The study of physical systems under extreme conditions allows scientists to gain valuable insights into their organization and structure. In nuclear physics, the examination of neutron-rich isotopes provides crucial tests for modern nuclear structure theories. These isotopes, characterized by a significantly different neutron-to-proton ratio compared to stable nuclei, exist as short-lived resonances and decay through spontaneous neutron emission. A recent study published in Nature reports the first observation of two such isotopes, oxygen-28 (28O) and oxygen-27 (27O), as they decay into oxygen-24 with four and three neutrons, respectively.
The nucleus 28O, consisting of eight protons and 20 neutrons, is particularly interesting as it is expected to be one of the few ‘doubly magic’ nuclei according to the standard shell-model picture of nuclear structure. The international collaboration of researchers, led by Yosuke Kondo from the Department of Physics at Tokyo Institute of Technology, achieved this groundbreaking observation by utilizing the RIKEN RI Beam Factory. This facility generated intense beams of unstable nuclei linked to an active target of thick liquid hydrogen and multi-neutron detection arrays. The team employed proton-induced nucleon knockout reactions from a high-energy 29F beam to produce the neutron-unbound isotopes 27O and 28O. Through direct detection of their decay products, they were able to observe these isotopes and study their properties.
Comparing Experimental Results and Theoretical Predictions
The study’s researchers compared the decay energies of 27O and 28O to the predictions of sophisticated theoretical models, including a large-scale shell model calculation and a newly developed statistical approach based on effective field theories of quantum chromodynamics. Interestingly, most theoretical approaches initially anticipated higher energies for both isotopes. This disparity between experimental results and theoretical expectations highlights the need for further refinement in understanding nuclear structure.
Insights into Neutron-rich Nuclei and the Island of Inversion
The investigation also involved the cross-section for the production of 28O from the 29F beam. The results indicated that 28O does not exhibit a closed N = 20 shell structure, contradicting previous assumptions. This finding suggests that the ‘island of inversion,’ where the energy gap between neutron orbitals weakens or disappears, extends beyond fluorine isotopes 28F and 29F and includes oxygen isotopes. Dr. Kondo, the lead researcher, emphasizes the importance of these findings in providing new insights into interactions considered in “ab initio” approaches, particularly through the valuable constraints offered by the energies of 27O and 28O.
By offering new insights into nuclear structure, the discovery of oxygen-28 and oxygen-27 resonances contributes to our overall understanding of extremely neutron-rich nuclei. This breakthrough also paves the way for detailed investigations into multi-neutron correlations and the exploration of other exotic systems using the multi-neutron-decay spectroscopy technique employed in this study.
The observation of neutron-rich isotopes oxygen-28 and oxygen-27 and the subsequent comparison of their properties with theoretical models provide significant advancements in nuclear physics. This research sheds light on the organization and structure of physical systems under extreme conditions, broadening our understanding of nuclear structure and enabling further investigations into exotic systems.