Hyperpolarized 129Xe: Physics and Applications

该奖项是根据2009年《美国复苏和再投资法案》(公法111-5)资助的。惰性气体之所以这样命名，是因为它们通常不会与其他物质发生太多相互作用。然而，像许多其他元素一样，这些气体中的某些气体具有一种称为自旋的性质，即每个原子中心的原子核的行为就像一块非常微小的旋转磁铁。可以检测到这些旋转磁铁的存在，因为当它们协同作用时，它们可以通过一种被称为核磁共振(核磁共振)的技术在导线线圈中产生可检测到的电流。在这个项目中，使用了一种被称为“自旋交换光学泵浦”的激光技术来产生同位素Xe-129中的自旋的异常排列(“偏振”)。Xe-129是稳定的(非放射性的)，并且富含自然生成的氙气。氙气的这种所谓的“超极化”使其对核磁共振的敏感性提高了10,000倍或更多，使各种基础和应用的磁共振实验成为可能。该项目有两个主要推动力。一是改进和优化目前最先进的产生大量超极化氙气的方法。在这种方法中，气体流过一个长长的玻璃池，激光也通过这个玻璃池传播。激光被碱金属(通常是Rb)的蒸气吸收。碱金属原子的单价电子也具有自旋，并被光排列或极化。然后碱金属原子与氙原子碰撞，自旋极化转移到氙核。基于碱金属电子磁共振的几种技术被用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的是将超极化的氙气应用于基础物理中的一个长期存在的问题：预测相互作用的大粒子系统的行为方式的能力。在这种情况下，大系统是大约1020个Xe-129核，在-200°C冻结在适当的位置，它们的核自旋相互作用。这是一个研究混沌对所有这些原子核产生的核磁共振信号影响的理想系统。从基本的角度来看，这是一个特别令人信服的系统，因为混沌效应只被理解为所谓的经典系统，因此人们原则上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原子核系统显然受到量子力学理论的支配，因此禁止对每个粒子进行如此精确的了解。尽管这看似自相矛盾，但一位合作者预测了一种普遍的核磁共振信号行为，与使用量子领域经典混沌的数学模拟进行的实验显著匹配。几乎不可能夸大超极化稀有气体研究的多学科覆盖范围。除了基础物理，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。所有这些应用都依赖于对自旋交换光学泵浦过程的基本物理的理解，以便优化偏振和产生率。医学成像应用可能是最引人注目的：超极化惰性气体是理想的，因为它们无毒，可以吸入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。在该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，这与这两家公司都非常相关。因此，该项目涉及物理学中的多个学科(从AMO到凝聚态，再到量子力学和混沌之间的关系)，影响用于医学成像和其他应用的超极化Xe-129的商业开发，并涉及所有水平的学生。特别是，让本科生，特别是女性，参与这一研究计划的记录是很好的，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.