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The level structure of I124 (Z=53, N=71) has been studied via the Sn122(Li7,5n)I124 reaction with a beam energy of 54 MeV. Through in-beam and out-of-beam ?-ray spectroscopy, the sophisticated low-lying levels including isomeric states and numerous collective states have been established for the first time. A positive-parity collective band built on the 10+ state at 1297 keV is interpreted as being associated with the combination of a proton and a neutron in the same intruder h11/2 orbital, namely the ph11/2?h11/2 configuration. This band shows a typical quadrupole vibrational character. In contrast, the negative parity bands based on the pg7/2?h11/2 configuration show a soft triaxial rotation. An isomeric 8- state at 689 keV with a half-life of 14 ns can be explained as a K isomer due to a proton with an ?p=9/2 in the g9/2 orbital coupled to a neutron with an ?n=7/2 in the h11/2 orbital. The excited states based on this pg9/2?h11/2 configuration show a coupled rotational structure. Another coupled rotational band built on the 6+ state at 714 keV is thought to be based on the pg9/2?d3/2 configuration.

Mass-separated ${^{187}\mathrm{Ta}}_{114}$ in a high-spin isomeric state has been produced for the first time by multinucleon transfer reactions, employing an argon gas-stopping cell and laser ionization. Internal $\ensuremath{\gamma}$ rays revealed a ${T}_{1/2}=7.3\ifmmode\pm\else\textpm\fi{}0.9\text{ }\text{ }\mathrm{s}$ isomer at $1778\ifmmode\pm\else\textpm\fi{}1\text{ }\text{ }\mathrm{keV}$, which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer-decay reduced $E2$ hindrance factor ${f}_{\ensuremath{\nu}}=27\ifmmode\pm\else\textpm\fi{}1$ supports the interpretation that axial symmetry is approximately conserved.

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.

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.

week ending 16 OCTOBER 2015 PHYSICAL REVIEW LETTERS PRL 115, 169902 (2015) Publisher’s Note: Decay and Fission Hindrance of Two- and Four-Quasiparticle K Isomers in 254 Rf [Phys. Rev. Lett. 115, 132502 (2015)] H. M. David, J. Chen, D. Seweryniak, F. G. Kondev, J. M. Gates, K. E. Gregorich, I. Ahmad, M. Albers, M. Alcorta, B. B. Back, B. Baartman, P. F. Bertone, L. A. Bernstein, C. M. Campbell, M. P. Carpenter, C. J. Chiara, R. M. Clark, M. Cromaz, D. T. Doherty, G. D. Dracoulis, N. E. Esker, P. Fallon, O. R. Gothe, J. P. Greene, P. T. Greenlees, D. J. Hartley, K. Hauschild, C. R. Hoffman, S. S. Hota, R. V. F. Janssens, T. L. Khoo, J. Konki, J. T. Kwarsick, T. Lauritsen, A. O. Macchiavelli, P. R. Mudder, C. Nair, Y. Qiu, J. Rissanen, A. M. Rogers, P. Ruotsalainen, G. Savard, S. Stolze, A. Wiens, and S. Zhu (Received 7 October 2015; published 16 October 2015) DOI: 10.1103/PhysRevLett.115.169902 PACS numbers: 23.20.Lv, 21.10.Tg, 23.35.+g, 27.90.+b, 99.10.Fg This paper was published online on 24 September 2015 with a tagging error resulting in an incorrect author footnote. J. Chen’s footnote should read as “ ‡ Present address: National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA.” The paper has been corrected as of 29 September 2015. The footnote is correct in the printed version of the journal. © 2015 American Physical Society

An excited metastable nuclear state of 192Os in a hydrogen-like charge state has been studied for the first time. It was populated in projectile fragmentation of a of 197Au beam on a of 9Be target with the UNILAC-SIS accelerators at GSI. Fragmentation products in the region of interest were passed through the fragment separator and injected into the experimental storage ring (ESR). Cooling of the injected beam particles enabled Schottky mass spectrometry to be performed. Analysis shows the lifetime of the state to be considerably longer than that of the neutral ion [τneut=8.5(14)s]; this change is attributed to hindrance of internal conversion in hydrogen-like of 192Os. Calculations have been performed to estimate the lifetime, and the result has been compared with that measured experimentally. There is good agreement between the expected [τH−like=13.0(24)s] and measured lifetimes (τrest=15.1+1.5−1.3 s) from the internal decay of of 192mOs. This provides a test for the reliability of the values obtained from internal conversion coefficient calculations in highly ionized systems and is the first measurement of its kind to be performed using the ESR setup.

