The origin of many p-nuclei remains an unsolved problem in nuclear astrophysics. While photo-dissociation reactions in the γ -process can explain the production of many p-nuclei, some, notably ^92,94 Mo and ^96,98 Ru are underproduced in network calculations. The ^90 Zr(p, γ ) ^91 Nb reaction is part of a possible (p, γ ) reaction chain for the production of the p-nucleus ^92 Mo. Available data show a large disagreement between the different reported cross sections measurements for the ^90 Zr(p, γ ) ^91 Nb reaction. We measured proton capture cross sections with an enriched ^90 Zr target using in-beam γ -ray spectroscopy for proton energies between 2.75 MeV and 5.1 MeV. The emitted γ -rays were detected using the HORUS ( H igh efficiency O bservatory for γ - R ay U nique S pectroscopy) detector array at the University of Cologne, Germany. To account for the possible contribution of the ^91 Zr(p,n) reaction, an enriched ^91 Zr was irradiated. We measured production cross sections for the ground and isomeric state of ^91 Nb as well as partial cross section for up to ten high-energy primary transitions. The results are in good agreement with a former in-beam γ -ray spectroscopy measurement by Laird et al.. We provide a possible explanation for the discrepancies between our data and other available measurements.

The predictions for reaction rates from the Hauser-Feshbach statistical model as they are needed for γ-process reaction-network calculations crucially depend on the accuracy of the nuclear physics input-parameters like nuclear-level densities, γ-ray strength functions, and particle+nucleus optical-model potentials. At the Institute for Nuclear Physics of the University of Cologne, several charged-particle induced reactions were investigated via the in-beam method with HPGe detectors at the high-efficiency γ-ray spectrometer HORUS. One outstanding feature of this experimental method is the access to partial cross-sections, thus, cross-sections for the population of excited states in the reaction product via a radiative capture of an charged particle. Here, recent results for the reactions 89Y(p,γ)90Zr and 112Sn(α,γ)116Te will be presented.

The predictions of reaction rates for the gamma process in the scope of the Hauser-Feshbach statistical model crucially depend on nuclear physics input-parameters as optical-model potentials (OMP) or gamma-ray strength functions. Precise cross-section measurements at astrophysically relevant energies help to constrain adopted models and, therefore, to reduce the uncertainties in the theoretically predicted reaction rates. During the last years, several cross-sections of charged-particle induced reactions on heavy nuclei have been measured at the University of Cologne. Either by means of the in-beam method at the HORUS gamma-ray spectrometer or the activation technique using the Cologne Clover Counting Setup, total and partial cross-sections could be used to further constrain different models for nuclear physics input-parameters. It could be shown that modifications on the alpha-OMP in the case of the Sn-112(alpha,gamma) reaction also improve the description of the recently measured cross sections of the Cd-108(alpha,gamma) and Cd-108(alpha,n) reaction and other reactions as well. Partial cross-sections of the Mo-92(p, gamma) reaction were used to improve the gamma-strength function model in Tc-93 in the same way as it was done for the Y-89(p,gamma) reaction.

Background: Mo-92 is the most abundant nucleus of the p nuclei, with an isotopic abundance of more than 14 %. The gamma-process nucleosynthesis is believed to produce Mo-92 but fails to explain its large abundance, especially with respect to the other p nuclei produced in the same stellar environment. Further studies require precise nuclear models for the calculation of reaction cross sections.Purpose: A measurement of the total and partial cross sections of the Mo-92(p,gamma) Tc-93 reaction allows for a stringent test of statistical-model predictions. Not only different proton + nucleus optical model potentials, but also the gamma-ray strength function of Tc-93 can be investigated. In addition, high-resolution in-beam gamma-ray spectroscopy enables the determination of new precise nuclear structure data for Tc-93.Method: Total and partial cross-section values were measured by using the in-beam method. Prompt. rays emitted during the irradiation of Mo-92 with protons at seven different energies between 3.7 and 5.3 MeV were detected by using the high-purity germanium (HPGe) detector array HORUS at the Institute for Nuclear Physics, University of Cologne. The gamma gamma-coincidence method was applied to correlate gamma-ray cascades in 93Tc with their origin in the Mo-92 + p compound state.Results: The measured cross sections are compared to Hauser-Feshbach calculations by using the statistical-model code TALYS on the basis of different nuclear physics input models. Using default settings based on standard phenomenological models, the experimental values cannot be reproduced. A shell-model calculation was carried out to predict the low-energy M1 strength in Tc-93. Together with Gogny-Hartree-Fock-Bogoliubov (Gogny-HFB) or Skyrme-HFB plus quasi-particle random-phase approximation (QRPA) models for the gamma-ray strength function, the agreement between experimental data and theoretical predictions could be significantly improved. In addition, deviations from the adopted level scheme were found.Conclusions: By using Gogny-or Skyrme-HFB + QRPA E1 and shell-model M1 strength functions, statisticalmodel predictions can be significantly improved. Partial cross sections provide a valuable testing ground for gamma-ray strength functions for nuclear astrophysics applications. In addition, they can be used to investigate nuclear-structure properties of the compound nucleus.

