We measure the decoherence of a spatially separated atomic superposition du
e to spontaneous photon scattering. We observe a qualitative change in deco
herence versus separation as the number of scattered photons increases, and
verify quantitatively the decoherence rate constant in the many-photon lim
it. Our results illustrate an evolution of decoherence consistent with gene
ral models developed for a broad class of decoherence phenomena.