Direct experimental verification of neutron acceleration by the material optical potential of solid 2H2

We have measured the acceleration of neutrons by the material optical potential of solid (2)H(2). Using a gravitational spectrometer, we find a minimal kinetic energy E(c)=(99 +/- 7) neV of neutrons from a superthermal ultracold neutron (UCN) source with solid (2)H(2) as an UCN converter. The result is in excellent agreement with theoretical predictions, E(c)=106 neV.