H. Kaiser

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.

The new photoactivation facility at the 4.3 MV Stuttgart DYNAMITRON: setup, performance, and first applications

At the Stuttgart 4: 3M V DYNAMITRON accelerator a newbremsstrahlung irradiation facility w as installed for photoactivation experiments to complement the well-established arrangement for systematic photon-scattering studies taking advantage of the high photon fluxes achievable at this high-current accelerator. The population or depopulation of isomeric states in low-energy photon-induced reactions ðEg ¼ 0:5–4:0 MeV) can be studied with sensitivities far beyond previous limits. The setup of this facility, its performance, and the absolute calibrations are described in detail. First exemplary results for a test case, the reaction 115 In ðg;g 0 Þ 115m In, are presented and analyzed in combination with data from low-energy photon-scattering experiments ( 115 In ðg;g 0 Þ 115 In). The quality of the results demonstrates the

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.

Photoactivation of 180Tam: Does it affect s-process nucleosynthesis of nature’s rarest stable isotope?

The depopulation of the quasistable isomer (Jπ=9−, Ex=75 keV) in 180Ta through resonant photoabsorption was studied with intense bremsstrahlung. The results show evidence for a deexciting level with an energy of 1.085 MeV or below, leading to a dramatic acceleration of the isomer decay to the short-lived Jπ=1+ ground state under stellar s-process conditions. Consequences for a possible nucleosynthesis of nature’s rarest isotope 180Ta within the s process are discussed.

Photoactivation of 180MTa at Astrophysically Relevant Energies

A possible connection of the isomeric state in 180Ta to its ground state band via low-lying intermediate states (IS) was investigated by means of photoactivation using bremsstrahlung. A depopulation of the isomer has been found down to a bremsstrahlung endpoint-energy of 1 MeV but not at the lowest bombarding energy of 0.8 MeV. Therefore an IS slightly lower than 1 MeV has to be assumed. Further IS were observed at about 1.2, 1.5, 2.1, 2.5, and 2.8 MeV, respectively.