摘要:
基于Ru (Z = 44) 丰中子同位素中存在最大三轴形变的理论预言和实验证据，综述了近年来Rh (Z = 45),Pd (Z =46), Ag (Z =47), Cd (Z =48)(质子数Z 位于Ru，Z =44 之上)及Zr (Z =40), Nb (Z =41), Mo (Z =42),Tc (Z =43)(质子数Z 位于Ru，Z =44 之下) 的A~ (100~126) 丰中子同位素中关于三轴形变的形状变迁和形状共存系统性研究的重要进展。252Cf 自发裂变瞬发 射线-- 三重符合、特别是新建立的--- 四重符合数据的系统观测和研究，在Ru, Pd, Cd 和Nb 丰中子同位素中显著扩展或首次观测到了一系列能带，为这个核区原子核形状的研究提供了新的、重要的实验数据。联系此前报道的有关进展，使用PES, TRS, PSM, CCCSM 和SCTAC 理论模型计算拟合新的实验数据，在该核区沿同中素和同位素链，并随自旋和激发能变化各自由度，跟踪原子核形状渐进变化，获得了新的系统性研究成果，显著扩展和深化了人们对原子核形状变迁和形状共存的认知。对于Ru 及其上的Rh (Z = 45), Pd (Z = 46), Ag (Z = 47) 和Cd (Z = 48) 丰中子同位素的研究表明：Rh 丰中子核具有比最大值稍小的三轴形变，γ = 28°，并在103{106Rh 同位素链上鉴别出了手征对称破缺；在三轴形变核112Ru和114Pd(N = 68)中发现了三轴原子核的摆动运动，该摆动运动也可能在114Ru (N = 70)中存在；观察到了从具有最大三轴形变的110,112Ru 中手征破缺到稍小三轴形变的112,114,116Pd 中扰动的手征破缺的过渡；在较软的Ag 核中观察到了丰富的谱学结构，在^{104,105}Ag 中鉴别出了可能的手征对称破缺，在较重的^{115,117}Ag 中提出了趋于三轴形变的软度；具有小形变的Cd核的能级结构被解释为准粒子耦合、准转动和软三轴形变；最近的库伦激发的研究提供了Z = 50, N = 82满壳附近^{122,124,126}Cd 核中出现核集体性的实验和理论证据；上述研究成果展现出从Ru中的最大三轴形变(γ=30°，三轴形变极小增益为0.67 MeV), 经具有大三轴形变的Rh核γ=28°)，到Pd核中的稍小、但稳定于中等自旋到高自旋区的三轴形变(γ~41°,三轴形变极小增益为0.32 MeV)，再经Ag核中的软度，最后到具有很小形变、但仍出现集体性质、包括软三轴形变的Cd核的过渡。对于Pd核转动带交叉系统性的研究揭示了其第一带交叉(νh_{11/2})2 中子转动顺排的上行驱动，和第二带交叉(πg_{9/2})2质子转动顺排的下行驱动效应，成功地解释了^{114}Pd 中的三轴摆动运动，并给出了^{110-118}Pd同位素链中理论早已预言、而比早期理论预言更为完整准确的形状渐进变迁和形状共存的图像。根据该核区的系统研究，发现最大三轴形变出现在^{112}Ru，而在相邻的偶Z(Pd)同位素链，三轴形变极小的中心在^{114}Pd, 两者均为N = 68。上述系统性研究沿相邻的Ru和Pd偶Z同位素链，在N =68同中素中鉴别出最大三轴形变，均比理论预言的108Ru 和110Pd 多4个中子。在Z值位于Ru (Z = 44) 之下的Zr (Z = 40), Nb (Z = 41), Mo (Z = 42) and Tc (Z = 43) 丰中子同位素中，Y和Zr核具有很强的轴对称四极形变，而在较重的Zr同位素中出现了自由度；较重的Nb核(A = 104~106) 基态具有中等程度的软三轴形变和强四极形变，随着自旋和激发能的增加，过渡到接近于轴对称的强四极形变；而较轻的Nb核(A≤103) 基态均接近轴对称形状；在Nb同位素链上基态由球形到强四极形变的形状突变发生在^{100}Nb(N = 59)，在^{100-106}Nb同位素链中基态的软三轴形变随中子数增加而增加；在Nb核中还观察到关于软三轴形变的形状共存；Mo核具有大的三轴形变，观察到了振动和手征对称破缺；Tc核具有比最大值稍小的三轴形变，γ=26°，并观察到了手征对称破缺。质子数Z从41到48的A~(100~126)丰中子同位素，特别是Pd和Nb 同位素，呈现出关于三轴形变的过渡特征。This paper reviews the systematic investigations and understanding for the shape transitions and coexistence with regard to triaxial deformations in A s 100 to 126 neutron-rich Rh (Z = 45), Pd (Z = 46), Ag (Z = 47), Cd (Z = 48) and Zr (Z = 40), Nb (Z = 41), Mo (Z = 42), Tc (Z = 43) isotopes with Z beyond and below Ru (Z = 44), respectively, in Ru the maximal triaxial deformation having been predicted and deduced. The recent measurements and studies of prompt triple- and four-fold, γ-γ-γ and γ-γ-γ-γ, coincidence data from the spontaneous fission of 252Cf using Gammasphere have yielded considerable expansion and extension or first observation of the bands in Ru, Pd, Cd, and Nb isotopes,which provided important data for the studies of nuclear shapes in this region. Combined with previous investigations, recent systematic studies of the new data well reproduced by PES, TRS, PSM, CCCSM and SCTAC model calculations have traced shape changes along the isotonic and isotopic chains, respectively,and with changing excitations/spins as well, significantly expanding our knowledge of shape transitions/coexistence in nuclei.For the neutron-rich Ru and beyond, Rh, Pd, Ag and Cd isotopes, triaxial deformations γ= 28°,slightly smaller than the maximal value, were deduced in Rh (Z = 45) isotopes, with chiral symmetry breaking proposed in ^{103-106}Rh; onset