The reaction Y-89(p,gamma)Zr-90 was studied at five proton energies close to the Gamow window. This reaction is of astrophysical importance, since it is located in a mass region, where the p-nuclei abundances are not well reproduced by network calculations. For this purpose, the in-beam technique utilizing the high-efficiency high-purity germanium (HPGe) detector array HORUS at the Tandem ion accelerator at the University of Cologne was used. The excellent agreement of the measured total cross sections with previous data shows, that the setup in Cologne is well suited for such measurements. An additional interesting outcome of this measurement are partial cross sections of the de-excitation of the Zr-90 compund nucleus up to the 15th excited state, an observable only accessible in this kind of high-resolution in-beam experiments. The experimental setup and preliminary results of the total and partial cross sections obtained for the Y-89(p,gamma) reaction are presented. Additionally, we show results of a first test measurement of the alpha-capture reaction on the p-nucleus Mo-92 using the in-beam technique with HPGe detectors.

J. Mayer,1,* S. Goriely,2 L. Netterdon,1 S. Péru,3 P. Scholz,1 R. Schwengner,4 and A. Zilges1 1Institute for Nuclear Physics, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany 2Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP226, 1050 Brussels, Belgium 3CEA, DAM, DIF, F-91297 Arpajon, France 4Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany (Received 28 November 2015; published 20 April 2016)

The activation technique is a widely used method for the determination of cross-section values for charged-particle induced reactions at astrophysically relevant energies. Since network calculations of nucleosynthesis processes often depend on reaction rates calculated in the scope of the Hauser-Feshbach statistical model, these cross-sections can be used to improve the nuclear-physics input-parameters like optical-model potentials (OMP), gamma-ray strength functions, and nuclear level densities.In order to extend the available experimental database, the Cd-108(alpha,n)Sn-111 reaction cross section was investigated at ten energies between 10.2 MeV and 13.5 MeV. As this reaction at these energies is almost only sensitive on the alpha-decay width, the results were compared to statistical model calculations using different models for the alpha-OMP. The irradiation as well as the consecutive gamma-ray counting were performed at the Institute for Nuclear Physics of the University of Cologne using the 10 MV FN-Tandem accelerator and the Cologne Clover Counting Setup. This setup consists of two clover-type high purity germanium (HPGe) detectors in a close face-to-face geometry to cover a solid angle of almost 4 pi.

The reaction 89Y(p, γ)90Zr was studied at five proton energies close to the Gamow window. This reaction is of astrophysical importance, since it is located in a mass region, where the p-nuclei abundances are not well reproduced by network calculations. For this purpose, the in-beam technique utilizing the high-efficiency high-purity germanium (HPGe) detector array HORUS at the Tandem ion accelerator at the University of Cologne was used. The excellent agreement of the measured total cross sections with previous data shows, that the setup in Cologne is well suited for such measurements. An additional interesting outcome of this measurement are partial cross sections of the de-excitation of the 90 Zr compund nucleus up to the 15th excited state, an observable only accessible in this kind of high-resolution inbeam experiments. The experimental setup and preliminary results of the total and partial cross sections obtained for the 89Y(p, γ) reaction are presented. Additionally, we show results of a first test measurement of the a-capture reaction on the p-nucleus 92Mo using the in-beam technique with HPGe detectors.

A big part in understanding the nucleosynthesis of heavy nuclei is a proper description of the effective interaction between an α-particle and a target nucleus. Information about the so-called α+nucleus optical-model potential is achieved by precise cross-section measurements at sub-Coulomb energies aiming to constrain the theoretical models for the nuclear physics input-parameters. The cross sections of the 108Cd(α,γ) and 108Cd(α,n) reaction have been measured for the first time close to the astrophysically relevant energy region via the in-beam method at the high-efficiency γ-ray spectrometer HORUS and via the activation technique at the Cologne Clover Counting Setup at the Institute for Nuclear Physics in Cologne, Germany. Comparisons between experimental results and theoretical predictions calculated in the scope of the Hauser–Feshbach statistical model confirm the need for a exponentially decreasing imaginary part of the potential. Moreover, it is shown that the results presented here together with already published data indicate that a systematic investigation of the real part of the potential could help to further improve the understanding of reactions involving α-particles.

