一些近期发现的同核异能态的壳模型解释

Shell-Model Explanation on Some Newly Discovered Isomers

近期,在^(101)In、^(123,125)Ag和^(218)Pa等核中,首次观测到同核异能态。本工作通过原子核壳模型解释In、Ag同位素和N=127同中素中的这些同核异能态及相关的同核异能态背后的物理原因。^(101-109)In这五个奇A核In同位素中,观测到的_(1/2)^(-)同核异能态的激发能非常接近。这可以通过引入中子0g_(7/2)和1d_(5/2)轨道间的很强的组态混合来解释。更进一步分析表明,这些奇A核In同位素中,从9/2^(+)基态到_(1/2)^(-)同核异能态,一个质子从1p_(1/2)轨道激发到0g_(9/2)轨道。这一质子组态变化可能引发中子0g_(7/2)和1d_(5/2)轨道的单粒子能变化。这样一个原子核内的组态依赖的壳演化被称为第二类壳演化。与In同位素类似,^(123,125)Ag的同核异能态被发现是_(1/2)^(-)态,对应着一个质子空穴在1p_(1/2)轨道。但之前观测到的^(115,117)Ag的_(1/2)^(-)态是基态。这意味着质子1p_(1/2)轨道和0g_(9/2)轨道在N=72附近发生了反转。壳模型分析表明张量力是造成这两个轨道反转的决定性原因。之前观测到的奇奇核N=127同中素^(210)Bi、^(212)At、^(214)Fr和^(216)Ac中,基态是1-态,同时存在高自旋的同核异能态。然而,基于α衰变性质和壳模型计算,推荐^(218)Pa中的基态和新发现的同核异能态分别为8-态和1-态。奇奇核N=127同中素基态和同核异能态的演化是由质子中子相互作用从粒子粒子形式转化为空穴粒子形式以及质子组态混合所导致。总的来说,壳模型对这些双幻核^(100)Sn、^(132)Sn和^(208)Pb附近核中新发现的同核异能态有较好的描述。双幻核附近核中的同核异能态,也称为壳模型同核异能态,是核结构研究中非常重要的。因为这些同核异能态常常提供了中重质量区域极端丰中子和缺中子原子核中的第一个谱学性质,并包含了丰富的物理信息,比如质子中子相互作用及其在壳演化中的作用。

Recently,isomeric states are discovered for the first time in ^(101)In,^(123,125)Ag,and ^(218)Pa.The nuclear shell model is used to explain the underlying physics in these and related isomers in In,Ag isotopes,and the N=127 isotones.The observed excitation energies of the _(1/2)^(-)isomeric states in odd-A In isotopes,^(101-109)In,are rather similar among five isotopes,which can be explained by introducing the strong neutron configuration mixing between the 0 g_(7/2) and 1 d_(5/2) orbitals.In addition,from the 9/2^(+)ground state to the _(1/2)^(-)isomeric state in these odd-A In isotopes,a proton moves from the 1 p_(1/2) orbital to the 0 g_(9/2) orbital,which may induce the change on the single particle energies of the neutron 0 g_(7/2) and1 d_(5/2) orbitals.Such configuration dependent shell evolution in one nucleus is called the type Ⅱ shell evolution.Similar to In isotopes,the isomeric states in ^(123,125)Ag are found to be te _(1/2)^(-)ates,whcih correspond to a proton hole in 1 P_(1/2) orbital.But _(1/2)^(-)states are ground states in ^(115,117) Ag,which indicates an inversion of the proton 1 p_(1/2) and 0 g_(9/2) orbitals around N=72.The shell-model analysis shows that the tensor force is the key reason of the inversion of the two orbitals.A 1-ground state and a high spin isomeric state are observed previously along the odd-odd N=127 isotones,^(210)Bi,^(212)At,^(214)Fr,and ^(216)Ac.However,the ground state and the newly discovered isomeric state of ^(218)Pa are suggested to be 8-and 1-,respectively,based on the properties of a decay and the shell-model calculations.The evolution of the ground states and isomeric states along the odd-odd N=127 isotones are caused by the transition of the proton-neutron interaction from particle-particle type to hole-particle type and the proton configuration mixing.In general,the nuclear shell model gives nice descriptions on these newly discovered isomeric states in nuclei around the doubly magic nuclei ^(100) Sn,^(132) Sn,and ^(208)Pb.The isomeric states in nuclei around doubly magic nuclei,so called the shell-model isomers,are of high importance in the nuclear structure study,because they often provide the first spectroscopic properties in the extreme neutron-rich and neutron-deficient nuclei in the medium and heavy mass region and include a plenty of information in physics,such as the proton-neutron interaction and its role in shell evolution.