Purification of $^{4}$He through Differential Evaporation

The neutron electric dipole moment (nEDM) experiment, to be housed at the Spallation Neutron Source at Oak Ridge National Laboratories, will probe for a dipole moment at the level of 10$^{-28}$ e cm. As part of the measurement process, neutrons precess in an environment of isotopically pure helium, doped with polarized $^{3}$He. After this $^{3}$He depolarizes it must be removed. We are developing an evaporative purification technique for this removal, lowering the concentration of $^{3}$He in $^{4}$He from 10$^{-8}$ to 10$^{-10}$, at an operating temperature of 300 -- 350 mK. Because the vapor pressure of $^{3}$He is enhanced at temperatures below 500mK, $^{3}$He atoms can be preferentially removed from the solution. The purifier requires a large liquid surface area, while minimizing superfluid film flow. The evaporated atoms are adsorbed on activated charcoal. We have built a device to measure $^{3}$He/$^{4}$He ratios using a leak detector mass spectrometer and a residual gas analyzer.