Diamondlike carbon can replace beryllium in physics with ultracold neutrons

To complete our study of ultracold neutron (UCN) storage-vessel coatings, we have measured the Fermi potential for neutrons on diamondlike carbon coatings produced by laser induced vacuum arc deposition. A sample with an sp3 content of 0.45, measured using, for the first time, neutron transmission had a Fermi potential of (249±14)neV. A second sample with an sp3 fraction of 0.67, measured using cold neutron reflectometry, gave (271±13)neV. These values complete the demonstration that there is a viable alternative to Be in UCN physics.