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.

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: 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.

Background: Mass-asymmetric fission has been observed in low energy fission of 180 Hg. Calculations predicted the persistence of asymmetric fission in this region even at excitation energies of 30-40 MeV. Purpose: To investigate fission mass distributions by populating different isotopes of Hg using heavy ion fusion reactions. Methods: Fission fragment mass-angle distributions have been measured for two reactions, 40 Ca+ 142 Nd and 13 C+ 182 W, populating 182 Hg and 195 Hg, respectively, using the Heavy Ion Accelerator Facility and CUBE spectrometer at the Australian National University. Measurements were made at beam energies around the capture barrier for the two reactions and mass ratio distributions were obtained using the kinematic reconstruction method. Results: Asymmetric fission has been observed following the population of 182 Hg at an excitation energy of 22.8 MeV above the saddle point. A symmetric peaked mass ratio distribution was observed for 195 Hg nuclei at a similar excitation energy above the saddle point. Conclusions: Mass-asymmetric fission has been observed in neutron deficient Hg nuclei populated via heavy ion fusion for the first time. The results are consistent with observations from beta-delayed fission measurements and provide a proof-of-principle for expanding experimental studies of the influence of shell effects on the fission processes.

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