Neutron Economy and Freeze-out Dynamics in the Late-Time Cold r-Process
Abstract
Standard r-process models often treat the freeze-out phase as a passive, uniform smoothing of the global abundance pattern. We demonstrate, however, that the final pattern is instead actively sculpted by a dynamic ``neutron economy'' that operates well after the free neutron source is exhausted. By analyzing cold neutron star merger outflows, we identify a distinct neutron exporter-importer relationship: the second-peak region (A ∼ 130) transitions early into a β-decay dominated regime, acting as a net exporter, while the third-peak (A ∼ 195) and Rare Earth (A ∼ 164) regions remain capture-dominated neutron importers. Targeted simulations reveal that the final peak structures emerge from a competition between two opposing forces: a global ``push'' driven by neutron capture shifting mass toward heavier nuclei, and a local ``pull'' from beta delayed neutron emission returning material back toward lighter masses. We identify 136Pd, 135Ag, and 130Ru as the primary exporters driving this economy, while third-peak fine structure is governed by N=126 isotones (Sm to Tb). Physically, this transport reflects a fundamental drive toward shell closure, where nuclei that overshoot the N=82 closed shell shed mass to migrate back toward this stable configuration, while nuclei in the third peak capture these redistributed neutrons to anchor themselves at the N=126 shell. Crucially, our models overproduce the odd-even staggering in the third peak compared to solar data. This discrepancy suggests current models likely overestimate the β-delayed neutron emission probabilities (Pn) for these N=126 nuclei, highlighting them as high-priority targets for theoretical refinement and future FRIB experiments.
Versions
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BibTeX
@misc{li2026neutron,
title = {{Neutron Economy and Freeze-out Dynamics in the Late-Time Cold r-Process}},
author = {Mengke Li and Bradley Meyer},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.31465},
eprint = {2609.31465},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.31465},
}
RIS
TY - UNPB
TI - Neutron Economy and Freeze-out Dynamics in the Late-Time Cold r-Process
AU - Mengke Li
AU - Bradley Meyer
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.31465
UR - https://arxiv.org/abs/2609.31465
AB - Standard r-process models often treat the freeze-out phase as a passive, uniform smoothing of the global abundance pattern. We demonstrate, however, that the final pattern is instead actively sculpted by a dynamic ``neutron economy'' that operates well after the free neutron source is exhausted. By analyzing cold neutron star merger outflows, we identify a distinct neutron exporter-importer relationship: the second-peak region (A ∼ 130) transitions early into a β-decay dominated regime, acting as a net exporter, while the third-peak (A ∼ 195) and Rare Earth (A ∼ 164) regions remain capture-dominated neutron importers. Targeted simulations reveal that the final peak structures emerge from a competition between two opposing forces: a global ``push'' driven by neutron capture shifting mass toward heavier nuclei, and a local ``pull'' from beta delayed neutron emission returning material back toward lighter masses. We identify ^136Pd, ^135Ag, and ^130Ru as the primary exporters driving this economy, while third-peak fine structure is governed by N=126 isotones (Sm to Tb). Physically, this transport reflects a fundamental drive toward shell closure, where nuclei that overshoot the N=82 closed shell shed mass to migrate back toward this stable configuration, while nuclei in the third peak capture these redistributed neutrons to anchor themselves at the N=126 shell. Crucially, our models overproduce the odd-even staggering in the third peak compared to solar data. This discrepancy suggests current models likely overestimate the β-delayed neutron emission probabilities (P_n) for these N=126 nuclei, highlighting them as high-priority targets for theoretical refinement and future FRIB experiments.
ER -