Depolarization of nuclear spin polarized ¹²⁹Xe gas by dark rubidium during spin-exchange optical pumping

Continuous-flow spin-exchange optical pumping (SEOP) continues to serve as the most widespread method of polarizing ¹²⁹Xe for magnetic resonance experiments. Unfortunately, continuous-flow SEOP still suffers from as-yet unidentified inefficiencies that prevent the production of large volumes of xenon with a nuclear spin polarization close to theoretically calculated values. In this work we use a combination of ultra-low field nuclear magnetic resonance spectroscopy and atomic absorption spectroscopy (AAS) measurements to study the effects of dark Rb vapor on hyperpolarized ¹²⁹Xe in situ during continuous-flow SEOP. We find that dark Rb vapor in the optical cell outlet has negligible impact on the final ¹²⁹Xe polarization at typical experimental conditions, but can become significant at higher oven temperatures and lower flow rates. Additionally, in the AAS spectra we also look for a signature of paramagnetic Rb clusters, previously identified as a source of xenon depolarization and a cause for SEOP inefficiency, for which we are able to set an upper limit of 8.3×10¹⁵ Rb dimers per cm³.