Strong Neutron Pairing in core+4n Nuclei.

Area of Science:

• Nuclear Physics
• Exotic Nuclei Research
• Quantum Correlations

Background:

• Investigating neutron-rich isotopes is crucial for understanding nuclear structure and the limits of nuclear stability.
• Neutron pair emission is a key decay mode that provides insights into nuclear correlations.
• The N=12 isotones, such as Carbon-18 and Oxygen-20, represent interesting systems for studying the effects of excess neutrons.

Purpose of the Study:

• To investigate the mechanism of neutron pair emission from neutron-rich N=12 isotones, specifically Carbon-18 and Oxygen-20.
• To analyze the correlations between emitted neutrons to understand the underlying nuclear structure.
• To explore the role of valence neutrons and nuclear core structure in two-neutron emission.

Main Methods:

• Utilized high-energy nucleon knockout reactions from secondary beams of Nitrogen-19 and Oxygen-21.
• Populated unbound states of Carbon-18 and Oxygen-20 up to 15 MeV above their two-neutron emission thresholds.
• Analyzed triple fragment-neutron-neutron correlations to determine decay mechanisms.

Main Results:

• The decay of Carbon-18 (from Nitrogen-19 knockout) is dominated by direct neutron pair emission, with the largest observed two-neutron correlation strength.
• This strong correlation suggests a $^{14}$C core with four valence neutrons arranged in strongly correlated pairs.
• The decay of Oxygen-20 (from Oxygen-21 knockout) shows significant competition between direct emission and sequential decay, attributed to the knockout of a deeply bound neutron.

Conclusions:

• Direct neutron pair emission is a dominant process in neutron-rich systems like Carbon-18.
• The observed correlations provide evidence for cluster-like structures and strong pairing correlations in the outermost valence neutrons.
• The difference in decay mechanisms between Carbon-18 and Oxygen-20 highlights the influence of the nuclear core and the nature of the knocked-out nucleon on the decay pathway.