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.

Experimental studies of strongly deformed nuclei

Present experimental nuclear research has three main frontiers. The search for anticipated new phenomena requires the study of exotic nuclear states, where either the excitation energy, the deformation or the so-called isospin takes extreme values. The recently developed, highly-efficient Germanium detector arrays have provided great opportunities for the high-resolution studies of nuclear states associated with extremely elongated nuclear shapes. In the mass region of A=130, more than a hundred superdeformed (with a nuclear axis ratio of 2:1) rotational bands have been identified so far. In this mass region, the centrifugal force related to the fast rotating motion is responsible for the large deformation which can be stabilized by strong nuclear shell effects (like in the case of the atomic shell closures in the noble gases). Contrary to the observations of discrete superdeformed (SD) transitions, the identification of discrete γ rays from hyperdeformed (HD) nuclear states, having an axis ratio of 3:1, represents one of the last frontiers of nuclear physics. Although a large community with such arrays have been searching for HD states in very long experiments, no discrete HD states have been identified so far.

In the actinide (Uranium-Plutonium-Thorium) region, however, the appearance of fission resonances gives a unique possibility for the identification and examination of extremely deformed nuclear states. The appearance of a deep and local 2^nd minimum in the potential energy surface (PES) of the nucleus at large quadrupole deformations (β₂=0.6) is a typical feature of the actinide nuclei. It can be described within the macroscopic-microscopic theoretical framework, in which the liquid drop deformation energy of the fissioning nucleus is superposed by periodically varying shell corrections. The ground state of the 2^nd potential minimum is called the fission isomeric state. So far 33 fission isomers have been identified in the region between U (Z=92) and Bk (Z=97) (''island of fission isomers'') having a decay lifetime in the ps-ms range [2].

Soon after the double-humped fission barrier concept was well-established having two minima in the PES, unexpected features of the fission cross-sections of the light actinides (called ''Thorium-anomaly'') pointed to extend the existing picture and introduce the triple-humped fission barrier concept with a shallow 3^rd (hyperdeformed) minimum in the PES at very large quadrupole and large octupole deformations. Almost two decades later we found experimentally in several measurements on ²³⁴U, on ²³⁶U and most recently on ²³²U, that the 3^rd minimum is in fact as deep as the 2^nd minimum for the uranium isotopes.

Our experimental approach to investigate extremely deformed nuclear states of the light actinides is based on the observation of resonances in the fission cross section. Resonances appear when states in the 1^st minimum energetically coincide with (β vibrational) states in the 2^nd (SD) or in the 3^rd (HD) potential minimum. Observing resonances as a function of the excitation energy caused by resonant tunnelling through excited states in the 3^rd minimum of the potential barrier, allows us to identify the excitation energies of the HD states. Moreover, the observed states can be ordered into rotational bands, with moments of inertia proving that the underlying nuclear shape of these states is indeed a HD configuration. For the identification of the rotational bands, the spins and their projections onto the nuclear symmetry axis (K values) can be obtained by measuring the angular distribution of the fission fragments. Furthermore, by analysing the overall structure of the fission cross section and by fitting it with nuclear reaction code calculations, the fission barrier parameters can be extracted very precisely.

Introduction: a portrait

Welcome Dear Reader! My name is Lorant Csige, and I would like to present my research work which is supported (and financed) by European Union and the State of Hungary, co-financed by the European Social Fund in the framework of TÁMOP-4.2.4.A/ 2-11/1-2012-0001 ‘National Excellence Program’. First, let's start with a short introduction.

I graduated at the University of Debrecen as a Master of Science in Physics in 2004. I had my first contact to nuclear physics in the 4th year of my studies, when I started to work on a student project at the Institute of Nuclear Research of the Hungarian Academy of Sciences (MTA Atomki). My main research interest is the nuclear fission process, and the structure of nuclear states having extreme deformation. I use particle accelerators for the production of such states and to induce fission, and quite some time I have to design, contruct, and build particle detectors and other devices "with my bare hands".

From 2004 to 2009 I was a PhD student at MTA Atomki. I defended my theses in 2009 with the best, summa cum laude qualification. In January 2009 I left Hungary, and the alma mater to be a post-doc at the Ludwig Maximilians University in Munich, Germany. I spent 4 years here, and after another one year at the Max-Plack Institute of Quantum Optics (where I was a research fellow), I came back to Hungary in 2013 with the help of the above mentioned research grant.