Search

Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic 208Pb, generally attributed to enhanced binding energy causing a valley in the potential energy surface, which attracts quasifission trajectories. Measurements of binary quasifission mass spectra and cross-sections have been made for reactions of 50Ti with actinide nuclides from 232Th to 249Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from 70% of calculated capture cross-sections for 232Th to only 40% for 249Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the peak in yield near 208Pb originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above 208Pb. The important role of sequential fission calls for re-interpretation of quasifission observables and dynamics in superheavy element synthesis reactions.

Nuclear physics in the 21st century is driven by a quest to understand the properties of ever more exotic nuclear systems. Unusual structural phenom- ena are observed to arise in light weakly bound nuclei such as 8Li. 8Li has a cluster structure core of 7Li surrounded by a loosely bound neutron which is observed to influence reaction mechanisms near the fusion barrier. Elastic scat- tering provides a vital step towards understanding more complicated reaction mechanisms. In this work, elastic scattering was measured for 8Li + 2°9Bi at energies 2% to 34% above the barrier, allowing extraction of reaction cross- sections. The systematics of the reaction cross-sections of 8Li compared to neighbouring nuclei 6,7,9,11Li are discussed.

Superheavy elements are created through the fusion of two heavy nuclei. The large Coulomb energy that makes superheavy elements unstable also makes fusion forming a compact compound nucleus very unlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process called quasifission. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission. A systematic study of carefully chosen mass-angle distributions has provided information on the global trends of quasifission. Large deviations from these systematics at beam energies near the capture barrier reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and thus implicitly in hindering or favouring superheavy element synthesis.

A quantitative understanding of fusion dynamics at high angular momentum is attempted employing experimental fission cross sections as a probe and carrying out a simultaneous description of the fusion and fission cross sections at above barrier energies. For this, experimental fission fragment angular distributions for three systems: 16O+148Sm, 28Si+136Ba and 40Ca+124Sn, all forming the same compound nucleus 164Yb at similar excitation energies, have been measured at four beam energies above their respective capture barriers. A simultaneous description of the angle integrated fission cross sections and evaporation residue/fusion cross sections available in literature for the systems is carried out using coupled-channels and statistical model calculations. Fission cross sections, which are most sensitive to the changes in angular momentum, provide very stringent constraints for model calculations thus indicating the need of precision evaporation residue as well as fission cross sections in such studies. A large diffuseness (ao>0.65 fm) of the nuclear potential gives the best reproduction of the experimental data. In addition, different coupling schemes give very different angular momentum distributions, which, in turn, give very different fission cross section predictions. Both these observations hint at the explanation that depending on energy dissipation of the interacting nuclei occurring inside or outside the fusion pocket, very different fission cross sections can result due to heavily altered angular momentum and thus justifies the sensitivity of fission cross sections used as probes in the present work.

Superheavy elements are created in the laboratory by the fusion of two heavy nuclei. The large Coulomb repulsion that makes superheavy elements decay also makes the fusion process that forms them very unlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process called quasifission. Mass-angle distributions give the most direct information on the characteristics and time scales of quasifission. A systematic study of carefully chosen mass-angle distributions has provided information on the global trends of quasifission. Large deviations from these systematics reveal the major role played by the nuclear structure of the two colliding nuclei in determining the reaction outcome, and thus implicitly in hindering or favouring superheavy element production.

Current coupled channels (CC) models treat fusion as a coherent quantum-mechanical process, in which coupling between the collective states of the colliding nuclei influences the probability of fusion in near-barrier reactions. While CC models have been used to successfully describe many experimental fusion barrier distribution (BD) measurements, the CC approach has failed in the notable case of 16O+208Pb. The reason for this is poorly understood; however, it has been postulated that dissipative processes may play a role. Traditional BD experiments can only probe the physics of fusion for collisions at the top of the Coulomb barrier (L = 0ħ). In this work, we will present results using a novel method of probing dissipative processes inside the Coulomb barrier. The method exploits the predicted sharp onset of fission at L ~ 60ħ for reactions forming compound nuclei with A < 160. Using the ANU’s 14UD tandem accelerator and CUBE spectrometer, reaction outcomes have been measured for the 58Ni+60Ni reaction at a range of energies, in order to explore dissipative processes at high angular momentum. In this reaction, deep inelastic processes have been found to set in before the onset fission at high angular momentum following fusion. The results will be discussed in relation to the need for a dynamical model of fusion.

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.

Nucleon and cluster transfer probabilities have been measured in the collisions of 16,18O, 19F with 204,208Pb, 209Bi for charge stripping channels down to ΔZ = - 3. Strong evidence of correlated nucleon transfer has been observed in particular channels, and neighbouring systems are seen to differ significantly in their behaviour. New measurements were made using an improved ΔE-E telescope. The back-scattered projectilelike fragments were measured in the telescope at θlab = 160.6°, and in combination with monitor detectors at forward angles allowed determination of absolute transfer probabilities. The improved design allows isotopic yields to be measured with greater precision.

