Nucleus ^{26}O: A Barely Unbound System beyond the Drip Line

Area of Science:

• Nuclear Physics
• Nuclear Structure
• Exotic Nuclei

Background:

• Investigating nuclei beyond the neutron drip line is crucial for understanding nuclear forces.
• Neutron-rich oxygen isotopes provide a unique laboratory for studying nuclear structure phenomena.

Purpose of the Study:

• To investigate the unbound nucleus ^{26}O.
• To determine the energy of the ^{26}O ground-state resonance relative to the decay threshold.
• To search for excited states in ^{26}O.

Main Methods:

• Invariant-mass spectroscopy was employed.
• A one-proton removal reaction from a ^{27}F beam at 201 MeV/nucleon was used.
• Decay products ^{24}O and two neutrons were detected in coincidence using the SAMURAI spectrometer.

Main Results:

• The ^{26}O ground-state resonance was observed 18±3(stat)±4(syst) keV above the decay threshold.
• A new excited state, likely the first 2^{+} state, was identified at 1.28_{-0.08}^{+0.11} MeV above threshold.
• Experimental data were compared with theoretical predictions.

Conclusions:

• The proximity of the ^{26}O ground state to the threshold indicates a weakly bound system.
• Three-nucleon forces, pf-shell intruder configurations, and continuum effects are essential for describing neutron-rich oxygen isotopes.
• This study advances our understanding of nuclear structure at the limits of stability.