Scattering of ultracold neutrons in condensed deuterium and on material surfaces

Ultracold neutrons (UCNs) are a versatile tool for fundamental physics experiments, such as the exact determination of the free neutron lifetime or the search for a possible non-zero neutron electric dipole moment. For several decades, the ultracold neutron facility PF2 (also called "Turbine") at Institut Laue–Langevin, Grenoble, has been providing the highest density of ultracold neutrons worldwide. This density is a crucial parameter in any UCN storage experiment. Nevertheless, the accuracy of most experiments performed at PF2 has been limited by UCN counting statistics, which triggered the development of new UCN converters based on solid deuterium (sD${}_2$) across Europe and North America. Initial simulations and test measurements predicted a significant increase in UCN density compared to PF2. The actual UCN output of these new converters is lower than expected. This discrepancy can be explained by the scattering of UCNs from rough surfaces and from defects inside the sD${}_2$ crystal. Both were taken into account here quantitatively for the first time in UCN transmission experiments. Within this thesis, a sample container for cryogenic liquids and solids that addresses both these issues was conceived, constructed and tested. With the help of this sample container, the proper preparation of the sample can be verified optically before the neutron measurement is started. Its highly polished silica windows allow for growing a sample with a smooth surface, and thus reduce surface scattering as much a possible. Due to the high rigidity of silica compared to commonly used aluminum windows, the container does not bulge under pressure differentials of 1 to 2 bar. This sample container was used in the work on this thesis in time-of-flight (TOF) experiments and allowed to measure the total scattering cross sections of liquid and solid deuterium for ultracold neutrons. The new data for liquid deuterium overlap well with a theoretical model that has been published previously. The data on solid deuterium allowed for the first time to estimate the size and concentration of defects inside the crystal. These results will help gain a realistic understanding of the scattering processes inside sD${}_2$-based UCN converters. In addition, ultracoldneutron transmission experiments were performed on various metal foils, which proved conclusively the large effect that rough sample surfaces have on the transmission of ultracold neutrons. The results allow to draw conclusions as to how the mean free path ${\lambda }_{mfp}$ of UCNs in solid deuterium can be increased, which may be interesting to the operators of sD${}_2$-based UCN converters and may help to increase their extractable UCN density.