Frequency shifts in the EPR spectrum of ${}^{39}$K due to spin-exchange collisions with polarized ${}^3$He and precise ${}^3$He polarimetry

Abstract

The Zeeman splittings and EPR frequencies of alkali-metal atoms are shifted in the presence of a polarized noble gas. For a spherical geometry, the shift is enhanced over what is expected classically by a dimensionless atomic parameter ${\kappa }_0$ that is unique to each alkali-metal atom - noble-gas pair. We present a precise measurement of ${\kappa }_0$ for the ${}^{39}$K-${}^3$He system with a relative accuracy of better than 1\%. A critical component of achieving sub-percent accuracy involved characterizing the shape of our samples using both MRI and CT medical-imaging techniques. The parameter ${\kappa }_0$ plays an important role in establishing the absolute polarization of ${}^3$He in a variety of contexts, including polarized targets for electron scattering experiments and MRI of the gas space of the lungs. Our measurement more than doubles the accuracy possible when using ${\kappa }_0$ for polarimetry purposes. Just as important, the work presented here represents the first {\it direct} measurement of ${\kappa }_0$ for the ${}^{39}$K-${}^3$He system; previous values for ${\kappa }_0$ in the ${}^{39}$K-${}^3$He system relied on a chain of measurements that were benchmarked by previous measurements of ${\kappa }_0$ in the Rb-${}^3$He system.

18 pages, 15 figures