S. G. Pickstone

Mixed-symmetry octupole and hexadecapole excitations in the N = 52 isotones

Background: Excitations with mixed proton-neutron symmetry have been previously observed in the N=52 isotones. Besides the well-established quadrupole mixed-symmetry states (MSS), octupole and hexadecapole MSS have been recently proposed for the nuclei Zr92 and Mo94. Purpose: The heaviest stable N=52 isotone Ru96 was investigated to study the evolution of octupole and hexadecapole MSS with increasing proton number. Methods: Two inelastic proton-scattering experiments on Ru96 were performed to extract branching ratios, multipole mixing ratios, and level lifetimes. From the combined data, absolute transition strengths were calculated. Results: Strong M1 transitions between the lowest-lying 3- and 4+ states were observed, providing evidence for a one-phonon mixed-symmetry character of the 32(-) and 42+ states. Conclusions: sdg-IBM-2 calculations were performed for Ru96. The results are in excellent agreement with the experimental data, pointing out a one-phonon hexadecapole mixed-symmetry character of the 42+ state. The 31-||M1||32(-) matrix element is found to scale with the 2s+||M1||2ms+ matrix element.

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

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.

Lifetime measurement of excited low-spin states via the (p,p′γ) reaction

Abstract In this paper a method for lifetime measurements in the sub-picosecond regime via the Doppler-shift attenuation method (DSAM) following the inelastic proton scattering reaction is presented. In a pioneering experiment we extracted the lifetimes of 30 excited low-spin states of 96 Ru, taking advantage of the coincident detection of scattered protons and de-exciting γ-rays as well as the large number of particle and γ-ray detectors provided by the SONIC@HORUS setup at the University of Cologne. The large amount of new experimental data shows that this technique is suited for the measurement of lifetimes of excited low-spin states, especially for isotopes with a low isotopic abundance, where ( n , n ′ γ ) or – in case of investigating dipole excitations – ( γ , γ ′ ) experiments are not feasible due to the lack of sufficient isotopically enriched target material.

NeuLAND - from double-planes to the demonstrator

K. Boretzky†1, G.D. Alkhazov2, L. Atar3, T. Aumann3,1, C. Beinrucker4, D. Bemmerer5, D. Bertini1, A. Blanco6, C. Caesar3,1, T.E. Cowan5,7, G. Dentinger3, Z. Elekes8, A. Endres4, P. Fonte6, D. Galaviz9, I. Gašparić10, St. Gohl5,7, V.L. Golovtsov2, T. Heftrich4, M. Heil1, M. Heine3, A. Heinz11, M. Holl3, A. Horvat3, A. Horvath12, H. Johansson11, J. Kahlbow3, A. Kelić-Heil1, R. Kissel3, D. Körper1, D. Kresan3, A.G. Krivshich2, V. Kuznetsov2, S. Lindberg11, L. Lopes6, J. Machado9, J. Mayer13, K. Miki3, L. Netterdon13, T. Nilsson11, E.M. Orischin2, S.G. Pickstone13, R. Plag1, M. Pohl4, R. Reifarth4, T.P. Reinhardt7, S. Reinicke5,7, M. Röder5,7, D. Rossi1, H. Scheit3, F. Schindler3, H. Simon1, M. Sobiella5, K. Sonnabend4, D. Stach5, P. Teubig9, R. Thies11, H. Toernqvist3, L.N. Uvarov2, V.V. Vikhrov2, S.S. Volkov2, A. Wagner5, A.A. Zhdanov2, A. Zilges13, K. Zuber7, the RB collaboration , and the FAIR@GSI division1 1GSI, Darmstadt, Germany; 2PNPI St. Petersburg, Russia; 3TU Darmstadt, Germany; 4Univ. Frankfurt, Germany; 5HZDR, Dresden-Rossendorf, Germany; 6LIP, Coimbra, Portugal; 7TU Dresden, Germany; 8MTA ATOMKI, Debrecen, Hungary; 9Univ. Lisbon, Portugal; 10RBI, Zagreb, Croatia; 11Chalmers Univ. of Technology, Göteborg, Sweden; 12Eötvös Univ., Budapest, Hungary; 13Univ. of Cologne, Germany

Study of mixed-symmetry excitations in 96Ru via inelastic proton-scattering

Mixed-symmetry states of octupole (L = 3) and hexadecapole (L = 4) character have been recently proposed in the N = 52 isotones Zr-92 and Mo-94, 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 Ru-96, 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 E-1, M-1, 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.

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

The HORUS spectrometer at the 10MV Tandem accelerator at the Institute for Nuclear Physics in Cologne consists of 14 high-purity germanium γ-detectors. To extend the experimental opportunities, the new silicon-detector array SONIC was designed, housing eight ΔE-E sandwich silicon detectors for ion spectroscopy. In order to process all 30 detector signals, the analog data acquisition was replaced by a digital one using the commercially available DGF-4C modules from the company XIA. The new data acquisition system was tested in two experiments.

The Pygmy Dipole Resonance – status and new developments

At energies well below the isovector electric Giant Dipole Resonance, a concentration of electric dipole strength is observed in many stable and radioactive nuclei. This low-lying excitation mode is usually denoted as Pygmy Dipole Resonance (PDR). Different theoretical approaches can reproduce the gross features, like the energetic position and partly the excitation strength of the PDR, but they differ considerably in its underlying structural description. More sophisticated experiments looking for the isospin character, the detailed decay pattern, and the single-particle structure of the low-lying E1 excitations are therefore mandatory to gain a deeper understanding. This manuscript will give an overview about the most recent experimental results and present new experimental tools for the future.