Four-quasiparticle States Research Articles

Multi-quasiparticle sub-nanosecond isomers in 178W

We report on the first measurement of the half-lives of Kπ=11− and 12+ four-quasiparticle states in the even-even nucleus 178W. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3(Ce) scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became experimentally accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array. The measured half-lives are 476(44)ps and 275(65)ps for the Iπ=11− and 12+ states, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.

Hindered α decays of heaviest high- K isomers

To find candidates for long-lived high-$K$ isomers in even-even $Z=106\text{--}112$ superheavy nuclei, we study dominant $\ensuremath{\alpha}$-decay channels of two- and four-quasiparticle configurations at a low excitation. Energies are calculated within the microscopic-macroscopic approach with the deformed Woods-Saxon potential. Configurations are fixed by a standard blocking procedure and their energy found by a subsequent minimization over deformations. Different excitation energies of a high-$K$ configuration in parent and daughter nucleus seem particularly important for hindrance of the $\ensuremath{\alpha}$ decay. A strong hindrance is found for some four-quasiparticle states, particularly ${K}^{\ensuremath{\pi}}={20}^{+}$ and/or ${19}^{+}$ states in $^{264\text{--}270}\mathrm{Ds}$. Contrary to what was suggested in experimental papers, it is rather a proton configuration that leads to this strong hindrance. If not shortened by the electromagnetic decay, $\ensuremath{\alpha}$ half-lives of $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{s}$ could open new possibilities for studies of chemical/atomic properties of related elements.

Decay spectroscopy of 160Sm: The lightest four-quasiparticle K isomer

The decay of a new four-quasiparticle isomeric state in 160Sm has been observed using γ-ray spectroscopy at the RIBF, RIKEN. The four-quasiparticle state is assigned a 2π⊗2νπ52−[532], π52+[413], ν52−[523], ν72+[633] configuration. The half-life of this (11+) state is measured to be 1.8(4) μs. The (11+) isomer decays into a rotational band structure, based on a (6−) ν52−[523]⊗ν72+[633] bandhead, consistent with the gK−gR values. This decays to a (5−) two-proton quasiparticle state, which in turn decays to the ground state band. Potential energy surface and blocked-BCS calculations were performed in the deformed midshell region around 160Sm. They reveal a significant influence from β6 deformation and that 160Sm is the best candidate for the lightest four-quasiparticle K isomer to exist in this region. The relationship between reduced hindrance and isomer excitation energy for E1 transitions from multiquasiparticle states is considered with the new data from 160Sm. The E1 data are found to agree with the existing relationship for E2 transitions.

Favored configurations for four-quasiparticleKisomerism in the heaviest nuclei

Configuration-constrained potential-energy-surface calculations are performed including ${\ensuremath{\beta}}_{6}$ deformation to investigate high-$K$ isomeric states in nuclei around ${}^{254}$No and ${}^{270}$Ds, the heaviest nuclei where there have been some observations of two-quasiparticle isomers, while data for four-quasiparticle isomers are scarce. We predict the prevalent occurrence of four-quasiparticle isomeric states in these nuclei, together with their favored configurations. The most notable examples, among others, are ${K}^{\ensuremath{\pi}}={20}^{+}$ states in ${}^{266,268}$Ds and ${}^{268,270}$Cn having very high $K$ value, relatively low excitation energy, and well-deformed axially symmetric shape. The predicted isomeric states, with hindered spontaneous fission and $\ensuremath{\alpha}$ decay, could play a significant role in the future study of superheavy nuclei.

Nuclear Structure of the Heaviest Elements – Investigated at SHIP-GSI

The quest for the heaviest nuclei that can exist is a basic topic in natural science as their stability is characterized by a delicate interplay of short range nuclear forces acting between the nucleons (protons and neutrons) and long-range Coulomb forces acting solely between charged particles, i.e. the protons. As the stability of a nucleus is strongly correlated to its structure, understanding the nuclear structure of heaviest nuclei is presently a main challenge of experimental and theoretical investigations concerning the field of Superheavy Elements. At the velocity filter SHIP at GSI Darmstadt an extensive program on nuclear structure investigations has been started about a decade ago. The project covered both as well systematic investigations of single particle levels in odd-mass isotopes populated by α-decay as investigation of two- or fourquasi-particle states forming K isomers and was supplemented by direct mass measurements at SHIPTRAP and investigation of spontaneous fission properties. Recent experimental studies allowed to extend the systematics of low lying levels in N = 151 and N = 153 up to 255Rf and 259Sg, investigation of possible relations between nuclear structure and fission properties of odd-mass nuclei and investigation of shell strengths at N = 152 and towards N = 162.

