How complex is the nuclear fission barrier? A challenge for nuclear fission theory

nuclear fission
Pre-neutron fission fragment mass and total energy distribution; in the upper projection the fission mode contributions are indicated. © EU, 2016

The investigation of fission fragment properties as a function of nuclear temperature shows that fragment mass and total kinetic energy depend on the angle relative to the incident neutron vector. This might be interpreted as the first experimental evidence for a nuclear fission barrier, which is more complex than hitherto believed.

Fission occurs when a compound system has gained enough energy to overcome the ridge of the potential, which holds the ensemble together. In the traditional picture, we regard the barrier as a one-dimensional path in the nuclear energy landscape crossing an inner and one or two outer peaks separated by a deep valley, which hosts a meta-stable state called shape isomer. The characteristics of the higher of the inner or outer peaks determine the angular distribution of the final fission fragments relative to the incident neutron beam direction. In the present picture, the angular distribution is considered to be the same for all possible fragmentations at a given nuclear temperature.

One possible explanation is that the outer barrier – in this case the higher of the two barriers – is not one- but, at least, two-dimensional. As a consequence, transition states may have different spin and parity leading to distinct angular distributions with different fission fragment mass and TKE properties. This hypothesis could be tested experimentally by investigating similar fission systems, e.g. 232Th(n,f) and 236U(n,f), for which the new MONNET facility at IRMM will provide excellent conditions.

The joint work performed by the Uppsala University and the JRC-Institute for Reference Materials and Measurements (IRMM) resulted in a PhD-thesis and is a perfect example of the successful European Commission fellowship programme for the training and education of the next generation of researchers.

Read more in: A. Al-Adili et al.: “Fragment mass-, kinetic energy- and angular distributions for 234U(n, f) at incident neutron energies from En = 0.2 MeV to 5.0 MeV“,  Phys. Rev. C 93 (2016) 034603, doi:10.1103/PhysRevC.93.034603

JRC publication: “Fragment mass-, kinetic energy- and angular distributions for 234U(n, f) at incident neutron energies from En = 0.2 MeV to 5.0 MeV