Background: Astrophysical models studying the origin of the neutron-deficient p nuclides require the knowledge of proton capture cross sections at low energy. The production site of the p nuclei is still under discussion but a firm basis of nuclear reaction rates is required to address the astrophysical uncertainties. Data at astrophysically relevant interaction energies are scarce. Problems with the prediction of charged particle capture cross sections at low energy were found in the comparisons between previous data and calculations in the Hauser-Feshbach statistical model of compound reactions.Purpose: A measurement of 74 Ge(p,γ) 75 As at low proton energies, inside the astrophysically relevant energy region, is important in several respects. The reaction is directly important as it is a bottleneck in the reaction flow which produces the lightest p nucleus 74 Se. It is also an important addition to the data set required to test reaction-rate predictions and to allow an improvement in the global p+nucleus optical potential required in such calculations.Method: An in-beam experiment was performed, making it possible to measure in the range 2.1 ≤ Ep ≤ 3.7 MeV, which is for the most part inside the astrophysically relevant energy window. Angular distributions of the γ-ray transitions were measured with high-purity germanium detectors at eight angles relative to the beam axis. In addition to the total cross sections, partial cross sections for the direct population of twelve levels were determined. Results:The resulting cross sections were compared to Hauser-Feshbach calculations using the code SMARAGD. Only a constant renormalization factor of the calculated proton widths allowed a good reproduction of both total and partial cross sections. The accuracy of the calculation made it possible to check the spin assignment of some states in 75 As. In the case of the 1075 keV state, a double state with spins and parities of 3/2 − and 5/2 − is needed to explain the experimental partial cross sections. A change in parity from 5/2 + to 5/2 − is required for the state at 401 keV. Furthermore, in the case of 74 Ge, studying the combination of total and partial cross sections made it possible to test the γ width, which is essential in the calculation of the astrophysical 74 As(n,γ) 75 As rate. Conclusions:Between data and statistical model prediction a factor of about two was found. Nevertheless, the improved astrophysical reaction rate of 74 Ge(p,γ) (and its reverse reaction) is only 28% larger than the previous standard rate. The prediction of the 74 As(n,γ) 75 As rate (and its reverse) was confirmed, the newly calculated rate differs only by a few percent from the previous prediction. The in-beam method with high efficiency detectors proved to be a powerful tool for studies in nuclear astrophysics and nuclear structure.
Background: Most of the heavier p isotopes are believed to be produced in the γ process whose reaction path crucially depends on the proton and α-particle penetrability at sub-Coulomb energies. Both nuclei of the samarium p-process chronometer, 146 Sm and 144 Sm, are produced in the γ process, and their initial abundance ratio is very sensitive to the (γ , n) and (γ , α) branching ratio on 148 Gd. The 148 Gd(γ , α) 144 Sm reaction rate was measured roughly 20 years ago by means of the activation technique and its surprising results triggered adjustments to the global low-energy α+nucleus optical-model potentials (OMPs). Purpose: We want to take advantage of modern α-particle spectroscopy techniques in order to constrain the controversial previous results on the 148 Gd(γ , α) 144 Sm reaction rate. Method: The 148 Gd(γ , α) 144 Sm reaction rate has been determined by measuring the cross section of the reverse reaction 144 Sm(α, γ) 148 Gd, applying the activation technique to the α decay of 148 Gd. Targets have been irradiated at the cyclotron of the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany. The α-particle spectroscopy has been carried out with a state-of-the-art low-background ionization chamber of the Technische Universität Dresden, Germany. Results: Cross sections for the 144 Sm(α, γ) 148 Gd reaction have been measured between 10.66 and 12.66 MeV with much higher precision than in the previous measurement. The results agree with earlier results within their uncertainties. The statistical-model analysis has been carried out using the TALYS code on the basis of the latest parametrizations of α-OMPs. The best reproductions of the experimental results within the statistical model have been used to calculate the reaction rates. Conclusion: The values presented here suggest a steeper increase in the astrophysical S factor towards lower center-of-mass energies. Different parametrizations of the α-OMP were able to describe the experimental values sufficiently. Further measurements at energies below 11.0 MeV are suggested.
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