We present the first results for a new supersonic nozzle that decouples nucleation and droplet growth, and closely controls the supersaturation and temperature during nucleation. We characterize the expansions using pressure trace measurements, and the aerosol properties using light scattering and small angle neutron scattering. We show that when nucleation and droplet growth are separated, the aerosol number density decreases, the average particle size increases, and the aerosol can be more monodisperse than that formed in a conventional nozzle. Under these conditions, we can estimate the nucleation rate J as a function of supersaturation S and temperature T directly from the experimental data. For D2O we find that the nucleation rate is 4.3x10(15)less than or equal toJ/cm(-3) s(-)less than or equal to6.0x10(15) at 230.1less than or equal toT/Kless than or equal to230.4 and 29.2less than or equal toSless than or equal to32.4.