Physics letters / B 747, 88 - 92 (2015). doi:10.1016/j.physletb.2015.04.055, Published by North-Holland Publ., Amsterdam

High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work, the high-spin structure of Er-156,Er-157,Er-158 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently four strongly or superdeformed (SD) sequences have been observed in Hf-174, and cranking calculations using the Ultimate Cranker code predict that such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment took place to search for linking transitions between the SD bands, possible wobbling modes, and new SD band structures.

High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of special terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work the high-spin structure of Er-157 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently, four strongly or superdeformed (SD) sequences have been observed in Hf-174 and ultimate cranker calculations predict, such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment was run to search for linking transitions between the SD bands and possible wobbling modes.

The lifetimes of states with spins 26h greater than or equal to I greater than or equal to 18h in the yrast superdeformed band of Pb-192 have been measured using the Doppler shift attenuation method. The results are consistent with a constant quadrupole moment (Q(0)) = 19.6(-0.4)(+0.5)(stat) +/- 2.0(sys) eb. This result is comparable with the values obtained for SD bands in Pb isotopes with N greater than or equal to 112, but could also be onsistent with the smaller quadrupole moment measured in Pb-193. Reasons for the apparently lower deformation of SD Pb-193 (compared to its heavier neighbours) are considered in the light of total Routhian surface calculations, and the question of whether a similarly reduced deformation should be expected in Pb-192 is addressed. (C) 2004 Elsevier B.V. All rights reserved.

The level scheme of 18176Os has been investigated with the 150Nd(36S,5n) reaction. The low-K rotational bands built upon the 9/2+[624], 7/2−[514] and 1/2−[521] neutron configurations have been extended and other new bands established. The configurations of these low-K bands are discussed within the framework of the cranked-shell model. The lifetimes for some of the states in the 9/2+[624] and 1/2−[521] collective rotational bands were also measured using the Doppler Shift Attenuation method. The large deformations deduced are found to be consistent with those predicted from theoretical Total Routhian Surface calculations. These results support the idea that for these low-K states the nuclear shape is axially symmetric and allows the K quantum number to be defined and the associated K-selection rule to be upheld. This behaviour apparently contrasts with that of the higher-K states in 181Os. In the higher-spin regime, two new high-K intrinsic states, with Kπ=37/2+ and Kπ=41/2+, were established, along with the fragmented decay of a Kπ=33/2− intrinsic state. The configurations and excitation energies of these experimentally determined intrinsic states are found to be in excellent agreement with theoretical calculations based on a fixed shape Nilsson model plus BCS pairing. The structures on top of these intrinsic states do, however, show very different behaviour. A relatively regular high-K rotational band was observed on top of the Kπ=41/2+ state but not for the other newly-established intrinsic states. Theoretical configuration-constrained potential energy surface calculations suggest that the irregular transition sequence above the Kπ=37/2+ intrinsic bandhead state, the limited excitations observed above the other intrinsic states and the observation of fragmented and non-hindered decays, are due to these configurations being subject to an appreciable γ softness. These calculations reveal that the Kπ=41/2+ configuration is less susceptible to distortions in the γ plane than any of the other high-K states.

The He-8 + Pb-208 reaction was studied in the first experiment with the EXOGAM germanium detector array using beam delivered by the SPIRAL facility. gamma-rays from direct and fusion-evaporation reactions were observed with high resolution. gamma-gamma coincidence data were obtained at a beam intensity level of 105 8He particles per second. Specially designed absorbers and beam detectors could further reduce the background radiation by orders of magnitude.

