The level structure of 158Er has been studied using the Gammasphere spectrometer via the 114Cd(48Ca, 4n) reaction at 215 MeV with both thin (self-supporting) and thick (backed) targets. The level scheme has been con-siderably extended with more than 200 new transitions and six new rotational structures, including two strongly coupled high -K bands. Configuration assignments for the new structures are based on their observed alignments, B(M1)/B(E2) ratios of reduced transition probabilities, excitation energies, and comparisons with neighboring nuclei and theoretical calculations. With increasing angular momentum, this nucleus exhibits Coriolis-induced alignments of both neutrons and protons before it then undergoes a rotation-induced transition from near-prolate collective rotation to a noncollective oblate configuration. This transition occurs via the mechanism of band termination around spin 45 h over bar in three rotational structures. Two distinct lifetime branches, consistent with the crossing of a collective "fast" rotational structure by an energetically favored "slow" terminating sequence, are confirmed for the positive-parity states, and similar behavior is established in the negative-parity states. Weak-intensity, high-energy transitions are observed to feed into the terminating states. At the highest spins,

Rotational structures have been measured using the Jurogam II and GAMMASPHERE arrays at low spin following the Gd-155(alpha, 2n) Dy-157 and Nd-148(C-12, 5n) Dy-155 reactions at 25 and 65 MeV, respectively. We report high-K bands, which are conjectured to be the first candidates of a K-pi = 2(+)gamma vibrational band, built on the [505]11/2(-) neutron orbital, in both odd-A Dy-155,Dy-157 isotopes. The coupling of the first excited K = 0(+) states or the so-called beta vibrational bands at 661 and 676 keV in Dy-154 and Dy-156 to the [505]11/2(-) orbital, to produce a K-pi = 11/2(-) band, was not observed in both Dy-155 and Dy-157, respectively. The implication of these findings on the interpretation of the first excited 0(+) states in the core nuclei Dy-154 and Dy-156 are also discussed.

Low-spin states of Dy-157 have been studied using the JUROGAM II array, following the Gd-155 (alpha, 2n) reaction at a beam energy of 25 MeV. The level scheme of Dy-157 has been expanded with four new bands. Rotational structures built on the [523]5/2(-) and [402]3/2(+) neutron orbitals constitute new additions to the level scheme as do many of the inter- and intraband transitions. This manuscript also reports the observation of cross I+ -> (I-1)(-) and I- -> (I-1)(+) E1 dipole transitions interlinking structures built on the [523]5/2(-) (band 5) and [402]3/2(+) (band 7) neutron orbitals. These interlacing band structures are interpreted as the bands of parity doublets with simplex quantum number s = -i related to possible octupole correlations.

A comprehensive systematic study is made for the collective beta and gamma bands in even-even isotopes with neutron numbers N = 88 to 92 and proton numbers Z = 62 (Sm) to 70 (Yb). Data, including excitation energies, B(E0) and B(E2) values, and branching ratios from previously published experiments are collated with new data presented for the first time in this study. The experimental data are compared to calculations using a five-dimensional collective Hamiltonian (5DCH) based on the covariant density functional theory (CDFT). A realistic potential in the quadrupole shape parameters V (beta, gamma) is determined from potential energy surfaces (PES) calculated using the CDFT. The parameters of the 5DCH are fixed and contained within the CDFT. Overall, a satisfactory agreement is found between the data and the calculations. In line with the energy staggering S(I) of the levels in the 2(gamma)+ bands, the potential energy surfaces of the CDFT calculations indicate gamma-soft shapes in the N = 88 nuclides, which become gamma rigid for N = 90 and N = 92. The nature of the 0(2)(+) bands changes with atomic number. In the isotopes of Sm to Dy, they can be understood as beta vibrations, but in the Er and Yb isotopes the 0(2)(+) bands have wave functions with large components in a triaxial superdeformed minimum. In the vicinity of Sm-152, the present calculations predict a soft potential in the beta direction but do not find two coexisting minima This is reminiscent of Sm-152 exhibiting an X(5) behavior. The model also predicts that the 0(3)(+) bands are of two-phonon nature, having an energy twice that of the 0(2)(+) band. This is in contradiction with the data and implies that other excitation modes must be invoked to explain their origin.