Competition between collective oblate rotation and non-collective prolate K isomerism in neutron-rich tungsten isotopes

Recent experiments open up the possibility to investigate oblate rotation-aligned states and prolate high-K isomers in neutron-rich tungsten isotopes. In the present work, we perform the projected-shell-model calculations for A ∼ 190 tungsten nuclei. The 190W results are compared with experimental data. The observed 8+ isomer is assigned as a two-quasiproton Kπ = 8+ configuration. Low-lying high-K four-quasiparticle states are predicted. Of particular interest is the prediction of the Kπ = 20+ state in 190,192W, which may form a long-lived spin trap. In competition with the prolate high-K states, rotational alignment leads to near-yrast collective oblate rotation.

Four-Quasiparticle High-K States in Neutron-Deficient Lead and Polonium Nuclei

Configuration-constrained potential energy surface calculations have been performed to investigate four-quasiparticle high-K configurations in neutron-deficient lead and polonium isotopes. A good agreement between the calculations and the experimental data has been found for the excitation energy of the observed Kπ = 19− state in 188Pb. Several lowly excited high-K states are predicted, and the large oblate deformation and low energy indicate high-K isomerism in these nuclei.

Identification of a New Four-Quasiparticle State in Odd-Odd 186Au

The high-spin level structure of 186Au is re-investigated by means of in-beam γ-ray spectroscopy via the 172Yb(19F, 5nγ)186Au reaction. The oblate bands previously reported are revised and extended to higher-spin states. A new Iπ = (20+) excited state has been identified and assigned to the πh−111/2 ⊗ νi−213/2h−19/2 configuration. The total Routhian surface and cranked shell model calculations suggest that the πh−111/2 ⊗ νi−213/2h−19/2 band in 186Au has a non-axial deformation.

High-Kfour-quasiparticle states inGd138

States above the known ${K}^{\ensuremath{\pi}}={8}^{\ensuremath{-}}$ 6 $\ensuremath{\mu}$s isomer in $^{138}\mathrm{Gd}$ have been populated with the $^{106}\mathrm{Cd}$($^{36}\mathrm{Ar}$,$2p2n$) reaction at a beam energy of 180 MeV at the University of Jyv\"askyl\"a, Finland. The recoil-isomer tagging technique was utilized to correlate delayed $\ensuremath{\gamma}$-ray decays, detected in the GREAT focal plane spectrometer, with prompt decays measured in the JUROGAM II spectrometer at the target position. The lifetime of the ${K}^{\ensuremath{\pi}}={8}^{\ensuremath{-}}$ isomeric state has been remeasured as 6.2(2) $\ensuremath{\mu}$s. Two high-lying strongly coupled bands have been established with ${K}^{\ensuremath{\pi}}\ensuremath{\geqslant}{12}^{\ensuremath{-}}$. Potential-energy surface calculations, in conjunction with $g$ factor measurements, reveal that they are built upon four-quasiparticle structures comprising two-quasineutron plus two-quasiproton configurations. The short half-life or lack of hindrance for the decays from these four-quasiparticle band-head states is reasoned to be a consequence of increased triaxial deformation and mixing due to the high density of states relative to the lower two-quasiparticle 6-$\ensuremath{\mu}$s isomeric state.

Multi-quasiparticle isomers inLu174

Four-, six-, and eight-quasiparticle isomers with ${K}^{\ensuremath{\pi}}={14}^{\ensuremath{-}}$, (${21}^{+}$) and (${26}^{\ensuremath{-}}$) have been identified in the deformed nucleus $^{174}\mathrm{Lu}$, in addition to the previously reported ${K}^{\ensuremath{\pi}}={13}^{{}^{+}}$, 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 ${K}^{\ensuremath{\pi}}={14}^{\ensuremath{-}}$ 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 ($I=13\text{\ensuremath{-}}14\ensuremath{\hbar}$) and the highest spin ($Ig20\ensuremath{\hbar}$) states observed, but there remain ambiguities in assigning configurations in the $I=15\text{\ensuremath{-}}19\ensuremath{\hbar}$ region.

