P. Scholz

Experimental constraints on the γ-ray strength function in

Partial cross sections of the 89 Y(p, ) 90 Zr reaction have been measured to investigate the -ray strength function in the neutron-magic nucleus 90 Zr. For five proton energies between Ep = 3:65 MeV and Ep = 4:70 MeV, partial cross sections for the population of seven discrete states in 90 Zr have been determined by means of in-beam -ray spectroscopy. Since these -ray transitions are dominantly of E1 character, the present measurement allows an access to the low-lying dipole strength in 90 Zr. A -ray strength function based on the experimental data could be extracted, which is used to describe the total and partial cross sections of this reaction by Hauser-Feshbach calculations successfully. Significant di erences with respect to previously measured strength functions from photoabsorption data point towards deviations from the Brink-Axel hypothesis relating the photo-excitation and de-excitation strength functions.

^{90}Zr

Uncertainties in adopted models of particle+nucleus optical-model potentials directly influence the accuracy in the theoretical predictions of reaction rates as they are needed for reaction-network calculations in, for instance, {\gamma}-process nucleosynthesis. The improvement of the {\alpha}+nucleus optical-model potential is hampered by the lack of experimental data at astrophysically relevant energies especially for heavier nuclei. Measuring the Re187({\alpha},n)Ir190 reaction cross section at sub-Coulomb energies extends the scarce experimental data available in this mass region and helps understanding the energy dependence of the imaginary part of the {\alpha}+nucleus optical-model potential at low energies. Applying the activation method, after the irradiation of natural rhenium targets with {\alpha}-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the reaction cross section were determined via {\gamma}-ray spectroscopy by using the Cologne Clover Counting Setup and the method of {\gamma}{\gamma} coincidences. Cross-section values at five energies close to the astrophysically relevant energy region were measured. Statistical model calculations revealed discrepancies between the experimental values and predictions based on widely used {\alpha}+nucleus optical-model potentials. However, an excellent reproduction of the measured cross-section values could be achieved from calculations based on the so-called Sauerwein-Rauscher {\alpha}+nucleus optical-model potential. The results obtained indicate that the energy dependence of the imaginary part of the {\alpha}+nucleus optical-model potential can be described by an exponential decrease. Successful reproductions of measured cross sections at low energies for {\alpha}-induced reactions in the mass range 141{\leq}A{\leq}187 confirm the global character of the Sauerwein-Rauscher potential.

using partial cross sections of the

An extended database of experimental data is needed to address uncertainties of the nuclear-physics input parameters for Hauser-Feshbach calculations. Especially

^{89}Y(p,γ)^{90}Zr

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reaction

+nucleus optical model potentials at low energies are not well known. The in-beam technique with an array of high-purity germanium (HPGe) detectors was successfully applied to the measurement of absolute cross sections of an (

Measurement of the Re187(α,n)Ir190 reaction cross section at sub-Coulomb energies using the Cologne Clover Counting Setup

\alpha

Total and partial cross sections of the Sn 112 ( α , γ ) Te 116 reaction measured via in-beam γ -ray spectroscopy

,

Cross-section measurement of the Ba 130 (p,γ) La 131 reaction for γ -process nucleosynthesis

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First test results of the digital data acquisition at the HORUS spectrometer

) reaction on a heavy nucleus at sub-Coulomb energies. The total and partial cross-section values were measured by means of in-beam

γ-process reaction studies via in-beam γ-ray spectroscopy at HORUS

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