Partial cross sections of the Y-89(p,gamma)Zr-90 reaction have been measured to investigate the gamma-ray strength function in the neutron-magic nucleus Zr-90. For five proton energies between E-p = 3.65 MeV and E-p = 4.70 MeV, partial cross sections for the population of seven discrete states in Zr-90 have been determined by means of in-beam gamma-ray spectroscopy. Since these gamma-ray transitions are dominantly of E1 character, the present measurement allows an access to the low-lying dipole strength in Zr-90. A gamma-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 differences 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. (C) 2015 The Authors. Published by Elsevier B.V.

The p nucleus 92Mo is believed to be mainly produced through photodisintegration reactions in type II supernovae. However, this production scenario cannot solely account for the observed solar relative isotopic abundance of 92Mo. Additional production scenarios have been suggested to explain this discrepancy. One of these scenarios could be the production of 92Mo in type Ia supernovae via a chain of proton-capture reactions. To verify this scenario, an accurate knowledge of the involved reaction rates is important. We measured the cross section of 90Zr(p,γ) reaction using an enriched 90Zr target by means of in-beam γ-ray spectroscopy in the energy range between 3.6MeV and 5.1MeV. Since the reactions 90Zr(p,γ) and 91Zr(p,n) produce the same nucleus, the contributions of both reactions have to be disentangled. This procedure is explained in this contribution in detail.

An extended database of experimental data is needed to address uncertainties of the nuclear-physics input parameters for Hauser-Feshbach calculations. Especially $\alpha$+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 ($\alpha$,$\gamma$) reaction on a heavy nucleus at sub-Coulomb energies. The total and partial cross-section values were measured by means of in-beam $\gamma$-ray spectroscopy. Total and partial cross sections were measured at four different $\alpha$-particle energies from $E_\alpha = 10.5$ MeV to $E_\alpha = 12$ MeV. The measured total cross-section values are in excellent agreement with previous results obtained with the activation technique, which proves the validity of the applied method. The experimental data was compared to Hauser-Feshbach calculations using the nuclear reaction code TALYS. A modified version of the semi-microscopic $\alpha$+nucleus optical model potential OMP 3, as well as modified proton and $\gamma$ widths, are needed in order to obtain a good agreement between experimental data and theory. It is found, that a model using a local modification of the nuclear-physics input parameters simultaneously reproduces total cross sections of the $^{112}$Sn($\alpha$,$\gamma$) and $^{112}$Sn($\alpha$,p) reactions. The measurement of partial cross sections turns out to be very important in this case in order to apply the correct $\gamma$-ray strength function in the Hauser-Feshbach calculations. The model also reproduces cross-section values of $\alpha$-induced reactions on $^{106}$Cd, as well as of ($\alpha$,n) reactions on $^{115,116}$Sn, hinting at a more global character of the obtained nuclear-physics input.

The low-lying positive-and negative-parity states in Yb-168 have been investigated by means of the (alpha, 2n gamma) fusion evaporation reaction. Using the coincidence method, the level scheme was corrected and extended up to 3MeV, for both the positive-and negative-parity states. Using the new branching ratios determined in the present experiment, the K quantum number was proposed for two negative-parity bands by direct comparison with the Alaga rule. Like in some other nuclei, one negative-parity band was established, decaying predominantly to the gamma-vibrational band. In a second experiment, the lifetimes of the low-lying excited states up to J(pi) = 6(+) in the ground-state band were measured by using the in-beam fast-timing method with the Bucharest mixed high-purity germanium (HPGe) and LaBr3:Ce detector array using the triple-gamma coincidence method. Reduced E2 transition probabilities were extracted from the measured lifetimes and compared with the corresponding observables in neighboring isotopes, showing a smooth behavior with increasing mass. The positive-and negative-parity states as well as E1 and E2 transition probability ratios revealed by these experiments are compared with the interacting boson model in the sd and spdf boson space, and with the confined beta-soft rotor model, and are found to be in good agreement.

