The study of physical systems under extreme conditions provides valuable insights into their organization and structure. Neutron-rich isotopes, particularly the light ones with a neutron-to-proton ratio significantly different from stable nuclei, serve as stringent tests for current nuclear structure theories. Recently, an international collaboration of researchers led by Yosuke Kondo from the Department of Physics at Tokyo Institute of Technology published a groundbreaking study in Nature, reporting the first observation of two neutron-rich isotopes: oxygen-28 (28O) and oxygen-27 (27O). These isotopes decay into oxygen-24 with four and three neutrons, respectively.
To successfully observe the isotopes 28O and 27O, the researchers utilized the capabilities of the RIKEN RI Beam Factory. This facility could produce intense beams of unstable nuclei, coupled with an active target of thick liquid hydrogen and multi-neutron detection arrays. By employing proton-induced nucleon knockout reactions from a high-energy 29F beam, the researchers generated the neutron-unbound isotopes 27O and 28O. They then observed and studied their properties by directly detecting their decay products.
The study revealed the existence of both 27O and 28O as narrow low-lying resonances. The decay energies of these isotopes were compared to the results 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 predicted higher energies for both isotopes. This discrepancy highlights the significance of experimental observations in providing valuable constraints for interactions considered in theoretical models.
Implications for Nuclear Structure
The discovery of these neutron-rich isotopes has significant implications for our understanding of nuclear structure, especially for extremely neutron-rich nuclei. The nucleus 28O, composed of eight protons and 20 neutrons, is particularly fascinating as it is expected to be one of the few “doubly magic” nuclei according to the standard shell-model picture of nuclear structure. The study’s findings suggest that the energy gap between neutron orbitals weakens or vanishes, extending beyond the fluorine isotopes 28F and 29F into the oxygen isotopes. This phenomenon, known as the “island of inversion,” had not been observed in oxygen isotopes until now.
The successful observation and analysis of these neutron-rich isotopes open up new possibilities for further research. The detailed investigation of multi-neutron correlations and the study of other exotic systems become feasible with the multi-neutron-decay spectroscopy technique employed in this study. Additionally, the energies of 27O and 28O, obtained through experimental analysis, can provide valuable constraints for the interactions considered in “ab initio” approaches, as suggested by statistical coupled-cluster calculations.
The study conducted by Yosuke Kondo and his international team of researchers represents a significant advancement in the study of neutron-rich isotopes. Through their innovative experimental design and analysis, they were able to observe and study the decay properties of two neutron-rich oxygen isotopes. The findings not only enhance our understanding of nuclear structure, particularly for extremely neutron-rich nuclei, but also pave the way for future investigations into multi-neutron correlations and other exotic systems. This research exemplifies the importance of experimental observations in refining and validating theoretical models in nuclear physics.