An apparatus to control and monitor the para- D₂ concentration in a solid deuterium, superthermal source of ultra-cold neutrons

Controlling and measuring the concentration of para-D₂ is an essential step toward realizing solid deuterium as an intense ultra-cold neutron source. To this end, we implemented an experimental technique to convert para- to ortho-deuterium molecules by flowing D₂ gas through a cryogenic cell filled with paramagnetic hydrous ferric oxide granules. This process efficiently reduced the para-D₂ concentration from 33.3% to 1.5%. Rotational Raman spectroscopy was applied to measure the residual para-D₂ contamination to better than 2 parts in 10³, and the hydrogen contamination to 1 part in 10³. We also contrast our optical technique to conventional thermal conductivity measurements of the para-D₂ concentration, reporting some of the relevant strengths and weaknesses of our implementation of each technique.