are given below. In vacuum, the current distribution from BSE on the electrode is given by where η is the BSE yield coefficient. In the presence of gas at low electric field the corresponding equations become: where S is the ionization coefficient of the gas and p its pressure Finally, for a high electric field we get For practical purposes, the BSE predominantly fall outside the volume acted upon by predominantly the SE, while there is an intermediate volume of comparable fraction of the two signals. The interplay of the various parameters involved has been studied in the main, but it also constitutes a new field for further research and development, especially as we move outside the plane electrode geometry. Electron and ion induction Prior to practical implementations, it is helpful to consider a more esoteric aspect (principle), namely, the fundamental physical process taking place in the GDD. The signal in the external circuit is a displacement current i created by induction of charge on the electrodes by a moving charge e with velocity υ in the space between them: At the point in time when the charge arrives at the electrode, there is no current flowing in the circuit since υ=0, only when the charge is in motion between the electrodes do we have a signal current. This is important in the case, for example, when a new electron-ion pair is generated at any point in the space between anode-cathode, say at x distance from the anode. Then, only a fraction ex/d of charge is induced by the electron during its transit to the anode, whilst the remainder fraction of e(d–x)/d charge is induced by the ion during its transit to the cathode. Addition of those two fractions gives a charge equal to the charge of one electron. Thus by counting the electrons arriving at the anode or the ions at the cathode we derive the same figure in current measurement. However, since the electrons have a drift velocity about three orders of magnitude greater (in nanosecond range) than the ions, the induced signal may be separated in two components of different significance when the ion transit time may become greater than the pixel time on the scanned image. The GDD has thus two inherent time-constants, a very short one due to the electrons and a longer one due to the ions. When the ion transit time is greater