Yrast Band Research Articles

Emergence of principal axis rotation in 110Ag

The negative-parity yrast band of 110Ag has been extended significantly and the lifetimes of the high spin levels of this band have been measured. The experimentally observed level scheme and measured electromagnetic transition rates have been compared with the theoretical predictions of a model with two quasiparticles coupled to a triaxially deformed core. This calculation successfully reproduces the energy spectra and electromagnetic transition rates beyond Iπ=12−ℏ. These observations indicate that the principal axis of rotation is responsible for the generation of high angular momentum states along the yrast cascade in 110Ag. In all the other lighter isotopes, these states are generated through titled axis rotation. Thus, 110Ag is the first nucleus where the boundary between tilted and principal axis rotation could be established.

Non-yrast nuclear spectra in a model of coherent quadrupole-octupole motion

A model assuming coherent quadrupole-octupole vibrations and rotations is applied to describe non-yrast energy sequences with alternating parity in several even-even nuclei from different regions, namely $^{152,154}$Sm, $^{154,156,158}$Gd, $^{236}$U and $^{100}$Mo. Within the model scheme the yrast alternating-parity band is composed by the members of the ground-state band and the lowest negative-parity levels with odd angular momenta. The non-yrast alternating-parity sequences unite levels of $\beta$-bands with higher negative-parity levels. The model description reproduces the structure of the considered alternating-parity spectra together with the observed B(E1), B(E2) and B(E3) transition probabilities within and between the different level-sequences. B(E1) and B(E3) reduced probabilities for transitions connecting states with opposite parity in the non-yrast alternating-parity bands are predicted. The implemented study outlines the limits of the considered band-coupling scheme and provides estimations about the collective energy potential which governs the quadrupole-octupole properties of the considered nuclei.

Candidate chiral doublet bands in128La

The candidate chiral doublet bands in ${}^{128}$La have been searched for through the ${}^{118}$Sn(${}^{14}$N, 4n)${}^{128}$La reaction at a beam energy of 69 MeV. A positive-parity sideband with the same configuration as that of the yrast band has been identified. The positive-parity and spins of the sideband have been assigned based on the $\ensuremath{\Delta}I=1$ mixed $M1/E2$ and $E2$ characters of the linking transitions between the sideband and the yrast band. The properties of the sideband and the yrast band are discussed. The newly identified sideband and the yrast band are suggested to form the candidate of the near degenerate chiral doublet bands.

Yrast electromagnetic transition strengths and shape coexistence in 182Pt

Abstract Using the reaction 170Yb(16O, 4n) at a beam energy of 87 MeV and the Recoil Distance Doppler-Shift Method, five lifetimes in the yrast band of 182Pt have been determined for the first time. For the data analysis, a dedicated version of the Differential Decay Curve Method has been employed. Calculations within the Interacting Boson Model and the General Collective Model indicate shape coexistence in the investigated nucleus.

Investigation of octupole vibrational states in150Nd via inelastic proton scattering (p,p′γ)

Octupole vibrational states were studied in the nucleus $^{150}\mathrm{Nd}$ via inelastic proton scattering with $\unit[10.9]{MeV}$ protons which are an excellent probe to excite natural parity states. For the first time in $^{150}\mathrm{Nd}$, both the scattered protons and the $\gamma$ rays were detected in coincidence giving the possibility to measure branching ratios in detail. Using the coincidence technique, the $B(E1)$ ratios of the decaying transitions for 10 octupole vibrational states and other negative-parity states to the yrast band were determined and compared to the Alaga rule. The positive and negative-parity states revealed by this experiment are compared with Interacting Boson Approximation (IBA) calculations performed in the (spdf) boson space. The calculations are found to be in good agreement with the experimental data, both for positive and negative-parity states.

Shape Evolution in Rotating 178Os

In order to investigate the evolution of X(5) in rotating 178Os, an experiment populating 178Os via the fusion evaporation reaction 154Sm(29Si,5n)178Os was performed at the HI-13 tandem accelerator at the China Institute of Atomic Energy (CIAE). Lifetimes of excited states above 8+ in the yrast band in 178Os have been measured using the Doppler shift attenuation method. Lifetimes above 12+ states were measured for the first time. The deduced transitional quadruple moments (Qt), together with the previous data using the recoil distance Doppler shift (RDDS) method are compared with theoretical calculations based on the X(5) model and the interaction Boson model (IBM). Above the 10+ states, the Qt values fit well with the X(5) predictions. The present result suggests that the shape of a nucleus 178Os keeps X(5) critical point symmetry as spin increases to at least 14+. The shape change of 178Os with spin increasing is similar to that of 176Os.

High-spin yrast states in theγ-soft nuclei135Pr and134Ce

High-spin states have been studied in Pr-135(59), populated through the Cd-116(Na-23,4n) reaction at 115 MeV, using the Gammasphere gamma-ray spectrometer. The negative-parity yrast band has been significantly extended to spin similar to 45 (h) over bar and excitation energy 21.5 MeV, showing evidence for several rotational alignments. The positive-parity yrast band of Ce-135(58), populated through the p4n channel of this reaction, was also populated to spin similar to 38 (h) over bar and excitation energy 18 MeV. Cranking calculations indicate that these nuclei are soft with respect to the triaxiality parameter gamma and that several competing nuclear shapes occur at high spin.

