The oxygen-28 (28O) nucleus is of particular interest as, with the Z = 8 (protons) and N = 20 (neutrons) magic numbers, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei.
“The oxygen-28 nucleus has long been of interest as, in the standard shell-model picture of nuclear structure, it is expected to be ‘doubly magic’,” said Tokyo Institute of Technology physicist Yosuke Kondo and colleagues.
“Indeed, it is very well established that for stable and near-stable nuclei, the proton and neutron numbers 2, 8, 20, 28, 50, 82 and 126 correspond to spherical closed shells.”
“Such nuclei represent a cornerstone in our understanding of the structure of the many-body nuclear system.”
“In particular, as substantial energy is required to excite them owing to the large shell gaps, they can be considered, when modeling nuclei in their mass region, as an ‘inert’ core with no internal degrees of freedom.”
In their research, the physicists observed two such nuclei — oxygen-28 and oxygen-27 — through their decay into oxygen-24 with four and three neutrons, respectively.
To do this, they used RIKEN’s RI Beam Factory, which could produce intense beams of unstable nuclei coupled to an active target of thick liquid hydrogen and multi-neutron detection arrays.
Proton-induced nucleon knockout reactions from a high-energy beam of fluorine-29 generated the neutron-unbound isotopes oxygen-27 and oxygen-28.
The researchers observed these isotopes and studied their properties by directly detecting their decay products.
They found that both oxygen-27 and oxygen-28 exist as narrow low-lying resonances and compared their decay energies to the results of sophisticated theoretical models — a large-scale shell model calculation and a newly developed statistical approach — based on effective field theories of quantum chromodynamics. Most theoretical…
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