of SE that arrives at the anode inside radius r, V the potential difference between the electrodes placed at distance d, k is Boltzmann's constant, T the absolute gas temperature, e the electron charge and ε is the ratio of the thermal (agitation and kinetic) energy of the electrons divided by the thermal energy of the host gas; I is the corresponding current collected by the anode inside r, δ is the SE yield coefficient and Ib the incident electron beam current. This provides the spatial distribution of the initial electrons SE as they are acted upon by the uniform electric field that moves them from the cathode to the anode, while the electrons also diffuse away due to thermal collisions with the gas molecules. Plots are provided in the accompanying efficiency characteristics of the GDD, for a set of operating conditions of pressure p and distance d. We note that a 100% collection efficiency is fast approached within a small radius even at moderate field strength. At high bias, a nearly complete collection is achieved within a very small radius, a fact that has favorable design implications. The above radial distribution is valid also in the presence of formation of electron avalanches at high electric field, but it must be multiplied by an appropriate gain factor. In its simplest form for parallel electrodes, the gain factor is the exponential in the current equation: where α is the first Townsend coefficient. This gives the total signal amplification due to both electrons and ions. The spatial charge distribution and gain factor varies with electrode configuration and geometry and by additional discharge processes described in the referenced theory of the GDD. BSE distribution The BSE usually have energies in the kV range so that the much lower electrode bias has only a secondary effect on their trajectory. For the same reason, the finite number of collisions with the gas also results in a second order deflection from their trajectory they would have in vacuum. Therefore, their distribution is practically the same as has been worked out by SEM workers, the variation of which depends on the specimen surface properties (geometry and material composition). For a polished specimen surface the BSE distribution assumes a nearly cosine function but for a rough surface we may take it to be spherical (i.e. uniform in all directions). For brevity, the equations of the second case only