Two-quasiparticle Configuration Research Articles

Multiple chiral doublet bands in 104Rh

Two new negative-parity bands have been identified in the odd-odd 104Rh nucleus. According to their experimentally observed properties, they have the same π(1g9/2)−1⊗ν(1h11/2)1 high-j configuration as the previously known negative-parity chiral doublet bands. This observation raises the possibility of the existence of multiple chiral doublet bands, MχD, in this nucleus. Comparing the properties of the observed bands with results of detailed theoretical calculations, one can conclude that the lower energy parts of bands 1 and 2 are chiral partner bands with the π(1g9/2)−1⊗ν(1h11/2)1 two-quasiparticle configuration, while bands 3 and 4 are chiral partner bands with the π(1g9/2)−1⊗ν(1g7/2)−2(1h11/2)1 four-quasiparticle configuration. Thus, MχD based on different configurations is observed in 104Rh.

Octupole deformation in two-quasiparticle states of even–even deformed nuclei

Following the Skyrme-Hartree–Fock-BCS approach with blocking to two-quasiparticle states describing K-isomeric states of deformed even–even nuclei as presented in an earlier work of some of us Minkov N et al (2022 Phys. Rev. C 105 044 329), we study the emergence of axial-octupole deformation in excited states of this nature. We select two representative nuclei in the rare-earth and actinide mass regions, namely 148Ce (octupole-deformed in its ground state) and 234U (with a left-right symmetric ground-state shape), and several low-lying two-quasiparticle configurations illustrating cases where, in the same nucleus, some such excited states are calculated to be octupole deformed and some others are not. Then we investigate the mechanism favoring the appearance of left-right asymmetric shapes in the two-quasiparticle excited states of one-particle-one-hole type. We find in particular that strong octupole coupling between the blocked hole and particle states of a two-quasiparticle configuration does not favor pear-like shapes. In contrast, octupole coupling of the blocked particle (resp. hole) state with another particle (resp. hole) state located slightly above (resp. below) it in the single-particle spectrum favors pear-like shapes in the two-quasiparticle state.

Two quasiparticle wobbling in the even-even nucleus 130Ba

Two newly observed bands built on a two-quasiparticle configuration in 130Ba have been investigated for the first time with the microscopic projected shell model. The experimental energy spectra and the available electromagnetic transition probabilities are well reproduced. The wobbling character of the higher band is revealed by the angular momentum projected wavefunctions via the K plot and the azimuthal plot. This provides the first strong microscopic evidence for wobbling motion based on a two-quasiparticle configuration in even-even nuclei.

Transverse wobbling in an even-even nucleus

Two new bands built on the two-quasiparticle $\pi(h_{11/2})^2$ configuration of the even-even nucleus $^{130}$Ba are investigated using constrained triaxial covariant density functional theory combined with quantum particle rotor model calculations. The energy difference between the two bands, as well as the available electromagnetic transition probabilities $B(M1)_{\textrm{out}}/B(E2)_{\textrm{in}}$ and $B(E2)_{\textrm{out}}/B(E2)_{\rm{in}}$, are well reproduced. The analysis of the angular momentum geometry reveals that the higher band represents transverse wobbling motion of a two-quasiparticle configuration. This is the first example of two-quasiparticle wobbling bands in an even-even nucleus.

Characterization of long-lived isomers in the odd-odd heavy actinide Md254

Bandhead energies of all the physically admissible low-lying two-quasiparticle configuration states in the doubly-odd heavy actinide $_{101}^{254}\mathrm{Md}_{153}$ are evaluated using the well-tested two-quasiparticle rotor model with explicit inclusion of the residual proton-neutron interaction. A critical examination of these results, aimed at characterization of the long-lived (${t}_{1/2}=10$ min and 28 min; $%\ensuremath{\varepsilon}\ensuremath{\le}100$) isomer pair, conclusively rules out a high-spin ($J\ensuremath{\ge}5$) assignment for either of the isomers. Our analysis leads to ${J}^{\ensuremath{\pi}}K={1}^{\ensuremath{-}}0{p:1/{2}^{\ensuremath{-}}[521]\ensuremath{\bigotimes}n:1/{2}^{+})[620]}$ and ${3}^{\ensuremath{-}}3{p:7/{2}^{\ensuremath{-}}[514]\ensuremath{\bigotimes}n:1/{2}^{+}[620]}$ assignments, respectively, to these isomers and designates the 10-min isomer as its ground state. Our study reveals a ``landmark'' position for $^{254}\mathrm{Md}$ in the decay path of super heavy elements. The as-yet unobserved electron capture decay branches from each of the two $^{254}\mathrm{Md}$ isomers to $^{254}\mathrm{Fm}$ levels are specified.