We examined the 40Ca + 238U reaction experimentally using the Mass-Angle Distribution (MAD) technique and within the Time Dependent Hartree Fock (TDHF) theory, using the TDHF3D code. A new, TDHF based approach has been developed to construct mass distributions as well. The results revealed that the orientation of the heavy deformed prolate nucleus plays a major role in the reaction outcome. It was found that aligned collisions lead to quasifission and short contact times of 5-10 zs, whilst anti-aligned collisions lead to longer contact times (> 23 zs). TDHF accurately predicted the presence of quasifission and the average mass splits in this reaction. The influence of shell effects around 208Pb in the calculated quasifission characteristics was observed in both experiment and theory.

Mass-angle distributions carry detailed information on the characteristics of quasifission, and thus of the dynamics of heavy element formation reactions. Recent experimental results are presented and discussed.

A well known issue with concordance cosmology is the cosmological lithium problem, where models of Big Bang Nucleosynthesis indicate abundances of 7Li three to four times larger than values inferred via spectroscopic measurements of metal-poor halo stars [1]. Since the source of this discrepancy remains unclear [2], it is vital to fully understand the nuclear reactions that affect the production of 7Li during the Big Bang [3]. At the Australian National University, experimental equipment and analysis techniques have been developed for nuclear reaction studies at energies near the fusion barrier, exploiting large solid angle detectors to enable the investigation of breakup without a priori assumption of the breakup kinematics. The extension to reactions of astrophysical interest may help shed light on these reactions. Recent experiments, using these new techniques, have provided a complete picture of the breakup mechanisms of light nuclei in collisions with heavy targets [4]. The present work focuses on obtaining a complete picture of breakup mechanisms of 7Li following interactions with 27Al. It has been found that breakup is almost exclusively triggered by nucleon transfer between the colliding partners, to a larger extent than was found for heavier targets. The findings of these experiments, as well as progress towards extensions to astrophysically relevant reactions, such as d + 7Be [5] will be presented.

Mass-angle distributions carry detailed information on the characteristics of quasifission, and thus of the dynamics of heavy element formation reactions. Recent experimental results are presented and discussed.

The understanding of quantum effects in determining nuclear reaction outcomes is evolving as improved experimental techniques reveal new facets of interaction dynamics. Whilst the phenomenon of coupling-enhanced quantum tunnelling is understood to arise due to quantum superposition, the observed inhibition of fusion at energies well below the barrier is not yet quantitatively understood. Collisions involving weakly-bound nuclei, which have low energy thresholds against breakup, present further challenges. Recent coincidence measurements for reactions of weakly bound stable nuclei have not only provided a complete picture of the physical mechanisms triggering breakup, but have also shown how information on reaction dynamics occurring on time-scales of ~zepto-seconds can be obtained experimentally. These new experimental findings demand major developments in quantum models of near-barrier nuclear reactions.

The formation of a compound nucleus in different projectile and target combinations is a powerful method for investigating the fusion process. Recently, the dominance of quasi-fission over fusion-fission has been inferred for 34S+208Pb in comparison to 36S+206Pb; both reactions lead to the compound nucleus 242Cf*.The mass and angle distributions of the fission fragments from these reactions were studied in order to further investigate the presence of quasi-fission.

A significant challenge to nuclear astrophysics is the cosmological lithium problem, where models of Big Bang nucleosynthesis indicate abundances of 7Li two to four times larger than what is inferred via spectroscopic measurements of metal-poor stars. Recent experimental techniques developed for nuclear reaction studies at energies near the fusion barrier, if extended to reactions of astrophysical interest, may help understand nuclear reactions that can affect the production of 7Li during the Big Bang. Experiments at the ANU, using new experimental techniques, have provided complete pictures of the breakup mechanisms of light nuclei in collisions with heavy targets, such as 208Pb and 209Bi [1]. These experiments revealed dominant breakup mechanisms which had not even been considered in theoretical models. The study of the breakup of 6Li and 7Li following interactions with 58,64Ni and 27Al acts as a stepping stone from this previous work towards future experimental studies of breakup reactions of astrophysical relevance. In all cases studied, breakup is dominantly triggered by nucleon transfer between the colliding partners, but the transfer mechanisms are different. The findings of these experiments and experimental considerations for extensions to reactions of light nuclei, such as d +7Be, will be presented.

Coincidence measurements of breakup fragments were carried out for the 7Li + 197Au and 204Pb systems at sub-barrier energies. The mechanisms triggering breakup, and time-scales of each process, were identified through the reaction Q-values and the relative energy of the breakup fragments. Binary breakup of 7Li were found to be predominantly triggered by nucleon transfer, with p-pickup leading to 8Be → α + α decay being the preferred breakup mode. From the time-scales of each process, the coincidence yields were separated into prompt and delayed components, allowing the identification of breakup process important in the suppression of complete fusion of 7Li at above-barrier energies.

