Neutron‑capture reactions play a central role in the synthesis of elements heavier than iron, proceeding through the slow (s), intermediate (i), and rapid (r) neutron‑capture processes in stellar environments. Despite decades of experimental effort, direct measurements of (n,γ) reactions remain largely restricted to stable or very long‑lived nuclei. Of the thousands of neutron‑rich isotopes involved in astrophysical reaction networks, only about 350 stable and fewer than 20 unstable isotopes have been studied directly. For nuclei with half‑lives of days or seconds, direct measurements are generally considered impossible with existing techniques. Consequently, nucleosynthesis models rely heavily on theoretical predictions, which can differ by orders of magnitude only a few steps away from stability, making neutron‑capture rates one of the dominant uncertainties in nuclear astrophysics.
Heavy‑ion storage rings coupled to radioactive‑beam facilities offer a promising path forward. Over the past decade, the ESR and CRYRING at GSI Darmstadt have demonstrated the power of inverse‑kinematics measurements of astrophysically relevant reaction rates, although so far this approach has been limited to charged‑particle reactions.
In this talk, I will outline two initiatives aimed at enabling direct neutron‑capture measurements in storage rings. The NRING project at CRYRING proposes to install a novel neutron target at an existing storage ring and to establish the required experimental methods. Ultimately, fully exploiting neutron‑capture studies with stored radioactive beams will require a purpose‑built facility. The TRISR project aims to design and, if feasible, install a low‑energy heavy‑ion storage ring with an integrated neutron target at the ISAC‑I facility at TRIUMF. Such a dedicated neutron‑capture storage ring, optimized for the ISAC beam characteristics, is expected to increase the achievable luminosity by up to eight orders of magnitude compared to CRYRING, allowing for the first time to measure directly neutron‑capture reactions on nuclei with half‑lives down to seconds.
[1] Ariel Tarifeño-Saldivia, César Domingo-Pardo, Iris Dillmann, Yuri A. Litvinov, "Direct Neutron Reactions in Storage Rings Utilizing a Supercompact Cyclotron Neutron Target", https://arxiv.org/abs/2508.15465, accepted for Phys. Rev. Acc.and Beams (2026)