The p nucleus Mo-92 is believed to be mainly produced through photodisintegration reactions in type II supernovae. However, this production scenario cannot solely account for the observed solar relative isotopic abundance of Mo-92. Additional production scenarios have been suggested to explain this discrepancy. One of these scenarios could be the production of Mo-92 in type Ia supernovae via a chain of proton-capture reactions. To verify this scenario, an accurate knowledge of the involved reaction rates is important. We measured the cross section of Zr-90(p,y) reaction using an enriched Zr-90 target by means of in-beam y-ray spectroscopy in the energy range between 3.6 MeV and 5.1 MeV. Since the reactions Zr-90(p,y) and Zr-91(p,n) produce the same nucleus, the contributions of both reactions have to be disentangled. This procedure is explained in this contribution in detail.

The accuracy of predicted isotopic abundance distributions of heavy elements stemming from reaction-network calculations depends on uncertainties regarding the astrophysical scenarios as well as the nuclear physics input. For the p process, i.e., for the synthesis of the nuclei which are not originating from neutron-capture processes, thousands of reactions on mainly unstable nuclei in an explosive astrophysical scenario have to be considered. Therefore, reducing the uncertainties of nuclear-physics input means either to measure key-reaction rates as precise as possible or performing systematic studies to improve theoretical predictions of reaction-rates. Using the Cologne Clover Counting Setup, cross sections for charged-particle capture reactions at astrophysically relevant energies can be investigated via the activation method. Moreover, the combination of the 10 MV FN Tandem accelerator and the high-efficient γ-ray spectrometer HORUS in Cologne allows the study cross sections of radiative-capture reactions in-beam via the spectroscopy of prompt γ-rays with high-purity germanium detectors (HPGe). Besides the presentation of the different experimental setups we will show recent experimental results on the key-reactions as 92Mo(p,γ), 130Ba(p,γ), and 112Sn(α ,γ) as well as on other reactions which are sensitive to different nuclear-physics input for the theoretical predictions of reaction-rates.

Background: The nucleosynthesis of the neutron-deficient p nuclei remains an open question in nuclear astrophysics. Beside uncertainties on the astrophysical side, the nuclear-physics input parameters entering Hauser-Feshbach calculations for the nucleosynthesis of the p nuclei must be put on a firm basis. Purpose: An extended database of experimental data is needed to address uncertainties of the nuclear-physics input parameters for Hauser-Feshbach calculations. Especially α + 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 (α,γ ) reaction on a heavy nucleus at sub-Coulomb energies. Method: The total and partial cross-section values were measured by means of in-beam γ -ray spectroscopy. For this purpose, the absolute reaction yield was measured using the HPGe detector array HORUS at the FN tandem accelerator at the University of Cologne. Total and partial cross sections were measured at four different α-particle energies from Eα = 10.5 MeV to Eα = 12 MeV. Results: The measured total cross-section values are in excellent agreement with previous results obtained with the activation technique, which proves the validity of the applied method. With the present measurement, the discrepancy between two older data sets is removed. The experimental data was compared to Hauser-Feshbach calculations using the nuclear reaction code TALYS. With a modification of the semi-microscopic α + nucleus optical model potential OMP 3, the measured cross-section values are reproduced well. Moreover, partial cross sections could be measured for the first time for an (α,γ ) reaction. Conclusions: A modified version of the semimicroscopic α + nucleus optical model potential OMP3, as well as modified proton and γ widths, are needed in order to obtain a good agreement between experimental data and theory. It is found that a model using a local modification of the nuclear-physics input parameters simultaneously reproduces total cross sections of the 112Sn(α,γ ) and 112Sn(α,p) reactions. The measurement of partial cross sections turns out to be very important in this case in order to apply the correct γ -ray strength function in the Hauser-Feshbach calculations. The model also reproduces cross-section values of α-induced reactions on 106Cd, as well as of (α,n) reactions on 115,116Sn, hinting at a more global character of the obtained nuclear-physics input.

