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Low Bias GDD and CL (1986)

The ESEM with the gaseous detection device (GDD) yield information never seen before. For example, the contrast below emanates from the same surface area of a mineral in different detection modes (11, 12, 36 ,  26). It is not always necessary to apply a high voltage on the GDD electrode, as low bias, of the order of 10 Volts, is also possible to use and still obtain a variety of contrasts.  For example, strong BSE contrast is evident as we increase the gas pressure, since the BSE have an amplification factor arising from their own high energy creating electron-ion pairs.  The secondary electron signal predominates at the lower pressure, where the BSE escape through the gas without many collisions.  The overall contrast is determined by the interplay of gas pressure, electrode bias and electron position and configuration as it was further demonstrated elsewhere (34 ,36, 37, 38).  Low electrode bias results in strong BSE imaging by the high yield of initial gaseous ionization by the signal electrons at very high pressure towards atmospheric levels. However, the low voltage operation of the GDD providing some special/additional benefits in the imaging arsenal of the technology, has not been exploited by commercial ESEM yet.