Deep-inelastic collisions of a 208Pb beam on a 208Pb target were performed using the ATLAS accelerator at Argonne National Laboratory. The Gammasphere detector array was used for the detection of prompt and delayed gamma-rays of the reaction products.207Tl is one proton away from the 208Pb doubly-magic nucleus. Its low-energy level structure is dominated by the single proton-hole states πs1/2−1, πd3/2−1 and πh11/2−1. The 11/2− state is isomeric with T1/2 = 1.33(11) s. The reaction partner of 207Tl is 209Bi, which has arelatively well established level scheme compared to 207Tl. Cross-coincidences between these two nuclei were used to confirm or establish levels above the 11/2− isomeric state in 207Tl. These states are obtained via breaking of the neutron core. Angular correlation analysis was performed on known transitions in 208Pb, proving the applicability of this method for multipolarity assignment.

Excited states in Ta-180 have been identified using the Yb-176(B-11,alpha 3n)Ta-180 and Yb-176(Li-7,3n)Ta-180 reactions and associated time-correlated gamma-ray spectroscopy, including particle-gamma coincidences for channel selection. As well as identifying the rotational band based on the 9(-) two-quasiparticle state at 75 keV, at least eight other low-lying two-quasiparticle states and associated rotational bands have been established. Lifetimes in the few nanosecond region were isolated using gamma-gamma-time techniques. Most of the observed two-quasiparticle states and some of the band members can be identified with states known from particle transfer studies. The properties of the observed Omega(n)+/-Omega(p) partners of 1(+) and 8(+) bands from the nu 9/2(+)[624]circle times pi 7/2(+)[404] configuration and the 0(-) and 9(-) pair from the nu 9/2(+)[624]circle times pi 9/2(-)[514] configuration are discussed. High-K structures identified include the band based on the four-quasiparticle 45 mu s, 15(-) isomer, a 32 ns, four-quasiparticle 18((+)) isomer, and a six-quasiparticle 19((-)) intrinsic state and its band. Configuration assignments are aided by analysis of the in-band decay properties, which confirm, for example, a predominantly nu pi(3) configuration for the 15- isomer. The results are compared with multiquasiparticle calculations. A number of yrast high-K six- and eight-quasiparticle states which could be accessible in future studies are predicted.

Excited states in Ta-181 have been identified using time-correlated gamma-ray spectroscopy and particle-gamma techniques following the incomplete fusion reaction Yb-176(B-11,alpha 2n) Ta-181. As well as observing the rotational band based on the 11 ns isomer at 482 keV from the 5/2(+)[402] proton configuration, which is found to be essentially identical to that associated with its 7/2(+)[404] pseudospin partner, several multi-quasiparticle isomers are observed including a 33 mu s state, of probable spin and parity 21/2(-), at 1483 keV. Experimental states are compared with multi-quasiparticle calculations.

The stable nucleus ${}^{177}$Hf was studied with time-correlated particle-$\ensuremath{\gamma}$-$\ensuremath{\gamma}$ techniques following the reaction ${}^{176}\mathrm{Yb}{(}^{9}\mathrm{Be},\ensuremath{\alpha}{4n)}^{177}$Hf at a beam energy of 70 MeV. New high-spin states include extensions to three-quasiparticle bands based on the ${K}^{\ensuremath{\pi}}{=23/2}^{+}$ and ${25/2}^{\ensuremath{-}}$ configurations, together with a new band associated with the ${K}^{\ensuremath{\pi}}{=19/2}^{\ensuremath{-}}$, ${T}_{1/2}=56$ \ensuremath{\mu}s isomer. Moreover, the band based on the five-quasiparticle ${T}_{1/2}$=51 minute, ${K}^{\ensuremath{\pi}}{=37/2}^{\ensuremath{-}}$ state has been found, and a band assigned to a new ${K}^{\ensuremath{\pi}}{=39/2}^{+}$, five-quasiparticle state. Values of ${(g}_{K}\ensuremath{-}{g}_{R}{)/Q}_{0}$ agree well with the configuration assignments made to new and previously known bands. Comparison with multiquasiparticle calculations that incorporate a Lipkin-Nogami treatment of pairing correlations with blocking, suggests that all of the most favored three- and five-quasiparticle states have been observed. The dependence of the $\ensuremath{\gamma}$-ray intensities on the angular yields of the $\ensuremath{\alpha}$ particles suggests that the population arises from two entry distributions: one associated with complete fusion of the beam with the target, the other with incomplete fusion.

