Nuclear fission, one of the most complex processes in nuclear physics, results in the splitting of a heavy nucleus into two fragments.

Fragment kinetic energies show this occurs at a very deformed (elongated) shape. Fission of heavy (actinide) nuclei has been extensively studied, showing at least two mass-asymmetric “modes”, believed to follow different paths in deformation space. These are caused by nuclear shell effects on the path to fission, but where the two modes bifurcate is controversial.

Fission can be modelled as following trajectories on a multidimensional potential energy surface (PES), with one or more potential barriers (saddle-points) and subsequent valleys on the fission path. Do different fission modes result from different barriers, or later bifurcations in a valley? Experimentally, this can be probed by observing how fission fragment mass and kinetic energy distributions evolve with the excitation energy of the nucleus about to undergo fission. Different barriers should result in rapid changes in fission observables as each barrier is exceeded. The challenge is to measure fission characteristics at a large number of closely spaced excitation energies at and above the fission barrier energies.

This has been achieved at the Heavy Ion Accelerator Facility (ANU), where transfer-induced fission from (d,p) reactions on 232Th, 235U, 238U, 244Pu, 243Am, and 248Cm targets were used to induce fission at an 8 MeV wide range of excitation energies just above the fission barrier. Precise reconstruction of the excitation energy was achieved by measuring the energy of the outgoing proton with the BALiN ΔE–E silicon array, in coincidence with the fission fragments measured using the CUBE multi-wire proportional counters.

In this talk I will present the analysis methods developed to investigate fission modes from the combined detector systems and discusses the excitation energy dependence of shell-driven fission modes across the actinides.