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

Abstract

We present a new approach for calculating electromagnetic excitation of nuclei as well as probabilities of emission and distributions of neutrons from decays of excited nuclear systems for ultrarelativistic, ultraperipheral heavy ion collisions. Excitation functions for $\gamma$ + Pb $\to$ Pb$^*$ are parametrized using physics-motivated components: excitation of giant resonances, quasi-deuteron absorption mechanism, excitation of nucleon resonances as well as high-energy dissociation of protons and neutrons. Neutron emission (up to 10 neutrons) from low-energy excitations of $^{208}$Pb is modelled 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 functions are parametrized by smooth analytical functions. The results are compared to appropriate data for the $\gamma$ Pb $\to$ Pb$^*$ $\to$ k n reaction. As an example the approach is used for calculating electromagnetic excitation in UPC processes. Both single photon and double photon excitations are included and discussed. Topological cross sections with a given number of neutrons in forward and backward directions are calculated in the calculation at the LHC energies. Excitation functions are presented. The results of calculation are compared with SPS and more recent ALICE experimental data and good agreement is achieved.