Diabatic crossing of chiral “twins” in the odd-oddAg106nucleus: A theoretical perspective

A systematic study of both the observed positive-parity magnetic rotation band and the negative-parity $\mathrm{\ensuremath{\Delta}}I=1$ doublet bands in an odd-odd $^{106}\mathrm{Ag}$ nucleus is carried out. The negative-parity doublet bands depict some unusual features that have not been observed in any isotope in the mass $A=100$ region. For instance, (i) the moment of inertia of the partner band is quite different from that of the yrast band, and (ii) these bands cross each other at an angular momentum of $I=14\ensuremath{\hbar}$. Also, the observed significantly large but constant $B(M1)$ transitions confirm that the strong $M1$ transitions are being reinforced by the contributions from collective rotation. To explain these features, a collective model has been developed whose kinetic and potential energies are extracted from the tilted-axis cranking model. Instead of the triaxial parameter $\ensuremath{\gamma}$, a second-order phase transition is found to be responsible for the spontaneous breakdown of chiral symmetry. Analytical solution of the Schr\odinger equation has generated a doublet nondegenerate eigenvalue spectrum. The ensuing model results based on the two-quasiparticle configuration $\ensuremath{\pi}{g}_{\frac{9}{2}}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{\frac{11}{2}}$ exhibit similarities with many observed features of the negative-parity doublet bands and hence confirm their chiral character. The cranking mass parameter in kinetic energy plays an important role in diabatic crossing between these emerged chiral twin bands.

Observation of aγband based on a two-quasiparticle configuration inGe70

The structure of $^{70}$Ge has been studied through in-beam gamma ray spectroscopy. A new band structure is identified that leads to forking of the ground-state band into two excited bands. Band structures have been investigated using the microscopic triaxial projected shell model approach. The observed forking is demonstrated to result from almost simultaneous band crossing of the two neutron aligned and the \gamma-band built on this two-quasiparticle configuration with the ground-state band.

Effects of 2p-2h configurations on low-energy dipole states in neutron-rich N=80, 82 and 84 isotones

Starting from the Skyrme interaction SLy4 we study the effects of phonon-phonon coupling on the low-energy electric dipole response in 130−134 Sn, 132−136 Te and 134−138 Xe. Our calculations are performed within the finite-rank separable approximation, which enables one to perform quasiparticle random phase approximation calculations in very large two-quasiparticle configuration spaces. A dependence of the pygmy dipole resonance strengths on the neutron skin thickness is found. The inclusion of the two-phonon configurations gives a considerable contribution to the low-lying strength.

Nature of isomerism in exotic sulfur isotopes.

We clarify the origin of the anomalously hindered E2 decay from the 4_{1}^{+} level in ^{44}S by performing a novel many-body analysis in the shell model. Within a unified picture about the occurrence of isomerism in neutron-rich sulfur isotopes, the 4_{1}^{+} state is demonstrated to be a K=4 isomer dominated by the two-quasiparticle configuration νΩ^{π}=1/2^{-}⊗νΩ^{π}=7/2^{-}. The 4_{1}^{+} state in ^{44}S is a new type of high-K isomer which has significant triaxiality.

High-spin level scheme of doubly odd128I

High-spin states in the odd-odd ${}^{128}$I have been studied experimentally using the ${}^{124}$Sn(${}^{7}$Li, 3n$\ensuremath{\gamma}$)${}^{128}$I reaction at beam energies of 28 and 32 MeV. A level scheme built on the known ${T}_{1/2}=175$ ns, (${6}^{\ensuremath{-}})$ isomer has been established. The low-lying states in the level scheme have been proposed to be associated with the $\ensuremath{\pi}{d}_{5/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ and $\ensuremath{\pi}{g}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ two-quasiparticle excitations. The strongly populated negative-parity states, forming a bandlike structure, have been assigned to be based on the $\ensuremath{\pi}{g}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ configuration. The $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ multiplet ${9}^{+}$, ${10}^{+}$, and ${11}^{+}$ members have been identified according to the systematics of the corresponding level structures observed in the lighter odd-odd iodine isotopes. The two-quasiparticle configuration assignments are supported by the nucleon pair approximation and empirical shell-model calculations.

