On oblate-prolate transition in the ground state rotational band of light mercury isotopes

The transition from an oblate to a prolate shape in the ground state band of even mass Hg isotopes and in the ground states of the chain of odd mass Hg isotopes are studied. The shape is found by minimizing the deformation energy which is calculated by means of Strutinsky's shell correction method. The rotational energy corresponds to the axialsymmetric rotator. The moment of inertia is calculated with the help of the cranking model. Pairing and hexadecupole deformation are included. The results are in good agreement with the available experimental data.     

Microscopic description of collective motion in pf shell nuclei

Rotational and vibrational excitations in pf shell nuclei are studied by means of the generator coordinate method. The generator coordinates are the pairing energies and the quadrupole moments of constrained Hartree-Bogoliubov states, projected onto good angular momenta and particle numbers. The Kuo interaction and the one modified by McGrory are used. The vibrational character of the yrast energies appears to be produced by mixing prolate and oblate wave functions. Pairing correlations are essential for this mixing. In contrast to the yrast states the excitation energies of the higher states depend strongly on the interaction used. They show good agreement with experiment, particularly in the case of ⁴⁸Ti with the Kuo force. The calculated B(E2) values exhibit a rotational band structure in general, even if the energies look more vibrational. The force dependence of the excitation energies can qualitatively be understood by inspection of the intrinsic energy surface.     

Effect of periodic changes in the shape of a superconducting body on its dynamics in a noncontact magnetic suspension

The effect of periodic variations of the shape of a homogeneous superconductor on its dynamics in an axisymmetric magnetic field produced by permanent magnets is studied in the case in which the gravity and inertia forces are directed along the axis of the suspension. The shape of the body varies in accordance with a harmonic law from an oblong to an oblate ellipsoid. The dependences of the forces and moments acting on the body on its position in the suspension and shape are obtained. It is shown that the periodic variation of the shape is responsible for excitation of vibrations and may lead to instability of the body. Calculations are performed in the quasi-stationary approximation.     

The shape of a drop in a constant electric field

The variation of the shape of a drop immersed in an immiscible liquid under the action of an electric field is calculated. The charge is transferred both by ohmic current through the interface and by the convective component over the interface. A solution quadratic in the parameter that is the ratio of the electric pressure to the capillary pressure is analyzed. Conditions where the drop transforms into a spheroid that is prolate or oblate along the electric field vector are found. An experimental study of the drop deformation by electric forces is carried out.     

Possible bubble nuclei -36Ar and 200Hg

The question of bubble configurations for ³⁶Ar and ²⁰⁰Hg is studied by Hartree Fock calculations using the density dependent force G-0. By imposing a constraint the solutions can be studied as a function of bubble deformation. Pairing is included in a self consistent manner via a B.C.S. calculation. Factors leading to bubble versus uniform distribution are discussed.     

Pairing isomers

The formalism for quadrupole pairing forces acting simultaneously with monopole pairing and quadrupole particle-hole forces has been worked out in the framework of BCS and RPA. The low-lying 0⁺ states are studied. Expressions for E0 and E2 transition probabilities as well as spectroscopic amplitudes for (t, p) and (p, t) reactions are given. The formalism is applied to the actinide region and it is shown that the low-lying 0⁺ states strongly populated in (p, t) but not in (t, p) reactions can be explained as pairing isomers having an appreciably smaller value of the pairing gap Δ than the ground state. This phenomenon is similar to the phenomenon of gapless superconductivity in solid state physics, i.e. the pairing isomers are formed because of the inhomogeneity of oblate and prolate levels in the vicinity of the Fermi surface.     

On the Difference of High Spin Band Structure Between I and Ce Isotopes

It has been predicted that in nuclei with Z=52－56,there are many low-lying oblate and prolate rotational bands.Experimental investigation has been carrying out for the high spin band structure in serial I and Ce isotopes recently.It is found along with the prolate bands,there do exist oblate bands for proton h_11/2,g_7/2 and d_5/2,configurations in I.The band termination (non-collective oblate shape) was found in ^119,121I as well.All the prolate band structure was found in Ce.Further theoretical calculation was done with newly fitted Nilsson κ and μ parameters.Results show that the absolute values of shell energy and self-consistent pairing energy for oblate shape are always lager for I than for Ce.Besides,the band head oblateprolate energy difference is much in favor of oblate shape for I than for Ce.All these results are related to the existence of an oblate 54 gap in proton single particle diagram.     

