. These probabilities are parametrized by smooth analytical functions. The results are compared to appropriate data for the γ Pb → Pb*→kn and γ Au → Au*→kn reaction. As an example, the approach is used for calculating electromagnetic excitation in ultraperipheral heavy ion collisions (UPC) processes. Both single-photon and double-photon excitations are included and discussed. Topological cross sections with a given number of neutrons in the forward and backward directions are calculated at the Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies. Excitation functions are presented. The results of the calculation are compared to Super Proton Synchrotron (SPS), RHIC, and more recent A Large Ion Collider Experiment (ALICE) experimental data and good agreement is achieved.