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

对肺部或其他类似空间内气体的磁共振成像利用磁核对齐指向同一方向的稀有气体作为活跃介质，这一过程被称为稀有气体磁共振。只有通过自旋交换光学泵浦(SEOP)来制备用于成像程序的自旋极化惰性气体样品(氦-3或氙气-129)，才有可能在今天有效地利用该过程的灵敏度。这项研究的重点是SEOP过程本身，在这个过程中，角动量首先从光转移到碱金属原子，碱金属原子吸收光，然后通过碰撞将动量转移到惰性气体原子。碰撞使磁核对准(或极化)，使它们的南北磁极相互对准，从而使它们对磁共振研究有用。这个项目将解决一些关于SEOP效率如何依赖于激光功率、激光带宽、碱金属原子的选择以及混合气体中缓冲气体的压力的公开问题。氙气-129(129Xe)的SEOP特别令人感兴趣，因为人们对它知之甚少，而且它可能会取代氦-3，成为稀有气体核磁共振的首选原子。该Goali项目的工业合作伙伴Polarean，Inc.处于129Xe-MRI技术开发的前沿。该项目将使大学研究人员能够与Polarean，Inc.合作，研究与129Xe的SEOP相关的物理问题，并利用所获得的知识来改进商业偏振设备。三个主要的研究目标是：(1)推动Rb-129Xe Seop走向超快速率：目标是证明并实现在较少研究的低缓冲气体密度区域(~0.1 amagat)自旋交换效率的量化增益。超窄的OptiGrate激光器允许大量的光沉积，即使在这种情况下，对于更窄的Rbd1吸收线也是如此。(我们注意到Optigrate是该项目的第二个工业合作者。)(2)用于SEOP的Rb和Cs的直接比较：目标是使用双头高功率Rb/Cs半导体激光器的公共平台对Rb-129Xe和Cs-129Xe的SEOP参数进行定量测量，以确定Cs是否有利以及在多大程度上有利。(3)碱金属团簇在限制SEOP效率中的作用：目标是获得这些潜在有害纳米团簇在SEOP偏振器中的直接证据，并将它们的影响纳入129Xe SEOP的“标准模型”。实现这些目标中的每一个都直接影响到129Xe Seop的效率，阐明了一些有趣的(也是迄今难以捉摸的)物理，同时也导致了用于肺部MRI等应用的HP 129Xe产量的提高。

Magnetic resonance imaging of gas inside the lungs or other similar spaces where gases can be admitted utilizes as the active medium noble gases that have had their magnetic nuclei aligned to point in the same direction, a process known as noble-gas MRI. Effective use of the process today is only possible if the sensitivity is enhanced by spin-exchange optical pumping (SEOP) for preparing the spin-polarized noble gas samples (helium-3 or xenon-129) to be used in the imaging procedure. This research focuses on the SEOP process itself, in which angular momentum is transferred first from light to alkali-metal atoms that absorb the light and then transfer the momentum to the noble-gas atoms through collisions. The collisions align (or polarize) the magnetic nuclei, bringing their north and south magnetic poles in mutual alignment, thereby rendering them useful for MRI studies. This project will address some open questions relating to how the efficiency of SEOP depends on the laser power, the laser bandwidth, the choice of alkali-metal atoms, and the pressure of the buffer gas in the gas mixtures. SEOP of xenon-129 (129Xe) is of particular interest because it is less well understood, and because it may supersede helium-3 as the atom of choice for noble-gas MRI. The industrial partner for this GOALI project, Polarean, Inc., is at the forefront of 129Xe-MRI technological development. This project will enable university researchers to collaborate with Polarean, Inc. to study physics issues related to SEOP of 129Xe, and to use the resulting knowledge to improve a commercial polarization device. The three main research aims are: (1) Pushing Rubidium-129Xe SEOP toward ultra-fast rates: The goal is to demonstrate and implement quantitative gains in the spin-exchange efficiency in the less-studied low-buffer-gas-density regime (~ 0.1 amagat). The ultra-narrow OptiGrate laser allows significant light deposition even for the much narrower rubidium D1 absorption line in this regime. (We note that Optigrate is a second industrial collaborator in this project.) (2) Direct comparison of rubidium to cesium for use in SEOP: The goal is to make quantitative measurements of Rb-129Xe and Cs-129Xe SEOP parameters using the common platform of the dual-head high-power Rb/Cs diode laser, in order to determine whether and to what degree Cs is advantageous. (3) The role of alkali-metal clusters in limiting SEOP efficiency: The goal is to obtain direct evidence for these potentially deleterious nanoclusters in a SEOP polarizer and incorporate their effects into the "standard model" for 129Xe SEOP. Meeting each of these goals has a direct impact on the efficiency of 129Xe SEOP, elucidating some interesting (and heretofore elusive) physics while also leading to improved production of HP 129Xe for applications such as lung MRI.