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 the feasibility by incorporating readily available technologies. Using a commercial supercompact cyclotron delivering a proton beam of 130 $μ$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 Darmstadt. This autonomous accelerator-target assembly design enables deployment at both, in-flight and ISOL facilities, to exploit their complementary production mechanisms. Potential upgrades based on higher-energy and/or higher-current cyclotrons will 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 short-lived nuclei, thereby opening a new avenue for understanding the synthesis of heavy elements in our universe.

Figures (13)

• Figure 1: Cross-sectional schematic showing the storage-ring beam pipe ($\varnothing$ 100 mm) and the cyclotron beam pipe ($\varnothing$ 50 mm) with a 5 mm gap between them. The $^9$Be$(p,xn)$ neutron source is positioned 25 mm from the cyclotron beam pipe end cap. The central scoring region ($\varnothing$ 10 mm) indicates the Monte Carlo simulation volume for neutron density calculations.
• Figure 2: Cross-sectional schematic of the single-material moderator configuration studies. The moderator consists of a 200 cm × $a$ × $a$ solid parallelepiped containing both beam pipe assemblies, with $a$ systematically varied to determine optimal dimensions for each material.
• Figure 3: Monte Carlo calculation results for single-material moderator design. The dotted black line represents the threshold areal density for a compact and cost-effective moderator.
• Figure 4: Cross-sectional schematic for the moderator-reflector configuration studies. The neutron target assembly consists of a 200 cm × ($a+2R_s$) × ($a+2R_s$) solid parallelepiped containing both beam pipe assemblies, with $a$, $l$, and $R_s$ systematically varied to determine optimal dimensions for each material.
• Figure 5: Monte Carlo optimization results for moderator/reflector combinations keeping $l=200$ cm. Blue contours represent the asymptotic optimization threshold of $9 \times 10^{-7}$ n/cm$^2$/prim for 5, 7, and 10 MeV proton energies. Gray shaded regions show configurations exceeding the threshold for D2O + graphite and BeO + graphite combinations across the parameter space $a = 40–120$ cm and $R_s = 20–60$ cm.