Garrett et al. systematically investigated band-crossing frequencies resulting from the rotational alignment of the first pair of i(13/2 )neutrons (AB) in rare-earth nuclei. In that study, evidence was found for an odd-even neutron number dependence attributed to changes in the strength of neutron pairing correlations. The present paper carries out a similar investigation at higher rotational frequencies for the second pair of aligning i(13/2 ) neutrons (BC). Again, a systematic difference in band-crossing frequencies is observed between odd-N and even-N Er, Yb, Hf, and W nuclei, but in the BC case, it is opposite to the AB neutron-number dependence. These results are discussed in terms of a reduction of neutron pairing correlations at high rotational frequencies and of the effects of Pauli blocking on the pairing field by higher-seniority configurations. Also playing a significant role are the changes in deformation with proton and neutron numbers, the changes in location of single-particle orbitals as a function of quadrupole deformation, and the position of the Fermi surface with regard to the various Omega components of the neutron i(13/2 ) shell.

© 2019 American Physical Society Published version Majola, S. N. T.; Bark, R. A.; Bianco, L.; Bucher, T. D.; Bvumbi, S. P.; Cullen, D. M.; Garrett, P. E.; Greenlees, P. T.; Hartley, D.; Hirvonen, J.; Jakobsson, U.; Jones, P. M.; Julin, R.; Juutinen, S.; Ketelhut, S.; Kheswa, B. V.; Korichi, A.; Lawrie, E. A.; Masiteng, P. L.; Maqabuka, B.; Mdletshe, L.; Minkova, A.; Ndayishimye, J.; Nieminen, P.; Newman, R.; Nyakó, B. M.; Ntshangase, S. S.; Peura, P.; Rahkila, P.; Riedinger, L. L.; Riley, M.; Roux, D.; Ruotsalainen, P.; Saren, J.; Sharpey-Schafer, J. F.; Scholey, C.; Shirinda, O.; Sithole, A.; Sorri, J.; Stolze, S.; Timár, J.; Uusitalo, J.; Zimba, G.

An experiment populating low/medium-spin states in Dy-156 was performed to investigate the possibility of tetrahedral symmetry in this nucleus. In particular, focus was placed on the low-spin, negative-parity states since recent theoretical studies suggest that these may be good candidates for this high-rank symmetry. The states were produced in the Nd-148(C-12, 4n) reaction and the Gammasphere array was utilized to detect the emitted. rays. B(E2)/B(E1) ratios of transition probabilities from the low-spin, negative-parity bands were determined and used to interpret whether these structures are best associated with tetrahedral symmetry or, as previously assigned, to octupole vibrations. In addition, several other negative-parity structures were observed to higher spin and two new sequences were established.

The lifetimes of the first excited 2^{+} and 4^{+} states in ^{72}Ni were measured at the National Superconducting Cyclotron Laboratory with the recoil-distance Doppler-shift method, a model-independent probe to obtain the reduced transition probability. Excited states in ^{72}Ni were populated by the one-proton knockout reaction of an intermediate energy ^{73}Cu beam. γ-ray-recoil coincidences were detected with the γ-ray tracking array GRETINA and the S800 spectrograph. Our results provide evidence of enhanced transition probability B(E2;2^{+}→0^{+}) as compared to ^{68}Ni, but do not confirm the trend of large B(E2) values reported in the neighboring isotope ^{70}Ni obtained from Coulomb excitation measurement. The results are compared to shell model calculations. The lifetime obtained for the excited 4_{1}^{+} state is consistent with models showing decay of a seniority ν=4, 4^{+} state, which is consistent with the disappearance of the 8^{+} isomer in ^{72}Ni.

The first rotational sequences have been assigned to the odd-odd nucleus Re-168. Coincidence relationships of these structures with rhenium x rays confirm the isotopic assignment, while arguments based on the gamma-ray multiplicity (K-fold) distributions observed with the new bands lead to the mass assignment. Configurations for the two bands were determined through analysis of the rotational alignments of the structures and a comparison of the experimental B(M1)/B(E2) ratios with theory. Tentative spin assignments are proposed for the pi h(11/2)nu i(13/2) band, based on energy level systematics for other known sequences in neighboring odd-odd rhenium nuclei, as well as on systematics seen for the signature inversion feature that is well known in this region. The spin assignment for the pi h(11/2)nu(h(9/2)/f(7/2)) structure provides additional validation of the proposed spins and configurations for isomers in the Au-176 -> Ir-172 -> Re-168 alpha-decay chain.

