Ultracold neutron upscattering rates in a molecular deuterium crystal

A calculation of ultracold neutron (UCN) upscattering rates in molecular deuterium solids has been carried out, taking into account intramolecular excitations and phonons. The different molecular species ortho-D₂ (with even rotational quantum number J) and para-D₂ (with odd J) exhibit significantly different UCN-phonon annihilation cross sections. Para- to ortho-D₂ conversion, furthermore, couples UCN to an energy bath of excited rotational states without mediating phonons. This anomalous upscattering mechanism restricts the UCN lifetime to 4.6 msec in a normal-D₂ solid with 33% para content.