Effects of Nuclear Structure on Quasi-fission

The quasi-fission mechanism hinders fusion of heavy systems because of a mass flow between the reactants, leading to a re-separation of more symmetric fragments in the exit channel. A good understanding of the competition between fusion and quasi-fission mechanisms is expected to be of great help to optimize the formation and study of heavy and superheavy nuclei. Quantum microscopic models, such as the time-dependent Hartree-Fock approach, allow for a treatment of all degrees of freedom associated to the dynamics of each nucleon. This provides a description of the complex reaction mechanisms, such as quasi-fission, with no parameter adjusted on reaction mechanisms. In particular, the role of the deformation and orientation of a heavy target, as well as the entrance channel magicity and isospin are investigated with theoretical and experimental approaches.Comment: Invited talk to NSRT12. To be published in Eur. Phys. J. Web of Con

Measured g factors and the tidal-wave description of transitional nuclei near A = 100

The transient-field technique has been used in both conventional kinematics and inverse kinematics to measure the g factors of the 2+ states in the stable even isotopes of Ru, Pd and Cd. The statistical precision of the g(2+) values has been significantly improved, allowing a critical comparison with the tidal-wave version of the cranking model recently proposed for transitional nuclei in this region.Comment: Accepted for publication in Physical Review C, April 201

Sub-barrier quasifission in heavy element formation reactions with deformed actinide target nuclei

Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Low excitation energies favor SHE survival against fusion-fission competition. In “cold” fusion with spherical target nuclei near 208Pb, SHE yields are largest at beam energies significantly below the average capture barrier. In “hot” fusion with statically deformed actinide nuclei, this is not the case. Here the elongated deformation-aligned configurations in sub-barrier capture reactions inhibits fusion (formation of a compact compound nucleus), instead favoring rapid reseparation through quasifission. Purpose: To determine the probabilities of fast and slow quasifission in reactions with prolate statically deformed actinide nuclei, through measurement and quantitative analysis of the dependence of quasifission characteristics at beam energies spanning the average capture barrier energy. Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission and quasifission mass and angle distributions for reactions with projectiles from C to S, bombarding Th and U target nuclei. Results: Mass-asymmetric quasifission occurring on a fast time scale, associated with collisions with the tips of the prolate actinide nuclei, shows a rapid increase in probability with increasing projectile charge, the transition being centered around projectile atomic number ZP = 14. For mass-symmetric fission events, deviations of angular anisotropies from expectations for fusion fission, indicating a component of slower quasifission, suggest a similar transition, but centered around ZP ∼ 8. Conclusions: Collisions with the tips of statically deformed prolate actinide nuclei show evidence for two distinct quasifission processes of different time scales. Their probabilities both increase rapidly with the projectile charge. The probability of fusion can be severely suppressed by these two quasifission processes, since the sub-barrier heavy element yield is likely to be determined by the product of the probabilities of surviving each quasifission process.The authors acknowledge support from ARC Grants No. FL110100098, No. DP130101569, No. FT120100760, No. DE140100784, No. DP140101337, No. DP160101254, and No. DP170102318, and support by the Federal Government NCRIS program for operations of the ANU Heavy Ion Accelerator Facility

Influence of entrance-channel magicity and isospin on quasi-fission

The role of spherical quantum shells in the competition between fusion and quasi-fission is studied for reactions forming heavy elements. Measurements of fission fragment mass distributions for different reactions leading to similar compound nuclei have been made near the fusion barrier. In general, more quasi-fission is observed for reactions with non-magic nuclei. However, the $^{40}$Ca+$^{208}$Pb reaction is an exception, showing strong evidence for quasi-fission, though both nuclei are doubly magic. Time-dependent Hartree-Fock calculations predict fast equilibration of $N/Z$ in the two fragments early in the collision. This transfer of nucleons breaks the shell effect, causing this reaction to behave more like a non-magic one in the competition between fusion and quasi-fission. Future measurements of fission in reactions with exotic beams should be able to test this idea with larger $N/Z$ asymmetries.Comment: accepted for publication in Physics Letters

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

Examining the role of transfer coupling in sub-barrier fusion of ⁴⁶,⁵⁰Ti+¹²⁴Sn

Background: The presence of neutron transfer channels with positive Q values can enhance sub-barrier fusion cross sections. Recent measurements of the fusion excitation functions for 58Ni +132,124Sn found that the fusion enhancement due to the influence of neutron transfer is smaller than that in 40Ca +132,124Sn although the Q values for multineutron transfer are comparable. Purpose: To investigate the differences observed between the fusion of Sn + Ni and Sn + Ca. Methods: Fusion excitation functions for 46,50Ti +124Sn have been measured at energies near the Coulomb barrier. Results: A comparison of the barrier distributions for 46Ti +124Sn and 40Ca +124Sn shows that the 40Ca +124Sn system has a barrier strength resulting from the coupling to the very collective octupole state in 40Ca at an energy significantly lower than the uncoupled barrier. Conclusions: The large sub-barrier fusion enhancement in 40Ca induced reactions is attributed to both couplings to neutron transfer and inelastic excitation, with the octupole vibration of 40Ca playing a major role.Research at ANU was supported by the Australian Research Council Grants DP130101569, FT120100760, DP140101337, FL110100098, DE140100784 and by National Collaborative Research Infrastructure Strategy (NCRIS) for the operation of the Heavy Ion Accelerator Facility

Quasifission and Shell Effects in Reactions Forming 266Sg

The role of shell effects in reactions forming the heavy element 266Sg was investigated using the Mass Angle Distribution technique. For the 34S + 232Th reaction the doubly magic shell closure at 208Pb was found to strongly influence asymmetric quasifission, the exit channel at sub barrier energies. The evolution of the dinuclear system is arrested as it passes through this mass region. Mass splits corresponding to AL/AH ≈ 58/208 are seen for a large range of angles indicating a long timescale for this process. The more mass asymmetric 28Si + 238U reaction has a much smaller quasifission cross section. Therefore the shell effects around 208Pb are not dominant here

Quasifission and Shell Effects in Reactions Forming

The role of shell effects in reactions forming the heavy element 266Sg was investigated using the Mass Angle Distribution technique. For the 34S + 232Th reaction the doubly magic shell closure at 208Pb was found to strongly influence asymmetric quasifission, the exit channel at sub barrier energies. The evolution of the dinuclear system is arrested as it passes through this mass region. Mass splits corresponding to AL/AH ≈ 58/208 are seen for a large range of angles indicating a long timescale for this process. The more mass asymmetric 28Si + 238U reaction has a much smaller quasifission cross section. Therefore the shell effects around 208Pb are not dominant here