of wobbling motions were identified in 112Ru and ^{114}Pd (N =68),and probably also in ^{114}Ru (N =70); evolution from chiral symmetry breaking in ^{110,112}Ru with maximal triaxial deformations to disturbed chirality in ^{112,114,116}Pd with less pronounced triaxial deformations was proposed; rich nuclear structure was proposed in soft Ag isotopes with possible chiral doubling structure suggested in ^{104,105}Ag, and softness towards triaxial deformation proposed in heavier ^{115,117}Ag;quasi-particle couplings, quasi-rotations and soft triaxiality were suggested in Cd (Z =48) isotopes with small deformations; onset of collectivity was recently suggested in ^{122,124,126}Cd in the vicinity of Z =50 and N = 82 closed shells by studies of Coulomb excitations; shape evolutions from maximal triaxial deformations in Ru (γ=30°, with triaxial minimum energy gain of 0.67 MeV), through Rh with large triaxial deformations ( γ=28°), to less pronounced triaxiality in Pd (with triaxial minimum energy gain of 0.32 MeV), then soft triaxiality in Ag, and finally to slightly deformed Cd isotopes but with emergence of collectivity and soft triaxiality were proposed. The systematic studies of the band crossings in Pd revealed up-rising drivings of the first band crossings caused by (νh_{11/2})2 and down-sloping drivings of the second band crossings by (πg_{9/2})2, explained the onset of wobbling motions in ^{114}Pd,and showed a long-sought picture of shape evolution and coexistence in the Pd isotopic chain which is more complete but complex than earlier predictions. Based on the systematic studies in the mass region,maximal triaxial deformation is found to be reached in ^{112}Ru and less-pronounced triaxiality centered at ^{114}Pd, both for N =68, four neutrons more than predicted in earlier theoretical calculations.In the neutron-rich Zr (Z =40), Nb (Z =41), Mo (Z =42) and Tc (Z =43) isotopes with Z just below Ru, large quadrupole deformations of axially symmetric shapes were deduced in Y and Zr isotopes, with emergence of the degree of freedom having been suggested for heavier Zr isotopes; medium triaxial deformations were deduced for the ground states of heavier (A > 104) Nb isotopes, and, with increasing excitations and spins, evolution from medium triaxial deformations with strong quadrupole deformations at ground states to nearly axially-symmetric shapes were deduced; light Nb isotopes (A6103) have near axially-symmetric shapes with strong quadrupole deformations; combining with the identification of onset of strong quadrupole deformation at 100Nb in the Nb isotopic chain, an increase of soft triaxiality with increasing neutron number was proposed in ^{100-106}Nb. Shape coexistence with regard to soft triaxiality is also proposed in Nb isotopes; large triaxial deformations, vibrations and chiral doublets were proposed in Mo isotopes; chiral doubling and large triaxial deformations (γ ~26°) slightly smaller than the maximal triaxiality were suggested in Tc isotopes.The neutron-rich nuclei with Z ranging from 41 through 48 and A ~100 to 126, especially the Pd and Nb isotopes are thus found to be transitional nuclei with regard to triaxiality.
