Self-consistent description of electromagnetic excitations for astrophysical applications

在过去的几十年里，基于能量密度泛函(EDF)的微观核模型获得了很高水平的描述能力。这一发展的动力和挑战是，首先，在获取新的奇异原子核方面取得了显著的实验进展，其次，天体物理学要求从核模型中获得高质量的输入。然而，仍有一个方面尚未被自洽微观模型很好地控制：电磁激励的描述。它们是确定r过程核合成计算中辐射中子俘获率的基础。该计划旨在从两个方向扩展现有的EDF：一个是提供对磁激发模式的适当描述，另一个是考虑开放壳层球形核。在形式上，磁激发的描述使用与电模式(自然奇偶)相同的方法。基函数是广泛使用的随机相近似(RPA)，它产生强度分布的总体结构。在时间阻塞近似(TBA)下，通过声子耦合获得了精细结构。实际上，我们使用了最近实现的TBA，它包括单粒子连续体，准粒子-声子相互作用，以及非线性模型中声子的重整化，这确保了方法随着声子数量的增加而自然收敛。本项目针对的另一个新特征是准粒子RPA(QRPA)的扩展，它也将适用范围扩展到开壳层核。其结果将是对电磁激发截面的更可靠和更完整的描述，为核合成计算提供改进的输入。该项目分四个步骤进行：1.我们仔细检查208Pb中所有可能的电和磁模式，包括高自旋通道。我们利用这些结果来修正核EDF，以适应磁模和电模，这主要意味着在EDF中校准迄今尚未确定的自旋参数。现在有了对磁模的可靠描述，我们将把这些模包括到TBA的声子空间中，这将进一步细化光谱分布的精细结构4。最后，将该方法扩展到描述球形开壳核。这意味着首先将RPA扩展到qRPA，然后仔细研究如果现在更大的声子基数在TBA中的影响。在第一阶段，我们考虑非相对论Skyrme EDF，对于它我们已经有了相当多的经验。在第二阶段，我们向前迈进到相对论领域，首先通过增加对磁通道中的pi介子和Rho介子交换的显式处理，第二通过将上述步骤应用于相对论性EDF，包括那些从局部手征势导出的EDF。

Microscopic nuclear models based on energy-density functionals (EDF) have acquired over the past decades a high level of descriptive power. This development was motivated and challenged, first, by remarkable experimental progress in accessing new exotic nuclei, and second, by astrophysical demands calling for high quality input from nuclear models. However, there is still one aspect which is not yet well under control by self-consistent microscopic models: thedescription of electromagnetic excitations. They are fundamental for the determination of radiative neutron capture rates for r-process nucleosynthesis calculations.The proposed project aims at an extension of existing EDFs in two directions: one is to provide a pertinent description of magnetic excitation modes and the other is to consider open shell spherical nuclei. Formally, the description of magnetic excitations uses the same approaches as for electrical modes (natural parity). Basis is the widely used Random-Phase-Approximation (RPA) which produces the gross structure of strength distributions. The fine structure isobtained by phonon-coupling within the Time-Blocking Approximation (TBA). Actually, we use recent implementation of TBA which includes the single-particle continuum, the quasiparticle-phonon interaction, and renormalization of phonons within the framework of a nonlinearmodel, which ensures a natural convergence of the method with increasing number of phonons. A further new feature, aimed at in this project, is the extension to quasi-particle RPA (QRPA) which extends the range of applicability also to open-shell nuclei. The outcome willbe a more reliable and more complete description of electromagnetic excitation cross sections that delivers improved input for nucleosynthesis calculations.The project proceeds in four steps:1. we scrutinize all possible electrical and magnetic modes in 208Pb including high-spin channels.2. we use the results to modify the nuclear EDF to accommodate magnetic modes as well as the electrical ones which means mainly calibration of hitherto undetermined spin parameters in the EDF.3. having now a reliable description of magnetic modes, we will include these modes into the phonon space for TBA which will further refine the fine structure of spectral distributions4. Finally the approach will be extended to describe spherical open-shell nuclei. This means first to extend RPA to QRPA and after that to scrutinize the effects if the now larger basis of phonons in TBA.In a first phase, we consider non-relativistic Skyrme EDF for which we have already a considerable body of experience. In the second phase, we step forward into the relativistic domain, first by adding explicit treatment of pi- and rho- meson exchange in the magnetic channel, and second by applying the above sequence of steps to relativistic EDFsincluding also those which are derived from local chiral potentials.