The level scheme of 190 Pb identified in the 166 Er( 28 Si,4n) 190 Pb reaction includes several long isomers and exhibits structures which can be associated with co-existing spherical, oblate and prolate configurations. A chance near-degeneracy between 8 + states from the competing configurations causes mixing and fragmentation of the observed E3 and E2 decays from the yrast 11 − and 10 + states, through interference. The candidate identified for the prolate band is approximately 500 keV higher than that observed in 188 Pb.

A high spin isomer with spin ${25}^{\ensuremath{-}}$ and mean lifetime of $220\ensuremath{\mu}\mathrm{s}$ has been observed at an excitation energy of 4772 keV in ${}^{212}\mathrm{At}$. The measured $E3$ decay rate for the 224 keV transition deexciting this level is enhanced but significantly lower than those observed in the neighboring isotopes or predicted using a multiparticle octupole coupling model. The relatively simple configurations involved in this decay and the close relationship to neighboring states allow a clear association to be made between the occupation of a specific single particle orbital and a blocking of the octupole vibrational correlation.

A superconducting solenoid electron spectrometer operated in the lens mode was adapted to search for the β- decay of 54Mn. The Compton electron and other instrumental backgrounds were largely reduced by special shielding of the absorber system. An improved procedure was developed to select the events by momenta. An upper limit of less than 3.9 × 10-5 has been established for the intensity of the β- branch. This would define the partial half-life of the 54Mn β- decay to be greater than 2.2 × 104 yr. The implications of this result for the 54Mn cosmic-ray chronometry problem are discussed.

High-spin states in 179Ta have been studied by a variety of γ-ray spectroscopic techniques following the 176Yb(7Li,4n) reaction. The new results independently confirm and extend the one-, three- and five- quasiparticle bands proposed previously. The more comprehensive results obtained allow firm spin assignments to be made and also the identification of rotational bands associated with the higher seniority intrinsic states. Configuration assignments follow from analysis of these rotational band properties. To explain changes in the yrast line across the chain of isotopes from 173Ta to 179Ta, multi-quasiparticle calculations have been performed and compared with experiment. The calculations, which treat pairing correlations using the Lipkin-Nogami approach and include blocking, incorporate single-particle energies adjusted to reproduce the observed one-quasiparticle bandheads. Properties specific to 179Ta include evidence for configuration mixing in the 212− isomer configuration. Its decay is one of several anomalously fast K-hindered transitions observed and discussed in terms of configuration changes and other mechanisms.

The properties of high-spin states in 208 Po have been investigated using the 204 Hg( 9 Be,5n) 208 Po reaction at a bombarding energy of 62 MeV. Decay properties for states in 208 Po up to an excitation energy of 9643 keV and spins up to J = 24 have been deduced. The level scheme is compared with predictions of multi-particle shell-model calculations and evidence is presented that the coreexcited states intrude into the decay sequence at an excitation energy near 6500 keV, although they do not form isomers. Above 7.5 MeV the yrast line is dominated by core-excited states.

An isomer with a half-life of 6{plus_minus}1 {mu}s has been observed in the N=74 nucleus {sup 138}Gd, populated following the reaction {sup 106}Cd({sup 35}Cl,p2n){sup 138}Gd. The isomer decays via a 583 keV E1 transition with a hindrance per degree of K forbiddenness, f{sub {nu}}=24. This value is similar to the values measured for the N=74 isotones {sup 134}Nd and {sup 136}Sm but markedly different from that measured for {sup 130}Ba. This suggests that there is some change in structure across the N=74 isotones and possible explanations of this feature are discussed. {copyright} {ital 1997} {ital The American Physical Society}

