Search

Metastable states with T$_{1/2}$ = 8(2) ms in $^{205}$Bi and T$_{1/2}$ = 0.22(2) ms in $^{204}$Pb, with ≈ 8 MeV excitation energy and angular momentum ≥ 22 ħ, have been established. These represent, by up to two orders of magnitude, the longest-lived nuclear states above an excitation energy of 7 MeV, ever identified in the nuclear chart. Additionally, the half-life of the 10.17 MeV state in $^{206}$Bi has been determined to be 0.027(2) ms, the next highest value in this highly excited regime. These observations indicate the emergence of an island of extreme nuclear isomerism arising from core-excited configurations at high excitation in the vicinity of the doubly closed-shell nucleus $^{208}$Pb. These results are expected to provide discriminating tests of the effective interactions used in current large-scale shell-model calculations.

Isomers with three- and five-nucleon-hole configurations have been established in Tl203. These include newly identified levels with a three-nucleon-hole structure: Iπ = (15/2-) with T1/2 = 7.9(5) ns and Iπ = (35/2-) with T1/2 = 4.0(5) ns. In addition, five-nucleon-hole states have also been established: Iπ = (39/2-) with T1/2 = 1.9(2) ns and Iπ = (49/2+) with T1/2 = 3.4(4) ns. The previously determined long-lived decay, T1/2 = 6.6(3) μs from this work, is associated with isomerism of the Iπ = (29/2+) state. Levels above this long-lived isomer have been identified through a delayed-prompt γ-γ coincidence measurement. Five-nucleon-hole states with excitation energies Ex≈ 7 MeV have been established as well as possible octupole excitations of the Pb208 core built on these levels. The level scheme of Tl203 is extended up to Ex≈ 11 MeV with the inclusion of 25 new transitions. Empirical and shell-model calculations have been performed to aid in the description of the observed states which are found to be of intrinsic character.

Isomeric states and associated collective structures have been studied up to high spin in Hg198,200,202 using multinucleon transfer reactions and the Gammasphere array. A coupled rotational band, with possible four-quasiparticle character, is established in Hg198. Sequences built on two-quasiparticle, positive-and negative-parity levels are assigned to Hg202. New isomers in Hg202 with Iπ=(7-) and (9-), and T1/2 = 10.4(4) ns and 1.4(3) ns, respectively, have been identified. A half-life of 1.0(3) ns is established for the Iπ=12+ state in Hg200. B(E2) values deduced from isomeric transitions in Hg isotopes indicate that, while collectivity near the ground state gradually diminishes from N = 112 to N = 124, it is found to increase for the 12+ and 9-states up to N = 118, followed by a reduction for higher neutron numbers. Calculations using the ultimate cranker code provide insight into the variation of deformation with spin and allow for an understanding of observed band crossings. The evolution of collectivity with spin, and along the isotopic chain, is described.

International audience; Nonyrast, excited states in neutron-rich W186 were populated via inelastic-scattering reactions using beams of Xe136 nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particle-γ coincidence techniques using the Gammasphere array of high-purity germanium detectors and the compact heavy-ion counter, CHICO2. The Kπ=2+ (γ), Kπ=0+ and Kπ=2− (octupole) rotational side bands were extended to spins 14ℏ, 12ℏ, and 13ℏ, respectively. A staggering pattern observed in the energies of levels in the Kπ=2+ band was found to be consistent with a potential that gets softer to vibration in the γ degree of freedom with increasing spin. The odd-even staggering of states in the Kπ=2− band was found to exhibit a phase opposite to that seen in the γ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in W186.

