Isotope Separation by Condensation Reduction of Laser-Excited Molecules in Wall-Cooled Subsonic Gas Streams

Abstract

Explicit relations are developed to estimate isotope enrichment factors for ⁱQF₆ vapors diluted in a carrier gas G, which are isotope selectively laser-excited and flow subsonically through a wall-cooled cylindrical cell. At gas mix pressures below 100 millitorr, laser-assisted condensation repression on cold walls can induce isotope separations for some vapors at certain cryogenic temperatures. For example, for ⁱSF₆/N₂ mixtures, narrow temperature “windows” are found in the 70 to 90 K region where enrichments exceed β_i = β₃₃ = 1.7. For ⁱUF₆/G gas mixes, enrichment under full condensation conditions is not possible since the surface potential well (~1150 cm^–1) of a UF₆ condensate layer is higher than the vibration-to-translation conversion quantum of the v₃ vibration (~628 cm^–1). However, for UF₆^* adsorptions on a bare surface of F₂-passivated gold with well depth of 400 cm^–1 or less, initial isotope enrichments with β ~ 1.1 are possible before the surface is covered with UF₆ condensate. Throughputs in cold-wall isotope separations are low because of low operating pressures. For enrichments of milligrams of a radioactive isotope in nuclear medicine, this is still useful and offers a low-footprint alternative to calutron or ultracentrifuge separations. Since feed and product streams are the same, the method lends itself to multistaging, with one laser irradiating four or more chambers in series.