第一性原理研究三体力对^(28)F和^(29)Ne低激发谱的影响

Effects of three-nucleon force on the low-lying states of ^(28)F and ^(29)Ne within ab initio calculations

从手征有效场论的N^(3)LO两体和N^(2)LO三体核力出发,采用第一性原理方法(开壳原子核多体微扰理论)计算了^(28)F和^(29)Ne的低激发能谱,并以此研究手征三体力对反转岛区域原子核性质的影响.通过对比采用纯两体与两体加三体力计算的^(28)F和^(29)Ne的低激发能谱,发现手征三体力对反转岛内原子核基态宇称反转现象有着重要的贡献.为了进一步探究三体力的影响,系统计算了丰中子Ne同位素的偶偶核基态波函数中的组态成分.结果显示,三体力显著地加强了跨壳激发效应,使理论计算与实验结果更加符合.

One of the most interesting anomalies in nuclear physics is the island of inversion(IOI),which indicates that the magic numbers disappear.It provides a good platform to understand single-particle motion,many-body correlation,and nucleonnucleon interaction.In neutron-rich isotopes near N=20 area,many experiments have found that the spectra,electromagnetic transitions,radii,and two-neutron separation energies of these nuclei are abnormal.This region is called the N=20 IOI,because it is characterized by neutron particle-hole excitations related to deformation from the 1s0d shell into the 1p0f shell across the N=20 shell gap.Recent experiments show that the ground states of ^(29)Ne and ^(28)F have negative parities,indicating that these two nuclei are within the N=20 IOI.This provides new opportunities and challenges for theoretical models in this area.With the standard shell model,the N=20 IOI has been proved by many works to have a strong cross-shell effect between1s0d and 1p0f shells.They proposed that the tensor force and three-nucleon force should have an important influence on the microscopic description of neutron-rich nuclei in this area,especially in Ne and Mg isotopes.However,the N=20 IOI is still rarely studied in ab initio calculations.Recently,an extension of the ab initio valence-space in-medium similarity renormalization group method was presented to study the physics of neutron-rich nuclei by Miyagi et al.In the present paper,based on chiral force including nucleon-nucleon and three-nucleon force,we have performed ab initio calculations for the low-lying states of ^(28)F and ^(29)Ne,focusing on the relative excitation energies of the positive-and negative-parity states in these nuclei.In order to study the effect of three-nucleon force on the N=20 IOI,we use nucleonnucleon force at N^(3)LO with or without three-nucleon force at N^(2)LO in our calculations.Using the open-shell many-body perturbation theory,the cross shell sd-pf realistic effective Hamiltonian,including single-particle energies and effective two-body interactions,is constructed self-consistently without introducing parameters.The constructed effective Hamiltonian is diagonalized with the shell model method.In shell model calculation,we use a truncation with up to five nucleons(including protons and neutrons)occupying the pf orbit.The low-lying states of ^(28)F and ^(29)Ne are calculated with NN and NN+3N interactions.The results show that three-nucleon force is important in the descriptions of low-lying states of ^(28)F and^(29)Ne,especially in ^(29)Ne while the first negative state turned to be ground state due to the three-nucleon force.Our calculations give that three-nucleon force gives large effects for the relative position between the 0d_(3/2) and 1p_(3/2) singleparticle orbitals,causing the two orbitals to reverse.Furthermore,the configurations of low-lying states of ^(28)F and ^(29)Ne are calculated within our ab initio calculations with NN+3N interaction.The results give that the lowest positive and negative parity states in ^(28)F and ^(29)Ne are dominated by 2 and 1 components,respectively.We have also calculated the configurations of the ground state in neutron-rich even-even Ne isotopes,and the results indicate that the three-nucleon force could enhance the cross-shell excitations in the nuclei in the vicinity of IOI,which is essential for the exotic properties of these nuclei.Furthermore,we have also performed the phenomenological shell model calculations with SDPF-MU effective interactions,and the results are similar to our ab initio calculations including threenucleon force.