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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Activations with neutrons in the keV energy range were routinely performed at the Karlsruhe Institute of Technology (KIT) in Germany in order to simulate stellar conditions for neutron-capture cross sections. A quasi-Maxwell-Boltzmann neutron spectrum of kT = 25 keV, being of interest for the astrophysical s-process, was produced by the 7Li(p,n) reaction utilizing a 1912 keV proton beam at the Karlsruhe Van de Graaff accelerator. Activated samples resulting in long-lived nuclear reaction products with half-lives in the order of yr 100 Myr were analyzed by Accelerator Mass Spectrometry (AMS). Comparison of the obtained reaction cross sections to literature data from previous Time-of-Flight (ToF) measurements showed that the selected AMS data are systematically lower than the ToF data. To investigate this discrepancy, 54Fe(n,γ)55Fe and 35Cl(n,γ)36Cl reaction cross sections were newly measured at the Frankfurt Neutron Source (FRANZ) in Germany. To complement the existing data, an additional neutron activation of 54Fe and 35Cl at a proton energy of 2 MeV was performed. The results will give implications for the stellar environment at kT = 90 keV, reaching the not yet experimentally explored high-energy s-process range. AMS measurements of the activated samples are scheduled.

We present first results and plans for future neutron activation measurements at the Goethe University Frankfurt. The measurements were performed at the Van-de-Graaff accelerator employing the 7 Li(p,n) reaction.

Experimental determination of the cross sections of proton capture on radioactive nuclei is extremely difficult. Therefore, it is of substantial interest for the understanding of the production of the p-nuclei. For the first time, a direct measurement of proton-capture cross sections on stored, radioactive ions became possible in an energy range of interest for nuclear astrophysics. The experiment was performed at the Experimental Storage Ring (ESR) at GSI by making use of a sensitive method to measure (p,γ) and (p,n) reactions in inverse kinematics. These reaction channels are of high relevance for the nucleosyn-thesis processes in supernovae, which are among the most violent explosions in the universe and are not yet well understood. The cross section of the 118Te(p,γ) reaction has been measured at energies of 6 MeV/u and 7 MeV/u. The heavy ions interacted with a hydrogen gas jet target. The radiative recombination process of the fully stripped 118Te ions and electrons from the hydrogen target was used as a luminosity monitor. An overview of the experimental method and preliminary results from the ongoing analysis will be presented.

We measured the Coulomb dissociation of 16O into 4He and 12C at the R3B setup in a first campaign within FAIR Phase 0 at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The goal was to improve the accuracy of the experimental data for the 12C(α,γ)16O fusion reaction and to reach lower center-ofmass energies than measured so far. The experiment required beam intensities of 109 16O ions per second at an energy of 500 MeV/nucleon. The rare case of Coulomb breakup into 12C and 4He posed another challenge: The magnetic rigidities of the particles are so close because of the same mass-to-charge-number ratio A/Z = 2 for 16O, 12C and 4He. Hence, radical changes of the R3B setup were necessary. All detectors had slits to allow the passage of the unreacted 16O ions, while 4He and 12C would hit the detectors’ active areas depending on the scattering angle and their relative energies. We developed and built detectors based on organic scintillators to track and identify the reaction products with sufficient precision.