  We measure the decoherence of a spatially separated atomic superposition due
to spontaneous photon scattering. We observe a qualitative change in
decoherence versus separation as the number of scattered photons increases, and
verify quantitatively the decoherence rate constant in the many-photon limit.
Our results illustrate an evolution of decoherence consistent with general
models developed for a broad class of decoherence phenomenon.
