Using Coherent Pion Photoproduction to Measure Neutron Distributions in Nuclei

原子核中中子分布的性质目前仅对少数原子核是已知的。中子在原子核中的分布通常被量化为“中子皮”，定义为中子和质子均方根半径之差，位于德国美因茨的MAMI加速器设施产生的高能光子将允许测量几个原子核靶上相干光生中性π介子的衰变，因此可以建立这些原子核的中子分布。近年来，光生π介子方法已经成功地解释了重核208 Pb的中子分布，其中一个靶是双魔核48 Ca，由于质子-中子的不对称性，它可以解释一些定义不明确的核参数。48 Ca中子分布模型的结果差异很大，因此中子皮测量将有助于建立最接近实际的模型。此外，由中子皮测量建立的参数，包括中子皮大小对核对称能的依赖性，将对核子物质状态方程模型产生很大影响。这提供了一些物理现象的信息，包括中子星的结构和冷却机制，最近被证实是r-过程元素的主要来源。因此，在这个实验中测量的中子皮将产生广泛的影响，计划于2018年夏天运行。

The properties of neutron distributions in atomic nuclei are currently only known for a small number of nuclei. The neutron distribution in a nucleus is often quantified as a "neutron skin", defined as the difference between the neutron and proton root mean square radii.High energy photons produced by the MAMI accelerator facility in Mainz, Germany, will allow measurements of the decay of coherently photoproduced neutral pions on several nuclei targets, so neutron distributions for these nuclei can be established. In recent years, the method of pion photoproduction has successfully elucidated the neutron distribution of several nuclei, including the size and shape of the neutron skin of the heavy nucleus, 208Pb.One of the targets to be used is the doubly magic nucleus, 48Ca, of significant interest due to the proton-neutron asymmetry in the system, allowing insight into several poorly defined nuclear parameters. Models of the 48Ca neutron distribution have given widely varying results, and thus a neutron skin measurement would help to establish which models are closest to reality.In addition, parameters established from the neutron skin measurements, including the dependency of the neutron skin size on the nuclear symmetry energy, will have a strong impact on models of the equation of state of nucleonic matter. This gives information on several physical phenomena, including the structure and cooling mechanisms of neutron stars, which have recently been confirmed to be a major source of the r-process elements. The neutron skins measured in this experiment will thus have a wide impact, with the run scheduled for Summer 2018.