Dipole Response of Medium-Mass Nuclei

在MeV能量范围内的光子与原子核之间的电磁相互作用由核偶极响应控制。它已被证明，一个精确的知识的偶极响应是重要的理解在宇宙中的核合成，连接到对称参数的状态方程的核物质，并显示出相关的中子皮的核。由于中子和质子之间的不对称性的增加，在低能量下的电偶极强度的进一步增强，预计在奇异的，富中子的核。尽管核结构模型取得了巨大的进步，但对偶极强度性质的预测仍然是一个挑战，本项目的目标是利用互补的实验探针测量Z=20和Z=28壳层闭合时中等质量核偶极响应的各种观测值。这样的核是一个理想的试验场，因为由于封闭的质子壳层和相当低的质量数，达到粒子阈值的能级密度足够低，可以进行详细的分析。另一方面，在这些核的理论计算是非常发达的，这将是重要的，看看，如何可靠，他们可以预测性质的激发。为此，我们计划在不同的国家和国际实验设施使用光子，质子和α束的实验组合，并将结果与最先进的理论计算进行比较。这些测量将填补知识的空白，在中等质量系统学中提供额外的点，并将为理论家提供非常需要的反馈。

The electromagnetic interaction between photons in the MeV energy range and atomic nuclei is governed by the nuclear dipole response. It has been shown that a precise knowledge of the dipole response is important to understand the nucleosynthesis in the universe, is connected to the symmetry parameter in the equation of state of nuclear matter, and shows a correlation with the neutron skin of nuclei. Because of the increasing asymmetry between neutrons and protons, further enhancement of electric dipole strength at low energies is expected in exotic, neutron-rich nuclei. In spite of the enormous general progress in models for nuclear structure, a robust prediction of the properties of dipole strength is still a challenge.The goal of the project is to measure various observables of the dipole response in medium-mass nuclei at the Z=20 and Z=28 shell closures with complementary experimental probes. Such nuclei are an ideal testing ground because the level density up to the particle threshold is low enough for a detailed analysis due to the closed proton shell and the rather low mass number. On the other hand, theoretical calculations in those nuclei are very well developed and it will be important to see, how reliable they can predict properties of the excitations. For this purpose, we plan a combination of experiments using photon, proton, and alpha beams at different national and international experimental facilities and to compare the results with state-of-the-art theoretical calculations. These measurements will fill the gaps in the knowledge, providing additional points in the mid-mass systematics, and will give a very much needed feedback to theoreticians.