Physics:Diffusion in gases
During the drift in electric fields, charged particles diffuse according to a Gaussian distribution
It is convenient to define a reduced drift velocity, the mobility at atmospheric pressure
From classical arguments it can be shown that the diffusion coefficient is given by the Nernst-Einstein relation
|height12pt width0pt Gas||[cm]||D [cm2/s]||[cm2 sec 53x28px|
|height12pt width0pt H2||70x28px||0.34||13.0|
For electrons, the neutralization by ions and the attachment by molecules with electron affinity must be considered. Except for very low fields the mobility of electrons is not a constant; the mean free path varies in some gases with the electric field (Ramsauer effect), all resulting in a diffusion coefficient dependent on the electric field.
Note that the limiting accuracy is not given by the standard deviation from ft(x), but depends on the number of electrons necessary to trigger the shift-line electronics. If n electrons are produced and k electrons are needed to overcome the electronics threshold, the following formula holds: