Spectroscopic factors of neutron-hole and proton-hole states in ^{131}Sn and ^{131}In, respectively, were measured using one-nucleon removal reactions from doubly magic ^{132}Sn at relativistic energies. For ^{131}In, a 2910(50)-keV γ ray was observed for the first time and tentatively assigned to a decay from a 5/2^{-} state at 3275(50) keV to the known 1/2^{-} level at 365 keV. The spectroscopic factors determined for this new excited state and three other single-hole states provide first evidence for a strong fragmentation of single-hole strength in ^{131}Sn and ^{131}In. The experimental results are compared to theoretical calculations based on the relativistic particle-vibration coupling model and to experimental information for single-hole states in the stable doubly magic nucleus ^{208}Pb.

We report on the first (p,p′γ) experiments at Ep=80 MeV to investigate the Pygmy Dipole Resonance (PDR) in the semi-magic nucleus 140Ce. This experiment is the latest in a series of experiments to investigate the PDR with different complementary probes to provide a multi-messenger data set on the properties of the PDR in 140Ce. In addition, calculations within the Quasi-particle Phonon Model (QPM) have been performed. Cross sections have been calculated for proton- as well as α-scattering reactions based on the transition densities obtained from the QPM, not only at the RPA level, but including the full model space of up to 3p–3h configurations. This allows for the first time to compare the calculations to the experimental results on an absolute scale for single excitations. Agreement between QPM and experiment is observed, which proves the high accuracy of the calculated transition densities for individual PDR states.

The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.

The decay properties of the Pygmy Dipole Resonance (PDR) have been investigated in the semi-magic N=82 nucleus 140Ce using a novel combination of nuclear resonance fluorescence and γ–γ coincidence techniques. Branching ratios for transitions to low-lying excited states are determined in a direct and model-independent way both for individual excited states and for excitation energy intervals. Comparison of the experimental results to microscopic calculations in the quasi-particle phonon model exhibits an excellent agreement, supporting the observation that the Pygmy Dipole Resonance couples to the ground state as well as to low-lying excited states. A 10% mixing of the PDR and the [21+×PDR] is extracted.

V. Derya,1,* N. Tsoneva,2,3,4 T. Aumann,5 M. Bhike,6 J. Endres,1 M. Gooden,6 A. Hennig,1 J. Isaak,2,7 H. Lenske,3 B. Löher,7 N. Pietralla,5 D. Savran,7 W. Tornow,6,8 V. Werner,5 and A. Zilges1 1Institut für Kernphysik, Universität zu Köln, 50937 Köln, Germany 2Frankfurt Institute for Advanced Studies (FIAS), 60438 Frankfurt am Main, Germany 3Institut für Theoretische Physik, Universität Gießen, 35392 Gießen, Germany 4Institute for Nuclear Research and Nuclear Energy, 1784 Sofia, Bulgaria 5Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany 6Department of Physics, Duke University, Durham, North Carolina 27708, USA 7GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany 8Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA (Received 27 January 2016; published 10 March 2016)

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.

Background: Two-phonon excitations originating from the coupling of two collective one-phonon states are of great interest in nuclear structure physics. One possibility to generate low-lying E1 excitations is the coupling of quadrupole and octupole phonons. Purpose: In this work, the γ-decay behavior of candidates for the (2_1^+⊗ 3_1^-)_1^- state in the doubly-magic nucleus ^40Ca and in the heavier and semi-magic nucleus ^140Ce is investigated. Methods: (γ⃗,γ') experiments have been carried out at the High Intensity γ-ray Source (HIγS) facility in combination with the high-efficiency γ-ray spectroscopy setup γ^3 consisting of HPGe and LaBr_3 detectors. The setup enables the acquisition of γ-γ coincidence data and, hence, the detection of direct decay paths. Results: In addition to the known ground-state decays, for ^40Ca the decay into the 3^-_1 state was observed, while for ^140Ce the direct decays into the 2^+_1 and the 0^+_2 state were detected. The experimentally deduced transition strengths and excitation energies are compared to theoretical calculations in the framework of EDF theory plus QPM approach and systematically analyzed for N=82 isotones. In addition, negative parities for two J=1 states in ^44Ca were deduced simultaneously. Conclusions: The experimental findings together with the theoretical calculations support the two-phonon character of the 1^-_1 excitation in the light-to-medium-mass nucleus ^40Ca as well as in the stable even-even N=82 nuclei.

