Hyperpolarized 129Xe: Physics and Applications

该奖项根据 2009 年美国复苏和再投资法案（公法 111-5）提供资金。惰性（或“惰性”）气体之所以如此命名，是因为它们通常不会与其他材料发生太多相互作用。然而，与许多其他元素一样，其中某些气体具有一种称为“自旋”的特性，也就是说，每个原子中心的原子核的行为就像一个非常小的旋转磁铁。这些旋转磁铁的存在可以被检测到，因为当它们协同作用时，它们可以通过一种称为核磁共振 (NMR) 的技术在线圈中产生可检测的电流。在该项目中，一种称为“自旋交换光泵浦”的激光技术用于在同位素 Xe-129 中产生非凡的自旋排列（“偏振”），该同位素是稳定的（非放射性）并且富含天然存在的氙。氙气的这种所谓的“超极化”将其对 NMR 的灵敏度提高了 10,000 倍或更多，从而使各种基础和应用磁共振实验成为可能。该项目有两个主要推动力。首先是改进和优化产生大量超极化氙的最先进方法。在这种方法中，气体流经长玻璃池，激光也穿过该玻璃池。激光被碱金属（通常是铷）蒸气吸收。碱金属原子的单价电子也具有自旋并被光排列或偏振。然后碱金属原子与氙原子碰撞，自旋极化转移到氙核。应用几种基于碱金属电子磁共振的技术来定量评估碱金属极化程度和氙极化程度。这些是优化系统性能的关键诊断。该项目的第二个主要目标是将超极化氙应用于基础物理学中一个长期存在的问题：预测相互相互作用的粒子的大型系统将如何表现的能力。在这种情况下，大型系统由 1020 个左右的 Xe-129 原子核组成，在 -200 °C 下冻结，它们的核自旋彼此磁性相互作用。 这是一个理想的系统，可用于研究混沌对所有这些原子核产生的 NMR 信号的影响。从基本角度来看，这是一个特别引人注目的系统，因为只有所谓的经典系统才能理解混沌效应，原则上人们可以同时知道相互作用粒子的每个位置和速度。相互作用的 Xe-129 原子核系统显然受量子力学理论支配，因此禁止对每个粒子进行如此精确的了解。尽管存在这种看似矛盾的现象，一位合作者还是预测了一种普遍的核磁共振信号行为，该行为与使用量子领域经典混沌的数学模拟进行的实验非常吻合。超极化惰性气体研究的多学科影响力几乎不可能被夸大。除了基础物理学之外，它们还应用于医学成像、生物化学和分子成像以及表面科学。所有这些应用都取决于对自旋交换光泵浦过程的基本物理原理的理解，以优化偏振和生产率。医学成像应用也许是最引人注目的：超极化惰性气体是理想的选择，因为它们无毒，并且可以吸入以产生动物和人类肺部的美丽磁共振图像（MRI）。研究肺部疾病的医生和研究潜在治疗方法的制药公司都对这项技术非常感兴趣。在此奖项之前的研究已经产生了一项关于超极化氙气存储单元的专利，该专利与这两家公司都非常相关。因此，该项目跨越了物理学领域的各个学科（从 AMO 到凝聚态物质，再到量子力学与混沌之间的关系），影响了用于医学成像和其他应用的超极化 Xe-129 的商业开发，并涉及各个级别的学生。特别是，让本科生，特别是女性本科生参与该研究项目的记录已经建立，并将继续下去。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The inert (or 'noble') gases are so named because they generally do not interact much with other materials. Like many other elements, however, certain of these gases possess a property called 'spin,' that is, the nucleus at the center of each atom behaves like a very tiny spinning magnet. The presence of these spinning magnets can be detected, because when acting in concert, they can produce a detectable electrical current in a coil of wire by a technique is known as nuclear magnetic resonance (NMR). In this project, a laser technique known as 'spin-exchange optical pumping' is used to generate an extraordinary alignment ('polarization') of the spins in the isotope Xe-129, which is stable (non-radioactive) and is abundant in naturally occurring xenon. This so-called 'hyperpolarization' of xenon gas enhances its sensitivity to NMR by a factor of 10,000 or more, making possible a wide variety of both fundamental and applied magnetic resonance experiments.There are two main thrusts to the project. The first is to improve and optimize the state-of-the-art method for generating large quantities of hyperpolarized xenon. In this method, the gas flows through a long glass cell through which the laser light also travels. The laser light is absorbed by a vapor of alkali metal (usually rubidium). The single valence electron of the alkali-metal atom also possesses spin and is aligned or polarized by the light. The alkali-metal atoms then collide with the xenon atoms and the spin polarization is transferred to the xenon nuclei. Several techniques based on magnetic resonance of the alkali-metal electron are applied to quantitatively assess both the degree of alkali-metal polarization and the degree of xenon polarization. These are crucial diagnostics for optimizing performance of the system. The second main thrust of this project applies hyperpolarized xenon to a long standing problem in fundamental physics: the ability to predict how a large system of mutually interacting particles will behave. In this case, the large system is 1020 or so Xe-129 nuclei, frozen in place at -200 °C with their nuclear spins interacting magnetically with each other. This is an ideal system in which to study the effects of chaos on the NMR signal generated by all of these nuclei. It is an especially compelling system from a fundamental perspective, since chaotic effects are only understood for so-called classical systems, whereby one can in principle know simultaneously each of the positions and velocities of the interacting particles. The system of interacting Xe-129 nuclei is clearly governed by quantum mechanical theory, whereby such precise knowledge of each particle is forbidden. Despite this seeming paradox, a collaborator predicted a universal NMR signal behavior that is remarkably matched by experiments using a mathematical analog of classical chaos in the quantum realm.It is nearly impossible to overstate the multidisciplinary reach of research into hyperpolarized noble gases. In additional to fundamental physics, they are applied in medical imaging, biochemistry and molecular imaging, and surface science. All of these applications depend on an understanding of the basic physics of the spin-exchange optical pumping process in order to optimize the polarization and production rate. The medical imaging application is perhaps most compelling: hyperpolarized noble gases are ideal because they are non-toxic and can be inhaled to produce beautiful magnetic resonance images (MRI) of animal and human lungs. Physicians studying lung disease and drug companies studying potential treatments are all keenly interested in this technology. Research previous to this award has already produced a patent on storage cells for hyperpolarized xenon that are quite relevant to both of these companies. Hence, the project reaches across disciplines within physics (from AMO to condensed matter, to the relationship between quantum mechanics and chaos), impacts commercial development of hyperpolarized Xe-129 for medical imaging and other applications, and involves students at all levels. In particular, a track record of involving undergraduates, especially women, in this research program is well established and will continue.