E2 collective properties for the low-lying states in 186,188,190,192 Os and 194 Pt have been investigated experimentally by means of Coulomb excitation using 3.3–4.8 MeV/nucleon 40 Ca, 58 Ni, 136 Xe and 208 Pb beams. The deexcitation γ rays following Coulomb excitation were detected in coincidence with the scattered particles. Levels with excitation energies up to 3–4 MeV of the ground-state, γ and 4 + collective bands as well as excited 0 + states were populated in each nucleus studied. A semiclassical Coulomb-excitation least-squares search code GOSIA was used to extract E2 matrix elements from the measured γ-ray yields. For each nucleus studied, a unique and almost complete set of E2 matrix elements for the low-lying states has been determined, which includes both the magnitudes and signs of the transitional and diagonal matrix elements. The completeness of the set of measured E2 matrix elements makes it possible to determine the intrinsic quadrupole deformation for the low-lying states in these nuclei via a model-independent method. The results indicate clearly that the E2 properties for the low-lying states in these nuclei are correlated well using only the quadrupole collective degrees of freedom. The extracted E2 matrix elements are compared with the prediction of various collective models such as the asymmetric rigid rotor model, the γ-soft model of Leander, and the IBA-2 model. These particular models do not reproduce the data satisfactorily, however the general trends of the data are consistent with the descriptions of γ-soft type collective models through a prolate to oblate shape-transition region. That the enhanced B (E2) values between the I ,quasi- K π = 4,4 + state and members of the I ,quasi- K π = 2,2 + band are well reproduced by the γ-soft model is consistent with the interpretation of the I ,quasi- K π = 4,4 + state being a two-phonon γ-vibration excitation.

A six-quasiparticle isomer with ${K}^{\ensuremath{\pi}}={21}^{\ensuremath{-}}$, a half-life of 290(12) ms, and the ${\ensuremath{\pi}}^{3}{\ensuremath{\nu}}^{3}$ configuration has been identified in the odd-odd nucleus $^{178}\mathrm{Ta}$, at an excitation energy of 2902 keV. The rotational bands built on the known ${K}^{\ensuremath{\pi}}={15}^{\ensuremath{-}}$ isomer and on the newly found ${16}^{+}$ four-quasiparticle and ${22}^{+}$ six-quasiparticle states, have also been identified, allowing characterization of the configurations. The ${15}^{\ensuremath{-}}$ band is predominantly of $\ensuremath{\pi}{\ensuremath{\nu}}^{3}$ character with a ${\ensuremath{\pi}}^{3}\ensuremath{\nu}$ admixture. When the mixing is taken into account the excitation energies of the main yrast multi-quasiparticle states can be reproduced. The multi-quasiparticle states observed are related essentially through the addition of the two-quasiparticle component ${\ensuremath{\nu}}^{2}[{6}^{+}]$ or ${\ensuremath{\pi}}^{2}[{6}^{+}]$. Depending on whether the transition between the states involves the change ${\ensuremath{\nu}}^{2}[{6}^{+}]\ensuremath{\rightarrow}[0]$ or ${\ensuremath{\pi}}^{2}[{6}^{+}]\ensuremath{\rightarrow}[0]$, the $E2$ hindrance factors for decays between the six- and four-quasiparticle states are relatively large or small. This dependence mimics the pattern observed in the two-quasiparticle core transitions and, because the ${15}^{\ensuremath{-}}$ isomer is mainly $\ensuremath{\pi}{\ensuremath{\nu}}^{3}$, the magnitude sequence is inverted compared to that observed in $^{176}\mathrm{Ta}$.

A novel implantation-decay technique has been employed to measure the magnetic moments of the ${2}_{1}^{+}$ states in the neutron-deficient nuclei ${}^{184}\mathrm{Pt}$, ${}^{186}\mathrm{Pt}$, and ${}^{188}\mathrm{Pt}$. The magnetic moment systematics for the even Pt isotopes now extend from ${}_{78}^{184}{\mathrm{Pt}}_{106}$ to ${}_{78}^{198}{\mathrm{Pt}}_{120}$, spanning the upper half of the valence neutron shell. Despite the prolate-to-oblate shape transition near $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}190$, they remain remarkably constant. The $g({2}_{1}^{+})$ values for ${}^{184,186,188}\mathrm{Pt}$ are consistent with shape-coexistence models in which the deformed configuration has a larger effective number of valence protons than the less-deformed configuration.

