{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"291a227025e1d9c6376543299570cb058c4b9fadd0e86c8ba45d00d12f381bb9","created":"2026-10-03T07:18:07Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"5598468e3e774955a86f50a5375614608e637aefb3ab4562bc2b498f65361897","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"nuc.limits-of-existence.oxygen-28-unbound","kind":"phenomenon","literature_status":"partially-resolved","n":"1","parents":[],"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.","posed_since":"1997","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.","problem_ref":null,"references":"","settled_by":"Ab initio continuum calculations reproducing the measured 28O and 26O energies together with the 24O drip line using one interaction.","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.","title":"Why oxygen-28 is unbound despite magic proton and neutron 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