Fission of Relativistic Nuclei with Fragment Excitation and Reorientation

Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of $\sim 2$, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of fragments, but fail within a factor of 2–10 for a large number of individual yields. Here, we present a novel approach that provides an account of the additional excitation of primary fragments due to final state interaction with the target. Our method is applied to the ${}^{238}\mathrm{U}+{}^{208}\mathrm{Pb}$ reaction at $1\mathrm{GeV}/\mathrm{nucleon}$ (and is applicable to other energies), an archetype case of fission studies with relativistic heavy ions, where we find that the large probability of energy absorption through final state excitation of giant resonances in the fragments can substantially modify the isotopic distribution of final fragments in a better agreement with data. Finally, we demonstrate that large angular momentum transfers to the projectile and to the primary fragments via the same mechanism imply the need of more elaborate theoretical methods than the presently existing ones.

Figure 1

Average excitation energies of a few uranium fragments due to abrasion (open circles) and when the average electromagnetic excitation energy of the outgoing fragments is added to them (triangles). Inset: cross sections for production of primary uranium fragments due to abrasion.

Figure 2

Mass distribution of projectile fragments. The blue diamonds correspond to the yields obtained with the abrasion-ablation model without fission, while the red circles include fission decays. The black squares include the electromagnetic excitation leading to particle evaporation and fission products.

Figure 3

Isotopic distribution of niobium (upper panel) and ruthenium (lower panel) fragments. Data (open circles) are from Ref. [23]. The solid (dashed) lines correspond to calculations with (without) inclusion of final state electromagnetic excitation of abraded fragments. The arrows point to the region (shaded area) of increasing contribution of fragments decaying by fission.

Figure 4

Transverse, $J_{\perp }$, and longitudinal, $J_{\parallel }$, angular momentum transfer (see text for definitions) to primary fragments due to nucleon abrasion as a function of the fragment mass number.