Nuclear fission is the violent separation of a single nucleus into two nuclei, and this complex process offers extreme tests of our understanding of fundamental nuclear physics. However, since its discovery in 1939, a complete understanding of this process has yet to be reached and is currently the subject of active debate.
that remains is why some heavy (actinide) nuclei break apart symmetrically (producing two equal mass nuclei) and others fission asymmetrically into a heavy and a light fragment. It is understood that this must be due to the presence of quantum shell effects, which offer an increased stability to nuclei with certain numbers of protons and/or neutrons, but the key question is which shell effects are responsible.
This question is further complicated by the recent (2010) observation of mass-asymmetric fission in neutron-deficient mercury, which was long expected to fission symmetrically. Mercury lies far from the actinide region, and greatly extends the known locations of mass-asymmetric fission observed across the nuclear chart. Is this asymmetric fission caused by the same shell effects as those seen in the mass-asymmetric fission of actinide nuclei? The only way to find out is through extensive and detailed experimental studies of fission fragment distributions of nuclei in this mass region, such that the shell effects that are responsible in the fission process can be rigorously tested.
To this end, I will present a recent experimental study performed at the ANU investigating the fission of bismuth isotopes at the lowest energies studied to date. These low energy studies are important as they maximise the sensitivity to the quantum shell effects. Preliminary results of a wider systematic study of fission of nuclei in the sub-lead region will also be presented, where we take advantage of heavy-ion induced fission reactions to map out previously uncharted regions of the nuclear chart. These wide-ranging experimental studies will offer a unique and thorough method to hopefully answer one of the major questions of nuclear physics – what shell effects dictate the fission process?