Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, UK.
Nature. 2013 May 9;497(7448):199-204. doi: 10.1038/nature12073.
There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are 'octupole deformed', that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on (220)Rn and (224)Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.
有强有力的间接证据表明，某些重的、不稳定的原子核是“八极变形的”，也就是说，它们被扭曲成了梨形。这与更常见的具有反射对称、四极变形的橄榄球形状的原子核形成了对比。难以捉摸的八极变形原子核对于核结构理论很重要，对于寻找超越标准模型的物理也很重要；任何可测量的电偶极矩（后者的特征）预计都会在这样的原子核中被放大。在这里，我们确定了氡和镭的短寿命同位素的电八极跃迁强度（八极关联的直接测量）。使用重放射性离子的加速束进行了库仑激发实验。我们在 (220)Rn 和 (224)Ra 上的数据清楚地表明，后者的八极变形更强。这些结果可以在不同的八极关联理论方法之间进行区分，并有助于限制适合实验研究原子电偶极矩的候选者，这些研究可能会揭示对标准模型的扩展。
