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

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.