A four quasiparticle high-{ital K} isomer with a meanlife of 45(2) {mu}s has been identified at 1451 keV in {sup 180}Ta, populated in the {sup 176}Yb({sup 11}B,{alpha}3{ital n}){sup 180}Ta reaction. The isomer decays into a rotational band which is associated with the two-quasiparticle 9{sup {minus}} isomer at 75.3 keV. Analysis of the branching ratios within that band and the magnetic moment for the 9{sup {minus}} isomer, supports the configuration assignment to the 9{sup {minus}} isomer. The {ital K} hindrance for the {ital E}2 decay of the 15{sup {minus}} isomer to the 9{sup {minus}} band is substantially lower than that for an apparently similar 15{sup {minus}} isomer in {sup 178}Ta, a difference which can be attributed partly to a change from the {pi}9/2{sup {minus}}[514]{nu}{sup 3}9/2{sup +} [624]7/2{sup {minus}}[514]5/2{sup {minus}}[512] configuration in {sup 178}Ta to the {pi}{sup 3}9/2{sup {minus}}[514]7/2{sup +}[404]5/2{sup +}[402]{nu}9/2{sup +}[624] configuration in {sup 180}Ta. The reduced hindrance factors for {ital E}2 decays from related four-quasiparticle isomers in the isotopes {sup 176,178,180}Ta match the hindrances of the corresponding {ital E}2 decays from component 6{sup +} core states in the hafnium isotopes, {sup 174,176,178}Hf. {copyright} {ital 1996 The American Physical Society.}

The lifetimes of the 21 2 − state (1416 keV) and the 29 2 + state (2641 keV) in 211At were measured to be τm = 50.7(10) ns and τm = 73.3(10) ns, respectively, using the γ-time technique. Using these lifetime measurements a value for the quadrupole moment of the 29 2 + state was derived. The explicit inclusion of octupole coupling effects into a description of the configuration for the 29 2 + level allows the extraction of an orbital independent effective charge of eeff = 1.5e. From this a nominal value for the quadrupole moment of the πi13/2 orbital can be deduced. New levels identified have allowed a determination of the strength of the πi13/2−πf7/2 residual interaction.

Nuclei in the mass-180 region have many high-Ω single-particle levels close to the Fermi energy and are, therefore, prime candidates for high- K isomers. Since both neutron and proton level densities are rather low, one should expect blocking and particle-number fluctuations to be rather important. We have performed good-particle-number calculations and have shown that the simpler blocked BCS theory gives a good approximation to the multi-quasiparticle spectra if the pairing strength is chosen appropriately. This has allowed us to perform a systematic theoretical study of this mass region. Residual spin-spin interactions are shown to be essential in reproducing the energies and even the correct order of known states. Good agreement has been found for 175 Hf, 176 Hf and 177 Ta, where extensive data already exist. Predictions for new high- K states near the yrast line are made for these nuclei and for 178 W.

A {ital T}{sub 1/2}=220 ns, eight-quasiparticle isomer, with four unpaired neutrons and four unpaired protons, has been established at an excitation energy of 6576 keV in the prolate deformed nucleus, {sup 178}W. The associated rotational band has also been identified, revealing the collective properties in the presence of blocked pairing correlations, with expected quenching of the nuclear superfluidity. The band retains a small degree of rotational alignment, and has a less-than-rigid dynamic moment of inertia.

High spin gamma-ray spectroscopy of 185 Tl and 187 Tl has been performed with the reactions 154 Gd( 35 Cl, 4n) and 159 Tb( 32 S, 4n). Positive γ-ray identification with the thallium isotopes was made via X-ray coincidences, and supported by mass selected γ-ray spectra, the latter obtained with the reactions 154 Gd( 36 Ar, p4n) and 155 Gd( 36 Ar, p3n). Rotational bands associated with both prolate and oblate shape were observed. The bandheads of the proposed oblate 13 2 + [606] states were found to be isomeric, with meanlives of 12 ± 2 ns in 185 Tl and 1.0 ± 0.2 ns in 187 Tl. Prolate deformed i 13 2 bands were observed in both nuclei, while in 187 Tl, bands due to h 9 2 and f 7 2 protons coupled to the prolate shape are also assigned. An h 9 2 band is tentatively assigned in 185 Tl. The observation of these rotation-aligned bands at low excitation energy implies that the development of prolate deformed minima in the odd nuclei is not necessarily blocked by occupation of a single deformation-driving orbital. Equilibrium deformation calculations for intrinsic states in a range of thallium nuclei are presented. Experimental trends with mass number are reproduced, but absolute excitation energies, and energy differences between the prolate and oblate states are not, continuing the persistent discrepancy between theory and experiment in the mercury region. Theoretical calculations of intruder orbital occupation probabilities show a correlation between prolate deformation and h 9 2 and f 7 2 proton pair population, in particular of the 1 2 − [541] orbital from the h 9 2 proton shell. They also show that blocking of the 1 2 − [541] orbital significantly suppresses the prolate deformation. Implications for the structure of the prolate deformed mercury and thallium isotopes are considered, leading to the conclusion that the prolate mercury core nuclei consist of a mixture of low-Ω proton intruder excitations.

