原子核层次的手征对称性

对称性及其破缺是基本的科学问题。手征对称性在自然界中广泛存在,大至星系旋臂、行星自转,小到矿物晶体、有机分子、基本粒子,都与手征对称性密切相关。原子核层次的手征对称性概念于1997年提出,随后成为核物理研究的热点问题。目前,实验上已经在核素图上的80,100,130和190质量核区发现了30多例可能具有手征对称性的原子核。简要介绍原子核中的手征对称性概念;手性原子核的预言、识别以及实验验证;并通过展示手性原子核结构的多样性（M_χD）,回顾了理论和实验研究进展;介绍最新发现的原子核中手性和空间反射对称性的联立自发破缺,对未来手性原子核研究的前景进行了展望。Symmetry and its breaking are basic scientific problems. Chiral symmetries are common in nature, for example, the macroscopic spiral arms of galaxies and the rotation of planets; the microscopic spirals of the mineral crystalline, the organic molecules and the elementary particles. The concept of chirality in atomic nuclei was first proposed in 1997. Since then many efforts have been made to understand chiral symmetry and its spontaneous breaking in atomic nuclei. Up to now, more than 30 candidates of chiral nuclei have been reported in the 80, 100, 130, and 190 mass regions. The concept of the chirality in atomic nuclei, the prediction, the signal, and the experimental verification of the chiral nuclei are briefly introduced; the recent theoretical and experimental progress are reviewed, in particular the existence of multiple chiral doublets (M_χD), i.e., more than one pair of chiral doublet bands in one single nucleus; the simultaneous spontaneous breaking of chiral and reflection symmetry in the newly observed atomic nuclei is introduced, together with a prospect on the future study on nuclear chiral symmetry.     

Candidate chiral nuclei in bromine isotopes based on triaxial relativistic mean field theory

Following the observation of candidate chiral doublet bands in ~(78,80)Br, triaxial deformations with corresponding configurations in odd-odd bromine isotopes have been investigated using adiabatic and configuration-fixed constrained triaxial relativistic-meanfield calculations with an aim of finding the boundary of chirality in the chain of Br isotopes. Several minima with triaxial deformation and the proper particle-hole configuration were obtained in ~(74,76,78,80,82)Br, where the chiral doublet bands have the possibility of occurrence. Furthermore, the possible existence of multiple chiral doublet(MχD) bands is demonstrated in ~(74,76,78,80,82)Br.Experiments to explore the chirality and MχD properties of Br isotopes are conducted to verify the predictions.     

Microscopic analysis of octupole shape transitions in neutron-rich actinides with relativistic energy density functional

Quadrupole and octupole deformation energy surfaces, low-energy excitation spectra, and electric transition rates in eight neutron-rich isotopic chains – Ra, Th, U, Pu, Cm, Cf, Fm, and No – are systematically analyzed using a quadrupole-octupole collective Hamiltonian model, with parameters determined by constrained reflectionasymmetric and axially-symmetric relativistic mean-field calculations based on the PC-PK1 energy density functional.The theoretical results of low-lying negative-parity bands, odd-even staggering, average octupole deformations β_3,and B(E3;3_1~- →0_1~+) show evidence of a shape transition from nearly spherical to stable octupole-deformed, and finally octupole-soft equilibrium shapes in the neutron-rich actinides. A microscopic mechanism for the onset of stable octupole deformation is also discussed in terms of the evolution of single-nucleon orbitals with deformation.     

Examining the Chiral Geometry in l04Rh and l06Rh

The criteria for chiral doublet bands based on one particle and one hole coupled to a triaxial rotor have been summarized. Two representative cases in A - 100 odd-odd nuclei, nearly degenerate △I = 1 doublet bands in 104Rh and 106Rh, are checked against these chiral criteria. It is shown that 106Rh possesses better chiral geometry than 104Rh, although the energy near degeneracy is achieved in 104Rh in comparison with the constant energy separation of doublet bands in 106Rh.     

Chiral doublet bands in odd-odd nuclei

The criteria for chiral doublet bands based on one particle and one hole coupled to a triaxial rotor have been summarized. Candidate chiral doublet bands in A~100 and 130 odd-odd nuclei were checked against these chiral criteria. Two representative cases, nearly degenerate ΔI =1 doublet bands in ¹²⁶Cs and ¹⁰⁶Rh were investigated in the two quasiparticles coupled with a triaxial rotor model. After including the pairing correlation, good agreement has been obtained between the calculated results and the data available, which supports the interpretation of observed doublet bands as chiral bands.     