Structure of the yrast band in the odd-odd deformed nucleusPm156

The six-level sequence deduced for the odd-odd nucleus $^{156}\mathrm{Pm}$ in the high-spin studies following spontaneous fission of $^{252}\mathrm{Cf}$ is shown to constitute the ${K}^{\ensuremath{\pi}}={4}^{+}$ yrast band having the two-quasiparticle configuration {$p$:5/2[532] $+$ $n$:3/2[521]}. Spin parities ${I}^{\ensuremath{\pi}}={4}^{+}$ through 9${}^{+}$ are assigned to the earlier suggested six levels. The location and the decay $\ensuremath{\gamma}$'s of the 10${}^{+}$ level of this band are indicated. It is also pointed out that there are no $\ensuremath{\gamma}$ rays common to these postfission high-spin spectra and those seen in the $^{156}\mathrm{Nd}$ $\ensuremath{\beta}$-decay studies.

Spectroscopy ofHo144using recoil-isomer tagging

Excited states in the proton-unbound odd-odd nucleus $^{144}\mathrm{Ho}$ have been populated using the $^{92}\mathrm{Mo}(^{54}\mathrm{Fe},\mathit{pn})^{144}\mathrm{Ho}$ reaction and studied using the recoil-isomer-tagging technique. The alignment properties and signature splitting of the rotational band above the ${I}^{\ensuremath{\pi}}=({8}^{+})^{144m}\mathrm{Ho}$ isomer have been analyzed and the isomer confirmed to have a $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ two-quasiparticle configuration. The configuration-constrained blocking method has been used to calculate the shapes of the ground and isomeric states, which are both predicted to have triaxial nuclear shapes with $|\ensuremath{\gamma}|\ensuremath{\approx}{24}^{\ifmmode^\circ\else\textdegree\fi{}}$.

Shape-driving effects in the triaxial nucleus,Xe128

An extended decay scheme for Xe-128 has been constructed by using data from the Sn-124(Be-9, 5n)Xe-128 reaction at a beam energy of 58 MeV. Bands have been identified as being built on several intrinsic states, including a proposed 9/2(-)[514] circle times 1/2(+)[400] two-quasineutron configuration that forms the K-pi=5(-) intrinsic state at 2228 keV, and on a previously assigned K-pi=8(-) intrinsic state at 2786 keV. A half-life of 73(3) ns has been measured for the latter. Theoretical calculations have been performed by using the configuration-constrained blocking method based on a nonaxial Woods-Saxon potential. Large gamma deformation and gamma softness are predicted for the ground state and the K-pi=5(-) intrinsic state, whereas a nearly axially symmetric shape is predicted for the K-pi=8(-) two-quasiparticle configuration. The low value of the hindrance factor for the E1 transition depopulating the K-pi=8(-) intrinsic state is discussed in the context of analogous transitions in neighbouring N=74 isotones.

Observation of theπh11∕2⊗νi13∕2band in odd-oddRe170

High-spin states in doubly odd {sup 170}Re have been populated via the {sup 142}Nd({sup 32}S,p3n{gamma}) reaction at beam energies of 155 and 166 MeV. K X-{gamma} and {gamma}-{gamma} coincidences have been measured using a detector array consisting of 12 high-purity germanium detectors with BGO suppressors. A rotational band is identified and assigned to {sup 170}Re for the first time. This band is proposed to be built on the {pi}h{sub 11/2}x{nu}i{sub 13/2} two-quasiparticle configuration by comparing the band properties with known bands in neighboring nuclei and the cranked-shell model calculations.

