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受核偶极子响应的支配。研究表明，精确地了解偶极子响应对理解宇宙中的核合成具有重要意义，它与核物质状态方程中的对称参数有关，并与原子核的中子表皮有关。由于中子和质子之间的不对称性的增加，在低能量下电偶极子强度的进一步增强有望在奇异的，中子丰富的核。尽管在核结构模型方面取得了巨大的总体进展，但对偶极子强度特性的可靠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中质量原子核在Z=20和Z=28壳包层处的偶极子响应的各种可观测值。这样的原子核是一个理想的试验场，因为接近粒子阈值的能级密度足够低，可以进行详细的分析，这是由于封闭的质子壳和相当低的质量数。另一方面，这些原子核的理论计算已经发展得很好，很重要的一点是，它们预测激发的性质有多可靠。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组合实验，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，为中等质量系统提供额外的点，并将给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.