Photofission studies of extremely deformed nuclear states: a new era in nuclear physics

Photofission measurements enable selective investigation of the extremely deformed nuclear states in the light actinides and can be utilized to better understand the landscape of the multiple-humped potential energy surface in these nuclei. The selectivity of these measurements originates from the low and reasonably well-defined amount of angular momentum transferred during the photoabsorption process. So far, fission resonances have been studied primarily in light-particle-induced nuclear reactions. These studies do not benefit from the same selectivity found in photonuclear excitation and consequently they are complicated by statistical population of the states in the 2^nd and 3^rd minima with a very limited probability of. These measurements have also suffered from dominating prompt-fission background.

Until now, sub-barrier photofission experiments have been performed only with bremsstrahlung photons and have determined only the integrated fission yield. In these experiments, the fission cross section is convolved with the spectral intensity of the photon beam, resulting in a typical effective γ-ray bandwidth ΔE/E between only 300-400 keV. These experiments observe a plateau, referred to as the "isomeric shelf", in the fission cross section, presumably as a result of the competition between prompt and delayed photofission. However, due to the lack of high resolution photofission studies in the corresponding energy region (E≈4-5 MeV), no experimental information exists to confirm this concept.

Higher-resolution studies could be performed at tagged-photon facilities (e.g. NEPTUN at Darmstadt, Germany), though only with marginal statistics, due to the limited beam intensities realizable through tagging, ~10⁴ γ/(keVs) [13]. This beam intensity cannot be significantly improved, since it is determined by the random coincidence contribution in the electron-tagging process. Thus, high statistics photofission experiments in the deep sub-barrier energy region, where cross sections are typically as low as σ=1 nb-10 μb, cannot be performed with tagged-photon beams.

The relatively recent development of inverse-Compton scattering γ-ray sources, capable of producing tunable, high-flux, quasi-monoenergetic γ-ray beams by Compton-backscattering of eV-range photons off a relativistic electron beam, offers an opportunity to overcome previous limitations. It has to be emphasized that a measurement of the photofission cross section in the deep sub-barrier energy region will be a crucial step towards a reliable characterization of the PES, including unambiguous determination of the double- or triple-humped nature of the surface and precise evaluation of the barrier parameters.

Currently, the most intense Compton-backscattered γ-ray source is the High Intensity γ-ray Source (HIγS) at the Duke University (USA) with a bandwidth of ΔE=150-200 keV and a spectral flux of 10² γ/(eVs). Next-generation Compton-backscattering γ-ray sources, such as the upgrade of HIγS (HIγS2), and the Extreme LIght Infrastructure - Nuclear Physics (ELI-NP, Bucharest, Romania), are anticipated to provide beams with spectral fluxes up to ~10⁶ γ/(eVs) and energy resolutions down to ΔE~1 keV, far superior to those currently available at presently available γ sources. The capabilities of these next-generation sources allow one to aim at an identification of low-amplitude fission resonances. The narrow energy bandwidth expected for the new γ beam facilities will also allow for a significant reduction of the presently dominant background from non-resonant processes. Thus, next-generation γ-ray sources are expected to enable observation of the fine structure in the isomeric shelf. This may open the perspective towards a new era of photofission studies.