Mapping quasifission characteristics and timescales in heavy element formation reactions

Abstract

Background: The formation of superheavy elements by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Purpose: Through extensive mass-angle distribution measurements, we map out the systematic dependence of quasifission characteristics as a function of the identity of the colliding nuclei. Methods: The Australian National University's Heavy Ion Accelerator Facility and CUBE spectrometer have been used to measure mass-angle distributions for 42 reactions forming heavy elements. Beam energies above their respective capture barriers were used to minimize the known influence of nuclear structure effects. Results: Different mappings of mass-angle distribution characteristics (including timescales) over the nuclear landscape show a systematic dependence on entrance channel and compound nucleus fissilities. Conclusions: The results provide an empirical baseline to assess effects of nuclear structure at lower beam energies, and motivate the testing and validation of complete dynamical models of heavy element fusion through comparison of mass-angle distributions.

Shape evolution and collective dynamics of quasifission in the time-dependent Hartree-Fock approach

“…Although this approach has been widely used, it could have limitations due to the fact that quasifission is not a statistical decay but a dynamical process, and that fusion-fission could be asymmetric due to, e.g., late-chance fusion-fission at low excitation energies [28] as well as shell structure of pre-scission configurations [29]. Correlations between mass and scattering angle of the fragments have also been measured extensively [25,[30][31][32][33]. In particular, they can be used to disentangle fast quasi-fission processes (few zeptoseconds) to longer reaction mechanisms associated with contact times between the fragments exceeding 10 − 20 zs (i.e., long-time quasifission and fusion-fission).…”

Systematic study of quasifission characteristics and timescales in heavy element formation reactions

“…Quasifission mass-angle distributions (MAD) first measured at GSI in the 1980s [2,5] showed that quasifission timescales could often be shorter than the rotation time of ∼10 −20 s. However, subsequently only a few measurements [6,7] were made until recent years, when an extensive series of experiments (using the Australian National University Heavy Ion Accelerator Facility and CUBE spectrometer) were carried out [3,[8][9][10][11][12][13][14][15][16]. The kinematic coincidence technique used in the measurements [2,3,17] provides direct information on the mass-ratio of the fragments at scission; thus, the data are represented in terms of mass ratio M R , rather than pre-or According to the characteristics of the MAD (minimum mass yield at symmetry, mass-angle correlation with peak yield at symmetry, and no significant mass-angle correlation), they are assigned as type MAD1, MAD2 and MAD3 respectively [3]. There is a clear correlation between the MAD class and the entrance channel charge product.…”

“…Examples of MAD and deduced quasifission sticking time distributions are shown in Fig.1, for reactions forming the compound nucleus 234 Cm [3]. According to the characteristics of the MAD (minimum mass yield at symmetry, mass-angle correlation with peak yield at symmetry, and no significant mass-angle correlation), they are assigned as type MAD1, MAD2 and MAD3 respectively [3].…”

“…The assigned MAD category for each reaction, and associated approximate mean sticking times are also shown (adapted from Ref. [3]). …”

Dynamical approach to heavy ion-induced fission

“…Table 1 provides target details and energy ranges for the reactions studied. Coincident fission fragments were detected in the CUBE detector, which consists of two large-area position-sensitive multi-wire proportional counters (MWPCs) [12]. A schematic view of the experimental setup is given in Figure 1.…”