Temperature Dependence Of Ultracold Neutron Loss Rates

The discrepancy between measured ultracold neutron (UCN) loss rates and those predicted from pure materials has been the subject of study for over 30 years. Nevertheless, the data about UCN upscattering to the meV range, which is the main cause of UCN losses over a wide temperature range, are still contradictory and rather poor, especially at low temperatures. The low-temperature behavior of the upscattering loss rate is crucial for distinguishing between different models of this process. Here we report a study of UCN upscattering in a well controlled sample environment (ultrahigh vacuum) and at temperatures down to 4 K. We studied UCN interaction with chemically bound hydrogen. In our data interpretation we used data for the surface chemical content of our storage bottle that were measured with elastic recoil detection analysis as well as phonon data from neutron inelastic scattering. The complex analysis allows us to rule out the model of sub-barrier upscattering inside the bulk material and demonstrate a good agreement between experimental data and a "1/v" model consisting of a thin hydrogenous film or clusters with low Fermi potential. The phonon spectrum of ice can easily explain the observed low-temperature losses. Finally, we discusse possible application of UCN upscattering to condensed matter study.