The temporal and spatial variations of the relative fluxes in a binary mixture how consisting of particles of different size but almost equal density are investigated using a dual photon gamma-ray transmission technique. Experiments are carried out using the dual photon gamma-ray technique in a mass flow hopper containing a binary mixture of 20% per weight turnip seeds (fine phase) and 80% per weight ABS particles coarse phase of size ratio 2.11:1 and of nearly equal densities. Measurements are taken in a "snap-shot" mode to identify the solids fraction of the individual components in the mixture and a statistical analysis is applied to the solids fraction values to calculate self-diffusion velocities for each of the individual components of the mixture. Significant phase separation is observed within zones of high bulk shear gradients such as close to the hopper walls and close to the hopper outlet. Large interstitial voidage changes observed during the initial transient stage of flow are also found to give rise to significant self-diffusion velocities.