标题： $^{12}$的第一激发态存在极大的扁椭变形：现代核理论面临的新挑战 显示英文标题

标题： Extremely large oblate deformation of the first excited state in $^{12}$C: a new challenge to modern nuclear theory

摘要： 利用56 MeV的 $^{208}$Pb($^{12}$C,$^{12}$C$^*$)$^{208}$Pb$^*$反应以及慕尼黑Maier-Leibnitz实验室的 {\sc Q3D}磁谱仪，对核素$^{12}$C中4.439 MeV的高激发第一激发态进行了库仑激发研究。 使用大约 10$^{11}$个离子/秒的平均束流强度，并结合最先进的{\it 从头计算}核磁偶极极化率计算，实现了高统计数据，从而精确确定了光谱四极矩$Q_{_S}(2_{_1}^+) = +0.076(30)$≈ eb，与先前的测量结果一致。 结合之前的工作，得出加权平均值$Q_{_S}(2_{_1}^+) = +0.090(14)$≈ eb，其中包括 Vermeer 及其合作者对类似实验的重新分析，$Q_{_S}(2_{_1}^+) = +0.103(20)$≈ eb。 如此大的扁椭形变形挑战了现代核理论，并强调了需要$\alpha$聚类和相关的三轴性效应来实现$E2$集体现象的完全收敛。 显示英文摘要

摘要： A Coulomb-excitation study of the high-lying first excited state at 4.439 MeV in the nucleus $^{12}$C has been carried out using the $^{208}$Pb($^{12}$C,$^{12}$C$^*$)$^{208}$Pb$^*$ reaction at 56 MeV and the {\sc Q3D} magnetic spectrograph at the Maier-Leibnitz Laboratorium in Munich. High-statistics achieved with an average beam intensity of approximately 10$^{11}$ ions/s together with state-of-the-art {\it ab initio} calculations of the nuclear dipole polarizability permitted the accurate determination of the spectroscopic quadrupole moment, $Q_{_S}(2_{_1}^+) = +0.076(30)$~eb, in agreement with previous measurements. Combined with previous work, a weighted average of $Q_{_S}(2_{_1}^+) = +0.090(14)$ eb is determined, which includes the re-analysis of a similar experiment by Vermeer and collaborators, $Q_{_S}(2_{_1}^+) = +0.103(20)$~eb. Such a large oblate deformation challenges modern nuclear theory and emphasizes the need of $\alpha$ clustering and associated triaxiality effects for full convergence of $E2$ collective properties.