Excited states in the N = 88 nucleus Tb-153 were observed up to spin similar to 40 in an experiment utilizing the Gammasphere array. The Tb-153 states were populated in a weak alpha 4n evaporation channel of the Cl-37 + Sn-124 reaction. Two previously known sequences were extended to higher spins, and a new decoupled structure was identified. The pi h(11/2) band was observed in the spin region where other N = 88 isotopes exhibit effects of prolate to oblate shape changes leading to band termination along the yrast line, whereas Tb-153 displays a persistent collective behavior. However, minor perturbations of the very highest state in both signatures of this h(11/2) band are observed, which perhaps signal the start of the transition towards band termination. (Less)

The exact nature of the lowest K-pi = 2(+) rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the. degree of freedom perpendicular to the nuclear symmetry axis. Very few such. bands have been traced past the usual backbending rotational alignments of high-j nucleons. We have investigated the structure of positive-parity bands in the N = 90 nucleus Dy-156, using the Nd-148(C-12,4n) Dy-156 reaction at 65 MeV, observing the resulting. gamma-ray transitions with the Gammasphere array. The even-and odd-spin members of the K-pi = 2(+)gamma band are observed up to 32(+) and 31(+), respectively. This rotational band faithfully tracks the ground-state configuration to the highest spins. The members of a possible. vibration built on the aligned yrast S band are observed up to spins 28(+) and 27(+). An even-spin positive-parity band, observed up to spin 24(+), is a candidate for an aligned S band built on the seniority-zero configuration of the 0(2)(+) state at 676 keV. The crossing of this band with the 0(2)(+) band is at h omega(c) = 0.28(1) MeV and is consistent with the configuration of the 0(2)(+) band not producing any blocking of the monopole pairing.

A significant extension of the level scheme for the odd-odd nucleus Re-172 was accomplished through the use of the Gammasphere spectrometer. States up to a tentative spin assignment of 39 were observed and two new structures were identified. Configuration assignments are proposed based on alignment properties and observed band crossings.

Excited states of Hf-163 were populated using the Zr-94(Ge-74,5n) reaction and the decay gamma rays were measured with the Gammasphere spectrometer. Two previously known bands were extended to higher spins, and nine new bands were identified. In addition to bands associated with three- and five-quasiparticle configurations, two gamma-vibrational bands coupled to the i(13/2) excitation were also observed. The lowest level of a newly identified, negative-parity band is proposed to be the ground state of the nucleus. A systematic delay of the high-spin proton crossing frequency with increasing quadrupole deformation from Hf-162 to Hf-172 was established. Extensive band searches failed to reveal a triaxial, strongly deformed structure in Hf-163 similar to the one observed in several nuclei around A similar to 165.

Knowledge of the exact microscopic structure of the 01+ ground state and first excited 02+ state in 150Sm is required to understand the branching of double β decay to these states from 150Nd. The detailed spectroscopy of 150Sm and 152Gd has been studied using (α,xn) reactions and the γ-ray arrays AFRODITE and JUROGAM II. Consistently strong E1 transitions are observed between the excited Kπ = 02+ bands and the lowest negative parity bands in both nuclei. These results are discussed in terms of the possible permanent octupole deformation in the first excited Kπ = 02+ band and also in terms of the “tidal wave” model of Frauendorf.Received 10 December 2012DOI:https://doi.org/10.1103/PhysRevC.87.044333©2013 American Physical Society

The phenomenon of wobbling can only occur for a nuclear shape with stable triaxial deformation. To date, only a few examples of this exotic collective mode have been observed in lutetium and tantalum isotopes. A search for a wobbling sequence was performed in Re-171 to determine if this feature can be observed in Z > 73 nuclei. No evidence was found for wobbling; however, an interaction between the pi i(13/2) sequence and another positive-parity band may give an indication on why wobbling may not occur in this nucleus. The level scheme for Re-171 was significantly extended and interpretations for the decay sequences are proposed within the context of the cranked shell model.

Although the observation of wobbling was once thought to be possibly confined to lutetium isotopes in N approximate to 94 nuclei, the identification of this exotic collective mode in Ta-167 has raised the question of the role of the proton Fermi surface with regard to this phenomenon. To investigate this issue, an experiment was performed to populate high-spin states in the N = 94 nucleus Re-169. The heavy-ion reaction Mn-55 + Sn-118 was used in conjunction with Gammasphere to detect the emitted gamma rays. More than 130 new transitions were added to the Re-169 level scheme, including the first identification of the pi i(13/2) rotational sequence in this nucleus. This configuration is the structure on which all known wobbling sequences are based, but no wobbling band was observed, likely owing to the fact that the pi i(13/2) sequence is located at a relatively high energy in comparison with the other structures found in Re-169. Nine decay sequences are now established in this nucleus and are described within the context of the cranked shell model. In addition, significant extension of the level scheme of the odd-odd Re-170 nucleus was possible and a discussion of the residual interactions for the pi h(9/2)nu i(13/2) and pi i(13/2)nu i(13/2) configurations in this region is given as well. DOI: 10.1103/PhysRevC.87.024315