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

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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.