Nonyrast, excited states in neutron-rich 186W were populated via inelastic-scattering reactions using beams of 136Xe nuclei accelerated to 725 and 800 MeV. Levels populated in the reactions were investigated via particleγ coincidence techniques using the Gammasphere array of high-purity germanium detectors and the compact heavy-ion counter, CHICO2. The Kπ = 2+ (γ), Kπ = 0+ and Kπ = 2− (octupole) rotational side bands were extended to spins 14¯h,12¯ h, and 13¯h, respectively. A staggering pattern observed in the energies of levels in the Kπ = 2+ band was found to be consistent with a potential that gets softer to vibration in the γ degree of freedom with increasing spin. The odd-even staggering of states in the Kπ = 2− band was found to exhibit a phase opposite to that seen in the γ band; an effect most probably associated with Coriolis coupling to other, unobserved octupole vibrational bands in 186W.

The excited level structures of Tl202 and Pb203, above the 7+ and 29/2- isomers, respectively, have been studied. An isomer with Iπ=20+ and T1/2=215(10)μs has been established in Tl202, and the level scheme extended from I=10 to 20ℏ with the placement of fifteen new transitions. In Pb203, the Iπ=37/2+ state is established to be metastable, with T1/2=2.5(3)ns. Levels in both nuclei arise from intrinsic excitations, with likely particle-hole character for the higher-lying states in Pb203. The 20+ isomer in Tl202 is most likely associated with a πh11/2-1 - ν(i13/2-2,f5/2-1) configuration, while the 37/2+ state in Pb203 results from the excitation of five neutrons. Calculations, using both an empirical approach and the oxbash code, have been performed to aid in the description of the excited level structure.

The structure of decoupled positive-parity and semidecoupled negative-parity bands up to high spin in Hg197,199 has been studied using multinucleon transfer reactions with the Gammasphere array. A positive-parity band in Hg199 with the (νi13/2)3 decoupled configuration that deexcites to levels in the negative-parity sequences is newly established. The semidecoupled sequences in both isotopes are extended to higher spin. Rotation-aligned angular momenta and crossing frequencies determined from experiment are in fair agreement with those from cranking calculations. A transition from oblate collective rotation at low spin to triaxial noncollective shapes at high spin is indicated.

A six-quasiparticle isomer with T1/2 = 57(2) ns has been established in the doubly-odd isotope Tl200 and its level scheme is significantly extended. Half-lives of previously reported isomers in Tl200 and Pb201,202 have been determined and revised values are reported in a few cases, with T1/2 = 397(17) ns and 7.0(5) ns for the Iπ=5+ and 7- states in Tl200, 52(2) ns for the 41/2+ level in Pb201, and 93(4) ns for the 16+ state in Pb202. Configurations for the isomers have been assigned and these predominantly involve intrinsic excitations of neutrons from the i13/2 subshell. The inferred transition rates for the decay of these isomers compare well with single-particle estimates attesting to their intrinsic character.

The structure of the odd-A isotopes Pt193,195,197, which lie along the line of stability, has been studied up to high spin through multinucleon transfer reactions. Positive- and negative-parity sequences in Pt193,195 have been considerably extended and multiple band crossings established. An isomer with T1/2=5.0(5) ns and Iπ=25/2- is present in Pt195. The isotopes Pt193,195 are characterized by moderate oblate deformation, and angular momentum generation at high spin in the yrast, positive-parity sequences is attributed to the rotation alignment of i13/2 neutrons and h11/2 protons. A detailed understanding of the observed features is presented using calculations based on tilted axis cranking covariant density functional theory as well as others with the ultimate cranker code.

