Ground-state Rotational Band Research Articles

Low-lying, excitedK=0bands in238U

The lowest excited $K=0$ bands in ${}^{238}\mathrm{U}$ have been studied by internal conversion electron spectroscopy following Coulomb excitation with $\ensuremath{\alpha}$ particles at 19 MeV. Strong $E0$ enhancement was found for transitions between the ${2}^{+}$ members of the ${0}_{2}^{+},$ ${0}_{3}^{+}$ bands and of the ground state rotational band, and $E0$ strength values of 0.0099(18) and 0.175(26) were determined. Based on our more accurate energy value of the ${0}_{3}^{+}$ state, 997.23(24) keV, the moments of inertia for the excited $K=0$ bands were studied.

Alignment delays in the N = Z nuclei (72)Kr, (76)Sr, and (80)Zr.

The ground state rotational bands of the N = Z nuclei (72)Kr, (76)Sr, and (80)Zr have been extended into the angular momentum region where rotation alignment of particles is normally expected. By measuring the moments of inertia of these bands we have observed a consistent increase in the rotational frequency required to start pair breaking, when compared to neighboring nuclei. (72)Kr shows the most marked effect. It has been widely suggested that these "delayed alignments" arise from np-pairing correlations. However, alignment frequencies are very sensitive to shape degrees of freedom and normal pairing, so the new experimental observations are still open to interpretation.

Fast-neutrons incident on holmium

Differential neutron-scattering cross-sections of 165Ho are measured at 40 or more scattering angles and at ≈0.5 MeV incident-energy intervals from ≈4.5–10.0 MeV. These results are combined with previously-reported neutron total and scattering cross-sections to construct a comprehensive experimental data base which is interpreted in the context of a coupled-channels model, with attention to the collective excitation of the K=7/2 − ground-state rotational band. Physical aspects are discussed and comparisons made with previous models reported in the literature and with the relevant portions of the ENDF/B-6 evaluated nuclear data file.

Effects of instantons on the YN interaction

Neutron inelastic scattering from thulium-169 has been studied for states above 100 keV via the (n,n′γ) reaction at incident energies in the 0.2- to 1.0-MeV range. A high-resolution Ge spectrometer in conjunction with the time-of-flight technique was utilized. Thirty-six gamma-ray transitions from 16 levels were observed. Gamma-ray angular distributions were measured at En=750 keV and excitation functions at 125 degrees were measured in 50 keV steps over the range of incident energies. Differential gamma-ray production cross sections and gamma-ray branching ratios were obtained. Inferred neutron inelastic level cross sections of the four lowest ground-state rotational band (Kπ=1/2+) members are compared to the sum of calculated compound nucleus and direct interaction cross sections. For the remaining levels, measurements are compared to compound nucleus calculations only. The comparison shows generally good agreement particularly near threshold.

Relativistic coupled channel analyses of 800 MeV proton inelastic scattering from26Mg

Elastic and inelastic scattering of 800 MeV protons from the axially deformed nucleus ${}^{26}\mathrm{Mg}$ is analyzed using optical potentials obtained from the relativistic impulse approximation (RIA) in Dirac coupled channel formalism. A rotational collective model is used to obtain the transition optical potentials for the low-lying excited states of the ground-state rotational band. The theoretical results agree with the experimental data for the elastic and the ${2}^{+}$ state, and reproduce the ${4}^{+}$ excited state reasonably well. In order to check these RIA results we perform Dirac phenomenological (DP) analysis of the data and obtain the best fit Lorentz scalar and vector optical potentials. RIA and DP optical potentials are compared with those of a similar treatment for 800 MeV protons scattering from ${}^{24}\mathrm{Mg}.$

Thulium-169 neutron inelastic scattering cross section measurements via the [formula omitted]Tm [formula omitted] reaction

Neutron inelastic scattering from thulium-169 has been studied for states above 100 keV via the (n,n′γ) reaction at incident energies in the 0.2- to 1.0-MeV range. A high-resolution Ge spectrometer in conjunction with the time-of-flight technique was utilized. Thirty-six gamma-ray transitions from 16 levels were observed. Gamma-ray angular distributions were measured at E n=750 keV and excitation functions at 125 degrees were measured in 50 keV steps over the range of incident energies. Differential gamma-ray production cross sections and gamma-ray branching ratios were obtained. Inferred neutron inelastic level cross sections of the four lowest ground-state rotational band (K π=1/2 +) members are compared to the sum of calculated compound nucleus and direct interaction cross sections. For the remaining levels, measurements are compared to compound nucleus calculations only. The comparison shows generally good agreement particularly near threshold.

