Abstract
The production of superheavy elements through the fusion of two heavy nuclei is severely hindered by the quasifission process, which results in the fission of heavy systems before an equilibrated compound nucleus (CN) can be formed. The heaviest elements have been synthesised using 48Ca as the projectile nucleus. However, the use of 48Ca in the formation of new superheavy elements has been exhausted, thus a detailed understanding of the properties that made 48Ca so successful is required. Measurements of mass-angle distributions allow fission fragment mass distribution widths to be determined. The effect of the orientation of prolate deformed target nuclei is presented. Closed shells in the entrance channel are also shown to be more important than the stability of the formed CN in reducing the quasifission component, with reduced mass widths for reactions with the closed shell target nuclei 144Sm and 208Pb. Comparison to mass widths for 48Ti-induced reactions show a significant increase in the mass width compared to 48Ca-induced reactions, highlighting the difficulty faced in forming new superheavy elements using projectiles with higher atomic number than 48Ca.
Highlights
Superheavy elements (SHEs) mark the upper boundary of the existence of atomic nuclei. Their production offers an insight into exotic nuclear structure, and provides thorough tests of established nuclear models
For a SHE to be produced, two heavy nuclei must come into contact and form an equilibrated compound nucleus (CN), by evolving from the di-nuclear shape at contact, to that of a compact system [3, 4]
This immediately indicates that the shell effects in the entrance channel are far more significant than the fissility of the CN
Summary
Superheavy elements (SHEs) mark the upper boundary of the existence of atomic nuclei. Their production offers an insight into exotic nuclear structure, and provides thorough tests of established nuclear models. Due to the large repulsive Coulomb force, the system often instead breaks apart before this equilibrated system is reached. This process is known as quasifission (QF) [5,6,7]. The difficulty in forming SHEs is further compounded by the need for heavier projectile nuclei than 48Ca, as production of targets of elements heavier than Cf is currently not achievable. The role of closed shells is demonstrated due to the fact that the 48Ca projectile and 208Pb target both have full proton and neutron shells, whilst 144Sm has a closed neutron shell
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