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Whether it comes from nuclear deformation (nuclei becoming non-spherical) late in the r-process or from fission fragments is not known.","posed_since":"1997","precise":"Determine whether the r-process rare-earth peak at $A$ about 160 forms by trapping of material at deformed nuclei near $N$ about 104 during freeze-out (Surman et al., 1997) or by deposition of fission fragments, and identify the nuclear properties (masses and half-lives for $Z = 60\\ \\text{to}\\ 66, N = 100\\ \\text{to}\\ 110$) that control it. Answer: the mechanism and the specific nuclei whose properties set the peak.","problem_ref":null,"references":"","settled_by":"Mass and half-life measurements across the neutron-rich lanthanides at FRIB and CARIBU, fed into network calculations for merger and collapsar conditions.","status_note":"Mass measurements of neutron-rich lanthanides at Argonne since about 2016 began to test the deformation mechanism without deciding between the two pictures as of 2026.","title":"Origin of the rare-earth abundance 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