S. M. Mullins

Rotational band on the 31 yr 16+ isomer in 178Hf

Abstract High-spin states in 178Hf have been identified using particle-γ-γ-time techniques and the incomplete fusion reaction, 176Yb(9Be,α3n)178Hf. The rotational band associated with the four-quasiparticle T 1 2 = 31 year, Kπ = 16+ isomer in 178Hf has been established. The gK−gR values obtained from the in-band decay properties confirm the configuration of the isomer as does its alignment which matches the sum of the alignments of the two Kπ = 8− two-quasiparticle bands that contain the components of the 16+ configuration. New information has also been obtained on the four-quasiparticle 14− band and on the two-quasiparticle 6+ and 8− bands.

The first candidate for chiral nuclei in the A~80 mass region: 80Br

Abstract Excited states of 80Br have been investigated via the 76Ge(11B, α 3 n ) and 76Ge(7Li, 3n) reactions and a new Δ I = 1 band has been identified which resides ∼ 400 keV above the yrast band. Based on the experimental results and their comparison with the triaxial particle rotor model calculated ones, a chiral character of the two bands within the π g 9 / 2 ⊗ ν g 9 / 2 configuration is proposed, which provides the first evidence for chirality in the A ∼ 80 region.

Incomplete fusion as a spectroscopic tool

Particle- coincidence studies have been a promising but not well exploited means of using incomplete fusion reactions to gain access to states in heavy nuclei near stability. At beam energies near the Coulomb barrier, fusion of the heavy fragment from break-up of the projectile with emission of non-equilibrated particles or other charged particles can be used to study relatively neutron-rich nuclei which cannot be reached by fusion, evaporation reactions with stable beams. Qualitative features which make the reactions attractive as a spectroscopic tool include a spin input which is higher than that achievable if fusion reactions were carried out with beams equivalent to the massive fragment and a correlation between the angle of emission of the light fragment and the number of evaporated neutrons which assists channel identification.

Measured magnetic moments in 169Tm and the particle-rotor model; implications for transient field calibration

Abstract Gyromagnetic ratios were measured in the 1 2 + [411] ground-state band of 169 Tm by the transient field technique in such a manner that the extracted g -factors are independent of assumptions concerning the strength and velocity dependence of the transient field. Precise γ-ray branching intensities and multipolarity mixing ratios were determined from measured particle-γ-ray angular correlations. The electromagnetic properties of the low-lying natural parity states in 169 Tm are compared with particle-rotor calculations based on the Woods-Saxon potential and the implications of the measured magnetic moments for the calibration of the transient field strength are discussed.

Enhanced deformation in light Pr nuclei

Abstract We observe for the first time rotational bands of enhanced deformation (ED) in 127 Pr and 128 Pr, and suggest that these bands are built upon πg9/2 ( [404] 9 2 ) in 127 Pr and on the πg9/2νh11/2 orbitals in 128 Pr. These are the lightest isotopes of Pr for which ED bands have been observed. A contrasting trend in the dynamical moments of inertia ( J (2) ) versus N is observed at low frequencies for the “normal-deformed” πh11/2 and “enhanced-deformed” πg9/2 bands of 127 – 131 Pr both experimentally and in self-consistent Woods-Saxon calculations. The trends in J (2) and β2 are correlated with a decreasing probability for pair scattering into the deformation-driving νh9/2/f7/2 ( [541] 1 2 ) orbital.

Competing phenomena: high-seniority excitations and γ-softness in 184Os

Abstract The nuclear structure of the stable nucleus 184 76 Os has been studied including five newly identified rotational bands built on 2-quasiparticle K π =0 + , 6 − , 8 − couplings. The excitation energies of the proposed multi-quasiparticle excitations are in good agreement with predictions of blocked BCS calculations. The interaction responsible for the first band crossing, caused by a high- K tilted 2-neutron configuration, is calculated and a new interpretation presented for the yrast states after the interaction. A second crossing with an aligned s– β band is also reported. The yrast behaviour at high spins is dominated by a wealth of short-lived (nanoseconds and shorter) high- K levels. The low reduced hindrances for intrinsic state decays are discussed, and attributed to the triaxial shapes and fluctuations predicted by configuration-constrained potential-energy-surface calculations. Comparisons with neighbouring nuclei are made.

High-spin states, yrast isomers and residual interactions in the odd-odd nucleus 212At

Abstract Excited states in the odd-odd nucleus 212At have been studied using γ-ray and electron spectroscopy following the 208Pb(7Li,3n) reaction. Levels were identified to spins of 25 h and to excitation energies of about 4.8 MeV. New isomeric states which decay by enhanced E3 transitions have been established at spins 18+, 22− and 25−. Shell model calculations reproduce well the main features of the level scheme and show the configuration dependence of certain empirical residual interaction matrix elements.

Perturbed alignments within an [ital i][sub 13/2] neutron intruder band in [sup 141]Gd

The nucleus [sup 141]Gd was investigated at high spins for the first time via the reactions [sup 112]Sn([sup 32]S,2[ital pn])[sup 141]Gd at 155 MeV and [sup 112]Sn([sup 33]S,2[ital p]2[ital n])[sup 141]Gd at 170 MeV. The methods of in-beam [gamma]-ray spectroscopy were used to establish a number of different structures. These include a band assigned as the [nu][ital i][sub 13/2][660]1/2[sup +] intruder in the Nilsson scheme. This is the heaviest nucleus thus far in which this type of intruder band has been identified, and is the first case in which the 13/2[sup +] bandhead has been clearly observed. Cranked shell model (CSM) calculations predict the occurrence of backbends due to [ital h][sub 11/2] proton and [ital h][sub 9/2] neutron band crossings. Gradual upbends are observed in the experimental alignments, which suggests the interaction strengths are stronger than those predicted by the CSM.

Bands of enhanced deformation in light Pr nuclei

SummaryRotational bands based on the πg9=2 structure have been assigned for the first time in127Pr and128Pr, through GAMMASPHERE measurements. This extends the region of these bands of enhanced deformation down toN = 68 and 69, the lightest Pr isotopes for which this structure has been observed.

Triaxiality and tilted-bands in heavy osmium nuclei

Nuclei in the A ≈ 180 region around Z = 72 exhibit well deformed prolate shapes, giving rise to isomeric states as a result of the partial conservation of K, the projection of the total angular momentum on the nuclear symmetry axis. As more protons are added, away from Z = 72, the prolate deformation decreases and the nuclear potential becomes susceptible to triaxial fluctuations, governed by the γ degree of freedom. In these so called γ-soft nuclei, K is no longer a conserved quantity, resulting in the reduction or even loss of K-isomerism. Based on the excitation energies of the 2+ γ-vibrational bandheads, osmium isotopes (Z = 76) have the most γ-soft potentials in the A ≈ 180 region. Here, new results are reported for the highspin structure of 184Os and comparisons with neighbouring even-even osmium nuclei are made.