基于HI-13串列加速器在束γ终端原子核激发态寿命测量的核结构研究

利用在HI-13串列加速器的在束γ终端上建立的多普勒线移衰减法、反冲距离多普勒线移法和快时间延迟符合法等3种寿命测量方法,开展了原子核手征对称性、磁转动等核结构热点课题的研究工作。实验结果表明,130Cs的伙伴带具有很好的手征特性,而106Ag的候选带并非手征双带。根据测量的能级寿命提取的B（M1）值表明,106Ag的正宇称晕带和107Ag的正、负宇称晕带都具有典型的磁转动特性。此外,寿命测量数据还为解释和理解不同的同位素链中展现出的形状演化、形状共存、形状相变及临界点对称性等物理现象提供了有价值的实验证据。 The significance and principle of lifetime measurements of excited states in nuclei are briefly described. By using the recoil-distance Doppler-shift method, the Doppler-shift attenuation method, and the fast-timing technique established at the in-beam γ terminal of HI-13 tandem accelerator, nuclear structure studies on topical subjects including chiral symmetries and magnetic rotation have been performed. Our experimental results indicate that 130Cs shows better chiral characteristics, however, the two candidate bands of 106Ag could not be a pair of chiral doublet bands. For magnetic rotation, the deduced B(M1) values deduced from the measured level lifetimes clearly demonstrate that the yrast positive-parity band in 106Ag and both the yrast positiveand negative-parity bands in 107Ag are magnetic rotation bands. In addition, our lifetime measurement data also provide valuable experimental evidences for the interpretation and understanding of the shape evolution, the shape coexistence, and the critical-point symmetries of shape phase transition in the different isotope chains.     

爆发性核合成与核结构

讨论了天体物理环境中爆发性核合成及其与核结构的紧密关系.用反射不对称壳模型计算了远离稳定线丰中子核148Ba的低能八极转动带,结果与实验很好符合.展示了其应用于不稳定核低能态计算的有效性 ,以及在核天体物理研究中的应用前景.The explosive nucleosynthesis in the astrophysical environment and its close link to nuclear structure are discussed. The low lying octupole bands for neutron rich nucleus 148 Ba far from the stability line have been calculated by the reflection asymmetric shell model and the results are in good agreement with the experimental data, presenting the capability of the model for calculations of the low lying states of unstable nuclei as well as the foreground of application in nuclear astrophysics.     

πg9/2νh11/2 doublet bands in A～100 mass region

Using the model with one particle and one hole coupled with a triaxial rotor, the πg9/2/1/2νh11/2 doublet bands in the A～100 mass region are studied, and compared with the πg9/2/1/2νh11/2 doublet bands. It is found that the calculated results for the configuration of πg9/2/1/2νh11/2 are very similar the results for a pure h11/2 proton particle and a neutron quasiparticle with λn = ε5. After including the pair correlation, the model describes the candidate chiral doublet bands in 106Rh successfully, which supports the interpretation of chirality geometry.     

Reflection-Asymmetric Shell Model Description of the Neutron-Rich^142，145Ba Nuclei

The reflection asymmetric shell model has been applied to describe the octupole deformed bands in neutron-rich even-even ^142Ba and odd-N^145Ba nuclei. The alternating parity bands of ^142Ba and the simplex s = -i bands of^145Ba are calculated and compared with the available experimental data. The calculated results are in good agreement with the experimental data. The spin and parity assignments of ground-state of^145Ba are discussed.The results show that the present work is a useful attempt to further explore the nuclear reflection asymmetry in neutron rich region.     

