Science at Felsenkeller

The scientific motivation for the new underground accelerator laboratory at Felsenkeller is to understand the production of the chemical elements in the universe.

The plot shows the observed abundance of the chemical elements in the solar system, on a relative scale. The lightest elements hydrogen, helium, and lithium have been produced in Big Bang nucleosynthesis in the very first minutes of the universe. Two of them, hydrogen and helium, are the most abundant elements observed. The third and fourth most abundant elements are carbon and oxygen. They originate in hydrostatic stellar helium burning, and they form the basis for many subsequent processes of nucleosynthesis as outlined in the plot. The light elements up to iron are mainly produced by nuclear fusion processes, whereas the heavy elements beyond iron are mostly the product of neutron capture processes in stars and stellar explosions.

A number of other nucleosynthesis processes play a role, as well. Overall, nuclear astrophysics aims to explain the patterns observed in the abundance plot: Why does the world around us consist of exactly those chemical elements that we see? Which astronuclear processes have generated exactly the right mix of elements that are needed for human life to develop?

The Felsenkeller underground accelerator laboratory will be used both for in-house research by the two institutions HZDR and TU Dresden that have installed the laboratory and as a facility that will be made available free of charge to scientific users from any country, based on the recommendations of an independent international scientific advisory board.

Felsenkeller link to astronuclear research in the HZDR Nuclear Physics division

The Felsenkeller low-energy ion accelerator is synergetic to the nELBE and γELBE installations at HZDR, both of which are operated by the HZDR Nuclear Physics Division. Here, just one example for these synergies is given, namely the nuclear reactions around neon isotopes:

At Felsenkeller, owing to the low background and high beam intensity, resonances in the ²²Ne(α,γ)²⁶Mg nuclear reaction may be studied by in-beam γ-ray spectroscopy. The corresponding states in the ²⁶Mg nucleus may be studied by nuclear resonance fluorescence using the γELBE bremsstrahlung beam. The ²²Ne(α,γ)²⁶Mg reaction is important as a competitor to a main neutron source for the astrophysical s-process:  ²²Ne(α,n)²⁵Mg. At nELBE, yet another reaction on neon can be studied, namely ²⁰Ne(n,α)¹⁷O which is the time-inverted partner of the ¹⁷O(α,n)²⁰Ne reaction that restores neutrons consumed by a so-called neutron poison reaction to the stellar scenario.

In brief: Higher ²²Ne(α,γ)²⁶Mg yields measured at Felsenkeller and higher ²⁶Mg γ-widths measured at γELBE would lead to a lower number of neutrons in the star. Higher ²⁰Ne(n,α)¹⁷O yields measured at nELBE, by contrast, lead to a higher number of neutrons in the star. 

In-house research at Felsenkeller

Two "day one" nuclear reactions have been identified for the in-house research: The ³He(α,γ)⁷Be and ¹²C(α,γ)¹⁶O reactions. 

The 3He(α,γ)7Be reaction is important in two distinct astrophysical scenarios: It controls the pp-2 and pp-3 parts of the proton-proton chain of hydrogen burning in the Sun, at a temperature of 0.016 GK (Gamow energy 0.025 MeV). It affects lithium production in Big Bang Nucleosynthesis (BBN), because the 7Be produced in this reaction decays to 7Li with 53 days half-life (Gamow energy 0.2 MeV). Going from right to left in the plot, the 3He(α,γ)7Be  cross section (shown parameterized as the astrophysical S-factor S34) has been measured precisely by several independent groups for E ≥ 0.3 MeV, again precisely by the LUNA collaboration (including HZDR scientists) for E = 0.1-0.2 MeV, and only indirectly for E ≤ 0.1 MeV.

At Felsenkeller, the entire ³He(α,γ)⁷Be excitation function shall be covered from 0.1-1.0 MeV, in order to connect the various experiments in one study, measuring bith the emitted in-beam γ-rays and the induced ⁷Be activity. The new data will decisively improve the precisions of the Big Bang predictions of ⁷Li and of the predicted ⁷Be and ⁸B solar neutrino fluxes.

The 12C(α,γ)16O reaction affects the ratio of abundances of the two elements carbon and oxygen. It has been studied many times in the past, but at energies that are much higher than the Gamow peak for the relevant astrophysical scenario.  Due to two subthreshold resonances, one affecting the electric dipole (E1) and another the electric quadrupole (E2) component of the S-factor, the extrapolation from the energies where data exist to the Gamow peak is fraught with uncertainty.

Taking advantage of the ultra-low background at Felsenkeller, the ¹²C(α,γ)¹⁶O excitation function shall be measured from 0.6-2.0 MeV, entering the Gamow peak area at least for the highest helium burning temperatures. 

Felsenkeller as a facility

Once the accelerator is fully commissioned and has been tested successfully in some in-house research campaign, a significant number of beam time hours will be made available to the worldwide scientific community for the purposes of pure scientific research, free of charge and based on the recommendations by an independent scientific advisory board.

The scientific advisory board of Felsenkeller consists of:

• Prof. Brian Fulton, University of York
• Prof. Zsolt Fülöp, MTA ATOMKI, Debrecen
• Prof. Alessandra Guglielmetti, University of Milan and INFN
• Prof. Jordi José, UPC Polytechnical University of Catalonia, Barcelona

The time when the Felsenkeller accelerator will open for outside users will be announced in due course.