Investigating neutron-rich pre-fragment distributions in projectile fragmentation reactions

Projectile fragmentation is one of the primary methods used to produce radioactive beams at facilities such as the Facility for Rare Isotope Beams (FRIB). An empirical parameterization of the production cross sections is widely used to calculate the beam intensities of such radioactive nuclides which is critical for the scheduling and running of successful experiments at these facilities. However, the details of the reaction mechanism are not yet well-understood. The reaction involves an initial fast removal of nucleons (abrasion) creating an excited pre-fragment, which then undergoes an evaporative de-excitation (ablation) to form the final observed nucleus. Particularly, very little experimental information is known about the excited, projectile-like pre-fragment that is generated from the abrasion step. An experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL) to provide more information about the intermediate state(s) produced in fragmentation reactions. A ³²Mg secondary beam was fragmented on a beryllium target at 86 MeV/u, producing many neutron-rich nuclides that were detected in coincidence with one or more neutrons using the Modular Neutron Array (MoNA)-Sweeper setup. The neutron hit multiplicity spectra associated with each observed nuclide are being compared with the predictions of various reaction models to extract information about the pre-fragments that are generated in the reaction.