Sources and emission time scales in E/A=50 MeV ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$${+}^{\mathrm{nat}}$Cu reactions

Velocity distributions and fragment-fragment correlation functions have been measured as a function of charged particle multiplicity for ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$${+}^{\mathrm{nat}}$Cu collisions at E/A=50 MeV. With increasing multiplicity, the velocity distributions evolve from a pattern exhibiting targetlike and projectilelike sources to an anisotropic distribution centered near the projectile-target center-of-mass velocity. Emission times extracted from the correlation functions are consistent with the decay of a projectilelike source in low multiplicity collisions, but the time scale for fragment emission in the high multiplicity collisions is too short for a separation of sources to occur.