International audience; Triaxial Strongly Deformed (TSD) band structures have been observed in several LuHf-Ta isotopes in the A∼165 region. The occurrence of TSD bands in Lu (Z=71) and Hf (Z=72) isotopes is not consistent with predicted proton and neutron shell gaps, probably due to inadequate knowledge of energies of high-j orbitals at large triaxiality. Based on calculated shell gaps, among the Lu isotopes, 165Lu94 and 168Lu97 are good candidates for TSD structures. However, the strongest TSD band in 163Lu92 is observed to be three times more strongly populated than the corresponding one in 165Lu94, and TSD structures have not yet been observed in 168Lu97. Further, no TSD bands have been observed in 166Hf94, which is located at the centre of the predicted TSD island. The above observations appear to indicate that, at large triaxiality, TSD structures may be favored at N=92 than at N=94. Calculations by R. Bengtsson indicate that TSD band in 166W, with a positive γ value should become yrast at spin, I∼44~. The aforesaid systematic calculations of TSD and Normal Deformed (ND) structures in Hf, Yb and W isotopes further suggest that the N=92 isotones in these three elements have higher excitation energies at normal deformation in the spin range of 30-50 ~ [1]. So, the favorable position of TSD bands in the N=92 isotones may be an outcome of the fact that the band at normal deformation goes up in energy for this neutron number. There were two previous measurements to study high-spin states in 166W. In the first measurement, a 28Si beam was used by Gerl et al. [2] and four HPGe detectors were used to record data. In the second measurement by Simpson et al. [3], the p2n reaction channel was used, and the deexciting γ rays were detected using the POLYTESSA array. The positive-parity yrast band was observed up to an excitation energy of 9 MeV, and spin, I = 30~, well below where possible TSD bands are expected to be yrast. Two other negativeparity rotational bands were also reported up to spin, I = 23~.

Fast-rotating N=151 isotones 245Pu, 247Cm and 249Cf have been studied through inelastic excitation and transfer reactions with radioactive targets. While all have a ground-state band built on a νj15/2[734]9/2− Nilsson configuration, new excited bands have also been observed in each isotone. These odd-N excited bands allow a comparison of the alignment behavior for two different configurations, where the νj15/2 alignment is either blocked or allowed. The effect of higher order deformations is explored through cranking calculations, which help clarify the elusive nature of νj15/2 alignments. Keywords: Superheavy, Neutron-rich, Inelastic and transfer reactions, Rotational alignments, Higher-order deformations

Expérience GANIL/VAMOS/EXOGAM; International audience; High-spin states in 132 Te, an isotope with two proton particles and two neutron holes outside of the 132 Sn doubly magic core, have been extended up to an excitation energy of 6.17 MeV. The prompt-delayed coincidence technique has been used to correlate states above the T 1/2 = 3.70(9) μs isomer in 132 Te to the lower states using 232 Th(7 Li ,f) at 5.4 MeV/u and the Indian National Gamma Array (INGA). With 9 Be(238 U ,f) at 6.2 MeV/u and EXOGAM γ-array coupled with the VAMOS++ spectrometer, the level scheme was extended to higher excitation energies. The high-spin positive-parity states, above J π = 10 + , in 132 Te are expected to arise from the alignment of the particles in the high-j orbitals lying close to the Fermi surface, the πg 2 7/2 , and the νh −2 11/2 configurations. The experimental level scheme has been compared with the large scale shell model calculations. A reduction in the p-n interaction strength resulted in an improved agreement with the measurements up to the spin of 15. In contrast, the comparison of the differences between the experiment and these calculations for the N = 76,78 isotones of Te and Sn shows the increasing disagreement as a function of spin, where the magnitude is larger in Te than in Sn. This behavior could possibly be attributed to the deficiencies in the p-n correlations, in addition to the n-n correlations in Sn.

The decay of a Kπ=8- isomer in Pu244 and the collective band structures populating the isomer were studied using deep inelastic excitations with Ti47 and Pb208 beams, respectively. Precise measurements of M1/E2 branching ratios in the band confirm a 9/2-[734]ν - 7/2+[624]ν configuration assignment for the isomer, validating the systematics of Kπ=8-, two-quasineutron isomers observed in even-Z, N=150 isotones. These isomers around the deformed shell gap at N=152 provide critical benchmarks for theoretical predictions of single-particle energies in this gateway region to superheavy nuclei.

