Influence of entrance-channel magicity and isospin on quasi-fission

TL;DR

This study investigates how nuclear shell effects and isospin influence the competition between fusion and quasi-fission in heavy element formation, highlighting the complex role of magic nuclei and nucleon transfer.

Contribution

It provides experimental measurements and theoretical insights into how shell effects and isospin affect quasi-fission, especially in reactions involving doubly magic nuclei.

Findings

More quasi-fission occurs with non-magic nuclei

The $^{40}$Ca+$^{208}$Pb reaction shows strong quasi-fission despite both nuclei being doubly magic

Nucleon transfer breaks shell effects, influencing reaction dynamics

Abstract

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries.