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High Bias GDD-Contrast Mechanisms (1986)

The gaseous detection device (GDD) yields variable contrast with variation of gas pressure and electrode configuration, also at high electrode bias, i.e. in the order of a few hundred Volts. This creates a gaseous avalanche amplification of all ionizing signals.  The contrast principles are the same as with low electrode bias but the relative and absolute values of signal distribution and gaseous ionization takes place over a different scale. The SE are confined much closer to the optical axis whilst the crossover to BSE contrast occurs at higher pressure. Furthermore, the signal-to-noise ratio and the frequency response of the detector is greatly enhanced.  An extensive study of the distribution of charge in the gas is presented by (38).  This has been shown also by experimental work using concentric electrodes under varying conditions of pressure.  Thus, the same field of view shown below is an example of gradual contrast inversion by the interplay of the system variables. For an explanation of the imaging mechanisms involved reference should be made to paper (37).  The full possibilities and benefits of this type of image formation are yet to be understood and implemented by the commercial world.

Detector electrode configuration used(37) .


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