Approximately 75 % of the primary electrons will be scattered when you use a working distance of 12 mm and a pressure of 3.5 torr, and a significant fraction of these scattered electrons will hit the sample farther away from the beam target than 50 pm. (The scattered intensity is approximately given by Is/ lo = 1 – exp(-ρσL/kT) where p is the pressure, σ the total scattering cross section for electron scattering on the gas used, L the distance between the last pressure limiting aperture and the sample, k the Boltzmann constant and T the absolute temperature).