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.

Background: Two-phonon excitations originating from the coupling of two collective one-phonon states are of great interest in nuclear structure physics. One possibility to generate low-lying $E1$ excitations is the coupling of quadrupole and octupole phonons. Purpose: In this work, the $\gamma$-decay behavior of candidates for the $(2_1^+\otimes 3_1^-)_{1^-}$ state in the doubly-magic nucleus $^{40}$Ca and in the heavier and semi-magic nucleus $^{140}$Ce is investigated. Methods: $(\vec{\gamma},\gamma')$ experiments have been carried out at the High Intensity $\gamma$-ray Source (HI${\gamma}$S) facility in combination with the high-efficiency $\gamma$-ray spectroscopy setup $\gamma^3$ consisting of HPGe and LaBr$_3$ detectors. The setup enables the acquisition of $\gamma$-$\gamma$ coincidence data and, hence, the detection of direct decay paths. Results: In addition to the known ground-state decays, for $^{40}$Ca the decay into the $3^-_1$ state was observed, while for $^{140}$Ce the direct decays into the $2^+_1$ and the $0^+_2$ state were detected. The experimentally deduced transition strengths and excitation energies are compared to theoretical calculations in the framework of EDF theory plus QPM approach and systematically analyzed for $N=82$ isotones. In addition, negative parities for two $J=1$ states in $^{44}$Ca were deduced simultaneously. Conclusions: The experimental findings together with the theoretical calculations support the two-phonon character of the $1^-_1$ excitation in the light-to-medium-mass nucleus $^{40}$Ca as well as in the stable even-even $N=82$ nuclei.

In addition to the well-established quadrupole mixed-symmetry states, octupole and hexadecapole excitations with mixed-symmetry character have been recently proposed for the N = 52 isotones 92Zr and 94Mo. We performed two inelastic proton-scattering experiments to study this kind of excitations in the heaviest stable N = 52 isotone 96Ru. From the combined experimental data of both experiments absolute transition strengths were extracted.

In addition to the well-established quadrupole mixed-symmetry states, octupole and hexadecapole excitations with mixed-symmetry character have been recently proposed for the N = 52 isotones 92Zr and 94Mo. We performed two inelastic proton-scattering experiments to study this kind of excitations in the heaviest stable N = 52 isotone 96Ru. From the combined experimental data of both experiments absolute transition strengths were extracted.

Mixed-symmetry states of octupole (L = 3) and hexadecapole (L = 4) character have been recently proposed in the N = 52 isotones 92 Zr and 94Mo, based on strong M1 transitions to the lowest-lying 3− and 4+ states, respectively. In order to investigate similar excitations in the heaviest stable N = 52 isotone 96Ru, two inelastic proton-scattering experiments have been performed at the Wright Nuclear Structure Laboratory (WNSL), Yale University, USA and the Institute for Nuclear Physics, University of Cologne, Germany. From the combined data of both experiments, absolute E1, M1, and E2 transition strengths were extracted, allowing for the identification of candidates for MS octupole and hexadecapole states. The structure of the low-lying 4+ states is investigated by means of sdg-IBM-2 calculations.

The decay behavior of low-lying dipole states in 140Ce was investigated exploiting the γ3-setup at the HIγS facility using quasi-monochromatic photon beams. Branching ratios of individual excited states as well as average branching ratios to low-lying states have been extracted using γ – γ coincidence measurements. The comparison of the average branching ratios to QPM calculations shows a remarkable agreement between experiment and theory in the energy range from 5.0 to 8.5 MeV.

