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arXiv 1107.4593nucl-th
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稳定$^{40,48}$ Ca核在第二随机相位近似下的低激发态偶极响应

Low-lying dipole response in the stable $^{40,48}$Ca nuclei with the second random-phase approximation

D. Gambacurta, M. Grasso, F. Catara

摘要:

在Skyrme第二无规相位近似的框架下分析了稳定同位素$^{40}$ Ca和$^{48}$ Ca的低能偶极激发。相应的随机相位近似计算提供了一个可以忽略不计的强度分布的两个核的能量区域从5到10兆电子伏。在第二无规相位近似下,2粒子2空穴组态的包含和耦合导致了富中子核$^{48}$ Ca在低能时的偶极响应.在核$^{48}$ Ca中存在中子皮,这表明可以用侏儒激发来解释这种低位响应。分析了激发模式的组成(1粒子-1空穴和2粒子-2空穴结构的含量)、跃迁密度和集体性(不同贡献的数目和相干性)。该分析表明,一般而言,这些激发不能清楚地解释为中子皮对堆芯的振荡,除了具有最大$B(E1)$值的峰值,其位于9.09 MeV。对于该峰,中子跃迁密度占主导地位,并且中子和质子跃迁密度在核内部异相振荡,导致等标度分量和等矢量分量的强烈混合。因此,这种状态显示出一些通常与侏儒共振相关的特征。

英文摘要:

Low-energy dipole excitations are analyzed for the stable isotopes $^{40}$Ca and $^{48}$Ca in the framework of the Skyrme-second random-phase approximation. The corresponding random-phase approximation calculations provide a negligible strength distribution for both nuclei in the energy region from 5 to 10 MeV. The inclusion and the coupling of 2 particle-2 hole configurations in the second random-phase approximation lead to an appreciable dipole response at low energies for the neutron-rich nucleus $^{48}$Ca. The presence of a neutron skin in the nucleus $^{48}$Ca would suggest the interpretation of the low-lying response in terms of a pygmy excitation. The composition of the excitation modes (content of 1 particle-1 hole and 2 particle-2 hole configurations), their transition densities and their collectivity (number and coherence of the different contributions) are analyzed. This analysis indicates that, in general, these excitations cannot be clearly interpreted in terms of oscillations of the neutron skin against the core with the exception of the peak with the largest $B(E1)$ value, which is located at 9.09 MeV. For this peak the neutron transition density dominates and the neutron and proton transition densities oscillate out of phase in the internal part of the nucleus leading to a strong mixing of isoscalar and isovector components. Therefore, this state shows some features usually associated to pygmy resonances.

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