Richard Guy Helmer

Richard Guy Helmer

Nature of excited0+states inGd158described by the projected shell model

Excited 0+ states are studied in the framework of the projected shell model, aiming at understanding the nature of these states in deformed nuclei in general, and the recently observed 13 excited 0+ states in 158Gd in particular. The model, which contains projected two- and four-quasiparticle states as building blocks in the basis, is able to reproduce reasonably well the energies for all the observed 0+ states. The obtained B(E2) values however tend to suggest that these 0+ states might have a mixed nature of quasiparticle excitations coupled to collective vibrations.

Identification of a four-particle state in the N = 152 nucleus 250Cf

Examination of the first-forbidden beta decays in very heavy (A ⩾ 245) nuclei leads to the first identification of a four-quasiparticle (4qp) state in a nucleus of this ‘frontier’ region. The 1658.0 keV 2+ state in the N = 152 nucleus 250Cf, fed in β− decay from the 3.21 h 2− 250Bk parent and in ϵ-decay from the 2.22 h 1− 250Es parent, is assigned the 4qp configuration 2+(p1:7/2[633] ⊗ p2:3/2 [521] ⊗ n1:1/2[620] ⊗ n2:9/2 [734]) on the basis of the observed beta decay features. This assignment is supported by the observed gamma decays of this 250Cf state, excitation energies of the constituent one-, two- and three-quasiparticle bands in core nuclei and the experimental log ft values for the corresponding n:1/2 [620] ⇌ p:3/2[521] and the n:9/2 [734] ⇌ p:7/2[633] transformations in nuclei of this region. It is further shown that the same underlying single-particle transformation populates the 1qp, 2qp, 3qp and 4qp states with comparable log ft values in fast (log ft = 6.5 ± 0.6) first forbidden beta transitions in this ‘frontier’ region. An operative selection rule for such transitions is suggested.

β decay of theT=0isomer in theN=Zproton drip-line nucleus70Br

We have investigated the \ensuremath{\beta} decay of the $T=0,$ ${T}_{1/2}\ensuremath{\approx}2.2\mathrm{s},$ isomeric level in the self-conjugate odd-odd nucleus ${}^{70}\mathrm{Br}.$ The observed \ensuremath{\beta}-delayed \ensuremath{\gamma} rays in the daughter nucleus ${}^{70}\mathrm{Se}$ and the deduced decay properties of the isomeric level allow, in conjunction with results from the deformed shell model, a spin and parity assignment of ${J}^{\ensuremath{\pi}}{=9}^{+}$ and suggest a structure of ${\ensuremath{\pi}[404]{9/2}^{+}\ensuremath{\nu}[404]{9/2}^{+}}{9}^{+}.$ The allowed unhindered \ensuremath{\beta} decay of the isomer, possibly oblate deformed, proceeds to four-quasiparticle states in ${}^{70}\mathrm{Se}.$

Configuration and unusually fast decay of theKπ=16−four-quasiparticle isomer in182Re

High-spin states in ${}^{182}\mathrm{Re}$ were studied using the ${}^{176}\mathrm{Yb}{(}^{11}\mathrm{B},5n)$ reaction at 70 MeV. The previously known four-quasiparticle isomer at 2256 keV is assigned spin and parity of ${16}^{\ensuremath{-}}.$ The rotational band built upon this multiquasiparticle state was identified and examination of the ${(g}_{K}\ensuremath{-}{g}_{R}{)/Q}_{0}$ values revealed that it arises from the $\ensuremath{\pi}{(9/2}^{\ensuremath{-}}[514])\ensuremath{\bigotimes}{\ensuremath{\nu}}^{3}{(7/2}^{\ensuremath{-}}[514]{,7/2}^{\ensuremath{-}}[503]{,9/2}^{+}[624])$ configuration. The decay of the ${16}^{\ensuremath{-}}$ isomer is found to be less hindered when compared to those observed for similar four-quasiparticle long-lived states in neighboring odd-odd ${}^{176,178,180}\mathrm{Ta}$ isotopes. A possible explanation of this feature, involving a ``local'' mixing with a nearby ${16}^{\ensuremath{-}}$ collective state, is discussed.

