Nuclear structure in extremely exotic systems explored by laser spectroscopy of pure ion beams.

原子核构成了我们周围所看到的大多数东西的基本构件。了解质子和中子在原子核内的量子排列、它们的稳定性、将它们聚集在一起的力的性质，甚至是这些元素在宇宙中是如何形成的，都是核结构研究的主题。在原子核周围，轨道电子以离散的能量占据量子“架”，其排列在很大程度上取决于质子数(即。元素)正在研究中。然而，在超精细水平上，这些能级的移动和分裂是由于添加中子以产生不同的同位素而改变核属性，如大小、形状、磁化和量子自旋。精密激光可以用来激发这些能级之间的电子，以揭示这样的性质。为了更全面地了解核力的性质，我们需要知道质子和中子数(Z和N)的非自然组合的奇异核的核性质是如何变化的。这样的原子核的寿命可能不到一毫秒。它们是在“同位素工厂”的核反应中以N和Z的分布产生的，通过静电将其作为一束离子束输送到光谱分析站。使用飞行中的磁偏转选择特定质量(N+Z)，但我们如何选择单个元素？我们想要研究的同位素(N，Z)可能只是按质量选择的总束流的百万分之一。在一百万只山羊的田野里，很难看到几只绵羊。新的国际设施旨在生产更多产品，但如果两者都生产更多，就不会发挥其潜力。这是核物理研究中一个长期存在的问题。对于一束激光，量子电子能级提供了每种元素的指纹。在光的特征频率下，电子仅在一种元素的能级之间被激发。此外，激光和激发步骤的组合将完全消除一个额外的电子。一束离子(从陷阱释放，有充足的时间与激光相互作用)，其中两个电子从中性原子态而不是一个电子移出，因此电荷是中性原子态的两倍，在静电加速下，离子将更快地到达光谱站。较早到达时，只会出现一个N和Z的原子核，而其他组合(都是较晚到达的)将被踢开。使用激光来研究(以及提纯)原子核光束，通过检测激发后弛豫回来的电子发出的光子作为频率的函数，揭示了上述所有性质。提纯将允许激光研究以低于每秒一个的速度产生的原子核(目前需要1000/S)，而不考虑存在哪些具有相同质量的同位素的其他元素。在原子核内部，中子和质子各自占据各自的量子架子，处于离散能量状态。这些粒子是按顺序填充的，核子之间的相互作用会随着能级的填充而提高或降低它们的能量。能级迁移，即由此引起的它们之间的能量差距的变化，甚至重新排序，从根本上影响着核的性质。这些测量将为了解核相互作用提供一个远离稳定性的物种的灵敏探测器。核科学位于自然(和非自然)世界的基石上，它的应用超出了它的范围。长期以来，人们一直怀疑钍-229含有一种异构体--即一种以能量激发的核态，至少是暂时存在的。如果观察到，这将是自然界中能量最低的一次，可能是第一次用激光进行核激发的演示。人们有相当大的兴趣将异构体用作精确的时钟(来自振荡的原子核跃迁)，测试爱因斯坦的相对论，以及基本物理常数到底有多恒定--这是物理学中最大的悬而未决的问题之一。

Atomic nuclei form the fundamental building blocks for most of what we see around us. Understanding the quantum arrangement of protons and neutrons within the nucleus, their stability, the nature of the forces which hold them together and even how the elements were formed in the universe are the subject of nuclear structure research. Surrounding the nucleus, orbiting electrons occupy quantum 'shelves' at discrete energies, with an arrangement largely dependent upon the proton number (ie. element) under study. However, on a hyperfine level, these energy levels move and split as a result of changing nuclear properties such as size, shape, magnetization and quantum spin as neutrons are added to create different isotopes. Precision lasers can be used to excite electrons between these levels to reveal such properties. To more fully understand the nature of the nuclear forces we need to know how nuclear properties change for exotic nuclei with unnatural combinations of proton and neutron numbers (Z and N). Such nuclei may live for less than a millisecond. Produced with a distribution in N and Z from nuclear reactions at 'isotope factories', they are electrostatically transported as a beam of ions to a station for spectroscopy. A specific mass (N+Z) is selected using in-flight magnetic deflection, but how can we choose a single element? The isotope (N,Z) we wish to study may be a part per million of the total beam selected by mass only. A few sheep would be hard to see in a field of a million goats. New international facilities aim to produce more, but will not fulfill their potential if they produce more of both. This is a long standing problem in nuclear physics research. To a laser beam, quantum electron levels provide a fingerprint for each element. At a characteristic frequency of light, the electrons are excited between levels in one element alone. Moreover, a combination of laser beams and excitation steps will remove an additional electron altogether. A bunch of ions (released from a trap, with ample time to interact with the laser) with two electrons removed from their neutral atomic state rather than one, and therefore twice the charge, will travel to the spectroscopy station faster under electrostatic acceleration. Arriving earlier, only nuclei of a single N and Z will be present, and other combinations (all arriving later) kicked away. Using a laser to study (as well as purify) the beam of nuclei reveals all the properties above by detecting photons emitted by electrons relaxing back after excitation as a function of frequency. Purification will allow lasers to study nuclei which are produced at a rate of less than one per second (compared with 1000/s required today), and irrespective of what other elements with isotopes of the same mass are present. Within the nucleus, neutrons and protons each occupy their own quantum shelves, lying at discrete energies. These are filled sequentially, and the interaction between nucleons raises or lowers their energy as the levels are filled. Level migrations, the consequent changes to the energy gaps between them or even a reordering, fundamentally affect the nuclear properties. These measurements will provide a sensitive probe of species far from stability in order to understand nuclear interactions. Lying at the corner stone of the natural (and unnatural) world nuclear science finds applications beyond its chapter. It has long been suspected that thorium-229 contains an isomer - that is, a nuclear state excited in energy which lives at least momentarily. If observed, this would have the lowest energy of any seen in nature and could be the first demonstration of nuclear excitation with a laser. Considerable interest has been gathered to use the isomer as an accurate clock (from an oscillating nuclear transition), testing Einstein's theory of relativity and how constant the fundamental physical constants really are - one of the greatest unanswered problems in physics.