Mid Term Review

Examining thermalisation in heavy ion fusion using precision cross section measurements of the compound nucleus 220Th

Ms Lauren Bezzina
Department of Nuclear Physics

Heavy-ion fusion is a complex, many-body quantum process, whereby two separate nuclei merge to form a single, compact compound nucleus. It is intrinsically dissipative, requiring the kinetic energy of the collision to be dispersed into a multitude of internal nucleonic excitations. Existing models of fusion, accounting for the coherent superposition of collective excited states [1], have been quite successful in predicting the outcome of fusion at energies near and below the fusion barrier. Crucially, however, these models do not explicitly treat the progression of the system from a fully coherent quantum state to the thermalised, compact compound nucleus. As a consequence, predictions of fusion cross sections at above barrier energies with these models may disagree with experiment by up to a factor of 2 [2].

Determining the variables which control this thermalisation is a key step in understanding the progression towards a fully energy-dissipated compound nucleus. One variable thought to be important is the amount of nuclear matter overlap at barrier radius. This matter overlap is controlled by the entrance channel charge product, ZpZt since for the same compound nucleus, increasing ZpZt increases the Coulomb repulsion between the reactants. This then requires a stronger attractive nuclear potential to overcome the repulsion, resulting in a reduced radius of the barrier. Studies of the same compound nucleus formed using differing ZpZt will reveal how this variable influences compound nucleus formation and is the basis for the systematic measurements begun in this PhD project.

This seminar will outline the experimental program designed to measure the outcomes following compound nucleus formation: evaporation residue (ER) formation and fusion-fission. Measuring the cross section of compound nucleus decay modes will then allow quantification of other collision outcomes that are otherwise indistinguishable from the fusion-fission mode, in particular, quasifission, which is known to suppress fusion. A presentation of the development of the method to extract high-precision ER cross sections will be included, along with benchmarking reactions and initial data from the new 8T version of the SOLITAIRE experiment [3]. Preliminary fission cross sections measured with the ANU CUBE fission spectrometer will also be presented.

[1] M. Dasgupta et al., Measuring barriers to fusion, Annu. Rev. Nuc. Part. Sci. 48, 401 (1998).
[2] J. O. Newton et al., Systematic failure of the Woods-Saxon nuclear potential to describe both fusion and elastic scattering: Possible need for a new dynamical approach to fusion, Phys. Rev. C., 024605 (2004).
[3] M. D. Rodrı́guez et al., SOLITAIRE: A new generation solenoidal fusion product separator, Nucl. Instrum. Meth. A 614, 119 (2010).

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