超越平均场方法研究原子核性质

The mean field theory has been successfully applied in describing the global properties of finite nuclei, such as the binding energy, charge radii, the excited energies of various collective states. It is applied not only for nuclei along the beta-stability line, but also for nuclei far from the beta-stability line. However, some properties can not be described in the framework of mean field theory due to the weakness of independent-particle assumption, such as the spectroscopic factor, one has to go beyond mean field theory to solve the problems. Study of beyond mean field theory is still a challenging work in nuclear many-body theory, it needs more effort. Based on the microscopic Skyrme energy density functional, we have extended the Skyrme Hartree-Fock model to go beyond mean field framework, called self-consistent particle-vibration coupling approach, which is a method to consider the fluctuation of the mean field, in this case the self-energy of the nucleon is no more static but energy dependent. In this proposal, we will first extend our present particle-vibration coupling approach to include the contributions from the non-natural parity excitations in the vertex function, such as the magnetic dipole and Gamow-Teller excitations, then study the microscopic optical potential for finite nuclei and apply the obtained optical potential in description of the experimental data of neutron/proton scattering off nuclei, not only for stable nuclei but also for weakly bound nuclei. Meanwhile, it is also naturally to extend this method to study the properties of unstable nuclei, such as the single-particle energy, level density, and spectroscopic factor, which are paid more attention experimentally. Another goal of this proposal is to study the reaction induced by electron on the finite nuclei, especially for the weakly bound nuclei. The medium effect provided by the beyond mean field calculation will be extensively discussed in the elastic or quasi-elastic scattering of electron from nuclei. It can help us to deduce the information on the distribution of the charge and current density in weakly bound nuclei by electron scattering reaction.

基于平均场方法之上的超越平均场理论方法来研究原子核的性质一直是一项很有挑战性的理论研究，是核理论研究的基本课题之一。它在描述原子核的谱因子以及光学势等性质方面具有一定的优势，比如可以直接计算不同能量下核子在有限核中的自能，既光学势，该方法具有能够细致反映目标核的结构特性、没有可调参数、易于推广到非稳定原子核等优点。我们将首先基于微观的Skyrme能量密度泛函，通过自洽的Hartree-Fock和无规位相近似计算给出原子核的基态和各种声子态，再通过粒子-声子耦合模型计算得到核子-核微观光学势，研究核子与原子核的反应，特别是非稳定原子核的微观光学势和反应特性。同时，也可以扩展来研究原子核特别是非稳定原子核的单粒子能、能级密度以及谱因子等实验上比较关心的物理量。继而研究超越平均场计算产生的核介质效应对电子与原子核发生弹性以及非弹性散射过程的影响。通过理论研究为国内相关实验提供参考。

在项目的执行过程中，密切结合国内外核物理实验的最新进展，加强国际合作与交流以确保项目的顺利进行。项目执行期间，我们开展了以下研究：采用完全自洽的Skyrme Hartree-Fock和无规位相近似模型研究了原子核的电荷交换巨共振并研究它对对称能参数的依赖性，特别是我们研究了AGDR与IAS巨共振能量差对对称能参数的敏感性，并结合实验数据来约束核物质状态方程；在核物质中采用超越平均场近似方法研究了介质效应对反对称核物质中对相互作用的影响，发现介质效应在1S0态中从屏蔽效应改变为反屏蔽效应，而且排斥变吸引相互作用为密度和对称参数依赖；采用完全自洽的Skyrme Hartree-Fock+BCS和准粒子无规位相近似模型研究了丰质子原子核，包括17Ne、18Ne以及Ar和Ca丰质子同位素链核的巨偶极共振，发现在低能区这些丰质子原子核的响应方程存在矮共振模式，进一步的研究表明在轻丰质子核区，矮共振的集体性较弱；采用BHF理论研究核物质中同位旋相关的微观光学势，通过计算核物质中的核子自能和采用定域密度近似方法得到有限原子核16O、40Ca、90Zr以及208Pb在中子或者质子不同入射能量下的光学势，并用于分析弹性散射实验数据，发现采用所得到微观光学势能比较好的再现弹性散射实验结果；采用完全自洽的Skyrme Hartree-Fock和无规位相近似模型研究了含超子原子核巨共振性质，对含超子原子核的巨单极共振做了研究，发现含超子原子核的巨单极共振能量比相应的不含超子原子核的巨单极能量有所增加，进而计算得到不可压缩性系数变大；采用完全自洽的Skyrme Hartree-Fock和无规位相近似模型我们研究了有限原子核48Ca, 90Zr, 132Sn以及208Pb电荷交换GT和SD共振的响应方程分布，和实验数据的对比，得到了原子核电荷交换GT和SD的淬火因子，对于理解实验强度分布的物理机制有很大帮助。