Competition of breakup and dissipative processes in peripheral collisions at Fermi energies

TL;DR

This paper investigates the interplay of breakup and dissipative processes in peripheral heavy ion collisions at Fermi energies, analyzing isotope and velocity distributions to understand reaction mechanisms and equilibration.

Contribution

It provides a detailed empirical decomposition of velocity distributions into direct and dissipative components and compares them with transport calculations, offering new insights into collision dynamics.

Findings

Velocity distributions decompose into two components: breakup and dissipative.

Transport calculations successfully describe the dissipative component.

Isotope distributions qualitatively match data, highlighting the role of secondary de-excitation.

Abstract

Heavy ion collisions in the Fermi energy regime may simultaneously show features of direct and dissipative processes. To investigate this behavior in detail, we study isotope and velocity distributions of projectile-like fragments in the reactions $^{18}$O (35 $A\cdot$MeV) + $^9$Be($^{181}$Ta) at forward angles. We decompose the experimental velocity distributions empirically into two contributions: a direct, `breakup' component centered at beam velocity and a dissipative component at lower velocities leading to a tail of the velocity distributions. The direct component is interpreted in the Goldhaber model, and the widths of the velocity distributions are extracted. The dissipative component is then successfully described by transport calculations. The ratio of the yields of the direct and the dissipative contributions can be understood from the behavior of the deflection functions. The isotope distributions of the dissipative component agree qualitatively with the data, but the modification due to secondary de-excitation needs to be considered. We conclude, that such reactions are of interest to study the equilibration mechanism in heavy ion collisions.