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Background: About 50% of heavy elements are produced by the slow neutron capture process (s process) in stars. The element gallium is mostly produced during the weak s process in massive stars.Purpose: Our activation at kBT ≈ 25 keV is the first experiment in a series of activation and time-of-flight measurements on 69Ga and 71Ga relevant for astrophysics.Methods: We activated 69Ga and 71Ga with a neutron distribution that corresponds to a quasistellar distribution with kBT = 25 keV at the Joint Research Centre (JRC), Geel, Belgium. Protons were provided by an electrostaticVan de Graaff accelerator to produce neutrons via the reaction 7Li(p, n). The produced activity was measured via the γ emission by the decaying product nuclei by high-purity germanium detectors.Results: We provide spectrum-averaged cross sections (SACS) and ratios of the cross sections σGa/σAu for the neutron spectrum of the activation. We obtain values of σ69Ga,SACS = (186 ± 12) mb and σ71Ga,SACS = (112 ±7) mb, and cross-section ratios of σ69Ga/σAu = 0.29 ± 0.02 and σ71Ga/σAu = 0.17 ± 0.01.Conclusions: Our data disagree with the available evaluated data provided by KADoNiS v0.3, our cross-section ratio is about 20% higher for 69Ga and about 20% lower for 71Ga.

BMBF (05P15RFFN1) DFG (RE 3461/4-1) Background: About 50% of heavy elements are produced by the slow neutron capture process (s process) in stars. The element gallium is mostly produced during the weak s process in massive stars. Purpose: Our activation at kBT≈25 keV is the first experiment in a series of activation and time-of-flight measurements on Ga69 and Ga71 relevant for astrophysics. Methods: We activated Ga69 and Ga71 with a neutron distribution that corresponds to a quasistellar distribution with kBT=25keV at the Joint Research Centre (JRC), Geel, Belgium. Protons were provided by an electrostatic Van de Graaff accelerator to produce neutrons via the reaction Li7(p,n). The produced activity was measured via the γ emission by the decaying product nuclei by high-purity germanium detectors. Results: We provide spectrum-averaged cross sections (SACS) and ratios of the cross sections σGa/σAu for the neutron spectrum of the activation. We obtain values of σ69Ga,SACS=(186±12)mb and σ71Ga,SACS=(112±7)mb, and cross-section ratios of σ69Ga/σAu=0.29±0.02 and σ71Ga/σAu=0.17±0.01. Conclusions: Our data disagree with the available evaluated data provided by KADoNiS v0.3, our cross-section ratio is about 20% higher for Ga69 and about 20% lower for Ga71. publishersversion published

In the nuclear mass range A≈60 to 90 of the solar abundance distribution the weak s-process component is the dominant contributor. In this scenario, which is related to massive stars, the overall neutron exposure is not sufficient for the s process to reach mass flow equilibrium. Hence, abundances and isotopic ratios are very sensitive to the neutron capture cross sections of single isotopes, and nucleosynthesis models need accurate experimental data. In this work we report on a new measurement of the 63Cu(n,γ) cross section for which the existing experimental data show large discrepancies. The 63Cu(n,γ) cross section at kBT=25 keV was determined via activation with a quasistellar neutron spectrum at the Joint Research Centre (JRC) in Geel, and the energy dependence was determined with the time-of-flight technique and the calorimetric 4πBaF2 detector array DANCE at the Los Alamos National Laboratory. We provide new cross section data for the whole astrophysically relevant energy range.

