Observation of ultracold-neutron production by 9-Å cold neutrons in superfluid helium

Using velocity-analyzed cold neutrons we observed the production of ultracold neutrons in superfluid $^{4}\mathrm{He}$ at 0.45 to 1.5 K. Ultracold neutrons are produced for an incident neutron wavelength of 8.78\ifmmode\pm\else\textpm\fi{}0.06 \AA{} which agrees with the single-phonon emission theory. The temperature variation of the ultracold-neutron storage lifetime is also discussed.