P.D. Forsyth

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

Abstract Rotational bands, characteristic of a superdeformed prolate shape ( e ≈0.6) and extending to above spin 60 h , have been observed in both 150 Gd and 151 Tb. The magnitudes of the moments of inertia I (2) were found to vary with frequency and the variation greatly exceeded that seen in 148,149 Gd and 151,152 Dy. The differences in the I (2) s are attributed to the occupation of particular high- N orbitals. Moreover, contrary to the previous examples the bands in both 150 Gd and 151 Tb de-excited at a much higher rotational frequency of h ω≈0.4 MeV and this may indicate that the pair gap extends to higher frequencies in 150 Gd and 151 Tb.

Hg and Their Dynamical Moments of Inertia

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.

Superdeformed bands in 150Gd and 151Tb: Evidence for the influence of high-N intruder states at large deformations

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.

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

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.

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

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+.

Discrete levels in 154Dy above spin 30ħ: Evidence for terminating band structures

High-spin states in 158Er have been populated with the reactions 145Nd(16O,3n) and 146Nd(16O,4n) and high-spin states in 159Er by the reaction 145Nd(18O,4n). Gamma-gamma coincidence and angular correlation data were obtained using an array of five Ge(Li)-NaI(Tl) escape-suppressed spectrometers. Linear polarisation data were obtained using a sectored Ge(Li) detector and internal conversion data were obtained using a mini-orange spectrometer. In 158Er the yrast band has been observed up to Jpi =26+ and three negative-parity side bands have been observed up to spins Jpi =23-, 22- and 15- respectively. The ground-state band has been observed to continue after the first backbend in the yrast sequence to Jpi =20+ and the odd- and even-spin members of the gamma-vibrational band were observed up to 7+ and 10+, respectively. In 159Er the yrast band has been observed up to Jpi =57/2+ and four side bands have been identified to spins Jpi =15/2-, 39/2+, 49/2-, and 51/2- respectively. The bands observed are interpreted as configurations of quasiparticles using the cranked shell model. Residual quasiparticle interactions are calculated as a function of rotational frequency.

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

Abstract Rotational band structures in 156Dy have been populated via the reactions 148Nd(12C, 4n) and 124Sn(36S, 4n) at beam energies of 65 MeV and 155 MeV, respectively. High-spin states (I ≳ 40kh) have been observed and a detailed level scheme is presented. Two positive-parity bands have been observed to spins I = 42 and 36. Four negative-parity bands have been identified up to or above spin I = 36. New levels are also observed in octupole and gamma vibrational bands. The band structures are understood in terms of quasiparticle configurations using the cranked-shell model. The second and third i 13 2 neutron alignments, the systematics of the first h 11 2 proton alignments in even-even N = 90 isotones and the observation of a third discontinuity in the lowest energy positive- and negative-parity bands near spin 40 in 156Dy are discussed.

Rotational alignment in 158Er and 159Er

Abstract Two new negative-parity and two new positive-parity decay sequences are established to high spin for 161 Lu 90 . The resulting systematics of the routhians, alignments, and B (M1)/ B (E2) ratios indicate that 161 Lu has both a smaller and less stable quadrupole deformation than the lighter N = 90 isotones and the heavier lutetium isotopes. At low spin, the data are consistent with a sizeable negative γ deformation. Above the band crossing associated with the excitation of a pair of i 3 2 quasineutrons, the evidence for γ deformation disappears; however, the configuration-dependent B (M1)/ B (E2) ratios indicate that the nuclear shape may not be completely stable. The systematics of the band crossings based on the excitation of a pair of h 1 2 quasiprotons observed at higher spins in 161 Lu and other odd- and even- Z , N = 90 isotones provides the first experimental evidence from high-spin data for proton quadrupole pair correlations.

Near yrast discrete line gamma-ray spectroscopy in 156Dy up to spin 40kh

High-spin states in 159Er and 160Er have been populated using the 116Cd+48Ca reaction at a 48Ca bombarding energy of 210 MeV. Gamma rays were detected in an array (TESSA2) of six escape suppressed germanium spectrometers and a 50 element BGO multiplicity and summed energy detector. Decay scheme and angular correlation data for these nuclei were deduced from the gamma - gamma -BGO coincidence data. In 159Er the yrast band has been identified up to Ipi =89/2+ and three side bands have been observed to 69/2-(77/2-), 43/2+ and 51/2-(59/2-). In 160Er the lowest energy positive-parity band has been observed up to 38+(40+) and the two lowest energy negative-parity side bands up to 40-(44-) and 39-(41-). The ground state level sequence has been observed up to (28+). The spin and parity of the low-lying states (I<20h(cross)) in the bands observed in 160Er were established by measuring gamma -ray angular distributions and internal conversion coefficients following the 148Nd(16O, 4n) reaction at a beam energy of 80 MeV. The band structures can be understood in terms of aligning quasiparticles using the cranked shell model. The alignment of the first pair of h11/2 protons is observed in the three lowest energy bands in each nucleus. The increase in crossing frequency of this alignment with neutron number is further evidence for the increase in deformation for the light Er isotopes. Evidence is presented for the competition of favoured single-particle configurations with the collective structures at the highest spins.