M. Spieker

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.

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

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.

using partial cross sections of the

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.

^{89}Y(p,γ)^{90}Zr

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.