Distillation of hydrogen isotopes for polarized HD targets

Abstract

We have developed a new cryogenic distillation system to purify Hydrogen-Deuteride (HD) gas for polarized HD targets in LEPS experiments at SPring-8. A small amount of ortho-H2 (∼0.01%) in the HD gas plays an important role in efficiently polarizing the HD target. Since there are 1-5% impurities of H2 and D2 in commercially available HD gases, it is necessary to purify the HD gas up to ∼99.99%. The distillation system is equipped with a cryogenic distillation unit filled with many small stainless steel cells called "Heli-pack". The distillation unit consists of a condenser part, a rectification part, and a reboiler part. The unit is kept at the temperature of 17-21 K. The Heli-pack has a large surface area that makes a good contact between gases and liquids. An amount of 5.2 mol of commercial HD gas is fed into the distillation unit. Three trials were carried out to purify the HD gas 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 using a gas analyzer system combining a quadrupole mass spectrometer with a gas chromatograph. One mol of HD gas with a purity better than 99.99% has been successfully obtained for the first time. The effective NTP (Number of Theoretical Plates), which is an indication of the distillation performances, is obtained to be 37.2±0.6. This value is in good agreement with a designed value of 37.9. The HD target is expected to be efficiently polarized under a well-controlled condition by adding an optimal amount of ortho-H2 to the purified HD gas

In-Line Calibration of Raman Systems for Analysis of Gas Mixtures of Hydrogen Isotopologues with Sub-Percent Accuracy

“…However, such a well-known mixture of the three gases is not easy to produce by mixing, since, as stated above, high purity HD gas is in general not commercially available and can only be produced with great efforts. 6 Therefore, the method discussed here is based on the catalytic production of HD from well-known amounts of H 2 and D 2 via the isotopic exchange reaction…”

“…The targets are made of high purity HD gas in which the residual H 2 and D 2 concentrations need to be known. 5,6 For these applications, Raman spectroscopy is recommended as the method of choice, being a noncontact, in-line, multispecies analysis technique with high sensitivity. 7 KATRIN already employs Raman spectroscopy successfully for the compositional in-line analysis of the source gas.…”

“…HD is not commercially available in high purity, and HD gas dissociates when kept in a container for a long time. 6 Thus, the heteronuclear isotopologue needs to be produced in a controlled manner within the mixing device. Our device, the HYdrogen Deuterium Equilibration loop (HYDE), is inspired by a technique used for the characterization of a gas chromatograph by Kawamura et al 15 In our gas mixing device, two instead of one mixing vessel are installed and the whole system is designed as a loop rather than a "once-through" line.…”

HD gas purification for polarized HDice targets production at Jefferson Lab

“…Because the vapor pressure differences are on the order of 10-15% even at temperatures between the triple point and the boiling point, a packed distillation column is used to enhance the separation. The column, approximately 5 cm in diameter, is filled with the commercial packing material, heli-pak [6] to increase its effective surface area. This permits repeated condensations and evaporations, each contributing another enhancement of ≈ 10% to the isotopic separation resulting in approximately an order of magnitude enhancement in the concentration.…”

Polarimetry of the polarized hydrogen deuteride HDice target under an electron beam

“…The purification is done by distillation [35] [36] and enables the extraction of H 2 and D 2 from the HD mixture because the vapor pressures of H 2 ,D 2 and HD differ at low temperature.…”