Decay and Fission Hindrance of Two- and Four-Quasiparticle K Isomers in (254)Rf

Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73)μs have been discovered in the heavy Rf254 nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the Kπ=8-, ν2(7/2+[624],9/2-[734]) two-quasineutron and the Kπ=16+, 8-ν2(7/2+[624],9/2-[734])⊗ - 8-π2(7/2-[514],9/2+[624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. The four-quasiparticle isomer is longer lived than the Rf254 ground state that decays exclusively by spontaneous fission with a half-life of 23.2(1.1)μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state

Improved precision on the experimental E0 decay branching ratio of the Hoyle state

Background: Stellar carbon synthesis occurs exclusively via the 3α process, in which three α particles fuse to form 12C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the α threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the E0 decay branching ratio of the Hoyle state, and the current 10% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The rate of the 3α process is an important input parameter in astrophysical calculations on stellar evolution, and a high precision is imperative to constrain the possible outcomes of astrophysical models.The project was supported by the Australian Research Council Discovery Grants No. DP140102986, No. DP170101673, and No. DP170102423. Operation of the ANU Heavy Ion Accelerator Facility is supported by the NCRIS HIA capability. The support from technical staff for the development of the pair spectrometer, as well as during the long experimental runs, is greatly appreciated. This work was partially supported by the International Joint Research Promotion Program of Osaka University and JSPS KAKENHI Grant No. JP 17H02893, the Natural Sciences and Engineering Research Council of Canada, the National Research Foundation (NRF), South Africa, under Grants No. 93533 and No. 118645

High-spin spectroscopy and shell-model interpretation of the N < 126 radium isotopes 212Ra and 213Ra

The level structures of Ra212 and Ra213 have been established via time-correlated γ-ray spectroscopy following the Pb204(C12,4n)Ra212 and Pb204(C13,4n)Ra213 reactions. In Ra212, levels up to ∼6.2MeV were identified and firm spin-parity assignments were achieved to a Jπ=19+ isomer with a mean life of 31(3) ns. For Ra213 the corresponding values were ∼4.5MeV in excitation energy and Jπ=33/2+. Two isomeric states with Jπ=23/2+, τ=27(3) ns and Jπ=33/2+, τ=50(3) ns were discovered in Ra213. The experimental data were compared with semiempirical shell-model calculations, which allowed dominant configurations to be assigned to most of the observed levels.This research was supported in part by the Australian Research Council, Grants No. DP120101417, No.DP130104176, No. DP140102986, No. DP140103317, and No. FT100100991. A.A. and M.S.M.G. acknowledge support of the Australian Government Research Training Program

Possible deformation evolution in the Πi13/2 structure of 171Re

The phenomenon of wobbling can only occur for a nuclear shape with stable triaxial deformation. To date, only a few examples of this exotic collective mode have been observed in lutetium and tantalum isotopes. A search for a wobbling sequence was performed in 171Re to determine if this feature can be observed in Z&gt;73 nuclei. No evidence was found for wobbling; however, an interaction between the ?i13/2 sequence and another positive-parity band may give an indication on why wobbling may not occur in this nucleus. The level scheme for 171Re was significantly extended and interpretations for the decay sequences are proposed within the context of the cranked shell model. �2013 American Physical Society

gamma-soft Ba-146 and the role of nonaxial shapes at N approximate to 90

Low-spin states in the neutron-rich, N=90 nuclide Ba146 were populated following β decay of Cs146, with the goal of clarifying the development of deformation in barium isotopes through delineation of their nonyrast structures. Fission fragments of Cs146 were extracted from a 1.7-Ci Cf252 source and mass selected using the CAlifornium Rare Ion Breeder Upgrade (CARIBU) facility. Low-energy ions were deposited at the center of a box of thin β detectors, surrounded by a highly efficient high-purity Ge array. The new Ba146 decay scheme now contains 31 excited levels extending up to ∼2.5 MeV excitation energy, double what was previously known. These data are compared to predictions from the interacting boson approximation (IBA) model. It appears that the abrupt shape change found at N=90 in Sm and Gd is much more gradual in Ba and Ce, due to an enhanced role of the γ degree of freedom

N=151 Pu, Cm and Cf nuclei under rotational stress: Role of higher-order deformations

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 ha

Quadrupole moment measurements for strongly deformed bands in 171,172Hf

A lifetime experiment, using the Doppler-shift attenuation method, has been performed at Gammasphere to measure the transition quadrupole moments Q t of strongly deformed bands in Hf171 and Hf172. The measured value of Qt ~ 9.5 e b for the band labeled ED in Hf171 strongly supports the recent suggestion that this sequence and several structures with similar properties in neighboring Hf isotopes are associated with a near-prolate shape with a deformation enhanced relative to that of normal deformed structures. The measured values of Qt- 14 e b for the bands labeled SD1 and SD3 in Hf172 confirm that these sequences are associated with a prolate superdeformed shape, a property inferred in earlier work from other measured characteristics of the bands. Similar bands in Hf173-175 are also likely to be associated with superdeformed shapes. The observations are in contrast to predictions of cranking calculations performed with the ultimate cranker code. � 2011 American Physical Society

Multiple band structures in 169,170Re: Search for the wobbling mode in 169Re, and residual-interaction analysis of structures in 170Re

Although the observation of wobbling was once thought to be possibly confined to lutetium isotopes in N?94 nuclei, the identification of this exotic collective mode in 167Ta has raised the question of the role of the proton Fermi surface with regard to this phenomenon. To investigate this issue, an experiment was performed to populate high-spin states in the N=94 nucleus 169Re. The heavy-ion reaction 55Mn+118Sn was used in conjunction with Gammasphere to detect the emitted ? rays. More than 130 new transitions were added to the 169Re level scheme, including the first identification of the ?i13/2 rotational sequence in this nucleus. This configuration is the structure on which all known wobbling sequences are based, but no wobbling band was observed, likely owing to the fact that the ?i13/2 sequence is located at a relatively high energy in comparison with the other structures found in 169Re. Nine decay sequences are now established in this nucleus and are described within the context of the cranked shell model. In addition, significant extension of the level scheme of the odd-odd 170Re nucleus was possible and a discussion of the residual interactions for the ?h 9/2?i13/2 and ?i13/2?i13/2 configurations in this region is given as well. �2013 American Physical Society

Three-quasiparticle isomer in Ta 173 and the excitation energy dependence of K -forbidden transition rates

Using the Er168(B10,5n) reaction at a beam energy of 68 MeV, new data have been obtained for the population and decay of a T1/2=148 ns, Kπ=21/2- three-quasiparticle isomer at 1717 keV in Ta173. Revised decay energies and intensities have been determined, together with newly observed members of a rotational band associated with the isomer. By comparison with other isomers in the A≈180 deformed region, the Ta173 isomer properties help to specify the key degrees of freedom that determine K-forbidden transition rates. In particular, when all three quasiparticles are of the same nucleon type, there is a strong dependence of the E2 reduced hindrance factor on the isomer excitation energ