R Wadsworth

The competition between collective and single particle behaviour in the odd-odd isotopes 128,130La

Excited states in the odd-odd nuclei 128,130La have been investigated to high spin using gamma -ray spectroscopy. Two distinct bands are seen, one based on a pi h11/2(X) nu h11/2 structure and the other on a pi h11/2(X) nu g7/2 configuration. Backbends are observed in both configurations for both nuclei and the nature of the particles responsible can be conclusively proved. Experimental variation in the energy-level signature splitting and B(M1)/B(E2) ratios are discussed in terms of the polarising effects of the odd proton and odd neutron on the soft collective core. The results are discussed in terms of the cranked shell model, odd-odd particle-rotor calculations and total Routhian surface calculations. They are also compared to neighbouring odd and odd-odd systems.

Collective and non-collective high-spin states in 115I

The 92Mo(27Al, 2p2n gamma ) reaction has been used to populate high-spin states in 115I. The dominant structures are associated with pi h112/(550)1/2- and pi g92/(404)9/2+ orbitals. At the highest spins, favoured states with Ipi =43/2- and 51/2-, associated with non-collective oblate configurations are yrast. The systematics of such states in the odd-A 115-121I isotopes are discussed.

Collective oblate structure in the odd-odd isotopes 128,130La

Excited states in the nuclei 128,130La have been investigated to high spin using gamma -ray spectroscopy techniques. In addition to the main rotational structures based on a prolate collective rotor, and a superdeformed band in 130La, two bands are seen with properties similar to the collective oblate structure proposed for 131La. The experimental evidence for these bands is discussed in terms of the variation in energy-level signature splitting, B(M1)/B(E2) ratios and mixing ratios as a function of rotational frequency and single-particle configuration. These results are interpreted using odd-odd particle-rotor model calculations and self-consistent cranking calculations. They are compared with the proposed collective oblate bands in other nuclei in the A=130 region and possible evidence for collective oblate shape co-existence is presented.

Quadruple moment of the i13/2 intruder band in 139Gd

The Doppler shift attenuation method has been used to extract the mean quadruple moment of the nu i13/2 intruder band in 139Gd. From a centroid-shift analysis of the data, and using Braune stopping powers, a value for the intrinsic nuclear quadrupole moment of Q0 approximately=7.0 eb is suggested, corresponding to a deformation beta 2 approximately=0.35. Because of appreciable side-feeding into the band at each level, a lineshape analysis of the data has also been attempted, in which the time structure of the side-feeding has been allowed to vary. This analysis indicates that the side-feeding is significantly slower than the in-band cascade.

Studies of octupole structures in the isotopes 220,222Rn populated via α decay.

The nuclei 220,222Rn have been studied, using the alpha -radioactive sources 226Ra and 228Th, via the technique of alpha - gamma angular correlation. This has enabled Jpi assignments to be made to the previously known low-lying levels in 220,222Rn and alpha hindrance factors to be interpreted. The systematics of alpha hindrance factors for these Z=86 nuclei are examined and compared with those of Z=88, 90 nuclei. The subsequent alpha decays to 216,218Po were observed and 2+ assignments confirmed for the first excited states in these nuclei.

Shape coexistence in 138Sm and evidence for the rotational alignment of a pair of N=6 neutrons

The level scheme of 138Sm has been extended up to Ipi =(32+) using gamma -ray coincidence data collected with the Eurogam array. The 106Cd(35Cl,3p)138Sm fusion-evaporation reaction was used at a bombarding energy of 150 MeV. Several new bands have been observed and the known bands extended to higher spin. Band crossings and shape coexistence are discussed with the aid of Woods-Saxon cranking calculations (CMS and TRS). Evidence is presented for a neutron (i132/)2 pair alignment which is predicted to drive the nucleus to a prolate shape with enhanced quadrupole deformation beta 2=0.32. A strongly coupled band was also established and is interpreted as a collectively rotating oblate structure.

Rotational band structures in 129La

High spin states in the odd proton nucleus 129La have been investigated using the techniques of in-beam gamma -ray spectroscopy. Nine rotational bands have been observed in 129La, seven for the first time. Various band crossings were observed at rotational frequencies in the range h(cross) omega =0.28-0.57 MeV. The data are mainly discussed within the framework of the cranked shell model. The rotational bands are assigned quasiparticle configurations originating from pi h11/2, pi g7/2, pi (h11/2)2(X) pi g7/2 and pi 11/2 (X) nu (h11/2g7/2) states, respectively. Four of the band crossings are thought to be associated with the rotational alignments of the first pair of h11/2 neutrons or protons.

A Study of Th-223 Levels Populated By Alpha-decay

Low spin states of Th223 have been populated via alpha-decay. The parent nucleus (U227, t 1/2 approximately 1 min) was formed in the reaction Pb-208(Ne-22, 3n)U227, E(beam) = 110 MeV. Out-of-beam measurements of alpha-gamma and alpha-e- coincidences were performed with the aid of a rotating target mechanism. These measurements have enabled a limited level scheme of the low-spin states in Th223 to be constructed, along with tentative J-pi assignments and alpha hindrance factors. The data are interpreted in terms of the geometric model assuming a static octupole deformation and compared to that for the N = 133 isotones Ra-221 and Rn-219.

Coexistence of Collective Oblate and Superdeformed Prolate Shapes in

The 196Pb nucleus was populated via the 184W(16O,4n)196Pb and 186W(16O,6n)196Pb reactions at beam energies of 98 MeV and 120 MeV, respectively. A new Delta I=1 magnetic dipole band has been observed. This band is believed to be built on a collective oblate configuration involving high-K proton orbitals coupled to two rotationally aligned i13/2 neutrons. The previously observed 196Pb superdeformed band was also populated in these reactions.

^196

Six rotational bands have been established in 136Sm following heavy-ion induced reactions carried out at the Nuclear Structure Facility, Daresbury. The yrast band has been observed up to Ipi =(24+) and undergoes three band-crossings. A band was observed built on the low-lying 2+ gamma-vibrational state, in addition to two negative parity sidebands built on two-quasiproton excitations. A dipole band showing the characteristics of collective oblate rotation was also established. The results are discussed in terms of total Routhian surface and cranked Woods-Saxon model calculations.