Small Angle Neutron Scattering from Nanodroplet Aerosols

We report the first measurements of small angle neutron scattering from an aerosol. The aerosol was produced by expanding a ${\mathrm{D}}_2\mathrm{O}$- ${\mathrm{N}}_2$ vapor mixture in a supersonic Laval nozzle. The neutron wavelength (0.5 nm) is less than the typical particle size, and we can therefore derive the average particle size (5–8 nm), number density $\left(\sim {10}^{12}{\mathrm{cm}}^{-3\mathrm{}}\right)$, and polydispersity of the size distribution directly from the experimental data rather than by inferring them from complex models of particle formation and growth. We also predict and observe a Doppler shift-induced anisotropy in the scattering pattern due to the directed motion of the aerosol in the nozzle. Further applications of this new technique are discussed.