Measuring the Transmission of Ultracold Neutrons Through Deuterium and Hydrogen – Challenges and Latest Results

The latest scattering cross section data (σ^scatt) for ultracold neutrons (UCN) in liquid and solid deuterium and, depending on the outcome of ongoing experiments, in hydrogen will be presented. The deuterium cross sections are pivotal to the design and improvement of new UCN sources which promise to provide higher UCN densities than the current frontrunner, the “turbine” at ILL. Higher UCN densities will greatly benefit nuclear and particle physics experiments such as the exact determination of the free neutron lifetime and the search for a possible non-zero electrical dipole moment of the neutron.

We measure total cross sections in transmission geometry, since we are convinced this delivers the best results. However, our data are not free from side effects like surface scattering, which have to be corrected for. I will explain these effects and their treatment in the presentation.