A program to study the γ-ray decay from the region of high-level density has been established at iThemba LABS, where a high-resolution gamma-ray detector array is used in conjunction with silicon ...particle-telescopes. Results from two recent projects are presented: 1) The 74Ge(α,α′γ) reaction was used to investigate the Pygmy Dipole Resonance. The results were compared to (γ,γ′) data and indicate that the dipole states split into mixed isospin and relatively pure isovector excitations. 2) Data from the 95Mo(d,p) reaction were used to develop a novel method for the determination of spins for low-lying discrete levels utilizing statistical γ-ray decay in the vicinity of the neutron separation energy. These results provide insight into the competition of (γ,n) and (γ,γ′) reactions and highlights the need to correct for angular momentum barrier effects.
The shape method, a novel approach to obtain the functional form of the γ-ray strength function (γSF), is introduced. In connection with the Oslo method the slope of the nuclear level density (NLD) ...and γSF can be obtained simultaneously even in the absence of neutron resonance spacing data. The foundation of the shape method lies in the primary γ-ray transitions which preserve information on the functional form of the γSF. The shape method has been applied to 56Fe, 92Zr, and 164Dy, which are representative cases for the variety of situations encountered in typical NLD and γSF studies. The comparisons of results from the shape method to those from the Oslo method demonstrate that the functional form of the γSF is retained regardless of nuclear structure details or Jπ values of the states fed by the primary transitions.
The γ-ray strength functions and nuclear level densities of 138La and 139La have been measured below the neutron separation energies. These new data were used to calculate astrophysical ...Maxwellian-averaged (n,γ) cross-sections to investigate the production and destruction of the p-nucleus 138La in the photodisintegration process. The results confirm the underproduction of 138La in the p-process with respect to the observed abundances and strongly support the ν-process through νe capture on 138Ba as the main contributor to the synthesis of 138La in Type II supernovae.
The high-spin structures and isomers of the N = 81 isotones Xe-135 and Ba-137 are investigated after multinucleon-transfer (MNT) and fusion-evaporation reactions. Both nuclei are populated (i) in ...Xe-136+ U-238 and (ii) Xe-136+ Pb-208 MNT reactions employing the high-resolution Advanced Gamma Tracking Array (AGATA) coupled to the magnetic spectrometer PRISMA, (iii) in the Xe-136+ Pt-198 MNT reaction employing the gamma-ray array GAMMASPHERE in combination with the gas-detector array CHICO, and (iv) via a B-11+ Te-130 fusion-evaporation reaction with the HORUS gamma-ray array at the University of Cologne. The high-spin level schemes of Xe-135 and Ba-137 are considerably extended to higher energies. The 2058-keV (19/2(-)) state in Xe-135 is identified as an isomer, closing a gap in the systematics along the N = 81 isotones. Its half-life is measured to be 9.0(9) ns, corresponding to a reduced transition probability of B(E2,19/2(-) -> 15/2(-)) = 0.52(6) W.u. The experimentally deduced reduced transition probabilities of the isomeric states are compared to shell-model predictions. Latest shell-model calculations reproduce the experimental findings generally well and provide guidance to the interpretation of the new levels.
.
Lifetimes of excited states in
107
Tc,
109
Rh,
111
Rh, and
113
Rh were measured at GANIL using the Recoil-Distance Doppler Shift method. The neutron-rich nuclei were produced in fission reactions ...in inverse kinematics with a
238
U beam impinging on a
9
Be target. Fission fragments were identified event-by-event in the ray-tracing spectrometer VAMOS++ and correlated with prompt
γ
rays observed around the target position with the EXOGAM Ge detector array. Several lifetimes were obtained for states in the positive-parity yrast bands in the four nuclei and compared to triaxial particle-rotor calculations. The results clarify the configuration for the strongest positive-parity band in
107
Tc and suggest a gradual increase of triaxial deformation with atomic number
Z
, reaching almost maximum triaxiality for the neutron-rich Rh nuclei.
Shape evolution and magnetic rotation in 141Nd Zerrouki, T.; Petrache, C. M.; Leguillon, R. ...
The European physical journal. A, Hadrons and nuclei,
04/2015, Letnik:
51, Številka:
4
Journal Article
Recenzirano
The high-spin states in
141
Nd were investigated using the
96
Zr(
48
Ca, 3n) reaction and the EUROBALL array. The level scheme has been extended up to an excitation energy of around 16MeV and spin ...81/2. Two new bands of dipole transitions and three bands presumably of quadrupole transitions were identified and their connections to low-lying states were established. Cranked Nilsson-Strutinsky and tilted axis cranking calculations are combined in the interpretation of the observed dipole bands. The high-spin bands with assigned quadrupole transitions are interpreted as triaxial bands, while the dipole bands appear in the calculations to exhibit a shape evolution from low-deformation triaxial to spherical shape. They can be classified as magnetic rotation, with transition probabilities that show the characteristic decrease with angular momentum caused by the shears mechanism.
The electromagnetic structure of 140 Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The 2 + and 4 + states of the ...ground-state band and a second 2 + state were populated by multistep excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the 2 + 1 state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and algebraic models are also employed to interpret the experimental data. The results indicate that 140 Sm shows considerable γ softness, but in contrast to earlier speculation no signs of shape coexistence at low excitation energy. This work sheds more light on the onset of deformation and collectivity in this mass region.
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