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多数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，使它们结合在一起的力的性质，甚至是宇宙中元素是如何形成的，这些都是核结构研究的主题。在原子核周围，绕轨道运行的电子以离散能量占据量子“架子”，其排列在很大程度上取决于质子数（即质子数）。正在研究的元素。然而，在超精细的水平上，随着中子的加入产生不同的同位素，这些能级会随着核性质的改变而移动和分裂，比如大小、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来揭示这些特性。为了更充分地理解核力的本质，我们需要知道质子和中子数（Z和N）非自然组合的奇异核的核性质是如何变化的。这种核的寿命可能不到一毫秒。在“同位素工厂”的核反应中，它们以N和Z的分布产生，它们以离子束的形式静电传输到一个站进行光谱分析。一个特定的质量（N+Z）是使用飞行中的磁偏转选择的，但我们如何选择一个单一的元素？我们希望研究的同位素（N，Z）可能只是按质量选择的总光束的百万分之一。在一百万只山羊的田野里，很难看到几只绵羊。新的国际设施的目标是生产更多，但如果两者都生产得更多，就无法发挥其潜力。这是核物理学研究中一个长期存在的问题。对于激光束来说，量子电子能级为每个元素提供了指纹。在光的特征频率下，电子单独在一个元素的能级之间被激发。此外，激光束和激发步骤的结合将完全去除一个额外的电子。一束离子（从陷阱中释放出来，有足够的时间与激光相互作用）从中性原子状态中移除两个电子，而不是一个，因此电荷增加了一倍，在静电加速下，它们将更快地到达光谱站。到达较早的时候，只有单个N和Z的原子核会出现，其他的组合（都到达较晚）会被踢走。利用激光来研究（以及净化）原子核束，通过探测电子在激发后作为频率函数放松所发射的光子，揭示了上述所有特性。净化将使激光能够研究以每秒不到一个的速度产生的原子核（相比之下，今天需要1000个/秒），而不管是否存在其他具有相同质量同位素的元素。在原子核内，中子和质子各自占据着各自的量子架，处于离散的能量状态。这些能级依次被填满，核子之间的相互作用随着能级的填满而提高或降低它们的能量。能级迁移，以及由此引起的能级间能量间隙的变化，甚至是重新排序，从根本上影响了核的性质。这些测量将为了解核相互作用提供一个远离稳定的物种的敏感探针。作为自然（和非自然）世界的基石，核科学的应用超越了它的章节。长期以来，人们一直怀疑钍-229含有一种同分异构体——也就是说，一种被激发出能量的核状态，至少可以暂时存活。如果观测到，这将是自然界中所见的能量最低的，并且可能是第一次用激光证明核激发。利用同分异构体作为一个精确的时钟（来自振荡的核跃迁），测试爱因斯坦的相对论和基本物理常数到底有多恒定——这是物理学中最大的未解之谜之一，已经引起了相当大的兴趣。

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.