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The neutron-rich dysprosium isotopes $^{168}$Dy$_{102}$ and $^{169}$Dy$_{103}$ have been investigated using the EURICA γ-ray spectrometer, following production via in-flight fission of a high-intensity uranium beam in conjunction with isotope separation through the BigRIPS separator at RIBF in RIKEN Nishina Center. For $^{168}$Dy, a previously unreported isomer with a half-life of 0.57(7) μs has been identified at an excitation energy of 1378 keV, and its presence affirmed independently using γ-γ-γ coincidence data taken with Gammasphere via two-proton transfer from an enriched $^{170}$Er target performed at Argonne National Laboratory. This isomer is assigned Jπ=Kπ=(4−) based on the measured transition strengths, decay patterns, and the energy systematics for two-quasiparticle states in N=102 isotones. The underlying mechanism of two-quasiparticle excitations in the doubly midshell region is discussed in comparison with the deformed QRPA and multi-quasiparticle calculations. In $^{169}$Dy, the B(E2) value for the transition de-exciting the previously unreported Kπ=(1/2−) isomeric state at 166 keV to the Kπ=(5/2−) ground state is approximately two orders of magnitude larger than the E2 strength for the corresponding isomeric-decay transition in the N=103 isotone $^{173}$Yb, suggesting the presence of a significant γ-vibrational admixture with a dominant neutron one-quasiparticle component in the isomeric state.

The neutron-rich dysprosium isotopes ¹⁶⁸Dy₁₀₂ and ¹⁶⁹Dy₁₀₃ have been investigated using the EURICA γ-ray spectrometer, following production via in-flight fission of a high-intensity uranium beam in conjunction with isotope separation through the BigRIPS separator at RIBF in RIKEN Nishina Center. For ¹⁶⁸Dy, a previously unreported isomer with a half-life of 0.57(7) μs has been identified at an excitation energy of 1378 keV, and its presence affirmed independently using γ-γ-γ coincidence data taken with Gammasphere via two-proton transfer from an enriched ¹⁷⁰Er target performed at Argonne National Laboratory. This isomer is assigned Jπ = Kπ = (4⁻) based on the measured transition strengths, decay patterns, and the energy systematics for two-quasiparticle states in N = 102 isotones. The underlying mechanism of two-quasiparticle excitations in the doubly midshell region is discussed in comparison with the deformed QRPA and multi-quasiparticle calculations. In ¹⁶⁹Dy, the B(E2) value for the transition de-exciting the previously unreported Kπ = (1/2⁻) isomeric state at 166 keV to the Kπ = (5/2⁻) ground state is approximately two orders of magnitude larger than the E2 strength for the corresponding isomeric-decay transition in the N = 103 isotone ¹⁷³Yb, suggesting the presence of a significant γ-vibrational admixture with a dominant neutron one-quasiparticle component in the isomeric state.

We report for the first time two distinctive features in the odd–odd nucleus $^{120}$I: a pair of doublet bands and a high-spin isomer built on the πh11/2νh11/2 configuration. For producing the excited states of $^{120}$I, a fusion-evaporation reaction $^{118}$Sn($^{6}$Li, 4n) at E=lab48 MeV was employed. The beam was provided by the 14UD tandem accelerator of the Heavy Ion Accelerator Facility at the Australian National University. The observed doublet structure built on the positive-parity states is the first case and unique in isotopes with Z=53. The emerging properties are indicative of the known chiral characteristics, leading to a doubling of states for the πh11/2νh11/2 configuration. In contrast, the high-spin isomer with a half-life of 49(2) ns at spin-parity Jπ=25+ can be explained in terms of a noncollective oblate structure with the full alignment of six valence nucleons outside the $^{114}$Sn core: three protons (g7/2)1(d5/2)1(h11/2)1 and three neutrons (h11/2)3. This is an outstanding case that reveals a pure single-particle structure consisting of equal numbers of valence protons and neutrons outside the semi-double shell closure of $^{114}$Sn with Z=50 and N=64.

