Distillation of hydrogen isotopes for polarized HD target

We have developed a cryogenic distillation system to purify Hydrogen-Deuteride (HD) gas for a polarized HD target in LEPS experiments at SPring-8. A small amount of ortho-H$_2$ ($\sim$0.01%) in the HD gas plays an important role in efficiently polarizing the HD target. Since there are 1$\sim$5% impurities of H$_2$ and D$_2$ in commercially available HD gases, it is inevitable that the HD gas is purified up to $\sim$99.99%. The distillation system has a cryogenic pot (17$\sim$21 K) containing many small stainless steel cells called Heli-pack. Commercial HD gas with an amount of 5.2 mol is fed into the pot. We carried out three distillation runs by changing temperatures (17.5 K and 20.5 K) and gas extraction speeds (1.3 ml/min and 5.2 ml/min). The extracted gas was analyzed by using a gas analyzer system combining a quadrupole mass spectrometer with a gas chromatograph. The HD gas of 1 mol with a purity better than 99.99% has been successfully obtained. The effective NTS (Number of Theoretical Stages), which is an indicator of the distillator performances, is obtained as 37.2$\pm$0.6. This value is in reasonable agreement with a designed value of 37.9. The HD target is expected to be efficiently polarized under a well-controlled condition by doping an optimal amount of ortho-H$_2$ to the purified HD gas.