High-spin yrast structure of159Ho

An investigation of the yrast structure of the odd-$Z$ ${}^{159}$Ho nucleus to high spin has been performed. The ${}^{159}$Ho nucleus was populated by the reaction ${}^{116}$Cd(${}^{48}$Ca,$p4n\ensuremath{\gamma}$) at a beam energy of 215 MeV, and resulting $\ensuremath{\gamma}$ decays were detected by the Gammasphere spectrometer. The ${h}_{11/2}$ yrast band has been significantly extended up to ${I}^{\ensuremath{\pi}}=75/{2}^{\ensuremath{-}}$ (tentatively $79/{2}^{\ensuremath{-}}$). A lower frequency limit for the second ${({h}_{11/2})}^{2}$ proton alignment was extracted consistent with the systematics of this alignment frequency, indicating an increased deformation with neutron number in the Ho isotopes. The energy-level splitting between the signature partners in the ${h}_{11/2}$ structures of the Ho isotopes and the neighboring $N=92$ isotones is discussed.

Investigation into the rotational bands of 185Pt with the particle-rotor model

Theoretical calculations have been performed for the ν9/2+[624](i13/2) and ν7/2−[503]( f7/2) bands of 185Pt in the framework of particle-rotor model. The band properties of signature splitting and configuration mixing have been analyzed. The level energy and signature splitting before the band crossing can be well interpreted by introducing triaxiality. The positive-parity yrast band is proposed to be dominated by the ν9/2+[624](i13/2) component, while the negative-parity band shows strong mixing of ν7/2−[503](f7/2) and ν9/2−[505](h9/2) configurations.

The first candidate for chiral nuclei in the [formula omitted] mass region: 80Br

Excited states of 80Br have been investigated via the 76Ge(11B, α3n) and 76Ge(7Li, 3n) reactions and a new ΔI=1 band has been identified which resides ∼ 400 keV above the yrast band. Based on the experimental results and their comparison with the triaxial particle rotor model calculated ones, a chiral character of the two bands within the πg9/2⊗νg9/2 configuration is proposed, which provides the first evidence for chirality in the A∼80 region.

High-spin level structure ofRh115: Evolution of triaxiality in odd-even Rh isotopes

High-spin excited states in the neutron-rich nucleus $^{115}\mathrm{Rh}$ have been identified for the first time by studying prompt $\ensuremath{\gamma}$ rays from the spontaneous fission of $^{252}\mathrm{Cf}$ with the Gammasphere detector array. A new yrast band and a sideband are built in $^{115}\mathrm{Rh}$. This level scheme is proposed to be built on the $7/{2}^{+}$ ground state. The existence of a large signature splitting and an yrare band in $^{115}\mathrm{Rh}$ shows typical features of a triaxially deformed nucleus. The rigid triaxial rotor plus particle model is used to interpret the level structure of $^{115}\mathrm{Rh}$. The level energies, the $\ensuremath{\gamma}$ branching ratios, the large signature splitting in the yrast band, and the inverted signature splitting in the yrare band in $^{115}\mathrm{Rh}$ are reproduced very well. Strong $K$ mixing occurs in $^{115}\mathrm{Rh}$ at high spin.

Microscopic structure of 126Ba in IBM terms

The yrast band of the nonaxially deformed 126Ba nucleus is described by the Hamiltonian of the interaction boson model. Its parameters are calculated on the basis of a microscopic theory within a spherical mean field, and residual interactions that include pairing and multipole factorized forces. Each state of the yrast band is considered independently of others, allowing us to study variations in the superfluid properties of the nucleus and the quasiparticle structure of collective D phonons with spin. The calculations are performed in an expanded configuration space that includes the collective D phonon states, and noncollective states in which an additional phonon of positive parity whose spin assumes values of 0 to 6 is present along with the D phonons. It is shown that the collective Hamiltonian parameters cannot be reproduced without considering the effect of the noncollective states.

Signature inversion in odd-oddRh114: First identification of high-spin states in very neutron-richRh114and application of the triaxial projected shell model

High-spin excited states in the very neutron-rich nucleus $^{114}\mathrm{Rh}$ have been studied by examining the prompt $\ensuremath{\gamma}$ rays emitted in the spontaneous fission of $^{252}\mathrm{Cf}$ with the Gammasphere detector array. A high-spin level scheme of $^{114}\mathrm{Rh}$ has been established for the first time with 13 new levels. The level scheme is proposed to be built on a ${7}^{\ensuremath{-}}$ state. The existence of a relatively large signature splitting and an yrare band show features which may indicate triaxial deformation. The phenomenon of signature inversion has been observed in $^{114}\mathrm{Rh}$ at $I=12 \ensuremath{\hbar}$. The observed signature inversion of $^{114}\mathrm{Rh}$ is interpreted successfully in terms of the triaxial projected shell model. Theoretical calculations suggest that the negative-parity, yrast band of $^{114}\mathrm{Rh}$ has the two-quasi-particle configuration of $\ensuremath{\pi}{g}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$, consistent with the systematics of odd-odd Rh isotopes. The signature inversion at spin $12 \ensuremath{\hbar}$ may be attributed to the change of rotational mode, from quasi-particle aligned rotation at low spins to collective rotation at high spins.

