A new model is presented for spin-exchange optical pumping using an open-source code, ElmerFEM-CSC. The model builds on previous models by adding the effects of alkali-vapor heterogeneity in optical pumping cells and by modeling the effects of hyperpolarized-gas wall-relaxation using a diffusion model. The code supports full, three-dimensional solutions to optical-pumping models, and solves for (1) laser absorption, (2) alkali vapor concentration, (3) fluid flow parameters, (4) thermal affects due to the pumping laser, and (5) noble gas polarization. The source code for the model is available for researchers to utilize and modify.
We present a comprehensive theory of nuclear spin polarization of
3He and
129Xe gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are (1) spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; (2) spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; (3) electron-nuclear spin-exchange collisions between alkali-metal atoms and
3He or
129Xe atoms; (4) spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; (5) optical pumping by laser photons; (6) spatial diffusion. The static magnetic field is assumed to be small enough that the nuclear spin of the alkali-metal atom is well coupled to the electron spin and the total spin is very nearly a good quantum number. Conditions appropriate for the production of large quantities of spin-polarized
3He or
129Xe gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with
N2 or
H2 gas. Some of the more important results of this work are as follows: (1) Most of the pumping and relaxation processes are sudden with respect to the nuclear polarization. Consequently, the steady-state population distribution of alkali-metal atoms is well described by a spin temperature, whether the rate of spin-exchange collisions between alkali-metal atoms is large or small compared to the optical pumping rate or the collisional spin-relaxation rates. (2) The population distributions that characterize the response to sudden changes in the intensity of the pumping light are not described by a spin temperature, except in the limit of very rapid spin exchange. (3) Expressions given for the radio-frequency (rf) resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. (4) Diffusion effects for these high-pressure conditions are mainly limited to thin layers at the cell surface and at internal resonant surfaces generated by radio-frequency magnetic fields when the static magnetic field has substantial spatial inhomogeneities. The highly localized effects of diffusion at these surfaces are described with closed-form analytic functions instead of the spatial eigenmode expansions that are appropriate for lower-pressure cells.
Abstract In an optically pumped alkali vapor cell with a high density of atoms, the attenuation of the pump light generates a spatially non-uniform distribution of the electronic spin polarization of alkali atoms, which is detrimental to biomagnetism applications of magnetometers as well as the hyperpolarization of noble gas atoms. Therefore, in this study, we propose a new scheme to generate a nearly uniform, unsaturated spin polarization region based on counter-propagating pump beams and atomic diffusion. A finite element method-based simulation is used to demonstrate the three-dimensional distribution of the spin polarization in a spherical cell. The effects of cell temperature and pump light power on the homogeneity of the spin polarization are studied. The distribution of spin polarization near the center of the cell is experimentally measured and a 1 cm uniform spin polarization region is achieved in the center of the cell. The uniformity of spin polarization in the center region of the cell increased by 50% compared with single beam pumping. The advantage of our proposed scheme is that it can generate an unsaturated uniform region of spin polarization in the center of a cell using a single species of alkali atoms.
The applicability of commonly used 129Xe nuclear polarization (PXe) buildup models to large optical cell polarizers, which operate at lower gas pressures when compared to small cell polarizers for which spin-exchange optical pumping (SEOP) models were originally developed, has yet to be systematically evaluated. In addition, spatial heterogeneity of Rb vapor density ([Rb]) and its impact on SEOP in low pressure, large optical cell polarizers remains poorly understood. In this theoretical study, it was shown that 1D analytical and numerical PXe buildup models yield diverging PXe and critical flow rate values at low flow rates and high laser absorption, contributing to the discrepancy between theoretical and experimentally measured production rates. Additionally, it was found that [Rb] heterogeneity leads to lower PXe buildup and an undesirable increase in optimal cell temperature, while the optimal laser absorption remains relatively unchanged. The updated simulation framework presented here can be utilized to model other SEOP systems.
The ability to produce liter volumes of highly-spin-polarized
129Xe enables a wide range of investigations, most notably in the fields of materials science and biomedical magnetic resonance imaging. However, for nearly all polarizers built to date, both peak
129Xe polarization and the rate at which it is produced fall far below those predicted by the standard model of Rb metal vapor, spin-exchange optical pumping (SEOP). In this work we comprehensively characterized a high-volume flow-through
129Xe polarizer using three different SEOP cells with internal volumes of 100, 200, and
300cm3 and two types of optical sources: a broad-spectrum 111-W laser [full width at half maximum (FWHM) equal to 1.92 nm] and a line-narrowed 71-W laser (FWHM equal to 0.39 nm). By measuring
129Xe polarization as a function of gas flow rate, we extracted the peak polarization and polarization production rate across a wide range of laser absorption levels. Peak polarization for all cells consistently remained a factor of 2--3 times lower than predicted at all absorption levels. Moreover, although production rates increased with laser absorption, they did so much more slowly than predicted by the standard theoretical model and basic spin-exchange efficiency arguments. Underperformance was most notable in the smallest optical cells. We propose that all these systematic deviations from theory can be explained by invoking the presence of paramagnetic Rb clusters within the vapor. Cluster formation within saturated alkali-metal vapors is well established and their interaction with resonant laser light was recently shown to create plasmalike conditions. Such cluster systems cause both Rb and
129Xe depolarization, as well as excess photon scattering. These effects were incorporated into the SEOP model by assuming that clusters are activated in proportion to excited-state Rb number density and by further estimating physically reasonable values for the nanocluster-induced velocity-averaged spin-destruction cross section for Rb $(\ensuremath{\langle}{\ensuremath{\sigma}}_{\text{cluster-Rb}}v\ensuremath{\rangle}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, the
129Xe relaxation cross section $(\ensuremath{\langle}{\ensuremath{\sigma}}_{\text{cluster-Xe}}v\ensuremath{\rangle}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, and a non-wavelength-specific photon-scattering cross section $({\ensuremath{\sigma}}_{\mathrm{cluster}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2})$. The resulting modified SEOP model now closely matches experimental observations.
While the method of Spin-exchange optical pumping, SEOP, of noble gases has existed since 1960 [Bouchiat60], many fundamental aspects of spin exchange optical pumping have gone largely unexplained. These questions included the values of the rate coefficients for spin-exchange, many aspects of the alkali-metal polarization and density inside the SEOP cells, the mechanisms of He wall relaxation, non-ideal values of the observed He polarization, and efficiency to produce it. This thesis describes the results of our efforts to quantity all parameters in SEOP and to begin to determine some of these unknowns. This thesis presents measurements of spin-exchange rate coefficients and efficiencies. A method to extend SEOP to multiple alkali species is presented. In addition, limits to the efficiency of optical pumping, the efficiency of spin-exchange, and the observed He polarization are explored.