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arXiv · 2610.11095

Production Memory of the $^{225}\mathrm{Ra}$-$^{225}\mathrm{Ac}$ Clock in Neutron-Star Merger Ejecta

Hisham Anwer
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Abstract

Radioactive nuclei in neutron-star merger ejecta carry information about both their abundance and their production history. Using the $^{225}\mathrm{Ra}$-$^{225}\mathrm{Ac}$ sequence, we establish exact conditions linking a surviving isotope partition to the time available for parent decay. An Ac fraction of 0.1 produced solely through Ra decay requires at least 2.456 days, providing a direct consistency test for proposed source states. We show that subsequent activity retains additional production memory: two histories with the same mean birth time can produce different response crossings. This distinction is captured analytically and separates mean chronology from the influence of the production-time distribution. A controlled radiation calculation then shows how deposition and photon escape transform the nuclear contrast into a common light-curve pivot for fixed ejecta properties. The results connect nuclear production chronology to late-time merger emission and provide benchmarks for nucleosynthesis and radiation-transport calculations.

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Hisham Anwer. 2026-10-08. Production Memory of the $^{225}\mathrm{Ra}$-$^{225}\mathrm{Ac}$ Clock in Neutron-Star Merger Ejecta. https://arxiv.org/abs/2610.11095

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