Exploring Zeptosecond Quantum Equilibration Dynamics: From Deep-Inelastic to Fusion-Fission Outcomes in 58^{58}Ni+60^{60}Ni Reactions

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 many-body dynamics of this evolutionary process are poorly understood. To explore energy dissipative phenomena and equilibration dynamics in one such system, an experimental investigation of deep-inelastic 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 one-body 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.Comment: 11 pages, 9 figures, 1 table, including Supplemental Material - Version accepted for publication in Physical Review Letter

Reduced quasifission competition in fusion reactions forming neutron-rich heavy elements

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.Comment: Accepted to PRC as a Rapid Communicatio

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

Superheavy elements can only be created in the laboratory by the fusion of two massive nuclei. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission, the major competitor to fusion in these reactions. The systematics of 42 mass-angle distributions provide information on the global characteristics of quasifission. Deviations from the systematics reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and in hindering or favouring heavy element production.The authors acknowledge operations support for the ANU Heavy Ion Accelerator Facility from NCRIS, and support from Dr. N. Lobanov and Dr. T. Kibedi and the ANU Heavy Ion Accelerator Facility staff in operating the Linac. Financial support from ARC grants DP130101569, DP140101337, FL110100098, FT120100760 and DE140100784 is acknowledged

Fusion and quasifission studies for the ⁴⁰Ca40+¹⁸⁶W, ¹⁹²Os reactions

Background: All elements above atomic number 113 have been synthesized using hot fusion reactions with calcium beams on statically deformed actinide target nuclei. Quasifission and fusion-fission are the two major mechanisms responsible for the very low production cross sections of superheavy elements. Purpose: To achieve a quantitative measurement of capture and quasifission characteristics as a function of beam energy in reactions forming heavy compound systems using calcium beams as projectiles. Methods: Fission fragment mass-angle distributions were measured for the two reactions 40Ca+186W and 40C+192Os, populating 226Pu and 232Cm compound nuclei, respectively, using the Heavy Ion Accelerator Facility and CUBE spectrometer at the Australian National University. Mass ratio distributions, angular distributions, and total fission cross sections were obtained from the experimental data. Simulations to match the features of the experimental mass-angle distributions were performed using a classical phenomenological approach. Results: Both 40Ca+186W and 40C+192Os reactions show strong mass-angle correlations at all energies measured. A maximum fusion probability of 60−70% is estimated for the two reactions in the energy range of the present study. Coupled-channels calculations assuming standard Woods-Saxon potential parameters overpredict the capture cross sections. Large nuclear potential diffuseness parameters ∼1.5 fm are required to fit the total capture cross sections. The presence of a weak mass-asymmetric quasifission component attributed to the higher angular momentum events can be reproduced with a shorter average sticking time but longer mass-equilibration time constant. Conclusions: The deduced above-barrier capture cross sections suggest that the dissipative processes are already occurring outside the capture barrier. The mass-angle correlations indicate that a compact shape is not achieved for deformation aligned collisions with lower capture barriers. The average sticking time of fast quasifission events is 10−20 s.The authors are grateful to the accelerator staff of the ANU Heavy Ion Accelerator Facility for their excellent support during the experiments. The authors acknowledge support from the Australian Research Council through the Grants No. DP140101337, No. FL110100098, No. FT120100760, No. DP160101254, No. DP170102318, and No. DE140100784. Support for accelerator operations through the NCRIS program is acknowledged

Observation of rotation about the longest principal axis in Zr 89

High-spin states in Zr89 were populated in the Se80(C13,4n) reaction, and γ-ray coincidences were measured using the Indian National Gamma Array. The level scheme of Zr89 has been extended up to spin I=49/2 with the observation of a new dipole band. Directional correlation and polarization asymmetries of the γ rays have been measured to determine spin and parity of the levels. Line shapes of several transitions have been analyzed to determine lifetimes of the levels. Possible configurations of the band have been discussed using the cranked Nilsson-Strutinsky model. The calculations suggest a triaxial shape of the nucleus at high spins, and the band may represent rotation of the nucleus about the longest axis