Accumulation and extraction of ultracold neutrons from a superfluid   helium converter coated with fluorinated grease
  We report experiments on the production of ultracold neutrons (UCN) in a converter of superfluid helium coated with fluorinated grease. We employed our technique of window-free extraction of accumulated UCN from the helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. The time constant for UCN passage through the same extraction aperture as in a previous experiment was a factor two shorter, despite a lower mean velocity of the accumulated UCN in the present experiments. A time-of-flight measurement of the cold neutron spectrum incident on the converter allowed us to estimate the multi-phonon contribution to the UCN production. The UCN production rate inferred from two methods agrees with the theoretical expectation. 
O. Zimmer, P. Schmidt-Wellenburg, M. Assmann, M. Fertl, J. Klenke, S.   Mironov, H.-F. Wirth, B. van den Brandt