A schematic of the fission path that occurs during the photofission of an even-even nucleus when excited to near-barrier energies. The double-humped barrier is depicted as predicted for mass-symmetric fission by the Strutinsky hybrid model.

A Fission Introduction

The absorption of a photon by a heavy, even-even causes an excitation to an unstable, low-spin state. The states will subsequently undergo a nuclear reaction or decay to reach a more stable configuration. One likely reaction available to heavy nuclei is fission, the splitting of the original nucleus into two distinct nuclei, also known as fragments. Fission occurs when the nuclear shape deforms to the point when the Coulomb repulsion overcomes the nuclear surface tension; the nucleus is then unbound and splits into two nuclei, referred to as fragments. It is convenient to frame the process in the language of energy transformation because the changing surface tension is associable with a changing deformation energy. Fission then takes place after the nucleus has transformed all or some of its initial excitation energy into deformation energy.

As displayed in the figure, the deformation energy is strongly dependent on the shape of the nucleus, shown here as the deformation parameter β. This double-humped fission barrier is predicted by the Strutinsky hybrid model, a model that corrects the deformation energy computed as if the nucleus were a liquid drop for quantum mechanical effects, and sheds light on the path taken by a nucleus to reach fission. The nucleus, originally excited in the first potential well, reaches the point of instability, the saddle point, following complicated nuclear dynamics that are not fully understood. However, at the saddle point, the conservation total angular momentum provides a foothold for the understanding of the impending fission.

Fission Angular Distributions

The description of the nucleus at the saddle point is credited to the angular momentum quantum numbers, J, K, and M. Together these describe the orientation of the nuclear symmetry axis, which is important in understanding the trajectory of the fragments following scission, the point at which the fragments separate. The fragments are understood to repel each other along this symmetry axis and thus knowledge of its orientation gives direct information of the angular distribution of the fission fragments. The direct connection between the angular distribution and the description of the transition state makes the understanding of the fission process by studying the angular distributions feasible.

The following is plotted based on the data presented in N.S. Rabotnov et al.. Yad. Fiz. 11, 508 (1970).

It was postulated by Aage Bohr that when the nucleus absorbs a photon with energy near the fission barrier height the majority of the excitation energy would be converted into deformation energy. As a result, the nucleus at the saddle point is thermodynamically cold and should have a low level density. The fission process then only proceeds through a relatively few number of states that each have a distinct angular distribution associated with them. If fission through one of these states be more probable than the others, its characteristic angular distribution will be pronounced. In addition, if the number of states is sufficiently small, the individual contributions to the composite angular distribution from each state shouldn't be diluted to the point of unrecognition. The goal of fragment angular distributions is then to measure and fit the angular distribution to extract observables and then disentangle from it the relative contributions of each state involved. In this manner, insight into the transition state can be gained.

The experimental arrangement of 4 silicon strip detectors around a target.

One of six Micron Semiconductor type YY1 silicon strip detectors used in the experiment.

Fission Fragment Detection

A recent experiment at UNC seeking to measure these fragment angular distributions was conducted using the High-Intensity γ-ray Source at TUNL. Silicon strip detectors were position about a bulk target as shown in the figure to measure yields with respect θ, the polar angle measured with respect to the beam axis, and φ, the azimuthal angle measured with respect to the polarization plane. Though these angular distributions have been studied in the past, never before have they been measured with a fully linearly-polarized photon beam as they have by our group.