Underground nuclear astrophysics: Status and recent results from Felsenkeller laboratory

TL;DR

The paper surveys underground nuclear astrophysics with a focus on the Felsenkeller 5 MV shallow-underground laboratory, detailing its shielding, accelerator, and detector setup that suppress backgrounds to levels competitive with deep underground facilities. It reports on ongoing measurements of key reactions relevant to Big Bang Nucleosynthesis and solar hydrogen burning, including $^3$He($\alpha$,$\gamma$)$^7$Be, $^2$H($p$,$\gamma$)$^3$He, and $^{12}$C($p$,$\gamma$)$^{13}$N, highlighting how the facility’s low-background environment and diverse detector configurations enable high-precision cross-section and S-factor determinations. The results illustrate convergence with established underground experiments (e.g., LUNA) and demonstrate the utility of Felsenkeller as an open, EU-accessible platform for astrophysical reaction studies, with future plans to extend measurements to carbon-burning and heavier-element reactions via novel targets such as a $^{4}$He gas-jet and solid carbon targets. These efforts advance constraints on stellar and Big Bang nucleosynthesis models and improve the reliability of astrophysical reaction rates in challenging energy regimes.

Abstract

For almost three decades it has been known that the study of astro-physically important nuclear reactions between stable nuclei requires the use of low-background, underground accelerator laboratories. The Felsenkeller shallow-underground laboratory in Dresden, shielded by a 45 m thick rock cover, hosts a 5 MV Pelletron ion accelerator with an external sputter ion source (mainly able to provide carbon and oxygen beams) and an internal radio-frequency ion source (providing proton and alpha beams). The reduced muon, neutron and gamma-ray background achieved both with natural and active shielding situate the laboratory well in line with deep underground accelerator labs worldwide and allows highly sensitive nuclear reaction experiments. Currently, measurements affecting the solar fusion and Big Bang nucleosynthesis are ongoing. In addition to in-house research by HZDR and TU Dresden, the lab is an open facility for scientific users worldwide, with beam time applications reviewed by an independent science advisory board. Furthermore, EU-supported transnational access is available via the ChETEC- INFRA network for nuclear astrophysics. A brief introduction to underground nuclear astrophysics, status of the Felsenkeller shallow-underground laboratory and some preliminary results are discussed.

Figures (3)

• Figure 1: Layout of the underground installations in Felsenkeller tunnels VIII and IX. Area A is an enclosed radiation controlled area, the other areas are open tunnel but closed to public access. Thick black lines denotes the in-beam experimental and the activation experiment areas. The thick red line indicates the ion beam path in tandem mode. Red triangles denote the specific areas where muon flux in the lab has been measured.
• Figure 2: $\mathrm{^2H(p,\gamma)^3He}$$\gamma$-ray spectrum measured at 450 keV beam energy. The full energy peak together with first and second escape peaks are given by the red arrows.
• Figure 3: $\mathrm{^{12}C(p,\gamma)^{13}N}$ S-factors. In blue the data measured at LUNA Skow_luna23, in red the results from the Felsenkeller laboratory Skow_fels23. The light blue line is the R-matrix extrapolation computed using all the available literature data for the $\mathrm{^{12}C(p,\gamma)^{13}N}$ reaction.