The complementary (γ, γ′) and (α, α′γ) reactions were used to study the isospin properties of low-lying E1 excitations in the doubly-magic nucleus 48Ca. In contrast to heavier nuclei, a state-to-state change in isospin character was revealed in 48Ca and a dominant isoscalar excitation was found which is interpreted as an isoscalar oscillation. Recently, protons at 80 MeV were used as an additional hadronic probe in a p-γ coincidence experiment on 140Ce for the first time. Results of the experiments on 48Ca and first results of the 140Ce will be presented in this contribution.

We present an extensive experimental study of the recently predicted pygmy quadrupole resonance (PQR) in Sn isotopes, where complementary probes were used. In this study, (α,α′γ) and (γ,γ′) experiments were performed on 124Sn. In both reactions, Jπ=2+ states below an excitation energy of 5 MeV were populated. The E2 strength integrated over the full transition densities could be extracted from the (γ,γ′) experiment, while the (α,α′γ) experiment at the chosen kinematics strongly favors the excitation of surface modes because of the strong α-particle absorption in the nuclear interior. The excitation of such modes is in accordance with the quadrupole-type oscillation of the neutron skin predicted by a microscopic approach based on self-consistent density functional theory and the quasiparticle-phonon model (QPM). The newly determined γ-decay branching ratios hint at a non-statistical character of the E2 strength, as it has also been recently pointed out for the case of the pygmy dipole resonance (PDR). This allows us to distinguish between PQR-type and multiphonon excitations and, consequently, supports the recent first experimental indications of a PQR in 124Sn.

Background: One-phonon mixed-symmetry quadrupole excitations are a well-known feature of near-spherical, vibrational nuclei. Their interpretation as a fundamental building block of vibrational structures is supported by the identification of multiphonon states resulting from a coupling of fully-symmetric and mixed-symmetric quadrupole phonons. In addition, the observation of strong M1 transitions between low-lying 3(-) and 4(+) states has been interpreted as an evidence for one-phonon mixed-symmetry excitations of octupole and hexadecapole character.Purpose: The aim of the present study is to identify collective one-and two-phonon excitations in the heaviest stable N = 52 isotone Ru-96 based on a measurement of absolute M1, E1, and E2 transition strengths.Methods: Inelastic proton-scattering experiments have been performed at the Wright Nuclear Structure Laboratory (WNSL), Yale University, and the Institute for Nuclear Physics (IKP), University of Cologne. From the acquired proton-gamma and gamma gamma coincidence data we deduced spins of excited states, gamma-decay branching ratios, and multipole mixing ratios, as well as lifetimes of excited states via the Doppler-shift attenuation method (DSAM).Results: Based on the new experimental data on absolute transition strengths, we identified the 2(+) and 3(+) members of the two-phonon mixed-symmetry quintuplet (2(1,ms)(+) circle times 2(1,s)(+)). Furthermore, we observed strong M1 transitions between low-lying 3(-) and 4(+) states suggesting one-phonon symmetric andmixed-symmetric octupole and hexadecapole components in their wave functions, respectively. The experimental results are compared to sdg-IBM-2 and shell-model calculations.Conclusions: Both the sdg-IBM-2 and the shell-model calculations are able to describe key features of mixed-symmetry excitations of Ru-96. Moreover, they support the one-phonon mixed-symmetry hexadecapole assignment of the experimental 4(2)(+) state.

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.

A dedicated setup for the in-beam measurement of absolute cross-sections of astrophysically relevant charged-particle induced reactions is presented. These, usually very low, cross-sections at energies of astrophysical interest are important to improve the modeling of the nucleosynthesis processes of heavy nuclei. Particular emphasis is put on the production of the p nuclei during the astrophysical γ process. The recently developed setup utilizes the high-efficiency γ-ray spectrometer HORUS, which is located at the 10 MV FN tandem ion accelerator of the Institute for Nuclear Physics in Cologne. The design of this setup will be presented and results of the recently measured 89Y(p,γ)90Zr reaction will be discussed. The excellent agreement with existing data shows that the HORUS spectrometer is a powerful tool to determine total and partial cross-sections using the in-beam method with high-purity germanium detectors.