Energy dissipation in multinucleon transfer reactions

Abstract

Nuclear reactions are incredibly complex, involving collisions between composite sys- tems where many-body dynamics determine outcomes. Successful models have been de- veloped to explain particular behaviour of reactions in distinct energy and mass regimes, but a unifying picture remains elusive. Particular problems have become evident in standard Coupled channels approaches to calculating fusion cross sections, with hindrance effects having been identified both above and below the barrier. Recent works [1, 2] have demonstrated inadequacies in these ap- proaches using static internuclear potentials, and have shown the need to address some hidden physics. The dissipation of energy from the relative motion of the collision part- ners to internal states is known to be important in these processes, but is yet to be suc- cessfully incorporated into reaction models. Multinucleon transfer reactions are a useful tool to examine these aspects, as they span the transition from the quasielastic regime, where the colliding nuclei barely overlap, and the deep inelastic regime, where collisions are violent with significant redistribution of mass and charge between the fragments, as well as large losses of kinetic energy from the relative motion [3]. This PhD thesis examines the onset of dissipation as the bombarding energy approaches the Coulomb barrier over a range of light to medium mass projectiles incident on heavy targets. This investigation has focussed on studying the quasielastic scattering yields that are de- tected at backward angles. These products are of interest to the question of mechanisms to hinder fusion, as they represent the flux that fails to penetrate the fusion barrier at low angular momentum and is instead reflected. By identifying these products uniquely in mass and charge, together with an accurate measurement of kinetic energy losses, we are able to establish which reaction modes are the most important "doorways" for energy dissipation to proceed through. We present in this work the systematic trends in dissipative effects as the mass asymme- try of the reaction system changes, with measurements at bombarding energies spanning the fusion barrier. This work illustrates the growing importance of dissipative effects as the charge product of the reactants grows, and identifies a stark distinction in the nature of the dominant transfer mechanism between light and medium mass projectiles. While direct transfer involving clusters is very important in light nuclei, deep-inelastic transfer involving mutual nucleon exchange becomes much more important for reactions involv- ing medium-mass projectiles. There is a growing realisation in the nuclear reactions community that energy dissipa- tion is an important effect in fusion dynamics both above and below the barrier, yet this remains an area that has not been studied in detail. This work makes a first attempt to identify the important parameters and steps towards a phenomenological footing for understanding dissipative processes in nuclear reactions.

References:

[1] M. Evers, M. Dasgupta, D. J. Hinde, L. R. Gasques, M. L. Brown, R. Rafiei, and R. G. Thomas. "Systematic study of the nuclear potential diffuseness through high precision back-angle quasi-elastic scattering". In: Phys. Rev. C 78 (3 2008), p. 034614.

[2] M. Dasgupta, D. J. Hinde, A. Diaz-Torres, B. Bouriquet, C. I. Low, G. J. Milburn, and J. O. Newton. "Beyond the coherent coupled channels description of nuclear fusion". In: Physical review letters 99.19 (2007), p. 192701.

[3] K. E. Rehm, A. M. van den Berg, J. J. Kolata, D. G. Kovar, W Kutschera, G Rosner, G. S. F. Stephans, and J. L. Yntema. "Transition from quasi-elastic to deep-inelastic reactions in the 48 Ti+ 208 Pb system". In: Phys. Rev. C 37.6 (1988), p. 2629. Show more