超越平均场模型对Ba同位素链八极形状演化研究

本文采用超越平均场模型研究了缺中子Ba同位素链114-124Ba的八极形变及其演化规律。计算了Ba链的位能曲面、低能激发谱、电四极跃迁几率、电八极跃迁几率及集体波函数的几率密度分布,并与实验相比较。理论计算较好地再现了Ba核的低能激发结构。位能曲面、低能的负宇称带、较大的B（E3;3₁- → 0₁+）和展宽的几率密度分布表明,114Ba中存在较强的八极关联。116,118Ba具有过渡核特征,而120-124Ba则为典型的四极形变原子核。     

Octupole instability induced by rotation in the nuclei Nd

Levels in ^146,148Nd have been populated in the ¹⁵⁰Nd(, ′ xn) reactions. Cascades of enhanced E1 transitions have been observed. B(E1)/B(E2) branching ratios, of the order of 10⁻⁶ fm⁻² as well as the presence of low energy negative-parity bands indicate strong octupole effects. Shell correction type calculations suggest an explanation in terms of octupole correlations at medium spins.     

Microscopic structures of parity doublets in the151Pm,153Eu and155Eu nuclei

Microscopic structures of the proposed parity doublet bands in the odd-proton Z=61¹⁵¹Pm and Z=63^{153, 155}Eu nuclei are examined using the quasiparticle phonon nuclear model Hamiltonian including the residual pairing and multipole-multipole interactions. Octupole correlations between intrinsic nonrotational states are studied to check the suggested assignments of parity doublet partners in these nuclei. We conclude significant octupole correlations between a pair ofK ^π = 1/2^± doublets in each of these nuclei, with somewhat weaker correlations betweenK ^π = 3/2^± bands in both the Eu isotopes. No octupole connection between the lowestK ^π = 5/2^± bands is found; however, significant octupole collectivity, built on the low-lying 5/2^? bands, is predicted in the higher-lying 5/2⁺ bands which contain the 5/2⁺ [402] fragmented strength.     

A microscopic description of boson and fermion alignment in octupole bands of actinide nuclei

The rotational alignment of the octupole boson in negative-parity bands of actinide nuclei is investigated in the framework of the cranked random-phase approximation (CRPA).     

Self-consistent calculation of the quadrupole-octupole deformation energy surface of 222Ra

The Hartree-Fock + BCS quadrupole-octupole deformation energy surface of ²²²Ra, calculated with the Skyrme SIII interaction, presents a minimum for a non-zero value of the octupole moment. Our results suggest the existence of a quadrupole coupling which may account for the observed differences between the moments of inertia of the positive- and negative-parity bands.     

Experimental evidence for low-spin members of “upper” bands in156Dy

Study of the energy levels of¹⁵⁶Dy through the¹⁵⁹Tb(p, 4n)¹⁵⁶Dy reaction has revealed the existence of six states with excitation energies between 1.8 and 2.8 MeV and spins between 6 and 12. Some of them can tentatively be assigned as low-spin members of “upper” bands which are thought to be responsible for the backbending phenomenon experimentally observed in the ground-state andβ-vibrational bands of this nucleus. Others could be levels of a negative-parity octupole band.     

E2 transitions between positive- and negative-parity states of the ground-state alternating-parity bands

Experimental transition probabilities between states of the ground-state alternating-parity bands of ¹⁴⁴Ba and their theoretical analysis are presented. Lifetimes of states in ¹⁴⁴Ba have been measured using the recoil distance method following spontaneous fission of ²⁵²Cf. The experiment was performed at the Lawrence Berkeley National Laboratory employing the Gammasphere array and the New Yale Plunger Device. The experimental data show a significantly larger value of the E2 transition probability between the negative-parity states compared to the positive-parity ones. It is shown that this effect can be explained by a higher weight of the deformed component in the wave functions of the odd-I states. In the framework of the cluster approach it is explained by a higher weight of the alpha-cluster component in the wave function of the negative-parity states compared to the positive-parity ones. In the framework of the traditional collective model with the quadrupole and octupole degrees of freedom the same effect is explained by a higher value of the quadrupole deformation at the minima of the potential energy as a function of β₂₀ and β₃₀ compared to its value at the top of the barrier separating two physically equivalent minima, having opposite signs of the octupole deformation. Additionally, the dependence on parity of the E2 transition probability is analyzed qualitatively in nuclei with a minimum at β₃₀ = 0 in the collective potential energy and compared to experimental data for ¹⁴⁸Nd.     

On the description of collective negative parity bands in even-even deformed nuclei

The collective bands of negative parity are studied via a microscopic approach where Coriolis forces and octupole correlations are treated on an equal footing. The apparent moment of inertia of these bands are nicely reproduced.