A recent experiment using projectile fragmentation of a Au beam on a Be target, combined with the fragment recoil separator and experimental storage ring at ring at GSI, has uncovered an isomeric state in Re at 267(10) keV with a half-life of ∼60 s. The data analysis technique used to resolve the isomeric state from the ground state is discussed. © Published under licence by IOP Publishing Ltd.

The properties of low-lying high-spin states in 204 Hg and 205 Hg have been investigated using the 204 Hg( 9 Be, 9 Be′) 204 Hg and 204 Hg( 9 Be, 2α) 205 Hg reactions at a bombarding energy of 62 MeV. Decay properties for states up to an excitation energy of 3688 keV and spin of (8 + ) have been deduced for 204 Hg, and up to an excitation energy of 2579 keV for 205 Hg. The level schemes are compared with predictions of empirical shell-model calculations.

Level lifetimes for the low-Ω π h 9 2 and π i 13 2 intruder bands in 179Ir up to spin 37 2 and 49 2 , respectively, have been determined using the recoil-distance and Doppler-shift attenuation methods. The results reveal that transition quadrupole moments of the i 13 2 band are almost 30% larger than those of the h 9 2 band. Furthermore, there is no indication of rotational stretching. Lifetime results and total Routhian surface calculations are discussed in conjunction with the deformation-driving effects of the different proton orbitals.

A recoil-shadow measurement of isomer decays in $^{172}\mathrm{Hf}$ has revealed many weak \ensuremath{\gamma}-ray transitions. One of these is a sevenfold K-forbidden E1 transition from the ${\mathit{K}}^{\mathrm{\ensuremath{\pi}}}$=${8}^{\mathrm{\ensuremath{-}}}$ isomer (${\mathit{T}}_{1/2}$=163 ns) to the ${\mathit{K}}^{\mathrm{\ensuremath{\pi}}}$=${0}^{+}$ ground-state band. The low hindrance factor for this transition in $^{172}\mathrm{Hf}$ is compared with the hindrance factors for other \ensuremath{\Delta}K=8 E1 transitions.

High-spin states in 179 W have been studied following the 170 Er(su13C, 4n) reaction. Rotational bands up to ifI 53 2 have been identified, based on 1-, 3-, 5- and 7-quasiparticle structures. Different alignment mechanisms compete in the generation of angular momentum at the yrast line. A Fermi-aligned if( i 13 2 ) 2 structure, coupled to high- K and to the if 7 2 − [514] orbital, forms the negative-parity yrast sequence above if I π = 31 2 − . This may be called a “t-band” since its description within the cranking model requires a “tilting” of the cranking axis. The anomalous decay of the if K π = 35 2 − 5-quasiparticle isomer is explained as arising from destructive interference of transition amplitudes coupling to the Fermi-aligned structure. Detailed analysis of the excitation energies of the multi-quasiparticle states indicates the quenching of both neutron and proton pair correlations, by comparison with blocking calculations.

Previous studies of 172 Os noted an anomaly in the behaviour of the moment of inertia of the yrast band at low spin. A phenomenological model of shape coexistence based on interacting rotational bands was proposed to explain this anomaly and this model predicted low-lying non-yrast states. In order to test these predictions, the β-decay of 172 Ir has been used to populate 172 Os. Excited states have been observed and classified into positive-parity “quasi-β” and “quasi-γ” bands and a negative-parity band. The energies of the quasi-β band states are seen to be in general agreement with the predictions of the phenomenological model and the model is refined to take into account the new data. The bands involved are determined to have significantly different moments of inertia.

