The electron-ion scattering experiment ELISe is part of the installations envisaged at the new experimental storage ring at the international Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany. It offers an unique opportunity to use electrons as probe in investigations of the structure of exotic nuclei. The conceptual design and the scientific challenges of ELISe are presented.

We report the first measurement of low-energy proton-capture cross sections of 124 Xe in a heavyion storage ring. 124 Xe 54+ ions of five different beam energies between 5.5 AMeV and 8 AMeV were stored to collide with a windowless hydrogen target. The 125 Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and X-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron-and γ-widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.Charged-particle induced reactions like (p,γ) and (α,γ) and their reverse reactions play a central role in the quantitative description of explosive scenarios like supernovae [1] or X-ray binaries [2], where temperatures above 1 GK can be reached. The energy interval in which the reactions most likely occur under astrophysical conditions is called the Gamow window [3,4]. Experimentalists usually face two major challenges when approaching the Gamow window: firstly, the relatively low center-of-mass energies of only a few MeV or less, and secondly, the rapid decrease of cross sections with energy. The high stopping power connected to low-energy beams typically limits the amount of target material, and thus the achievable luminosity. A measurement of small cross sections, on the contrary, requires high luminosities.The description of charged-particle processes in explosive nucleosynthesis -e.g., the γ process occurring in core-collapse and thermonuclear supernovae [5-7] and the rp process on the surface of mass-accreting neutron stars [8] -requires large reaction networks including very short-lived nuclei. Experimental data are extremely scarce [9], especially in the mass region A > 70, and the modelling relies on calculated cross sections. It is therefore essential to test the theory and its central input parameters. In this Letter we report the first study of the 124 Xe(p,γ) 125 Cs reaction. The cross section is measured on the high energy tail of the Gamow peak, which is located between 2.74 and 5.42 MeV at 3.5 GK in the γ process [4]. While the 124 Xe(p,γ) reaction serves as a major milestone for improving the experimental technique

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