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Karl-Heinz Schmidt
Proton removal in peripheral collisions at relativistic energies (Cold fragmentation) In very peripheral heavy-ion collisions at relativistic energies, a few nucleons are abraded from the projectile in the overlap zone of projectile and target, and a slightly lighter prefragment is formed. Due to the high kinetic energy of the projectile, the nuclear binding does not have any influence on this process; it is just determined by the nuclear density distributions and the nucleon-nucleon cross sections. This is why the N/Z ratio of the remaining prefragment is subject to a large fluctuation. At the extreme, the abrasion process may only remove protons, leading to very neutron-rich prefragments. Since the abrasion process induces an excitation energy of 27 MeV per abraded nucleon on the average, most of these prefragments do not survive: Most proton-removal products which are very neutron-rich deexcite by emitting neutrons and thus become less neutron rich. Only the extreme low-energy tail of the excitation-energy distribution, i. e. "cold fragmentation", leads to the production of final projectile fragments with large neutron excess. Thus, detailed information on the excitation-energy acquired in relativistic heavy-ion collisions can be deduced from the cross sections of proton-removal products. Cold fragmentation provides extraordinarily good conditions for the production of very neutron-rich isotopes. For heavy masses (A > 160), which cannot be produced by fission, the cold fragmentation seems to be the most promising mechanism for the production of extremely neutron-rich secondary beams.
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