Ultracold neutron upscattering rates in a molecular deuterium crystal

A calculation of ultracold neutron (UCN) upscattering rates in molecular de uterium solids has been carried out, taking into account intramolecular exc itations and phonons. The different molecular species ortho-D-2 (with even rotational quantum number J) and para-D-2, (with odd J) exhibit significant ly different UCN-phonon annihilation cross sections. Para- to ortho-D-2 con version, furthermore, couples UCN to an energy bath of excited rotational s tates without mediating phonons. This anomalous upscattering mechanism rest ricts the UCN lifetime to 4.6 msec in a normal-D-2 solid with 33% para cont ent.