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It has been mapped experimentally only up to neon, element 10.","posed_since":"","precise":"For $Z=11 \\text{ to } 20$ (sodium to calcium), determine the most neutron-rich isotope bound against one- and multi-neutron emission. 31F and 34Ne are the heaviest bound F and Ne isotopes (Ahn et al., PRL 2019), and 39Na has been observed as bound. Answer: the drip-line nucleus for each $Z$ from Na to Ca, with experimental evidence that the next isotopes are unbound.","problem_ref":null,"references":"","settled_by":"Production cross-section measurements at FRIB at full beam power and at the RIKEN RIBF establishing non-observation limits for the next isotopes.","status_note":"RIKEN reported 39Na as bound (around 2022-2023); the drip line for $Z \\ge 11$ remained unestablished as of 2026.","title":"Location of the neutron drip line beyond 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