hence the type of signal detected. The energetic BSE traverse a long distance, whereas the SE travel a much shorter lateral distance mainly by way of diffusion in the gas. Correspondingly, an electrode placed further away from the beam axis will have a predominantly BSE component in comparison to the predominant SE component collected by an electrode placed close to the axis. The precise proportion of signal mix and intensity depends on the additional parameters of gas nature and pressure in conjunction with electrode configurations and bias, bearing in mind that there is no abrupt physical distinction between SE and BSE, apart from the conventional definition of the 50 eV boundary between them. In another form, the GDD involves one or more electrodes as above but biased with a generally high voltage (e.g. 20–500 V). The processes involved are the same as in the low voltage case with the addition of an amplification of signal along the principle of a proportional amplifier as used in particle physics. That is, all slow electrons in the gas emanating either from the ionizing BSE or directly from the specimen (i.e. the SE) are multiplied in an avalanche form. The energy imparted on the traveling slow electrons by the external electrode field is sufficient to ionize the gas molecules through successive (cascade) collisions. The discharge is controlled in proportion by the applied electrode bias below the breakdown point. This form of detection is referred as ionization-GDD. Parallel to the ionization, there is also excitation of the gas in both cases above. The gaseous photons are produced both by BSE and SE both directly and by cascade avalanche with the ionization electrons. These photons are detected by appropriate means, like photo-multipliers. By positioning Light tubes strategically, using filters and other light optics means, the SE can again be separated from the BSE and corresponding images formed. This form of detection is referred as scintillation-GDD. SE distribution and separation The principles outlined above are best described by considering plane electrodes biased to form a uniform electric field, such as shown in the accompanying diagram of GDD-principle. The electron beam striking the specimen at the cathode effectively creates a point source of SE and BSE. The distribution of slow electrons emitted from a point source inside a gas acted upon by a uniform field is given from the equations (low field): with where R is the fraction