Tilted and helical columnar phases for an axially symmetric discoidal system

We report novel phase behavior for a system of disclike ellipsoidal particles interacting via a pair potential. We identify a structural phase transition between two hexagonal columnar phases, both tilted, induced by spatial ordering of the tilt about the columnar axis and positional correlations between neighboring columns upon cooling. The local minima of the potential energy surface support irregular helical arrangements of the discoids about the columnar axis for the high-temperature hexagonal columnar phase, and a tilted arrangement for both phases. Our study demonstrates that dispersion-repulsion forces corresponding to oblate ellipsoids are sufficient to produce a columnar phase that is tilted and helical.     

Systematics of ground state multiplets of atomic nuclei in the delta-interaction approach

Pairing forces between nucleons in an atomic nucleus strongly influence its structure. One of the manifestations of pair interaction is the ground state multiplet (GSM) formation in the spectrum of low-lying excited states of even–even nuclei. The value of GSM splitting is determined by the value of pair interaction of nucleons; for each isotope, it can be estimated on the basis of experimental nuclear masses. The quality of this estimate is characterized by the degree of reproduction of GSM levels in the nucleus. The GSM systematics in even–even nuclei with a pair of identical nucleons in addition to the filled nuclear core is considered on the basis of delta interaction.     

Shapes of nuclear configurations in a cranked harmonic oscillator model

The shapes of nuclear configurations are calculated using Slater determinants built with cranked harmonic oscillator single particle states. The nuclear forces role is played by a volume conservation condition (of the potential or of the density) in a first part. In a second part, we have used the finite range, density dependent interaction of Gogny. A very simple classification of configurations emerges in the first part, the relevant parameter being the equatorial eccentricity of the nuclear density. A critical equatorial eccentricity is obtained which governs the accession to the case for which the nucleus is oblate and symmetric around its axis of rotation. Nuclear configurations calculated in the second part observe remarkably well these behaviours.     

丰中子A~100质量区内的扁椭球高K同核异能态

远离核素图上“稳定谷”的丰中子核一直是核物理学研究的热点。作为形变丰中子核的一种特殊的亚稳定激发态,高K同核异能态的形状大多数为长椭球,扁椭球的高K同核异能态非常少见。近期的一项实验认为丰中子核94Se上的$ {K}^{\mathrm{\pi }}={7}^{-} $两准粒子态为扁椭球。这是形变原子核上存在扁椭球高K同核异能态的第一个实验证据。结合相关实验,我们猜测可能有其它尚未发现的扁椭球高K同核异能态存在于丰中子A~100质量区。利用组态限制势能面计算方法,本文对丰中子A~100质量区内的$ {K}^{\mathrm{\pi }}={9}^{-} $和$ {K}^{\mathrm{\pi }}={7}^{-} $两准粒子态进行了研究,并预言了此质量区内扁椭球高K同核异能态的可能位置。根据Nilsson模型,扁椭球高K同核异能态的存在与费米能级附近的高Ω单粒子轨道有关。这些高Ω单粒子轨道来源于高j闯入态在扁椭球形变时的退简并。扁椭球高K同核异能态是研究丰中子核形变参数、激发能等物理性质的理想对象,有助于加深我们对于形变原子核能级结构的理解。     

Collapse of the N=28 shell closure in (42)Si

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.     

A theoretical study of the spheroidal droplet evaporation in forced convection

In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time.     

