SUPA, School of Physics, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom.
Institut für Kernphysik, University of Mainz, Germany.
Phys Rev Lett. 2014 Jun 20;112(24):242502. doi: 10.1103/PhysRevLett.112.242502. Epub 2014 Jun 18.
Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.) fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.) fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.) fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of (208)Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.
从在 MAMI 电子束设施上使用 Crystal Ball 探测器和格拉斯哥标记器测量的相干π介子光生产截面中提取了关于 (208)Pb 中中子皮的大小和形状的信息。通过对理论模型的插值拟合来利用测量的截面，发现中子分布的半高宽和扩散度分别为 c(n)=6.70±0.03(stat.) fm 和 a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.) fm，这对应于中子皮厚度 Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.) fm。这些结果首次成功地用电磁探针提取了中子皮的厚度，并表明 (208)Pb 的皮具有晕特征。该测量为核结构和富含中子物质的物态方程提供了宝贵的新约束。
