C. Angell

Dipole‐Strength Distributions Below the Giant Dipole Resonance in the Stable Even‐Mass Molybdenum Isotopes

Dipole‐strength distributions in the stable even‐mass molybdenum isotopes up to the neutron‐separation energies have been studied in photon‐scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE at the Research Center Dresden‐Rossendorf, Germany, and with mono‐energetic photon beams at the High Intensity Gamma‐ray Source facility at Triangle Universities Nuclear Laboratory. In order to determine the dipole‐strength distribution, statistical methods were developed for the analysis of the measured spectra. The data obtained for the stable even‐mass molybdenum isotopes from the present (γ,γ’) experiments are combined with (γ,n) cross sections from the literature resulting in a photoabsorption cross section covering the full range from about 4 to 15 MeV, which is of interest for nuclear structure as well as for nuclear astrophysics network calculations. Novel information about the low‐energy tail of the Giant Dipole Resonance and the energy spreading of its strength is der...

Recent Results from the Excitation of Dipole States at the HIγS Facility

High‐sensitivity studies of E1 and M1 excitations observed in the 138Ba(γ,γ′) reaction at energies below the neutron emission threshold have been performed. The electric dipole character of the so‐called “pygmy” mode was experimentally verified for excitations from 4.0–8.6 MeV. The fine structure of the M1 “spin‐flip” mode was observed for the first time in N = 82 nuclei.

Study of Collective Dipole Excitations below the Giant Dipole Resonance at HIγS

The High‐Intensity Gamma‐ray Source utilizing intra‐cavity back‐scattering of free electron laser photons from relativistic electrons allows one to produce a unique beam of high‐flux gamma rays with 100% polarization and selectable energy and energy resolution which is ideal for low‐energy γ‐ray scattering experiments. Nuclear resonance fluorescence experiments have been performed on N=82 nuclei. High sensitivity studies of E1 and M1 excitations at energies close to the neutron emission threshold have been performed. The method allows the determination of excitation energies, spin, parities, and decay branching ratios of the pygmy dipole mode of excitation. The observations are compared with calculations using statistical and quasi‐particle random‐phase approximations.

Low-Energy Dipole Modes of Excitation Below the Neutron Separation Energy

The nuclear resonance fluorescence experiments have been performed at the High Intensity Gamma Source (HIγS) on 138Ba nuclei using four 60% efficient HPGe detectors. Excitation energies, spin, parities, and decay branching ratios were measured for the low‐energy dipole modes of excitations. Experimental results on the parity measurement below the neutron separation energy shows that all dipole states in this energy region exhibit E1 excitation. These results are consistent with theoretical prediction of the collective isoscalar nature of this low‐energy mode of excitation.

Photo‐Induced Population of the h11/2 Isomeric States in (γ, n) Reactions

The mechanism of excitation of isomeric h11/2 states in nuclei around the closed shell at N=82 have been studied at the High‐Intensity Gamma Source (HIγS). We have taken advantage of the monoenergetic (ΔE/E=1.5%) and pulsed γ‐ray‐beam from HIγS to perform in‐beam spectroscopy measurements with an improved level of precision and sensitivity. The giant dipole resonances at 15 MeV in the N=82 target isotones 138Ba, 140Ce, and 142Nd have been excited, and following neutron emission the γ‐ray cascades leading to the isomeric h11/2 state and the ground state were observed in the N=81 isotones. For all three nuclei a very similar de‐excitation scheme was found. The only level observed from which the isomeric state was populated was found to be Jπ = 7/2−. The ground state is principally populated from the states with spin and parities Jπ = 1/2+, 5/2+, and 7/2+. The structure of the N=81 isotones and the role of the gateway states in isomer population will be discussed. The results of the measurements will be comp...