Dipole Response of Medium-Mass Nuclei

MeV能量范围内的光子与原子核之间的电磁相互作用受原子核偶极响应的支配。结果表明，对偶极子响应的精确了解对于理解宇宙中的核合成是重要的，它与核物质状态方程中的对称性参数有关，并与核的中子皮相关。由于中子和质子之间的不对称性越来越强，在奇异的、丰含中子的原子核中，电偶极子强度在低能下有望进一步增强。尽管核结构模型取得了巨大的总体进展，但对偶极子强度性质的稳健预测仍然是一个挑战。该项目的目标是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的偶极响应的各种观测数据。这样的原子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，直到粒子阈值的能级密度足够低，足以进行详细分析。另一方面，这些原子核中的理论计算非常发达，重要的是看看它们预测激发性质的可靠性有多高。为此，我们计划在不同的国家和国际实验设施中使用光子、质子和阿尔法光束进行实验，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，并将给理论家提供非常必要的反馈。

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.