Penalty functions for combining coupled-cluster and perturbation amplitudes in local correlation methods with optimized orbitals

Local active space correlation models based on the coupled-cluster doubles (CCD) model like Generalized Valence Bond Perfect Pairing (GVB-PP) and Imperfect Pairing (IP) are attractive methods for treating electron correlation, because they are computationally inexpensive and can describe strong correlations. However, they suffer from symmetry-breaking (SB) in systems with multiple resonance structures, which arises due to neglected correlations. We investigate the extent to which these problems can be removed by using second-order perturbation theory (PT) for weak correlations coupling three different electron pairs, and (infinite-order) coupled-cluster (CC) theory for stronger correlations involving electrons in only one or two pairs. The resulting Three-Pair Corrected Imperfect Pairing (TIP) method is explored here, and it is shown that to robustly combine CC and PT it is necessary to modify several aspects of the basic method. Most importantly, a penalty function term is introduced to ensure the PT amplitudes remain small. Comparison against CC treatment of the three-pair correlations suggests penalty terms will be beneficial for any hybrid CC/PT method that includes orbital optimization. The TIP method greatly reduces SB in aromatic hydrocarbons and recovers a significantly higher fraction of the valence electron correlation energy than IP.     

Relative population of the oblate and prolate structures in187Au

The population of the oblate and the prolate structures in¹⁸⁷Au has been studied using the “Château de Cristal” set-up through the¹⁷²Yb(¹⁹F, 4n) reaction at 90, 95 and 100 MeV beam energy. γ — γ, γ-Fold and γ -Sum energy coincidence measurements were performed, γ-ray intensity measurements showed that as the beam energy increases the prolate system is less populated than the oblate one. Fold and Sum-energy associated with oblate and prolate structures showed a different behaviour with increasing beam energy. Two mechanisms are suggested to explain these results.     

On the orientational characteristics and transport coefficients of an oblate spheroidal hematite particle in a simple shear flow

We have developed the basic equation of the orientational distribution function of oblate spheroidal hematite particles with rotational Brownian motion in a simple shear flow under an applied magnetic field. An oblate spheroidal hematite particle has an important characteristic in that it is magnetized in a direction normal to the particle axis. Since a dilute dispersion is addressed in the present study, we have taken into account only the friction force (torque) whilst neglecting the hydrodynamic interactions among the particles. This basic equation has been solved numerically in order that we may investigate the dependence of the orientational distribution on the magnetic field strength, shear rate and rotational Brownian motion and the relationship between the orientational distribution and the transport coefficients such as viscosity and diffusion coefficient. We found that if the effect of the magnetic field is more dominant, the particle inclines in such a way that the oblate surface aligns in the magnetic field direction. If the Peclet number increases and the effect of the shear flow becomes more dominant, the particle inclines such that the oblate surface tilts in the shear flow direction. The viscosity due to the magnetic torque is shown to increase as the magnetic field increases, since the magnetic torque due to the applied magnetic field becomes the more dominant effect. Moreover, the viscosity increase is shown to be more significant for a larger aspect ratio or for a more oblate hematite particle. We have applied the analysis to the problem of particle sedimentation under gravity in the presence of a magnetic field applied in the sedimentation direction. The particles are found to sediment with the oblate surface aligning more significantly in the sedimentation direction as the applied magnetic field strength increases.     

Observation of signature inversion in the πh9/2(×)νi13/2 oblate band

High-spin states in 190Tl have been studied via the 160Gd(35Cl, 5nγ) reaction. The level scheme, consisting of the πh9/2(×)νi13/2 oblate band and a cascade with character of single particle excitations, has been established. Spin values have been firmly assigned to the oblate band in 190Tl, resulting in low-spin signature inversion in the πh9/2(×)νi13/2 oblate band for the first time. Based on the similarity of the level structure in doubly odd Tl nuclei, spin values for the oblate bands in 192―200Tl should be re-assigned, and a consistent low-spin signature inversion has occurred in these oblate deformed nuclei. The low-spin signature inversion phenomena can be interpreted qualitatively by using the 2-quasiparticle plus rotor model including p-n residual interactions.     

The influence of 1QP-state equilibrium deformations on E1 transition probabilities

E1 transition probabilities in ^{171, 173}Lu are calculated using equilibrium deformations of one-quasiparticle states deduced within the model applied. Pairing correlations and Coriolis coupling are also taken into account.