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

原子核构成了我们周围大部分事物的基本组成部分。了解原子核内质子和中子的量子排列、它们的稳定性、将它们结合在一起的力的性质，甚至是元素在宇宙中是如何形成的，都是核结构研究的主题。围绕着原子核，轨道电子以离散的能量占据量子“架子”，其排列在很大程度上取决于质子数（即质子数）。元素）进行研究。然而，在超精细水平上，这些能级移动和分裂是由于改变核性质，如大小，形状，磁化和量子自旋，因为中子被添加以产生不同的同位素。精密激光器可以用来激发这些能级之间的电子，以揭示这些性质。为了更全面地理解核力的性质，我们需要知道质子数和中子数（Z和N）的非自然组合的奇异核的核性质如何变化。这种原子核的寿命可能不到一毫秒。在“同位素工厂”的核反应中产生的N和Z分布，它们作为离子束静电传输到光谱站。使用飞行中的磁偏转选择特定的质量（N+Z），但是我们如何选择单个元素呢？我们希望研究的同位素（N，Z）可以是仅按质量选择的总束的百万分之一。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，但如果它们生产更多的两种产品，将无法发挥其潜力。这是核物理研究中一个长期存在的问题。对于激光束，量子电子能级为每个元素提供了指纹。在光的特征频率下，电子在一种元素的能级之间被激发。此外，激光束和激发步骤的组合将完全去除额外的电子。一束离子（从阱中释放出来，有足够的时间与激光相互作用），其中两个电子从中性原子状态中被移除，而不是一个，因此电荷是两倍，在静电加速下将更快地到达光谱站。到达得更早，只有一个N和Z的原子核会出现，其他的组合（都是后来到达的）被踢走了。使用激光研究（以及纯化）原子核束，通过检测激发后弛豫的电子发射的光子作为频率的函数，揭示了上述所有性质。纯化将允许激光器研究以每秒不到一个的速度产生的原子核（与今天所需的1000/s相比），并且无论存在具有相同质量同位素的其他元素。在原子核内，中子和质子各自占据自己的量子架，处于离散的能量。这些能级被依次填充，随着能级被填充，核子之间的相互作用会提高或降低它们的能量。能级迁移，其结果是它们之间能隙的变化，甚至是重新排序，从根本上影响核的性质。这些测量将为了解核相互作用提供远离稳定性的物种的灵敏探针。躺在自然（和非自然）世界的基石上，核科学发现了超出其章节的应用。长期以来，人们一直怀疑钍-229含有一种异构体，即一种至少暂时存在的能量激发态。如果被观测到，这将是自然界中能量最低的，并且可能是第一次用激光激发核。人们对使用这种异构体作为一个精确的时钟（来自振荡的核跃迁）产生了相当大的兴趣，以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未解问题之一。

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.