M. A. Riley

Equilibrium deformations and excitation energies of single-quasiproton band heads of rare-earth nuclei

Abstract Noncollective single-proton states in odd- Z (Eu, Tb, Ho, Tm, Lu, Ta, Ir and Au) rare-earth nuclei have been calculated using the shell correction method with an average Woods-Saxon potential and a monopole pairing residual interaction. Calculated equilibrium deformations of the lowest single-proton states are presented, and calculated band head excitation energies are compared with experimental proton band heads for odd- Z rare-earth nuclei. Good agreement is found between the experimental and calculated band heads. We find that strong polarisation effects due to the odd proton explain many of the systematic trends of known band heads. Different deformation driving forces of the odd-proton orbitals can also partly explain deviations seen in high-spin data. Shape co-existence effects in Ir and Au isotopes are discussed. In addition, equilibrium deformations of even-even rare-earth nuclei are computed and compared with experimental values.

Multiple Superdeformed Bands in

Abstract Three superdeformed bands have been observed in 194Hg. The dynamical moment of inertia J (2) of all three bands is observed to increase by 30–40% over the frequency range ħω = 0.1–0.4 MeV. This phenomena can be understood in terms of the gradual alignment of pairs of high-j intruder orbitals within the framework of the cranked Woods-Saxon and Nilsson models including pairing. The calculations together with the observed J (2) behaviour of the three bands indicate that pairing correlations in the superdeformed minimum are rather weak.

^194

General properties of terminating bands are briefly reviewed and exemplified on the observed high-spin properties of 158Er and 156Er. The very similar features of the positive parity high-spin spectra of the N = 88 isotones of Dy, Er and Yb are pointed out and discussed. The possibility to estimate B(E2)-values in terminating bands from measured feeding times is explored. The spin contribution from different orbitals is calculated in terminating bands as well as more collective bands. One aim is to get some idea of the interaction between different configurations and to this end we also consider a single-j shell model. The limits of terminating bands in the yrast region when the number of valence nucleons increases is discussed. The usefulness of these results when planning future experimental studies is considered.

Hg and Their Dynamical Moments of Inertia

Abstract Rotational bands of 157 Ho have been populated via the 124 Sn( 37 Cl, 4n) reaction at beam energies of 155 and 165 MeV. Gamma-ray spectroscopy was performed using the 8 π spectrometer at Chalk River. Many rotational bands have been observed for the first time. A detailed level scheme is presented, containing approximately 380 transitions, and the quasiparticle structure of the various bands is discussed. Band termination has been observed in the yrast states. For strongly coupled bands, B (M1)/ B (E2) transition strength ratios are extracted and compared with previous measurements and theoretical expectations. Branching ratios for out-of-band E2 transitions are analysed to extract band mixing interaction strengths. Implications for rotational damping are considered. The interaction at the first backbend in the ground band is found to be strongly signature dependent; this is evidence for a signature-dependent triaxial shape of the nucleus.

Properties of Terminating Bands in Nuclei

Abstract Two weakly populated rotational bands have been observed in 191 Hg with properties (energy spacings, moments of inertia and lifetimes) very similar to those of the previously reported superdeformed band. Based on cranked Woods-Saxon calculations, these structures are interpreted as the first excited bands in the superdeformed minimum of 191 Hg. Comparisons between the data and the calculations highlight the role of specific orbitals at large deformations.

Multiple band structure and band termination in 157Ho towards complete high-spin spectroscopy

Abstract Rotational bands have been observed in 157, 158, 159Er to very high spin ( J ∼ 41 h ). Upbends are found, due to the alignment of two h 11 2 protons at 0.40 ⪅ h ω ⪅ 0.46 MeV in all bands. A systematic shift with neutron number of the band-crossing frequency is observed and is related to a change in quadrupole deformation ϵ2.

Excited superdeformed bands in 191Hg

The neutron-rich nucleus ^{144}Ba (t_{1/2}=11.5  s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV ^{144}Ba beam on a 1.0-mg/cm^{2} ^{208}Pb target. The measured value of the matrix element, ⟨3_{1}^{-}∥M(E3)∥0_{1}^{+}⟩=0.65(+17/-23) eb^{3/2}, corresponds to a reduced B(E3) transition probability of 48(+25/-34)  W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

The systematics of h112 proton alignment in 157, 158, 159Er

Abstract The high spin states of 162 Yb and 164 Hf have been studied using 122 Sn( 44 Ca, 4n) and 120 Sn( 48 Sn( 48 Ti, 4n) reactions respectively. The resulting new level schemes for 162 Yb and 164 Hf are presented and compared with existing data for 160 Er. The systematic behaviour of these three N = 92 isotones at large angular momentum yields information both on modifications of the single-neutron spectrum of states resulting from the polarisation of the nuclear field and on the single-proton spectrum of states.

Direct evidence of octupole deformation in neutron-rich 144Ba

Abstract High-spin states in 154 Dy have been studied using the TESSA2 γ-rays spectrometer following the 110 Pd( 48 Ca,4n) 154 Dy reaction at a beam energy of 210 MeV. States up to 44 + and 37 − have been observed. Below spin 30 the data display regular rotational behaviour which can be interpreted in terms of the cranked shell model. Above spin 30, sequences of levels connected by stretched E2 transitions, which show large gains in energy when compared to a rotating liquid drop reference, are lowest in energy for both parities. Particularly low energy levels are observed for spin I π = 36 + and 42 + and in addition dipole transitions are found connecting negative-parity states around spin I = 35. The experimental data for I ≳ 30 are compared with calculations, based on the Nilsson-Strutinsky cranking formalism, in which it is possible to trace fixed configurations through a sequence of spins. For the high-spin positive-parity sequence, the similarity with the 156 Er spectrum is discussed.

High-spin, discrete line spectroscopy of 162Yb and 164Hf and the systematics of N = 92 isotones

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+.