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Title: Why oxygen-28 is unbound despite magic proton and neutron numbers
Plain: Oxygen-28 has 8 protons and 20 neutrons, both 'magic' numbers that usually make nuclei extra stable, yet it has been known since the late 1990s not to hold together; a 2023 experiment observed it directly as a short-lived state. Explaining this tests how shell structure changes far from stable nuclei.
Precise: 28O was observed in 2023 as a state unbound with respect to $24\mathrm{O} + 4\mathrm{n}$ (Kondo et al., Nature 620, 965, 2023), without evidence of an $N = 20$ shell closure; fragmentation searches had found no bound 28O since 1997. Determine the mechanism (continuum coupling, 3N forces, deformation, intruder pf-shell configurations) that removes $N = 20$ magicity and places the oxygen drip line at 24O. Answer: an ab initio calculation with quantified uncertainty reproducing the 28O decay energy and 24O as the last bound isotope, with the dominant contribution identified.
Settled by: Ab initio continuum calculations reproducing the measured 28O and 26O energies together with the 24O drip line using one interaction.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
Status note: Three-nucleon forces explain the drip line at 24O (Otsuka et al., 2010); ab initio calculations did not agree on the structure of 28O relative to the 2023 data.
Posed since: 1997