Two-Quasiparticle Bands and Isomers in 98Sr

Rotational bands in neutron-rich 98Sr nucleus havebeen investigated by measuring high-fold prompt γ-raycoincidence events of the spontaneous fission of 252Cfwith the Gammasphere detector array. A deformed K = 3 band built on the1838 keV level has been confirmed and extended. Another deformed K = 6band based on the 2535 keV level has been established. Both the bandsoriginate most probably from the ν9/2[404]⊗ν3/2[411]two-quasiparticle configuration with Ω = |Ω1−Ω2|and Ω = |Ω1+Ω2|, respectively. Based on thedelay-coincidence measurements, the half-lives for the K = 3 and K = 6band head levels have been obtained to be 13±3 ns and 4.5±1.0 ns, respectively.

Identification of excited states in doubly odd63140Eu77by recoil-isomer tagging

The 36Ar + 107Ag fusion-evapn. reaction was used to search for isomeric states in the N = 77 isotope 140Eu near the proton-drip line. The recoiling nuclei were implanted into a Si detector, at the focal plane of a gas-filled separator, where prompt and delayed g-ray transitions were correlated across isomeric states using recoil-isomer tagging. The feeding and decay of a new 299(3) ns isomeric state was established. This measurement represents the first observation of excited high-spin states in 140Eu. The behavior of the new states above the isomer is discussed in terms of theor. calcns. based upon the cranked-shell model and upon the exptl. systematics of other N = 77 isotones. Within this framework, the isomer was interpreted to be built upon a ph11/2 ? nh11/2 two-quasiparticle configuration. [on SciFinder (R)]

Nuclear structure of neutron-rich78As

Excited states in the neutron-rich doubly-odd nucleus78As have been identified for the first time by proton-γ and γ-γ coincidence measurements via the76Ge(α, pn) reaction at 32, 36, and 40 MeV beam energy. Four levels have been found to decay with lifetimes in the nanosecond region. The 5(+) to (10+) states are ascribed to the (πg 9/2⊗νg 9/2) intruder two-quasiparticle configuration with some collective components in the 9(+) and (10+) states.

In-beam spectroscopy of 110Te

The neutron-deficient nucleus 110Te has been investigated by in-beam spectroscopic methods using the NORDBALL multi-detector array. Except for the energy of one level observed in α-decay, excited states in 110Te were previously unknown. The level scheme constructed from γγ-coincidence relationships is presented with tentative spin assignments up to I π = 21 −. Above spin 8 + the yrast states have negative parity. They are most likely the members of a band predominantly based on the gn h 11 2 g 7 2 ) two-quasiparticle configuration. A band crossing is observed at I π ≈ 17 −. It is suggested to be caused by the alignment of two h 11 2 neutrons. The cranked shell model correctly predicts the spin alignment, but fails to reproduce the observed band crossing frequency, possibly because of octupole effects.

Onset of collectivity in the odd-odd nucleus 72As.

High-spin states in the odd-odd nucleus $^{72}\mathrm{As}$ were investigated by means of coincidence measurements via the $^{59}\mathrm{Co}$${(}^{19}$F,\ensuremath{\alpha}pn) and the $^{65}\mathrm{Cu}$${(}^{12}$C,\ensuremath{\alpha}n) reactions at beam energies of 55 MeV and 50 MeV, respectively. Two new rotational-like level sequences of probably opposite parities have been found to feed into the known ${7}^{(\mathrm{\ensuremath{-}})}$ isomer at 562.8 keV. Lifetime measurements for some states of the positive-parity sequence using the Doppler shift attenuation method revealed transition strengths of B(E2)\ensuremath{\approxeq}40 Weisskopf units. Thus, this band is interpreted as a collective structure built on the two-quasiparticle intruder configuration (\ensuremath{\pi}${\mathit{g}}_{9/2}$\ensuremath{\bigotimes}\ensuremath{\nu}${\mathit{g}}_{9/2}$).

Non-collective degrees of freedom in 192Pb

Two positive-parity and two negative-parity groups of states of non-collective character with excitation energies ranging from 3 to 7 MeV and spins as high as 21 ħ have been established in 192Pb. The main features of each group are described phenomenologically as four-quasiparticle configurations, arising from coupling of the 2 + 1, 4 + 1 and 6 + 1 states to a specific aligned neutron two-quasiparticle configuration. Supporting evidence for this interpretation and the importance of proton excitations in this singly closed-shell nucleus is obtained from the predictions of the quasiparticle multi-step shell model (QMSM). In addition, a four-quasiparticle structure of predominant neutron character is proposed on the basis of comparisons with the experimental data on 194Pb and with the results of the QMSM calculations.