T. Lönnroth

Levels in 110Pd, 112Pd, 114Pd and 116Pd from the beta decays of the on-line mass separated Rh isotopes☆

Abstract The energy levels of even 110–116 pd nuclei have been studied from β-decays of odd-odd 110–116 Rh isotopes produced at the on-line isotope separator IGISOL. Two β-decaying states with I π = 1 + and I ⩾4 have been identified in all the cases. New level schemes with preliminary spin assignments have been constructed for 112 Pd, 114 Pd and 116 Pd. Similarities with the level structures of their Xe isotones have been pointed out.

Gamma-ray spectroscopy of 110Cd

Abstract The nucleus 110Cd has been studied using the 13C and 18O induced reactions and the NORDBALL multi-detector array. The yrast band was observed to Iπ = 28+ and a large number of side bands were identified. The h 11 2 neutrons were found to play a dominant role in the observed rotational structures. The weakly populated yrast 3187 keV 8+ state is interpreted as arising from the proton g 9 2 −2 configuration. Collective bands, where the band members are connected by ΔI = 1 transitions and where the ΔI = 2 transitions are not observed, are also reported. These bands are interpreted to have prolate configurations involving both the h 11 2 neutrons and the g 9 2 protons. The experimental data are discussed in terms of the total routhian surface and cranked shell model calculations.

Coexistence of Vibrational and Quasiparticle Structures in Neutron-Deficient 114,115,116,117Te

Excited states in 114-117Te were populated in the bombardment of 114,116Sn targets with 3He ions at energies Elab = 20.9-27.5 MeV. The subsequent de-excitation was studied using in-beam spectroscopic methods, including measurements of γ-ray excitation, γ-γ coincidences, γ-RF timing, γ-ray angular distributions and conversion electrons. States with angular momenta up to J 12 h were observed, but no isomers with T1/2 1 ns were located. The excited structures are discussed in terms of vibrational bands coupled to one- and two-quasiparticle shell-model structures. The systematical behaviour of the excited states in Te isotopes is studied in the mass range A = 115-123 for the odd, and A = 112-134 for the even isotopes.

Lifetimes of yrast states in 110Cd

Abstract Lifetimes or lifetime limits of 20 yrast levels in 110 Cd have been measured using the recoil-distance method and the NORDBALL array of Compton-suppressed Ge detectors. From the reduced transition probabilities competing rotational and vibrational behaviour of 110 Cd is revealed. Also non-collective states, associated with proton configurations, have been observed.

In-Beam Study of 213Rn, 215Ra and 217Ra: Strongly Enhanced E3 Transitions in Odd-A N = 127 Isotones

With the aim of searching for high-spin configurations involving the high-j neutron orbitals, excited states in 213Rn, 215Ra and 217Ra were populated in the bombardment of 208Pb with 13C ions. Gamma-ray singles, excitation functions and angular distributions, pulsed-beam-γ timing and γ-γ(t) coincidences were measured. Six isomeric levels were observed in 213Rn, three in 215Ra while none in 217Ra. The lowest-lying 15/2- states in all odd-A N = 127 isotones are de-excited by E3 transitions with a strength of about 23 W.u, suggesting that they are all coupled to the octupole phonon. In 217Ra no evidence for collective behavior is found despite its closeness to the actinide region. The low-lying states in the level scheme can be interpreted as arising from three-neutron configurations. The wave function of a Jπ = (37/2+) state in 213Rn is proposed to include about 30% admixture from the two-octupole configuration πh9/23f7/2 ⊗ vg9/2 ⊗ (3-)2 since the de-exciting E3 transition has a strength of about 40 W.u. This is the first indication of such an excitation in the lead region.

Neutron and proton bands in 122Xe at high spins

High-spin bands in 122,123,124Xe and 123Cs were populated using 3He- and 16, 18O-induced reactions on Te, Pd and Ag targets. The subsequent de-excitation was studied using gamma -ray and conversion-electron spectroscopic methods. States with spins up to Jpi =(25-), (35/2-), (21-) and (51/2-), respectively, were identified. In 122Xe four Delta I=2 bands were identified in addition to the ground state and gamma bands, and furthermore a Delta J=1 band with a tentative (K,I)pi =(10, 10)- band head was observed. In 123Xe the previously observed 11/2- and 7/2+ bands were extended to Jpi =(35/-) and (23/2+), respectively. The yrast cascade in 123Cs was extended to spin 51/2-. In 124Xe the recently presented level scheme was completed with some high-lying levels. The band structures were interpreted within the cranked shell model (CSM) framework. A consistent description of band crossing frequencies in both odd and even nuclides was obtained, and configurations were assigned to most of the observed bands. Some relative transition rates were obtained from branching intensities.