Shell effects can play a prominent role in fission fragment mass distributions. For lighter systems in the region of A~180-200, mass distributions were generally expected to be symmetric. However, a recent experiment showed that fission of 180Hg following electron capture of 180Tl leads to an asymmetric mass split. Recent calculations by various groups indicate that the mechanism of asymmetric fission could be very different in this mass region compared to the actinide region. To investigate the role of shell effects in this mass region, we have measured the fission fragment mass distribution for the 13C+182W,176Yb reactions forming the compound nuclei 195Hg and 189Os respectively, at lab bombarding energies of 60, 63 and 66 MeV using the CUBE detector setup located at the ANU Heavy Ion Accelerator Facility. The experimental data were fitted with single and double Gaussian distributions. The results indicate an asymmetric mass split for 195Hg, whereas for 189Os, the mass distribution is well fitted with a single Gaussian distribution.

For nuclear reactions in which super-heavy nuclei can be formed, the essential difference between the fusion process followed by fission and non-equilibrium processes leading to fission-like fragments is there action time. Quite probable non-equilibrium processes, characterized by very short reaction times, are highlighted thanks to mass-angle correlations. However, long lifetime components associated with fission following fusion have been observed with two independent experimental techniques, providing evidence for the formation of compound nuclei with Z = 120 and 124, followed by mass asymmetric fission.

The influence of breakup channels on the complete fusion of weakly bound systems is investigated in terms of dynamic polarization potentials. The systematic enhancement of the cross section at sub-barrier energies seems to be consistent with recent experimental observations that nucleon transfer, often leading to breakup, is dominant compared to direct breakup at this energy regime. At energies above the barrier, the repulsive polarization potential due to direct breakup predominates and complete fusion is suppressed.

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.

A highly efficient superconducting solenoidal fusion product separator has been developed at the Australian National University in order to enable separation and detection of evaporation residues following heavy-ion collisions. The determination of absolute fusion cross-sections requires an accurate knowledge of the transmission efficiency of evaporation residues through the superconducting solenoid, which in turn depends on the angular distribution of evaporation residues exiting the target. Two methods have been developed to extract the angular distributions using the radial distribution and the velocity distribution of the evaporation residues exiting the solenoid. The angular distributions are compared with existing direct measurements of evaporation residue angular distributions.

A 6.5 Tesla superconducting gas-filled solenoid (SOLITAIRE) has been developed at the Heavy Ion Accelerator Facility at the ANU as a reaction product separator. Key features of the device allowing its application for precise measurement of heavy ion fusion cross sections are described. The physical separation of beam particles and the high efficiency (~80%) transport of heavy ion fusion products open up applications in nuclear structure physics, and in materials science. Finally, the developments to allow its application to providing beams of light radioactive isotopes (SOLEROO) are described.

Using a position sensitive detector array, back-angle coincidence measurements of breakup fragments at sub-barrier energies has enabled the complete characterisation of the breakup processes in the reactions of 6,7Liw ith 208PbB. reakup processes and their time-scales are identified through the reaction Q-values and the relative energy of the captured breakup fragments. The majority of breakup processes fast enough (~10-22s) to affect fusion are triggered by transfer of a neutron from 6Li, and of a proton to 7Li. These breakup mechanisms should therefore have a major contribution to the ~30% suppression of complete fusion observed at above-barrier energies.

The time scales of the dynamical nuclear rearrangement processes of fission following fusion, and of quasifission, are significant tests of models of nuclear dynamics. Experimental approaches to determine characteristic times, and their consistency, are discussed.

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16O,32 S+208Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ∆Z = 1 and ∆Z = 2 were extracted, indicating surprisingly large absolute probabilities at subbarrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments ~15 MeV and ~25 MeV for the 16O and 32S-induced reactions, respectively, indicate the population of highly excited states in the residual nuclei. A comparison with expected optimum Q-values suggests large losses in kinetic energy of the projectile-like fragments. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.

Experiments with weakly-bound nuclei have demonstrated that breakup significantly affects the reaction outcomes. Coincidence measurements of breakup fragments at sub-barrier energies, using a position sensitive back-angle detector array covering 117° to 167° , have enabled the complete characterisation of the breakup processes in the reactions of the weakly-bound 6,7Li with 208Pb. The timescales of different breakup processes were also extracted from the fragments kinematics, enabling a clear characterization of prompt and delayed breakup. The majority of these prompt breakup events are triggered by n-stripping for 6Li, and p-pickup for 7Li. The demonstration that the reaction dynamics and outcomes can be significantly determined not only by the properties of the two colliding nuclei, but by the ground-state and excited state properties of their neighbours, is a key insight for understanding and predicting reactions of weakly-bound nuclei near the limits of nuclear existence.