Excited states in the neutron-deficient nuclides W8874162 and W9074164 were investigated by using the ?-ray spectrometer Jurogam. A change in structure is apparent from the first rotational alignments in W162 and W164, whose rotationally aligned bands are interpreted as ?(h9/2)2 and ?(i13/2)2 configurations, respectively. The level schemes have been extended using recoil (-decay) correlations with the observation of excited collective structures. Configuration assignments have been made on the basis of comparisons of the deduced aligned angular momentum, as a function of rotational frequency, with the predictions of the cranked shell model. © 2016 American Physical Society.

Excited states in Re186 with spins up to J=12 were investigated in two separate experiments using W186(d,2n) reactions at beam energies of 12.5 and 14.5 MeV. Two- and threefold γ-ray coincidence data were collected using the CAESAR and CAGRA spectrometers, respectively, each composed of Compton-suppressed high-purity germanium detectors. Analysis of the data revealed rotational bands built on several two-quasiparticle intrinsic states, including a long-lived Kπ=(8+) isomer. Configuration assignments were supported by an analysis of in-band properties, such as gK-gR values. The excitation energies of the observed intrinsic states were compared with results from multi-quasiparticle blocking calculations, based on the Lipkin-Nogami pairing approach, that included contributions from the residual proton-neutron interactions.

© 2015 American Physical Society. A high-spin isomer with τ>700 ns has been found in Hg204, populated in reactions of 1360-MeV Pb208 and 330-MeV Ca48 beams with a thick U238 target and a 1450-MeV Pb208 beam on a thick Pb208 target. The observed γ-ray decay of the isomer has established the yrast states below it, including another isomer with τ=33(3) ns. The experimental results are compared with shell-model calculations that include four holes in the configuration space between Sn132 and Pb208. The available spectroscopic information, including transition strengths, total conversion, and angular correlation coefficients, together with the observed agreement with the calculations, allows spin, parity, and configuration assignments to be proposed for the experimental states. The τ>700 ns isomer is the 22+ state of maximum spin available from the alignment of the four valence holes with the configuration πh11/2-2νi13/2-2.

The 208Pb region is characterised by the existence of collective octupole states. Here we populated such states in 208Pb + 208Pb deep-inelastic reactions. γ-ray angular distribution measurements were used to infer the octupole character of several E3 transitions. The octupole character of the 2318 keV 17- 14+ in 208Pb, 2485 keV 19/2- 13/2+ in 207Pb, 2419 keV 15/2- 9/2+ in 209Pb and 2465 keV 17/2+ 11/2- in 207Tl transitions was demonstrated for the first time. In addition, shell model calculations were performed using two different sets of two-body matrix elements. Their predictions were compared with emphasis on collective octupole states.

Deep-inelastic reactions and γ-ray spectroscopy have been used to study excited states in 195Au. A three-quasiparticle isomer with a mean-life of 18.6(3) μs has been assigned at 2461+Δ keV, with decays into newly identified structures. Possible configurations for the isomer are discussed including a Jπ =31/2- intrinsic state produced by coupling the 11/2-[505] proton hole to the 10+ state obtained from the 9/2+[624],11/2+[615] two-neutron holes, expected from configuration-constrained potential-energy-surface calculations. Residual interactions are evaluated within a semi-empirical shell model basis. New lifetime information is also obtained for the 21/2+ and 25/2 + states at 1813 and 1980 keV. The relationship between these and other newly identified states and the negative-parity states in the even-even neighbors is discussed.

Deep inelastic and transfer reactions with an 820-MeV, 136Xe beam and various ytterbium and lutetium targets have been employed to study high-spin structures in the neutron-rich thulium isotopes beyond 171Tm. Results in the doubly odd nucleus, 174Tm, include the identification of numerous new two- and four-quasiparticle intrinsic states including several isomers below 1 MeV, and the observation of the Kπ=4- ground state rotational band populated via direct decay from a τ=153(10)-μs, Kπ=14- isomer at 2092 keV. The 398-keV, M1 transition linking the isomer and ground state band is abnormally fast for a highly forbidden, ΔK=10 decay. This relative enhancement is explained in terms of mixing of the 13- level with the nearby 13- member of a Kπ=8- rotational band, with an interaction strength of V ≈ 1.4 keV. Multiquasiparticle calculations are compared with the observed states.

