D. Howe

Decay of high-spin isomers in Os nuclei by barrier penetration

Abstract A decay scheme for the 130±20 ns high-spin isomer in 182 Os has been established. The excitation energy of the isomer is 7049±1 keV and it has I π = 25 (+) . A 2.4% decay directly to the yrast 24 + level at 5988 keV is observed. In 184 Os a 20±5 ns isomer is observed at 2366±1 keV excitation energy with I π = 10 + . Again, direct transitions into the yrast 8 + and 10 + levels are observed. Contrary to previous speculations, there is no compelling evidence for stable triaxial shapes in the structure of the levels through which the isomers decay. The abnormally short half-lives observed, as well as the unusual decay patterns, are best understood in terms of a γ-soft nuclear potential. Motion in the γ-direction allows the isomer to decay via barrier penetration from an axially symmetric prolate shape with the angular momentum along the nuclear symmetry axis (deformation aligned state) via oblate shapes to another prolate shape with the angular momentum perpendicular to the nuclear symmetry axis (rotation-aligned state).

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

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.

Study of high-spin states in 181, 182Os

Abstract High-spin states in the nuclei 181, 182Os have been populated in the 150 Nd( 36 S ,x n ) reactions and studied with the ESSA30 array. The nucleus 181Os has also been studied at the NBI tandem accelerator using the 167Er(18O,4n) reaction. The previously known bands in both nuclei have been extended to higher spins and two new side bands have been found in 181Os. In the latter nucleus the ground state has been established to have I π = 1 2 − . The extraction of the ratios of reduced transition probabilities B(M1) B(E2) from branching and E2 M1 mixing ratios permitted configuration assignments for most of the bands in both nuclei. The analysis has been carried out within the semiclassical vector model for M1 radiation. The positive-parity yrare sequences in 182Os and the band based on the I π = K π = 23 2 − state in 181Os have been interpreted as t-bands arising from a rotation about a tilted axis. The alignment behaviour and the crossing frequencies are for most of the bands consistent with predictions of the cranked shell model.

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

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.

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

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.

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

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.

Near-yrast discrete line γ-ray spectroscopy of 159Er and 160Er to I⩾ or ≈40h̄

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.

Rotational band structures in 154Gd

The N=90 rare earth nucleus 154Gd has been studied in the spin regime of I<or=26h(cross) by using the reaction 150Nd(9Be, 5n)154Gd at 55 MeV. Gamma-gamma coincidence and angular correlation data were obtained using the TESSA2 array. Two positive parity sequences have been extended to 26+ and four negative parity structures have each been observed up to spins of I=22-24h(cross). The rotational structures observed in 154Gd are interpreted within the framework of the cranked shell model (CSM) in terms of quasiparticle excitations. The characteristics of the rotational band sequences and the band crossings that occur are compared with those in neighbouring nuclei and to the predictions of this model. The first, second and third i13/2 neutron alignments in 154Gd and the systematics of the neutron alignments in the negative parity rotational bands in the even-even N=90 nuclei are discussed. Evidence for quasiproton excitations at the highest spins is discussed.

Double crossing in 174Hf: A deformation jump?

Abstract At I π =22 + , the yrast band of 174 Hf is observed to have a second rotational anomaly, which may be linked with the high-spin extension of the K π =0 + “β-band”. A possible explanation of the anomaly is that the nucleus is more deformed in the excited band than in the yrast band.

Search for correlations between prolate-shape collective and oblate-shape non-collective nuclear rotation: High-spin states in 159,160Yb

Abstract High-spin states of 159,160 Yb have been studied using the escape-suppressed array TESSA 2. Extensions of yrast and lateral bands have been found up to I ~ 40. Experimental data suggest strong correlations between maximum alignment configurations of the valence nucleons and related collective states. Theoretical analysis fully supports the idea of prolate-collective versus oblate-noncollective correlations. Band termination interpretation is discussed.