Moments of inertia of nuclei in the rare earth region: A relativistic versus nonrelativistic investigation

A parameter-free investigation of the moments of inertia of ground-state rotational bands in well-deformed rare-earth nuclei is carried out using cranked relativistic Hartree-Bogoliubov (CRHB) and nonrelativistic cranked Hartree-Fock-Bogoliubov (CHFB) theories. In CRHB theory, the relativistic fields are determined by the nonlinear Lagrangian with the NL1 force and the pairing interaction by the central part of a finite-range Gogny D1S force. In CHFB theory, the properties in particle-hole and particle-particle channels are defined solely by Gogny D1S forces. Using an approximate particle number projection before variation by means of the Lipkin-Nogami method improves the agreement with the experimental data, especially in CRHB theory. The effect of the particle number projection on the moments of inertia and pairing energies is larger in relativistic than in nonrelativistic theory.

Isomer spectroscopy of neutron rich 190W 116

Gamma-rays de-exciting a millisecond isomer in the neutron-rich nucleus 190 74W 116 have been observed following relativistic projectile fragmentation of a 1 GeV per nucleon 208Pb beam. The isomeric decay populates the ground-state rotational band, with energies that indicate a significant deformation change at Z=74 for the N=116 isotones. The successful application of the projectile fragmentation technique in a region of deformed nuclei opens up the prospect of exploiting K-isomer decays to probe the evolution of collective nuclear structure far from the valley of beta stability.

Fission barriers at high angular momentum and the ground state rotational band of the nucleus 254No

We study the superheavy nucleus 254No in the framework of the Hartree-Fock-Bogoliubov approximation with the finite-range density-dependent Gogny force, at zero and high angular momentum. The properties of the ground state rotational band and the fission barriers are discussed as a function of angular momentum. We found a two-humped barrier up to spin values of (30-40)Planck's over 2pi and a one-humped barrier for higher spins. We reproduce fairly well with the binding energy, the ground state deformation, the gamma-ray energies, and the bound on the fission barrier height measured at high spin.

Diabatic Mean-Field Description of Rotational Bands in Terms of the Selfconsistent Collective Coordinate Method

Diabatic description of rotational bands provides a clear-cut picture for understanding the back-bending phenomena, where the internal structure of the yrast band changes dramatically as a function of angular momentum. A microscopic framework to obtain the diabatic bands within the mean-field approximation is presented by making use of the selfconsistent collective coordinate method. Applying the framework, both the ground state rotational bands and the Stockholm bands are studied systematically for the rare-earth deformed nuclei. An overall agreement has been achieved between the calculated and observed rotational spectra. It is also shown that the inclusion of the double-stretched quadrupole-pairing interaction is crucial to obtain an overall agreement for the even-odd mass differences and the rotational spectra simultaneously.

Magnetic moments in the 1/2−[521] ground-state band of 171Yb and Coriolis-induced renormalization of rotational g-factors in odd-A rare-earth nuclei

Gyromagnetic ratios of the states with spins between 7/2− and 21/2− (inclusive) in the 1/2−[521] ground-state band of 171Yb were measured by the transient-field technique following Coulomb excitation. Multipolarity mixing ratios were determined from measured particle-γ-ray angular correlations and total branching ratios were determined from intensity balances in a γ-γ coincidence measurement. The electromagnetic properties of the low-lying natural parity states in 171Yb are compared with particle-rotor calculations and can be well described in the limit of a pure K=1/2 band. A comparison of measured g-factors in the ground-state rotational bands of 155Gd, 169Tm and 171Yb with particle-rotor model calculations suggests that the renormalization of the rotational g-factor in the low-K, single-quasiparticle bands of odd rare-earth nuclei, compared with their even neighbours, is predominantly due to Coriolis interactions.

Systematics of the Quadrupole–Quadrupole Interaction and Convergence Properties

Our main concern in this work is to show how higher shell admixtures affect the spectrum of a Q·Q interaction. We first review how, in the valence space, the familiar SU(3) result for the energy spectrum can be obtained using a coordinate space Q·Q interaction rather than the Elliott one which is symmetric in r and p. We then reemphasize that the Elliott spectrum goes as L(L+1) where L is the orbital angular momentum. While in many cases this is compatible with the rotational formula which involves I(I+1), where I is the total angular momentum, there are cases, e.g., odd–odd nuclei, where there is disagreement. Finally, we consider higher shell admixtures and devise a scheme so as to obtain results, with the Q·Q interaction, which converge as the model spaces are increased. We consider not only ground state rotational bands but also those that involve intruder states.

