H. Tiesler

Collectivity of the intruder bands in 114Sn

Abstract For the investigation of the collectivity and the structure of the intruder bands with positive and negative parity in 114 Sn, coincidence lifetime measurements using the recoil distance Doppler shift (RDDS) and the Doppler-shift attenuation (DSA) methods were performed at the GASP spectrometer at the INFN-Laboratori in Legnaro. Excited nuclear states were populated with the reaction 100 Mo( 18 O,4n) 114 Sn. For analysis the differential decay curve method was used in the γγ -coincidence mode. In total 31 lifetimes could be obtained, belonging to 11 and 13 states in the positive- and negative-parity intruder bands, respectively, and to 7 spherical states. This work enhances our knowledge of low-lying collectivity among the Sn isotopes near the mid-shell region.

Structure and evolution of electric dipole strength in 204,206,208Pb below the neutron emission threshold

Abstract The electric dipole strength distributions below threshold in stable even-mass Pb isotopes have been extracted from photon scattering experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Between 4 and 6.5 MeV excitation energy a resonance-like clustering of strength is observed. A strong evolution of a fragmentation of the dipole strength with opening the neutron shell is found. The fine structure of the strength is compared to microscopic quasiparticle-phonon model calculations that quantitatively reproduce the data quite well and explain the fragmentation of the dipole strength. Models suggesting the oscillation of excess neutrons with respect to an N ≃ Z core as a possible origin of low-lying E1 strength are critically examined.

Note on a search for the two-octupole phonon 2+ state in 208 Pb with resonant photon scattering

Abstract Results of a 208 Pb (γ,γ′) experiment are presented aiming at an identification of the 2 + member of the long-sought two-octupole phonon multiplet. Four E2 excitations have been observed below 6.5 MeV excitation energy, two of them for the first time. However, in contrast to new results of calculations within the quasiparticle–phonon nuclear model (QPM), no obvious candidate for the two-octupole phonon vibration could be found in the present study. We discuss the J π =2 + states detected in this as well as previous experiments with respect to their possible two-octupole phonon structure.

Octupole excitations in146Sm

SummaryThe mean lives of the lowest 9- and 12+ states of146Sm have been measured by means of the RDM. Their (preliminary) values are τm(9-) = 0.97 ± 0.05 ns and τm(12+) = 15 ± 2 ps, respectively. The strengths of the collectiveE3 transitions of the 12+ → 9- → 6+ cascade are compared with the corresponding ones in148Gd.

Double and triple octupole excitations in the A≈150 region

The results of recent experiments performed at L.N.L. on multiple octupole excitations in the N=84 isotones are reviewed. In particular, the results on B(E3) strengths in 146Sm are discussed and expectations for higher-order octupole excitations are compared with the present experimental situation.

Electromagnetic Transition Strengths in 104CD and 101AG and Their Interpretation within the Interacting Boson (Fermion) Plus Broken Pair Model

The structures of high spin states in neutron-deficient nuclei below the Sn isotope chain close to doubly-magic 100Sn are dominated by the interplay between proton holes in the g9/2 orbit and neutrons distributed over several single-particle orbits between the N = 50 and N = 82 shell closures. In nuclei close to 100Sn, this interplay can be either competitive, leading to well-separated families of either neutron particle or proton-hole configurations or cooperative at higher spin values, where both spin-aligned protons and neutrons contribute to the total angular momentum of the states. Given this situation, measurements of electromagnetic transition strengths are most appropriate to deduce information about the structure of high-spin states,namely to disentangle the contributions of the different configurations to the individual states. In the present work, detailed lifetime measurements in 101Ag and 104Cd have been performed, resulting in a large number of reduced electromagnetic transition strengths in both nuclei. These results as well as the level energies are compared to calculations within the interacting boson (fermion) plus broken pair model.

Evidence for double-octupole excitation in146Sm

The mean life of the 12+ level at 4195 keV in146Sm has been measured with the Recoil Distance method. The reduced strength of the E3 decay 12+→9−, about 60 W.u., confirms the collectivedouble-octupole character of the 12+ level.