Low-temperature catalysis of para-deuterium to ortho-deuterium conversion on Grafoil.

Abstract

Para-deuterium to ortho-deuterium conversion rates were determined on Grafoil in the temperature range of 10 to 27 K. Above 10 K the measured conversion rates in the two-dimensional (2D) solid regimes are almost independent of temperature, with a gradual slow-down occurring at higher temperatures in ``2D gas'' and ``2D fluid'' phases. The observed weak temperature dependence as well as the magnitude of the conversion rate are consistent with a recent theoretical analysis of low-temperature ortho-para conversion via emission of 2D solid-${\mathrm{D}}_{2}$ phonon(s). Measurements in the ``2D gas'' phase suggest that this phase is a lattice gas. This is supported by a calculation of the ratio of the conversion rate in a lattice gas to that in a Debye solid, which is in reasonable agreement with our measured ratio. Application of our results to catalytic purification of ortho-${\mathrm{D}}_{2}$ is discussed in terms of ortho-${\mathrm{D}}_{2}$ purity requirements for metastably nuclear-spin-polarized solid ${\mathrm{D}}_{2}$, useful as a nuclear fusion fuel or a polarized target for particle-physics experiments.