High-spin states in the odd-proton thulium isotopes 173Tm and 175Tm have been studied using deep-inelastic reactions and γ-ray spectroscopy. In 173Tm, the low-lying structure has been confirmed and numerous new states have been identified, including a three-quasiparticle Kπ= 19/2- isomer with a lifetime of τ=360(100)ns at 1906keV and a five-quasiparticle Kπ=35/2- isomer with a lifetime of τ= 175(40)ns at 4048keV. The Kπ=35/2- state is interpreted as a t-band configuration that shows anomalously fast decays. In 175Tm, the low-lying structure has been reevaluated, a candidate state for the 9/2-[514] orbital has been identified at 1175keV, and the 7/2-[523] bandhead has been measured to have a lifetime of τ= 460(50)ns. Newly identified high-K structures in 175Tm include a Kπ=15/2- isomer with a lifetime of τ= 64(3)ns at 947keV and a Kπ= 23/2+ isomer with a lifetime of τ= 30(20) μs at 1518keV. The Kπ=15/2- isomer shows relatively enhanced decays to the 7/2-[523] band that can be explained by chance mixing with the 15/2- member of the 7/2- band. Multiquasiparticle calculations have been performed for 173Tm and 175Tm, the results of which compare well with the experimentally observed high-spin states.

Deep-Inelastic reactions have been used to populate high-spin states in the even-even osmium isotopes and in the iridium neighbors. New isomers have been identified in 190Os, 192Os, 194Os, 191Ir and 193Ir. These include a 2 ns 12+ state at 2865 keV and a 295 ns, 20+ state at 4580 keV in 192Os. Although a number of multi-quasiparticle states arising from prolate and triaxial deformations are expected in these nuclei, the main structures in 192Os can be interpreted as a two-stage alignment of i13/2 neutrons at oblate deformation, in close analogy with similar structures in the isotones 194Pt and 196Hg. The isomers are attributed to low-energy E2 transitions at the point of the alignment gains. The isomer observed in 191Ir is long-lived (τm ∼8s) and probably arises from coupling of the h11/2 proton to the 10 -ν/9/2- [505]11/2+ [615] prolate configuration that gives rise to long-lived isomers in 190Os and 192Os, although potential-energy-surface calculations indicate that the resultant three-quasiparticle state will be triaxial.

Gamma rays in the π-2ν2 nucleus 208Hg and π-1ν2 nucleus 209Tl have been studied at Gammasphere using deep-inelastic reactions induced by a 1360 MeV208 Pb beam on a thick 238U target. Previously unknown yrast γ-ray cascades above the 8+ and 17/2+ nanosecond isomers in 208Hg and 209Tl, respectively, were identified in coincidence with known γ rays deexciting the isomers. Yrast levels up to spin 13- in 208Hg have been located, and they are interpreted in light of the structure of the 210Pb isotone and with the help of shell model calculations. Shell model calculations by using the Vlow-k realistic interaction have been performed for 210Pb and 208Hg, and compared with experiment. The V low-k effective Hamiltonian seems to account well for the properties of these neutron-rich shell model nuclei.

A three-quasiparticle isomer with τ=170(8) μs and Kπ= (19/2 +) has been identified in the neutron-rich isotope Ho169. The isomer decays with K-forbidden transitions to members of a band associated with the 7/2-[523] proton configuration, whose structure is characterized through analysis of the in-band γ-ray branching ratios. In the isotone Tm171, the rotational band based on the known 19/2+, three-quasiparticle isomer has also been observed. Alternative one-proton two-neutron configurations for the isomer in Ho169 are discussed in terms of multiquasiparticle calculations and through a comparison with the structures observed in Tm171.

Gamma-ray branches that connect high-K states to low-K states in the s-process nucleus Lu176 were observed, thus providing a link between the 58 Gyr, 7- ground state and the 5.3 h, 1- isomeric state. High sensitivity and unambiguous placement were achieved through the study of the decay of the 58 μs Kπ=14+ isomer using γ-γ-coincidence measurements. The large number of decay paths from the isomer provides a means of populating a broad selection of states from above, resulting, paradoxically, in higher sensitivity than in cases where low-spin input reactions are used. The out-of band decay widths important for excitation processes in stars are quantified.

