GOALI: Resolving Outstanding Questions in Spin-Exchange Optical Pumping of 129Xe

摘要：自旋是20世纪初第一次量子革命开始时发现的许多原子性质之一。自旋导致一些原子粒子（如电子、质子和许多原子核）像旋转的磁铁一样运动。就像旋转的磁体在附近的线圈中产生电流（如水力发电）一样，如果磁体都排列在一起（“极化”），就可以用附近的线圈检测和操纵具有自旋的原子样本。PI使用“光泵”，这是一个涉及激光的过程，将自旋从光转移到铷或铯原子，然后与氙原子碰撞，产生自旋极化的氙核样本。本项目的活动和目标集中在理解和最大限度地提高自旋转移到氙核的效率。PI的GOALI合作伙伴Polarean公司（北卡罗来纳州达勒姆）负责设计和销售氙气偏振光片，该偏振光片可用于磁共振成像（MRI），特别是人体肺部。核磁共振成像通常依赖于人体组织中存在的水来产生图像，但是肺的气体空间中很少有水。氙气是无毒的，不会被人体代谢，但自旋极化导致气体在核磁共振成像上“发光”，就像水在其他组织中的作用一样。这项技术有巨大的潜力来回答关于肺功能和肺病（美国第四大死亡原因）的重要问题，但使这项技术可靠和成本效益取决于在这个项目中所做的工作，以了解自旋转移的物理学。除了这个应用之外，自旋在正在进行的第二次量子革命中扮演着重要的角色，我们现在所说的“自旋电子学”（电子的自旋版本），自旋特性在量子计算和相关技术的实现中变得重要。学生们在实验室“动手”学习电子、真空技术、管道，甚至一些玻璃吹制；他们在技术上受过良好的训练，但也拥有可以在学院内外的许多其他职业中服务的技能。促进实验室环境的公平、多样性和包容性是一个目标。从历史上看，该小组中大约一半的本科研究人员是女性。该项目还将补充并受益于新授予的NSF-REU部门项目。摘要：稳定的自旋为1/2的同位素3He和129Xe通过自旋交换光泵浦（SEOP）很容易超极化（HP）到超过10%的能级。129Xe的SEOP特别令人感兴趣，因为它既不太为人所了解，又有可能取代3He作为惰性气体核磁共振的选择。与GOALI合作伙伴Polarean一起，PI寻求解决129Xe SEOP中持续存在的关键物理问题，并将这些知识应用于商用偏振设备中。主要有三个研究目标：1)在低Xe部分密度和低总气体密度（数十torr及以下）的研究较少的区域测量Rb和Cs自旋破坏。2)在低气体密度下测量Rb-129Xe和Cs-129Xe的自旋交换速率，在相同的低密度状态下，范德瓦尔斯分子具有更长的寿命，自旋交换时间应该很快（几分钟到几秒钟）。目标1和目标2可以共同确定自旋交换效率，这是评估高效生产高压气体的关键指标。该项目还旨在表征含有Rb和Cs的“混合”电池，以确定特定的蒸汽压比是否能优化SEOP效率。3)寻找在SEOP偏振器中存在碱金属纳米团簇的直接证据，将其影响纳入超极化129Xe生产的“标准模型”，并开始制定消除或减轻其影响的策略。主要的实验方法是使用法拉第旋转的碱金属原子的EPR光谱，其中已经开发了两种特定的技术：一种连续波“频率锁定”技术来测量极化129Xe磁场，以及一种脉冲技术（类似于脉冲核磁共振）来快速获取整个超精细光谱。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

General audience abstract:One of the many atomic properties discovered at the start of the first quantum revolution in the early twentieth century is spin. Spin causes some atomic particles (e.g., electrons, protons, and many atomic nuclei) to behave like spinning magnets. Much the way rotating magnets produce currents in a nearby coil (as with hydroelectric power), samples of atoms possessing spin, if the magnets are all aligned together (``polarized”), can be detected and manipulated with a nearby coil of wire. The PI uses “optical pumping,” a process involving lasers that transfers spin from light to rubidium or cesium atoms, which then collide with xenon atoms to create samples of spin-polarized xenon nuclei. The activities and goals in this project are centered on understanding and maximizing the efficiency of spin transfer to xenon nuclei. The PI’s GOALI partner, Polarean, Inc. (Durham, North Carolina), engineers and markets xenon polarizers, which can be used in magnetic resonance imaging (MRI), particularly of the human lung. MRI normally relies on the presence of water in human tissues to produce an image, but there is very little water in the gas space of the lung. Xenon gas is non-toxic and is not metabolized by the body, but the spin polarization causes the gas to “light up” on an MRI the same way that water does in other tissues. This technique has enormous potential to answer important questions about lung function and lung disease (the fourth leading cause of death in the United States) but making the technique reliable and cost-effective depends on the work done in this project to understand the physics of spin transfer. Beyond this application, spin is playing an important role in the ongoing second quantum revolution, where we now speak of “spintronics” (the spin version of electronics), and spin properties have become important in implementations of quantum computing and related technologies. Students are “hands-on” in the lab learning electronics, vacuum technology, plumbing, and even some glass blowing; they are technically well trained but also come out in possession of skills that can serve them in a host of other occupations, both in and outside of the academy. Promoting equity, diversity, and inclusion in the laboratory environment is a goal. Historically about half of the undergraduate researchers in the group have been women. This project will also both complement and benefit from a newly awarded departmental NSF-REU project. Technical audience abstract:The stable spin-1/2 isotopes 3He and 129Xe are readily hyperpolarized (HP) to levels exceeding 10% via spin-exchange optical pumping (SEOP). SEOP of 129Xe is of particular interest because it is both less well understood and likely to supersede 3He as the nucleus of choice for noble-gas MRI. Together with the GOALI partner, Polarean, the PI seeks both to address persistent yet critical physics issues in SEOP of 129Xe and to implement this knowledge in a commercial polarization device. The three main research aims are: 1) Measurement of Rb and Cs spin destruction due to Xe in the less-studied regime of low Xe partial density and low total gas density (tens of torr and below). 2) Measurement of 129Xe spin-exchange rates at low gas densities for both Rb-129Xe and Cs-129Xe in the same low-density regime as in Aim 1, where van der Waals molecules have longer lifetimes and spin-exchange times should be fast (minutes to seconds). Together, Aims 1 and 2 allow determination of the spin-exchange efficiency, the crucial figure of merit for evaluating efficient production of HP gases. The project also aims to characterize “hybrid” cells, containing both Rb and Cs, to determine whether specific vapor-pressure ratios optimize SEOP efficiency. 3) Search for direct evidence of the presence of alkali-metal nanoclusters in a SEOP polarizer, incorporate their effects into the “standard model” for hyperpolarized 129Xe production, and begin to develop strategies to eliminate them or mitigate their effects. The main experimental approach is EPR spectroscopy of the alkali-metal atoms using Faraday rotation, whereby two specific techniques have been developed: a cw “frequency locking” technique to measure the polarized 129Xe magnetic field, and a pulsed technique (analogous to pulsed NMR) to rapidly acquire the entire hyperfine spectrum.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.