NERVA irradiation program. GTR test 21. Volume II. Nuclear-radiation-induced conversion of parahydrogen to orthohydrogen

NIST liquid hydrogen cold source

“…This would imply that the optimum thickness could be quite large. However, there is a great deal of uncertainty about the actual state, with contradictory evidence 6.7 and most designers have assumed that the actual state is an equal mixture of ortho and para hydrogen 8 . Our studies indicate that the optimum thickness is approximately 30 mm, so long as the ortho content is greater than 40 % to 50 %, and the close agreement of our measured and calculated cold neutron intensities and energy dependence seems to confirm that the concentration is closer to normal (75 % ortho) than to para for our source.…”