This thesis describes experimental investigations and astrophysical network calculations relevant for the nucleosynthesis of the p nuclei. These 35 proton-rich isotopes cannot be produced by neutron-capture reactions which is the general production mechanism for elements heavier than iron in the r and s processes. Therefore, other mechanisms like photo-disintegration reactions on heavy seed nuclei (γ process) or proton-capture reactions are taken into account. The modelling of these processes relies on a hugh amount of reactions which mostly occur for unstable isotopes. This demands, in combination with the contribution of excited states to the stellar rate, the prediction of the rates by a suited theoretical approach: the Hauser-Feshbach statistical model. To improve the reliability of the predictions, systematic experimental investigations are performed within this work for the nuclear input to the calculations. The study of charged-particle optical model potentials using the activation approach for the investigation of (α,n) and (p,n) reactions is described as well as the investigation of (γ,n) reactions in a broad mass range of 140 ≤ A ≤ 210. However, there are also key reactions which are of special interest for the nucleosynthesis of individual p nuclei. An impressive example is the puzzle about the production of the most abundant p nucleus 92Mo. Within this work, the results of an experiment using high-resolution in-beam γ-spectroscopy for the study of the 90Zr(p,γ) reaction are summarized. In addition, the efforts to investigate the 91Nb(p,γ) reaction in standard kinematics by the production of target of the unstable isotope 91Nb to be used with the high-intensity proton-beam provided by the accelerator of FRANZ, Frankfurt, are discussed. Finally, the influence of experimental results in astrophysical network calculations is discussed using post-processing nucleosynthesis methods for the γ process in type II supernovae. Diese Arbeit beschreibt experimentelle Untersuchungen und astrophysikalische Netzwerkrechnungen, die dem Verständnis der Nukleosynthese der p-Kerne dienen. Diese 35 protonenreichen Isotope können nicht mittels Neutroneneinfangreaktionen produziert werden, die der generelle Mechanismus zur Erzeugung von Elementen schwerer als Eisen sind. Daher werden andere Mechanismen vorgeschlagen wie die Photodesintegration einer Verteilung schwerer Saatkerne (γ-Prozess) oder Protoneneinfangreaktionen. Die Modellierung dieser Prozesse basiert auf der Kenntnis einer großen Anzahl von Reaktionen, die meistens an instabilen Kernen auftreten. Um außerdem die Beiträge angeregter Kernzustände zur stellaren Reaktionsrate zu bestimmen, ist deren Vorhersage mit einem geeigneten theoretischen Ansatz notwendig: dem Hauser-Feshbach Statistischen Modell. Um die Verlässlichkeit der Vorhersagen zu verbessern, sind systematische experimentelle Untersuchungen in dieser Arbeit durchgeführt worden. Optische Kernpotentiale für geladene Teilchen wurden durch Experimente mit der Aktivierungsmethode zu (α,n)- und (p,n)-Reaktionen bestimmt und in einem breiten Massenbereich von 140 ≤ A ≤ 210 wurden (γ,n)-Reaktionen untersucht. Allerdings gibt es auch Schlüsselreaktionen, die von besonderem Interesse für die Nukleosynthese einzelner p-Kerne sind. Ein beeindruckender Fall ist das Rätsel um die Produktion des häufigsten p-Kerns 92Mo. In dieser Arbeit werden die Resultate eines Experiments mit hochauflösender γ-Spektroskopie zur Reaktion 90Zr(p,γ) zusammengefasst. Außerdem werden die aufwändigen Schritte beschrieben, die notwendig sind, um die Reaktion 91Nb(p,γ) in direkter Kinematik zu untersuchen: die Produktion eines Targets aus dem instabilen Isotop 91Nb und sein Einsatz an der Hochstrom-Protonenquelle von FRANZ, Frankfurt. Abschließend wird der Einfluss experimenteller Resultate in astrophysikalischen Netzwerkrechnungen diskutiert. Hierzu wurde der γ-Prozess in einer Typ II Supernova mit Methoden der ”post-processing nucleosynthesis“ untersucht.

This work presents a direct measurement of the Ru-96(p,gamma)Rh-97 cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating Ru-96 ions interacted repeatedly with a hydrogen target. The Ru-96(p,gamma)Rh-97 cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the gamma-ray strength function as well as the level density model can be pinned down with the measured (p,gamma) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the gamma-ray strength function and the level density have been removed. As a result, a constrained Ru-96(p,gamma)Rh-97 reaction rate over a wide temperature range is recommended for p-process network calculations.

XIII Nuclei in the Cosmos, 7-11 July, 2014 Debrecen, Hungary. As an alternative production scenario to the so-called g process, the most abundant p nucleus 92Mo may be produced by a chain of proton-capture reactions in supernovae type Ia. The reactions 90Zr(p,g) and 91Nb(p,g) are the most important reactions in this chain. We have measured the first reaction using high-resolution in-beam g-spectroscopy at HORUS, Cologne, Germany, to contribute to the existing experimental data base. So far, we only investigated the high-energy part of the Gamow window and the analysis is still in progress. We plan to study the second reaction in standard kinematics at the FRANZ facility, Frankfurt, Germany. Current developments at FRANZ will be explained in detail.

Background: Optical potentials are crucial ingredients for the prediction of nuclear reaction rates needed in simulations of the astrophysical ��-process. Associated uncertainties are particularly large for reactions involving ��-particles. This includes (��,��) reactions which are of special importance in the ��-process. Purpose: The measurement of (��,n) reactions allows for an optimization of currently used ��-nucleus potentials. The reactions 165Ho(��,n) and 166Er(��,n) probe the optical model in a mass region where ��-process calculations exhibit an underproduction of p nuclei which is not yet understood. Method: To investigate the energy-dependent cross sections of the reactions 165Ho(��,n) and 166Er(��,n) close to the reaction threshold, self-supporting metallic foils were irradiated with ��-particles using the FN tandem Van de Graaff accelerator at University of Notre Dame. The induced activity was determined afterwards by monitoring the specific ��-decay channels. Results: Hauser-Feshbach predictions with a widely used global ��-potential describe the data well at energies where the cross sections are almost exclusively sensitive to the ��-widths. Increasing discrepancies appear towards the reaction threshold at lower energy. Conclusions: The tested global ��-potential is suitable at energies above 14 MeV, while a modification seems necessary close to the reaction threshold. Since the ��- and neutron width show non-negligible impact on the predictions, complementary data are required to judge whether or not the discrepancies found can to be solely assigned to the ��-width.