Zeptosecond dynamics of transfer-triggered breakup: mechanisms, timescales, and consequences for fusion

Abstract

Above-barrier complete fusion cross-sections for reactions with light, weakly-bound nuclei such as 6,7Li and 9Be are suppressed relative to expectations from theory and experiment. This has been interpreted to be a result of the weakly-bound nucleus breaking up into its cluster constituents, reducing the probability of complete charge capture. However, experiments to probe mechanisms of breakup in below-barrier reactions of 9Be and 6,7Li with high atomic number targets have shown that breakup of unbound states formed following nucleon transfer dominates over direct breakup of the projectile into its cluster constituents. This thesis extends the study of breakup following transfer in interactions of 9Be and 7Li with light targets of 6 ≤ Z ≤ 28. Below-barrier coincidence measurements of breakup fragments produced in these reactions show a vanishing amount of direct breakup, and the dominance of transfer-triggered breakup.

Since breakup can only suppress complete fusion if it occurs prior to the collision partners reaching the fusion barrier, the location of breakup is crucial. In turn, the location of breakup is intimately related to the lifetime of the unbound state populated. Nuclei produced in long-lived states cannot suppress complete fusion, since they will pass the barrier before breakup can occur. Conversely, nuclei produced in states with lifetimes comparable to the zeptosecond (10^−21 s) timescale of the collision may break up before reaching the fusion barrier. Through the use of experimental observables that are sensitive to the location of breakup, the importance of a realistic treatment of resonance lifetimes to correctly reproduce experimental results with theoretical modelling will be established.

Below-barrier measurements of transfer-triggered breakup, where capture is minimised, are used to determine the breakup probability as a function of distance of closest approach for reactions of 7 Li and 9 Be with light targets of 13 ≤ Z ≤ 28, as well for reactions of 9Be with heavy targets of 62 ≤ Z ≤ 83. These probability functions are used as input into classical dynamical trajectory models to predict above-barrier complete and incomplete fusion cross-sections. These fusion cross-sections are found to be sensitive to the lifetime of the weakly-bound nucleus produced after transfer. When realistically modelled, the inclusion of lifetime leads to the conclusion that breakup alone cannot account for the observed suppression of complete fusion in reactions 9Be with 144Sm to 209Bi.

Experimental groundwork is laid for measurement of the 7Be(d,p)8Be reaction at the Australian National University, relevant to Big Bang nucleosynthesis. The efficacy of using a large solid angle array and kinematic reconstruction techniques for such studies is demonstrated through a measurement of α particles produced in the mirror reaction 7Li(d,n)8Be. In this reaction, a high population of the broad 4+ resonance in 8Be is observed, totalling 69% of the coincidence yield after efficiency correction. It is therefore crucial to investigate the excitation of 8Be in the 7Be(d,p)8Be reaction. Test measurements of 7Be production via the 10B(6Li,7Be)9Be reaction are made using the SOLEROO RIB facility. Normalised secondary beam intensities above 10 4 cts/s/mg/cm^−2/μeA are achieved with beam purity of ∼ 96%.