Exotic nuclear shapes with radioactive ion beams

原子核由质子和中子组成，是大自然的基本组成部分，已知存在了100多年。然而，无论是从原子核的集体行为还是从单个核子相互作用的角度来看，原子核的结构都还远未被理解。我们最常把原子核想象成一个球体，位于绕轨道运行的电子的中心，完成了教科书中的原子版本。然而，我们所知道的是，位于中心的这个由质子和中子组成的复杂量子系统几乎从来不是球形的，而是变形的。原子核有各种形状，从橄榄球(长椭圆形)、聪明人(扁圆形)到梨形(八极)。预测形状很困难，原子核理论也不能总是预测实验发现中出现的奇异形状现象。事实证明，这种形状是对核结构模型的一次非常敏感的测试。形状可以帮助区分哪些模型执行得最好，哪些模型在预测最重(和最新！)的实验结果时最可靠。最有趣的形状通常是在半衰期非常短的放射性同位素中找到的。不过，这些奇异的原子核可以在欧洲核子研究中心的Isolde等设施中进行研究。在这里，高能和高强度的质子束轰击碳化铀等材料的目标，产生近千种不同的同位素。然后，这些物质被按质量和元素过滤，然后被运输并加速到各种试验站，在那里它们可以进行研究。在这项研究中，放射性束流需要有能量，这样才能引发二次反应。这是通过将提取的同位素重新加速到约10%的光速，并将光束聚焦到同位素纯材料的薄片上来完成的。核形状不仅对核结构物理很重要，而且对物理学中的基本对称性的研究也是如此。对梨形原子核的理解在寻找原子电偶极矩(EDM)方面具有重要意义，在EDM中，原子核中反射不对称的电荷分布增强了EDM。非零电火花加工将意味着对电荷-宇称对称性的破坏(CP破坏)，并预示着一个超越当前标准模型的物理测试的新时代。寻找这样的发现的竞赛正在进行中，特别是在奇数质量的氡和镭同位素方面，这是这项研究的一部分。事实上，理解CP破坏被认为是理解早期宇宙中观察到的物质/反物质不对称的一条途径，正是这种不对称导致了我们今天的存在。

Atomic nuclei, made up of protons and neutrons, are fundamental building blocks of nature that have been known to exist for more than 100 years. Yet, the structure of the nucleus is not even close to being understood, either in terms of its collective behaviour or individual nucleon interactions. We most often picture atomic nuclei as a spherical ball at the centre of orbiting electrons, completing the textbook version of the atom. However, what we do know is that this complex quantum system of protons and neutrons at the centre is almost never spherical, but deformed. Nuclei have shapes, from rugby-balls (prolate) and Smarties (oblate) to pears (octupole).Predicting shapes is difficult, and nuclear theory cannot always predict the exotic shape phenomena that arise in experimental discoveries. The shape turns out to be a very sensitive test of nuclear-structure models. Shape can help distinguish which models perform best, and which ones are most reliable at predicting experimental findings in the heaviest (and newest!) elements, for example.The most interesting shapes are often to be found in radioactive isotopes with very short half-lives. These exotic nuclei can be studied though, at facilities such as ISOLDE at CERN. Here, high-energy and high-intensity proton beams bombard targets of materials such as uranium carbide, to produce almost a thousand different isotopes. These are then filtered by mass and element before being transported and accelerated to various experimental stations where they can be studied. In this research, the radioactive beams need to be energetic so that secondary reactions can be induced. This is done by re-accelerating the extracted isotopes to around 10% of the speed of light and focusing the beam onto thin foils of isotopically pure materials.Nuclear shape is important not only for nuclear-structure physics, but also for the study of fundamental symmetries in physics. The understanding of pear-shaped nuclei has strong implications in the search for atomic electric dipole moments (EDMs), where the reflection-asymmetric charge distribution in the nucleus enhances the EDM. A non-zero EDM would imply a violation of the charge-parity symmetry (CP-violation) and herald a new era of testing physics beyond the current standard model. The race is on for such a discovery, particularly in the odd-mass radon and radium isotopes, which form part of this study. Indeed, understanding CP-violation is seen as a route to the understanding of the observed matter/anti-matter asymmetry in the early universe, which led to our existence today.