Abstract
Theoretical models for continuous-flow and stopped-flow spin-exchange optical pumping of 129Xe have long predicted much higher 129Xe polarization values than are measured experimentally, leading to a search for additional depolarization mechanisms. In this work, we show that a misapplication of the general theory of spin-exchange optical pumping along with the incorrect use of previously measured spin-exchange constants has been perpetuated in the past 20 years and is the main cause of the long-held discrepancy between theoretical and experimental 129Xe polarization values. Following the standard theory of spin-exchange optical pumping developed almost 40 years ago by Happer et al., we outline the common mistake made in the application of this theory in modern theoretical models and derive a simplified expression of the spin-exchange cross section that can be used to correctly predict 129Xe polarization values under any set of experimental conditions. We show that the complete expression of the spin-exchange cross section derived using the work of Happer et al. predicts spin-exchange rates tenfold higher than those previously assumed in theoretical models of continuous-flow and stopped-flow spin-exchange optical pumping and can fully rectify the long-standing discrepancy between theoretical and experimental polarization values.
Highlights
Nuclear spin hyperpolarization is used to increase the sensitivity of nuclear magnetic resonance (NMR) measurements by several orders of magnitude
We show that the complete expression of the spin-exchange cross section derived using the work of Happer et al predicts spinexchange rates tenfold higher than those previously assumed in theoretical models of continuous-flow and stopped-flow spin-exchange optical pumping and can fully rectify the long-standing discrepancy between theoretical and experimental polarization values
For a nuclear spin 1/2, hyperpolarization consists of bringing the nuclear spin system out of thermal equilibrium, depopulating one of the two energy states in favor of the other, creating a difference in populations between the two energy levels far greater than what is dictated by the Boltzmann distribution at thermal equilibrium
Summary
Nuclear spin hyperpolarization is used to increase the sensitivity of nuclear magnetic resonance (NMR) measurements by several orders of magnitude. A re-evaluation of some of the assumptions commonly made in these theoretical models for predicting HP 129Xe polarization values revealed major issues with the way the standard theory of SEOP19 has been applied in these models.[4,21,23,25,26,27,28] simulated and experimental Rb vapor densities for continuous-flow SEOP were found to be an order of magnitude smaller than those expected in a closed cell at thermal equilibrium which have been assumed in these models. The newly derived expression, along with the true Rb vapor density, is used to correctly predict experimental 129Xe polarization values
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