J. N. Wilson

Excited bands in the doubly-magic superdeformed 152Dy nucleus: evidence for the first N = 7 proton hyper-intruder orbital

Abstract Five excited superdeformed bands have been observed in the doubly closed-shell superdeformed nucleus 152 Dy. Three of the new bands are interpreted in terms of single neutron excitations across the N = 86 shell gap. One of these bands appears to partly decay into the yrast superdeformed band. Another band involves a single proton excitation across the Z = 66 shell gap into the N = 7 hyper-intruder orbital. This is the first evidence of the influence of this deformation driving orbital on superdeformed structures in any mass region.

Intruder bands in 108Sn

Abstract The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

Single particle excitations and properties of multiple band terminations near spin 50ħ in 158Er

The competition between collective rotation and single-particle behaviour at high spin in 158Er has been investigated with the Eurogam spectrometer. Band terminating states have been observed in three structures at Iπ = 46+, 48− and 49− and specific single-particle configurations are assigned by comparison with cranked Nilsson-Strutinsky calculations. These special states are found to be related by very simple single particle excitations. These data indicate that an oblate mean field (ϵ ∼ −0.14) is established for a nuclear core plus 12 aligned valence nucleons which is stable against single-particle rearrangements. In the (π, α) = (+, 0) states the collective and weakly collective terminating structures are observed to coexist between 30+ and 46+.

Magnetic rotation in 197Pb and 198Pb

Abstract High-spin states in 197Pb and 198Pb were populated in the 186W(18O ,x n) reactions. In-beam γ -ray coincidences were measured in two experiments using the Gammasphere and the Eurogam II spectrometer arrays, respectively. In both nuclei new bands of enhanced magnetic dipole transitions were found and the known cascades were partly reordered and extended to higher spins. In most cases, γ -ray transitions connecting the magnetic rotational bands to lower-lying states have been identified. Configuration assignments are suggested for the bands. The systematic behavior confirms the shears mechanism. An effective interaction between the main high-spin proton and neutron orbitals is derived.

Measurement of transition quadrupole moments of high-spin states in the N=74 isotones 133Pr, 132Ce and 131La

Abstract The Doppler-Shift Attenuation Method has been used to extract transition quadrupole moments of high-spin bands in the N=74 isotones 133 Pr, 132 Ce and 131 La, produced in the 37 Cl + 100 Mo reaction. The results appear to be configuration dependent and, for 133 Pr and 132 Ce, the involvement of Ω=1/2 νh 9/2 and νf 7/2 intruder orbitals appears to enhance the collectivity at high spin ( I>25 ℏ ).

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.

High-spin rotational bands in doubly odd

Three rotational bands have been established to high spin in doubly odd using high-fold -ray coincidence data collected with the Eurogam II spectrometer. Two of the bands are linked by dipole transitions at low spin and are believed to be signature partners built on the structure. A low-spin signature inversion is observed and taken as evidence for a triaxial shape with , which is supported by TRS cranking calculations. The third band, seen at higher spin/frequency, is tentatively associated with a deformed five-particle - two-hole configuration, whereby two proton holes result from two-particle - two-hole excitations across the Z = 50 shell gap.

Evidence for the pi h11/2(X) nu h11/2 configuration in doubly odd 116I

Two (signature partner) rotational bands have been extended to high spin in doubly odd 116 using high-fold gamma -ray coincidence data collected with the Eurogam II spectrometer. The observation of the bands to a high rotational frequency (0.7 MeV/h(cross)), without evidence of low-frequency particle alignments, suggests a pi h11/2(X) nu h11/2 structure with double blocking of the alignments. This represents the first observation of such a structure in doubly odd iodine isotopes.

Band termination spectroscopy in 157Er

The level scheme of 157Er has been extended from a spin region where the nucleus behaves as a prolate rotor to a region where the spin is produced by the alignment of all or most of the available valence nucleons along the symmetry axis of a weakly deformed oblate shape. The level scheme was established at high spin using up to four-fold gamma -ray coincidences detected in the Eurogam spectrometer following the reaction 114Cd(48Ca,5n)157Er at a bombarding energy of 210 MeV. Particularly favoured states have been established at IK=69/2+, 81/2+, 71/2+, 77/2-, 87-/2 and 89-/2. Specific single-particle configurations are assigned to these special states by comparison with cranked Nilsson-Strutinsky calculations. These states are related to structures observed in the neighbouring nuclei 158Er and 157Ho. These data provide the spectrum of single-particle states for the lowest lying valence orbitals above the 146Gd closed core.

Oblate bands in A approximately 200 bismuth nuclei

The nuclei 198-200Bi were populated via the 186W(19F,xn)198-200Bi reaction at beam energies of 115 MeV and 105 MeV. Another experiment, aimed at investigating the high-spin-level structure of 203,204Bi, used the 198Pt(11B,xn) reaction at a beam energy of 74 MeV. Five new Delta I=1 rotational structures, consisting of stretched magnetic dipole transitions, have been observed. One of these bands is assigned to 198Bi, one to 199Bi, two to 200Bi, and one to 203Bi. The behaviour of the dynamic moments of inertia of these oblate bands is compared with other bands in neighbouring Pb and Bi nuclei.