K. O. Zell

Photoexcitation of dipole modes in 160,162,164Dy

Abstract High resolution photon scattering experiments on 160,162,164 Dy have revealed considerable fragmentation of dipole strength into two groups around 2.5 MeV and around 3 MeV. In assuming that the new J =1 states near 3 MeV correspond to magnetic excitations, the B (M1)↑ sums are effectively twice the sd-IBA-2 SU (3) value for g -factors obtained from magnetic moments. The new data also show a sudden drop in B (M1)↑ strength from Dy to Er.

Band crossings in 128Ba

Abstract Excited states of 128 Ba have been studied by in beam γ-ray and conversion electron spectroscopy using the 118 Sn( 13 C, 3n) 128 Ba reaction. 28 new levels were found, 16 spin and parity assignments were made and 4 new bands were identified. The low-lying two-quasiparticle bands were compared with cranked shell model calculations. The low-lying collective levels are discussed in the framework of the interacting boson model.

Enhanced quadrupole collectivity at N = 40: the case of neutron-rich Fe isotopes.

The transition rates for the 2_{1}^{+} states in 62,64,66Fe were studied using the Recoil Distance Doppler-Shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N=40. The results are interpreted by the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N=40 as governed by the number of valence protons with respect to Z~30. The deformation suggested by the experimental data is reproduced by state-of-the-art shell calculations with a new effective interaction developed for the fpgd valence space.

Lifetime measurements in g92 decoupled bands

Abstract Lifetimes of positive-parity states in 71,73 As produced by the reactions 56,58 Fe( 18 O,p2n) 71,73 As have been measured by the Doppler recoil-distance method. The ratios of the measured reduced transition rates to the corresponding ones in the core nuclei 70,72 Ge can be explained by assuming that the coupling of a proton in the 1 2 [440] state increases the deformation of the core.

Excited states in124Xe

Excited states of124Xe have been investigated via the122Te(α, 2n),114Cd(13C, 3n) and108Pd(19F, p2n) reactions using in-beam gamma-ray and electron spectroscopy methods. 36 new levels and several spin and parity assignments were established, revealing the existence of at least one positive parity superband and two negative parity bands, one of them showing backbending.

Superdeformed band in 146Gd. First observation of band crossing

Abstract A discrete superdeformed band is found in 146 Gd. Lifetime measurements yield an average quadrupole moment of 12±2 e b. A band crossing corresponding to an alignment of ≈2ħ is observed at a rotational frequency of 0.65 MeV. A theoretical analysis suggests that the band has a π6 2 v7 1 configuration with odd spins and odd parity. Arguments are given that the [651 1/2] and [642 5/2] single-particle states are involved in the band crossing.

High-spin states and shape evolution in 70,72Se

High-spin states in 70,72Se have been extended up to spin 14+ and 28+, respectively. The yrast bands in both nuclei exhibit a very regular rotational pattern up to the highest spins observed, with no indication of band crossings above hw approximately=0.5 MeV. According to deformed shell modeling calculations the regular yrast structures can be associated with aligned configurations involving g9/2 quasiparticles.

Study of band structures and crossings in 179Os

Abstract High-spin states in 180 Os have been populated in the 166 Er( 18 O, 4n) reaction and studied with in-beam spectroscopy techniques. A γ-coincidence measurement with 5 anti-Compton spectrometers allowed to establish eleven bands, the ground band up to spin-32, the highest spin identified in this work. The extraction of g K factors, based on the analysis of branching ratios and M1/E2 mixing ratios, permitted configuration assignments for some of the bands. Calculations in the framework of the pairing self-consistent rotating BCS model, taking into account shape evolution have been carried out to interpret the observed band crossing features. The two crossings found in most of the bands are considered to result from the rotation alignment of ν( i 13 2 ) 2 and π( h 9 2 ) 2 or π( i 13 2 ) 2 configurations, respectively. The signature splitting observed in certain negative-parity bands indicates that the nucleus may have a triaxial shape. The staggering effect in the low-lying γ-band suggests that the nucleus is γ-soft.

Picosecond lifetime measurements and collective transition strengths in 128Ba

Abstract Mean lifetimes of 14 excited states in 128 Ba populated via the 118 Sn( 13 C, 3n) and 114 Cd( 18 O, 4n) reactions were measured using the recoil-distance Doppler-shift (RDDS) technique and the pulsed- beam method. The differential decay-curve method (DDCM) was applied to extract the lifetimes from the experimental RDDS data. The nuclear deorientation effect on the transitions depopulating the 2 1 + and 4 1 + levels was measured and taken into account in the lifetime determination. The resulting B (E2) values in the ground-, gamma- and two-quasiproton bands are compared to theoretical predictions of collective models.

Electric quadrupole transition strengths of the type 6+1 → 4+1 in 106–112Sn

Abstract Nanosecond isomers in 106,110,112Sn and 107Cd are investigated by the bombardment of 106,110,112Cd with 29MeV 3He ions. Thereby, the following half-lives of states with Jπ = 6+1 are determined: T 1 2 (2323.6 keV in 106 Sn ) = 2.8 ±0.5 ns , T 1 2 (2477.0 keV in 110 Sn ) = 5.6 ± 0.4 ns and T 1 2 (2548.9 keV in 112 Sn ) = 13.8 ±0.4 ns . Further, T 1 2 (205.0 keV in 107 Cd ) = 0.7 ±0.1 ns was derived. Calculations of level energies and transition rates are performed within the quasiparticle-phonon model. The experimental B(E2, 6+1 → 4+1) values in 106–112Sn are reasonably reproduced within this model revealing thereby the importance of weak two-phonon components. Some level energies and transition rates in 106–112Sn (Z = 50) and their corresponding valence mirror partners with N = 82 are compared and discussed.