N=Z原子核^(64)Ge可能存在的三轴形变

Possible triaxial deformation in N=Z nucleus germanium-64

为寻找核态可能存在的三轴形变,用对力-形变-转动频率自洽推转壳模型对锗和硒同位素进行了总转动能面计算.计算是在四极形变(β_2,γ)网格中进行的,且十六极形变β_4可变.在锗同位素中发现了由64Ge的三轴、^(66)Ge的扁椭、再经三轴、向长椭形变的形状相变.一般来说Ge和Se同位素具有γ软性形状,导致了显著的动力学三轴效应,计算中没有证据表明存在基态下的刚性三轴性.在^(64,74)Ge中发现基态和集体转动态下γ=-30°的三轴形变,这是三轴形变的极限.本文重点讨论N=Z核^(64)Ge可能存在的三轴形变,给出了基于唯象Woods-Saxon势下的单粒子能级信息,并对N=Z核^(64)Ge三轴形变的产生机理进行了讨论.

Evidence for nonaxialγdeformations has been widely found in collective rotational states.Theγdeformation has led to very interesting characteristics of nuclear motions,such as wobbling,chiral band,and signature inversion in rotational states.There is an interesting question;why the nonaxialγdeformation is not favored in the ground states of even-even(e-e)nuclei.The quest for stable triaxial shapes in the ground states of e-e nuclei,with a maximum triaxial deformation of|γ|≈30°,is still a major theme in nuclear structure.In the present work,we use the cranked Woods-Saxon(WS)shell model to investigate possible triaxial shapes in ground and collective rotational states.Total-Routhian-surface calculations by means of the pairingdeformation-frequency self-consistent cranked shell model are carried out for even-even germanium and selenium isotopes,in order to search for possible triaxial deformations of nuclear states.Calculations are performed in the lattice of quadrupole(β_(2),γ)deformations with the hexadecapoleβ_(4) variation.In fact,at each grid point of the quadrupole deformation(β_(2),γ)lattice,the calculated energy is minimized with respect to the hexadecapole deformationβ_(4).The shape phase transition from triaxial shape in 64Ge,oblate shape in ^(66)Ge,again through triaxiality,to prolate deformations is found in germanium isotopes.In general,the Ge and Se isotopes haveγ-soft shapes,resulting in significant dynamical triaxial effect.There is no evidence in the calculations pointing toward rigid triaxiality in ground states.The triaxiality ofγ=-30°for the ground and collective rotational states,that is the limit of triaxial shape,is found in 64,74 Ge.One should also note that the depth of the triaxial minimum increases with rotational frequency increasing in these two nuclei.The present work focuses on the possible triaxial deformation of N=Z nucleus 64 Ge.Single-particle level diagrams can give a further understanding of the origin of the triaxiality.Based on the information about single-particle levels obtained with the phenomenological Woods-Saxon(WS)potential,the mechanism of triaxial deformation in N=Z nucleus 64Ge is discussed,and caused surely by a deformedγ≈30°shell gap at Z(N)=32.At N=34,however,an oblate shell gap appears,which results in an oblate shape in 66 Ge(N=34).With neutron number increasing,the effect from the N=34 oblate gap decreases,and hence the deformations of heavier Ge isotopes change toward the triaxiality(or prolate).