Isomers in the neutron-rich nucleus W190 have been characterized. A 10 - state from the 9/2-[505] 11/2+[615] two-neutron configuration with a 240-μs lifetime decays via a K-allowed, 97-keV, M2 transition to an 8+ state with a 160-ns lifetime from the 9/2-[505] 7/2-[503] neutron configuration. New states have also been identified in W188, including a Kπ=8-, 158-ns isomer from the 9/2-[614] 7/2+[404] two-proton configuration. The K hindrance is observed to decrease with increasing neutron number, consistent with a trend toward increasing triaxial softness.

The stable and neutron-rich isotopes Er168, Er170, and Er172 have been studied with Gammasphere using inelastic excitation with energetic Xe136 beams. The previously assigned structures based on the proposed Kπ=4- isomeric intrinsic states in both Er168 and Er170 have been re-evaluated and an equivalent band identified in Er172. In Er170, the identification of a Kπ=6- band with transitions close in energy to those of the 4 - band leads to a modified interpretation, since the overlap would have compromised previous analyses. The gK-gR values for the 4- bands deduced from the in-band γ-ray intensities for the sequence of isotopes suggest a predominantly two-neutron configuration in Er168, an equally mixed two-neutron, two-proton configuration in Er170, and a two-proton configuration in Er172. A comprehensive decay scheme for the previously proposed 6+ isomer in Er172 has also been established, as well as band structures built on this isomer that closely resemble the 6+ and 7- two-neutron structures known in the isotone Yb174. The implied K hindrances are discussed. The main decay path of the 6+ isomer occurs through the newly identified 4- isomer. The measured lifetimes of the 4 - and 6+ isomers in Er172 are 57(3) and 822(90) ns, respectively. Multiquasiparticle calculations support the suggested configuration changes across the isotopic chain.

Chopped-beam techniques and γ-ray spectroscopy with Gammasphere have been used to measure the lifetime of the 1112-keV 8- isomeric state in Er174. The value obtained of τ=5.8(4) s corresponds to a reduced hindrance of fν=98 for the 163-keV E1 transition to the 8+ state of the ground-state band, in good agreement with the systematics of the corresponding E1 strengths in the N=106 isotones. The K-mixing in the 8- states is calculated in the context of the particle-rotor model and used to extract the underlying reduced hindrances.

High-spin states in the neutron-rich nucleus Ta185, populated in the decay of a long-lived, three-quasiparticle state, have been studied using deep-inelastic reactions with Xe136 ions and a W186 target. The lifetime of the isomer has been measured as 17(2) ms and the spin and parity determined to be Kπ=21/2-, leading to a π7/2+[404]ν3/2-[512]11/2+[615] configuration assignment. The isomer decays into the rotational band built upon the π9/2-[514] intrinsic state via K-forbidden transitions with reduced hindrances of 52 and 71. The π9/2-[514] state is itself an isomer with a lifetime of 17(3) ns. It decays via K-allowed E1 transitions to states in the π7/2+[404] band with strengths that are similar to equivalent transitions in the lighter tantalum isotopes.

High-spin states populated in the decay of microsecond isomers in the transitional nuclei Sb121 and Sb123 have been investigated in detail in several experiments using γ-ray and electron spectroscopy. The nuclei were formed using multinucleon transfer and fusion-fission reactions with Xe136 beams and also using the Sn120(Li7,α2n)Sb121 and Sn122(Li7,α2n)Sb123 incomplete-fusion reactions. Isomeric half-lives ranging from several nanoseconds to a few hundred microseconds were determined by means of conventional decay curve analyses, whereas very short-lived isomers (T1/2~1 ns) were identified using the generalized centroid-shift method. A number of new transitions were observed, including a branch through spherical states from the 19/2+ member of the 9/2+ deformed band in Sb121, in competition with the main decay path through the rotational band. This is attributed to mixing between the 19/2+ band member and a 19/2+ spherical state. Both levels are predicted to coincide approximately in energy in Sb121. The fact that a 25/2+ isomer occurs for A=121 and the lighter isotopes, while a 23/2+ isomer is observed for A=123-131 is explained through a multistate mixing calculation, taking into account the gradual shift of the 2d5/2 and 1g7/2 proton orbitals and the change in proton-neutron effective interactions from an attractive particle-particle type in the lower part of the shell to a repulsive particle-hole type with increasing the neutron number toward the N=82 shell closure. The observed enhancement of the B(E2;19/2-→15/2-) values in Sb121 and Sb123 over the B(E2;7-→5-) values in the corresponding Sn cores is discussed in terms of configuration mixing between spherical and deformed states.

