Influence of atomic screening on fragmentation of ultrarelativistic lead ions in LHC collimators

When a lead beam is collimated in the CERN LHC some of the ions fragment in the collimators causing problems downstream. For design purposes the fragmentation probability needs to be assessed. At LHC energies (γ≃3000; the Lorentz-factor γ is the total energy of an ion in units of its rest energy) electromagnetic dissociation dominates the fragmentation in medium to heavy target materials. Due to the extended range of the interaction at high energies, atomic screening affects the dissociation in the LHC collimators. We determine the magnitude of the reduction in cross section relative to the unscreened case for various collimator materials and find a significant effect for tungsten, the heaviest candidate planned for use.