  We have measured the Ca isotope effect in the newly discovered superconductor
CaC6. The isotope effect coefficient is 0.50(7). If one assumes that this
material is a conventional electron-phonon coupled superconductor, this result
shows that the superconductivity is dominated by coupling of the electrons by
Ca phonon modes and that C phonons contribute very little. Thus, in contrast to
MgB2, where phonons in the B layers are responsible for the superconductivity,
in CaC6 the phonons are primarily modes of the intercalated Ca.
