Tunnelling in the Presence of an Environment: Insights from the Fusion of Heavy Nuclei

Abstract

A great many physical processes are mediated by the penetration or surmounting of a potential barrier. Some notable examples are the thermionic emission of electrons, chemical reactions, tunnelling of Cooper pairs through Josephson junctions, nuclear ‘burning’ in stars etc. Often the potential in question is not simply a function of the position of the intruding particle, but is a complicated landscape determined by the static structure of the physical objects concerned as well as by their dynamical response to their mutual interactions. For example, in a Josephson junction, the penetration of the barrier is strongly influenced by the excitation of lattice vibrations while catalysed chemical reactions proceed via complex routes which present a series of lower activation energies. In a physical system with a well-defined temperature, the details of the potential landscape may be obscured by the Maxwell-Boltzmann distribution of energies. The fusion of heavy nuclei can, however, be achieved over a controlled range of well-defined beam energies to reveal fine details of some of these effects. To date the major new experimental work in this area has been performed at the ANU, Canberra and at the INFN, Legnaro. As well as yielding valuable information on nuclear structure (detailed information on nuclear shapes etc.) and some totally unexpected reaction dynamics (complexity of the induced surface oscillations, neutron flow), these experiments and their theoretical analysis give considerable insights into the general problem of ‘tunnelling in the presence of an environment’.