Chlorine Isotope Separation by Thermal Diffusion

From "two-bulb experiments" the thermal diffusion constant $\alpha$ of HCl has been determined to be +0.010 at $T_r=685\mathrm{}°$K and -0.009 at $T_r=229\mathrm{}°$K. Such a rapid change of $\alpha$ and $R_T$ with temperature and an inversion temperature at or above room temperatures seem to be characteristic of strongly polar molecules. The troublesome "memory effects" of HCl in a mass spectrometer can be eliminated by using a pin-hole leak right in the box of the ion source plus a baking out, flushing and repeated-analysis routine. Some details are given of a 6.8-meter hot-wire thermal diffusion column apparatus, having an equilibrium separation factor of 7, and which has provided fairly large samples of HCl enriched to 94 percent H${\mathrm{Cl}}^{35}$ and 62 percent H${\mathrm{Cl}}^{37}$ for cyclotron beam targets and other experiments.