[en] The response of Pilot 425 to heavy ions with energies less than 600 MeV/amu (β approximately 0.8) is examined both theoretically and experimentally. Measurements are presented from an experiment which employed a ²⁰Ne beam at many energies below 575 MeV/amu. The signal is assumed to come from three sources: (1) Cherenkov light from the heavy ion; (2) Cherenkov light from secondary electrons; and (3) scintillation of the radiator. It is found that the effective index of refraction is 1.518 and that scintillation is present at a level of approximately 2.7% of the Cherenkov signal for β=1 for ²⁰Ne. The first of these values differs from values previously quoted in the literature. (Auth.)