Developing Techniques for High Fidelity Studies of Reactions with Light Weakly Bound Nuclei
Creators: name: Carter, Ian Paul
Publisher: name: The Australian National University
Year: 2016
Subjects: Radioactive Ion Beams
RIB
breakup
elastic scattering
PPACs
gas detectors
Li-8
Be-9
SOLEROO
BALiN

description: Research capabilities in nuclear physics have greatly expanded in recent years with the availability of radioactive ion beams and exotic nuclei near the drip line. As a result, new phenomena are being discovered in areas of nuclear reactions and nuclear structure. This thesis work is focused on studies of reaction mechanisms of light weakly bound nuclei at energies near the Coulomb barrier, where nuclear structure influences nuclear reactions outcomes. Two strands towards this end were followed concurrently; the first, to develop a radioactive beam capability to enable reaction studies with 6 He and 8 Li nuclei, and the second, to study the systematics of breakup mechanisms of the stable but weakly bound nucleus 9 Be in interactions with targets of mass A = 40-124. The radioactive beam capability at the Australian National University uses in-flight transfer reactions to produce light unstable beams. The radioactive ion species of interest are then transported and focused onto a secondary target using the magnetic field generated by a superconducting solenoid. The relatively low purities of the unstable beam obtained using a single solenoid (typically 30%) normally necessitates the use of two solenoids in tandem to further purify the radioactive ion beam as done at the TwinSol (USA) and RIBRAS (BRAZIL) facilities. A unique feature of the Australian National University (ANU) radioactive beam capability is a pair of tracking detectors placed at the exit of the solenoid that allows identification and determination of the trajectories of the radioactive species, and electronic tagging event-by-event. These detectors were developed and successfully implemented as part of this thesis work. The reconstruction of ion trajectories using these detectors aids in rejection of contaminant species. Effective beam purities of greater than 90% have been achieved for 6 He and 8 Li, with most impurities being tritons. The tracking detectors have demonstrated rate handling capability of 3×10 6 particles per second. The trajectory reconstruction also provides information on the point of interaction and the angle of incidence of the ion on the secondary target, allowing precise reconstruction of reaction kinematics which is necessary for high fidelity studies of nuclear reactions. Details of the ion transport, tracking detector performance and secondary beam characteristics are described in this thesis, along with the results of the first experiment using a radioactive beam of 8 Li from the ANU capability. Parallel to developing the tracking detectors, experiments with 9 Be, identifying and characterising all breakup mechanisms of 9 Be incident on targets of mass A = 40-124 were carried out. These experiments were done at several energies below the fusion barrier to minimise absorption of breakup fragments by the target. The charged breakup fragments were detected in BALiN, a highly pixelated double sided silicon detector array. The dominance of n-transfer from 9 Be to the target, forming 8 Be, is observed over the entire target mass-region studied in this thesis. Following transfer the 8 Be formed breaks up into two alpha particles. The relative energies of the two coincident alpha particles are used to separate breakup following population of the long-lived ground state of 8 Be from the shorter-lived excited states. This separation is significant since complete fusion cannot be affected by breakup occurring on a time-scale slower than fusion. Selecting the near-target breakup events, and presenting their probability as a function of the radial separation of the projectile and target, can be used in a classical trajectory model to predict suppression of complete fusion at above-barrier energies. The experimental results obtained in this work, combined with the previous studies of 9 Be on heavy targets, give the systematics of breakup in reactions with masses ranging from 40 to 209 u. Such systematics should aid in the developments of models of reactions with weakly bound nuclei.