“Resolving the discrepancy between theoretically predicted and experimentally measured ¹²⁹Xe nuclear spin polarization”

By bringing spins out of thermal equilibrium, nuclear spin hyperpolarization techniques can enhance the nuclear magnetic resonance signal by several orders of magnitude, creating a bulk magnetization that would typically require magnetic field strengths greater than 100,000 T! This non-equilibrium magnetization enables studies at magnetic field strengths 3 orders of magnitude lower than those typically produced with superconducting magnets and the detection of nuclei at micromolar and sub-micromolar concentrations. Hyperpolarization methods generally require a source of spin order that can be transferred to the nuclei of interest. In my work, I focus on spin-exchange optical pumping (SEOP), a method whereby the spin order of photons produced by high-power diode laser are first transferred to the electrons of an alkali atom (optical pumping) and then to the nuclei of ¹²⁹Xe (spin-exchange).

Xenon is a noble gas with a relatively high solubility in tissues. Its large electron cloud confers to the ¹²⁹Xe NMR-visible isotope a strong sensitivity to its physical and chemical environment, making it an ideal NMR probe. Hyperpolarized ¹²⁹Xe is widely used as contrast agent in magnetic resonance experiments for probing lung ventilation function and recently gained approval by the United States Food and Drug Administration for clinical use. For biomedical imaging applications, the ability to obtain high quantities of Xe with high levels of nuclear spin polarization is paramount. Theoretical models of SEOP are generally used to optimize experimental conditions, but these models have fallen short, overestimating experimentally produced ¹²⁹Xe polarization values by 2-3 folds and leading to a search for additional depolarization mechanisms.

The first part of my work addresses this discrepancy between theoretically predicted and experimentally obtained ¹²⁹Xe polarization values obtained via SEOP. I show that an overestimation of the alkali vapor density, along with an underestimation of the spin-exchange cross section between the alkali atoms and the noble gas atoms, led to this long-standing discrepancy between theoretically predicted and experimentally obtained ¹²⁹Xe polarization values. I then present an experimental setup to measure alkali electronic polarization developed under optical pumping.  Finally, I will describe how to directly transfer spin order from ¹²⁹Xe to other nuclei.

Topic: Michele Kelley Thesis Defense

Time: Mar 24, 2023 09:30 AM Eastern Time (US and Canada)

Join Zoom Meeting

https://unc.zoom.us/j/9716303024?pwd=L0tjZGpCRWRxdGxMTE1pUXpLNXVYZz09