Background: The nucleosynthesis of the neutron-deficient p nuclei remains an open question in nuclear astrophysics. Beside uncertainties on the astrophysical side, the nuclear-physics input parameters entering Hauser-Feshbach calculations for the nucleosynthesis of the p nuclei must be put on a firm basis.Purpose: An extended database of experimental data is needed to address uncertainties of the nuclear-physics input parameters for Hauser-Feshbach calculations. Especially alpha + 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 (alpha,gamma) reaction on a heavy nucleus at sub-Coulomb energies.Method: The total and partial cross-section values were measured by means of in-beam gamma-ray spectroscopy. For this purpose, the absolute reaction yield was measured using the HPGe detector array HORUS at the FN tandem accelerator at the University of Cologne. Total and partial cross sections were measured at four different alpha-particle energies from E-alpha = 10.5 MeV to E-alpha = 12 MeV.Results: The measured total cross-section values are in excellent agreement with previous results obtained with the activation technique, which proves the validity of the applied method. With the present measurement, the discrepancy between two older data sets is removed. The experimental data was compared to Hauser-Feshbach calculations using the nuclear reaction code TALYS. With a modification of the semi-microscopic alpha + nucleus optical model potential OMP 3, the measured cross-section values are reproduced well. Moreover, partial cross sections could be measured for the first time for an (alpha,gamma) reaction.Conclusions: A modified version of the semimicroscopic alpha + nucleus optical model potential OMP3, as well as modified proton and gamma widths, are needed in order to obtain a good agreement between experimental data and theory. It is found that a model using a local modification of the nuclear-physics input parameters simultaneously reproduces total cross sections of the Sn-112(alpha,gamma) and Sn-112(alpha,p) reactions. The measurement of partial cross sections turns out to be very important in this case in order to apply the correct gamma-ray strength function in the Hauser-Feshbach calculations. The model also reproduces cross-section values of alpha-induced reactions on Cd-106, as well as of (alpha,n) reactions on Sn-115,Sn-116, hinting at a more global character of the obtained nuclear-physics input.

Background: Deviations between experimental data of charged-particle-induced reactions and calculations within the statistical model are frequently found. An extended data base is needed to address the uncertainties regarding the nuclear-physics input parameters in order to understand the nucleosynthesis of the neutron-deficient p nuclei.Purpose: A measurement of total cross-section values of the Ba-130(p,gamma)La-131 reaction at low proton energies allows a stringent test of statistical model predictions with different proton+nucleus optical model potentials. Since no experimental data are available for proton-capture reactions in this mass region around A approximate to 130, this measurement can be an important input to test the global applicability of proton+nucleus optical model potentials.Method: The total reaction cross-section values were measured by means of the activation method. After the irradiation with protons, the reaction yield was determined by use of gamma-ray spectroscopy using two clover-type high-purity germanium detectors. In total, cross-section values for eight different proton energies could be determined in the energy range between 3.6 MeV <= E-p <= 5.0 MeV, thus, inside the astrophysically relevant energy region.Results: The measured cross-section values were compared to Hauser-Feshbach calculations using the statistical model codes TALYS and SMARAGD with different proton+nucleus optical model potentials. With the semimicroscopic JLM proton+nucleus optical model potential used in the SMARAGD code, the absolute cross-section values are reproduced well, but the energy dependence is too steep at the lowest energies. The best description is given by a TALYS calculation using the semimicroscopic Bauge proton+nucleus optical model potential using a constant renormalization factor.Conclusions: The statistical model calculation using the Bauge semimicroscopic proton+nucleus optical model potential deviates by a constant factor of 2.1 from the experimental data. Using this model, an experimentally supported stellar reaction rate for proton capture on the p nucleus Ba-130 was calculated. At astrophysical temperatures, an increase in the stellar reaction rate of 68% compared to rates obtained from the widely used NONSMOKER code is found. This measurement extends the scarce experimental data base for charged-particle-induced reactions, which can be helpful to derive a more globally applicable proton+nucleus optical model potential.

Background: Uncertainties in adopted models of $\text{particle}+\text{nucleu}\mathrm{s}$ 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, $\ensuremath{\gamma}$-process nucleosynthesis. The improvement of the $\ensuremath{\alpha}+\text{nucleu}\mathrm{s}$ optical-model potential is hampered by the lack of experimental data at astrophysically relevant energies especially for heavier nuclei.Purpose: Measuring the $^{187}\mathrm{Re}(\ensuremath{\alpha},n)^{190}\mathrm{Ir}$ 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 $\ensuremath{\alpha}+\text{nucleus}$ optical-model potential at low energies.Method: Applying the activation method, after the irradiation of natural rhenium targets with $\ensuremath{\alpha}$-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the reaction cross section were determined via $\ensuremath{\gamma}$-ray spectroscopy by using the Cologne Clover Counting Setup and the method of $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences.Results: 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 $\ensuremath{\alpha}$+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 $\ensuremath{\alpha}+\text{nucleus}$ optical-model potential.Conclusion: The results obtained indicate that the energy dependence of the imaginary part of the $\ensuremath{\alpha}+\text{nucleus}$ optical-model potential can be described by an exponential decrease. Successful reproductions of measured cross sections at low energies for $\ensuremath{\alpha}$-induced reactions in the mass range $141\ensuremath{\le}A\ensuremath{\le}187$ confirm the global character of the Sauerwein--Rauscher potential.