Rotational bands of 155Gd

High spin states of 155Gd have been investigated using the 150Nd( 12C, α3n) reaction. Three rotational bands have been observed. Two of them have been extended up to (51/2) + and 45/2 −, respectively, and another band has been newly observed. The backbending and blocking effects are discussed in comparison with the calculation of cranked shell model. The routhians and the B(M1/BE2 ratio of the high-Ω rotational band are compared with the calculation by the tilted axis craking model. In addition the ground state rotational band of 156Gd has been extended up to 22 +.

Dirac coupled channel analyses of 800 MeV proton inelastic scattering from24Mg

Elastic and inelastic scattering of 800 MeV protons from the axially deformed nucleus ${}^{24}\mathrm{Mg}$ is investigated using microscopic relativistic impulse approximation optical potentials in Dirac-coupled channel formalism. A rotational collective model is used to obtain the transition optical potentials for the low-lying excited states of the ground state rotational band. The effect of multistep processes is investigated by including the channel coupling between two excited states in addition to the coupling between the ground state and excited states. The theoretical results agree with the experimental data for the elastic and the ${2}^{+}$ and ${4}^{+}$ excited states reasonably well.

A model of nuclear rotation

We construct a model which describes approximately ground-state, superdeformed and fission-isomeric rotational bands of even-even nuclei. The model is based on a three-term energy expression corresponding to: (a) a rigid rotation of part of the nucleus, (b) the energy of initially counter-rotating gyroscopes that the overall rotation gradually aligns in the direction of the total angular momentum, and (c) a potential energy resisting such alignment. The model can be used for a macroscopic description of the angular momentum dependence of nuclear fission barriers.

Multipole mixing parameters of γ-transitions between the γ-vibrational band and the ground state rotational band of 168Er and F-spin violation

The M1/E2 multipole mixing parameters of two interband transitions in 168Er were precisely remeasured with the twelve detector γ-γ angular correlation apparatus of our laboratory [1] as 1 δ(2 +γ- vib → 2 +rot ) = +0.003(8) and 1 δ(3 +γ- vib → 2 +rot ) = +0.005(15) . Our results disagree with most results of previous investigations but they are certainly more reliable. The new result for 1 δ(2 +γ- vib → 2 +rot ) together with adopted values for 1 gd for other strongly deformed even-even nuclei exhibit an interesting correlation between 1 δ(2 +γ- vib → 2 +rot ) and the g-factor difference g(2 + γ-vib)- g(2 +rot). This correlation is discussed within the frame of F-spin violation.

Identification of spherical states at low energy in the deformed nucleus185Pt

The nucleus {sup 185}Pt, which has a strongly deformed ground state and numerous low-lying rotational bands, is shown to possess low-lying excited states which are nonrotational. This is demonstrated through the absence of rotational excitations associated with two low-spin (J= (3) /(2) ; (5) /(2) ) states at excitations between 500 and 600 keV. The implications of these observations for shape changes and shape coexistence in this region are discussed. {copyright} {ital 1998} {ital The American Physical Society}

Concluding remarks

Concluding remarks

ΔI=4Bifurcation in Ground Bands of Even-Even Nuclei and the Interacting Boson Model

We find a {Delta}I=4 bifurcation phenomenon in many ground state rotational bands by performing a staggering parameter analysis, which has been employed often for superdeformed bands. We find also that a small modification of the intrinsic state of the interacting boson model wave function in the {ital sd}-boson space, which has been used extensively for ground bands, is able to produce the regular {Delta}I=4 bifurcation pattern. Adding a {ital g} boson, we are able to generate even an irregular pattern in the {Delta}I=4 bifurcation phenomenon, which seems to be observed in some ground bands. {copyright} {ital 1997} {ital The American Physical Society}

Reflection asymmetric deformation and clustering in heavy nuclei

The relationship between the cluster properties of heavy nuclei and the appearance of low-lying collective negative-parity states in these nuclei is discussed. Energies of the cluster typenp-nh particle-hole excited states, wheren is the number of nucleons in a cluster, are calculated. It is shown that the energies of these states take minimal values in nuclei having low-lying collective 1- states and ground-state alternating-parity rotational bands with ΔI = 1. The correlation between the difference of the quadrupole deformations in the potential energy minimum and at the barrier separating two octupole minima and the value of the angular momentum, at which parity splitting disappears, is studied.