Nuclear shell evolution towards the terra incognita

The most attention in current nuclear physics studies, therefore, is devoted to the understanding of the limits of existence for neutron-rich nuclei and our task is not only to prove their existence but to produce these short-lived exotic species sufficiently enough to be able to study their nuclear properties via experimental techniques which have only been possible for stable nuclei since 1950s. From the theoretical point of view, the main interest in studying nuclei lying near the neutron drip-line has been generated by the fact that nuclear shell structure will differ from those of normal nuclei lying close to the valley of stability due to the loosely bound neutrons. Any significant changes in proton or neutron single-particle energies can even result with new magic numbers instead of the traditional ones (i.e. 2, 8, 20, 28, 50, 82, and 126) thus, with so-called "Nuclear Shell Evolution". The present project is mainly focused on studying Nuclear Shell Evolution in the 78Ni mass region. Among all doubly-magic nuclei along the nuclear chart, 78Ni (with Z=28 and N=50) represents a unique stepping stone towards the physics of extremely neutron-rich nuclei because of its extreme neutron-to-proton ratio (N/Z ~ 1.8) thus, the focus of large number of experimental and theoretical efforts. When moving from the stable 68Ni (Z=28, N=40) isotope to 78Ni (Z=28, N=50) in the nickel chain, due to the increasing of neutrons it is predicted that proton single-particle energies will be modified or even some will be inverted within the same shell. Furthermore, the Z=28 shell gap is predicted to be smaller as a result of the increasing effect of the tensor interaction, one of the non-central components of the nucleon-nucleon interaction (i.e. nuclear force). However, the shell structure in the Ni chain is less known due to its large N/Z ratio, thus difficult to reach via existing spectroscopic techniques. In such context, neutron-rich Cu nuclei with one proton outside of the Z = 28 shell and lying between N = 40 and N = 50 shells, i.e. 69-79Cu, emerge as crucial probes. Characterization of their excited levels and comparison with shell model predictions will provide the experimental basis to search for changes into the shell structure around 78Ni, allowing to explore the predictive power of nuclear interactions. Significantly less is known on the excited states of Cu isotopes starting from 75Cu up to 79Cu. We propose to study the nuclear properties of the neutron-rich 75Cu (Z=29, N=46) and 77Cu (Z=29, N=48) isotopes towards the N = 50 shell, which represents a particular case just a step before 78Ni. The experimental study is being performed at RIBF-RIKEN radioactive beam facility in Japan. With its largest production rates of neutron-rich species via fragmentation or fissioning of different projectiles at very high intensities, RIKEN is the world-leading laboratory at present, pushing the borders of neutron-rich nuclei in the "Terra incognita" region. Different experimental approaches have been currently applied in order to investigate the shell structure along the Cu chain. The excited states up to 4 MeV in the 75,77Cu nuclei have been populated first time in the present work via beta decay of 75,77Ni. Main outcome of the study showed that the Z=28 shell gap is reduced less drastically than previously thought and the work has been recently published as a letter in the most prestigious nuclear physics journal after Nature (E. Sahin et al. Phys.Rev.Lett. 118, 242502 (2017)). A second experimental program in order to identify the single-particle nature of the excited states in the Cu nuclei is being conducted via direct reaction mechanism within SEASTAR large international collaboration at RIKEN. Finally, the collective properties in 77Cu in connection with the 76Ni nucleus, which is presently unaccessible due to the experimental limitations will be studied by a third experimental campaign at RIKEN within the SUNFLOWER collaboration. The reaction mechanism to populate the excited states of 77Cu is coulomb excitation at intermediate energy of 200 MeV/nucleon. The analysis of these various data sets are currently ongoing and the results will be published in several high-rank scientific journals in 2019.

The physics of exotic nuclei constitutes a diverse research field from understanding fundamental properties of a very complex, quantum mechanical system to how the universe was created and developed to where it is today. Any discovery about the nature of subatomic matter will enable a new understanding of our world. The contribution of EVOLUTION is to investigate important characteristics of the structure of nuclei lying far away from stability, in particular near the 78Ni mass region. EVOLUTION constitutes a unique example of obtaining experimental information on very exotic nuclei using gamma-ray spectroscopic techniques which were available only for the stable nuclei in the past. The obtained data using radioactive ion beams greatly revealed the shell evolution and the magicity of the proton Z=28 shell gap in the present research project. Experimental results were delivered to the nuclear theory and greatly contributed to a better description of the structure of very exotic nuclei.

The present research project is focused on investigating the shell evolution of neutron-rich nuclei near 78Ni. Information on the shell structure in this mass region is limited due to the fact that 78Ni is the most exotic doubly-magic nucleus nucleus with its largest neutron-to-proton ratio, ~1.79. In another word, these nuclei are shorter-lived and are located much further from the stability line thus more difficult to access experimentally. Fragmentation or fission of stable projectiles is presently the only approach to observe limits of existence in these nuclei. The production of these nuclei via fragmentation or fission is followed by their in-flight separation and detection in less than 1 micro seconds time range. There are several radioactive-beam facilities in the world applying this technique: GANIL (France), GSI (Germany), NSCL-MSU (USA), and RIBF-RIKEN (Japan). However, due to the long-term upgrading purposes, only the laboratories at NSCL-MSU, USA and RIBF-RIKEN, Japan are currently available. Therefore, I am planing to perform the experimental activity, described detailed in the ?Project Description?, in the international large-scale facilities EURICA/DALI2 and BigRIPS at RIKEN and GRETINA and A1900 at NSCL-MSU. Accurate theoretical calculations from different perspectives in order to define the physics results of the proposal will be an irrefutable gain of the collaboration during the project working plan period. In this context, I have established fruitful collaborations with leading theorists in the field of nuclear structure research, in particular with Morten Hjorth-Jensen, who has developed techniques that are highly relevant for the interpretation of experiments along the lines of those addressed in the present proposal. Finally, the presented research project will extend the state of the art and greatly contribute to the scientific excellence and competition.