Spin-exchange rate coefficients in Rb-Xe and Cs-Xe systems using repolarization

We measure and compare the spin-exchange rate coefficients Κ_se for Rb and Cs in a He (94%)-N₂ (3%)-Xe (3%) gas mixture, similar to that used in flow-through Xe polarizers. The measurements are crucial to understanding which alkali-metal is the best partner for polarizing ¹²⁹Xe; they are carried out in two matched sets (one each for Rb and Cs) of four sealed vapor cells. Combining these results with our recent spin-destruction rate results from the same samples points to which alkali-metal is intrinsically most efficient for producing large quantities of hyperpolarized Xe for applications such as medical imaging. We employed the repolarization method [1,2], where Κ_se = (P_Aεγ_A) / (P_Xe[Xe]-εP_A[Xe]), and where ε is the nuclear slowing-down factor. Using optically detected pulsed EPR of the alkali-metal hyperfine structure, we measured the alkali-metal polarization P_A , the alkali-metal spin-destruction rate γ_A due to Xe, and the ¹²⁹Xe polarization P_Xe . The spin-destruction rate is measured “in the dark” at long delay times, in order to isolate the slowest polarization-decay component; at even longer delay times, when P_A reaches a steady state due to the non-zero P_Xe , the so-called “back-polarization” of the alkali-metal can be determined and then calibrated by examining hyperfine peak ratios at much higher P_A during active optical pumping. We calculated P_Xe[Xe] by using the known Rb-Xe and Cs-Xe enhancement factors (κ₀)_Rb and (κ₀)_Cs [3] and measuring the respective alkali-metal EPR frequency shifts due to polarized ¹²⁹Xe. The repolarization method has the advantage of not requiring a determination of the alkali-metal number density. [1] R.K. Ghosh and M.V. Romalis, Phys. Rev. A 81, 043415 (2010); [2] B. Chann, et al., Phys. Rev. A 66, 032703 (2002); [3] S. Zou, et al., Phys. Rev. A 106, 012801 (2022).