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

In a recent investigation of solid D2 as a UCN source[1], it was surmised that the yield of UCN is correlated with the spin states of the deuterium molecules. Subsequently, detailed neutron scattering calculations and measurements of the UCN lifetime in mixtures of paraand ortho-deuterium quantitatively verified this conjecture.[2,3,4,5,6] The work we report here was motivated by our attempt to control the spin state of deuterium and develop a superthermal ultracold neutron source capable of the extremely high UCN yields predicted for pure ortho-D2 crystals by Golub et al.[7,8] At least three solid deuterium sources are currently under construction.[4,9,10] Our superthermal solid D2 source at Los Alamos Neutron Science Center (LANSCE) can be envisioned as roughly one liter of solid deuterium held at temperatures below 10 K. This block of deuterium is then immersed in a cold neutron flux. UCN are produced when the cold neutrons are downscattered, losing almost all of their energy. Because the source is operated at very low temperatures, the loss rates due to thermal upscattering are very small for UCN and the produced UCN can be efficiently extracted from