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.

Publisher’s Note: Microscopic formulation of the interacting boson model for rotational nuclei [Phys. Rev. C83, 041302(R) (2011)

We propose a novel formulation of the Interacting Boson Model (IBM) for rotational nuclei with axially-symmetric strong deformation. The intrinsic structure represented by the potential energy surface (PES) of a given multi-nucleon system has a certain similarity to that of the corresponding multi-boson system. Based on this feature, one can derive an appropriate boson Hamiltonian as already reported. This prescription, however, has a major difficulty in rotational spectra of strongly deformed nuclei: the bosonic moment of inertia is significantly smaller than the corresponding nucleonic one. We present that this difficulty originates in the difference between the rotational response of a nucleon system and that of the corresponding boson system, and could arise even if the PESs of the two systems were identical. We further suggest that the problem can be cured by implementing $\hat{L} \cdot \hat{L}$ term into the IBM Hamiltonian, with coupling constant derived from the cranking approach of Skyrme mean-field models. The validity of the method is confirmed for rare-earth and actinoid nuclei, as their experimental rotational yrast bands are reproduced nicely.

MICROSCOPIC STUDY OF NEGATIVE PARITY YRAST STATES IN NEUTRON-DEFICIENT 119–127Ba ISOTOPES

The negative parity yrast bands of neutron-deficient 119–127 Ba nuclei are studied by using the Projected Shell Model approach. Energy levels, transition energies and B(M1)/B(E2) ratios are calculated and compared with the available experimental data. The calculations reproduce the band head spins of negative parity yrast bands and indicate the multi-quasiparticle structure for these bands.

Microscopic formulation of the interacting boson model for rotational nuclei

We propose a novel formulation of the interacting boson model (IBM) for rotational nuclei with axially symmetric, strong deformation. The intrinsic structure represented by the potential-energy surface (PES) of a given multinucleon system has a certain similarity to that of the corresponding multiboson system. Based on this feature, one can derive an appropriate boson Hamiltonian, as already reported. This prescription, however, has a major difficulty in the rotational spectra of strongly deformed nuclei: the bosonic moment of inertia is significantly smaller than the corresponding nucleonic one. We present that this difficulty originates in the difference between the rotational response of a nucleon system and that of the corresponding boson system, and could arise even if the PESs of the two systems were identical. We further suggest that the problem can be solved by implementing the $\mathrm{L\ifmmode \hat{}\else \^{}\fi{}}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{L\ifmmode \hat{}\else \^{}\fi{}}$ term into the IBM Hamiltonian, with the coupling constant derived from the cranking approach of Skyrme mean-field models. The validity of the method is confirmed for rare-earth and actinoid nuclei, as their experimental rotational yrast bands are reproduced nicely.

174Hf and 174Yb by the Projected Shell Model with Improved 4-quasiparticle basis

The rare-earth nuclei 174Hf and 174Yb are studied in the framework of the projected shell model. The calculations are carried out with an improved version of code which contains all types of four-quasiparticle basis in the configuration space. Both the yrast band and the multi-quasiparticle excited bands can be well reproduced. Compared with the experimental data, our calculations improve the previous results, especially for high spin states. This improvement suggests that the newly added configurations are important. In 174Yb, a new possible configuration is suggested for the observed Kπ = 14+ band, and the decay of the Kπ = 6+ isomeric state to the yrast band is discussed.

Combined in-beam electron andγ-ray spectroscopy ofHg184,186

By exploiting the SAGE spectrometer a simultaneous measurement of conversion electrons and {gamma} rays emitted in the de-excitation of excited levels in the neutron-deficient nuclei {sup 184,186}Hg was performed. The light Hg isotopes under investigation were produced using the 4n channels of the fusion-evaporation reactions of {sup 40}Ar and {sup 148,150}Sm. The measured K- and L-conversion electron ratios confirmed the stretched E2 nature of several transitions of the yrast bands in {sup 184,186}Hg. Additional information on the E0 component of the 2{sub 2}{sup +}{yields}2{sub 1}{sup +} transition in {sup 186}Hg was obtained.

CHIRALITY IN THE MASS 80 REGION: 79Kr

The high spin states of 79 Kr were studied via the 70 Zn (13 C , 4 n ) reaction to search for chiral doublet bands based on the three-quasi-particle configuration, πg9/22 ⊗ νg9/2-1. The 13 C beam of 65 MeV was provided by the 930 AVF Cyclotron at CYclotron and RadioIsotope Center (CYRIC) facility at Tohoku University. The triple coincidence γ rays were detected by the Hyperball2 array. The side band structure to the πg9/22 ⊗ νg9/2-1 yrast band has been identified in 79 Kr . Spin and parity assignments are made based on the DCO ratio and linear polarization analysis.