Multi-quasiparticle high-K states, several of which are isomeric, were observed in Hf180 with the Gammasphere array. Lifetimes in the ns-μs range were determined using centroid-shift and decay measurements within a μs coincidence time window. The configurations of high-K states involve two and four quasiparticles, with states up to Kπ=(18-) established. High-K excitations are found to be progressively more favored with increasing excitation energy. The K quantum number is quite robust up to the highest spins observed, as evidenced by the large values of the reduced hindrance for isomeric decays. Rotational bands built on three high-K states are identified, and the measured branching ratios in these sequences enable the assignment of underlying configurations. Multi-quasiparticle calculations using the Lipkin-Nogami approach for pairing, with blocking included, reproduce the observed high-K energies quite well.

An oblate rotational sequence, built on an aligned, two-quasineutron isomeric state has been established in Pt196. The isomer has a half-life of 7.7(7) ns and is associated with the Iπ=12+,(i13/2)2 neutron configuration. Excited states, with angular momentum generated primarily through successive nucleon alignments, have been populated through 1p transfer from Au197. The nucleus Pt196 is the most neutron-rich Pt isotope for which high-spin states, beyond the 12+ isomeric state, have been established thus far. Cranked shell model calculations have been performed to understand shape evolution with spin, and the role of nucleons occupying specific Nilsson orbitals in generating aligned angular momentum for both prolate and oblate deformations has been explored.

A pair of transverse wobbling bands is observed in the nucleus Pr135. The wobbling is characterized by ΔI=1, E2 transitions between the bands, and a decrease in the wobbling energy confirms its transverse nature. Additionally, a transition from transverse wobbling to a three-quasiparticle band comprised of strong magnetic dipole transitions is observed. These observations conform well to results from calculations with the tilted axis cranking model and the quasiparticle rotor model.

Fast-rotating N=151 isotones 245Pu, 247Cm and 249Cf have been studied through inelastic excitation and transfer reactions with radioactive targets. While all have a ground-state band built on a νj15/2[734]9/2- Nilsson configuration, new excited bands have also been observed in each isotone. These odd-N excited bands allow a comparison of the alignment behavior for two different configurations, where the νj15/2 alignment is either blocked or allowed. The effect of higher order deformations is explored through cranking calculations, which help clarify the elusive nature of νj15/2 alignments.

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in Rf256, which suggest that a long-lived, low-energy 8- isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a Rf256 nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-μs isomer was identified. However, its low population, ~5(2)% that of the ground state instead of the expected ~30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z=104, which describes nuclei with Z=92-103 very well.

The energies of 2-quasiparticle (2-qp) states in heavy shell-stabilized nuclei provide information on the single-particle states that are responsible for the stability of superheavy nuclei. We have calculated the energies of 2-qp states in (256)Rf, which suggest that a long-lived, low-energy 8(-) isomer should exist. A search was conducted for this isomer through a calorimetric conversion electron signal, sandwiched in time between implantation of a (256)Rf nucleus and its fission decay, all within the same pixel of a double-sided Si strip detector. A 17(5)-mu s isomer was identified. However, its low population, similar to 5(2)% that of the ground state instead of the expected similar to 30%, suggests that it is more likely a 4-qp isomer. Possible reasons for the absence of an electromagnetic signature of a 2-qp isomer decay are discussed. These include the favored possibility that the isomer decays by fission, with a half-life indistinguishably close to that of the ground state. Another possibility, that there is no 2-qp isomer at all, would imply an abrupt termination of axially symmetric deformed shapes at Z = 104, which describes nuclei with Z = 92-103 very well.

Excited states in the neutron-deficient nucleus Ta167 were studied through the Sn120(V51,4n) reaction. Twelve rotational bands have been observed and the relative excitation energy of each sequence is now known owing to the multiple interband connections. Several quasineutron alignments were observed that aided in the quasiparticle assignments of these bands. The resulting interpretation is in line with observations in neighboring nuclei. Trends in the wobbling phonon energy seen in Lu161,163,165,167 and Ta167 are also discussed and particle-rotor model calculations (assuming constant moments of inertia) are found to be inconsistent with the experimental data.

