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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.

The particle spectrometer SONIC for particle-γ coincidence measurements was commissioned at the Institute for Nuclear Physics in Cologne, Germany. SONIC consists of up to 12 silicon ∆E-E telescopes with a total solid angle coverage of 9 %, and will complement HORUS, a γ-ray spectrometer with 14 HPGe detectors. The combined setup SONIC@HORUS is used to investigate the γ-decay behaviour of low-spin states up to the neutron separation threshold excited by light-ion inelastic scattering and transfer reactions using beams provided by a 10 MV FN Tandem accelerator. The particle-γ coincidence method will be presented using data from a 92 Mo(p,p'γ) experiment. In a 119 Sn(d,X) experiment, excellent particle identification has been achieved because of the good energy resolution of the silicon detectors of approximately 20 keV. Due to the non-negligible momentum transfer in the reaction, a Doppler correction of the detected γ-ray energy has to be performed, using the additional information from measuring the ejectile energy and direction. The high sensitivity of the setup is demonstrated by the results from a 94 Mo(p,p'γ) experiment, where small γ-decay branching ratios have been deduced.

Background: Reaction rates of radiative capture reactions can play a crucial role in the nucleosynthesis of heavy nuclei in explosive stellar environments. These reaction rates depend strongly on γ -ray decay widths in the reaction products, which are, for nonresonant capture reactions at high excitation energies, derived from the γ -ray strength function and the nuclear level density. Recently, the ratio method was applied to primary γ rays observed from (d, p) reactions and nuclear resonance fluorescence measurements to extract the dipole strength in atomic nuclei and to test the generalized Brink-Axel hypothesis. Purpose: The purpose of this work is to apply the ratio method to primary γ -ray intensities of the 63,65 Cu(p,γ ) reactions to extract γ -ray strength information on the nuclei 64,66 Zn. The impact of spin distribution, total γ -ray decay widths, level densities, and width fluctuations on the application of the ratio method will be discussed. Additionally, by comparing the relative γ -ray strength at different excitation energies, conclusions on the validity of the generalized Brink-Axel hypothesis can be made. Method: The radiative proton capture reaction measurements have been performed at the HORUS γ -ray spectrometer of the University of Cologne at one excitation energy for each reaction. Primary γ -ray intensities have been determined by normalizing secondary γ -ray transitions in two-step cascades using their absolute branching ratio. The ratio method was applied to the measured primary γ -ray intensities as well as to previous measurements by Erlandsson et al. at different excitation energies. Results: The relative strength function curve for 64 Zn from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for 66 Zn where both measurements were at almost the same excitation energy. Absolute γ -strength function values have been obtained by normalizing the relative curves to quasiparticle random phase approximation calculations because of the absence of experimental data in the respective energy region. Conclusion:The generalized Brink-Axel hypothesis, i.e., the independence of the strength function on the excitation energy, seems to hold in the studied energy region and nuclei. The method to obtain primary γ -ray intensities from two-step cascade spectra was shown to be a valuable and sensitive tool although its uncertainties are connected to the knowledge of the low-energy level scheme of the investigated nucleus. The scaling in the ratio method should be taken with care, because the relative strength is not a simple sum of f E 1 and f M1 but a somewhat complex linear combination dependent on the excitation energy of the nucleus.

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