Signature inversion and the first observation of a magnetic dipole band in odd-odd rubidium isotopes:82Rb

High-spin states in ${}^{82}\mathrm{Rb}$ were studied through the ${}^{68}\mathrm{Zn}{(}^{18}\mathrm{O},p3n)$ reaction at 56 MeV beam energy via a thin target coincidence measurement. The known sequence of positive-parity states built on the ${6}^{+}$ state at 191.3 keV was extended up to spin 14, and signature inversion was observed around spin 11. Above spin 8 the observed properties of this band indicate a rotationlike structure. A new high-lying level sequence connected by $\ensuremath{\Delta}I=1$ transitions was identified. This magnetic dipole sequence is built on a new ${(11}^{\ensuremath{-}})$ level at 2617.3 keV excitation energy which is interpreted as a $\ensuremath{\pi}{(g}_{9/2}{)}^{2}\ensuremath{\bigotimes}\ensuremath{\pi}{(p}_{3/2}{,f}_{5/2})\ensuremath{\bigotimes}\ensuremath{\nu}{g}_{9/2}$ four-quasiparticle state at small quadrupole deformation.

Nuclear structure in odd-odd nuclei,144<~A<~194

A comprehensive review of the present understanding, both theoretical and experimental, of intrinsic and rotational level structures in medium-heavy deformed odd-odd nuclei is presented. A discussion of the various experimental methods is presented, emphasizing the need for a variety of experimental approaches. The odd-odd nuclei that are immediately amenable to fruitful additional study are pointed out. A discussion of the intrinsic level structures, Gallagher-Moszkowski (GM) splittings, Newby (N) shifts, and role of the residual $p\ensuremath{-}n$ interaction is presented. Currently available data in the rare-earth region allow the empirical determination of 137 GM splittings and 36 N shifts for 25 odd-odd nuclei in the mass region 152\ensuremath{\leqslant}$A$\ensuremath{\leqslant}188. A new parametrization of the residual $p\ensuremath{-}n$ interaction is presented which also takes into account the 27 GM splittings and 12 N shifts from the actinide region. Newly discovered features of rotational bands, such as odd-even staggering, and other high-spin phenomena, such as signature inversion and delay in bandcrossing frequency, are discussed. The role of higher-order Coriolis coupling is pointed out. Systematics of the two-quasiparticle excitations, shape coexistence, isomers, and four-quasiparticle states are presented. Calculated results of the two-quasiparticle intrinsic excitations using two methods, the intrinsic level spacings for odd-A neighboring nuclei and the quasiparticle-plus-phonon coupling model, are compared with experiment.

Intrinsic states and rotational bands in 176Ta and 178Ta

High-spin states in the deformed, doubly odd nuclei, 176Ta and 178Ta have been studied by time-correlated γ-ray and electron spectroscopy techniques following (HI, xn) reactions. The new results independently confirm and extend in spin the previously known two- and four-quasiparticle states, albeit with some differences. Many high-seniority structures, most of them with associated rotational bands, have been identified for the first time. Several high- K isomers with half-lives ranging from a few nanoseconds up to hundreds of milliseconds have been discovered. Observation of rotational bands, built upon the intrinsic states has allowed characterization of the configurations, through both the in-band decay properties and alignments. The excitation energies of the observed multi-quasiparticle structures are generally reproduced using calculations based on the Lipkin-Nogami pairing model with inclusion of the effects of blocking and empirical nucleon-nucleon interactions. Some discrepancies in the energies, such as for the 14 − and 15 − isomers in 176Ta and 178Ta, respectively, are attributed to specific configuration mixing. Several anomalously fast K-forbidden transitions are discussed.

Configuration changes and hindered decays in four- and six-quasiparticle isomers in 178Ta.

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 $^{176}\mathrm{Ta}$.

Low-lying four-quasiparticle state in 82Rb

The g-factor of the 6+ 191.3 keV excited state with a half-life T1/2=12.3(6) ns in 82Rb populated in the 79Br(4He,n) and 81Rb(3He,2n) reactions has been measured by the TDPAD method. On the basis of the experimental g-factor gexp=+0.670(8) the four-quasiparticle configuration \(\left[ {\pi g_{9/2} \otimes \nu \left( {g_{9/2} } \right)_{7/2}^3 } \right]_{6^ + }\) involving an anomalous neutron coupling has been assigned to this state.