High-spin states in the odd-odd nucleus Ta168 have been populated in the Sn120(V51,3n) reaction. Two multi-quasiparticle structures have been extended significantly from spin ~20 to above 40. As a result, the first rotational alignment has been fully delineated and a second band crossing has been observed for the first time in this nucleus. Configurations for these strongly coupled rotational bands are proposed based on signature splitting, B(M1)/B(E2) ratio information, and observed rotation-alignment behavior. Properties of the observed bands in Ta168 are compared to related structures in the neighboring odd-Z, odd-N, and odd-odd nuclei and are discussed within the framework of the cranked shell model.

High-spin states in the odd-odd nucleus Ta166 are investigated through the 5n channel of the V51+Sn120 reaction. Four new bands are observed and linked into the previous level scheme. Configurations for the bands are proposed, based on measured alignments and B(M1)/B(E2) transition strength ratios.

High-spin states (I 50) of the odd-odd nucleus Ta170 have been investigated with the Sn124(51V,5n) reaction. The resolving power of Gammasphere has allowed for the observation of eleven rotational bands (eight of which are new) and over 430 transitions (~350 of which are new) in this nucleus. Many interband transitions have been observed such that the relative spins and excitation energies of the 11 bands have been established. This is an unusual circumstance in an odd-odd study. Configurations have been assigned to most of these bands based upon features such as alignment properties, band crossings, B(M1)/B(E2) ratios, and the additivity of Routhians. A systematic study of the frequency at which normal signature ordering occurs in the πh9/2νi13/2 band has been performed and it is found that its trend is opposite to that observed in the πh11/2νi13/2 bands. A possible interpretation of these trends is discussed based on a proton-neutron interaction.

Rotational bands have been identified up to high spins (≈28-h) in the odd-A nuclei 247,249Cm and 249Cf through inelastic excitation and transfer reactions around the Z = 100 region where stability results from shell effects. The [620]1/2 Nilsson configuration in 249Cm is the highest-lying neutron orbital, from above the N = 164 spherical subshell gap, for which high-spin rotational behavior has been established. The data allow for an unambiguous experimental assignment of configurations to the observed bands, unusual for odd-A nuclei near Z = 100. The high-spin properties are described in terms of Woods-Saxon cranking calculations.

The collective wobbling mode, the strongest signature for the rotation of a triaxial nucleus, has previously been seen only in a few Lu isotopes in spite of extensive searches in nearby isotopes. A sequence of transitions in the N=94 Ta167 nucleus exhibiting features similar to those attributed to the wobbling bands in the Lu nuclei has now been found. This band feeds into the πi13/2 band at a relative energy similar to that seen in the established wobbling bands and its dynamic moment of inertia and alignment properties are nearly identical to the i13/2 structure over a significant frequency range. Given these characteristics, it is likely that the wobbling mode has been observed for the first time in a nucleus other than Lu, making this collective motion a more general phenomenon.

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 have been populated in Cm246 and No252 with quantum numbers Kπ=8-, which decay through Kπ=2- rotational bands built on octupole vibrational states. For N=150 isotones with (even) atomic number Z=94-102, the Kπ=8- and 2- states have remarkably stable energies, indicating neutron excitations. An exception is a singular minimum in the 2- energy at Z=98, due to the additional role of proton configurations. The nearly constant energies, in isotones spanning an 18% increase in Coulomb energy near the Coulomb limit, provide a test for theory. The two-quasiparticle Kπ=8- energies are described with single-particle energies given by the Woods-Saxon potential and the Kπ=2- vibrational energies by quasiparticle random-phase approximation calculations. Ramifications for self-consistent mean-field theory are discussed.

