【摘要】 近年来,人们在很多原子核中发现了具有强M1跃迁、较弱E2跃迁的磁转动结构,这已经成为原子核高自旋态的研究热点.通过在束γ谱学实验,北京大学实验核物理研究组在中国原子能科学研究院HI-13串列加速器和南非iThemba国家实验室回旋加速器上先后开展了对A～80和A～60质量区原子核磁转动结构的研究.在⁷⁵As、⁷⁹Se和⁶²Cu核中发现了高激发磁偶极带结构,利用基于协变密度泛函理论的倾斜轴推转模型分析了它们的结构特点.研究表明,⁷⁵As和⁷⁹Se核中磁偶极带的角动量形成机制不同于传统的磁转动剪刀机制,随着转动频率的增加,它们只有中子角动量J_ν向总角动量方向出现了较大角度的闭合,而质子角动量J_π基本上保持不动,总角动量的倾斜角出现了一个较大的变化（向x轴方向闭合）.这种只有一个剪刀片闭合的角动量形成机制被形象地称为订书机机制.更多还原

【Abstract】 Nuclear collective rotation,which involves coherent contributions from many nucleons,has been well known for a long time.It is ascribed as a consequence of deformation and gives rise to regular rotational bands,which are characterized by strong electric quadrupole （E2） transitions.Studies of the rotational bands in nuclei have been in the forefront of nuclear structure physics and have led to many interesting phenomena including the backbending,superdeformed bands,and chiral doublet bands.In 1990s,a new type of rotational-like sequences,which have strong M1 transitions and weak or vanishing E2 transitions,have been discovered in weakly deformed or near-spherical nuclei and attracted a lot of interests.This new type of rotational structure cannot be understood in terms of conventional rotation of deformed nuclei but has been successfully interpreted in terms of the shears mechanism.In this interpretation,nuclear orientation is specified by the current distribution rather than the deformation.The magnetic dipole vector arises from proton particles （holes） and neutron holes （particles） in high-j orbitals,and rotates around the total angular momentum vector.In the subsequent experimental studies,the magnetic rotation phenomenon were found in the A～60,A～80,A～110,A～140,and A～190 mass regions.Here,we report the studies of magnetic rotation in the A～80 and A～60 mass regions.The high-spin structures of⁷⁵As,⁷⁹Se and⁶²Cu were respectively populated via⁷⁰Zn（⁹Be,1p3n）⁷⁵As,⁸²Se（α,α3n）⁷⁹Se and⁵⁴Cr(¹²C,1p3n)⁶²Cu heavy ion fusion–evaporation reactions.The dipole bands were established for the first time in these three nuclei.The properties of these dipole bands are investigated in terms of the self-consistent tilted axis cranking covariant density functional theory.In the calculation for⁷⁵As,⁷⁹Se and⁶²Cu,the valence nucleon configurations ofπ[(1g_9/2)¹(1f_5/2)^-2]■υ[(1g_9/2)⁵（fp）^-3],π[(1g_9/2)¹（fp）⁵]■υ[(1g_9/2)⁵]andπ[(f_7/2)^-1(p_3/2f_5/2)²]■υ[(g_9/2)¹(p_3/2f_5/2)⁴]are used,respectively.The calculated energy spectra reasonably reproduce the experimental excitation energies of⁷⁵As,⁷⁹Se,and⁶²Cu.The evolutions of deformation parameters β and γ of these dipole bands driven by increasing rotational frequency are discussed.In contrast to the relatively large triaxial deformation of the dipole bands in⁷⁵As and⁶²Cu,the dipole band of⁷⁹Se has a relatively axially symmetric and small prolate deformation.With the increase of rotational frequency,the β deformations for⁷⁵As,⁷⁹Se and⁶²Cu behave in a similar way,i.e.,a smooth decrease in β.Meanwhile,both γ values of⁷⁵As and⁶²Cu show a smoothly increasing tendency.Based on the examination of the composition of the proton and neutron angular momentum vectors J_πand J_υas well as the total angular momentum J_tot=J_π+J_υat both the bandhead and the maximum rotational frequency,the dipole bands in⁷⁵As and⁷⁹Se can be interpreted as novel stapler bands,where the valence nucleons in (1g_9/2) orbital rather than the collective core are responsible for the closing of the stapler of angular momentum.Although not firmly confirmed in experiments,the dipole structure in⁶²Cu may be a candidate of magnetic rotational bands.To unambiguously confirm this,further experimental investigations are strongly desirable.更多还原