Direct Neutron Reactions in Storage Rings Utilizing a Supercompact Cyclotron Neutron Target

Abstract: We propose a new approach for a high-density free-neutron target, primarily aimed at nuclear astrophysics reaction studies in inverse kinematics with radioactive ions circulating in a storage ring. The target concept integrates four key subsystems: a neutron production source driven by a supercompact cyclotron utilizing $^9$Be($p,xn$) reactions, an optimized moderator/reflector assembly using either heavy-water or beryllium oxide with a graphite reflector shell to thermalize fast neutrons, a cryogenic liquid hydrogen moderator to maximize thermal neutron density in the interaction region, and beam pipe geometries that enable neutron-ion interactions while maintaining vacuum conditions for ion circulation. This integrated approach focuses on feasibility by incorporating readily available technology. Using a commercial supercompact cyclotron delivering 130 $\mu$A, the design achieves thermal neutron areal densities of $\sim3.4\times10^{6}$ n/cm$^2$ for a proof-of-concept demonstrator at the CRYRING ion-storage ring at GSI. This autonomous accelerator-target assembly design enables deployment at both in-flight and ISOL facilities to exploit their complementary production yields. Potential upgrades based on higher-energy and/or higher-current isochronous cyclotrons should enable an increase in areal density to $\sim$10$^9$ n/cm$^2$. In combination with a customized low-energy storage ring and a radioactive ion-beam facility, the proposed solution could deliver luminosities above 10$^{23}$ cm$^{-2}$ s$^{-1}$, thereby enabling neutron capture measurements of $\sim$mb cross sections within a few days of experiment. The proposed system represents a significant milestone towards enabling large neutron-capture surveys on exotic nuclei, thereby opening a new avenue for understanding the synthesis of heavy elements in our universe.