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±0.06 Å which agrees with the single-phonon emission theory. The temperature variation of the ultracold-neutron storage lifetime is also discussed.