Excited states in the range of isotopes 174, 176, 178, 180, 182, 184 Os, populated in the β-decay of 174, 176, 178, 180, 182, 184 Ir parent activities have been identified using γ-ray singles and coincidence techniques, utilising the high efficiency of a Compton-suppressed array, and conversion electron techniques. Many new states at low excitation energies have been identified, complementing the level schemes previously established from in-beam studies. The new states include excited 0 + states in 174 Os, 176 Os 178 Os and 180 Os. A large body of data on decay properties, spins and parities, and relative E2 and E0 matrix elements has been obtained. The systematics of the quasi-β and quasi-γ bands is discussed. Some detailed analyses in terms of the schematic band-mixing model are presented, incorporating reproduction of the yrare states in the fits. In particular, the proposed interpretation of the anomalous low-frequency alignment gains in the yrast positive-parity states, as a consequence of mixing with a low-lying intruder band, is confronted. The B (E0)/ B (E2) ratios are over-estimated by this model although their form (spin and isotope dependence) is reproduced. Better agreement is obtained within the IBM, which uses however reduced effective charges to match experiment. Absolute measurements of the B (E0) and B (E2) values may be necessary to distinguish between the models and test conjectures of shape differences.

High-spin states of the stable isotope 182W have been studied using the reactions 176Yb(13C, α3n)182W and 176Yb(9Be, 3n)182W at beam energies of 65 and 40 MeV, respectively. Three, possibly four, new intrinsic states at high spins have been observed corresponding to different high-K four-quasi-particle structures. The Kπ = 15+ and 17− levels have lifetimes of 78(15) and 25(10) ns, respectively. Rotational bands are observed built on the Kπ = 16+ and 17− yrast states. The coupling of Nilsson orbitals which give rise to high-K states at the yrast line and the K-forbiddenness of the K π = 15 + isomeric decay are discussed.

Excited states in the odd-odd nucleus 214 Fr have been studied using γ-ray and electron spectroscopy following 208 Pb( 11 B, 5n) and 205 Tl( 13 C, 4n) reactions. Levels were identified to spins of around 36 ħ and excitation energies of ∼ 8.6 MeV. A number of isomeric states have been measured and g -factors obtained using the TDPAD method. At low spin, semi-empirical shell-model calculations appear to provide a good description of the states observed. An understanding of the structure of higher spin states in terms of the probable yrast configurations requires the addition of two units of spin at an energy of around 2 MeV. States with spins around 30 ħ are formed by core-excited configurations, with double core-excitation suggested for the highest states observed. The properties of many of the isomeric states can be understood in terms of multiparticle octupole coupling, with the properties of these states well reproduced by multiparticle octupole-coupled shell-model calculations.

High-spin states of the N=57 isotones, 103Pd and 105Cd, have been studied using the reactions 94Zr(16O, alpha 3n)103Pd and 94Zr(16O,5n)105Cd. The nu h11/2 yrast bands in both nuclei are extended up to spin 47/2h(cross), past the first band-crossing. The observed increase in alignment in both bands is significantly lower than in analogous bands in the heavier odd-N isotopes, where the alignment is thought to be due to a pair of low- Omega h11/2 neutrons. The relatively low increase in alignment in 103Pd and 105Cd is consistent with cranked shell model calculations which predict that at low deformation both the (g9/2)2 proton and the (g7/2)2 neutron alignments are favoured in these N=57 isotones, over the (h11/2)2 neutron alignment.

Transitions in the very neutron-deficient ${\mathrm{isotope}}^{186}$Pb have been identified in mass-gated, recoil-\ensuremath{\gamma}, and recoil-\ensuremath{\gamma}-\ensuremath{\gamma} coincidence data obtained with a fragment mass analyzer and Compton-suppressed Ge-detector array. The results of the present work confirm and extend a band of levels tentatively proposed in earlier work done elsewhere, and provide a definitive mass assignment of the observed transitions. The band observed in $^{186}\mathrm{Pb}$ bears a very close resemblance to the yrast band in the isotone $^{184}\mathrm{Hg}$, supporting the view that the $^{186}\mathrm{Pb}$ band is built upon a prolate structure.