Highly excited states, up to spin 39, have been established in W174, using the Gammasphere array. Ultimate cranker calculations predict the appearance of triaxial, strongly deformed structures above spin 30 in W174. A new approach was developed for a comprehensive search of the data for such structures, similar to those observed in the Lu and Hf isotopes. No evidence was found for strongly deformed bands in the W isotopes populated in this experiment. Existing rotational structures have been considerably extended, allowing for the observation of both neutron and proton alignments in a number of bands. There is evidence for the i13/2 neutron and possibly both the h9/2 and h11/2 proton crossings. The observed neutron and proton crossing frequencies are in good agreement with predictions of Woods-Saxon cranking calculations using an empirical pair-gap energy, and they lead to an improved understanding of the underlying structure of the bands.

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.

The fragment mass analyzer at the ATLAS facility has been used to unambiguously identify the mass number associated with different decay modes of the nobelium isotopes produced via Pb204(Ca48,xn)No252-x reactions. Isotopically pure (>99.7%) Pb204 targets were used to reduce background from more favored reactions on heavier lead isotopes. Two spontaneous fission half-lives (t1/2=3.7-0.8+1.1 and 43-15+22 μs) were deduced from a total of 158 fission events. Both decays originate from No250 rather than from neighboring isotopes as previously suggested. The longer activity most likely corresponds to a K isomer in this nucleus. No conclusive evidence for an α branch was observed, resulting in upper limits of 2.1% for the shorter lifetime and 3.4% for the longer activity.

Rotational structures in the Ta169 nucleus were studied via the Sn124(V51, 6n) reaction. These data were obtained as a side channel of an experiment focusing on Ta171, but the sensitivity provided by the Gammasphere spectrometer proved sufficient for a significant extension of the level scheme of this rare-earth nucleus. Over 170 new transitions and four new band structures were placed in Ta169, including the intruder πi13/2 structure. Linking transitions between all of the sequences were identified, and the relative excitation energies between the different configurations were determined for the first time. The rotational sequences were interpreted within the framework of the cranked shell model.

High-spin states in W174 (Z = 74) have been populated using the reaction Te128(Ti50, 4n)W174 at beam energies of 215 and 225 MeV. The Gammasphere array was used to detect the γ rays emitted by the evaporation residues. Four previously known collective band structures have been extended, and 16 new rotational sequences observed. Two are built upon isomeric states, one corresponding to a two-quasiparticle K = 8 isomer, the other to a four-quasiparticle K = 12 isomer, with the latter exhibiting strong K-violating ΔK=12 decays to the ground state band. Nucleonic configurations for the two- and four-quasiparticle excitations are proposed, and Woods-Saxon cranking calculations are presented to understand the rotational structures. Decay mechanisms of multi-quasiparticle K isomers are discussed in terms of the prevalent phenomenological models, with special emphasis on γ-tunneling calculations. Surprisingly, the latter underpredict the decay hindrance for the K = 12 isomer by three orders of magnitude, unlike all other isomer decays in this mass region.

We have identified two isomers in No254, built on two- and four-quasiparticle excitations, with quantum numbers Kπ=8- and (14+), as well as a low-energy 2-quasiparticle Kπ=3+ state. The occurrence of isomers establishes that K is a good quantum number and therefore that the nucleus has an axial prolate shape. The 2-quasiparticle states probe the energies of the proton levels that govern the stability of superheavy nuclei, test 2-quasiparticle energies from theory, and thereby check their predictions of magic gaps.

High-spin states in Ta171 were populated in the Sn124(V51,4n) reaction at 228 MeV to search for evidence of stable triaxial deformation. Identification of a wobbling sequence based on the previously known πi13/2 structure would provide a unique signature for this rarely observed shape. No such sequence was identified in these data, which suggests that the island of triaxial strongly deformed bands may be smaller than once thought. However, over 200 new transitions and two new bands were placed in the level scheme and the sequence based on the πi13/2 orbital could be observed up to spin and parity Iπ=(101/2+). The relative excitations of all the sequences were determined and the ground state of Ta171 was found to have 5/2+ quantum numbers, contrary to previous reports. All of the previously known structures were extended to much higher spin and their high-frequency band crossings are interpreted within the framework of the cranked shell model.

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.

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.