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The neutron activation technique is a well established method to measure neutron capture cross sections relevant for the s-process. The 7Li(p,n) reaction at Ep = 1912 keV is often used as a neutron source since the energy distribution of the emitted neutrons closely resembles a Maxwell-Boltzmann spectrum of kBT = 25 keV, mimicking the 22Ne(α,n) phase in TP-AGB stars. The weak s-process, which takes place in massive stars, can reach energies up to kBT = 90 keV. Neutron spectra corresponding to a Maxwell-Boltzmann distribution with kBT > 25 keV cannot be produced by the 7Li(p,n) reaction directly. We developed a method to obtain quasi-Maxwellian neutron capture cross sections over a wide energy range by combining a set of spectrum average cross sections measured at six different proton energies and distances between the lithium target and the sample. The measured spectrum averaged cross section can be used to calculate the Maxwellian-Averaged cross-section (MACS) from kBT = 25 keV to kBT = 90 keV. Over the last two years neutron capture cross sections on over 20 isotopes have been measured at Goethe University Frankfurt using this methodology. An overview of the current experimental method, challenges during data analysis and the first results are presented.

The slow neutron capture process produces heavy elements in different stellar sites at different temperatures. Neutron capture cross sections for stellar temperatures between kBT = 5 keV and kBT = 100 keV are crucial for a quantitative understanding of the s-process abundance distribution. Over the last decade activation measurements were performed at the Goethe University Frankfurt to study cross sections at a temperature of kBT = 25 keV. We developed a new method to measure neutron capture cross sections at kBT = 6 keV.

We measured the Coulomb dissociation of 16O into 4He and 12C within the FAIR Phase-0 program at GSI Helmholtzzentrum für Schwerionenforschung Darmstadt, Germany. From this we will extract the photon dissociation cross section 16O(α,γ)12C, which is the time reversed reaction to 12C(α,γ)16O. With this indirect method, we aim to improve on the accuracy of the experimental data at lower energies than measured so far. The expected low cross section for the Coulomb dissociation reaction and close magnetic rigidity of beam and fragments demand a high precision measurement. Hence, new detector systems were built and radical changes to the R3B setup were necessary to cope with the high-intensity 16O beam. All tracking detectors were designed to let the unreacted 16O ions pass, while detecting the 12C and 4He.

The neutron activation method is well-suited to investigate neutron-capture cross sections relevant for the main s-process component. Neutrons can be produced via the 7Li(p,n) reaction with proton energies of 1912 keV at e.g. Van de Graaff accelerators, which results in a quasi-Maxwellian spectrum of neutrons corresponding to a temperature of kBT = 25 keV. However, the weak s-process takes place in massive stars at temperatures between 25 and 90 keV. Simulations using the PINO code [2] suggest that a Maxwellian spectrum for higher energies, e.g. kBT = 90 keV, can be approximated by a linear combination of different neutron spectra. To validate the PINO code at proton energies Ep ≠ 1912 keV, neutron time-of-flight measurements were carried out at the PTB Ion Accelerator Facility (PIAF) at the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany.

About 50% of the elements heavier than iron are produced during the slow neutron capture process. This process occurs in different stellar sites at various energies. To understand the ongoing nucleosynthesis, the probability of a neutron capture for different temperatures and therefore for different stellar sites is essential. Activation experiments using the 7Li(p,n) reaction as neutron source were performed. At a temperature of kBT = 25 keV the cross sections were determined for 27Al, 37Cl and 41K. A new method was developed to perform activation experiments at even lower temperatures. For a proof of principle, the cross section for 64Ni was measured at kBT = 25 keV as well as for kBT = 6 keV. To study the impact of isomeric states at higher energies, activations of 181Ta were performed using two different proton energies.

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.