Abstract:
This paper reviews the systematic investigations and understanding for the shape transitions and coexistence with regard to triaxial deformations in A s 100 to 126 neutron-rich Rh (Z = 45), Pd (Z = 46), Ag (Z = 47), Cd (Z = 48) and Zr (Z = 40), Nb (Z = 41), Mo (Z = 42), Tc (Z = 43) isotopes with Z beyond and below Ru (Z = 44), respectively, in Ru the maximal triaxial deformation having been predicted and deduced. The recent measurements and studies of prompt triple- and four-fold, γ-γ-γ and γ-γ-γ-γ, coincidence data from the spontaneous fission of 252Cf using Gammasphere have yielded considerable expansion and extension or first observation of the bands in Ru, Pd, Cd, and Nb isotopes,which provided important data for the studies of nuclear shapes in this region. Combined with previous investigations, recent systematic studies of the new data well reproduced by PES, TRS, PSM, CCCSM and SCTAC model calculations have traced shape changes along the isotonic and isotopic chains, respectively,and with changing excitations/spins as well, significantly expanding our knowledge of shape transitions/coexistence in nuclei.For the neutron-rich Ru and beyond, Rh, Pd, Ag and Cd isotopes, triaxial deformations γ= 28°,slightly smaller than the maximal value, were deduced in Rh (Z = 45) isotopes, with chiral symmetry breaking proposed in ^{103-106}Rh; onset of wobbling motions were identified in 112Ru and ^{114}Pd (N =68),and probably also in ^{114}Ru (N =70); evolution from chiral symmetry breaking in ^{110,112}Ru with maximal triaxial deformations to disturbed chirality in ^{112,114,116}Pd with less pronounced triaxial deformations was proposed; rich nuclear structure was proposed in soft Ag isotopes with possible chiral doubling structure suggested in ^{104,105}Ag, and softness towards triaxial deformation proposed in heavier ^{115,117}Ag;quasi-particle couplings, quasi-rotations and soft triaxiality were suggested in Cd (Z =48) isotopes with small deformations; onset of collectivity was recently suggested in ^{122,124,126}Cd in the vicinity of Z =50 and N = 82 closed shells by studies of Coulomb excitations; shape evolutions from maximal triaxial deformations in Ru (γ=30°, with triaxial minimum energy gain of 0.67 MeV), through Rh with large triaxial deformations ( γ=28°), to less pronounced triaxiality in Pd (with triaxial minimum energy gain of 0.32 MeV), then soft triaxiality in Ag, and finally to slightly deformed Cd isotopes but with emergence of collectivity and soft triaxiality were proposed. The systematic studies of the band crossings in Pd revealed up-rising drivings of the first band crossings caused by (νh_{11/2})2 and down-sloping drivings of the second band crossings by (πg_{9/2})2, explained the onset of wobbling motions in ^{114}Pd,and showed a long-sought picture of shape evolution and coexistence in the Pd isotopic chain which is more complete but complex than earlier predictions. Based on the systematic studies in the mass region,maximal triaxial deformation is found to be reached in ^{112}Ru and less-pronounced triaxiality centered at ^{114}Pd, both for N =68, four neutrons more than predicted in earlier theoretical calculations.In the neutron-rich Zr (Z =40), Nb (Z =41), Mo (Z =42) and Tc (Z =43) isotopes with Z just below Ru, large quadrupole deformations of axially symmetric shapes were deduced in Y and Zr isotopes, with emergence of the degree of freedom having been suggested for heavier Zr isotopes; medium triaxial deformations were deduced for the ground states of heavier (A > 104) Nb isotopes, and, with increasing excitations and spins, evolution from medium triaxial deformations with strong quadrupole deformations at ground states to nearly axially-symmetric shapes were deduced; light Nb isotopes (A6103) have near axially-symmetric shapes with strong quadrupole deformations; combining with the identification of onset of strong quadrupole deformation at 100Nb in the Nb isotopic chain, an increase of soft triaxiality with increasing neutron number was proposed in ^{100-106}Nb. Shape coexistence with regard to soft triaxiality is also proposed in Nb isotopes; large triaxial deformations, vibrations and chiral doublets were proposed in Mo isotopes; chiral doubling and large triaxial deformations (γ ~26°) slightly smaller than the maximal triaxiality were suggested in Tc isotopes.The neutron-rich nuclei with Z ranging from 41 through 48 and A ~100 to 126, especially the Pd and Nb isotopes are thus found to be transitional nuclei with regard to triaxiality.