Photofission Studies: Past and Future

Abstract

Employing electromagnetic probes, in particular high-energy γ photons, offers an outstanding advantage over neutron- or particle-induced reactions in the investigation of the fission process. Photon-induced reactions have well-established selection rules and thus can be characterized by a high spin selectivity. In contrast to the generally very complex excitation spectra of the compound nuclei in particle-induced fission, the interpretation of photofission is rather simple due to the predominant E1 absorption of γ photons. Besides, in neutron-induced fission reactions, a lower limit on the excitation energy of the fissioning system is set by the released separation energy, which could be overcome by the use of transfer reactions, e.g., (d,pf) and (³He,df). However, these coincidence measurements generally suffer from a dominant prompt fission background. With photons, none of these limitations occur, which triggered the scientific community to perform low-energy photofission experiments already in the early 1960s. Such measurements were performed primarily with bremsstrahlung radiation and its variants with high flux, but with very large energy uncertainty. Very recently, photonuclear and photofission physics has just entered a new era by the development of Compton-backscattered γ-ray sources offering an unprecedented tool to investigate the fission process both with high flux and at the same time with high-energy resolution.