Physics colloquium: Brownian Shape Motion in Fission - A marriage between structure & dynamics

Ever since its discovery 85 years ago, nuclear fission has posed serious theoretical challenges but, at the same time, the phenomenon presents unique opportunities to gain insights into the nuclear system. Over the past dozen years or so, the field has experienced a renaissance due to significant improvements in both instrumentation and modeling. This talk focusses on the latter.

Although the nuclear many-body system resembles a (charged) liquid drop, its nucleons have long mean free paths and move in a 'mean' field, occupying specific single-particle orbitals. Consequently, the nuclear system displays both macroscopic and microscopic features.

When the nucleus undergoes a shape change, as during the fission process, the associated change of the mean field agitates the nucleons, thus heating up the system. This mechanism causes the dynamics to be strongly dissipative, so the shape evolution is akin to Brownian motion. The fission process can therefore be simulated as a random walk in the space of nuclear shapes, guided by the shape-dependent density of states.

The resulting treatment, which in effect constitutes a ‘marriage' between macroscopic dynamics and microscopic structure, provides a novel and powerful theoretical tool for calculating a variety of fission observables. For the first time, the fission fragment mass (and charge) yields can now be fairly reliably calculated for any fissionable nucleus. Moreover, when the shape-dependent microscopic structure in the density of states is incorporated, more detailed features can be understood, such as the possibility of a non-monotonic energy dependence of the mass yields and the sawtooth fragment-mass dependence of the mean neutron multiplicity.