Nuclear Structure

Nuclear Structure
Contact persons: Constantin MIHAI

The atomic nucleus is a complex quantum mechanical system that lacks a deep theoretical understanding to describe its structure in a fundamental way. The many-body interacting building blocks of the nucleus, protons and neutrons, give rise to a very rich variety of phenomena, from single particle characteristics to collective features, with a strong interplay between the two descriptions. Although an underlying unitary mechanism that accounts for all the observed nuclear phenomena must be present, it is difficult to comprehend it and to embody into a single theoretical description. Therefore, researchers all over the world are hoping that by providing a detailed experimental knowledge of as many nuclei as possible in different regions of the nuclear chart, one can provide a final understanding of the nuclear structure.

In the Department of Nuclear Physics, we perform a wide range of experimental investigations, using both the local infrastructure and various facilities around the world in different international collaborations. The local program is concentrated on detailed studies of the nuclear shapes, which includes the identification of shape isomers, shape coexistence and nuclear deformation. One of the most successful idea that was investigated in our laboratory was the first identification of a shape isomer in 66Ni using gamma spectroscopy and heavy-ion transfer reactions below the Coulomb barrier [1]. This discovery was inferred from lifetime measurements of the first three excited 0+ states, pointing to oblate, spherical, and prolate nature of these excitations. The confirmation of this new paradigm in light nuclei has attracted a considerable interest from the international community and several experimental studies are ongoing or expected to be performed in the near feature in order to identify this phenomenon in nearby nuclei.

The investigations of collective phenomena are carried out intensively in our laboratory. The main tool that we have developed is that of lifetime measurements, observables that are closely related to the determination of the reduced matrix elements of nuclear transitions. These quantities are very sensitive to details of the nuclear structure and their knowledge is crucial for testing different theoretical approaches that often provide contradictory values. Therefore, we have employed a wide range of experimental techniques to cover the relevant interval for lifetimes of excited states in nuclear systems: the fast-timing technique [2], the Recoil Distance Doppler Shift (RDDS) method (or plunger method) [3] and the Doppler Shift Attenuation Method (DSAM) [4].

Other studies are also performed at several international facilities (FAIR, ILL, ISOLDE, GANIL, IPN Orsay, FLNR-IUCN, PARIS/AGATA) using a variety of reaction types, including Coulomb excitation, beta decay, proton and neutron beta-delayed reactions, thermal neutron capture or fission reactions. One prominent example is the experimental study of dipole resonances performed at iThemba Labs using inelastic scattering reactions of light particles. These studies are focusing on the study of Pygmy Dipole Resonances (PDR) and of the Giant Dipole Resonance (GDR) and provide complementary information to the ones obtain via inelastic scattering of gamma rays, similar with the ones envisaged to be obtained at the future ELI-NP facility.

All these investigations of nuclear structure performed by various groups around the world provide a massive volume of experimental data. The scientific community has therefore gathered all the relevant information into the Evaluated Nuclear Structure Data File (ENSDF) database which is maintained by the Nuclear Structure and Decay Data (NSDD) network of evaluators. In our department it was established a new Data Center responsible with the evaluation of nuclei with mass numbers A=57, 58, 59, 117, 118, and 119 [5].

[1] S. Leoni et al., Phys. Rev. Lett. 118, 162502 (2017).
[2] J. Wiederhold et al., Phys. Rev. C 99, 024316 (2019).
[3] F. L. B. Garrote et al., Phys. Rev. C 97, 064310 (2018).
[4] M. Ionescu-Bujor et al., Phys. Rev. C 98, 054305 (2018).
[5] A. Negret, B. Singh, Nuclear Data Sheets 124, 1-156 (2015).