Dynamics of breakup and quasifission : mechanisms inhibiting fusion

The study of reaction dynamics involving weakly bound stable and halo nuclei, is currently one of the most intriguing and challenging problems in nuclear physics. The influence of weakly bound nuclei on the fusion process is not solely due to coupling to collective degrees of freedom, as with well bound nuclei, but requires consideration of the strong couplings to the low lying unbound states, present due to the small binding energy of these nuclei. The breakup process, and its effect on the fusion cross section is poorly understood, and to help solve this puzzle, this thesis presents a quantitative understanding of breakup and its impact on suppression of fusion at above-barrier energies. This thesis achieves the complete identification and characterization of all breakup mechanisms for the beryllium-9 weakly bound nucleus, by measuring the properties of the two a fragments resulting from breakup, across a wide angular range. To suppress the absorption of breakup fragments by the target nucleus, which can cause complexity in interpretation, the measurements were carried out at beam energies below the fusion barrier. A large-area pixelated silicon detector array, commissioned and used for the first time to make the measurements presented in this thesis, has allowed for the kinematic reconstruction of each breakup event. The experimentally reconstructed Q-value and relative energy of the breakup fragments, when utilized together, provided the first complete picture of the breakup of this nucleus. Contrary to the simple expectation of direct breakup into the {u03B1}-{u03B1}-n partition, in this work it is shown that breakup of beryllium-9 is a two-step process, triggered predominantly by transfer of the valence neutron. By exploiting the concept of relative energy, breakup timescales have been obtained and breakup processes which are fast enough to affect fusion, called "prompt breakup", have been separated from the total breakup yield. By describing the probability of prompt breakup as a function of the classical radial separation of the projectile and target, the systematics of breakup have been obtained, and shown to depend strongly on the surface separation of the interacting nuclei. Measured prompt breakup probability functions have been used together with a classical trajectory model to predict the above-barrier suppression of complete fusion. Agreement of the sub-barrier no-capture breakup measurements with measured incomplete fusion cross sections at above-barrier energies have demonstrated the power of this technique in understanding the dynamics of fusion. Existing and new quantal models for describing the breakup of weakly bound nuclei will directly benefit from the detailed understanding of the breakup mechanism of beryllium-9. To extend these studies, tagged beams of helium-6 at the Australian National University will be used for experiments to understand the interplay between breakup of halo nuclei and fusion. Superheavy elements can only be formed by fusing two heavy nuclei. The repulsive electrostatic energy of such systems is extremely large, so rather than fusing, the system re-separates prematurely into two heavy fragments in a non-equilibrium process called "quasifission". However, occasionally fusion can occur resulting in the formation of heavy elements. Finding the variables determining the competition between quasifission and fusion is a problem currently challenging experimentalists and theoreticians. The many shape degrees of freedom and quantum chaotic nature of the system means that reliable calculation of this competition is beyond current theoretical capabilities. Prediction of the most favorable reactions to form superheavy elements, thus currently relies on empirical information. To aid in the development of realistic models, it is important to determine which degrees of freedom are critical in quasifission dynamics, and what are the threshold values beyond which fusion starts to be suppressed by quasifission. Heavy elements (uranium and heavier) readily fission even from a near-spherical shape. The experimental problem then is to identify the onset of quasifission amongst a large yield of fission-events. This thesis addressed this problem by forming the same isotope of the element polonium, using four different combinations of nuclei, and presents a detailed comparison of the properties of the fission-like events. Utilizing the large angular coverage of the CUBE fission spectrometer, wide-ranging mass-angle spectra were obtained for each reaction, at near-barrier beam energies. This thesis has applied an analysis technique which proved extremely sensitive to determine the onset of quasifission in a system of target-projectile combinations forming the same heavy/superheavy composite nucleus. This technique conclusively identified the reaction of sulphur with erbium as being at the threshold of quasifission in reactions forming polonium isotopes. As accepted nuclear reaction models predict that quasifission should not occur for such light asymmetric combinations, this finding is a challenge to any theoretical model aiming to describe the complex multi-dimensional fusion dynamics