Measurements of mass-angle distributions (MADs) for Cr + W reactions, providing a wide range in the neutron-to-proton ratio of the compound system, (N/Z)CN, have allowed for the dependence of quasifission on the (N/Z)CN to be determined in a model-independent way. Previous experimental and theoretical studies had produced conflicting conclusions. The experimental MADs reveal an increase in contact time and mass evolution of the quasifission fragments with increasing (N/Z)CN, which is indicative of an increase in the fusion probability. The experimental results are in agreement with microscopic time-dependent Hartree-Fock calculations of the quasifission process. The experimental and theoretical results favor the use of the most neutron-rich projectiles and targets for the production of heavy and superheavy nuclei.

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of 48 Ca, 50 Ti, and 54 Cr with 208 Pb. Moving from 48 Ca to 54 Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured massangle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P CN (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for 50 Ti and 15 for 54 Cr in comparison to 48 Ca. The energy dependence of P CN indicates that cold fusion reactions (involving 208 Pb) are not driven by a diffusion process.

Energy dissipative processes play a key role in how quantum many-body systems dynamically evolve towards equilibrium. In closed quantum systems, such processes are attributed to the transfer of energy from collective motion to single-particle degrees of freedom; however, the quantum manybody dynamics of this evolutionary process are poorly understood. To explore energy dissipative phenomena and equilibration dynamics in one such system, an experimental investigation of deepinelastic and fusion-fission outcomes in the 58 Ni+ 60 Ni reaction has been carried out. Experimental outcomes have been compared to theoretical predictions using Time Dependent Hartree Fock and Time Dependent Random Phase Approximation approaches, which respectively incorporate onebody energy dissipation and fluctuations. Excellent quantitative agreement has been found between experiment and calculations, indicating that microscopic models incorporating one-body dissipation and fluctuations provide a potential tool for exploring dissipation in low-energy heavy ion collisions.The dynamic evolution of perturbed quantum manybody systems towards equilibrium is a topic of great interest in many fields, including quantum information [1], condensed matter [2-5], and nuclear physics [6][7][8][9][10]. Energy dissipation-the transfer of energy from collective motion to internal or external degrees of freedomshapes this dynamic evolution, playing a significant role in whether and how such complex systems achieve full equilibration. To date, a great deal of effort has focused on quantum systems in which energy dissipation is brought about via contact with an external environment (e.g., gas, photons, etc.) [11,12]. Much less is known about energy dissipation that arises from internal degrees of freedom [2, 5,13].One testing ground for the exploration of energy dissipation due to internal degrees of freedom can be found in heavy ion collisions. The nuclear collision process results in a closed composite quantum system that is isolated from external environments during the time of the collision (a timescale of several zeptoseconds, prior to particle emission), rapidly evolves towards equilibration in many degrees of freedom, and undergoes significant excitation and internal rearrangement throughout the equilibration process. Through the manipulation of collision entrance channel parameters (projectile-target combinations and energies), a range of factors with the potential to affect energy dissipation can be explored. Typical timescales for energy dissipation in such systems could in principle vary from isospin and mass equilibration times on the order of 0.3-0.5 zs [14,15] and ∼5 zs [16,17], respectively.In nuclear reactions, the observation of the total kinetic energy of the reaction products (TKE) offers a direct measure of energy dissipation. The observation of the masses of reaction products via direct or indirect methods offers a measure of system equilibration in a key degree of freedom, and can be used to explore fluctuations in reaction product mas...

Background: The fusion-evaporation reaction at energies around the Coulomb barrier is presently the only way to produce the heaviest elements. However, formation of evaporation residues is strongly hindered due to the competing fusion-fission and quasifission processes. Presently, a full understanding of these processes and their relationships has not been reached. Purpose: This work aims to use new fission measurements and existing evaporation residue and fission excitation function data for reactions forming Cf isotopes to investigate the dependence of the quasifission probability and characteristics on the identities of the two colliding nuclei in heavy element formation reactions. Method: Using the Australian National University's 14UD electrostatic accelerator and CUBE detector array, fission fragments from the 12 C + 235 U, 34 S + 208 Pb, 36 S + 206 Pb, 36 S + 208 Pb, and 44 Ca + 198 Pt reactions were measured. Mass and angle distributions of fission fragments were extracted and compared to investigate the presence and characteristics of quasifission. Results: Mass-angle-correlated fission fragments were observed for the 44 Ca + 198 Pt reaction; no correlation was observed in the other reactions measured. Flat-topped fission-fragment mass distributions were observed for 12 C + 235 U at compound-nucleus excitation energies from 28 to 52 MeV. Less pronounced flat-topped distributions were observed, with very similar shapes, for all three sulfur-induced reactions at excitation energies lower than 45 MeV. Conclusions: A high probability of long-timescale quasifission seems necessary to explain both the fission and evaporation residue data for the 34 S + 208 Pb and 36 S + 206 Pb reactions. Flat-topped mass distributions observed for 12 C-and 34,36 S-induced reactions are suggested to originate both from late-chance fusion-fission at low excitation energies and the persistence of shell effects at the higher energies associated with quasifission.

Background: Quasifission, a fission-like reaction outcome in which no compound nucleus forms, is an important competitor to fusion in reactions leading to superheavy elements. The precise mechanisms driving the competition between quasifission and fusion are not well understood. Purpose: To understand the influence reaction parameters have on quasifission probabilities, an investigation into the evolution of quasifission signatures as a function of entrance channel parameters is required. Methods: Using the Australian National University's (ANU) CUBE detector for two-body fission studies, measurements were made for a wide range of reactions forming isotopes of curium. Important quasifission signatures-namely, mass-ratio spectra, mass-angle distributions, and angular anisotropies-were extracted. Results: Evidence of quasifission was observed in all reactions, even for those using the lightest projectile (12 C + 232 Th). But the observables showing evidence of quasifission were not the same for all reactions. In all cases, mass distributions provided some evidence of the possible presence of quasifission but were not sufficient in most cases to clearly identify reactions for which quasifission was important. For reactions using light projectiles (12 C, 28,30 Si, 32 S), experimental angular anisotropies provided the clearest signature of quasifission. For reactions using heavier projectiles (48 Ti, 64 Ni), the presence of mass-angle correlations in the mass-angle distributions provided strong evidence of quasifission and also provided information about quasifission timescales. Conclusions: The observable offering the clearest signature of quasifission differs depending on the reaction timescale.