Ultracold Neutron Sources: An Essentially Enhanced Optimal Operation Temperature for Ortho-Deuterium-Based Superthermal Converters

Abstract

There is a longstanding dogma in the physics of ultracold neutrons (UCN): The optimal temperature of ortho-deuterium based UCN-sources must not exceed $5\,\mathrm{K}$. This value has been established by the pioneering work of Golub et al. decades ago, utilizing previously established cross sections for solid ortho-deuterium. But recently, an improved calculation of the crucial 1-phonon up-scattering cross section - using the so-called corrected Incoherent Approximation - leads to a new upper limit for the optimal operation temperature of ortho-deuterium based UCN sources. After a discussion of the relevant cross sections of the \(\mathrm{D}_2\)-molecule we conclude that the optimal temperature lies between $10\,\mathrm{K}$ and $12\,\mathrm{K}$, a factor 2 higher than estimated earlier. This offers significantly relaxed cryogenic conditions for UCN experiments with solid ortho-deuterium and especially for the operation of ortho-deuterium based ultracold neutron sources.