Enhanced backward emission of heavy fragments in high-energy proton-nucleus collisions

The conventional description of high energy proton-nucleus collisions in terms of an intranuclear cascade and compound nuclear decay (the two-step model) breaks down at projectile energies $E_p>10\mathrm{}$ GeV. Unusual backward enhancements are found in the angular distribution of heavy fragments (Sc, Cu) from uranium targets. This effect is explained in a fast breakup model of deep spallation/cleavage processes. The observed backward emission originates in the competition between (backward-directed) Coulomb repulsion and (forward-directed) momentum components from recoiling nucleons.