The nuclei I131 and I133 have been populated in multinucleon transfer reactions between Xe136 ions and various targets, and their structure has been investigated by time-correlated γ-ray coincidence spectroscopy and the measurement of γ-ray angular correlations. A 19/2- isomer at 1918 keV, with a half-life of 24(1) μs, has been identified in I131, as well as nanosecond isomers with Jπ=23/2+ in both isotopes. A T1/2=25(3) ns isomer at 4308 keV in I131 is suggested to have Jπ=(31/2-,33/2-) and is primarily attributed to the coupling of an odd proton in the d5/2 or g7/2 orbit with the (π2)0+(νh11/2-3d3/2-1)15- configuration in Te130 responsible for the 15- isomer in that nucleus. The observed level properties are compared with predictions of a shell-model calculation.

Four-, six-, and eight-quasiparticle isomers with Kπ=14-, (21+) and (26-) have been identified in the deformed nucleus Lu174, in addition to the previously reported Kπ=13+, four-quasiparticle isomeric state. Analysis of alignments and in-band crossover-to-cascade branching ratios lead to the characterization of the configurations associated with the Kπ=14- and (21+) isomers. The excitation energies of the observed states are compared with results from multi-quasiparticle calculations that include effects of blocking and residual nucleon-nucleon interactions. Good agreement is found for medium-spin (I=13-14) and the highest spin (I>20) states observed, but there remain ambiguities in assigning configurations in the I=15-19 region.

Isomers in near-spherical Z=51, antimony isotopes are reported here for the first time using fusion-fission reactions between Al27 and a pulsed Hf178 beam of energy, 1150 MeV. γ rays were observed from the decay of isomeric states with half-lives, T1/2=200(30) and 52(3)μs, and angular momenta I=(252) and Iπ=232+, in Sb121,123, respectively. These states are proposed to correspond to ν(h112)2 configurations, coupled to an odd d52 or g72 proton. Nanosecond isomers were also identified at Iπ=192- [T1/2=8.5(5) ns] in Sb121 and Iπ=(152-) [T1/2=37(4) ns] in Sb123. Information on spins and parities of states in these nuclei was obtained using a combination of angular correlation and intensity-balance measurements. The configurations of states in these nuclei are discussed using a combination of spin/energy systematics and shell-model calculations for neighboring tin isotones and antimony isotopes.

A Kπ=13+, 280 ns four-quasiparticle isomer in the odd-odd nucleus Lu174 has been identified and characterized. The isomer decays to both Kπ=7+ and Kπ=0+ rotational bands obtained from the parallel and antiparallel coupling of the proton 7/2+[404] and neutron 7/2+[633] orbitals. K mixing caused by particle-rotation coupling explains the anomalously fast transition rates to the 7+ band but those to the 0+ band are caused by a chance degeneracy between the isomer and a collective state, allowing the mixing matrix element for a large K difference to be deduced.

Two Gammasphere experiments have been performed in order to establish the possible triaxial nature of strongly deformed (SD) bands in 174Hf. A lifetime measurement, using the Doppler-shift attenuation method, confirmed the large deformation of the four previously observed bands in this nucleus with transition quadrupole moments ranging from 12.6 to 13.8 b. These values are significantly larger than those predicted for triaxial minima by ultimate cranker (UC) calculations. A thin-target, high-statistics experiment was also carried out to search for linking transitions between the SD bands. No such transitions, which represent an experimental signature for wobbling modes, were observed. Four additional SD bands were found in 174Hf together with a single SD band in 173Hf. These results indicate that the strongly deformed sequences of N≈102 Hf isotopes behave differently than the triaxial strongly deformed (TSD) bands found in Lu nuclei near N=92. The interpretation of these bands in terms of possible stable triaxial deformation is confronted with the experimental findings and UC predictions.

