Cluster formation in pre-compound nuclei in the time-dependent framework

Abstract

Background: Applications of nuclear time-dependent density functional theory (TDDFT) are often capable of providing quantitative description of heavy ion reactions. However, the structure of pre-compound states produced in heavy ion reactions are difficult to assess theoretically in TDDFT as the s.p. density alone is a weak indicator of shell structure and cluster states. Purpose: We employ the time-dependent nucleon localization function (NLF) to reveal structure of pre-compound states in nuclear reactions involving light and medium-mass ions. We primarily focus on spin saturated systems with N = Z. Furthermore, we study reactions with oxygen and carbon ions, for which experimental evidence for α clustering in pre-compound states exists. Method: We utilize the symmetry-free TDDFT approach and compute the NLFs to describe ${}^{16}$O + ${}^{16}$O, ${}^{40}$Ca + ${}^{16}$O, ${}^{40}$Ca + ${}^{40}$Ca, and ${}^{16,18}$O + ${}^{12}$C collisions at energies above the Coulomb barrier. Results: We show that NLFs reveal a variety of time-dependent modes involving cluster structures. For instance, the ${}^{16}$O + ${}^{16}$O collision results in a vibrational mode of a quasi-molecular α-${}^{12}$C-${}^{12}$C-α state. For heavier ions, a variety of cluster configurations are predicted. For the collision of ${}^{16,18}$O + ${}^{12}$C, we showed that the pre-compound system has a tendency to form α clusters. This result supports the experimental findings that the presence of cluster structures in the projectile and target nuclei gives rise to strong entrance channel effects and enhanced α emission. Conclusion: The time-dependent NLF is a good indicator of clusters structures in complex pre-compound states formed in heavy-ion fusion reactions. The localization reveals the presence of collective vibrations involving cluster structures, which dominate the initial dynamics of the fusing system.

10 pages, 8 figures

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• Nucleon localization function in rotating nuclei
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• Extraction of ground-state nuclear deformations from ultra-relativistic heavy-ion collisions: Nuclear structure physics context
• Production of twisted particles in heavy-ion collisions
• Examining the justification for the introduction of a fermion localization function
• Continuum damping effects in nuclear collisions associated with twisted boundary conditions