Band structures and proton-neutron interactions in 174Ta

Abstract Six rotational bands in the odd-odd nucleus 174Ta have been populated with the 160Gd(19F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π 1 2 − [541] ⊗ ν 1 2 − [521] band and semi-decoupled π 1 2 − [541] ⊗ ν 7 2 − [633] band are observed, in addition to the high-K couplings of the π 9 2 − [514] ⊗ ν 7 2 + [633] , π 9 2 − [514] ⊗ ν 5 2 − [512] , π 7 2 + [404] ⊗ ν 7 2 + [633] , and π 5 2 + [402] ⊗ ν 5 2 − [512] configurations. The signature splitting of the π 1 2 − [541] ⊗ ν 7 2 + [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i 13 2 neutrons in the π 1 2 − [541] bands of odd-Z nuclei.

THE DISTRIBUTION OF THE ROTATIONAL TRANSITION STRENGTH IN WARM NUCLEI STUDIED THROUGH GAMMA-RAY CORRELATIONS

Abstract The study of damping of rotational motion applying te rotational plane mapping (RPM) method is presented and discussed. The aim of this technique is to extract the distribution of the rotational transition strength from an analysis of the shape of the “central valley” of two- and three-dimensional γ-ray spectra. The method is applied to a triple γ-coincidence data set of 162,163Tm nuclei formed in 37Cl+130Te reactions. The rotational transition strength is obtained as a function of rotational frequency for selected regions of entry states, and the width is found to be rather constant and approximately equal to 80 keV. This value is significantly smaller than the value predicted theoretically for the rotational damping width Γrot. Also the ratio between the observed depth and width of both the 2D and 3D valleys does not agree with the simple model adopted in the RPM method. These discrepancies point to the presence of both a wide and a narrow component in the distribution of rotational strength as extracted by the RPM method. The analysis of simulated spectra obtained on the basis of realistic band-mixing calculations, including residual interactions, confirms this behaviour.

The role of the shape driving h12 neutron orbital in 108Cd

Abstract The nucleus 108Cd has been studied using a heavy-ion reaction and the NORDBALL multi-detector array. The yrast band has been extended to Ip = 24+, and nine quasi-rotational side bands have been identified. The n h 11 2 negative-parity orbital is found to play a dominant role both in the low-spin as well as in the high-spin structure of 108Cd. The first band crossing in the yrast band is caused by the h 11 2 neutrons. The 10+ state of the aligned n( h 11 2 ) 2 configuration decays strongly into a low-lying low-spin intruder-like positive-parity band. It is suggested that this intruder structure is dominated by a non-aligned n( h 11 2 ) 2 configuration. For the first time in this mass region, we report a collective, possibly prolate, M1 band built on the p( g 9 2 ) −2 configuration. The experimental results are discussed in terms of total-routhian-surface and cranked-shell-model calculations.

Yrast levels in 198Pb and the structure of neutron-deficient Pb isotopes

Abstract Excited states in 198 Pb were populated through the reaction 192 Os( 12 C, 6n) 198 Pb at E lab = 100–106 MeV. The subsequent de-excitation was studied using conventional in-beam γ-ray and conversion-electron spectroscopic methods including γγ coincidence, pulsed-beam γ and angular distribution measurements. Levels up to a possible spin of (23 − ) were observed. A new isomer with J = 16 and T 1 2 = 6.4 ns is observed at 4724.8 + e keV. The weak 5 − → 2 + cross-over E3 transition was observed and its intensity corresponds to a rate of 0.8 ± 0.2 W.u. The structure of the excited states is interpreted to arise from two- and four-quasiparticle neutron shell-model configurations. A Hartree-Fock calculation was performed to describe the shapes of the ground states of light Pb isotopes. Two shallow energy minima with deformation values of e ~ 0 and e ~ −0.2 are observed in 198 Pb.