A study of high spin states in the odd-neutron isotones 239Pu and 237U is reported. Striking differences were found in the high-spin properties of rotational bands built on the 1/2+[631] ground states in these two nuclei. These differences mirror those observed in the even-even Pu and U immediate neighbors and appear to be related to the strength of octupole correlations.

Matière Nucléaire NESTER; International audience; The excitation energy of the lowest-energy superdeformed band in $^{196}$Pb is established using the techniques of time-correlated $\gamma$-ray spectroscopy. Together with previous measurements on $^{192}$Pb and $^{194}$Pb, this result allows superdeformed excitation energies, binding energies, and two-proton and two-neutron separation energies to be studied systematically, providing stringent tests for current nuclear models. The results are examined for evidence of a "superdeformed shell gap."

γ-ray spectroscopy of high-spin states of the neutron-deficient nucleus Pb189 has been conducted with the Gd158(Ar36,5n) and Er164(Si29,4n) reactions. With the first of these, detection of evaporation residues and mass gating were used to unambiguously assign a number of prompt γ-ray transitions to Pb189. With the second reaction and a pulsed beam, an isomer with a mean life of 32 μs was found. Although inconclusive, the available evidence favors identification of the isomer with the 332+ state of the ν(i13/2)-3 configuration. The levels observed below the isomer can be identified with states involving three different structures: the neutron (i13/2)-3 multiplet in the spherical well; a prolate-deformed band involving mixed i13/2 neutron orbitals; and a state with the oblate π(2p-2h)0+ν(i13/ 2)-1 configuration. The evidence for structures associated with different shapes is supported by the observation of E0 components in some of the Jπ→Jπ transitions linking them.

The stable nucleus 174Yb has been studied using deep-inelastic reactions and time-correlated γ-ray spectroscopy. New intrinsic states assigned include a 370-ns isomer at 1765 keV, which we associate with a predicted Kπ = 7- two-quasineutron configuration. Analysis of the alignment and in-band properties of its rotational band, identified using time-correlated coincidences, allows characterization of the configuration. The properties of a newly identified rotational band built on the known 830-μs isomer at 1518 keV support the 6+, 2-quasineutron configuration assignment proposed previously. The 6+ band is fed by a four-quasiparticle, Kπ = 14+ isomer at 3699 keV and several higher multiquasiparticle states, including a six-quasiparticle isomer at 6147 keV with K = (22,23). The results are discussed in terms of the states predicted on the basis of multiquasiparticle calculations. The anomalously fast K-forbidden transition strengths from the 14+ isomer are attributed to either K mixing in the neutron configuration or to random mixing in the high-level-density region. The 7- isomer decays are not abnormal, whereas the very hindered E2 transition from the 6+ isomer to the ground-state band remains unexplained.

Deep-inelastic reactions between a beam of 1360 MeV 208Pb ions and a thick 238U target have been used to populate the neutron-rich nucleus 211Pb. The observation of its γ decay has allowed identification of excited states up to the highest spin which can be formed from the three valence neutrons, including identification of three high-spin isomers. Level energies and transition strengths are compared to shell-model calculations with empirical interactions and predictions are made for the expected behaviour of more neutron-rich lead isotopes. The evidence for a possible increase in the neutron effective charge moving away from the N=126 shell gap is evaluated.

Two high-spin regularly spaced rotational bands with large dynamical moments of inertia have been identified in 175Hf with the Gammasphere spectrometer. These new bands are very similar to the previously identified triaxial superdeformed bands in the hafnium nuclei. However, the new bands in 175Hf have been linked into the known level scheme and thereby provide the first firm spin assignments for these structures in this region. In order to understand the new bands, theoretical calculations have been performed based on the ULTIMATE CRANKER code. The new bands in 175Hf are deduced to be built upon highly deformed structures. No experimental evidence for triaxiality was established and this work suggests that the structure of the so-called "triaxial" superdeformed bands in the Hf nuclei may be quite different from those identified in the lighter mass Lu nuclei. Since the two highly deformed bands in 175Hf are associated with different deformations, this work also identifies the role of the intruder orbits in polarizing the nuclear shape.

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