P. J. Daly

Particle hole yrast states in146Gd and147Gd and the Z=64 shell closure

The level structures of146Gd and147Gd have been investigated by in-beamγ-ray ande− spectroscopy with (α, xn) reactions on enriched Sm targets. Detailed level schemes up to ∼4 MeV, which differ radically from earlier schemes, are reported. The energy levels are characterized as particle-hole excitations using empirical single particle energies and two nucleon interactions. Analysis of pure 1p 1h proton excitations demonstrates that theZ=64 andN=82 energy gaps are about equally large.

Structure of 52,54Ti and shell closures in neutron-rich nuclei above 48Ca

Abstract The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Particle plus octupoleM2/E3 isomers and high-spin particle-hole states in147Tb and148Tb

The level structures of theN=82 andN=83 nuclei147Tb and148Tb have been studied by means of (α, 8n) and (α,7n) reactions induced by 68 to 110 MeVα particle bombardments of151Eu targets. In-beam conversion electron measurements have established that isomers withT1/2=4.8(6)ns in147Tb andT1/2=22(1)ns in148Tb decay byM2+E3 transitions to the ground states. The measuredB(E3) values show that the isomeric states arise from the coupling of the valence nucleon(s) to the146Gd core octupole. Particlephonon coupling in these nuclei and in the one-neutron nucleus147Gd is discussed and compared with well known cases involving the208Pb core. The higher lying yrast states in the two Tb nuclei are described as shell-model particle-hole excitations using empirical single particle energies and nucleon-nucleon interactions.

Feeding times of high-spin states in 152, 154Dy: Probes of nuclear structure above the yrast line

Abstract Measurements of feeding times of high-spin yrast states up to spin 30 ℏ in 154 Dy and 36 ℏ in 152 Dy were utilized to obtain information about possible spin-dependent shape changes. The reactions 25 Mg( 134 Xe, 5n), 124 Sn( 34 S, 4n) and 25 Mg( 132 Xe, 5n), 122 Sn( 34 S, 4n) were used to populate the high-spin states in 154 Dy and 152 Dy, respectively. Feeding times as well as lifetimes were determined with the recoil-distance technique. In 152 Dy only long feeding times (⩾ 10 ps) could be identified, indicating that the aligned-particle yrast states are fed through configurations of similar character, with little direct population from collective cascades in the continuum region. In 154 Dy discrete states with I ⩽ 30 ℏ have lifetimes which are characteristically collective, whereas the preyrast cascades exhibit both fast (≲1 ps) and slow (∼10 ps) feeding components. The latter imply a change with increasing spin from collective to aligned-particle character, probably associated with a prolate to oblate shape transition.

Aligned νi132 bands coupled to different shapes in 186Hg

Abstract The structure of 186Hg has been studied through the 156Gd(34S, 4n) reaction. Three bands with even spin and parity are observed; two of them are established up to high spin and are found to cross with little interaction at I= 16+. Cranked shell model calculations suggest that these two bands can be associated with a decoupled ν i 13 2 pair based on a prolate and on an oblate or triaxial shape, respectively.

Lifetime measurements of terminating and collective high-spin states in 155DY and 156DY

Abstract Lifetime measurements for the isotopes 155 Dy and 156 Dy have been extended up to spins around 40ħ by the Doppler shift attenuation method in conjunction with γ-γ-coincidence measurements. Results have been obtained for the yrast band in 156 Dy and for three bands in 155 Dy. Strong retardations of the E2-strengt were observed for intruding terminating sequences, for which specific single particle configurations are suggested.

High-spin neutron hole excitations in light odd-A Pb nuclei☆

Abstract The high-spin level structures of 195,197,199,201,203 Pb have been investigated by ( α , x n γ ) and ( 3 He , x n γ) spectroscopy. The resulting level schemes are interpreted in terms of the weak coupling of an ν i 13 2 hole to known states of the even- A core nuclei.

Superdeformed bands in 189,190Hg

Abstract Experiments using the 160 Gd ( 34 S , x n ) reaction at 159 and 162 MeV have revealed two γ-ray cascades which have properties similar to those of the previously reported superdeformed bands in the heavier Hg isotopes. These sequences have been identified as superdeformed structures in 189Hg and 190Hg. For the band in 190Hg, an average quadrupole moment Q0 = 18 ± 3 e · b was obtained from a DSAM measurement. The data are interpreted within the cranked Woods-Saxon formalism and the rise in the dynamical moment of inertia for both bands is linked to the presence of pair correlations and quasiparticle alignments. For the 189–194Hg superdeformed bands, the variations in the observed population intensity and in the rotational frequencies at which decay out of the bands occur are found to be consistent with the calculated change in the well depth of the superdeformed minimum with mass.

Level structure of 148Gd up to I=44☆

Abstract The level scheme of 148Gd has been extended to I=44 by use of a Compton-suppressed Ge spectrometer array. Up to I=38 the observed level spectrum consists of spherical and oblate states of aligned-particle type. A change in structure along the yrast line is indicated at higher spins, where fast E2 transitions suggest the onset of collectivity.

Evidence for triaxial shapes in Pt nuclei

Abstract Positive parity levels in 191Pt obtained from (α, xnγ) reactions and β-decay are presented as a first example of a rather complete i 13 2 level family. The spectrum confirms triaxial shapes found before from h 11 2 and h 9 2 proton structures in this mass region. In addition to the usual decoupled yrast band, a second ΔI = 2 band within the i 13 2 family, built on a low-lying j−1 = 11 2 state, is observed in agreement with theory.