S. J. Williams

Isovector pairing in odd-odd N = Z 50Mn

High-spin states in the odd–odd N=Z nucleus 5025Mn have been investigated. A sequence of states up to Jπ=6+ has been assigned as the T=1 analogue of the yrast band in 5024Cr for the first time. The differences in energy between levels in these bands are interpreted in terms of rotational alignments and the effect they have on the Coulomb energy of the nucleus. Comparisons with shell model calculations show that the Coulomb energy difference between the T=1 analogue structures is an important indicator of the competition between isovector pairing modes in N=Z nuclei and their isobars.

Rotational bands in the semi-magic nucleus 5728Ni29

Two rotational bands have been identified and characterized in the proton-magic N = Z + 1 nucleus 57Ni. These bands complete the systematics of well- and superdeformed rotational bands in the light nickel isotopes starting from doubly magic 56Ni to 60Ni. High-spin states in 57Ni have been produced in the fusion-evaporation reaction 28Si(32S,2p1n)57Ni and studied with the γ-ray detection array Gammasphere operated in conjunction with detectors for evaporated light charged particles and neutrons. The features of the rotational bands in 57Ni are compared to those of neighbouring isotopes and interpreted by means of configuration-dependent cranked Nilsson–Strutinsky calculations. The two observed high-spin bands are considered signature partners and assigned to configurations with one 1g9/2 proton and one 1g9/2 neutron, resulting in an unambiguous understanding of the energetically favoured signature α = −1/2 band but a somewhat less satisfactory description of the signature α = +1/2 band.

High-spin, multi-particle isomers in {sup 121,123}Sb.

Isomers in near-spherical Z = 51, antimony isotopes are reported here for the first time using fusion-fission reactions between {sup 27}Al and a pulsed {sup 178}Hf beam of energy, 1150 MeV. {gamma} rays were observed from the decay of isomeric states with half-lives, T{sub 1/2} = 200(30) and 52(3){micro}s, and angular momenta I = (25/2) and I{pi} = 23/2 +, in {sup 121,123}Sb, respectively. These states are proposed to correspond to {nu}(h 11/2){sup 2} configurations, coupled to an odd d 5/2 or g 7/2 proton. Nanosecond isomers were also identified at I {pi} = 19/2 - [T{sub 1/2} = 8.5(5) ns] in {sup 121}Sb and I{pi} = (15/2 -) [T{sub 1/2} = 37(4) ns] in {sup 123}Sb. Information on spins and parities of states in these nuclei was obtained using a combination of angular correlation and intensity-balance measurements. The configurations of states in these nuclei are discussed using a combination of spin/energy systematics and shell-model calculations for neighboring tin isotones and antimony isotopes.

High-Spin, multi-particle isomers in 121,123 Sb

Isomers in near-spherical Z = 51, antimony isotopes are reported here for the first time using fusion-fission reactions between {sup 27}Al and a pulsed {sup 178}Hf beam of energy, 1150 MeV. {gamma} rays were observed from the decay of isomeric states with half-lives, T{sub 1/2} = 200(30) and 52(3){micro}s, and angular momenta I = (25/2) and I{pi} = 23/2 +, in {sup 121,123}Sb, respectively. These states are proposed to correspond to {nu}(h 11/2){sup 2} configurations, coupled to an odd d 5/2 or g 7/2 proton. Nanosecond isomers were also identified at I {pi} = 19/2 - [T{sub 1/2} = 8.5(5) ns] in {sup 121}Sb and I{pi} = (15/2 -) [T{sub 1/2} = 37(4) ns] in {sup 123}Sb. Information on spins and parities of states in these nuclei was obtained using a combination of angular correlation and intensity-balance measurements. The configurations of states in these nuclei are discussed using a combination of spin/energy systematics and shell-model calculations for neighboring tin isotones and antimony isotopes.

Probing structural changes in the very neutron-deficient Os isotopes with recoil-decay tagging

In recent years, the exploitation of the iecoil-decay tagging (RDT) technique with,large arrays of germanium detectors has revealed much information about the structure of heavy nuclei approaching the proton drip line. The yrast bands of the N <= 93 osmium isotopes have been identified in a campaign of tagging experiments using various spectrometer arrays coupled to the RITU gas-filled separator based at the University of Jyvaskyla. Trends in the yrast state excitation energies have indicated a transition from gamma-soft triaxial to near-spherical shapes with decreasing neutron number. Recent experimental results for Os-162 obtained with the JUROGAM and GREAT spectrometers also indicate the importance of configurations involving the h(9/2) neutron states as the N = Z 82 shell gaps are approached.

Spectroscopy of212Po and213At using a8He radioactive beam and EXOGAM

A beam of 28 MeV 8He particles with an average intensity 2 × 105 ions s−1, produced by the SPIRAL facility at GANIL, was used to initiate fusion–evaporation reactions in thick targets of 208Pb and 209Bi. Excited states of 212Po and 213At were populated in (8He,4n) reactions. The emitted γ-rays were detected using the EXOGAM array composed of four Compton suppressed germanium Clover detectors in a close geometry configuration. Preliminary results show that 212Po has been populated to high spin (I 14) with a significantly increased yield compared with our previous 8He beam experiment. An apparently increased relative population of states up to 12+ was observed compared with published results from a 208Pb(9Be,αn)212Po reaction at 48 MeV. Evidence is presented for a previously unreported de-excitation pathway: 69 keV (13−→ 12+). States up to Iπ = (39/2−) have also been observed in 213At.